Evaluation of high temperature gas cleaning options for biomass gasification product gas for Solid Oxide Fuel Cells

Progress in Energy and Combustion Science 38 (2012) 737e764

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Review

Evaluation of high temperature gas cleaning options for biomass gasification product gas for Solid Oxide Fuel Cells P.V. Aravind*, Wiebren de Jong Process & Energy Department, 3mE Faculty, Delft University of Technology, Leeghwaterstraat 44, 2628 CA, Delft, The Netherlands

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 March 2011 Accepted 15 March 2012 Available online 3 July 2012

An analysis of high temperature gas cleaning systems for cleaning the product gas of biomass gasification for fueling solid oxide fuel cells (SOFCs) is presented. Influence of biomass derived contaminants on SOFCs is briefly presented and the removal of potential contaminants such as tar, particulates, H2S and HCl, alkali compounds from biosyngas is reviewed. It appears that the gasification product gas can be cleaned to meet the requirements of SOFCs based on Ni/GDC anodes at high temperatures (typically in the range of 1023e1223 K) by using currently known gas cleaning methods. Although information from literature, results from chemical equilibrium studies and preliminary experiments were sufficient to put forward a conceptual design for a high temperature gas cleaning system, detailed experimental investigations are still required. This is needed to obtain detailed information on contaminant tolerance of SOFCs, and to arrive at detailed designs of gas cleaning units that are economically viable for biomass gasifier-SOFC systems. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: SOFC Biomass Gasifier Hot gas cleaning

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .738 1.1. Biomass gasification derived product gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739 Biomass derived contaminants, their influence on SOFC performance and cleaning options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .739 2.1. Particulate matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740 2.1.1. The influence of particles in biosyngas on SOFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740 2.1.2. High temperature particle cleaning options for SOFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741 2.1.3. High temperature particle filtration and a discussion of the system conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743 2.2. Tars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743 2.2.1. The influence of tars on SOFC operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744 2.2.2. Tar removal options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 2.2.2.1. Non-catalytic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 2.2.2.2. Catalytic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745 2.2.2.2.1. In-bed addition of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746 2.2.2.2.2. Downstream catalyst beds and monoliths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748 2.2.2.3. Discussion of the choice of tar cleaning for SOFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751 2.3. Sulfur compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751 2.3.1. The influence of H2S on SOFC anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752 2.3.2. Cleaning of hydrogen sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753 2.3.3. Conclusions regarding the choice for H2S cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754 2.4. Hydrogen chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 2.4.1. The influence of HCl on SOFC performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 2.4.2. Cleaning of hydrogen chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755 2.4.3. Conclusions regarding the choice of sorbent for HCl cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755

* Corresponding author. Tel.: þ31 15 2783550; fax: þ31 15 2782460. E-mail address: [email protected] (P.V. Aravind). 0360-1285/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2012.03.006

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2.5.

3. 4.

Alkali metal compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756 2.5.1. The influence of alkali compounds on SOFC performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756 2.5.2. Alkali cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756 2.5.3. Conclusion regarding the choice for alkali cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758 2.6. Nitrogen-containing contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758 2.7. Other contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758 Summary of the results and a suggested configuration for a cleaning system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .760 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760

1. Introduction Solid Oxide Fuel Cells (SOFC) are power producing devices that electrochemically oxidize gaseous fuels with very high electrical efficiency. They have higher fuel flexibility than other types of fuel cells such as the Polymer Electrolyte Membrane Fuel cell (PEMFC), Phosphoric Acid Fuel Cell (PAFC) [1] and probably Molten Carbonate Fuel Cells (MCFC). While low temperature fuel cells such as PEMFCs efficiently operate mainly with hydrogen as fuel, hydrogen, carbon monoxide, methane and their mixtures are considered as good fuels for high temperature fuel cells such as SOFCs. SOFCs operate at high temperature when compared to other types of fuel cells. SOFCs are also often considered to be relatively tolerant to several fuel gas impurities, when compared to other fuel cell types. A simplified picture of the working principle is shown in Fig. 1. In the SOFC, the fuel enters the anode chamber and is oxidised. Oxygen enters the cathode chamber and is ionized and transported through the electrolyte to the anode. The anode of the fuel cell disperses the fuel gas over its interface with the electrolyte, catalyzes the electrochemical reactions, and conducts the electrons that are freed from the fuel molecules. These electrons flow through an external circuit producing electric power. The cathode of the fuel cell distributes the oxygen at its interface with the solid electrolyte, and conducts the electrons from the external circuit where they reduce the oxygen molecules, producing oxide ions. Oxide ions diffuse through the solid electrolyte to the anode to form H2O or CO2, depending on the type of fuel. The solid electrolyte contains

Electrical Energy e-

Fuel outlet H2 , H2 O, CO, CO 2

Air outlet

←

→ O2

O 2-

H 2 , CO Fuel inlet

← O2

→ anode

electrolyte

cathode

Fig. 1. Working principle of an SOFC.

Air inlet

oxygen vacancies, which allows oxygen ions to diffuse. The solid electrolyte also prevents the two electrodes to come into electronic contact. Electrolytes also play an important role in determining the operating temperature of the fuel cell. Biomass is a versatile, renewable, widely available and potentially sustainable energy source. It is also practically carbon neutral. Thermo-chemical gasification of this abundantly available fuel generates a product gas which can be used in a variety of prime movers, including fuel cells [2,3]. This product gas is sometimes called (raw) biosyngas. It consists of a mixture of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), hydrogen (H2), nitrogen (N2), water vapor (H2O), and minor impurities. The composition of this gas varies with the characteristics of the gasification process such as the reactor type, the gasification media used (e.g., air, oxygen, or steam) and different gasifier operation parameters such as the gasification stoichiometry. As the biomass derived product gas contains the above mentioned major components, it can be a good fuel for SOFCs provided that the gas is sufficiently cleaned. Gasifier-SOFC systems are expected to show very high efficiencies even at low power levels of a few hundred kilowatts, when compared to competing systems such as gasifier-IC engine systems. Efficiencies can be significantly enhanced if Gas Turbines (GT) are connected downstream from SOFCs because high quality heat is produced in SOFCs and the fuel utilization is significantly less than 100%. GasifierSOFC-GT systems are expected to have electric efficiencies around 50e60% even at low power levels, see Aravind et al. [4]. The following are the main impurities contained in biomass gasification product gas: 1) 2) 3) 4) 5) 6)

Particulate matter; Tar (aromatic hydrocarbon species); Sulfur compounds; HCl; Alkali metal species; Nitrogen containing compounds, especially ammonia.

The presence of contaminants in such gases will also vary when different types of biomass are used, furthermore when different gasification agents are used, when conversion stoichiometry is varied and finally when different gasification techniques are employed. These contaminants should be removed to a certain level, so that the product gas can be used as a fuel for SOFCs [5,6]. While the gas can be cleaned at high temperature or at low temperatures (near ambient temperatures), high temperature gas cleaning is expected to help to yield higher efficiencies. It has to be noted that biomass gasification and coal gasification have many similarities in terms of gas composition, and know-how developed for biomass gasifier-SOFC systems can be applied to a certain extent to coal gasifier-SOFC systems. However, this paper considers only biomass derived fuel gas.

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In an SOFC, the fuel gas passes through the anode compartment. SOFCs are being developed with many different types of anodes with different structures and materials. These alternative types of SOFCs will show diverse kinds of interactions with the contaminants. Hence, the tolerance levels for these contaminants may vary considerably based upon SOFC anodes that are used to oxidize biomass derived product gas. For this reason, the choices of the gas cleaning systems will depend on the types of SOFCs and the product gases from the different gasification systems being considered. Proper selection of the SOFCs with suitable anodes and gas cleaning systems and the appropriate operation parameters for them will be of critical significance in the development of feasible and efficient designs of biomass gasifier-SOFC systems. However, SOFC tolerance towards several trace constituents of biomass gasification product gas has not been studied well yet. Hence any discussion on the selection of suitable anodes will be rather preliminary and requires further detailed research. Conventional SOFC anodes contain substantial amounts of nickel. Nickel is widely considered as an attractive material for SOFC anodes, because it is a good electronic conductor and is catalytically active for the electrochemical oxidation of hydrogen. The nickel is supported on an ion-conducting material, most commonly yttria stabilized zirconia (YSZ). Doped ceria, like Gadolinia Doped Ceria (GDC) is a mixed conductor under reducing conditions. It offers an increased surface area for electrochemical reactions, unlike materials that offer only ionic conductivity, which limits the reactions to fuel/electrode/electrolyte boundaries. Ceria based anodes are also are also expected to result in better performance with fuels containing hydrocarbons [7]. Moreover, SOFCs with Ni/GDC anodes have been successfully tested with biomass derived product gas. While Ni/GDC anodes are already used in industrially produced SOFC systems, Samaria Doped Ceria (SDC) based SOFC anodes are also becoming increasingly popular among SOFC scientists and developers [8]. However, they are yet to be matured sufficiently for industrial production and their contaminant tolerance levels have not yet been studied in any detail and hence are not considered in this study. This paper presents a brief review of the influence of biosyngas derived contaminants on SOFCs, mainly the ones with Ni/YSZ and Ni/GDC anodes as they have been thoroughly studied and industrially produced. This paper also offers a detailed evaluation of the cleaning options for such a gas with an example of the product gas derived from airblown circulating fluidized bed (CFB) gasification for feeding to SOFCs. While the influence of the contaminants on the anode materials were studied, their influence on other parts of the systems, to be made of conventional materials such as stainless steel are not addressed in this manuscript. This is done mainly due to the following reasons; 1) Gas cleaning is expected to be stringent for SOFCs when compared to other conventional systems and contaminants present at very low levels may not cause serious problems for conventional materials, and 2) the improvement of the contaminant tolerance of conventional materials is being addressed by a wider scientific community for a variety of applications and the intention of the authors with this paper is to present the very recent and highly interesting developments with biomass gasifier-SOFC systems and gas cleaning for the same. 1.1. Biomass gasification derived product gas The composition and properties of the product gas generated in the gasifier will vary according to the technology used. Air is the commonly used gasifying agent, whereas oxygen and steam are the other two; in some processes CO2 is used [9]. As the technology is foreseen for small to medium scale of thermal input operation in

739

the near future, both fixed-bed gasifier and fluidized bed gasifier concepts are considered in this paper. Entrained flow reactors are aimed at much larger capacities and are not easily downscaled efficiently. Moreover, the limited experimental research on coupling biomass gasifiers with SOFCs has so far been carried out with the above two types of gasifiers. There are a few examples in the literature so far of biomass gasifiers coupled with SOFCs. One example is the Fast Internally Circulating Fluidized Bed (FICFB) gasification e CHP unit of Güssing in Austria, which was connected to SOFC test equipment with hot gas particulate cleanup and with an intermediate temperature sulphur removal [10,11]. In the EU Framework 6 ‘Biocellus’ project [12] more gasifier types were connected with SOFCs for testing. These are a small-scale updraft wood gasifier at the Paul Scherrer Institute in Switzerland [13], a heat pipe reformer pilot unit at Technical University Munich [14,15], a larger two-stage downdraft ‘Viking’ gasifier at the Danish Technical University (DTU) [16] using a low temperature gas cleaning configuration and the Delft 100 kWth circulating fluidized bed gasifier (CFBG) [17]. The last mentioned gasifier consists of a main gasification reactor (including a riser and a downcomer), a cyclone and a hot gas ceramic filter. Air, steam, oxygen, or a mix of any of these gases can be used for gasification. Details of the process are reported by Siedlecki et al. [18]. Fig. 2 presents a typical product gas composition (major gas constituents) that is representative of this gasifier when air is used as the gasification agent. For a detailed comparison with gas compositions from other gasification facilities, please see Munasinghe and Khanal [19]. The gas cleaning system proposed in this paper is expected to work with a fuel gas of this typical composition after having evaluated the cleaning requirements per component. This gas composition is obtained from the CFBG with clean wood as the feedstock and with the gasification process running at atmospheric pressure. SOFC anode performance results using various synthetic gas compositions that represent biomass derived product gases from different gasifiers show that, when the gases are completely clean i.e. free from harmful contaminants, will present no insurmountable problem for SOFCs [20e22]. However, cleaning the real biomass derived gas to the required level for long-term operation is a topic that still needs further research. 2. Biomass derived contaminants, their influence on SOFC performance and cleaning options Biomass gasification derived product gas can be cleaned after cooling to ambient temperatures or at high temperatures near the gasifier outlet temperature, which is usually in the range of

10.99%

14.00% 54.39% 5.24%

2.14% 0.65%

12.59%

Fig. 2. Typical main gas composition of airblown CFB gasification.

CO CO2 CH4 C2H4 H2 H2O N2

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973e1173 K. Since the SOFC inlet temperatures are often approximately 1123e1173 K, employing a high-temperature gas cleaning system will avoid the cooling and subsequent reheating of the gas, which is required if low-temperature cleaning systems are employed. High-temperature gas cleaning potentially increases the efficiency of the system depending on the system design, because heat loss during the cooling of the gas is avoided. With the help of chemical equilibrium calculations it has been shown that the tendency for carbon deposition increases when the gas temperature is reduced from the typical gasifier outlet temperatures [4,23]. High-temperature cleaning will help to avoid such issues. On the negative side, high temperature gas cleaning is considered to be expensive and still less reliable, compared to low temperature cleaning systems. Therefore further research and development efforts are required for developing reliable and economically viable high temperature gas cleaning units. Selection of a proper gasification system will help to reduce the concentration of contaminants from the gasifier to a certain extent. However, to meet the requirements of SOFCs, it is expected that substantial downstream gas cleaning is necessary. Gas cleanup systems may consist of several components such as cyclones, scrubbers or filters. Specific contaminants in biomass gasified product gas, their possible influence on SOFC operation and the technologies for removing them to meet the requirements of SOFCs, are discussed in the following sections.

2.1. Particulate matter Solid particles belong to category of contaminants that were the first to undergo measures for reducing the emissions. Particulate matter is always present in the raw biomass derived product gas generated by gasifiers. The size of the particulates present in such a gas could range from a few microns to sub-micron levels. Particulates generally include the inorganic material derived from mineral constituents in the biomass feedstock, unconverted biomass in the form of (attrited) char, soot, and attrited bed inventory material from the gasifier bed, if there are bed materials or additives employed in the gasification process [3]. Total mineral concentrations in clean wood are up to around 1e2 wt%. On the other hand, agricultural crop residues, such as straw or rice hulls, typically contain up to 10e20 wt% inorganic material. Mineral matter from other sources, such as silica from soil that is mixed with the biomass during its handling quite often contributes to the ash content in the product gas. Char entrained in the product gas, in addition to the problems it causes as particulates in the stream, also lowers the conversion efficiencies in gasification systems. Largescale gasifiers can attain carbon conversion efficiencies of 98e99%, leaving approximately 1e2% of the carbon bound in the feedstock as solids in the product gas. Collection of this material and subsequent re-injection of the char into the gasifier can increase overall gasification efficiencies. Mineral matter present in biosyngas can abrade and damage downstream equipment and could cause problems for SOFC operation. The organic fraction can clog the system or form deposits that reduce the cell performance. Particulate matter composition varies with temperature. Many of these components, which may appear in gaseous form at high temperatures, may become liquid or solidify and appear as aerosols/particles at slightly lower temperatures. Alkali compounds are very good examples of this. They are present as gaseous components above 873e973 K and will condense at lower temperatures. Solid carbon, which may separate out from the gasification product gas during cooling if conditions are thermodynamically favorable, is another example. Due to these reasons, discussions about particulates in biomass derived gasification product gas have to be

specific and should always include the temperature ranges considered. A detailed analysis carried out on the particles obtained from biomass gasification product gas generated by a two-stage gasifier after cooling the gas to 313 K by Hindsgaul et al. [24] provides indications about the nature of the particles. The researchers have observed that the major part (85%) of the total particulate mass collected before the plant’s cleaning system consists of primary soot particles with a diameter of less than 70 nm. Ash particles with diameters up to 3.6 micron were observed using a scanning Electron Microscope (approximately 3% of the sample mass). Less than 1% of the particles had aerodynamic diameters larger than 1.1 micron. However, the composition of the particulate matter will vary significantly with temperature, and at common gasifier outlet conditions of approximately 1073 K, a significant fraction of these particulates are expected to be gas-phase compounds. Fluidized-bed gasifiers produce high particulate counts in the product gases because they often carry bed material and the solid particles are continuously in a state of motion supported by the fluid flow. Fluidized-bed gasification systems include cyclones to separate the bed material from the product gases before the gas leaves the gasification system. In this case the cyclone mainly serves as the initial particulate removal technology and removes the bulk of the coarse particulates. However, finer particulates of a few microns or sub-micron sizes are seldom separated in cyclones [3] and will remain in the gas stream. Usually, some of the larger diameter particulates also escape from cyclones. These materials, if not separated by further cleaning processes, can create operational problems for downstream equipment. The particle size distribution of the particulate matter in the product gas of the 100 kWth Delft test CFBG downstream a cyclone is given in Fig. 3. The concentration of ash in the product gas is dependent on the reactor design, the gasifier operation parameters, the mineral content of the biomass feedstock, and the temperature ranges involved. Its impact and cleaning options are dealt with in the following section. 2.1.1. The influence of particles in biosyngas on SOFCs The particle sizes mentioned match well with the pore sizes of SOFC anodes which range from sub-micron levels to a few microns [26]. Hence, it can be expected that, if the particles are in solid form or are tiny droplets at SOFC operation temperatures, they can block the micro pores of the anodes and negatively affect the SOFC

Fig. 3. Example of particle size distribution for different biomass fuels during steam/ oxygen-blown gasification downstream the cyclone of a CFB gasifier [25].

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performance. Fig. 4 shows an example of SOFC anode microstructure. Moreover, it has been reported that significant quantities of the particles at room temperature are carbonaceous compounds and that most of the sub-micron particles are carbon-rich compounds, which contain little ash. This provides the possibility that these hydrocarbon compounds are being oxidized on the anode surface. Influence of the particles on SOFC performance is scarcely available in detail in the literature. It is, therefore, not clear beforehand how such particles can influence the cell performance. However, several of the experiments and results described in Section 2.2 show successful SOFC operation when the SOFC is fed with cleaned real gasification product gas [16,17,27e29]. These experiments employed currently available particle filtration technologies, such as ceramic or sintered metal filters. Hofman et al. reported the effect of particulates from biomass gasifiers on SOFCs [27]. The aim of this work was to experimentally assess the feasibility of feeding real biomass product gas to SOFCs for efficient and clean power production. Planar SOFC membranes were operated at two gasification sites. In all experiments, the gas was hot-cleaned to remove particulates, HCl, and H2S. SOFCs with Ni/GDC anodes were used. When particle filtration was not sufficient, severe contamination of the Ni-GDC anode was reported because the gas containing ash and char particulates reached the SOFC anode. This contamination was only apparent near the fuel inlet area because the experiment was stopped early to avoid further contamination of the gas cleaning unit and the SOFC test rig gas lines. There is no solid information available, as of now, on the exact tolerance levels of SOFCs to particulates in biosyngas [30]. However, it is likely that, particulates must be removed to the minimum possible levels, even down to a few ppmw levels, for smooth and long-term SOFC operation on biomass gasification derived product gas. 2.1.2. High temperature particle cleaning options for SOFCs Cyclones: Cyclones have been used for quite some time and have been further perfected with time. They use centrifugal force to separate solids from the gas. Cyclones are used quite often as an initial gas cleanup device. They are reasonably effective and are inexpensive to build and operate. In (circulating) fluidized bed

Fig. 4. Pores in the anode active layer of an SOFC. Left to the anode layer is the current collection layer.

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gasifiers, cyclones are an integral part of the reactor design to separate the bed material and other particulates from the gas stream. Cyclones are effective in removing larger particles and can operate within a wide range of temperatures. They can remove more than 90% of particulates larger than approximately 5 mm in diameter at minimal pressure drops of 0.01 atm [3]. Partial removal of particulates in the 1e5 mm range is also possible. They are not effective with sub-micron particles. Cyclones also remove condensed tars and alkali material from the gas stream, if they are operated at lower temperatures. However, vapor forms of those constituents remain in the gas stream and their separation is highly dependent on the gas temperature. Issues that reduce cyclone efficiency are illustrated in Fig. 5. An overview of the state-of-the art in cyclone design and modeling is given by Cortes and Gil [31]. Multi-stage cyclones may reduce particle concentrations to a range acceptable for gas turbine operation. However, low efficiency for sub-micron particle removal is (still) their main drawback [33]. Therefore, cyclone usage upstream of highly sophisticated systems like fuel cells requires further removal of the particulates from the outlet stream. One way to improve the performance of a cyclone is to include a cylindrical rotating metal gauze part in it; this equipment is called a Rotating Particle Separator (RPS) [34,35]. RPS equipment is claimed to be functional at temperatures up to 773 K. It has applications in small and medium sized coal and wood combustion facilities as well as gasification installations [36]. Tests were performed under gasification conditions at ECN (The Netherlands) downstream of a CFB gasifier operated at 1123 K. However, the operation was at a relatively low temperature and the filter was flushed with water for regeneration. Tar clogging was observed in the RPS [37]. It is doubtful whether this technology is the most optimal for particulate matter cleanup because SOFCs work at higher temperature. Electrostatic filters: Electrostatic filters have also been used extensively in a variety of gas-cleaning operations. They employ an electric field to separate the particles once they are ionized during their passage between the electrodes with a very high potential

Fig. 5. In-cyclone effects reducing the dust capture efficiency [32] (figure reproduced with permission).

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difference between them. The electrically charged particulates migrate to the collector plate and are deposited on the surface. Particulates are removed from the scrubber plates by either washing or by mechanical means. Dry scrubbers with mechanical dust removers can operate at temperatures of 773 K or more. Wet scrubbers remove the caught particulates with flowing water and are limited to temperatures of about 338 K [3]. These devices have comparatively expensive investment and operational costs, making them less attractive for small-scale combined heat and power (CHP) applications. Barrier filters: Filtration can be defined as the process of separating dispersed particles from a dispersing fluid by means of a porous medium. The quality of the separation process depends on the choice of the filter medium. Some of the criteria used for filter selection are presented below: 1. The size of particles to be retained by the filter. 2. The permeability of the clean medium. 3. The solids-holding capacity of the medium. Barrier filters obstruct the flow of the solid particles and will allow the gas to pass through smaller cavities. They can have simple designs such as sand filters, or possess rather complicated structures, like high-temperature ceramic filters. Barrier filters effectively remove small-diameter particulates from fuel gas in the range of w0.5e100 mm in diameter from fuel gas depending upon their design. Thus filter systems offer very high collection efficiencies, typically well above 99%, over rather large size ranges. They can be designed to remove very small particles, including those in the sub-micron range, and are enhanced by the buildup of a filter cake. As the pore size decreases, the pressure drop across the filter increases. They have to be cleaned periodically to avoid an excessive buildup of the filter cake. Pulsing clean gas through the filter in the reverse direction of normal gas flow cleans barrier filters. Nitrogen or steam is quite often employed for this purpose [38]. To reduce the overall particulate load, these filters are typically placed downstream of cyclones. Barrier filters are effective for removing dry particulates, but are less suitable for wet or sticky contaminants such as tars. Tars stick to the filter surface and can cause deposition due to cracking or soot formation. This can lead to fouling and plugging. Even in the absence of further decomposition, tars are difficult to remove from these materials. Barrier filters can be divided into different subgroups based on their construction as follows: 1) packed bed or granular bed filters (these are ‘depth filters’); 2) bag filters (they make use of ‘surface filtration’); 3) hot gas filters (ceramic and metallic, also ‘surface filtration’). Sand filters represent a typical example of a packed bed filter. The major advantages of granular bed filters are their potential for use in hot gas cleaning operations and for combined removal of particles and harmful gaseous components. Compared to bag filters somewhat increased face velocities can be applied. The disadvantage of sand filters is the complexity of the solid handling and the associated regeneration process. For example, a Panel Bed Filter (PBF) has recently been applied in biomass derived product gas cleaning at w550  C upstream of an SOFC test unit at the gasification site in Güssing (Austria) [10,39]. A granular bed filter developed by Taiwan Industrial Technology Research Institute, for cleaning syngas derived from biomass gasification is reported to show about 99.3% particle separation efficiency in a countercurrent flow regime [40]. While packed or granular bed filters provide adequate filtration of tars, they still create operational problems related to the cleaning of the filter and to waste disposal. These

filters may be appropriate for small systems operating in remote locations where labor is relatively inexpensive, but are not incorporated into designs for larger-scale commercial facilities except as a guard bed due to operational and cost considerations [3,41]. Baghouse filters are composed of woven material that intercepts small particles on the filter surface. They operate at relatively low temperatures of approximately 623 K or lower. Because they are not suitable for temperatures above 873 K, they are not further addressed here in detail. Rigid hot gas filters are made of porous rigid materials. The gas passes through the pores in the filter material, while the solid particles are blocked [42]. In biomass gasification systems, these devices can potentially operate at moderate to high temperatures, even as high as 1073 K or beyond, depending on their construction material. In most cases, when hot gas filters are employed in gasification systems, cyclones are used before the filter to reduce the overall loading of particulates to the hot gas filter. Both metallic and ceramic filters are available [3]. Metal filters may get sintered at high temperatures and are also susceptible to corrosion. Wheeldon reports that iron aluminide filters are used to filter coal gasification product gas from the Kellogg Brown & Root “KBR” transport reactor, an advanced circulating fluidized bed gasifier/combustor. The gasifier forms the heart of the Power Systems Development Facility (PSDF) in Birmingham, Alabama (United States). No evidence of damage was seen after w2000 h; however, for longer exposure times in the temperature range 623e773 K increasing amounts of corrosion (iron oxide, suggesting that the alumina protective layer was penetrated) was observed [43], though it was claimed that >8000 h operation should be feasible with a good dust capture system. In the late nineties, at a commercial demonstration biomass gasification facility at Värnamo in Sweden, long-term tests (w2500 h) were conducted with metallic candle filter elements, which were suitable for use with the warm gas at 350  C. Successful operation in its full gasifier/gas turbine power generation mode for extended periods using these filters was reported. For biomass gasifier-SOFC systems, sinter metal filter combined with an upstream cyclone was successfully used for experiments in the EU framework 6 “Biocellus” project at several locations [15]. A novel development is the use of metal foam, as shown by Ghidossi et al. for sewage sludge gasification in a fluidized bed [33]. The authors performed only short-term experiments and regeneration appears to be an issue in the longer term development. Iron-Chrome-Aluminum filters of GST System were used downstream of the Güssing FICFB gasifier and upstream of an SOFC test set up [11]. The filters were operated at 723e773 K with stable filtration behaviour for 140 h on biosyngas. Dust loads downstream with tar exclusion were reported to vary between 0 and 26 mg/m3n . The maximum operating temperature of the filter unit was claimed to be 873 K. Reported temperatures for metal filters are still limited. Ceramic filters are suitable for high-temperature operation, but are fragile and have the potential to break due to thermal stresses during temperature cycling. Ceramic filters are also susceptible to reactions with alkali vapors in gasification systems, which can lead to the filter decomposition or plugging. Such barrier filters have been tested and shows promising potential in several recent gasification demonstration systems [38,43,44]. Another advantage of ceramic filters is that they offer the possibility of combining different cleaning applications by adding different catalysts to the ceramic material. They may require upstream dust removal, but too rigorous upstream cleaning is not advantageous, despite the decrease in pressure drop buildup, because it does not facilitate the buildup of a less dense filter cake of mixed fine and somewhat coarser (e.g., carbonaceous) materials. Such a filter cake texture can act as a shield for tar and soot blinding [45].

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A ceramic filter unit with wall-flow filters (‘Ceramem’ type) integrated in the former 1.5 MWth Delft PFBG test rig has been operated for more than 100 h under gasification conditions [38,46]. The filters have been operated without the upstream cyclone during long stable set point periods. Due to biomass feeding issues, oxygen breakthrough led to filter cake oxidation and subsequent thermal stresses that caused cracking. Later this could be prevented via strict on-line monitoring and fail safe filter operation. The observed pressure drop was between 10 and 30 mbar under industrially relevant process conditions. Pelletized miscanthus, clean wood and brown coal were gasified. The ceramic filter consists of three honeycomb-like elements that were cleaned online, one at a time, using pulses of heated nitrogen. Filter temperatures during the stable set points were typically in the 923e973 K range. Gas cleaning efficiencies of approximately 99.95% were obtained with typical filter outlet dust loads of 5e10 mg/m3n low calorific value (LCV) gas. Candle filters were tested in the coal derived syngas environment at the PSDF demonstration unit referred to previously. Successful gas cleaning at high temperatures for several thousand hours was reported while the facility was being used for both coal combustion and gasification tests. Typical operating temperatures were 1023 K for the combustion mode and w673 K for the gasification mode experiments. Successful gasification tests were carried out for over 2700 h. A particle separation efficiency of 99.999% was reported [47]. High levels of particulate removals were reported with various ceramic and metal filters (monolithic oxide, monolithic SiC, continuous fiber reinforced ceramic composite (CFCC) with particle loadings of <0.1 ppmw (size < 100 mm) in the outlet stream. Such filters were also used at lower temperatures of approximately 523e558 K (‘warm gas cleaning’) at the Buggenum integrated gasification combined cycle (IGCC) power plant in The Netherlands operated by NUON [48]. The gas entered the ceramic filter after having been cooled down and passed through a cyclone. Successful filter operations for more than 25,000 h were reported. Although this facility was operating on coal, introducing biomass into the feed did not influence the particulate cleaning capability. Ceramic candle-type filters were also tested at the aforementioned biomass gasification demonstration site at Värnamo, Sweden [49,50]. The facility was in operation until late 1999. The 6 MWe IGCC facility consisted of an airblown, pressurized fluidized bed gasifier coupled to an Alstom Typhoon gas turbine. In the plant, the biosyngas was first cooled to approximately 623 K and then passed through the candle filters. Ceramic candle elements broke after 1200 h, for no obvious reason. A new set was installed but failed after w350 h; in this case mechanical fatigue was found to be the cause because micro-cracking was found in the tested elements. However no chemical corrosion had occurred [51]. Improvements were made in the design and implementation of the grids that hold the filters, but somehow these could not completely prevent filter failure. The large scale experiences with ceramic candle elements were diverse, and further developments have been made in the technology, which are considered to be highly promising. Moreover, by using ceramic filters, not only can particles be captured with ultra-high efficiency, but such filters can be further improved by using a catalyst inside (clean side). This development will be dealt with in the next section, which covers tar conversion. Similarly, ceramic filters operating at 673 K were successfully (w1200 h) used for operating an SOFC stack connected to biomass gasifiers at the Paul Scherrer Institute in Switzerland [13]. There has been significant progress in rigid particle filter testing and development, and it appears that they may become a reliable option for medium to long-term high temperature cleaning of biomass derived product gases for gasifier-SOFC systems.

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2.1.3. High temperature particle filtration and a discussion of the system conditions It is well known that biomass gasification derived tar components will start condensing at temperatures lower than 673e723 K. Therefore, if the particle removal temperature is below this range, it is important that tar removal is carried out before particle filtration to avoid tar condensation problems. Similarly, cooling biomass gasification product gas may cause carbon deposition problems. A thermodynamic analysis of the effect of cooling this type of gas from a fixed-bed biomass gasification unit with air as the gasification agent was reported before by Aravind et al. [4] It was shown that, in the case evaluated, cooling the gas below about 1023 K caused carbon deposition. It is possible to avoid this problem by the addition of sufficient steam to the stream so that, at equilibrium, no solid carbon precipitates for the given temperature. However, a higher percentage of steam in the fuel gas can cause reduction in the fuel cell voltage thus decreasing the system efficiency. Moreover, alkali compounds in biosyngas are known to condense in the temperature range of 973e823 K. Hence, if the particle cleaning is carried out above these temperatures, it will be required to employ alkali getters either inside the gasifier or further down the cleaning chain, unless the fuel is very low in alkali content. For temperatures near to gasifier operation, ceramic filters seem to offer the best particulate cleaning option, with particulate cleaning up to a few ppmw levels being possible. Even though there are no clear research results indicating that these levels are sufficient for SOFCs, initial biosyngas cleaning systems for SOFCs may have to employ them as the best available option. However, detailed research, including the effect of particulates on long-term SOFC operation is required for the development of gas cleaning units for gasifier-SOFC systems in the future. The selection of the operation temperature for these filters also needs to be carefully studied. While lower temperatures may offer increased endurance for these devices, higher operation temperatures will lead to less heat loss and increased system efficiencies [4]. For this reason, detailed biomass gasification experiments with particle filters kept at high temperatures close to the gas cleaning temperature are desirable. Such conditions are also helpful in avoiding carbon deposition and tar condensation problems. However, the employment of a separate alkali getter will be probably required in this case. 2.2. Tars Tars are polyaromatic hydrocarbon (PAH) compounds that plug ceramic filters and pipelines. They are condensable and also cause soot formation during combustion. A widely accepted definition of tar is ‘all organic molecules with molecular weights greater than that of benzene’ [52]. Tars are present in fuel gas as vapors and as aerosols. Detailed descriptions of biomass derived tars are available in the literature [3,52e54]. As biomass is heated, moisture evaporates and the organic matter volatilizes. The volatilized material can either undergo further decomposition, or it can undergo dehydration, condensation, and polymerization reactions, which leads to the formation of different tar molecules [3]. The composition of tar depends on the reaction conditions inside the gasifier, including the gasification temperature, pressure and residence time. Composition variations usually range from primary oxygenated pyrolysis products at lower gasification temperatures to high molecular weight, deoxygenated products at higher temperatures and severe reaction conditions. Based on the complexity of the molecules, tars are also classified as primary, secondary, and tertiary tars, as listed below along with complex tertiary tars, such as the simple two-ring PAH compound naphthalene capable of surviving at severe reaction conditions [53].

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1. Primary tars: Cellulose-derived products such as levoglucosan, hydroxyacetaldehyde, and furfurals; analogous hemicellulosederived products; lignin-derived methoxyphenols etc. 2. Secondary tars: Phenolics and olefins etc. 3. Alkyl tertiary tars: Methyl derivatives of aromatics, such as methyl acenaphthylene, methylnaphthalene, toluene, and indene etc. 4. Condensed tertiary tars: PAH series without substituents; benzene, naphthalene, acenaphthylene, anthracene/phenanthrene, and pyrene etc. Devi et al. [55] formulated an alternative classification as shown in Table 1. Tars formed at high temperatures in CFBGs contain heterocyclic ethers and polyaromatic hydrocarbons, depending on the reactor temperature. Turbulent bed gasifiers typically show tar concentrations in the range of 1e15 g/m3n . In comparison, fixed-bed updraft reactor designs produce tar concentrations of 20e100 g/ m3n and will need comparatively more extensive cleaning systems. Fixed-bed downdraft gasifiers on the other hand produce much less tar in the gas with typical concentrations of 0.01e1.5 g/ m3n [3]. This is due to the fact that the volatiles from the pyrolysis stage usually pass through a very high temperature combustion zone in the downdraft configuration. High gasification temperatures (approximately 1273 K) result in mostly tertiary tars and low temperatures (approximately 773 K) result predominantly in primary tars. Thermodynamic equilibrium calculations performed for typical gasifier temperatures do not show the relatively high tar concentrations experimentally observed. (Van Paasen et al. [56]). The limited residence time and comparatively slow conversions determine their fate. When lower temperatures are involved downstream of the gasifier, tar components usually condense, thereby plugging and fouling the pipes and other equipment. At temperatures above 673 K, tars can undergo reactions to form solid char that further plugs the system. Tars can deactivate catalysts/sorbents used for reforming and gas cleaning, and, hence, usually have to be (almost) completely removed from the gasification product gas even before the gas is fed through other cleaning devices. Tars are still considered to be the major bottleneck or even stumbling block in industrial biomass gasification [57]. This holds for fluidized bed and fixed bed updraft based gasification performed at temperatures well below 1273 K because tar content in the raw gas can be several g/m3n . 2.2.1. The influence of tars on SOFC operation The influence of tars on SOFCs is yet to be well understood. In the fuel cell-gasifier systems considered here, tar molecules potentially impact the SOFC in several ways, including the deactivation of the catalysts and the degradation of the fuel cells with

carbon deposits. They may also be reformed and subsequently oxidized, contributing to electricity production. Direct oxidation of certain tar components cannot be ruled out. It is also possible that they may simply pass through the anode without any significant influence. The fate of tars on the SOFC anodes may significantly depend upon the type of SOFC employed and its operating conditions, such as temperature, product gas moisture content, current density, etc. It also may depend upon several other factors, such as 1) the thermodynamic possibility of carbon deposition and 2) the kinetics of carbon formation and subsequent reaction steps in their conversion, to name some of the most important factors. Thus, very detailed theoretical and experimental studies are required to understand the fate of tars at the SOFC anodes. Several recent publications describe the influence of tar components on SOFC performance [5,6,17,27]. Most of them provided a rather preliminary discussion but showed promising results regarding the tolerance of SOFCs during comparatively short time exposure to tarloaded gases. Further investigations are needed to develop a detailed understanding of the fate of tars when fed to SOFCs. Singh et al. [23] carried out thermodynamic calculations to study the risk of carbon deposition due to the tars present in the biosyngas and the effect of various parameters like current density, steam, and temperature on carbon deposition. Calculations were carried out at temperatures between 873 K and 1473 K using a typical gasification product gas composition, representing product gas from air-fed gasification with varying steam content. Tar was represented by a mixture of toluene, naphthalene, phenol, and pyrene. A total of 32 species was considered for the thermodynamic analysis carried out using Gibbs energy minimization technique. Carbon deposition was shown to decrease with increasing current density, and it became zero after a critical current density. Steam in the anode feed stream also decreases the amount of carbon deposition. With an increase in temperature, the amount of carbon first decreases and then increases. The minimum deposition is observed in the usual SOFC operation temperature ranges (1073 Ke1273 K). It has been shown by Aravind et al. that few ppms of tar (with naphthalene as a model compound) does not affect the performance of the SOFCs with (planar) Ni/GDC anodes during shortterm operation [5,6]. Ceria based anodes were selected for the studies because of the expected advantages (with doped ceria) of mixed conductivity and the ability to suppress carbon deposition with carbonaceous fuels. Electrochemical impedance measurements were carried out on symmetric Ni/GDC test cells in a single gas atmosphere. Experiments carried out at 1023 K and 1123 K with humidified hydrogen mixed with naphthalene resulted in the following observations: 110 ppmv of naphthalene did not show any significant impact on the electrochemical performance of the anode for hydrogen oxidation for an exposure time of 90 min. However, there were indications that tars could possibly be reformed on SOFC anodes. Dekker et al. carried out SOFC tests using

Table 1 Tar classification adapted from Devi et al. [55] (Table reproduced with permission). Tar class

Class name

Property

Representative example compounds

1 2

GC-undetectable Heterocyclic

3

Light aromatic

None Pyridine, phenol, cresols, quinoline, isoquinoline, dibenzophenol Toluene, ethylbenzene, xylenes, styrene

4

Light polyaromatic

5

Heavy polyaromatic

Very heavy tars, cannot be detected by GC Tars containing hetero atoms; highly water soluble compounds Usually light hydrocarbons with single ring; do not pose a problem regarding condensability and solubility Two and three ring compounds; condense at low temperature even at very low concentration Larger than three-rings, these components condense at high temperatures at low concentrations

Indene, naphthalene, methylnaphthalene, biphenyl, acenaphthalene, fluorene, phenanthrene, anthracene Fluoranthene, pyrene, chrysene, perylene, coronene

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synthetic gas with different tar components [58]. They concluded that lighter hydrocarbons, such as acetylene and ethylene, as well lighter tar compounds, such as toluene, did not affect the cell operation. However, the introduction of low concentrations of heavier polyaromatic hydrocarbons, such as naphthalene, phenanthrene and pyrene, caused a strong reduction in the amount of internal methane reforming, resulting in voltage drop. Using real biomass derived gas as SOFC feed, Oudhuis et al. [29] coupled a two-stage gasifier to a downscaled Sulzer HEXIS stack for a duration of 48 h, showing that the system works, but the authors observed soot formation during fuel heating, which negatively influenced the cell performance. Similarly, Nagel et al. [28] operated a 1 kW Hexis stack with gas from an updraft wood gasifier (tar load w 8 g/m3n ) and observed a performance loss of 6% within 30 h of operation. A Ni-GDC anode SOFC was operated successfully for 150 h on a tar-free cold-cleaned wood gas from a two-stage gasifier with a low S/C ¼ 0.5 [16]. Experiments were also carried out at Delft with similar SOFCs connected to the CFBG with real gas containing several thousand ppms of tar fed to the SOFCs [5,17]. No significant degradation of SOFC performance was observed during an experiment lasting for several hours. Biollaz et al. have shown that, during a 1200 h test, biomass derived tars did not cause problems for a tubular SOFC [13]. They used syngas from updraft wood gasification, diluted at a ratio of 1:20 with synthetic syngas. The sulfur concentration was kept below 1 mg/m3, and the designed tar concentration in the fuel gas, which was fed to SOFC after heating, was approximately 5 mg/m3. The cell only showed a performance degradation rate of 1% per thousand hours. This was the same as the case with synthetic gas containing the same amount of sulfur but no tar. Martini et al. reported the results of a 67 h long test on SOFC operation using syngas from an allothermal gasification unit, which uses steam for gasification [11]. No serious performance degradation was reported. While no details about the test or the SOFC used are given, a description of the warm gas cleaning unit (723 K or above) was provided. Rather detailed results of tars fed to SOFC anodes were reported by Mermelstein and others [59]. They have shown with experiments that carbon deposition is less of a problem with Ni/ GDC anodes compared to Ni/YSZ anodes. They carried out 30 min experiments at 1038 K on SOFCs with both types of anodes fed with humidified H2-N2 mixtures and benzene used as a model tar. In addition to the advantages of Ni/GDC anodes, it was also shown that current density has a significant impact on reducing carbon deposition. This observation is in agreement with the results from the thermodynamic calculations carried out by the authors and also with the results from the detailed thermodynamic calculations previously presented by Singh et al. [23], in which the carbon deposition is shown to decrease with an increase in current density and becomes zero beyond a critical current density. These developments introduce a significant chance of realizing efficient gasifier-SOFC systems operating with biomass gasification product gas containing tar in the future, when sufficient knowledge and experience is developed on the topic. Though SOFCs have shown high tolerance levels for tars for short-term operation, longer term experiments are required to know if indeed tar can be withstood by SOFCs during long-term operation lasting several tens of thousands of hours, which is the expected life span of SOFCs. Additionally, most of these studies were performed at rather safe operation conditions, such as low fuel utilization levels. Previously reported scientific results indicate that, with the current level of available information, the level of tars is probably required to be brought down to a few ppm in the real biomass gasification derived product gas that is fed to SOFC stacks for long-

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term operation. While there are indications that even larger quantities of tars may not affect SOFC operation, or even that they may get reformed at the SOFC anodes, further detailed research is required to solidly confirm this and to select suitable SOFC operation conditions for long duration power production, when being fed with a tar-containing gasification product gas. 2.2.2. Tar removal options As the long-term impact of different tar components in real product gas on different SOFC anode materials is still not completely known, current hot gas removal options are reviewed here. Basically, two classes can be identified: non-catalytic methods and catalytic techniques. 2.2.2.1. Non-catalytic methods. Tars derived from biomass gasification can be converted using non-catalytic partial oxidation, which aims to reduce their concentration significantly [60,61]. Experiments were performed within the temperature range of 1173e1423 K with residence time varying between 1 and 12 s. Naphthalene was used as the model tar species and it was reduced by 98e99% at 1173 K with an excess air ratio of 0.5. For this tar reduction technique, substantially higher temperatures are needed to sufficiently remove these aromatic species compared to a catalytic process. Brandt and Henriksen [62] mention that the temperature and residence time needed are 1523 K and 0.5 s, respectively. In-line with this statement is the study of noncatalytic partial oxidation by Jess et al. [63,64]. This has the disadvantages that expensive alloys must be used in constructing the tar cracker and significant energy losses would be incurred in the system. Also, soot is reported to be produced when applying this tar cleaning method [65], which might lead to depositions on downstream gas cleaning units or on the SOFC anode parts. Moreover, partial oxidation, performed by adding air or oxygen, could increase CO levels at the expense of the conversion efficiency and operational cost [66]. Research on the use of plasmas for tar reduction has also been conducted, as an alternative to non-catalytic tar conversion method. Examples are the Pyroarc, Glidarc and (pulsed) Corona [67,68]. With the pulsed corona technique at gas temperatures lower than 773 K, promising tar reductions (a few % tar remaining at >100 kJ/l input, with lower amounts at increasing temperatures) have been realized, though only model component (naphthalene) studies were presented. The effect of particulate matter or other contaminants on the plasma’s performance is unclear. No economic evaluation was reported to the authors’ knowledge. 2.2.2.2. Catalytic methods. Catalytic tar cleaning is potentially attractive [69e71] because no additional energy input is necessary, efficiency and heating value losses are kept at a minimum, and no tarry waste streams are generated that need to be disposed of or recycled to the gasifier. Criteria for successful catalysts are [65]: 1) They must be effective at removing tars. 2) They should be resistant to deactivation as a result of carbon fouling or sintering. 3) They should be easily regenerated. 4) They should be strong. 5) They should be inexpensive. Catalytic tar cleanup related to biomass gasifiers can be further divided into the following different technologies: * in-bed addition of catalysts (fluidized bed gasifiers only); * downstream situated catalyst beds; * catalytic filtration.

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These techniques are all based on the chemical reactions of tars, which are thermodynamically favored at high temperatures. At the same time, the need for the collection, disposal, or returning of the tar (containing solvents) is also eliminated. The chemical reactions occurring during catalytic tar reforming are in several publications. Some of the reported research initiatives are only focused on steam reforming [72e74]. However, gasified biomass consists of H2O, CO2, H2 and CO; therefore, dry reforming and other reactions are important in this case as well. An overview of the important reactions playing a role in these tar reduction strategies is given below [55,75]: CnHm þ nH2O ¼ nCO þ (n þ m/2)H2 (R1; steam reforming) CnHm þ nCO2 ¼ 2nCO þ (m/2)H2 (R2; dry reforming) CnHm / C* þ CxHy þ gas (R3; thermal cracking) CnHm þ H2 / CO þ H2 þ CH4 þ . þ coke (R4; hydrocracking/ hydroreforming) CO þ H2O ¼ CO2 þ H2 (R5; water-gas shift reaction)

Calcined rocks

Olivine

Clay minerals

Ferrous metal oxides

Fig. 6. One lump model for tar conversion [76].

…

mineral

Chars

+H2 (hydroreforming)

FCC Catalysts

+H2O (steam reforming)

CO H2 CH4

Alkali metal carbonates

Tar

+CO2 (dry reforming)

synthetic Activated alumina

Catalytic and thermal cracking

catalytic

Transition metals

Numerous intermediate reactions also occur. Often several reactions are lumped into one overall reaction as shown in Fig. 6. Most catalysts, however, are subject to deactivation, which refers to the decline of the activity and/or the selectivity as time progresses. The mechanisms of deactivation can be divided into 3 main categories, poisoning, fouling and thermal degradation [77]. Other mechanisms of deactivation include erosion, attrition and phase transformation. Poisoning of tar reforming catalysts: Poisoning is the strong chemisorption of reactants, products or impurities on the active sites of the catalysts. During steam reforming of methane and naphtha over nickel-based catalysts, H2S is often suggested to be the typical poison. Several researchers have investigated the influence of H2S on tar reforming catalysts [78e80]. Sulfur ad/ absorbs strongly on metals, influencing the activity of the catalyst substantially. The rate of sulfur poisoning varies from catalyst to catalyst and depends on the catalyst composition and reaction conditions. Fouling and coking of tar reforming catalysts: Fouling is the physical deposition of species from the fluid phase onto the catalyst surface. In contrast with poisoning, no adsorption or absorption takes place at the active sites, but the catalyst is covered with a layer that can plug the pores. The effect of fouling on the catalyst activity strongly depends on the pore structure [81]. Fouling can be caused by particles in the feed stream; however, in the case of reactions involving hydrocarbons (like tar reforming), fouling is frequently caused by the deposition of coke and/or carbon. Coking is actually a combination of fouling and poisoning because some carbonaceous components are chemisorbed by the active sites. The mechanisms of the formation and deposition of carbon and coke on

metal catalysts have been discussed by several researchers [73,79,81e85]. To prevent coke formation, Forzatti and Lieti [82] suggest employing high hydrogen partial pressure and a high steam/carbon ratio. In general, operating conditions should be such that carbon formation is minimized. Thermal degradation of tar reforming catalysts: Thermal degradation is the loss of catalytic surface area, support area and/or active phase support due to several factors including sintering, chemical transformation, evaporation [79,86]. Sintering mainly occurs at temperatures above 773 K and is caused by crystallite growth. This can be subdivided into three principal mechanisms [79]: crystallite migration, atomic migration, and (at very high temperatures) vapor transport. Little is known about the interaction between these mechanisms. Sintering processes are known to be slow and irreversible; therefore, they should be avoided. The sintering rate increases exponentially with temperature due to the increasing mobility of the atoms. The temperature at which the solid phase becomes mobile also depends on several other factors, such as, shape and size of the crystallite, support roughness, pore size, impurities present and atmosphere [79,82,86]. The tar conversion catalysts used can be applied upstream in the process, e.g., in-bed (additive) materials downstream of the gasifier. The materials used as catalysts for tar conversion can be classified into mineral, naturally occurring materials and synthetic catalysts, see Fig. 7. The following sections present an overview and evaluation of the different catalytic tar cleaning strategies, with a focus on demonstrated concepts based on real biomass gasification derived gases. 2.2.2.2.1. In-bed addition of catalysts. In fluidised bed gasifiers tars are generated, but they can be partially decomposed into syngas components using in-situ catalysts as a bed material or an additive, like dolomite [87], treated olivine sand [88] and artificially made/modified catalysts. This is the primary method to destroy tars and simplifies downstream processing. However, the conditions in the gasifier are most severe of any in the system. This means that the catalyst here are most prone to abrasion, chemical fouling and coking. Additionally, there is not much reported information on the conversion of methane and C2e3 hydrocarbons in the produced gas [65].

Cokes Fig. 7. Overview of catalytic tar conversion material classes, adapted from Abu El-Rub et al. [75].

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Natural minerals: Minerals are naturally occurring, homogeneous solids with a definite, but generally not fixed, chemical composition and an ordered atomic arrangement [75,89]. Rock materials like Dolomite and Limestone, in particular, in their porous calcined form, are very well known bed materials/additives for fluidised bed gasification using different oxidizing media. As early as the 1980s, research was carried out using limestone as additive material [90,91]. Karlsson et al. [92] reported tar contents of 1e2 g/ m3n of light tars (excluding benzene) and 100e300 mg/m3n of heavy tars on a demonstration scale IGCC with dolomite as the bed material. Depending on the CaO/MgO ratio, Simell et al. (VTT, Finland) classified these mineral rocks [93]. Siedlecki et al. have shown with experiments in a CFBG that the addition of magnesite reduces tars, in this case the sum of PAHs and phenolics, to 1.7 g/m3n , on a raw gas basis [18]. The dolomites are among the most active and the most widely used [87,94e101]. They are comparatively active in tar conversion (up to 95%), inexpensive and are considered to be disposable, which is advantageous and explains their popularity. Disadvantages are that the material is heterogeneous in nature (differing per region). The catalytic tar reduction potential was reported to depend strongly on morphological factors (pore size and surface area) and on the content of other metals. Alkalis act as promotors by enhancing the gasification of the C-intermediates; the reactivity differences was attributed to the iron content [93,97]. In particular, dolomites are soft and, thus, have relatively high attrition rates [65,99,101], which leads to performance degradation and an increase in the amount of solids that need to be removed during gas cleaning. These must be removed to reduce downstream fouling and plugging of the equipment. Furthermore, calcination is necessary with accompanied energy input. Deactivation of calcined rock material is attributed to two main factors: carbon deposition and recarbonation when CO2 partial pressures are too high [93,102] in the system. Partial recarbonation with the resulting lower tar conversion activity may even happen when the material is stored in the air [103]. Corella et al. found that in-bed use of dolomite was somewhat less effective than its use in a downstream bed [87]. However, tar concentrations lower than 2 g/m3n were obtained with a bed with dolomite [101,104,105], which helps to prevent coking in the additional downstream tar polishing units. Ising reports that the use of fresh dolomite led to tar concentrations in the gas of about 300 mg/m3n , whereas used dolomite resulted in values up to 2.5 g/m3n on a 500 kWth airblown CFB gasifier (UMSICHT, Germany) operating at 1183e1193 K [103]. Recently, from CUTEC (Germany), a report [106] was published on the use of natural bed materials in a steam/O2 blown 400 kWth CFB gasifier. It was indicated that compared to the use of sand, dolomite resulted in better tar reduction. However, values of 3.5 g/m3n were reported in the raw gas. The difference between this value and the value reported by Ising is just an example of the heterogeneous nature of the dolomite used. The difference in the oxidizer used could play an important role and may explain the differences observed. The use of dolomite in the 1.5 MWth pressurized bubbling fluidized bed (BFB) at Delft resulted in only limited tar reduction and concentrations of the major tar species of the order of a few g/m3n [107,108]. On the 8 MWth fast internally circulating fluidised bed (FICFB) gasification plant at Güssing, Austria, tests lasting about one week were performed with limestone as the bed material. The tar content of the primary gasification gas was approximately 1 g/m3n with a high hydrogen content in the gas (50%vol. on a dry basis) [109]. Olivine sand is another naturally occurring mineral with a silicate structure, in which Mg and Fe cations are incorporated in the silicate tetrahedral structure [110]. It can be represented by the chemical formula (Mg,Fe)2SiO4, and as a mineral it is between fayalite and forsterite. This mineral has also demonstrated an

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ability to convert tar when used in beds, both in atmospheric and pressurised FB applications with biomass (see e.g., [100,106,111]) and also for biomass-plastic mixtures in a lab-scale BFB gasifier (see Aznar et al. [112]). It appeared that applying a heat treatment to this mineral material under oxidizing conditions (air) had a significant positive impact on its activity [113,114]. Iron oxide, reduced (to Fe3þ in the form of a and g oxides) and migrated to the outside of the mineral, is believed to play an important role. Ca is also considered to be important in tar reduction [115]. This catalyst’s advantages are its low price (of the order of dolomite), natural abundance and its high resistance against attrition compared to dolomite. The demonstration plant in Güssing uses in-bed olivines and clearly shows good catalytic activity. There are differences in the different batches of the material depending on the source and environment of the (pre-)treatment [116]. The mineral becomes more active after an initial period of operation under reducing conditions [115]. The Güssing plant currently runs for 7000 h per year. This technology is regarded as commercially viable [117] and new plants based on similar technology have been built and are in operation in Oberwart and Villach (Austria). Bauxite and natural alumina were reported by Ising to be effective in reducing tar contents with in-situ use in a 500 kWth CFB gasifier; tar values of between 100 and 300 mg/m3n were reported [103] Clay minerals have been reported as possible in-bed tar reducing materials. They can be classified as kaolinites, monmorillonites and illites. These clays are comparatively cheap, abundantly available and their disposal is less of a problem than transition metal based inactivated catalysts. Lower tar reducing activity as compared to Nibased catalysts and dolomites, have been measured. The main mechanism of tar destruction for these minerals was attributed to the tars’ reaction (R3, thermal cracking) [118] with acid sites [119] of the otherwise amorphous material. Catalytic activity for tar destruction was attributed to a combination of properties: effective pore diameter, internal surface area and the number of strongly active sites [120]. However, above 1123 K, the activity of aluminosilicates drops, as reported by Simell and Bredenberg working at VTT (Finland) [118]. Iron ores have also been suggested as tar reduction agents. Tar is better reduced by the metallic form than by using oxides. In addition, iron catalyzes reactions involving the main species of the product gas, e.g., the water-gas shift reaction (R5). Although these are also quite abundant, available and still cheap, they show lower activity than dolomite and are prone to deactivation as a result of coke formation [121]. Synthetic catalysts: Commercial Ni steam reforming catalysts are designed for use in fixed bed applications and are not considered to be robust enough for in-bed fluidized bed applications because coke formation and catalyst attrition lead to a rapid drop in tar conversion activity [122,123]. Coke formation is associated with the acidity of the catalyst surface. It can be mitigated by (earth)alkali oxides [124]. Modifications to Ni catalysts have been made to cope with the above mentioned disadvantages to allow their in-bed use. Garcia et al. [125] used Ni aluminate as an in-bed catalyst with La and Co as promotors. Toluene was used as the model tar species. For temperatures higher than 873 K, more than 80% conversion was obtained. The Co/Ni molar ratio seriously influenced the conversion. The catalyst activity showed the following order: Ni-AlLa > Ni-Co-Al > Ni-Al. Arauzo et al. [126] applied a Ni aluminate catalyst, also for pyrolysis of poplar, under different oxidizer environments at temperatures of 773e973 K. Pfeifer et al. [127] reported on a Nickel-enriched olivine, which can be considered a hybrid between a natural material and an artificial catalyst because it is more stable than the conventional

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artificial catalysts. Using up to 40 wt% of this material in a bed of olivine led to a decrease of tar content in the gas from approximately 2 g/m3n to 0.5 g/m3n . The catalyst showed no noticeable deactivation in two tests of 30 and 45 h, respectively. The catalyst was developed at the University of Strasbourg (ECPM, France) and was first applied on a bench-scale fluidized bed biomass steam gasifier at the University of L’Aquila (Italy). Some related publications [128,129] and patents [130,131] have been published. A FCC catalyst was used in a fluidized bed biomass gasifier at the end of the 1980s. However, the tar content was not reduced much. Corella et al. attributed this later to the limited residence time of the gas in the gasifier and to the low temperature that was used [132,133]. Asadullah et al. [134e136] (University of Tsukuba, Japan) used a Rh/CeO2/SiO2 catalyst as an in-bed agent to efficiently convert tar species. This catalyst was much better than previously the tested Rh/CeO2 catalysts [137,138], as these all showed sintering behaviour [139]. The authors observed practically no coke formation and a completely negligible tar concentration in the product gas. Although no long-term tests were performed, the indications were positive for this catalyst. 2.2.2.2.2. Downstream catalyst beds and monoliths. The tars can further be cracked or reformed downstream the gasifier in separate beds at high temperature, yielding additional syngas. This has proven to be an effective approach to catalytic tar destruction, see e.g., [65,87,140]. Natural minerals: Naturally occurring minerals can be used insitu as described previously; however, they can also be used in a downstream reactors. As indicated before, they are cheap and disposable. Their softness and resulting attrition is a disadvantage. Another disadvantage is that the chlorine content in the biomass fuel may react with CaO to produce CaCl2, and the catalyst may be consumed chemically. The former Swedish Company TPS has applied this technology commercially for tar reduction using calcined dolomite (together with oxygen supply) in a circulating fluidized bed situated downstream of the main airblown biomass CFB gasifier [141]. A substantial amount of research on fixed beds situated downstream of biomass gasifiers with dolomite and limestone has been carried out, see e.g., [87,96,98,118,142e153]. Dolomite-based downstream reactors are expected to reduce the tar presence in biomass gasification product gas to a few hundred ppmw. Other natural minerals applied for downstream cleaning of tar components are Bauxite (Al2O3/Fe2O3), Bentonite (CaO/Al2O3/SiO2) and natural mixed oxides [103]. Using dolomites and bauxite, inlet concentrations of real tar from a 500 kWth CFB gasifier were reduced by 95%. Bentonite in this configuration was less effective: about 75% tar conversion was observed. The residence time in the catalyst section was kept at 1.2 s. Char: A cheap material that is able to significantly reduce tar from biomass gasification gas is char [154]. This, however, is not a catalyst in the strictest definition because it is consumed by gasification reactions with the steam and CO2. Char gasification impacts pore size, structure, number of particles, etc., resulting in increased performance with regard to tar removal. El-Rub et al. showed that naphthalene conversions at 1173 K were practically 100%, and that, at 1023 K and with typical airblown gasification gas compositions, above 95% tar conversions were obtained with only a few mass% of the char being consumed [155]. In an EU Framework 6 project, GREEN FUEL CELL, aimed at SOFC integration with biomass gasifiers, tar conversion using char was addressed [156]. In addition, Köppel et al. showed that the use of char obtained from gasification as a tar reforming agent is an interesting option for tar cleanup and for model tars because well above 90% conversion is obtained [157].

Metallic and metal oxide synthetic catalysts. Nickel-based catalysts: Among the artificial catalysts applied in downstream beds, Ni based ones are the most popular. Commercial steam reforming catalysts supplied by BASF, ICI, UCI, Haldor Topsoe, Sudchemie, and Toyo CCI all contain a large amount of this element. The commercial catalysts are mainly available in the form of rings, spheres, pellets and extrudates. Corella et al. [158] tested seven of these for biomass gasification gas upgrading. All of the catalysts were Ni-based with varying amounts of Mg, Ca, and K and multiple supports (Al2O3, SiO2, and MgAl2O4). They indicated changes in the main gas constituents that came from the formation and destruction of methane. Specifically, naphtha catalysts showed higher reforming activity than did steam-methane reforming catalysts. When applied at temperatures significantly lower than 1173 K, sulphur species in the gasification gas have a serious poisonous impact on the catalyst’s activity. However, this is far more severe for the decomposition of NH3 than for tar decomposition [159]. Köppel et al. [160] show that a change in the H2S content of the gas of one order of magnitude leads to an approximately 100 K increase in the temperature needed to obtain complete naphthalene conversion. Also, the commercial reforming catalyst materials are sensitive to other trace compounds in the gasification product gas, like alkali and chlorine species. Moreover, loss of material has been reported [161]. Furthermore, rapid deactivation of the catalysts from coking has been mentioned by many researchers. Tar conversions that initially are in the range of 95e99.99% are followed by a fast decay to substantially lower residual activity [162]. With high steam/tar ratios, far less coking occurs [163]. The deactivation is insignificant when a dolomite guard bed is used [164] to crack the heavier tar species, typically down to values below 2 g/m3n . Due to these observations, several efforts were made to stabilize the Ni catalysts, to overcome the problems from sulphur species poisoning (e.g., by incorporating Ca in the structure) at lower temperatures and to prevent or reduce coke deposition (e.g., by including alkali metals in the catalyst matrix). Basically, the catalyst was modified to include promoters and support modifiers. In addition to pure model studies, which are not dealt with in detail in this review (see e.g., good overviews by Dayton [123] and Milne [152]), a substantial amount of research work has been dedicated to investigating the performance of conventional, commercial Ni-based catalysts situated downstream of a biomass or peat gasifier [57,63,70,73,74,122,158,160e174]. In addition, advances were made in the reactor designs to accommodate such catalysts. In the Netherlands, at BTG, commercial Ni-based catalysts have been applied in a Reverse-Flow, Catalytic Tar Converter (RFTC), developed to remove tar from producer gas in an energy efficient way [175]. Raw producer gas from a biomass gasifier is fed to the RFTC at a temperature between 623 and 923 K. In the entrance section the producer, the gas is heated up to the desired reaction temperature of 1173e1223 K. In the central section, there is a commercial Ni catalyst and steam reforming reactions take place. Tar components - and also light hydrocarbons including methane - are converted into CO and H2. Additionally, nearly all NH3 is removed (forming N2 and H2). To counterbalance these endothermic reactions, a small amount of air is added to the reactor (about 5% of the producer gas flow). Hence, the heating value of the gas is slightly reduced, but this is fully compensated for by the increased amount of gas. Experiments have been performed with the catalytic reactor. The catalyst used in the RFTC has been tested for over 6000 h with wood-derived producer gas. During this period, no detectable change in catalyst activity was observed. The addition of extra sulphur reduced the activity (to a new stable level), but tar was still completely removed. However, ammonia and methane conversion was reduced. After stopping the

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additional sulphur supply, the original catalyst activity was restored again [176]. A recent development is the application of a monolithic reactor with a Ni-based coating. Monoliths are ceramic blocks with parallel, straight channels on the walls, on which a thin layer of catalytically active material is deposited [177,178]. Fig. 8 shows a typical monolith element. The honeycomb structure of these monoliths tolerates gas loaded with a certain amount of particulate matter. Research on this novel catalyst type has been described by Simell et al. [179], Ising [103], Corella, Toledo and co-workers [180e183], and Pfeifer et al. [184,185]. A Ni-based monolith process was patented [186] and tested for 500h downstream of the 500 kWth CFB gasifier on real gas. It reached the goal of tar levels lower than 50 mg/m3n [103] and showed no significant deactivation. This was claimed to be due to the periodic cleaning of the monolith unit. As a follow up study, this monolith type catalyst was installed at the Güssing plant for long-term testing [184]. Slip streams taken from this plant were fed to 3 nickel-based monolith type catalysts in a laboratory-scale reactor. Almost complete tar reduction and considerable ammonia decomposition was achieved by this catalyst at temperatures above 850  C and space velocities of about 1100 h1. Inlet tar content was on the order of 1.5 g/m3n . At VTT, pressurised tests were reported in the 1990s [70] using forest waste wood, bark, wood chips and peat for the fluidised bed gasification. The ceramic monoliths were placed in a slipstream containing dust. The monoliths consisted of Ni/Al2O3 with dimensions 300  50  50 mm having square channels. Two of them were placed on top of each other with a spacing of 31  31 mm. At temperatures of approximately 1183e1193 K, tars were practically completely removed at 1 s residence time (SV ¼ 2500 l/h), and the equilibrium composition was approached. In comparison to fixed beds of catalyst, somewhat higher residence times are needed for the same conversion of tars. The differences are due to less effective heat and mass transfer to the catalytically active surface [177]. In particular, the highly endothermic reforming reactions can lower the catalyst temperatures at the surface, leading to comparatively low reaction rates. Tests of the order of 100 h (4 runs totalling 500 h) were completed with virtually 100% tar conversion and no clogging problems from solids. For these tests, the fuels were probably not the most difficult (low alkali/chlorine, ash). The H2S content of the gas varied between 35 and 110 ppmv; the HCl concentration had a maximum of 90 ppmv, and dust contents were in the range of 0.2e5.8 g/m3n [179]. VTT was involved in a European FP5 project, NOVACAT [187], in which monolith tar cracking was tested. The (prototype) monoliths were obtained from BASF AG. Toledo et al. [183] concluded that tar levels below 200 mg/m3n can be attained, but

Fig. 8. Schematic of a tar reforming monolith and picture of a monolith reactor unit [103] (figure reproduced with permission).

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the lifetime of the monoliths is very much dependent on the configuration that is chosen to ensure a temperature profile that prevents the occurrence of too high or too low temperatures at the entrance and exit of the monoliths. Also, the feedstock should contain low alkali content, or at least the alkalis should be removed before reaching the monolith’s faces, as problems arising from stickiness can occur due to the presence of these trace metal species. In the European FP6 project, BIGPower [188,189], in Skive (Denmark), a monolith catalytic tar converter is positioned downstream of the 30 MWth Carbona pressurised gasifier and upstream of a product gas cooler and a lower temperature filter. Fe based catalysts: In the 1980s, Tamhankar et al. [121] studied the conversion of benzene on an iron oxide/silica catalyst. Although the conversion was limited (40%) without hydrogen, a nearly complete conversion was realized in the presence of 10%vol. hydrogen. It was found that without H2, the original hematite (Fe2O3) was reduced to magnetite (Fe3O4), whereas with hydrogen, the magnetite phase was further reduced to metallic Fe. Recently, Nordgreen et al. [190] performed thermodynamic equilibrium calculations to define optimal conditions in which no coking takes place and in which the iron is in metallic form. Experiments with biomass gasification that used iron in a downstream fixed bed at temperatures in the range of 1023 Ke1173 K showed a significant reduction of tar, although the decrease was much lower than what has been observed for, e.g., dolomites. In addition, the methane content decreased significantly. The observation has been explained by the ability of dolomite to catalyze other chemical reactions, although the authors are not explicit about which ones [191]. Uddin et al. [192] report on a two-stage gasification of woody biomass using a specially designed flow-type double beds micro reactor. For the study, temperature-programmed non-catalytic steam gasification of biomass was performed in a top layer bed at 473 Ke1123 K, followed by catalytic gasification of volatiles (including tars) in a bottom bed at a constant temperature, primarily 873 K. Iron oxide catalysts, which were transformed to Fe3O4 after use, possessed catalytic activity in biomass tar decomposition. More than 90% of the volatile matter was gasified by the use of an iron oxide catalyst at a superficial velocity of 4.5  103 h1. Tar was decomposed over the iron oxide catalysts, followed by a water-gas shift reaction. The surface area of the iron oxide appeared to be an important factor for the catalytic tar decomposition. The activity of the iron oxide catalysts for tar decomposition remained stable with cyclic use, but the activity of the catalysts for the water-gas shift reaction decreased with repeated use. An associated patent was released [193]. Other non-Ni-based or Fe-based metal and metal oxide catalysts: To overcome the disadvantages of Ni-based catalysts, other metallic catalysts have also been investigated. In the 1980s, Battelle Columbus (now SilvaGas) developed a catalyst, DN-34, that was found to perform better than a conventional ICI-46-1 steam reforming catalyst [194] in destroying a variety of aromatic hydrocarbons. The catalyst exhibited significant water-gas shift activity but was unsatisfactory for methane destruction. Tar removal rates determined in the slipstream of the company’s Burlington Vermont (USA) plant (capacity of operation of the whole plant: 273 dry tons/day) were at 90% with an operation temperature of 1063 K [195]. Ising [103] used a Co/Mo catalyst obtained from Tricat in the slipstream of a 500 kWth CFB gasifier at UMSICHT (Germany). The initial conversion of tars was at approximately 97%, but after a few hours of operation, the tar conversion rate was reduced to 35%. To overcome coke formation and sintering, Furusawa and Tsutsumi [196,197] studied Co/MgO catalysts. The catalytic performance data showed that the Co/MgO catalyst had higher activity than the Ni/MgO catalysts tested in this study; however, they

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performed tests at a lower steam/carbon ratio and a higher concentration of naphthalene than those in other studies. At the Georgia Institute of Technology (Georgia, USA), a sulfided cobalt oxide catalyst on an alumina carrier was developed for tar removal that is tolerant to alkali and sulfur attack. Experiments with benzene as a model tar and with real black liquor tar up to 1000 h without significant decrease in activity have been previously reported [198]. Black liquor gasification and gas upgrading are among the most demanding conditions. Regarding real black liquor gasification gas, 130 h of testing was claimed without significant catalyst activity for which an associated US patent application was issued [199]. Catator (a Swedish company) has developed Pt-Rhebased catalysts for steam reforming. In addition, Albertazzi et al. [200] used such a catalyst for reforming that is exposed to biomass gasification gas. Rh has shown exceptional ability for aromatic species degradation [201]; therefore, it is a suitable choice as an active phase for the type of gas coming from a biomass gasifier. A Pt-Rh catalyst appears to be a good choice because it can handle both alkanes and aromatics, which is needed in this application. Recently, this type of catalyst has been studied in an environment of exposure to typical fly ash from a biomass gasifier and real gas. A fluid catalytic cracking (FCC) catalyst was tested by El-Rub et al. [155] using phenol and naphthalene. The conversion of phenol was quite good (w87% conversion) at 973 K with a 0.3 s residence time, but with naphthalene as the model compound, the FCC catalyst performed only moderately well, with approximately 60% tar conversion mixed in a bed of sand at a temperatures as high as 973 K with a 0.3 s residence time. VTT started research on (doped) zirconia through oxidative catalytic cleaning of biomass gasification gas. Zirconia is a metal oxide that possesses acidic, basic, oxidizing and reducing surface properties [202]. A phase transition from a monoclinic to a tetragonal structure takes place above 1273 K. This might be a cause of sintering at high temperature applications. Stability increases by the use of dopants [203], of which Al, Ce and Y are the most commonly encountered. The application of the material was described by Juutilanen et al. [204,205], and they have recently filed a European patent [206]. Monoliths were dip-coated with the material. Synthetic gasification gases with toluene and naphthalene as model tar compounds were used in experiments, and the decomposition of both toluene and NH3 took place in the presence of O2, even when 100 ppm H2S was present in the gases. Carbon deposits were not observed on the surface of the catalytic material. Selectivities of zirconia for tar and ammonia oxidation were improved by incorporating Al into the ZrO2 material. Reaction temperatures were reported to be low compared with those required by Ni and dolomite [65,123]. Ten-hour tests were reported at 973 K; the Ce-doped zirconia showed an increase in conversion of naphthalene from 61% to 73%; for Y-doped zirconia under the same conditions, naphthalene conversion increased from 71% to 75%. Typical space velocities were 5000 h1. The highest naphthalene conversions were observed when the temperature reached a level at which nearly all O2 is consumed. The tests of silica-doped zirconia had inferior conversion behavior as compared to other tests. Ceria-doped zirconia resulted in the highest naphthalene conversions (nearly 90%) in conditions of up to 873 K. One drawback to using these zirconia-based catalysts is the necessary use of oxygen (or air), which the catalytic unit requires. Catalytic filtration: A novel development in tar removal is the combination of ceramic gas filtration and catalytic tar conversion; see Heidenreich and Nacken from Pall Filtersystems (“Werk Schumacher”) [207]. A schematic of the working principle is presented in Fig. 9. High-temperature filtration is an attractive method for particulate removal of hot gases because, when this method is used,

Fig. 9. Working principle of catalytic filtration (adapted from Nacken et al. [208]) (figure reproduced with permission).

the gas flow can maintain its sensible heat, resulting in higher thermal efficiency than that of other methods. Till now, the removal of particulates and tars is performed in two separate units: a catalyst unit followed by a filtration unit, or vice versa. A filter to remove particles from gasification processes appeared to be necessary, as cyclones are insufficient to reduce the particulates to an acceptable level for sophisticated downstream devices such as a fuel cell. The disadvantage of placing the catalyst unit upstream of the filter is the rapid deactivation of the catalyst by particle deposition. The disadvantage of placing the catalyst unit downstream of the filter is the necessity of reheating, depending on the filtration temperature. In case the temperature drops below the condensation point of the tar components, there is a risk of pipes clogging downstream of the filter [209]. Therefore, technically and economically, a promising way to remove tars and particulates is by combining filtration and catalysis in one cleaning unit, a catalytic filter. Most studies have focused on catalysis and have not examined the filter behavior. To some extent, catalytic filters can be compared to membrane reactors, especially in the case of the type of filter shown in Fig. 10, a flow-through membrane in which the permeation of a premixed feed stream takes place. The concept for such a filter is from Saracco et al. [210], who used catalytically

Fig. 10. Catalytic flow-through membrane reactor [211] (figure reproduced with permission).

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modified filters for the dehydration of alcohol and the reduction of NOx. There are three methods to realize a catalytic filter [212]: -

Developing a catalytic coating on the porous support. Inserting the catalytic component in the ceramic grain and binder mixture. Modifying the candle filter design by using a porous inner tube fixed at the head of the candle to allow for the integration of a catalyst particle layer.

The first option is most frequently used; however, the disadvantages of this method are the low freedom in the thickness of the catalyst layer and the complex manufacturing. The second option has the disadvantage of losing active surface after grain sintering at the relatively high temperature used during the manufacturing of the hot gas filter. The third option has the advantage that it is more flexible with regard to potential applications because the development of an appropriate highly active catalyst particle system is less time consuming than the development of a catalytic coating. However, mechanical challenges and contacting efficiency could hamper the development of the third method. The latest development in the development of catalytic filters is a multi-function filter, which has recently been integrated in the freeboard of a wood-fired laboratory-scale atmospheric bubbling fluidized bed gasifier [213,214]. The advantages of a catalytic filter are reduced investment costs and space efficiency. Furthermore, the combined process can be performed at high temperatures, which obviates the necessity of reheating and thus saves energy and eliminates the risk of plugging. 2.2.2.3. Discussion of the choice of tar cleaning for SOFCs. To this point, the analysis indicates the following. Different studies on the influence of tar in biomass gasification product gas on SOFC performance yielded mixed results. Although there are several indications that tar might not affect SOFC performance and might even become reformed at the anode, such arguments have not yet been solidly proven for long duration operation. This fact makes the selection of proper gas cleaning units difficult. Hence, at this stage, we propose three options: one for an optimistic case, a second for an intermediate case and a third for a conservative case. For the optimistic case, it can be assumed that tars can be reformed at SOFC anodes, or at least that they will not affect the SOFC performance. For the intermediate case, it can be assumed that a few hundred ppm of tars can be tolerated but not full tar loads from biomass gasifiers. For the conservative case, it can be assumed that the tar is required to be removed to low ppm levels for safe and long-term SOFC operation. These three scenarios will require three different approaches for tar removal. As shown in above literature review, tars can be converted to non-condensable gaseous constituents at high temperatures. For thermal cracking without catalysts, temperatures above or near 1273 K are usually required. However, the use of catalysts reduces these temperatures to approximately 1173 K or lower. Because gasifier outlet temperatures range from approximately 1073 Ke1173 K, catalytic conversion appears to be advantageous for energy systems based on biomass gasification. Although employing the catalyst in the gasification bed is also considered as an option for tar reduction, employing a separate downstream reactor is considered to be more effective. For this reason, it is desirable to employ a separate reactor for the tar cleaning in the gas-cleaning system proposed here. To prevent excessive coke formation, the upstream in-bed addition of, e.g., dolomite, is needed to reduce the tar load to w2 g/m3n [87]. Additionally, it is expected that hightemperature tar-cleaning technologies will help remove tar

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molecules while keeping a major portion of their chemical and sensible energy in the gas. As shown by literature, several materials are considered to be suitable catalysts for tar cracking in downstream catalytic beds. The most tested materials are as follows:  Nickel-based catalysts.  Natural minerals such as calcined carbonate rocks (minerals containing mainly CaO and MgO). Calcined carbonate rocks and supported nickel-containing catalysts show good catalytic activity near 1173 K and are well established in the literature, which indicates that nickel-based catalysts have the best performance and remove tars nearly completely. The disadvantage, however, is that nickel-based catalysts are gradually deactivated by coke deposition. It is also well known that nickel is easily affected by other contaminants in the syngas, e.g., HCl and H2S. Therefore, nickel-based catalysts can likely be used only in combination with other cleaning devices and only if it can be assured that the gas entering the nickel catalyst is relatively clean. Laboratory tests have shown that calcium carbonate rocks, such as limestone, ankerite and dolomite, are also capable of decomposing tars to very low levels at temperatures ranging from 973 K to 1173 K [65]. It was suggested that dolomite is a better catalyst, although olivine has better attrition resistance than dolomite. However, the results reported in the literature indicate that dolomite-based reactors generally ended up with a few hundred ppm of tars at the outlet, while nickel-based reactors could reach levels of a few ppm. As discussed, studies with SOFCs and fuel gases containing tar components indicate less stringent requirements for tar cleaning than previously thought. It is therefore concluded that dolomite can be a good material as an initial choice for tar reduction in gasifierSOFC systems. Dolomite has the following advantages: 1) it is not easily deactivated by other contaminants when compared to nickel, 2) dolomite is cheaper than nickel, and 3) dolomite is expected to perform better than olivine. The problem of attrition can largely be avoided by employing fixed-bed reactors. A second nickel-based cleaning device may still be considered for use after other contaminants such as H2S and HCl are removed. In a conservative case, a nickel-based reactor can be used in combination with a dolomite-based reactor if the system design necessitates significant gas cleaning. For the intermediate case, a dolomite-based tar reformer can be used without a nickel-based one. If the current indications regarding the insignificant influence of tar on SOFC anodes are confirmed through long-term experiments and detailed studies, it is possible that a tar reformer could be avoided if biomass gasifiers are connected with SOFCs when suitable SOFCs and their operation parameters are chosen. Combined tar and particle cleaning using catalytic filters is an attractive development that needs to be studied in detail and further developed and demonstrated for future applications. This approach is a promising example of process intensification. 2.3. Sulfur compounds In general, biomass fuel contains less sulfur than fuels such as coal. Sulfur in the biomass feedstock will cause the production of sulfur compounds such as H2S and COS during gasification, depending on the gasification system used. Most biomass feedstocks contain low percentages of sulfur. Wood typically contains less than 0.1% sulfur by weight, and herbaceous crops might contain 0.3%e0.4% [215]. A few feedstocks, such as refuse-derived fuel (RDF), might contain 1% or more sulfur, which is approximately the

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same as bituminous coal. As a result of the low levels of sulfur in the biomass, the concentrations of H2S in the product gases are below those requiring cleanup in most applications. COS is another sulfur compound present in biosyngas at lower levels. For sensitive systems such as fuel cells, the removal of sulfur is required. It is expected that sulfur in biosyngas is present predominantly as H2S [216]. H2S is a colorless gas with an offensive smell similar to that of rotten eggs and dissolves in water to make a solution that is weakly acidic. The presence of H2S in biosyngas varies as much as 20 ppme200 ppm [216,217]. A total sulfur content of 200 ppmv (190 ppm H2S and 10 ppm COS) was reported for straw gasification without any in-bed sorbent. Use of in-bed sorbents such as limestone can reduce H2S presence in the gas generated to even lower values. 2.3.1. The influence of H2S on SOFC anodes The influence of H2S on SOFC performance has been widely studied. H2S is said to cause considerable problems in SOFC operation. It has been suggested that, even at low ppmv levels, H2S is adsorbed at active sites of the anode, thus inhibiting the fuel molecules from getting adsorbed and, in turn, affecting the fuel oxidation reactions. It is also widely considered that such an influence is reversible when H2S is fed at low ppm levels. At higher concentrations, sulfur will react with nickel and cause irreversible damage. The reported tolerance level of SOFCs for the presence of sulfur in the fuel gas varies with fuel composition and SOFC operating conditions. A presence of even 1 ppm of sulfur is likely to affect the performance of cells with Ni/YSZ anodes. This does not necessarily mean that all nickel-based anodes with all fuel compositions will equally be vulnerable to sulfur contamination. With a standard state-of-the-art SOFC with a Ni/YSZ anode, it has been shown that 1 ppmv of H2S can cause a significant performance decrease [218,219]. With a state-of-the-art Ni/YSZ anode in an actual measurement, Veyo reported that the presence of even a single ppm of sulfur caused a dip in the voltage of a tubular SOFC operated with simulated syngas [219]. In the presence of H2S, cell resistances increase at a lower rate at 1298 K than at 1273 K. This was interpreted as primarily a H2S desorption effect, in which more nickel surface areas are increasingly exposed as temperatures increase. For SOFCs employing Ni/YSZ anodes operating with sulfurcontaining impurities, a temperature dependence on the cell impedance was also reported by Matsuzaki and Yasuda [218]. The polarization impedance and DC overvoltage increased when H2S concentrations exceeded 0.05, 0.5, and 2 ppm increase in the fuel gas at 1023 K, 1173 K and 1273 K, respectively. The performance loss, because of sulfur contamination, was recoverable when the H2S was removed from the fuel gas. This recovery indicates that SOFCs may have to be operated at higher temperatures when biosyngas gas from biomass gasifiers is used as fuel and when Ni/YSZ anodes are employed. Results obtained by Rasmussen and Hagen from experiments on SOFCs with Ni/YSZ anodes fed with H2S in moist H2 [220] indicated cell voltage drops from 2 ppm to 100 ppm H2S when H2S was periodically added to the fuel gas (over 24-h periods). Tests on single anode-supported planar SOFCs were performed under the current load at 1123 K. Voltage returned to its initial value after the H2S supply was turned off. Evaluation of the changes in the cell voltage suggests that saturation coverage was reached at approximately 40 ppm H2S. A front-like movement of sulfur contamination over the anode was observed by monitoring the in-plane voltage in the anode. Furthermore, impedance spectra showed that the polarization resistance increased when adding H2S. It was also suggested that the contaminating effect of H2S was primarily

caused by the adsorption of the H2S at active sites, blocking Niparticles, not because of a significant change in the microstructure of the anode or the formation of an insulating layer. Results from studies on SOFCs with Ni/YSZ anodes that were conducted to evaluate the influence of H2S that is usually present in biosyngas were recently reported by Norheim et al. [221]. The experiments were carried out by feeding the anode a H2/CO2 mixture containing H2S and using single-cell SOFC setups operated at 1073 K. When H2S was mixed into the fuel gas in concentrations ranging from 20 to 100 ppm, the performance decreased with increasing sulfur concentration up to 80 ppm. The performance losses at 80 ppm and 100 ppm sulfur were equal. At a constant current density, the operating voltage was reduced when H2S was added. An increase in the area-specific cell resistivity was also reported. In another experiment, removing the sulfur impurity during a 240 ppm H2S exposure test was resulted in the full recovery of the cell performance. Again, the results indicated no irreversible changes in the cell structure at the levels at which the H2S was added. Several other studies have also evaluated the influence of H2S on SOFC anodes. Studies of the catalytic activity of Ni/YSZ cermets revealed an increased influence of H2S on methane reformation and the water-gas shift reaction when compared to CO oxidation [222]. Studies using in-situ Raman spectroscopy with 50 ppmv H2S at SOFC operating temperatures also revealed that the influence of H2S is not attributable to the formation of bulk nickel sulfides [223]. Studies of a micro tubular SOFC with an Ni/YSZ anode with both HCl and H2S added to humidified hydrogen indicated cell performance degradation at 1123 K and 1173 K when H2S is introduced (up to 3 ppmv) [224]. Anodes based on Ni/Ceria appear to show higher tolerance levels for sulfur for hydrogen oxidation, as reported in the literature. Aravind et al. recently demonstrated that a few ppmv of H2S does not affect the performance of the SOFCs with Ni/GDC anodes for shortterm operations. Experiments carried out at 1023 K and 1123 K with humidified hydrogen mixed with H2S up to 9 ppmv did not show significant impact on the electrochemical performance of the anode for hydrogen oxidation [5,6]. The same group (Ouweltjes et al.) also showed that although hydrogen oxidation on Ni/GDC anodes is not influenced by several ppm of H2S, methane oxidation on Ni/GDC anodes appears to be influenced by sulfur, even in very low concentrations. In this study, 3 ppmv H2S in methane-containing gas caused a significant drop in the SOFC performance [22]. The effect of H2S on a single-cell stack using simulated reformate gas and hydrogen/steam mixture as a fuel was reported by Schubert et al. [225]. They used electrolyte supported cells with Ni/GDC anodes. Different concentrations of H2S (2 ppme50 ppm) were added to fuel gases containing H2, CO, CO2, H2O, CH4 and N2 during cell operation. Tests were conducted at 1123 K. It was found that, at 1123 K, the contamination of the cell with up to 50 ppm H2S caused no continuous voltage degradation in the cell with reformate gas composition and at 45, 5 and 50 Vol. % with H2, H2O and N2, respectively. The cell voltages decreased initially after the H2S additions, followed by a stable cell voltage under these contaminating conditions. With reformate gas, in one experiment, the addition of 40 ppm H2S caused a drop of 9 mV (from 720 mV). In another experiment, addition of 8 ppm H2S caused a drop of 20 mV. When the H2S addition was ceased, a slower voltage recovery followed. The effect of the sulfur poisoning was observed mostly with the initial H2S addition and was explained by the rapid increase in coverage of the anode surface by sulfur atoms reaching high values even at lower H2S concentrations. These results indicate a rather stable performance with SOFCs with Ni/ GDC-based anodes when fuel containing H2S is used. Trembly et al. studied the performance of single-cell planar SOFCs with Ni/GDC anodes using coal syngas containing H2S [226].

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Cells were tested at 1123 K with a constant current load. Results with syngas in the presence of H2S in the range of 200 ppme240 ppm indicated good performance over 570 h, however, with a 10%e12.5% potential based degradation. These results suggest that these cells can be used for extended periods of time for syngas applications and that, in the presence of H2S, the cells show no major degradation after reaching stable operation. Based on comparisons with results in the literature, the authors suggest that the Ni-GDC cell seems to be a reasonable candidate for operating with syngas in the presence of H2S. This is in contrast with other studies showing that Ni-YSZ degrades significantly in the presence of sulfur. However, at low temperatures, approximately 873 Ke830 K, low ppm of H2S is shown to have an influence on Ni/GDC anodes [227]. Increasing the H2S concentration from 1 ppm to 3 ppm in moist H2 significantly increased the anode degradation. A decrease in H2 content in the fuel (97e9.7% H2) was also reported to increase the degree of sulfur interaction with the anodes. An influence of temperature variation on poisoning was also reported. Lowering the operating temperature (from 873 K to 830 K) caused an increase in anode poisoning. These results were compared with thermodynamic predictions, which showed the dependency of Ni-S interaction on partial pressure pS2 and temperature, as well as the effect of the ceria-S interaction, on pO2 and pS2. For biosyngas, sulfur also might be carried by sulfur-containing hydrocarbons such as thiophene. Although not directly connected with biosyngas oxidation, the influence of thiophene on SOFC stack performance has been studied by Nagel and others [228]. Thiophene is considered to be a sulfur-containing tar component. Experiments were conducted with a 1 kW SOFC stack to which natural gas was added after a catalytic partial oxidation reactor. The stack was operated at 1223 K. The sound stack operation was reported with natural gas with thiophene, although with a decrease in performance. Thiophene was added up to 400 ppmv in the natural gas, and a stack degradation of 6% was found. Thiophene’s possible influence on the SOFC could be attributable to the sulfur in the structure. However, its removal might require hydrocarbon reforming, which could lead to H2S formation and subsequent separate H2S removal. Although much more information is required for a full understanding of the effects of all such contaminants on SOFCs and for designing gas-cleaning systems considering all such contaminants in detail, in the present study, it is assumed that combining tar reforming and H2S removal will be sufficient to remove contaminants such as thiophene. A few other electrodes, based on different materials, are also being developed for greater sulfur tolerance [229]. Because such anodes are in the early stages of development, a detailed discussion of such anodes is not presented in this paper. For the gas-cleaning system being designed, it was decided that the sulfur levels should be brought down to approximately a few ppmv. This relatively high level is suggested because the methane concentration in biosyngas from air-fed gasifiers is very low. However, when steam gasifiers are employed, more stringent H2S cleaning might be advisable because the methane content in product gas is often high. 2.3.2. Cleaning of hydrogen sulfide There are considerable limitations on the temperature requirements for high-temperature sulfur-cleaning devices. In general, H2S removal down to the sub-ppm levels is necessary for chemical production and fuel cell power generation, and this requires sulfur capture in external reactors downstream of the gasification units. At higher temperatures, metal oxides are considered as a suitable option for H2S removal. With zinc oxide sorbents (573 Ke823 K), or with ceria sorbents at temperatures near 1073 K, it is feasible to

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reduce the sulfur content in the fuel gas from approximately 200 ppmv to 300 ppmv in H2S concentration to approximately 1 ppm [230,231]. If the sulfur is present as COS, it can be converted to H2S for its removal. At elevated temperatures, H2S removal is usually done with metal oxide sorbents [232]. In general, the reaction equation of the desulfurization reaction is:

Mex Oy þ yH2 S/Mex Sy þ yH2 O

(1)

The metal oxide sorbents can be separated into regenerable and disposable ones. The disposable sorbents are typically based on calcium and are injected into the gasifier to serve as bulk gas desulfurization agents. Regenerable sorbents are primarily used in a separate fixed-bed reactor after the gasifier, which makes regeneration of the used sorbent easier. In general, the metal oxide can be regenerated with the following reaction:

Mex Sy ðsÞ þ 1:5yO2 ðgÞ/Mex Oy ðsÞ þ ySO2 ðgÞ

(2)

There are several candidate metal oxides that can be used for desulfurization of the fuel gas. Ayala and Venkataramani [230] studied the thermo-chemical equilibrium of several metal oxides. They observed that at elevated temperatures, only the sorbents based on zinc, cesium or copper oxide are capable of cleaning H2S below 1 ppmv. This is confirmed by several other studies [233,234]. In general, the cleaning capability of various oxides in a temperature window of 350 Ke923 K are reported as follows: Sn < Ni < Fe < Mn < Mo < Co < Zn < Cu and Ce, with oxides of molybdenum and onward being capable of cleaning up to a level of less than 1 ppm in the gas within the given temperature window [233]. A recent review of high-temperature desulfurization of biosyngas was presented by Meng et al. [235], and therefore a detailed literature review is not presented in this paper. The brief discussion on H2S cleaning sorbents below draws largely from the MSc thesis of van Loon carried out at Delft [236]. Zinc oxide: Zinc oxide sorbents theoretically show good results for H2S cleaning at elevated temperatures (723 Ke1023 K). A major disadvantage of ZnO is that it quickly reduces in reducing fuel gas atmospheres at high temperatures. In addition to the stoppage of cleaning, this presents serious problems because pure zinc starts to vaporize. Instead of cleaning the gas, this approach will cause the gas to become polluted with zinc. Therefore, attempts were made to combine ZnO with other metal oxides to prevent the sorbent reduction. It was reported that zinc ferrite and zinc titanate are useful in this regard, and thus they are discussed below. Zinc titanate: Zinc titanate is formed by a reaction of zinc oxide with titanium dioxide. The exact chemical formulation depends on the molar ratio. With the addition of TiO2, an increase in the operating temperature can be achieved because zinc reduction is prevented [237]. Based on theoretical studies [233], it was reported in the literature that zinc titanate was able to reach the same residual H2S levels as pure ZnO. H2S is removed as per the reaction below.

Zn2 TiO4 þ 2H2 S/2ZnS þ TiO2 þ 2H2 O:

(3)

The sorbent is stabilized against reduction. In the literature, in general, zinc titanate was mentioned as the best option for H2S removal at high temperatures (up to 1123 K) [230,233,234]. It has the same favorable thermodynamic properties as the zinc oxide sorbent, but it was more stable. Another advantage of zinc titanate is that it has shown better attrition resistance than other sorbents. H2S emission variations after the chemical reactions with zinc titanate sorbent were analyzed using chemical equilibrium computations, which were carried out with biosyngas based on a composition obtained from Delft CFBG.

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Equilibrium computations were carried out at Delft with the following input concentrations: 1) biosyngas: 1 mol, 2) H2S: 100 ppmv, and 3) Zn2TiO4: 10 mol, at atmospheric pressure in a temperature range of 773 Ke1073 K. The results shown in Fig. 11 indicate the following. H2S emissions at a sorbent temperature of 773 K or more will be higher than the 1 ppmv often suggested for SOFCs in the past. However, recent experiments indicate that H2S with few ppm had no significant influence on hydrogen and carbon monoxide oxidation on Ni/GDC anodes [5,20]. However, the influence of H2S on methane reforming was observed, and this can only be ignored if the methane presence in the gas entering the SOFC is not significant, which is the case here. Zinc ferrite: Zinc ferrite is based on zinc and iron oxides. The iron oxide is added to the zinc oxide to lower the reduction rate of ZnO. In addition, iron oxide itself has desulfurizing ability, which is an added advantage. However, iron oxide does not offer sufficient cleaning when compared to zinc oxide, as indicated by the higher H2S levels in the gas at chemical equilibrium at high temperatures in the range of 673 Ke873 K. Kobayashi et al. [238] developed a zinc ferrite sorbent capable of reducing the H2S level to 1 ppme3 ppm at 723 K, and 5 Vol% moisture. Pineda et al. [239] reported that zinc ferrites can clean fuel gas to a concentration of 1 ppmv to 3 ppmv H2S at 873 K. A disadvantage of zinc ferrite sorbents is that they can have problems with excessive attrition [240]. Gangwal and Gupta [241] reported that the use of zinc ferrite sorbents must be limited to 823 K. Copper-based sorbents: Another candidate with favorable thermo-chemical equilibrium for sulfur removal is copper oxide. However, copper-based sorbents are rarely used in practice at high temperatures. The problem is that, similar to zinc oxide, copper oxide is easily reduced to elemental copper in the reducing fuel gas environment at high temperatures, which, in turn, results in insufficient levels of desulfurization. Preventing the reduction of copper oxides is difficult [235,242]; therefore, copper oxide is not a realistic option for H2S cleaning at high temperature unless it is supported by other materials. Although the development of such sorbent materials is being pursued by several researchers, it appears that more work is required on these types of durable sorbents. Cerium-based sorbents: Ayala and Venkataramani [230] have demonstrated the favorable thermo-chemical equilibrium properties of ceria for sulfur removal. Although CeO2 will react with H2S, the reaction thermodynamics do not permit high levels of sulfur

cleaning [231]. Reduced cerium oxide has been shown to be a highly efficient and durable second-generation high-temperature desulfurization sorbent. During the sulfidation phase, the H2S concentration was reduced to nearly 1 ppmv at a temperature of 973 K. This is well beyond the capability of first-generation zinc-based sorbents. The authors have conducted experiments with gas compositions similar to those generated in the coal gasification units in IGCC power plants. The reported experiments were performed at pressures of 5 atm or higher. However, the sulfidation reaction appeared to be slow at temperatures near 873 K. Elemental sulfur was produced directly when Ce2O2S, was regenerated using SO2. There was no apparent deterioration in sorbent performance in an extended test consisting of 25 reductionesulfidationeregeneration cycles, where the reduction and sulfidation temperature was 1073 K and the regeneration temperature was 873 K. However, the oxidation state of ceria depends heavily on the partial pressure of oxygen in the syngas. With the gas composition considered in this paper, there is a chance that the Ce2O3 was oxidized to CeO2 very rapidly; however, detailed studies are required to confirm this for different gas compositions. Equilibrium computations were carried out with biosyngas from Delft CFBG fed with H2S and the sorbent used as reduced states of ceria, namely, Ce2O3 and Ce18O31. Following are the input parameters given for the equilibrium calculations. Equilibrium computations were carried out at Delft with the following input concentrations: 1) biosyngas: 1 mol, 2) H2S: 100 ppmv, 3) Ce2O3/Ce18O31: 10 mol. Computations were conducted at atmospheric pressure in a temperature range of 773 Ke1073 K. The results are presented in Fig. 12. It is seen that Ce2O3 offers the best option, with H2S presence in the gas always remaining below sub-ppm. Non-stoichiometric cerium oxide at reduced conditions appears to be a possible candidate for H2S removal for SOFC applications at high temperatures (near 973 K) under certain conditions. However, further studies, especially on the oxidation to CeO2, are required to confirm Ce2O3 as a suitable choice for biosyngas applications.

Fig. 11. Presence of H2S in cleaned biosyngas after reaction with Zn2TiO4 in the gas based on chemical equilibrium calculations.

Fig. 12. Presence of H2S in cleaned biosyngas after reaction with Ce2O3/Ce18O31 in the gas based on chemical equilibrium calculations.

2.3.3. Conclusions regarding the choice for H2S cleaning It appears that a suitable option for H2S cleaning with the available information is offered by zinc-based sorbents. Zinc titanate is considered stable at temperatures of 873 K. However, chemical equilibrium calculations with the biosyngas composition obtained from the Delft CFBG, with 100 ppm H2S as input and using

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zinc titanate as the sorbent, showed that the equilibrium concentration of H2S was approximately 4 ppmv. Because the literature indicates that H2S levels up to 9 ppm caused no problems for hydrogen oxidation, zinc titanate was the first choice for sorbent for high-temperature sulfur cleaning, although H2S at 4 ppmv in the gas is expected to influence methane oxidation at the SOFC surface. However, because methane presence was only approximately 2% in the gas and part of this 2% was expected to be reformed during tar reforming, it was assumed that 4 ppmv H2S was acceptable in this case. The use of mixed sorbents and reduced ceria for this application was also suggested as a topic for further research. 2.4. Hydrogen chloride The halide gases obtained from gasification primarily contain HCl, with HF and HBr being the other two constituents of which HCl is considered as present in higher concentrations. Few reports present precise quantities of HCl in biosyngas. In the case of biomass gasification, depending on the biomass fed, several ppm of HCl could be present in the producer gas. Simell et al. recorded HCl peaks up to 90 ppm in biosyngas [179]. Thermodynamic calculations carried out for a typical fuel gas of autothermal fluidized bed wood gasification systems showed that the gas contains 200 ppm HCl [243]. HCl at such concentrations is likely to affect the SOFCs and other gas-cleaning devices. Therefore, it is often assumed that HCl will be removed from the syngas to a level of approximately a few ppm before it is fed to the SOFCs. 2.4.1. The influence of HCl on SOFC performance The impact of HCl on SOFC performance has not been studied in detail. In general, HCl is reactive to catalysts like nickel and therefore could cause problems with SOFC operation. The HCl has to be removed before the gas is fed to a SOFCs for two reasons: 1) HCl can cause corrosion of system components, 2) It can react with components of the SOFC anodes and cause cell degradation, although the expected impact would not be severe. PV Aravind et al. recently demonstrated that a few ppm of HCl does not affect the performance of the SOFCs with Ni/GDC anodes for short-term operation. Experiments carried out at 1023 K and 1123 K with humidified H2 mixed with HCl up to 9 ppmv did not show any significant impact on the electrochemical performance of the anode for hydrogen oxidation [5,6]. Buchinger et al. showed that a stable SOFC operation can be achieved with hydrogen gas containing HCl at approximately 50 ppm for several tens of hours [224]. Trembly et al. reported steady SOFC operation with HCl containing fuel through 20 ppme160 ppm, which HCl in the fuel caused a recoverable loss in performance [244]. These results indicate that, with the currently available information, cleaning biosyngas to reduce the HCl levels to a few ppm is required before it is fed to the SOFCs. Further research is required for more in-depth elucidation on the influence of HCl on SOFC anodes and to develop HCl-tolerant anodes. 2.4.2. Cleaning of hydrogen chloride There are basically two methods for removing HCl at high temperatures: 1. By injecting alkali compounds into the gasifier to form salts that can be removed by particulate control. 2. by removal by sorbents in a separate reactor. Alkali metal compound injection is not discussed because it can cause higher alkali emissions in the gas, and it has been preferable to use separate downstream cleaning reactors for all of the contaminants in the proposed gas-cleaning system.

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Krishnan et al. [245] calculated the equilibrium partial pressures of both HCl and metal chloride vapor as a function of temperatures for various alkali and alkaline earth compounds under coal gasification product gas conditions. These calculations predict the theoretical limit to which the concentration of HCl can be reduced by various oxide and carbonate sorbents. Among the alkali and alkaline earth carbonates, only the sodium and potassium compounds have equilibrium HCl pressures less than 1 ppm at 773 K. With Na2CO3 at 873 K, Krishnan et al. reported 1.3 ppmv HCl and 1.4 ppmv NaCl emissions. With K2CO3 at 873 K, these values were reported as 0.11 ppmv HCl and 3.4 ppmv KCl. With Ba2CO3 at the same temperature, these values were reported as 22 ppmv HCl and 3.5  1012 ppmv BaCl2. Of the alkali chlorides, potassium chloride provides the lowest level of HCl vapor, but NaCl (g) has a lower vapor pressure than KCl (g). The alkaline earth oxides provide very low equilibrium HCl vapor levels if they are not converted to the corresponding carbonates. However, the typical biosyngas gas streams contain CO2, and under thermodynamic equilibrium conditions, alkaline earth oxides will then be converted to the corresponding carbonates. It has been shown by thermodynamic equilibrium calculations that sodium and potassium compounds are capable of reducing HCl vapor to a level of less than 1 ppm at elevated temperatures (temperatures near 873 K) [245]. Krishnan et al. also reported that the mineral nahcolite (NaHCO3) could be used as an inexpensive HCl-scavenging sorbent suitable for fixed and fluidized bed reactors [246]. It has been demonstrated that these sorbents are capable of reducing HCl levels to below 1 ppm in the temperature range of 873 Ke973 K. Dou et al. [247] performed experiments with sorbents based on alkali and alkali earth metal compounds (NaHCO3, Ca(OH)2 and Mg(OH)2) supported on Al2O3. All of the sorbents were capable of reducing the HCl level from 1000 ppm to 1 ppm at a temperature of 823 K. Nunokawa et al. [248] found that the sorbents prepared by sodium carbonate and alumina give better results than pure sodium carbonate. Li et al. [249] studied hightemperature HCl removal and found a sorbent capable of scavenging the fuel gas to levels below 1 ppm HCl at 923 K. They used a sorbent called GH1, which is composed of 45% Meerschaum, 30% Ca(OH)2, 7% NaHCO3, 10% NaCO3 and some clay and cellulose. They also performed tests at 1023 K in which a purity level of 1 ppm of HCl was reached. When nahcolite is used as a sorbent, the following reaction will take place:

NaHCO3 ðsÞ þ HClðgÞ/NaClðsÞ þ H2 OðgÞ þ CO2 ðgÞ

(4)

However, water molecules are separated when the temperature of nahcolite is raised to approximately 823 K. Nahcolite can then disintegrate to sodium carbonate, and HCl will react with Na2CO3 to form sodium chloride.

2NaHCO3 ðsÞ/Na2 CO3 ðsÞ þ H2 OðgÞ þ CO2 ðgÞ

(5)

2.4.3. Conclusions regarding the choice of sorbent for HCl cleaning Various sorbents based on alkali or alkaline earth metal compounds are suggested for high-temperature capturing of halides. However, with these metallic compounds, metal vapor can form at high temperatures, which can cause problems if it gets into the fuel gas. A compromise between the evaporation of the metal and the capability of the sorbent to remove HCl is the key issue in the selection of a proper sorbent. Sodium- and potassium-based sorbents offer good acid removal capabilities, with sodium carbonate as one of the best options. These sorbents can reduce levels of HCl to sub-ppm levels [245].

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Based on the above discussion, Na2CO3 will be used as the sorbent for the present studies at 873 K because it offers reasonable HCl cleaning capability with minimum emission of alkyl halide (in this case, NaCl). In this case, with sodium carbonate, the cleaning reaction will be:

Na2 CO3 ðsÞ þ 2HClðgÞ/2NaClðsÞ þ CO2 ðgÞ þ H2 OðgÞ

(6)

Variations in the emissions of HCl and sodium chloride downstream of the sorbent, which occur when sodium carbonate is used to remove HCl from a gas with a composition same as that from Delft CFBG (as given in Fig. 2), mixed with 50 ppm HCl, were analyzed using chemical equilibrium computations. Chemical equilibrium computations were performed at Delft with biosyngas with a composition the same as that from Delft CFBG. The software tool Factsage was used for the calculations. Equilibrium computations were performed with the following input concentrations: 1) biosyngas: 1 mol, 2) HCl: 50 ppmv, and 3) Na2CO3: 10 mol. Computations were carried out at atmospheric pressure in a temperature range of 773 Ke1073 K. Results from the calculations are presented in Fig. 13. It can be observed that, to have a HCl level of approximately 1 ppm, the sorbent temperature has to be lowered to approximately 873 K. The NaCl emission is then slightly above 1 ppm at 873 K. Because the high temperature causes the evaporation of alkali chlorides, in this case approximately 1.4 ppm, it was decided that an alkali getter would be used after the sodium carbonate reactor in the gascleaning series. 2.5. Alkali metal compounds Biomass feedstocks can contain significant amounts of alkali compounds, mainly comprising sodium and potassium, with the potassium content usually being considerably higher than the sodium content. Potassium is an element required for plant growth, and higher concentrations are found in fast-growing plants. Eutectic sodium and potassium salts in the ash material can vaporize at gasification temperatures above 973 K [3]. Unlike solid particulates, which can be separated by physical means such as barrier filters, vaporized alkali compounds will remain in the product gas at high temperature and thus cannot always be removed by simple filtration. Condensation of the vaporized alkali typically begins at approximately 923 K on particles in the gas

stream, with subsequent deposition on cooler surfaces in the system such as the heat exchangers and the turbine expansion blades. Of the total alkali contents of a given biomass, only a minor fraction remains in the gas phase after the gasification process. Various studies have reported widely varying amounts of alkali metal in the gaseous phase. With sawdust as an input to a pressurized fluidized bed gasifier, the alkali concentration in the gas was reported at sub-ppm levels, even at temperatures as high as 1088 K [44]. For syngas from other fuels such as various grasses and bagasse, the presence of alkali metal was reported at levels of a few ppm at temperatures above 773 K [216,250,251]. During tests with a fluidized bed gasification facility, it was reported that the alkali presence from sawdust gasification after cyclones at temperatures of 1046e1068 K [44] was at sub-ppm levels. But even this is significantly higher than 0.1 ppmw, which is considered to be the limit for gas turbine operation. However, for the gasification of straw, the total alkali content went up to a few ppm levels before cleaning [216]. 2.5.1. The influence of alkali compounds on SOFC performance Alkali evaporation and its subsequent deposition in biomass gasifierebased systems have been well studied [44,216,252e258]. Alkali metal compounds cause corrosion in gas turbines, heat exchangers and so forth [216]. They are said to have deleterious effects on fuel-reforming catalysts [3] and probably may also have a negative influence on fuel cell electrodes. The nature of impacts of these compounds on SOFC electrodes has not been studied in detail. Impurities such as Na2O and SiO2, which are present in the anode structure and introduced during the cell fabrication and affect its durability, have been described in the literature [259]. It has been observed that the SOFCs containing impurities of several hundred ppm in the anode structure degrade much faster (within a few hundred hours) than SOFCs with a few tens of ppm of contaminants. Although these contaminants are not introduced through the fuel gas, this could give some indication about the possible impact of similar contaminants in the gas supply. This might indicate that a rather high level of alkali cleaning will be required before biosyngas is fed into the SOFCs. The impact on SOFCs of alkali compounds in the fuel gas has not been studied in detail, and the preliminary experimental results reported indicate that some impact at levels of a few ppm [5]. However, this has to be confirmed with detailed experiments. 2.5.2. Alkali cleaning Two principal methods are employed for cleaning vapor-phase alkali compounds: 1. Regarding cleaning at low temperatures, when the gas temperature is lowered below 823 Ke873 K, alkali vapors will become condensed and will be able to be removed with particle removal systems. Because this method is not suitable for cleaning alkali compounds to a few ppm or sub-ppm levels at or above 873 K, it is not discussed in detail in this paper. 2. Syngas with alkali compounds can also be cleaned by passing through alkali getters, such as activated bauxite or activated alumina. Alkali compounds are then physisorbed or chemisorbed on the getter surface, with chemisorption suggested to dominate when moisture is present.

Fig. 13. Emission of gaseous NaCl, HCl and (NaCl)2 at various temperatures with the representative biosyngas composition from the Delft CFBG with 50 ppmv of HCl in the syngas based on chemical equilibrium calculations.

When alkali getters are used, if the removal is by chemisorption, the general reaction is:

2AlkCl þ H2 O þ Al2 O3 :xSiO2 þ O2 /2AlkAlO2 :xSiO2 þ 2HCl

(7)

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The H2O content of the gas plays an important role. In the absence of water, the chemisorption will be completely stopped. Similarly, the presence of HCl also plays a role in the reaction and affects the cleaning. In the absence of water, the possibility for chemisorption is minimal. However, because biosyngas inevitably contains moisture, it will always promote chemisorption when the equilibrium conditions are favorable. If there is a large amount of HCl present in the feed gas, chemisorption is again not favored. If the removal is by chemisorption, HCl will be released in the process, which will need to be removed. This can cause additional problems when a series of reactors are employed to remove different contaminants. This problem is avoided if the mechanism employed is physisorption. Studies have indicated that it is possible to remove alkali contaminants by physisorption on alkali getters, as described in the following section. Possible sorbents for alkali absorption: Wolf et al. [260] tested four possible solid sorbents for the removal of alkali species. They found that bauxite is capable of removing alkali below 1 ppmv. The researchers were able to reduce the alkali content to values lower than 100 ppb during tests performed in a packed-bed filter at 1023 K under a reducing environment. The experiments were conducted at atmospheric pressure, and NaCl was used as the alkali. The literature also indicates that sorbents such as bauxite, kaolinite and activated alumina can clean the gases to below 1 ppm alkali [261e263]. In general, the research focus has been on removing the sodium content in the fuel gas. For biomass, potassium is more important than sodium because potassium often has a higher concentration. Turn et al. [250] performed experiments with biomass derived gases. They concluded that with activated bauxite as a sorbent, the potassium concentration could be reduced to 0.03e0.07 ppmw and sodium to 0.2e0.9 ppmw. The gas was fed through a fixed-bed rector at 973 K. Dou et al. [247] tested several sorbents for alkali metal removal from high-temperature coal-derived gas. They carried out tests using a fixed-bed reactor at 1113 K with a gas composition simulated from that of an IGCC plant with compositions similar to that of biosyngas. NaCl was used as an alkali metal model compound. These authors found that, of the five sorbents being tested, including second-grade alumina, bauxite, kaoline, acidic white clay and activated alumina, Al2O3 shows the highest adsorption efficiency. For an experiment that was 3 h long, the sodium adsorption efficiency of Al2O3was reported to be as high as 98.20%. The tests also indicated that the removal of alkali metal vapor with activated Al2O3is a physical adsorption process in the absence of water that supports the chemical reaction involved. From these studies, it is inferred that a) adsorption of alkali compounds on activated alumina will occur and that, b) when the conditions are favorable for chemical reactions to take place, such as the presence of moisture and certain reaction conditions, alkali is chemisorbed with the release of HCl. However, in gasifier-SOFC systems, re-emission of HCl is undesirable and should be avoided if possible. To explore this possibility, a detailed analysis of cleaning reactions is required. To evaluate the use of activated alumina as a sorbent for cleaning biosyngas from the Delft CFBG with clean wood as fuel, equilibrium calculations were carried out under the following assumptions. The biosyngas composition that is used is presented in Table 1. Prior to alkali cleaning, HCl is cleaned from the gas, as discussed previously, using sodium carbonate as a sorbent, and 1 ppm HCl remains in the gas after cleaning. For the calculations, 1 ppm KCl from the biomass gasifier is assumed to be present in the gas. NaCl evaporation is expected during the HClcleaning stage (from the Na2CO3 used for HCl removal), and therefore, it is assumed for the calculations that 2 ppm NaCl is present in the gas.

757

Two sets of equilibrium computations were carried out at Delft with the input concentrations as given below. Case 1: 1) biosyngas: 1 mol, 2) KCl: 1 ppmv and 3) Al2O3: 10 mol. Computations were carried out at atmospheric pressure in a temperature range of 773 Ke1073 K. Case 2: 1) biosyngas: 1 mol, 2) KCl: 1 ppmv, 3) HCl: 1 ppmv, 4) NaCl: 2 ppmv and 5) Al2O3: 10 mol. Computations were carried out at atmospheric pressure in a temperature range of 773 Ke1073 K. Results for the calculations, as shown in Figs. 14 and 15, indicate the following. First, the possibility exists for the chemisorption of alkali (as given in reaction 7), and hence HCl evaporation was higher at lower temperatures of approximately 873 Ke1073 K. Second, at high temperatures of approximately 1123 Ke1143 K, chemisorption was not favored, and the fraction of HCl in the gas significantly decreased, even when moisture was present. However, the possibility of the alkali chloride being physisorbed to reach the sub-ppm level of cleaning at high temperatures was previously observed in the literature [247]. This possibility provides an opportunity for alkali cleaning from biosyngas by physisorption at high temperatures without significant chemisorption and thus re-emission of HCl into the fuel gas, when the equilibrium conditions provided are such that they prevent chemisorption. Under these conditions, the alkali chlorides are expected to become adsorbed on the adsorbent surface without being involved in the chemical reactions. To evaluate this assumption, preliminary tests were performed using activated alumina as a sorbent to remove KCl from a hydrogen stream [5,264]. The experiments indicated that alkali cleaning of more than 80% is probably achievable at temperatures of 1073 Ke1123 K. It appears that the cleaning mechanism is a combination of physisorption and chemisorption. With the given gas conditions at temperatures above 1073 K, re-emission of HCl will be significantly reduced because chemisorption is not favored at these temperatures, and physisorption appears to be sufficient for removing the alkalis. If these results are confirmed in detailed experiments, it would suggest an excellent opportunity for employing high temperature alkali getters for biosyngas cleaning without significant reemission of HCl. This also would allow the use of alkali-based sorbents at high temperatures above 873 K for HCl cleaning as a part of a total cleaning system when the sorbents are used in the series before the alkali getters.

Fig. 14. Presence of gas phase KCl and HCl in cleaned biosyngas after reaction with Al2O3 in the gas based on chemical equilibrium calculations (with 1 ppmv KCl as alkali input).

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used. Fixed-bed downdraft gasifiers give rather low ammonia concentrations in the gas, and reported concentrations are approximately a few hundred ppm [266]. Ammonia can be a fuel for SOFCs [267e269]. At the anode, ammonia dissociates into N2 and H2, and consequently, H2 is oxidized. The reaction is endothermic and can increase the efficiency of the cell operation. The total breakup of the ammonia on the nickel cermet anodes above 863 K has been reported [269]. Dekker et al. indicated that the decomposition of ammonia on the nickel cermet anode will depend on the availability of the catalyst present and the temperature. It is reported that total decomposition of ammonia takes place at higher temperatures and low NOx formation, less than 0.5 ppm was observed till an SOFC temperature of 1123 K [269]. Because ammonia could be a fuel for SOFCs, options for removing ammonia are not discussed here. 2.7. Other contaminants Fig. 15. Presence of gas phase KCl, NaCl, and HCl in cleaned biosyngas after reaction with Al2O3 in the gas based on chemical equilibrium calculations (with 1 ppmv HCl, 1 ppmv KCl, and 2 ppmv NaCl together with biosyngas as input).

In general, for biomass gasification systems, alkali getters can be used in two conditions:  By injecting sorbents into the gasification reactor  In a separate reactor, in which the gas stream is forced through a bed of sorbents As with other sorbents, a separate reactor is suggested for the gas-cleaning system being designed because it provides much better control over the operation parameters of the reactor with the getter materials. 2.5.3. Conclusion regarding the choice for alkali cleaning It appears that the best way to clean alkali salts at temperatures exceeding 873 K is to use an alkali getter. The literature indicates that activated alumina is one of the best sorbents, with physisorption combined with chemisorption involved as the cleaning mechanism. When this sorbent is employed at temperatures of approximately 1123 Ke1143 K, and when the fuel gas has a composition representing that of the Delft CFBG (Fig. 2), the alkali salts will probably be removed from the gas by physisorption without any significant chemical reactions involved. This removal enables alkali cleaning without the re-emission of any significant amount of HCl. Hence, a fixed-bed reactor with activated alumina was the first choice for cleaning alkali compounds from biosyngas for the purpose of this research. 2.6. Nitrogen-containing contaminants The primary nitrogen-containing contaminant in biosyngas is ammonia (NH3) [38,108,216]. The acceptable levels of ammonia in the outlet flue gas streams from power plants are typically dictated by civil regulations. Ammonia removal is usually performed to meet these environmental regulations. HCN is another nitrogencontaining component that can be found in biosyngas, where it can be present at levels of up to a few hundred ppm [216]. There is no detailed information available about its impact on the performance of commercially produced SOFCs. However cells with special anodes are even suggested for HCN production using SOFCs with the anode off gas containing HCN [265]. HCN influence on SOFCs and their cleaning is not considered in the present study but is suggested as a topic for future research. In biosyngas, ammonia can be present at levels of up to a few thousand ppm [38,108], depending on the gasification method

Several contaminants are present in biosyngas in very small quantities. These contaminants originate from the biomass or from components of the gasifier system or cooling and cleaning systems. Mercury, cadmium, lead, manganese, cobalt, antimony, selenium, beryllium, arsenic, chromium, nickel and silicon are examples of such contaminants. Their presence is often limited to levels of a few ppm or sub-ppm [258]. The impact of trace contaminants in biomass gasification product gas on SOFCs with Ni/GDC anodes has rarely been studied; therefore, future investigations are required to understand their impact on long-term operations of different types of SOFCs. However, limited studies using coal derived contaminants and SOFC with Ni/YSZ anodes indicate possible impact from several trace contaminants on SOFCs [270]. The presence of the above mentioned trace elements in clean wood-derived biomass gasification product gas can be significantly low whereas the gas from leafy biomass gasification may have them at higher levels. Hence, in-depth studies for each combination of biomass, gasifier type, SOFC type and gas cleaning method is necessary to determine safe levels of trace elements in the fuel gas for long-term operation of SOFC fed with biosyngas and is strongly suggested as a topic for future research. 3. Summary of the results and a suggested configuration for a cleaning system Evaluation of the available information on the influence of biomass derived contaminants on SOFCs indicates that, as anticipated, SOFCs with Ni/GDC anodes might have less stringent cleaning requirements, especially when they are operated above 1123 K. Therefore, it will be easier to design gas-cleaning systems for these types of SOFCs than for other SOFC options. In this section, we attempt to present gas-cleaning system concepts primarily for these types of SOFCs, especially for a gas-cleaning chain presented with 873 K as the lowest gas cleaning temperature. This scheme was used at Delft University for building an experimental gascleaning unit. An analysis of the options available for hightemperature gas cleaning indicates that cleaning of biosyngas at various temperatures, while meeting the requirements of SOFCs with a Ni/GDC anode using available technologies, is likely to be feasible. Various possible gas-cleaning options are briefly presented that could create temperature limitations (and, in turn, result in different lowest temperatures in the cleaning chain). Although several of the gas-cleaning methods described below can also be used to develop systems for cleaning the biomass gasification product gas for SOFCs with state-of-the-art Ni/YSZ anodes, in general, more stringent cleaning might be required. Gas cleaning concepts with different lowest gas cleaning temperatures are

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discussed below, including the conceptual design for the system with 873 K as the lowest temperature in it is presented. (1) The lowest temperature in the gas-cleaning unit varies between 1023 K and 823 K. If HCl cleaning is avoided altogether and if a few tens of ppm of H2S in biosyngas is no problem for the SOFCs, zinc titanate could be used for sulfur cleaning at higher temperatures. The possibility of rather high tolerance of the SOFCs for HCl and H2S in fuel is discussed above [1,3]. If some of the initial results can be confirmed with long-term experiments, or suitable anodes are developed, such possibilities of high tolerance levels for hydrogen oxidation, for several ppms of these contaminants, with or without recoverable performance losses, might emerge in the future. Tar reforming and particulate cleaning with ceramic filters is suggested at gasification temperature. An alkali getter can be suggested at the SOFC inlet temperature. (2) Gas cleaning units with the lowest gas-cleaning temperature in the range of 823 Ke673 K; lower temperatures down to 673 K may represent systems in which ZnO is used for H2S cleaning at various temperatures. At these temperatures, sodium carbonate can be used for HCl cleaning at the same temperatures as for the ZnO sorbent. Tar and particulate cleaning is suggested to be performed at the gasification temperature. An alkali getter can be suggested at the SOFC inlet temperature. Alternately, particles can also be removed near the H2S and HCl cleaning temperatures, which will help remove a portion of the alkalis as well. Cooling the syngas to such temperatures may lead to carbon deposition. As discussed before steam addition will help to avoid this. (3) Nonetheless, lower temperatures, and hence higher steam addition values, such as 20e30%, may represent cases in which low-temperature filters such as bag filters are used for particulate cleaning after a tar reformer working at near gasifier temperature. With other gas-cleaning devices, HCl and H2S cleaning can either be working at lower temperatures or avoided altogether. Alkalis are expected to be removed at the particle filter operating at temperatures lower than 673 K. No detailed discussion on such systems is provided because the system components were not discussed in detail in this paper. Details of the proposed gas-cleaning chain with 873 K as the lowest gas-cleaning temperature in the system that is used to build and experimental test rig are given below. For the SOFCs with Ni/GDC anodes considered in this study, operating on the biosyngas produced from the Delft CFBG airblown gasification, the following contaminant tolerance limits are assumed for designing this gas cleaning system. Particulate: 1 ppmw Tar: Several tens to few hundred ppmv H2S: Few ppmv HCl: Few ppmv Alkali Chlorides: 1 ppmv For cleaning tars, alkali compounds, HCl and H2S at high temperatures, separate fixed-bed reactors for each of the sorbents/ catalysts are easier design options. For particulate removal, ceramic filters will be employed at approximately 1123 K, in addition to the cyclone existing as a part of the circulating fluidized bed gasifier. Tar removal is carried out immediately after the ceramic filter, so that the following sorbents are not affected by tar cracking. For tar removal, cleaning between 1123 K and 1173 K, with dolomite as a catalyst, is probably sufficient if the tars present at a few tens of ppm do not cause any problem for SOFC operation, as discussed

759

before (1173 K is probably required for better cleaning). In this case, if required, the gas has to be heated from a gasification temperature in the range of 1123 Ke1173 K. If this is not sufficient, an additional fixed-bed reactor with nickel-based sorbents operating at 1123 K is required, which can bring the tar level down to a few ppm. This reactor should be placed after the H2S, HCl and alkali removal units preferably before the final particulate filter. When a nickelcontaining reactor is employed, even the dolomite bed shall be operated at 1123 K. For the removal of HCl, the best option is cleaning with nahcolite or sodium carbonate as sorbent. The highest possible operating temperature for reaching the 1 ppm level is expected to be approximately 873 K. Many of the alkali compounds are condensed out at this stage as well. HCl removal is done before alkali removal so that the alkali getter removes evaporated sodium chloride from the HCl cleaning stage. For hydrogen sulfide cleaning, zinc titanate will be used at 873 K. It is suggested that this sorbent be placed downstream of the HCl removal stage. Hence, a rather clean gas enters the reactor containing comparatively expensive zinc titanate. For alkali removal, the best option is cleaning with an activated alumina sorbent. The operating temperature suggested is 1123 K. With the present configuration, it is suggested that alkali removal is performed as a final stage, except for the optional final tar removal unit and the final particulate filter. The cleaning system proposed (Fig. 16) is capable of cleaning tars to levels of a few hundred ppm if dolomite is used as a catalyst for tar cracking and to levels of a few ppmv if nickel-based sorbents are used. Particulates are expected to be cleaned to approximately 1 ppmw. HCl emission is expected to be approximately 1 ppmv to 2 ppmv. KCl is expected to be at sub-ppm levels in the cleaned gas and H2S is expected to be at a few ppm levels. Steam addition to the syngas that is being cleaned is not required in the proposed system because carbon deposition is not expected thermodynamically at the temperature ranges involved, including the lowest temperature of the gas flow in the HCl cleaning reactor, i.e., 873 K. However, when gases with different compositions are employed, steam may have to be added to suppress carbon deposition, which will slightly alter the cleaning levels that can be achieved by the system, depending on the composition of the fuel gas.

From the gasifier 1123 K

Ceramic filter at 1123 K

Tar cracking with Dolomite at 1173 K

HCl cleaning with Na 2CO3 at 873 K

H 2 S cleaning with Zinc Titanate at 873 K

Alkali cleaning with activated Alumina at 1123 K

Ceramic filter at 1123 K

To the SOFC at 1123 K Fig. 16. Flow scheme for the proposed gas-cleaning system with a series of fixed-bed reactors and two ceramic filters.

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4. Conclusions This extensive literature survey indicates that the biomass gasification product gas can be cleaned to meet the requirements for use in SOFCs with different possible system schemes applying different possible lowest temperatures in the gas cleaning chain. Combined with the results on rather high contaminants tolerance of SOFCs developed in the past couple of years, this is a significant development which points to the necessity of intensified efforts for the development of highly efficient and sustainable power plants based on biomass gasifiers and SOFCs. Although information from literature, and results from chemical equilibrium studies were sufficient to put forward a conceptual design for the gas cleaning system, further detailed experiments are necessary to finalize the design for the cleaning series and to evaluate its long-term performance. A final design for a commercial high temperature gas cleaning system can be achieved only after a thorough technoeconomical evaluation of all of the available alternatives. Acknowledgement The authors wish to thank Ir. N. Woudstra (Delft University of Technology), Prof. H. Spliethoff (Technical University Munich), and Prof. J. Schoonman (Delft University of Technology), for their kind support and guidance during the work. M. van Loon and G. Protopapas are thanked for their contributions. European Commission and AgentschapNL (NL Agency) are thanked for partial financial support (with Biocellus project from European Commission and EOS-LT Gas Cleaning project from AgentschapNL). References [1] Larminie J, Dicks A. Fuel cell systems explained. 2nd ed. John Wiley & Sons; 2003. [2] Fuel cell handbook. 7th ed. EG&G Technical Services, Inc; 2004. [3] Stevens JD. Hot gas conditioning: recent progress with larger-scale biomass gasification systems. NREL/SR-510-29952. Available from: Golden, Colorado: National Renewable Energy Laboratory; 2001. [4] Aravind PV, Woudstra T, Woudstra N, Spliethoff H. Thermodynamic evaluation of small-scale systems with biomass gasifiers, solid oxide fuel cells with Ni/GDC anodes and gas turbines. Journal of Power Sources 2009;190(2): 461e75. [5] Aravind PV. Studies on high efficiency energy systems based on biomass gasifiers and solid oxide fuel cells with Ni/GDC Anodes (PhD thesis). Delft: TU Delft; 2007. [6] Aravind PV, Ouweltjes JP, Woudstra N, Rietveld G. Impact of biomass derived contaminants on SOFCs with Ni/GDC anodes. Electrochemical and Solid-State Letters 2007;11(2):B24e8. 2008. [7] Zhu WZ, Deevi SC. A review on the status of anode materials for solid oxide fuel cells. Materials Science and Engineering A 2003;362(1e2):228e39. [8] Wang JB, Jang J-C, Huang T- J. Study of Ni-samaria-doped ceria anode for direct oxidation of methane in solid oxide fuel cells. Journal of Power Sources 2003;122(2):122e31. [9] De Jong W. Sustainable hydrogen production by thermochemical biomass processing. In: Gupta RB, editor. Hydrogen fuel: production, transport and storage. Boca Raton (USA, Florida): CRC Press (Taylor & Francis); 2008. [10] Stanghelle D, Slungaard T, Sønju OK. Granular bed filtration of high temperature biomass gasification gas. Journal of Hazardous Materials 2007; 144:668e72. [11] Martini S, Kleinhappl M, Hofbauer H. High temperature gas treatment for the operation of a solid oxide fuel cell (SOFC). 17th European biomass conference & exhibition. Hamburg (Germany): ETA Florence; 2009. p. 652e660. [12] Frank N, Saule M, Karl J. Biocellus e Biomass fuel cell utility system, project summary (D810). Munich (Germany): Technical university of Munich; 2008. [13] Biollaz SMA, Hottinger P, Pitta C, Karl J. Results from a 1200 hour test of a tubular SOFC with woodgas. 17th European biomass conference & exhibition. Hamburg: ETA Florence; 2009. 635e638. [14] Karellas S, Karl J, Kakaras E. An innovative biomass gasification process and its coupling with microturbine and fuel cell systems. Energy 2008;33: 284e91. [15] Schweiger A, Hohenwarter U. Small scale hot gas cleaning device for SOFC utilisation of woody biomass product gas. 15th European biomass conference & exhibition - from research to market deployment. Berlin: ETA Florence; 2007. 879e882.

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