Recent advances in syngas production from biomass catalytic gasification: A critical review on reactors, catalysts, catalytic mechanisms and mathematical models

Renewable and Sustainable Energy Reviews 116 (2019) 109426

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Recent advances in syngas production from biomass catalytic gasification: A critical review on reactors, catalysts, catalytic mechanisms and mathematical models Jie Ren, Jing-Pei Cao *, Xiao-Yan Zhao, Fei-Long Yang, Xian-Yong Wei Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Biomass gasification Heterogeneous catalyst Reactors Mathematical models Tar removal

Biomass gasification converts into syngas, then into other chemicals via Fischer-Tropsch (F-T) synthesis is promising for renewable energy utilization. Although gasification is a sustainable and environmental-friendly technology for value-added utilization of biomass, tar formation is the major problem during the biomass gasification. Tar could condense on the reactor then block and foul equipment. An optimized gasifier and highly active catalyst were proved to be effective for biomass tar elimination. Furthermore, tar formation mechanism and the decomposition pathway were also important to advance the optimization of gasification reactors and catalyst design. This paper summarized the fundamentals, such as gasifier types, Ni-based catalyst, and reaction and deactivation mechanism. This review also sheds light on other excellent catalysts, effective gasifiers and mathematical models of biomass catalytic gasification, and catalyst reaction mechanisms and mathematical models are also discussed in detail. At last, the paper ends with a conclusion and prospective discussion to the latter lab and industrial-scale research.

1. Introduction 1.1. Biomass energy and utilization Energy is an indispensable resource for the national economic con­ struction, a driving force for development [1,2]. Although traditional fossil energy (coal, oil and nature gas) still occupied the most part of energy consumption, CO2, SO2, etc. releasing from traditional energy overuse could lead to global warming, environmental pollution, and other problems [3,4]. Recently, the research focus has gradually shifted to efficient use of clean renewables and improved the development of energy structure. Among these major renewable natural resources, biomass energy as a representative carbon energy is sustainable and low pollution, which has a wide range of sources. Biomass is the organic materials originated from plants or animals. Traditionally, biomass just used as an energy source for cooking and heating in developing coun­ tries. Biomass as a source of energy or chemicals is a significant research topic through thermochemical and biochemical transformation. There are three main thermochemical conversion methods for biomass, i.e., combustion [5,6], pyrolysis [7–10] and gasification [11–13]. Biomass combustion produces heat and electric power

traditionally used in the process industry. However, NO2, CO2, etc. release should be considered during the actual utilization [14,15]. The limited utilization and difficulty in downstream processing of bio-oil produced from biomass pyrolysis, which restricted the large-scale application of pyrolysis technology [10,16,17]. Biomass gasification is a promising thermochemical process that converts biomass into gas under different atmospheres (air, steam, O2, CO2, etc.), and produced syngas could also be used for highly valued chemicals production via Fischer-Tropsch (F-T) synthesis. 1.2. Biomass non-catalytic gasification As Table 1 listed, biomass consists of cellulose, hemicellulose, lignin, and other components. Wood, straw and plant biomass contains more cellulose, while shell has a high content of lignin than others. Biomass selection is important for syngas production during gasification. Generally, the high (hemicellulose þ cellulose)/lignin ratio could pro­ duce a high content of syngas. For the distribution of the gaseous product from different types of biomass gasification, we will discuss in Section 3. Apart from biomass types, the quality of gaseous products also de­ pends on the gasifying agent types, operation of reactor and activity of

* Corresponding author. E-mail address: [email protected] (J.-P. Cao). https://doi.org/10.1016/j.rser.2019.109426 Received 11 March 2019; Received in revised form 26 August 2019; Accepted 24 September 2019 Available online 7 October 2019 1364-0321/© 2019 Elsevier Ltd. All rights reserved.

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Nomenclature CR CRF DRIFT FESEM F-T GC GC-MS GHSV IR KAS L-RA L-CA L-RE MSW

MS Mass spectrometry OFW Ozawa-Flynn-Wall SEM Scanning electron microscope S/B Steam/biomass S/C Steam/carbon TG Thermogravimetry TOF Turn over frequency TPSR Temperature-programmed surface reaction TEM Transmission electron microscope TPR Temperature programmed reduction TPD Temperature programmed desorption UV-vis-NIR Ultraviolet–visible-and near infra-red WGSR Water-gas shift reaction WHSV Weight hourly space velocity XRD X-ray diffraction XPS X-ray photoelectron spectroscopy

Coats-Redfern Char reactivity factor Diffuse reflectance infrared Fourier transformed spectroscopy Field emission scanning electron microscopy Fischer-Tropsch Gas chromatograph Gas chromatography-mass spectrometry Gas hourly space velocity Infrared spectroscopy Kissinger-Akahira-Sunose Limonite raw material Limonite calcined at 650 � C Limonite reduced at 650 � C Municipal solid waste

catalytic cracking. For tar thermal cracking, the gaseous products derived from gasifi­ cation were heated to a high temperature, and then heavy tar molecules can be cracked into gases or light hydrocarbons [20,21]. During the biomass gasification, different gaseous products and concentrations could be obtained from gasification in the specific reactor. Reactor for gasification could be concluded to fixed bed, fluidized bed, entrained flow reactor, rotary kiln reactor, plasma reactor, etc. based on the biomass size, ash, moisture, and the feed way. Bridegwater [22] in­ dicates that tar could be thermally cracked in a fluidized bed reactor. However, refractory biomass tar is hard to remove by thermal cracking alone. To further increase the efficiency of tar thermal cracking, increasing residence time, rising temperature and adding oxidant are required. Brandt and Henriksen [23] mentioned that tar content could be reduced until 1290 � C. Houben [24] also discussed naphthalene thermal cracking when residence time from 1 to 2 s at a temperature from 900 to 1150 � C, and the result shows that tar reduction reached to 99% at 900 � C with air ratio of 0.5. The biomass characteristics and other gasification parameters have a great influence on the result of products, which were summarized in Table 2. In actual, partial pressure of gasifying agent and gasification, gasi­ fication temperature and heat rate have an important effect on gasifi­ cation performance, gaseous products composition and yield. High heating rate and gasification temperature could increase syngas (H2 and CO) yield and decrease tar yield. However, ash melting and specific

Table 1 The main compositions of some biomass [18,19]. Biomass

Cellulose (%)

Hemicellulose (%)

Lignin (%)

Others (%)

Softwood Hardwood Wheat straw Rice straw Bagasse Oak wood Pine wood Birch wood Spruce wood Sunflower seed hull Coconut shell Almond shell Poultry litter Deciduous plant Coniferous plant Willow plant Larch plant

41 39 40 30 38 35 42 35 41 27

24 35 28 25 39 19 18 25 21 18

28 20 17 12 20 28 25 19 28 27

7 7 15 33 3 – – – – –

24 25 27 42 42 50 26

25 27 18 25 26 19 27

35 27 11 22 30 25 35

– – 20 12 2 6 12

the catalyst. H2, CO, CH4, and CO2, light and heavy hydrocarbons are the main component in gaseous products, which can be used for the gen­ eration of heat/electricity, chemicals, and biofuels. Commercial methods for tar removal can be divided into thermal cracking and Table 2 Influences of biomass characteristics on the performance of the gasification process. Parameters

Observation

Biomass type

1. 2. 3. 1. 2. 3.

Moisture content

Ash content

1.

Particle size

1. 2. 3. 4.

The main components of biomass are cellulose, hemicellulose, and lignin, which will influence the composition of the products. Normally, the higher the (hemicellulose þ cellulose)/lignin ratio, the higher the syngas yield. The main compositions of some biomass are listed in Table 1. Low content of moisture, then energy efficiency and syngas quality would be increased. The tar content could be increased when moisture content higher than 30–40%, then lead to a decrease of gasifier temperature and gas yield. The optional moisture content is in the range of 10–20% for gasification, which could make bed temperatures more stable. In general, “updraft” gasifiers can be operated at moisture content about 60%, and maximum moisture content should be 25% on “downdraft” gasifiers. Although the plasma and supercritical water gasification reactor could be used for high-moisture content of biomass, high installation costs and others must be considered. Biomass could be employed as a great material in fixed bed updraft gasifiers when ash content lower than 2%. Especially, biomass, oil seed crops, grasses, flower, etc. with ash content (>10%), which would cause high slag formation. The small particle size of biomass could increase the surface area and diffusion resistance during the gasification, then improve the heat and mass transfer and enhance the gasification efficiency. Finally, increasing H2 concentration in syngas and carbon conversion. Generally, the particle size of biomass between 0.15 and 51 mm is better for gasification. A particle size smaller than 0.15 mm is optional for entrained flow gasifiers, particle size (>6 mm) is better in bubbling bed reactors, fixed bed reactors could tolerate >51 mm particle size because of the longer residence time. Decreasing the biomass particle size may increase the pretreatment cost of the biomass materials. The influence of biomass particle size might be reduced when used for gasification at high temperature.

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requirements of the reactor are the main problems when the tempera­ ture above 1000 � C. Normally, the gasification temperatures of agri­ cultural wastes and woody biomass are 750–850 � C and 850–950 � C, respectively [25]. Although high pressures are more technologically complex, light hydrocarbons and tar yield reduction and high carbon conversion are in favor of downstream applications of syngas, e.g., biofuels, turbine and engine fuel. There are some other operating con­ ditions (bed material, gasification agents, equivalence ratio, etc.) play­ ing a key role in gasification process performance, and they were presented in Table 3. The main processes involved in the biomass gasification could be classified as upstream processing, gasification and downstream pro­ cessing [26], which are presented in Fig. 1. Biomass is processed to make it suitable for next operations during upstream processing. Including processing, size reduction, drying, and densification are also required to obtain appropriate particle sizes, moisture and low density of biomass to make the gasification efficient. Gasification processes will take place in the presence of gasifying agents (steam/air/CO2/N2) at high temperature (600–1000 � C). If the gasifying agents are employed at high temperature, the heavy tars of biomass primary pyrolysis could be converted into light hydrocarbons, char, ash, even the permanent gases (H2, CO, CO2, CH4, and C2þ) and minor contaminants. During the downstream processing, tar, alkali compounds and other contaminants (N and S containing compounds) were removed from gaseous products [27]. Furthermore, reforming re­ actions and F-T synthesis could change the composition of gaseous products then make it successfully used for syngas convert to other value-added chemicals. However, for the reason of the high alkali contents in biomass, tar (heavy hydrocarbons) formation is a serious problem in the biomass gasification, which can contaminate equipment and lead to increase of the maintenance costs [28,29]. Considering the huge consumption of energy, tar thermal removal is not an economical method to realize syngas production. In addition to the gasification parameters mentioned above, a highly active catalyst is extraordinarily important and useful for biomass tar reforming.

been taken to reduce the tar production during the biomass gasification, the tar amounts are still far beyond the allowable level in practical application. In order to reduce the tar in the gas and improve the syngas yield, it is an essential process to realize the effective conversion of biomass tar that addition of catalyst with the special structure and high activity. The tar cracking catalyst mainly plays following roles during the process of biomass gasification: (1) Reducing the activation energy of cracking reaction, and then reducing the resource consumption; (2) Reducing the input of gasification medium; (3) Achieving directional catalytic cracking of tar and obtain more useful products or highly valueadded chemicals via F-T synthesis. Sutton et al. [30] claimed that biomass tar cracking catalyst should be effective for tar elimination and CH4 reforming, and it should be easily regenerated and economical for industrial application, then the coke-resistant ability is also important. Recently, natural mineral, alkali, precious metal and other synthetic heterogeneous catalysts are applied to biomass, coal and syngas con­ version [31–33]. However, a series of advanced characterizations, such as x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) should be employed for catalyst design and catalytic pathways investigation, which are very helpful to know the catalyst structure and active metal state. To compare the difference in biomass gasification under non-catalytic and catalytic conditions, Fig. 2 was drawn for explanation. As it was shown, unlike the high amount of heavy or light tar products under non-catalytic conditions, heavy tar can be pyrolyzed into directional highly value-added gases in the presence of a catalyst. Especially, the active metal site and special pore structure could provide excellent adsorption site, and then promote the complex heavy or light tar reforming. The simplified mechanism for tar catalytic cracking was described as follows: 1. Hydrocarbons and CH4 dissociated, and absorbed on the metal site; 2. Hydrocarbons and CH4 dehydrogenation; 3. Water was hydroxylated to OH radicals and result in oxidation of hydrocarbon fragments; 4. Syngas, CH4, and light hydrocarbons production. Heterogeneous catalyst with excellent carrier and active metal ad­ ditions are proved to be feasible for syngas production. In general, heterogeneous catalysts, dividing into the six categories including single

1.3. Biomass catalytic gasification Although various gasification parameters mentioned above have Table 3 Effects of operating condition on the performance of biomass gasification. Gasification Parameter

Observation

Bed material

Bed material could be inert and active materials. 1. Inert material could be energy transfer medium for biomass gasification 2. Active bed material could improve the syngas quality, promote gas reforming and tar cracking. 1. An optimal S/B ratio varies in the range 0.3–1.0 for biomass gasification 2. Higher H2 and CO2 concentrations could be obtained when S/B ratio was in the range 1.35-4.04, and the tar and CO amount decrease for the reason of the water-gas shift reaction (WGSR). 3. S/B capacity of the gasifiers was as the sequence: fixed bed gasifiers > fluidized reactors > entrained flow gasifiers. 4. An excess of steam leads to the tar formation because of the temperature decrease. Furthermore, the higher the S/B of the biomass gasification, the higher energy required during the gasification process. 1. Optional gasification agents, air, O2, steam, CO2, etc. would influence the syngas quality. 2. The heating value was between 4 and 7 MJ/Nm3 when biomass was gasified under air as gasification agent. 3. As a result of the dilution by N2, the concentrations of CO and H2 was low. In addition, CO2 concentration was increased because of the H2 and CO combustion. 4. Steam, as a gasification agent, could obtain a high H2 concentration result from WGSR. Although O2 gasification could result in syngas with a high heat value, high CO and H2 concentrations and low concentration of tar, the expensive costs on gasification agent are the main problem. 5. CO2 gasification could produce syngas with a high CO concentration result from the reaction between carbon and CO2. However, external heat is required during the gasification. 1. The optimal equivalence ratio is between 0.2 and 0.3 for fixed bed and fluidized bed gasifiers during biomass gasification, and entrained flow gasifiers usually require 20% equivalence ratio. 2. H2 and CO concentrations in syngas would increase when the equivalence ratio decreased. 3. Gasification could complete at equivalence ratio above 0.4, while it is incomplete when below 0.2. 4. High equivalence ratio could promote tar cracking due to high level of O2 involved in reforming process. 5. Equivalence ratio would be affected by the contents of moisture and volatile.

Steam to biomass ratio (S/B)

Gasification agents

Equivalence ratio

3

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Fig. 1. Main processes involved in biomass gasification.

Fig. 2. Schematic diagram of the difference between biomass non-catalytic and catalytic gasification.

metal, bimetallic catalysts, alkali metal catalysts and other catalysts (basic catalysts, acid catalysts, etc.) have the advantages for H2-rich gas production from tar catalytic cracking and gas compositions adjusting during the gasification. Normally, the synthesized methods of hetero­ geneous catalyst have a certain influence on the catalyst activity. Ion-

exchange, impregnation, precipitation method are the common methods for metal-based catalyst preparation. High metal dispersion could be produced when the ion-exchange method is employed for catalyst preparation [34]. To load active metal on the support, the precipitation method depends on the optimally external environment

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(temperature, pH, stirring speed, etc.). Although active metal just dispersed on the surface on the support, the impregnation method easily maintains the shape and structure of the catalyst. To update, many re­ searchers reported some effective catalysts for biomass catalytic gasifi­ cation. Apart from commercial and economical Ni-based catalyst, some researchers focus on the active precious metal, such as Pt, Ru, and Pd based catalyst [35,36]. Furthermore, there have other researchers concentrate on the investigation of support with different structures [37, 38]. Until now, the research on gasification catalyst is gradually focused on adding metal auxiliaries and designing effective catalysts with special structure, high catalytic activity and long lifetime. To save energy and experiment time, some mathematical models were proposed to simulate the various operating conditions, i.e., experimental pressure, temperature, flow, etc. Furthermore, models can be employed to obtain the best conditions and maximum limits of gas­ ifiers at increased pressures and temperatures. It is also very useful and safe in evaluating various biomass materials and their gasified behavior in different gasifiers without building them. However, conventional single compound models, kinetic models and lumped models make it difficult to accurately predict complex tar reforming reactions due to various conditions in actual use. Although some books and review articles have been published on different topics such as (1) Biomass tar removal, (2) Catalyst design for gasification, (3) Utilization of biomass, there are few reviews on biomass catalytic gasification with high activity catalyst design, catalyst prop­ erties, gasifier types, tar crack mechanism and mathematical models. We anticipate that this review will give the latter researchers and scale-up implementation of industry an important reference in biomass cata­ lytic gasification. In this article, recent advance and main idea are re­ ported according to the following order:

(iii) (iv) (v)

(vi)

gasifiers and discuss their applications. This is done to provide guidance for reactor optimization in the future. Part 3, we give an overview of all types of tar reforming catalysts. Main emphasis is given to the economical char catalyst and nickel catalyst. Part 4, we discuss possible catalytic and deactivate mechanism of biomass catalytic gasification by using tar model compounds and real tar, and some thermodynamic results are presented. Part 5, we employed some gasification models to compare and predict the gas yield and composition with experimental data. The advantages and disadvantages of every model are summa­ rized by discussing some literature. Part 6, we provide a conclusion and perspective.

2. Gasifiers Biomass gasification in a different structure of reactor will produce various products and amounts. According to the shape or size, ash, moisture and the requirements of the users, gasifier for biomass gasifi­ cation can be divided into fixed bed and fluidized bed [39,40]. Other types of gasifier, such as entrained flow reactor, rotary kiln reactor, and plasma reactor were also normally used in biomass gasification [41–43]. In order to compare the main advantages and disadvantages of biomass catalytic gasification gasifier, the brief comparison and sche­ matic diagrams of different reactors were presented in Fig. 3. 2.1. Fixed bed As Fig. 3a shows, the fixed bed can be further classified based on the interaction between biomass and atmosphere [44]. (1) “Updraft” gasifier, biomass and gasification agent moves downwards and then the gaseous products move upward. (2) “Downdraft” gasifier, biomass and gaseous products are moved downward [45]. (3) “Crossdraft” gasifier, which has the simplest structure. As the biomass descends from the top of reactor, and thermochemical reactions of biomass occur progres­ sively. In addition, the air will be poured into the “crossdraft” reactor

(i) Part 1, we give an introduction of biomass utilization and biomass gasification, and we emphasize the importance of the catalyst during biomass gasification. (ii) Part 2, we review the main gasifier types and their advantages and disadvantages, and we also describe some other effective

Fig. 3. Schematic view of different types of reactors. 5

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from the sides, which is different from the other fixed bed reactor as Fig. 3a shows.

2.1.3. “Crossdraft” Comparing with “updraft” and “downdraft” fixed bed reactor, “crossdraft” gasifier is the simplest reactor types for biomass gasifica­ tion. Biomass is introduced from the top to bottom, and biomass ther­ mochemical conversion continually occurs inside the reactor. As Fig. 3a shows, downdraft gasifier equipped with ash bin, fire and reduction zone separately compare with “updraft” and “downdraft”, and air was poured into the downdraft reactor from the sides. Crossdraft gasifier has excellent response against the load, flexibility of syngas production, start-up time and reactor height and compatibility for dry air blast. However, it is difficult for biomass fuel with high content and small size gasification, besides, this kind of gasifier has a poor reduction of CO2 in syngas. There have few applications and publication so far result from the great permeability of the gasification bed [50].

2.1.1. “Updraft” For “updraft” gasifier, the gasifying agent is introduced from the bottom and then reacts with biomass and other combustible gases [46]. For the reason of hot gas passes through the fuel bed and leaves at low temperature, some heats from the gas production could further used for biomass drying and steam generation. Therefore, high thermal effi­ ciency, low-pressure drop, and low slag formation are the main strengths for “updraft” gasifier. Updraft gasifier also has the ability for decreasing dust content in syngas at high flame temperature. However, low syngas yield, long engine startup time, poor capability of gasification, high tar sensitivity, and high moisture of biomass are the main disadvantages of this type of fixed bed reactor [47].

2.2. Fluidized bed

2.1.2. “Downdraft” For “downdraft” gasifier, it is also called as “co-current” gasifier result from biomass and react gas move to downward together, then wastes and gaseous products moved to the same direction. All the decomposition products including gas and tar would pass through the oxidation zone and cracking continually. The high quality of syngas with low tar content would be obtained in the “downdraft” gasifier. Down­ draft fixed bed reactor is favorable to use for power generation in a small scale [48,49]. Due to cellulose, hemicellulose and lignin are the main compositions of the biomass, and possible schematic diagram of biomass gasification is presented in Fig. 4. H2, CO2, CH4, and H2O are usually generated from the hydroxyl and methoxy reaction.

2.2.1. “Bubbling” and “circulating” As Fig. 3b shows, bubbling fluidized bed reactor is made of inert material (sand, SiO2, olivine, dolomite, etc.) hold on fluidization. Gasi­ fication agent is introduced from the bottom of the bed to top at the rate of 1–3 m/s through a distribution grid. In this case, every phase of the biomass gasification could react at inner bed. The liquid-like inert ma­ terials are continuously stirred in the presence of gas bubbles, which provide a uniformly heat exchange conditions for biomass and gas. At this point, circulating gasifier is similar to bubbling. Gas cyclone sepa­ rator is installed at the outlet of reactor, which can carry biomass into bubbling reactor. Energy for high-speed gas (normally 5–10 m/s) can be given from combustion for dragging the biomass pyrolysis and gasification.

Fig. 4. Possible pathways of biomass gasification into H2-rich gas in the fixed bed reactor (cellulose, hemicellulose, and lignin as the representative structure of biomass). 6

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Currently, the fluidized bed is a promising reactor for tar reforming or biomass gasification [51–53]. It has fuel and loads flexibility, high rate of mass and heat transfer, uniform temperature, and high efficiency of cold gas. Biomass (rice, grasses, wheat straws, etc.) with high contents of ash and alkali metals, which could result in broken equipment and decrease quality of gaseous products [54]. Operating temperature of fluidized bed is generally set between 800 and 900 � C, which based on the melting point of inert materials. For the reason of gasification re­ actions usually occur at high temperature, high activity catalyst should be designed for biomass gasification.

Table 4 Comparison of different gasifier types. Types of gasifier

Advantages

Disadvantages

Fixed bed (“updraft”, “downdraft” and “crossdraft”)

1. High of thermal efficiency, solid residence time and carbon conversion;

1. High content of tar in the syngas, difficulty in operation and temperature control, and uniform sizes and low moisture for biomass is required; 2. Low production of CO and H2;

2. Capacity of handling biomass with high humidity and different sizes; 3. Excellent contact between the biomass and the react atmosphere;

2.3. Entrained flow reactor Biomass materials (<1 mm) and gasification agent are introduced in co-current (Fig. 3d). Entrained flow reactors are commonly operated at the pressures of 25–30 bar and the temperature between 1300 and 1500 � C. For the reason of wastes could be easily slurred, then atomized and poured into gasifier, entrained flow gasifiers are usually employed and used for coal, residues and mixed plastic waste gasification. Most importantly, moisture removal from the heat treatment is required before biomass used as the material for gasification [55–57].

4. Feasibility of large-scale production; 5. Lower production of biomass tar; 6. Low entrainment of dust and ash; 7. Simple construction, etc.

2.4. Rotary kiln Rotary kiln reactor is an important gasifier and widely used in in­ dustrial waste combustion and cement production. Rotary kiln reactor typically has a steel shell with the structure of abrasion-resistant re­ fractory, which could reduce the temperature of metal shell. As Fig. 3c shows, the dried and fixed-size biomass materials were controlled and fed by the favorable rotation speed. The produced gas and slag generally came out from the upper and lower of the reactor outlet. The react gas and biomass could be contacted completely due to the rotation of reactor continually, and new biomass surfaces exposed to the gasification agent. However, the flue gas produced from the gasification could take abun­ dant heat away. Accordingly, the heat exchange between the biomass and react gas is low effective unless barriers were installed inside the reactor [39].

Fluidized bed (“bubbling” and “circulating”)

1. High of contact between biomass and atmosphere, carbon conversion and thermal loads; 2. Excellent for temperature control, feed and process, handling biomass with different properties; 3. Suitable for biomass and other pre-treated waste;

4. Possibility for large-scale catalysts uses; reduced residence times, etc.

2.5. Plasma reactor The two electrodes, usually Cu or carbon electrodes were installed in the plasma reactor. Plasma reactor will give an electric discharge when inner temperature of reactor reached up to 10000 � C, which normally namely “arc”. An atomic degradation of the matter would take place during the plasma process when biomass is treated in reactor under different atmospheres. The energy for rising temperature of biomass gasification will be supplied from plasma process, while in the absence of an oxidizing agent. The plasma reactor for the thermochemical conversion of biomass can be applied in following two situations: (1). During the upstream process of biomass gasification, plasma reactor can make the gaseous products clean, then improve the degradation of tar into light compo­ nents and the content of syngas with light components; (2). Thermal destruction for adequate size solid treatment. Although the flow rate, moisture content, size and composition of biomass are fluctuant, plasma reactor can control the gasification temperature with different solid and gasification agent quality independently. Above all, the main advan­ tages and disadvantages of different gasifiers are summarized in Table 4.

Entrained flow reactor

Rotary kiln

2.6. Other gasifiers Although some researchers obtain a high tar conversion in conven­ tional gasifier, there are other publications reported biomass gasifica­ tion in the well-designed reactor. Pr€ oll and Hofbauer [58] employed CaO/CaCO3 as bed material to selective removal of CO2 from the gasification reactor in a dual fluidized

1. Materials flexibility, uniform temperature, high carbon conversion, extremely low tar concentration, short reactor residence time, easy to control the parameters process; 2. High-temperature slagging operation (vitrified slag); 3. Possibility for large-scale use, etc.

1. Excellent capacity of handling biomass with different properties, flexible loading; Suitable for waste easily be melted; Possibility for large-scale use;

3. Limited flexibility to feed and process (Biomass should have similar properties) 4. Starting difficulties and temperature control; 5. Mobile grates should be installed (Avoid the formation of preferential paths in the fixed bed); 6. Catalysts may be poisoned and deactivated (High activity catalyst is required), etc. 1. Carbon loss in the ashes;

2. Dragging of dust and ashes; 3. Relatively low process temperature is required to avoid phenomena of de fluidization of the bed; Restrictions on the size; 4. High costs of investment and maintenance; 5. Technology complex and difficult to control, et4. The influence of biomass particle size might be reduced when used for gasification at high temperature.c. 1. Requirements of abundant oxidants;

2. Reducing size and preparation supply are required; 3. Heat recovery is required to improve efficiency 4. Low cold gas efficiency; 5. Short life of system components and high costs of investment and maintenance, etc. 1. Significant hard to control the starting temperature and the leakage and wear of movement part; 2. High of refractory consumption, dust and (continued on next page)

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Table 4 (continued ) Types of gasifier

Advantages 2. Simplicity of construction and high reliability of operation; 3. Low investment costs, etc.

Plasma reactor

1. Production of vitrified completely inert and nonleachable slag, which include heavy metals; 2. Recovery of leachable waste, which can be used as a building material; 3. Low flow rate of the syngas and low content pollutants in syngas; 4. Extremely short reaction times; possibility for large-scale use, etc.

Disadvantages tar content; Low capacity of heat exchange; 3. Low efficiency of heat; Limited flexibility process; 4. High costs of maintenance, etc. 1. Presence of nanoparticles in the syngas, the maintenance of movement part and heat shocking for the start-up and shout-down; 2. Consumption of refractory and electrodes; 3. Removal of molten material in the ducts is required; safety problems, etc.

Fig. 6. Schematic diagram of two-stage fixed bed reactor.

Wiinikka et al. [63] described the pressurized entrained-flow high-temperature black liquor gasification system and used for black liquor gasification (Fig. 7). This reactor could run under reducing con­ ditions, corrosive environment, 30 bar, and 1000 � C. O2, CO2, and steam could produce syngas through a liquid smelt. Syngas and smelt are quenched and separated in a quench cooler (below the reactor). Small particles will suspend in the gas and larger particles could be dissolved in the quench water. Small particles become heavy and separated when syngas passes through a counter current condenser. Low/medium pressure steam could be produced from the heat recovered from the gas, and used in the paper process, and then black liquor will be originated. High electricity consumption in turn results in low gasification effi­ ciency. It is a limitation that generation of plasma will take high costs of construction and maintenance. There are a lot of investigations were performed at the lab scale, Rutberg et al. [64] obtained 6% net electric energy conversion through calculations from wood plasma gasification. Janajreh et al. [42] compared traditional biomass gasification with plasma gasification, and then chemical thermodynamic modeling was conducted further to evaluate the gasification efficiency. For the reason of high electricity consumption, plasma gasification efficiency was 42%, which was lower than biomass air gasification (72%). Microwave plasma was used to gasification of glycerol, and a high syngas content (57% H2, 35% CO) and carbon conversion (80%) with no O2 feeding were obtained. Japan, Canada, and India have commercial plasma gasifier, and Japanese plasma gasification station produced 7.9 MW/h electricity from 268 t of municipal solid waste (MSW) gasification every day from 2002 to 2014 [65]. Currently, a new 2000 t/d MSW gasifica­ tion station was built in the UK, which will be the largest plasma gasifier all over the world [19]. Accordingly, it is difficult to realize large-scale and commercial syngas production. Next step for scientific research must focus on energy decrease and further enhance gasification efficiency. An excellent catalyst could reduce energy input and promote tar cracking, which has become the focus of gasification technology development in recent years.

Fig. 5. Dual fluidized bed steam gasification principle [58].

bed gasifier (Fig. 5). The result shows high H2 contents in the syngas products and high efficiency of energy conversion could be obtained at a low operating temperature. Li et al. [59] reported a two-stage fixed bed to reform the hot coke oven gas, whose temperatures of pyrolysis and catalytic bed could be controlled respectively by using furnaces with two thermocouple wires (Fig. 6). Two additional thermocouples are placed outside of reactor to read the real temperature. With the temperature rising, the biomass will be gasified and poured into the second stage carried by the Ar. Wang et al. [60] added a steam generator in this reactor to introduce different react atmosphere, and they investigated the corncob tar reforming over Ni/lignite char in this reactor, they found that catalytic bed temperature fixed at 650 � C and temperature of first bed increased from 100 to 900 � C at 10 � C/min are the best conditions for tar reforming. They obtained the high tar-free syngas (43.9 and 85.1 mmol/g) and low water-soluble tar under Ar and steam atmo­ spheres. Zhao et al. [33] and Cao et al. [61,62] also used this reactor to realize biomass gasification and obtained excellently reformed results.

3. Heterogeneous catalysts Heterogeneous catalysts can be classed as natural minerals and synthetic catalysts. Nature minerals, such as dolomite, olivine, and limonite were proved to be active for biomass gasification [33,66–72]. Synthetic heterogeneous catalysts can be further divided into alkaline earth metal and transition metal (Ni, Pt, Ru, Zr, and Rh based) catalysts. 8

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Fig. 7. Schematic diagram of pressurized entrained-flow high-temperature black liquor gasification system [63].

3.1. Mineral catalysts

size around 1.5 mm exhibited the highest tar conversion (94%) and lowest tar content (74 g/Nm3), which is calculated by the Arrhenius formula and has apparent energy of 42 kJ/mol [90]. Low-cost limonite (Fe2O3) with abundant reserves is potential substitutes for commercial catalysts. Besides, waste limonite also can be directly used for iron production [91,92]. From the results of coal volatile cracking over Indonesian natural limonite at 600 and 800 � C, Li et al. [91] found that limonite has wonderful surface structure and high specific surface area and showed great activity for coal volatiles pyrolysis. Zhao et al. [33] studied reforming of corncob volatiles over raw limonite and treated limonite at a calcined temperature between 400 and 650 � C under different atmospheres (Ar and steam). The results indicated that steam could inhibit the carbon deposition and promote syngas production via the reaction with volatiles (C þ H2O→CO þ H2, CH4þ2H2O→4H2þCO2). In addition, the raw limonite has a lowest gas yield (25.7 mmol/g) for corncob tar reforming. As the temperature rises, the chemical composition of limonite changes (FeOOH to Fe2O3), which is confirmed by the XRD. Fe2O3 could exhibit a high activity than FeOOH, and the gas yields reached to maximum (46.9 mmol/g) when reduced limonite was used for tar reforming at 650 � C. Furthermore, steam could eliminate the carbon deposition via WGSR, and H2-rich gas yield could reach to 74.1 mmol/g at 700 � C when steam was poured into. The changes of limonite composition are as Fig. 8 shows.

Mineral catalyst contains metal oxides (Al2O3, Fe2O3, MgO, CaO, etc.), which have certain activity for biomass gasification [73–79]. As previous literature reported, hydroxides, zeolites, anatase, rutile, cer­ ianite, and baddeleyite were commonly used as the carrier/catalyst for biomass tar cracking, which mainly consisted of Al2O3, TiO2, CeO2, ZrO2, and SiO2 [80–83]. Al2O3 is a cheap support for catalyst prepara­ tion, while it is unstable and easily deactivated in practical utilization especially when Ni is used as the active metal [84]. To extend the life­ time and activity of the tar cracked catalyst, other efficient metal oxides (CeO2, ZrO2, SiO2, etc.) were also employed for catalyst synthesis and used for catalytic cracking of biomass tar or tar model compounds. Miyazawa et al. [85] reported that the activities of these metal oxides are as follows: Ni/MgO < Ni/CeO2
3.2. Alkali metal catalysts Alkali metals is belonging to group IA (Li, Na, K, Rb, etc.), which are the primary catalyst and just be thought to enhance the biomass gasi­ fication reactions (Biomass þ O2/H2O→H2,CO, CO2, CH4, H2O, char, ash and other hydrocarbons), and it is not suitable for direct use as a catalyst for tar reforming. Generally, alkali metals are used as a gasification or catalyst additives and mixed to feed into the gasified reactor directly with the biomass materials. Gall et al. [93], Jiang et al. [94], Hu et al. [95], and Masnadi et al. [96] reported that alkali metals could enhance tar cracking during biomass thermochemical conversion. Mitsuoka et al. [97], Hognon et al. [98] and Murata et al. [99] found alkali metals are effective to improve the syngas quality in tar steam reforming, which 9

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Fig. 8. The changes of limonite composition. XRD characterization of limonite under different conditions (a), gas yields of different limonite (c), TEM images of L-CA (c) and L-RE (d). L-RA: Limonite raw material; L-CA: Limonite calcined at 650 � C; L-RE: Limonite reduced at 650 � C. Data are collected from Ref. [33].

could form a reactive char [100,101]. In addition, biomass ash contains alkali metals, which also can be used as catalyst itself to solve the problem of ash treatment and improved the yield of the value-added gaseous products. However, alkali metals are easy to be evaporated and resulted in particle agglomeration, catalyst activity lost, and the recovery difficulties are also a problem during the tar reforming. Due to particle agglomeration, the potential activity of these catalysts is easily

lost. According to the study of coal and biomass co-gasification, some researchers found that the addition of alkali salts into active catalysts could enhance the activity of biomass catalytic gasification [102]. Mandal et al. [40] employed K2CO3 supported on Al2O3 or SiO2–Al2O3 for wood sawdust, castor stalk, etc. gasification in a dual fluidized bed. They proved that formed tar was eliminated completely and obtained a high yield of H2-rich gas (H2/CO > 10).

Table 5 Performance comparison of different catalysts and synthetic methods. Biomass

Catalyst

Methods

H2 yields

Conditions

Ref.

Sawdust Wood sawdust Rice hull Sawdust Pig manure Pine wood Pine sawdust Pine sawdust Maize stalk Corncob Corncob Corncob Corn stalk Wood sawdust Wood sawdust

Ni/MgO Ni/MCM-41 Ni/CeO2–ZrO2 Ni/dolomite Ni/lignite char Ni/Al catalysts Ni/La2O3-αAl2O3 Ni/Co–Al–Mg Ni–Ce/Al2O3 Ni/dolomite Ni/lignite char Ni/Resin Ni–Mg–Al NiO/MgO NiZnAlOx

Commercial Impregnation Impregnation Impregnation Ion-exchange Co-precipitation Impregnation Co-precipitation Co-impregnation Ion-exchange Ion-exchange Ion-exchange Co-impregnation Commercial Co-precipitation

81% 51 vol% 70% 73% 69 mmol/g 77% 96% – 71% 22 mmol/g 60 mmol/g 61 mmol/g 56% 51 vol% 48 vol%

S/CH4 ¼ 2; 800 C; Catalyst: 15.0 g; GHSV ¼ 3600 h Ni loading: 40 wt%, 0.25 g; 800 � C; water: 5.0 mL/h Ni loading: 12%, Ce loading: 7.5%; W/B ¼ 4.9, 800 � C S/C ¼ 5.0; WHSV ¼ 1.5 h 1; 800 � C Ni loading: 19 � 1 wt%; 650 � C; Ar S/C ¼ 5.58; 650 � C; Ni loading: 28% Ni loading: 9.92%; 700 � C; S/C ¼ 12 T ¼ 650 � C; GHSV ¼ 13000 h 1; S/C ¼ 7.6; Mg/Al ¼ 0.26; Co/Ni ¼ 0.10 Ni, Ce loading: 14.9%, 2.0%; 900 � C, S/C ¼ 6; WHSV ¼ 12 h 1 Ni loading: 6%; Ar atmosphere; 650 � C; 1 g biomass Ni loading: 17.32%; 1 h; Steam: 30 kPa; 650 � C; 1 g biomass Ni loading: 18.0%; 1 h; 650 � C; 1 g biomass; Steam: 30 kPa Ni:Mg:Al ¼ 1:1:1; T1 ¼ 400 � C, T2 ¼ 800 � C; S/C ¼ 3.54; 30 min NiO loading: 7.2 wt%; 850 � C T1 ¼ 535 � C, T2 ¼ 800 � C �

10

1

[104] [105] [106] [107] [108] [109] [110] [111] [112] [33] [113] [61] [114] [115] [116]

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[123–125] for the reason of transformation between Ce3þ and Ce4þ when reacting with oxygen from tar reforming [126]. Miao et al. [127] indicated that rare metal could improve the stability and dispersion of active constituent in the catalyst. Previous researchers claimed that Ce could oxidize surface coke and increase the interaction between active metal and support [128–130]. Zhang et al. [131] reported the effects of Ce and Mg promoter on Ni/olivine catalyst during toluene catalytic reforming. They proved that Ce promoted Ni/olivine (Ni–Ce/olivine) and Mg promoted Ni–Ce/olivine (Ni–Ce–Mg/olivine) catalyst show a higher performance than Ni/olivine catalyst alone. Through a series of characterizations (XRD, Fourier-transform infrared spectroscopy (FTIR), and thermogravimetry analyzer (TG)), no carbon deposition can be found on the Ni–Ce/olivine and Ni–Ce–Mg/olivine catalyst. Interest­ ingly, Ni–Ce–Mg/olivine catalyst not only exhibits the coke resistance but also the anti-H2S (10 ppm) activity. Garbarino et al. [132] investi­ gated the catalytic activity of Ni–La/Al2O3 and Ni/Al2O3 catalysts through temperature-programmed surface reaction (TPSR) of ethanol and phenol. They preceded a series of characterizations (UV-vis-NIR, field emission scanning electron microscopy (FESEM), skeletal infrared spectroscopy (IR), etc.) and the results show the existence of Ni–La in­ teractions. La reduced the support acidity and significant decrease the ethanol dehydration.

3.3. Ni-based catalysts Among all of the transition metals (group VIII), commercial and economical Ni-based catalysts are widely reported in the publications [103]. Ni-based catalysts are usually used to tar removal for H2-rich gas production through CH4 reforming [109], which are also the best catalyst in dry and steam reforming of methane. Ni-based catalyst favors syngas production at the high temperature (>740 � C), especially it could promote tar cracking under steam atmosphere. Table 5 listed activity comparison of different Ni-based catalysts under different gas hourly space velocity (GHSV), weight hourly space velocity (WHSV) and steam/carbon ratio (S/C) and their synthetic methods. As Table 5 listed, the suitable additives could improve the performance or stability of the Ni-based catalyst, especially the carrier with an excellent structure or anti-coke ability. Lanthanides metal, like Ce, and La, could promote the reducibility and renewability of the catalyst, and then extend the life­ time of the Ni-based catalyst. Furthermore, alkaline metals promoters, Mg, and K were added into the catalyst for the consideration of costs. Promoters from transition metal, such as Mo, Zr, and Mn were also proved to show the high activity and stability during the biomass cata­ lytic gasification. Aznar et al. [117] employed commercial holder tropsch catalyst for biomass gasification, which has 12–14% Ni loading and <0.5 wt% Mg loading on Al2O3. They indicated that catalyst has a high tar reduction (4 mg/m3) and CH4 reduction (0.5 vol%) at the temperature of 790–820 � C and residence time of 0.1 s. Wang et al. [118] investigated preparation and performance of Ni–Fe/Al2O3, and they found Ni–Fe/Al2O3 has a high activity than Ni/Al2O3 and Fe/Al2O3 catalysts during steam reforming of cedar wood tar. According to their mention, the formation of Ni–Fe alloys is the main reason for its high catalytic performance. The Fe atoms could improve tar cracking and suppress the carbon deposition. Optional material could prevent Ni sintering and activity lost, and noble metal doping will improve the catalytic activity of Ni catalyst. In this context, Nishikawa et al. [119] introduced Pt, Rh, and Pd to Ni/CeO–Al2O3 and used for woody biomass steam gasification. The noble metal doped catalysts are prepared via co-precipitation, precipi­ tation, and impregnation method, respectively. The results showed that the Pt and Fe doped Ni/CeO–Al2O3 catalysts have a high gas yield during gasification and Co catalyst showed low gas composition. They further studied the Pt promoter doped into Ni/CeO2/Al2O3, they proved that Pt modification for Ni/CeO2/Al2O3 could exhibit high activity without H2 reduction, and Pt/Ni/CeO2/Al2O3 catalyst could be activated by the tar compounds. Moreover, Pt–Ni/CeO2/Al2O3 with 0.1% Pt, 4 wt% Ni and 30 wt% CeO2 could enhance the activity during the steam gasification and produce a higher gas yield and lower tar yield than Ni/Al2O3 and Ni/CeO2/Al2O3 [120]. Chaiprasert and Vitidsant [121] studied the ef­ fects of the promoter (Pt, Co, and Fe) on Ni/dolomite for biomass gasification, these catalysts are synthesized by different methods, and impregnation method showed better performance from the results. The author claimed Pt, Fe and Co promoters enhanced the tar reforming, WGSR, and methanation reaction, respectively, and noble metal doping could inhibit carbon deposition on the surface of catalyst. Oh et al. [122] investigated the catalytic steam reforming of toluene over Ru–Mn-pro­ moted Ni-base catalysts and commercial Ni-based catalysts at temper­ atures of 400–800 � C, then Ni/Ru–Mn/Al2O3 catalysts showed high activity for toluene conversion when temperature higher than 600 � C. They mentioned the Mn on the Ni/Ru–Mn/Al2O3 catalyst have high stability and coking resistance. Toluene conversion reached 100% over Ni/Ru–Mn (2.6 wt%)/Al2O3 catalyst when temperature at 600 � C. Based on the result of reforming data and catalyst characterization, they confirmed H2 generation mechanism under steam atmosphere. Recently, rare earth metals doped catalysts proved to have an excellent activity for tar reforming and other tar model compounds. For example, Ce, an electronic and structural promoter, which can enhance the activity of Ni-based catalysts and used in many catalytic reactions

3.4. Other supported catalysts As reviewed above, the natural mineral, alkali metal catalysts, and Ni-based catalysts have activity for biomass gasification. However, they are easily deactivated and sintered significantly by carbon deposition and particles growth. Many researchers found that the novel metal catalysts could overcome the disadvantages of conventional mineral, alkali metal, and Ni-based catalysts, and show a high efficiency for tar conversion. Lu et al. [133] reported that H2 production from biomass gasification over CeO2, nCeO2, n(CeZr)xO2, Pd/C and Ru/C catalyst, and the effects of reactor types, heating rate and biomass types and particle size are systematically investigated. The authors pointed out that long residence time, high heating rate at 700 � C and 25 MPa are favorable to H2 pro­ duction. Tomishige et al. [134] investigated the catalytic activity of CeO2/SiO2 loaded on Rh, Pd, Pt, Ru and Ni for cellulose catalytic gasi­ fication. The result shows the activity order is Pt ¼ Ni < Pd < Rh at 550 � C. With the temperature increasing, the tar conversion from 88% (550 � C) reached 97% (600 � C) when Rh/CeO2/SiO2 catalyst was employed for tar reforming. Furthermore, low carbon deposition and deactivation could be observed on the surface of CeO2/SiO2 loaded on Rh (Rh/CeO2/SiO2) catalyst when tar reforming proceeded at the low temperature. Furthermore, Tomishige et al. [135] employed Rh/CeO2/SiO2 catalyst for partial oxidation of cedar wood tar. Comparing with commercial Ni/Al2O3, Rh/CeO2/SiO2 exhibited highly stable activity when 280 ppm H2S were poured into gasifier. From the XPS results, the surface S species over the Rh/CeO2/SiO2 was less than Ni/Al2O3 after the reaction. The adsorbed S species on the Rh/CeO2/­ SiO2 catalyst could be formed to SO2 and H2S under oxidation and reduction atmospheres. Asadullah et al. [136] compared the activity of Ru/CeO2with Ru/CeO2/SiO2, Ru/CeO2/Al2O3, and Ru/CeO2/ZrO2 in the continuous-feeding system for biomass gasification. They found Ru/CeO2 catalyst drastically deactivated for the reason of decrease of specific surface area (60–13 m2/g). SiO2 with a higher specific surface area than Al2O3 and ZrO2 could inhibit Ni/CeO2 aggregation, and then promoted the catalytic cracking performance. Based on the result of conversion, they proved CeO2/SiO2 is a great support for Ru based catalyst preparation and H2-rich gas production. The authors investi­ gated the loading influence of Rh/CeO2/SiO2, and the result shows that 35 wt% CeO2 on SiO2 will be better to load Rh based on the results of gas yield and tar conversion. Virginie et al. [137] designed Fe/olivine catalyst with different Fe loading (10 and 20 wt%) and calcined tem­ peratures (400, 900, 1000, 1100 or 1400 � C) by using impregnation. 11

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Combining with the results of mass spectrometry (MS), SEM/EDX, XPS and temperature programmed reduction (TPR), etc. Fe (II) will be oxidized and left the olivine after they are calcined between 1000 and 1400 � C, and Fe/olivine with the 10 wt% of Fe loading should be pre­ pared at 1000 � C. Virginie et al. [138] mentioned that Fe/olivine cata­ lyst is effective for tar reduction in dual fluidized bed, and tar conversion reduced by 65% at 850 � C. They found that Fe/olivine materials not only for tar and hydrocarbon reforming but also could be an oxygen carrier that transfers oxygen to the gasifier and burn volatile compounds. The catalysts are excellently stable during 48 h reforming. A series of char­ acterization revealed that the carbon deposition on the surface of cata­ lyst is low and easily oxidized. Wang et al. [139] studied the activity of Fe–Co/Al2O3 for steam reforming of tar and toluene, they found that Fe–Co with face-centered cubic and body-centered cubic structure will be formed when Fe/Co ¼ 0.25. H2 addition could maintain the Fe–Co/Al2O3 metallic state and further inhibit catalyst oxida­ tion/deactivation and exhibit high activity during the steam reforming of toluene. Keller et al. [140] selected La, Sr, Fe and their mixtures supported on ZrO2, and benzene and ethylene are employed as tar model compounds during chemical looping reforming. They found La and Fe combinations show significant conversion of benzene, and addition of Sr is also proved to effective for benzene conversion. However, La0.8Sr0.2­ FeO3 is not stable for benzene and ethylene cracking especially under reducing conditions, then formed La2Zr2O7 and some SrZrO3 perovskite would interact with the ZrO2 support. Accordingly, the interaction be­ tween different metals must be considered when designing doped metal catalysts for biomass gasification. Dagle et al. [141] presented a publi­ cation on steam reforming of hydrocarbons over Ni, Rh, Ir, Ru, Pt, and Pd supported on MgAl2O4 catalyst. They designed a novel bimetallic IrNi

catalyst and found that IrNi catalysts were more stable than mono­ metallic catalysts in this test. As Fig. 9 shows, theoretical calculations (ab initio molecular dynamics simulations) also indicated that small Ir clusters supported on large Ni particles could provide more anti-coke activity. As they reported, small Ir clusters (around 2–3 atoms) sup­ ported on Ni particles (�5 nm) could show the best coke and Ni sintering resistance than Ir/Ni clusters alone. Combining with Fig. 9a, c, and d, it shows that IrNi catalyst with the electron-rich Ir sites could enhance the coke resistance and activity during the hydrocarbons steam reforming. In order to achieve large-scale utilization of biomass gasification in industry, more economical catalyst carriers should be designed. As a promising material, biomass or coal chars have been reported to be the catalyst for biomass gasification, and they showed a potential perfor­ mance during tar reforming [142–146]. The biomass/coal char-supported catalysts could easily be gasified and recover the inherent energy of the char without regeneration after deactivation. Recently, Wang et al. [147] investigated Ni/biomass-char and Ni/coal-char prepared by mechanically mixing NiO and char particles for sawdust gasification in a laboratory-scale updraft biomass gasifier. They concluded that 15% NiO loading, 0.3 s gas residence time and 800 � C were the best conditions for tar removal (>97%). Furthermore, Ni/coal-char catalyst showed the stable activity for sawdust tar removal gasification during 8 h tests. Shen et al. [148] developed char-supported Ni–Fe catalysts by using rice husk char for biomass gasification. Under their optimized conditions, Ni–Fe char catalyst has a high condensable tar conversion about 92.3% and 93% before and after calcination. Althrough coal with abundant storage usually used for power produc­ tion through combustion, air pollution and environmental destruction are the main disadvantages of coal utilization [149,150]. Lignite with

Fig. 9. Simulated equilibrium structures and distribution densities (along the Z direction) of different NiIr ratio clusters on MgAl2O4 (111) support (a); Catalytic activity and stability of the different catalysts for model syngas containing tars reforming under steam atmosphere (b); Binding energies (c) and reaction energy (d) for the intermediates during methane dissociation and CO formation on Ir clusters and bulk (111) surface [141]. 12

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porous structure and high content of oxygen-containing species proved to be a potential carrier for loading Ni via ion-exchange [60,151,152]. Cao et al. [153] investigated the sewage sludge catalytic gasification over Ni/lignite char in a two-stage fixed-bed reactor, the H2 yields and nitrogen transformations are discussed at different temperature and atmosphere. Afterwards, Wang et al. [60] prepared the Ni/lignite char via ion-exchange and applied for corncob gasification. The tar-free syngas yield reached to 43.9 mmol/g under Ar atmosphere, which is higher than commercial Ni/Al2O3 catalyst. To further study the influ­ ence of minerals on Ni/lignite char preparation, Ren et al. [113] removed the internal and external minerals of lignite to increase the amount of oxygen-containing groups. The lignite treated with HCl/HF and H2O2 then loaded Ni showed a high specific space area of 291.1 m2/g and Ni crystallite size of 3.4 nm, which are highly dispersed and most active for corncob volatile reforming. To improve the stability and coke-assistance of Ni/lignite char, they added Ce into Ni/lignite char with various ratios of Ni and Ce (1:1, 10:1, 20:1, 50:1 and 100:1). Comparing with commercial Ni/Al2O3 catalyst, Ni–Ce/lignite char with Ni/Ce ratio of 50 showed a uniformly dispersed Ni component and a most stable gas yield (69.1%) due to the formation of Ni–Ce alloy [154]. Although lignite char is easily gasified under steam atmosphere, it can expand the use of traditional chemical energy. Lignite could exchange Ni from the industrial wastewater and prepare highly dispersed catalyst for biomass catalytic gasification. After gasification, waste char could be used for power generation. Furthermore, gaseous products could pre­ pare other value-added chemicals via F-T synthesis. An integrated concept of biomass catalytic gasification over lignite char supported environment-friendly catalyst is presented in Fig. 10. Zeolites with a complex and variable three-dimensional network structure, high surface area, surface acidity, and unique channels have been widely used in heterogeneous catalysis [155–159]. Metal dispersed on modified zeolites and used for hydrogenation, bio-oil upgrading, and tar reforming are focused by many researches recently [160,161]. Many researchers employed various zeolites and modified ones for different reactions, and they proved it is a promising catalyst for tar removal [162–165]. Buchireddy et al. [166] also compared the activity of tar

reforming over Ni supported on commercial zeolites with different pore size and acidity. Their results showed that zeolites with larger pore size loaded Ni via wet impregnation and could exhibit a high activity for tar cracking. Furthermore, the activity and stability of zeolite catalysts depend on their acidity. High acidity zeolite catalysts are easily be re­ generated and show excellent nitrogen and sulfur resistance. However, it is the main shortcoming that carbon deposition results in rapid deacti­ vation of catalyst [167,168]. Chen et al. [169] studied the catalytic performance of Ni, Ni–MgO loaded on commercial HZSM-5 (Si/Al ¼ 25) for biomass gasification. Combining with the results of Transmission electron microscope (TEM), Gas chromatograph (GC), Gas chromatography-mass spectrometry (GC-MS), etc., and it indicated that Ni–MgO/HZSM-5 (6 and 2 wt%) have a better activity and stability than Ni/HZSM-5, which have a high tar conversion (91.03 wt%) and gas release (7.64 MJ/Nm3). In summary, the advantages and disadvantages of natural minerals, alkali metal-based, Ni-based catalyst, other transi­ tion metal-based, etc. are presented in Table 6. 4. Catalytic and deactivate mechanism 4.1. Tar model compounds As the representative and simple aromatic model compounds, toluene, naphthalene, and benzene are usually employed to test and proposed the cracking mechanism of complex tar [73,170–175]. Ren et al. [176] designed a layered carbon load Ni catalyst and employed for corncob tar reforming, and then study the reaction activation energy of toluene reforming. They claimed the Ni (111) plane of layered carbon load Ni catalyst is the main reason of superior activity during tar and toluene reforming. Layered carbon supported Ni catalyst with the low activation energy (24.5 kJ/mol) and high TOF of 2.88 h 1 has the excellent ability for toluene cracking at low temperature. However, carbon supported Ni catalyst was easily gasified and deactivated for the reason of steam existence. The toluene conversion and turn over fre­ quency (TOF) was presented in Fig. 11. Cao et al. [177] compared this kind of layered carbon load Ni catalyst

Fig. 10. Integrated concept of value-added products production from biomass gasification over Ni/lignite char catalyst. 13

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Table 6 Advantages and disadvantages of different catalysts. Catalyst Nature minerals

Limonite

Advantages

Disadvantages

Abundant, inexpensive

Easily deactivated in the absence of hydrogen; lower catalytic activity than dolomite Have a lower catalytic activity than dolomite Lower catalytic activity than dolomite

Olivine Clay minerals Alkali metal-based

High attrition resistance; cheap Inexpensive; abundant; fewer disposal problems

Ni-based catalyst Other transition-metal-based

Cheap; abundant; 8–10 times more active than dolomite Able to attain complete tar elimination at around 900 � C; increase the yield of CO2 and H2; Cheap; abundant; production inside the gasifier high tar conversion; comparable to dolomite High activity

Char Zeolite

Natural production inside gasifier reduce ash-handling problems

Particle agglomeration at high temperatures; lower catalytic activity than dolomite Rapid deactivation by coke Rapid deactivation because of sulfur and high tar content in the feed; relatively expensive Consumption because of gasification reactions Complex synthesis; relatively expensive

Fig. 11. Toluene conversion (a) and Arrhenius plots (b) of layered carbon supported Ni catalyst at different temperatures [176].

with commercial Ni/Al2O3, and the results indicated this kind of catalyst is more stable than Ni/Al2O3 under H2 atmosphere, which shows the high anti-coke ability. Oemar et al. [178] investigated steam reforming of toluene over La0.8Sr0.2Ni0.8Fe0.2O3 perovskite catalyst, and the reformed mechanism was proposed from various characterization

(Temperature programmed desorption (TPD), in situ DRIFT (Diffuse reflectance infrared Fourier transformed spectroscopy), TPSR of water, TPSR of toluene). The reaction between adsorbed aldehyde and adsor­ bed oxygen are the rate-determining step. As they presented, a mecha­ nism diagram is drawn (Fig. 12) according to the description in the publication. For the adsorption phase, water is dissociated on the sup­ port site, and then formed adsorbed OH and adsorbed H. The adsorbed OH is further dissociated to adsorbed H and O to replace the oxygen of the support structure. Toluene is decomposed on the metal site of catalyst into adsorbed �CH2 and benzene, then benzene will be cracked into �C2H2. Subsequently, �C2H2 and �CH2 will react with adsorbed oxygen. Finally, the adsorbed H2, adsorbed CO and adsorbed CO2 desorbed into H2, CO, and CO2 gaseous products, respectively. Gai et al. [179] also investigated the pyrolysis behavior of toluene in fluidized bed reactor. Model-free and model-fitting methods were employed to analyze the pyrolysis kinetics of H2-rich gas. They mentioned that CH4 and C2H4 are obtained 650–800 � C, while C3H8 and C2H4 will release at higher temperatures (800–850 � C). In the range of conversion fraction (20–80%), the apparent activation energy of pro­ pane H2, CH4, C2H4, and C3H8 are 69.55, 23.27, 17.59 and 16.34 kJ/mol, respectively. Gai et al. also claimed the H2 production result from three-dimensional diffusion, and CH4 evolution could be due to nucleation and growth. The most probable reaction mechanism for C2H4 and C3H8 production was formed via chemical reaction. Devi et al. [180] reported the reforming of naphthalene over olivine catalyst, and they mentioned a possible mechanism for the syngas production (Fig. 13). The results indicate atmosphere has a great influence on naphthalene

Fig. 12. Proposed catalytic mechanism of toluene reforming. 14

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Fig. 13. Possible reaction pathways for naphthalene decomposition. Redrawn from Ref. [180].

Fig. 14. Simplified thermal conversion pathways of toluene, xylenes, and naphthalene under H2 and steam atmospheres. Redrawn from Ref. [183]. 15

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decomposition, and H2O and CO2 enhance naphthalene decomposition, whereas H2 inhibits this effect. They claimed C–H or C–C bond cleavage is the first step for non-substituted aromatic compound, rings of the aromatic compound are opened, and then lower aliphatic or aromatic hydrocarbons are formed. The permanent gases of syngas are produced from consecutive reactions of the lower hydrocarbons. Indene is the first stable decomposition product for the reason of just one carbon is sub­ tracted. A simple power law kinetic model is employed to analyze the kinetic parameters under different atmosphere, and pretreated olivine €ber and with an activation energy of 213 kJ/mol is calculated. Gra Hüttinger [181] propose that indene is produced from naphthalene hydrogenation. The formation mechanism also described by Mebel et al. [182] formed by the naphthoxy radical production. Jess [183] employed toluene, benzene, and naphthalene as model tars to study the kinetics of the thermal conversion under H2/steam atmosphere. They obtained the following reactivity order: toluene > naphthalene > benzene. As the possible pathways presented in Fig. 14, except for the for­ mation of CH4 and C2H6, condensed products and soot are also produced from naphthalene cracking. H2 could inhibit the formation of soot and steam has few influence on the aromatics conversion. Under certain conditions, soot and organic cracking products could not be cracked under steam atmosphere even temperature increased. Author proposed possible reaction pathway of toluene, benzene and naphthalene are shown in Fig. 14, and the calculated results of thermal conversion are also shown in Table 7. Above all, the above researchers investigated polycyclic aromatic hydrocarbons cracking from monocyclic aromatic hydrocarbons, which provided ideas for cracking mechanism of the real tar with complex components. Although cracking mechanism was easily investigated when benzene, naphthalene, etc. were employed as the tar model for gasification, steam usually was poured into reaction system and result in catalyst deactivation. A catalyst with strong steam resistance should be considered when it used for long-term running in actual use.

The recognized tar cracking mechanism is partial combustion re­ actions and a series of catalytic reforming reactions [185]. Some possible catalytic reforming reactions were shown as below (Eqs. (1)-(14)). H2 and CO are easily produced from catalytic cleave of the C–C bonds of the carbohydrate backbone (Eqs. (3)–(5)) and more H2 can be generated in the presence of steam. (1)

Tars→C þ CnHm þ H2þCO þ CO2

(2)

Dry reforming: CnHm þ nCO→2nCOþ(m/2)H2þQ

(3)

Steam reforming: CnHm þ nH2O→nCOþ(n þ m/2)H2þQ

(4)

CnHmþ2nH2O→CO2þ(m/2þ2n)H2þQ

(5)

Methanation: CH4þH2O→2COþ3H2

(6)

CO2þ4H2→2CH4þ2H2O

(7)

Cþ2H2→2CH4

(8)

C þ O2→CO2

(10)

Boudouard reaction: C þ CO2→2CO

(11)

Water-gas reaction: C þ H2O→CO þ H2

(12)

Cþ2H2O→CO2þ2H2

(13)

WGSR: CO þ H2O→2CO2þH2

(14)

To understand the pathways of biomass catalytic gasification, many researchers performed their investigation from simple biomass compo­ nents (lignin, cellulose, sugars, etc.). Cortright et al. [186] reported the H2 and CO2 production over bimetallic catalyst for sugars and sugar alcohols reforming under steam atmosphere. Huber et al. [187] mentioned the H2 selectivity could be controlled over different metal composites and alloys catalysts. How­ ever, CO and H2 production are the endothermic processes, and external energy is required. Except for the above mechanism, Lignin, cellulose, and hemicellulose are the main component in biomass, mechanism of these components would close to the real tar. Fig. 15 shows the conversion of short substituents of the aromatic rings in primary products of biomass pyrolysis. Above 380 � C, the –OCH3, especially in the ortho position of the –OH will be reactive [188]. Different types of reactions and catalyst result in the substitution of the –OCH3 by –H, –OH or –CH3 group [188,189]. It was reported that active metal can attack the C–O bond at high temperatures and shows excellent catalytic properties for hydrogenation [190,191]. As illus­ trated in Fig. 15a, it explains the formation of –OH groups and H2O at the ortho position of the –OH. The –OCH3 is also the source of CH4 production [192], but this hydroxylation reaction of the –OCH3 requires the involvement of H2 and other donor groups. CO, CO2, and H2 for­ mation were as Fig. 15e presented, and absorbed H atom could be brought by H2 in primary pyrolysis of biomass and hydrogen overflow on the surface of the catalyst and carrier as illustrated in Fig. 15f. When the temperature was further increased above 450 � C, most of the initial bonds were broken, except for the existence of stable ether linkage and phenyl linkage [193]. As a consequence, the –CH3 or –OH in aromatic rings could produce CH4 according to a mechanism of deme­ thylation at 550 and 580 � C [194]. Fig. 15d suggested the formation of H2O molecule, which could react with the carbon deposition to produce syngas. Finally, the addition H2 production should be due to the aro­ matic rings rearrangement in the polycyclic structure when the tem­ perature was higher than 500 � C [195]. Steam is an ideal gasifier to achieve maximum syngas production at high temperature, low pressure, and high S/C ratio. As Eqs. (4) and (5) presented, high temperature is favor of the exothermic products release based on Le Chatelier’s principle [196]. High temperature could pro­ mote the endothermic reactions (Eq. (4)) and (Eq. (5)), and remove tar, and increase syngas yield. Other syngas could be produced from tar and light hydrocarbons conversion via the reactions (Eq. (2))-(Eq. (7)). If the biomass/coal chars are used as the carrier, reactions (Eq. (8))-(Eq. (13)) will take place and reduce catalytic performance of the catalyst. However, gasification of support should be considered when char catalyst was used for tar cracking. As Fig. 16 presented, Abu El-Rub [197] proposed that tars absorbed on the surface of char active site, then reformed into syngas under inert or steam atmosphere. Moreover,

4.2. Real tar

Gasification: CxHyOz→Tar þ Char þ H2þCO þ CO2þCH4þC2

(9)

Oxidation reaction: Cþ1/2O2→CO

Table 7 Kinetic data of toluene, benzene, and naphthalene conversion [184]. Hydrocarbon

Pre-exponential factor

Naphthalene Toluene Benzene

1.7 � 1014m0.3mol 3.3 � 1010 m1.5s 1 2.0 � 1016mol0.1m

0.1

s

0.3 -l

s

Activation energy 1

350 kJ/mol 247 kJ/mol 443 kJ/mol

16

Reaction order Hydrocarbon

Hydrogen

Steam

1.6 1.0 1.3

-0.5 0.5 -0.4

0 0 0.2

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Renewable and Sustainable Energy Reviews 116 (2019) 109426

Fig. 15. Proposed part of conversion mechanisms of lignin, cellulose, and hemicellulose.

mainly due to the active metal lost and oxidation, carbon deposition and sintering during longtime running. In addition, carrier structure changes are also a critical issue especially when biomass/coal char is used for tar cracking. However, fly ash, various feed rate, S poisoning, and other contaminants are the main reasons for catalyst deactivation during large-scale utilization. 5. Mathematical model of biomass gasification Tar models design for biomass gasification could be divided into three models: single-compound models, kinetic models, and lumped models. Single-compound model is the simplest model of biomass gasification and they reflect how biomass reacts with gasification agents. Lumped and kinetic models used in the biomass gasification is an excellent model to calculate heat and mass transformation. The single compound model, toluene, benzene, naphthalene, etc. were considered as the best tar model to investigate biomass gasifica­ tion. To obtain the mechanism of tar cracking, reducing the tar com­ ponents to develop the single compound model is a feasible method during gasification [18]. Toluene [198] and phenol [199] were used as a single compound model to study a thermodynamic model during tar formation. Benzene and naphthalene were the only products when toluene was chosen as the tar model compounds. Furthermore, naph­ thalene and benzene were obtained from naphthalene, benzene phenol, then crack into non-condensable gases and H2O. Zhao et al. [198] concluded the possibility to understand the mechanism of tar model compounds reforming, which minimized time-consuming and expensive costs and experiments design. To understand the simulation of bubbling fluidized bed gasifier, previous researchers build tar model using a lumped model method. Particularly, Abdelouahed et al. [200] designed another lumped model during dual fluidized bed gasification. Their simulations were the same as the experimental results.

Fig. 16. Schematic diagram of tar cracking on active site of char catalyst.

polymerizations will occur between free radicals and then produced carbon deposition during tar decomposition. Especially, the active sur­ face area of the char catalyst will be refreshed at the high temperature (800 � C). More H2, CO or CO2 could be formed under steam atmosphere (Eqs. (11)-(13)) in the presence of char catalyst. After reviewed litera­ ture, catalyst deactivation in tar model compounds reforming was 17

Renewable and Sustainable Energy Reviews 116 (2019) 109426

J. Ren et al.

Table 8 Gasification related reactions and kinetics constants.

(21)

ri ¼ ki CA CB 8:05r2 þ1:15r3 þ3r4 þr5

(22)

r5 ¼ k5 ðCCO CH2 O þ ðCCO2 2CH2 =KW ÞÞ

(23)

RCO ¼ 3:4r1 þ4:4r3 þr4

(24)

rH2 ¼ 2:15r1

RCH4 ¼ 3:4r2 RCO2 ¼

r5

(25)

r4

r3 þr5

(26)

r6

Reaction

Kinetics constant

Ref

5

Methanation: C3.4H4.1O3.3þ8.05H2→3.4CH4þ3.3H2O

16

Boudouard: C3.4H4.1O3.3þCO2→4.4COþ0.9H2Oþ1.15H217 Methane reforming: CH4þH2O→COþ3H2

18

Water gas shift: CO þ H2O ↔ CO2þH2

19

Carbonation: CO2þCaO→CaCO3

20

[202]

2.0 � 10 exp(-6000/T)

Char gasification: C3.4H4.1O3.3þ0.1H2O→2.15H2þ3.4CO 15

2.4 � 10

11

1.2 � 10

3

exp(-13670/T)

[203]

exp(-16840/T)

[203]

3.0 � 105exp(-15000/T)

[202,204]

1.0 � 106exp(-6370/T)

[205]

1.7 � 10

3

exp(-3485/T)

[206]

A suitable reactor, concentrations, and types of feedstock, then actual gasification conditions such as pressure, temperature, and flow rate should be optimized to obtain the maximum performance. It is a commercial and quick method to build mathematical modeling to evaluate the biomass gasification progress. Models are helpful to eval­ uate various biomass feedstocks and gasified behavior without actually different kinds of reactors at wanted temperatures and pressures. In general, the simulation of biomass gasification could be classed as 5 categories: (1) Kinetic Models (2) Tar Models (3) Computational Fluid Dynamics Models (4) Artificial Neural Network Models (5) Thermody­ namic Equilibrium Models.

The calculation for mass and energy balances were listed as Eqs. (27) and (28), respectively, which is available without the assumption of heat. The values heat capacity and standard heat of formation for the components were listed as Table 9. X X mi ¼ mo (27)

5.1. Kinetic models

ΔH ¼

i

o

X X Hi þ Qr ¼ Ho i

(28)

o

(29)

H ¼ n � Hf þ ΔH Z

T2

(30)

Cp dT T1

Here, m: The mass flow rate; H: Enthalpy; Qr: The energy for gasi­ fication; ΔH: Enthalpy change. The results of experiment and model calculation were presented in Fig. 17. As shown, high temperature and S/B ratio could promote the H2 production, and the CO yields decreased with the decrement of reaction temperature and S/B ratio. Their prediction results of the model are consistent with the experimental results, and these conclusions provide an excellent theory guidance for future research. Sreejith et al. [209] provided a kinetic model to study wood gasifi­ cation in a fluidized bed gasifier under air-steam atmosphere. With the increase of sorbent/biomass ratio from 0.75 to 1.5, the heating value of

The common computational software packages, such as Matlab, DFT and Aspen Plus, are useful for time, money and resources saving. Some models were previously utilized to obtain great ideas before experiment measurement. Due to kinetic modeling considers reaction kinetics and hydrodynamics of the gasifier, and it is more accurate than thermody­ namic models in obtaining gas yield and composition at lowtemperature. The only advance of kinetic modeling is a precise mathe­ matical approach, and its complexity would be increased with the in­ tricacy of reactor design. The Arrhenius (k ¼ Aexp( Ea/RT)) plots are significant to this model, which reflects the kinetic parameters. This equation contains the temperature, conservation of energy and mass and momentum. Inayat et al. [201] employed oil palm empty fruit bunch and steam as the gasification agent to apply to CO2 capture. They also claimed that the effects of temperature were more important than S/B ratio. They developed a kinetic model to study the effects of temperature and concentration at steady and dynamic states. Considering the occurrence of the char gasification, methanation and Boudouard reac­ tion, they combined previous research and employed kinetic and ther­ modynamic equations to calculate mass and energy balances. The related methods and calculations were listed as follows. Here, ri: The rate of reaction i; C: The concentration of reactant in the reaction; ki: The respective Arrhenius constant; Kw: Equilibrium constant for WGSR. R: Volumetric rate of each component, which determined by Eqs. (22)–(26) according to the kinetics parameters used for the calcu­ lations (Table 8).

Table 9 Heat capacity and standard heat of H2O, H2, CO, etc. formation [207,208].

18

Chemicals

Heat capacity (J/mol/K)

Hf (kJ/ mol)

H2O H2 CO CO2 CH4 CaO CaCO3 Cellulose

72.43þ(10.39 � 10 3)(T) (1.50 � 10 6)(T2) 27.01þ(3.51 � 10 3)(T)þ (0.69 � 105)(T 2) 28.07þ(4.63 � 10 3)(T) (0.26 � 105)(T 2) 45.37þ(8.69 � 10 3)(T) (9.62 � 105)(T 2) 14.15þ(75.5 � 10 3)(T) (18 � 10 6)(T2) 41.84þ(2.03 � 10 2)(T) (4.52 � 105)(T 2) 82.34þ(4.975 � 10 2)(T) (12.87 � 105)(T 2) 176.667þ(406.843 � 10 3)(T) (59.818 � 105)(T (151.538 � 10 6)(T2)

-241.8 0 -110.5 -393.5 -74.9 -635.6 1206.9 -1256.9

2

)

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Renewable and Sustainable Energy Reviews 116 (2019) 109426

Fig. 17. Comparison of model and experiment: Effects of temperature (a) and S/B ratio (b) on product gas composition. Redrawn from Ref. [201]. Table 10 Single-reaction model: Best kinetic parameters of secondary products from the previous literature. Product

Log A (s

Tar CH4 CO CO2 H2

1

)

Ea (kJ/mol)

Ref. [211]

Ref. [210]

Ref. [211]

Ref. [210]

Ref. [211]

Ref. [210]

5.0 4.9 4.7 2.6 6.6

5.1 5.0 5.1 4.1 5.0

93.3 94.2 87.9 49.0 129.0

105.0 125.8 121.8 82.5 116.4

2.2 0.1 1.8 0.8 0.1

5.5 4.5 3.7 2.8 3.4

the syngas was reached to 6.12 MJ/Nm3. Furthermore, H2 yield was reached to 53% at an equivalence ratio of 0.25, S/C of 1.5, sorbent-to-biomass ratio of 2.7 and 727 � C when sorbent was poured into the gasification system. In addition, they also proved that air-steam atmosphere is better than steam atmosphere. To investigate the individual gases kinetics and tar cracking, Khonde et al. [210] designed another single-reaction model and activation en­ ergy model for rice husk gasification. They found that H2-rich gas yield and tar conversion reached to maximum (91%) with the temperature and residence time increasing. For the accuracy in correlating tar con­ version under any conditions, Khonde et al. [192] claimed that N2 and air were more important for secondary gas cracking, and distributed activation energy model was more suitable than the single-reaction model. They employed distributed activation energy model at low residence times and performed many simulations with different A (pre-exponential factor) and Ea (activation energy) to study the kinetic parameters. The related kinetic parameters for their models were listed in Table 10. In addition, they further modified previous calculations to best fit of kinetic parameters, and solved differential equation by using RungeKutta fourth order method (Eq. (31) and (32)). � dVi =dT ¼ ki ðV*i Vi (31) Vi ¼ V *i

�Z

�

∞

Z

exp 0

δ (%)

0

t

� � ki dt f ðEiÞdEi

Vi: Initial yield of tar or gas at t (time); V�i : Final yield of Vi; ki: Reaction rate equation; f(Ei): Gaussian distribution of activation energy (mean); Eδi with standard deviation δi; f(Ei)dEi: Fraction of tar or gas formed with the activation energies from E to E þ dE. Fig. 18 presents the tar and CO yields obtained from experimental and simulated data, which shows a great correlation to the singlereaction model. Single-reaction model correlation indicates that this model is suitable at high residence times. The possible standard error and activation energy were shown in Fig. 18c and d, and they compared with Boroson et al. [211] results with multiple-reaction distributed activation energy model, which is the minimum standard error (δ) of simulation. However, they could not obtain the time-temperature data due to the limitations of this distributed activation energy model. 5.2. Other mathematical models Antonopoulos et al. [212] employed the olive wood, miscanthus, and cardoon as the biomass materials to investigate the effects of tempera­ ture and biomass moisture on gasification by using the thermodynamic models. The gaseous products (H2, CO, and CO2) concentration was simulated and compared with experimental results. Furthermore, they obtained the relation between heating value of the outlet, gas yields with the reactor temperature. Huang et al. [213] employed thermogravi­ metric analyzer to investigate thermal cracking behavior of soybean straw, and they compared the obtained kinetic parameters with

(32) 19

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Renewable and Sustainable Energy Reviews 116 (2019) 109426

Fig. 18. Single-reaction model correlation of (a) tar yield, (b) CO formed, and simulation for tar yield with distributed activation energy model (c) standard error versus number of iterations (d) activation energy versus number of iterations. Redrawn from Ref. [210].

Fig. 19. TG curves (a), activation energy changes with different conversion (b), and kinetic plot of KAS (c) and OFW (d) models of soybean straw cracking. α: conversion rate; β: heating rate (K/min). Redrawn from Ref. [213].

experimental data. They carried out pyrolysis experiments at a heating rate of 5, 10, 20, 30 K/min, and utilized iso-conversional Kissinger-­ Akahira-Sunose (KAS), Ozawa-Flynn-Wall (OFW), and Coats-Redfern (CR) method to calculate the activation energy (154.15 and

156.22 kJ/mol for KAS and OFW models). The kinetic parameters of tar decomposition were simulated by using these three models, and the results showed the same trend as the experimental data. The related results were shown in Fig. 19. 20

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Renewable and Sustainable Energy Reviews 116 (2019) 109426

Giltrap et al. [214] introduced char reactivity factor and provided a downdraft gasified model for the reduction zone of the biomass gasifier under steady-state operation. For convenient calculation in this model, they ignored the possible pyrolysis products from pyrolysis and cracking reactions. This model assumes CO2 was completely cracked products and solid carbon was presented as char throughout the reduction region. The reaction (C þ CO2→CO, C þ H2O→CO þ H2, Cþ2H2→CH4, and CH4þH2O→COþ3H2) was adopted in this model. Although their model predicted the gas composition, gaseous production concentrations were wrongly predicted. The initial condition and data of the reduction zone in the reactor are important for this model accuracy. � � dnx 1 dv Rx nx (33) ¼ dz dz v dT 1 ¼ P dz v x nx cx

�

X ri △Hi

v

i

dP dz

P

dv dz

� � dP v2 ¼ 1183 ρgas þ 388:19v dz ρair

79:896

� �� E1 r1 ¼ nCRFA1 exp ⋅ PCO2 RT

P2CO K1

� �� E2 r2 ¼ nCRFA2 exp ⋅ P H2 O RT

PCO ⋅PH2 K2

�� � E3 r3 ¼ nCRFA3 exp ⋅ P2H2 RT

PCH4 K3

� � E4 r4 ¼ nCRFA4 exp ⋅ PCH4 ⋅ PH2 O RT

� X Rx nx cx

process variables could increase the process efficiency. Although artifi­ cial neural network modeling is a very promising method for biomass gasification, abundant data and large database are required to produce this model development for artificial neural network modeling. 6. Summary and perspectives In addition to wind, hydro, wave, solar energy, etc., biomass is one of the main renewable energy sources. Biomass catalytic gasification is a key technology for environment production and chemical production. The gaseous products enable synthesis of high-added biofuels and chemical precursors by F-T synthesis. As reviewed above, the advan­ tages and disadvantage of developed catalysts are summarized as the following. Natural mineral catalysts are widely applied for biomass gasification result from its economical and abundant. However, natural catalysts with the low mechanical strength in fluidized bed reactor result in their low activities than synthetic ones. Furthermore, biomass con­ tains alkali species could be gasification catalyst, make it easier to handle the ash problem. However, Alkali metals are not suitable as a secondary catalyst result from the low hydrocarbon conversion. Biomass themselves as the catalyst support also have catalytic activity in gasifi­ cation, and then achieve efficient utilization of biomass. Expensive noble metal catalysts have high activity, stability, and coke resistance ability for biomass gasificaition. Other transition metal (group VIII) such as Fe, Co, Ni, Pt, and Cu based catalyst also exhibit a certain performance in biomass gasification. Due to the high activity and economy, Ni-based catalysts are extensively used for biomass catalytic gasification under different at­ mospheres. However, Ni-based catalysts are easily deactivated result from coke formation and Ni particles growth. To improve the activity and stability of Ni-based catalyst, transition metals, noble metals, and rare-earth metals could be doped into Ni-based catalyst. Although many researchers successfully developed supported high activity catalysts for decomposition of model tars (benzene, toluene, naphthalene, etc.), and it is difficult and almost impossible to exactly predict the mechanism of tar cracking for the reason of complexity of the biomass tar. In order to obtain a highly efficient decomposition of tar, the following points should be considered in the future:

(34)

x

(35)

� (36) � (37)

� (38) PCO ⋅P3H2 K4

! (39)

Babu et al. [215] further modified the study of Giltrap et al. [196] by incorporating the exponentially varying char reactivity factor (CRF), they calculated and linearly simulated results at CRF from 1 to 10000. Vakalis et al. [216] proposed a novel approach and divided the reactor into multiple boxes, and then they evaluated solid-vapor balance in downdraft gasifiers. The H2-rich gas and char yields and compositions were compared with common modeling results. The results were closer than the result of single-stage equilibrium modeling during the tar real cracking. Researchers have also made this approach more accurate by introducing data to empirical correlations and alleviated the previous restrictions of equilibrium models. Computational fluid dynamics modeling is also a vital tool in the simulation of modeling thermochemical gasifiers. To study the crack characteristics of dispersed phase and gas phase, 2D computational fluid dynamics modeling was proposed based on the Eulerian-Eulerian method. Coute et al. [217] investigated the effects of O2 on tempera­ ture, S/B ratio, and gaseous products composition. They found that experimental data were in agreement with simulations. Previous re­ searchers build some computational fluid dynamics models to investi­ gate cold gas efficiency, conversion efficiency, products composition and temperature profile of biomass (wood) gasification [218–220]. However, the simulated results are different from experimental data for the reason of high complexity of gasifier inside. More recently, artificial neural network modeling is a new tool for biomass gasification, which proposed for analysis of complex processes. Sreejith et al. [221] constructed this model to predict gas concentra­ tions, heat content and temperature profile in fluent bed gasification. The predicted H2 yields (28.2%) were close to the experimental data (29.1%) at the S/C of 2.53. Another control strategy, called as “feed forward-feedback” was employed to evaluate the biomass gasification [222], and operational data was collected and operated from fixed-bed reactor in Technical University of Dresden. Due to the changes of air and fuel distribution, the introductions of advanced control systems with

1) Optimization of scale-up gasifier. Recently, a large number of laboratory-scale catalysts and reactors are investigated in detail. Biomass can be gasified in a fixed bed, fluidized bed, entrained flow, rotary kiln and plasma reactor under small-scale experiment condi­ tions. However, few laboratory-scale catalysts are usually used for commercial use. Due to the high purity of biomass and model tar were used in the lab scale, catalyst maybe shows high activity for tar cracking. However, the factors on the activity of catalyst become very complex in large-scale reactors, . In fact, unstable flow rate, space velocity, temperature, and pressure will be the main consid­ eration for future researchers. In addition, dynamic biomass feeding, fly ash and catalyst broken in the presence of HCN, NOx, SOx and fly ash also should be considered. 2) Design of highly efficient and easily regenerated catalyst. In order to reduce the energy consumption, a suitably active metal site design for tar cracking is possible over specific structure catalyst. Although some highly active catalysts are designed for tar cracking in the lab scale, it is easily deactivated due to the complexity of the tar com­ positions in actual use. Carbon deposition and poisoning are the main reason for affecting catalyst activity. During the large-scale biomass gasification of industry, nature and cheap catalyst would be discarded when they are deactivated from carbon deposition or poisoning. Accordingly, it is important to develop a cheap catalyst with specific composition and structure, which could easily be re­ generated and decompose tar effectively. 3) Understanding of tar crack mechanism. For the reason of the complexity of biomass tar, most researchers just selected model 21

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Renewable and Sustainable Energy Reviews 116 (2019) 109426

compounds (toluene, naphthalene, etc.) as the tar representatives. They considered tar model compounds cracked reaction is a single first-order reaction and employed them for kinetics analysis, How­ ever, it is not enough to surmise the real reaction rules and interac­ tion of the tar components. Next step should concentrate on the reforming the mixtures with different kind and amount of model compounds to explore the cracking mechanism of real tar.

[18] [19] [20] [21]

Declaration of interest

[22]

The authors declare no conflict of interest.

[23]

Acknowledgements

[24]

This work was subsidized by the National Key R&D Program of China (Grant 2017YFE0124200), the National Natural Science Foundation of China (Grants U1710103, 21676292 and 21978317), and the Priority Academic Program Development of Jiangsu Higher Education In­ stitutions. We are grateful to Elsevier, ACS and RSC publishers provide copyrights for related Figures and Tables. We are also thankful to China Scholarship Council who provides scholarship for Jie Ren (No. 201806420028) to develop his PhD research at the RWTH Aachen University.

[25] [26] [27] [28] [29] [30]

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