Hot gas filtration – A review

Fuel 104 (2013) 83–94

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Review article

Hot gas filtration – A review Steffen Heidenreich ⇑ Pall Filtersystems GmbH, Werk Schumacher Crailsheim, Zur Flügelau 70, D-74564 Crailsheim, Germany

a r t i c l e

i n f o

Article history: Received 19 December 2011 Received in revised form 27 June 2012 Accepted 25 July 2012 Available online 13 August 2012 Keywords: Hot gas filtration Particle removal Gas cleaning Fly ash High temperature filter elements

a b s t r a c t This paper provides a detailed survey on hot gas filtration. Fundamental aspects of filtration at higher temperatures are described first, including the influence of the temperature on dust properties and filtration behaviour. The main focus is on the review of hot gas filter media as well as hot gas filter systems. Moreover, applications of hot gas filtration are presented and discussed in detail, for example advanced coal gasification as well as biomass gasification and pyrolysis, incineration of low-level contaminated radioactive waste from nuclear power generation, waste incineration, fluid catalytic cracking in oil refineries and other processes. By using hot gas filters, downstream equipment, such as heat exchangers, catalyst units, turbines and scrubbers, are protected from erosion and fouling, processes can be intensified or simplified as well as blocking by condensation or desublimation can be prevented. Energy efficiency, process intensification, PM 10 and PM 2.5 emission values, water shortage and water quality as well as overall process costs are topics which raise an increasing interest in hot gas filtration. Ó 2012 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4.

5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filtration at high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hot gas filter media and filter elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Ceramic filter media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Metal filter media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Hot gas filter elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Catalytically activated hot gas filter elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hot gas filter systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Single tubesheet design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Multi stage designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Filter tube design of Asahi Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Cross flow filtration concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Filter system integration into a biomass gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Further research and development requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Incineration and pyrolysis of low-level contaminated radioactive waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Coal gasification and combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Biomass gasification and pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Fluid catalytic cracking units in refineries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Waste incineration and pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Other processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Tel.: +49 7951 302 172. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.07.059

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1. Introduction Hot gas filtration has been attracting more and more attention in a wide variety of processes over the last years. Main reasons for that are higher demands on emission levels, such as PM 10 and PM 2.5, the possibility to simplify or to intensify processes, problems to supply and to clean wash waters for wet scrubbing as well as the development and application of new processes, such as e.g. biomass gasification. In advanced power generation systems, in chemical industry, in oil refineries, in incinerations, metal refining and metal recycling, the need for hot gas cleaning is driven by the requirements of increased process efficiency, process intensification, product quality and environmental legislation. It has been shown that product quality and process efficiency and economics can be improved by using hot gas filtration. For example using hot gas filtration for fly ash removal instead of wet scrubbing, increases the efficiency of a coal gasification process by about 3%. In many processes, high filtration temperatures are required to avoid undesirable condensation or desublimation reactions, which result in the fouling and blocking of filters or the polluting of products. One example for this is the condensation of tars in biomass gasification (see Section 5). The filtration of high temperature process gases also generates the opportunity to utilise particle free, high temperature gas in coupled or subsequent process steps. Furthermore, the economic and process-technological advantages of hot gas filtration are obvious in high temperature processes that involve the circular movement of substances or process gases as well as the recovery of products or expensive materials, such as catalysts or noble metals. Protection of downstream heat exchangers or catalyst units by hot gas filters increases the energy efficiency or the reaction performance, respectively. Moreover, some processes can first be realized by using hot gas filters. The only disadvantage of hot gas filtration is the higher investments for the filter system due to the increased demands on the materials and the higher gas volume at higher temperatures. Furthermore, the pressure drop is higher caused by the increased viscosity of the gas with temperature. However, in most cases the advantages dominate as well as higher investments for the filter system are compensated by a reduction of the overall process costs. The first technical application of hot gas filtration was for the incineration of low-level contaminated radioactive waste from nuclear power generation. At the beginning of the 1970s, hot gas filtration was used for example at the former nuclear research centre of Karlsruhe in Germany [1] and in some incineration plants in France [2]. Since 1978 hot gas filtration is used in nuclear power plants in Japan [3]. The development of advanced coal based power generation techniques such as the pressurized fluidized bed combustion (PFBC) and the pressurized integrated gasification combined cycles (IGCC) from the end of the 1980s until the end of the 1990s has significantly influenced the development of hot gas filtration. At this time, many big research and demonstration projects have been started in the USA, in Japan and in Europe for development, investigation and testing of different hot gas filter media and filter systems. Meanwhile more than 25 large hot gas filter units are in operation or in commissioning in coal gasification plants worldwide. About 20 of them were installed in China in the last 8 years. The hot gas filter with the longest operating time in an IGCC plant was installed in 1994 in Buggenum in the Netherlands. The total number of hot gas filters worldwide installed in many different applications is in the order of some hundreds. This paper aims to provide a detailed survey on hot gas filtration and to raise the awareness of hot gas filtration. Hot gas filter

media and filter systems are reviewed and described as well as details and particularities of filtration at higher temperatures are discussed. An overview of applications and some detailed examples are presented.

2. Filtration at high temperature Filtration at temperatures above 260 °C is called hot gas filtration according to the draft of the VDI guideline 3677-3 [4]. High temperatures place high demands on the properties and the mechanical, thermal and chemical stability of the materials which are used. The filter media as well as the vessel material have to be stable against temperature, pressure and chemical composition of gas and dust. The higher the temperature is the higher the demands on the materials are. Hot gas filtration is performed at temperatures ranging typically up to 900 °C, at pressures up to 8 MPa, in both oxidising and reducing atmospheres, and often with chemically aggressive compounds. At low filtration temperatures, dust properties, such as particle size distribution, adhesion and cohesion forces as well as compressibility of the dust determine the pressure drop and the detachment behaviour of the dust cake. At higher temperatures, the thermal properties of the dust influence the filtration behaviour. Dust softening or sintering of the dust can occur at higher temperatures which results in a sticking dust layer on the filter element surface and correspondingly in an instable filtration. Dilatometer [5] or high temperature rheometer measurements [6] of dust samples can show at which temperature softening of the dust starts. Softening temperature depends on the chemical composition of the dust. Chlorides, such as NaCl, KCl or CaCl2 decrease the softening temperature. In case of a eutectic mixture, softening temperature can be decreased significantly. By an increase of the sticking force of the dust with increasing temperature, also dust bridging can occur. This can cause an instable filtration due to incomplete regeneration of the filter elements. In the worst case, breakage of filter elements can result from bridging. Measurement of the tensile strength of a dust cake as a function of the temperature can give a rough indication whether a dust cake tends to bridging or not. However, a better measurement for the likelihood of dust bridging, proposed by Hurley and Dockter [7], is the ratio of the tensile strength to the density of the dust cake as a function of the temperature. They called this ratio the critical thickness index (CTI) and showed by measurements with different dusts that this index is a better indicator of the likelihood of a dust cake to form bridges. If the index is high, the strength of the dust cake corresponding to his weight is high and it is more likely that the dust cake tends to bridging. The investigation of two different fly ashes from a PFBC coal combustion by Hurley and Dockter showed a significant increase of the CTI index for one of these ashes from 700 to 750 °C which indicates an increase of the tendency to form bridges at the higher temperature. This result was in good agreement with the filtration experience with this dust. The second fly ash which had a much higher density showed a lower CTI index and no increase of the CTI index by increasing the temperature from 700 to 750 °C. The filtration experience with this dust showed no bridging at these temperatures. Filtration tests at a 4 MWth PFBC coal combustion of other researchers showed no bridging and no problems of regeneration at 800 °C, however, at 950 °C bridging of the dust was detected [8]. At high temperature, the dust cake properties can be changed by chemical solid phase reactions in the dust. By means of dilatometer, thermo-gravimetric analysis and differential scanning calorimetry, thermal expansion, loss of weight and relative heat flux of the dust can be analysed, respectively. Using these analysis

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methods, Hemmer [9,10] could show clear relations between thermal changes of different dusts – bark ash, lignite ash, incineration ash and sodium bicarbonate – and their filtration behaviour. For one lignite ash from PFBC combustion as an example, he could determine stable filtration up to 600 °C by filtration tests. Corresponding investigations of the thermal changes of the ash showed first major instability at 600 °C. Sintering of this lignite ash started at 820 °C. Filtration tests and analysis of the thermal changes of a second lignite ash showed a first instable operating region between 350 and 450 °C as well as instable filtration above 575 °C. Sintering of this lignite ash started at 875 °C. Furthermore, the dust can react with the gas. Kanaoka et al. could show that the reaction of lime with CO2 for example changes the dust properties significantly [11]. The differential pressure of the dust cake depends strongly on the porosity of the cake. A decrease of the porosity by compression for example raises the pressure drop of the cake. At higher temperatures, the structure and the porosity of the dust cake can change due to the increase of the sticking force or by plastic deformation of the dust particles. Measurements with different dusts, such as lime, quartz and bark ash, have shown higher porosities of the dust cake at high temperatures than at ambient temperature [10,12]. Other investigations with fly ash from coal combustion showed no change of the porosity of the dust cake with increasing temperature [13]. Particle collection is determined by different mechanisms which depend differently on temperature. For particles smaller than 1 lm, an increase of temperature improves particle collection. In this size range diffusional forces dominate the collection. Collection by inertial forces is reduced with increasing temperature since the impaction is reduced according to theoretical consideration for a single fibre [12,14]. However, the influence of inertial impaction is negligible compared to the barrier effect for typical filtration velocities [12,14]. Thus, collection efficiency increases only for small particles with increasing temperature. Grade efficiency measurements with different filter media and dusts have confirmed this general tendency [12,15,16].

at the contact points by inorganic and/or organic binders. By the vacuum-forming manufacturing process high porosity of the filter elements is achieved. Fig. 1 shows the structure of a low density ceramic filter element made of alumosilicate fibres. Low density ceramics have high fracture toughness due to the loose structure. However, the mechanical strength is very low according to the loose structure of the fibres and the risk of candle breakages is correspondingly high. The intensity of the back pulse for regeneration needs to be controlled so that no fibres are released from the filter structure. The differential pressure of low density ceramics is relatively low. High density ceramics have a mechanically very stable structure with a high mechanical strength. High density ceramic filter elements of Pall Corporation made of silicon carbide have bursting pressures of higher than 5 MPa and O-ring pressure strength of more than 20 MPa. High density ceramics are manufactured by pressing or extrusion followed by sintering at high temperature. The pore size and the size distribution can be very exactly adjusted by selecting the right grain sizes. Furthermore, an option to adjust the pore size and porosity is to add pore-forming materials which burn out during the sintering process, such as e.g. polymers, sawdust or graphite. By the amount and particle size of the pore-forming material, the pore size and the porosity can be controlled. High density ceramics are available with symmetric or asymmetric structure. A symmetric structure has a uniform pore size distribution over the whole thickness of the material. An asymmetric structure has a thin membrane layer with smaller pores on top of the support material, as shown in Fig. 2. The membrane collects small particles due to its fine pores. By making the membrane thin, the differential pressure of the filter element is limited. An optimum is to have a layer which is so thin that the support is just covered by the layer without defects. Surface filtration is achieved by the membrane. A penetration of particle in the support structure of the filter element is prevented and the element can be effectively regenerated by back pulsing. This is an advantage with regard to the long term behaviour of the filter elements and their life time.

3. Hot gas filter media and filter elements The filter medium has to be resistant against the temperature and needs to have a sufficient high mechanical stability. Furthermore, the filter medium has to have a good thermal shock resistance and chemical stability against gases, such as H2S, HCl, NH3, Cl2 and H2O, and against alkalis. With increasing temperature the requirements for the materials increase. When selecting the material it has to be taken into account that the properties of the porous filter material with the high surface area will be different compared to the compact material. Only rigid self supporting filter elements made of ceramic or metal are applied at high temperatures. Flexible filter media which are applied at low temperatures would be destroyed at high temperatures due to the high mechanical stress caused by the movement during back pulsing. 3.1. Ceramic filter media High density and low density ceramics are available as filter media. High density ceramics are made of sintered grains preferably of silicon carbide, alumina or cordierite. Low density ceramics are mostly made of alumosilicate fibres. The porosity of high density ceramics is in the range of about 40%. Low density ceramic filter elements have a high porosity of up to about 90% and a high internal surface due to the fibre structure. They are typically vacuum-formed from fibre slurry and the fibres are bonded together

Fig. 1. Structure of a fibre ceramic.

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which generate a protection surface of alumina [21,22]. The maximal operating temperature of these elements is at about 780 °C [23]. Other filter elements with a protective alumina surface layer are made of FeCralloy (Footnote 1). By heat treatment at about 1000 °C under oxidising atmosphere, aluminium moves to the surface and generates a protection surface of alumina [24]. This protection layer is compact and very corrosion resistant. The maximal operating temperature of these elements is at about 1000 °C [25]. Investigations in coal gasification environment have shown corrosion for FeCralloy as well as for iron aluminide media [26,27]. It is supposed that by cracking and spalling of the alumina layer due to different thermal expansion of the alumina and the underlying alloy, the iron in the alloy is attacked by hydrogen sulphide and water vapour [27]. Filter elements of HR 160 (Footnote 1) and Hastelloy X showed no noticeable corrosion after an exposure of 1500–2500 h in coal gasification environment [28]. Maximal operating temperature of filter elements of HR alloys is at about 600 °C [25]. 3.3. Hot gas filter elements

Fig. 2. Asymmetric structure of a high density ceramic filter element with a support having coarse pores and a filtering membrane having fine pores.

The collection efficiency of high density ceramic filter elements is very high and reaches nearly 100% [17], and even for submicron particles this performance can be achieved [18]. Thermal stability of ceramic filter media depends on the material used. Temperatures up to 1000 °C and higher are possible [19,20].

The preferred geometry of hot gas filter elements is the form of a candle which is closed at one end (see Figs. 3 and 4). This geometry enables a very reliable operation. The cake is typically buildup at the outside of the candles and can be easily and reliably detached from the outside surface of the candle. The filter candle lengths range from 1 to 3 m depending on the material. Typical candle outer diameters are between 60 and 150 mm. In some cases also inverted candles with a flow direction from the inside to the outside having the dust cake on the inside surface of the candle as well as filter tubes were used. Other geometries with a higher density of the filtration area have also been tested. One example are ceramic filter elements

3.2. Metal filter media Only metallic filter elements are an alternative to the ceramic at high temperatures. Sintered porous metal filter media, manufactured from either fibre metals or powder metals are used. Additionally, metal fabrics are available as filter media. Sintered powder metal media are typically manufactured by pressing metal powders of selected particle size distribution and subsequent sintering in hydrogen or under vacuum. The porosity of sintered powder metal filter media ranges from about 20% up to about 40%. Sintered fibre metal media are non-woven structures formed from short fine metal fibres with diameters typically ranging from 2 to 40 lm. These non-woven structures are carefully sintered in hydrogen or under vacuum. Porosity of these sintered fibre structures can be up to about 90%. Metallic filter elements are made of different steel grades and various metal alloys. The material applied is selected by its thermal and chemical stability. Oxidation and corrosion lead to irreversible plugging of the pores due to the fact that the volume of the reaction products is higher than that of the pure metal. Filter elements made of stainless steel are typically applied for temperatures up to 420 °C. Filter media of high temperature steels can be applied up to 650 °C [19]. In most hot gas applications, sulphur or chlorides are present in the gas. In this case, only special metal alloys, such as Inconel1 600, Monel (Footnote 1) or Hastelloy (Footnote 1) X, can be applied. For applications with high levels of hydrogen sulphide, special filter elements have been developed in the recent years. These filter elements are made of special iron aluminide alloys 1 Dia-Schumalith is a trademark of Pall Corporation. Inconel und Monel are trademarks of Special Metals Corporation. Hastelloy and HR 160 are trademarks of Haynes International Inc. Fecralloy is a trademark of UKEA, UK.

Fig. 3. High density ceramic filter candles with lengths of 1.25 m, 1.5 m, 2 m and 2.5 m.

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commercial hot gas filtration applications so far. A further design to increase the filtration area density was the development of a cross flow hot gas filter element [30]. The raw and clean gas flow channels are placed above each other with a change in the flow direction of 90° for the clean gas channels. Particles are collected by flowing through the porous walls. However, also for this design the main drawback is the risk of plugging the raw gas channels and an insufficient detachment of the dust by back pulsing. This made this design not attractive for technical application. 3.4. Catalytically activated hot gas filter elements

Fig. 4. Metal filter candle.

with a honeycomb monolith structure. The monolith has a plurality of cells aligned in parallel. In order to get a wall flow, the cells are alternately plugged either at the upstream end or at the downstream side of the monolith. The gas flow enters in the cells open at the upstream side and flows through the walls and the cells open at the downstream side (see Fig. 5). The particles are collected at the walls of the cells open at the upstream side. Nowadays, this filter geometry is well known as diesel soot filter applied in vehicles. Such hot gas filter honeycomb monoliths have been successfully tested in a PFBC application at 863 °C during a 300 h test [29]. However, the main disadvantage of this filter element design is the risk of plugging the upstream cells which cannot be removed by back pulsing. Due to this risk, this design has not been used in

Fig. 5. Scheme of a wall flow filter with honeycomb structure.

Recent developments have been hot gas filter elements which combine filtration and a catalytic function. One example for such developments are catalytically activated ceramic hot gas filter elements for the combined removal of particles and the reduction of nitrogen oxides [31–33]. A catalyst based on TiO2–V2O5–WO3 is integrated into the inner structure of the filter element. A fine filtering membrane on the outer side of the filter element protects the catalyst of any particle deposition. Filtration as well as selective catalytic reduction (SCR) of the nitrogen oxides is performed at temperatures between 280 and 320 °C. A second example for the development of catalytic filter elements is the combined removal of particles and tars from syngas in biomass gasification. Different tar reforming catalytic filter elements having the catalyst integrated as fixed bed or as catalytic coating of the porous inner structure of the filter elements have been developed [34–36]. Filtration as well as tar reforming is performed at temperatures between 800 and 850 °C. By using catalytic filter candles two functions can be combined in one unit instead of having two. This simplifies the process flow and reduces costs and footprint. Furthermore, the combination allows the use of the energy of the gas as well as prevents plugging of the catalyst by particle deposition. 4. Hot gas filter systems The filter vessel has also to fulfill special requirements at high temperatures. For the combination of high temperature and high pressure a vessel with refractory lining is essential. Different filter system designs have been developed in the past. The main reasons for all these developments have been to increase the robustness and reliability of the systems or to reduce the system costs, respectively. Some of these system designs with their advantages and disadvantages shall be highlighted in the following. 4.1. Single tubesheet design The single tubesheet design is the one mostly applied for hot gas filtration. Conventionally, filter candles closed at one end are installed vertically hanging in a tubesheet which separates the filter vessel in a raw and a clean gas room (see Figs. 6a and 6b). The gas to be filtered flows from the outside of the candles inwards building a dust cake on the outer surface of the candles. The filter candles are arranged into groups. The different groups are sequentially cleaned on-line by a back pulse with clean gas from a gas tank. Conventional jet pulse systems are used for back pulsing or special blowback systems developed for the regeneration of rigid hot gas filter elements, such as the jet pulse system with a common venturi ejector [37] or the Coupled Pressure Pulse (CPP) system [38,39], both of Pall Corporation. Air or nitrogen as well as cleaned process gas can be used for regeneration. Sometimes the blowback gas needs to be preheated to overcome problems with condensation. The vessel can be either circular (for higher operating pressures) or rectangular (for applications at atmospheric pressure). In rectangular vessels, sometimes

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Gas Tank

Blowpipe

Venturi Ejector

Blowback Valve

Clean Gas

Tube Sheet

Raw Gas

Filter Element

Fig. 6a. Scheme of a filter system with single tubesheet design. Fig. 7. Schematic drawing showing the relative position of the filters in the multistage filter system developed by Lurgi Lentjes Babcock [40].

Fig. 6b. View of high density ceramic filter candles hanging in a tubesheet from the bottom of a filter vessel.

the filter elements are also horizontally installed in a vertically arranged tubesheet (see e.g. [40–43]). The main advantage of the single tubesheet design is that the installation of the filter elements and maintenance is performed easily from the clean gas side of the filter vessel. The main disadvantage for circular vessels is the restricted number of filter elements that can be installed for a given vessel diameter. However, despite of this disadvantage the single tubesheet design is still the preferred applied one for hot gas filtration. 4.2. Multi stage designs In order to increase the filtration area in circular vessels and to overcome the limitation of the number of filter elements of the sin-

gle tubesheet design, alternative hot gas filter systems with multi stages have been developed in the past, e.g. the systems developed by Lurgi Lentjes Babcock (LLB) (see Fig. 7) [44] and by Westinghouse (see Fig. 8) [45]. Main feature of the LLB filter system is that the candles are mounted standing instead of hanging. The filter candles are pressed by a weight with the open end in a metal seat. The weight shall keep them tightly in the seat during filtration and specially during back pulsing. The candle seats are fixed on horizontal manifolds which can be placed in multiple stages in the filter vessel. An advantage of the standing installation of ceramic filter candles has been seen that the candles are kept under compression by their own weight and are therefore less likely to break. LLB filter systems were installed and operated in the past at the High Temperature Winkler gasification demonstration plant in Berrenrath (Germany) and at the 300 MWel IGCC coal gasification plant in Puertollano (Spain). The filter system in Berrenrath contained 578 ceramic filter candles [46] and the one in Puertollano had two filter vessels each equipped with 1036 ceramic filter candles [47]. In the Westinghouse filter system, the filter candles are clustered in groups which are positioned above each other (see Fig. 8). Each group has 30–60 filter candles. The candles are installed hanging in single holders. The head of each candle is enclosed by a sealing material and fixed by screwing a counterpart to the holder. Installation has to be carried from the raw gas side of the vessel. A group of holders are connected to a common plenum. This plenum has its own back pulsing arrangement and is in flow connection to the clean gas side to collect the clean gas from the candles and to discharge it. Two or more plenums are stacked vertically and supported by a common pipe which is connected to the main tubesheet. The Westinghouse filter system has been mainly tested in different pilot and demonstration projects in the USA. A 384 filter candle containing system has been operated

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Fig. 9. Inverted candle filter system concept of Siemens-Westinghouse [47].

Fig. 8. Schematic drawing showing the relative position of the filters in the multistage hot gas filter system developed by Westinghouse [41].

in the 70 MW Tidd PFBC demonstration plant in Brilliant (Ohio, USA) from 1992 to 1996 [48]. A filter system containing 748 filter candles was installed at the 107 MW Pinon Pine coal gasification plant in the USA [49]. This plant was completed in 1997, but has never been in real operation due to problems and was closed in 2001 [50]. A filter system containing 91 candles is installed and operated since 1996 at the Power System Development Facility of Southern Company Services in Wilsonville (Alabama, USA) [28]. Siemens-Westinghouse developed and tested two advanced hot gas filter system configurations based on the experiences with the afore-described Westinghouse system [51]. Both systems followed the principle of having vertically stacked plenums which are supported from a tubesheet as in the case of the Westinghouse design. However, one concept used so called ‘‘inverted candle’’ installation with a flow of the raw gas to the inner side of the candles (see Fig. 9). Each individual filter candle is contained in a metal housing that protects the filter element from ash bridging and vibration. Each filter element is fixed with its open end at the bottom of the individual metal housing. Dust cake is build-up on the inner surface of the candles. Installation has to be carried from the raw gas side of the vessel. One of the advantages of this design is that the possibility of ash bridging between the candles and corresponded candle breakage is avoided. A second advantage is that if an element would crack it will stay in its housing, maybe still providing partial filtering, without the risk to damage neighbouring elements by dropping on them. The second concept used filter sheets installed in the vessel as shown in Fig. 10. The filter sheets were installed on vertical clean-gas pipe manifolds. The main advantage of this second concept is the compact installation of the filter elements which results in an increase of the filtration area within a given vessel compared to the candle design. So far, both concepts have not been applied in commercial applications even if testing revealed the potential of the ‘‘inverted candle’’ design to be a highly reliable, commercially successful hot gas filter system [51].

Fig. 10. Sheet filter system concept of Siemens-Westinghouse [47].

4.3. Filter tube design of Asahi Glass A further hot gas filter design has been developed in Japan by Asahi Glass Co. Ltd. In this design, shown in Fig. 11, ceramic filter tubes with a length of 3 m and an outer and inner diameter of 168 mm and 140 mm, respectively, were used [52]. The filter tubes were made of cordierite. The raw gas entered into the inside of the filter tubes downwards from the top. The tubes were open at both ends. The dust cake which was built-up at the inside surface of the tubes was detached by a back pulse from the outside to the inside and discharged at the bottom of the tubes. The tubes were supported by water cooled tubesheets which divided the filter vessel into several clean gas chambers. The main advantages of this filter design have been seen in the fact that dust bridging between the filter tubes and thus the risk of candle breakage is avoided and that the tubes can be installed with high packing density. Testing of this filter system has been performed in the 10 MW PFBC test facility in Karhula (Finland) at the beginning of the 1990s. Later in the 1990s, it was applied in the 71 MW PFBC demonstration plant at

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Raw Gas

Filter Tube Clean Gas Clean Gas

Cross Flow Filter Module

Cyclone

Fig. 12. Scheme of the cross flow concept investigated by Sibanda et al. [56].

Fig. 11. Scheme of the tube filter system developed by Asahi Glass [48].

Wakamatsu in Japan where it achieved more than 11,000 h of operation at temperatures between 650 and 850 °C and clean gas concentrations of below 1 mg/N m3 [53,54]. The filter system suffered from several problems in both applications. Breakages of tubes apparently caused by the different thermal expansions of the ceramic tubes and the metal connection parts at the tubesheets as well as dust leakages at the sealing of the joint portion of the filter tubes and the metal parts occurred [55]. 4.4. Cross flow filtration concepts Recent hot gas filter design concepts try to use the cross flow filtration principle which is a state of the art method in liquid filtration. These concept studies have been performed just on laboratory scale yet. Main advantages of such systems have been seen in the reduction of the filter size by using higher filtration velocities as well as in limited growth of the filter cake by the shear stress generated by the axial flow and less frequent backpulsing of the filter elements. Sibanda et al. [56,57] investigated a concept (see Fig. 12) in which the raw gas flow enters the inside of a filter tube. Part of the gas flow passes radially through the wall of the filter tube and the remaining part flows axially through the tube and is directed to a downstream cyclone. The particles need to agglomerate at the surface of the filter tube and the shear forces resulting from the axial flow need to remove the agglomerates from the surface and carry them to the downstream cyclone so that the concept works. Agglomeration and detachment of the particles depend strongly on their sticking properties. Sibanda et al. could achieve collection efficiencies of 99% for optimal operating conditions. The collection efficiency of the cyclone alone was 90% in this case [56]. Sharma et al. [58,59] tested a similar concept in which a part of the gas flow entering the filter vessel passes through a filter candle wall from the outside to the inside and exits as clean gas. The remaining part of the gas flow is recycled and mixed with the inlet flow (see Fig. 13). The gap between the housing and the filter candle

is kept small in order to have high enough shear forces resulting from the downward flow. The shear force on the filter surface removes the agglomerates from the surface and the recycled flow carry them to a downstream cyclone where they are separated. Sharma et al. reported successful testing of their concept on laboratory scale. The requirement that this concept works is the same as in the case of the concept of Sinbanda, described above. Furthermore, if fine particles are in the gas which cannot be agglomerated, then particles smaller than the cut size of the cyclone will be concentrated in the recycled gas stream. As long as agglomeration can be achieved or just coarse dusts have to be removed then the cross flow concepts should work. However, scale-up of these interesting concepts has not been evaluated so far. 4.5. Filter system integration into a biomass gasifier A new concept of a compact hot gas cleaning and conditioning system for biomass gasification syngas has recently been

Jet Ejector/ Pump

Clean Gas

Raw Gas Filter Candle Recycled Gas Stream

Cyclone

Fig. 13. Scheme of the cross flow concept investigated by Sharma et al. [58].

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Syngas

Table 1 General overview of hot gas filtration applications. Coal gasification Coal combustion Biomass gasification

Blowback Gas

Biomass pyrolysis Biomass combustion Refineries Low-level radioactive waste incineration and pyrolysis Waste incineration and pyrolysis

Catalytic Filter Candles

Gasification Zone

Biomass Steam/ Gas

Smelting processes Metal production Metal recycling (e.g. Aluminium recycling Glass industry Cement industry Steel industry Chemical industry (e.g. production and recovery of catalysts Production of metal oxide powders, pigments and nanoparticles

filter installations worldwide based on references of three leading companies in hot gas filtration (Pall Corporation, Clear Edge Ltd., Glosfume Ltd.). 5.1. Incineration and pyrolysis of low-level contaminated radioactive waste

Slag Fig. 14. Schematic principle of the integration of a catalytic filter system into the freeboard of a fluidized bed gasifier.

developed [60]. Catalytic filter elements for particle and tar removal are directly integrated into the freeboard of a gasifier. The integration of the filter system within the gasifier results in a very compact unit. By this cost-effective approach, the investment costs of the gas cleaning equipment for a biomass gasification plant can be reduced as well as the needed space for the installation. An additional advantage of this concept is that the gas temperature is in the right range for the catalytic tar reforming reaction and no reheating or auxiliary electric heating of pipes and filter vessel is necessary. The temperature in a fluidized bed biomass gasifier is typically around 800–900 °C, which is the temperature required for catalytic tar reforming. Fig. 14 shows schematically the principle of this new compact cleaning concept. Remarkable system simplification and process intensification can be achieved by this concept. Investigations at a bench-scaled fluidized bed biomass gasifier, operated at atmospheric pressure and temperatures between 800 and 820 °C with a catalytic filter candle integrated in the freeboard of the gasifier has demonstrated the general feasibility of the concept [61,62]. However, tests at a scaled-up gasifier unit are needed for reliable proof of this concept. 4.6. Further research and development requirements Even if the single tubesheet design has been being well proven as reliable hot gas filter system already applied in hundreds of installations for many years and even if some of the afore described design concepts are very interesting, the mainly required approach for further developments is to increase the competitiveness of hot gas filtration by providing cost-reduced filter systems. This requires the development of new compact filter system designs with high packing density of the filtration area. 5. Applications Hot gas filtration has already been applied in many processes. A general overview is given in Table 1. There are hundreds of hot gas

First technical application where hot gas filtration has been used was incineration of low-level contaminated radioactive waste from nuclear power generation. From the beginning of the 1970s until the mid of the 1990s, a two stage hot gas filter system was used at the former nuclear research centre of Karlsruhe in Germany. The filter system was operated at temperatures between 650 and 900 °C. Each filter stage comprised a refractory lined vessel containing 85 ceramic filter elements of silicon carbide [1]. At the beginning of the 1970s, hot gas filters were also used in some incineration plants for low contaminated radioactive wastes in France. Filter elements made of asbestos fibres installed in refractory lined vessels were operated at 700–800 °C [2]. Since 1978 hot gas filtration is used for incineration of low contaminated radioactive wastes in nuclear power plants in Japan. About 30 hot gas filter systems have been installed. Typically, the hot gas filter systems have two filter stages. Each stage comprises a refractory lined vessel containing about 100 ceramic filter candles. The first stage is operated at 600–800 °C, the second one at 500–600 °C [3]. Hot gas filters are also applied for the pyrolysis of low contaminated radioactive waste [63–66]. 5.2. Coal gasification and combustion The development of advanced coal based power generation techniques, such as PFBC and IGCC, has required, stimulated and significantly influenced the development of hot gas filter elements and systems for high temperatures and high pressures. The conditions in PFBC are temperatures of up to 850 °C and pressures of 1– 1.2 MPa and in IGCC pressures are up to 8 MPa and temperatures are between 250 and 400 °C. Several big research and demonstration projects have been started in the USA, in Japan and in Europe from the end of the 1980s until the end of the 1990s. Some examples are the 250 MWel IGCC demonstration plant in Buggenum (in the Netherlands), the 300 MWel IGCC demonstration plant in Puertollano (Spain), the 10 MWth PFBC test facility in Karhula (Finland), the 71 MWel PFBC demonstration plant in Wakamatsu (Japan), the 262 MWel IGCC demonstration plant in Wabash River (USA) and the 15 MWth Power System Development Facility in Wilsonville (Alabama, USA). While the PFBC technology has not been applied commercially yet, the IGCC has got significant interest in the last 10 years. Worldwide, more than 25 large hot gas filter units are in operation or in commissioning in coal gasification plants. The largest filter vessels have a diameter of about 6.5 m, a height of more than 25 m and a weight of about 300 t containing about 1200 filter candles. About 90% of the filter units are equipped with

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Wet Gas Cleaning Columns Hot Gas Filter

Cyclone Syngas Cooler

Gasifier Oxygen

Fig. 15. Simplified principal process flow scheme of the gasifier and gas cleaning of the IGCC plant in Buggenum.

ceramic filter candles of silicon carbide. The remaining 10% are operated with iron aluminide or FeCralloy metal filter candles. The filter unit with the longest operating time is running at the IGCC plant in Buggenum. Start-up of this filter was in 1994. More than 800 ceramic silicon carbide Dia-Schumalith filter elements of Pall Corporation are installed in the vessel with single tubesheet design. The ceramic filter elements achieve service life times of more than 4 years. The filtration temperature is between 250 and 285 °C and the operating pressure is about 2.6 MPa. The filtration velocity varies between 60 and 90 m/h depending on the syngas quantity and the temperature. Fig. 15 shows a simplified principal flow scheme of the IGCC plant which is operated according to the Shell gasification process [67]. 5.3. Biomass gasification and pyrolysis Interesting applications for the employment of hot gas filtration are gasification and pyrolysis of biomass. Many small hot gas filter units containing 1–100 candles have been operated in lab and demonstration plants around the world since the beginning of the 1990s. With regard to biomass as a regenerable energy resource, plans are currently existing to build large biomass gasification plants. So far, filtration has been performed preferably in the temperature range of 500–600 °C [68–71]. At these temperatures, on the one hand condensation of tars can be prevented and on the other the likelihood of softening and stickiness of the dusts which have typically high alkaline content is low. New advanced concepts in biomass gasification have the goal to perform the hot gas filtration at temperatures of 800–900 °C [72]. Since gasification in a fluid bed reactor is typically performed at these temperatures and downstream catalyst units for gas and tar reforming require at least these reaction temperatures, filtration shall be performed at these temperatures in order to simplify the process by avoiding gas cool down and heat up. This process concept, as shown in Fig. 16, is considered as the most economical and efficient protection of the catalyst unit from particle deposition and fouling. Tests on the filtration of syngas from biomass gasification of different feedstocks at high temperatures of about 800 °C have shown first promising results that stable filtration can be achieved at these high temperatures [73,74]. However, additional long-term tests are needed to prove these results. 5.4. Fluid catalytic cracking units in refineries Hot gas filters are also used in refineries for flue gas cleaning of fluid catalytic cracking units (FCCUs). By means of the hot gas

Biomass Gasification 800 - 900 ºC

Hot Gas Filtration 800 - 900 ºC

Gas und Tar Reforming 800 - 900 ºC

Fig. 16. General process steps for an energy efficient biomass gasification.

filters increased particulate emission standards are accomplished as well as turbo expanders and heat exchangers are efficiently protected from erosion and fouling. For many years hot gas filters of smaller size containing up to about 100 candles have been being used as 4th stage underflow filters of the 3rd stage separator. About 3–5% of the gas flow of the third separator stage is directed as underflow to the 4th stage filter (see the simplified flow scheme in Fig. 17). The filtration temperature of 4th stage filters is typically between 500 and 600 °C and the filtration velocity varies between 90 and 150 m/h. Metal as well as ceramic filter elements are applied in 4th stage filters. A typical regenerator of a FCC unit has two internal cyclones and an external third cyclone. Due to more stringent emission standards some refineries have replaced their third stage cyclone by a third stage hot gas filter unit in the recent years. Such filter units have typically a filtration surface area of 500–1000 m2 and contain 1000–2000 metal filter elements. The filtration temperature of 3rd stage filters is typically between 300 and 400 °C and the operating pressure at 0.3 MPa. Hot gas filters are also used in FCC units as vent filters on catalyst storage hoppers (see Fig. 17). 5.5. Waste incineration and pyrolysis Furthermore, hot gas filters are broadly applied for flue gas cleaning in incinerations. Low density ceramic filter candles are mostly used in these applications. It is reported on hot gas filters in hazardous waste, industrial waste and clinical waste incineration, in incineration and pyrolysis of municipal waste and sewage sludge as well as in pyrolysis of plastic waste [75–79]. Filtration temperature in the incineration applications is usually between 200 and 350 °C. Filtration surface areas of up to 1000 m2 are installed and the filtration velocity ranges usually between 60 and 70 m/h. Filtration temperature in pyrolysis is between 350 and 500 °C. Startin reported on more than 30 hot gas filtration applications in pyrolysis where low density ceramics are employed [80]. Most of these applications are in Japan. The largest filter unit was commissioned in 2002 containing 600 filter candles with a

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Spent Catalyst

Vent Filter

FCCReactor

3rd Stage Separator

Fresh Catalyst Hopper

4th Stage Filter

Regenerator

Regenerated Catalyst Air Spent Catalyst Hopper Fig. 17. Process flow scheme of the separation units in a typical fluid catalytic cracking unit.

filtration surface of 840 m2. The filtration velocity is in the range of 72–90 m/h.

process costs are aspects which increase the interest in hot gas filtration significantly.

5.6. Other processes

References

Hot gas filters are often applied in the production of metal oxide powders, catalysts, pigments and metals, where they can show significant process and economical advantages. Moreover, hot gas filters are also employed for the recycling of metals, for example the recycling of aluminium [81,82,77]. In case of recycling aluminium scrap with a high PVC content, hot gas filtration above 400 °C combined with the injection of sodium bicarbonate upstream of the filter achieves an efficient control of dioxin emissions [83].

[1] Leibold H, Dirks F, Rüdinger V. Waste Manage 1989;9:87–94. [2] Perkins BL. Incineration facilities for treatment of radioactive wastes – a review. LA-6252 report. US Department of Commerce; 1976. [3] Torii A. Ceramic filter for radioactive waste treatment system. In: Kanaoka C, Makino H, Kamiya H, editors. Advanced gas cleaning technology. Tokyo: Jugei Shobo; 2005. p. 58–64. [4] VDI guideline 3677 sheet 3 draft; 2010. [5] Berbner S, Pilz T. Powder Technol 1996;86:103–11. [6] Zimmerlin B, Leibold H, Seifert H. Powder Technol 2008;180:17–20. [7] Hurley JP, Dockter BA. Adv Powder Technol 2003;14:695–705. [8] Ninomiya Y, Zhang L, Sato A. J Jpn Inst Energy 2005;84:359 [English Abstract]. [9] Hemmer G, Hoff D, Kasper G. Adv Powder Technol 2003;14:631–55. [10] Hemmer G. Grundlagenuntersuchungen zur kombinierten Abscheidung von Stäuben und gasförmigen Schadstoffen aus Biomasseverbrennungsanlagen. Aachen: Shaker Verlag; 2002. [11] Kanaoka C, Hata M, Makino H. Powder Technol 2001;118:107–12. [12] Löffler F, Gäng P. KfK-PEF Bericht 70, Karlsruhe; 1990. ISSN 0931-2749. [13] Kim JH, Liang Y, Sakong KM, Choi JH, Bak YC. Powder Technol 2008;181:67–73. [14] Gäng P, Löffler F. In: Proceedings of the partec first European symposium separation of particles from gases, Nürnberg; 1989. p. 39–58. [15] Durst M. BWK 1990;42:610–4. [16] Hajek S, Peukert W. Chem Ing Tech 1997;69:341–5. [17] Cummer KR, Brown RC. Biomass Bioenergy 2002;23:113–28. [18] Heidenreich S, Scheibner B. Filtr sep 2002(May):22–5. [19] www.pall.com. [20] www.filtraguide.com. [21] Tortorelli PF, Lara-Curzio E, McKamey CG, Pint BA, Wright IG, Judkins RR. Evaluation of iron aluminides for hot gas filter applications. In: Proceedings of the advanced coal-based power and environmental systems ‘98, paper PB.7. U.S. Department of Energy; 1998. [22] June M, Sawyer JW. Iron aluminide hot gas filter development. In: Proceedings of the advanced coal-based power and environmental systems ‘98, paper PB.16. U.S. Department of Energy; 1998. [23] Ondrey G. Chem Eng 2001:29–39. [24] Succi M, Zilio S, Bonucci A. US patent application 2008/0209898. [25] www.mottcorp.com. [26] McKamey CG, Tortorelli PF, Lara-Curzio E, McCleary D, Sawyer J, Judkins RR. Characterization of field-exposed iron aluminide hot-gas filter. In: 5th International symposium on gas cleaning at high temperature, Morgantown; September 2002. [27] Dahlin RS, Landham EC, Guan X. Corrosion and flow resistance of metal filter elements used in the cleanup of syngas from the transport gasifier at the

6. Conclusions Hot gas filtration is a reliable and well proven technology already applied in hundreds of installations in different processes all around the world for many years. Due to the particular requirements and conditions at high temperatures, hot gas filtration is a special field of filtration. Rigid self supporting ceramic or metal filter elements are applied at high temperatures. The preferred filter system design for hot gas filtration is the single tubesheet design. Hot gas filters have shown to be able to separate particles efficiently down to the submicron range. Clean gas concentrations down to below 1 mg/m3 can be achieved. Hot gas filtration offers various advantages in many industrial processes. The protection of downstream equipment, such as catalyst units, heat exchangers, turbines and scrubbers from erosion, fouling and pollution by using hot gas filters has been well proven. High filtration temperatures can prevent undesired condensation or desublimation reactions or can avoid the de novo synthesis of dioxins. Process intensification and simplification, process and energy efficiency, product quality, low emission levels as well as overall

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[28] [29]

[30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]

[44] [45] [46]

[47]

[48] [49] [50] [51]

[52]

[53]

[54]

S. Heidenreich / Fuel 104 (2013) 83–94 power systems development facility. In: 7th International symposium on gas cleaning at high temperature, Newcastle; June 2008. Guan X, Gardner B, Martin RA, Spain J. Powder Technol 2008;180:122–8. Bishop B, Raskin N. CeraMem filter development program. Advanced coal-fired power systems ’96 review meeting, Morgantown, paper DOE/ER80896-97/ C0720. U.S. Department of Energy; 1996. Ciliberti DF, Lippert TE. IChemE Symp Ser 1986;99:193–213. Nacken M, Heidenreich S, Hackel M, Schaub G. Appl Catal B: Environ 2007;70:370–6. Heidenreich S, Nacken M, Hackel M, Schaub G. Powder Technol 2008;180:86–90. Hackel M, Schaub G, Nacken M, Heidenreich S. Powder Technol 2008;180:239–44. Heidenreich S, Nacken M. Chem Ing Tech 2004;76:1508–12. Nacken M, Ma L, Engelen K, Heidenreich S, Baron GV. Ind Eng Chem Res 2007;46:1945–51. Nacken M, Ma L, Heidenreich S, Baron GV. Appl Catal B: Environ 2009;88:292–8. Heidenreich S, Haag W, Salinger M. Fuel 2011. http://dx.doi.org/10.1016/ j.fuel.2011.03.007. Mai R, Leibold H, Seifert H, Heidenreich S, Haag W, Walch A. Chem Ing Tech 2002;74:1438–41. Heidenreich S, Haag W, Mai R, Leibold H. Gefahrst – Reinhalt Luft 2010;70:237–41. Smolders K, Baeyens J. Powder Technol 2000;111:240–4. Cerafil product information, Cerafil Division., Madison Filter Ltd.; 2005. Hajek S. CIT Plus 2003;5:90–2. Tomonori A. The field test of hot gas filter at MSW incineration plant. In: Kanaoka C, Makino H, Kamiya H, editors. Advanced gas cleaning technology. Tokyo: Jugei Shobo; 2005. p. 340–7. Dehn G, Möllenhoff H, Wegelin R, Krein J. US patent 5769915. Haldipur GB, Dilmore WJ. US patent 5143530. Schumacher HJ, Renzenbrink W. Gas cleaning with the ceramic candle filter of the HTW demonstration plant at Berrenrath. In: Schmidt E, Gäng P, Pilz T, Dittler A, editors. High temperature gas cleaning. Karlsruhe; 1996. p. 794–805. Krein J. LLB candle filter for the elcogas IGCC power plant puertollano. In: Dittler A, Hemmer G, Kasper G, editors. High temperature gas cleaning II. Karlsruhe; 1999. p. 253–60. Ahmadi G, Smith DH. Aerosol Sci Technol 1998;29:206–23. Ahmadi G, Smith DH. Powder Technol 2002;128:1–10. www.netl.doe.gov. DOE/NETL-2003/1183 report; 2002. Newby RA, Alvin MA, Bruck GJ, Lippert TE, Smeltzer EE, Stampahar ME. Optimization of advanced filter systems. DOE final. Report DE-AC2697FT33007; 2002. Oda N, Hanada T. Performance of the Advanced Ceramic Tube Filter (ACTF) for the Wakamatsu 71 MW PFBC and further improvements for commercial plants. In: Schmidt E, Gäng P, Pilz T, Dittler A, editors. High temperature gas cleaning. Karlsruhe; 1996, p. 794–805. Sasatsu H, Misawa N, Abe R, Mochida I. Prediction for pressure drop across CTF at Wakamatsu 71 MWe PFBC combined cycle power plant. In: Dittler A, Hemmer G, Kasper G, editors. High temperature gas cleaning II. Karlsruhe; 1999, p. 261–75. Sasatsu H, Misawa N, Shimizu M, Abe R. Powder Technol 2001;118:58–67.

[55] Sasatsu H, Misawa N, Kobori K, Iritani J. Hot gas particulate cleaning technology applied for PFBC/IGCC. In: Proceedings of the 5th international symposium on gas cleaning at high temperature, Morgantown, paper 1.6; 2002. [56] Sibanda V, Greenwood RW, Seville JPK. Powder Technol 2001;118:193–202. [57] Sibanda V, Greenwood RW, Seville JPK, Ding Y, Iyuke S. Powder Technol 2010;203:419–27. [58] Sharma SD, Dolan M, Ilyushechkin AY, McLennan KG, Nguyen T, Chase D. Fuel 2010;89:817–26. [59] Sharma SD, McLennan K, Dolan M, Nguyen T, Chase D. Fuel 2011. http:// dx.doi.org/10.1016/j.fuel.2011.02.041. [60] Heidenreich S, Nacken M, Foscolo PU, Rapagna S. US patent application 2010223848. [61] Heidenreich S, Nacken M, Salinger M, Foscolo PU, Rapagnà S. Integration of a catalytic filter into a gasifier for combined particle separation and tar removal from biomass gasification gas. In: Proceedings of the 7th international symposium on gas cleaning at high temperature, Newcastle, Australia; 2008. [62] Rapagna S, Gallucci K, Di Marcello M, Matt M, Nacken M, Heidenreich S, et al. Bioresour Technol 2010;101:7134–41. [63] Hesbol R, Mason JB. US patent 5909654; 1999. [64] Mason JB, Oliver TW, Carson MP, Hill GM. Studsvik processing facility pyrolysis/steam reforming technology for volume and weight reduction and stabilization of LLRW and Mixed Wastes. In: Proceedings of the waste management conference 99, Tucson; 28 February–4 März 1999. [65] www.studsvik.com. [66] Pyrolysis of radioactive organic waste. . [67] Heidenreich S, Wolters C. Filtr separat 2004;June:22–4. [68] Kurkela E, Stahlberg P, Laatikainen J, Simell P. Bioresour Technol 1993;46:37–47. [69] Nagel FP, Ghosh S, Pitta C, Schildhauer TJ, Biollaz S. Biomass Bioenergy 2011;35:354–62. [70] Stahl K, Neergaard M. Biomass Bioenergy 1998;15:205–11. [71] Siedlecki M, Nieuwstraten R, Simeone E, de Jong W, Verkooijen AHM. Energy Fuels 2009;23:5643–54. [72] Bengtsson S. Biomass Bioenergy 2011. http://dx.doi.org/10.1016/ j.biombioe.2011.08.01. [73] Heidenreich S, Simeone E, Haag W, Nacken M, de Jong W, Salinger M. Gefahrst – Reinhalt Luft 2011;71:281–5. [74] Simeone E, Siedlecki M, Nacken M, Heidenreich S, de Jong W. Fuel 2011. http:// dx.doi.org/10.1016/j.fuel.2011.10.030. [75] www.caldo.com. [76] www.tenmat.com. [77] www.glosfume.com. [78] www.clear-edge.com. [79] www.incoteco.com/gcg/references.htm. [80] Startin A. The application of ceramic filters to pyrolysis gas cleaning. thermalnet newsletter, Issue 5; September 2007. . [81] Beattie CJC, Withers CJ. Applications of low density ceramic filters for gas cleaning at high temperatures. In: Clift R, Seville JPK, editors. Gas cleaning at high temperatures. London; 1993. p. 173–89. [82] Startin A, Elliott G. Filtr Sep 2001;November:38–40. [83] Williams R. Filtration 2009;9(1):45–7.