Tar and particulate removal methods for the producer gas obtained from biomass gasification

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Alternative energy production through biomass gasification produces combustible gases, such as carbon monoxide, hydrogen, and methane. These gases can be used for generation of direct heat, electricity, or liquid fuels through the Fischer Tropsch process.

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Nội dung Text: Tar and particulate removal methods for the producer gas obtained from biomass gasification

Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 6 Number 10 (2017) pp. 269-284 Journal homepage: http://www.ijcmas.com Review Article https://doi.org/10.20546/ijcmas.2017.610.034 Tar and Particulate Removal Methods for the Producer Gas Obtained from Biomass Gasification R. Preetha Devi1*, S. Kamaraj1, R. Angeeswaran2 and S. Pugalendhi1 1 Department of Bioenergy, AEC & RI, TNAU, Coimbatore-03, T.N., India 2 International Institute of Renewable Energy, NERD Society, Vadavalli, Coimbatore, T.N., India *Corresponding author ABSTRACT Keywords Alternative energy production through biomass gasification produces combustible gases, such as carbon monoxide, hydrogen, and methane. Biomass gasification, These gases can be used for generation of direct heat, electricity, or liquid Limitation. fuels through the Fischer Tropsch process. However, a major limitation of Article Info the overall process is the purity of the generated synthesis gas. The tars and particulates generated in the gasification process constitute a major Accepted: 04 September 2017 impediment to the commercial use of this technology because they may Available Online: condense on valves, fittings, and therefore, hinder the smooth running of an 10 October 2017 engine. Introduction The gasification of carbon-containing result of the photosynthetic conversion materials to produce combustible gas is an process excluding materials embedded in established technology. Coal gasification has geological formations and transformed to been the primary focus due to its higher fossil. In principle, biomass is a less energy density and ease of transportation in damaging and environmentally benign fuel as comparison to renewable biomass resources. the carbon dioxide released from the Currently, environmental issues and the need combustion process is captured during the to augment or replace existing power plant growth. One of the most important generation facilities have shifted the focus of biomass fuels is wood, however, wood is gasification development from non-renewable often too valuable to be used for power fossil fuel sources to renewable fuel sources, generation and the timber industry is able to mainly biomass. make better use of trees by processing them into construction materials. Therefore, Biomass residues such as bark, sawdust, and odd-sized pieces are frequently used as fuel. Many The term “biomass” represents material of agricultural residues can, indeed, be used as biological origin derived from plants as a fuels. They include straw from grains, husks 269
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 from rice, coconuts or coffee, stalks from Tar maize or cotton, bagasse from sugarcane, and animal manure. In addition to these, dedicated The organics, produced under thermal or energy crops such as switch grass are being partial-oxidation regimes (gasification) of any used as fuel sources. Using these biomass organic materials are called “tars”. Tar residues as fuels may solve the environmental includes the variety of oxygenated aromatics problem of how to dispose of them. formed in the pyrolysis step of the gasification process. Molecular weight of the Biomass gasification tar is greater than the Benzene (molecular weight of benzene is 78). The actual Biomass gasification is a thermochemical composition of tar is complex, it dependent process that produces relatively clean and on the severity of the reaction condition, combustible gases through pyrolytic reaction. including gasifier temperature and reaction The synthesis gas (also known as syngas or time in the reactor. The biomass feedstock is producer gas) generated can be an important heated, it dehydrates and then volatilizes as it resource suitable for direct combustion, thermally decomposes. The volatilized application in prime movers such as engines material either can undergo further and turbines, or for the production of decomposition to form permanent gases, or it synthetic natural gas (SNG) and can undergo dehydration, condensation and transportation fuels (e.g. Fischer-Tropsch polymerization reactions that result in tar diesel). Producing high quality syngas to meet formation. operational requirements of turbines or internal combustion engines is critical to the Tar is undesirable because of various successful implementation of biomass problems associated with condensation, gasification. Specifically, the efficient and formation of tar aerosols and polymerization economic removal of tars and particulates to form more complex structures, which can from the syngas are the major obstacles to be damage internal combustion engines (ICEs), overcome (Table 3). gas turbines, hot gas application and other machinery. Therefore, before the syngas can For energy production, the major concerns be used in a gas engine or turbine, it must be about syngas are its heating value, cleaned of impurities, especially tars, a major composition, and possible contamination. The impediment to widespread use of biomass proportion of the combustible gases hydrogen gasification technology. Currently, there is no (H2), methane (CH4), carbon monoxide (CO) specific method for determining the and steam in the syngas determines the concentration of tar and particulates from heating value of the gas. The composition of biomass gasification. Developing a simple syngas depends on the biomass properties and and effective protocol for quantifying the gasifier operating conditions. For a particular gravimetric tar and particulate in biomass gasification system, operating conditions play gasification is an important goal of this study. a vital role in all aspects of biomass Gas cleaning and tar reduction have been the gasification. These include carbon subject of research on thermochemical conversion, syngas composition, tar formation conversion of biomass for the production of and reduction. Syngas from biomass energy and chemicals. Catalytic tar gasification contains tar and particulates as destruction for coal gasification has been impurities which can cause severe operational studied for several decades and a number of problems. reviews have been written on biomass 270
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 gasification hot gas cleanup emphasizing the excessive, the combustion process may not be use of dolomites and nickel based catalyst self-sustaining and supplemental fuel must be (Stevens, (2001); Sutton et al., (2001); Milne used, which could defeat the objective of et al., (1998)). Physical treatment of syngas producing energy by biomass combustion for using mechanical methods such as cyclone, captive use or market (Klass, 1998). scrubber, and particulate filters has also been identified (Devi et al., 2003). This study will Ash #ontent also look at the effects of temperature on the performance of physical methods and This refers to the inorganic component in chemical proprietary catalysts for tar removal biomass. It is expressed in the same format as in an industry. the moisture content. This property is especially important under high temperature Biomass as a fuel gasification as melted ash may cause problems in the reactor (Quaak et al., 1999). Biomass simply refers to organic materials originated from plants (wood, crops etc.) and Elemental composition animal wastes. Different biomass conversion processes produce heat, electricity and fuels. The ash-free organic components of biomass Among all biomass conversion processes, are relatively uniform. The major components gasification is one of the most promising are carbon, oxygen, and hydrogen. Most (Devi et al., 2003). An assessment of the use biomass may also contain a small amount of of biomass as a fuel requires a basic nitrogen. understanding of their composition, characteristics, and performance. Each type of Volatile matter content biomass has specific properties that determine its performance as a fuel in combustion or The part of the biomass that is released when gasification devices or both (Quaak et al., the biomass is heated is referred to as the 1999). The most important properties relating volatile matter. Biomass feedstock contains a to the thermal conversion of biomass are very high proportion of volatile organic moisture content, ash content, volatile matter, material, 70 to 90% for wood (Klass, 1998). and energy density. In addition to high temperature gasification, biomass can be used Energy density to produce energy via low temperature microbial gasification process where methane The energy density refers to the potential is mainly produced anaerobically. energy available per unit volume of the biomass. It is dependent on the feedstock Moisture content heating value and bulk density. In general, the biomass energy density of biomass is about This is the amount of water in the material, one-tenth of that of fossil fuels. expressed as a percentage of the material’s weight. This weight can be on a wet basis, on Biomass gasification principle a dry basis, and on a dry-and-ash basis. Biomass materials exhibit a wide range of The chemistry of biomass gasification is moisture content and since this affects its similar to that of coal gasification in the sense value as a fuel source, it is important that the that thermal decomposition of both solids basis be stated whenever moisture content is occurs to yield a mixture of essentially the measured. If the moisture content is same gases. However, biomass gasification 271
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 occurs under much less severe operating CO + 1/2 O2 → CO2 + Heat conditions than for coal feedstock because its CH4 + 3/2 O2 → CO + 2H2O main constituents, the high-oxygen cellulosics and hemicellulosics, have higher reactivity Reduction than the oxygen-deficient, carbonaceous materials in coal. The thermo-chemical The reaction products of the oxidation zone processes involved in gasification are drying, continually move into the reduction zone pyrolysis, oxidation, and reduction. where there is insufficient oxygen, leading to reduction reactions between the hot gases and Drying char. The principal reactions are as follows. This phase involves evaporation of the CO2 + C + Heat →2CO moisture contained in the biomass. At C +H2O + Heat → CO + H2 temperatures above 100˚C, water in the bio- CO + H2O + Heat → CO + H2 fuel is converted to steam. Part of this vapor may be reduced to hydrogen during In this zone, the sensible heat of the gases and gasification and the rest ends up as moisture char is converted into the stored chemical in the produced syngas. energy in the syngas. Therefore, the temperature of the gases is reduced during Pyrolysis this process. The bio-fuels begin to pyrolyze at Gasification systems temperatures above 200˚C (Klass, 1998). This is the thermal decomposition of the fuel into Gasification is a form of incomplete volatile gases and char. The proportion of combustion; heat from the burning solid fuel these components is influenced by the creates gases which are unable to burn chemical compositions of bio-fuels being fed completely, due to insufficient amounts of and the operating conditions of the gasifier. oxygen from the available supply of air. By The main process of thermal decomposition weight, syngas from gasification of wood of biomass can be represented as follows: contains approximately 15-21% hydrogen (H2), 10-20% carbon monoxide (CO), 11-13% C6H10O5 + Heat → yCxHz + qCxHnOk + CO carbon dioxide (CO2), and 1-5% of methane, +C all of which are combustible plus nitrogen (N2). The nitrogen is not combustible; Oxidation however, it does occupy volume and dilutes the syngas as it enters and burns in an engine. After pyrolysis, there is an oxidation zone A generalized reaction describing biomass where the pyrolysis products move into the gasification is as follows (Dayton, 2002): hotterzones of the gasifier. Air is introduced into the oxidation zone under starved oxygen Biomass + air (or H2O) → CO, CO2, H2O, H2, conditions. The oxidation takes place at CH4, and N+ tars + particulates temperatures ranging from 700- 1000˚Coxidation reactions is as follows The actual biomass syngas composition (Klass, 1998) depends on the gasification process, the C + O2 →CO2 + Heat gasifying agent, and the feedstock H2 +1/2 O2 → H2O + Heat composition. Various gasification 272
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 technologies have been under investigation the high amounts of tar produced. for converting biomass into a gaseous fuel. A characteristic of the various gasifiers is the Downdraft or co-current gasifiers way in which the fuel is brought into contact at the gasification stage. Four types of In the downdraft gasifier, air is introduced reactors exist: into downward flowing packed bed or solid fuels and gas is drawn off at the bottom. The Updraft or counter current gasifiers zones are similar to those in the updraft Downdraft or co-current gasifiers gasifier; but the order is somewhat different Cross-draft gasifiers and (Quaak et al., 1999). A lower overall Fluidized-bed gasifiers efficiency and difficulties in handling higher moisture and ash content are common Fixed bed gasifiers problems in small downdraft gas producers. In addition to these drawbacks, it is important Fixed bed gasifiers have grates built in to for downdraft gasifiers to maintain uniform support the feedstock and maintain a high temperatures over a given cross-sectional stationary reaction bed. They are relatively area in the reaction chamber. These factors easy to design and operate but have limited limit the use of downdraft gasifiers to a power capacity. Therefore, fixed bed gasifiers are range of less than 1 MW. This gasifier is, preferred for small to medium scale however, preferred to updraft gasifier for applications with thermal requirements up to internal combustion engines because of the 1 MW (Klein, 2002). Fixed bed gasifiers can low tar content associated with the syngas. be classified as either updraft or downdraft depending on the method of air introduction. Fluidized-bed gasifiers Updraft or countercurrent gasifiers Fluidized-bed gasification was initially developed to overcome operational problems In this type of reactor, air is taken in at the of fixed-bed gasification of fuels with high bottom, and the gas leaves at the top. The ash content, but is suitable for large capacities biomass moves counter to the gas flow and (more than 10MW) in general. The fuel is fed passes successively through drying, into a suspended (bubbling fluidized-bed) or pyrolization, reduction, and hearth zones. In circulating (circulating fluidized-bed) hot the drying zone, the biomass is dried. In the sand bed. The bed behaves like a fluid and is pyrolization zone, it is decomposed into characterized by high turbulence. Fuel volatile gases and solid char. The heat for particles mix quickly with the bed material, pyrolization is mainly delivered by the resulting in rapid pyrolysis and a relatively upward-flowing producer gas and partly by large amount of gases. Major problems with radiation from the hearth zone. The fluidized bed gasification are the resulting advantages of this type of gasifier are its high tar content (up to 500mg/Nm3), simplicity, relatively low gas-exit incomplete carbon combustion, and poor temperature, high thermal efficiency and as a response to load changes. Problems with result, biomass with high moisture content (up feeding, instability of the reaction bed, and to 60% wb) (Quaak et al., 1999) can be fly-ash sintering in the gas channels can occur gasified without any pre-drying of the feed. with some bio-fuels. There are two principal Moreover, size specifications are not very types of fluidized bed gasifiers namely, critical for this gasifier. Major drawbacks are bubbling fluidized bed (BFB) and circulating 273
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 fluidized bed (CFB). The circulating type controlling the tar content in gasifier product separates and recycles fly-ash from the gas depending on where tar is removed; either reaction bed while the bubbling type does not. in the gasifier itself (primary measures) or Fluidized bed gasifiers have been the focus of outside the gasifier (secondary measures) appreciable research and development for (Devi et al., 2003). large scale generation (Klein, 2002; and Spliethoff, 2001) (Table 1). Primary methods Gas quality requirements This can be achieved by optimizing biomass fuel properties and/or gasifier design and The product gas formed from biomass operating conditions. An ideal primary gasification contains both combustible and method concept eliminates the use of noncombustible components. The secondary treatments. The primary measures combustible gases include CH4, CO and H2. include: proper selection of the operating The major noncombustible components are conditions, the use of catalysts during CO2, H2O and N2, in addition to organic (tars) gasification, and proper gasifier design (Devi and inorganic impurities (Alkali metals, H2S, et al., 2003). HCl, NH3), and particulates. The generation of H2S is of little importance in biomass Temperature gasification as long as the biomass contains less than 0.5% sulfur content. NH3 is Biomass gasification is carried out at dependent on the nitrogen content of the relatively high temperatures (above 800). biomass and biomass with less than 2% Increasing the temperature in the gasification nitrogen is safe for gasification (Table 2). of sawdust in a fixed bed gasifier produces a decrease in the total number of detectable tar In gasification, tar is defined as a mixture of species (Kinoshita et al., 1994). organic compounds in the product stream that are condensable in the gasifier or in Pressure downstream processing steps or conversion devices (Milne et al., 1998). The gas quality Pressurized and atmospheric gasifiers are indicates the extent to which the gas is currently used in advanced biomass suitable for end use equipment or process and gasification designs. Experiments involving is represented by several parameters including gasification of Wisconsin whole tree chips chemical composition, tar and particulate indicated that when pressure was increased to concentration, and Lower Heating Value 21.4 bar, the amount of total tar decreased (LHV) and is dependent upon the (Knight, 2000). requirements of the end use itself. The gas quality for power generation is tabulated Gasifying medium below. Air, steam, steam-oxygen mixture and carbon Gas conditioning dioxide have been used as gasifying media. Heating value of the producer gas with air as Before the producer gas can be used in a gas the gasifying is lower because of the high engine or turbine, it must be cooled and percentage of nitrogen produced. Steam cleaned of tars, alkali metals, and dust. gasification produces a gas with a lower Basically, there are two main options for percentage of nitrogen and a higher 274
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 percentage of hydrogen. However, steam heating value with low tar content, and it gasification is endothermic and hence should be economically feasible. sometimes requires complex design for heat supply in the process (Devi et al., 2003). Secondary methods Equivalence Ratio (ER) This is achieved by applying downstream cleaning processes. These methods can be Equivalence ratio can be defined as the ratio physical or chemical and include the of the actual air fuel ratio to the air fuel ratio following: needed for complete combustion. This is an important factor in biomass gasification using The use of cyclone, baffle filter, ceramic air as gasifying medium. Tar yield decreases filter, fabric filter, rotating particle separator, as ER increases because of more availability electrostatic filter and/or scrubbers. These are of oxygen to react with volatiles in the normally placed external to the gasifier. flaming pyrolysis zone. However, a higher ER value tends to favor high carbon dioxide Tar cracking downstream the gasifier either content in the producer gas at the expense of thermally or catalytically. Although, these hydrogen and carbon monoxide, and therefore methods are reported to be very effective in a lower heating value. tar reduction, in some cases they are not economically viable (Devi et al., 2003). Catalysts Figure 3 illustrates the secondary method of gas cleaning and conditioning. The use of catalysts during biomass gasification affects the producer gas Cyclone composition and reduces the tar yield. Three group of catalyst materials have been applied The Cyclone is the most widely used in biomass gasification systems-alkali metals, technique to separate the Syngas from the non-metallic oxides, and supported metallic dust and ash entrained in the gas stream. The oxides. Alkali metals are considered as basic principle behind cyclone separators is to primary catalysts. They enhance char use centrifugal force to make it possible to formation reactions during thermo-chemical separate dust particles from a gas stream (Fig. conversion. 4). A cone section causes the vortex diameter to decrease until the gas reverses on itself and Gasifier design spins up the center to the outlet pipe or vortex finder. The shape of the cone induces the A two-stage gasifier has been studied in the stream to spin, creating a vortex. Larger or stage gasifier (Figure 1) (Devi et al., 2003), denser particles are forced outward to the tars formed during pyrolysis (first stage) are walls of the cyclone where the drag of the decomposed in the reduction zone (second spinning air as well as the force of gravity stage). The Technical University of Denmark causes them to fall down the sides of the cone also designed a two-stage gasifier (Figure 2) into an outlet (Seinfeld, 1975; Svarovsky, where a combination of pyrolysis of the 1984). The separation efficiency of a cyclone biomass feed with subsequent partial is usually expressed as the particle size that oxidation of the volatile products in the will be separated with 50% efficiency presence of a charcoal bed was achieved. The (Fredriksson, 1999). modifications made in the gasifier design Particulate filters should be able to produce a gas of high 275
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 To separate particles from a flowing gas, particles from a contaminated gas passing some type of filter may be used. A filter has through. Bag-filters, cartridge filters and two important characteristics: its efficiency granular filters belong to this category. Filter and resource consumption. The efficiency is materials may be of the surface collection quantified as the fraction of incoming type (e.g. Gore-TEX and Tetra-TEX particles which are retained by the filter. The membranes) or depth collection type (glass efficiency of a filter depends on many fibers and granular filters) (Hindsgaul, parameters, of which the particle size is often 2000).The particles are collected on the fibers the most important. The resource by interception and diffusion. Interception is consumption can be divided into initial costs when a particle hits a fiber due to inertia and costs of operation (e.g. pressure drop and effects or because the particle is large enough use of materials) as well as maintenance to touch the fiber as it passes. Interception is costs. The pressure drop often depends on the the most important effect for larger particles resource consumption can be divided into (>1µm) (Hindsgaul, 2000). Diffusionis when initial costs and costs of operation (e.g. the Brownian motions of the particle bring it pressure drop and use of materials) as well as in contact with the filter material. Diffusion is maintenance costs. The pressure drop often the major collection effect for submicron depends on the accumulated amount of particles (
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 Techniques Temperature Particles reduction (%) Tars reduction (%) Sand bed filter 10-20 70-99 50-97 Wash tower 50-60 0-98 10-25 Venturi scrubber n/a n/a 50-90 Rotational atomizer 99 0-60 Precipitator Fabric filter 130 70-95 0-50 Rotational particle 130 85-90 30-70 separator Fixed bed tar adsorber 80 n/a 50 Catalytic tar cracker 900 n/a >95 Fig.1 Two-stage gasifier concept Fig.2 Two-stage gasifier (Devi et al., 2003) Fig.3 Tars reduction by secondary methods 277
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 Fig.4 Cyclone separator (Global Air Filtration Systems Inc.) Fig.5 Baghouse filters Baghouse filters 278
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 In baghouse collectors, the dust filled air 1998). stream passes through fabric bags that filter the dust particles. Bags are made of different Rotating Particle Separator (RPS) material such as woven or felted cotton, synthetic, or glass-fiber and the choice of one This is a technique used to separate small over the other may depend on the temperature particles from a gas or liquid. The filter of the raw gas. Figure 5 shows a baghouse consists of a large number of small parallel filter arrangement. An advantage of this setup channels, which rotate around a common axis. is the ability to do maintenance on one filter The specific shape of the channels is not while in operation. important. Centrifugal forces drive the solid or liquid particles towards the walls, where Cartridge filters the particles stick as a result of the centrifugal force, Van der Waals force, or surface Cartridge filter can be surface or depth-type tension. The particles collected and filter. Depth-type filters capture particles and agglomerated on the channel walls are contaminant through the total thickness of the removed periodically by injecting pressurized medium, while in surface filters (usually air at high velocity in reverse flow direction made of thin materials like papers, woven into the channels. This is done by a nozzle wire, and cloths) particles are blocked on the moving over the rotating filter element at surface of the filter. The membrane and periodic intervals without disturbing the fibrous type of filters have been used for operation of the RPS. The technique is proven gasification (Hindsgaul, 2000). It can be for removal of small particles or droplets, generally stated that if the size of filter down to 0.1µm from gases at ambient surface is increased, higher flows are temperature (van Kemenade, 2003). possible, the filter last longer, and the dirt holding capacity increases. Cartridge filters Cooling towers and venturi scrubbers are normally designed disposable: this means that they have to be replaced when the filter is Cooling/scrubbing towers are usually used clogged. after cyclones as the first wet scrubbing units. All “heavy tar” components condense there. Electrostatic precipitators (ESP) However, tar droplets and gas/liquid mists are entrained by the gas flow, thus rendering the Electrostatic precipitators operate by charging tar removal rather inefficient. Venturi and collecting particles in a strong electric scrubbers are usually the next step (Milne et field between a central electrode and the wall. al., 1998). A venturi scrubber accelerates the Gravity forces the mixture of tar and dust to waste gas stream to atomize the scrubbing the bottom of the precipitator where it can be liquid and improve the gas-liquid contact. In a removed. venturi scrubber, a throat section is built into the duct that forces the gas stream to Only wet ESP can be used to remove tar from accelerate as the duct narrows and then a biomass gasifier gas, because tar expands. As the gas enters the venturi throat, condensation on dry ESPs precipitation both gas velocity and turbulence increase. electrode would progressively inhibit particle Depending upon the scrubber design, the removal. With ESPs, particle removal scrubbing liquid is sprayed into the gas stream efficiencies of more than 99% are possible for before the gas encounters the venturi throat, particles as small as 0.05µm (Milne et al., or in the throat, or upwards against the gas 279
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 flow in the throat. reform catalytically (Milne et al., 1998). The scrubbing liquid is then atomized into Partial oxidation small droplets by the turbulence in the throat and droplet-particle interaction is increased. Oxygen or air added to steam seems to After the throat section, the mixture produce more refractory tars but, while decelerates, and further impacts occur causing enhancing the conversion of primary tars. the droplets to agglomerate. Once the When oxygen is added selectively to different particles have been captured by the liquid, the stages, such as in secondary zones of a wetted particulate matter and excess liquid pyrolysis-cracker reactor, tars can be droplets are separated from the gas stream by preferentially oxidized (Milne et al., 1998). an entrainment section which usually consists of a cyclonic separator or mist eliminator Catalytic cracking of tars (Corbitt, 1990). The correct selection and dimensioning of wet gas cleaning systems The research on catalytic, hot-gas cleanup has requires information on the particle size involved (a) incorporating or mixing the distribution in the gas. There are no reliable catalyst with the feed biomass to achieve so- sets of tar droplet size distributions from called catalytic gasification or pyrolysis; and biomass producer gases (Corbitt, 1990). (b) treatment of gasifier raw gas in a second bed or beds. Two main classes of catalyst A major issue with using wet gas cleaning have been studied: non-metallic and metallic systems is the wastewater generated and this oxides. The most widely used non-metallic economic effect needs to be taken into catalysts are calcined dolomites and consideration when recommending such magnesites, zeolites, and olivine (Dayton, techniques (Milne et al., 1998). 2002). Metallic oxides used as catalyst are generally Nickel based catalysts because they Thermal cracking of tars have proved to be efficient for tars and ammonia decomposition in laboratory-scale Thermal processes raise the temperature of gas purification experiments (Hepola, 1993). the producer gas to the levels that “crack” the Other transition metals such as Cobalt and heavy aromatic tar species into lighter and Molybdenum may be used as well (Milne et less problematic species, such as hydrogen, al., 1998). carbon monoxide and methane. For this process, it is suggested that temperatures Although dolomite is the most widely used exceed 1000to reduce tars effectively (Milne catalyst and has been proven to be a very et al., 1998). effective bed additive in terms of tar reduction, it has some critical limitations. Steam reforming Dolomite is softer and thus gets eroded by the silica sand particles. Also, some dolomite The addition of steam, over and above that particles break during the calcinations and formed from the water and oxygen in the result in a large production of fines leading to feedstock, has been reported to produce fewer increased carryover of solids from the bed. refractory tars, enhance phenol formation, Dolomite is a calcium magnesium ore with reduce the concentration of other oxygenates, the general chemical formula CaMg(CO3)2 have only a small effect on the conversion of that contains approximately 20% MgO, 30% aromatics, and produce tars that are easier to CaO, and 45% COon a weight basis (Dayton, 280
Int.J.Curr.Microbiol.App.Sci (2017) 6(10): 269-284 2002).The use of calcined dolomites in olivine and hence more attention should be biomass gasification for tar cracking and given to find out whether olivine could removal has been the subject of interest in hot produce a clean gas with very low tar content gas cleaning. Delgado et al., (1996) studied (Devi et al., 2005). the use of calcined dolomites in biomass gasification with steam. Nickel based catalysts have been found to almost completely remove the tar and are also The catalytic decomposition of biomass tars very effective for NH3 removal at using calcined dolomites was also reported by temperatures above 800˚C (Wang et al., Devi et al., (2005). Calcination of dolomite 1999). The main limitation of using Nickel involves decomposition of the carbonate based catalysts is severe deactivation of the mineral, eliminating CO2to form MgO-CaO. catalyst. This deactivation occurs mainly Complete dolomite calcination occurs at fairly when the catalyst is placed right after the high temperatures and is usually performed at gasifier; the high tar concentration has a 800-900˚C (Dayton, 2002) and restricts its devastating effect on catalyst activity. More effective use to these relatively high recent work has included dual systems with temperatures. Aznar et al., (1997) performed catalysts such as dolomite serving as a guard experiments involving a bed of calcined bed for highly active catalysts such as Nickel dolomite placed after a biomass fluidized bed based reforming catalysts. Catalytic processes gasifier in which gasification was made with can operate at much lower temperatures (600- steam-oxygen mixtures to clean the raw 800˚C) processes, alleviating the need for syngas. The dolomite was calcined for 2 expensive alloys for reactor construction hours at 900˚C in an external oven and (Zhang et al., 2003). Also, unlike physical weighed before its introduction into the processes, catalytic cleaning converts the tar, reactor. The temperature of the catalytic bed eliminating waste disposal problems. reactor was measured at both the center and at Potentially, catalytic cracking processes its wall. Experimental results showed a tar provide the simplest and most effective means elimination of 90-95% with space time of of removing tars while retaining the sensible 0.06-0.15 kg calcined dolomite h-1and an heat required for efficient use of the producer increase in the gas yield by 0.15-0.40 m3at gas in close coupled applications. The use of a standard temperature and pressure(STP) per catalytic reactor downstream of the kg dry, ash free (daf) biomass fed (Aznar et gasification reactor has proven to be a more al., 1997). effective approach to tar destruction (Kurkela et al., 1993). In using catalysts as gas cleaning An alternative of dolomite can be naturally technique, there is almost no difference in the occurring particles of olivine which is a lower heating value of the gas produced as the mineral containing magnesium, iron oxide increase in the hydrogen production is and silica. Rapagna et al., (2000) have found compensated by a decrease in carbon the tar reforming activity of Olivine monoxide, and there is hardly any change in comparable to calcined dolomite. Olivine is methane production (Corella et al., 1999). advantageous in terms of its ability to withstand friction and does not easily break Gasification of woodchips and pine pellets at (Devi et al., 2005). However, there is still atmospheric pressure showed that it was ambiguity on the prospective use of olivine as possible to produce a combustible gas from a a tar decomposing catalyst. It is not yet well t gasifier. The syngas composition and an known how tars behave in the presence of estimation of the resulting heating value were 281
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