BioWeb

Click to Print

Gasification of BiomassGasification is the controlled partial oxidation of organic materials (e.g., biomass resources, fossil fuels and theirrefinery wastes, etc.) achieved by supplying less oxygen than the stoichiometric (ideal combustion processduring which a fuel is completely burned) requirement for complete combustion. Intermediate betweencombustion (thermal degradation with excess oxygen) and pyrolysis (thermal degradation in the absence ofoxygen), gasification reactions occur at temperatures between 600 and 1500oC and produce a low or mediumBtu gas (a combination of combustible and non-combustible gases) depending upon the process type andoperating conditions.

Gasification technology is already being used to produce bioenergy and bioproducts such as use in dual-modeengines to produce power (e.g. for irrigation); use in internal combustion engines (Wander, 2004); to produceheat, steam, and electricity (Yin, 2002); to produce hydrogen used in petroleum refineries; and to producemethanol (Brown, 2003). It has been evaluated as a direct source of transportation fuel in cars and trucks(Camacho, 1988) and research is on-going to use the syngas produced by gasification to produce ethanol fortransportation use and to produce organic chemicals.

Gasification ReactionsDuring gasification, the fuel (e.g., biomass resources, fossil fuels) is heated to a high temperature which resultsin the production of volatile compounds (gases) and solid residues (char). The quantity and composition of thevolatile compounds depends on the reactor temperature and type, the characteristics of the fuel, and the degreeof equilibrium achieved by the various gas-phase reactions (gas gas reaction), particularly the water-gas shiftreaction (carbon monoxide and water vapor). Table 1 summarizes the gasification reactions. The primarycombustion reaction (equation 1) occurs in the presence of free oxygen, and is highly exothermic and very fast.It provides the energy needed to sustain the other gasification reactions. The gasification of solid materials(char) with reactive agents such as carbon dioxide, steam, and hydrogen (equations 3-5) occur at hightemperatures (>600oC) and produce gas, tar, and ash. The char-gas reactions (C-CO2 and C-H2O) control theultimate conversion of the char and the products of these reactions dominate the final composition of thesyngas. The occurrence and extent of the gas-phase reactions (homogeneous reactions) depend on theoperating conditions within the biomass gasifier. Secondary reactions (i.e., the water-gas shift reaction,methanation, tar cracking and the reforming of tars and heavy hydrocarbons) (hydrocarbons containing 3 ormore carbon atoms) occur at high temperatures (>600oC) and appropriate pressure conditions and involve thedecomposition of heavy hydrocarbons and tars to carbon and low molecular weight syngas. The composition ofthe syngas produced during gasification is dependent on the degree of equilibrium attained by the variousgas-phase reactions.

BioWeb http://bioweb.sungrant.org/SGBioWeb/Templates/Technical/TechnicalPrin...

1 dari 19 4/7/2014 9:39 AM

The gasification process involves different reactive agents such as air, oxygen, hydrogen or steam to convertthe fuel to gases (figure 1). The simplest gasification process uses air as a reactive agent. Excess char formedby the pyrolysis process within the gasifier is burned with limited air (usually an equivalence ratio of 0.25) toproduce a low energy syngas (93 209 Btu/ft3; 3.5 - 7.8 MJ/Nm3) containing primarily hydrogen and carbonmonoxide diluted with nitrogen from the air. The syngas is suitable for boiler and engine applications, butcannot be transported through pipelines due to its low Btu content.


2 dari 19 4/7/2014 9:39 AM

Steam can be added from an external source, or obtained from the dehydration of water in the fuel. The use ofsteam as the sole reactive agent requires an external heat source, but the use of steam in combination with airdoes not require an external heat source as the reaction becomes exothermic due to the oxygen in the air. Thehigher temperature of the reactions helps to devolatize the fuel (an industrial process in which low-molecular-weight components such as unreacted monomers, solvents, water, and various polymerization by-products areseparated from a polymeric system) to produce syngas. Gasification of char in the presence of steam produces asyngas consisting mainly of CO2, CO, H2, and CH4. Steam reacts with carbon monoxide to produce hydrogen andcarbon dioxide in a water-gas shift reaction (equation 6). Under conditions of low temperatures, low heat rates,and high pressure, secondary reactions become important due to the long residence time of volatile compounds.Under conditions of low pressure, high temperature, and high heat rates, most of the volatile compounds escapefrom the fuel particles which reduces the chances of a solid char-gas interaction. The use of a steam reactiveagent produces a higher energy syngas relative to using air as the reactive agent.

Limiting the amount of nitrogen supplied to the gasification reactions (oxygen gasification) results in a mediumenergy syngas (approximately 12-21 MJ/Nm3; about 320 563 Btu/ft3) that does not contain nitrogen and issuitable for distribution through a pipeline due its medium energy content. Increasing the oxygen contentincreases the energy content of the syngas and increases the percent of methane, hydrogen, and carbon


3 dari 19 4/7/2014 9:39 AM

monoxide in the syngas (Bailie, 1979). In an oxygen-steam mixture, increasing the oxygen to carbon ratiosignificantly increases the energy content of the syngas (Watkinson, 1987). Oxygen gasification requires anearby source of oxygen which may increase capital costs.

The use of hydrogen as a reactive agent is conducted under high pressure conditions and requires that stringentreaction conditions be maintained as most of the products are in a gaseous phase. The high degree of controlnecessary and the availability of a hydrogen source limit the use of this process.

A number of factors affect the gasification reactions and influence the syngas composition and distributionincluding the temperature, pressure, and height of the reactor fuel bed; the fluidization velocity; the gasifyingmedium; the equivalence ratio; the air to steam ratio; the presence of catalysts; and the fuel moisture andparticle size. These variables are interrelated and affect the gasification rate and process efficiency, and theenergy content and composition of the syngas that is produced.

The rate of gasification and the performance of the gasification reactor is temperature dependent. Gasificationreactions are generally reversible and the equilibrium point can be shifted by changing the temperature. Attemperatures between 600-900oC, gasification reactions are rate controlling, but above 900oC, heat and masstransfer rates (the rate of heat or mass addition into a system) are controlling and below 600oC, completegasification does not occur (Brink, 1981). Increasing the temperature increases the formation of combustiblegases, decreases the yield of char and liquids, and leads to more complete conversion of the fuel (Harris, 2005;Scott, 1988; Voloch, 1983; Elliot, 1985; Font, 1988). Tar production at low temperatures (< 500oC) increasesinitially with increases in temperature and then declines with further temperature increases (Alves, 1989).Hydrocarbon gases, especially methane and ethylene, increase with temperature while the yields of higherhydrocarbons (C3-C8; compounds with 3 to 8 carbons) decrease at temperatures above 650oC probably due tothe onset of cracking/reforming reactions that break down the high molecular weight hydrocarbons into lightercomponents (Utioh, 1989). The energy content of the syngas increases steadily up to 700oC (due to theincrease in concentrations of CO, H2 and hydrocarbon gases), then decreases (probably due to cracking of thehydrocarbons) (Sadakata, 1987).

Increases in the reactor bed pressure result in decreases in fuel weight loss during devolatilization (Nandi,1985), but at a constant temperature, the first-order rate constant (k) (the rate of reaction k is directlyproportional to the concentration of one of the reactants) for char gasification increases with increasingpressure. Gasification rates of char increase with increasing pressure and are most significant at hightemperatures (900-950oC) (Plante, 1988). Methane yields increase as pressure increases (Liinanki, 1985) andH2/CO and CO2/CO molar ratios (number of moles of carbon dioxide divided by the number of moles of carbonmonoxide) increase with the higher total pressure of a char-steam gasification process (Richard, 1985). Highpressure appears to increase the heat transport properties of mixed coal/biomass fuels (McLendon, 2003).

For a given reactor temperature, higher fuel bed heights increase the residence time, increase total syngasyields, and increase formation of H2, CO, CO2, CH4 and C2H4 (Font, 1988; Beaumont, 1984). Conversionefficiencies (energy output in the gas form divided by the energy input in the fuel) increase, bed temperaturedecreases (due to the increase in heat capacity), and fuel bed pressure (pressure difference between the bottomand top of the bed) decreases in the dense bed (bed of sand) of the reactor but is unaffected in the freeboardregion (the region above the sand) with increasing fuel bed height (Sadaka, 1998).

Fluidization velocity (the velocity that maintains the particles suspended in the gasifying media) plays animportant role in the mixing of particles in the fluidized bed. In air gasification systems, higher fluidizationvelocity increases fuel bed temperature and decreases the energy content of the syngas (due to increasedamounts of oxygen and nitrogen in the inlet gas). Studies differ on the impact of gas velocity on syngas yield,composition, and energy content with one study showing no difference over the range evaluated (Raman,1980), and one study finding a higher energy content at a fluidization velocity of 0.28 m/s but remaining fairlyconstant at velocities of 0.33 and 0.37 m/s (Sadaka, 2002).


4 dari 19 4/7/2014 9:39 AM

The equivalence ratio (actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio) has the strongestinfluence on the performance of gasification reactors because it affects the fuel bed temperature, syngasquality, and the thermal efficiency of the reactions. Higher equivalence ratios result in smaller pressure drops inboth the dense bed and the freeboard regions of the reactor when the reactor operates at different fluidizationvelocities and fuel bed heights. High equivalence ratios increase the rate of syngas production and increasereactor temperature due to more exothermic reactions (Schoeters, 1989). Low equivalence ratios reduce fuelbed temperatures resulting in the production of less syngas, more tar, and a lower energy content of the syngas(Ergudenler, 1992).

Increasing the air to steam ratio increases the energy content of the syngas up to a peak level of 0.25 kg/kg.The impact is particularly strong at low ratios because the steam released during the devolatilization stagecontributes to the gasification process even when steam is not added (Tomeczek, 1987).

Catalysts are used in the gasification process to enhance the quality of the syngas and reduce the rate of tarproduction (Baker, 1985). A major problem that occurs with the use of conventionally supported Ni catalystsduring the catalytic steam reforming of tar is the deposition of carbon on the catalyst due to the high aromaticcharacter of the carbon. A Ni/dolomite catalyst has been shown to provide high activity and stability under ashort contact time during the steam gasification of tar, and produces negligible carbon deposition (Srinakruang,2005). The activity and properties of the Ni/dolomite catalyst are significantly affected by the temperature ofthe calcinations process (conversion of limestone into calcium oxide and carbon dioxide at temperatures inexcess of 900C).

The characteristics of the fuel affect the gasification process. High fuel moisture content reduces the gasificationtemperature (due to the energy required to evaporate the water in the fuel) which leads to the production ofhigher volumes of char (Elliot, 1985). The rate of thermal diffusion within the fuel particles decreases withincreased particle size, resulting in a lower heating rate and the production of more char and less tar. At a giventemperature, syngas yield increases as particle size decreases (Raman, 1980). Heating value versustemperature plots for different particle sizes are parabolic with the maximum heating value aligned with thesmallest particle size. Gasification rates increase as particle size decreases (Edrich, 1985).

Steam can be added from an external source, or obtained from the dehydration of water in the fuel. The use ofsteam as the sole reactive agent requires an external heat source, but the use of steam in combination with airdoes not require an external heat source as the reaction becomes exothermic due to the oxygen in the air. Thehigher temperature of the reactions helps to devolatize the fuel (an industrial process in which low-molecular-weight components such as unreacted monomers, solvents, water, and various polymerization by-products areseparated from a polymeric system) to produce syngas. Gasification of char in the presence of steam produces asyngas consisting mainly of CO2, CO, H2, and CH4. Steam reacts with carbon monoxide to produce hydrogen andcarbon dioxide in a water-gas shift reaction (equation 6). Under conditions of low temperatures, low heat rates,and high pressure, secondary reactions become important due to the long residence time of volatile compounds.Under conditions of low pressure, high temperature, and high heat rates, most of the volatile compounds escapefrom the fuel particles which reduces the chances of a solid char-gas interaction. The use of a steam reactiveagent produces a higher energy syngas relative to using air as the reactive agent.

Limiting the amount of nitrogen supplied to the gasification reactions (oxygen gasification) results in a mediumenergy syngas (approximately 12-21 MJ/Nm3; about 320 563 Btu/ft3) that does not contain nitrogen and issuitable for distribution through a pipeline due its medium energy content. Increasing the oxygen contentincreases the energy content of the syngas and increases the percent of methane, hydrogen, and carbonmonoxide in the syngas (Bailie, 1979). In an oxygen-steam mixture, increasing the oxygen to carbon ratiosignificantly increases the energy content of the syngas (Watkinson, 1987). Oxygen gasification requires anearby source of oxygen which may increase capital costs.

The use of hydrogen as a reactive agent is conducted under high pressure conditions and requires that stringentreaction conditions be maintained as most of the products are in a gaseous phase. The high degree of controlnecessary and the availability of a hydrogen source limit the use of this process.


5 dari 19 4/7/2014 9:39 AM

A number of factors affect the gasification reactions and influence the syngas composition and distributionincluding the temperature, pressure, and height of the reactor fuel bed; the fluidization velocity; the gasifyingmedium; the equivalence ratio; the air to steam ratio; the presence of catalysts; and the fuel moisture andparticle size. These variables are interrelated and affect the gasification rate and process efficiency, and theenergy content and composition of the syngas that is produced.

The rate of gasification and the performance of the gasification reactor is temperature dependent. Gasificationreactions are generally reversible and the equilibrium point can be shifted by changing the temperature. Attemperatures between 600-900oC, gasification reactions are rate controlling, but above 900oC, heat and masstransfer rates (the rate of heat or mass addition into a system) are controlling and below 600oC, completegasification does not occur (Brink, 1981). Increasing the temperature increases the formation of combustiblegases, decreases the yield of char and liquids, and leads to more complete conversion of the fuel (Harris, 2005;Scott, 1988; Voloch, 1983; Elliot, 1985; Font, 1988). Tar production at low temperatures (< 500oC) increasesinitially with increases in temperature and then declines with further temperature increases (Alves, 1989).Hydrocarbon gases, especially methane and ethylene, increase with temperature while the yields of higherhydrocarbons (C3-C8; compounds with 3 to 8 carbons) decrease at temperatures above 650oC probably due tothe onset of cracking/reforming reactions that break down the high molecular weight hydrocarbons into lightercomponents (Utioh, 1989). The energy content of the syngas increases steadily up to 700oC (due to theincrease in concentrations of CO, H2 and hydrocarbon gases), then decreases (probably due to cracking of thehydrocarbons) (Sadakata, 1987).

Increases in the reactor bed pressure result in decreases in fuel weight loss during devolatilization (Nandi,1985), but at a constant temperature, the first-order rate constant (k) (the rate of reaction k is directlyproportional to the concentration of one of the reactants) for char gasification increases with increasingpressure. Gasification rates of char increase with increasing pressure and are most significant at hightemperatures (900-950oC) (Plante, 1988). Methane yields increase as pressure increases (Liinanki, 1985) andH2/CO and CO2/CO molar ratios (number of moles of carbon dioxide divided by the number of moles of carbonmonoxide) increase with the higher total pressure of a char-steam gasification process (Richard, 1985). Highpressure appears to increase the heat transport properties of mixed coal/biomass fuels (McLendon, 2003).

For a given reactor temperature, higher fuel bed heights increase the residence time, increase total syngasyields, and increase formation of H2, CO, CO2, CH4 and C2H4 (Font, 1988; Beaumont, 1984). Conversionefficiencies (energy output in the gas form divided by the energy input in the fuel) increase, bed temperaturedecreases (due to the increase in heat capacity), and fuel bed pressure (pressure difference between the bottomand top of the bed) decreases in the dense bed (bed of sand) of the reactor but is unaffected in the freeboardregion (the region above the sand) with increasing fuel bed height (Sadaka, 1998).

Fluidization velocity (the velocity that maintains the particles suspended in the gasifying media) plays animportant role in the mixing of particles in the fluidized bed. In air gasification systems, higher fluidizationvelocity increases fuel bed temperature and decreases the energy content of the syngas (due to increasedamounts of oxygen and nitrogen in the inlet gas). Studies differ on the impact of gas velocity on syngas yield,composition, and energy content with one study showing no difference over the range evaluated (Raman,1980), and one study finding a higher energy content at a fluidization velocity of 0.28 m/s but remaining fairlyconstant at velocities of 0.33 and 0.37 m/s (Sadaka, 2002).

The equivalence ratio (actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio) has the strongestinfluence on the performance of gasification reactors because it affects the fuel bed temperature, syngasquality, and the thermal efficiency of the reactions. Higher equivalence ratios result in smaller pressure drops inboth the dense bed and the freeboard regions of the reactor when the reactor operates at different fluidizationvelocities and fuel bed heights. High equivalence ratios increase the rate of syngas production and increasereactor temperature due to more exothermic reactions (Schoeters, 1989). Low equivalence ratios reduce fuelbed temperatures resulting in the production of less syngas, more tar, and a lower energy content of the syngas


6 dari 19 4/7/2014 9:39 AM

(Ergudenler, 1992).

Increasing the air to steam ratio increases the energy content of the syngas up to a peak level of 0.25 kg/kg.The impact is particularly strong at low ratios because the steam released during the devolatilization stagecontributes to the gasification process even when steam is not added (Tomeczek, 1987).

Catalysts are used in the gasification process to enhance the quality of the syngas and reduce the rate of tarproduction (Baker, 1985). A major problem that occurs with the use of conventionally supported Ni catalystsduring the catalytic steam reforming of tar is the deposition of carbon on the catalyst due to the high aromaticcharacter of the carbon. A Ni/dolomite catalyst has been shown to provide high activity and stability under ashort contact time during the steam gasification of tar, and produces negligible carbon deposition (Srinakruang,2005). The activity and properties of the Ni/dolomite catalyst are significantly affected by the temperature ofthe calcinations process (conversion of limestone into calcium oxide and carbon dioxide at temperatures inexcess of 900C).

The characteristics of the fuel affect the gasification process. High fuel moisture content reduces the gasificationtemperature (due to the energy required to evaporate the water in the fuel) which leads to the production ofhigher volumes of char (Elliot, 1985). The rate of thermal diffusion within the fuel particles decreases withincreased particle size, resulting in a lower heating rate and the production of more char and less tar. At a giventemperature, syngas yield increases as particle size decreases (Raman, 1980). Heating value versustemperature plots for different particle sizes are parabolic with the maximum heating value aligned with thesmallest particle size. Gasification rates increase as particle size decreases (Edrich, 1985).

Gasification ProcessDuring gasification, as air is passed through the fuel bed, relatively discrete drying, pyrolysis, gasification andoxidation (combustion) zones develop within the reactor. The fuel is dried and moisture removed in the dryingzone. In the pyrolysis zone, fuel is converted to volatile compounds and char. The char is gasified in thepresences of reactive agents such as carbon dioxide, steam, hydrogen, and oxygen in the gasification zone.Secondary reactions of primary gases and tars take place in the oxidation zone. Because the major product ofbiomass at temperatures below 600oC is char, biomass gasification requires high temperatures in order to gasifychar. The location of these zones within a reactor depends on the relative movement of fuel and air, and thezones are differentiated by the variety of reactions and their temperatures. The depth and relative importanceof each zone depend on the chemical composition of the fuel, its moisture content and particle size, the massflow rate of the reactive agent, and the bed temperature.

The drying zone receives its energy through heat transfer from other zones. The rate of drying depends uponthe temperature, velocity, and moisture content of the drying gas, as well as the external surface area of thefeed material, the internal diffusivity of moisture and the nature of bonding of moisture to the feed material,and the radioactive heat transfer. As the fuels enter the drying zone, their internal temperature is increased to100-150oC. Low density materials change dimensions slightly due to shrinkage and compression whereasnegligible size changes are experienced by fuels with high density. No chemical reactions take place in thiszone.

Heat transfer from the adjacent hot reduction zone (pyrolysis zone) causes devolatilization of the fuel.Temperature in the devolatilization zone increases rapidly due to the large temperature difference between therelatively cold fuel material and hot gases. The rate of temperature rise is controlled by heat transfer. As fuelspass through this zone, rapid charring and reduction in volume occur, causing changes in the structure andphysical and thermal properties of the fuel. Gases, liquids (tars and oils), and solids (char) are produced in thiszone. Liquid production is undesirable in gasification, and needs to be controlled. The product mix depends ontemperature, heat rate, and the structure, composition and size of catalysts.

In the oxidation zone, physical and chemical changes are inhibited as the oxygen carrier (i.e., usually air), isintroduced into the fuel bed. The oxygen burns the carbon in the fuel until nearly all free carbon is exhausted.


7 dari 19 4/7/2014 9:39 AM

Limited penetration of the fuel by oxygen occurs. If air is the reactive agent, the oxygen content decreases untilgone and the carbon dioxide percent increases proportionately. The oxidation zone has the highest temperaturedue to the exothermic nature of the reactions.

In the ash cooling zone, the remainder particles (ash) cool faster than particles in other zones. The ash coolingzone protects the reactor grate from intense heat in fixed bed reactors and distributes the air over the bed.Practically no chemical reactions take place here, although in some fixed bed designs, this zone acts as a filterfor the resulting syngas and in some reactor designs, this zone preheats the incoming air stream.

Although gasification processes have been highly developed, there are still several limitations. Some are relatedto feedstock characteristics while others are related to the overall design of the reactors. Limitations include themoisture content of the fuel, the feeding system, ash deformation temperature, particle mixing and segregation,and entrainment and elutriation.

The moisture content of the fuel significantly affects the gasifier operation. The moisture mass fraction that isthe limiting value, differs by fuel source. Fuels with high moisture content lower the reactor bed temperaturedue to the energy required to evaporate the water contained in the fuel. Heat output has been shown todecrease as moisture content of the fuel increases (Butuk, 1987) and the energy content of the syngas alsodeclines with increased moisture (Black, 1980).

The type of fuel feeding mechanism required is determined by the size, shape, density, moisture content, andcomposition of the fuel. Several mechanisms have been developed to accommodate the wide variety of biomassfuels used, including direct feeding to the bed and over-the-bed feeding. In direct feeding, the feedingmechanism must be isolated from the reactor. If not, tar can flow back, condense in the feeding mechanism,and stick to the screw and shaft. Also, toxic and combustible gases back-flow from the gasifier and might causeexplosions. A combination of purge gas, airlocks and lock hoppers are used to seal the feeding mechanismagainst the slight positive pressure of the fuel bed, minimizing the possibility of gas back-flow. Over-the-bedfeed systems are usually less troublesome because there is no direct contact between the hot fuel bed materialand the feeder. However, this type of feeding mechanism is restricted to higher density and/or larger sizedparticles which are less likely to be carried out of the reactor by the outgoing gases. Due to particle emissions,over-the-bed feeding usually results in a dirty syngas, which increases syngas clean-up costs and fuel loss. Dueto their low density and irregular shape, fluffy fuels are difficult to feed into a gasifier (Bilbao, 1987; Ghaly,1989).

The ash and slag deformation temperatures (the temperature that causes changes in shape) are affected by thecomposition of the ash and its concentration. Melted ash can clog the grate and ash handling is a criticalproblem. A major constraint in the efficiency of gasifiers is operating below the initial deformation temperaturewhich can be as low as 900oC. The deformation temperature varies depending on the type of fuel and thepresence of mineral oxides (e.g., Na2O and K2O) (Ghaly, 1990; Perkins, 1984). The most desirable chemicalconstituent of ash is Al and the most undesirable constituents are K, Ca and Fe (Huffman, 1981). Rapidagglomeration (fusion of bed materials and feedstocks to form very large particles) of the ash/fuel bed materialmixture have been reported at high temperatures resulting in fusion of the bed material (Imc, 1980). Highvolatile content of fuels can cause severe slag formation and makes it difficult to keep the reactor temperaturebelow the ash melting temperature (Carre, 1988). The use of automatic ash removal systems and/or movablegrates may reduce these problems. Studies show that K2O (which has low melting temperature and is a highpercent of the weight of the ash in some biomass resources such as wheat straw), is a major cause ofagglomeration (Ergudenler, 1992).

The design of fluidized bed reactors is extremely important because both the axial and radial transport of solidswithin the bed influence gas-solid contact, the thermal gradient, and the heat transfer coefficient. Segregationin a fluidized bed is affected by the particle density, shape, size, superficial gas velocity, mixture composition,bed aspect ratio (the ratio of the static bed height divided by the dynamic or expanded bed height). Cranfield(1978) investigated solid mixing of large sized fuel particles in three-dimensional fluidized beds and concludedthat the concentration of the jetsam in the upper region (compared with its overall value) is a good measure of


8 dari 19 4/7/2014 9:39 AM

the degree of mixing achieved. Variations in the size, shape and density of the fuel particles can cause severemixing problems which result in changes in temperature gradients within the reactor, increase tar formationand agglomeration, and decrease the conversion efficiency (Bilbao, 1988). Effective mixing of fuels of varioussizes is needed to maintain uniform temperature and a good mix depends on the relative concentrations of thesolids in the bed and the velocity of the gas (Ghaly, 1989; Bilbao, 1988).

Entrainment, elutriation, and carryover are similar terms used interchangeably to describe the ejection ofparticles from the surface of a bubbling bed, fractionation in the freeboard region (i.e., the region above the bedmaterial), and removal of particles from fluidized beds in the gas stream. Entrainment is affected by manyfactors including fluidizing gas properties (superficial gas velocity, gas density, viscosity and relative humidity),fuel properties (particle size, particle size distribution and particle density), and other factors (bed diameter,bed depth, gas distribution and internal surfaces). Clean-up of the syngas to prevent particle emissions isgenerally required, adding to the cost of gasification. The design of the clean-up equipment is difficult becausethe information needed is not readily apparent from basic principles due to the complexity of the phase flow inthe freeboard (Horio, 1980; Geldart, 1972).

Gasification ReactorsA number of different gasifier reactor designs have been developed and evaluated. The designs can generally beclassified into two broad categoriesfixed bed reactors and fluidized bed reactors--based on the relativemovement of the fuel and the gasifying medium.

Fixed bed reactors. Fixed bed reactors are those in which the fuels move either countercurrent or concurrentto the flow of gas as the fuel is converted to syngas. They are well suited to situations requiring solid fuelcontacting operations (gas solid contact) and close temperature control, and for situations where carryoverparticles need to be revolved from the reaction zone. They are relatively simple to operate and generallyexperience minimum erosion of the reactor body. There are three basic fixed bed designsupdraft, downdraft,and crossdraft gasifiers.

In an updraft fixed bed gasifier (figure 2), the flow of the fuel and gases are countercurrent to each other.


9 dari 19 4/7/2014 9:39 AM

The high temperature combustion zone (oxidation zone) is located at the bottom of the gasifier where part ofthe fuel is burned. The reactive agent is injected at the bottom of the reactor and ascends to the top while thefuel is introduced at the top and descends to the bottom. The fuel descends through four zones (drying,pyrolysis reduction and combustion) of progressively increasing temperatures, reaching temperatures in excessof 1500oC in the combustion zone. The heat from the combustion zone and the reduction (gasification) zoneabove it are forced upward by convection and radiation to the pyrolysis and drying zones, providing the heatrequired for drying, pyrolysis, and endothermic char gasification processes. Gases, tar and other volatilecompounds are dispersed at the top of the reactor, while ashes are removed at the bottom. The syngasproduced by an updraft gasifier usually exits at low temperatures (approximately 400oC) and is rich inhydrocarbons and tar which contains as much as 30% of the energy of the original fuel. High tar content is nota problem for use in direct heat applications, but must be cleaned for other applications. The syngas fromupdraft gasifiers contains more CO and less CH4, ethane and acetylene than syngas from other gasifiers.Updraft gasifiers are widely used to gasify biomass resources and generally use steam as the reactive agent tocontrol the oxidation zone temperature. Exhaust gas re-circulation is an alternative approach to temperaturecontrol in updraft gasifiers. The design and construction of updraft gasifiers is simple relative to other gasifiertypes and they are characterized by low syngas exit temperature, high charcoal burnout, and high thermalefficiency. However, slagging (deposit formation on heat transfer surfaces) can be very severe in updraftgasifiers, especially when high ash fuels are used (e.g., cereal straws and corn cobs). Updraft gasifiers alsoproduce syngas with high tar content and are unsuitable for use with fluffy, low density fuels.

In downdraft fixed bed reactors, the reaction zones are similar to those in the updraft unit, except the locationsof the combustion (oxidation) and reduction zones are reversed (figure 3).


10 dari 19 4/7/2014 9:39 AM

The fuel is introduced at the top and the reactive agent is introduced through a set of nozzles on the side of thereactor. The most important difference is that the pyrolysis products in a downdraft gasifier are allowed to passthrough the high temperature combustion zone where they undergo further decomposition. Also, the moistureevaporated from the biomass fuel enters the gasification zone and serves as a reactive agent. The syngas leavesthe gasifier from the bottom at a temperature of about 700oC and contains substantially less oil and tar than inupdraft gasifiers, which requires less cleaning and can be used in a wider array of applications. Fuels with highash content and low ash fusion temperatures (e.g., crop residues) can lead to slagging and clinker formation(clinker formation occurs when high concentrations of chlorine and potassium in the ash melt at 600C and fusewith silica). Downdraft gasifiers are inappropriate for fluffy (low density) fuels.

Cross-draft fixed bed gasifiers exhibit many of the operating characteristics of downdraft gasifiers. Air orair/steam mixtures are introduced from the side near the bottom while the syngas is drawn off on the oppositeside (figure 4).


11 dari 19 4/7/2014 9:39 AM

An inlet nozzle is used to bring air to the center of the combustion zone at high velocity which increases thetemperature of the zone. The combustion (oxidation) and reduction zones are concentrated around the sides ofthe unit and are smaller in volume than other fixed bed gasifiers due to the rapid consumption of oxygen.Cross-draft gasifiers respond rapidly to load changes, are relatively simpler to construct, and produce syngassuitable for a number of applications, but are sensitive to changes in the fuel composition and moisture content.

Fluidized bed reactors. A fluidized bed gasifier has a bed made of an inert material (such as sand, ash, orchar) which acts as a heat transfer medium. Studies using fluidized beds to convert biomass date back to the1950s (Morgani, 1953). In this design, the bed is initially heated and the fuel introduced when the temperaturehas reached the appropriate level. The bed material transfers heat to the fuel and simulates a fluid by blowingthe reactive agent through the distributor plate at a controlled rate. Unlike fixed bed reactors, gasifiers withfluidized beds have no distinct reaction zones and drying, pyrolysis and gasification occur simultaneously duringmixing and are thus close to isothermal (the temperature values are almost the same within the bed).Compared to other gasifiers, fluidized bed gasifiers has strong gas to solids contact (due to the bubblingphenomena), excellent heat transfer characteristics, better temperature control, large heat storage capacity,good degree of turbulence, and high volumetric capacity. Their disadvantages include large pressure drop,particle entrainment, and erosion of the reactor body. Because fluidized bed reactors operate at pressuresslightly above atmospheric levels, their design and construction must prevent leakage and the fuel feedingsystem must be equipped with a pressure-locking device. Because the fuel is immediately gasified as it is fedinto the bed, these gasifiers respond slowly to load changes because they have no buffer stock of gas to supplyfluctuating demands. Due to their complicated and expensive control systems, fluidized bed gasifiers appear tobe commercially viable at larger sizes (> 30 MW thermal output). Fluidized bed reactors are classified by theirconfiguration and the velocity of the reactive agent and consist of bubbling, circulating, spouted, and swirlingfluidized beds.

In bubbling fluidized bed gasifiers, granular material is fed into a vessel where gases are introduced at a flowrate that maintains pressure at a level sufficient to keep the fuel particles in suspension (incipient fluidization)(figure 5).


12 dari 19 4/7/2014 9:39 AM

At low fluidization velocities (i.e., just above the minimum fluidization velocity), gas in excess of what is neededfor minimum fluidization passes through the bed in the form of bubbles (the bed looks like a bubbling fluid). Asthey rise in the bed, the bubbles coalesce and grow, and at the bed surface, they burst causing solids in the bedto disperse and enter the freeboard (the space above the sand bed) where carryover (particles leaving the bed)occurs. If the height at which this occurs is above the transfer disengaging height (TDH), carryover ismaintained at a constant level (i.e., saturation gas carrying capacity). Maintaining pressure across the bed is animportant consideration in the design of a bubbling bed gasifier. The characteristics of the bed particlesdetermine the size and rating of the blower needed to supply the air

Bubbling fluidized bed reactors are categorized as either a single fluidized bed or dual (multi) fluidized beds.Single fluidized bubbling bed gasifiers have only one bed where the fuel and the reactive agent enter and fromwhich the syngas and char exit. This design results in lower cost and less maintenance relative to multi-beddesigns, and the syngas is ready for utilization. However, the energy content of the syngas is lower thanachieved in dual bed designs, inorganic materials in the fuel cannot be separated, and pyrolysis occurs at thebottom of the gasifier leading to non-uniform temperature distribution. Dual (or multi) bed bubbling gasifiershave more than one bed. The first bed is usually used to burn some of the char to produce the energy for thesecond bed where pyrolysis occurs. Dual bed systems produce syngas with higher energy content due to thecombustion of the char in a separate chamber which prevents the combustion gas from diluting the pyrolysisgas. Additionally, inorganic materials in the fuel can be separated and the heat of pyrolytic reactions is evenlydistributed allowing pyrolysis to occur at a relatively uniform temperature. Higher construction costs andgreater maintenance are the disadvantages of a dual system.

A circulating fluidized bed gasifier (also called a fast fluidized bed gasifier) is a modified bubbling bed gasifier inwhich the solids leaving the reactor vessel are returned through an external collection system (figure 6).


13 dari 19 4/7/2014 9:39 AM

As the gas velocity in a bubbling fluidized bed is increased, more particles are entrained (escape from the bed)in the gas stream and leave the reactor. Eventually the transport velocity (the velocity required to empty thebed) for most of the particles is reached, and the vessel can be quickly emptied of solids which are returned tothe reactor. The stream of particles moving upward in the reactor is at solid concentrations well above that fordilute phase transport. Compared to other gasifiers, circulating fluidized bed gasifiers have a higher processingcapacity, better gas-solid contact, and the ability to handle cohesive solids that might otherwise be difficult tofluidize in bubbling fluidized beds. Despite these advantages, circulating fluidized beds are still less commonlyused because their height significantly increases their cost.A spouted fluidized bed gasifier has a bed of coarse particles partly filling the vessel and a relatively largecontrol opening at the base where gas is injected. With a sufficient flow of gas, particles in the gas can be madeto rise as a fountain in the center of the bed and to develop a circling motion on the bed. Additional air added tothe base can produce a spouted bed. The minimum particle diameter practical for spouting is about 1mm whichis close to the value at which gas-solid contact effectiveness is impaired due to the formation of large gasbubbles. The total pressure drop across a fully spouting bed is lower by at least 20% relative to what is requiredfor good quality aggregative fluidization (fluidization of larger particles). This type of gasifier has been used togasify coal.

A swirling fluidized bed consists of a bed of granular material in a cylindrical column. Primary air is introducedat the bottom of the bed through a distributor plate at a velocity sufficient to fluidize the bed material in abubbling regime. Secondary air is introduced through at least one pair of openings into the freeboard region ofthe column to create swirl or vortex flow (which allows air to move upward in circles). The injection ofsecondary swirling air into the freeboard (the space above the bed) helps to achieve high relative movementbetween the air and the fuel particles. A centrifugal action ensures that all fuel particles above a certainminimum size are retained in the combustion chamber and prevented from being elutriated (escape from thebed). This increases the gas residence time and the degree of mixing (gas-gas and gas-solids) which increasesthe rate of reactions in the freeboard. Swirling fluidized beds meet the requirements of combustion by achievingcomplete combustion and have limited application for gasification. They may be more suited to steamgasification.

Gasification of Biomass Resources. Due to its simplicity, the use of air as the reactive agent in the


14 dari 19 4/7/2014 9:39 AM

gasification of biomass is being studied by several researchers. In this process, the reactor temperature isdependent on the air flow rate and the biomass feed ratea low flow rate of air into the system reduces the fuelbed temperature which results in lower syngas yields and higher tar yields. Studies have examined the impactof temperature when using air as the reactive agent for biomass gasification, and found that highertemperatures resulted in higher syngas yields, decreased tar and char yields, higher energy content of thesyngas, and greater energy recovery (Groves, 1979 for cotton gin trash; Lian, 1982 for oak sawdust;Walawender, 1978 for feedlot manure; Ergudenler, 1993 for wheat straw; and Cao, 2005 for sawdust).

Compared to air gasification, steam gasification of biomass resources produces a higher energy content syngas.Increasing temperatures increase syngas yields and the energy content of the syngas which exhibits a parabolictemperature function. The syngas is particularly rich in H2, and contains significant quantities of CO2, CO, andCH4, and some higher hydrocarbons (e.g., ethane, ethylene, and propylene). Some biomass resources (e.g.straw and sawdust) exhibited higher syngas and lower tar yields than others (e.g., wood chips and thistle)under the same temperature and reaction conditions (Corella, 1989). Studies examining the effect oftemperature to steam gasify biomass include Boateng, 1992; Hoveland, 1982 (corn grain dust); Walawender,1981 (alpha cellulose); Walawender, 1982 (straw); Slapak, 2000 (recycling waste); and Chen, 1982 (ricehusks). Experiments also confirm that oxidant partial pressure influences steam biomass gasification (Mermoud,2005; beech wood charcoal).

Few studies examine the use of oxygen or hydrogen as the reactive agent for biomass gasification. Tillman(1987) used oxygen to gasify municipal solid waste to produce a medium Btu syngas (10.6 MJ/Nm3) composedmainly of CO, H2, CO2 and CH4. Weil (1978) used preheated hydrogen to gasify peat under conditions ofincreasing temperature producing syngas with high carbon monoxide and hydrocarbons.

An increase in the air-to-steam ratio using wood shavings increased the syngas yield and energy content in anexperiment that kept the temperature constant by heating the reactor from the outside (Schoeters, 1989).Using feedlot manure as the biomass resource, increasing the air-to-steam ratio over a range of temperatureswas shown to increase carbon conversion and increase the volume and energy content of the syngas (Halligan,1975).

The effects of a number of different catalysts on biomass gasification have been examined. Addition ofcabonates (Na2CO3, K2CO3, CaCO3, NaHCO3 and KHCO3) increase syngas production with potassium carbonatebeing the most efficient when gasifying wood (Fung, 1980; Rolin, 1983; Hallen, 1985). Other catalysts thatincrease syngas production include limestone and calcium oxide (Fung, 1980; Walawender, 1981). Limestonewas also found to prevent agglomeration and affects the syngas composition and energy content (Walawender,1981). Inorganic salts (e.g. sodium tetraborate, potassium chloride, and lithium chloride) mixed with coffeehusks and spruce sawdust, significantly affected the yield of syngas and its chemical distribution. Additionallychar yields increased, while tar yields and the moisture content of the syngas decreased relative to gasificationof untreated fuels under similar conditions (Laichena, 1993). Alkali carbonates (Na2CO3 and K2CO3) increasechar formation, reduce tar formation, and increase CH4 reformation. The Na2CO3 catalyst did not reduce CH4and other hydrocarbon yields as has been reported with the use of other alkali carbonates (Brown, 1984;Mudge, 1988).

The gasification of a number of different biomass resources has been evaluated. The gasification of wheat strawwas studied by Sadaka (2002) who showed that the performance of the fluidized bed gasifier (temperature,pressure drop, syngas yields and energy content) was affected by fluidization velocity, steam flow rate andbiomass to steam ratio; by Risens (2003) who showed that adding calcium affects the ash chemistry andsintering tendency; and Walawender (1982) who gasified straw over a range of temperatures and demonstratedthat the energy content of the syngas exhibited parabolic variations with respect to temperature. Chen (1980)evaluated the gasification of rice husks from 600 to 700oC and found that the syngas yields and its energycontent increased as temperature increased. Singh (1986) gasified cotton stalks and found that the energycontent of the syngas was the same as for pure cellulose, but that the syngas yields, carbon conversion rates,and mass yield of the syngas were lower. Gas and tar yields, and the energy content of the syngas, were shownto increase with increasing temperatures for corncobs (Epstein, 1978) and the syngas contained high levels of


15 dari 19 4/7/2014 9:39 AM

CO and H2. Beck (1980), using oak sawdust, reported increased syngas yields and higher energy content withincreasing temperatures. Hoveland (1985) demonstrated that volatile cracking of alpha cellulose dominates atbelow 567oC and that water-gas shift reactions dominate above 567oC. Walawender (1981) gasified alphacellulose to obtain a medium Btu syngas composed mostly of H2, CO2, CO and CH4. Sweeten (2003) reportedthat feedlot manure has approximately half the heating value of coal, twice the volatile matter of coal, fourtimes the N content of coal on a heat basis, and due to soil contamination during collection, has 9-10 times theamount of ash as low ash (5%) coal. Raman (1981) found that the superficial velocity did not have a significantinfluence on syngas yield, composition, or energy content using feedlot manure. Halligan (1971) reportedincreasing syngas yield, energy content, and carbon conversion of feedlot manure over a temperature range of693oC to 796oC. Pan (2000) examined blends of biomass and coal and showed that blending improved theperformance of gasifying low-grade coal, and the possibility of converting refuse coal to a low-Btu gas. A blendratio of at least 20% pine chips for low-grade coal and 40% pine chips for refuse coal was most appropriate. Athermal efficiency of about 50% was achieved for the blends. The gasification of pyrolysis chars from severalbiomass resources (sunflower seed husks, pinecones, rapeseed, cotton refuse, and olive refuse) indicated thatthe gasification characteristics of biomass chars were dependent on the biomass properties (i.e., ash quantityand composition and fixed carbon contents).

ReferencesAlves, S.S. and J.L. Figueiredo. 1989. pyrolysis of wet wood. Chemical Engineering Science, 44(12):

2861-2869.Bailie, R.C. 1979. Hessleman Gas Generator Testing for Solar Energy Research Institute. Contract No.

AH-8-1077-1.Baker, E. L. Mudge and W. Wilcox. 1985. Catalysis of gas-phase reactions in steam gasification of biomass.

Fundamentals of Thermochemical Biomass Conversion. R. P. Overend, T. A. Milne and K. L. Mudge (eds).Elsevier Applied Science Publishers, London, UK, pp. 863-874

Beaumont, O. and Y. Schwob. 1984. Influence of physical and chemical parameters on wood pyrolysis. Ind. Eng.Chem. Process. Des. Dev., 23: 637-641.

Beck, S.R. and M.J. Wang. 1980. Wood gasification in a fluidized bed. Ind. Eng. Chem. Process. Des. Dev.,19(2): 312-317.

Bilbao, R., J.L. Lezaun, M. Menendez and J.C. Abanades. 1988. Model of mixing / segregation for sand - strawmixtures in fluidized beds. Powder Technology, 56: 149-151.

Black, J., K. Bircher and K. Chisholm. 1980. Fluidized bed gasification of solid wastes and biomass. Thermalconversion of solid wastes and biomass, Am. Chem. Socity, Washington D.C.: 351-361.

Brown, R. 2003. Biorenewable Resources. Engineering New Products from Agriculture. Iowa State Press. ABackwell Publishing Company.

Boateng, A.A., W.P. Walawender, L.T. Fan and C.S. Chee. 1992. Fluidized bed gasification of rice hull.Bioresource Technology, 40(3): 235-239.

Brink, D. 1981. Gasification. Organic Chemicals from biomass. Goldstein, I.S. (ed.). CRC Press. Boca Raton, Ch.4: 45.

Brown, M. L. Mudge and E. Baker. 1984. Catalysts for gasification of biomass. Biotechnology and BioengineeringSymmp. No. 14:125-136..

Butuk, N. R. Morey. 1987. Fluidized bed combustion and gasification of corn cobs. Transactions of the ASAE.32(2): 543-547.

Cao, Y., Y.Wang, J. Rieley and W. Pan 2005. A novel biomass air gasification process for producing tar-freehigher heating value fuel gas. Fuel Processing Technology, Volume 87, Issue 4, April 2006, Pages 343-353

Chen, C.C. and M.H. Rei. 1980. Gasification of rice husk. Presented at Bio-Energy 80 World Congress andExposition, Atlanta, GA, April 21.

Chen, T.H. and D.L. Day. 1982. Effect of Temperature change on the stability of thermophilic digestion of swinemanure. Agricultural Waste. 16:313 317.

Corella, J., J. Herguido and J. Gonzalez-Saiz. 1989. Steam gasification of biomass in fluidized bed-Effect of thetype of feedstock. In: Pyrolysis and Gasification. Ferrero, G.L., K. Maniatis, A. Buekens and A.V. Bridgwater(eds). Elsevier Applied Science, London. pp. 618-623.

Cranfield, R. 1978. solids mixing in fluidized beds of large particles, American Institute of Chemical Engineers


16 dari 19 4/7/2014 9:39 AM

Journal. 74(176):54-59.Edrich, R., T. Bradley and M. Grabboski. 1985. The gasification of ponderosa pine charcoal. Fundamentals of

Thermochemical Biomass Conversion. R. P. Overend, T. A. Milne and K. L. Mudge (eds). Elsevier AppliedScience Publishers, London, UK, pp. 557-566.

Elliot, D.C. and L.J. Sealock. 1985. Low Temperature Gasification of Biomass Under Pressure. In: Fundamentalsof Thermochemical Biomass Conversion. R. P. Overend, T. A. Milne and K. L. Mudge (eds). Elsevier AppliedScience Publishers, London, UK, pp. 937-951.

Epstein, E., H. Kosstrin and J. Alpert. 1978. Potential energy production in rural communities from biomass andwastes using a fluidized-bed pyrolysis system. IGT Symposium on Energy from Biomass and Wastes,Washington, D. C., Aug. 14-18.

Ergudenler, A. 1993. Gasification of Wheat Straw in a Dual-Distributor Type Fluidized Bed Reactor. UnpublishedPh.D. Thesis. Technical University of Nova Scotia. Nova Scotia. Canada.

Ergudenler, A. and A.E. Ghaly. 1992. Determination of reaction kinetics of wheat straw using thermogravimetricanalysis. Journal of Applied Biochemistry and Biotechnology, 34/35: 75-91.

Font, R., A. Marcilla, J. Deversa and E. Verdu. 1988. Gaseous hydrocarbons from flash pyrolysis of almondshells. Ind. Eng. Chem. Res., Vol. 27: 1143-1149.

Fung, D. and R. Graham. 1980. The role of catalyst in woood gasification. Thermal conversion of solid wastesand biomass. Jones, J.L. and Randing, S.B. (eds.) Am. Chem. Soc. Washington, D.C. 367-377.

Geldart, D. 1972. The effects of particle size and size distribution on the behavior of gas fluidized beds. PowderTechnology. 6:201-215.

Ghaly, A.E. and A.M. Al-Taweel. 1990. Physical and Thermochemical Properties of Cereal Straws, EnergySources 12:131-141.

Ghaly, A.E., A.M. Al-Taweel, F. Hamdullahpur and I. Ugwu. 1989. Physical and chemical properties of cerealstraw as related to thermochemical conversion. In: Proceedings of 7th. Bioenergy R&D Seminar. E.N. Hogan(eds). Ottawa, Ontario, pp. 655-661.

Groves, J.D., J.D. Craig, W.A. Le Pori and R. G. Anthony. 1979. Fluidized bed gasification of cotton gin waste.ASAE paper No. 79-4547. Presented at ASAE Wniter meeting, New Orleans, Louisiana.

Hallen, R.T., L.J. Sealock and R. Culleo. 1981. Influnce of alkali carbonates on biomass volatilization. In:Fundamentals of Thermochemical Biomass Conversion. R.P. Overend, T.A. Milne and L.K. Mudge (eds).Elsevier Applied Science Publishers, London. pp. 157-162.

Halligan, J.E., K.L. Herzog and H. W. Parker. 1971. Synthesis gas from bovine wastes. Ind. Eng. Chem. ProcessDes. Dev. 14 (1): 64-69.

Harris, D., D.Roberts, D. Henderson. 2006. Gasification behavior of Australian coals at high temperature andpressure. Fuel 85 (2006) 134142.

Horio, M., A. Nonaka, Y. Sawa and I. Muchi. 1986. A new similarity rule for fluidized bed scale up. Am. Inst. OfChem. Eng. Journal. 32(9): 1466-1482.

Hoveland, A.D., W.P. Walawender, L.T. Fan and F.S. Lai. 1982. Steam gasification of grain dust in a fluidizedbed reactor. Transactions of ASAE, 25(4): 1074-1080.

Huffman, G.P. F.E. Huggins and G.R. Dunmyre. 1981. Investigation of the high temperature behaviour of coalash in reducing and oxidizing atmospheres. Fuel, 60: 585-597.

Imc. 1980. Application of fluid bed technology to the gasification of waste wood. Final report. Volume I. ENFORProject c-12. DSS Contract 05SS. KL006-8-0249.

Laichena, J.K.1993. The Effect of Inorganic Salts on the Gasification of Biomass in a Fluidized Bed. UnpublishedPh.D. thesis. Technical University of Nova Scotia. Nova Scotia. Canada.

Lian, C.K. and M.E. Findley. 1982. Air blown wood gasification in large fluidized bed reactor. Ind. Eng. Chem.Process Dec. Dev. Vol. 21. : 699-701.

Liinanki, L., P.J. Svenningsson and G. Thessen. 1981. Gasification of agricultural residues in a downdraughtgasifier. In: Energy From Biomass. Palz, W., J.Coombs and D.O. Hall (eds). Elsevier Applied SciencePublishers, London, UK, pp. 832-832.

McLendon, T, A. Lui, R. Pineault, S. Beer and S. Richardson. 2004. High-pressure co-gasi_cation of coalandbiomass. Biomass and Bioenergy 26 (2004) 377 388

Mermoud, F., F. Golfier, S. Salvador,Van de Steene and J Dirion. 2006. Experimental and numerical study ofsteam gasification of a single charcoal particle. Combustion and Flame 145 (2006) 5979.

Mudge, L., M. Brown and W. Wilcox. 1988. Bench scale studies on fluid bed pyrolysis of wood. Report PNL-6114,


17 dari 19 4/7/2014 9:39 AM

Bacific Northwest Laboratory, Richland, Washington.Nandi, S.P. and M. Onischak. 1981. Gasification of Chars Obtained from Maple and Jack Pine Woods. In:

Fundamentals of Thermochemical Biomass Conversion. R.P. Overend, T.A. Milne and K.L. Mudge (eds).Elsevier Applied Science Publishers, London, UK, pp. 567-578.

Pan, Y. G., E. Velo, X. Roca, J. J. Manya, L. Puigjaner. 2000. Fluidized-bed co-gasification of residualbiomass/poor coal blends for fuel gas production. Fuel. Vol. 79:1317:1322.

Plante, P. C. Roy and E. Chornet. 1988. CO2 Gasification of Wood Charcoals Derived from Vacuum andAtmospheric Pyrolysis. Canadian Journal of Chemical Engineering, 66: 307-312.

Raman, K.P., W.P. Walawender, L.T. Fan and C.C. Chang. 1981. Mathematical model for fluid bed gasification ofbiomass material. Application to feedlot manure. Ind. Eng. Chem. Process Des. Dev., 20: 686-692.

Richard. J., M. Cathonnet and J. Rowan. 1985. Gasification of charcoal: Influence of water vapor. Fundamentalsof Thermochemical Biomass Conversion. R. P. Overend, T. A. Milne and K. L. Mudge (eds). Elsevier AppliedScience Publishers, London, UK, pp. 589-599.

Rolin, A.C. Richard, G. Martin and X. Deglise. 1983. Influence of Catalysts on Gasification of Biomass at ShortResidence Time. In: Energy From Biomass. Palz, W., J.Coombs and D.O. Hall (eds). Elsevier Applied SciencePublishers, London, UK, pp. 901-901.

Sadaka, S. S., A. E. Ghaly and M. A. Sabbah. 1998. Development of an air-steam fluidized bed gasifier. MisrJournal of Agricultural Engineering. Vol. 15(1): 47:52.

Sadaka, S. S., A. E. Ghaly and M. A. Sabbah. 2002. Two phase biomass air-steam gasification model forfluidized bed reactor: Part I, II, III. Biomass and Bioenergy. 22: 439-487

Sadakata, M., K. Takahashi, M. Saito and S. Takeshi. 1987. Production of gas fuel and char from wood, ligninand holocellulose by carbonization. Fuel, 66: 1667-1671.

Schoeters, J., K. Maniatis and A. Buekens. 1989. The fluidized bed gasification of biomass: Experimental studieson bench scale reactor. Biomass 19: 129-143.

Scott, D.S, J. Piscorz, M.A. Bergougnou, R. Graham and R.P. Overend. 1988. The Role of Temperature in FastPyrolysis of Cellulose and Wood. Industrial Engineering Chemical Research., 27: 8-11.

Singh, S.K., W.P. Walawender, L.T. Fan and W.A. Geyer. 1986. Steam gasification of cotton wood (branches) ina fluidized bed. Wood Fiber Sci., 18: 327-344.

Slapak, M. J., J. M. van Kasteren, A.A.Drinkenburg. 2000. Design of a process for steam gasification of PVCwaste. Resources, Conservation and Recycling. Vol. 30:8193.

Tillman, D.A. 1987. Biomass Combustion. Biomass: Regenerable Energy. Hall, D.O. and Overened, R. P. (eds).John Wiley and Sons, pp. 203-219.

Tomeczek, J., W.Kudzia, B. Gradon and L. Remarczyk. 1987. The Influence of Geometrical Factors andFeedstock on Gasification in a High Temperature Fluidized Bed. Canadian Journal of Chemical Engineering.65: 785-790.

Utioh, A.C, N.N. Bakshi and D.G MacDonald. 1989. Pyrolysis of grain screening in a batch reactor. CanadianJournal of Chemical Engineering, 67: 439-442.

Voloch, M., R. Neuman, M.R. Ladisch, R.M. Peart and G.T. Tsao. 1983. The Effects of Oxygen and Temperatureon Gas Composition from Gasification of Corn Cobs. Paper presented at the 1983 winter Meeting of ASAE,Chicago, Paper No. 83-3544.

Walawender, W.P. and L.T. Fan. 1978. Gasification of dried feedlot manure in a fluidized bed - preliminary pilotplant tests. Paper presented at the 84th National Meeting of American Institute of Chemical EngineersJournal , Atlanta, Ga, Feb. 27.

Walawender, W.P., D.A. Hoveland and L.T. Fan. 1982. Steam gasification of Alpha- cellulose in a fluid bedreactor, presented at Fundamentals of Thermochemical Biomass Conversion Conference, Estes Park, CO.

Walawender, W.P., S. Ganesan and L.T. Fan. 1981. Steam gasification of manure in a fluid bed: Influence oflimestone as a bed additive. IGT Symposium on Energy from Biomass and Wastes V, Lake Buena Vista, FA,Jan. 26-30.

Risnes, H., J. Fjellerup, U. Henriksen, A. Moilanen, P. Norby, K. Papadakis, D. Posselt, L. Srensen. 2003.Calcium addition in straw gasification. Fuel 82 (2003) 641651

Srinakruang, J., K. Sato, T. Vitidsant and K. Fujimoto. 2005. A highly efficient catalyst for tar gasification withsteam. Catalysis Communications 6 : 437440

Sweeten, J., K. Annamalai, B. Thien, L. McDonald. 2003. Co-firing of coal and cattle feedlot biomass (FB) fuels.Part I. Feedlot biomass (cattle manure) fuel quality and characteristics. Fuel 82: 11671182


18 dari 19 4/7/2014 9:39 AM

Watkinson, A., C. Cheng and C. Lim. 1987. Oxygen-steam gasification of coals in a spouted bed. Canadian.Journal of Chemical Engineers. Vol. 65: 791-798.

Weil, S.A., S.P. Nandi , D.V. Punwani and J.L. Johnson. 1978. Peat hydrogasification. Presented at 176th

National Meeting of ACS, Maiami, Fl.

Copyright 2007 Sun Grant Initiative and the University of Tennessee. The informationcontained in BioWeb is intended for educational use only. This resource is not intended to beused as a guide to investment, or for any purpose other than public education. Full disclaimeravailable at http://Bioweb.sungrant.org/disclaimer

Author: Samy Sadaka (edited by MarieWalsh)

Last Modified: 11/15/2008

Wander, P., C. Altafini and R. Barreto. 2004. Assessment of a small sawdust gasification unit. Biomass andBioenergy 27: 467476.

Yin, X., C. Zhi, S. Zheng and Y. Chen. 2002. Design and operation of a CFB gasi_cation and power generationsystem for rice husk Biomass and Bioenergy 23: 181 187.


19 dari 19 4/7/2014 9:39 AM

BioWeb

Documents

Transcript of BioWeb