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BC Bugwood Envirochem Services Inc. The co-current fixed bed ("down draft") gasifier is similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name "down draft gasifier"). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added at the top of the bed, resulting in the same efficiency as the counter-current type. Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the counter-current type. Fluidized Bed Gasifiers - Fluidized bed gasifiers come in two forms, direct and indirect. In the direct fluid bed gasifier, the fuel is fluidised in air, oxygen or steam. The ash is removed dry or as heavy agglomerates that de-fluidize. Fuel throughput is higher than for the fixed bed, but not as high as for the entrained flow gasifier. The conversion efficiency is rather low, so recycle or subsequent combustion of solids is necessary to increase conversion. Fluidised bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of gasifiers. Biomass generally contains high levels of such ashes. In indirect fluidized gasifiers, gasification is accomplished using steam as an oxidant. However, steam reforming of biomass is endothermic and often heat transfer limited. Endothermic gasification generates more methane than direct gasification per volume of gas, so the energy density may be higher. However, additional methane conversion may be required for methanol applications. The thermal input required for steam reforming of biomass means that a high heat transfer rate is required. Known methods of indirect heat transfer include heat exchangers embedded in the gasification zone (MTCI/Thermochem Recovery Intl.), circulating preheated sand (Battelle/FERCO, Inc.), and cycling of phase change materials (Iowa State University). Steam gasification is thermodynamically more efficient than direct gasification, but practical heat transfer limitations and thermodynamic availability requirements for high-temperature heat exchange often makes reality a bit different. Even so, the main benefit of indirect gasification is that it can eliminate the need for an oxygen plant in a syngas application. Entrained Flow Gasifiers - In an entrained flow gasifier, a dry pulverized solid, an atomized liquid fuel or a fuel slurry is gasified with oxygen (much less frequently with air) in a co-current flow. The gasification reactions take place in a dense cloud of very fine particles. The high temperatures and pressures also mean that a higher throughput can be achieved; however thermal efficiency is somewhat lower as the gas must be cooled before it can be cleaned with existing technology. The high temperatures also mean that tar and methane are not present in the product gas; however the oxygen requirement is higher than for the other types of gasifiers. All entrained flow gasifiers remove the major part of the ash as a slag as the operating temperature is well above the ash fusion temperature. A smaller fraction of the ash is produced either as a very fine dry fly ash or as a black-coloured fly ash slurry. Some fuels, in particular certain types of biomass, can form slag that is corrosive for ceramic inner walls that serve to protect the gasifier outer wall. However, some entrained bed type of gasifiers do not possess a ceramic inner wall but have an inner water or steam-cooled wall covered with partially solidified slag. These types of gasifiers are resistant to corrosive slags. Some fuels have ashes with very high ash fusion temperatures. In this case, limestone is usually mixed into the fuel prior to gasification. Addition of a little limestone will usually suffice for the lowering the fusion temperatures. The fuel particles must be much smaller than for other types of gasifiers. This means the fuel must be pulverised, which requires somewhat more energy than for the other types of gasifiers. By far the most energy consumption related to entrained bed gasification is not the milling of the fuel but the production of oxygen used for the gasification. Figure 4.6.6 shows the various gasifier configurations. Page 63PDF Image | IDENTIFYING ENVIRONMENTALLY PREFERABLE USES FOR BIOMASS
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