Indirect gasifiers

Indirect gasifiers give rise to complete conversion of the fuel without the need for high temperatures (<900°C to avoid ash melting) and can operate at high pressures. However, some limitations still need to be addressed such as the potential need for external heat in the gasification reactor and the excess of steam fed to this chamber. 

SL/RN Process. In the SL/RN process (Fig. 4), sized iron ore, coal, and dolomite are fed to the rotary kiln wherein the coal is gasified and the iron ore is reduced. The endothermic heat of reduction and the sensible energy that is required to heat the reactants is provided by combustion of volatiles and carbon monoxide leaving the bed with air introduced into the free space above the bed. The temperature profile in the kiln is controlled by radial air ports in the preheat zone and axial air ports in the reduction zone. Part of the coal is injected through the centerline of the kiln at the discharge end. The hot reduced iron and char is discharged into an indirect rotary dmm cooler. The cooled product is screened and magnetically separated to remove char and ash. 

Background Indirect coal liquefaction differs fundamentally from direct coal hquefaction in that the coal is first converted to a synthesis gas (a mixture of H9 and CO) which is then converted over a catalyst to the final product. Figure 27-9 presents a simplified process flow diagram for a typical indirect coal hquefaction process. The synthesis gas is produced in a gasifier (see a description of coal gasifiers earlier in this section), where the coal is partially combusted at high temperature and moderate pressure with a mixture of oxygen and steam. In addition to H9 and CO, the raw synthesis gas contains other constituents (such as CO9, H9S, NH3, N9, and CHJ, as well as particulates. 

Status of Indirect Liquefaction Technology The only commercial indirect coal liquefaction plants for the production of transportation fuels are operated by SASOL in South Africa. Construction of the original plant was begun in 1950, and operations began in 1955. This plant employs both fixed-bed (Arge) and entrained-bed (Synthol) reactors. Two additional plants were later constructed with start-ups in 1980 and 1983. These latter plants employ dry-ash Lurgi Mark IV coal gasifiers and entrained-bed (Synthol) reactors for synthesis gas conversion. These plants currently produce 45 percent of South Africa s transportation fuel requirements, and, in addition, they produce more than 120 other products from coal. 

Liquids can also be synthesized via an indirect scheme where the coal is first gasified in an intermediate step. The coal is pulverized and reacted with steam to produce water gas, an equimolar mixture of carbon monoxide and hydrogen ... 

Indirect coal liquefaction is a technology in which coal is first gasified to synthesis gas ("syngas," CO + H2), which is used to synthesize liquid fuel by the Fischer—Tropsch process. 

The gasifiers considered in this overview can be differentiated into direct (autothermal) and indirect (allothermal) processes. In the indirect processes, the heat needed to drive the endothermic heterogeneous reactions is provided by inert (circulating) solid material or by heat exchangers. The processes are... 

Two major types of gasifiers currently in development are direct-fired gasifiers, using air, and the indirect-fired method, where heated sand surrounds biomass and gasifies it. These design schemes are described in Chapter 3. 

The indirect-fired process gasifies biomass at low pressure, using indirect gasification to produce gases with medium... 

The D.M.2 gasifier is also based on an indirectly heated gasification process. The heat needed for the gasification stage is introduced into the process by a heat carrier (e.g., spheres of corundum). One possible process layout for hydrogen production based on the staged-reforming process is shown in Fig. 10.4. 

Spath, P., Aden, A., Eggeman, T. el al. (2005). Biomass to Hydrogen Production -Detailed Design and Economics Utilizing the Battelle Columbus Laboratory Indirectly-heated Gasifier. Battelle National Renewable Energy Laboratory (NREL). www.nrel.gov/docs/fy05osti/37408.pdf. 

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