Gasification

Twin Fluid-Bed Steam Gasification The SilvaGas Process

This two-stage atmospheric biomass gasification process was developed by Battelle, and the first commercial demonstration unit with a feed capacity of 200t/d was built in Burlington, Vermont. Commercialization of the process has been taken over by Future Energy Resources (FERCO), who market it under the name of SilvaGas. The medium Btu gas at the demonstration unit is fired in an existing biomass fired boiler and is planned to be used later in a combustion turbine (Paisley, and Overend 2002).

The principle of the SilvaGas process (see Figure 5-37) is similar to that of a catalytic cracker in an oil refinery or of the Exxon Flexicoker process. In all these processes two fluid-bed reactors are used. In one, an endothermic process takes place; in the SilvaGas process, for the gasification of biomass. The necessary heat for the reaction is supplied by a hot solid (sand, catalyst, or coke), which is heated by an exothermic reaction in the second reactor.

As in all biomass gasification processes, a feed preparation stage is necessary in which the biomass is reduced to 30-70 mm-length chips and oversize or foreign material such as metals are removed. The biomass is fed to the gasifier where it is mixed with hot sand (at about 980°C) and steam. During the ensuing endothermic cracking reaction, light gaseous hydrocarbons are formed together with hydrogen and carbon monoxide. After separating the heat carrier and the gas in cyclones, the relatively cold heat carrier and residual unreacted char are discharged to the combustor or regenerator. The sand is reheated in the combustor by burning the char with air. The reheated sand is removed from the flue gas by a cyclone separator and returned to the gasifier.

The syngas from the gasifier still contains typically about 16g/m3 tars. Depending on the application (e. g., for gas turbine fuel), these must be removed. Cracking catalysts, as used in the petroleum industry, are used to break down the heavy hydrocarbons. Work is continuing to find lower-cost disposable catalysts for this application. The syngas is cleaned up in a scrubber for alkali and particulate removal. A typical gas composition from the Burlington demonstration unit is shown in Table 5-17.

The flue gas is a valuable source of heat. Using it for pre-drying of the biomass feed helps increase the efficiency of the process, but alternative uses such as steam production may be applied if site-specific conditions favor this.

The FICFB Process

The FICFB (fast internal circulating fluid-bed) process developed by the Vienna University of Technology in Austria is another process that separates steam gasification of the biomass from combustion of char as a source of heat for the former (see Figure 5-38). A 42t/d feed commercial demonstration combined heat and power (CHP) unit has been built in the town of Giissing, where it is integrated into the operations of the local district heating utility. The synthesis gas is fired in a gas motor generating 2MWe and 4.5 MW heat is supplied to domestic and industrial consumers. The plant was taken on stream in December 2001.

The gasifier operates as a stationary fluid-bed reactor with sand as the fluidizing medium. The sand and ungasified char leave the reactor at the bottom and are trans­ferred to the combustor where the char is burnt to heat the sand. The hot sand is separated from the flue gas in a cyclone and returned to the gasifier via a seal leg bringing in the necessary heat for the gasification reaction, which takes place at about 850°C. The synthesis gas is cooled and cleaned for use in a gas motor. Of note is the use of an oil wash to remove tars. In the demonstration unit in Giissing RME (Rape methylester) is used as washing oil (Hofbauer 2002). Gas compositions are given in Tables 5-18 and 5-19.