A dual fluid-bed process for biomass was developed in the United States in the 1970s and early 1980s by the EEE Corporation (Bailie, 1980, 1981). It was commercialized in Japan by EBARA Corporation. This process, called the Bailie process after its inventor, consists of two circulating fluid-bed reactors that permit the use of air instead of oxygen for conversion of biomass to medium-energy gas. In one bed, feedstock combustion occurs with air to heat the sand bed. The hot sand is circulated to the other reactor where steam gasification of fresh feed and recycled char occurs. The cooled sand is recircu­lated to the combustion reactor for reheating. This configuration produces a product gas with a heating value of 11.8 MJ/m3 (n) or more. The composition of the gas from the gasifier operated at 650 to 750°C in one of the pilot plants fed with RDF in Japan was 34.7 mol % carbon monoxide, 11.2 mol % carbon dioxide, 12.7 mol % methane, 8.0 mol % other hydrocarbons, 30.0 mol % hydrogen, 2.5 mol % nitrogen, and 0.9 mol % oxygen. The heating value was 17.6 MJ/m3 (n). The pilot plant data indicated that 60% of the carbon resided in the medium-energy gas, 30% was converted to char, and the remaining 10% formed liquid and char. The energy yield as medium-energy gas was between 50 and 60%. The plants operated with RDF feedstocks in Japan were a 36-t/ day pilot plant, a 91-t/d demonstration plant, and a 408-t/day commercial plant. These plants have been shut down.

Indirectly Heated, Dual Fluid-Bed, Steam Gasification

This process was developed by Battelle in the 1980s in a dual-bed PDU having a capacity of 20 to 25 t/day (Paisley, Feldmann, and Appelbaum, 1984; Paisley, Litt, and Creamer, 1991). Heat is supplied by recirculating a stream of hot sand between the separate combustion vessel and the gasifier. The PDU used a conventional fluid-bed combustor. In a commercial plant, both the gasifier and the combustor would be operated in the entrained mode to achieve higher throughputs. Tests have been conducted with wood and RDF. The operating ranges of the gasifier in the PDU were 630 to 1015°C at near-atmospheric pressure. The largest gasifier used was 0.25 m inside diameter and had a maximum wood throughput of 1.7 t/h. The heating value of the product gas was 17.7 to 19.6 MJ/m3 (n) and was reported to be independent of the moisture level of the feed. A thermally balanced operation with wood feedstock was achieved at throughputs of 1.5 t/h. Combustor carbon utilization was complete at temperatures above 980°C, and gasifier carbon conversion to gas was 50 to 80% at temperatures above 705°C. Typical nitrogen-free gas compositions were 50.4 mol % carbon monoxide, 9.4 mol % carbon dioxide, 15.5 mol % methane, 7.2 mol % ethane and ethylene, and 17.5 mol % hydrogen. Carbon conversions with RDF were similar to those of wood over a temperature range of 650 to 870°C. The heating values of the product gases were about 21.6 to 23.6 MJ/ m3 (n). A commercial plant based on this process has been built to supply

fuel gas to a central station power plant in Vermont (Paisley and Farris, 1995; Farris and Weeks, 1996).