In the 1970s and early 1980s, about 40 companies worldwide offered to build biomass gasification plants for different applications. Since then, many of the smaller companies and some of the larger ones have gone out of business, discontinued biomass gasification projects, or emphasized established biomass combustion technologies. The problems encountered in first-of-a-kind biomass gasification plants and the low prices of petroleum and natural gas all had an adverse impact on the marketability of biomass gasification technologies. Sev­eral of the plants built in North America in the 1970s and 1980s have been

shut down, dismantled, or placed on standby. A survey of commercial thermal biomass gasification showed that few gasifiers have been installed in the United States (Miles and Miles, 1989). Most of the units in use are retrofitted to small boilers, dryers, and kilns. The majority of the existing units operate at rates of 0.14 to 1.0 t/h of wood wastes on updraft moving grates. In the United States, many purveyors of biomass gasification technologies have gone out of business or are focusing their marketing activities in other countries or on other conversion technologies, particularly combustion for power generation, in states where combined federal and state incentives make the economic factors attractive. Some existing gasification installations have also been shut down and placed in a standby mode until natural gas prices make biomass gasification competitive again.

Examination of state-of-the-art thermal biomass gasification technology shows that moving-bed gasifiers have been studied and extensively tested

(Babu and Whaley, 1992). Nine atmospheric-pressure updraft gasifiers were commercialized from 1982 to 1986 in Europe and have been successfully operated with wood and peat. Six plants were placed in operation for close — coupled district heating purposes in Finland, while three plants were built in Sweden for district heating and drying wood chips (Kurkela, 1991). In general, the moving-bed systems require close control of feedstock size and moisture content and appropriate means to handle the high tar content of the raw product gases.

The Winkler fluid-bed coal gasifier was successfully scaled up to gasify 25 dry t/h of peat in 1988 by Kemira Oy in Finland (Babu and Whaley, 1992). The product gas was used for the manufacture of ammonia. Major mechanical and process modifications included improvements to the peat lockhopper feeding system, and the control of naphthalene formation by using higher gasifier temperatures and the addition of a benzene scrubber for naphthalene removal. The application of fluid-bed gasifiers to wood and other types of biomass has been commercialized in North America by Omnifuel in Canada and by Southern Electric International, Inc., in Florida, both of which are described later, and Energy Products of Idaho, Inc. The largest and most successful fluid-bed biomass gasification plants to date have been attributed to the Ahlstrom and Gotavarken circulating, fluid-bed gasifiers employed in close-coupled operation with lime kilns in Sweden, Finland, and Portugal (Babu and Whaley, 1992). The gasifiers are about 2 m in diameter and range in height from 15 to 22 m. They are operated at near atmospheric pressure at about 700°C with circulating limestone and are capable of handling mixtures of sawdust, screening residues, and bark. A large-scale circulating fluid-bed gasifier was built in 1992 by Studsvik AB for gasifying RDF (refuse-derived fuel) (Rensfelt, 1991).

These biomass gasifiers are representative low-pressure technologies, which when combined with current state-of-the-art gas-cleanup systems render them­selves suitable for close-coupled operation with lime kilns, furnaces, boilers, and probably advanced, combined-cycle power systems. However, from the standpoint of producing methanol, gasification under elevated pressure and temperature is preferred because the equipment size is reduced for the same throughput, the cost of recompression before methanol synthesis should be less, and the noncondensable hydrocarbons and tars are only present in low concentrations in the spent water. The opposing effects of temperature and pressure on CrC4 light hydrocarbon yields can be optimized to afford low yields of these products by careful selection of operating temperature and pressure.

At least five industrial-scale biomass gasifiers were available commercially from U. S. manufacturers in the 1990s. A two-stage stirred-bed gasifier is avail­able from Producers Rice Mill Energy. The company built three gasifiers of 11 to 18 t/day capacity in Malaysia for rice hull feedstocks. Sur-Lite Corporation built small-scale, fluid-bed gasifiers of up to 10 GJ/h capacity for cotton-gin trash in California and Arizona, for rice husks in Indonesia, and for wood and coal in White Horse, Canada. Morbark Industries, Inc. supplies two-stage, starved-air gasification-combustion systems. The units in operation include a 4-GJ/h system for a nursing home in Michigan and a 1-GJ/h system for heating private facilities. Energy Products of Idaho supplies fluid-bed gasifiers to pro­duce low-energy gas. The company has constructed a 57-GJ/h plant in Califor­nia to fuel a boiler, a 99-GJ/h plant in Missouri to fuel a dryer, and an 85-GJ/ h plant in Oregon to generate 5 MW of power from steam. Southern Electric International, a subsidiary of The Southern Company, coordinated the design and construction of a 264-GJ/h, fluid-bed, wood gasification plant in Florida, which has since been dismantled and moved to Georgia. It is described later.

A few representative biomass gasification processes that have been commer­cialized or that are near commercialization are described here to illustrate some of the details of gasifier designs and the operating results. The biomass pyrolysis plants described in Chapter 8 are not discussed here because the major products are liquids and charcoals, and the by-product gases are used for plant fuel.

Pyrolysis and Partial Oxidation with Air of MSW in a Rotating Kiln

Monsanto Enviro-Chem Systems, Inc., developed an MSW pyrolysis process called the Landgard process through the commercial stage (U. S. Environmental Protection Agency, 1975; Klass, 1982). A full-scale, 1050-t/day plant was built in Maryland and placed in operation in the mid-1970s. The plant was designed to operate for 10 h/day and to accept residential and commercial solid waste typical of U. S. cities. MSW disposal was the primary objective of the plant, not energy recovery. Large household appliances, occasional tires, and similar materials were acceptable feeds; automobiles and industrial wastes were ex­cluded. The process included several operations: shredding of the MSW from storage in 900-HP hammer mills to provide particles small enough (4-cm diameter) to fall through the grates, storage of the shredded MSW which had a heating value of 10.7 MJ/kg, feeding of the shredded MSW to the pyrolysis reactor by twin hydraulic rams, pyrolysis, gas processing, and gas utilization in two waste heat boilers which generated 90,700 kg/h of steam, and processing of the ungasified residue to remove ferrous metals. Pyrolysis took place in a refractory-lined, horizontal, rotary kiln, which was 5.8 m in diameter and 30.5 m long. The kiln was rotated at 2 r/min. The heat required for pyrolysis was provided by burning the MSW with 40% of the theoretical air needed for complete combustion, and supplemental fuel (No. 2 fuel oil) was supplied at a rate of 24.4 L/t of waste. The fuel oil burner was located at the discharge end of the kiln. Pyrolysis gases moved countercurrent to the waste and exited at the feed end of the kiln. The gas temperature was controlled to 650°C, and the residue was kept below 1100°C to prevent slagging. The product gas on a dry basis had a heating value of 4.7 MJ/m3 (n) and consisted of about 6.6 mol % hydrogen, 6.6 mol % carbon monoxide, 11.4 mol % carbon dioxide, 2.8 mol % methane, 1.7 mol % ethylene, 1.6 mol % oxygen, and 69.3 mol % nitrogen. The plant was shut down in January 1981 and was scheduled to be replaced by a direct combustion system. Cost and reliability were cited as the reasons for the change.