The gasification of subbituminous coal, activated carbon, coke and a mixture of coal and biomass by direct solar irradiation in a solar furnace is investigated. Sunlight concentrated by a 23-kW solar furnace was focused directly on the fuel being gasified in a gravity-fed gasifier through a window in the reactor, and steam or CO2 was passed through the bed to react with the fuel and form a combustible product gas. Experiments performed with coal and steam resulted in the conversion of more than 40% of the sunlight arriving at the reactor focus into chemical fuel, with production rate increasing with solar power and product gas composition and thus gas heating value remaining constant. A typical moisture-free gas composition obtained consists of 54% H2, 25% CO, 16% CO2, 4% CH4 and 1% higher hydrocarbons. Experiments with activated carbon and a uniform mixture of coal and biomass resulted in similar conversion efficiencies but slightly different product gas compositions, while coke showed a lower efficiency. Advantages of solar gasification over conventional oxygen-blown gasifiers are indicated.

This paper describes the BIOTHERMGAS process for conversion of biomass and wastes to substitute natural gas (SNG) and/or medium-Btu gas. This process combines biological and thermochemical unit operations into a scheme that can efficiency convert the full spectrum of biomass or waste feedstocks (regardless of moisture and nutrient contents) to methane or other fuel products with minimum process residues. The BIOTHERMGAS process employs biogasification followed by thermochemical gasification of dewatered refractory digester residues. Nitrogen and other inorganic nutrients are recycled from the thermal process effluent to the biogasification unit. The product gas from the thermochemical gasifier can be converted to methane either by catalytic methanation or by biomethanation. The waste heat from thermal product gases is used to supply the heat requirement of the bioconversion component. The preliminary systems analyses were conducted on five applications of the BIOTHERMGAS process: three using biomass (Bermuda grass) and two using municipal wastes as feedstocks.

Methane and methanol synthesis gas can be produced by steam gasification of biomass in the presence of appropriate catalysts. This concept is to use catalysts in a fluidized bed reactor which is heated indirectly. The objective is to determine the technical and economic feasibility of the concept. Technically the concept has been demonstrated on a 50 lb per hr scale. Potential advantages over conventional processes include: no oxygen plant is needed, little tar is produced so gas and water treatment are simplified, and yields and efficiencies are greater than obtained by conventional gasification. Economic studies for a plant processing 2000 T/per day dry wood show that the cost of methanol from wood by catalytic gasification is competitive with the current price of methanol. Similar studies show the cost of methane from wood is competitive with projected future costs of synthetic natural gas. When the plant capacity is decreased to 200 T per day dry wood, neither product is very attractive in today's market.

The advantages of biomass as a feedstock are examined and biomass conversion techniques are described. Combustion is the simplest method of producing heat from biomass, using either the traditional fixed bed combustion on a grate or the fluidized bed and suspended combustion techniques now being developed. Pyrolysis of biomass is a particularly attractive process if all three products gas, wood tars, and charcoal can be used. Gasification of biomass with air is perhaps the most flexible and best developed process for conversion of biomass to fuel, yielding a low energy gas that can be burned in existing gas/oil boilers or in engines. Oxygen gasification yields a gas with higher energy content that can be used in pipelines or to fire turbines. In addition, this gas can be used for producing methanol, ammonia, or gasoline by indirect liquefaction. Fast pyrolysis of biomass produces a gas rich in ethylene that can be used to make alcohols or gasoline. Finally, treatment of biomass with high pressure hydrogen can yield liquid fuels through direct liquefaction.

Results of the experimental work conducted thus far show that wood can be readily gasified in a steam environment into a hydrocarbon rich fuel gas that can be used as a replacement for petroleum based fuels or natural gas with minimal boiler retrofit. Further, this conversion can be achieved in a compact gasification reactor with heat supplied by a circulating entrained phase, thereby eliminating the need for an oxygen plant. Tars were found except at the lowest gasifier temperatures employed, and therefore heat recovery from the product gas should be much simpler than that from commercially available fixed bed gasification systems where product gas contains significant quantities of tar. The data generated were used in a preliminary conceptual design. Evaluation of this design shows that a medium Btu gas can be produced from wood at a cost competitive with natural gas or petroleum based fuels.