Thermal Conversion Processes

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A variety of pyrolysis processes have been developed to convert municipal solid wastes and agricultural and wood residues to solid, liquid and gaseous fuels. The thermal process can be designed to maximize the desired end products since the quantity of char, bitumen-like liquid and gas produced varies as a function of the time and temperature sequence used during pyrolysis. Lower heating rates and lower temperature produce higher amounts of solid fuel or char.

Various processes for producing gas from refuse are also in various stages of technological development. Basically, gas is produced either by pyrolysis or biological conversion. Among the more successful pyrolysis systems is the Purox process developed by Union Carbide while the Dynatech Process biologically converts domestic refuse into high-grade methane gas.

Other technologies that are undergoing rapid development include the conversion of garbage and other organic solid wastes into gasoline (trash-togasoline process). A series of non-catalytic petrochemical processes can convert organic wastes to a synthetic hydrocarbon crude oil containing about 90 percent high-octane gasoline and 10 percent fuel and lubricating oils. The process involves pyrolysis, gas purification, and polymerization.

Wood wastes and other agricultural residues can also be converted to oil. This can be achieved by biomass liquefaction or by reductive formylation. The oils obtained are basically heavy fuel oils comparable to no. 6 fuel oil. They are largely aliphatic, have a low sulfur content and a heat value of about 35,000 J/g (15,000 Btu/lb).

Organic municipal waste and other biomass wastes, such as sawdust and animal manure, can be liquefied to produce an oil-like product. In one process, 30 parts of dry wood flour is blended with 70 parts of vehicle oil, and 7.5 parts of 20 percent aqueous solution of sodium carbonate. The mixture is heated to over 370°C under carbon monoxide or synthesis gas pressure to a final total pressure of about 17,000 kPa (2,500 psi) and for a total residence time ranging from 45 to 60 minutes. If completely converted, 30 parts of wood would yield about 17.4 parts oil having a heating value of about 31,300 J/g (13,500 Btu/lb).

The production of fuel oil through reductive formylation involves heating the solid waste slurry to 300-350°C range at pressures ranging from 1,300 to 3,000 psig for 5 to 10 minutes in the presence of a source of formate, usually formic acid or its compounds. The oils produced have heat of combustion of 35,000 J/g (15,000 Btu/lb) compared to about 40,000 J/g (17,500 Btu/lb) for no.6 fuel oil.

Another approach to the conversion of cellulosic wastes to liquid fuels uses indirect liquefaction, that is, thermal gasification followed by liquid fuel synthesis. The use of catalysts is incorporated in the reaction steps and the process is capable of using a variety of feedstocks from urban, agricultural forest and industrial sources. Quality products can be produced with anticipated yields of 200 to 400 liters of liquid fuel per ton (dry, ash-free) of feedstock.

The scope for research and development in the field of solid waste conversion to liquid fuels is broad and offers a lot of opportunities. However, many of the technologies are in the early experimental stage and have not reached full pilot scale trials.

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