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Thermal gasification is a new process for the co-disposal of densified sludge and solid waste in a co-current flow, fixed bed reactor (also called a downdraft gasifier). The advantages of this technology include lower costs than other incineration or pyrolysis technologies, simple construction and operation, and the ability to use a variety of fuels including agricultural wastes and other biomass materials in addition to densified sludge and solid waste. Essentially, the gasification process involves the partial combustion of a carbonaceous fuel to generate a low energy combustible gas and a char. Operationally, fuel flow is by gravity with air and fuel moving co-currently through the reactor. The low energy gas is composed primarily of carbon monoxide, hydrogen, and nitrogen and of trace amounts of methane and other hydrocarbons. Although fixed bed gasifiers are mechanically simpler than other codisposal reactors, such as multiple hearth furnaces or mass fired incinerators, they have more exacting fuel requirements which include: moisture content, <20 percent; ash content, <6 percent; and relatively uniform grain size. Without front end processing, neither municipal solid waste nor dewatered sludge meet these criteria. Demonstrating that a suitable gasifier fuel could be made with a simple front end system consisting of source separation for solid waste, a sludge dewatering system, and fuel densification system has been one of the objectives of this project. To demonstrate the gasification process, a pilot scale gasifier was constructed. A broad range of fuels have been tested with the gasifier including an agricultural residue, densified waste paper, and densified waste paper and sludge mixtures containing up to 25 percent sludge by weight. The sludge fuels were made from mixtures of lagoon-dried primary and secondary sludge and from recycled newsprint (in full scale systems a mixed paper fraction of solid waste could be used). Mixtures were densified using commercially available agricultural cubing equipment. The gasifier was operated with each fuel, and measurements of the variables needed to characterize the process were made. The results of gas, fuel, and char analyses were used to compute energy balances. These data were also used to calculate efficiencies for each run. Hot gas efficiency. which include the sensible heat of the gas, ranged from 40.0 to 85.2 percent. The cold gas efficiency, which does not include the gas sensible heat, ranged from 37.1 to 80.7 percent. The dry low energy gas produced during the tests ranged in a higher heating value (HHV) from 4.52 to 6.79 MJ/m3. |
