Gasification of Densified Sludge and Wastepaper in a Downdraft Packed-Bed Gasifier

The co-disposal of densified sludge and wastepaper in a co-current flow packed bed gasifier represents new application of the thermal gasification process. Advantages of this technolo a gy include lower costs than other incineration or pyrolysis technologies, simple construction and operation, and the ability to a variety of fuels including agricultural wastes and other b.iomass materials in additio use n to densified sludge and wastepaper. The disposal of sewage sludge and ωlid wastes in an economic and environmentally acceptable manner is a problem common to many communities. The co-d isposal of sludge and wastepaper in a common facility is a potentialωlution to both of these problems. The feasibility of the gasification process for the co-d isposal of densified sludge andωurce separated wastepaper is examined in this review. THE GASIFICATION PROCESS The gasification of densified sludge and wastewater , a detailed description of a downdraft gasifier, and the important operating parameters that affect the gasification process are reviewed in this section. Gasification of Densified Sludge and Wastepaper Gasification involves the partial combustion of a carbonaceous fuel to generate a combustible gas (producer gas) containing carbon monoxide, hydrogen, and some hydrocarbon gases, and a char rich in carbon. A process flow diagram for a complete sludge and wastepaper gasification system is shown in Figure 1. The key elements of the system are: fuel processing, gasification, and gas utilization. To gasify sludge and wastepaper in a downdraft gasifier, fuel processing is required. A suitable gasifier fuel can be made from source separated wastepaper, sludge, and. woodchips by shredding, mixing, and densifying the fuel components. A densification system operated by the Papakube Corporation of San Diego, California was utilized to produce sludge/wastepaper fuels. The Papakube densification system includes an integral shredder, a metering system that allows moistening of the fuel 的 the optimum moisture content, and a modified agricultural feed cubero The producer gas can either be used to power an internal combustion engine to generate power or as a fuel for a boiler. Description of Downdraft Gjlsifier Developed originally to reduce the quantities of tar in the producer gas, a downdraft gasifier typically is composed of three subassemblies: 1) a fuel hopper, 2) a firebox, and 3) a charpit. Fuel flow in downdraft gasifiers is by gravity with air and fuel moving co-currently through the reactor. The University of Ca lifornia, Department of Civil Engineering pilot scale batch-fed downdraft gasifier is shown in Figure 2a. The fuel hopper is constructed as double wall cylinder. The double wall acts as a condensor to remove the water vapor from the fuel prior to gasification. The inner cylinder is in the form of a truncated cone to reduce the tendency for fuel bridging. The firebox is alωa double walled cylinder. The inner cylinder is the actual firebox. Ai r is supplied by four tubes to the annular space between the walls which 兒rves as an air plenum to distribute air evenly to the six tuyeres (air nozzles) that supply air for partial combustion of the fue l. The choke plate is essentially a large SOURCE SEPARATED ~ WASTEPAPER BOILER STEAM DEWATERED WASTEWATER SLUDGE DRIED WOODCHIPS STORAGE ENGINE/ DENSIFIER • .. AND • • .. GASIFIER SHREDDER ←寸, ELECTRICITY GENERATOR DRYING