Final disposal of enormous wastewater sludge in Taiwan becomes an environmental issue since most landfill sites will be closed in the next decade. The low heat value and high ash content in sludge restrict its energy application using thermochemical processes. Co-firing of sludge with the abundant forestry wastes like wood chips in Taiwan seems a feasible alternative. The heat value of mixture can be elevated accordingly. The existence of woody materials mitigates the typical operation problems like sand sintering and clogging in commercial fluidized reactors due to the sludge ashes. This study conducts the financial analysis and life cycle assessment of the co-firing process, pyrolysis and gasification, based on the running data in a commercial cogeneration power plant in Taichung (central Taiwan), along with the pilot-scale tests of pyrolysis (at 500oC) and gasification (at 800oC). In the scenario, the sludge and wood chips are conveyed from one wastewater treatment plant in Taichung and an experimental forest in Nantou, respectively, to the cogeneration power plant. The cost and environmental impacts from the major stages of this process are considered, including dewatering, pelleting, logistics, furnace operation, derived pollution control (wastewater and flue gas) and product application. The results show that among the three processes, the conventional co-firing in the cogeneration power plant is the most economically feasible, though the environmental impacts are highest, mainly from the aspects of greenhouse effects and acid rain. The pyrolysis costs less than the gasification, and the derived environmental impacts are also fewer than gasification.
Yu-De Huang, Sinotech Engineering Consultants, Taiwan
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