ENERGY FROM MUNICIPAL SOLID-WASTE - A COMPARISON WITH COAL COMBUSTIONTECHNOLOGY

The conversion of municipal solid waste (MSW) to energy can conserve m ore valuable fuels and improve the environment by lessening the amount of waste that must be landfilled and by conserving energy and natural resources. The importance of utilizing MSW was recognized in the 1991 U.S. National Energy Strategy, which sought to ''support the conversi on of municipal solid waste to energy.'' One route to utilizing the en ergy value of MSW is to burn it in a steam power plant to generate ele ctricity. Coal has long been the predominant source of energy for elec tricity production in the U.S.; therefore, a considerable science and technology base related to coal combustion and emissions control can b e, and has been, applied with substantial benefit to MSW combustion. T his paper compares the combustion of coal and MSW in terms of fuel cha racteristics, combustion technology, emissions, and ash utilization/di sposal. Go-combustion of coal and MSW is also discussed. MSW issues th at can be addressed by research and development are provided. The majo r environmental issues that designers of MSW combustion systems have h ad to address are emissions of trace organic compounds, particularly p olychlorinated dioxins and furans, and trace elements such as mercury, lead, and cadmium. Emission of trace organics is generally the result of a poorly designed and/or operated combustion system; modern MSW sy stems use good combustion practices that destroy organic compounds dur ing the combustion process. Proper control of air/fuel mixing and temp erature, and avoidance of ''quench'' zones in the furnace, help to ens ure that potentially harmful organics are not emitted. Computer codes and other design and troubleshooting tools that were developed for coa l combustion systems have been applied to improve the performance of w aste-to-energy systems. Trace element emissions from both coal and MSW combustion result primarily from vaporization of elements during the combustion process. Most of the trace elements that are vaporized cond ense on fly ash as the combustion products cool downstream of the furn ace and can be effectively controlled by using an efficient particulat e removal device. However, volatile elements, particularly mercury, ar e emitted as a vapor. Several mechanisms are available to capture merc ury vapor and some are in use. The development of satisfactory control technology for mercury is a topic currently of high interest in coal burning. The potential for leaching of trace elements and organics fro m MSW residues after disposal raises issues about the classification a nd management of ash. Results of laboratory leaching tests, especially for lead and cadmium, have not been consistently supported by field e xperience. Careful interpretation of the available test protocols is n eeded to make sure that residues are properly managed. Because of the large scale of coal-fired boilers for electricity production, ed-firin g of MSW with coal in such boilers could consume large quantities of w aste. Several short-term demonstrations have shown that co-firing is f easible. The issues involved in co-firing are emissions of trace eleme nts, trace organics, and acid gases; boiler slagging and fouling; and long-term effects, such as corrosion and erosion of boiler tubes. Area s where research and development has contributed to improved MSW combu stion include (a) the formation mechanisms of polychlorinated dioxins/ furans, especially low-temperature, catalytic mechanisms, (b) methods of combustion air distribution in incinerators that result in better c ombustion and reduced emission of organic compounds, (c) the use of ga s reburning to control NOx and reduce emission of organic compounds, ( d) practical methods for removing organic compounds and mercury from M SW flue gas, (e) the performance of electrostatic precipitators in rem oving MSW fly ash, particularly when co-firing MSW and coal in existin g coal-fired boilers, and (f) burning MSW in fluidized beds or of pulv erizing refuse-derived fuel and firing it in suspension-fired, pulveri zed coal boilers. (C) 1998 Published by Elsevier Science Ltd.