Comparison of Material Flows in Sewage-free and Sewage-generating Flue-Gas Purification Systems of Municipal Waste Incineration Plants

During incineration of wastes in waste incineration plants, polluted flue gases are generated which have to be subjected to flue gas purification. Although the legal requirements are nearly unambiguous, the question of whether wet flue gas purification is to be performed in a sewage-free or sewage-generating manner is discussed controversially by experts in the Federal Republic of Germany. As a contribution to this discussion, material flow studies of sewage-free and sewage-generating flue gas purification processes in waste incineration plants were performed by IT AS in cooperation with ITC­ TAB. The study covered three waste incineration plants, two of which were operated in a sewage­ generating and one in a sewage-free manner. The results obtained can be summ arized as follows: The data and information submitted by most of the plant operators are not sufficient for a comprehensive balancing of flue gas purification systems in waste incineration plants. For this reason, plant operation often is not optimally tailored to the substances prevailing. During operation, at least temporary strong superstoichiometric dosage of auxiliary chemicals cannot be excluded. By means of plausibility assumptions and model calculations, closed balancing of most plants could be achieved. Moreover, it was demonstrated by the balancing of technical-scale waste incineration plants that the material flows in "wet" flue gas purification are less dependent on the design of the flue gas purification section (sewage-free/sewage-generating), but considerably affected by the operation of the flue gas purification system (e.g. volume of absorption agents used). Hence, material flows can be controlled in a certain range. Introduction Thermal waste treatment generates flue gases which must be cleaned of pollutants before being released into the atmosphere. A legal regulation (17th German Federal Environmental Impact Control O rdinance) issued in 1990 defined new limits for municipal solid waste incinerators (MSWI) which are below the corresponding requirements of power plants. In flue gas purification devices, a dust removal system may be followed by multistage scrubbers in which the pollutants are absorbed by scrubbing liquids. The scrubbing liquids can either be evaporated internally or passed to a sewage treatment plant. As a consequence of relatively high heavy metal concentrations in the waste water of MSWI detected in the seventies, the possibility of the release of waste water from MSWI should be excluded in general. Waste water from MSWI may no longer be passed to sewage treatment plants in Germany since the late eighties. Although the legal requirements are nearly unambiguous, the question of whether wet flue gas purification is to be performed in a 281 sewage-free or sewage-generating manner is discussed controversially by experts in the Federal Republic of Germany. Arguments used in favour of discharging such waste waters are the lower pollutant loads arising from modem waste incineration plants, and the added processing expense in sewage-free operations. In this study, material flow analyses are established for sewage-generating and sewage-free flue gas purification systems of technical MSWI. Material flow analyses are a tool used to trace and make transparent the use and whereabouts of specific substances, by type, volume and mass, with all ramifications in the process taken into account. The basic data of these material flow analyses were conducted from informations received from technical-scale MSWI. Approach Three technical-scale MSWI were investigated. All plants, for which balances were established, have electrostatic precipitators for dust removal downstream of the boiler, followed by two stage scrubbing systems. One of the plants selected is equipped with a sewage-free flue gas purification system. The others discharge their waste water into a municipal sewage treatment plant. Additional information was used for plausibility assessments. The bounderies of the system selected for the material balances do not extend over the entire MSWI. The area of coverage begins with the raw gas downstream of the boiler and ends downstream of the scrubbers of the flue gas purification system. In all plants, the offgas referred to as clean gas in the figures in this study is passed through additional flue gas purification devices to remain under the limits under the 17th German Federal Environmental Impact Control O rdinance. These additional flue gas cleaning devices have no impact on the flows of the pollutants considered. The results described here are limited to S02' HCI, and mercury. Conclusions are derived from a comparison of the balances and, finally, the findings are evaluated. Description of the Plants The plants studied were grate firing systems incinerating more than 75% of solid municipal waste and, in addition, bulky waste and sewage sludge. No detailed description of these common technical systems for waste incineration and the required flue gas purification devices will be given at this point. In all plants considered, the concentrations of the pollutants in the raw gas are on the order of magnitude typically found in German MSWI. Table 1 lists some important data about the raw gas in the plants for which balances were established. In addition to flue gas data, also extensive information was available about the use of auxiliary chemicals for neutralization and heavy metal precipitation. Balance Plant A There is excellent information available about Plant A, which allowed an exact balance of the plant including the waste water treatment plant. As a consequence of the greatly varying concentration data, ranges of values are presented in some of the diagrams below. According to figure 1, 1 t of waste contains 4.2 to 10.7 kg of chlorine which passes almost entirely into the raw gas during incineration on the grate. In the first flue gas purification step, the electrostatic pre�ipitator, the dust separation removes the chlorides bound in the filter ash. The first scrubber removes between 3 and 6.8 kg of chlorine per 1 t of waste absorbed by the scrubbing liquid. The second scrubber absorbes small quantities of chlorine, on the order of 0.2 to 0.8 kg per 1 t of waste. O nly small quantities