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Stack emission Dust Dust as an emission constituent is influenced by the precipitation behaviour of the exhaust gas cleaning device. Dust emissions have been sharply reduced in the past decades by the use of electrostatic precipitators and fabric filters. Trace elements As trace elements are contained in the raw materials and fuels they may also appear in emitted dust. Trace elements such as arsenic, cadmium, lead, nickel or vanadium, are practically completely incorporated in the clinker. Mercury or thallium behave differently because of their greater volatility. These elements are collected and removed selectively from the system. Mercury is the only element that may require input to be limited in individual cases. Nitrogen oxides The clinker burning process is a high-temperature process with temperatures in the sintering zone at around 1450 °C and flame temperatures of about 2000 °C. These high temperatures result in the formation of nitrogen oxides (NOX). Therefore, the generation of nitrogen oxides is inherent to the process and not influenced by the use of alternative fuels, unless nitrogen-rich alternative materials are used. Nitrogen monoxide (NO) accounts for 95 %, and nitrogen dioxide (NO2) for about 5 % of this compound present in the exhaust gas of rotary kiln plants. Several measures are evaluated or have been installed e.g. low-NOX calciners, selective non-catalytic reduction technique (SNCR), catalytic exhaust gas cleaning. The cost will be of decisive importance. Total carbon, CO, SO2, NH3 The raw material (and fuel ?) composition(s) and the burning conditions are decisive for the content of total carbon and carbon monoxide in the exhaust gas. Moreover, depending on the respective locations, the raw material situation can effect the emissions of sulfur dioxide and ammonia and ammonium compounds. Sulfur dioxide emissions result from the input of sulfur compounds into the kiln with the raw meal, while the fuel-related input is practically completely combined in the cement clinker, a high amount of readily volatile sulfides in the raw material, e. g. in the form of pyrites or marcasite, can lead to higher SO2 emissions. If process engineering or operational measures are not sufficient, then the SO2 emissions can be lowered by the dry additive process, i. e. usually by adding hydrated lime. More extensive abatement measures, such as the use of wet scrubbers, are as a rule not economically justifiable. Only in those cases where the SO2 emissions caused by the raw materials exceed a few thousand mg/m3 this method of SO2 abatement can be seen as a standard technique. Dioxin and furans Dioxin and furan emissions are often discussed in conjunction with the use of alternative fuels. However, organic constituents in the feed materials are completely destroyed at the extremely high temperatures and the long residence times in the rotary kiln firing system. All the emission data compiled by the Research Institute of the German Cement Industry and other notified measuring bodies indicate, that dioxin and furan emission concentrations are very low regardless of the type of fuel used or the way the kiln is operated. References:
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