Cement kiln emissions
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Emissions from cement works can be determined both by continuous and discontinuous measuring methods, which are described in corresponding national guidelines and standards. Continuous measurement is primarily used for dust, NOx and SO2, while the remaining parameters relevant pursuant to ambient pollution legislation are usually determined discontinuously by individual measurements.
The following explanations on emissions refer to modern kiln plants based on dry process technology.
[edit] Climatically relevant gases / carbon dioxide
During the clinker burning process climatically relevant gases are emitted. CO2 accounts for the main share of these gases. Other climatically relevant gases, such as dinitrogen monoxide (N2O) or methane (CH4), are emitted in very small quantities only. CO2 emissions are both raw material-related and energy-related. Raw material-related emissions are produced during limestone decarbonation (CaCO3) and account for about 60 % of total CO2 emissions. Energy-related emissions are generated both directly through fuel combustion, and indirectly through the use of electrical power.
[edit] Dust
To manufacture 1 t of Portland cement, about 1.5 to 1.7 t raw materials, 0.1 t coal and 1 t clinker (minus other main constituents and sulfate agents) must be ground to dust fineness during production. In this process, the steps of raw material preparatory processing, fuel preparation, clinker burning and cement grinding constitute major emission sources for particulate components. While particulate emissions of up to 3,000 mg/m3 were measured at the stack of cement rotary kiln plants as recently as in the 50ies, these can be limited to 30 mg/m3 today.
[edit] Nitrogen oxides (NOx)
The clinker burning process is a high-temperature process resulting in the formation of nitrogen oxides (NOx). Nitrogen monoxide (NO) accounts for about 95 %, and nitrogen dioxide (NO2) for about 5 % of this compound present in the exhaust gas of rotary kiln plants. As most of the NO is converted to NO2 in the atmosphere, emissions are given as NO2 per m³ exhaust gas.
Without reduction measures, process-related NOx contents in the exhaust gas of rotary kiln plants would in most cases considerably exceed the specifications of e. g. European legislation for waste burning plants (0.50 g/m3 for new plants and 0.80 g/m3 for existing plants). Reduction measures are aimed at smoothing and optimising plant operation. Technically, staged combustion and Selective Non-Catalytic NO Reduction (SNCR) are applied to cope with the emission limit values.
High process temperatures are required to convert the raw material mix to Portland cement clinker. Kiln charge temperatures in the sintering zone of rotary kilns range at around 1,450 °C. To reach these, flame temperatures of about 2,000 °C are necessary. For reasons of clinker quality the burning process takes place under oxidising conditions, under which the partial oxidation of the molecular nitrogen in the combustion air resulting in the formation of nitrogen monoxide dominates. This reaction is also called thermal NO formation. At the lower temperatures prevailing in a secondary firing unit, however, thermal NO formation is negligible: here, the nitrogen bound in the fuel can result in the formation of what is known as fuel-related NO.
[edit] Sulphur dioxide (SO2)
Sulphur is input into the clinker burning process via raw materials and fuels. Depending on their respective deposits, the raw materials may contain sulphur bound as sulphide or sulphate. Higher SO2 emissions by rotary kiln systems of the cement industry might be attributable to the sulphides contained in the raw material, which become oxidised to form SO2 at the temperatures between 370 °C and 420 °C prevailing during the kiln feed preheating process. Most of the sulphides are pyrite or marcasite contained in the raw materials. Given the sulphide concentrations found e. g. in German raw material deposits, SO2 emission concentrations can total up to 1.2 g/m3 depending on the site location. In such cases, lime hydrate may be utilised to lower SO2 emissions.
The sulphur input with the fuels is completely converted to SO2 during combustion in the rotary kiln. In the area of the preheater and the kiln, this SO2 reacts to form alkali sulphates, which are bound in the clinker.
[edit] Carbon monoxide (CO) and total carbon
The exhaust gas concentrations of CO and organically bound carbon are a yardstick for the burn-out rate of the fuels utilised in energy conversion plants, such as power stations. By contrast, the clinker burning process is a material conversion process that must always be operated with excess air for reasons of clinker quality. In concert with long residence times in the high-temperature range, this leads to complete fuel burn-up.
The emissions of CO and organically bound carbon during the clinker burning process are caused by the small quantities of organic constituents input via the natural raw materials (remnants of organisms and plants incorporated in the rock in the course of geological history). These are converted during kiln feed preheating and become oxidised to form CO and CO2. In this process, small portions of organic trace gases (total organic carbon) are formed as well. In case of the clinker burning process, the content of CO and organic trace gases in the clean gas therefore does not permit any conclusions on combustion conditions.
[edit] Dioxins and furans (PCDD/F)
Rotary kilns of the cement industry and classic incineration plants mainly differ in terms of the combustion conditions prevailing during clinker burning. Kiln feed and rotary kiln exhaust gases are conveyed in counter-flow and mixed thoroughly. Thus, temperature distribution and residence time in rotary kilns afford particularly favourable conditions for organic compounds, introduced either via fuels or derived from them, to be completely destroyed. For that reason, only very low concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (in short: dioxins and furans) can be found in the exhaust gas from cement rotary kilns. Investigations have shown that their emissions are independent of the type of input materials used and cannot be influenced by process technology measures.
[edit] Polychlorinated biphenyls (PCB)
The emission behaviour of PCB is comparable to that of dioxins and furans. PCB may be introduced into the process via alternative raw materials and fuels. The rotary kiln systems of the cement industry destroy these trace components virtually completely.
[edit] Polycyclic aromatic hydrocarbons (PAH)
PAHs (according to EPA 610) in the exhaust gas of rotary kilns usually appear at a distribution dominated by naphthalene, which accounts for a share of more than 90 % by mass. The rotary kiln systems of the cement industry destroy virtually completely the PAHs input via fuels. Emissions are caused by organic constituents in the raw material.
[edit] Benzene, toluene, ethylbenzene, xylene (BTEX)
As a rule benzene, toluene, ethylbenzene and xylene are present in the exhaust gas of rotary kilns in a characteristic ratio. BTEX is formed during the thermal decomposition of organic raw material constituents in the preheater. They account for about 10 % of total carbon emissions.
[edit] Gaseous inorganic chlorine compounds (HCl)
Chlorides are minor additional constituents contained in the raw materials and fuels of the clinker burning process. They are released when the fuels are burnt or the kiln feed is heated, and primarily react with the alkalis from the kiln feed to form alkali chlorides. These compounds, which are initially vaporous, condense on the kiln feed or the kiln dust, respectively, at temperatures between 700 °C and 900 °C, subsequently re-enter the rotary kiln system and evaporate again. This cycle in the area between the rotary kiln and the preheater can result in coating formation. A bypass at the kiln inlet allows to effectively reduce alkali chloride cycles and to thus diminish operational malfunctions. During the clinker burning process, gaseous inorganic chlorine compounds are either not emitted at all or in very small quantities only. Owing to the alkaline kiln gas atmosphere, the formation of hydrogen chloride (HCl) in the exhaust gas can be virtually ruled out.
[edit] Gaseous inorganic fluorine compounds (HF)
Of the fluorine present in rotary kilns, 90 to 95 % is bound in the clinker, and the remainder is bound with dust in the form of calcium fluoride stable under the conditions of the burning process. Owing to the great calcium excess, the emission of gaseous fluorine compounds, and of hydrogen fluoride in particular, is virtually excluded. Ultra-fine dust fractions that pass through the measuring gas filter may feign low contents of gaseous fluorine compounds in rotary kiln systems of the cement industry.
[edit] Trace elements
The emission behaviour of the individual elements in the clinker burning process is determined by the input scenario, the behaviour in the plant and the precipitation efficiency of the dust collection device. The trace elements introduced into the burning process via the raw materials and fuels may evaporate completely or partially in the hot zones of the preheater and/or rotary kiln depending on their volatility, react with the constituents present in the gas phase, and condense on the kiln feed in the cooler sections of the kiln system. Depending on the volatility and the operating conditions, this may result in the formation of cycles that are either restricted to the kiln and the preheater or include the combined drying and grinding plant as well. Trace elements from the fuels initially enter the combustion gases, but are emitted to an extremely small extent only owing to the retention capacity of the kiln and the preheater.
Under the conditions prevailing in the clinker burning process, non-volatile elements (e.g. arsenic, vanadium, nickel) are completely bound in the clinker.
Elements such as lead and cadmium preferably react with the excess chlorides and sulphates in the section between the rotary kiln and the preheater, forming low-volatile compounds. Owing to the large surface area available, these compounds condense on the kiln feed particles at temperatures between 700 °C and 900 °C. In this way, the low-volatile elements accumulated in the kiln-preheater-system are precipitated again in the cyclone preheater, remaining almost completely in the clinker.
Thallium and its compounds condense in the upper zone of the cyclone preheater at temperatures between 450 °C and 500 °C. As a consequence, a cycle can be formed between preheater, raw material drying and exhaust gas purification.
Mercury and its compounds are not precipitated in the kiln and the preheater. They condense on the exhaust gas route due to the cooling of the gas and are partially adsorbed by the raw material particles. This portion is precipitated in the kiln exhaust gas filter.
Owing to trace element behaviour during the clinker burning process and the high precipitation efficiency of the dust collection devices trace element emission concentrations are on a low overall level.
