Cement Energy and Environment

The reaction normally takes place at relatively lower temperature, less than 1200°C. Fuel NOx formation normally depends on: • Nitrogen content in the fuel • Volatile content in the (solid) fuel • Oxygen level in the combustion zone • Initial NO concentration in the combustion gas • Temperature in the secondary combustion zone A higher volatile content in the fuel reduces fuel nitrogen conversion to NO. At tt:!mperatures between 800 -:c and 1100 -:c. tt1e following reactions may take place: N + 0? NO (1) N + NO ? N2 + 0 (2) Since the rate of reaction 2 increases more rapidly than the rate of reaction 1 as the temperature increases, higher t•:!mperatures (between 800°C and 11OOOC) may reduce NOx emissions in secondary combustion zones. Prompt NOx Formation Prompt NOx is formed by fuel-derived radicals , such as CH and CH2, reacting with N2 in hydrocarbon flames. The overall contribution of prompt NOx to total NO is relatively small. In rotary kiln , thermal NOx gHneration is dominant whereas in the calciner and in the secondary combustion zone where combustion temperature is up to 1200 -:c fuel NOx is major contributor. Influence of Kiln System on NOx Emission Kiln system in cement plant is normally one of the following. • Pre-heater kiln with grate or planetary cooler • In-Line Calciner (ILC) kiln • Separate Line Calciner (SLC) kiln. In pre-heater kiln the NOx Hmission is determined exclusively by the condition in the kiln burning zone. In ILC kiln system the kiln exit gases having NO pass through the calciner. CH radicals and nitrogen from the calciner fuel reacts with kiln NOx to reduce it to free nitrogen. Balance nitrogen compound in calciner fuel during combustion forms NOx. The result may be a net production as well as net reduction of NO in calciner. In SLC kiln system the combustion in calciner takes place in pure air. When using solid fuel like coal up to 50 per cent of nitrogen compounds in the fuel may get converted into NO. Thermal NO from kiln leaves kiln string without any opportunity to reduce/reburn and gets added to NOx from calciner string. SLC kilns therefore , have higher NOx emissions from stack compared to ILC kiln system. Control Techniques for NOx Reduction Typical NOx emission in older technologies can be as high as 1800 - 2000 mg/Nm3, while average emission value in modern plants is around 1200 mg/Nm3. The reduction of NOx emissions from cement pyro system can be done in two ways. Primary NOx Reduction Measures In primary reduction measures existi ng process is modified to reduce the formation of NOx, The following ways are very common. • Optimisation of clinker burning process. • Automatic kiln control system or Expert system. • Use of Low NOx burner to allow low primary air and to control flame flow pattern. • Addition of water to the flame or fuel of the main burner. • Staged Combustion in Precalciner. In calciner staged combustion, fuel is first burned under reducing conditions to reduce NOx and then remaining fuel burns under oxidising conditions to complete the combustion. Introduction of raw meal allows control of calciner temperature. Through these mechanisms, both fuel NOx and thermal NOx are controlled. The reaction: 2CO+ 2NO ? 2C02 + N2 Primary reduction measures can reduce NOx level up to 20 per cent. Secondary NOx Reduction Measures (SNCR) In Secondary reduction measure a separate gas cleaning unit is added. Selective Non Catalytic Reduction of NO with NH3 was developed by Exxon Research & Engineering Co., USA. The reagent, typically NH3 or urea, is injected into the kiln system at a location with an appropriate temperature window (870°C to 11 00°C). The temperature is critical, at higher temperatures the 40