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Show I (j {< Reduction of NOx Emissions from Cement KilniCalciner through the Use of the NOxOUT® Process By W.H. Sun, M.J. Bisnett, and D.W. Kirk, Nalco Fuel Tech and H.E. Steuch and J. Hille, Ash Grove Cement Company ABSTRACT Post-combustion urea injection has proven to be an effective method in controlling NOx from various combustion sources. This process, a selective non-catalytic reduction process known as NOxOU~, has been successfully demonstrated in a cement kiln / calciner operated by Ash Grove Cement. Testing was done under ten different kiln / calciner operating conditions. Using three to four injectors, NOx was efficiently reduced from 350 to 600 pounds per hour (3.5 to 6.0 lb / ton of clinker) to less than 100 pounds per hour (1.0 lb / ton of clinker). This calculates to a NOx reduction of >80% for most cases. Chemical utilization was greater than 50%. A high degree of mixing and a long residence time at an appropriate temperature present in the preheater tower contributed to these excellent results. An average ammonia slip was four ppm above a baseline level at normalized stoichiometric ratio of 1. Based on this demonstration, cement kiln / calciners have been identified as an ideal application for the NOxOUT Process. NOx was efficiently and effectively reduced with minimal by-product emissions and virtually no effect on plant operations. INTRODUCTION The oxides of nitrogen (NOx) affect the environment by contributing to acid rain, ground level ozone, and photochemical smog. The Clean Air Act Amendments (CAA) of 1990 address the control of NO x emissions in Titles I (non-attainment) and IV (acid rain) and state regulators are required to submit plans (SIPs) to comply with the CAA. In addition to fossil-fuel fired utility boilers, which are the major source, industrial sources of NOx such as process heaters, incinerators and kilns may be regulated depending on their emission rate and the level of attainment of the National Ambient Air Qualilty Standards in the area of these sources. NOx emissions from cement kilns are significant (>1000 TPY) due to the process requirements. In a cement kiln, gas temperature is raised above 2700°F to form "clinker". This heat is provided by burners whose peak flame temperatures can exceed 3000°F. Since NOx formation during combustion increases with temperature and oxygen content, NOx concentrations resulting from such high temperature process can exceed 1000 ppm. The thermodynamic equilibrium concentration of NOx at 3000°F and 1% oxygen with natural gas is 1500 ppm. In addition, NOx emission increases with nitrogen content in the fuel termed, fuel-NOx. The NOx generation from fuel bound nitrogen is also sensitive to oxygen content in the flame zone. Utilizing its sensitivity to oxygen content, NOx emission can be reduced through combustion modification. While maintaining the flame temperature, combustion air can be staged to provide a slightly fuel-rich condition at the flame zone. At 3000°F, the equilibrium NOx concentration decreases to 500 ppm at the stoichiometric condition and even lower under oxygen-starved conditions. Additional air is injected downstream of the peak temperature zone to complete the combustion. A combustion modification is an effective method to control NOx but the extent of NOx control is limited because a high temperature zone, which generates NOx, must still be provided for the process. SNCR Process Selective non-catalytic NOx reduction (SNCR) processes have effectively controlled NOx emissions in various types of boilers firing all types of fossil fuel or waste-fuel.1,2,3 Unlike combustion modifications, ammonia4 or urea5 is injected in the post-combustion zone, typically at temperatures between 1600 and 2200°F. Therefore, the process does not affect the burner operations. The level of NO x removal depends on the reagent feedrate and can range from 30 to 70%. Some critical process parameters are temperature, residence time, mixing between the reagent and flue gas, and uncontrolled NOx level. Presented at the International Specialty Conference on Waste Combustion in Boilers and Industrial Furnaces sponsored by Air & Waste Management Association, Kansas City, Missouri, April 21, 1994 ' |