NOx Control with Lime-Urea Hydrate on Coal-Fired Boilers

Coal-fired boilers have generally relied upon combustion modifications and/or lowNOx burners to achieve initial NOx reductions on the order of 15%-50%. Selective noncatalytic reduction (SNCR) technologies (e.g. Thermal DeNOx and urea injection) can potentially produce higher levels of NOx removal, however, with the SNCR processes it is necessary to address the reactions between ammonia (NH~ emissions from the process and sulfur trioxide (SO~. The NH3 can react with S03 forming ammonium sulfates, which in turn, can lead to air heater fouling. NOx reductions of 80%-90% will thus likely require the use of selective catalytic reduction (SCR) unless some of the other technologies are used in combination. Recently, however, a cost effective dry S02/NOx control technology has been developed utilizing lime-urea hydrate injection in the upper furnace or convective pass regions of coal-fired boilers. Although primarily intended for simultaneous reductions of S02 and NOx, the proportion of urea to lime in the hydrate may be adjusted so as to focus the emissions reduction potential on NOx. The presence of the lime in the hydrate serves to react with, and remove any S03 prior to the flue gas reaching temperature conditions favorable for reaction with ammonia emissions. In this manner, air heater fouling attributable to ammonium sulfates formation is potentially alleviated. Pilot-scale test results have demonstrated simultaneous removals of S02 and NOx on the order of 65% at a Ca/S ratio of 3 and N/NOx ratio of 1.5. The current paper focuses on the use of the lime-urea hydrate principally for NOx control. By being able to optimize the temperature window for NOx control, removals of approximately 60% can be achieved at N/NOx ratios of 1. Further emissions reduction potential for NOx are possible by implementing the technology in combination with combustion modifications and/or low NOx burners. An economic assessment of the lime-urea hydrate technology for NOx control has indicated capital costs of less than 40 S/kW for a 100 MW unit. Levellzed busbar costs for a plant with a 65% capacity factor and 20 years remaining plant life were calculated to be on the order of 3.6 mills/kWh. Cost comparisons with alternative NOx reduction technologies demonstrated the lime-urea hydrate, in combination with low NOx burners, to be a cost effective alternative to SCR while achieving similar overall NOx emissions reductions.