Calcium magnesium acetate and urea advanced reburning for NO control with simultaneous SO_2 reduction

摘要

Calcium magnesium acetate (CMA) shows potential as a reductant for simultaneous NO_x and SO_x removal from coal-fired combustion plant. The performance of urea co-injection with CMA on NO reduction in an 'advanced reburn' (AR) configuration has been investigated with a view to optimization of the process in a pulverized coal-fired furnace operating at 80 kW. The impact on SO_2 reduction has also been investigated. Urea/CMA solution was sprayed into the reburn zone of the furnace using twin-fluid atomizers over a range of reductant/NO stoichiometric ratios (NSR). The influence on NO reductions of primary zone stoichiometry (lambda_1) was investigated for a range of CMA reborn feed rates (Rff) and reburn zone stoichiometry (lambda_2). In addition, the effect of temperature on the SNCR performance of urea was investigated. Optimum process conditions were categorized either by maximizing NO and SO_2 reductions (Modes A and B, respectively) or maximizing reductant utilization efficiencies (Modes C and D). NO control was best performed at lambda_1=1.05, but SO_2 reductions were greatest at more fuel-lean primary zone conditions (lambda_1= 1.15). Highest NO reductions of 85 percent under AR-rich conditions were achieved under Mode A, but were only slightly higher compared with reductions of 79 percent under Mode B, where SO_2 reductions were optimized at 85 percent. N-utilization was also at an acceptable level of 25 percent compared to the maximum utilization efficiency which was obtained at NSR = 1.5 of 30 percent for the same conditions of stoichiometry operating in Mode C. Operation at this lower level of reburn (9.6 percent) could significantly reduce the consumption of CMA with some impact on NO reduction (73 percent). SO_2 removal performance would be compromised severely with reductions lowered from 75 percent at Mode A to 35 percent at Mode C. Optimizing Ca utilization (Mode D) resulted in poor NO and SO_2 reductions, at 61 and 22 percent, respectively, and can be discounted as a viable option. The technique offers flexibility of operation depending on the emission control requirements.