The Effectiveness of Calcium (Magnesium) Acetate and Calcium Benzoate as NO_x Reduction Agents in Coal Combustion

Abstract

A laboratory study was conducted to assess the effectiveness of the chemicals calcium magnesium acetate (CMA), calcium acetate (CA), and calcium benzoate (CB) in reducing NO_x emission from coal-fired furnaces. CMA, which is currently being used as a non-corrosive road deicing salt, has demonstrated exceptional effectiveness in our laboratory as an in-boiler S02 removal agent (Levendis et al., 1993). This is due to the affinity of the calcium and magnesium components of CM A for sulfur; the reaction results in solid sulfate salts. The current investigation explored the role of the organic component of CMA, as well as that in CA and CB, in removing NO_x emissions that are typically found in post-flame regions of furnaces burning pulverized coal or residual oil. Bench-scale experiments were conducted at gas temperatures ranging from 450 to 1450°C and oxygen concentrations between 0 to 5%. The residence time was in the order of 4 s, Injection of the CMA reagent at a mass ratio of CMA/NO of 41 (or an equivalent acetate/NO molar ratio of 18) reduced NO, by more than 90% (at 0% O₂ and temperatures above 850°C) to 50% (at 5% O₂ and 650 to 850°C). At an oxygen concentration of 2%, NO_x reduction seemed to be most effective between 1000 and 1350°C with NO_x reductions of 70 to 80%. The efficacy of calcium acetate (CA) and calcium benzoate (CB) as NO, reducers was also explored, with CA (CA/NO mass ratio of 41 and acetate/NO molar ratio of 15) being less efficient than CMA at a similar φ with oxygen present but as efficient in 0% oxygen. CB (CB/NO mass ratio of 41 with benzoic acid/NO molar ratio of 9) was as efficient as CMA, regardless of O₂ concentration, but at temperatures higher by 200°C

Key Words:

• Calcium acetate
• CMA
• NO
• reduction
• coal combustion