OCR Text |
Show NO. Ob/MMBtu) = (8.02*N) + (0.364*GSR) + (0.054*SR) (12) Where NO. emissions are expressed in Ib/million Btu, and where N is the fraction of nitrogen in the fuel. N is a direct input to PREP. This equation is appropriate if the nitrogen content in the coal is in the 0.7 - 1.5 percent (.007 -.015 fraction) region. Biomass. Nitrogen in the biomass fuels is virtually always in amine form, as it is the result of protein in the plant matter [13]. The nitrogen is highly volatile, and nitrogen fragments evolve according to an "Sit shaped curve. For the dominant portion of the curve, nitrogen volatilizes more rapidly than carbon [55]. The evolution of nitrogen volatiles is governed by the solid panicle pyrolysis mechanism generally, but is not unusually sensitive to reactor temperature [55]. Conversion of fuel nitrogen to NO. is governed by nitrogen content of the biomass fuel, the particle size distribution of the fuel (specifically the fraction of fuel reporting as fmes), the grate stoichiometry and the degree of staging, and the total stoichiometric ratio of the biomass reactor. The model, therefore, calculates nitrogen oxide emissions from biomass fired spreader-stokers by the following stages of calculation, based upon the research of Munro [38], Tillman [56, 57], and Winter, Clough, and Pershing [63]: 1) calculate the nitrogen content of the fmes fraction; 2) calculate the nitrogen content of the large fuel particle fraction; 3) treat the nitrogen conversion rate in the fines as 40% of the fuel nitrogen going to NO.; 4) treat the nitrogen conversion rate in the large fuel particle fraction as ranging from 8% to 11 % of the fuel nitrogen going to NO.t depending upon the bed stoichiometry, with the most favorable results being achieved when GSR = 0.8; and 5) sum the NO. as formed, and conven it to lb NOJMMBtu. If pile burning systems such as inclined grate burners, Dietrich cells, or wet cells are employed, the nitrogen conversion rate is considered to be 11 %. These conversion factors hold assuming the fuel contains 0.1 - 0.4 percent nitrogen. It should be noted that Kester [31] measured an average nitrogen conversion rate in a wood fired spreader-stoker of 18.5%. This corresponds to a feed consisting of 26% fines and 74% large fuel particles. Such a composition is not uncommon in wood fire spreader-stokers. It should also be noted that successful NO. control has, in several instances, involved reducing the fines content of a spreader-stoker or overbed feed pile burning system. 12 |