| OCR Text |
Show pyrolysis products are not considered. The release of fuel-N species follows the approach of Abbas et al [21] where: • The rate of release of fuel nitrogen is proportional to the devolatilisation rate. • The distribution of N between char and volatile matter is uniform. • Volatile-N evolves instantaneously as H C N. • Thermal and prompt N O x are considered small and not taken into account. Using a global mechanism approach [7], the rates of oxidation of HCN to NO and the rate of reduction of NO to N2 by HCN are determined from: d[ XN0]/ dt = 4.0 xlO11 X„CN Xo2 b exp( - 67,000/RT) (III) d[ XN2 ]/dt = 3.0 xlO12 XHCN XNO exp( - 60,000/RT) (IV) where Xi is a mole fraction, T is the temperature (K), and R is the universal gas constant. The reaction order, b in equation (IV) varies according to the oxygen mole fraction [7]. On the assumption that HCN is the major fuel-N derived intermediate species from coal devolatilisation [22], the role of N H „ derived directly from the fuel-N (or via H C N ) as a N O reducing agent is not considered in the present model, given that this is less important to the rebum case than H C N [8,9] (although in comparison to other rebum fuels such as natural gas, N H 3 may be more significant). The char-N is assumed to convert to NO with a conversion rate proportional to carbon burnout [18,21], 7*NO,CHAR = rcfil,CHAR (p (V) where TNCCHAR is the rate of formation of NO from char nitrogen, Vc is the calculated combustion rate, ^.CHAR is the mass fraction of nitrogen in the char and cp is the N O yield coefficient [18]. The rate of NO reduction by char particles is obtained from: d[NO]/dt = A exp{ - E/RT) x AE PNO (VI) where A is the frequency factor, E is the activation energy, AE is the external surface area of the char (m2/g) and PNo is the N O partial pressure. There is some uncertainty in the published rates for N O reduction at the char particle surface. Consequently, and two sets of data were incorporated into the model for comparison, the first after Levy et al [10] where A = 4.18 x IO4 g-mol/m2satm, E = 34,700 cal/g-mol and the second after Chan et al [19], where A = 3.14 x IO7 g-mol/m2satm, E = 43,700 cal/g-mol. Coal devolatilisation products include several major gaseous species (CO, C02, CH4, CJri^ , Hj, etc.). The present model only considers C H 4 as the main devolatilisation product and hence the major source of C H fragments during coal pyrolysis. The following mechanism has been proposed for N O reduction by this route [8,9]: |
