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Show ,t has been assumed that the release of volatilcs is fast compared to the lime required to heat-up a particle. When T p is reached, the particle gives off its volatilcs in amount determined by the above formulae. The maximum volatile yield V M m a x and consequently the Q factor arc dependent on the specific coal and it should be established experimentally. Figure 3 shows the dcvolatilizalion data for hvb Coal Valley coal together with the approximation provided by Eqs (11). Char Combustion. The combustion of char occurs predominantly after volatilcs are given off. Char produced from rapid pyrolysis arc microporous solids whose properties can be described by their size, true and apparent density, porosity, pore volume distribution and surface area distribution (sec for example Smith 1982; Wall, 1987). Although the process of carbon combustion is relatively well understood, there has been little success in predicting the char combustion rales using the knowledge of the coal. Al present, modeling of the char combustion rates requires that these rales have to be measured for each type of char, under conditions representative of the flame. It is assumed that char consists of pure carbon and carbon monoxide is the product of char oxidation. The char reaction submodel takes into account the bulk diffusion of oxygen to the particle external surface, the chemical reaction and pore diffusion. The overall reaction rate (q) is: ( \05 (12) where kj is the oxygen diffusion rate to a spherical particle which is calculated as follows: „.„.- (iiif-L (13) In these equations, P Q 2 is the bulk oxygen partial pressure, T p and Tg are the particle and surrounding gas temperatures. Dp is the particle diameter and kc is an Arrhenius type expression representing the psudo-reaction rale which incorporates the chemical reaction and pore-diffusion : ke = Aetxp[-Ee/RTp) (14) It was recognised (Knill et al., 1990) that the char burnout model as shown above could be used for the prediction of the first 5 0 % of the burnout curve. Beyond this point, the burnout rates decrease substantially and the following impcrical formula for a hvb Coal Valley coal was applied (sec also Sayrc et al., 1992): q'= 4{l-B)2q for B> 0.5 (15) where q* is the lower burnout rate and B stands for the degree of burnout. -- {pUjmNO ) - - OXj Ox, ' I'cff dm NO ' k Sc Ox, J (In) Z sk - NO ~ Y,*NO-red k=t.PJ where $*_ NO ^ = t,p,f arc the timc-avcraged N O sources for thermal-, prompt- and fucl-NO. and * NO-red arc lnc time-averaged N O sinks due to N O reduction reactions. The sections below describe the basis of the NOx post-processor while details arc given in Peters and Weber (1991). Thermal-NO. In combustion of lean and near stoichiometric fuel-air mixtures, the principal reactions governing thcrmal-NO are known as Zcldovich mechanism: O + N 2 <=> N O + N N +O2 <=* N O + O (Rl) (R2) The above mechanism is often complemented by the reaction N + OH <=> NO + H (R3) which is neglected in the calculations presented in this paper. Under the assumption that the N-radicals concentration can be calculated using a steady stale approximation, the rate of thermal N O formation is (in gmol/cm3 s): 1-NO=2*l[0] [Q2][N2]-(Kg > N O)~ [NO] [02] + (*-i/*2)[NO] 07) where [ ] indicates concentrations of chemical species in mol/cm3 and K ^ N O denotes the equilibrium constant of the overall reaction 0.5 02+0.5 N 2 <=> N O (R4) The forward and reverse rate constants appearing in Eq.(17) are taken from Bowman (1975). Furthermore, the O-radical concentration is assumed to be in equilibrium with molecular oxygen and is calculated following Wcslenbcrg (1975). Eq. (17) is applicable to pre-mixed laminar combustion and shows the strong dependence of thcrmal-NO formation on the combustion gas temperature and the lesser dependence on the oxygen concentration. In order to obtain the lime-mean NO formation rate appropriate for turbulent combustion, a presumed single-variable pdf approach of Hand et a!.,(1989) is used with the gas temperature being the only fluctuating variable: n st-NO =1° M N Q lr,_N0(T)Bpdf(T'.a,b\ dT n -TU (18) Emissions Of Nitric Oxide (NOx Post-Processor) The stationary, time-mean balance equation for the total-NO mass fraction (m^o) reads: where a and b are parameters of the Bcla-pdf function. The expectation E I of the fluctuating temperature is set to its time-average value calculated from the enthalpy equation, 11-11 |