(1 − .1)R1 + .1V1; (R1)(1 − .2)R2 + .2V2; (R2)where .1 takes the value of volatile matter percentage ob-tained in proximate analysis of coal, and .2 is given thevalue of 0.8 to re4ect the characteristics of devolatilizationat high temperature. The daf coal devolatilization rate is pro-portional to the mass of daf coal and takes the 9rst-orderkinetic model. The volatile release rate can be given as˙ mv;p = −.1md;pBv1 exp − Ev1RTp −.2md;pBv2 exp − Ev2RTp : (8)The rate of mass reduction of daf coal can be given as˙ md;p = −md;pBv1 exp − Ev1RTp −md;pBv2 exp − Ev2RTp : (9)2.3.3. Char reaction rateThere are three kinds of heterogeneous char reactions onthe surface of coal particles. They are assumed as follows:C+O2 → CO2; (R1)2C+O2 → 2CO; (R2)C+CO2 → 2CO; (R3) A di>usion-kinetic model is used to simulate the overall re-action rate (Zhou, 1993). The heterogeneous char reactionrate is assumed to be of 9rst order in oxygen concentra-tion and carbon dioxide concentration. The char reactionrate for the above three kinds of surface reactions can bewritten as˙ m1c;p = − 1#c1&d2pS YO2;SB1 exp − E1RTp ; (10)˙ m2c;p = − 1#c2&d2pS YO2;SB2 exp − E2RTp ; (11)˙ m3c;p = − 1#c3&d2dS YCO2;SB3 exp − E3RTp : (12)The total species reaction rate is then obtained as˙ mc;p =˙ m1c;p +˙ m2c;p +˙ m3c;p; (13)˙ mo2;p = #c1 ˙ m1c;p + #c2 ˙ m2c;p; (14)˙ mco2;p = − ˙ m1c;p − #c1 ˙ m1c;p + #c3 ˙ m3c;p; (15)˙ mco;p = −(˙ m2c;p + #c2 ˙ m2c;p +˙ m3c;p + #c3 ˙ m3c;p): (16)The oxygen and carbon dioxide concentration YO2;S andYCO2;S at the coal particle surface can be calculated us-ing a stagnant 9lm theory, by solving the species di>u-sion equations near the surface of the coal particles andobtained asYo2;S = − ˙ mo2;p˙ mp+ YO2;g + ˙ mO2;p˙ mp exp(−Bp); (17)Yco2;S = − ˙ mco2;p˙ mp+ YCO2;g + ˙ mCO2;p˙ mp exp(−Bp); (18)where YO2;g and YCO2;g are mass fractions of oxygen and car-bon dioxide surrounding the coal particles which are equalto the oxygen and carbon dioxide concentrations at the grid.
2.4. Reaction kinetic constantsFor the coal type calculated in this paper, the reactionkinetic constants for the gas phase species of CH4 and CO,together with kinetic constants of coal particle moistureevaporation, devolatilization and carbon surface reactionsare given in Table 1 (Zhou, 1986).2.5. NOX formation modelFor NOX formation the Zel’dovich mechanism of thermalNO and the DeSoete mechanism of fuel NO are considered(Smoot, 1985). The mechanism of fuel NO formation isTable 1Reaction kinetic constantsChemical or physical process K = B exp(−E=RT)B unit E(J=mol)CH4 +2O2 → CO2 +2H2O1:6 × 1010 m3 kg−1s−1 1:081 × 1052CO + O2 → 2CO2 7:0 × 104 m3 kg−1s−1 6:651 × 104Moisture evaporation 8:32 × 105 4:228 × 104Devolatilizationreaction (1) 3:7 × 105 s−1 7:366 × 104reaction (2) 1:46 × 1013 s−1 2:511 × 105Char reaction 1 1:225 × 103 ms−1 9:977 × 104Char reaction 2 1:813 × 103 ms−1 1:089 × 105Char reaction 3 7:375 × 103 ms−1 1:380 × 105expressed byHCNNO O2N2NO(R1)  (R2)Coal nitrogen Char surface reaction (R3)(∗)The amount of NO produced in the combustion is char-acterized using the following steady-state transfer equationfor the mass 4ow of NO:@@x(uYS )+ @r@r(vYS )+ @r@ 

(rwYS )= @@x   e1Y@YS@x + @r@r r e1Y@YS@r + @r2@