OCR Text |
Show between the formation and reduction of N O in the char's surface. The formation mechanism of char-NO is controlled by the carbon atom sites capable of producing N O . The fraction of carbon atom sites capable of producing N O (Xc ) can be calculated from the overall carbon active sites, i.e. -C{t)=(-Cf) + (-CH) + (-CN) During the oxidation process, other possible active sites formed by oxygen absorption, i.e. (-C0) and {-CN0) should also be included;. -C{l)=(-Cf) + (-C0) + (-CN) + (-CN0) + (-CH) Therefore the reaction mechanism for NO formation and reduction can be represented in terms of carbon active sites: (-cN) + (-cf)] + o2 -L>(-c0) + (-cN0) i-CNO)-^rNO + {-Cf) and NO + (-Cf)-^-> Vi N2+(-C0) and the fraction of the CN0 active sites can be related to the total active sites: XCN0 =(-CN0)/-C(t) Therefore, by calculating the time variation of the active site fraction, the net rate of production of N O can be predicted, i.e. (6N0 = k2 Xc - k3 PNO. 3. General Conclusions The combustion of the volatiles poses a number of problems in relation to the mixing and the soot burnout stages, although both of these are subject to useful approximations. Char burning is also complex due to two principal factors. Firstly, the role of the original fuel-nitrogen functionalities on N O x release is not fully resolved. Secondly, the kinetic regimes may change as the particle sizes get smaller and cooler. The flamelet model has proved useful for addressing the first of these problems, namely combustion of the volatiles, by using detailed nitrogen chemistry. Certain factors are not accurately quantified at the present time because of the non-homogeneous nature of coal. These are (i) the influence of rank on devolatilisation, (ii) the efficiency of N O x release from the char under normal and reburning conditions, and (iii) combustion of the unburned carbon, and 10 |