OCR Text |
Show D. Marlow and T. S. Norton through HeN. Production of NO occurs via the Fenimore mechanism chiefly by the pathway: CH+ N2 -+ HCN-+ NC()-+ NH-+ N-+ NO In the absence of oxygen, the HeN is not oxidized to NO, but remains as a stable product. For more fuel rich situations, recycle of NO to HeN and N2 becomes important: NO+CH-+HCN-+N2 This pathway causes the reduction of NO observed in reburning. Our domain of interest also includes lo'w-temperature, fuel-lean combustion. Under these conditions, the N20 mechanism for NO formation becomes significant: N2 +O+M-+ N2 ()-+ NO III. Reduced Mechanism The skeletal mechanism is reduced to seven rate equations by applying partial equilibrium and steady state assumptions. Application of the steady-state assumption here assumes that the change in concentration from a species' initial value is small relative to the rates of production and destruction. The assumption leads to an algebraic expression for the concentration of the steady-state species. Since there are no derivatives in the conservation equations for the PSR, the equations for all species are algebraic equations, and little or no reduction in computational effort results from the steady-state assumptions. When convection and diffusion are important to a problem, finite differencing is applied. Then the steady-state assumption decouples the species equations for each node, resulting in significant simplifications to the numerical problem. The following species are assumed in steady state: 0, CH), 3CH2, lCH2, CH20H, CH20, CH, HCO, C, H02, CN, NCO, NH, N, N20, NH1, NNH, HNO. The first 15 of these were assumed in steady-state by Glarborg et ale (1992). The last three species are added here for NH3 conversion. In addition, OH is assumed in partial equilibrium. The steady-state assumptions for the first 15 species listed above have been tested thoroughly for combustion in a PSR by Glarborg et ale (1992), as has the partial equilibrium assumption for OH. The validity of the steady-state assumptions for the 3 remaining species (NH1, NNH, HNO) has been established in low-pressure, premixed flames by Lindstedt et al. The validity of these assumptions for PSR conditions is established by the agreement between the skeletal and reduced mechanisms observed 5 |