| Oxy-Coal Combustion Steam System of Near-zero Emissions（OCCSS）is one of the effective methods to achieve high efficiency and near zero emissions of CO2.The syngas from coal burns with pure oxygen with the presence of steam under high pressure,high preheat temperature conditions（namely“four high”condition）.In order to achieve combustion reliabilly and rapidly burnout under O2/H2O atmosphere,the statistical narrow band correlated-k radiation model（SNBCK）and optical thin model（OPT）was used,which improved the one-dimensional,adiabatic,counterflow diffusion flame model.The improved radiation model was verified.The results show that the two radiation models are basically consistent with the results in the literature,which proves that the modified OPPDIF code accurately simulates the combustion characteristics of the hedging diffusion flame.We selected the reaction mechanism under high water conditions for chemical kinetics calculation of syngas,and analyzed the effects of O2/H2O ratio（0%60%）,CO/H2 ratio（5%H295%H2）,and the strain rate(30150 s-1)on CO emissions.The radiative,thermal,transport and chemical effect of H2O were investigated in the combustion process,and explored its impacts on CO emissions.The maximum flame temperature was calculated using the SNBCK and OPT radiation models.It was found that the maximum flame temperature simulated by the OPT model is lower than that of the SNBCK model,which is because the SNBCK radiation model considers the radiation reabsorption to reduce the radiave heat loss.As the strain rate increases,the difference between the maximum flame temperatures decreases.For H2-lean syngas,when the stretch rate is 150 s-1,the difference between them is small,decreasing by 0.54%to 0.22%,that is,radiation reabsorption is negligible.With the increase of the strain rate,the accuracy of the OPT model is improved.When the CO/H2 ratio is less than 50%,in contrast with air combustion,the maximum flame temperature under O2/H2O conditions is higher than the air condition temperature,and when the CO/H2 ratio is above 50%,the change is the opposite.The role of H2O in O2/H2O atmosphere is to broaden the flame temperature reaction zone,and the flame front is slightly closer to the oxidant side.As the H2 content increase,the CO emissions under both conditions decrease accordingly and the difference reduces.The oxidation of CO is mainly through CO+OH=CO2+H.The chemical and radiative effects of H2O have a significant impact on the emission of CO,which inhibit CO production and decrease CO concentration.The thermal and radiative effects of H2O have little effect on CO formation.In addition,the chemical effect of H2O progresses the flame temperature,the thermal and radiative actions reduce the flame temperature,and the transport effect on the temperature is almost negligible.Chemical kinetic analysis shows that the effect of H2O on flame temperature is achieved through the effect of heat release rate.The addition of H2O inhibits flame temperature and OH and H radicals.The CO content at the flame front decreases with the increase of H2O.The higher the H2O content,the more obvious the dilution effect of H2O,the stronger the effect of H2O on reducing CO emissions in the diffusion flame of syngas.With the increase of H2 content,temperature and OH and H radicals progress,and both of the flame front and the max of OH and H radicals both move toward the oxidant side.The CO concentration reduces at the flame front with the increase of H2 content.The higher the H2 content,the lower the inhibition effect of H2 content on the CO emission in the diffusion flame of syngas.The increase of the strain rate promotes the emission of CO at the flame front,and the difference between the CO content at the flame front simulated by the SNBCK and OPT radiation models decreases with the stretch rate increasing,when the strain reaches150 s-1,the difference is small.For the lean-H2 syngas,the difference decreases from7.29%to 2.23%. |
