Influence Mechanisms Of CO₂/H₂O Gasification During Pulverized Coal MILD-Oxy Combustion

The integration of moderate or intense low-oxygen dilution（MILD）combustion with Oxy-fuel combustion is so-called MILD-Oxy combustion,which has the advantages of both carbon emission reduction and low NO_x emission and is considered as next generation Oxy-fuel combustion technology.The characteristic of pulverized coal combustion differs from gaseous or liquid fuels that is coal char heterogeneous reactions（C+0.5O₂→CO,C+CO₂→2CO,C+H₂O→CO+H₂）.Under MILD-Oxy combustion regime,there exist low O₂ and high CO₂ and H₂O levels at main reaction zone,which result in the significant gasification reaction.Thus,it is of great significance to deeply study the influence mechanism of gasification reaction on MILD-Oxy combustion of pulverized coal in order to better understand the characteristics and theories of MILD-Oxy combustion technology.In order to evaluate the effects of steam concentration on char gasification reaction under different O₂ levels,the single particle model of coal char is selected to be research objective.Results show that,the sensibility of O₂ and steam content on the homogeneous reactions follows:R1（CO+0.5O₂+H₂O→CO₂+H₂O）>R3（CO₂+H₂→CO+H₂O）>R2（CO+H₂O→CO₂+H₂）.The share of coal char reacting with CO₂（char-CO₂）affected by that reacting with H₂O（char-H₂O）is dependent on steam level.Strengthening char-H₂O is benefit to the share of char-CO₂ at below 20%steam level,while adverse to that at above 20%steam level.As O₂ concentration increases,due to the shares of chare-O₂and char-CO₂ are promoted,that of char-H₂O is decreased.Considering the turbulence diffusion is also a key factor for char gasification reaction,the effects of the velocities and temperatures of secondary stream on the heterogeneous reaction behaviors are studied based on IFRF’s 0.58 MW furnace.Results shows that,as jet velocity is increased,due to the pulverized coal particle residence times are decreased and the char burnout rates are shortened.For all cases,the shares of gasification reaction exceed28%,which are increased with the velocities and temperatures are decreases.As the share of gasification reaction increases,more reductive gases are produced and more NO_x are reduced at furnace exit.Under MILD combustion regime,the gasification reaction should not be omitted.Thereby,the effects of gasification reaction on pulverized coal MILD combustion under air and O₂/CO₂ atmospheres are compared and studied.Results show that,under air and O₂/CO₂atmospheres,due to the existence of gasification reaction,the ignition time is delayed 3.15%and 6.78%,and burnout time is decreased 10.5%and 16.8%,respectively.Besides,gasification reaction also results in low NO_x emissions because of the production of reductive gases（CO and H₂）.On the one hand,reductive gases are benefit to NO reduction;on the other hand,they reduce local O₂ level and suppress Fuel-N conversion through Fuel-N?+?O₂?→?NO?+?products.During pulverized coal MILD-Oxy combustion,CO₂/H₂O mixtures affect flame temperature and heat transfer due to their gasification reactivity,heat capacity and heat radiation.Gasification reaction evidently affects the temperature,but only in a narrow region and its effect on heat transfer is negligible.As H₂O level increases from 20%to 50%,heat capacity affects average temperature within 5 K and enhances convective heat transfer.Heat radiation leads to a decrease in the temperature from-0.88%to-2.2%,and the variation of radiative heat transfer increase from 2%to 4%.In the end,the effects of H₂O gasification reaction on char reaction behavior under MILD-Oxy combustion are studied.As O₂ and steam concentration are increased,the share of gasification reaction is increased.For 30%O₂/40%H₂O case,the coal char consumed by gasification reaction accounts for 62%.Besides,the effect of water-gas shifting reaction（CO+H₂O→CO₂+H₂）on generating H₂ becomes evident and is easy to occur under low H₂O concentration case when O₂content is>21%.When O₂ concentration exceeds 24%,increasing H₂O concentration makes a reduction on the char burnout rate.Gasification reaction are enhanced and more reductive gases（CO and H₂）are produced,which is favor of the reduction on NO_x.