Study on NO Formation and Reduction during Coal Combustion under O2/CO2/H2O Condition

Author:Sun Zhi Jun

Supervisor:xiang jun su sheng


Degree Year:2019





Oxy-fuel combustion technology is one of the main carbon capture technologies and a hot issue in recent years.Studies have also shown that NOx emissions can also be greatly reduced by oxy-fuel combustion.At present,research on the mechanism of NOx conversion in oxy-fuel combustion is mainly focused on the formation and conversion of NOx during dry oxy-fuel recycle conditions.However,there is a large amount of water vapor in the recycled flue gas during the oxy-combustion process,and the combination of high-concentration water vapor and CO2 is still very lacking in the research on the mechanism of NOx formation and conversion,and detailed research is urgently needed.This has important theoretical and practical value for profoundly revealing the essential mechanism of the oxy-fuel combustion.The main work of this thesis is as following:NH3/CH4 was used as the simulated pulverized coal volatiles on the newly designed rig,and the effect of high concentration H2O on NO homogeneous formation and reduction was studied.The results show that,with or without oxygen,the oxidative function of OH increases with the increase of H2O concentration,the concentration of NO increases,and the trend of NO formation is the same as the increase of H2O concentration.However,under oxygen-lean conditions,although the NO formation trend is the same as that of oxygen-enriched conditions,the amount of NO produced under oxygen-lean conditions is greatly reduced.The main reason is that oxidative functional groups such as OH are produced by a large amount of incomplete combustion under oxygen-poor conditions.The CO,H2 and CHi originating molecules,groups and the like are consumed.Under the same oxygen conditions,the order of NO formation is:H2O/Ar>H2O/CO2>CO2/Ar,which is the highest and lowest in Ar/H2O and CO2/Ar atmospheres,and CO2 and H2O participate in the reaction of NO.It will create a competition phenomenon for NO generation.Therefore,compared with the H2O/Ar atmosphere,the combination of H2O/CO2 is more conducive to reducing NO emissions.The main reason is that the high CO2 concentration promotes the formation of reducing function groups,and the increase of H2O concentration promotes the increase of the concentration of oxidized function groups.Furthermore,the effect of H2O on NO was studied by using NH3/CO as the simulated volatiles of coal powder.The results show that the formation and reduction of NO in this condition are the same as those under NH3/CH4.That is,NO increases as the H2O content increases,but the amount of NO produced is greatly reduced.The order of NO formation under high H2O conditions is H2O/Ar>H2O/CO2>CO2/Ar.At the same H2O concentration,the NO concentration in the NH3/CO atmosphere was significantly lower than that in the NH3/CH4 atmosphere.The main reasons include two aspects:on the one hand,high concentration of CO inhibits oxidation of NH3,and on the other hand,CO is consumed by OH formed by high water concentration.Then,the evolution of nitrogen in coal char was studied by using coal char produced in different atmospheres and different residence time of LY coal and ZD coal.As a result,it was found that,in the atmosphere containing H2O,CO2 or H2O/CO2,the nitrogen content in the coal char decreased with increasing the residence time for both coals.Due to the gasification reaction of H2O,CO2 with coal char,the nitrogen content in coal char is lower in H2O and CO2 than that in inert atmosphere.The effect of nitrogen conversion in H2O focusing is stronger than that of CO2,especially for low-order ZD coal.The effect of H2O and CO2 on the evolution of nitrogen is also reflected in the combustion of coal char.The effect of H2O and CO2 on the conversion of nitrogen to NO and the conversion of H2O/CO2to coal to nitrogen to NO are very different.H2O/CO2 co-impact will inhibit NO generation more than individual effects.Based on the above research,the experimental bench of high temperature dropper furnace was built,and the influence of H2O and H2O/CO2 on the formation and transformation of NO during pulverized coal combustion was studied.The results show that high concentration of CO2 contributes to the reduction of NO emissions during coal combustion,and the NO conversion increases with decreasing CO2 concentration.For low-order ZD coal,the change of CO2 concentration is more sensitive to the formation and reduction of NO,and the increase of CO2 concentration contributes to the decrease of NO.For the oxygen-rich wet cycle,the H2O concentration increases and the relative CO2content decreases accordingly.The increase of H2O will promote the formation of NO to a certain extent,and the NO emission increases with the increase of H2O content.Under H2O/CO2 conditions,H2O and CO2 have a competitive effect on the formation and reduction of NO.The increase of H2O promotes the formation of oxidative groups such as OH,and the decrease of CO2 concentration reduces the formation of reducing substances.The study also found that the reduction rate of recycled NO decreased with increasing H2O concentration.At a given initial NO concentration,high concentrations of H2O inhibit NO reduction.By adding 900 ppm NO to the simulated circulating flue gas,it was found that about 20%of the recycled NO was reduced even under conditions of excess oxygen and high concentration of H2O(Vol.40%).There is no clear effect of increased H2O/CO2 molar ratio on NO reburning was observed.In summary,this study firstly studied the homogeneous and heterogeneous reaction mechanisms of NO under wet oxy-fuel combustion with high concentration of H2O addition.And then studied the mechanism of the formation and conversion of NO during pulverized coal combustion at the same conditions.It was found that NO was promoted in wet cycle oxy-fuel combustion with high concentration of H2O addition.There is an optimal H2O/CO2 ratio for both low-rank and high-rank coal,which makes the NO emission the lowest during the combustion.In addition,the study also clarified that,the key reaction step of H2O for NO formation and conversion is H2O+O?2OH under the condition of high concentration of H2O.With the increase of concentration of H2O,and the formation of oxidizing groups such as OH groups will be promoted,thereby promoting NO production.It is expected to provide a theoretical basis for the study of NO emission during oxy-fuel combustion with wet recycle.