NO Emission And Control During Co-combustion Of Coal And Petroleum Coke

With the development of the petroleum refining industry,the output of petroleum coke has increased year by year.Due to the wide adaptability of the fuel and the low cost of pollution control,the co-combustion of petroleum coke and coal in a circulating fluidized bed（CFB）has become an effective way to utilize high-sulfur petroleum coke.The method of spraying calcium in the furnace to desulfurize the combustion process of high-sulfur petroleum coke are usually adopted in CFB,but the addition of limestone can cause changes in nitrogen oxide emissions.Based on the phenomenon of“sudden drop and slow rise”of NOx during the process of adding and stopping limestone in the industrial field,this paper study in-depth on the NOx emission characteristics and the influence of limestone in the combustion process of coal and petroleum coke by using various experimental methods（including CFB combustion experiments,tube furnace mechanism experiments,physical phase analysis,etc.）.Finally,the key substances that add limestone to inhibit NO emission were found out,and the mechanism of action was clarified.Firstly,the combustion characteristics and NO emission characteristics of the co-combustion of petroleum coke and coal were studied by using the combination of thermogravimetric analysis experiment and tube furnace combustion experiment.The study found that the combustion characteristics of petroleum coke and bituminous coal are relatively similar.When the two are mixed and burned,not only the burning speed is faster,but also the heat release is better than that of lignite and gangue.Due to the improvement of the comprehensive combustion characteristics,the NO emission concentration of the mixed fuel is higher than that of the single fuel,regardless of the blending ratio and the blended coal type.Limestone will promote the formation of NO during the co-firing of petroleum coke and coal.With the increase of Ca/S,the increase of NO is more obvious.This phenomenon has nothing to do with blended coal types.Through further research,it was found that Ca O catalyzes the conversion of coke nitrogen to NO,which is the main reason for the increase in NO emission concentration.Considering the difference in the form of the reactor,the phenomenon that the NO concentration drops sharply when limestone is added has not been reproduced in the tube furnace.Therefore,considering the difference in the form of the reactor,the phenomenon that the NO concentration drops sharply when limestone is added has never been reproduced in the tube furnace.Therefore,the concentration distribution characteristics and variation rules of NO and N₂O in each typical area of the furnace when limestone is added are carried out on the 20k WCFB.The study found that the area with the highest NO concentration was located in the transition area near the secondary air outlet,and N₂O gradually increased along the flue gas flow.The addition of limestone can significantly inhibit the emission of NO,with a reduction rate of 20%,but at the same time promote the emission of N₂O.The area where limestone inhibits NO emission most significantly coincides with the area where the sulfur fixation reaction occurs,and both are in the dilute phase area of the furnace.At the same time,the addition of limestone caused a significant decrease in CO concentration in the dense phase zone of the furnace.Under the strong reducing atmosphere in the furnace,Ca S,as an important intermediate product of sulfur fixation,may have an important impact on NO emission.Combined with the experimental results of the circulating fluidized bed,in the vertical tube furnace,two paths of Ca S generation in the reducing atmosphere of the circulating fluidized bed were confirmed,namely the thermal decomposition of Ca SO₄and the direct generation of the limestone sulfur fixation reaction.Ca S can not only catalyze the reduction of NO by CO,but also participate in the chemical reaction of reducing NO and be consumed,so it exhibits a very strong inhibitory effect on NO.The chemical reaction equation for the possible involvement of Ca S in the reduction of NO was calculated by Fact Sage software.Finally,the phenomenon of"sudden decline and slow rise"of NO in industrial sites is explained rationally by using the research conclusions,and specific technical ideas and implementation points are put forward for the coordinated removal of SO₂and NOx by industrial CFB.