Experimental Study On Ignition Combustion And NO_x Emission Characteristics Of Pyrolyzed Char/Coal Under Blending Condition

The world economy is entering a period of high-speed development,there is a greater demand for energy,and at the same time,it also put forward higher requirements for the utilization of energy.Although the proportion of new energy and renewable energy is increasing year by year,coal,as a traditional fossil fuel,can provide stable power output and guarantee energy security.In addition,coal also plays a very important role in energy by-products and chemical industry.Coal pyrolysis produces gas and oil,residual char has a huge scale and high calorific value.Co-combustion with higher volatile coal is a measure with great application prospect,so as to realize the classification and quality utilization of coal.Large-scale consumption of carbon-based fuels generates a large amount of polluting gas NO_x and greenhouse gas CO₂,causing great harm to the environment and leading to global climate warming.Oxy-steam combustion technology,as a new combustion technology,adjust flame temperature with steam,and steam also participates in the combustion reaction.The generated reducing gas can well reduce the concentration of NO_x emissions,and is also a good CO₂ capture technology.In this research,the ignition,burnout and NO_x emission of Shenhua bituminous（SH）,pyrolyzed bituminous char（PB char）and blended fuel under air and Oxy-steam conditions were studied using advanced optical diagnostic technology in a flat-flame entrained flow high temperature reactor.The influence of temperature,oxygen concentration,steam concentration and other factors was investigated.This will make contribution to achieve China’s carbon peak,carbon neutral goal,to achieve clean and efficient utilization of coal,to achieve intelligent energy and technology energy.The ignition behavior and combustion characteristics of pyrolyzed bituminous char particle stream were studied in a flat-flame entrained-flow pulverized coal/char reactor（EFR）.A highly volatile content bituminous pulverized coal was also tested for comparison.The OH planar laser-induced fluorescence（OH-PLIF）,CH^*/NO^*chemiluminescence,full spectrum spontaneous emission,and three-color high-temperature pyrometer study were performed to determine SH coal and PB char ignition process,flame temperature,and flame structure.In the EFR,the heat-up and ignition of the PB char were retarded with a decrease in both ambient oxygen concentration from30%to 5%and ambient temperature from 1800 K to 1600 K.Under 30%oxygen concentration,the PB char combustion was particularly intensified.The rapid volatile evolution at initial stage appeared to have a little effects on PB char ignition,and the heterogeneous combustion mode is dominant in the pyrolyzed char particle stream.The volatile combustion and NO release characteristics of pulverized fuel（PF）flames in an optical flat-flame entrained-flow reactor were investigated by CH^*/NO^*chemiluminescence imaging.Pyrolyzed bituminous char with an ultralow volatile content(on a air dried base V_ad of approximately 10%)and a comparative coal sample of Shenhua（SH）bituminous coal with a high volatile content(V_ad=30.25%)were studied under different ambient O₂ concentration（10%～30%）.The initiation of the combustion region of PB char occurs later than SH bituminous coal,owing to its low volatile contents,slow devolatilization rate,and homogeneous-heterogeneous ignition/combustion mode.The conversion ratios of fuel-N to NO and the releasing ratios of volatile-N/char-N were determined during ignition process.Semiquantitative NO release integrated intensity and volatile combustion intensity results can be obtained by integrating the NO^*or CH^*chemiluminescence intensity in the corresponding reaction region.The volatile combustion and NO release show good synchronous for SH coal,which means that the NO releasing originated predominantly and showed higher conversion ratio from the volatile-N in the ignition region.NO releasing originated mainly from char-N during ignition late stage for PB char.The homogeneous oxidation/reduction reactions for NO plays a significant role during the ignition initial stage and the char heterogeneous reactions for NO is predominant during late stage especially for ultralow volatile PB char.The enriched-O₂ environment is beneficial for SH bituminous to decrease NO_x emission during ignition stage.The ignition behavior,burnout and interaction between fuels of pulverized pyrolyzed bituminous char blended with pulverized bituminous coal at different mass fractions were studied.Experimental studies were conducted with OH planar laser-induced fluorescence（OH-PLIF）diagnosis technology in a flat-flame optical entrained flow reactor.An interaction occurred between pyrolyzed char and bituminous coal,which was indicated by comparing the mass fraction mean values（MFMVs）and the experimental values of the blended fuels.The addition of bituminous coal enhances the competitive effects of the two fuels on ignition and burnout.The synergistic effect observed upon ignition of the blends is mainly attributed to the high volatile contents and the potential catalysis effects from the alkali and alkaline earth metal species in the blends.The important factor leading to this synergistic effect is that the alkali and alkaline earth metal species in the ash can reduce the ignition temperature and accelerate the release of volatiles,thereby shortening the ignition delay.The high flame temperature and the potential catalysis effects from the alkali metal and transition metal species promote blended fuel burnout.Considering both the ignition delay and the residual char burnout,the optimal blending ratio of pyrolyzed char is approximately 20%because the fixed carbon burnout of the blends was considerably promoted and the ignition delay was significantly shortened.The devolatilization,ignition and volatiles burnout of single bituminous coal particles in particle stream with a size fraction of 53-80μm were investigated with a high spatial and temporal resolution,large field-of-view OH planar laser-induced fluorescence（OH-PLIF）diagnosis technology in an entirely premixed laminar flow flat-flame reactor.OH-PLIF diagnostic images with high temporal and spatial resolutions and a large-field-of-view cover the overall space from ignition to the end of volatile combustion of individual coal particles in the particle stream.The radial OH distribution surrounding single coal particle and the corresponding distances of its peak value to the coal particle center were identified.With the high temporal resolution of OH-PLIF images of a single coal particle,Equivalent diameter of flame front,growth rate of equivalent diameter of flame front,and entire volatile combustion time were identified and measured.By comparing the ignition and volatile combustion of single particle and group coal particle,the influence of the interaction among particles on ignition delay and initial volatile combustion can be negligible.The volatile combustion duration of particle stream increased due to the interaction among particles.The growth rate of equivalent diameter of flame front depended strongly on the oxygen concentration of the ambient flue gas atmosphere.When the ambient oxygen concentration increased from 10%to 30%,the growth rate of equivalent diameter of flame front decreased with the increase of oxygen content.The NO_x emission and ignition behaviors of pyrolyzed char,blended fuel and Shenhua bituminous coal in Oxy-steam atmosphere were studied using OH-PLIF diagnostic technology and FT-IR infrared flue gas analyzer.The influence of steam concentration,oxygen concentration and the interaction between the two fuels in the blended fuel on ignition and NO_x emission were investigated.As steam concentration increased,the ignition delay distance of Shenhua bituminous coal,pyrolyzed char and blended fuel increased,and the NO emission concentration decreased.As oxygen concentration increased from 5%to 15%,NO emission concentration decreased in 0-20%steam atmosphere.As oxygen concentration increased from 5%to 15%,NO emission concentration increased in 30%-50%steam atmosphere.As oxygen concentration increased from 3%to 40%,the ignition delay distance of Shenhua bituminous coal and blended fuel decreased monotonically,while the ignition delay distance of pyrolyzed char increased first and then decreased.When oxygen concentration is 5%and steam concentration is 30%-50%,there exists a synergistic effect between pyrolyzed char and Shenhua bituminous coal,which can promote the ignition and reduce NO_x emission of the blended fuels.