Optical Diagnostics On The Ignition And Co-combustion Characteristics Of Pulverized Coal And Biomass Particles

Facing the severe situation of carbon emission reduction,China must give consideration to the clean and efficient utilization of coal resources and the development of renewable energy.The co-combustion of coal and biomass is an important way of the joint utilization of fossil fuels and renewable energy.Biomass is a carbon-neutral fuel,but it also has the characteristics of low energy density and high alkali content.The differences in microstructure and chemical composition lead to great differences in the characteristics of ignition and combustion between biomass and coal.To directly use biomass fuel in existing pulverized coal combustion equipment,it is necessary to have a clearer understanding on the combustion characteristics of coal and biomass.In addition,the existing research on the mixed combustion characteristics of coal and biomass mostly uses fixed-bed reactors and offline measurement methods,and lacks online measurement data in complex combustion environment.Aiming at the above problems,the work in this paper built a carrying flow reaction system based on a Hencken burner,which provided an experimental environment close to the real industrial furnace.Combining with optical diagnosis technology,the research work of multi-scale and multi-working conditions was carried out.First of all,with coal and biomass single particles as experimental objects,a single-particle combustion optical measurement system with spectrum resolution and time resolution was built to comprehensively analyze the full cycle process of volatile and coke combustion of coal and biomass single particles.The results display that the ignition delay time of fuel decreases linearly with the increase of volatile content.In most conditions,the ignition mode of biomass and lignite particles is homogeneous ignition,while that of single bituminous coal particles is heterogeneous ignition.The influence of fuel pyrolysis characteristics on the ignition mode was analyzed by chemical permeation devolatilization（CPD）model.For the oxy-fuel combustion conditions,it was found that CO₂ atmosphere has a significant inhibiting effect on the ignition and combustion of particle,and the increase of flue gas temperature and oxygen content can reduce the ignition delay time.For single particles of biomass and lignite,the reduction of particle size will shorten the ignition delay time,but for single particles of Zhundong coal,in certain particle size ranges,the reduction of particle size will lead to ignition lag.Subsequently,the research object was expanded to the combustion of pulverized coal and biomass particles clouds.OH-PLIF was used to observe the volatile flame structure of particles clouds.Based on the flame image analysis,the influence of Reynolds number,temperature,oxygen and biomass blending ratio on the ignition and combustion stability of fuel particles clouds were systematically evaluated.The results show that in turbulent conditions,the entrainment of air flow and the dispersion effect of particles enhance the heat flow and material transfer between fuel particles and background flue gas,which greatly reduces the ignition delay time of fuel particles clouds.The increase of ambient temperature and primary O₂ fraction can also improve the ignition characteristics of particle clouds,but once once a certain threshold is exceeded,its influence on the ignition delay of particle groups will become weak,and the dominant factors controlling the ignition behavior will change.For the mixed combustion of coal and biomass,it was found that the characteristics of high volatile content,low devolatilization and ignition temperature of biomass does promote the ignition of mixed fuel,but in turbulent conditions,if the biomass is directly used to replace pulverized coal with equal weight,the gounp flame will be significantly weakened.The characteristics of biomass with low density and easy dispersion in the radial direction of jet will lead to the deterioration of jet rigidity,which is not conducive to particle agglomeration and group combustion.Meanwhile,the combustion temperature of biomass particles is lower than that of coal,which further leads to the decline of mixed combustion stability.Based on the experimental data,this paper gives some reasonable suggestions on the design of coal and biomass mixed combustion conditions.Furthermore,in view of the high content of alkali metals in Zhundong coal and biomass fuels,combined with single particle optical measurement and thermogravimetric analysis,the releasing and catalytic combustion characteristics of different types of alkali metal were evaluated,further verifying the synergistic effect during the process of the mixed combustion of coal and biomass.The results show that the release of K and Na from biomass particles has two distinct stages:volatilization stage and coke stage,while only one Na release peak exists in the whole combustion process of Zhongdong coal particles.The optical measurement results show that alkali metals have little effect on the combustion intensity of volatile matter,but can significantly improve the combustion reaction activity and combustion intensity of coke.Especially for biomass,organic alkali metals with lower content play an important catalytic role in the combustion stage of coke.Combined with thermogravimetric experiments,it was found that in the process of fuel pyrolysis,alkali metals play a role in regulating the types of pyrolysis products.In the process of coke combustion,alkali metals promote coke burning through-CA（C stands for coke,and A stands for alkali metal）oxidation-reduction circulation mechanism,reducing the reaction activation energy.The experimental data fully proves that in the co-combustion process of coal and biomass,K released from biomass can improve the ignition and combustion characteristics of pulverized coal coke and promote the burnout of mixed fuel.In addition,the actual combustion equipment usually uses the preheated air as oxidant.Aiming at the problem that gas preheating will lead to the enhancement of soot formation during the combustion process,this paper established a series of gas preheating type（293 K～723 K）sooting diffusion flames.The soot volume fraction and soot temperature distribution in the flame were measured by LII and two-color pyrometry.The influence mechanisms of gas preheating and nitrogen addition on soot formation were explored and the database of sooting flames was enriched.The results show that,with the increase of preheating temperature of fuel and co-flow air,the overall temperature of flame rises and the generation of soot precursors accelerates.Further,the evolution rate of soot accelerates and the total soot loading increases.However,this enhancement effect is more significant in the flame with weak soot formation character（such as methane and ethane flames）.Gas preheating brings about the temperature rise in reaction front areas of all flames,but in the central area of propane and ethylene flames,incomplete combustion heat loss and soot radiant heat loss caused by extremely high soot loading result in some abnormal temperature changes.The addition of nitrogen can significantly inhibit soot formation,mainly through dilution effect（reducing reactant concentration）and thermal effect（changing the overall temperature of the flame）.The former is dominant only when N₂ is added to the flames with extremely high soot loading.