Investigation Of Compressed Biomass Pellets Combustion Characteristics Based On Spectral Analysis

Biomass energy is an important renewable energy with large reserves,and it can achieve "zero carbon cycle" in the combustion process.Therefore,biomass energy plays an increasingly important role in the energy structure.However,biomass energy has the characteristics of high K content,high water content and low calorific value.A large amount of K is released into the gas phase during biomass combustion,resulting in problems such as corrosion,slagging and coking that endanger combustion equipment.The measurement method of K release characteristics based on selfradiation spectrum can help optimize the combustion process in biomass boilers,including laser spectroscopy(LIBS)and self-emission spectroscopy.Laser spectroscopy can not realize temperature measurement at the same time,but the temperature measurement by other means(thermocouple,etc.)will affect the spectral signal.On the other hand,the temperature is not taken as a variable into the calculation process of K concentration in the current research on the detection of K release characteristics of self-emission spectroscopy,and the accuracy of the measurement of K release concentration is limited.Compared with fossil fuels,the combustion heat value of biomass is lower,resulting in low boiler heat load and difficulty to meet energy needs;Torrefaction pretreatment can help to improve biomass calorific value and reduce water content.The combustion characteristics and potassium release characteristics of biomass after torrefaction will change.Therefore,the study on the combustion characteristics and potassium release characteristics of torrefied biomass is significant for guiding the torrefaction pretreatment of biomass and the boiler combustion process of torrefied biomass.Based on the above characteristics and research status of biomass energy,this paper first carried out the K characteristic spectral radiation intensity measurement in diffusion flame,and studied the influence of K concentration and flame temperature on K characteristic radiation intensity considering the self-absorption effect.Secondly,this paper carried out biomass single pellet combustion experiments with pine and rice straw as fuel,and explored the combustion characteristics and K release characteristics of different types of biomass compressed pellets.Finally,based on the multi-factor orthogonal torrefaction experiment,this paper carried out the compressed pellets combustion experiments of primary and torrefied biomass,and studied the effects of different torrefaction factors on the effect,as well as the differences in biomass combustion characteristics and K release characteristics before and after torrefaction.The main conclusions of this paper are as follows:(1)the radiation function model among K ion concentration,K characteristic spectral radiation intensity and flame temperature was established:Iκ766(T,CK)=0.821·Ib(λ766,T)·CK·(2)The biomass single Pellet combustion experiment showed that the combustion temperature and K release of straw and pine were bimodal,and the volatilization stage duration was 9.30 s and 10.10 s,and the peak temperature was 1756 K and 1774 K,respectively.The duration of char stage is 54.00 s and 30.48 s respectively,and the peak temperature is 1143 K and 1287 K respectively.The K release concentration in the char stage was much higher than that in the volatilization stage.The K release concentration of straw in the three combustion stages was higher than that of pine wood,which was 10.03 ppm,110.43 ppm,23.59 ppm and 2.09 ppm,27.83 ppm,5.20 ppm,respectively.(3)It was found in the biomass torrefaction experiment that biomass type and oxygen concentration were the most important factors affecting torrefaction,and oxygen atmosphere torrefaction was more suitable for low temperature torrefaction of woody biomass or herbaceous biomass.Further primary and torrefied pellets combustion experiments showed that the ignition time of torrefied rice straw and pine wood decreased from 1.56 s and 0.78 s to 0.94 s and 0.54 s,respectively.Due to the precipitation of part of hemicellulose and moisture in mild torrefaction,the staged combustion time and combustion peak temperature of rice straw and pine wood increased slightly(the combustion time increased from 7.00 s and 7.28 s to 7.84 s and 9.52 s,respectively,and the combustion peak temperature increased by 10.77 K and 11.64 K,respectively).The duration of char stage prolonged(increased from 51.88 s and 35.96 s to 68.76 s and 47.52 s,respectively),but the peak temperature decreased slightly(decreased by 20.48 K and 20.82 K,respectively).