Laser Diagnostic Measurement And Numerical Simulation Of Alkali Release From A Burning Coal/Biomass Pellet

The clean utilization of fossil fuels and the development of renewable energy are both needed to be taken into account in the world nowadays.In the present study,the release of alkali in the combustion process of coal and biomass was investigated.A self-built advanced laser online diagnostic system was employed to simultaneously measure the combustion characteristics of the coal and biomass pellets and the release characteristics of alkali.According to the experimental results,modeling and numerical simulation approaches were used to develop the models of char combustion and alkali release.Through the combined experimental and numerical methods,a better understanding of particle combustion and alkali metal release characteristics can be achieved,which provides the theoretical basis for the practical utilization of coal and biomass.Firstly,based on the self-built single-pellet combustion system,the combustion and co-firing characteristics of Zhundong coal and corn stalk were investigated.The effects of temperature,particle size and alkali on char combustion were studied via the system.The experimental results show that biomass has an obvious acceleration effect on combustion during the char stage of the blended fuels;increasing temperature,reducing particle size or adding alkali can enhance the reactivity of char.Based on the experimental results,a shrinking char combustion model was developed,and the kinetic parameters of Zhundong char combustion were obtained.The simulation results of Zhundong char combustion using the developed shrinking model agree well with the experimental results under different temperatures and particle sizes.The release and transformation characteristics of alkali during coal/biomass combustion were then investigated by a multi-point LIBS system coupled with temperature and mass sensors and chemical analysis methods,respectively.These methods were also used to investigate the alkali release and transformation characteristics during the co-firing of Zhundong coal and corn stalk.Quantitative LIBS measurements can be achieved according to the Bill-Lambert law.The sodium release during Zhundong coal combustion was found to have three characteristic stages:?1?the devolatilization stage;?2?the char burnout stage;and?3?the ash cooking stage.The potassium release from the poplar also has the above three characteristics,but the potassium release from the corn stalk only has the first devolatilization stage.The main release stage of the sodium from Zhundong coal is the char burnout combustion stage,while that of the potassium from the biomass is the devolatilization stage.During the co-firing process,a higher proportion of biomass will enhance the release of potassium while a higher proportion of Zhundong coal will bring a higher release of sodium.It was found that biomass in the blends can accelerate the combustion and outgassing processes.Thus,if the proportion of biomass is higher in the blends,the alkali release peak appears at an earlier time,while the peak concentration of the released sodium and potassium decreases and increases,respectively.Furthermore,the interaction between coal and biomass has generated crystals causing eutectic melting phenomenon,which results in a sharp decline of ash fusion temperatures of the blends.The alkali release was found to be closely related to the particle burning stages by analyzing the correlation between sodium release,particle surface temperature and particle diameter.A linear relationship was found between the residual sodium mass in the particle and the volume of the particle.The volatile sodium release rate obeys a two-step Arrhenius expression.Predictions by the developed two-step kinetics model agree well with the measured sodium release profiles in all the three burning stages.The gas phase distribution of alkali compositions were further studied by PLIF in this study.A 330 nm Na-PLIF and a coupled polarizer K-PLIF system can effectively eliminate the interference from the flame radiation and scattering during the combustion of coal and biomass particles.Based on the Beer-Lambert law,the quantitative distribution of alkali atoms can be obtained from the absorption of laser inside the measurement zone.The release characteristics of atomic alkali are very similar to that of the volatile alkali.Under the high temperature of flame,the atomic sodium contributes about 24%of the total sodium,while the atomic potassium accounts for about 0.8%of the total potassium.The major component of the gas phase alkali is hydroxide alkali.Finally,a map of potassium transformation processes during the combustion is developed.Starting with inorganic and organic potassium,there are eight proposed transformation pathways and five proposed release pathways during the combustion.The pathways describe the transformation of potassium between the fuel volatile matter,char,and ash.Finally,an open-source Lattice Boltzmann?LB?code DL_MESO has been further developed for gas-solid two-phase combustion.A counter-flow premixed propane flame was simulated using the developed Lattice Boltzmann method?LBM?and the results agree well with previous published data.Based on the validated code,a char combustion model,which considers the ash inhibition effect on oxygen diffusion and incorporates a sodium release model,was adapted in the LBM to simulate the combustion of a single char pellet.The simulation results well predict the shrinking of the char pellet,sodium release and diffusion,and the reactions in both the solid and gas phases.The simulation results of the particle mass,particle temperature and sodium release agreed well with the optical measurements,which validates the developed LB approach.The validated LB approach was then used to simulate the combustion of porous char and sodium release inside the porous medium.The simulation well presents the pore-structure destruction and flow path variation as combustion proceeds.The combustion products?CO and CO₂?show different distribution characteristics from the volatile products?sodium vapor?inside the porous char.The combustion products are mostly generated at the flame front and diffuse with flow towards inside the porous char.However,volatile sodium vapor forms in the entire porous char and tend to accumulate in regions where the flow motion is weak as in internal flow microchannels or blocked as in closed pores.