Process And Operational Parameters For Low-temperature Combustion Of Maize-stover-char

Utilization of biomass is a good choice for reducing the greenhouse effect and meeting the increasing demand for energy because it is carbon neutral and abundant in nature.Combustion is one of the most promising technologies to utilize biomass.However,the high temperature of the solid phase during combustion causes many problems in current combustion technology including slagging,fouling and corrosion of heat exchanger and high particulate-matter emissions.If the low-temperature combustion of solid phase can be achieved,the above problems can be minimized or avoided.The ratio of CO/CO₂and combustion kinetics at char combustion stage are key parameters in the design of low-temperature combustion apparatus for industrial application.Study on CO/CO₂ ratio during char combustion shows that the effect of char type on this ratio is obvious.However,result on this ratio during the combustion of biomass char at low temperature has been rarely reported.Derivation of combustion kinetics of biomass char is mainly based on volume model.However,the assumption in this model that the reaction takes place homogeneously inside particle does not agree with the situation in industrial application.Model based on surface reaction which is widely used for analyzing the combustion of coal char can be efficiently applied to relevant industrial calculation.Therefore,it’s necessary to know the combustion kinetics of biomass char at low temperature based on surface reaction.To the best of our knowledge,few investigations on this have been conducted.This study focuses on the CO/CO₂ ratio and combustion kinetic parameters based on surface reaction during combustion of biomass char at low temperature.In this study,maize stover was chosen for experiment because it is abundant in China.The ratio of CO/CO₂ and combustion kinetics during combustion of maize-stover-char at low temperature were obtained and the smoldering process（one typical low-temperature combustion）of maize-stover-char was modelled based on obtained parameters.Relevant results can provide theoretical support for the design and optimization of low-temperature combustion technology.Firstly,the pre-experiments with maize-stover-char（byproduct in the production of bio-oil using fluidized bed）were performed in a simultaneous thermal analyzer（STA）.Different types of crucibles（one aluminum crucible/a pair of crucibles）,different sample sizes and air flow rates were used in experiments.The CO/CO₂ ratio and combustion kinetics were obtained based on mass loss rate（DTG）and heat flow（DSC）data.Through the analysis of experimental results,the disadvantages in the design of experiments were found out.Main conclusions are shown as the followings:（1）A pair of crucibles should be used in order to get accurate DSC information.（2）Big air flow rate should be used in order to make combustion kinetically-controlled so as to obtain accurate kinetic data.（3）Combustion kinetics based on surface reaction are more appropriate for industrial calculation of combustion rate compared with kinetics based on volume model.（4）The results based on the type of char used in pre-experiment cannot be directly applied to industrial calculation because of its low carbonization degree.Despite this,the relevant results can provide theoretical support for the improvement of experiment design and the treatment of similar byproduct.Secondly,the experimental design was optimized based on above results.The combustion experiments with maize-stover-char produced using new method（peak pyrolysis temperature of 500℃ and holding time of 2 h）were carried out in STA.The CO/CO₂ ratio and combustion kinetics based on surface reaction were obtained based on DTG and DSC data.The reliability of kinetic data was validated via calculating the ratio of oxygen transport resistance to kinetic resistance.Results are shown as the followings:（1）The temperature has little effect on the CO/CO₂ ratio at low temperature and the ratio is around 0.45 at the range of 400℃～500℃.（2）There is a temperature difference between reaction zone and measured point,which can result in a significant shift of DTG data towards low-temperature zone and further influence the accuracy of kinetic data.Therefore,the correction of the temperature should be made before calculation.（3）Combustion kinetics of maize-stover-char at low temperature based on surface reaction were then derived from the corrected DTG data.The pre-frequency factor（A_s）is 6655 m·s^-1 and the activation energy（E）is 91.5 k J·mol^-1.（4）At the initial stage of char combustion in STA,oxygen transport resistance is negligible compared with kinetic resistance.This shows that the DTG data used to derive kinetics are from kinetically-controlled zone,which validates the reliability of the obtained kinetics.Lastly,the temperature distribution inside piled char during its combustion in STA was analyzed and the smoldering rate of maize-stover-char was calculated using obtained CO/CO₂ ratio and combustion kinetics.Results are shown as the followings:（1）The temperature difference（tens of Celsius degree）between the reaction front and measured point does exist.For sample size of 30 mg（full crucible）,the calculated result is 70℃ and the experimental result is 75℃.（2）The shape of mass-loss curve based on calculated smoldering rate is similar to that of experimental curve in the smoldering of maize-stover-char.Besides,the calculated combustion rate is close to the experimental one（relative error is below 20%）.This shows that the combustion kinetics of maize-stover-char at low temperature based on surface reaction can be used to efficiently predict the burnout time of piled combustion of biomass.