Study On The Coupling Combustion Law Of Bio-syngas And Coal And The Determination Method Of Blending Ratio

Due to the serious emission problem of the gaseous pollutants in coal-fired power plants,such as the fossil carbon dioxide(CO₂)and nitrogen oxide(NO_x),and the low utilization ratio of the biomass energy,the co-combustion power generation technology of bio-syngas and coal is regarded as one of the effective technologies for reducing pollution emissions of coal-fired power plants and improving utilization rate of biomass energy.Among them,the sufficient co-combustion of bio-syngas and coal and the accurate determination of the biomass blending ratio are the keys to the stable,efficient and economical operation of coupled power generation units of bio-syngas and coal,and also the precondition for the large-scale application of co-combustion power generation technology of biomass and coal.At present,the effects of the bio-syngas blending on coal combustion characters and reaction kinetics remain obscure in the co-combustion process of bio-syngas and coal,and the accuracy of determine methods of the existing biomass blending ratio is low,whic h have curbed the large-scale application of co-combustion technology of biomass and coal.In this paper,the high efficiency co-combustion and NO ultra-low emissions of the bio-syngas and coal,and the accurate determination of biomass blending ratio are taken as the research objectives.By combining theoretical,experimental,and numerical simulation,the relevant research work is carried out from two aspects:revealing the co-combustion raw of the bio-syngas and coal,and establishing ¹⁴C detection method of the biomass blending ratio.The main contents and conclusions of the research are shown as follows:(1)The co-combustion mechanism of bio-syngas and coal is investigated by combining experimental research and numerical simulation.Firstly,based on the constructed micro-fluidized bed reactor of the bio-syngas and coal co-combustion,the experimental studies of the co-combustion of bio-syngas and coal under different temperature,pulverized coal particle size and heat blending ratio of bio-syngas were carried out.The effects of temperature,pulverized coal particle size and heat blending ratio of bio-syngas on combustion characteristics of coal in co-combustion of bio-syngas and coal were analyzed,and the kinetic mechanism of pulverized coal combustion reaction was investigated.Experimental results reveal that rising the temperature,refining particle s ize of pulverized coal and increasing heat blending ratio of bio-syngas can significantly improve the combustion characteristics of pulverized coal,and the homogeneous ignition mechanism of coal is found in the co-combustion;the first-order reaction model(F1 model)can reasonably describe the combustion process of pulverized coal in the micro-fluidized bed reactor,and the apparent activation energy and pre-exponential factor of pulverized coal combustion reaction are significantly reduced by the bio-syngas blending,among them,when the heat blending ratio of bio-syngas are 0,5,10,20 and 30%,the apparent activation energy,respectively,are 64.01,48.70,45.45,46.75 and 42.35k J·mol-1,and the pre-exponential factor,respectively,are 57.88,9.26,7.10,8.95 and6.7s-1.Besides,the co-combustion process of bio-syngas and coal in micro-fluidized bed reactor is analyzed by numerical simulation.The simulation results indicate that as increase of heat blending ratio of bio-syngas,the peak concentration of CO increases,while the peak concentration of CO₂ and NO decreases.The blending of bio-syngas promoted the homogeneous reaction of volatiles,gasification reaction of coal char and NO reduction reaction,and inhibited oxidation reaction of coal char,thus affecting the distribution of temperature field,distribution of CO,CO₂ and NO component in the reactor.(2)Experiments were carried out on the NO formation law in the coupled combustion of bio-syngas and coal under different temperature,excess air coefficient and heat blending ratio of bio-syngas.Experimental results suggest that the conversion rate of fuel nitrogen displays a first increase and then decrease trend with the rising temperature firstly.Secondly,the conversion rate of fuel nitrogen increases with the increasing of the excess air coefficient.As the coefficient of excess air increases from 1.0 to 1.1,the conversion rate of fuel nitrogen only increases by a small amplitude,while the conversion rate of fuel nitrogen grows obviously as the coefficient increases from 1.1 to 1.25.Thirdly,as the blending of bio-syngas,the NO peak changes from‘single peak'to‘double peak'and the combustion of char-N forms the second peak,the conversion rate of fuel nitrogen decreases with the increasing of heat blending ratio of bio-syngas.Besides,at a bio-syngas blending ratio of 15%,the reduction rate of nitrogen reaches 45.19%.(3)Based on the ¹⁴C quantitative analysis method,the accurate determination method of the biomass blending ratio is establish by theoretical analysis and experimental study.Firstly,a small experimental platform for determination of biomass blending ratio is built and the calculation model for the biomass blending ratio is established,the influences on the uncertainty and accuracy of the determination method of biomass blending ratio were studied mainly from the combustion atmosphere,measurement method of ¹⁴C content of flue gas,and calculation model of the biomass blending ratio.Experimental results indicate that the liquid scintillation counter has an extremely low background of 0.4733±0.0308 CPM and the system detection efficiency is 75.8%,which indicates that the designed and established experimental platform is reasonable and stable.The combustion atmosphere has significant influence for the uncertainty of the determination method of biomass blending ratio,but the accuracy of it is inapparent.The ¹⁴C measuring method of flue gas and calculation model of biomass blending ration have a great impact on the uncertainty and accuracy of the determination method of biomass blending ratio.In addition,the accuracy of the ¹⁴C quantitative analysis method in the determination of biomass blending ratio is evaluated.According to the analysis results,as the biomass blending ratio increases,the uncertainty of determination method of biomass blending ratio increases,but the accuracy of it decreases.When the biomass carbon-based blending ratio is greater than 6.53%,the relative standard error and relative error of the biomass blending ratio detection method are both within 10.00%.(4)The determination method of the biomass heat blending ratio in co-combustion power plants is established.Firstly,a sampling system of flue gas applicable to the ¹⁴C quantitative analysis method is proposed and a medium-sized experimental platform for determination of the biomass heat blending ratio is constructed.Secondly,the prediction model of the net calorific value of coal and biomass fuel is established,the calculation model of the biomass heat blending ratio for the co-combustion power plants is derived,and the verification experiments for the determination of biomass heat blending ratio are carried out.In addition,the effects of reference substance,background substance and ¹⁴C content of air on the accuracy of determination method of the biomass heat blending ratio are analyzed.The analysis results reveal that the as-received basis net calorific values of coal and biomass fuel are highly linearly correlated with the percentage content of carbon element.The established prediction models for the low calorific values of biomass and coal have errors separately lower than 5%and 8%.The reference substance exerts large influences on the accuracy of the determination method for the biomass heat blending ratio,the influence of ¹⁴C content of air takes second place,while the background substance only shows an influence within 1.00%.Secondly,combined with the case of a co-combustion power plant,the accuracy of the established determination method of biomass heat blending ratio was verified.The results show that the determination method established here is found to have higher accuracy,and the relative error is reduced by 17.3%.