The Combustion Characteristics Of Blend Biomass In A Fixed Bed Reactor

Power generation by direct biomass combustion is the most mature technology and can realize large-scale biomass energy utilization,among which combustion technology in grate boiler has the advantages such like simple operation,intuitive,and small investment.This technology has attracted wide attention from China and “One Belt and One Road” cooperation countries with China.However,the energy density of agricultural biomass waste is low and the content of alkali metal and chlorine is high.Therefore,how to effectively improve the combustion efficiency of biomass,and investigate the emissions of alkali metal and chlorine,are the key points to develop power generation technology by biomass combustion.In this work,corn straw and pine wood,the most productive agricultural and forestry wastes in northeast China,were selected as model biomass.Several experiments were operated in fixed bed reactor to investigate the combustion characteristics:（1）The combustion characteristic of corn straw in fixed bed reactor;（2）The co-combustion characteristics of blend biomass of corn straw and pine wood;（3）The release of alkali metals and chlorine during the combustion process of single and blend biomass waste;（4）Coupling verification of experimental research and theoretical simulation using Euler-two-dimensional model.Based on the above studies,this work proposed the optimal combustion conditions of biomass from the comprehensive consideration of combustion efficiency,pollutant emission（NO_x,SO₂,and chloride,etc.）and alkali metal release,which can provide important data and theoretical support for the practical application of biomass in power generation technology.The typical agricultural biomass corn straw in north China was selected.Effect of parameters such as primary air flow and fuel characteristics（corn straw length,moisture content,and ash content）on bed temperature,flame propagation velocity,burnig rate and gas release（CO₂,CO,NO_x,HCN,and SO₂）during combustion in fixed bed were studied.Results showed that in a critical range of primary air volume,the burnout of corn straw increases with the increase of primary air volume.With the increase of the corn straw length,the ignition delay time becomes longer,the flame propagation speed and the maximum bed temperature increase,but the lamination regularity of corn straw becomes worse when the length is too long.If the moisture content is appropriately increased,the flame front temperature will increase,and the local flame propagation speed will increase,resulted in the decrease of conversion rate of C to CO,and the increase of conversion rate of S to SO₂.The increase of ash content increased the maximum temperature of the bed,which leads to serious slagging phenomenon in combustion and reduced the burnout of fuel.The energy density of corn straw is low,and the combustion efficiency is also relatively low.To solve this problem,the mixed combustion method of corn straw and forest biomass pine wood with high energy density and compactness was presented.The experimental study was carried out on the variation of the mixing ratio of corn straw to pine wood chips,the air supply condition and the fuel length under the typical mixing ratio.Results show that when the mixing ratio of corn straw was 30%-50%,there was obvious interaction between corn straw and pine in the combustion process,the stability of combustion in bed was enhanced,the flame propagation speed was the fastest,the ignition delay time was reduced,an d the emission of NO and other nitrogen oxides was the minimum.The combustion efficiency and NOx emission are considered comprehensively to propose the best parameters,that is,the mixed fuel with a mixing ratio of ～50% of corn straw and pine wood blocks with a length of 5 cm can be burned under an air-rich component with a primary air temperature of 85 °C,which can effectively improve the combustion stability and combustion efficiency.Meanwhile,the release amount of pollutants in the flue gas is relatively low.Based on the results of layer combustion of biomass in fixed bed reactor,this work further investigated the release and conversion of alkali and alkaline earth metals（K,Na,Ca,and Mg）.Results show that with the increase of primary air flow rate,the migration of alkali metal K and Na to gas phase was obviously enhanced,and the release amount of K was about 5 times that of Na.About 88% of the K in pine is in the form of inorganic salts.Under the condition of high primary air volume,most of the K in the lamination process is released into the gas phase as KCl,accompanied by a small amount of K₂CO₃.Additionally,the increase of primary air temperature（20 °C-135 °C）leads to the release of a large number of alkali metals,and also promotes the release of HCl.The mixture of corn straw and pine wood with low alkali metal and chlorine content leads to the increase of alkali metal chloride release and the continuous release of fly ash particles.However,mixed combustion inhibited the release of HCl.Based on the experimental study of biomass combustion in fixed bed reactor,this work further uses Euler two-dimensional model to simulate and verify the experimental results.The temperature distribution of fixed bed and the gas generated during combustion were compared and analyzed.Furthermore,the complex layer combustion process and combustion structure were further simulated（evaporation,volatile matter release,volatile matter and coke combustion）.The simulation results obtained by Euler-two-dimensional model are in well agreement with experimental results on fixed bed temperature,flame propagation velocity,combustion velocity and gas evolution trends.The simulation results well reflect the stability of biomass combustion in fixed bed.The bed mass decreases linearly and the mass loss changes uniformly.However,the non-uniformity of fuel local structure in the bed makes the concentration of CO calculated theoretically lower than the measured value.The model has high precision and is an effective method which can assist the application of large biomass boiler.