Study On Straw Biomass Fusion Gasification And Its High-Temperature Produced Gas Low-Nitrogen Combustion Technology

Gasification is hailed as the most potential utilization method for the straw biomass,however,the existence of tar limits the development of the technology.Tar not only block the pipeline and make the gasification system unable to operate normally,but also affect the subsequent combustion utilization of the produced gas.As the produced gas contains a large number of non-combustible components,its characteristics such as low calorific value and high volume lead to the difficulty for the ignition and poor unstability combustion.By increasing the gasification temperature,tar can be further converted into non-condensable light gas,which fundamentally solves the tar problem.At the same time,the high-temperature produced gas without tar can be combusted directly,which not only avoids the loss of energy carried by the produced gas itself,but also facilitates the ignition and the increase of combustion stability.However,with the increase of temperature of the produced gas,the NO_x emission increases.In this work,based on the effect of high temperature on tar conversion and ash melting during the straw biomass gasification process,combined with the suppression principle of the high temperature and low oxygen on NO_x emission,the technical scheme of straw biomass fusion gasification and its high-temperature produced gas low-nitrogen combustion is proposed.According to the proposed technical scheme,the following research works have been carried out.The effects of the gasification conditions on the inorganic components,pore structure,carbon structure as well as the ash melting behavior at different ash fusion temperature range during the straw biomass fusion gasification are investigated by building the high-temperature thermogravimetry testing system and high-temperature visualization testing system.Results show that:The ash melting is mainly caused by the muscovite formation.The fusion ash could form the small spherical crystal particles.When the gasification temperature is higher than the ash flow temperature,the small spherical crystal particles on the pellet surface fuse into the large spherical crystal particles firstly,and then peel off from the pellet surface.Under the gasification temperature range,the pores inside pellet are mainly composed of mesopores,and the specific surface area of the micropores does not exceed 4 m~2/g at most.The increase of the gasification temperature promotes the reaction of the gasification agent with the carbon of the cross-linked structure,but inhibits the reaction of the gasification agent with the carbon of the small aromatic ring structure.Considering the evolution behavior of fusion ash in different ash fusion temperature range,combined with the heat and mass transfer as well as the reaction kinetics mechanism,a fusion gasification model for single straw biomass pellet is established.Based on the model,the effects of gasification condition on the heat and mass transfer,reaction rate and reaction time during the fusion gasification are investigated.Results show that:when the gasification temperature is higher than the ash flow temperature,the exfoliation of the fusion ash on the pellet surface reduces the heat and mass transfer resistance of the pellet,which leads to a sharp increase in the temperature and gaseous component content inside pellet in the late stage of the reaction.When the gasification temperature is higher than the ash flow temperature by 50 K,the total gasification time of the pellet decreases rapidly.However,when the gasification temperature is higher than1600 K,the decrease trend reduces.When the gasification equivalent ratio(ER)is less than 0.5,the effect of ER on the pyrolysis rate can be ignored.The process model of straw biomass fusion gasification scheme is established.Based on the model,the influence of operation parameters on the gasification characteristics are investigated.Combined with the straw biomass fusion gasification characteristics,a preliminary design of the straw biomass fusion gasification device is carried out.The final optimal design scheme is determined by exploring the influence of the structural parameters of the device on the operation characteristics.Results show that:the matching relationship between the air preheating temperature and ER plays a decisive role in the realization of the straw biomass fusion gasification scheme.When ER increases from 0.32to 0.47,the minimum air preheating temperature reduces from 1273 K to 300 K,and the obtainable heat value of produced gas is in the range of 3.5-5.5 MJ/Nm~3.In the straw biomass fusion gasification scheme,when ER is greater than 0.32,the fixed carbon in the straw has been completely converted,and the influence of air preheating temperature on the produced gas components can be ignored.When designing the straw biomass fusion gasification device,the secondary air inlet velocity is 100 m/s,r/R is 0.7,and l/L is 5.Taking the high-temperature produced gas as the research object,the effect of operating parameters on the characteristics of ignition and pollutant emission for high-temperature produced gas are investigated.Results show that with the increase of produced gas temperature from 1073 K to 1273 K,the minimum excess air coefficient required for ignition reduces from 0.73 to 0.59,but the highest air excess coefficient required for ignition increases from 0.78 to 1.10.CO emission increases exponentially with the flue gas circulation ratio.The flue gas circulation ratio corresponding to the inflection point of the curve increases with the produced gas temperature.As the produced gas temperature increases from 300 K to 1073 K,the flue gas circulation ratio corresponding to the inflection point increases from 0.15 to 0.68.Under the condition of taking CO emissions into consideration,NO_x emissions can be reduced to about 30 mg/m~3.According to the pollutant emission characteristics for high-temperature produced gas,the design condition of high-temperature produced gas low-nitrogen combustion device is selected and the preliminary design of high-temperature produced gas low-nitrogen combustion device is carried out.The final optimal design scheme is determined by exploring the effect of the structural parameters on the operation characteristics.Results show that when the oxidizer inlet series is 3 and the gas inlet velocity is 15 m/s,the combustion efficiency and NO_x emission can be obtained.The maximum volumetric heat load of the device can reach 28.8 MW/m~3.When the produced gas temperature is above 973 K,the combustion device can run stably,and the minimum NO_x emission can reach 33 mg/m~3.Through the above research,the physical and chemical characteristics of char particle in the process of straw biomass fusion gasification were clarified,and the evolution behavior of ash in different fusion temperature range was obtained.A single straw biomass fusion gasification model was established to reveal the influence mechanism of ash melting on heat and mass transfer.The matching relationship of operating parameters in low-nitrogen combustion for high-temperature produced gas was obtained.The optimal design scheme of straw biomass fusion gasification and low-nitrogen combustion for produced gas were determined.It lays a foundation for the application of straw biomass fusion gasification and its high-temperature produced gas low-nitrogen combustion.