Straw Gasification With Low-temperature Fluidization And High-temperature Melting Coupled Process

The"double carbon"goal has far-reaching impacts for China’s energy strategy.New energy resources,such as solar and wind power,can well compensate for the shortage of secondary energy—mainly electricity.However,reviewing the energy consumption structure of China,there is still a large part that depends on other energy forms,especially carbon-based one.Biomass,as a kind of renewable energy with vast reserves and"carbon neutral",can fit well into the current energy consumption structure of China.Gasification is an important way for efficient energy utilization of biomass.The current biomass gasification techniques enable little operating flexibility while disposing of rice and wheat and straw,which are the main biomass resources in China.Therefore,in this paper,a novel two-stage gasification concept is presented to efficiently convert rice-wheat straw fuel into clean,medium-high calorific value gas.The concept has two gasification stage,comprising a medium-low temperature gasification stage and a high temperature one.The gasification performance was investigated in detail in multiple scale systems.Firstly,a series of experiments were conducted on a laboratory-scale fluidized bed gasifier to obtain quantitative data of the main products.The accuracy of the data was verified by mass and carbon balance.The results show that the net calorific value of the product gas in the first stage ranges from 2.0-6.6 MJ/Nm³.The energy share in the product gas increases from 27.2%to 64%as the reaction temperature increases from 650 ℃ to 800 ℃.By studying the migration pattern of alkali and alkaline earth metals in the fuel,it was found that a large amount of K elements in the fuel were released between 750 and 800 ℃,which implies a high risk of alkali metal-induced slagging at this temperature.Therefore,the operating temperature of the fluidized bed gasifier,i.e.the first stage,should be lower than 750 ℃.Based on quantitative data,the mass and energy flows of the main products of RWS in bubbling fluidized bed gasification were established.The correlation of temperature and equivalence ratio was established under adiabatic conditions.In order to investigate the mechanism of rice straw gasification,the effects of gasification temperatures,equivalence ratios,steam/biomass ratios and oxygen concentrations on the low-temperature gasification section were investigated in this paper.Based on 34 sets of experimental data under different conditions,a GA-BP neural network was constructed to predict the experimental data.The results showed that the GA-BP neural network could well predict fluidized bed gasification for rice straw.The correlation coefficient between the predicted and true values reached 0.932,and the relative errors of each dependent variable were lower than 10%.With the neural network,the gasification data sets under different operating parameters were established and the effects of each operating parameter on the product gas were analyzed.The results showed that the equivalence ratio and oxygen concentration had a low impact on the cold gas efficiency from rice straw fluidized bed gasification,but had a great impact on the heat value of the produced gas.The gasification temperature and steam/biomass ratio has a significant influence on the cold gas efficiency.According to the data set,the function between the heat transfer coefficient of the gasification system and the four operating parameters was obtained by the second-order spreading equation with cross-phase,and the relationship between the reaction temperature T and SBR and ER under adiabatic conditions was derived.Accordingly,it is possible to determine the reaction temperature of the adiabatic gasification system by adjusting different SBR and ER.On the basis of the above research,a series of coupled gasification experiments were carried out on a 200 k W_th pilot plant to investigate the gasification performance of two-stage coupled gasification.The results show that the two-stage coupled gasification system can operate stably for a long time,with more stable gasification performance.The lower heating value of the produced gas varies from 5.50 to 6.67 MJ/Nm³.The cold gas efficiency varies from56.7 to 70.8%,and the carbon conversion rate reaches more than 87.9%.It was proven that this gasification technology was able to completely remove the tar by thermal cracking,significantly improving the delivery performance of the produced gas.In view of the experimental conditions,the highest cold gas efficiency was achieved at the equivalence ratio of 0.18 and 0.3 for the two gasification stages,respectively.Thermogravimetric studies were carried out on the residual char collected from the first stage of the pilot system,detailed kinetic data were obtained under O₂ and CO₂ atmospheres.The results show that the reaction of char with O₂ can be divided into two stages—200-600℃ and 600-1000 ℃,respectively,which follow the Avrami-Erofeev equation at n=2 and the power function law at n=1/2,respectively.The reaction of gasification char with CO₂ follows the Avrami-Erofeev equation at n=4 and the Avrami-Erofeev equation at n=1/2.Avrami-Erofeev equation at n=4.In addition,the activation energy of the char reacting with both O₂ and CO₂increases with the reaction depth of the first stage,standing for the reactivity decreases slightly.A 1.5 MW_th two-stage coupled gasification demonstration system was established based on the pilot test results,to provide a stable heat source for grain drying.Pre-thermal commissioning work was carried out with rice husk as the feedstock,and the stability of the system was verified by stable operation for more than 60 days.Under the typical working conditions of rice husk gasification,the cold gas efficiency and carbon conversion rate of the demonstration system could reach 67.5%and 87.0%,respectively,while the lower heating value of gas produced reached 6.7 MJ/Nm³.The tar content is below the detection limit.After that,coupled gasification experiments were performed using rice straw briquette pellets as fuel.The results showed that the heat value of gas produced by rice straw gasification was lower than that of rice husk for 5.82 MJ/Nm³.In all experimental conditions,the cold gas efficiency of two-stage coupled gasification reached the highest 56.8%when the equivalent ratio of 0.1850.177 for the two stages,respectively.