Numerical Simulation Of Coal Gasification Process In A High-density Circulating Fluidized Bed Gasifier

High-density circulating fluidized bed(HDCFB)is a noval gasification reactor suitable for low-rank coal gasification.It has high solid content and large particle circulation in the bed,which is conducive to gas-solid mixing,heat/mass transfer and gasification reaction,and achieves high carbon conversion and syngas production.However,there are some problems in the practical application,such as low carbon conversion,low gasification efficiency and poor product selectivity.Based on the computational particle fluid dynamics(CPFD)method,a three-dimensional mathematical model of high density circulating fluidized bed coal gasification process was established,the flow and gasification reaction characteristics were simulated,and the strengthening and regulating effects of particle size distribution(PSD)and reactor bottom diameter enlargement on the gasification process were investigated.It provides basic data and theoretical guidance for the design and optimization of the new gasification process.(1)Based on the CPFD method,coupled with the flow,heat transfer and gasification kinetic model,a mathematical model of high-density circulating fluidized bed coal gasification process was established,the validity of model was examined,and the flow and reaction characteristics were analyzed.The results show that the solid circulating flow rate in the riser reaches up to 303 kg/(m2·s),and the particle volume fraction in the lower part of the riser is greater than 0.1.The "core-annulus fluid" phenomenon in the traditional circulating fluidized bed no longer exists,and the gasification index is in the error range.(2)Residence time distribution of different PSD systems were analyzed statistically.The results show that the average residence time of particles decreases and the distribution range of residence time widens with the increase of PSD width.For the particle systems with different peak number of PSD curves,the bimodal distribution system has the longest average residence time and the most concentrated residence time distribution.(3)The effects of PSD on flow and gasification reaction characteristics in the reactor were investigated.The results show that the particle circulation decreases with increasing the PSD width for the single-peak particle distribution system.The circulation flow rate of extremely narrow sieve particle system reached up to the maximum(384 kg/(m2·s)),and the carbon conversion first increased and then decreased.The carbon conversion rate of narrow sieve particle system was 88.2%.For the particle systems with different peak number of PSD curves,the bimodal distribution system has the highest particle circulation amount and gasification efficiency.(4)On the flow and gasification performance of the reactor,the influence of the PSD width is more obvious than that of the peak number of PSD curve.The gasification performance of the unimodal narrow sieve system is the best in the above particle system.(5)Two bottom-expanding riser models were established,and the influence of bottom enlargement on the flow and gasification performance was investigated.The results show that the bottom enlargement reduces the particle volume fraction,but helps the particle circulation.The bottom diameter is expanded to optimize the proportion of combustion and gasification,and the carbon conversion rate is improved.However,the diameter of the expanding section should not be too large,which will lead to increased bottom fluidization wind resistance,poor fluidization effect and lower gasification efficiency.