Experimental And Kinetic Studies Of Potassium-Catalyzed And Pressurized Gasification Of Coal Char

From the current situation of resources in China,coal will still be an indispensable part of China's energy consumption for a long time.As a new type of gasification method,the catalytic gasification of coal has many advantages,including lower operating temperature,high efficiency and high product selectivity.The kinetic study of the pressurized catalytic coal char gasification has received considerable attention from the domestic and foreign scholars due to its importance and complexity.In this dissertation,a special vertically-installed tubular reactor was established and employed to study the kinetic and process of potassium catalyzed,steam or steam/hydrogen mixture pressurized gasification of coal char.Both an overall kinetic model with unified parameters and a complex kinetic model based on elementary reactions were proposed to describe the experimental results.The research contents are outlined as follows.(1)A vertically-installed tubular pressurized reactor was established.This reactor has distinct feature in conducting the gasification of fine char under pressurized fluidized conditions.The reactor is also characteristic of an availability of eliminating the internal and external diffusion,a fast steam switching and complying with the differential reaction condition.These characteristics allow a study of intrinsic kinetics of catalytic gasification and the influence of external diffusion on the gasification rate.Experimental result showed that the rate of catalytic char gasification obtained under the fluidized condition was 3.2 times as fast as that obtained in a horizontal boat reactor.The computational fluid dynamics software(Fluent 14.5)was used to simulate the dynamic characteristics of gas fluid in the tubular reactor and the watercondensation tank,by way to an experimental verification.The effect of gas transport from the sample bed to the outlet of the reactor on the measured gasification rate was investigated to estimate the in-situ gas release rate inside the sample bed.Simulation results showed that the effect of gas transport and backmixing could be ignored only when a space velocity was higher than 600 h-1.(2)The intrinsic reaction kinetics of the catalytic gasification of coal char under high steam pressure was studied using the vertically installed tubular reactor.Results showed that the gasification rate obtained under a steam partial pressure of 0.2 MPa was 3.3 times as fast as that obtained at 0.04 MPa.The higher pressure significantly promoted the production of H2 and suppressed the production of CO.The yield of CH4 was enhanced as the total pressure increased but it was insignificant as compared to other major gases.It was found that the gasification rate varied with carbon conversion with a maximum appearing in a carbon conversion of 0.4-0.6.The profiles of dimensionless gasification rate(the ratio of gasification rate to the average gasification rate)versus carbon conversion were virtually overlapped,regardless of temperature and pressure.This profile can be well described by the simplified random pore model.An overall kinetic model with unified parameters was proposed to describe the variations of gasification rate with temperature,pressure and total carbon conversion.(3)The influence of external diffusion on the catalytic gasification rate of coal char was studied.Experimental results showed that the effectiveness factor of the external diffusion was almost invariable with the carbon conversion changing from 0.2 to 0.7.It was further found that the relationship between the diffusion coefficient and the gas flow velocity,temperature,pressure can be described by the modified Chilton-Colburn correlation Sh=0.311Re2.83 Sc1/3(P/P0)-2.07.The effectiveness factors calculated from this correlation equation were in good agreement with those obtained experimentally.The combination of this correlation equation with the overall kinetic model can well describe the variations of gasification rate with the gas flow velocity,temperature,pressure and carbon conversion rate in the presence of external diffusion.(4)A new complex kinetic model(K-ASIM model)was proposed,based on the reaction mechanism of active sites/intermediates.This model can not only well describe the change of gasification rate,carbon conversion with the time,but also describe the generation rate of different gas product.The model can reasonably reflect the evolution of different potassium formations and active intermediates during the coal char gasification.It was revealed that the solid phase reaction between the coal char and the potassium was the rate controlling step for the whole gasification process.Furthermore,a comprehensive model(K-ASIMd),which coupled the external mass transfer with K-ASIM model was proposed.K-ASIMd can fairly predict the characteristic features of catalytic char gasification rates being changed by steam partial pressure and external diffusion,including the effectiveness factor,mass transfer coefficient and the steam partial pressure on the char surface.(5)The catalytic gasification of coal char was studied using the pressurized steam/hydrogen mixtures as a gasifying agent.It was observed that the gasification rate decreased with the increment of hydrogen partial pressure,with the point of maximum gasification rate being moved to a lower carbon conversion.At the fraction of hydrogen of 5%,the rate of gasification was reduced by 36%under the total pressure of 0.5 MPa.However,the inhibitory effect of hydrogen decreased gradually with the increment of total pressure from 0.5 MPa to 2.0 MPa.With the increment of the hydrogen partial pressure,the yield of methane increased linearly.The yield of methane reached 17%at the gasification pressure of 2.0 MPa and the hydrogen fraction of 50%.The mechanism of the hydrogen-inhibiting effect were discussed.