Mathematical Modeling for Coal Gasification Kinetics in Supercritical Water

Abstract

Supercritical water gasification of coal is a newly clean coal technology. In this study, we established a quantitative model for the reaction kinetics of coal gasification in supercritical water. The large number of real intermediates formed during gasification were lumped using volatile and fixed carbon, and mass balance was also ensured in kinetic equation which would facilitate access to a comprehensive model considering flow and heat transfer. The model focused on the formation of the individual gaseous species (H 2 , CO 2 , CH 4 , CO). Then the model was applied to fit the experimental data and kinetic parameters were estimated. It was found that predicted results of the model were in accordance with experiments. The model predicted that volatile completely converted into gas quickly when temperature reached above 650 °C, and the reforming reaction of fixed carbon became the control step of gasification. Hydrogen was mainly from volatile decomposition in the initial short stage and water gas shift reaction then contributed to the hydrogen production. Carbon reforming reaction dominated hydrogen generation after about 5 min. Methane generated from the direct decomposition of volatile and the effect of methanation reaction was negligible. Carbon monoxide was generated during the volatile decomposition and it was consumed simultaneously in water gas shift reaction with substantial carbon dioxide produced. As a result, carbon monoxide content accounted for a small percentage and hydrogen and carbon dioxide dominated the gas products of gasification.

Hydrogen Production by Coal Gasification in a Supercritical Water Fluidized Bed: Dynamic Modeling of an Industrial-Scale Reactor

“…The characteristics of coal gasification in supercritical water vary with the type of coal and reaction conditions. The kinetic model established by Su et al 45 for SCWG of coal is adopted in this study. This model eliminates the description of complex intermediate products and can be adapted to different types of coal.…”

“…The reaction rate for each reaction is shown in eq 18, where [r 1 ] and [r 2 ] represent the concentrations of the reactant in the chemical reaction. 45 Regarding the reverse reactions, the rate constants for the reserve reactions are determined by the forward rate constants and the equilibrium constant.…”

“…The weak bonds in the macromolecules will break up once the coal is in contact with SCW. Light hydrocarbons in the coal matrix will first be released and then decompose further into smaller hydrocarbon 45 This process is very fast, so the hydrogen yield increased rapidly in the first period. Carbon reforming is a relatively slow process, so the hydrogen yield increases gradually after a brief decrease.…”

Modeling of Single Porous Char Particle Gasification in Supercritical Water

“…where A j and E a,j are, respectively, the pre-exponential factor and activation energy for reaction R j , and R is the universal gas constant. Kinetic parameters are based on Su's work (Su et al 2016) as listed in Table 1.…”

“…Thus, this paper aims to develop a specialized model with the unique properties (density, specific heat, thermal conductivity and viscosity) of SCW implanted. Relevant parameters of radiation (Ren et al 2018), species diffusion (Zhao and Jin 2020) and reaction kinetics (Su et al 2016), which were obtained in our group and specifically used for SCW conditions, are adopted in the model. A CFD-based 2D transient numerical model coupling flow, heat transfer, species diffusion and chemical reactions is developed to reveal the gasification details of a single porous char particle in SCW, and the effects of inflow temperature, velocity and particle size are studied.…”