(324e) 2D Numerical Simulation of Coal Gasifier for Hydrogen Product and in-Situ Fixation of CO2

With the serious shortage and the incredible increasing price of the crude oil, coal resource as the largest fossil fuel resource in the world has attracted the increasing attentions. However, the vast emissions of CO2 and other harmful gases from coal utilization processes cause the serious environmental problems, such as green house effect, acid rain, and so on. Coal gasification process, as a core component of coal utilization technologies, plays a very important role in cleaning coal technologies. However, a large amount of energy and high equipment investment are needed to remove the CO2 in the coal syngas from the coal gasification, which evidently reduce the electricity efficiency and increase the cost of process. Thus, it is indispensable to develop new technologies to reduce the CO2 emission with high efficiency and the enhanced process. In the present study, the 2-D numerical simulation of coal gasification reactor for CO2 fixation is carried out using CaO as CO2 absorbent and H2O/O2 as reactant gases based on the Yitian Fang's experimental apparatus which is the traditional circulating fluidized bed gasifier with CO2/O2 as reactant gases [Yitian Fang, 2001]. The circulating fluidized bed gasifier mainly consists of a riser (48mm i.d. °Á 3560mm), a solid¨Cgas separation system and a recirculation column. Since the present study is focused on the flow and reaction of the gasifier, and for the purpose of the simplification, the gasifier is assumed to be the computation domain and the circulating part is neglected. In the present process with CO2 fixation in situ, N2, steam and O2 was preheated and injected into the gasifier and reacted with coal/CaO mixture fed into gasifier, when rising along gasifier, and then solid particles are sent to centrifugal separator at the top of the reactor, and separated with gas. In the present comprehensive model for the simulation of the gasifier with CO2 fixation in situ, besides the conservation equations of mass, momentum and energy, it also includes particle stochastic tracking model used as particle turbulent dispersion, standard ¦Ê&Å model used as gas turbulent flow, EDC(Eddy-Dissipation-Concept)Model adopted to describe the interaction between turbulence and reactions. As for the unsteady simulation, the intense interaction between the flow and the reactions during the start-up period induced to convergence difficulty. The simulation began with the calculation of cold flow field without reactions, and then the calculation of the flow with turbulent reaction is initiated with resulted cold flow field.The whole flow field was divided into 3564 standard grid mesh, and the resulting governing partial differential equations for all variables were discretized using the finite volume method and second-order upwind scheme was adopted for interpolation. A group of equations derived from the integration of the quantities over the every controlled volume. The SIMPLE algorithm is used to solve the set of algebraic equations. The flow field parameters (such as gas temperature distributions, pressure distribution, the components distributions of gas phase), carbon conversion, and particle movement properties (including particle trajectories, particle residue time and particle velocity) under different operating conditions in the gasification process are obtained. The simulation results have a good agreement with related literatures. At the outlet of the gasifier, more than 70 vol% hydrogen less than 10 vol% CO2 and CO are obtained. Near the bottom, a relatively high temperature distribution is presented for the reason that fast exothermic reactions( char combustion and water-shift reaction) occur mainly in this part, and at the presence of CO2 the product gas of the above two reactions, CaO carbonation reaction occurs swiftly, and the large amount of heat is emitted. The concentration of hydrogen and CO2 increased, while CO decreased when rising along the gasifier, because that with the decreased temperature when rising along the gasifer, water shift reaction occurred continuously and CO2 and hydrogen were produced continuously and the CO was consumed continuously.

Keywords coal gasification, numerical simulation, CO2 removal, hydrogen production