Modeling of a combined ITM-porous oxygen transport reactor: Towards a spatially uniform temperature ITM.

摘要

Almost all the fossil fuels, on combustion, emit CO 2 which is considered to be a greenhouse gas. Developing new power generation cycles that enables carbon-dioxide capture and sequestration are the limelight of current research. Present absorption technologies for carbon capture are energy-intensive and expensive. An alternative to these absorption technologies would be to combust fossil fuels in pure oxygen, where in the flue gas stream will have a much higher concentration of CO2, reducing or eliminating the need for costly CO2 capture. Oxy-fuel combustion is considered to be one of the new emerging technologiescapable of capturing and sequestrating CO2. In oxy-fuel combustion, the fossil fuel is burned in an environment of pure oxygen instead of air and the flue gas mainly consists of CO2 and H2O that can be easily separated through condensation processes. Ion Transport Membranes (ITMs) offer promising oxygen production technology with high purity (up to 99%) without adversely affecting the efficiency of the oxy-fired plants. The separation rate of such ITMs can be increased by replacing the conventional inert sweep gas with a reactant/diluent mixture (e.g. CO2, CH4) as this reduces the permeate partial pressure on the permeate side of the membrane, which, along with the temperature, governs the permeation flux. The significant limitation of this approach is that an uncontrolled, exothermic consumption of the permeated specie, can lead to membrane damage, and thus limits the potential of ITMs using reactive sweep gases (i.e. ITM reactors). By using a multichannel ITM reactor, it is proposed to operate the ITM reactor at, or near to isothermal conditions (i.e. a spatially uniform temperature). This may be achieved by introducing a reactant into the permeate stream uniformly across the entire ITM reactor length from an adjacent channel with porous walls.;The present work is aimed at predicting the oxy-combustion characteristics in an oxygen transport reactor with the objective of developing a nearly isothermal reactor. This is achieved by separation of oxygen from air through Ion Transport Membranes (ITM's) and by using a porous membrane, to achieve uniform stoichiometric ratio of fuel/O2 in order to have uniform combustion all along the length of the membrane. A two-dimensional, computational fluid dynamics (CFD) model is solved to study the combustion characteristics. The simulations are based on the numerical solution of the conservation of mass, momentum, energy and species transport equations of two dimensional flows. For the CFD calculations, the commercial solver FLUENT has been used. The mass transfer of oxygen through the membrane is modeled by user defined functions (UDF's) and the mass transfer of fuel through the porous layer is modeled using a 1D porous jump model. The membrane (ITM) is modeled as a selective layer, which allows the permeation of oxygen as a function of temperature and the difference of partial pressures of oxygen in the feed side and the permeate side. The flux through the porous layer is a function of permeability and thickness of the medium in addition to the pressure difference. The models used have been validated against the experimental results found in the literature and are found to be in good agreement. Influence on the performance of oxygen separation through the ITM has been studied by varying the flow conditions at the permeate side. Results show that for a constant mass flow rate of fuel mixture, the permeation rate of oxygen through ITM increases with increase in CH4/CO2 ratio. It was found that the oxygen permeation rate increased by approximately 3 times with reaction taking place on the permeate side compared to the separation only case. An improved uniform temperature distribution along the membrane was obtained by the combined ITM-porous oxygen transport reactor.