Characteristics of Oxy-fuel Combustion in an Oxygen Transport Reactor

Abstract

This study aims at investigating the characteristics of oxy-fuel combustion in an oxygen transport reactor (OTR). The cylindrical reactor walls are made of dense, nonporous, mixed-conducting ceramic membranes that only allow for oxygen permeation from the outside air into the combustion chamber. The oxygen permeate mixes with a mixture of CO2 and CH4 (sweep gas) that enters the reactor, resulting in combustion products composed of H2O and CO2. The modeling of the flow process considers a numerical solution of the conservation equations of mass, momentum, energy, and species in the axi-symmetric flow domain. The oxygen permeation across the membrane depends upon the prevailing temperatures and the oxygen partial pressure at both sides of the membrane. The simulations are performed for different compositions of CH4/CO2 mixtures and different mass flow rates. First, the comparison between the reactive and separation-only OTR units showed that combining reaction and separation increases the O2 permeation rate significantly to about 2.5 times under the assumptions given herein. Second, a mass flow rate of 1.625 × 10–7 kg/s with a CH4/CO2 mass ratio ranging from 0.5:0.5 to 1.0:0 gives an almost uniform axial temperature of about 1250 K in most of the reactor length with a high CH4 conversion of 75 to 35%, respectively. In all of the simulations, the total O2 permeation flux is almost the same, except for 1.625 × 10–7 kg/s with a CH4/CO2 mass ratio less than 0.3:0.7. The results indicate that the heat of reaction is mostly transferred to the air side with a portion used to heat the O2-permeating flux. For higher mass flow rates, the OTR operates with a rich mixture, resulting in low CH4 conversion. The combustion process in such cases can be improved by splitting the OTR into a series of units, where the fuel is added at stages along the reactor network.

Oxy-combustion of liquid fuel in an ion transport membrane reactor

“…Wang et al [18] has conducted experimental O 2 permeation study in a tubular BSCF membrane and found that for a constant temperature, the O 2 permeation flux is little affected by the feed flow rate. The numerical model studies of Hong et al [39] and BenMansour et al [1] also showed similar behavior. Coming back to our present simulation it appears that there is some effect on the permeated oxygen flux.…”

“…6. It must be mentioned that most of the published papers dealing with OTR have not treated the variation of feed flow rate effect including Ben-Mansour et al [1,2], Hong et al [39] and Habib et al [40]. The reason for this is due to the fact the variation of feed flow rate has very little effect on O 2 permeation in the case of separation only.…”

“…Many studies in the past have been conducted to evaluate the feasibility of using reactors comprising ion transport membrane for the oxy-fuel combustion applications with gaseous fuels [1,2]. However, to the author's knowledge there are no studies on utilizing liquid fuels for the same application.…”

The coupling effect of gas-phase chemistry and surface reactions on oxygen permeation and fuel conversion in ITM reactors

“…Their catalytic activity, coupled with oxygen permeation in a controlled manner, enables their use as catalytic membrane reactors with higher product selectivity [2]. Multiple applications have been extensively investigated using typically methane as a feedstock such as; oxidative coupling [7][8][9][10]; partial oxidation [5,6,11]; and oxy-fuel combustion [12][13][14][15][16]. In spite of these promising applications, the operating regimes, in which fuel conversion and oxygen permeation are predominantly attributed to catalytic or/and gas-phase reactions, have not been examined.…”

“…Although the results from these two studies have elucidated that fuel oxidation reactions and their chemical kinetic rates have substantial influences on oxygen permeation, their models considered a simple well-stirred reactor which does not resolve the state near the surface. Ben-Mansour et al [13], Habib et al [14] and Nemitallah et al [15,16] modeled two dimensional reactors and considered the local thermodynamic state to evaluate the permeation rate, but assumed one-or two-step global reactions without examining the coupling of homogeneous and heterogeneous chemistry to minimize computational costs. Wang and Lin [20] and Tan et al [12] assumed the kinetic parameters of perovskite membranes, but they ignored homogeneous-phase reactions that may take place at the temperature considered in their study.…”