Laminar oxy-fuel diffusion flame supported by an oxygen-permeable-ion-transport membrane

Abstract

A numerical model with detailed gas-phase chemistry and transport was used to predict homogeneous fuel conversion processes and to capture the important features (e.g., the location, temperature, thickness and structure of a flame) of laminar oxy-fuel diffusion flames stabilized on the sweep side of an oxygen permeable ion transport membrane (ITM). We assume that the membrane surface is not catalytic to hydrocarbon or syngas oxidation. It has been demonstrated that an ITM can be used for hydrocarbon conversion with enhanced reaction selectivity such as oxy-fuel combustion for carbon capture technologies and syngas production. Within an ITM unit, the oxidizer flow rate, i.e., the oxygen permeation flux, is not a pre-determined quantity, since it depends on the oxygen partial pressures on the feed and sweep sides and the membrane temperature. Instead, it is influenced by the oxidation reactions that are also dependent on the oxygen permeation rate, the initial conditions of the sweep gas, i.e., the fuel concentration, flow rate and temperature, and the diluent. In oxy-fuel combustion applications, the sweep side is fuel-diluted with CO2, and the entire unit is preheated to achieve a high oxygen permeation flux. This study focuses on the flame structure under these conditions and specifically on the chemical effect of CO2 dilution. Results show that, when the fuel diluent is CO2, a diffusion flame with a lower temperature and a larger thickness is established in the vicinity of the membrane, in comparison with the case in which N2 is used as a diluent. Enhanced OH-driven reactions and suppressed H radical chemistry result in the formation of products with larger CO and H2O and smaller H2 concentrations. Moreover, radical concentrations are reduced due to the high CO2 fraction in the sweep gas. CO2 dilution reduces CH3 formation and slows down the formation of soot precursors, C2H2 and * Corresponding author. Tel.: +1 617 253 2295; Fax: +1 617 253 5981 Email address: ghoniem@mit.edu (Ahmed F. Ghoniem) 2 C2H4. The flame location impacts the species diffusion and heat transfer from the reaction zone towards the membrane, which affects the oxygen permeation rate and the flame temperature.

Interactions between oxygen permeation and homogeneous-phase fuel conversion on the sweep side of an ion transport membrane

“…Because oxygen is introduced through permeation, the fuel and oxidizer in the sweep side of an ITM are not pre-mixed, leading to the establishment of a non-premixed or diffusion-controlled reaction zone [10]. In such diffusion-supported reactions, the local thermodynamic state including species concentration and temperature play significant roles in establishing stable reactions and determining the reaction products.…”

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

“…8(a). Under these conditions, it has been shown that the extent of homogeneous-phase reactions is significant leading to substantial fuel conversion in the gas-phase and the enhancement of oxygen permeation [17,27], when only homogeneous-phase reactions were modeled. When catalytic surface reactions are also considered, the change in CH4 conversion and oxygen permeation rate are negligible.…”

“…This is attributed to the exothermic reactions being moved from the gas-phase towards the membrane surface. The membrane conducts this reaction heat to the air side and radiates to the reactor walls [17,27]. Compared to conventional catalytic reactors which are essentially adiabatic, ITM reactors experience extensive thermal energy transfer through the membrane.…”

“…The sweep gas conditions for this gas-phase reaction regime referred to in [27], which include a methane concentration of 6% on a molar basis with the remainder being carbon dioxide and a sweep gas inlet temperature of 1300 K, corresponding to the largest CH4 conversion in Fig. 7(a) and Fig.…”

“…We formulated a spatially resolved physical model, which incorporates detailed gas-phase chemical kinetics and transport to parameterize an oxygen permeation rate expression in terms of the local thermodynamic state in the immediate vicinity of the membrane [26]. The numerical model was then used to characterize a diffusion-controlled reaction zone on the sweep side [27], as well as its interaction with the oxygen permeation rate through the local thermodynamic state [17]. Key features of the reaction zone stabilized on the sweep side including its location, temperature, thickness and structure were elucidated.…”