SUMMARYFossil fuels provide a significant fraction of the global energy resources, and this is likely to remain so for several decades. Carbon dioxide (CO 2 ) emissions have been correlated with climate change, and carbon capture is essential to enable the continuing use of fossil fuels while reducing the emissions of CO 2 into the atmosphere thereby mitigating global climate changes. Among the proposed methods of CO 2 capture, oxyfuel combustion technology provides a promising option, which is applicable to power generation systems. This technology is based on combustion with pure oxygen (O 2 ) instead of air, resulting in flue gas that consists mainly of CO 2 and water (H 2 O), that latter can be separated easily via condensation, while removing other contaminants leaving pure CO 2 for storage. However, fuel combustion in pure O 2 results in intolerably high combustion temperatures. In order to provide the dilution effect of the absent nitrogen (N 2 ) and to moderate the furnace/combustor temperatures, part of the flue gas is recycled back into the combustion chamber. An efficient source of O 2 is required to make oxycombustion a competitive CO 2 capture technology. Conventional O 2 production utilizing the cryogenic distillation process is energetically expensive. Ceramic membranes made from mixed ion-electronic conducting oxides have received increasing attention because of their potential to mitigate the cost of O 2 production, thus helping to promote these clean energy technologies. Some effort has also been expended in using these membranes to improve the performance of the O 2 separation processes by combining air separation and high-temperature oxidation into a single chamber. This paper provides a review of the performance of combustors utilizing oxy-fuel combustion process, materials utilized in ion-transport membranes and the integration of such reactors in power cycles. The review is focused on carbon capture potential, developments of oxyfuel applications and O 2 separation and combustion in membrane reactors. The recent developments in oxyfuel power cycles are discussed focusing on the main concepts of manipulating exergy flows within each cycle and the reported thermal efficiencies.
Ion transport membrane (ITM) based reactors have been suggested as a novel technology for several applications including fuel reforming and oxy-fuel combustion, which integrates air separation and fuel conversion while reducing complexity and the associated energy penalty. To utilize this technology more effectively, it is necessary to develop a better understanding of the fundamental processes of oxygen transport and fuel conversion in the immediate vicinity of the membrane. In this paper, a numerical model that spatially resolves the gas flow, transport and reactions is presented. The model incorporates detailed gas phase chemistry and transport. The model is used to express the oxygen permeation flux in terms of the oxygen concentrations at the membrane surface given data on the bulk concentration, which is necessary for cases when mass transfer limitations on the permeate side are important and for reactive flow modeling. The simulation results show the dependence of oxygen transport and fuel conversion on the geometry and flow parameters including the membrane temperature, feed and sweep gas flow, oxygen concentration in the feed and fuel concentration in the sweep gas.
Natural gas is an attractive fuel for internal combustion engines in light of its potential for reduced greenhouse gas and particulate emissions, and significant reserves. To facilitate natural gas use in compression ignition engines, pilot-ignited direct-injection natural gas combustion uses a small pilot injection of diesel to ignite a more significant direct injection of natural gas. Compared to modern diesel combustion, this strategy is a promising technology for the reduction of CO2 emissions while retaining diesel-like efficiency without a significant CH4 emission penalty. To further develop this technology, investigation of in-cylinder combustion processes is needed to identify the primary fuel conversion processes. The objective of this work was to provide a framework of conceptual understanding by identifying key processes in a typical pilot-ignited direct-injection natural gas combustion event and characterizing their sensitivity to fuel injection parameters. A parametric sweep of injection pressure, natural gas injection duration, and relative timing of the diesel pilot and natural gas injections was performed in an optically accessible 2 L single-cylinder engine. Combined heat release rate and OH*-chemiluminescence reaction zone analysis was used to demarcate the transition from ignition reactions to primary natural gas heat release. Five distinct combustion processes were identified: (1) pilot auto-ignition; (2) natural gas ignition; (3) rapid, distributed partially premixed natural gas combustion; (4) non-premixed combustion; and (5) late-cycle oxidation. While natural gas ignition was found to be insensitive to injection pressure, it was strongly affected by the time between pilot and natural gas injections. Reducing the relative injection timing from +8° to −6° resulted in the primary natural gas heat release transitioning from non-premixed, to distributed partially premixed, to stratified premixed flame propagation as a result of increasing natural gas premixing. The presented measurements and analysis serve to refine an initial conceptual model of the combustion process and lay the groundwork for future, more focused studies of pilot-ignited, direct-injection natural gas combustion.
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