A review of recent developments in carbon capture utilizing oxy‐fuel combustion in conventional and ion transport membrane systems

Fossil fuels provide a significant fraction of the global energy resources, and this is likely to remain so for several decades. Carbon dioxide (CO₂) 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 CO₂ into the atmosphere thereby mitigating global climate changes. Among the proposed methods of CO₂ capture, oxyfuel combustion technology provides a promising option, which is applicable to power generation systems. This technology is based on combustion with pure oxygen (O₂) instead of air, resulting in flue gas that consists mainly of CO₂ and water (H₂O), that latter can be separated easily via condensation, while removing other contaminants leaving pure CO₂ for storage. However, fuel combustion in pure O₂ results in intolerably high combustion temperatures. In order to provide the dilution effect of the absent nitrogen (N₂) and to moderate the furnace/combustor temperatures, part of the flue gas is recycled back into the combustion chamber. An efficient source of O₂ is required to make oxy‐combustion a competitive CO₂ capture technology. Conventional O₂ 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 O₂ 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 O₂ 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 O₂ 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. Copyright © 2010 John Wiley & Sons, Ltd.     

Characteristic evaluation of a CO2‐capturing repowering system based on oxy‐fuel combustion and exergetic flow analyses for improving efficiency

A CO₂‐capturing H₂O turbine power generation system based on oxy‐fuel combustion method is proposed to decrease CO₂ emission from an existing thermal power generation system (TPGS) by utilizing steam produced in the TPGS. A high efficient combined cycle power generation system (CCPS) with reheat cycle is adopted as an example of existing TPGSs into which the proposed system is retrofitted. First, power generation characteristics of the proposed CO₂‐capturing system, which requires no modification of the CCPS itself, are estimated. It is shown through simulation study that the proposed system can reduce 26.8% of CO₂ emission with an efficiency decrease by 1.20% and an increase power output by 23.2%, compared with the original CCPS. Second, in order to improve power generation characteristics and CO₂ reduction effect of the proposed system, modifications of the proposed system are investigated based on exergetic flow analyses, and revised systems are proposed based on the obtained results. Finally, it is shown that a revised proposed system, which has the same turbine inlet temperature as the CCPS, can increase power output by 33.6%, and reduce 32.5% of CO₂ emission with exergetic efficiency decrease by 1.58%, compared with the original CCPS. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental and numerical study of oxy‐methane flames in a porous‐plate reactor mimicking membrane reactor operation

The work investigates the reacting flow field, oxy‐methane flame characteristics and location, and the species distributions in a porous‐plate reactor mimicking the operation of oxygen transport membrane reactors (OTMRs). The study was performed experimentally and numerically considering ranges of operating equivalence ratio, from 0.5 to 1.0, and CO₂ concentrations in the total oxidizer flow (O₂ and CO₂), from 0% to 55% (by Vol). Oxygen was supplied through a slightly pressurized top and bottom chambers to cross the two porous plates to the central chamber, where a premixed mixture of CH₄ and CO₂ is introduced. ANSYS Fluent 17.1 software was used to solve for conservation and radiative transfer equations in the full three‐dimensional (3‐D) domain. The modified Westbrook‐Dryer (Oxy‐WD) two‐step reduced mechanism for oxy‐methane combustion was adapted for the calculations of chemical kinetics. The captured flame shapes using a high‐speed camera were compared with the calculated ones, and the results showed good agreements. At fixed equivalence ratio, elongated flames were obtained at higher CO₂ concentrations due to the increase in the mainstream Reynolds number and reduction in reaction rates, which delays the completeness of combustion. At fixed CO₂ concentration, the increase in equivalence ratio resulted in more compact and intense flames. The effective mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. Stable flames were located between the two porous plates at reasonable distance. This perfect flame location prevents the thermal fracture of the membranes and improves their oxygen permeation flux, resulting in better combustion characteristics when the results are projected on the case of OTMRs. This implies efficient and safe applicability of the OTMRs by the condition that membranes can provide sufficient oxygen flux for complete combustion. A warm outer recirculation zone (ORZ) was created beside each porous plate, which helps anchoring the flame at the leading edge of the porous plate. The range of temperature within the ORZ was 800 to 1600 K, which lies in the operability limits of membranes for the case of OTMRs. The effective complete mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. The temperature and species distributions within the reactor are presented in detail over wide ranges of operating conditions. The results recommended the reactor operation under stoichiometric combustion condition based on performance and economic points of views. The results are promising when projected on the application of the OTMRs under oxy‐combustion conditions for clean and efficient energy production.     

A flexible model integration approach for evaluating tradeoffs between CO2 emissions and cost in solid oxide fuel cell‐based building energy systems

A simulation framework for flexible evaluation of various distributed building energy systems based on the integration of component device simulation models is presented. Device technology models were constructed for a solid oxide fuel cell (SOFC), a gas turbine, a double pipe heat exchanger, and a compressor. A scheme is proposed for defining model interfaces in order to improve the flexibility and accessibility of the models. Based on that scheme, interfaces are defined for each device model. The component device models are integrated to construct system models of (1) a hybrid system combining an SOFC and a gas turbine (SOFC/GT system) and (2) a stand‐alone SOFC system. The integrated model of the SOFC/GT system is then used to carry out a multi‐objective optimization in order to study the tradeoffs between cost and CO₂ emissions of the SOFC system operation for a given electricity demand. Through these analyses, the optimal configuration of the SOFC/GT system and the optimal operation conditions of the SOFC system for the given electricity demand were explored. Copyright © 2005 John Wiley & Sons, Ltd.     

Additives in proton exchange membranes for low- and high-temperature fuel cell applications: A review

Polymer electrolyte membranes, also known as proton exchange membranes (PEMs), are a type of semipermeable membrane that exhibits the property of conducting ions while impeding the mixing of reactant materials across the membrane. Due to the large potential and substantial number of applications of these materials, the development of proton exchange membranes (PEMs) has been in progress for the last few decades to successfully replace the commercial Nafion^® membranes. In the course of this research, an alternate perspective of PEMs has been initiated with a desire to attain successful operations at higher working temperatures (120–200 °C) while retaining the physical properties, stability and high proton conductivity. Both low- and high-temperature PEMs have been fabricated by various processes, such as grafting, cross-linking, or combining polymer electrolytes with nanoparticles, additives and acid-base complexes by electrostatic interactions, or by employing layer-by-layer technologies. The current review suggests that the incorporation of additives such as plasticisers and fillers has proven potential to modify the physical and chemical properties of pristine and/or composite membranes. In many studies, additives have demonstrated a substantial role in ameliorating both the mechanical and electrical properties of PEMs to make them effective for fuel cell applications. It is notable that plasticiser additives are less desirable for the development of high-temperature PEMs, as their inherent highly hydrophilic properties may stiffen the membrane. Conversely, filler additives form an inorganic-organic composite with increased surface area to retain more bound water within the polymer matrices to overcome the drawbacks of ohmic losses at high operating temperatures.     

Expected impacts on greenhouse gas and air pollutant emissions due to a possible transition towards a hydrogen economy in German road transport

Transitioning German road transport partially to hydrogen energy is among the possibilities being discussed to help meet national climate targets. This study investigates impacts of a hypothetical, complete transition from conventionally-fueled to hydrogen-powered German transport through representative scenarios. Our results show that German emissions change between ?179 and +95 MtCO₂eq annually, depending on the scenario, with renewable-powered electrolysis leading to the greatest emissions reduction, while electrolysis using the fossil-intense current electricity mix leads to the greatest increase. German energy emissions of regulated pollutants decrease significantly, indicating the potential for simultaneous air quality improvements. Vehicular hydrogen demand is 1000 PJ annually, requiring 446–525 TWh for electrolysis, hydrogen transport and storage, which could be supplied by future German renewable generation, supporting the potential for CO₂-free hydrogen traffic and increased energy security. Thus hydrogen-powered transport could contribute significantly to climate and air quality goals, warranting further research and political discussion about this possibility.