A review of oxy‐fuel combustion in fluidized bed reactors

Presently, there is no detailed review that summarizes the current knowledge status on oxy‐fuel combustion in fluidized bed combustors. This paper reviewed the existing literature in heat transfer, char combustion and pollutant emissions oxy‐fuel combustion in fluidized beds, as well as modelling of oxy‐fuel in FB boiler and gaps were identified for further research direction. Copyright © 2016 John Wiley & Sons, Ltd.     

A review of low carbon fuel policies: Principles,program status and future directions

A low carbon fuel standard (LCFS) is a market-based policy that specifies declining standards for the average lifecycle fuel carbon intensity (AFCI) of transportation fuels sold in a region. This paper: (i) compares transportation fuel carbon policies in terms of their economic efficiency, fuel price impacts, greenhouse gas emission reductions, and incentives for innovation; (ii) discusses key regulatory design features of LCFS policies; and (iii) provides an update on the implementation status of LCFS policies in California, the European Union, British Columbia, and Oregon. The economics literature finds that an intensity standard implicitly taxes emissions and subsidizes output. The output subsidy results in an intensity standard being inferior to a carbon tax in a first-best world, although the inefficiency can be corrected with a properly designed consumption tax (or mitigated by a properly designed carbon tax or cap-and-trade program). In California, from 2011 to 2015 the share of alternative fuels in the regulated transportation fuels pool increased by 30%, and the reported AFCI of all alternative fuels declined 21%. LCFS credit prices have varied considerably, rising to above $100/credit in the first half of 2016. LCFS programs in other jurisdictions share many features with California's, but have distinct provisions as well.     

Oxy‐fuel combustion characteristics and kinetics of microalgae and its mixture with rice straw using thermogravimetric analysis

The thermal behavior of rice straw, microalgae, and their mixture was studied by thermogravimetric analysis (TGA). Co‐combustion of rice straw and microalgae broadened the temperature range of combustion, facilitated ignition, and promoted burnout. The blend ratio of microalgae should be less than or equal to 10% in a 20%O₂/80%N₂ atmosphere and 30% in a 20%O₂/80%CO₂ atmosphere to reach a higher comprehensive combustion index (CCI) value than the individual fuels. The co‐combustion with a small ratio of microalgae could remedy partial negative effects on combustion performance caused by the replacement of N₂ using CO₂. The interaction of blends depended on the atmosphere and temperature range. The prediction of the combustion performance of blends by a weighted sum of individual fuels worked better in an O₂/CO₂ atmosphere at low temperatures, while better in an O₂/N₂ atmosphere at high temperatures. The simulation using the model which contained 2 parallel multi‐order reactions matched with the thermogravimetric curves well, and blending reduced activation energies of the second stage.     

Gas Switching Reforming for syngas production with iron-based oxygen carrier-the performance under pressurized conditions

A four-stage Gas Switching Reforming for syngas production with integrated CO₂ capture using an iron-based oxygen carrier was investigated in this study. The oxygen carrier was first reduced using dry methane, where high methane conversion rate was achieved producing CO₂ and steam. Following the reduction stage is a transition to syngas production in an intermediate stage that begins with partial oxidation of methane while methane cracking dominates the rest of the stage. This results in substantial carbon deposition that gasifies in a subsequent reforming stage by cofeeding steam and methane, contributing to more syngas yield. Some of the deposited carbon that could not gasify during the reforming stage slip to the oxidation stage and get combusted by oxygen in the air feed to release CO₂, thereby reducing the CO₂ capture efficiency of the process. It is in this oxidation stage that heat is being generated for the whole cycle given the high exothermicity nature of this reaction. Methane conversion was found to drop substantially in the reforming stage as the pressure increases driven by the negative effect of pressure on both carbon gasification by steam and on the steam methane reforming. The intermediate stage (after reduction) was found less sensitive to the pressure in terms of methane conversion, but the mechanism of carbon deposition tends to change from methane cracking in the POX stage to Boudouard reaction in the reforming stage. However, methane cracking shows a tendency to reduce substantially at higher pressures. This is could be a promising result indicating that high-pressure operation would remove the need for the reforming stage with steam as no carbon would have been deposited in the POX stage.     

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.     

A cold‐start method and analysis for internal combustion engines particularly using a renewable fuel

A new cold‐start approach for internal combustion engines is described, which may be particularly suitable for an engine using an alternative fuel, such as ethanol or methanol. One of the cylinders in the engine may be equipped with a holding chamber in the cylinder head for the cold‐start purpose. The holding chamber may be opened or closed by a holding chamber valve to establish or block the fluid flow between the holding chamber and the cylinder space. The cold‐start procedure includes an intake stroke and a compression stroke, an expansion stroke that conserves the energy content of the earlier compressed charge without returning the compression work to the piston, and a subsequent recompression stroke to compress the charge to a much higher temperature. A thermodynamic analysis on the cold‐start process of a homogeneous charge internal combustion engine is undertaken, which includes the effect of fuel vaporization on the temperature change of the charge during the first compression stroke. It is found that the charge could be compressed effectively to a sufficiently higher temperature for ignition with pure ethanol as fuel. Copyright © 2010 John Wiley & Sons, Ltd.     

Waste to liquid fuels: potency,progress and challenges

The valorization of municipal solid waste (MSW) into liquid fuels is a multi‐beneficial global option for ensuring environmental and energy sustainability. The paper critically reviewed a wide range of recent literature on the potency, progress, and challenges associated with MSW upgrading into liquid fuels. Concise details on the various upgrading technologies involved such as gasification, pyrolysis, and syngas‐to‐fuels (i.e., syngas to gasoline and diesel) via a Fischer–Tropsch process were documented. Emphasis was critically given to the recent literature updates. The paper explored the role of heterogeneous catalyst systems in achieving the various processes with special considerations for optimizing fuel yield. Prospective technologies such as plasma gasification and nanoscale catalyst design with potentials to revolutionize the industry were also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Commercial process for mass production of synthetic natural gas through the adiabatic reactors: operational characteristics of a 50‐kW pilot‐plant,influence of steam,and CO2

In synthetic natural gas (SNG) reaction process, the water gas shift (WGS) reaction and methanation reaction take place simultaneously, and an insufficient supply of steam might deactivate the catalyst. In this study, the characteristics of the methanation reaction with a commercial catalyst and using a low [H₂]/[CO] mole ratio in SNG synthesis are evaluated. The reaction characteristics at various possible process parameters are evaluated varying different process parameters such as the [H₂O]/[CO] mole ratio, [H₂]/[CO] mole ratio, flow of different % CO₂, and reaction temperature. Temperature profiles on catalyst bed are monitored as a function of the [H₂O]/[CO] mole ratio, [H₂]/[CO] mole ratio, and flow of different % CO₂. Through a lab‐scale optimization process, suitable optimum conditions are selected and in the same condition a 50‐kW pilot‐scale SNG production process through adiabatic reactors is carried out. The pilot scale SNG reaction is stable through overnight and the CO conversion efficiency and CH₄ selectivity are 100% and 97.3%, respectively, while the maximum CH₄ productivity is 0.654 m³/kg_cat · h. Copyright © 2016 John Wiley & Sons, Ltd.     

Oxy-fuel combustion of pulverized coal: Characterization,fundamentals, stabilization and CFD modeling

Oxy-fuel combustion has generated significant interest since it was proposed as a carbon capture technology for newly built and retrofitted coal-fired power plants. Research, development and demonstration of oxy-fuel combustion technologies has been advancing in recent years; however, there are still fundamental issues and technological challenges that must be addressed before this technology can reach its full potential, especially in the areas of combustion in oxygen-carbon dioxide environments and potentially at elevated pressures. This paper presents a technical review of oxy-coal combustion covering the most recent experimental and simulation studies, and numerical models for sub-processes are also used to examine the differences between combustion in an oxidizing stream diluted by nitrogen and carbon dioxide. The evolution of this technology from its original inception for high temperature processes to its current form for carbon capture is introduced, followed by a discussion of various oxy-fuel systems proposed for carbon capture. Of all these oxy-fuel systems, recent research has primarily focused on atmospheric air-like oxy-fuel combustion in a CO₂-rich environment. Distinct heat and mass transfer, as well as reaction kinetics, have been reported in this environment because of the difference between the physical and chemical properties of CO₂ and N₂, which in turn changes the flame characteristics. By tracing the physical and chemical processes that coal particles experience during combustion, the characteristics of oxy-fuel combustion are reviewed in the context of heat and mass transfer, fuel delivery and injection, coal particle heating and moisture evaporation, devolatilization and ignition, char oxidation and gasification, as well as pollutants formation. Operation under elevated pressures has also been proposed for oxy-coal combustion systems in order to improve the overall energy efficiency. The potential impact of elevated pressures on oxy-fuel combustion is discussed when applicable. Narrower flammable regimes and lower laminar burning velocity under oxy-fuel combustion conditions may lead to new stability challenges in operating oxy-coal burners. Recent research on stabilization of oxy-fuel combustion is reviewed, and some guiding principles for retrofit are summarized. Distinct characteristics in oxy-coal combustion necessitate modifications of CFD sub-models because the approximations and assumptions for air-fuel combustion may no longer be valid. Advances in sub-models for turbulent flow, heat transfer and reactions in oxy-coal combustion simulations, and the results obtained using CFD are reviewed. Based on the review, research needs in this combustion technology are suggested.     

Membrane reactors for fuel cell quality hydrogen through WGSR – Review of their status, challenges and opportunities

The growing environmental concerns on the technologies being adopted for non-renewable energy generation and consumption has brought in a new dimension to the role of water gas shift reaction (WGSR) in providing pure hydrogen to the portable and stationery fuel cell systems. The review has focussed on the status of the membrane reactor technology for WGSR, the activity and deactivation of catalysts employed, efficiency of ceramic membrane systems, the membrane reactor configuration and design aspects, kinetics and mechanism of WGSR and modelling for simulation of membrane reactor performance specifically with reference to its application in fuel cell systems.     

A review on technology,configurations, and performance of cross‐flow hydrokinetic turbines

Increased demand of energy leads to exploration of new sources of energy. In the last few years, hydrokinetic energy technology emerged as an important area in the field of renewable energy generation. Various hydrokinetic turbines have been studied and used to harness the hydrokinetic potential. Among all hydrokinetic turbine technology, cross‐flow hydrokinetic turbine is considered as the most suitable approach for the riverine system. There are various configurations of cross‐flow hydrokinetic turbine exist for hydrokinetic energy extraction. A number of numerical and experimental studies were carried out on the performance enhancement and design optimization of different configurations under variable operating conditions. Under the present paper, review of different rotor for the configurations of the cross‐flow hydrokinetic turbines are discussed. The paper will be useful to understand the cross‐flow hydrokinetic technology in order to explore the methods for selection, performance enhancement, and design optimization of cross‐flow hydrokinetic turbines.     

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.     

Parametric study of chemical looping combustion for tri‐generation of hydrogen,heat, and electrical power with CO2 capture

In this article, a novel cycle configuration has been studied, termed the extended chemical looping combustion integrated in a steam‐injected gas turbine cycle. The products of this system are hydrogen, heat, and electrical power. Furthermore, the system inherently separates the CO₂ and hydrogen that is produced during the combustion. The core process is an extended chemical looping combustion (exCLC) process which is based on classical chemical looping combustion (CLC). In classical CLC, a solid oxygen carrier circulates between two fluidized bed reactors and transports oxygen from the combustion air to the fuel; thus, the fuel is not mixed with air and an inherent CO₂ separation occurs. In exCLC the oxygen carrier circulates along with a carbon carrier between three fluidized bed reactors, one to oxidize the oxygen carrier, one to produces and separate the hydrogen, and one to regenerate the carbon carrier. The impacts of process parameters, such as flowrates and temperatures have been studied on the efficiencies of producing electrical power, hydrogen, and district heating and on the degree of capturing CO₂. The result shows that this process has the potential to achieve a thermal efficiency of 54% while 96% of the CO₂ is captured and compressed to 110 bar. Copyright © 2005 John Wiley & Sons, Ltd.     

Natural gas‐based transportation in the USA: economic evaluation and policy implications based on MARKAL modeling

The recent divergence in domestic energy costs between oil and natural gas drives an investigation of the potential for further demand‐side utilization of natural gas in the USA. An economic assessment of the US transportation sector was conducted with a focus on the penetration of technologies that use natural gas as a fuel. Bottom‐up modeling of the US energy system using the market allocation framework enabled estimation of the optimal technology mix in both the light‐duty and heavy‐duty vehicle segments, over a 40‐year time horizon, under various scenarios of technical learning rates and natural gas prices. A modified functional form of Moore's law was developed and anchored to municipal transit bus data, to represent technical learning with regard to natural gas vehicle costs. Modeling results suggest that the present levels of natural gas vehicle penetration are suboptimal and a number of market failures were identified, which are most likely to propagate under‐adoption into the foreseeable future. Some policy guidelines, aimed at the federal level, were outlined as potential responses to the market failures discussed herein. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance,combustion and emission characteristics of a spark‐ignition engine with simultaneous injection of n‐butanol and gasoline in comparison to blended butanol and gasoline

Simultaneous injection of n‐butanol and gasoline through a new system of two injectors directing the sprays towards the back of the intake valve in a spark‐ignition engine was tried in lieu of injecting a blend of these fuels through a single injector. This system avoids the problem of phase separation, which is generally faced during the use of alcohol‐gasoline blends. Experiments were conducted on a spark‐ignition engine with this dual injection system using a fuel ratio of 1:1 (B50S) on the mass basis. High‐speed photographs indicated that the sprays from the injectors did not interfere till they reached the intake valve. Comparisons were made with pre‐blended butanol‐gasoline (B50) and neat (100%) gasoline at the best spark timing. All injection and spark parameters were controlled using a real time engine controller. Neat n‐butanol (B100) was superior only near full throttle with improved efficiency of the engine of about 1.2% (absolute). Heat release rates were observed to be higher and more advanced with B100 at wide open throttle. However, a reverse of this trend was observed at the throttle position of 15%. NO emission was also lower by 30% with B100 at wide open throttle as compared with gasoline. However, a small increase in carbon monoxide (CO) levels was observed because of lower post combustion temperatures as compared with gasoline and B50S. Simultaneous injection reduced hydrocarbon (HC) emissions by 13% to 50% as compared with B50 (blended fuel). HC emissions with gasoline and B50S were similar. Nitric oxide (NO) emission was lower with B50S as compared with gasoline; however it was higher than B50 because of better combustion. On the whole, the developed dual injection system was superior to the conventional method of blending in terms of performance, emissions and ability to change the fuel ratio as needed. B50S is suitable at all throttle positions, whereas B100 shows benefits at full throttle conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

High performance of the novel PdCeO2-NR/C (cerium oxide nanorods) nanocatalyst for the oxidation of C1, C2 and C3 organic molecules for fuel cells applications

Cerium oxide nanorods treated at 200 (CeO_2-NR2) and 400 °C (CeO_2-NR4) were used to synthesize of PdCeO_2-NR2/C and PdCeO_2-NR4/C nanocatalysts to promote the Methanol, Ethanol, Ethylene Glycol and Glycerol Oxidation Reactions (MOR, EOR, EGOR and GOR, respectively). The crystalline nanocatalysts had morphology of agglomerated Pd nanoparticles dispersed on Vulcan and on the nanorods as well. The polarization curves in 0.5 M KOH showed that the catalytic activity decreases in all reactions in the order PdCeO_2-NR4/C > PdCeO_2-NR2/C > Pd/C, which confirmed the synergetic effect of the nanorods on the catalytic behavior of Pd, particularly after heat treatment at 400 °C. The highest mass current density has been obtained from the EGOR (j_f = 6661 mA mg_Pd⁻¹) with the PdCeO_2-NR4/C nanocatalyst, which catalyzed the reaction with an onset potential (E_onset) of −0.26 V/SHE. Meanwhile, the EOR was promoted with the most negative E_onset (−0.40 V/SHE), delivering 5719 mA mg_Pd⁻¹, also using PdCeO_2-NR4/C. Therefore, considering electrocatalysis parameters for fuel cells applications, the oxidation of C₂H₆O₂ and C₂H₅OH showed advantages using PdCeO_2-NR4/C.     

Thermogravimetric analysis and process simulation of oxy‐fuel combustion of blended fuels including oil shale,semicoke, and biomass

Oxy‐fuel (OF) combustion is considered as one of the promising carbon capture and storage technologies for reducing CO₂ emissions from power plants. In the current work, the thermal behaviour of Estonian oil shale (EOS) and its semicoke (SC), pine saw dust, and their blends were studied comparatively under model air (21%O₂/79%Ar) and OF (30%O₂/70%CO₂) conditions using thermogravimetric analysis. Mass spectrometry analysis was applied to monitor the evolved gases. The effect of SC and pine saw dust addition on different combustion stages was analysed using kinetic analysis methods. In addition, different co‐firing cases were simulated using the ASPEN PLUS V8.6 (APV86) software tool to evaluate the effects of blending EOS with different biomass fuels of low and high moisture contents. The specific boiler temperatures of each simulated case with the same adjusted thermal fuel input were calculated while applying the operation conditions of air and OF combustion. According to the experiment and process simulation results, the low heating value and high carbonate content of SC brings along endothermic decomposition of carbonates, which negatively affects the heat balance during the conventional co‐combustion of EOS with SC. Instead, firing of EOS with SC and biomass in OF process can be an effective solution to reduce the environmental impact in terms of the reduction of CO₂ emissions and ash. Furthermore, the sensible heat from SC can positively affect the energy balance of the system as the endothermic effect of decomposition of CaCO₃ (for both EOS and SC) can be avoided in OF combustion.     

Thermo‐economic investigation: an insight tool to analyze NGCC with calcium‐looping process and with chemical‐looping combustion for CO2 capture

In this work, three kinds of natural gas‐based power generation processes for CO₂ capture and storage, that is, natural gas‐combined cycle with pre‐combustion decarburization (NGCC‐PRE), NGCC‐PRE with calcium‐looping process, and NGCC‐PRE with chemical‐looping combustion (NGCC‐CLC), are analyzed by Aspen Plus. The effects of two decisive variables (i.e., steam‐to‐natural gas (S/NG) ratio and oxygen‐to‐natural gas (O/NG) ratio) on the thermodynamic performances of individual process, such as energy and exergy efficiencies, are investigated systematically. Based on simulation outcomes, all the three processes are favored by operating at S/NG = 2.0 and O/NG = 0.65. Furthermore, comparisons of individual system efficiencies and exergy destruction contributor are herein involved. The results show that the highest system efficiencies and lowest exergy destruction are achieved in the NGCC‐CLC process. In addition, capital investment, dynamic payback period, net present value, and internal rate of return are used for deciding the economic feasibility and surely are involved in this work for comparison purpose. Copyright © 2016 John Wiley & Sons, Ltd.     

Vibration‐based piezoelectric,electromagnetic, and hybrid energy harvesters for microsystems applications: A contributed review

Wireless sensor nodes (WSNs) and embedded microsystems have recently gained tremendous traction from researchers due to their vast sensing and monitoring applications in various fields including healthcare, academic, finance, environment, military, agriculture, retail, and consumer electronics. An essential requirement for the sustainable operation of WSN is the presence of an uninterrupted power supply; which is currently obtained from electrochemical batteries that suffer from limited life cycles and are associated with serious environmental hazards. An alternative to replacing batteries of WSNs; either the direct replacement or to facilitate battery regular recharging, is by looking into energy harvesting for its sustainable drive. Energy harvesting is a technique by which ambient energy can be converted into useful electricity, particularly for low‐power WSNs and consumer electronics. In particular, vibration‐based energy harvesting has been a key focus area, due to the abundant availability of vibration‐based energy sources that can be easily harvested. In vibration‐based energy harvesters (VEHs), different optimization techniques and design considerations are taken in order to broaden the operation frequency range through multi‐resonant states, increase multi‐degree‐of‐freedom, provide nonlinear characteristics, and implement the hybrid conversion. This comprehensive review summarizes recent developments in VEHs with a focus on piezoelectric, electromagnetic, and hybrid piezoelectric‐electromagnetic energy harvesters. Various vibration and motion‐induced energy harvesting prototypes have been reviewed and discussed in detail with respect to device architecture, conversion mechanism, performance parameters, and implementation. Overall sizes of most of the reported piezoelectric energy harvesters are in the millimeter to centimeter scales, with resonant frequencies in the range of 2‐13 900 Hz. Maximum energy conversion for electromagnetic energy harvesters can potentially reach up to 778.01 μW/cm³. The power produced by the reported hybrid energy harvesters (HEHs) is in the range of 35.43‐4900 μW. Due to the combined piezoelectric‐electromagnetic energy conversion in HEHs, these systems are capable of producing the highest power densities.