Catalytic burner with internal steam generation for a fuel-cell-based auxiliary power unit for middle distillates

A catalytic burner (CAB) was developed, which utilizes the anode off-gas of a high temperature polymer electrolyte fuel cell (HT-PEFC). This CAB has two functions within the HT-PEFC-system: It has to convert completely all combustible components including methane and carbon monoxide, even in the low ppm range and it has to provide steam to the autothermal reformer (ATR). Thereby it increases the system's overall efficiency.     

Catalytic combustion of hydrogen for residential heat supply application

Hydrogen can be converted to thermal energy by combustion or to electricity energy by fuel cells. Considering the stringent requirements for safety from fire hazards and elimination of pollutants, the flameless catalytic combustion of hydrogen is favorable over conventional flame combustion for residential heat supply application. This paper reported an industrial‐scale heat acquisition system based on hydrogen catalytic combustion. The 1 wt% Pt‐loaded glass fiber felts prepared by an impregnation process were used as the combustion catalyst, and a catalytic combustion burner with a capacity of 1 kW was designed. It was found that 100% hydrogen conversion rate could be obtained during the stable combustion stage, and the stable combustion could be achieved by adjusting hydrogen flow rate. The change in H₂/air ratio would influence the initial combustion stage but has little impact on the stable combustion stage. A heat efficiency of 80% for hot water supply was obtained based on the present catalytic hydrogen combustion burner. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and kinetic modeling study of 2-butanol pyrolysis and combustion

2-Butanol (sC₄H₉OH) pyrolysis has been studied in a flow reactor with the synchrotron vacuum ultraviolet photoionization mass spectrometry combined with the molecular-beam sampling technique. The pyrolysis species were identified and their mole fractions were determined. Four pressures of 5, 30, 150 and 760 Torr were selected to study the pressure dependence of 2-butanol pyrolysis chemistry. The temperature- and pressure-dependent rate constants of unimolecular reactions of 2-butanol were calculated with the RRKM/Master Equation method. With the help of theoretical calculations, a detailed kinetic model consisting of 160 species and 1038 reactions was developed to simulate the 2-butanol pyrolysis. It is concluded that the mole fractions of pyrolysis species are very sensitive to the 2-butanol unimolecular reaction rates. To enhance the accuracy, the model is further validated by the species profiles in shock tube pyrolysis, a rich laminar premixed flame, oxidation data from jet-stirred reactor, ignition delay times, and laminar flame speed. Good agreements between the predicted and measured results were obtained.     

Oxidation of 1-butanol and a mixture of n-heptane/1-butanol in a motored engine

The oxidation of neat 1-butanol and a mixture of n-heptane and 1-butanol was studied in a modified CFR engine at an equivalence ratio of 0.25 and an intake temperature of 120 °C. The engine compression ratio was gradually increased from the lowest point to the point where significant high temperature heat release was observed. Heat release analyses showed that no noticeable low temperature heat release behavior was observed from the oxidation of neat 1-butanol while the n-heptane/1-butanol mixture exhibited pronounced cool flame behavior. Species concentration profiles were obtained via GC-MS and GC-FID/TCD. Quantitative analyses of the reaction products from the oxidation of neat 1-butanol indicate that 1-butanol is consumed mainly through H-atom abstraction. Among the H-atom abstraction reactions, it is observed that the H-atom abstraction from the α-carbon of 1-butanol is particularly favored. The investigation on the oxidation of the mixture of n-heptane/1-butanol showed that the oxidation of 1-butanol is facilitated at low temperatures through the radical pool generated from the oxidation of n-heptane.     

Catalytic combustion and system performance assessment of MCFC-MGT hybrid system

The catalytic combustor is applied as an off-gas and startup combustor for a molten carbonate fuel cell-micro-gas turbine (MCFC-MGT) hybrid system (HS) so as to utilize the waste energy of fuel cell off-gas. Three types of catalysts are prepared over a cordieritic honeycomb support. One is Pt catalyst which is not cost effective and less high temperature stability. CeZrO₂ and LaMnO₃ have been selected as an additive for another two Pt catalysts to improve the performance. Tests have been completed in realistic conditions and reaction feed close to the MCFC-MGT hybrid system. Simulations are carried out with a fluid mechanical code that incorporates detailed transport and heat loss mechanisms. The simulation results are compared with the Pt catalyst test results. The agreement confirms the accuracy of simulation. The model can be used to develop an MCFC-MGT hybrid system with an off-the-shelf gas turbine and assess the performances during part-load operation. From the experimental results, the reaction starts at 620 K for 1 vol.% CH₄ using Pt catalyst, while the temperature is above 800 K for the addition of additive. For the 50% CH₄ conversion, the preheated temperature of the three catalysts is 713 K, 870 K and 950 K respectively. While all of the catalysts exhibit good performance when using the MCFC off-gas as fuel. The results of performance analysis for part-load conditions show that the cell operation temperature and turbine inlet temperature (TIT) should be maintained as close as possible to the design value to prevent the performance degradation.     

Variation on anthracite combustion efficiency with CeO2 and Fe2O3 addition by Differential Thermal Analysis (DTA)

Effects of CeO₂ and Fe₂O₃ on anthracite combustion efficiency were investigated using differential thermal analysis (DTA). Based on heat release (Q_D) of anthracite as well as anthracite with CeO₂ and anthracite with Fe₂O₃ additions against α-Al₂O₃ in DTA experiment, effects of additives CeO₂ and Fe₂O₃ on anthracite combustion efficiency were evaluated. Under the same experimental conditions, heat releases of raw anthracite, anthracite with CeO₂ and anthracite with Fe₂O₃ were 11.04 kJ/g, 11.30 kJ/g and 11.42 kJ/g, respectively, indicating that anthracite combustion efficiency was improved by addition of CeO₂ and Fe₂O₃. To confirm the above results, carbon transfer was monitored using Thermogravimetric analysis Fourier transform infrared (TGA-FTIR) and Carbon-Sulfur analyzer during catalytic combustion process. The results indicated that CO₂ emission was increased, whereas CO emission and residual carbon of ash were decreased, being in accordance with the results of DTA. Finally, according to analyses of ignition temperature and catalytic combustion process, the possible mechanism of catalytic combustion of anthracite was proposed.     

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.     

Catalytic combustion for industrial gas turbines

This brief review provides a general account of work directed at the use of catalytic combustion in gas turbine engines. A major potential advantage of using catalytic combustion is that the fuel can be burnt efficiently at temperatures low enough (< 1500°C) to avoid significant oxidation of atmospheric nitrogen. This advantage was less important when catalytic combustion was demonstrated in the 1970s than it is today and received relatively little attention until the following decade. After discussion of the principles involved in the design of a combustor that must meet the mixing, size, performance and durability goals of a based gas turbine application, the review turns to accounts of experiments conducted on a laboratory scale with simple configurations. These established basic operating parameters for satisfactory combustion performance and led to larger scale work and to prototype design concepts for industrial gas turbines in the late 70s and early 80s. Test results were encouraging but were not pursued definitively in the U.S.A. Activity continued at several centres in Japan, with exploration of a number of different catalyst arrangements, geometries, and control systems, again with encouraging results. At the same time, there has been renewed interests in the U.S.A. and in Europe, spurred largely by the emphasis on reducing emissions of nitrogen oxides (NOx). The paper concludes with suggestions for further development of catalytically stabilized combustion systems for gas turbines. These systems must ensure adequate pre-catalyst temperature, with evenly premixed fuel and air, and sufficient temperature rise across the catalyst to ensure effective completion of reaction in a homogeneous reaction mode. The outstanding problems are largely concerned with questions of catalyst integrity and longevity in practical configurations and realistic engine operating conditions.     

Simulation on thermoelectric device with hydrogen catalytic combustion

The micro-thermoelectric-generator based on catalytic combustion of hydrogen and oxygen was designed. With the application of general finite reaction rate model in CFD software of FLUENT, the effect of inlet parameters on the highest temperature difference between the hot and cold plate of the generator was studied. Results showed that, the temperature in the heating and cooling channel of the micro-thermoelectric-generator was uniform; with the increasing of inlet reactant temperature, the highest temperature difference increased, but the total efficiency of the generator decreased. Results can be used to the further design and optimization of micro-thermoelectric-generator based on hydrogen catalytic combustion.     

Optimization of a counter-flow microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane

The objective of this study is to optimize a microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane. Hydrogen assisted catalytic combustion does not require preheating because the catalytic combustion of hydrogen occurs at room temperature. After start-up by hydrogen catalytic combustion, fuels of hydrogen and methane were changed to methane. The geometric configuration of the counter-flow reactor was optimized by the simulation model under steady state condition. The hydrogen flow rate in the counter-flow reactor was also optimized by transient simulations using the response surface methodology. As a result, the counter-flow reactor showed extremely short start-up time because of the optimized configuration and the optimized hydrogen flow rate. Hot spots were avoided because of the hydrogen shut-off after start-up. The operating characteristics of the counter-flow reactor were compared with those of the co-flow reactor.     

Linear analysis of an aerothermal instability occurring in diffusion-controlled premixed catalytic combustion

The onset of the unstable temperature distribution which may appear in plane axisymmetric catalytic burner is studied. The instability mechanism is assumed to be related with the temperature dependence of the viscosity, thus with the pressure drop in the porous combustor, governed by Darcy’s law. An area e.g., of lower temperature (dark zone) characterised by a smaller value of the kinetic viscosity gives rise to a locally increased gas flow mass velocity, the pressure drop remaining constant over the burner cross-section. The locally increased mass velocity produces an enhanced cooling of the area, whereby heat conduction from the hotter surrounding area tends to restore a homogeneous temperature distribution. A linear analysis of this thermal instability mechanism is carried out and yields a simple analytic solution for the state of neutral stability.     

Cylinder pressure, performance parameters, heat release, specific heats ratio and duration of combustion for spark ignition engine

An experimental work were conducted for investigating cylinder pressure, performance parameters, heat release, specific heat ratio and duration of combustion for multi cylinder spark ignition engine (SIE). Ccylinder pressure was measured for gasoline, kerosene and Liquefied Petroleum Gases (LPG) separately as a fuel for SIE. Fast Fourier Transformations (FFT) was used to cylinder pressure data transform from time domain into frequency domain to develop empirical correlation for calculating cylinder pressures at different engine speeds and different fuels. In addition, Inverse Fast Fourier Transformations (IFFT) was used to cylinder pressure reconstruct into time domain. The results gave good agreement between the measured cylinder pressure and the reconstructed cylinder pressure in time domain with different engine speeds and different fuels. The measured cylinder pressure and hydraulic dynamotor were the sours of data for calculating engine performance parameters. First law of thermodynamics and single zone heat release model with temperature dependant specific heat ratio γ(T) were the main tools for calculating heat release and heat transfer to cylinder walls. Third order empirical correlation for calculating γ(T) was one of the main gains of the present study. The correlation gave good agreement with other researchers with wide temperatures range. For kerosene, cylinder pressure is higher than for gasoline and LPG due to high volumetric efficiency where kerosene density (mass/volume ratio) is higher than gasoline and LPG. In addition, kerosene heating value is higher than gasoline that contributes in heat release rate and pressure increases. Duration of combustion for different engine speeds was determined using four different methods: (I) Mass fuel burnt, (II) Entropy change, (III) Temperature dependant specific heat ratio γ(T), and (IV) Logarithmic scale of (P&;V). The duration of combustion for kerosene is smaller than for gasoline and LPG due to high heat release rate. Cylinder pressure measuring technique is a useful tool for understanding and analyzing the combustion characteristics and determining reliable statistical data that cannot measure directly. The present work contributes in determining combustion characteristics, development and optimal operating conditions of SIE for different fuels.     

Numerical model of a thermoelectric generator with compact plate-fin heat exchanger for high temperature PEM fuel cell exhaust heat recovery

This paper presents a numerical model of an exhaust heat recovery system for a high temperature polymer electrolyte membrane fuel cell (HTPEMFC) stack. The system is designed as thermoelectric generators (TEGs) sandwiched in the walls of a compact plate-fin heat exchanger. Its model is based on a finite-element approach. On each discretized segment, fluid properties, heat transfer process and TEG performance are locally calculated for higher model precision. To benefit both the system design and fabrication, the way to model TEG modules is herein reconsidered; a database of commercialized compact plate-fin heat exchangers is adopted. Then the model is validated against experimental data and the main variables are identified by means of a sensitivity analysis. Finally, the system configuration is optimized for recovering heat from the exhaust gas. The results exhibit the crucial importance of the model accuracy and the optimization on system configuration. Future studies will concentrate on heat exchanger structures.     

CFD modeling of heat transfer and fluid flow inside a pent-roof type combustion chamber using dynamic model

A numerical work has been performed to analyze the heat transfer and fluid flow in a pent-roof type combustion chamber. Dynamic mesh model was used to simulation piston intake stroke. Revolution of piston (1000 ≤ n ≤ 5000) is the main governing parameter on heat and fluid flow. k–ε turbulence model was used to predict the flow in the cylinder of a non-compressing fluid. They were solved with finite volume method and FLUENT 12.0 commercial code. Velocity profiles, temperature distribution, pressure distribution and velocity vectors are presented. It is found that the inclined surface of pent-roof type of combustion chamber reduces the swirl effect and it can be a control parameter for heat and fluid flow.     

Investigation of reverse flow distributed combustion for gas turbine application

In this paper reverse flow modes of colorless distributed combustion (CDC) have been investigated for application to gas turbine combustors. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation reverse flow modes consisting of three configurations at thermal intensity of 28 MW/m³-atm and five configurations at thermal intensity of 57 MW/m³-atm have been investigated and these high thermal loadings represent characteristic gas turbine combustion conditions. In all the configurations the air injection port is positioned at the combustor exit end, whereas the location of fuel injection ports is changed to give different configurations. The results are presented on the exhaust emissions and radical emissions using experiments, and evaluation of flowfield using numerical simulations. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes investigated here. Cross-flow configuration, wherein the fuel is injected at high velocity cross stream to the air jet resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from closer to diffusion mode to distributed regime. This feature is beneficial for part load operation where higher stability limit is desirable.     

Hydrogen production by compact combined dimethyl ether reformer/combustor for automotive applications

A computational study of hydrogen production by a dimethyl ether reformer combined with a catalytic combustor is conducted to investigate its feasibility for on-board automotive applications. The combined reactor has a stacked channel structure consisting of alternating reformer monoliths and catalytic combustor monoliths on a heat-conducting substrate. The inner surfaces of the walls of each monolith are coated with reforming and combusting catalysts, respectively. The effects of the feeding flow rate, thermal conductivity of the substrate, and porosity of the catalyst layer on the hydrogen production efficiency of the proposed combined reactor are investigated to determine the optimal design and operating conditions.     

Improvement of skin friction and heat transfer prediction theory of turbulent boundary-layer combustion of hydrogen

The theory for skin friction and heat transfer prediction to a compressible turbulent boundary layer including hydrogen/air combustion, proposed by Stalker, is improved in this paper. The original theory is modified through two aspects. One is to enhance the accuracy of the relation between hydrogen surface mass fraction and streamwise Reynolds number by directly integrating the Kármán momentum integral relation without local similarity hypothesis. Consequently, a new skin friction formula is established. The other is to change the Prandtl number from constant value of unity in the original theory to a variable computed based on the molecular kinetic theory. The performance of the improved theory is evaluated by an experiment with a flow on a flat plate and a designed two-dimensional numerical experiment. The skin friction and heat transfer predicted by the improved theory are found to be better consistent with the experimental or numerical experimental data than the original theory.     

Comparison of the effect of heat release and products from heterogeneous reaction on homogeneous combustion of H2/O2 mixture in the catalytic micro combustor

Numerical simulation was carried out to investigate the hetero-/homogeneous combustion of stoichiometric H₂/O₂ premixed flames in a platinum-coated planar micro channel. Three-dimensional Computational Fluid Dynamics (CFD) models with detailed reaction mechanisms for homogeneous (gas phase, G) and heterogeneous (catalytic, C) reactions were adopted. The effects of the heat released and products from heterogeneous reaction (CR) on the homogeneous reaction (GR) were analyzed and compared systematically. In the presence of additional released heat, the flame temperature, intermediate concentrations and product yield increased, especially near the inner wall. The promotion effect of heat released from the CR on the GR was indicated, and it mainly reflected in the increase of chemical reaction rate and fuel consumption. When the heat released and the products from the CR were added simultaneously, the flame temperature and species (OH) concentration were still low as compared with the model without addition. The inhibition effect of the product on the GR was larger than the promotion effect of the additional heat on the GR. The increase in the chemical reaction rate and fuel conversion rate demonstrated that, the heat released from the CR could improve the combustion efficiency of the micro combustor.     

The dynamics of in-situ combustion fronts in porous media

The sustained propagation of combustion fronts in porous media is a necessary condition for the success of in situ combustion for oil recovery. Compared to other recovery methods, in situ combustion involves the complexity of exothermic reactions and temperature-dependent chemical kinetics. In the presence of heat losses, the possibility of ignition and extinction also exists. In this paper, we address some of these issues by studying the properties of forward combustion fronts propagating at a constant velocity in the presence of heat losses. We extend the analytical method used in smoldering combustion [7], to derive expressions for temperature and concentration profiles and the velocity of the combustion front, under both adiabatic and non-adiabatic conditions. Heat losses are assumed to be relatively weak and they are expressed using two modes: 1) a convective type, using an overall heat transfer coefficient; and, 2) a conductive type, for heat transfer by transverse conduction to infinitely large surrounding formations. In their presence we derive multiple steady-state solutions with stable low and high temperature branches, and an unstable intermediate branch. Conditions for self-sustaining front propagation are investigated as a function of injection and reservoir properties. The extinction threshold is expressed in terms of the system properties. An explicit expression is also obtained for the effective heat transfer coefficient in terms of the reservoir thickness and the front propagation speed. This coefficient is not only dependent on the thermal properties of the porous medium but also on the front dynamics.     

Enhanced CANDU reactor with heat upgrade for combined power and hydrogen production

Nuclear energy is considered a key alternative to overcome the environmental issues caused by fossil fuels. It offers opportunities with an improved operating efficiency and safety for producing power, synthetic fuels, delivering process heat and for multigeneration applications. The high-temperature nuclear reactors, although possess great potential for integration with thermochemical water-splitting cycles for hydrogen production, are not yet commercially established. Current nuclear reactor designs providing heat at relatively low temperature can be utilized to produce hydrogen using thermochemical cycles if the temperature of their thermal heat is increased. In this paper, a hybrid chemical-mechanical heat pump system is proposed for upgrading the heat of the Enhanced CANDU (EC6) reactor design to the quality required for the copper-chlorine (Cu–Cl) hybrid thermochemical water splitting cycle operating at 550–600 °C. A modification to the heat pump is proposed to bring the heat to temperature higher than 650 °C with operating coefficient of performance estimated as 0.65.