Thermodynamic analysis of ethanol processors for fuel cell applications

A thermodynamic analysis of hydrogen production from ethanol has been carried out with respect to solid polymer fuel cell applications. Ethanol processors incorporating either a steam reformer or a partial oxidation reactor connected to water gas shift and CO oxidation reactors were considered and the effect of operating parameters on hydrogen yield has been examined. Employment of feeds with high H₂O/EtOH ratio results in reduced energy efficiency of the system. When hydrogen, non-converted in the fuel cell, is used to supply heat in the steam reformer, the effective hydrogen yield is essentially independent of the temperature of the reformer and the water gas shift reactor. Optimal operating conditions of partial oxidation processors have been determined assuming an upper limit for the preheat temperature of the feed. Results are discussed along with other practical considerations in view of actual applications.     

Utilizing coke oven gases as a fuel for a cogeneration system based on high temperature proton exchange membrane fuel cell; energy,exergy and economic assessment

Based on a high temperature proton exchange membrane fuel cell (HT-PEMFC), a cogeneration system is proposed to produce heat and power. The system includes a coke oven gas steam reformer, a water gas shift reactor, and an afterburner. The system is analyzed in detail considering the energy, exergy and economic viewpoints. The analyses reveal the importance of HT-PEMFC in the system and according to the results, 9.03 kW power is generated with energy and exergy efficiencies of 88.2% and 26.2%, respectively and the total product unit cost is calculated as 91.8 $/GJ. Through a parametric study the effects on system performance are studied of such variables as the current density, fuel cell and reformer operating temperatures, and cathode stoichiometric ratio. It is found that an increase in the fuel cell temperature and/or a decrease in the reformer temperature enhance the exergy efficiency. The exergy efficiency is also maximized at the cathode stoichiometric ratio of 2.4. By performing a two-objective optimization using genetic algorithm, the best operating point is determined at which the exergy efficiency is (32.86%) and the total product unit cost is (78.68 $/GJ).     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

Investigations on the behaviour of 2 kW natural gas fuel processor

In this paper, the first experimental investigations on a pre-commercial natural gas steam reformer have been presented. The fuel processor unit contains the elements as follows: desulfurizer, steam reformer reactor, CO shift converter, CO preferential oxidation (PROX) reactor, steam generator, burner and heat exchangers.The fuel processor produces 45 Nl/min of syngas in which the hydrogen concentration is about 75 vol.% and the other chemical species are nitrogen, carbon dioxide and methane. The CO concentration is below 1 ppmv, so that this reforming system is suitable for the integration with a PEM fuel cell stack.The experimental activity has been conducted in a test station, properly designed to measure the behaviour of the fuel processor. The laboratory test facility is equipped by a National Instruments Compact DAQ real-time data acquisition and control system running Labview™ software. Several measurement instruments and controlling devices have been installed. Furthermore, a gas chromatograph is used to measure the product gas composition during the tests.The aim of this work has been to analyze the behaviour of this pre-commercial steam reforming unit during its operation cycle in different operating conditions (full and partial loads) in order to study its integration with a PEM fuel cell for developing a high efficiency microcogeneration system for residential applications.     

Modeling and optimization of an industrial hydrogen unit in a crude oil refinery

The main goal of this research is the modeling and optimization of an industrial hydrogen unit in a domestic oil refinery at steady state condition. The considered process consists of steam methane reforming furnace, low and high temperature shift converters, CO₂ absorption column and methanation reactor. In the first step, the reactors are heterogeneously modeled based on the mass and energy balance equations considering heat and mass transfer resistances in the gas and catalyst phases. The CO₂ absorption column is simulated based on the equilibrium non-ideal approach. In the second step, a single objective optimization problem is formulated to maximize hydrogen production in the plant considering operating and economic constraints. The feed temperature, firebox temperature, and steam flow rate in the reformer, feed temperature in shift converters, lean amine flow rate in the absorption column, and feed temperature in the methanator are selected as decision variables. The calculated effectiveness factors and mass transfer coefficients prove that the methane reforming is inertia-particle mass transfer control, while shift and methanation reactions are surface reaction control. The simulation results show that applying the optimal condition on the system increases hydrogen production capacity from 85.93 to 105.5 mol s⁻¹.     

Performance improvement of methanol steam reforming system with auxiliary heat recovery units

The thermal energy of a methanol steam reforming system is balanced with heat-up by a methanol burner, heat absorption by an evaporator, and an endothermic reforming reactor. As the thermal energy of a methanol steam reformer is delicately controlled, its thermal efficiency is significantly improved. In this study, three different system configurations are compared, namely, (1) a reference methanol steam reformer with an external evaporator, (2) a methanol steam reformer with an internal evaporator and type-1 auxiliary heat recovery unit (AHRU) with a heat source gas, and (3) a methanol steam reformer with an internal evaporator and type-2 AHRU with a heat source gas and reformed gas. These three configurations are analyzed, and the two heat recovery units are investigated. Results show that the internally evaporated methanol steam reformer efficiently converts the methanol to a hydrogen-rich mixture as exit gases are utilized to heat up the inlet methanol/water mixture.     

Thermal design of methane steam reformer with low-temperature non-reactive heat source for high efficiency engine-hybrid stationary fuel cell system

In hybrid fuel cell systems, the fuel-lean anode-off gas is very useful to improve the system efficiency via additional power generation or utilization of thermal energy for heating up of auxiliary devices. In this study, the thermal energy of the hybrid systems is firstly utilized in homogeneous charge combustion engine for additional power and is then supplied to heat up the external reformer. Different from other hybrid fuel cell systems, it is very difficult to utilize heat energy of exhausted gas from engine due to its low temperature characteristics. This study is concentrated on the computation analysis of external methane steam reformers with engine out exhausted gases. Computational model is validated with experiment and parametric study is conducted. Results show that the temperature uniformity of the longitudinal and radial directions is crucial for the methane conversion efficiency. Additionally, the methane conversion rate also depends on the performance of tube-side heat transfer. When the total methane flow is fixed, the methane conversion rate shows trade-off with increasing steam-to-carbon ratio (SCR). Finally, the sensitivity study shows that heat transfer area and reactor length are dominant parameters for steam reforming with engine out exhausted gases.     

Solar ground reformer

The possibility to couple a large-scale solar tubular reformer (a chemical reactor where a low hydrocarbon, e.g., methane, is reacting with steam and/or CO₂ to produce synthesis gas, a mixture of carbon monoxide and hydrogen), with a solar tower beam-down optics, is explored and analyzed. The reformer is installed on the ground and the concentrated solar energy enters through an aperture in the ceiling of the reformer enclosure and heats the reactor tubes, arranged within the enclosure along its walls. This arrangement enables, potentially, to adapt a large solar plant to a modified conventionally designed reformer. Through an example where about 50 MW of solar energy is introduced into an array of ground reformers, the optical path and the radiative heat exchange within the reformer cavity, coupled with the reactor tube side, are calculated and illustrated. Detailed calculation results describing the radiation flux profile inside the cavity, the surface temperatures of the reactor tubes and the composition of the process gas along the tubes, as well as the annual performance of the reformer, are shown. The results demonstrate that the solar reformer illustrated in this study can be connected to a combined cycle-power generation unit having a nominal capacity of 100-MW electricity. A power generation unit of this kind, if operating annually during roughly 2000 sunny hours, will produce about 25% of its power, supplied by the solar energy source.     

Simultaneous production of two types of synthesis gas by steam and tri‐reforming of methane using an integrated thermally coupled reactor: mathematical modeling

In this work, tri‐reforming and steam reforming processes have been coupled thermally together in a reactor for production of two types of synthesis gases. A multitubular reactor with 184 two‐concentric‐tubes has been proposed for coupling reactions of tri‐reforming and steam reforming of methane. Tri‐reforming reactions occur in outer tube side of the two‐concentric‐tube reactor and generate the needed energy for inner tube side, where steam reforming process is taking place. The cocurrent mode is investigated, and the simulation results of steam reforming side of the reactor are compared with corresponding predictions for thermally coupled steam reformer and also conventional fixed‐bed steam reformer reactor operated at the same feed conditions. This reactor produces two types of syngas with different H₂/CO ratios. Results revealed that H₂/CO ratio at the output of steam and tri‐reforming sides reached to 1.1 and 9.2, respectively. In this configuration, steam reforming reaction is proceeded by excess generated heat from tri‐reforming reaction instead of huge fired‐furnace in conventional steam reformer. Elimination of a low performance fired‐furnace and replacing it with a high performance reactor causes a reduction in full consumption with production of a new type of synthesis gas. The reactor performance is analyzed on the basis of methane conversion and hydrogen yield in both sides and is investigated numerically for various inlet temperature and molar flow rate of tri‐reforming side. A mathematical heterogeneous model is used to simulate both sides of the reactor. The optimum operating parameters for tri‐reforming side in thermally coupled tri‐reformer and steam reformer reactor are methane feed rate and temperature equal to 9264.4 kmol h^?1 and 1100 K, respectively. By increasing the feed flow rate of tri‐reforming side from 28,120 to 140,600 kmol h^?1, methane conversion and H₂ yield at the output of steam reforming side enhanced about 63.4% and 55.2%, respectively. Also by increasing the inlet temperature of tri‐reforming side from 900 to 1300 K, CH₄ conversion and H₂ yield at the output of steam reforming side enhanced about 82.5% and 71.5%, respectively. The results showed that methane conversion at the output of steam and tri‐reforming sides reached to 26.5% and 94%, respectively with the feed temperature of 1100 K of tri‐reforming side. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental and theoretical analysis of a natural gas fuel processor

In this study, a natural gas fuel processor was experimentally and theoretically investigated. The constructed 2.0 kWth fuel processor is suitable for a residential-scale high temperature proton exchange membrane fuel cell. The system consists of an autothermal reformer; gas clean-up units, namely high and low-temperature water-gas shift reactors; and utilities including feeding unit, burner, evaporator and heat exchangers. Commercial monolith catalysts were used in the reactors. The simulation was carried out by using ASPEN HYSYS program. A validated kinetic model and adiabatic equilibrium model were both presented and compared with experimental data. The nominal operating conditions which were determined by the kinetic model were the steam-to-carbon ratio of 3.0, the oxygen-to-carbon ratio of 0.5 and the inlet temperatures of 450 °C for autothermal reformer, 400 °C for high-temperature water-gas shift reactor and 310 °C for low-temperature water-gas shift reactor. Experimental results at the nominal condition showed that the performance criteria of the hydrogen yield, the fuel conversion and the efficiency were 2.53, 93.5% and 82.3% (higher heating value-HHV), respectively. The validated kinetic model was further used for the determination of 2–10kWthermal fuel processor efficiency which was increasing linearly up-to 86.3% (HHV).     

Performance analysis of a membrane‐based reformer‐combustor reactor for hydrogen generation

Hydrogen is mostly produced in conventional steam methane reforming plants. In this work, we proposed a membrane‐based reformer‐combustor reactor (MRCR) for hydrogen generation in order to improve heat recovery and overall thermal efficiency. The proposed configuration will also reduce the complexity in existing steam methane reforming (SMR) plants. The proposed MRCR comprises combustion zone, hydrogen permeate zone, and SMR zone. A computational fluid dynamics model was developed using ANSYS‐Fluent software to simulate and analyze the performance of the proposed MRCR. Results show that high hydrogen yields were observed at high reformer pressures (RPs) and low gas hourly space velocities (GHSVs). Furthermore, by increasing the steam to methane ratio and addition of excess air in the combustion side, the hydrogen yield from the MRCR decreases. This is attributed to the reduction in the effective temperature of the hydrogen membrane. High RP, low GHSV, and low steam to methane ratio that increased the hydrogen yield also decreased carbon monoxide (CO) emissions. For an increased RP from 1 to 10 bar, the CO emission decreased by about 99%. The reduction in CO emission at high RP would be attributed to the effect of water gas shift reaction in the MRCR. Results of the extensive parametric study presented in this work can be used to determine the operating conditions based on tradeoffs between hydrogen yield (mole H₂/mole CH₄), hydrogen production rate (kg of H₂/h), allowable CO emissions, and exhaust gas temperature for other applications such as gas turbine.     

Process simulation of hydrogen production by steam reforming of diluted bioethanol solutions: Effect of operating parameters on electrical and thermal cogeneration by using fuel cells

The possibility to exploit diluted bioethanol streams is discussed for hydrogen production by steam reforming. An integrated unit constituted by a steam reformer, a hydrogen purification section with high- and low-temperature water gas shift, a methanator reactor and a fuel cell were simulated to achieve residential size cogeneration of 5 kW electrical power + 5 kW thermal power as target output.Process simulation allowed to investigate the effect of the reformer temperature, of bioethanol concentration and of catalyst loading on the temperature and concentration profiles in the steam reformer. The net power output was also calculated on the basis of 27 different operating conditions.P_electrical output ranging from 3.3 to 6.0 kW were obtained, whereas the total heat output P_{thermal, total} ranged from 3.9 to 7.2 kW. The highest overall energy output corresponded to P_electrical = 4.8 kW, P_{Thermal, FC} = 3.1 kW, P_{heat recovery} = 4.1 kW, for a total 12 kW energy output. This was achieved by feeding a mixture with water/ethanol ratio = 11 (mol/mol), irrespectively of the catalyst mass, and setting the ref split temperature so to have an average temperature of 635 °C in the ESR reactor.     

Simulation of steam reforming of biogas in an industrial reformer for hydrogen production

The paper aims to investigate the steam reforming of biogas in an industrial-scale reformer for hydrogen production. A non-isothermal one dimensional reactor model has been constituted by using mass, momentum and energy balances. The model equations have been solved using MATLAB software. The developed model has been validated with the available modeling studies on industrial steam reforming of methane as well as with the those on lab-scale steam reforming of biogas. It demonstrates excellent agreement with them. Effect of change in biogas compositions on the performance of industrial steam reformer has been investigated in terms of methane conversion, yields of hydrogen and carbon monoxide, product gas compositions, reactor temperature and total pressure. For this, compositions of biogas (CH₄/CO₂ = 40/60 to 80/20), S/C ratio, reformer feed temperature and heat flux have been varied. Preferable feed conditions to the reformer are total molar feed rate of 21 kmol/h, steam to methane ratio of 4.0, temperature of 973 K and pressure of 25 bar. Under these conditions, industrial reformer fed with biogas, provides methane conversion (93.08–85.65%) and hydrogen yield (1.02–2.28), that are close to thermodynamic equilibrium condition.     

Four different configurations of a 5 kW class shell-and-tube methane steam reformer with a low-temperature heat source

The methane steam reforming reaction is an extremely high endothermic reaction that needs a high temperature heat source. Various fuel cell hybrid systems have been developed to improve the thermal efficiency of the entire system. This paper presents a low temperature steam reformer for those hybrid systems to maximize the utilization of energy from a low temperature waste heat source. In this study, the steam reformer has a shell and tube configuration that is divided into the following zones: the inlet heat exchanging zone, the reforming zone and the exit heat exchanging zone. Four different configurations for methane steam reformers are developed to examine the effect of heat transfer on the methane conversion performance of the low temperature steam reformer. The experimental results show that the overall heat transfer area is a critical parameter in achieving a high methane conversion rate. When the heat transfer area increases about 30%, the results showed elevated dry mole fractions of hydrogen about 3% with about 30 °C rise of reformer outlet temperature.     

Thermodynamic analysis of hydrogen production via sorption-enhanced steam methane reforming in a new class of variable volume batch-membrane reactor

Combined reaction–separation processes are a widely explored method to produce hydrogen from endothermic steam reforming of hydrocarbon feedstock at a reduced reaction temperature and with fewer unit operation steps, both of which are key requirements for energy efficient, distributed hydrogen production. This work introduces a new class of variable volume batch reactors for production of hydrogen from catalytic steam reforming of methane that operates in a cycle similar to that of an internal combustion engine. It incorporates a CO₂ adsorbent and a selectively permeable hydrogen membrane for in situ removal of the two major products of the reversible steam methane reforming reaction. Thermodynamic analysis is employed to define an envelope of ideal reactor performance and to explore the tradeoff between thermal efficiency and hydrogen yield density with respect to critical operating parameters, including sorbent mass, steam to methane ratio and fraction of product gas recycled. Particular attention is paid to contrasting the variable volume batch-membrane reactor approach to a conventional fixed bed reaction–separation approach. The results indicates that the proposed reactor is a viable option for low temperature distributed production of hydrogen from methane, the primary component of natural gas feedstock, motivating a detailed study of reaction/adsorption kinetics and heat/mass transfer effects.     

Thermodynamic analysis of H2 production from CaO sorption‐enhanced methane steam reforming thermally coupled with chemical looping combustion as a novel technology

In this novel paper, a technique for hydrogen production route of CaO sorption‐enhanced methane steam reforming (SEMSR) thermally coupled with chemical looping combustion (CLC) was presented (CLC‐SEMSR), which perceived as an improvement of previous methane steam reforming (MSR) thermally coupled with CLC technology (CLC‐MSR). The application of CLC instead of furnace achieves the inherent separation of CO₂ from flue gas without extra energy required. The required heat for the reformer is provided by thermally coupling CLC. The addition of CaO sorbents can capture CO₂ as it is formed from the reformer gas to the solid phase, displacing the normal MSR equilibrium restrictions and obtaining higher purity of H₂. The Aspen Plus was used to simulate this novel process on the basis of thermodynamics. The performances of this system examined included the composition of reformer gas, yield of reformer gas (Y_Rg), methane conversion (α_M), the overall energy efficiency (η), and exergy efficiency (φ) of this process. The effects of the molar ratio of CaO to methane for reforming (Ca/M) in the range of 0–1.2, the molar ratio of methane for combustion to methane for reforming (M(fuel)/M) in the range of 0.1–0.3, and the molar ratio of NiO to methane for reforming (Ni/M) in the range of 0.4–1.2 were investigated. It has been found to be favored by operating under the conditions of Ca/M = 1, M(fuel)/M = 0.2, and Ni/M = 0.8. The most excellent advantage of CLC‐SEMSR was that it could obtain higher purity of H₂ (95%) at lower operating temperature (655 °C), as against H₂ purity of 77.1% at higher temperature (900 °C) in previous CLC‐MSR. In addition, the energy efficiency of this process could reach 83.3% at the optimal conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

A novel integrated thermally coupled configuration for methane-steam reforming and hydrogenation of nitrobenzene to aniline

In this work, a novel thermally coupled reactor containing the steam reforming process in the endothermic side and the hydrogenation of nitrobenzene to aniline in the exothermic side has been investigated. In this novel configuration, the conventional steam reforming process has been substituted by the recuperative coupled reactors which contain the steam reforming reactions in the tube side, and the hydrogenation reaction in the shell side. The co-current mode is investigated and the simulation results are compared with corresponding predictions for an industrial fixed-bed steam reformer reactor operated at the same feed conditions. The results show that although synthesis gas productivity is the same as conventional steam reformer reactor, but aniline is also produced as an additional valuable product. Also it does not need to burn at the furnace of steam reformer. The performance of the reactor is numerically investigated for different inlet temperature and molar flow rate of exothermic side. The reactor performance is analyzed based on methane conversion, hydrogen yield and nitrobenzene conversion. The results show that exothermic feed temperature of 1270 K can produce synthesis gas with 26% methane conversion (the same as conventional) and nitrobenzene conversion in the outlet of the reactor is improved to 100%. This new configuration eliminates huge fired furnace with high energy consumption in steam reforming process.     

Performance evaluation of a combined power generation system integrated with thermochemical exhaust heat recuperation based on steam methane reforming

The present paper applies the thermodynamic analysis with the determining the efficiency of a combined cycle power plant with a chemically recuperated gas turbine. Thermochemical recuperation of exhaust heat after a gas turbine is realized via the steam methane reforming process. The main concept of combined cycle power plant (CCPP) with chemically recuperated gas turbine (CRGT) is based on the use of exhaust heat for endothermic reforming of the original hydrocarbon fuel in a reformer and for steam generation for a steam cycle. To understand the effect of operating variables such as temperature, pressure, and steam-to-methane ratio on the overall efficiency, the energy and mass balances were compiled. The energy flows were represented by a Sankey diagram. The results of the thermodynamic analysis show that efficiency of CCPP with CRGT is significantly higher (4–7%) than efficiency of CCPP with a conventional gas turbine without TCR. Maximum efficiency of CCPP with CRGT of 0.6412 is observed at inlet temperature of working gas of 1600 °C, pressure of 23 bar for a steam-to-methane ratio of 3.0. In the temperature of inlet working gas below 1200 °C the increase in the efficiency of CCPP with TCR is less than 2%.