Thermochemical waste-heat recuperation as on-board hydrogen production technology

Thermochemical waste-heat recuperation (TCR) as an on-board hydrogen production technology is considered. To determine the effectiveness of using TCR systems as an on-board hydrogen production technology and to assess the possibility of hydrogen production in TCR systems, a thermodynamic analysis of various hydrocarbon reforming reactions was carried out. The thermodynamic analysis has been realized via Aspen HYSYS software. Three steam reforming reactions with methane, methanol, and ethanol were investigated. It was established that the composition of the initial reaction mixture and the process temperature has a significant effect on the efficiency of the thermochemical heat recuperation system. The maximum efficiency of thermochemical heat recuperation systems due to steam reforming is achieved at 600 K for methanol; 700 K for ethanol and 900 K for methane.     

Forging furnace with thermochemical waste-heat recuperation by natural gas reforming: Fuel saving and heat balance

This paper considers thermochemical recuperation (TCR) of waste-heat using natural gas reforming by steam and combustion products. Combustion products contain steam (H₂O), carbon dioxide (CO₂), and ballast nitrogen (N₂). Because endothermic chemical reactions take place, methane steam-dry reforming creates new synthetic fuel that contains valuable combustion components: hydrogen (H₂), carbon monoxide (CO), and unreformed methane (CH₄). There are several advantages to performing TCR in the industrial furnaces: high energy efficiency, high regeneration rate (rate of waste-heat recovery), and low emission of greenhouse gases (CO₂, NO_x). As will be shown, the use of TCR is significantly increasing the efficiency of industrial furnaces – it has been observed that TCR is capable of reducing fuel consumption by nearly 25%. Additionally, increased energy efficiency has a beneficial effect on the environment as it leads to a reduction in greenhouse gas emissions.     

How to choose endothermic process for thermochemical waste-heat recuperation?

The thermochemical waste-heat recuperation (TCR) systems by steam reforming of various hydrocarbon fuels are considered. A method for determining the TCR systems efficiency is proposed. The methodology is based on the determination of the heat recuperation rate and heat transformation coefficient. TCR due to steam reforming of methanol, ethanol, glycerol, propane, and methane was analyzed. To obtain the initial data for energy analysis of TCR systems, the thermodynamic analysis was performed. With the help of Aspen HYSYS was determined the synthesis gas composition and reaction enthalpy for all investigated variants. The investigation was performed for a wide temperature range from 400 to 1200 K, for the steam-to-fuel ratio of 1, and pressures of 1 bar. It was established that TCR due to the steam reforming of ethanol, glycerol, and propane in the temperature range above 800 K, it is possible to achieve complete recuperation of the exhaust after the furnace. As a results of investigation, the practical recommendations were given for choosing endothermic reaction for the thermochemical waste-heat recuperation systems: in the temperature range up to 600 K for TCR it is necessary to choose a methanol steam reforming reaction; in the temperature range from 600 to 1000 K - the reaction of steam reforming of ethanol and glycerol; in the temperature range above 1000 K - the reaction of steam reforming propane and methane.     

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.     

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.     

Thermochemical recuperation by ethanol steam reforming: Thermodynamic analysis and heat balance

This article considers the scheme of fuel-consuming equipment with a thermochemical heat recuperation system by using ethanol steam reforming. The main concept of thermochemical recuperation (TCR) is the transformation of exhaust gases heat into chemical energy of a new synthetic fuel that has higher calorimetric properties such as low-heating value. Thermochemical recuperation can be considered as an on-board hydrogen production technology. To determine the efficiency of the thermochemical recuperation system, the thermodynamic analysis via Gibbs free energy minimization method was performed. The software Aspen-HYSYS was used for the thermodynamic analysis. The heat flows were calculated for a wide temperature range from 500 to 1000 K, for steam-to-ethanol ratio from 1 to 3, and for various pressures of 1, 5 and 10 bar. The results of the thermodynamic analysis were compared with the experimental results and the results of the thermodynamic analysis performed by other authors. All obtained results are in a good correlation. In the first law energy analysis was found that for a high steam-to-ethanol ratio (above 3), to perform thermochemical recuperation an external heat must be supplied to the TCR system. The heat deficit for steam-to-ethanol ratio 3 is from 1 to 2 MJ/kg_EtOH in the temperature range from 500 to 1000 K.     

Comparison study of thermochemical waste-heat recuperation by steam reforming of liquid biofuels

The concept of thermochemical exhaust heat recuperation by steam reforming of biofuels is considered. Thermochemical recuperation can be considered as an on-board hydrogen production technology. A schematic diagram of a fuel-consuming equipment with thermochemical heat recuperation is described. The thermodynamic analysis of the thermochemical recuperation systems was performed to determine the efficiency of using various fuels, in particular, methanol, ethanol, n-butanol, and glycerol. The thermodynamic analysis was performed by Gibbs free energy minimization method and implemented using the Aspen Hysys program. The thermodynamic analysis was performed for a wide temperature range from 400 to 900 K, for steam-to-fuel of 1, and pressures of 1 bar. The maximum fuel conversion reaches for the following temperatures: methanol - 600 K, ethanol - 730 K, n-butanol - 860 K, glycerol - 890 K. The dependence of the reforming enthalpy on temperature is determined. It was shown that the reaction enthalpy determines the heat transformation coefficient, which shows the ratio of the low heat value of synthetic fuel and the low heat value of the initial fuel. For all studied fuels, the maximum value of the transformation coefficient is observed for steam reforming of ethanol and the maximum heat transformation coefficient is 1.187. The temperature range is determined at which the maximum efficiency of the use of thermochemical recuperation occurs due to the reforming of biofuels. For methanol, the effective temperature is about 600 K, for ethanol is about 700 K, for n-butanol is 850 K, for glycerol is more than 900 K. The results obtained make it possible to efficiently select the type of fuel for thermochemical recuperation due to steam reforming.     

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%.     

Thermodynamic equilibrium analysis of combined dry and steam reforming of propane for thermochemical waste-heat recuperation

The thermochemical waste-heat recuperation is one for perspective way of increasing the energy efficiency of the fuel-consuming equipment. In this paper, the thermochemical waste-heat recuperation (TCR) by combined steam-dry propane reforming is described. To understand the influence of technological parameter such as temperature and composition of inlet gas mixture on TCR efficiency, thermodynamic equilibrium analysis of combined steam-dry propane reforming was investigated by Gibbs free energy minimization method upon a wide range of temperature (600–1200 K) and different feed compositions at atmospheric pressure. The carbon and methane formation was also calculated and shown. From a thermodynamic perspective, the TCR can be used for increasing energy efficiency at temperatures above 950 K because in this range the maximum conversion rate is reached (from 1.22 to 1.30 for the different feed composition). Approximately 10 mol of synthesis gas can be generated per mole of propane at the temperatures greater than 1000 K. Furthermore, the propane conversion rate and yield of hydrogen are increased with the addition of extra steam to the feed stock. Also, undesirable carbon formation can be eliminated by adding steam to the feed. The thermodynamic equilibrium analysis was accomplished by IVTANTHERMO which is a process simulator for thermodynamic modeling of complex chemically reacting systems and several results were checked by Aspen-HYSYS.     

Novel micro-channel methane reformer assisted combustion reaction for hydrogen production

A metal catalyst-containing, 80 ml, micro-channel reactor (MCR) with a section dedicated to combustion reaction was investigated for the potential application of on-board methane steam reforming (MSR) to hydrogen production. The metal catalyst was introduced into the MCR as a shape of a thin plate that was diffusion-bonded with the other micro-channel plates. The combustion reaction was performed on the other side of the MCR for direct provision of the necessary heat for the endothermic MSR and for miniaturizing the system volume. In the MCR, both the methane conversion and the hydrogen production rate are extremely high compared with those of the equilibrium under atmospheric pressure. The required heat of reaction is successfully provided by the combustion of either hydrogen or the methane mixture on the other side of the MCR without the need for any heating cartridges. This novel micro-channel reformer is suitable for application as a compact fuel processor due to its production of hydrogen-rich syn-gas, small volume, simple catalyst loading and use of an active and easily stackable catalyst.     

Analysis of chemical-reaction-coupled mass and heat transport phenomena in a methane reformer duct for PEMFCs

Mass, heat and momentum transport processes are coupled with catalytic chemical reactions in a methane steam reforming duct. It is often found that endothermic and exothermic reactions in the ducts are strongly integrated by heat transfer from adjacent catalytic combustion ducts. In this paper, a three-dimensional calculation method is developed to simulate and analyze reforming reactions of methane, and the effects on various transport processes in a steam reforming duct. The reformer conditions such as mass balances associated with the reforming reactions and gas permeation to/from the porous catalyst reforming layer are applied in the analysis. The predicted results are presented and discussed for a composite duct consisting of a porous catalyst reaction layer, the fuel gas flow duct and solid layers. Parametric studies are conducted to reveal the importance of reformer designs and operating conditions. The results show that the variables, such as porous layer configuration, temperature and catalyst loading, have significant effects on the transport processes and reformer performance.     

Thermodynamic analysis of hydrogen production via autothermal steam reforming of selected components of aqueous bio-oil fraction

From a technical and economic point of view, autothermal steam reforming offers many advantages, as it minimizes heat load demand in the reformer. Bio-oil, the liquid product of biomass pyrolysis, can be effectively converted to a hydrogen-rich stream. Autothermal steam reforming of selected compounds of bio-oil was investigated using thermodynamic analysis. Equilibrium calculations employing Gibbs free energy minimization were performed for acetic acid, acetone and ethylene glycol in a broad range of temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm) values. The optimal O₂/fuel ratio to achieve thermoneutral conditions was calculated under all operating conditions. Hydrogen-rich gas is produced at temperatures higher than 700 K with the maximum yield attained at 900 K. The ratio of steam to fuel and the pressure determine to a great extent the equilibrium hydrogen concentration. The heat demand of the reformer, as expressed by the required amount of oxygen, varies with temperature, steam to fuel ratio and pressure, as well as the type of oxygenate compound used. When the required oxygen enters the system at the reforming temperature, autothermal steam reforming results in hydrogen yield around 20% lower than the yield by steam reforming because part of the organic feed is consumed in the combustion reaction. Autothermicity was also calculated for the whole cycle, including preheating of the organic feed to the reactor temperature and the reforming reaction itself. The oxygen demand in such a case is much higher, while the amount of hydrogen produced is drastically reduced.     

Micro methanol reformer combined with a catalytic combustor for a PEM fuel cell

This paper presents the development of a micro methanol reformer for portable fuel cell applications. The micro reformer consists of a methanol steam reforming reactor, catalytic combustor, and heat exchanger in-between. Cu/ZnO was selected as a catalyst for a methanol steam reforming and Pt for a catalytic combustion of hydrogen with air. Porous ceramic material was used as a catalyst support due to the large surface area and thermal stability. Photosensitive glass wafer was selected as a structural material because of its thermal and chemical stabilities. Performance of the reformer was measured at various test conditions and the results showed a good agreement with the three-dimensional analysis of the reacting flow. Considering the energy balance of the reformer/combustor model, the off-gas of fuel cell can be recycled as a feed of the combustor. The catalytic combustor generated the sufficient amount of heat to sustain the steam reforming of methanol. The conversion of methanol was 95.7% and the hydrogen flow of 53.7 ml/min was produced including 1.24% carbon monoxide. The generated hydrogen was the sufficient amount to operate 4.5 W polymer electrolyte membrane fuel cells.     

Influence of methanol reformate injection strategy on performance,available exhaust gas enthalpy and emissions of a direct-injection spark ignition engine

This paper presents experimental study results of a direct injection engine fed with methanol steam reforming products and devised to work with a high-pressure thermochemical recuperation system. The influence of injection pressure and timing on heat release rate, fuel mass fraction burned, cycle-to-cycle variation, pollutant emissions, efficiency and exhaust gas energy available for methanol reforming is investigated and analyzed. Effect of injector flow area on the required injection pressure is discussed. End-of-injection (EOI) timing is shown to be the main influencing factor on engine efficiency and pollutant emissions. The obtained results indicate that there is a range of EOI timing where indicated efficiency is almost constant and NO_x emissions drop by a factor of 2.5. Particle number emissions can be reduced in this range by a factor of 4. We showed that engine exhaust gas possesses enough energy to sustain endothermic reforming reactions up to excess air ratio of 2.5.     

Exergy analysis on non-catalyzed partial oxidation reforming using homogeneous charge compression ignition engine in a solid oxide fuel cell system

Based on the recent improvements in high-temperature fuel cells, distributed power generation fuel cell system of small scale (～hundreds kilowatts) has been widely investigated. To improve the system efficiency, most developments focused on the fuel cell stack, but little was paid attention to the intrinsic exergy destructions at the other parts of a typical configuration. The main objective of this study is to investigate a feasibility of reducing the exergy destruction in the reforming process of fuel cell system, by using a homogeneous charge compression ignition (HCCI) engine as a replacement of existing reforming subsystems, i.e. steam methane reforming (SMR), partial oxidation (POX), or autothermal reforming (ATR), in a solid oxide fuel cell (SOFC) system. To do this, parametric studies with exergy analysis were conducted by using in-house 1-D SOFC and 0-D HCCI simulation models. In results, due to the work production from HCCI reforming engine in addition to the work of the fuel stack, it is demonstrated that HCCI-SOFC system has higher system efficiency than partial oxidation (POX) and autothermal reforming (ATR) systems, which use similar partial oxidation reaction for reformer operation. Furthermore, because of no requirement for catalyst, the HCCI system demonstrates wider operating range than that of POX and ATR systems. When compared to the steam methane reforming (SMR)-SOFC system, the HCCI-SOFC system has the lower total work but slightly higher exergetic system efficiency, mainly caused by large amount of heat exergy needed to operate endothermic reforming process in the SMR process. Based on our simulation data, the exergetic efficiency of the HCCI-SOFC system shows 6.0%, 2.1% and 0.4% higher than POX, ATR and SMR systems at the highest efficiency points of each strategy, while 5.5%, 5.8% and 3.8% higher than POX, ATR and SMR systems at 99% methane conversion points in each reformer, respectively.     

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.     

An experimental study on the reaction characteristics of a coupled reactor with a catalytic combustor and a steam reformer for SOFC systems

Steam reforming performance in a coupled reactor that consists of a steam reformer and a catalytic combustor is experimentally investigated in this study. Endothermic steam reforming can occur through the absorption of heat from the catalytic combustion of the anode offgas in a heat-exchanging coupled reactor. The reaction characteristics were observed by varying parameters such as the inlet temperature of the catalytic combustor, the excess air ratio for the catalytic combustion, the fuel utilization rate in the fuel cells, and the steam-to-carbon ratio in the steam reformer. The reactor temperature and reformate composition were measured to analyze the performance of the reactor. The results show the potential applicability and design technologies of the coupled reactor for the fuel processing of high temperature fuel cells using an external reformer.     

Performance enhancement of methanol reforming reactor through finned surfaces and diffused entry for on-board hydrogen generation

Hydrogen-rich combustion in engines helps in reducing pollutants significantly. But hydrogen usage on a moving vehicle is not getting large-scale user acceptance mainly due to its poor energy storage density resulting in shorter driving ranges. This storage issue led to the hunt for mediums that can efficiently produce on-board hydrogen. Methanol proves to be an efficient alcohol fuel for producing hydrogen through steam reforming reaction. The heat energy required for such endothermic reaction is obtained through exhaust engine waste energy and this process is collectively known as thermochemical recuperation. However, the conventional reactor used for this process faces a lot of problems in terms of efficiency and methanol conversion. In this study, an attempt has been made to improve the design of the reactor for on-board hydrogen generation using engine exhaust heat for addressing the challenges related to performance and hydrogen yield. For enhancing the heat transfer, a finned surface (straight & wavy) was introduced in the reactor which resulted in an increment in methanol conversion significantly. It was found that wavy fin improved the methanol conversion up to 96.8% at an exhaust inlet temperature of 673 K. Also, a diffusing inlet section was introduced to increase the residence time of reactant gases while passing through the catalyst zone. Under given inlet conditions, the methanol conversion for 6° diffuse inlet reactor goes up to 87.9% as compared to 75.4% for the conventional reactor.     

A numerical study on methanol steam reforming reactor utilizing engine exhaust heat for hydrogen generation

Internal combustion engines are used in most vehicles around the world to power the transport sector. Efficiency improvement, emission reduction, and utilization of alternative fuels are the main aspects of current IC engine research. Hydrogen-enhanced combustion proved to be one of the efficient ways to achieve such goals. But the problem lies in the storage of hydrogen for the transportation sector, and on-board fuel reforming is a promising option for solving this issue. It deals with transforming a suitable liquid fuel (methanol) into an H₂-rich gas using a catalytic conversion process. For sustaining the reforming reaction, the required heat energy is taken from engine exhaust waste heat, this process is known as thermochemical recuperation. Number of studies on the reformers utilized for on-board hydrogen generation using engine exhaust heat are limited in the literature. The present study investigates the performance of a reactor that uses the exhaust gas heat energy for sustaining the reforming reaction. A numerical analysis was performed over a selected reactor where exhaust gases were flowing at one side, while on the other, the reforming reaction was taking place with the help of heat provided by high-temperature exhaust gases. A packed bed-type reactor was chosen for the current study and a parametric study was conducted where the effects of various operating parameters on both reacting and heating sides on the reactor's performance were investigated. It was found that temperature was the most influential inlet parameter among others. Steam/Carbon ratio and flow configuration had a negligible effect on the hydrogen yield as well as methanol conversion. Reactant inlet velocity increment revealed a significant drop in methanol conversion as it reduces the residence time for reforming reaction in the catalyst zone.     

Experimental and computational investigations of a compact steam reformer for fuel oil and diesel fuel

The present work describes the optimisation of a compact steam reformer for light fuel oil and diesel fuel. The reformer is based upon a catalytically coated micro heat exchanger that thermally couples the reforming reaction with a catalytic combustion. Since the reforming process is sensitive to reaction temperatures and internal flow patterns, the reformer was modelled using a commercial CFD code in order to optimise its geometry. Fluid flow, heat transfer and chemical reactions were considered on both sides of the heat exchanger. The model was successfully validated with experimental data from reformer tests with 4 kW, 6 kW and 10 kW thermal inputs of light fuel oil. In further simulations the model was applied to investigate parallel flow, counter flow and cross flow conditions along with inlet geometry variations for the reformer. The experimental results show that the reformer design allows inlet temperatures below 773 K because of its internal superheating capability. The simulation results indicate that two parallel flow configurations provide fast superheating and high fuel conversion rates. The temperature increase inside the reactor is influenced by the inlet geometry on the combustion side.