Exergy analysis and optimization of methanol generating hydrogen system for PEMFC

An exergy analysis of methanol autothermal generating hydrogen system for PEMFC is presented. The process combines a catalytic combustion heat exchanger (CCHE), using partial off-gases containing hydrogen as feedstock, with an auto-thermal reformer (ATR), two water gas shift (WGS) reactors and four preferential oxidation (PROX) reactors. Energy and exergy of system were calculated and analyzed. The results demonstrated that inner exergy losses resulted from the irreversible heat transfer and reaction were the dominant factors. The most important destruction of exergy within the system was found to occur in the reformer and the catalytic combustion heat exchanger. Their ratios of exergy loss accounted for 25.03% and 24.95%, respectively, of the whole system. Based on results of thermodynamic and exergetic analysis, the reformer was optimized. The optimal W/M$W/M$ (molar water to methanol) is around 1.5–2.0 and A/M$A/M$ (molar air to methanol) is around 1.5. Certain recommendations were posed. The conclusions could help to optimize methanol autothermal generating hydrogen system for PEMFC.     

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.     

Investigation of a methanol processing system comprising of a steam reformer and two preferential oxidation reactors for fuel cells

Methanol steam reforming is able to produce hydrogen-rich syngas onsite for fuel cells and avoids the problems of hydrogen storage. Nevertheless, CO in the reformate needs to be further removed to ppm level before it can be fed into proton exchange membrane fuel cells. In this study, a methanol processing system consisting of a methanol reformer and two-stage preferential oxidation reactors is developed. The hydrogen production performance and scalability of the reformer are experimentally investigated under various operating conditions. The methanol reformer system shows stable methanol conversion rate and linearly increased H₂ flow rate as the number of repeating unit increases. Methanol conversion rate of 96.8% with CO concentration of 1.78% are achieved in the scaled-up system. CO cleanup ability of the two-stage preferential oxidation reactors is experimentally investigated based on the reformate compositions by varying the operating temperature and O₂ to CO ratios. The results demonstrate that the developed CO cleanup train can decrease the CO concentration from 1.6% to below 10 ppm, which meets the requirement of the fuel cell. Finally, stability of the integrated methanol processing system is tested for 180 h operation.     

Numerical prediction of operating characteristics of micro-gas turbine combustor with on-board reformed EGR system using co-simulation model and 1D reduced model

In this study, an on-board reforming gas turbine system was proposed to expend the combustion stability and operating points of as gas turbine combustor aiming for fuel lean condition. On-board reforming does not store the syngas unlike the existing conventional reforming device, but formed syngas as the operating load changes and participates in combustion. In previous research conducted for this study, a concept single nozzle combustor was designed that satisfies the thermal output of 150 kW and the turbine inlet temperature of 1200 K. In addition, by designing a non-catalytic partial oxidation-based concept reformer, syngas formation was confirmed in various operation points. In previous research, closed-loop analysis was performed to analyze the independent effects of combustor and reformer. However, in this study, open-loop analysis that simulates the combustor and reformer simultaneously was performed to analyze the effect of the combined system at various operating points. As a result, improved combustion was confirmed by the generation of OH radicals when the oxidizing agent was diluted with increasing hydrogen content. This is similar to the lean OH radical distribution in a low-oxidizing environment, which is the basic characteristics of moderate or intense low-oxygen dilution combustion. The reformer analyzed the reaction by changing the reformate fuel inlet velocity. Through this, it was confirmed that the mixedness inside the reformer improved as the reformate fuel inlet velocity. Finally, to calculate the efficiency of the hydrogen addition operating points under various conditions, suitable operating points were derived by comparison with conventional partial oxidation reforming. The operating range of moderate or intense low oxygen dilution combustion in an on-board reforming gas turbine system was numerically predicted. This is expected to greatly contribute to the study to improve the stability of moderate or intensive low oxygen dilution combustion in the future.     

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.     

Hydrogen production with a solar steam–methanol reformer and colloid nanocatalyst

In the present study a small steam–methanol reformer with a colloid nanocatalyst is utilized to produce hydrogen. Radiation from a focused continuous green light laser (514 nm wavelength) is used to provide the energy for steam–methanol reforming. Nanocatalyst particles, fabricated by using pulsed laser ablation technology, result in a highly active catalyst with high surface to volume ratio. A small novel reformer fabricated with a borosilicate capillary is employed to increase the local temperature of the reformer and thereby increase hydrogen production. The hydrogen production output efficiency is determined and a value of 5% is achieved. Experiments using concentrated solar simulator light as the radiation source are also carried out. The results show that hydrogen production by solar steam–methanol colloid nanocatalyst reforming is both feasible and promising.     

Thermodynamic study of hydrogen production from crude glycerol autothermal reforming for fuel cell applications

This study presents a thermodynamic analysis of hydrogen production from an autothermal reforming of crude glycerol derived from a biodiesel production process. As a composition of crude glycerol depends on feedstock and processes used in biodiesel production, a mixture of glycerol and methanol, major components in crude glycerol, at different ratios was used to investigate its effect on the autothermal reforming process. Equilibrium compositions of reforming gas obtained were determined as a function of temperature, steam to crude glycerol ratio, and oxygen to crude glycerol ratio. The results showed that at isothermal condition, raising operating temperature increases hydrogen yield, whereas increasing steam to crude glycerol and oxygen to crude glycerol ratios causes a reduction of hydrogen concentration. However, high temperature operation also promotes CO formation which would hinder the performance of low-temperature fuel cells. The steam to crude glycerol ratio is a key factor to reduce the extent of CO but a dilution effect of steam should be considered if reforming gas is fed to fuel cells. An increase in the ratio of glycerol to methanol in crude glycerol can increase the amount of hydrogen produced. In addition, an optimal operating condition of glycerol autothermal reforming at a thermoneutral condition that no external heat to sustain the reformer operation is required, was investigated.     

Hydrogen from coal-derived methanol via autothermal reforming processes

This paper reports on an investigation of hydrogen production via reformation of coal-based methanol. Through chemical analysis, coal-based methanol has shown to have higher amounts of trace hydrocarbons than chemical grade methanol derived from natural gas. While these trace hydrocarbons are typically insignificant for some energy conversion devices, fuel cell applications require ultra pure hydrogen. An autothermal reformer was investigated to find the optimal hydrogen production method with the existence of such trace impurities. Based on previous experimental results, steam reforming of coal-based methanol has shown significant catalyst degradation caused by the trace impurities. Autothermal reformation processes using coal-derived methanol avoids degradation with the trace impurities due to a higher operating temperature generated by the oxidation step. Autothermal reformation can also avoid some of the energy penalties of steam reformation but generally has a lower output concentration of hydrogen due to the diluent nature of using air as the oxidizer. This investigation shows that hydrogen production from coal-derived methanol via autothermal reformation is feasible when considering fuel cell applications.     

Mathematical simulation of hydrogen production via methanol steam reforming using double-jacketed membrane reactor

The hydrogen production and purification via methanol reforming reaction was studied in a double-jacketed Pd membrane reactor using a 1-D, non-isothermal mathematical model. Both mass and heat transfer behavior were evaluated simultaneously in three parts of the reactor, annular side, permeation tube and the oxidation side. The simulation results exhibited that increasing the volumetric flow rate of hydrogen in permeation side could enhance hydrogen permeation rate across the membrane. The optimum velocity ratio between permeation and annular sides is 10. However, hydrogen removal could lower the temperature in the reformer. The hydrogen production rate increases as temperature increases at a given Damköhler number, but the methanol conversion and hydrogen recovery yield decrease. In addition, the optimum molar ratio of air and methanol was 1.3 with three air inlet temperatures. The performance of a double-jacketed membrane reactor was compared with an autothermal reactor by judging against methanol conversion, hydrogen recovery yield and production rate. Under the same reaction conditions, the double-jacketed reactor can convert more methanol at a given reactor volume than that of an autothermal reactor.     

Mathematical modeling of hydrogen production using methanol steam reforming in the coupled membrane reactor when the output materials of the reformer section is used as feed for the combustion section

Hydrogen is one of the most abundant elements on Earth's surface. It is not in nature in its pure form, but it can produce by various methods such as methanol steam reforming, partial oxidation, electrolysis, etc. In the present study, using the mass and energy conservation law, hydrogen production in coupled membrane reactors has been modeled using the methanol steam reforming process using two different methods in the coupled membrane reactor. A separate (fresh) methanol is used as feed for the combustion section in the first method. While in the second method, the reformer reactor's output material is used as feed for the combustion section. After simplifying using the required assumptions, the governing equations solved using the ode45 (shooting method) numerical method using MATLAB software. The results of this study show that the conversion of methanol in the coupled membrane reactor when separate methanol is used as feed in the combustion reactor, while in the same reactor, the output material of the reformer section used as feed in the combustion section, is 92% and 88.5% respectively. In this study, the effect of different parameters on the methanol conversion rate is investigated. Finally, it found that with increasing temperature and pressure and decreasing membrane thickness in coupled membrane reactors, methanol conversion rate increases. The percentage of hydrogen recovery in the two coupled membrane reactors is almost equal to 92%.     

Hydrogen production using integrated methanol‐steam reforming reactor with various reformer designs for PEM fuel cells

A 95 mm × 40 mm × 15 mm compact reactor for hydrogen production from methanol‐steam reforming (MSR) is constructed by integrating a vaporizer, reformer, and combustor into a single unit. CuO/ZnO/Al₂O₃ is used as the catalyst for the MSR while the required heat is provided using Platinum (Pt) ‐catalytic methanol combustion. The reactor performance is measured using three reformer designs: patterned micro‐channel; inserted catalyst layer placed in a single plain channel; and catalyst coated directly on the bottom wall of single plain channel. Because of longer reactant residence time and more effective heat transfer, slightly higher methanol conversion can be obtained from the reformer with patterned microchannels. The experimental results show that there is no significant reactor performance difference in methanol conversion, hydrogen (H₂) production rate, and carbon monoxide (CO) composition among these three reformer designs. These results indicated that the flow and heat transfer may not play important roles in compact size reactors. The reformer design with inserted catalyst layer provides convenience in replacing the aged catalyst, which may be attractive in practical applications compared with the conventional packed bed and wall‐coated reformers. Copyright © 2011 John Wiley & Sons, Ltd.     

Study on a compact methanol reformer for a miniature fuel cell

A compact methanol reformer for hydrogen production has been successfully fabricated, which integrated one reforming chamber, one water gas shift reaction chamber, two preheating chambers and two combustion chambers. It can be started-up at room temperature by the combustion of liquid methanol in the combustion chamber within 7 min without any external heating. The cold start response of the methanol reformer has been investigated using different parameters including methanol and air supply rate, and the experiments revealed that the optimum methanol and air flow rate were 0.55 mL/min and 3 L/min respectively. The results indicated that this methanol reformer can provide a high concentration of hydrogen (more than 73%) and the system efficiency is always maintained above 74%. It is further demonstrated in more than 1600 h continuous performance that the reformer could be operated autothermally and exhibited good test stability.     

Thermodynamic analysis and heat integration for hydrogen production from bio-butanol for SOFC application: Steam reforming vs. autothermal reforming

Thermodynamic analysis of hydrogen production by steam reforming and autothermal reforming of bio-butanol was investigated for solid oxide fuel cell applications. The effects of reformer operating conditions, e.g., reformer temperature, steam to carbon molar ratio, and oxygen to carbon molar ratio, were investigated with the objective to maximize hydrogen production and to reduce utility requirements of the process and based on which favorable conditions of reformer were proposed. Process flow diagram for steam reforming and autothermal reforming integrated with solid oxide fuel cell was developed. Heat integration with pinch analysis method was carried out for both the processes at favorable reformer conditions. Power generation, electrical efficiency, useful energy for co-generation application, and utility requirements for both the processes were compared.     

Autothermal reforming of n-dodecane and desulfurized Jet-A fuel for producing hydrogen-rich syngas

Catalytic reforming is a technology to produce hydrogen and syngas from heavy hydrocarbon fuels in order to supply hydrogen to fuel cells. A lab-scale 2.5 kW_t autothermal reforming (ATR) system with a specially designed reformer and combined analysis of balance-of-plant was studied and tested in the present study. NiO–Rh based bimetallic catalysts with promoters of Ce, K, and La were used in the reformer. The performance of the reformer was studied by checking the hydrogen selectivity, CO_x selectivity, and energy conversion efficiency at various operating temperatures, steam to carbon ratios, oxygen to carbon ratios, and reactants' inlet temperatures. The experimental work firstly tested n-dodecane as the surrogate of Jet-A fuel to optimize operating conditions. After that, desulfurized commercial Jet-A fuel was tested at the optimized operating conditions. The design of the reformer and the catalyst are recommended for high performance Jet-A fuel reforming and hydrogen-rich syngas production.     

Reforming of natural gas—hydrogen generation for small scale stationary fuel cell systems

The reforming of natural gas to produce hydrogen for fuel cells is described, including the basic concepts (steam reforming or autothermal reforming) and the mechanisms of the chemical reactions. Experimental work has been done with a compact steam reformer, and a prototype of an experimental reactor for autothermal reforming was tested, both containing a Pt-catalyst on metallic substrate. Experimental results on the steam reforming system and a comparison of the steam reforming process with the autothermal process are given.     

A natural-gas fuel processor for a residential fuel cell system

A system model was used to develop an autothermal reforming fuel processor to meet the targets of 80% efficiency (higher heating value) and start-up energy consumption of less than 500 kJ when operated as part of a 1-kWe natural-gas fueled fuel cell system for cogeneration of heat and power. The key catalytic reactors of the fuel processor – namely the autothermal reformer, a two-stage water gas shift reactor and a preferential oxidation reactor – were configured and tested in a breadboard apparatus. Experimental results demonstrated a reformate containing ∼48% hydrogen (on a dry basis and with pure methane as fuel) and less than 5 ppm CO. The effects of steam-to-carbon and part load operations were explored.     

An onboard hydrogen generator for hydrogen enhanced combustion with internal combustion engine

Hydrogen enhanced combustion (HEC) for internal combustion engine is known to be a simple mean for improving engine efficiency in fuel saving and cleaner exhaust. An onboard compact and high efficient methanol steam reformer is made and installed in the tailpipe of a vehicle to produce hydrogen continuously onboard by using the waste heat of the engine for heating up the reformer; this provides a practical device for the HEC to become a reality. This use of waste heat from engine enables an extremely high process efficiency of 113% to convert methanol (8.68 MJ) for 1.0 NM of hydrogen (9.83 MJ) and low cost of using hydrogen as an enhancer or as a fuel itself. The test results of HEC from the onboard hydrogen production are presented with 2 gasoline engine vehicles and 2 diesel engines; the results indicate a hike of engine efficiency in 15–25% fuel saving and a 40–50% pollutants reduction including 70% reduction of exhaust smoke. The use of hydrogen as an enhancer brings about 2–3 fold of net reductions in energy, carbon dioxide emission and fuel cost expense over the input of methanol feed for hydrogen production.     

A reforming system for co-generation of hydrogen and mechanical work from methanol

The paper describes a reforming system for converting methanol into pure hydrogen. The system is based on an autothermal reforming reactor operating at elevated pressures followed by membrane-based hydrogen separation. The high-pressure membrane retentate stream is combusted and expanded through a turbine generating additional power. Process simulation illustrates the effects of the system operating parameters on performance and demonstrates system reforming efficiency up to ∼90%. When coupled with a PEM fuel cell and an electrical generator, overall fuel to electricity efficiency can be >48% depending upon the efficiency of a PEM fuel cell stack.     

A compact integrated fuel-processing system for proton exchange membrane fuel cells

A compact integrated fuel-processing system consisting of a plate-fin reformer (PFR) and a multi-stage preferential oxidation reactor is designed in this paper. The PFR, which was based on a plate-fin heat exchanger, is very compact, and reactant vaporization, methanol steam reforming and combustion are all integrated in it. Both internal plate-fins and external catalytic combustion were used to enhance heat transfer of the reformer, which offers both high methanol conversion ratio and low CO concentration, so that the water–gas shift reactor, which provides primary CO cleanup, is not necessary in this fuel-processing system. This will result in simplification of the fuel-processing system design and capital cost reduction. The performance of the main components in the fuel-processing system has been investigated. The axial temperatures of the different chambers in PFR were uniform, and the temperatures at the inlet and outlet of the PROX reactors were controlled strictly by plate-fin exchangers so that it could minimize parasitic hydrogen oxidation. In addition, the results indicated that this fuel-processing system can provide a high concentration of hydrogen and the system efficiency is always maintained above 75%. It is further demonstrated that the fuel-processing system could be operated autothermally and exhibited good test stability.     

Exergy analysis and CO2 emission evaluation for steam methane reforming

This paper investigates the industrial production of hydrogen through steam methane reforming (SMR) from both exergy efficiency and CO₂ emission aspects. An SMR model is constructed based on a practical flow diagram including desulfurizer, furnace, separation unit and heat exchangers. The influence of reformer temperature (Tr) and steam to carbon (S/C) ratio is analyzed to optimize exergy efficiency and CO₂ emission. A clear correlation is obtained between exergy efficiency and CO₂ emission. Results also show optimal S/C ratio decreases with Tr. An exergy load distribution analysis which evaluates interactions between the system and its subsystems with parameter variations is employed to find promising directions for efficiency improvement. Results show that the greatest improvement lies in increasing efficiency of furnace without increasing its relative exergy load. Integration of oxygen-enriched combustion (OEC) with SMR is also evaluated. The integration of OEC can increase the system efficiency greatly when the reformer operates above critical point, while in other cases the system efficiency may decrease.