Integrated solar combined cycle system with steam methane reforming: Thermodynamic analysis

The present paper considers an integrated solar combined cycle system (ISCCS) with an utilization of solar energy for steam methane reforming. The overall efficiency was compared with the efficiency of an integrated solar combined cycle system with the utilization of solar energy for steam generation for a steam turbine cycle. Utilization of solar energy for steam methane reforming gives the increase in an overall efficiency up to 3.5%. If water that used for steam methane reforming will be condensed from the exhaust gases, the overall efficiency of ISCCS with steam methane reforming will increase up to 6.2% and 8.9% for β = 1.0 and β = 2.0, respectively, in comparison with ISCCS where solar energy is utilized for generation of steam in steam turbine cycle. The Sankey diagrams were compiled based on the energy balance. Utilization of solar energy for steam methane reforming increases the share of power of a gas turbine cycle: two-thirds are in a gas turbine cycle, and one-third is in a steam turbine cycle. In parallel, if solar energy is used for steam generation for a steam turbine cycle, than the shares of power from a gas and steam turbine are almost equal.     

Solar enriched methane production by steam reforming process: Reactor design

A novel hybrid plant for a mixture of methane and hydrogen (enriched methane) production from a steam reforming reactor whose heat duty is supplied by a molten salt stream heated up by a concentrating solar power (CSP) plant developed by ENEA is here presented. By this way, a hydrogen stream, mixed with natural gas, is produced from solar energy by a consolidated production method as the steam reforming process and by a pre-commercial technology as molten salts parabolic mirrors solar plant. After the hydrogen production plant, the residual heat stored in molten salt stream is used to produce electricity and the plant is co-generative (hydrogen + electricity).The heat-exchanger-shaped reactor is dimensioned by a design tool developed in MatLab environment. A reactor 3.5 m long and with a diameter of 2″ is the most efficient in terms of methane conversion (14.8%) and catalyst efficiency (4.7 Nm³/h of hydrogen produced per kg_cat).     

Thermodynamic performance comparison of SOFC-MGT-CCHP systems coupled with two different solar methane steam reforming processes

Multi-energy complementary distributed energy system integrated with renewable energy is at the forefront of energy sustainable development and is an important way to achieve energy conservation and emission reduction. A comparative analysis of solid oxide fuel cell (SOFC)-micro gas turbine (MGT)-combined cooling, heating and power (CCHP) systems coupled with two solar methane steam reforming processes is presented in terms of energy, exergy, environmental and economic performances in this paper. The first is to couple with the traditional solar methane steam reforming process. Then the produced hydrogen-rich syngas is directly sent into the SOFC anode to produce electricity. The second is to couple with the medium-temperature solar methane membrane separation and reforming process. The produced pure hydrogen enters the SOFC anode to generate electricity, and the remaining small amount of fuel gas enters the afterburner to increase the exhaust gas enthalpy. Both systems transfer the low-grade solar energy to high-grade hydrogen, and then orderly release energy in the systems. The research results show that the solar thermochemical efficiency, energy efficiency and exergy efficiency of the second system reach 52.20%, 77.97% and 57.29%, respectively, 19.05%, 7.51% and 3.63% higher than those of the first system, respectively. Exergy analysis results indicate that both the solar heat collection process and the SOFC electrochemical process have larger exergy destruction. The levelized cost of products of the first system is about 0.0735$/h that is lower than that of the second system. And these two new systems have less environmental impact, with specific CO₂ emissions of 236.98 g/kWh and 249.89 g/kWh, respectively.     

Dry reforming of methane has no future for hydrogen production: Comparison with steam reforming at high pressure in standard and membrane reactors

The methane dry-reforming and steam reforming reactions were studied as a function of pressure (1–20 atm) at 973 K in conventional packed-bed reactors and a membrane reactors. For the dry-reforming reaction in a conventional reactor the production yield of hydrogen rose and then decreased with increasing pressure as a result of the reverse water-gas shift reaction in which the hydrogen reacted with the reactant CO₂ to produce water. For the steam reforming reaction the production yield of hydrogen kept increasing with pressure because the forward water-gas shift reaction produced additional hydrogen by the reaction of CO with water. In the membrane reactors the methane conversion and the hydrogen production yields were higher for both the dry-reforming and steam reforming reactions, but for the dry reforming at high pressure half of the hydrogen was transformed into water. Thus, the dry-reforming reaction is not practical for producing hydrogen.     

Hydrogen production from methane and solar energy – Process evaluations and comparison studies

Three conventional and novel hydrogen and liquid fuel production schemes, i.e. steam methane reforming (SMR), solar SMR, and hybrid solar-redox processes are investigated in the current study. H₂ (and liquid fuel) productivity, energy conversion efficiency, and associated CO₂ emissions are evaluated based on a consistent set of process conditions and assumptions. The conventional SMR is estimated to be 68.7% efficient (HHV) with 90% CO₂ capture. Integration of solar energy with methane in solar SMR and hybrid solar-redox processes is estimated to result in up to 85% reduction in life-cycle CO₂ emission for hydrogen production as well as 99–122% methane to fuel conversion efficiency. Compared to the reforming-based schemes, the hybrid solar-redox process offers flexibility and 6.5–8% higher equivalent efficiency for liquid fuel and hydrogen co-production. While a number of operational parameters such as solar absorption efficiency, steam to methane ratio, operating pressure, and steam conversion can affect the process performances, solar energy integrated methane conversion processes have the potential to be efficient and environmentally friendly for hydrogen (and liquid fuel) production.     

Production and purification of hydrogen by biogas combined reforming and steam-iron process

Cobalt ferrite and hematite with minor additives have been tested for production and purification of high purity hydrogen from a synthetic biogas by steam-iron process (SIP) in a fixed bed reactor. A catalyst based in nickel aluminate has been included in the bed of solids to enhance the rate of the reaction of methane dry reforming (MDR). The reductants resulting from MDR are responsible for reducing the oxides based on iron that will, in the following stage, be oxidized by steam to release hydrogen with less than 50 ppm of CO. Coke minimization along reduction stages forces to operate such reactors above 700 °C for reductions, and as low as 500 °C for oxidations to avoid coke gasification. To avoid problems such as reactor clogging by coke in reductions and/or contamination of hydrogen by gasification of coke along oxidations, steam in small proportions has been included in the feed with the aim of minimizing or even avoiding formation of carbonaceous depositions along the reduction stage of SIP. Since steam is an oxidant, it exerts an inhibiting effect upon reduction of the oxide, that slows down the efficiency of the process. It has been proved that co-feeding low proportions of steam with an equimolar mixture of CH₄ and CO₂ (simulating a poor heating value desulphurized biogas) is able to avoid coke deposition, allowing the operation of both, reductions and oxidations, in isothermal regime (700 °C). Empirical results have been contrasted with data found in literature for similar processes based in MDR and combined (or mixed) reforming process (CMR), concluding that the combination of MDR + SIP proposed in this work, taking apart economic aspects and complex engineering, shows similar yields towards hydrogen, but with the advantage of not requiring a subsequent purification process.     

Stepwise production of CO-rich syngas and hydrogen via solar methane reforming by using a Ni(II)–ferrite redox system

The methane reforming process combined with metal–oxide reduction was examined on iron-based oxides of Ni(II)–, Zn(II)–, and Co(II)–ferrites, for the purpose of converting solar high-temperature heat to chemical fuels of CO-rich syngas and reduced metal oxide as storage and transport of solar energy. It was found that the Ni(II)-doping effectively improves the reactivity of magnetite as an oxidant for methane reforming. A two-step cyclic steam reforming of methane, which produces CO-rich syngas and hydrogen uncontaminated with carbon oxides alternately in the separate steps, was successfully demonstrated by using a ZrO₂-supported Ni(II)–ferrite (Ni_0.39Fe_2.61O₄/ZrO₂) as a working material in the temperature range of 1073–1173 K. The produced CO-rich syngas had the H₂/CO ratio that was more suitable for methanol production than that produced by a conventional single-step steam reforming. This syngas production using the Ni_0.39Fe_2.61O₄/ZrO₂ as an oxidant was also demonstrated under direct irradiation by a solar-simulated, high-flux visible light in laboratory-scale fixed bed system. The directly-irradiated Ni_0.39Fe_2.61O₄/ZrO₂ particles acted simultaneously as good radiant absorbers and reactive chemical reactants to yield more than 90% of methane conversion to a 2:1 molar mixture of CO and H₂ under flux irradiation of 500 kW m⁻² in the residence time less than 1 s.     

Thermochemical waste‐heat recuperation by steam methane reforming with flue gas addition

The process flow schematic of fuel‐consuming equipment with thermochemical waste‐heat recuperation by steam methane reforming with an addition of flue gas to the reaction mixture is suggested. The advantages of such a thermochemical recuperation (TCR) system compared with the TCR system by steam methane reforming are shown and justified. Based on the first law energy analysis, the heat inputs and outputs of the TCR system were determined. To determine the exhaust gases heat transformed into chemical energy of a new synthetic fuel, the thermodynamic analysis by minimizing Gibbs energy via Aspen HYSYS was performed. It was found that with an increase in the mole fraction of combustion products in the reaction mixture, the enthalpy of the methane reforming reaction increases, especially noticeable at the temperature range above 1000 K. Based on the heat, balance of the TCR system was established that the addition of combustion products to the reaction mixture has the following effects: reducing the heat input for steam production in a steam generator; reduction of the steam generator size because of the need to produce a smaller amount of steam in comparison with TCR by pure steam methane reforming; and reducing the amount of heat transferred through the wall of the reformer and, as a consequence, reduction in size of the reformer.     

Thermochemical methane reforming using WO3 as an oxidant below 1173 K by a solar furnace simulator

Thermochemical methane reforming by a reactive redox system of WO₃ was demonstrated under direct irradiation of the metal oxide by a concentrated, solar-simulated Xe-lamp beam below 1173 K, for the purpose of converting solar high-temperature heat to chemical fuels. In the proposed cycling redox process, the metal oxide is expected to react with methane as an oxidant to produce syngas with a H₂/CO ratio of two, which is suitable for the production of methanol, and the reduced metal oxide which is oxidized back with steam in a separate step to generate hydrogen uncontaminated with carbon oxide. The ZrO₂-supported WO₃ gave about 45% of CO yield and 55% of H₂ yield with a H₂/CO ratio of about 2.4 in a temperature range of 1080–1160 K at a W/F ratio of 0.167 g min Ncm⁻³ (W is the weight of WO₃ phase and F is the flow rate of CH₄). The activity data under the solar simulation were compared to those for the WO₃/ZrO₂ heated by irradiation of an infrared light. This comparison indicated that the CO selectivity was much improved to 76–85% in the solar-simulated methane reforming, probably by photochemical effect due to WO₃ phase. The main solid product of WO₂ in the reduced WO₃/ZrO₂ was reoxidized to WO₃ with steam to generate hydrogen below 1173 K.     

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.     

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 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.     

Properties of thermodynamic equilibrium-based methane autothermal reforming to generate hydrogen

The characteristics of methane autothermal reforming to generate hydrogen were studied with thermodynamic equilibrium constant method. Results show that the methane steam reforming reaction is prone to backward at low temperature, and there is an inflection point temperature that the reaction turns forward. When steam–methane molar ratio is 2, the inflection point temperature increases with raising air–methane molar ratio. When air–methane molar ratio is 1, the inflection point temperature maintains between 700 and 800 K. Hydrogen yield increases firstly and then decreases with elevated temperature. The increase of air–methane molar ratio leads to a lower hydrogen production when temperature exceeds 1000 K. Increasing steam–methane molar ratio promotes the hydrogen production. Methane autothermal reforming occurs much more easily when temperature keeps at 1000 K and the molar ratio of air–methane and steam–methane is 1 and 2 respectively. Changing the steam–methane molar ratio can regulate H₂/CO molar ratio.     

Comparative exergy analysis of sorption enhanced and conventional methane steam reforming

Exergy efficiency analysis tool is used to evaluate sorption enhanced steam reforming in comparison with the industrial hydrogen production route, steam reforming. The study focuses on hydrogen production for use in high pressure processes. Thermodynamic sensitivity analysis (effect of reforming temperature on hydrogen yield and reforming enthalpy) was performed to indicate the optimum temperature (650 °C) for the sorption enhanced reforming. The pressure was selected to be, for both cases, 25 bar, a typical pressure used in the industrial (conventional) process. Atmospheric pressure, 1000 °C and CO₂ as inert gas were specified as the optimum operating parameters for the regeneration of the sorbent after performing exergy efficiency analysis of three realistic case scenarios. Aspen Plus simulation process schemes were built for conventional and sorption enhanced steam reforming processes to attain the mass and energy balances required to assess comparatively exergy analysis. Simulation results showed that sorption enhanced reforming can lead to a hydrogen purity increase by 17.3%, along with the recovery of pure and sequestration-ready carbon dioxide. The exergy benefit of sorption enhanced reforming was calculated equal to 3.2%. Analysis was extended by adding a CO₂ separation stage in conventional reforming to reach the hydrogen purity of sorption enhanced reforming and enable a more effective exergy efficiency comparison. Following that analysis, sorption enhanced reforming gained 10.8% in exergy efficiency.     

Hydrogen-rich syngas production and carbon dioxide formation using aqueous urea solution in biogas steam reforming by thermodynamic analysis

Biogas is a renewable biofuel that contains a lot of CH₄ and CO₂. Biogas can be used to produce heat and electric power while reducing CH₄, one of greenhouse gas emissions. As a result, it has been getting increasing academic attention. There are some application ways of biogas; biogas can produce hydrogen to feed a fuel cell by reforming process. Urea is also a hydrogen carrier and could produce hydrogen by steam reforming. This study then employes steam reforming of biogas and compares hydrogen-rich syngas production and carbon dioxide with various methane concentrations using steam and aqueous urea solution (AUS) by Thermodynamic analysis. The results show that the utilization of AUS as a replacement for steam enriches the production of H₂ and CO and has a slight CO₂ rise compared with pure biogas steam reforming at a temperature higher than 800 °C. However, CO₂ formation is less than the initial CO₂ in biogas. At the reaction temperature of 700 °C, carbon formation does not occur in the reforming process for steam/biogas ratios higher than 2. These conditions led to the highest H₂, CO production, and reforming efficiency (about 125%). The results can be used as operation data for systems that combine biogas reforming and applied to solid oxide fuel cell (SOFC), which usually operates between 700 °C to 900 °C to generate electric power in the future.     

Exploring the opportunities for carbon capture in modular,small-scale steam methane reforming: An energetic perspective

Small-scale steam methane reforming units produce more than 12% of all the CO₂-equivalent emissions from hydrogen production and, unlike large-scale units, are usually not integrated with other processes. In this article, the authors examine the hitherto under-explored potential to utilise the excess heat available in the small-scale steam methane reforming process for partial carbon dioxide capture. Reforming temperature has been identified as a critical operating parameter to affect the amount of excess heat available in the steam methane reforming process. Calculations suggest that reforming the natural gas at 850 °C, rather than 750 °C, increases the amount of excess heat available by about 28.4% (at 180 °C) while, sacrificing about 1.62% and 1.09% in the thermal and exergetic efficiency of the process, respectively. Preliminary calculations suggest that this heat could potentially be utilised for partial carbon capture from reformer flue gas, via structured adsorbents, in a compact capture unit. The reforming temperature can be adjusted in order to regulate the amount of excess heat, and thus the carbon capture rate.     

Refuse derived fuel hydrogasification coupled with methane steam reforming

The hydrogasification of Refuse Derived Fuel (RDF) consisting of non-recyclable plastic polymers was combined with methane steam reforming in a “hydrogen self-sustained” loop configuration. The hydrogasification unit fed by 1000 kg/h of RDF was initially modeled by Aspen plus to define best operating conditions, namely temperature, pressure and hydrogen feed flow rate. After the simulations, the temperature of the hydrogasification process has been fixed at 300 °C, the pressure at 10 bar and the hydrogen feed flow rate at 140 kg/h. The steam reforming unit operates at 850 °C while the water-gas shift is conducted at 350 °C. When all the methane produced by hydrogasification is used to feed the steam reformer, which yields H₂ that is recycled back to the hydrogasifier, the net hydrogen production is 222 kg/h with an amount of CO₂ released of 2265 kg/h. For the different process configurations adopted, the energy efficiency of the process ranges 84–89%.     

Methane membrane steam reforming: Heat duty assessment

Membrane reactors are an innovative technology with huge application potentialities for equilibrium limited endothermic reactions. Assembling a membrane selective to a reaction product avoids the equilibrium conditions to be achieved, supporting the reactions at lower operating temperatures. Taking as an example the natural gas steam reforming, a methane conversion around 98% can be reached imposing an operating temperature of 823 K, much lower than that of the traditional process. In the present paper, a stringent analysis of heat power requirement needed to carry out the natural gas steam reforming process by applying a membrane reactor is made. The simulations allows to understand how the main operating parameters (inlet temperature, inlet methane flow-rate, steam to carbon ratio, ratio between sweeping steam and inlet methane, operating reaction pressure) influence the total heat power required by the process, divided among power contributions for the reaction heat duty, reactant steam and permeation steam generation and preheating. Moreover, the specific thermal energy per mole of pure H₂ is computed and assessed. Optimizing the operating conditions set, a specific thermal energy per mole of pure hydrogen of 92.3 kWh kmol⁻¹ is obtained corresponding to a total thermal power of 687.4 kW required to convert, in a single membrane reactor, a methane flow-rate of 2 kmol h⁻¹ (GHSV = 9.590 h⁻¹) with a conversion around 98%.     

Redox reforming based,integrated solar-natural gas plants: Reforming and thermodynamic cycle efficiency

As demand for energy continues to rise, the concern over the increase in emissions grows, prompting much interest in using renewable energy resources such as solar energy. However, there are numerous issues with using solar energy including intermittency and the need for storage. A potential solution is the concept of hybrid solar-fossil fuel power generation. Previous work has shown that utilizing solar reforming in conventional power cycles has higher performance compared to other integration methods. Most previous studies have focused on steam or dry reforming and on specific component analysis rather than a systems level analysis. In this article, a system analysis of a hybrid cycle utilizing redox reforming is presented. Important cycle design and operation parameters such as the oxidation temperature and reformer operating pressure are identified and their effect on both the reformer and cycle performance is discussed. Simulation results show that increasing oxidation temperature can improve reformer and cycle efficiency. Also shown is that increasing the amount of reforming water leads to a higher reformer efficiency, but can be detrimental to cycle efficiency depending on how the reforming water is utilized.