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.     

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.     

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.     

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.     

Liquid organic hydrogen carriers (LOHCs) in the thermochemical waste heat recuperation systems: The energy and mass balances

This paper is presented a concept of thermochemical recuperation of waste heat based on hydrogen extraction from liquid organic hydrogen carriers (LOHC), on the example of methylcyclohexane-toluene system. The advantages of this concept is described, for example, a possibility to use a moderate low temperature of waste heat for generation high-exergy “green” hydrogen fuel. To understand the effect of operating parameters on the energy and mass balance, the thermodynamic analysis was performed. The chemical system for hydrogen generation was analyzed via Gibbs free energy minimization method. The thermodynamic analysis was conducted under various operating conditions: temperature of 100–400 °C, pressure of 1–4 bar. Aspen HYSYS software was used for the energy and mass conservation analysis. Sankey diagram for the energy flows is depicted. The results showed that the maximum energy efficiency the thermochemical waste heat recuperation system have in the temperature range above 300–350 °C. In this temperature range, the effect of pressure on the energy balance is negligible and it is recommended for the thermochemical recuperation system to use LOHC with a pressure of 1.5–2 bar. Based on the analysis, it was concluded that the temperature potential of waste heat for about 300–350 °C is enough for the investigated concept. An analysis of a mass balance showed that the decreasing in condensation temperature leads to a significant increasing in the share of condensed toluene from toluene-hydrogen mixture after a reactor. If temperature of a hydrogen-toluene mixture of 20 °C at pressure above 2 bar about 96% of toluene can be condensed after the first condenser.     

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.     

Optimization of operating parameters for methane steam reforming thermochemical process using Response Surface Methodology

The methane steam reforming to produce hydrogen under concentrated irradiation was proposed and the novel thermodynamic analysis interaction effects on the conversion of methane via Design Expert software was utilized in this paper. The four parameters (material porosity, inlet gas temperature, steam-to-methane ratio, inlet gas velocity) and three levels (low, center and high) were designed via the Box-Behnken Design in Response Surface Methodology. The response model was established to optimize and analyze the condition parameters, which showed effects on the methane conversion under concentrated irradiation. The results showed that the value of material porosity, inlet gas temperature and S/C had a positive effect on the methane conversion rate, while the value of inlet gas velocity had a negative effect on it. The analysis of variance of the methane conversion in the Response Surface Methodology was 0.9914. When the material porosity, gas inlet temperature, S/C and gas inlet velocity were 0.770, 579.925 K, 2.996 and 0.031, respectively, methane conversion was 94.03%; Among the above four factors, material porosity had the most significant effect on the reaction of methane steam reforming to produce hydrogen under concentrated irradiation. These results provided theoretical guidance for application of methane steam reforming under concentrated irradiation.     

Combination of thermodynamic analysis and regression analysis for steam and dry methane reforming

Hydrogen is predominantly produced via methane reforming. In this study, thermodynamic analysis and regression analysis of the steam reforming of methane (SRM) as well as dry methane reforming (DRM) are conducted. The method of Gibbs free energy minimization is applied for investigating the effect of factors, such as temperature, pressure, and inlet composition, on the performance of hydrogen production. Notably, this study is not restricted to the effect of a single factor, but to the combined results of all independent variables. Then, regression analysis is adopted for examining the quantitative relationship between response observed and conditions. As a result, different mathematical expressions are attempted, such as linear regression, second-order polynomial, and logarithmic form, for finding the optimal form to preferably illustrate the manner in which factors affect performance. In this process, the forms are compared in several ways from the perspective of not only regression parameters but also error bars on graphs curve images. Finally, a three-pieces-logarithmic model is proposed as the final form to explain the relation between factors and response with maximum error 7% and the most deviations range between 0% and 2%.     

Performance study of a solid oxide fuel cell and gas turbine hybrid system designed for methane operating with non-designed fuels

This paper presents an analysis of the fuel flexibility of a methane-based solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system. The simulation models of the system are mathematically defined. Special attention is paid to the development of an SOFC thermodynamic model that allows for the calculation of radial temperature gradients. Based on the simulation model, the new design point of system for new fuels is defined first; the steady-state performance of the system fed by different fuels is then discussed. When the hybrid system operates with hydrogen, the net power output at the new design point will decrease to 70% of the methane, while the design net efficiency will decrease to 55%. Similar to hydrogen, the net output power of the ethanol-fueled system will decrease to 88% of the methane value due to the lower cooling effect of steam reforming. However, the net efficiency can remain at 61% at high level due to increased heat recuperation from exhaust gas. To increase the power output of the hybrid system operating with non-design fuels without changing the system configuration, three different measures are introduced and investigated in this paper. The introduced measures can increase the system net power output operating with hydrogen to 94% of the original value at the cost of a lower efficiency of 45%.     

Performance improvement of combined cycle power plant based on the optimization of the bottom cycle and heat recuperation

Many F class gas turbine combined cycle(GTCC)power plants are built in China at present because of less emis-sion and high efficiency.It is of great interest to investigate the efficiency improvement of GTCC plant.A com-bined cycle with three-pressure reheat heat recovery steam generator(HRSG)is selected for study in this paper.In order to maximize the GTCC efficiency,the optimization of the HRSG operating parameters is performed.Theoperating parameters are determined by means of a thermodynamic analysis,i.e.the minimization of exergylosses.The influence of HRSG inlet gas temperature on the steam bottoming cycle efficiency is discussed.Theresult shows that increasing the HRSG inlet temperature has less improvement to steam cycle efficiency when itis over 590℃.Partial gas to gas recuperation in the topping cycle is studied.Joining HRSG optimization with theuse of gas to gas heat recuperation,the combined plant efficiency can rise up to 59.05% at base load.In addition,the part load performance of the GTCC power plant gets much better.The efficiency is increased by 2.11% at75% load and by 4.17% at 50% load.     

Performance evaluation of a geothermal based integrated system for power,hydrogen and heat generation

In the present study, an integrated system is proposed and thermodynamically analyzed to reduce greenhouse gas (GHG) emissions while improving overall system performance. The integrated system is comprised of a supercritical carbon dioxide (CO₂) Rankine cycle cascaded by an Organic (R600) Rankine cycle, an electrolyzer, and a heat recovery system. It is designed to utilize a medium-to-high temperature geothermal energy source for power and hydrogen production, and thermal energy utilization for space heating. Therefore, parametric studies for the supercritical CO₂ cycle, the Organic (R600) cycle, and the overall system are conducted. In addition, the effect of various operational conditions, such as geothermal source, ambient and cooling water temperatures on the performance of each cycle and the integrated system, is illustrated. It is found that increasing geothermal source temperature results in slight increases of the exergetic efficiency of the overall system. The energy efficiencies of the CO₂ and Organic Rankine cycles do not considerably vary with source temperature changes. The decay of the cooling water temperature leads to a decrease in the overall system exergetic efficiency. The system configuration, which is introduced, is capable of producing about 180 kg/h for the geothermal source of mass flow rate of 40 kg/s and a temperature of 473 K.     

ANOVA analysis of an integrated membrane reactor for hydrogen production by methane steam reforming

Steam methane reforming is an endothermic reaction and it used to produce hydrogen and syngas. In this research, a factorial design is developed for an integrated Pd-based membrane reactor, producing hydrogen by methane steam reaction. In literature, no analogous works are present, because a simple sensitivity analysis is carried out without finding significant factors for the process. The reactor is modelled in MATLAB software using the Numaguchi kinetic. The reactor does not use conventional catalysts, but a Ni(10)/CeLaZr catalyst supported on SSiC ceramic foam. In ANOVA analysis, inlet temperature (550 K-815 K), methane flow rate in the feed (0.1 kmol/h-1 kmol/h), hydrogen permeability (1000 m³μmm⁻²hrbar^0.5–3600 m³μmm⁻²hrbar^0.5), the thickness of membrane (0.003 m-0.02 m) are the chosen factors. The analyzed responses are: hydrogen yield, carbon dioxide conversion and methane conversion. Results show that only inlet temperature, methane flow rate, their interaction and the thickens of membrane are significant. Also, the optimal operating conditions are obtained with inlet temperature, methane flow rate, hydrogen permeability and thickness of membrane equal to 550 K, 0.1 kmol/h, 3600 m³μmm⁻²hrbar^0.5 and 0.003 m.     

Thermodynamic analysis of an integrated power generation system driven by solid oxide fuel cell

A new integrated power generation system driven by the solid oxide fuel cell (SOFC) is proposed to improve the conversion efficiency of conventional energy by using a Kalina cycle to recover the waste heat of exhaust from the SOFC-GT. The system using methane as main fuel consists an internal reforming SOFC, an after-burner, a gas turbine, preheaters, compressors and a Kalina cycle. The proposed system is simulated based on the developed mathematical models, and the overall system performance has been evaluated by the first and second law of thermodynamics. Exergy analysis is conducted to indicate the thermodynamic losses in each components. A parametric analysis is also carried out to examine the effects of some key thermodynamic parameters on the system performance. Results indicate that as compressor pressure ratio increases, SOFC electrical efficiency increases and there is an optimal compressor pressure ratio to reach the maximum overall electrical efficiency and exergy efficiency. It is also found that SOFC electrical efficiency, overall electrical efficiency and exergy efficiency can be improved by increasing air flow rate. Also, the largest exergy destruction occurs in the SOFC followed by the after-burner, the waste heat boiler, the gas turbine. The compressor pressure ratio and air flow rate have significant effects on the exergy destruction in some main components of system.     

Exergy analysis of the woody biomass Stirling engine and PEM-FC combined system with exhaust heat reforming

The woody biomass Stirling engine (WB-SEG) is an external combustion engine that outputs high-temperature exhaust gases. It is necessary to improve the exergy efficiency of WB-SEG from the viewpoint of energy cascade utilization. So, a combined system that uses the exhaust heat of WB-SEG for the steam reforming of city gas and that supplies the produced reformed gas to a proton exchange membrane fuel cell (PEM-FC) is proposed. The energy flow and the exergy flow were analyzed for each WB-SEG, PEM-FC, and WB-SEG/PEM-FC combined system. Exhaust heat recovery to preheat fuel and combustion air was investigated in each system. As a result, (a) improvement of the heat exchange performance of the woody biomass combustion gas and engine is observed, (b) reduction in difference in the reaction temperature of each unit, and (c) removal of rapid temperature change of reformed gas are required in order to reduce exergy loss of the system. The exergy efficiency of the WB-SEG/PEM-FC combined system is superior to EM-FC.     

Performance of a PEMFC system integrated with a biogas chemical looping reforming processor: A theoretical analysis and comparison with other fuel processors (steam reforming,partial oxidation and auto-thermal reforming)

In this work, the performance of a PEMFC (proton exchange membrane fuel cell) system integrated with a biogas chemical looping reforming processor is analyzed. The global efficiency is investigated by means of a thermodynamic study and the application of a generalized steady-state electrochemical model. The theoretical analysis is carried out for the commercial fuel cell BCS 500W stack. From literature, chemical looping reforming (CLR) is described as an attractive process only if the system operates at high pressure. However, the present research shows that advantages of the CLR process can be obtained at atmospheric pressure if this technology is integrated with a PEMFC system. The performance of a complete fuel cell system employing a fuel processor based on CLR technology is compared with those achieved when conventional fuel processors (steam reforming (SR), partial oxidation (PO) and auto-thermal reforming (ATR)) are used. In the first part of this paper, the Gibbs energy minimization method is applied to the unit comprising the fuel- and air-reactors in CLR or to the reformer (SR, PO, ATR). The goal is to investigate the characteristics of these different types of reforming process to generate hydrogen from clean model biogas and identify the optimized operating conditions for each process. Then, in the second part of this research, material and energy balances are solved for the complete fuel cell system processing biogas, taking into account the optimized conditions found in the first part. The overall efficiency of the PEMFC stack integrated with the fuel processor is found to be dependent on the required power demand. At low loads, efficiency is around 45%, whereas, at higher power demands, efficiencies around 25% are calculated for all the fuel processors. Simulation results show that, to generate the same molar flow-rate of H₂ to operate the PEMFC stack at a given current, the global process involving SR reactor is by far much more energy demanding than the other technologies. In this case, biogas is burnt in a catalytic combustor to supply the energy required, and there is a concern with respect to CO₂ emissions. The use of fuel processors based on CLR, PO or ATR results in an auto-thermal global process. If CLR based fuel processor is employed, CO₂ can be easily recovered, since air is not mixed with the reformate. In addition, the highest values of voltage and power are achieved when the PEMFC stack is fed on the stream coming from SR and CLR fuel processors. When a H₂ mixture is produced by reforming biogas through PO and ATR technologies, the relative anode overpotential of a single cell is about 55 mV, whereas, with the use of CLR and SR processes, this value is reduced to ∼37 and 24 mV, respectively. In this way, CLR can be seen as an advantageous reforming technology, since it allows that the global process can be operated under auto-thermal conditions and, at the same time, it allows the PEMFC stack to achieve values of voltage and power closer to those obtained when SR fuel processors are used. Thus, efforts on the development of fuel processors based on CLR technology operating at atmospheric pressure can be considered by future researchers. In the case of biogas, the CO₂ captured can produce additional economical benefits in a ‘carbon market’.     

Thermodynamic equilibrium analysis of combined carbon dioxide reforming with steam reforming of methane to synthesis gas

Thermodynamics equilibrium analysis of carbon dioxide reforming of methane combined with steam reforming to synthesis gas was studied by Gibbs free energy minimization method to understand the effects of process variables such as temperature, pressure and inlet CH₄/H₂O/CO₂ ratios on product distributions. For this purpose, the calculations were carried out at total pressures of 1 and 20 bar, and at ranges of temperature and steam-to-carbon ratios of 200–1200 °C and 0–0.50, respectively. The results revealed that carbon dioxide reforming of methane combined with steam reforming process was controlled by different reactions with regard to the operating temperature, pressure and varying feed compositions. The H₂/CO product ratio could be modified by changing the relative concentration of steam and CO₂ in the feed, temperature and pressure, depending on the downstream application.     

High efficiency chemical energy conversion system based on a methane catalytic decomposition reaction and two fuel cells: Part I. Process modeling and validation

A highly efficient integrated energy conversion system is built based on a methane catalytic decomposition reactor (MCDR) together with a direct carbon fuel cell (DCFC) and an internal reforming solid oxide fuel cell (IRSOFC). In the MCDR, methane is decomposed to pure carbon and hydrogen. Carbon is used as the fuel of DCFC to generate power and produce pure carbon dioxide. The hydrogen and unconverted methane are used as the fuel in the IRSOFC. A gas turbine cycle is also used to produce more power output from the thermal energy generated in the IRSOFC. The output performance and efficiency of both the DCFC and IRSOFC are investigated and compared by development of exact models of them. It is found that this system has a unique loading flexibility due to the good high-loading property of DCFC and the good low loading property of IRSOFC. The effects of temperature, pressure, current densities, and methane conversion on the performance of the fuel cells and the system are discussed. The CO₂ emission reduction is effective, up to 80%, can be reduced with the proposed system.     

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.     

Study of a deaerator location in triple-pressure reheat combined power cycle

Deaerator is an essential open feed water heater in the steam bottoming cycle to improve the efficiency and also to remove the dissolved gasses from the feed water. Heat recovery steam generator (HRSG) plays a key role on the performance of the combined cycle (CC). In this work, attention has been focused to improve the performance of a triple pressure (TP) CC with a deaerator location. In this work, two options for deaerator location, one at condenser (deaerator–condenser) and the other in between low pressure (LP) and intermediate pressure (IP) heaters have been studied to increase the heat recovery from the gas turbine exhaust. The compressor pressure ratio is not fixed initially and evaluated from HRSG inlet condition. The LP and IP in HRSG have been evaluated from the local flue gas temperature to get the minimum possible temperature difference in the heaters. The results show that the deaerator placed in between the LP and IP heaters, gives high efficiency compared to a deaerator–condenser arrangement. The optimum conditions for the HRSG, deaerator and steam reheater are evaluated through the thermodynamic study. The results are validated by comparing with the published results.     

High efficiency chemical energy conversion system based on a methane catalytic decomposition reaction and two fuel cells. Part II. Exergy analysis

A methane catalytic decomposition reactor-direct carbon fuel cell-internal reforming solid oxide fuel cell (MCDR-DCFC-IRSOFC) energy system is highly efficient for converting the chemical energy of methane into electrical energy. A gas turbine cycle is also used to output more power from the thermal energy generated in the IRSOFC. In part I of this work, models of the fuel cells and the system are proposed and validated. In this part, exergy conservation analysis is carried out based on the developed electrochemical and thermodynamic models. The ratio of the exergy destruction of each unit is examined. The results show that the electrical exergy efficiency of 68.24% is achieved with the system. The possibility of further recovery of the waste heat is discussed and the combined power-heat exergy efficiency is over 80%.