Effects of particle sizes on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system

The discrete element geometric model of the horizontally buried-pipe steam generator was set up. The effects of particle size (20 mm–80 mm) on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system was studied. When the particle size increases, the particle layer flatness decreases, the particle layer flow ununiformity increases. The volatility of the particle residence time distribution increases with the particle size increases, and the standard deviation of the particle residence time increases. When the particle size increases, the voidage of the particle system increases. So the particle thermal resistance in the steam generator increases with the particle size increases, the steam production of the generator decreases, and the system hydrogen production of decreases.     

Effect of the vacancy close to heat exchange wall on the heat transfer performance of the solid particles steam generator using waste heat in a methanol steam reforming hydrogen production system

With the massive consumption of fossil fuels and it resulted in significant carbon emissions, it is urgent to find an alternative clean energy source. Hydrogen has been regarded as one of the most promising energy candidates for the next generation. It is a great approach that methane steam reforming for hydrogen production by rational utilization of industrial waste heat, which significantly minimizes carbon emissions and develops methanol steam reforming technology. A solid particle steam generator based on the primary heat exchange method has been proposed, which can provide the heat and steam in the methanol steam reforming hydrogen production system. The quasi-two-dimensional packing heat transfer model of solid particles steam generator was set up.The effect of distance change between the vacancy and the cold wall and distance change between vacancies on heat transfer performance of the steam generator and hydrogen production capacity were studied. As the distance between the vacancy and the wall increases, the heat transfer performance of the steam generator gradually deteriorates, so the steam production of the steam generator decreases, and the system's hydrogen production capacity is reduced, the maximum of the heat flux and the minimum of the apparent thermal resistance are 34.67 kW/m² and 12.02 K/W, respectively. As the distance between vacancies increases, the heat transfer performance of the steam generator is gradually optimized slightly. To maintain the hydrogen production capacity, vacancies should be avoided to appear 2 layers of particles away from the heat exchange wall in the particles steam generator. From the results of the study, the farther the distance between vacancies, the better the steam production and hydrogen production capacity.     

Effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat

This paper reports the effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat. The unsteady model was set up, which has been applied to investigate the effects of particle sizes (1.77 mm–14.60 mm) on particle temperature, heat transfer quantity, overall coefficient of heat-transfer, etc. The heat transfer performance of waste heat recovery heat exchanger is improved when the particle size increases, which is conducive to increase hydrogen production. The particle temperature change rate, the specific enthalpy change rate, the moving velocity of the maximum heat release rate particle, the contribution rate of solid phases, the heat release rate and the overall coefficient of heat-transfer increase, but the effective time of heat transfer decreases. When the particle size increases from 1.77 mm to 14.60 mm, the solid phase average contribution rate increases from 89.43% to 94.03%, the overall coefficient of heat-transfer increases from 1.39 W m⁻² K⁻¹ to 13.41 W m⁻² K⁻¹, the heat release rate increases from 48.9% to 99.9% and the effective time of heat transfer reduces from 48 h to 6.7 h.     

System level heat integration and efficiency analysis of hydrogen production process based on solid oxide electrolysis cells

The solid oxide electrolysis cells (SOEC) technology is a promising solution for hydrogen production with the highest electrolysis efficiency. Compared with its counterparts, operating at high temperature means that SOEC requires both power and heat. To investigate the possibility of coupling external waste heat with the SOEC system, and the temperature & quantity requirement for the external waste heat, a universal SOEC system operating at atmospheric pressure is proposed, modeled and analyzed, without specific waste heat source assumption such as solar, geothermal or industrial waste heat. The SOEC system flow sheet is designed to create opportunity for external waste heat coupling. The results show that external waste heat is required for feed stock heating, while the recommended coupling location is the water evaporator. The temperature of the external waste heat should be above 130 °C. For an SOEC system with 1 MW electrolysis power input, the required external waste heat is about 200 kW. When the stack operates at thermoneutral state and 800 °C, the specific energy consumption is 3.77 kWh/Nm³-H₂, of which electric power accounts for 84% (3.16 kWh/Nm³-H₂) and external waste heat accounts for 16% (0.61 kWh/Nm³-H₂). The total specific energy consumption remains almost unchanged when operating the SOEC stack around the thermoneutral condition.     

Effects of fin structure size on methane-steam reforming for hydrogen production in a reactor heated by waste heat

This paper mainly describes the influence of changes in fin structure on the hydrogen production capacity of the methane-steam reforming system. The model of the triangular-fin-tube steam generator was set up. The effects of the fin height (34 mm–46 mm), fin root width (3 mm–6 mm) and the fin-type were studied. As the height of the fin increases (34 mm–46 mm), the CPC temperature at the outlet of the steam generator decreases (the maximum temperature decreases 23.6 K and the average temperature decreases 18.9 K). At the same time, the heat recovery efficiency increased from 96.3% to 98.4%, and so the system hydrogen production increases. As the fin root width increases (3 mm–6 mm), the CPC temperature at the outlet of the steam generator decreases (the maximum temperature decreases 3.7 K and the average temperature decreases 1.2 K). Meanwhile, the heat recovery efficiency increases from 97.5% to 98.1%, and so the system hydrogen production increases. When the fin type is changed from a straight fin to a triangular-fin, the average temperature of the solid particle decreases 30.5 K, the heat recovery efficiency increases by 7.9%, and the system hydrogen production increases.     

Effects of ellipsoidal and regular hexahedral particles on the performance of the waste heat recovery equipment in a methanol reforming hydrogen production system

Hydrogen production from methanol has attracted attention due to its wide range of raw material sources and mature technology. Using waste heat of industrial high temperature solid particles like blast slag and steel slag etc. To provide vaporization heat and reaction heat for the reaction between methanol and water is an emerging technology for hydrogen production from methanol, which can save additional thermal energy resources. Herein, the performances of equipment that uses the waste heat of ellipsoidal and regular hexahedral particles to provide a heat source for methanol to hydrogen were explored by the DEM-CFD method. Compared with spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles have poor fluidity in the stagnant area, and the empty area is enlarged and irregular in shape. The average velocity peaks of the ellipsoidal and regular hexahedron particles are larger than those of spherical particles, and the overall mean velocity fluctuation of ellipsoidal particles is similar to that of spherical particles while the regular hexahedron particles' is larger. The average temperature drop rate of the ellipsoidal and regular hexahedral particles is slower than that of spherical particles, the uniformity of temperature distribution is worse than that of spherical particles. The ellipsoidal and regular hexahedral particles’ average effective heat transfer coefficient is smaller than that of spherical particles, and the heat transfer effect is weaker than that of spherical particles. The effective heat transfer coefficient of ellipsoidal particles is 2.95 W/(m⁻²∙K⁻¹) lower than that of spherical particles and the effective heat transfer coefficient of hexahedral particles is 6.09 W/(m⁻²∙K⁻¹) lower than that of spherical particles. Therefore, compared with the spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles produce less hydrogen.     

Techno-economic feasibility assessment of sorption enhanced gasification of municipal solid waste for hydrogen production

Waste-to-fuel coupled with carbon capture and storage is forecasted to be an effective way to mitigate the greenhouse gas emissions, reduce the waste sent to landfill and, simultaneously, reduce the dependence of fossil fuels. This study evaluated the techno-economic feasibility of sorption enhanced gasification, which involves in-situ CO₂ capture, and benchmarked it with the conventional steam gasification of municipal solid waste for H₂ production. The impact of a gate fee and tax levied on the fossil CO₂ emissions in economic feasibility was assessed. The results showed that the hydrogen production was enhanced in sorption enhanced gasification, that achieved an optimum H₂ production efficiency of 48.7% (T = 650 °C and SBR = 1.8). This was 1.0% points higher than that of the conventional steam gasification (T = 900 °C and SBR = 1.2). However, the total efficiency, which accounts for H₂ production and net power output, for sorption enhanced gasification was estimated to be 49.3% (T = 650 °C and SBR = 1.8). This was 4.4% points lower than the figure estimated for the conventional gasification (T = 900 °C and SBR = 1.2). The economic performance assessment showed that the sorption enhanced gasification will result in a significantly higher levelised cost of hydrogen (5.0 €/kg) compared to that estimated for conventional steam gasification (2.7 €/kg). The levelised cost of hydrogen can be reduced to 4.5 €/kg on an introduction of the gate fee of 40.0 €/t_MSW. The cost of CO₂ avoided was estimated to be 114.9 €/tCO₂ (no gate fee and tax levied). However, this value can be reduced to 90.1 €/tCO₂ with the introduction of an emission allowance price of 39.6 €/tCO₂. Despite better environmental performance, the capital cost of sorption enhanced gasification needs to be reduced for this technology to become competitive with mature gasification technologies.     

Energy and exergy analysis of hydrogen production by solid oxide steam electrolyzer plant

Energy and exergy analysis has been conducted to investigate the thermodynamic–electrochemical characteristics of hydrogen production by a solid oxide steam electrolyzer (SOSE) plant. All overpotentials involved in the SOSE cell have been included in the thermodynamic model. The waste heat in the gas stream of the SOSE outlet is recovered to preheat the H₂O stream by a heat exchanger. The heat production by the SOSE cell due to irreversible losses has been investigated and compared with the SOSE cell's thermal energy demand. It is found that the SOSE cell normally operates in an endothermic mode at a high temperature while it is more likely to operate in an exothermic mode at a low temperature as the heat production due to overpotentials exceeds the thermal energy demand. A diagram of energy and exergy flows in the SOSE plant helps to identify the sources and quantify the energy and exergy losses. The exergy analysis reveals that the SOSE cell is the major source of exergy destruction. The energy analysis shows that the energy loss is mainly caused by inefficiency of the heat exchangers. The effects of some important operating parameters, such as temperature, current density, and H₂O flow rate, on the plant efficiency have been studied. Optimization of these parameters can achieve maximum energy and exergy efficiencies. The findings show that the difference between energy efficiency and exergy efficiency is small as the high-temperature thermal energy input is only a small fraction of the total energy input. In addition, the high-temperature waste heat is of high quality and can be recovered. In contrast, for a low-temperature electrolysis plant, the difference between the energy and exergy efficiencies is more apparent because considerable amount of low-temperature waste heat contains little exergy and cannot be recovered effectively. This study provides a better understanding of the energy and exergy flows in SOSE hydrogen production and demonstrates the importance of exergy analysis for identifying and quantifying the exergy destruction. The findings of the present study can further be applied to perform process optimization to maximize the cost-effectiveness of SOSE hydrogen production.     

Advances in hydrogen production by thermochemical water decomposition: A review

Hydrogen demand as an energy currency is anticipated to rise significantly in the future, with the emergence of a hydrogen economy. Hydrogen production is a key component of a hydrogen economy. Several production processes are commercially available, while others are under development including thermochemical water decomposition, which has numerous advantages over other hydrogen production processes. Recent advances in hydrogen production by thermochemical water decomposition are reviewed here. Hydrogen production from non-fossil energy sources such as nuclear and solar is emphasized, as are efforts to lower the temperatures required in thermochemical cycles so as to expand the range of potential heat supplies. Limiting efficiencies are explained and the need to apply exergy analysis is illustrated. The copper–chlorine thermochemical cycle is considered as a case study. It is concluded that developments of improved processes for hydrogen production via thermochemical water decomposition are likely to continue, thermochemical hydrogen production using such non-fossil energy will likely become commercial, and improved efficiencies are expected to be obtained with advanced methodologies like exergy analysis. Although numerous advances have been made on sulphur–iodine cycles, the copper–chlorine cycle has significant potential due to its requirement for process heat at lower temperatures than most other thermochemical processes.     

Effect of contact number on heat extraction of particle material for hydrogen production

Hydrogen production by steam thermal reforming with waste heat of industrial particle material is of good prospects. But the insufficient research on the heat extraction traits of particulate matter has set up obstacles to its application. This paper focused on the effect of contact number, as it has not yet been fully studied. A series of steady-state numerical models based on a face-centered cubic packing was established to simulate the effect of contact number by removing selected particles. The results show that the contact number has a marked influence on the thermal resistance. As the contact number decreases by 12, the thermal resistance increases by 13.9∼31.9%. The removal of particles causes redistribution of heat transfer in different heat transfer modes. The heat transfer from the solid layer to the next layer decreases by 57.8% at most, and the radiation heat transfer between them is strengthened (36% at most). The effect of heat redistribution due to the removal of particles is significantly weakened after the heat flows through the first particle layer without removal operation. With these results, a better interpret of particulate matter heat extraction may be established.     

Thermodynamic performance assessment of boron based thermochemical water splitting cycle for renewable hydrogen production

The present study is related with the thermodynamic performance assessment of renewable hydrogen production through Boron thermochemical water splitting cycle. Therefore, all step efficiencies and overall cycle efficiency are calculated based on complete reaction. Additionally, a parametric study is conducted to determine the effect of the reference environment temperature on the overall cycle efficiency. In this regard, exergy efficiencies, exergy destruction rates and also inlet and outlet exergy rates of the cycle are calculated and presented for various reference temperatures. The exergy efficiency of the cycle is calculated as 0.4393 based on complete reaction and occurs at 298 K. This study has shown that Boron thermochemical water splitting cycle has a great potential due to cycle performance. As a result, Boron based thermochemical water splitting cycle can help achieve better environment and sustainability due to high exergetic efficiency. By the way, economic and technical issues of the storage and transportation of the hydrogen can find a proper solution if the hydrogen production reaction of the Boron thermochemical water splitting cycle takes place on-board of a vehicle.     

Photocatalytic membrane reactors for hydrogen production from water

Hydrogen is considered today a promising environmental friendly energy carrier for the next future, since it produces no air pollutants or greenhouse gases when it burns in air, and it possesses high energy capacity. In the last decades great attention has been devoted to hydrogen production from water splitting by photocatalysis. This technology appears very attractive thanks to the possibility to work under mild conditions producing no harmful by-products with the possibility to use renewable solar energy. Besides, it can be combined with the technology of membrane separations making the so-called photocatalytic membrane reactors (PMRs) where the chemical reaction, the recovery of the photocatalyst and the separation of products and/or intermediates simultaneously occur. In this work the basic principles of photocatalytic hydrogen generation from water splitting are reported, giving particular attention on the use of modified photocatalysts able to work under visible light irradiation. Several devices to achieve the photocatalytic hydrogen generation are presented focusing on the possibility to obtain pure hydrogen employing membrane systems and visible light irradiation. Although many efforts are still necessary to improve the performance of the process, membrane photoreactors seem to be promising for hydrogen production by overall water splitting in a cost-effective and environmentally sustainable way.     

Valorization of the waste heat given off in a system alkaline electrolyzer-photovoltaic array to improve hydrogen production performance: Case study Antofagasta,Chile

The efficiency of an electrolyzer can be improved by preheating the water consumed, which is generally done by means of solar energy in PVT panels. In this research, the first objective is to determine whether it is possible to preheat the consumed water by using the residual heat given off by the electrolyzer itself fed by a PV array, and if the above is met, the second objective consists of quantify the benefits obtained in the performance of the system. The simulation is carried out over a period of one year, considering the meteorological conditions of the city of Antofagasta, Chile. The results indicate that it is possible to constantly maintain the water temperature consumed by the electrolyzer at its nominal value of 80 °C, since the energy contained in the waste heat is about 30 times higher than this hot water demand. Continuous operation at 80 °C compared to operation at variable temperature achieves an annual increase of 0.22% in hydrogen production and an average of 0.33% in electrolyzer efficiency. Moreover, by considering the thermal energy given off by the electrolyzer as useful output of the system, the overall energy efficiency increases by a relative percentage of 13%.     

Co-gasification of catechol and starch in supercritical water for hydrogen production

Hydrogen production from waste feedstocks using supercritical water gasification (SCWG) is a promising approach towards cleaner fuel production and a solution for hard to treat wastes. In this study, the catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated in a batch reactor at 28 MPa, 400–500 °C, from 10 to 30 min. The effects of reaction conditions, and the addition of calcium oxide (CaO) as a carbon dioxide (CO₂) sorbent and TiO₂ as catalyst on the gas yields and product distribution were investigated. Employing TiO₂ as a catalyst alone had no significant effect on the H₂ yield but when combined with CaO increased the hydrogen yield by 35% and promoted higher total organic carbon (TOC) reduction efficiencies. The process liquid effluent was characterized using GC–MS, with the results showing that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme is provided.     

Exergy analysis of hydrogen production by thermochemical water decomposition using the Ispra Mark-10 Cycle

An investigation is reported of the thermodynamic performance of the Ispra Mark-10 thermochemical water decomposition process for hydrogen production. Thermochemical water decomposition has been identified as a potentially important future process for the production of hydrogen, which is currently an important industrial commodity and has significant future potential as a fuel. Exergy analysis is used since energy analysis on its own does not pinpoint true process inefficiencies, and often does not provide rational efficiencies. The analysis indicates that the principle thermodynamic losses occur in the primary water decomposition reactors and are mainly due to internal irreversibilities associated with chemical reaction and heat transfer across large temperature differences, and that the losses associated with effluents (particularly cooling water) are not that significant. Energy and exergy efficiencies are provided and are observed to depend strongly on the main external process inputs, i.e., electricity and process heat, or heat, or the raw resource from which heat and electricity are produced.     

Assessment of hydrogen production from waste heat using hybrid systems of Rankine cycle with proton exchange membrane/solid oxide electrolyzer

A techno-economic assessment of hydrogen production from waste heat using a proton exchange membrane (PEM) electrolyzer and solid oxide electrolyzer cell (SOEC) integrated separately with the Rankine cycle via two different hybrid systems is investigated. The two systems run via three available cement waste heats of temperatures 360 °C, 432 °C, and 780 °C with the same energy input. The waste heat is used to run the Rankine cycle for the power production required for the PEM electrolyzer system, while in the case of SOEC, a portion of waste heat energy is used to supply the electrolyzer with the necessary steam. Firstly, the best parameters; Rankine working fluid for the two systems and inlet water flow rate and bleeding ratio for the SOEC system are selected. Then, the performance of the two systems (Rankine efficiency, total system efficiency, hydrogen production rate, and economic and CO₂ reduction) is investigated and compared. The results reveal that the two systems' performance is higher in the case of steam Rankine than organic, while a bleeding ratio of 1% is the best condition for the SOEC system. Rankine output power, total system efficiency, and hydrogen production rate rose with increasing waste heat temperature having the same energy. SOEC system produces higher hydrogen production and efficiency than the PEM system for all input waste heat conditions. SOEC can produce 36.9 kg/h of hydrogen with a total system efficiency of 23.8% at 780 °C compared with 27.4 kg/h and 14.45%, respectively, for the PEM system. The minimum hydrogen production cost of SOEC and PEM systems is 0.88 $/kg and 1.55 $/kg, respectively. The introduced systems reduce CO₂ emissions annually by about 3077 tons.     

Modeling of fractal heat conduction of semi-coke bed in waste heat recovery steam generator for hydrogen production

With the steam obtained from the waste heat of high temperature semi-coke, the hydrogen production through gasification method is considered more commercial. In order to improve the efficiency of waste heat recovery, the fractional model for heat conduction of semi-coke bed in waste heat recovery process was established. The non-destructive CT was employed to obtain the inner morphology of semi-coke bed and the image binarization processing was used to segment the CT image. With the MATLAB program, the box-counting method was used to calculate the fractal dimension of semi-coke bed. The fractional model for heat conduction of semi-coke bed was established by the fractal theory. The results showed that, the CT image and bit binary image of semi-coke bed can really reflect the inner morphology of semi-coke bed, and the inner morphology of semi-coke bed can be regarded as a fractal medium. The fractal dimension of semi-coke bed is 1.7537, which is very close to golden mean, 1.618, this could be the optimal structure for the heat conduction of semi-coke bed under the condition of natural accumulation. The one-dimensional heat conduction fractional equation of semi-coke bed was established and it can be accurately solved by fractal complex transformation and traveling wave transformation.     

Design of systems for hydrogen production based on the Cu-Cl thermochemical water decomposition cycle: Configurations and performance

In this study, we analyze several Cu-Cl cycles by examining various design schemes for an overall system and its components, in order to identify potential performance improvements. The factors that determine the number and effective grouping of steps for new design schemes are analyzed. A thermodynamic analysis and several parametric studies are presented for various configurations. The energy efficiency is found to be 44% for the five-step thermochemical process, 43% for the four-step process and 41% for the three-step process, based on the lower heating value of hydrogen. Also, conclusions regarding implementation of these new configurations are discussed and the potential benefits ascertained.     

Effect of molar ratio of water / ethanol on hydrogen selectivity in catalytic production of hydrogen using steam reforming of ethanol

As fossil energy resources are shrinking, the increase in global energy needs and environmental pollution paved the way to the search for new and renewable energy resources. Therefore, the future of energy technology is being built on the use of hydrogen, which is one of the cleanest and most efficient renewable energy sources, and steam reforming is becoming the utmost method to produce hydrogen. This study focuses on the operation condition of steam reforming of ethanol on catalyst materials, which were shaped using active metals such as Ni, Cu and Cs and supporting materials which were activated by carbon and LiAlO₂. These catalyst materials were tested to produce hydrogen gas using different water/ethanol mole ratio at different temperatures and a constant feed flow rate. The evaluation regarding hydrogen selectivity results and the percentage of hydrogen in the products revealed that NiCuCs/LiAlO₂ catalyst showed the highest performance at all water/ethanol ratios and temperatures between 300 and 600 °C.     

Effect of aspect ratio on the performance characteristics of free piston linear generator engine fueled by hydrogen

Free-piston linear generator (FPLG) engines currently gained great attention due to their capability to operate with variable fuel and compression ratio. This paper presents an experimental study on the effect of aspect ratio on the performance characteristics of the FPLG engine fueled by hydrogen. Three aspect ratios (i.e. 1.0, 1.5, and 2.0) are used to identify the engine combustion and performance parameters. The injection position is fixed in the middle of the stroke, while the equivalence ratio is kept at 1.0. The results indicate that the aspect ratio 2.0 produces the highest pressure, heat release, and shortest combustion duration. Whereas the aspect ratio 1.0 produces higher combustion efficiency and operating frequency. The piston speed decreases with the decrease in aspect ratio, which gives a negative effect on the indicated mean effective pressure and power output of the PFLG. Overall, the aspect ratio has a significant influence on engine performance characteristics.