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.     

Numerical study of heat transfer characteristics of semi-coke and steam 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 commercially. The heat transfer of semi-coke bed and steam was investigated using an unsteady convection heat transfer three-dimensional model of semi-coke. The effects of particle size, steam flow and particle bed thickness on heat transfer characteristics were considered. The particle temperature calculated by three-dimensional model was in good agreement with the corresponding particle temperature of experiment. The heat transfer characteristics of single particle, the particle temperature, the amount of heat recovery and the heat flux were investigated. The results show that, in the first 10 min of the heat transfer of semi-coke bed and steam, the bottom particle temperature decreases rapidly, but the top particle temperature is almost unchanged. The heat transfer rate evolution of the single particle in different positions is revealed. The heat transfer rate evolution of the bottom particle is different from that of the middle particle and top particle, and the heat transfer rate evolution of middle particle is similar to that of the top particle. The particle size, the steam flow and the particle bed thickness have great influence on the heat transfer mechanism of semi-coke and steam, and the 7.5 kg/h is considered to be the best steam flow for heat recovery. The intrinsic heat transfer mechanism between semi-coke bed and steam was revealed.     

Process integration in bioprocess indystry: waste heat recovery in yeast and ethyl alcohol plant

The process integration of the bioprocess plant for production of yeast and alcohol was studied. Preliminary energy audit of the plant identified the huge amount of thermal losses, caused by waste heat in exhausted process streams, and reviled the great potential for energy efficiency improvement by heat recovery system. Research roadmap, based on process integration approach, is divided on six phases, and the primary tool used for the design of heat recovery network was Pinch Analysis. Performance of preliminary design are obtained by targeting procedure, for three process stream sets, and evaluated by the economic criteria. The results of process integration study are presented in the form of heat exchanger networks which fulfilled the utilization of waste heat and enable considerable savings of energy in short payback period.     

Simulation-based modeling and optimization for refinery hydrogen network integration with light hydrocarbon recovery

Purge gases from hydrocrackers and hydrotreaters and refinery off-gases are important hydrogen sources. Some of these hydrogen sources are also rich in light hydrocarbons that are valuable energy resources and chemical materials. In this work, a systematic method is proposed to integrate hydrogen networks considering light hydrocarbon recovery. This work first develops a hydrogen network superstructure with light hydrocarbon recovery. Aspen HYSYS is employed for rigorous process and thermodynamic modeling of the light hydrocarbon recovery process, and a simulation-optimization model is then developed. To solve the simulation-optimization model efficiently, the genetic algorithm is used as the global solver to determine the feed to light hydrocarbon recovery unit, and the linprog and fmincon solvers are combined to determine the optimal hydrogen network design. The application and effectiveness of the proposed method is validated through a case study. The results show that fresh hydrogen consumption decreases by 13% and the total annualized cost reduces to 72% because of light hydrocarbon recovery. This method could provide useful guides for the management of hydrogen and light hydrocarbons in refineries.     

Utilization of waste heat from cement plant to generate hydrogen and blend it with natural gas

In this paper, a waste heat recovery system for a cement plant is developed and analyzed with the softwares of Engineering Equation Solver (EES) and Aspen Plus. This system is novel in a way that hydrogen is uniquely produced from waste heat obtained from the cement slag and blended with natural gas for domestic use. The presented system has a steam Rankine cycle combined with an organic Rankine cycle, an alkaline electrolyzer unit, oxygen and hydrogen storage tanks, a blending unit, and a combustor. Moreover, multiple useful outputs are obtained, such as power, hydrogen, and natural gas, as well as hydrogen blend. The power obtained from the organic Rankine cycle becomes the highest when the organic fluid R600a is used as a working fluid. The power generated from turbines is fed to the grid externally and the cement plant for internal use. Also, some power is utilized to produce hydrogen via an alkaline electrolyzer which has an efficiency of 62.94%. With the change of the percentage of hydrogen in the blend from 0% to 50%, the annual consumption of natural gas reduces from 48.261 billion m³ to 37.086 billion m³. Furthermore, the overall exergy and energy efficiencies for the plant are found at 55% and 22%, respectively. The carbon dioxide emissions in the released exhaust gas reduce from 34% to 28% when the same volumetric flow rates of the blend and oxygen gas are fed to the reactor. NO and NO₂ emissions increase from 4.06 g/day to 7.45 g/day, and from 0.02 g/day to 0.09 g/day when the hydrogen content is increased from 5% to 20%. Moreover, carbon monoxide emissions decrease from 0.05 g/day to 0.02 g/day, accordingly. As a result, both combustion energy and exergy efficiencies increase with the addition of hydrogen. Furthermore, CO and CO₂ emissions decrease with the hydrogen content increases.     

Efficiency of hydrogen internal combustion engine combined with open steam Rankine cycle recovering water and waste heat

A hydrogen internal combustion engine (HICE) wastes more heat, and producing nearly three times more water than a conventional engine. This paper describes the principle behind a novel waste heat recovery sub-system that exploits the water produced by an HICE as the working fluid for an open-cycle power generation system based on the Rankine cycle. Water from the HICE exhaust is superheated by the waste heat from the HICE and used to produce power in a steam expander. A fundamental thermodynamic model shows the contribution of the sub-system to the overall thermal efficiency of the HICE at various engine speeds, with and without a condenser. The results show that the condenser is not cost-effective and that the overall thermal efficiency with the proposed sub-system is 27.2% to 33.6%, representing improvements of 2.9% to 3.7%, at engine speeds of 1500 to 4500 rpm.     

Exergetic life cycle assessment of new waste aluminium treatment system with co-production of pressurized hydrogen and aluminium hydroxide

A new system is proposed for the treatment of waste aluminium. The total exergy loss (EXL) in the system for the co-production of 1 kg of hydrogen at 30 MPa and 26 kg of aluminium hydroxide is evaluated from the viewpoint of life cycle assessment (LCA) by comparison with the EXLs in conventional systems. The exergy flow diagram reveals that the exergy of waste aluminium, which contains only 15 mass% metal, is still large, while that of pure aluminium hydroxide is relatively small. Therefore, the EXL in the proposed system (150.9 MJ) is 55% less than that in the conventional system (337.7 MJ) in which the gas compressor and production of aluminium hydroxide consume significantly more exergy. The results also indicate that exergy analysis should be applied to the LCA as a critical measure for practical use, in addition to the conventional LCA of carbon dioxide emission.     

Production of dimethyl ether and hydrogen by methanol reforming for an HCCI engine system with waste heat recovery – Continuous control of fuel ignitability and utilization of exhaust gas heat

Homogeneous charge compression ignition (HCCI) is a promising technique to achieve high thermal efficiency and clean exhaust with internal combustion engines. However, the difficulty in ensuring optimal ignition timing control prevents its practical application. Previous research has shown that adjusting the proportion of dimethyl ether (DME) and hydrogen-containing methanol-reformed gas (MRG) can control the ignition timing in an HCCI combustion engine fueled with the two fuels. As both DME and MRG can be produced in endothermic methanol reforming reactions, onboard reforming utilizing the exhaust gas heat can recover the waste heat from the engine. A very high overall thermal efficiency can be achieved by combining the high engine efficiency with HCCI and the waste heat recovery. This research investigates the basic characteristics of methanol reforming in a reactor tube with different catalysts with the aim to produce fuels for the HCCI combustion system.     

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.     

Study of heat and mass transfer in integrated thermal management controller (ITMC) employed in waste heat recovery application

In the waste heat recovery process, heat source temperature control and thermal management are always required to ensure safety and high efficiency of the waste heat recovery system. To this aim, the conventional method is to establish a series of independent heat transfer units and adopt a complex control strategy, which makes the system very complex and only applicable for a specific object. The concept of “integrated thermal management controller” (ITMC) is presented in this work to provide a novel method to solve the above problems. A two-dimensional heat and mass transfer model is developed to analyze and predict the operation performance of the ITMC. The numerical analysis indicates good heat source temperature control and thermal management performance of the ITMC. In addition, an experimental prototype is established, and test data are presented, which agree well with the numerical results and verify the correctness of the model.     

Optimization experimental design of stable production life and estimated ultimate recovery in gaseous compound of hydrogen and carbon from shale reservoir

In this study, a mixed numerical model is established to simulate the production performance of gaseous compound of hydrogen and carbon in shale reservoir. Estimated ultimate recovery (EUR) and stable production life (SPL) of gaseous compound of hydrogen and carbon are affected by reservoir parameters and production control parameters, such as Langmuir pressure, Langmuir volume, critical desorption pressure, fracture half length, permeability in stimulated reservoir volume (SRV) area, bottom hole flow pressure and production of gaseous compound of hydrogen and carbon. Therefore, in the later research, in order to reduce the number of numerical experiments, the orthogonal experimental method is used to optimize the influence of hydrocarbon ERU and SPL, and analyze the main control factors affecting these two indices at the same time. The purpose of this paper is to optimize the production of hydrocarbons by orthogonal experimental method and find the main controlling factors affecting the production of compound of hydrogen and carbon. Two groups of orthogonal experiments were designed to find the best scheme in the process of compound of hydrogen and carbon development. The calculative results show that the output of hydrogen and carbon compounds has a highly significant impact on SPL, Langmuir volume and capability in SRV have a significant impact, and Langmuir pressure Critical destruction pressure and hydraulic fracture half-length had no significant effects.     

Hydrogen permeation in a palladium membrane tube: Impacts of outlet and vacuum degree

Palladium (Pd) membranes are a crucial device for separating hydrogen and are usually operated at normal pressure on the permeate side with a single outlet. Instead of these common operating conditions, the difference between using a double outlet and a single outlet is studied. Four different vacuum degrees (15–60 kPa) are applied on the permeate side, and the results are compared with the non-vacuum operations. Situations under the vacuum and the effects of temperatures (300–400 °C) on H₂ permeation are discussed. Finally, the influences of different feed gas mixtures (H₂/N_2, H₂/CO_2, and H₂/CO) on the Pd membrane performance are investigated. The results show that there is no difference in H₂ permeation impact the single outlet and the double outlet on the permeate side. When a vacuum is imposed on the permeate side, the H₂ permeation rate and H₂ recovery are efficiently intensified, that is, when the pressure difference is 9 atm, they increase from 73.21 to 84.51% and from 0.0035378 to 0.0040808 mol?s^?1, respectively. Moreover, the H₂ recovery can be improved to up to 68.44% under a vacuum degree of 60 kPa. At a given Reynolds number, an increase in temperature increases the H₂ permeation rate but lowers its recovery, stemming from more H₂ in the feed gas. This study also investigates the feed gas of H₂/N₂ under a vacuum to provide a useful insight into H₂ production and separation from ammonia, and the results are compared with two different feed gases of H₂/CO₂ and H₂/CO mixtures. The results suggest that the impurities (i.e., N₂, CO₂, and CO) have a negative influence on the Pd membrane, which causes the H₂ permeation rate to decrease, and the effect of N₂ is the least significant compared to the other two.     

Investigation of green hydrogen production and development of waste heat recovery system in biogas power plant for sustainable energy applications

In this study, biogas power production and green hydrogen potential as an energy carrier are evaluated from biomass. Integrating an Organic Rankine Cycle (ORC) to benefit from the waste exhaust gases is considered. The power obtained from the ORC is used to produce hydrogen by water electrolysis, eliminate the H₂S generated during the biogas production process and store the excess electricity. Thermodynamic and thermoeconomic analyses and optimization of the designed Combined Heat and Power (CHP) system for this purpose have been performed. The proposed study contains originality about the sustainability and efficiency of renewable energy resources. System design and analysis are performed with Engineering Equation Solver (EES) and Aspen Plus software. According to the results of thermodynamic analysis, the energy and exergy efficiency of the existing power plant is 28.69% and 25.15%. The new integrated system's energy, exergy efficiencies, and power capacity are calculated as 41.55%, 36.42%, and 5792 kW. The total hydrogen production from the system is 0.12412 kg/s. According to the results of the thermoeconomic analysis, the unit cost of the electricity produced in the existing power plant is 0.04323 $/kWh. The cost of electricity and hydrogen produced in the new proposed system is determined as 0.03922 $/kWh and 0.181 $/kg H₂, respectively.