An overview of solar decarbonization processes,reacting oxide materials,and thermochemical reactors for hydrogen and syngas production

Solar decarbonization processes are related to the different thermochemical conversion pathways of hydrocarbon feedstocks for solar fuels production using concentrated solar energy as the external source of high-temperature process heat. The main investigated routes aim to convert gaseous and solid feedstocks (methane, coal, biomass …) into hydrogen and syngas via solar cracking/pyrolysis, reforming/gasification, and two-step chemical looping processes using metal oxides as oxygen carriers, further associated with thermochemical H₂O/CO₂ splitting cycles. They can also be combined with metallurgical processes for production of energy-intensive metals via solar carbothermal reduction of metal oxides. Syngas can be further converted to liquid fuels while the produced metals can be used as energy storage media or commodities. Overall, such solar-driven processes allow for improvements of conversion yields, elimination of fossil fuel or partial feedstock combustion as heat source and associated CO₂ emissions, and storage of intermittent solar energy in storable and dispatchable chemical fuels, thereby outperforming the conventional processes. The different solar thermochemical pathways for hydrogen and syngas production from gaseous and solid carbonaceous feedstocks are presented, along with their possible combination with chemical looping or metallurgical processes. The considered routes encompass the cracking/pyrolysis (producing solid carbon and hydrogen) and the reforming/gasification (producing syngas). They are further extended to chemical looping processes involving redox materials as well as metallurgical processes when metal production is targeted. This review provides a broad overview of the solar decarbonization pathways based on solid or gaseous hydrocarbons for their conversion into clean hydrogen, syngas or metals. The involved metal oxides and oxygen carrier materials as well as the solar reactors developed to operate each decarbonization route are further described.     

Cost-benefit analysis of different hydrogen production technologies using AHP and Fuzzy AHP

In this paper, cost-benefit analysis is performed to compare eight different hydrogen production technologies using the classical analytic hierarchy process (AHP) and the Fuzzy AHP. The technologies considered are steam methane reforming, coal gasification, partial oxidation of hydrocarbons, biomass gasification, photovoltaic-based electrolysis, wind-based electrolysis, hydro-based electrolysis, and water splitting by chemical looping. For each of the hydrogen production technologies, five criteria are used for evaluation: greenhouse gas emissions, raw material and utilities consumption, energy efficiency, scalability, as well as waste disposal and atmospheric emissions. The results obtained for benefits category using AHP and Fuzzy AHP are plotted against the normalized equivalent annual costs of each technology. It is concluded that the fossil fuel based processes appear to have less beneficial qualities including greater environmental impacts, but are more cost-effective. On the other hand, the renewable based processes appear to have more benefits as well as being more expensive for hydrogen production. However, the cost-benefit analysis results imply that the process of water splitting by chemical looping among the renewable approaches is the most promising new technology.     

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.     

Life cycle assessment of various hydrogen production methods

A comprehensive life cycle assessment (LCA) is reported for five methods of hydrogen production, namely steam reforming of natural gas, coal gasification, water electrolysis via wind and solar electrolysis, and thermochemical water splitting with a Cu–Cl cycle. Carbon dioxide equivalent emissions and energy equivalents of each method are quantified and compared. A case study is presented for a hydrogen fueling station in Toronto, Canada, and nearby hydrogen resources close to the fueling station. In terms of carbon dioxide equivalent emissions, thermochemical water splitting with the Cu–Cl cycle is found to be advantageous over the other methods, followed by wind and solar electrolysis. In terms of hydrogen production capacities, natural gas steam reforming, coal gasification and thermochemical water splitting with the Cu–Cl cycle methods are found to be advantageous over the renewable energy methods.     

Potential importance of hydrogen as a future solution to environmental and transportation problems

Air pollution is a serious public health problem throughout the world, especially in industrialized and developing countries. In industrialized and developing countries, motor vehicle emissions are major contributors to urban air quality. Hydrogen is one of the clean fuel options for reducing motor vehicle emissions. Hydrogen is not an energy source. It is not a primary energy existing freely in nature. Hydrogen is a secondary form of energy that has to be manufactured like electricity. It is an energy carrier. Hydrogen has a strategic importance in the pursuit of a low-emission, environment-benign, cleaner and more sustainable energy system. Combustion product of hydrogen is clean, which consists of water and a little amount of nitrogen oxides. Hydrogen has very special properties as a transportation fuel, including a rapid burning speed, a high effective octane number, and no toxicity or ozone-forming potential. It has much wider limits of flammability in air than methane and gasoline. Hydrogen has become the dominant transport fuel, and is produced centrally from a mixture of clean coal and fossil fuels (with C-sequestration), nuclear power, and large-scale renewables. Large-scale hydrogen production is probable on the longer time scale. In the current and medium term the production options for hydrogen are first based on distributed hydrogen production from electrolysis of water and reforming of natural gas and coal. Each of centralized hydrogen production methods scenarios could produce 40 million tons per year of hydrogen. Hydrogen production using steam reforming of methane is the most economical method among the current commercial processes. In this method, natural gas feedstock costs generally contribute approximately 52–68% to the final hydrogen price for larger plants, and 40% for smaller plants, with remaining expenses composed of capital charges. The hydrogen production cost from natural gas via steam reforming of methane varies from about 1.25 US$/kg for large systems to about 3.50 US$/kg for small systems with a natural gas price of 6 US$/GJ. Hydrogen is cheap by using solar energy or by water electrolysis where electricity is cheap, etc.     

Well-to-Wheels analysis of hydrogen production from bio-oil reforming for use in internal combustion engines

The environmental profile of hydrogen depends greatly on the nature of the feedstock and the production process. In this Well-to-Wheels (WTW) study, the environmental impacts of hydrogen production from lignocellulosic biomass via pyrolysis and subsequent steam reforming of bio-oil were evaluated and compared to the conventional production of hydrogen from natural gas steam reforming. Hydrogen was assumed to be used as transportation fuel in an internal combustion engine vehicle. Two scenarios for the provision of lignocellulosic biomass were considered: wood waste and dedicated willow cultivation. The WTW analysis showed that the production of bio-hydrogen consumes less fossil energy in the total lifecycle, mainly due to the renewable nature of the fuel that results in zero energy consumption in the combustion step. The total (fossil and renewable) energy demand is however higher compared to fossil hydrogen, due to the higher process energy demands and methanol used to stabilize bio-oil. Improvements could occur if these are sourced from renewable energy sources. The overall benefit of using a CO₂ neutral renewable feedstock for the production of hydrogen is unquestionable. In terms of global warming, production of hydrogen from biomass through pyrolysis and reforming results in major GHG emissions, ranging from 40% to 50%, depending on the biomass source. The use of cultivated biomass aggravates the GHG emissions balance, mainly due to the N₂O emissions at the cultivation step.     

Impact of hydrogen on the environment

The present work considers the impact of hydrogen fuel on the environment within the cycles of its generation and combustion. Hydrogen has been portrayed by the media as a fuel that is environmentally clean because its combustion results in the formation of harmless water. However, hydrogen first must be generated. The effect of hydrogen generation on the environment depends on the production process and the related by-products. Hydrogen available on the market at present is mainly generated by using steam reforming of natural gas, which is a fossil fuel. Its by-product is CO₂, which is a greenhouse gas and its emission results in global warming and climate change. Therefore, hydrogen generated from fossil fuels is contributing to global warming to the similar extent as direct combustion of the fossil fuels. On the other hand hydrogen obtained from renewable energy, such solar energy, is environmentally clean during the cycles of its generation and combustion. Consequently, the introduction of hydrogen economy must be accompanied by the development of hydrogen that is environmentally friendly. The present work considers several aspects related to the generation and utilisation of hydrogen obtained by steam reforming and solar energy conversion (solar-hydrogen).     

Comparative analysis of H2/O2 cycle power plants based on different hydrogen production systems from fossil fuels

In this paper, two options for H₂ production by means of fossil fuels are presented and their performances are evaluated when integrated with H₂/O₂ cycles. The investigation has been developed with reference to two different schemes, both representative of advanced technology (TIT=1350°C) and of futurist technology (TIT=1700°C).The two methods, here considered, to produce H₂ are:

• •Coal gasification: it permits transformation of a solid fuel into a gaseous one, by means of partial combustion reactions;
• •Steam–methane reforming: it is the simplest and potentially the most economic method for producing hydrogen in the foreseeable future.

These hydrogen production plants require material and energy integrations with the power section, and the best connections must be investigated in order to obtain good overall performance.The overall efficiencies are very poor, especially those of the power plants coupled with the steam methane reforming; their mean values are about 21% for the first reference case and about 25% for the second one. The overall efficiencies of the power plants, coupled with the coal gasification, are little better than the previous ones but always rather low: their mean values are about 28% for the first reference case and about 33% for the second one.The CO₂ specific emissions depend on the fossil fuel typology and the overall efficiency: adopting a removal efficiency of 90% in the CO₂ absorption systems, the CO₂ emission reduction is 87% and 82% in the coal gasification and in the steam-methane reforming, respectively.     

High-temperature solar chemistry for converting solar heat to chemical fuels

This paper reviews the recent developments on thermochemical conversion of concentrated solar high temperature heat to chemical fuels. The conversion has the advantage of producing long term storable energy carriers from solar energy. This conversion also enables solar energy transportation from the sunbelt to remote population centers. The thermochemical pathway is characterized by a theoretical high efficiency. However, there are solar peculiarities in comparison to conventional thermochemical processes—high thermal flux density and frequent thermal transients because of the fluctuating insolation—, and conventional industrial thermochemical processes are generally not suitable for solar driven processes. Therefore, the adaptation to such peculiarities of solar thermochemical processes has been the important R&D task in this research field. Thermochemical water splitting, steam or CO₂ gasification of coal, steam or CO₂ reforming of methane, and hydrogenetive coupling of methane, are industrially important, endothermic processes to produce useful chemical fuels such as hydrogen, synthesis gas and C₂-hydrocarbons, which have been examined as solar thermochemical processes. The technical developments and feasibilities to conduct these endothermic processes by utilizing concentrated solar radiation as the process heat are discussed here. My recent experimental results to improve the advanced solar thermochemical technologies are also given.     

Solar photocatalytic reactor performance for hydrogen production from incident ultraviolet radiation

Solar based hydrogen production is a promising alternative to methods based on fossil fuels, such as steam methane reforming (SMR) and coal gasification. A more economically viable way of producing hydrogen from water is under active investigation by many researchers, to convert solar energy to chemical energy with higher efficiency. In this paper, supramolecular complexes developed by Brewer (2006) for photocatalytic hydrogen production are examined, particularly for larger scale engineering reactors that can use visible light to dissolve the photocatalysts in water, causing the splitting of water molecules into hydrogen and hydroxyl ions. This paper analyzes and optimizes the system parameters associated with this system. A predictive model for the reactor is developed for a batch type photocatalytic reactor. Results are presented and discussed to evaluate how the system parameters affect the hydrogen production rate, and solar to hydrogen efficiency, using a monochromatic LED array and Rhodium based photocatalysts.     

From coal towards renewables: Catalytic/synergistic effects during steam co-gasification of switchgrass and coal in a pilot-scale bubbling fluidized bed

Recent environmental sharp curbs on fossil fuel energy systems such as coal power plants due to their greenhouse gas emissions have compelled industries to include renewable fuels. Biomass/coal co-gasification could provide a transition from energy production based on fossil fuels to renewables. A low-ash coal and switchgrass rich in potassium were selected on the basis of previous thermogravimetric studies to steam co-gasify 50:50 wt% coal:switchgrass mixtures in a pilot scale bubbling fluidized bed reactor with silica sand as the bed material at ∼800 and 860 °C and 1 atm. With the switchgrass added to coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced remarkably relative to single-fuel gasification. The product gas tar yield also decreased considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. Switchgrass ash therefore can act as inexpensive natural catalysts for steam gasification and assist in operating at lower temperatures without being penalized by an increase in product tar yield. An equilibrium model over-predicted hydrogen and under-predicted methane concentrations. However, an empirically kinetically-modified model was able to predict the product gas compositions accurately.     

Hydrogen production and CO2 fixation by flue-gas treatment using methane tri-reforming or coke/coal gasification combined with lime carbonation

The production of hydrogen and the fixation of CO₂ can be achieved by treatment of flue gases derived from fossil fuel fired power plants via catalytic methane tri-reforming or by coal gasification in the presence of CaO. A two-step process is designed to be carried out in two reactors: a) a catalytic gasifier or steam-reformer, operating exothermally at 900–1000 K, with inputs of the flue gas, a carbonaceous source, steam and air, as well as CaO from the calciner, and outputs of H₂, and of “spent” CaCO₃ to the calciner; b) a calciner, operating endothermally at 1100–1300 K, with inputs of spent CaCO₃ from the gasifier, make-up fresh CaCO₃, and outputs of CO₂, as well as of CaO, partly recycled to the gasifier and partly processed in a cement plant. Thermochemical equilibrium calculations along with mass/energy balances indicate that for flue-gas treatment by tri-reforming, CO₂ emission avoidance of up to ∼59% and fossil fuel savings of up to ∼75% may be attained when concentrated solar energy is supplied as high-temperature process heat for the calcination step, all relative to conventional H₂ production by coal gasification. If instead fossil fuel would be used to drive the calcination step, the CO₂ emission avoidance and the fuel savings would be only 20% and 67%, respectively. Estimated annual H₂ production from a coal-fired 500 MWe burner by the proposed flue-gas treatment using either CH₄-tri-reforming or coal gasification would amount to 0.7 × 10⁶ or 0.6 × 10⁶ metric tons H₂, respectively. Estimated fossil fuel consumption for H₂ production by tri-reforming or coke gasification would be 149 or 143 GJ fuel/ton H₂.     

Renewable-powered hydrogen economy from Australia's perspective

This article broadly reviews the state-of-the-art technologies for hydrogen production routes, and methods of renewable integration. It outlines the main techno-economic enabler factors for Australia to transform and lead the regional energy market. Two main categories for competitive and commercial-scale hydrogen production routes in Australia are identified: 1) electrolysis powered by renewable, and 2) fossil fuel cracking via steam methane reforming (SMR) or coal gasification which must be coupled with carbon capture and sequestration (CCS). It is reported that Australia is able to competitively lower the levelized cost of hydrogen (LCOH) to a record $(1.88–2.30)/kg_H2 for SMR technologies, and $(2.02–2.47)/kg_H2 for black-coal gasification technologies. Comparatively, the LCOH via electrolysis technologies is in the range of $(4.78–5.84)/kg_H2 for the alkaline electrolysis (AE) and $(6.08–7.43)/kg_H2 for the proton exchange membrane (PEM) counterparts. Nevertheless, hydrogen production must be linked to the right infrastructure in transport-storage-conversion to demonstrate appealing business models.     

Political,economic and environmental impacts of biomass-based hydrogen

The purpose of this study is to assess the political, economic and environmental impacts of producing hydrogen from biomass. Hydrogen is a promising renewable fuel for transportation and domestic applications. Hydrogen is a secondary form of energy that has to be manufactured like electricity. The promise of hydrogen as an energy carrier that can provide pollution-free, carbon-free power and fuels for buildings, industry, and transport makes it a potentially critical player in our energy future. Currently, most hydrogen is derived from non-renewable resources by steam reforming in which fossil fuels, primarily natural gas, but could in principle be generated from renewable resources such as biomass by gasification. Hydrogen production from fossil fuels is not renewable and produces at least the same amount of CO₂ as the direct combustion of the fossil fuel. The production of hydrogen from biomass has several advantages compared to that of fossil fuels. The major problem in utilization of hydrogen gas as a fuel is its unavailability in nature and the need for inexpensive production methods. Hydrogen production using steam reforming methane is the most economical method among the current commercial processes. These processes use non-renewable energy sources to produce hydrogen and are not sustainable. It is believed that in the future biomass can become an important sustainable source of hydrogen. Several studies have shown that the cost of producing hydrogen from biomass is strongly dependent on the cost of the feedstock. Biomass, in particular, could be a low-cost option for some countries. Therefore, a cost-effective energy-production process could be achieved in which agricultural wastes and various other biomasses are recycled to produce hydrogen economically. Policy interest in moving towards a hydrogen-based economy is rising, largely because converting hydrogen into useable energy can be more efficient than fossil fuels and has the virtue of only producing water as the by-product of the process. Achieving large-scale changes to develop a sustained hydrogen economy requires a large amount of planning and cooperation at national and international alike levels.     

Economic analysis of hydrogen production from wastewater and wood for municipal bus system

The levelized cost of hydrogen for municipal fuel cell buses has been determined using the DOE H2A model for steam methane reforming (SMR), molten carbonate fuel cell reforming (MCFC), and wood gasification using wastewater biogas and willow wood chips as energy feedstocks. 300 kg H₂/day was chosen as the design capacity. Greenhouse gas emissions were calculated for each for the three processes and compared to diesel bus emissions in order to assess environmental impact. The levelized cost per kilogram for SMR, MCFC, and gasification is $5.12, $8.59, and $10.62, respectively. SMR provided the lowest sensitivity to feedstock price, and lowest levelized cost at various scales, with competitive cost to diesel on a cost/km basis. All three technologies provide a reduction in total greenhouse gases compared to diesel bus emissions, with MCFC providing the largest reduction. These results provide preliminary evidence that small scale distributed hydrogen production for public transportation can be relatively cost-effective and have minimal environmental impact.     

The impact of carbon sequestration on the production cost of electricity and hydrogen from coal and natural-gas technologies in Europe in the medium term

Carbon sequestration is a distinct technological option with a potential for controlling carbon emissions; it complements other measures, such as improvements in energy efficiency and utilization of renewable energy sources. The deployment of carbon sequestration technologies in electricity generation and hydrogen production will increase the production costs of these energy carriers. Our economic assessment has shown that the introduction of carbon sequestration technologies in Europe in 2020, will result in an increase in the production cost of electricity by coal and natural gas technologies of 30–55% depending on the electricity-generation technology used; gas turbines will remain the most competitive option for generating electricity; and integrated gasification combined cycle technology will become competitive. When carbon sequestration is coupled with natural-gas steam reforming or coal gasification for hydrogen production, the production cost of hydrogen will increase by 14–16%. Furthermore, natural-gas steam reforming with carbon sequestration is far more economically competitive than coal gasification.     

Design,modelling and optimal power and hydrogen management strategy of an off grid PV system for hydrogen production using methanol electrolysis

Hydrogen used as an energy carrier and chemical element can be produced by several processes such as gasification of coal and biomass, steam reforming of fossil fuel and electrolysis of water. Each of these methods has its own advantage and disadvantage. Electrolysis process is seen as the best option for quick hydrogen production. Hydrogen generation by methanol electrolysis process (MEP) gained much attention since it guarantees high purity gas and can be compatible with renewable energies. Furthermore, due to its very low theoretical potential (0.02 V), MEP can save more than 65% of electrical energy required to produce 1 kg of hydrogen compared to water electrolysis process (WEP). Electrolytic hydrogen production using solar photovoltaic (PV) energy is positioned to become as one of the preferred options due to the harmful environmental impacts of widely used methane steam reforming process and also since the prices of PV modules are more competitive.In this paper, hydrogen production by MEP using PV energy is investigated. A design of an off grid PV/battery/MethElec system is proposed. Mathematical models of each component of the system are presented. Semi-empirical relationship between hydrogen production rate and power consumption at 80 °C and 4 M concentration is developed. Optimal power and hydrogen management strategy (PHMS) is designed to achieve high system efficiency and safe operation. Case studies are carried out on two tilts of PV array: horizontal and tilted at 36° using measured meteorological data of solar irradiation and ambient temperature of Algiers site. Simulation results reveal great opportunities of hydrogen production using MEP compared to the WEP with 22.36 g/m² d and 24.38 g/m² d of hydrogen when using system with horizontal and tilted PV array position, respectively.     

Comparative assessment of hydrogen production methods from renewable and non-renewable sources

In this study, we present a comparative environmental impact assessment of possible hydrogen production methods from renewable and non-renewable sources with a special emphasis on their application in Turkey. It is aimed to study and compare the performances of hydrogen production methods and assess their economic, social and environmental impacts, The methods considered in this study are natural gas steam reforming, coal gasification, water electrolysis via wind and solar energies, biomass gasification, thermochemical water splitting with a Cu–Cl and S–I cycles, and high temperature electrolysis. Environmental impacts (global warming potential, GWP and acidification potential, AP), production costs, energy and exergy efficiencies of these eight methods are compared. Furthermore, the relationship between plant capacity and hydrogen production capital cost is studied. The social cost of carbon concept is used to present the relations between environmental impacts and economic factors. The results indicate that thermochemical water splitting with the Cu–Cl and S–I cycles become more environmentally benign than the other traditional methods in terms of emissions. The options with wind, solar and high temperature electrolysis also provide environmentally attractive results. Electrolysis methods are found to be least attractive when production costs are considered. Therefore, increasing the efficiencies and hence decreasing the costs of hydrogen production from solar and wind electrolysis bring them forefront as potential options. The energy and exergy efficiency comparison study indicates the advantages of biomass gasification over other methods. Overall rankings show that thermochemical Cu–Cl and S–I cycles are primarily promising candidates to produce hydrogen in an environmentally benign and cost-effective way.     

A comparison of electricity and hydrogen production systems with CO2 capture and storage. Part A: Review and selection of promising conversion and capture technologies

We performed a consistent comparison of state-of-the-art and advanced electricity and hydrogen production technologies with CO₂ capture using coal and natural gas, inspired by the large number of studies, of which the results can in fact not be compared due to specific assumptions made. After literature review, a standardisation and selection exercise has been performed to get figures on conversion efficiency, energy production costs and CO₂ avoidance costs of different technologies, the main parameters for comparison. On the short term, electricity can be produced with 85–90% CO₂ capture by means of NGCC and PC with chemical absorption and IGCC with physical absorption at 4.7–6.9 €ct/kWh, assuming a coal and natural gas price of 1.7 and 4.7 €/GJ. CO₂ avoidance costs are between 15 and 50 €/t CO₂ for IGCC and NGCC, respectively. On the longer term, both improvements in existing conversion and capture technologies are foreseen as well as new power cycles integrating advanced turbines, fuel cells and novel (high-temperature) separation technologies. Electricity production costs might be reduced to 4.5–5.3 €ct/kWh with advanced technologies. However, no clear ranking can be made due to large uncertainties pertaining to investment and O&M costs. Hydrogen production is more attractive for low-cost CO₂ capture than electricity production. Costs of large-scale hydrogen production by means of steam methane reforming and coal gasification with CO₂ capture from the shifted syngas are estimated at 9.5 and 7 €/GJ, respectively. Advanced autothermal reforming and coal gasification deploying ion transport membranes might further reduce production costs to 8.1 and 6.4 €/GJ. Membrane reformers enable small-scale hydrogen production at nearly 17 €/GJ with relatively low-cost CO₂ capture.     

Life cycle assessment of hydrogen fuel production processes

The use of hydrogen as an alternative fuel is gaining more and more acceptance as the environmental impact of hydrocarbons becomes more evident. A life cycle assessment study has been carried out to investigate the environmental aspects of hydrogen production. Production by natural gas steam reforming and production upon renewable energy sources are examined. Hydrogen is selected as a future alternative fuel because of the absence of CO₂ emissions from its use, its high-energy content and its combustion kinetics. A very large number of environmental burdens result from the operation of the different hydrogen production routes. A complete and accurate identification and quantification of the environmental emissions has been attempted. The use of wind, hydropower and solar thermal energy for the production of hydrogen are the most environmental benign methods. The benefits and the drawbacks of the competing hydrogen production systems are presented.