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Solar energy-driven hydrogen production

Nearly all current hydrogen generating processes use fossil fuels that lead to CO2 and GHG emissions. Towards a de-fossilization of its production, a distinction is now made between non-fossil-based "green H2" with very low CO2 emission, "gray H2" from hydrocarbons with CO2 emission, and "blue H2" where the carbon emission is captured and possibly utilized. The thermal fossil fuel-based hydrogen production technology includes steam reforming, partial oxidation, and ATR processes, respectively. Non-fossil feedstock production methods today mainly include fermentation, gasification or pyrolysis, and electrolysis.

Chapter 1 summarizes the different current and potential novel H2 production methods. Although these H2 production methods will be proven technically and economically viable, their moderate to high endothermicity requires the use of a “green” energy supply, where renewable energy appears to be the most indicated option. The application of renewable heat or power will therefore be investigated in the second part of the Ph.D. research, with a special focus on using Concentrated Solar Tower technology, both in a base-load and in a peaker operation mode.

Chapter 2 examines four H2 production methods using “gray” or “green” feedstock. Four experimental set-ups were used, including a pilot-scale concentrated solar rig. The reactors contained appropriate quantities of catalyst particles. Experiments were conducted in triplicate under the same operating conditions.

A first “gray” feedstock, CH4, was investigated mostly for its Hydrogen-Enriched Natural Gas (HENG) potential. Results demonstrate that the Fe/Al2O3 catalyst (with a 20 wt% loading of Fe) can yield nearly complete methane conversion at a temperature of 700 °C, whereas the Ni/MgO catalyst is less performing at the same temperature and residence time. The carbon formed by the decomposition reaction accumulates within the catalyst with a progressive reduction in H2 yield. A periodic regeneration is required. For both catalyst systems, the reaction rate determines the kinetics at temperatures below ~635 °C whereas external mass transfer becomes increasingly important at higher temperature. The present results confirm that the use of Fe/Al2O3 for CH4 decomposition is recommended.

A second “gray” (possibly “green” by future developments in bio-synthesis of methanol) feedstock was CH3OH. Experimental results demonstrate that the catalytic steam reforming of methanol (CSRM) by using Co/α-Al2O3 or MnFe2O4 provides a complete conversion of CH3OH at respectively 300 °C and 400 °C. The H2 yield is about 2.5 mol H2/mol CH3OH (slightly, lower than the stoichiometric yield of 3 mol H2/mol CH3OH). The CH3OH-carbon is transformed into mostly CO and CO2. Coke formation was not detected after long-duration catalyst use. A kinetic analysis demonstrated a high reaction rate, with optimum conversion already achieved within a contact time of 0.3 to 0.5 s. Longer contact times hardly change the product composition, and equilibrium is attained. Using Co/α-Al2O3 for scale-up and fuel cell application is recommended due to the lower CO formation and higher hydrogen yield.

Thirdly, the production of H2 from "green" or industrial NH3 was studied in the present research using non-noble metal catalysts. “Green” ammonia sources were investigated, such as ammonia produced by stripping from manure and digestate. Three catalysts were tested, wet-impregnated Fe-Al2O3 and dry-milled Fe-Al2O3, and wet-impregnated Ni-Al2O3. The mildly endothermic decomposition of NH3 into H2 and N2 is achieved with a 100% yield at 500 °C using a Fe/γ-Al2O3 catalyst. The alternative and more expensive Ni/γ-Al2O3 catalyst scores significantly lower. The reaction is very fast, with a rate constant of 7.5/s at 500 °C. Full conversion is obtained within 0.4 s. Very pure (> 95%) H2 can be produced and can be stored by several techniques. It is proven that the H2 production from NH3 has a high potential, and merits pilot-scale consideration.

Toward the Catalytic steam reforming of bio-ethanol (CSRE), the most promising catalysts were selected from an in-depth literature review and tested in the present research in isothermal electrically heated furnaces, and in a concentrated solar reactor. Over 30 successive hours were assessed, and the H2 production efficiencies were close to the stoichiometric expected values. Approximately 5.5 mol H2/mol ethanol was produced with CO and CO2 as main carbon-bearing reaction products. A kinetic analysis defined the reaction rate expressions and the reaction rate per unit per unit catalyst weight (0.58 mol H2 /gcat h). Moreover, the production system was critically assessed toward a further scale up with maximum heat recovery, H2 upgrading, and H2 use in a SOFC.

Chapter 3 focuses upon the thermal conversion of water by appropriate reversible redox reactions. Whereas thermal water splitting to H2 requires high temperatures, thermo-chemical water splitting cycles operate at moderate temperatures. These cycles were previously investigated, mostly by small-scale experiments and mainly to prove their concept without judgment of their practical, economic, environmental and cyclic performance. To facilitate the decision making and to guide future priority research, these multiple aspects were combined in a global screening system that applies the improved Analytic Hierarchy Process (AHP) and gray relational TOPSIS, together with the use of linear and non-linear combination weighing. The assessment is quantitative and comprehensive, emphasizing the complex relationship between energy efficiency, conversion, recyclability, economy and environmental quality. The screening index helps users, system manufacturers, researchers and governments to select the most appropriate future schemes. Twenty-four thermo-chemical water splitting cycles were investigated. After an objective screening, the selected priority redox systems using MnOx/Na2CO3 and MnFe2O4/Na2CO3 were experimentally investigated for short- and long-duration cycling.

For the MnOx/Na2CO3 redox water splitting system, tests are carried out both in an electric furnace, and in a concentrated solar furnace at 775 and 825 °C using 10 to 250 gram of redox reactants, respectively. In the solar furnace, the highest conversion (>95%) was obtained with the Mn3O4/ Na2CO3 system at 775 °C. The economic assessments revealed that at least 100 cycles would be needed to achieve competitive H2 prices below 2 €/kg for systems with a cheap energy supply, and provided CO2 could be separated from the reactant gas and reused in the reverse reaction.

The MnFe2O4/Na2CO3 cycles were subsequently experimentally investigated at lab- and solar reactor scale. Over 30 successive oxidation/reduction cycles were assessed, and the H2 production efficiencies exceeded 98 % for the coprecipitated reactant after these multiple cycles. Solar reactor experiments confirm the > 95% H2 efficiency. Tentative cost calculations showed a break-even operation for 30 consecutive cycles at H2 prices of 4 €/kg H2. At least 120 cycles before reactant re-activation will reduce the H2 production cost to ~1 €/kg H2. This implies the use of a cheap energy supply, and the complete reuse of CO2 in the reduction reaction. The results certainly foster a further improvement of the system. Coprecipitated MnFe2O4 is purchased at approximately 500 €/ton. The ball-milled reactant can be approximately 30% cheaper.

The heating costs are not considered since the system is supposed to use excess photovoltaic or wind turbine electricity, or to operate on concentrated solar heat. Heat recovery will be maximized by good heat management. CO2 should be separated, stored, and used in the reverse reaction. It is also proposed to use membrane modules to produce very pure H2. If the cheaper ball-milled reactant could be improved, the number of required cycles to break even will be reduced. The proposed MnFe2O4/Na2CO3 cycle, if realized at an industrial scale, can be competitive with the electrolysis of water using solar-generated electricity or heat. Further development and large-scale demonstration are needed.

Chapter 4 summarizes the contribution of the Ph.D. research into (i) the particle-driven receiver hydrodynamics, where long receiver tubes can now be implemented; and (ii) the determination of the hydrodynamics and thermal design parameters of an appropriate heat exchanger to recover the particle-stored heat. Auxiliary equipment will also be briefly discussed.

Since the research will promote hybrid PV-CST solar systems to supply energy to the H2-production reactors, it was deemed necessary to focus on the required improvements of the CST.  Essential results are summarized hereafter for the solar receiver. Introducing Bubble Rupture Promoters largely eliminates slugging. The wall-to-bed heat transfer is further increased from 200 W/m2K to > 600 W/m2K. A deemed key operation of the CST plant, is the storage of hot and cold particles. This was studied at the kWth and MWth scale. Heat losses will occur during storage. The temperature evolution of the hopper is predicted.

Particle conveying within the solar loop, is an important operation. The solids heated by the solar receiver are transferred to the power block and stored before their use. In turn, the cooled solids leaving the heat recovery are stored before being transferred back to the solar receiver to be reheated. It should be remembered that the power required to lift the particles by mechanical means (e.g. elevators) is increased by about 220 % because of the weight of chains and buckets and the excessive mechanical friction. It was hence decided to apply a pneumatic particle conveying in dense phase. A similar recommendation to use pneumatic conveying, even for capacities as high as 600 tph and lifting heights of 250 m were proposed in literature. The pneumatic conveying option is hence selected. Rather than applying the air-in-partcle dense phase system using discontinuous blow tanks (with additionlal problems of requiring high T valves), the proposed system will use a continuous particle-in-air dense phase conveying, with lower pressure drop along the conveying pipes. The solution is applicable at large scale only if a heat recovery is integrated to the loop.

The release of dust and the associated hot air is to be avoided. High-temperature particle/air separation was introduced. Filters are in service operating at temperatures above 800°C, and the technology is readily applied to the requirements of the solar plant. Filtration (face) velocities can be of the order of 3 m3/m2 min (against 1.2 m3/m2 min for common fabric filters). The blow back flow should achieve 0.15 m/s, and the blow-back pressure is commonly 10 bar. With a 1 μm fibre filter, the de-dusting efficiency will exceed 99 % for 0.1 μm particles. The residual emission will be << 1 mg/Nm3.

Chapter 5 combines the fundamental findings of previous chapters into a view of valorization. Special attention is paid to the scale-up and Technology Readiness Level of the investigated H2 production technologies, their costs, and associated process requirements such as H2-upgrading, the use of solid oxide fuel cells, and additional technical issues. Essential findings are summarized and used in scale-up procedures for both the thermo-catalytic conversion of CH4, CH3OH, C2H5OH and NH3, and for the MnFe2O4/Na2CO3 redox water splitting. Additional topics of H2 upgrading, safety issues associated with H2 production and its use, and high-temperature process gas filtration are discussed. A techno-economic assessment and a tentative discussion of the Technology Readiness Level (TRL) will be presented. Finally, the economic characteristics (expressed as levelized cost of hydrogen (LCOH)) and environmental parameters (expressed as GHG emission) were determined.

The 2015 United Nations General Assembly identified its Sustainable Development Goals (SDGs) with 17 objectives, to be achieved by 2030. Numerous countries are aiming to achieve a 100% renewable energy utilization by 2050. The assessment of chapter 5 proves that the solar-driven processes in general, and solar heat-driven H2 production, significantly contribute to meeting these goals.

Chapter 6 concludes the PhD research. It (i) links the results obtained with the set objectives of the research; (ii) summarizes all important finding; and (iii) recommends additional research. Within the H2 production scope, a more detailed knowledge of the interaction of the feedstock and the catalysts or reactants is certainly needed. Positron Emission Tomography could be a tool to provide insight in the gas (or vapor)/solid behavior. It should moreover be stressed that additional catalysts, either as single or mixed metal oxides should be investigated. Although the research did not reveal catalyst deactivation, experiments on long-term performance and catalyst life-time are needed. On a more practical level, a pilot-scale study under realistic conditions could be revealing how the catalyst and reactants behave when submitted to high feedstock feed rates.

With respect to the upflow particle-driven receiver, obtaining a very high outlet temperature of the particles could be a target objective preferably proven in the multi-tube set-up. Since scale-up to commercial capacities of 5 to 50 MWth is envisaged with novel thermodynamic cycles in heat recovery (combined gas/steam turbine, supercritical fluids, etc.), additional research should be examined, planned, and performed. Different powders, for example, olivine, cristobalite, alumina, or quartz, should moreover be tested on a large and continuous basis to obtain long-term characteristics of particle stability, equipment erosion, and replacement frequency. Within the particle loop of the CST, a special experimental confirmation of overall heat losses should be achieved.

Finally, the design recommendations and tentative economic balances should be proven at the pilot-scale. The experimental research of the present Ph.D. has provided conclusions positive enough to incite other researchers and even R&D centers to continue this research, and I sincerely hope that my research has started the break through both of H2 as an energy carrier, and of concentrated solar energy to supply reaction energies or power.

Date:23 Jun 2020 →  4 Apr 2023
Keywords:Thermo-chemical energy storage, Hydrogen production, Positron emission tomography, Pilot/Large-scale production of H2
Disciplines:Renewable power and energy systems engineering, Hydro energy, Solar energy, Chemical process design
Project type:PhD project