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Sulfur dioxide disproportionation is one of three reaction steps that make up the sulfur based thermochemical cycle used for thermal energy storage of concentrated solar power. The characteristics of this reaction were studied using thermodynamic modeling and laboratory measurements. Modeling results showed full disproportionation can only be achieved at pressure. The reaction driving force is enhanced by system pressure but declines with increasing temperature. Appropriate water to sulfur dioxide ratio also drives disproportionation. Batch experiments showed that reaction rate increases with temperature. A catalyst survey identified homogenous iodides as catalysts that can improve the reaction rate by up to twenty times while increasing the apparent extent of disproportionation. The dependence of disproportionation rate on sulfuric acid concentration was established via constant pressure experiments. Means to recover the iodide catalyst from sulfuric acid and molten sulfur for reuse were demonstrated. Modeling and test results were used to establish a design concept for a sulfur dioxide disproportionation reactor system capable of rapidly generating sulfur as required for the sulfur based thermochemical energy storage technology.

Hydrogen and electricity are expected to dominate the world energy system in the long term. The world currently consumes about 50 million metric tons of hydrogen per year, with the bulk of it being consumed by the chemical and refining industries. The demand for hydrogen is expected to increase, especially if the U.S. and other countries shift their energy usage towards a hydrogen economy, with hydrogen consumed as an energy commodity by the transportation, residential, and commercial sectors. However, there is strong motivation to not use fossil fuels in the future as a feedstock for hydrogen production, because the greenhouse gas carbon dioxide is a byproduct and fossil fuel prices are expected to increase significantly. For electricity and hydrogen production, an advanced reactor technology receiving considerable international interest is a modular, passively-safe version of the high-temperature, gas-cooled reactor (HTGR), known in the U.S. as the Modular Helium Reactor (MHR), which operates at a power level of 600 MW(t). For electricity production, the MHR operates with an outlet helium temperature of 850°C to drive a direct, Brayton-cycle power-conversion system (PCS) with a thermal-to-electrical conversion efficiency of 48 percent. This concept is referred to as the Gas Turbine MHR (GT-MHR). For hydrogen production, the process heat from the MHR is used to produce hydrogen. This concept is referred to as the H2-MHR. The growing international interest in the MHR concept is the direct result of MHR design features, which include: (1) Passive Safety, Competitive Economics, and Siting Flexibility. The MHR does not require active safety systems to ensure public and worker safety. The high-energy conversion efficiency of the MHR, combined with the elimination of active safety systems, result in a design that is passively safe and economically competitive with other non-passively safe reactor concepts. Because of its high efficiency, the MHR rejects less waste heat than other reactor concepts. This design feature, combined with passive safety, allows for more flexible siting options for the MHR. (2) High Temperature Capability and Flexible Energy Outputs. The MHR is capable of producing process-heat temperatures of 950°C and higher. This high-temperature capability translates into a high-energy conversion efficiency for a variety of energy outputs, including electricity, hydrogen production, and synthetic fuel production. (3) Flexible Fuel Cycles. The MHR can operate efficiently and economically with several different fuel cycles. MHR designs have been developed utilizing low-enriched (LEU) uranium fuels, high-enriched uranium (HEU) fuels, mixed uranium/thorium and plutonium/thorium fuels, and surplus weapons-grade plutonium fuels. The thermal neutron spectrum of the MHR, combined with robust, ceramic-coated particle fuel, allow for very high burnup in a single pass through the reactor. More recently, an MHR design has been developed to deeply burn plutonium and other transuranic (TRU) actinides recovered from lig...

iii EXECUTIVE SUMMARYHydrogen and electricity are expected to dominate the world energy system in the long term. The world currently consumes about 50 million metric tons of hydrogen per year, with the bulk of it being consumed by the chemical and refining industries. The demand for hydrogen is expected to increase, especially if the U.S. and other countries shift their energy usage towards a hydrogen economy, with hydrogen consumed as an energy commodity by the transportation, residential, and commercial sectors. However, there is strong motivation to not use fossil fuels in the future as a feedstock for hydrogen production, because the greenhouse gas carbon dioxide is a byproduct and fossil fuel prices are expected to increase significantly.For electricity and hydrogen production, an advanced reactor technology receiving considerable international interest is a modular, passively-safe version of the high-temperature, gas-cooled reactor (HTGR), known in the U.S. as the Modular Helium Reactor (MHR), which operates at a power level of 600 MW(t). For electricity production, the MHR operates with an outlet helium temperature of 850°C to drive a direct, Brayton-cycle power-conversion system (PCS) with a thermal-to-electrical conversion efficiency of 48 percent. This concept is referred to as the Gas Turbine MHR (GT-MHR). For hydrogen production, the process heat from the MHR is used to produce hydrogen. This concept is referred to as the H2-MHR.The growing international interest in the MHR concept is the direct result of MHR design features, which include:(1) Passive Safety, Competitive Economics, and Siting Flexibility. The MHR does not require active safety systems to ensure public and worker safety. The high-energy conversion efficiency of the MHR, combined with the elimination of active safety systems, result in a design that is passively safe and economically competitive with other non-passively safe reactor concepts. Because of its high efficiency, the MHR rejects less waste heat than other reactor concepts. This design feature, combined with passive safety, allows for more flexible siting options for the MHR.(2) High Temperature Capability and Flexible Energy Outputs. The MHR is capable of producing process-heat temperatures of 950°C and higher. This high-temperature capability translates into a high-energy conversion efficiency for a variety of energy outputs, including electricity, hydrogen production, and synthetic fuel production.(3) Flexible Fuel Cycles. The MHR can operate efficiently and economically with several different fuel cycles. MHR designs have been developed utilizing low-enriched (LEU) uranium fuels, high-enriched uranium (HEU) fuels, mixed uranium/thorium and plutonium/thorium fuels, and surplus weapons-grade plutonium fuels. The thermal neutron spectrum of the MHR, combined with robust, ceramic-coated particle fuel, allow for very high burnup in a single pass through the reactor. More recently, an MHR design has been developed to deeply burn plutonium and other transuranic (TRU) actinides r...

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A sulfur based thermochemical energy storage cycle for baseload power generation is being developed under the support of US DOE Sunshot program. Solar heat is stored in elemental sulfur via thermal decomposition of sulfuric acid and disproportionation of sulfur dioxide into elemental sulfur and sulfuric acid. Heat energy is recovered upon sulfur combustion. On-sun decomposition of sulfuric acid in a solar furnace has been demonstrated between 650 and 850°C. Near equilibrium conversion was obtained at high temperature but conversion was reduced due to catalyst poisoning at the lower temperatures. Sulfur dioxide disproportionation modeling showed the reaction driving force is maximized at the high system pressure and low system temperature. The effect of system pressure was validated experimentally. However, the disproportionation rate was found to increase with system temperature as a result of increased reaction kinetics. Homogenous iodide catalysts were used to further enhance the degree of disproportionation and the reaction rate. The process steps required to recover the catalyst for reuse have been verified.

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