Research Terms
This thermochemical process converts large-scale power plants’ flue exhaust into synthesis gas, which refines into high-quality synthetic fuels such as diesel or methanol. University of Florida researchers propose to do this via integration with a methane reformation driven redox cycle powered by solar energy, where the solar step and flue gas utilization step are separated in both space and time. This is vitally important for 24/7 operation and utilization of flue gas because at the industrial scales at which it is generated, storage is not a viable option. In total, this process affords the economical utilization and transformation of solar energy, flue gases and methane or natural gas to a valuable synthesis gas that can be further converted to a high quality, drop-in, diesel fuel and other added value chemicals that can be stored and transported.
Continuous conversion of industrial flue gases into valuable synthesis gas suitable for producing high quality liquid fuels and other value added chemicals
Natural gas and solar energy work in tandem continuously to process flue gas effluent from large-scale plants that consume fossil fuels. In a drop tube reactor, solar energy drives an endothermic reaction that reduces highly reactive ceria-based (CeO2) particles and oxidizes the methane found in natural gas to produce synthesis gas. A second reactor collects and stores the reduced ceria particles for later use in an exothermic reaction. The particles re-oxidize upon exposure to carbon dioxide and water vapor captured from flue gases, which themselves reduce selectively to generate synthesis gas. The re-oxidized ceria particles then return to the solar reactor for the synthesis gas production cycle to begin again.
These 3D-printed receiver tubes can survive the scorching temperatures of concentrated solar power systems while remaining cheap and lasting for decades. Concentrated solar power receivers contain heat transfer fluids, such as molten salt, absorbing energy from concentrated sunlight and then releasing the energy to generate electricity. Conventional receiver tubes operate at around 500°C, but advanced receivers operating at high temperatures offer improved efficiency, lower overall system costs, and expanded industrial applications. Refractory metals such as molybdenum possess notorious heat resistance, promising for concentrated solar power receiver tubing, but are overly liable to oxidize. Coatings based on borosilicate glass materials can prevent the oxidation of molybdenum. However, to achieve a high-temperature, fully oxidation-resistant material suitable for concentrated solar power applications, this coating must be applied to the inner and outer surfaces of all tubing, an onerous manufacturing requirement.
Researchers at the University of Florida have developed a laser engineered net shaping (LENS) 3D printer for fabricating receiver tubes capable of surviving high temperatures and resisting oxidation. The laser fuses the molybdenum, boron, and silicate into a thick three-dimensional coating in place, even for hard-to-reach regions such as the inside walls of long, narrow tubes.
Laser-based 3D printing for achieving oxidation-resistant coating on the hardest-to-reach parts of heat-resistant molybdenum tubing
Concentrated solar power is a renewable energy technology that stores thermal energy in fluids such as molten salt or supercritical carbon dioxide. The concentration of sunlight towards the fluids produces such high temperatures that the tubing must possess excellent heat resistance. Furthermore, the demands of high solar absorption, high thermal conductivity, oxidation resistance, and low-cost manufacturing present serious challenges to the selection of materials for concentrated solar power tubing. 3D printing unlocks powerful combinations of materials with desirable properties to meet this challenge. Molybdenum features a melting point above 2200 ?C while a mixture of molybdenum, silicon, and boron produces an oxidation-resistant coating that also offers creep resistance and fracture toughness. The laser engineered net shaping (LENS) 3D printer combines these into a heterostructure with both heat and oxidation resistance. The printing mechanism proceeds by depositing powders of pure molybdenum and a molybdenum-silicon-boron mixture simultaneously. A laser then irradiates the powders, generating a molybdenum substrate covered by an oxidation-resistant coating whose incorporation of both silicon and boron offers improved oxidation resistance compared to traditional molybdenum-silicon compositions.
This methane reforming pathway uses condensation to sequester either pure carbon or carbon dioxide. Currently available methods of industrial methane conversion to synthesis gas, or syngas (i.e., H2 and CO), primarily consists of the following techniques: steam reforming of methane, dry reforming of methane, and methane partial oxidation. These techniques, when coupled with well-documented catalytic pathways like Fischer-Tropsch synthesis (FTS), can produce fungible fuels such as diesel, methanol, and kerosene. However, since FTS requires stringent syngas ratios for fuel synthesis, shift reactors and gas-separation equipment must integrate to properly tune syngas ratios and simultaneously remove undesired products, such as CH4, H2O, and CO2. Notably, steam methane reforming is the most used method of hydrogen production globally, but also requires shifting reactors that convert CO to CO2 to increase the H2 yields. These processes do not readily afford the ability to utilize the methane in a carbon neutral manner without incurring a significant energy penalty. To address these issues, emergent research has leveraged the oxygen-exchange capacity of metal oxides to facilitate the conversion of methane to syngas. Known as chemical-looping reforming (CLR) of methane, typical steam, dry reforming and/or partial oxidation reactions are bisected into 2 steps with a redox cycle. The first step is a reduction of a metal oxide via methane partial oxidation in the absence of gas-phase oxygen to produce synthesis gas and the second step is oxidation of the reduced metal oxide via H2O and/or CO2 dissociation to produce syngas. By adjusting the amount of H2O and CO2 delivered in the second step, the syngas ratios may be tuned without a shifting reactor while affording even the possibility of ultra-pure hydrogen in the second step. Further, when compared to partial oxidation of methane there is no expensive O2 separations involved. Because the first step produces synthesis gas, these processes also do not lend themselves to utilizing methane in a carbon neutral manner.
Researchers at the University of Florida have developed a new gas-to-liquid (GTL) pathway for methane reforming that enables passive gas separation via condensation and thus allows for the sequestration of either pure C or CO2. This is achieved by altering the first step of CLR so that the methane-oxide reaction produces H2O and C or H2O and CO2 instead of syngas. In the second step, H2O and CO2 can similarly reoxidize the reduced oxide and produce pure streams of H2 and/or CO. This process also requires a substantially lower temperature, below 600 degrees C, thus enabling the potential use of inexpensive, trough-based solar concentrating technologies (compared to the more expensive 3-D concentrating systems) to provide process heat and yield a carbon-neutral fuel.
Methane reforming technology for carbon-neutral fuel synthesis
The proposed pathway for methane reforming leverages the favorable thermodynamics of ceria-based metal oxides to ensure complete combustion of methane, and thus enable facile gas-separation, in the endothermic first phase of the redox cycle. Our prior equilibrium thermodynamic analyses motivated by CLR over ceria indicate that the formation of H2O, CO2, CH4, and C is favorable at low temperatures (T < 600 degrees C). Inclusion of dopants, such as Zr, can further modify ceria's thermodynamic properties (e.g., decreasing the partial molar enthalpy), such that reaction 1 is more selective to H2O and CO2 formation. Controlling other operating conditions, such as system pressure and reaction duration, can also inhibit carbon deposition (if desired) and enhance reactant conversion. Finally, well-documented metal additives (e.g., Ni or Pt) will aid in improving reaction rates at lower operating temperatures. In the exothermic second stage of the cycle, reduced ceria, a well-documented media to facilitate H2O and/or CO2 dissociation, re-oxidizes to produce H2 and/or CO (depending upon the employed oxidant). Importantly, the cycle operates isothermally below 600 degrees, enabling cost-effective solar energy (e.g. parabolic trough system) or another renewable resource to supply process heat and thus providing a pathway for carbon-neutral fuel synthesis.
This solar energy collection and storage system enables operation at high temperatures (up to 1500 °C) that support more efficient power cycles and solar thermal technologies, providing lower-cost renewable energy. To become more cost-efficient, concentrating solar power (CSP) plants must employ collection and storage equipment that can operate at temperatures higher than 1000 °C. Molten metals would serve well as heat transfer fluids due to their high temperature stability, and thermal transport properties; however, thermal storage of them directly is costly. Additionally, available designs for molten metal thermal storage use components made of graphite or other oxidizable metals that are subject to corrosion.
Researchers at the University of Florida have developed high temperature materials of construction for solar receiver utilizing molten metals, brick thermal storage and flow components to achieve 24/7 thermal energy storage system that can produce heat at temperatures up to 1500 °C. This system provides high temperature, low-cost thermal storage and high-quality heat to fuel more efficient power plants.
High temperature solar receiver and thermal storage for efficient and affordable solar energy utilization for power production or other thermally driven technologies (e.g. chemical production, solar fuels)
Air-stabilized Mo components via MoSiB coating enable very high temperature solar energy conversion and low-cost thermal storage to improve the efficiency of concentrating solar power conversion and other thermal processes. The molten lead in the solar receiver transfers heat to a supercritical CO2 (sCO2) flow loop that serpentines through rocks or fire bricks that enable low-cost 24/7 operation. This thermal storage supplies high quality heat (T > 1000 °C and up to 1500 °C) able to drive an efficient supercritical CO2 power cycle or other thermal or chemical processes.
