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.
