Lukas Buelens

Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier’s principle

Upgrading CO2 with methane The use of carbon dioxide as a reactant could help to mitigate its impact on climate, but it is difficult to activate as an oxidant. Buelens et al. combined methane in a high-temperature “super-dry” reforming process that generates reactive carbon monoxide. Both molecules were fed into a reactor containing a nickel methane-reforming catalyst, an iron oxide solid oxygen carrier, and calcium oxide as a CO2 sorbent. The adsorbed CO2 was treated with an inert gas purge that shifted the equilibrium, releasing mainly CO. This isothermal process avoids carbon buildup and can be used with biogas methane that contains substantial levels of CO2. Science, this issue p. 449 A two-step isothermal process converts carbon dioxide and methane into carbon monoxide for chemical and fuel production Efficient CO2 transformation from a waste product to a carbon source for chemicals and fuels will require reaction conditions that effect its reduction. We developed a “super-dry” CH4 reforming reaction for enhanced CO production from CH4 and CO2. We used Ni/MgAl2O4 as a CH4-reforming catalyst, Fe2O3/MgAl2O4 as a solid oxygen carrier, and CaO/Al2O3 as a CO2 sorbent. The isothermal coupling of these three different processes resulted in higher CO production as compared with that of conventional dry reforming, by avoiding back reactions with water. The reduction of iron oxide was intensified through CH4 conversion to syngas over Ni and CO2 extraction and storage as CaCO3. CO2 is then used for iron reoxidation and CO production, exploiting equilibrium shifts effected with inert gas sweeping (Le Chatelier’s principle). Super-dry reforming uses up to three CO2 molecules per CH4 and offers a high CO space-time yield of 7.5 millimole CO per second per kilogram of iron at 1023 kelvin.

Mg–Fe–Al–O for advanced CO2 to CO conversion: carbon monoxide yield vs. oxygen storage capacity

A detailed study of new oxygen carrier materials, Mg–Fe–Al–O, with various loadings of iron oxide (10–100 wt% Fe2O3) is carried out in order to investigate the relationship between material transformation, stability and CO yield from CO2 conversion. In situ XRD during H2-TPR, CO2-TPO and isothermal chemical looping cycles as well as Mossbauer spectroscopy are employed. All samples show the formation of a spinel phase, MgFeAlOx. High loadings of iron oxide (50–90 wt%) lead to both spinel and Fe2O3 phases and show deactivation in cycling as a result of Fe2O3 particle sintering. During the reduction, reoxidation and cycling of the spinel MgFeAlOx phase, only limited sintering occurs. This is evidenced by the stable spinel crystallite sizes (∼15–20 nm) during isothermal cycling. The reduction of MgFe3+AlOx starts at 400 °C and proceeds via partial reduction to MgFe2+AlOx. Prolonged cycling and higher temperatures (>750 °C) lead to deeper reduction and segregation of Fe from the spinel structure. Very high stability and CO yield from CO2 conversion are found in Mg–Fe–Al–O materials with 10 wt% Fe2O3, i.e. the lowest oxygen storage capacity among the tested samples. Compared to 10 wt% Fe2O3 supported on Al2O3 or MgO, the CO yield of the 10 wt% Fe2O3–MgFeAlOx spinel is ten times higher.

Upgrading the value of anaerobic digestion via chemical production from grid injected biomethane

Anaerobic digestion can already at small scale effectively convert (waste) biomass to biogas. This biogas is typically combusted to generate electricity and heat, which is incentivized by regulatory support schemes. Because biogas can also be upgraded to biomethane and subsequently injected into the gas grid, the anaerobic digester can be considered as a means to connect decentralized biomass production to a centralized gas grid. We currently estimate the level of required government support to realize a profitable investment in Europe at 20–50 € MWhe−1 for the valorization of an average biogas in a combined heat and power unit, and at 15–25 € MWh−1 for the production of pipeline-quality biomethane, typically used as fuel. Here we explore, both technically and economically, an alternative scenario where biogas is upgraded to biomethane, injected into the existing gas grid, and used elsewhere to produce CO, syngas or H2. The super-dry reforming of CH4, a chemical looping approach using up to three CO2 molecules per CH4, allows an intensified production of CO as a feedstock for synthesis of platform chemicals and fuels through CO2 utilization. Even without subsidies, at present values and costs, this creates an economically positive case which can promote anaerobic digestion as an important driver for a new bio-industry. This approach avoids biomass transportation, in contrast with present biorefineries, while effectively valorizing decentralized biomass feedstocks such as agricultural waste or energy crops.