Following the global trend towards increased energy demand together with requirements for low greenhouse gas emissions, Adaptable Reactors for Resource- and Energy-Efficient Methane Valorisation (ADREM) focused on the development of modular reactors that can upgrade methane-rich sources to chemicals. Herein we summarise the main findings of the project, excluding in-depth technical analysis. The ADREM reactors include microwave technology for conversion of methane to benzene, toluene and xylenes (BTX) and ethylene; plasma for methane to ethylene; plasma dry methane reforming to syngas; and the gas solid vortex reactor (GSVR) for methane to ethylene. Two of the reactors (microwave to BTX and plasma to ethylene) have been tested at technology readiness level 5 (TRL 5). Compared to flaring, all the concepts have a clear environmental benefit, reducing significantly the direct carbon dioxide emissions. Their energy efficiency is still relatively low compared to conventional processes, and the costly and energy-demanding downstream processing should be replaced by scalable energy efficient alternatives. However, considering the changing market conditions with electrification becoming more relevant and the growing need to decrease greenhouse gas emissions, the ADREM technologies, utilising mostly electricity to achieve methane conversion, are promising candidates in the field of gas monetisation.

In this work, we report on air/N₂ gasification of a byproduct stream from an industrial fermenter in a tubular microwave plasma reactor to investigate the feasibility of the technology for organic compounds valorization, given the limited number of relevant works in the literature. In this context, an operating window regarding air/N₂/biomass flow rates and power input has been identified to enable stable and efficient operation. Up to 89% carbon conversion efficiency and 41% cold gas efficiency have been attained with syngas product composition H₂:CO:CO₂ = 41:53:6 on molar basis, fairly close to the calculated equilibrium composition values in the temperature range 973 K to 2173 K.

Non-thermal microwave plasma reactors can efficiently split the CO₂ molecule. However, big challenges remain before this technology can become a feasible industrial technology. Computer modelling can be very useful to tackle such challenges. Detailed kinetic modelling is commonly used to gain insights into the complex vibrational kinetics of CO₂, as vibrational excitation is strongly related to the energy efficiency in the dissociation process. The vibrational-to-translational temperature ratio has been identified as a key variable to achieve high energy efficiencies. This ratio has also been used to simplify detailed CO₂ vibrational kinetics, notably reducing the number of species and reactions required to model the non-thermal plasma. In this paper we use an isothermal reaction kinetics model to study the vibrational kinetics of CO₂ under the typical conditions used in non-thermal microwave plasma experiments. The importance of the different collisional processes is evaluated with respect to the different conditions and timescales at which CO₂ dissociation takes place. The long timescale behavior of the vibrational-to-translational temperature ratio under different conditions is discussed in detail. It is shown that the behavior at increasing gas temperatures can be fitted to an expression that incorporates the Landau-Teller temperature dependence. This is confirmed by the average adjusted R-square values higher than 0.99 and the average root mean square error values smaller than 0.22 at low gas temperatures. The limitations of the fitting expression are also discussed, especially the conditions and timescales at which it yields better results.

A series of ruthenium-doped strontium titanate (SrTiO₃) perovskite catalysts were synthesized by conventional and microwave-assisted hydrothermal methods. The structure was analyzed by X-Ray diffraction (XRD) confirming the formation of the perovskite phase with some TiO₂ anatase phase in all the catalysts. Microwave irradiation decreases the temperature and time of synthesis from 220 °C for 24 h (conventional heating) to 180 °C for 1h, without affecting the formation of perovskite. A 7 wt. % ruthenium-doped SrTiO₃ catalyst showed the best dielectric properties, and thus its catalytic activity was evaluated for the methane dry reforming reaction under microwave heating in a custom fixed-bed quartz reactor. Microwave power, CH₄:CO₂ vol. % feed ratio and gas hourly space velocity (GHSV) were varied in order to determine the best conditions for performing dry reforming with high reactants conversions and H₂/CO ratio. Stable maximum CH₄ and CO₂ conversions of ∼99.5% and ∼94%, respectively, at H₂/CO ∼0.9 were possible to reach with the 7 wt. % ruthenium-doped SrTiO₃ catalyst exposed to maximum temperatures in the vicinity of 940 °C. A comparative theoretical scale-up study shows significant improvement in H₂ production capability in the case of the perovskite catalyst compared to carbon-based catalysts.

The release of hydrogen from solid hydrides by thermolysis can be improved by nanoconfinement of the hydride in a suitable micro/mesoporous support, but the slow heat transfer by conduction through the support can be a limitation. In this work, a C/SiO₂ mesoporous material has been synthesized and employed as matrix for nanoconfinement of hydrides. The matrix showed high surface area and pore volume (386 m²/g and 1.41 cm³/g), which enabled the confinement of high concentrations of hydride. Furthermore, by modification of the proportion between C and SiO₂, the dielectric properties of the complex could be modified, making it susceptible to microwave heating. As with this heating method the entire sample is heated simultaneously, the heat transfer resistances associated to conduction were eliminated. To demonstrate this possibility, ethane 1,2-diaminoborane (EDAB) was embedded on the C/SiO₂ matrix at concentrations ranging from 11 to 31%wt using a wet impregnation method, and a device appropriate for hydrogen release from this material by application of microwaves was designed with the aid of a numerical simulation. Hydrogen liberation tests by conventional heating and microwaves were compared, showing that by microwave heating hydrogen release can be initiated and stopped in shorter times.

The complexity and challenges in noncontact temperature measurements inside microwave-heated catalytic reactors are presented in this paper. A custom-designed microwave cavity has been used to focus the microwave field on the catalyst and enable monitoring of the temperature field in 2D. A methodology to study the temperature distribution in the catalytic bed by using a thermal camera in combination with a thermocouple for a heterogeneous catalytic reaction (methane dry reforming) under microwave heating has been demonstrated. The effects of various variables that affect the accuracy of temperature recordings are discussed in detail. The necessity of having at least one contact sensor, such as a thermocouple, or some other microwave transparent sensor, is recommended to keep track of the temperature changes occurring in the catalytic bed during the reaction under microwave heating.

Microwave plasma (MWP) technology is currently being used in application fields such as semiconductor and material processing, diamond film deposition and waste remediation. Specific advantages of the technology include the enablement of a high energy density source and a highly reactive medium, operational flexibility, fast response time to inlet variations and low maintenance costs. These aspects make MWP a promising alternative technology to conventional thermal chemical reactors provided that certain technical and operational challenges related to scalability are overcome. Herein, an overview of state-of-the-art applications of MWP in chemical processing is presented (e.g. stripping of photo resist, UV-disinfection, waste gas treatment, plasma reforming, methane coupling to olefins, coal/biomass/waste pyrolysis/gasification and CO₂ conversion). In addition, two potential approaches to tackle scalability limitations are described, namely the development of a single unit microwave generator with high output power (>100 kW), and the coupling of multiple microwave generators with a single reactor chamber. Finally, the fundamental and engineering challenges to enable profitable implementation of the MWP technology at large scale are discussed.

A systematic study of the conventional and microwave (MW) kinetics of an industrially relevant demethylation reaction is presented. In using industrially relevant reaction conditions the dominant influence of the solvent on the MW energy dissipation is avoided. Below the boiling point, the effect of MWs on the activation energy E_a and k₀ is found nonexistent. Interestingly, under reflux conditions, the microwave-heated (MWH) reaction displays very pronounced zero-order kinetics, displaying a much higher reaction rate than observed for the conventionally thermal-heated (CTH) reaction. This is related to a different gas product (methyl bromide, MeBr) removal mechanism, changing from classic nucleation into gaseous bubbles to a facilitated removal through escaping gases/vapors. Additionally, the use of MWs compensates better for the strong heat losses in this reaction, associated with the boiling of HBr/water and the loss of MeBr, than under CTH. Through modeling, MWH was shown to occur inhomogeneously around gas/liquid interfaces, resulting in localized overheating in the very near vicinity of the bubbles, overall increasing the average heating rate in the bubble vicinity vis-à-vis the bulk of the liquid. Based on these observations and findings, a novel continuous reactor concept is proposed in which the escaping MeBr and the generated HBr/water vapors are the main driving forces for circulation. This reactor concept is generic in that it offers a viable and low cost option for the use of very strong acids and the managed removal/quenching of gaseous byproducts.

In the context of converting electricity into value-added chemicals, the reduction of carbon dioxide (CO₂) with hydrogen (H₂) in a surface-wave-induced microwave plasma discharge, so-called surfatron, was investigated. The effect of different input variables such as gas flow rate, feed gas composition ratio (H₂:CO₂) and specific energy input (SEI) on the reactor performance, i.e. the CO₂ conversion and energy efficiency, was assessed. A maximum CO₂ conversion of 85% is obtained when the feed gas mixture ratio (H₂:CO₂) was equal to 3. Moreover, a trade-off between CO₂ conversion and energy efficiency was clearly noticed when varying the supplied microwave power. High SEI resulted in high conversions and low energy efficiencies and vice-versa. Furthermore, the saturation of the carbon monoxide (CO) production was found at high SEI. These results were rationalized by means of a simplified reaction scheme and by optical emission spectroscopy analysis, which showed that the formation of hydrogen (H) and oxygen (O) atoms in the plasma are the dominant channels driving the reaction pathway. We also observed higher electron densities and temperatures at higher H₂ content, which may explain the high conversions achieved in the plasma reactor at high H₂:CO₂ ratios. H₂ is then not only capable of acting as a “catalyst” for CO₂ decomposition but also modifies the plasma properties, which seems to greatly enhance the potential of chemical reactions and thus the dissociation rates.

We have demonstrated that application of simple shear flow and heat in a Couette Cell is a scalable process concept that can induce fibrous structural patterns to a granular mixture of plant proteins at mild process conditions. In particular, a Couette Cell device with 7-L capacity was employed for the production of structured soy-based meat replacers. A reduced factorial experimental design was used to find the optimum process conditions between two relevant process parameters (process time and rotation rate), while the process temperature remained constant at 120 °C. Fibre-structured products with high anisotropy indices were produced. Fibrousness is favoured at 30 ± 5 min and 25 ± 5 RPM. The up-scaled Couette Cell can be operated in higher industrial values and yield 30 mm thick meat replacers, which emulate meat. Besides, the study did not reveal any barriers for further upscaling of this concept. The flexibility in design allows production of meat alternative products with sizes that are currently not available, but could have advantages when aiming at replacement of complete muscular parts of animals, for instance, chicken breast or beef meat.

Gasification technology may combine waste treatment with energy generation. Conventional gasification processes are bulky and inflexible. By using an external energy source, in the form of microwave-generated plasma, equipment size may be reduced and flexibility as regards to the feed composition may be increased. This type of gasification may be combined with fuel cell technology to generate electricity for on-site microwave generation. In this paper, we present short gasification experiments with cellulose, as model biomass compound, in air plasma. In order to optimize reaction rates, gasification and plasma generation are combined in the same volume in order to expose the solids to plasma of maximum intensity. The heating value of the fuel gas yield exceeds, up to 84%, the net microwave energy transmitted into the reactor over a range of operating conditions. As the system has not been optimized, in particular regarding residence time, the results give confidence that this concept can eventually be developed into a viable small-scale decentralized gasification technology.