Direct Biogas Reforming to Turquoise H2 and Carbon Material by Microwave Heated Catalytic Fluidized Bed Reactor

Dry reforming of biogas (a mixture of mainly CH4 and CO2) is seen as a sustainable way to produce decarbonized H2 which is accompanied by CO2 emission, yet. On the other hand, CH4 catalytic cracking can produce CO2-free hydrogen as the carbon is stored in solid form[1]. If biogas is the feedstock, prior CO2/CH4 separation from biogas would be required to produce bio-methane. Here, we investigate direct catalytic biogas reforming as a negative carbon emission technology, as the carbon source comes from biomass and the carbon is captured in a solid form, thereby preventing greenhouse gas emissions.

This approach faces several challenges, (i) the reaction is highly heat transport limited due to the endothermicity of the cracking and dry reforming reactions, (ii) catalyst stability at high temperature (T>900°C), (iii) the Boudouard reaction which can convert the solid carbon to CO. While methane conversion with stoichiometric or excess of CO2 (Dry Reforming) is well known[2], direct methane conversion in the absence of CO2 (Cracking) has been little addressed.

The objective of this work is to investigate the direct catalytic biogas reforming by microwave (MW) heating in a fluidized bed reactor in order to overcome heat transfer limitations and achieve high CH4 conversion [3],[4]. The challenge is to obtain a homogeneous and controlled temperature in the bed while the catalyst particles are moving and also producing carbon particles that also absorb MW. Methods The fluidized catalytic bed consists of a quartz tube filled with specific iron-based catalyst powder.

The synthetic biogas is a mixture of CH4:CO2:N2 in a ratio of 2:1:1. The MW heating device consists of a 750 W solid state generator operating at 915 MHz (Leanfa, Italy) and a standard WR975 waveguide and monomode cavity (Muegge, Germany). The reactor temperature is monitored by two devices: a pyrometer (Optris® / CTratio 2M) and an IR camera (Optris® PI 1M). The pyrometer is used to control the temperature and the IR camera is used to visualise the fluidisation and to check the temperature of the different zones of the reactor (Fig. 1). Both the camera and the pyrometer are connected to a digital interface that allows the signal to be recorded. The temperature of the two sources is very similar (±5°C at 900°C) and the fluidization temperature is homogeneous throughout the reactor. Upon MW heating, an iron-based catalyst achieving high CH4 and CO2 conversions and H2 yield. Conversion results are similar to those obtained in a fluidized bed heated by an external electric furnace (Fig. 2) which indicate that the bed temperatures are similar for MW and external heating. This catalyst showed stability over several hours on stream.

A high reaction temperature is required to achieve high conversion and also carbon formation. Below 850 °C, the carbon balance is above 100%, indicating that the carbon previously formed on the catalyst is gasified to CO by the Boudouard reaction (C(s) + CO2 → 2CO ΔH° = 172.3 kJ mol-1). This indicates that carbon formation from the methane cracking reaction (CH4 → C(s) + 2H2 ΔH° = 74.9 kJ mol-1) no longer compensates for carbon gasification from the Boudouard reaction in this temperature range.

In contrast, at higher temperatures (> 850°C), the carbon balance is less than 100%, indicating that solid carbon is produced and that the methane cracking reaction is faster than the Boudouard reaction. The reactor effluent at 950°C is composed of H2 and CO, with an H2/CO ratio close to 2, which could be used directly in the Fischer-Tropsh reaction for hydrocarbon synthesis directly from biogas. Catalytic biogas reforming in a fluidized bed reactor can achieve very high methane conversion into hydrogen and solid carbon by MW heating. These results are similar to the conventional heating and confirm good temperature control and homogeneous heating in the reactor. Equally important, the output stream is a mixture of H2 and CO in a ratio of 2:1, an ideal ratio for Fisher-Tropsch synthesis. This work opens up the prospect of converting biogas into liquid hydrocarbons in two catalytic steps without any gas separation (e.g., CH4/CO2) or recycling (CH4/H2).

References [1] Sánchez-Bastardo, N., Schlögl, R., Ruland, H. Ind. Eng. Chem. Res. 2021, 60, 11855–11881. [2] Lim, M. S., Chun, Y. N. Energy and Fuels, 2017, 31, 13761–13768. [3] Keller, M., Matsumura, A. & Sharma, A. Chem. Eng. J., 2020, 398, 125612. [4] Dadsetan, M., Khan, M. F., Salakhi, M., Bobicki, E. R., Thomson, M. J. Int. J. Hydrogen Energy, 2023, 48, 14565–14576.

Project Date:

• 24.09.2024

Link:

• https://hal.science/hal-04707534/