One of the biggest challenges that the world faces is climate change. According to the IPCC report 2022, the average temperature is set to reach 1.5 degrees Celsius above pre-defined levels, and this demands immediate attention to cut down on fossil fuels and shift towards greener solutions [1]. There are several candidates for alternative energy sources. Biogas, derived from organic matter such as animal or food waste, is one of the promising routes toward a greener future [2].

Studies have shown that the major setback in biogas utilization is its high carbon dioxide content. This drawback, consequently, results in lower energy efficiency and reduced conversion rates. Conventional pathways to upgrade biogas include carbon dioxide removal to produce methane or reforming the biogas to produce syngas via dry reforming or tri reforming process [3]. However, both these processes have high energy requirements, rendering the use of biogas ineffective. Hence, the whole process becomes highly energy intensive. The need of the hour is to design a carbon-neutral and highly efficient process.

This study proposes a chemical looping single reactor system for the conversion of biogas to liquid fuels using a modified Calcium-Iron based oxygen carrier. Biogas contains methane, carbon dioxide, nitrogen, and trace amounts of volatile organic compounds. A single reactor system can be divided into two zones: a) Particle reduction zone and b) Particle regeneration zone. Biogas along with the proposed oxygen carrier, i.e., Ca₂Fe₂O_5, is fed from the top of the moving bed reactor and reacts in a co-current fashion. Simultaneously, a stream of steam and carbon dioxide from the bottom reacts with reduced oxygen carrier particles. The product, which is a high-purity syngas, is removed from the middle of the reactor system. The regenerated material is fed back to the reactor through a riser. The selection of oxygen carriers accounts for the higher yields of syngas.

This process is further simulated in Aspen Plus to study the effect of temperature, pressure, gas composition, and oxygen carrier flow rate. Simulations have shown that the oxygen carrier flowrate influences the syngas purity, solids conversion as well as H₂/CO ratio. The highest syngas purity was observed in the feed ratio of 80:20 CH₄ and CO₂ mixture, whereas the solids conversion rate decreases with increasing flowrate. Moreover, the single reactor system can be operated under autothermal conditions with an oxy-combustion burner compensating for the endothermic heat. This has been confirmed through Aspen simulations.

This newly proposed system utilizing a single reactor has demonstrated impressive capability in producing high purity syngas, despite significant variations in the methane to carbon dioxide ratio. This system was compared with the traditional tri-reforming process for carbon dioxide utilization and syngas production for liquid fuels. The results revealed a remarkable improvement of 15-20% in carbon dioxide utilization using the single reactor system as compared to the tri-reforming process.

To further improve the oxygen carrier, Studies were conducted using Density Functional Theory (DFT) to find a suitable dopant with a strong affinity toward regeneration kinetics. Cobalt was found to increase the reactivity of carbon dioxide in oxidation, allowing the oxygen carrier to be regenerated entirely in CO₂/H₂O in the single reactor system [4]. Samples with different Cobalt compositions were prepared and tested in a Thermogravimetric analyzer for their reactivity and recyclability, aligning well with XRD and SEM analysis on the doped oxygen carrier.

References:

[1]D. R. M. T. E. P. K. M. A. A. M. C. S. L. H.-O. Pörtner, "IPCC, 2022: Climate Change 2022: Impacts, Adaptation and Vulnerability," Cambridge University Press. Cambridge University Press, Cambridge, UK and New York, NY, USA,, New York, NY, USA,, 2022.

[2]A. N. L. S. V. S. I. S. Vladimir Arutyunov, "Utilization of renewable sources of biogas for small-scale production of liquid fuels," Catalysis Today, 2020.

[3]B. J. S. XianhuiZhao, "Biogas Reforming to Syngas: A Review," iScience, 2020.

[4]Z. C. D. S. B. J. A. F. L.-S. F. Vedant Shah, "Highly Selective Production of Syngas from Chemical Looping Reforming of Methane with CO2 utilization on MgO- Supported Calcium Ferrite Redox material," Applied Energy, 2020.