Waste2Watts: Techno-Economic Feasibility of Biogas-Fed SOFC Power System Integrated with Biogas Cleaning Unit and Carbon Capture Technologies
Authors: Yu Hangyu , Wang Ligang , Van herle Jan , Pina Eduardo Antonio 
Published: September 2023

As a promising power generation technology, the solid oxide fuel cell (SOFC) system demonstrates high electrical efficiency and overall efficiency in cogeneration mode. Operating at high temperatures (> 680 ˚C), SOFCs cogenerate high-quality heat and can be integrated with different industry facilities. Furthermore, SOFC technology demonstrates high fuel tolerance, including hydrogen, natural gas, ammonia, LPG, etc. Unlike other fuel cell technologies, SOFC systems can also tolerate impurities in the fuel, e.g., CO2, N2 or even a small amount of O2. Besides, the compact system configuration makes SOFC system of great potential in the decentralized application.

In the context of the Paris Agreement, more and more solutions to replace fossil fuels have been proposed, where biogas is considered as one promising alternative. Biogas generally contains several impurities, making its application limited in the fuel cell technology field. Removing all the impurities and making it compatible for all the fuel cell technologies would be expensive. Whereas, since SOFC has high tolerance in fuel impurities, keeping certain harmful impurities under safe thresholds with a biogas cleaning unit and injecting the purified biogas into the SOFC system will be less costly. In this case, a biogas-fed SOFC system integrated with a biogas cleaning unit can be considered as a promising renewable technology by avoiding fossil fuel use and converting biowaste into electricity and heat.

Although biogas is produced from a renewable source, its use in SOFCs still generates CO2 emissions when it is fed with biogas. To realize carbon neutrality or even being carbon negative in a biogas-fed SOFC system, carbon capture (CC) technology is introduced to meet the net-zero carbon emission target in 2050. One conventional carbon capture storage (CCS) technology is the calcium looping system, which uses CaO/CaCO3 in a carbonation-calcination loop to separate and capture the CO2 from the exhaust gas. This type of technology is quite mature for large-scale applications and has been widely used in thermal power plants for the desulfurization and denitrification of exhaust. Novel technology based on sodium carbonate solution is capable of not only carbon capture, but also biogas cleaning. For the carbon capture utilization (CCU) technology, the Fischer-Tropsch (FT) synthesis can convert CO2 with hydrogen into syngas via electrochemical or thermochemical catalytically driven processes.

Focusing on the decentralized power system, small-scale biogas-fed SOFC system integrated with a biogas cleaning unit and CC system should be considered with capacity ranging from 20 kW to 200 kW. As a cogeneration system, the integration should not be limited to mass exchange. The electricity consumed by the biogas cleaning unit and CC system can be fulfilled via the SOFC system. Based on the system temperature pattern, a bigger heat exchange network can be formulated between three parts, which can avoid extra heat input. In this case, the optimality of overall system efficiency and revenue can be achieved.

In this study, one biogas-fed SOFC power system is proposed with capacity ranging from 20 kW to 200 kW. The techno-economic evaluation is carried out in two cases, with or without heat and electrical integration. Meanwhile, in different cases, the techno-economic analysis of different biogas cleaning technologies and CC technologies will be performed as well. Two operation modes of the SOFC system are considered: (1) hot recirculation mixed before the reformer, (2) cold recirculation mixed before the reformer. Without the biogas cleaning unit and the CC technology, the biogas-fed SOFC system's electrical efficiency can exceed 65% as is demonstrated in Figure 1. Furthermore, Detailed techno-economic analysis has been carried out by three indicators: capital expenditure (CAPEX), operating expenditure (OPEX) and levelized cost of electricity (LCOE). Integrated with biogas cleaning unit via only mass exchange, the LCOE of the biogas-fed system without CC technology ranges from around 0.11 EUR/kWh (20 kW) to 0.085 EUR/kWh (200 kW). The cost breakdown is shown in Figure 2(a). Based on the SOFC system techno-economic analysis, different CC technologies are added and compared. When the conventional CC technologies like Calcium looping (CaL) and Fischer-Tropsch (FT) synthesis are scaled down to 20-200 kW and applied, unaffordable LCOE prices (0.65 EUR/kWh for CaL and 5 EUR/kWh for FT) indicate that these large-scale conventional technologies are not feasible for our cases. However, a novel technology based on Sodium carbonate looping with 97% CC efficiency is rather promising and shows much lower cost. As is indicated in Figure 2(b), the LCOE cost of the whole system is only around 0.151 EUR/kWh at 200 kW system capacity, indicating that it is a techno-economically feasible solution for a carbon-neutral biogas-fed SOFC system.

Figure 1