SOFC-VPSA System for Power Generation with CO2 Separation
Authors: Wei Xinyi , Sharma Shivom , Marechal Francois , Van herle Jan 
Published: September 2023

Solid Oxide Fuel Cell (SOFC) has attracted huge scientific attentions lately, as it is a promising power production technology that can reduce user’s dependency on electricity-grid. SOFC system can generate electricity by using liquid or gaseous fuels. Small volume and external fuel storage make SOFC technology more compatible, and it can easily be adjusted for different industry power plant scales. Moreover, SOFC operates at high temperature (700 0C), and it cogenerates high-quality heat or steam, which can be used as a heat source within the system. As maximum fuel utilization for SOFC is around 85%, a burner is required for the combustion of unconverted fuels. The burner provides additional heat to the SOFC system, at the same time, it generates more CO2.

In order to meet “net zero” emission target for greenhouse gases in the year 2050, it is essential to introduce Carbon Capture and Storage (CCS) technology that can retrieve most of the produced CO2 from emission intensive activities and store it permanently in nature (e.g, sequestration and mineralization), leading to an almost carbon neutral activity. The stored CO2 can also be used in methanation process, to convert it into green methane. By integrating an existing CCS technology with SOFC, and using biofuels as energy source, SOFC system can be considered as a carbon negative technology. The most common and widely used CCS technology is Chemical Absorption (CA). The CA process presents good retrofitting options. However, corrosion and degradation are major issues.

The high efficiency of SOFC system is however penalised by the fact that the flue gases contain typically a mix of CO2 and N2, which makes it difficult and expensive to separate/capture the produced CO2. There is a possibility of directly injecting pure oxygen to the burner. For industrial scale, cryogenic distillation is commonly used air separation technology for producing O2. Cryogenic distillation is an energy intensive technology, and it is not suitable for small scale O2 production. In the recent times, other technologies such as Pressure Swing Adsorption (PSA) and membrane have evolved for O2 production from air. The non-cryogenic technologies for air separation are preferred to their lower operating cost and easier integration with other processes. PSA is usually suited for medium-range production capacity, and it can produce O2 with 94 % purity. However, it is still necessary to improve performance by reducing energy consumption. The energy consumption is 3.211 MJ/kg-O2 for PSA to produce oxygen. In the oxygen production process, energy consumption accounts for more than 90% of the operating cost.

This study considers integration of a SOFC system with PSA, which consists of a SOFC stack, balance of plant components for SOFC system, PSA bed and auxiliary components for PSA. The PSA produces O2 from air, that is injected to the combustion chamber (or burner). In the integrated system, the energy needed for the PSA directly comes from SOFC system. The right amount of oxygen is used to complete the oxidation of the unconverted fuel from the anodic side of SOFC, and CO2 is separated automatically after water condensation. Figure 1 shows layout of integrated system. The performance of PSA is explored for different materials, different pressure ratio and temperature, to achieve minimum energy consumption. Moreover, heat integration has been studied for SOFC system, the integrated system has waste heat available, especially at the downstream of the burner. In order to valorize the waste heat of the system, a steam cycle has been integrated for producing extra amount of electricity, to improve the overall efficiency of the system. Finally, the performance of integrated system has been optimized for maximization of electrical efficiency and minimization of exergy distraction, via multi-objective optimization.

In this study, the net electricity output from SOFC-PSA system is around 9 kW. The SOFC produces 9.8 kW electricity, and about 8% electricity is consumed by the PSA for producing O2 that is required for the combustion of unconverted fuel. The integrated system has an overall efficiency of more than 55%. A steam cycle has been integrated for producing extra amount of electricity, and it produces additional 2.5 kW of electricity. Hence, the overall efficiency of the integrated system reaches above 70% with automatic CO2 separation.

Figure 1