Analysis of an Integrated Downdraft Gasifier/SOFC System Biomass Fueled

Abstract

The connection between combined power and heat generation, and the concept of distributed generation is a relevant issue viz. improvement of the overall efficiency of energy conversion and distribution. In this context, the use of biomass may be of particular interest due to its potentially very low CO2 emissions as well as low kWh production cost with respect to other renewable energy sources. Traditional methods to convert biomass involve micro-turbines or steam power plants. However, both electrical and overall efficiencies may not be particularly high for both technological and thermodynamic reasons; the latter being hard to supersede. High temperature Solid Oxide Fuel Cells (SOFCs) offer an attractive solution because they are characterized by electrical efficiencies in the range of 35% to 48% [1], and present a significant potential for integrating with the biomass gasification process; thus, exploiting advantages of high operating temperatures. Moreover, thermal integration is of prime importance. In fact, optimal management of thermal integration may allow operation with high moisture content feedstocks and with high water concentration in the SOFC; thus, promoting further hydrogen production via the water gas shift reaction [2]. In this paper, an integrated solid oxide fuel cell-gasifier system is modeled to identify the main effects on fuel cell performance under several operating conditions in terms of biomass flow rates and moisture contents. The SOFC and the gasifier are modeled by a zero dimensional approach to perform a sensitivity analysis with a numerical tool which requires less computational effort compared to those previously presented. The thermochemical phenomena are taken into account with a high degree of detail, and model validation is also showed via comparison with literature available data. Results highlight the impact of biomass feedstock and its moisture content on the electrical current density value. Final performance depends on the interaction among gasifier thermal sustainability, SOFC operating temperature and chemical equilibrium concentrations, via water gas shift reactions. In particular, it has been shown that thermal integration allows operation with higher moisture content biomass, thereby further exploiting H2O conversion into H2 via water gas shift reactions and high fuel utilization operating conditions. The importance of thermal integration and advanced control of SOFC based biomass fueled systems is demonstrated by this study, as well as the use of a fast and reliable zero-dimensional modeling for system design and control purpose.

Biomass fueling of a SOFC by integrated gasifier: Study of the effect of operating conditions on system performance

“…2) Higher MCs allow the gasifier to produce syngas characterized by higher H 2 /CO ratio further giving much more favorable water gas shift behavior into the SOFC [19,20]. This is key to both have the SOFC operating much closer to pure H 2 fueling operating conditions, as well as having much lower off-gas waste.…”

“…The SOFC module, that is based as mentioned on a 0-D approach, is capable of representing a planar stack under steady state operating conditions [19,20]. The main assumptions are that the cell is isothermal, H 2 is the only gaseous species participating to electrochemical reactions, and the SOFC H 2 fuel utilization is a constant input parameter.…”

“…Among the others, thermal integration of the different components is a key aspect in the development of sustainable solutions as it potentially allows for a considerable increase of electric efficiency h el [19,20]. Depending on the design, gasification may be autothermal or allothermal whereas the SOFC may be operated under different fuel utilization conditions.…”