Syngas-fed SOFCs: Analysis of Performance Sensitivity to Fuel Composition

Solid Oxide Fuel Cells (SOFCs) are a promising technology for the electric power generation, because they are featuring high energy efficiency and fuel flexibility. Thus, unlike other fuel cell technologies (e.g. Proton Exchange Membrane fuel cells), SOFCs can be run on a wide variety of fuel types. In this context, the employment of RES-derived fuel gases offers an even more interesting solution for the development of a sustainable power generation system.

Owing to low energy density associated to RES primary sources, their use is not convenient in great-scale applications. However, SOFC technology does not suffer from the scale effect. For this, it appears competitive with the more mature power generation technologies, especially on small-scales.

SOFC systems are usually equipped with an external reformer interposed between the fuel supply and the SOFC. Yet, small-scale applications cost-effectiveness lies in the reduction of the system complexity. Consequently, the idea is to avoid the reforming section with relevant advantages in terms of simplicity, dimensions and costs.

Many technologies to produce the fuel exist, yielding synthetic gases (syngas) with different compositions. In addition to that, the composition obtained is influenced by some other factors. First of all, especially when RES are the raw materials (e.g. biomass or waste), the variability of the feedstock has to be considered. Moreover, even for a given technology, gas composition can undergo fluctuations throughout the synthesis process. In this frame, the investigation of fuel composition effect on SOFCs performance is of great interest.

In a previous study, the authors successfully built a model for the evaluation of SOFC performance (with a maximum error of 3% in the voltage range from OCV to 0.6V). Data for the model development were taken from the experimental activity carried out feeding the SOFC directly with a wide set of syngas typologies. In particular, fuels derived from co-electrolysis (solid oxide electrolysers combined with wind and solar plants) and several technologies of biomass gasification were considered.

Because of the restricted number of syngas compositions considered, a relevant limitation arose relative to the model applicability range, especially with regard to the fuel methane content (currently < 10%v).

Therefore, the present study aims to broaden the applicability range of the model to fuel mixtures with methane content up to about 50%. This corresponds to consider other syngas production technologies, such as plasma gasification, and to extend the model to the case of biogas feeding.

A wide experimental campaign has been carried out on SOFC single cells, through the determination of polarization curves and EIS spectra. Each test refers to a particular fuel mixture, which is simulated with technical gases to recall compositions that are likely to occur in real systems.

In particular, results from both polarization curves (in terms of OCV and ASR) and EIS analysis are correlated to the fuel features, such as the dilution factor and the CO₂ content.

All the information collected along the experimentation is used to improve the black-box numerical tool already developed, in order to forecast SOFC behaviour in a wider array of feeding conditions.