The need for increased fuel flexibility and greater resistance to impurities was a motivation for developing high temperature MCFC and SOFC systems [1]. Solid Oxide Fuel Cells (SOFC) operate at high temperatures and due to their robustness they offer the best opportunity for thermal integration with biomass gasification systems assisting in the reforming of hydrocarbon fuels, and to produce steam for thermal electric generation or other thermal load [2], [3].
High temperature fuel cells like molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) seem to be most promising for biomass-based fuel cell applications, since they allow for CO in the fuel gas stream as well as for internal reforming of hydrocarbons. This internal reforming might cause carbon deposits at the anode [4]. Investigation of the mechanism and preventing carbon formation caused by internal reforming of hydrocarbons is a major focus of recent SOFC research activities worldwide. Especially aromatic hydrocarbons and tars favour the development of carbon deposits. The carbon formation depends on the operating conditions like temperature, steam content and tar composition [5]. The allowable tar content in the fuel gas is one of the key questions for upcoming fuel cell concepts with integrated biomass gasification.
The high operating temperatures of SOFC and MCFCs translate into a greater tolerance for contaminants relative to other fuel cell technologies. SOFC systems offer fuel flexibility but require very stringent acid gas removal because they can only tolerate about 1 ppm H₂S and about 1 ppm halides in the fuel gases. Ammonia can be tolerated up to approximately 0.5 vol%. Other fuel cell concepts, above all the PEM fuel cell, make extremely high demands against the purity of the gaseous fuels, and carbon monoxide, hydrocarbons, dust and tars must be separated restrictively before [6].
The contaminant levels of biomass gasification product gas stream are highly dependent upon the input feedstock composition and the gasifier operating conditions. Nevertheless so far the existing knowledge remains a lot in a theoretical study of the gasification and SOFC coupling.
To date, fuel cells have not been demonstrated on biomass or even coal gasification product gases.
Several technoeconomic analyses of integrated biomass gasification fuel cell systems have been published [7]. PAFC and PEFC systems could also be integrated with gasification systems, however, the fuel processing would be significantly more complex and greater penalties in loss of efficiency will be incurred due to a worse heat balance between gasification and fuel cells.
High temperature fuel cells like molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) allow internal reforming of hydrocarbons and tars. This internal reforming might cause carbon deposits at the anode. Investigation of the mechanism and preventing carbon formation caused by internal reforming of methane is a major focus of recent SOFC research activities worldwide. Especially aromatic hydrocarbons and tars favor the development of carbon deposits. The carbon formation depends on the operating conditions like temperature, steam content and tar composition. The allowable tar content in the fuel gas is one of the key questions for upcoming fuel cell concepts with integrated biomass gasification.
SOFC fuel cells are already in operation with natural gas for several thousand hours. A demonstration plant of participant 10 has a rated power output of 110 kW at an electrical net efficiency of 46 %. The recent work is based on the use of natural gas. But also in this case the internal reaction at the anode takes place as oxidation of hydrogen and CO. Therefore the use of Biogas is no fundamental problem.
Anyhow the plant concept has to be adapted, at least because the cooling of the system by the endothermic reforming reaction is missing.
Most effort has to be put on the gas cleaning, because all catalytic processes are strongly affected by impurities like hydrogen-sulfur. A great importance comes to the selection of appropriate electrode materials. Common fuel cell concepts use nickel as electrode material due to its favourable electrical properties. But Nickel intensify carbon formation due to its catalytic effect on thermal cracking of hydrocarbons. A special meaning comes to the deposition of Nickel Sulphide at the anode if the fuel gas contains sulfur [8].