Intermediate Temperature SOFC coupled gasifier

Fuel cell
Fuel cell is an energy conversion device (chemical energy is converted to electrical energy) that utilises a gaseous fuel and oxidising gas to produce electricity and heat, having less or no other emissions, when compared with other power generation technologies. It consists of three components (anode, cathode and electrolyte) and depending on type, can operate at a wide range of temperatures with relatively high electric efficiencies. Fuel cells are currently manufactured by a number of fabrication techniques, such as dry pressing, tape casting, screen printing, slurry coating, depending on the type of fuel cell.
Fuel cells are used also in many applications, either stationary (power generation) or traction. Currently, there are six major types of fuel cells that are developed. Among these, the Alkaline and Polymer Electrolyte fuel cells are operated at low temperature, the Phosphoric Acid at intermediate, while Molten Carbonate and Solid Oxide fuel cells are mainly high temperature fuel cells. Demonstration activities all over the world are trying to bring the manufacturing and commercialisation closer to reality.
Solid Oxide Fuel Cells
Interest in Solid Oxide Fuel Cells (SOFCs) capability to operate at intermediate temperature range had led scientists and engineers to focus the research and Development (R&D) efforts on the design and fabrication techniques. The capability to fabricate such fuel cells having thin electrode structures has been demonstrated by a number of groups worldwide. Additionally to this “Thin Film Technology”, material characteristics, especially solid-state ionic and proton conduction at low temperatures, has created a new research field that has attracted attention and interest in recent years.
Of the various types of FCs, the SOFC is the most demanding from a materials point of view. However, because it operates at relatively high temperature, it offers the significant advantage of simple fuel pre-treatment. This advantage creates opportunities for SOFCs where natural gas, biomass, diesel, military fuels, and gasoline are the abundant fuels. Applications where SOFCs may find dominant positions include distributed power, and, military transport applications, heat generation for the home and auxiliary power units. For various applications, the technology must reach a reliable level sufficient to allow the plants to operate unattended.
Operating constraints
SOFC’s must operate at high temperatures to enable diffusion of oxygen ions through the electrolyte made possible by reason of oxygen vacancies in the electrolyte crystalline structure. With conventional designs the anode is a composite of nickel and yttria-stabilised zirconia (YSZ). This composite is an electronic conductor (due to nickel) and also an ionic conductor (due to YSZ). Nickel, however, catalyses the formation of graphite from hydrocarbons, except for a narrow range of operating temperatures and only for methane, thus carbon formation with nickel based anodes is unavoidable for the wider range of hydrocarbon fuels available. Research reports suggest that anodes made from a composite of copper and ceria, or samaria-doped ceria, may remove this barrier in the future.
Cell geometry and construction
Currently, R&D is also focused on the fabrication of fuel cell units with different geometry, depending mainly on different specific requirements. Basically, three different designs are under development, which differ only in cell geometry.
• Tubular Design
• Planar Design
• Monolithic Design
Cell is the repetitive electrochemical building block that is connected either in series or in parallel, forming the “stack” or the “unit” of fabrication. The basic SOFC cell consists of the following common parts:
• The Anode
• The Electrolyte
• The Cathode
• The Interconnect (bipolar) plate
• The support tube (only in tubular design)
Intermediate Temperature SOFC
When the SOFC operates at intermediate temperature range (below 700 °C), some of the problems raised from high temperature operation can be overcome. Such problems include material high cost and efficiency losses. Additionally, several changes need to be made to cell and stack design, cell materials, reformer design and operation, and operating conditions in order to operate at intermediate temperatures. On the other hand, low temperature operation brings additional benefits, which include:
• Low cost metallic materials, such as ferritic stainless steels can be used as interconnect and construction materials. This makes both the stack and balance of plant cheaper and more robust
• More rapid start up and shut down procedures
• Corrosion rates are significantly reduced
Biomass Integrated Gasification Fuel Cell Systems
The combination of biomass gasification with a Fuel Cell Systems such as SOFCs is a highly promising approach to exploit the potential of biomass in combined heat and power generation.
In a first step the solid biomass is converted to a combustible gas mixture. The composition of the gas mixture depends on the employed reactor type, gasification agent, feedstock and operating conditions of the gasification process. It consists to a major extent of hydrogen and carbon monoxide, the rest being carbon dioxide, methane, other hydrocarbon species, water, diverse impurities (e.g. tars, alkali salts, sulfur, soot particles etc.) and nitrogen in case of air as gasification agent. The impurities are potentially performance degrading and have to be removed to some extent in order to meet the requirements of the employed fuel cell. The requirements depend on the specific fuel cell (FC) type and its design, catalyst materials and the operating conditions. The strong interactions between the composition of the gas mixture obtained from the gasification process and the fuel cell entail that optimal system integration is crucial for overall energy efficient and cost effective system.