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.

Updraft gasifiers

Gasification
Gasification is a widely studied and applied technology to produce a mixture of combustible gases. It consists of several sequential processes which include: drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and, to a certain extent, oxidation of pyrolysis gases and gasification of char. Together with chemical processes and evaporation of moisture, transport phenomena also take place.
Applications
Typical applications might include using producer gas as a substitute for petroleum fuels in the standard gasoline or diesel engines that are commonly used in developing countries for electrical power production and water pumping or in local industries such as sawmills, rice mills and workshops. In addition, the gas can be used in standard heat appliances such as crop dryers and cement, lime, or brick kilns.
Fixed bed reactors are used in small-scale gasification while large biomass gasifiers are usually of the fluidized-bed or entrained –flow type. Fixed bed, updraft and downdraft reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverization.
Principle
In the updraft gasifier the downward-moving biomass is first dried by the up flowing hot product gas. After drying, the solid fuel is pyrolysed, giving char which continues to move down to be gasified, and pyrolysis vapours which are carried upward by the up flowing hot product gas. The tars in the vapour either condense on the cool descending fuel or are carried out of the reactor with the product gas, contributing to its high tar content. The product gas from an updraft gasifier thus contains a significant proportion of tars and hydrocarbons, which contribute to its high heating value. Usually the gases are directly used in a closely coupled furnace or boiler.
Gas quality
The fuel gas requires substantial cleanup if further processing is to be performed. There is interest in the cleaning of the updraft gas for electricity production, as low temperature tars are more reactive and thus easier to be removed, than the high-temperature tars produced in much lower amounts by downdraft and fluidized bed gasifiers.
Advantages
The principal advantages of updraft gasifiers are their simple construction and high thermal efficiency. The sensible heat of the gas produced is recovered by direct heat exchange with entering feed, which thus is dried, preheated and pyrolysed before entering the gasification zone. Updraft gasifiers can be used in the sizes between 2 and 20 MWe.

Torrefaction of biomass

Fuel wood is often difficult to use because of its poor combustion characteristics such as low heating value, variable moisture content which is often high, hydroscopic nature, smoking during combustion, etc. For a number of other applications, wood is often upgraded to charcoal. The charcoal-making process is inefficient with the product containing only about 55% of the energy of the original raw material in well-managed, commercial operations and as little as 20% in traditional processes.
Torrefaction
Torrefaction is a thermo chemical treatment of biomass at 200 to 320 °C. It is carried out under atmospheric conditions and in the absence of oxygen. During the process, the water contained in the biomass as well as superfluous volatiles are removed, and the biopolymers like cellulose, hemicellulose and lignin partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as “torrefied biomass” or “bio-coal”. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to further increase the density of the material and to improve its hydrophobic properties.
Benefits
Torrefaction appears to be an attractive option for upgrading wood to a product which retains about 90% of its energy and can be substituted for charcoal in a variety of applications. Biomass which is typically thermally unstable usually leads to formation of those condensable tars in gasifiers, making problems in down-stream equipment such as choking and blockage of piping. Torrefaction eliminates this problem.
The important advantages of torrefied wood include high energy yield and hydrophobicity so that it does not regain moisture during storage. Torrefaction achieves a stable low moisture content of 3%, reduction of mass by 30%, retention of 90% of original energy content and removal of smoke producing agents. All biological activity is eliminated reducing the risk of fire and stopping biological decomposition. Torrefied wood has a heating value of approximately 22,500 kJ/kg and highly friable and can be easily crumbled or pulverized. Torrefied biomass has excellent combustion properties; the fuel can be readily co-fired with coal, further gasified or fed to pyrolysis units.