Intermediate Temperature SOFC

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:
i)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

ii)More rapid start up and shut down procedures and
iii)Corrosion rates are significantly reduced.

Operating constraints of SOFC

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.

Solid Oxide Fuel Cells

Solid Oxide Fuel Cells

Interest in Solid Oxide Fuel Cells (SOFCs) which can 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, gasoline and other fuels are abundantly available. Applications where SOFCs may find dominant positions include distributed power, military transport applications, heat generation for the home and auxiliary power units.

Fuel cells

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 considered to be operated at low temperature, the Phosphoric Acid at intermediate, while Molten Carbonate and Solid Oxide fuel cells are mainly high temperature fuel cells.

Energy sources

Sun is the source of most of the energy and derives its energy from nuclear synthesis of protons into helium on its hot core. Solar radiation can be converted to heat through thermal solar devices and to electricity through photovoltaic cells. But most of the supply of energy received from sun is available through various sources as noted below.Fossil fuels like petroleum crude, natural gas and coal represent a huge source of chemical energy and have their origin in biomass buried under the earth and fossilized millions of years ago. These fossil fuels meet most of the global energy needs.Animate Energy is the useful energy delivered by human and work animals. India’s dependence on animate energy is high as compared to the developed countries with respect to human labour and draught animals.
Biomass includes energy crops, agricultural residues, marine plants, by products of forestry, food and agro processing industries, animal by products and waste, etc. Trees and plants use light energy of sun to convert atmospheric carbon dioxide and water into organic compoundsby a process called photosynthesis.
Wind energy derived from wind is a major source of kinetic energy, wind mills, also called wind turbines , convert the kinetic energy of wind into mechanical or electrical energy depending on the attachment.
Hydro energy can be both potential and kinetic. Turbines and water wheels can be used to convert it into mechanical or electrical energy.
Ocean currents generated as a result of the temperature difference between surface and deep waters of tropical oceans represent a large source of energy. In addition to the renewable sources listed above, Geo thermal energy obtained from hot core of earth is a source of heat energy and is used in some countries for power generation through steam cycle.

Applications of Gasifiers

For thermal applications, gasifiers are a good option as a gasifier can be retrofitted with existing devices such as ovens, furnaces, boilers, etc. Thermal energy of the order of 4.5 to 5.0 MJ is released by burning one cu.m of producer gas in the burner. Flame temperatures as high as 1200° C can be obtained by optimal air preheating and pre-mixing of air with gas. Producer gas can thus replace fossil fuels in a wide range of devices. A few of the devices which could be retrofitted with gasifiers are furnaces for melting non-ferrous metals and for heat treatment, tea dryers, ceramic kilns, boilers for process steam and thermal fluid heaters. A diesel engine can be operated on dual fuel mode using producer gas. Diesel substitution of over 80% at high loads and 70 - 80% under normal load variations can be achieved. The mechanical energy thus derived can be used either for driving water pumps for irrigation or for coupling with an alternator for electrical power generation. Alternatively, a gas engine can be operated with producer gas on 100% gas mode with suitably modified air / fuel mixing and control system.

Fluidized Bed gasifier

In a fluidized bed gasifier, inert material and solid fuel are fluidized by means of air distributed below the bed. A stream of gas (typically air or steam) is passed upward through a bed of solid fuel and material (such as coarse sand or limestone). The gas acts as the fluidizing medium and also provides the oxidant for combustion and tar cracking. The fluidized bed behaves like a boiling liquid and has some of the physical characteristics of a fluid. material is introduced either on top of the bed through a feed chute or into the bed through an auger. Fluidized-beds have the advantage of extremely good mixing and high heat transfer, resulting in very uniform bed conditions and efficient reactions. Fluidized bed technology is more suitable for generators with capacities greater than 10 MW because it can be used with different fuels, requires relatively compact combustion chambers and allows for good operational control. Fluidized bed gasifiers have been the focus of appreciable research and development and there have been several commercialization projects over the last ten years. The two main types of fluidized beds for power generation are bubbling and circulating fluidized beds.

Bubbling Fluidized Bed (BFB)

In a BFB, the gas velocity must be high enough so that the solid particles, comprising the bed material, are lifted, thus expanding the bed and causing it to bubble like a liquid. A bubbling fluidized bed reactor typically has a cylindrical or rectangular chamber designed so that contact between the gas and solids facilitates drying and size reduction (attrition). The large mass of sand (thermal inertia) in comparison with the gas stabilizes the bed temperature. The bed temperature is controlled to attain complete combustion while maintaining temperatures below the fusion temperature of the ash produced by combustion. As waste is introduced into the bed, most of the organics vaporize pyrolytically and are partially combusted in the bed. The exothermic combustion provides the heat to maintain the bed at temperature and to volatilize additional waste. The bed can be designed and operated by setting the feed rate high relative to the air supply, so that the air rate is lower than the theoretical oxygen quantity needed for full feed material oxidation. Under these conditions, the product gas and solids leave the bedcontaining unreacted fuel. The heating value of the gases and the char increases as the air input to the bed decreases relative to the theoretical oxygen demand. This is the gasification mode of operation. Typical desired operating temperatures range from 900° to 1000 °C. Bubbling fluidized-bed boilers are normally designed for complete ash carryover, necessitating the use of cyclones and electrostatic precipitators or baghouses for particulate control.