Production of Silicon Nitride from Rice Husk

Rice milling industry generates a lot of rice husk during milling of paddy which comes from the fields. This rice husk is mostly used as a fuel in the boilers for processing of paddy and is a carbon neutral green product. Rice Husk is burnt at controlled temperatures below 700 degrees centigrade to generate ash which is amorphous in nature . The transformation of this amorphous state to crystalline state takes place if the ash is exposed to high temperatures of above 850 degrees centigrade. Rice husk has a great potential as a raw material for the production of Si3N4. Prof. Concepción Real and others of Universidad de Sevilla, Spain report that silicon nitride (Si3N4) can be obtained from rice husks by the carbothermal reduction by applying the constant-rate-thermal-analysis (CRTA) method. In this method, the reaction rate of carbothermal reduction is controlled and while maintaining constant level of CO concentration generated during the process. By using this synthesis technique it has been possible to obtain ceramic powder from rice husks with a determined phase composition and a controlled microstructure.

Direct methanol fuel cell

One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means a larger quantity of expensive platinum catalyst is required than in conventional PEMFCs. This increased cost is, however, expected to be more than outweighed by the convenience of using a liquid fuel and the ability to function without a reforming unit. One other concern driving the development of alcohol-based fuel cells is the fact that methanol is toxic. Therefore, some companies have embarked on developing a Direct Ethanol Fuel Cell (DEFC). The performance of the DEFC is currently about half that of the DMFC, but this gap is expected to narrow with further development.

Gas cleaning and cooling

Gas cleaning and cooling for gasifier system is accomplished by a cyclone, a gas cooler with some scrubbing action and a packed bed filter. Gas cooling increases density of gas in order to maximize the amount of gas entering the engine cylinder. Wet scrubbers are used to remove gaseous pollutants and solid particles while cooling the gas at same time. There exist different kinds of scrubbers for small scale producer engine system. A Packing bed scrubber consists of packing, liquid, support grates and distributors plates. Packing can be made from wide range of commercial and home made materials-steel, wool, wood chips, coke, gravel etc. Gas is passed through bottom and removed at top.Fabric filter is considered to be one of the suitable filters for vehicle application. It is placed immediately after cyclone. In filter with glass-fiber cloth, it is possible to withstand a gas temperature up to 300 C. The performance of filter depends on type of gasifier, fuel moisture content and how vehicle is driven. It is recommended that gas flow rate through the filter box shall not exceed 65 m3/h. Pressure loss over filter is affected by load and amount of dust in the producer gas.

Gas quality for engine

For trouble free operation, engine must be supplied with producer gas that is sufficiently free from tars, dust and acids. For satisfactory IC engine operation, an acceptable particle content less than50 mg/Nm3 and a tar content less than100 mg/Nm3 is postulated. The cleaning of gas is necessary to avoid wear and tear in engine. Dust concentration in the gas depends upon the type of gasifier, intensity of load and type of fuel. As load increases, dust concentration in producer gas also increases. The removal of tar from gas producer is one of the more difficult problems in gas cleaning.

Clean biomass gas

Biomass gas leaves the gasifier as the mixture of combustible and non-combustible gases along with tar, water vapour, dust and mineral vapour. Sulphur compounds such as hydrogen sulphide (H2S) and nitrogen compounds (NH3, HCN) in producer gas are undesirable as their condensates are corrosive and pollutants in exhuast gases. The generation of H2S is of little importance in gasification of biomass as long as sulphur content does not exceed 0.5%. The amount of NH 3 and HCN in the gas depends on the nitrogen content of the fuel. Fuel with nitrogen content less than 2 % is safe for gasification. Silicon oxide (SiO2) and iron oxide (Fe2O 3) in dust are important because of their abrasive nature.

Transesterification processes

Transesterification (Alcoholysis)
Transesterification (also called alcoholysis) is the reaction of a fat or oil with an alcohol to form esters and glycerol. A catalyst is usually used to improve the reaction rate and yield. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the products side. Alcohols are primary and secondary monohydric aliphatic alcohols having 1±8 carbon atoms. Among the alcohols that can be used in the transesterification process are methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanol are used most frequently, especially methanol because of its low cost and its physical and chemical advantages (polar and shortest chain alcohol). It can quickly react with triglycerides and NaOH is easily dissolved in it. To complete a transesterification stoichiometrically, a 3:1 molar ratio of alcohol to triglycerides is needed. In practice, the ratio needs to be higher to drive the equilibrium to a maximum ester yield. The reaction can be catalyzed by alkalis, acids, or enzymes. The alkalis include NaOH, KOH, carbonates and corresponding sodium and potassium alkoxides such as sodium methoxide, sodium ethoxide, sodium propoxide and sodium butoxide. Sulfuric acid, sulfonic acids and hydrochloric acid are usually used as acid catalysts. Lipases also can be used as biocatalysts.

Mahua oil ethyl ester preparation

The mixture of Mahua oil (100 g), ethanol (20:1 molar ratio with Mahua oil) and sulfuric acid (5% w/w) is to be boiled in a reaction chamber fitted with condenser at a temperature range of 72–75C for 5 h. Then the top layer is separated and washed with alkali solution (saturated calcium carbonate solution) to reduce the pH to neutral. The ester is then washed with salt water (5% NaCl solution) and the product is dried at 110C in an oven for an hour to remove the traces of moisture. The comparison of MOEE with diesel in terms of engine performance and emission shows better results. The MOEE is found to burn more efficiently than diesel. The emission of carbon monoxide, hydrocarbon, oxides of nitrogen and smoke are decreased by 58, 63, 12 and 70%, respectively, in comparison with diesel suggesting that MOEE can be used as a substitute for diesel in diesel engine. With regards to other oils considerable research has been done on vegetable oils as diesel fuel. That research included palm oil, soybean oil, sunfower oil, coconut oil, rapeseed oil and tung oil. Animal fats, although mentioned frequently, have not been studied to the same extent as vegetable oils. Some methods applicable to vegetable oils are not applicable to animal fats because of natural property diferences. Oil from algae, bacteria and fungi also have been investigated. Microalgae have been examined as a source of methyl ester diesel fuel. Terpenes and latexes also were studied as diesel fuels.

Process of transesterification

The physical properties of the primary chemical products of transesterification indicate that the boiling points and melting points of the fatty acids, methyl esters, mono, di and triglycerides increase as the number of carbon atoms in the carbon chain increase, but decrease with increases in the number of double bonds. The melting points increase in the order of tri, di and monoglycerides due to the polarity of the molecules and hydrogen bonding. After transesterification of triglycerides, the products are a mixture of esters, glycerol, alcohol, catalyst and tri-, di- and monoglycerides. Obtaining pure esters isnot easy, since there are impurities in the esters, such as di- and monoglycerides. The monoglycerides cause turbidity (crystals) in the mixture of esters. This problem is very obvious, especially for transesterification of animal fats such as beef tallow. The impurities raise the cloud and pour points. On the other hand, there is a large proportion of saturated fatty acid esters in beef tallow esters (almost 50% w/w). This portion makes the cloud and pour points higher than that of vegetable oil esters. However, the saturated components have other value-added applications in foods, detergents and cosmetics. The co-product, glycerol, needs to be recovered because of its value as an industrial chemical such as CP glycerol, USP glycerol and dynamite glycerol. Glycerol is separated by gravitational settling or centrifuging. A small amount of water, generated in the reaction, may cause soap formation during transesterification which needs to be taken care of.

Energy scenario and Jatropha oil biodiesel in India

The consumption and demand for the petroleum products are increasing every year due to increase in population, standard of living and urbanization. In India, there is a deficit of 40 per cent in supply of petroleum products and the petroleum reserves are limited to 6 to 7 years only. The petroleum products continue to be the backbone of Indian economy with a share of 33 per cent of the energy basket. The increase in crude oil import affects the country’s economy and its development. Diesel consumption pattern in India has not varied much and is around 36 million tonnes as reported by the Ministry of Petroleum and Natural Gas. The diesel vehicles were banned in New Delhi and Bangalore cities for serious problem of air pollution due to higher emissions of polluted gases. The acid rain, global warming and health hazards are the results of ill effects of increased polluted gases like SOx, CO and particulate matter in atmosphere. Biodiesel is the only fuel to meet out increasing diesel demands. So even mixing of 20 per cent with diesel fuel by biodiesel can help India save 7.3 million tonnes of diesel per year. In India, about more than 14 million hectares land is cultural wasteland while more than 24 million hectare land is fallow land. The use of non edible oils compared to edible oils is very significant because of the increase in demand for edible oils as food and they are too expensive as compared with diesel fuel. Among the various non edible oil sources, Jatropha curcas oil has added advantages like pleasant smell, odorless, colorless and light yellowish and easily mixes with diesel fuel. Jatropha curcas oil cannot be used for food or feed because of its strong purgative effect. The Jatropha plant having advantages namely effectively yields oilseeds from the 3rd year onwards, rapid growth, easy propagation, life span of 40 years and suitable for tropical and subtropical countries like India. For producing biodiesel the oil extracted from the seeds of Jatropha is mixed with methanol at a proportion under a particular temperature. This solution is continuously stirred for two hours. During the above process, glycerol present in the solution separate out; which when settled can be separated out. Whatever is left after removing the glycerol is the liquid fuel. When the liquid fuel is washed twice, purified biodiesel is obtained. This could be used directly for running the engine. The direct use of raw jatropha oil in engine has been carried out by several researchers and they have reported formation of carbon deposits, incomplete combustions and reduction in life of engine due to high viscosity of curcas oil. Similar problems were reported by many researchers when using raw vegetable oils as engine fuel. Using refined curcas oil blends in precombustion chamber engine fair results onthermal efficiency and emission compared with diesel No.2 diesel can be obtained. It was reported that problems of filter blockage, carbon deposits and oil incompatibility with fuel line materials exist. It was found the jatropha oil can be blended up to 40 to 50 per cent with diesel fuel used in engine without modifications. However, acrolein is reported to be a high toxic substance released from the engine due to thermal decomposition of glycerol present in the oils. The problems encountered in raw oils are solved by forming biodiesel, which is non toxic, eco-friendly and have similar properties as that of diesel fuel.

Wood pellet technology in Europe

According to a report around 44 percent of all German wood pellets heating systems are found in Bavaria, with a further 19 percent installed in Baden-Württemberg. This is as per statistics from the Federal Office of Economics and Export Control (BAFA) which approves grant applications for pellets heating systems.The pellets are extremely dense and can be produced with a low humidity content (below 10%) that allows them to be burned with a very high combustion efficiency. Further, their regular geometry and small size allow automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying. Their high density also permits compact storage and rational transport over long distance. The market for wood pellets is currently experiencing strong international growth. For investors, producers and service providers, this opens up a wide range of opportunities, but also challenges. Currently the situation in the international pellets markets is very dynamic, new technologies are in demand worldwide, making high quality market information important for investment decisions. There is an increasing number of wood pellet manufacturers in Germany. In fact pellet-burning appliances are simpler to operate and more convenient than other wood-burning appliances. They are almost as easy to use as gas, oil, or electric heaters. According to current statistics from the organizer of the Interpellets 2008 trade fair and the 8th Pellets Industry Forum, Solar Promotion GmbH, 48 companies will produce modern wood pellet fuel at 55 sites this year. There are offer for investing in wood pellet projects in Ukraine by purchasing used machinery for manufacturing pellets from wood or saw dust . The machinery will give a production cost of 40 Euro per 1000 kg of pellets.

Pyrolysis can lead to less carbon emissions

Burning one ton of wood pellets emits 357 kilograms less carbon than burning coal with the same energy content. But turning those wood pellets into char would save 372 kilograms of carbon emissions. That is because 300 kilograms of carbon could be buried as char, and the burning of byproducts would produce 72 kilograms less carbon emissions than burning an equivalent amount of coal. This is a calculation presented in a paper by Malcolm Fowles of the Open University, United Kingdom. Such an approach could carry an extra benefit. Burying char, known as black carbon sequestration, enhances soils helping future crops and trees grow even faster, thus absorbing more carbon dioxide in the future. Researchers believe that the char, an inert and highly porous material, plays a key role in helping soil retain water and nutrients, and in sustaining microorganisms that maintain soil fertility and has significant and long-lasting positive effects on soil fertility. Pyrolysis process initiates on materials like wood and other biomass at around 230C and during pyrolysis thermally unstable components such as lignine in biomass are broken down and evaporate with other volatile components. The resulting pyrolysis gas consist mainly of tar, polycyclic aromatic hydrocarbons (PAH), methane (CH4), steam and CO2. The solid residual is carbon structures (coke) and ashes. Pyrolysis can play to offset greenhouse-gas emissions. Bioenergy through pyrolysis which gives bio oil in combination with biochar sequestration is a technology to obtain energy and improve the environment in multiple ways at the same time.

Diesel engine technology for biodiesel

Increased use of biodiesel are due to the useful properties such as less local air pollution, rapid biodegradability, low toxicity to people and the environment, and high flashpoint, reduction in greenhouse gas emissions in the transport sector and increase energy security by reducing dependence on imported oil. The only limitation to the production and use of biodiesel is generally the availability of feedstock. This need not have to be grown locally, but can be imported. Examples are North American soya oil, Malaysian palm oil, French sunflower oil, Greek cottonseed oil, Polish rapeseed oil and Danish cooking oil recycled from restaurants. Recent years have seen impressive improvements in diesel engine technology to improve energy efficiency and reduce emission levels. Modern diesel engines achieve their excellent performance through the use of high-pressure precision fuel injection equipment such as common rail and electronic injection systems. This requires fuels of correspondingly high quality, regardless of their origin. European fuel standard EN 14214, which was developed in close co-operation with the automotive, oil and biodiesel industries, ensures that biodiesel is suitable for even the most modern engines. The standard forms the basis for warranties from leading car manufacturers, including Audi, BMW, Daimler-Chrysler, MAN, Seat, Skoda, Volvo and Volkswagen. The latest technical development from vehicle manufacturers is a fuel sensor that measures the ratio of biodiesel to fossil diesel in the tank. By continuously optimizing the injection timing to suit the fuel mix, it reduces emissions.

Brief hystory of fuel cells

The present day fuel cell was called “gas battery” by the inventor Willium Grove . In 1889, chemists Ludwig Mond and Charles Langer first adopted the term “fuel cell” when they attempted to build the first practical device of fuel cell using industrial coal gas and air. Development of fuel cells was slow in their initial decades, but fuel cells had gained extensive attention since 1950s. In 1955, chemist Willard Thomas Groubb of U.S General Electric Company applied sulphonated polystyrene ion exchange membrane as electrolyte of fuel cells. Leonard Niedrach a chemist of General Electric Company invented a method of depositing platinum on to this membrane in 1958. Those fuel cells using solid polymer membrane as electrolyte and using platinum as catalyst were called “Grubb-Niedrach Fuel Cell” at that time. This was considered the beginning of PEMFC. First applied low temperature polymer electrolyte membrane fuel cells were developed by General Electric Company in the 1960’s for NASA’s Gemini space program. At that time, the PEMFC acted as auxiliary power, and byproduct of fuel cell reaction was pure water for astronauts. The proton exchange membrane fuel cell (PEMFC) was called firstly the ion exchange membrane fuel cell (IEMFC). It is also called as solid polymer electrolyte fuel cell (SPEFC), polymer electrolyte fuel cell (PEFC), solid polymer fuel cell (SPFC) and polymer electrolyte membrane fuel cell (PEMFC), etc. The proton exchange membrane fuel cell uses solid electrolyte membranes as its electrolyte. The membrane is not only an electronic insulator, but also excellent conductors of hydrogen ion (proton). At present, PEMFCs still use oxygen as oxidant and use hydrogen or methanol as fuels in general. According to different fuels used, PEMFC can be classified as three types: hydrogen proton exchange membrane fuel cells (H2 PEMFCs), methanol reforming fuel cells (MRFCs), direct methanol fuel cells (DMFCs). Earlier DMFC did not belong to PEMFC because alkaline or acidic liquid were used as electrolyte in the DMFC. The performance of the DMFC using such electrolyte is quite poor because the activity of electro-catalyzed oxidation of methanol is very low. Since 1990s, DMFC has gradually become a new member of PEMFC as solid polymer electrolyte membrane was adopted. Hydrogen PEMFCs and DMFCs generate electricity with high efficiency and low emission (pollution). In recent years, the development and commercialization of hydrogen PEMFCs and DMFCs for primary or auxiliary power for stationary, mobile, portable, and urban transportation systems have received increasing attention.

Proton exchange membrane fuel cell

The fuel cell of choice for a wide range of applications spanning portable, stationary and transportation markets is the proton exchange membrane fuel cell (PEMFC). This is principally because of the high power density and the relatively low temperature of operation. Today the PEMFC typically operates at close to 80ºC although there is a desire to move to higher temperatures close to 150ºC to mitigate the effects of carbon monoxide (CO) poisoning at the anode. The membrane electrode assembly (MEA) is the key component where hydrogen and air react electrochemically to generate electrical power. The MEA is typically located between a pair of flow field plates to give a single cell. The flow field plates are designed to distribute the reactant gases across the face of the MEA and also to collect the electrical current from the MEA. Sufficient unit cells are connected electrically to generate the desired power output. Depending on the application, a PEMFC system may contain from tens to a few thousand MEAs to produce from a few watts to several hundred kilowatts of power.

Direct oxidation.methanol fuel cell (DMFC)

DMFCs have lower weight and volume compared with indirect fuel cells. In this kind of fuel cells, solid polymers have been shown as an attractive alternative to the traditional liquid electrolytes. Nafion perfluorosulfonic acid polymers are the most commonly used fuel cell membranes. Although it would be desirable methanol could be spontaneously oxidized at the cathode methanol transport takes place across the membrane causing depolarization losses at the cathode and conversion losses in terms of lost fuel.In order to improve the performance of the DMFC, it is necessary to eliminate or reduce the loss of fuel across the cell, usually termed as methanol crossover. This is a major limitation at present for DMFCs to become a commercially viable alternative. Although nafion membranes are the most usually used as solid polymer electrolyte in DMFCs, methanol readily transports across perfluorosulfonic acid membranes and minimises the effects of methanol crossover. Materials such as acid doped PBI membranes seems to have a lower methanol permeability than nafion membranes.

R & D activities on gasifier development

Gasification has been receiving attention throughout the world. Work is being carried out on the gasification of variety of biomass such as municipal solid wastes, agricultural wastes and forest residues for different applications such as heat/power generation, production of syn-gas, methane, and hydrogen etc.
It is reported that in the near future, the use of gasification to convert solid wastes into ready-to use fuel could literally solve several environmental problems at the same time. A great number of small-scale fixed bed gasifiers are available around the world. So far successful applications have been seen in Finland, and Denmark where the gas is used for combustion in a boiler. Also other countries have demonstrated small fixed bed gasifiers with success for 1000 hours of operation in a year for power production. Biomass gasification devices aboard generally are large-scale, of high automation degree with complex techniques, and concentrated on power generation and thermal application. Their gasification efficiencies can reach 60-90%, and combustible gas has a caloric value of 17-25MJ/m3.
In the early 1980’s, rice husk-based gasification device was developed in China, using a down-draft fixed-bed gasifier of volume varying from 60 kW to 160 kW, which were applied in the local food industry and were also exported.
In other EU countries, electricity from biomass is an option only lately starting to be considered by Greek companies. However, the currently used gasification technologies are still far from satisfactory. The main challenges faced is non-stable gas production process caused by local hot spots existing in the gasifier, non-flexibility to diverse biomass types, difficulty of scale-up, and low quality of product gas. Despite the great number of developments at different industries and the pilot plants available around the world, there are only a few, that achieve a commercial operation.
Advanced technical level on the field of producer gas has been mastered by many countries such as Sweden, the United States, Italy, and Germany. In recent years, the United States had a breakthrough in biomass pyrolysis gasification, and researched and manufactured a set of biomass comprehensive biomass gasification set with gas turbine generation system for large-scale generation.

Biomass gasifiers

Gasification is the conversion of biomass into combustible gas, volatiles and ash in an enclosed reactor or gasifier. The gas produced can be used either for heat generation or for power generation. A wide range of biomass materials (wood, charcoal, coconut shells, rice husk, bagasse, etc.) can be used to fuel gasifiers. In most of agro industries thermal processing is one of the step involved in the production chain. Many industry segments also use high cost fuels such as diesel, LPG or electricity to meet their thermal requirements such as drying, sterilization, direct and indirect heating, steam generation in boilers, melting and other applications. With increasing cost of imported oil and electricity, industry is increasingly loosing its competitive edge, both in the local and global markets.
Biomass gasification process on the other hand offers an industrially proven, elegant, affordable and environment friendly way to meet this situation. Wood in drying and sizing mills, a major part of rice husk in rice mill, bagasse in gur / khandasari manufacturing units and agro residues such as groundnut shell etc., are used as furnace fuels via direct combustion. The operation of these furnaces, in general, has very low efficiency and results in a very serious air pollution and fly ash emissions. Alternate application of these residues via gasification route offers combustible gas, which can be used as fuel for all the above industrial thermal applications with relatively high efficiency.

Molten Carbonate Fuel Cells

Fuel cells are energy conversion devices that continuously transform the chemical energy of a fuel and an oxidant into electrical energy. This energy conversion process is accomplished by means of an electrochemical reaction whereby the reactants are consumed, by-products are expelled, and heat may be released or consumed. Fuel cells will continue to generate electricity as long as both fuel and oxidant are available. In a molten carbonate fuel cell (MCFC), carbonate salts are the electrolyte. Heated to 650 degrees C, the salts melt and conduct carbonate ions (CO3) from the cathode to the anode. At the anode, hydrogen reacts with the ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit, providing electrical power along the way, and return to the cathode. There, oxygen from air and carbon dioxide recycled from the anode react with the electrons to form CO3 ions that replenish the electrolyte and transfer current through the fuel cell. The operating principles for a carbonate fuel cell are simple in concept. The reactants fuel and an oxidant, in this case, air are fed to the cell’s electrodes. Ions are transported through the electrolyte sandwiched between the electrodes, creating a current equal to the amount of electric energy needed by the system connected to the fuel cell (also called load). The the overall reaction with hydrogen, is: H2+0.5O2+CO2(cathode)< == > H2O+CO2(anode)

Hydrogen storage

A critical barrier to the wide-spread use of hydrogen is how to effectively store hydrogen for various energy applications. Chemical hydrogen storage in particular through the use of sodium borohydride (NaBH4) is a way to combine high energy density and ease of hydrogen release. These characteristics are essential for any near-term commercial opportunities of hydrogen power sources. Sodium borohydride is a white solid at room temperature, stable in dry air and decomposing only at temperatures above 400C. When mixed with water, NaBH4 gives off pure hydrogen gas. The reaction can be catalyzed in a number of ways to give the desired hydrogen flow rates to match electrical output demand at the fuel cell. The highly flammable hydrogen gas is only generated immediately before use, therefore it is a much safer way of storing hydrogen compared to compressed or liquefied hydrogen. NaBH4’s stability in air also makes it a much safer choice than its pyrophoric reversible metal hydride counterpart, particularly for consumer applications.

Materials for the component of fuel cells

The future of fuel cell generators belongs to plastics. Modern chemistry affords component of fuel cells, except electrodes, electric elements, and heat exchangers, to be made of plastics. There exist fibre glass – reinforced plastics that are stronger than steel. some fluroplasts out do any metal in corrosion resistance in various aggressive media at temperatures up to 300 C. The insulation properties of fluroplasts are by no means inferior to that of best ceramic insulators. Some other remarkable synthetic materials are worth mentioning, namely, polyethylene, polysulfone, organo–silicone rubbers and sealing compounds, epoxy enamels, to name just a few suitable materials. The only limitations of these materials are operating temperature and their poisonous emissions.
Cheap ferrous metals have been lately used also for components operating in aggressive media. The required resistance have been achieved by lining the carbon steel products, say pipes or tanks, by polymeric materials (polyethylene, PTFE). The lined products offer the strength of steel, chemical resistance and insulation properties of the polymer and the last but not least are cheap.

Proton Electrolyte Membrane (PEM) fuel cell

A Proton Electrolyte Membrane (PEM) fuel cell consists of two bipolar plates (anode and cathode), a Membrane Electrode Assembly, and Diffusion Media. These elements when connected to an electric load produce DC power. Hydrogen fuel cells need two elements to generate power, oxygen from the air, and hydrogen. The hydrogen and oxygen react through the membrane assembly to produce the electric power. The only by-product of the fuel cell is pure water.

The US DOE Hydrogen Program emphasizes polymer electrolyte membrane (PEM) fuel cells in passenger vehicles.Liquid fuels are also attractive for portable and remote fuel cell applications due to their ease and convenience of handling. Family of portable PEMFC systems, rated at 250 We, that incorporate a compact methanol/water reformer and integral hydrogen purifier targetting both military and commercial applications are also under development. Efficiency for a PEM cell reaches about 40 to 50 percent. An external reformer is required to convert fuels such as methanol or gasoline to hydrogen. Currently, demonstration units of 50 kilowatt (kw) capacity are operating and units producing up to 250 kw are under development.

Fuel cells are grouped together in a "fuel cell stack." This stack then becomes the engine in a fuel cell automobile, or the power generator for electricity.

High efficiency solar systems developed

Conventional solar systems have an efficiency of only14 percent. Anna Dyson of Rensselaer Polytechnic Institute in Troy, New York has developed a system that gives combined heat and power with an efficiency of nearly 80 percent by efficiently capturing and transferring light into electricity and the solar heat into hot water. According to Anna Dyson the new system uses high-tech solar-concentrator technology and the system has stacks of pivoting lenses that senses the position of sun at any time and the modules are made to face the sun directly to focus sun rays onto high-tech solar cells. The key breakthrough is the miniaturized concentrator solar cell, which uses a lens with concentric grooves to focus collected light. Even though it is only the size of a postage stamp compared to the usual solar collector area that spans 4 x 4 feet, the cell is much more efficient in collecting and reusing solar energy. Micro channels at the base of the module transfer energy in the form of heat and light to wires contained inside. Each vertical stack of lenses rolls and tilts to track the sun. Incorporating these new cells into arrays could make solar energy an option that is competitive with other energy sources, reducing our dependency on fossil fuels. The lenses can be nestled between window panes and all of the pieces can be made of glass to lower the lighting needs of buildings, as it will provide usable light inside. It could supply as much as 50 percent of the energy needed for a building to operate. According to Anna Dyson the full-size prototype will be incorporated into a new building at The Center of Excellence in Syracuse, New York.

Polymer Electrolyte Fuel Cell

Polymer Electrolyte Fuel Cells (PEFC) are used as the vehicular power source to eventually replace gasoline and diesel internal combustion engines. First used in the 1960s for the NASA Gemini program, PEFCs are currently being developed and demonstrated for systems ranging from 1W to 2kW. PEFC fuel cells use a solid polymer membrane (a thin plastic film) as electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. The fuel for the PEFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:

Anode Reactions: 2H2 => 4H+ + 4e-

Cathode Reactions: O2 + 4H+ + 4e- => 2 H2O

Overall Cell Reactions: 2H2 + O2 => 2 H2O

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.

Cross Draft Gasifier

Cross draft gasifiers, although they have certain advantages over updraft and downdraft gasifiers, they are not of ideal type. The disadvantages such as high exit gas temperature, poor CO2 reduction and high velocity are the consequence of the design. Unlike downdraft and updraft gasifiers, the ash bin, fire and reduction zone in cross draft gasifiers are separated. The design characteristics limit the type of fuel for operation to low ash fuels such as wood, charcoal and coke. The load following ability of cross draft gasifier is quite good due to concentrated partial zones which operate at temperature up to 2000oC. Start up time (5 -10 minutes) is much faster than that of downdraft and updraft units. The relatively higher temperature in cross draft gasifier has an obvious effect on gas composition such as high carbon monoxide and low hydrogen and methane content when dry fuel such as charcoal is used. Cross draft gasifier operates well on dry blast and dry fuel.

Down draft gasifier

In a down draft gasifier, the primary gasification air is introduced at or above the oxidation zone in the gasifier. The producer gas is removed at the bottom of the apparatus, so that fuel and gas move in the same direction. The main advantage of down draft gasifiers lies in the possibility of producing a tar-free gas suitable for engine applications. A major drawback of down draft equipment lies in its inability to operate on a number of unprocessed fuels. Minor drawbacks of the down draft system, as compared to up draft, are somewhat lower efficiency resulting from the lack of internal heat exchange as well as the lower heating value of the gas. Besides this, the necessity to maintain uniform high temperatures over a given cross-sectional area makes impractical the use of down draft gasifiers in a power range above about 350 kW (shaft power).

Updraft Gasifier

In the Updraft Gasifier, the air intake is at the bottom and the gas leaves at the top. Near the grate at the bottom the combustion reactions occur, which are followed by reduction reactions somewhat higher up in the gasifier. In the upper part of the gasifier, heating and pyrolysis of the feedstock occur as a result of heat transfer by forced convection and radiation from the lower zones. The tars and volatiles produced during this process will be carried in the gas stream. Ashes are removed from the bottom of the gasifier. The major advantages of this type of gasifier are its simplicity, high charcoal burn-out and internal heat exchange leading to low gas exit temperatures and high equipment efficiency, as well as the possibility of operation with many types of feedstock.

Types of Gasifiers

Gasifiers are basically divided into two major types namely fixed bed and fluidized bed. Fixed bed gasifiers typically have a grate to support the feed material and maintain a stationary reaction zone. They are relatively easy to design and operate, and are therefore useful for small and medium scale power and thermal energy uses. It is difficult, however, to maintain uniform operating temperatures and ensure adequate gas mixing in the reaction zone. As a result, gas yields can be unpredictable and are not optimal for large-scale power purposes (i.e. over 1 MW). The primary types of fixed bed gasifiers are updraft, downdraft and crossdraft.

Energy conversion efficiency and labelling

The energy efficiency ranges of few conversion devices, %
Spark ignition engine 20- 25
Compression ignitionengine 30 -45

Electric motor 80-95
Electric generator 80-95
Steam turbine 7-40(Inclusive of boiler)
Hydro turbine 70-99
Battery 80-90
Solar cell 8 – 15
Water Pump Mechanical Potential 40-60
Countries the world over have tried to promote efficient use of energy through labelling programmes. Energy Efficiency Labelling is display of a label on a product depicting data in a standard format regarding energy use and a predefined energy efficiency measure for enabling comparison with the energy efficiency of similar products. Energy efficiency label provides relevant energy-use information to the purchaser for making an informed purchase decision. It seems to be an effective way to impart knowledge to the consumer regarding efficiency and life cycle costs. The basis of acceptance of an energy efficient product is that its life time cost is less and hence it makes sense to the consumers.A graded multi-level efficiency band similar to the one to five star rating of appliances has been used by many countries.

Driving forces for biodiesel

The key driving forces for biodiesel are the directive for the promotion of biofuels and the directive on fuel quality. The former is motivated by the need to cut greenhouse gas emissions in the transport sector and increase energy security by reducing dependence on imported oil. Also encouraging the growth of biodiesel are useful properties such as less local air pollution, rapid biodegradability, low toxicity to people and the environment, and high flashpoint. The supply of biodiesel is limited, however, by the availability of oilseed crops. Any plan for biodiesel should begin with a careful study of existing experience, followed by a survey of feedstock options including recycled cooking oil.

SOFC versus MCFC

The interest in the SOFC, compared with the MCFC, might be questioned in view of the thermodynamic limitation of operation at 1000o C, which reduces the cell operating potential by 100mV, or about 15 %. The answer is that the SOFC is electrochemically simpler than the MCFC, since it does not require CO2 feedback from the anode exit to the cathode inlet. Its solid electrolyte eliminates electrolyte management problems, and it may be more resistant to contaminants such as H2S. With the correct materials, it may be capable of a lifetimes up to 100,000 hours or more (tested with no apparent degradation to almost 35,000 hours operation at Brown Boveri). At the higher operating temperature, diffusion and kinetic limitations are minimized, and the cell is essentially limited only by IR drop.