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.