Bio Fuels

• The wood, saw dust, grass cuttings, domestic refuse, charcoal, dried manure etc. are the examples of solid bio fuel. They can be used for direct heating or raising of steam, by burning in a stove or furnace or chula e.g. fire wood can be directly burnt to release heat.
• When raw biomass is in an in convenient form, it is densified and cut into about 1 “-3” size in the form of pellet, cube or puck. These become easy to transport and in boilers.
• However, the direct burning of solid bio fuels is inefficient. Only 5% to 10% of heat released is utilized and remainder is lost in radiation and other losses.

• Most transportation fuels are liquid fuels because of high energy density. L.C. engines require clean burning fuels to keep the engine clean and minimize pollution. Liquid and gaseous fuels are easiest to burn in I.C. engines.
• Ethanol (C₂H₅OH) and Methanol (CH₃OH) can be used in petrol engines either directly or with some modifications depending upon blending with petrol or by their direct use respectively.
• Biobutanol is claimed to provide a direct replacement for gasoline without any modification.

(a) Ethanol :

• Ethanol has a high energy density of about 27000 kJ/kg and boiling point as 78°C.
• Ethanol can be used in petrol engines as a replacement for gasoline as it can be mixed with petrol in any percentage. Most existing cars can be run without modification with a 15% ethanol blend with petrol. The energy density of ethanol is about 45% less than gasoline per litre.
• Advantage of using ethanol is that it has higher octane rating, thus it can allow to increase compression ratio of the engine and its thermal efficiency. It also reduces engine emissions.
• Alcohol fuels are produced by fermentation of sugars components of plant material like wheat, corn, sugar beets, sugar cane, molasses and any sugar or starch material like potato, fruit waste etc.
• Bio ethanol production methods used are enzymes digestion (to release sugars from stored starches), fermentation of sugars, distillation and drying. It gives the by-product which can be used as cattle feed.

(b) Methanol :

• Methanol can be produced from municipal wastes. The wastes are first shredded and then passed under a magnet to remove ferrous materials.
• The shredded wasted are then gasified with O₂. The synthesis gas so produced is cleaned by water scrubbing and by other means to remove oils , H₂S, CO₂,. The clean gas undergoes a CO-shift conversion to obtain proper proportion of CO₂, CO and H₂, ratio. During this conversion, additional CO₂ is produced which is removed by amine scrubbing. Finally the methanol synthesis is carried out in presence of Cu/Zn/Cr catalyst to produce methanol.
• Methanol can be used as fuel for transport vehicles. The energy density of methanol is about 25% less than petrol per litre.

• The feed stocks used for production of biodiesel are animal fats, vegetable oils, soya, rapeseed, jatropha, mustard, sunflower, palm oil, algae etc.
• Biodiesel can be used as a fuel for vehicles in its pure form but it is usually used as a diesel additive to reduce the level of particulates, CO and HC from diesel powered vehicles. 15% blend of biodiesel with mineral diesel can be used in diesel engines.
• In certain countries like Germany, they permit 100% use of biodiesel in the engines. However, in cold climate, the biodiesel may need the use of heaters since they are viscous. The engine uses common rail direct injection (CRDI) electronically controlled engine technology.
• As indicated above, biodiesel is produced from vegetable oils and animal fats using transesterification process. Oils are mixed with sodium hydroxide and ethanol or methanol and the chemical reaction produces biodiesel and glycerol. The ratio of biodiesel to glycerol produced is about 10.

4. Biogas :

• The production of biogas from clury and agricultural waste is now in extensive use in many countries. It main constituent is methane and gases with traces of other impurities.
• Details of biogas production and its application in I.C. engines have been discussed in subsequent sections.