**2.1 Ethanol**

Ethanol is a clear, colorless, toxic liquid and has a characteristic odor. Ethanol is not classified as toxic to humans. Ethanol has a higher octane number than gasoline, providing premium blending properties as a liquid fuel. Ethanol contains less energy per volume than gasoline, and denatured ethanol (98% ethanol) contains about 30% less energy than gasoline per volume [7]. Since ethanol contains oxygen, using it as a gasoline additive results in up to 25% fewer carbon monoxide emissions than conventional gasoline [8].

Ethanol is soluble in polar and nonpolar solvents and has a clearly higher vapor pressure than gasoline and an oxygen content of approximately 35%. Ethanol itself is a good solvent and can be mixed with water in unlimited quantities. Because ethanol is a short-lived compound in surface water and subsurface aquifer, substantially limiting the risk to aquatic organisms, environmental problem is minimal even when it is spilled. Ethanol degrades quickly in the natural environment, and the biodegradation is rapid in soil, groundwater, and surface water, with predicted half-lives ranging from several hours to 10 days [9].

### **2.2 Methanol**

Methanol, or wood alcohol, is a colorless, odorless, toxic liquid and is the simplest form (CH3OH) among alcohols [8]. Methanol is corrosive to some materials. Methanol can be produced from several sources: synthetic gas (syngas), formic acid, formaldehyde, and methane. Methanol is classed as toxic so it requires additional considerations during usage to limit inhalation exposure and skin contact.

Methanol is hygroscopic, meaning that it will absorb water vapor directly from the atmosphere. Because absorbed water dilutes the fuel value of the methanol and may cause phase separation of methanol-gasoline blends, containers of methanol fuels must be kept tightly sealed [10].

### **2.3 Butanol**

Butanol has higher energy densities and could be distributed in the existing infrastructure [8]. The use of ethanol as an additive to gasoline to increase octane number has downside effects such as corrosion of metal component and vapor lock. Such troubleshooting can be remedied by modification of engine and fuel system, but addition of alcohols with high carbon number such as bio-butanol enables utilization in existing system without rendering any change.

Alcohols with high carbon contents such as butanol can be synthesized from syngas through catalytic reaction that employs modified catalysts used in Fischer-Tropsch or methanol synthesis.
