BIOFUEL TECHNOLOGIES

Fears about global warming, dwindling world oil supplies and the desire for independence from foreign oil sources have driven many countries to invest in alternatives to complement, and potentially replace, fossil fuels. Biofuels, derived from sustainable biological resources, are a renewable alternative to fossil fuels. Advances in technology have expanded the biofuel market to utilize different feedstocks and allow for improved production efficiencies, energy efficiencies and economics. While biodiesel and ethanol are the common forms of biofuel, there are emerging fuels that fall within the biofuel category.

Ethanol
Ethanol has had diverse applications over the years ranging from use as a solvent for human contact and consumption such as beverages scents, flavoring, colorings and medicines to use in the chemical world as an essential solvent and feedstock for the synthesis of other products. Today, most of the world's supply of ethanol is being used as fuel by blending with gasoline in concentrations ranging from 5% to 85% . Ethanol has been traditionally produced from corn, sugarcane, sugar beet and sorghum but developing technologies can utilize a wider variety of biomass as feedstock.

First Generation Ethanol
First generation ethanol utilizes corn, sugarcane, sugar beet and sorghum as feedstock for fermentation although any starchy plant can be used. The process is as follows:

• Separation of starch into sugars
  
• Microbial fermentation of sugars
  
• Distillation
  
• Dehydration

Second Generation Ethanol
Cellulosic ethanol is produced from cellulose, a naturally occurring complex carbohydrate polymer commonly found in plant cell walls. The process uses the large and diverse amounts of biomass which includes waste from urban, agricultural and forestry sources. There are two main methods of production of cellulosic ethanol:

1. Cellulolysis (Biological): hydrolysis followed by fermentation of the generated free sugars and distillation. The process is as follows:

• A "pretreatment" phase, to make the lignocellulosic material such as wood or straw amenable to hydrolysis
  
• Cellulose hydrolysis (cellulolysis), to break down the molecules into sugars. There are 2 major cellulose hydrolysis prcoesses:
  
  • Chemical Hydrolysis
    
  • Enzymatic Hydrolysis
    
• Separation of the sugar solution from the residual materials, notably lignin;
  
• Microbial fermentation of the sugar solution;
  
• Distillation to produce 99.5% pure alcohol.

2. Gasification (Thermochemical): The 2 types of gasification processes are as follows:

1. Synthesis gas fermentation (theoretical)
   
   • Gasification — Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
     
   • Fermentation — Converts the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
     
   • Distillation — Ethanol is separated from water
     
2. Catalysis
   
   • Gasification — Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
     
   • Catalysis — Converts the carbon monoxide and hydrogen into methanol, ethanol, propanol and butanol using a catalyst
     
   • Distillation and separation — The alcohols are separated from the water and from each other.
     

Methanol is most commonly produced from the methane component in natural gas using combinations of the following processes:

• Steam-methane Reforming (SMR) - production of CO and H2 from CH4 and H2O in the presence of a nickel catalyst
  
• Autothermal Reforming - combination of SMR and partial oxidation of methane
  
• Water-gas shift reaction - CO and H2O is reacted to adjust the quantity of H2 to provide the correct stoichiometry of CO and H2 for the methanol synthesis
  
• Methanol synthesis - Production of methanol and H2O from CO and H2 in the presence of a catalyst
  
• Methanol synthesis - Production of methanol and H2O from CO2 and H2 in the presence of a catalyst

Since the most common feedstocks for methanol synthesis are natural gas and light petroleum products, this methanol cannot be considered a biofuel. However, the methanol that is produced as one of the products thermochemical/catalysis route is a biofuel since biomass is used as a feedstock.

Butanol
The structure of butanol structure is more similar to gasoline than ethanol is which has been generating interest as a potential fuel for vehicles. Some vehicles can operate on butane without any engine modifications.

Currently butanol is obtained from non-renewable sources such as petroleum and natural gas by conversion of propene. Butanol can be produced from renewable sources by the following methods:

• Fermentation - the bacteria Clostridium acetobutylicum can convert the starch in various feedstocks to butanol
  
• Thermochemical/Catalysis - butanol is one of the products produced by the thermochemical and catalysis route discussed in the ethanol production.

Bio-oil
Biomass undergoes fast pyrolysis, with a limited oxygen supply, to form a condensed hydrocarbon liquid, char and a mixture of gases. The hydrocarbon liquid or bio-oil can be further processed by hydrocracking to produce fuels, can be used as feedstock for a thermochemical process to green diesel or feedstock for fermentation to ethanol.

Biodiesel and Green Diesel
Biodiesel is currently produced by transesterification of the fatty acids in animal or vegetable fats and oils. The process is as follows:

• Purification - removal of any non-oil material and water
  
• Neutralization of free fatty acids using base
  
• Transesterification - ethanol or methanol is used to create ethyl esters of fatty acids and glycerol
  
• Separation - the biodiesel and alcohol is decanted from the glycerol and other waste products and then distilled to separate the biodiesel from the alcohol

Biodiesel can be utilized by diesel engines in blends of B20 and even higher without any engine modification even though the structure of biodiesel differs from petroleum diesel. The presence of oxygen in the molecules of biodiesel increases the freeze point which makes it unsuitable for jet fuel.
Green diesel is structurally identical to petroleum diesel and therefore does not have the same problems as biodiesel. It can be produced by vegetable oil as a feedstock in an oil refinery in which hydrocracking or hydrogenation processes transform the oil to gasoline, diesel and propane. The thermochemical route can also be used to produce green diesel.

Jet Fuel
The use of a renewable fuel to replace petroleum jet fuel has different issues than the rest of the transportation industry. A new type of fuel must meet the threshold for energy content per unit volume and weight. Liquid hydrogen has too low of volume to energy content ratio and ethanol has too high of a weight to energy content ratio to be considered.
The focus has been on producing jet fuel that is the same chemical composition as petroleum jet fuel from renewable feedstocks. The use of biodiesel and green diesel are both being evaluated for their potential.