Anaerobic fermentation of biomass for fuel production

Anaerobic fermentation of biomass is a key process in biofuel production, offering a sustainable alternative to fossil fuels. This process involves the breakdown of organic material, such as agricultural waste, food scraps, and animal manure, by microorganisms in the absence of oxygen.

The result is the production of biofuel, primarily composed of ethanol, which can be used as a renewable energy source for electricity, heating, or vehicle fuel.

Production of biofuels such as bioethanol

Producing bioethanol is a complex multistep process, which is outlined below.

  1. Choice of raw material / biomass
    There are two key types of raw materials:
    a) Typical food sources such as corn, sugar cane and grains
    b) Non-food sources such as the residue from crops such as corn husks, wood and wheat straw.
    Currently most bioethanol is made from food sources. However, this raises ethical issues as in some parts of the world where food is scarce, a food source is being used to make fuel.
  2. Pretreatment
    This involves breaking down the biomass both chemically and physically. By breaking it down physically it creates a larger surface area to volume ratio for enzymes to act on.
  3. Enzymatic hydrolysis
    Enzymes then break down complex carbohydrates into simple carbohydrates such as glucose.
  4. Fermentation
    Yeast is then added to the sugar solution to ferment the sugars by anaerobic fermentation . This reaction breaks down the glucose into ethanol and carbon dioxide according to the following reaction.
    Word equation:  glucose → ethanol + carbon dioxide + 2 ATP
    Chemical equation:    C6​H12​O6​→ 2 C2​H5​OH + 2 CO2​ + 2 ATP
  5. Distillation
    Once fermentation is complete, the ethanol concentration in the mixture is around 8–15%. To purify it, the liquid undergoes distillation, where heat is used to separate ethanol from water and other impurities based on their boiling points.
    This process results in ethanol with a concentration of about 95%.
  6. Dehydration
    To produce ethanol of 99% purity suitable for blending with fuel, the remaining water is removed through methods such as molecular sieves or distillation.

A diagram of the production of biofuels. It commences with a biomass high in cellulose and hemicellulose which is pretrated and broken down into lignin, cellulose and hemicellulose. Enzymes then break down complex carbohydrates into simple carbohydrates during Enzymatic hydrolysis, creating fermentable sugar. The sugars then undergo advanced fermentation, distillation and dehydration to create bioethanol (C2H5OH), which is then used as fuel.