The Cellulosic

What is Cellulosic Ethanol?

Cellulosic Ethanol is a biofuel derived from non-food sources of biomass. These include agricultural waste, forest residues (such as dead wood), and energy crops.

How is Cellulosic Ethanol Produced?

Ethanol production involes 4 distinct steps: pretreatment, hydrolysis, fermentation, distillation. First, lignin is removed. Then hydrolysis breaks down the cellulose and hemicellulose into simple sugars. Fermentation produces ethanol as a byproduct. And finally, distillation extracts the ethanol from the fermentation broth.

Pretreatment:

First, the lignin must be removed. Lignin is a complex polymer that provides structural support to plant cell walls. Lignin prevents enzymes from breaking down the cellulose and hemicellulose – they key components of biomass which are converted to ethanol. Industry methods include chemical, thermal, or mechanical.

Hydrolysis:

Next comes Hydrolysis. This step involves the breakdown of cellulose and hemicellulose into simple sugars, such as glucose and xylose. These sugars can later be fermented into ethanol.

Hydrolysis can be acheived through different methods:

• Acid Hydrolysis
  • A chemical method that involves the use of strong acids, such as sulfuric acid, to break down the cellulose & hemicellulose.
  • Acid Hydrolysis is fast and efficient, but produces lots of wastewater.
• Enzymatic Hydrolysis
  • A biological method which uses enzymes to do the break down. Enzymes are typically produced by fungi, bacteria, or genetically engineered organisms.
  • While enzymatic hydrolysis is cleaner and more sustainable, it is slower and more expensive due to the cost of producing the enzymes.
• Steam Explosion
  • A physical method which involes heating the biomass with steam under high pressure which causes it to explode and break apart. Steam explosion produces high yields of sugars, but requires a high energy input and there is the possibility of loss of feedstock components.

The hydrolysis step is essential to the production of ethanol. And a crucial focus area for technology and research to reduce costs and make the whole production process economically viable.

Fermentation:

The fermentation step converts our hydrolyzed sugars into ethanol using microorganisms (typically yeast or bacteria). Fermentation releases carbon dioxide and other byproducts, but these are removed in the distillation step.

Typically anaerobic fermentation is used as it produces higher yields of ethanol than aerobic fermentation.

The fermentation step can be a complex process as several factors affect the yield and quality of ethanol produced. These factors include the type and concentration of microorganisms used, the fermentation conditions (temperature, pH, and nutrient levels), and the quality and composition of the feedstock.

Distillation:

For the final step, the ethanol is separated from the fermentation broth and purified through a distillation process. From here, the purified ethanol can be blended with gasoline or used as a standalone fuel.

There are 4 steps to acheiving distillation:

1. Concentration – removing water from the ethanol mixture to increase concentration. Usually done using evaporators or distillation columns.
2. Separation – separating the ethanol from any remaining impurities like water, acid, or other fermentation by-products. This is possible due to varying boiling points or the different compounds within the mixture (ethanol has a lower boiling point, allowing it to be separated).
3. Purification – removing any remaining contaminants – is typically done using molecular sieves or other purification techniques.
4. Dehydration – removing any remaining water from the ethanol to increase its purity and concentration.