Fermentation from biomass and waste
In order to produce ethanol from biomass and waste, two stages are required:
1. Hydrolysis to break the material down into simple sugar molecules
2. Fermentation to produce ethanol
Feedstock Composition
Feedstock may include purpose-grown crops (including maize and corn), crop waste, paper mill sludge, forest residues and household waste (including sewage). These are mostly lignocellulosic materials containing cellulose, hemicellulose and lignin. Cellulose and hemicellulose are long chain polymers that make up the bulk of plant material, and lignin is the chemical "glue" that holds them together.

photo: testing biomass for fermentation


diagram: Arkenol process for hydroysis of biomass

Sugar Separation
After hydrolysis, the sugar for fermentation must be separated from the acid. A new process developed by the company Arkenol in the United States, which is still at the pilot stage, makes use of ion exchange to improve the separation, allowing a greater proportion of the acid to be concentrated and re-used. Final traces of acid are precipitated as "gypsum" (calcium sulphate) by addition of lime.

 

Diagram: Structure of Glucose

 

Fermentation

Fermentation is a complex series of reactions, which convert carbohydrates, mainly sugars and starches, into ethanol and carbon dioxide. Several enzymes, such as zymase in yeasts, catalyse these reactions. Yeast is a living organism, and these are the products of anaerobic respiration.

chemical equation: glucose to ethanol

Conditions

  • Fermentation with yeast works best at temperatures in the range25 - 37°C, in the absence of oxygen (anaerobic) and will produce aqueous solutions of up to 14% ethanol.
  • Below 25°C the reaction rate is too slow, but at higher temperatures the enzymes start to denature and lose efficiency.
  • If oxygen is present, aerobic respiration will occur producing carboxylic acids, in this case ethanoic acid (vinegar).
  • The toxicity of ethanol to the organisms used limits the ethanol concentration possible.

Solving the 5-carbon problem

Conventional yeasts cannot make use of five-carbon sugars that arise from the hydrolysis of hemicellulose. Conventional methods for ethanol fermentation do not utilise this resource, which may count as 20% of the feedstock.
The process developed by Arkenol uses specially bred yeast (not genetically engineered) that feeds preferentially on C5 sugars, as well as on C6 sugars. In this way, a greater proportion of the feed is utilised.
Another approach is to use genetically engineering bacteria.

Hydrolysis
This process breaks the long cellulose and hemicellulose chains into simple sugars. Cellulose yields primarily glucose (a six-carbon sugar) whereas hemicellulose, in the region of 20% of the material, gives a mixture including several five-carbon sugars.

Methods of hydrolysis include using enzymes and using dilute or concentrated acids. Whereas in the past hydrochloric or hydrofluoric acid may have been used, sulphuric acid is found in newer processes.

photo: bacteria used in ethanol fermentation

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