Biological conversion processes

There are two main variations of the biological biomass conversion process. Anaerobic digestion produces methane gas, which can be used in a similar way to natural gas, while fermentation can be used to produce ethanol, which is being tested as a substitute for petrol.

Anaerobic digestion

Anaerobic digestion is a process by which organic matter is decomposed by bacteria in the absence of oxygen. Hydrolytic microorganisms, also including common food spoilage bacteria, break down complex organic wastes. These sub-units are then fermented into short-chain fatty acids, and carbon dioxide and hydrogen gases.

Syntrophic microorganisms then convert the complex mixture of short-chain fatty acids to acetic acid with the release of more carbon dioxide and hydrogen gases.

Finally, methanogenesis produces biogas from the acetic acid, hydrogen and carbon dioxide. The end product is biogas normally consisting of 50 to 60 % methane and CO₂. (See process chart below.)

Anaerobic digestion can occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs and the intestines of animals. The decay of biomass in landfills also produces gas (methane) that can be collected and burned in a combustion chamber to produce steam for electricity generation or for industrial processes.

All organic matter is suitable for biogas development - the prerequisite is the absence of oxygen and the presence of methanogenic bacteria - including wastewater, manure, domestic waste and biological by-products. Purified biogas developed by anaerobic digestion can be used both for heating purposes and for electricity generation.

   

 

Fermentation

Fermentation refers to the process by which plants of high sugar and starch content (sugar beet, wheat, corn, sugar cane etc.) are broken down with the help of microorganisms to produce ethyl and methyl alcohol. Although essentially the same process is used when making alcoholic beverages - i.e. yeast and heat is used in order to break down the biomass material into simple sugars - the changing of corn into ethanol actually takes many steps.

First starch in corn must be broken down into simple sugars before fermentation can occur. Once a simple sugar is obtained, yeast is added. Yeast is a single-celled fungus that feeds on the sugar and causes the fermentation. As the fungus feeds on the sugar, it produces alcohol (ethanol) and carbon dioxide.

The end product is a combustible fuel that can be used in vehicles. In fermentation, the ethanol retains much of the energy that was originally in the sugar, which explains why it is an excellent fuel. It blends with petrol up to 5% without the need to change fuel or engine specifications. It can also be easily distributed through existing distribution systems.

Chemical conversion

Biodiesel is produced by chemically upgrading oils, which can be:

-        oil seeds (sunflower, rapeseed, soya, castor etc.)

-        waste cooking oil or

-        waste animal fats.

By means of transesterification, the tryglycerides of the oils can be transformed into biodiesel. The production principles of biodiesel depend on its feedstock.

If the feedstock is oil seed, it is first extracted. The by-product of extraction, the remaining oil cake, can be used as animal feed. In the case of waste cooking oil or waste animal fats, the feedstock is mechanically cleaned for processing. In the next step, through the addition of methanol and a catalyst, transesterification takes place, which is followed by purification and the refining of the glycerine-layer. As a co-product the glycerine-layer can be processed into glycerine, methanol – which can be reused in the process – and potassium fertilizer.

The production principle of biodiesel is illustrated as follows:

 

The end product is biodiesel, which is – due to its ecological advantages - more and more common as motor fuel, especially in ecologically sensitive areas, like nature reservations, glaciers, lakes etc.