2.            STATE OF THE ART TECHNOLOGY

There are three main ways to convert biomass into biofuels:

-        thermal, thermo-chemical processes,

-        biological processes and

-        chemical conversion.

The thermal method involves heat treatment of the biomass material, the biological conversion uses microbiological action to convert the biomass material into usable fuel, while chemical conversion makes use of the technology of extraction and transesterification.

 

Thermal, thermo-chemical processes

The thermal conversion process takes two main forms, the difference between the two being the amount of air allowed to take part in the chemical reaction:

-        gasification

-        pyrolysis or carbonization.

Gasification

Gasification is a technology that converts solid fuel into a gaseous one through reaction with hot steam and oxygen. The process of heating biomass is carried out in the presence of some air or oxygen, which is insufficient to fully combust biomass. The fuel gas developed through the process is a mixture of methane, CO₂, N₂, CO and H₂, the rate of which is determined by the technology.

There are several different gasification techniques, which are for the most part still in the development stage, but which share certain general production characteristics. The feedstock used for gasification includes materials of organic origin that would otherwise be disposed of as waste.

The feedstock is prepared and fed to the gasifier in either dry or slurred form. It reacts in the gasifier with steam and oxygen at high temperature and pressure in an oxygen starved atmosphere. This produces the synthesis gas, or syngas, made up primarily of carbon monoxide and hydrogen and smaller quantities of carbon dioxide and methane.

The fuel gas is cleaner than biomass, because chemical pollutants are removed during the process. The gas thus produced can be used in many ways:

-        for direct combustion

-        in internal combustion engines

-        in gas turbines

-        for producing hydrocarbons (e.g. methane, methanol)

Pyrolysis or carbonisation

Pyrolysis is a form of incineration that refers to a process where biomass is exposed to high temperatures in the absence of air, causing the biomass to decompose. The end product of pyrolysis is a mixture of solids (char), liquids (oxygenated oils), and gases (methane, carbon monoxide, and carbon dioxide). The exact composition of this pyrolysis gas varies with the reaction conditions and the biomass feedstock employed. Normal pyrolysis means heating at a temperature between 300-350 °C, while fast pyrolysis involving high-speed heating (at a temperature of around 800-900 °C) results in less char, more gas and oil with a higher CO content.

With flash pyrolysis techniques (fast pyrolysis) the liquid fraction can be up to 70 percent of the thermal biomass input. Bio-oil contains about 40 weight-percent of oxygen and is corrosive and acidic. The oil can be upgraded to reduce the oxygen content, but that has economic and energy penalties. Pyrolysis and upgrading technology are still largely in the pilot phase.

Hydrothermal upgrading (HTU), originally developed by Shell, converts biomass at a high pressure and at moderate temperatures in water to biocrude. Biocrude contains far less oxygen than bio-oil produced through pyrolysis, but the process is still in a pre-pilot phase (Naber and others 1997).

The simplified process flow diagram of pyrolysis can be illustrated as below:

 

 

 

 

 

 

 

 

 

 

Pyrolysis of organic material resulting in combustible gases requires further treatment. The off-gases may be treated in a secondary combustion chamber, burnt, and partially condensed. Several types of pyrolysis units are available, including the rotary kiln, rotary hearth furnace, or fluidized bed furnace. These units are similar to incinerators except that they operate at lower temperatures and with less air supply.