Environmental Engineering Reference
In-Depth Information
The production of BtL fuels is relatively complex. The fi rst stage is a gasifi cation
of the biomass raw materials. Through the addition of oxygen and steam a synthesis
gas consisting of carbon monoxide (CO) and hydrogen (H
2
) is produced at high
temperatures. Different gas purifi cation stages separate out carbon dioxide (CO
2
),
dust and other impurities such as sulphur and nitrogen compounds. A synthesis
process converts the synthesis gas into fl uid hydrocarbons.
The best-known synthesis process is the Fischer-Tropsch process developed in
1925. Named after its developers Franz Fischer and Hans Tropsch, this process is
carried out at a pressure of around 30 bars and temperatures above 200 °C using
catalyzers. During the Second World War this process was widely used in oil-poor
Germany to extract much sought-after liquid fuels from coal. A different procedure
then produces methanol from the synthesis gas and processes it further into fuel.
During the fi nal production processing stage the liquid hydrocarbons are separated
into different fuel products and refi ned (Figure 12.15 ).
Figure 12.15
Principle of the production of BtL fuels.
BtL fuels have not yet reached a stage where they are ready for mass production.
Various companies are currently setting up prototype facilities for producing syn-
thetic biofuels. For example, Volkswagen and Daimler have reserved the brand
names SunFuel and SunDiesel and are collaborating with the manufacturing fi rm
Choren Industries. The main advantage of BtL fuels is that they can replace con-
ventional fuels directly without the need for any engine modifi cations. However,
BtL fuels are comparatively expensive because of the complex production
procedures involved.
12.4.5 Biogas
In addition to its use in the production of liquid fuels, biomass can also be used in
biogas plants to produce biogas. In this process bacteria ferments biomass raw
materials in a moist, hermetically sealed environment. The centrepiece of a biogas
plant is the heated fermenter (Figure 12.16). A stirring device mixes the substrate
and ensures that homogeneous conditions exist. The biological decomposition
