Gasification - The Cleanest Method Of Conversion

The newest method for energy generation is known as gasification. This method captures 65-70% of the energy present in solid fuels by first converting it to combustible gases. These gases are then burned, like we currently burn natural gas, to create energy. AESI utilizes gasification technology on every energy project we design.

The essence of gasification process is the conversion of solid carbon fuels into carbon monoxide by thermochemical process. The gasification of solid fuel is accomplished in closed chamber, under slight suction or pressure relative to ambient pressure. Gasification is quite complex thermochemical process. Splitting of the gasifier into strictly separate zones is not realistic, but nevertheless conceptually essential. Gasification stages occurs at the same time in different parts of gasifier.

The basic process elements are described below:

1. Drying - Biomass fuels consist of moisture ranging from 5 to 35%. At the tempearture above 100 C, the water is removed and converted into steam.  In the drying , fuels do not experience any kind of decomposition.
2. Pyrolysis - Pyrolysis is the thermal decomposition of biomass fuels in the absence of oxygen. Pyrolysis involves release of three kinds of products: solid, liquid and gases. The ratio of products is influenced by the chemical composition of biomass fuels and the operating conditions. The heating value of gas produced during the pyrolysis process is low.
3. Oxidation - Introduced air in the oxidation zone contains oxygen and water vapors, and inert gases such as nitrogen and argon. These inert gases are considered to be non-reactive with fuel constituents. The oxidation takes place at the temperature of 700-2000 C. Heterogenous reaction takes place between oxygen in the air and solid carbonized fuel, producing carbon monoxide.

Reduction

In reduction zone, a number of high temperature chemical reactions take place in the absence of oxygen. The principal reactions that take place in reduction are mentioned below.

Boudouard reaction CO₂ + C = 2 CO - 172.6 [MJ/kmol]

Water-gas reaction C + H₂O = CO + H₂ - 131.4 [MJ/kmol]

Water shift reaction CO₂ + H₂ = CO + H₂O + 41.2 [MJ/kmol]

Methane production reaction C + 2 H₂ = CH₄ + 75 [MJ/kmol]

Main reactions show that heat is required during the reduction process.Hence, the temperature of gas goes down during this stage. If complete gasification takes place, all the carbon is burned or reduced to carbon monoxide, a combustible gas and some other mineral matter is vaporized. The remains are ash unless the unit partially consumes the carbon thereby producing char, a byproduct of incomplete combustion.