Cover Image
close this bookBiomass Gasification - Technology and Utilisation (Individual Authors)
View the document(introduction...)
View the documentOverview of Gasification Technology
View the documentBiomass as Gasification Fuel
View the documentGasification for Energy Supply
View the documentGasification History and Development
View the documentGasification Process
View the documentProducer Gas and it´s Constituents
View the documentHazards with Producer Gas
close this folderGas Producers (Gasifiers)
View the document(introduction...)
View the documentUpdraft gas producer
View the documentDowndraft gas producer
View the documentTwin-fire gas producer
View the documentCrossdraft gas producer
View the documentOther gas producers
close this folderImpact of Fuel Properties on Gasification
View the document(introduction...)
View the documentEnergy content of fuel
View the documentFuel moisture content
View the documentParticle size and distribution
View the documentBulk density of fuel
View the documentFuel form
View the documentVolatile matter content of fuel
View the documentFuels and their ash content
View the documentReactivity of fuel
close this folderSuitability of Some Biomass Fuels
View the document(introduction...)
View the documentCharcoal
View the documentWood
View the documentSawdust
View the documentPeat
View the documentAgricultural residues
close this folderProducer Gas Drive Engines
View the document(introduction...)
View the documentPeformance of gasifier-engine system
View the documentOperational difference between diesel and gasoline engine
View the documentConversion of gasoline engine to producer gas
View the documentConversion of diesel engine to producer gas
View the documentConditioning of Producer Gas
View the documentTroubles with Gasification System
View the documentLiteratures on Gasification

Gasification Process

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 air sealed, closed chamber, under slight suction or pressure relative to ambient pressure. Gasification process occuring in general explained in this section.


Image

Gasification is quite complex thermochemical process. Splitting of the gasifier into strictly separate zones is not realistic, but neverthless conceptully essential. Gasification stages occurs at the same time in different parts of gasifier.


Image

Drying

Biomass fuels consist of moisture ranging from 5 to 35%. At the temperature above 100°C, the water is removed and converted into steam. In the drying , fuels do not experience any kind of decomposition.

Pyrolysis

Pyrolysis is the themal 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 (3.5 - 8.9 MJ/m 3 ).


Image

It is noted that no matter how gasifier is built, there will always be a low temperature zone, where pyrolysis takes place, generating condensable hydrocarbon.

Oxidation

Introduced air in the oxidation zone contains, besides oxygen and water vapours, 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 700-2000o c.

Heterogenous reaction takes place between oxygen in the air and solid carbonized fuel, producing carbon monoxide. Plus and minus sign indicate the release and supply of heat energy during the process respectively

C + O 2 = CO 2 + 406 [ MJ/kmol]

In reaction 12.01 kg of carbon is completely combusted with 22.39 m3 of oxygen supplied by air blast to yield 22.26 m 3 of carbon dioxide and 393.8 MJ of heat.

Hydrogen in fuel reacts with oxygen in the air blast, producing steam .
H 2 + ½ O 2 = H 2 O + 242 [ MJ/kmol]

Reduction

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

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

Water-gas reaction
C + H2 O = CO + H 2 - 131.4 [MJ/kmol]

Water shift reaction
CO 2 + H 2 = CO + H 2 O + 41.2 [MJ/kmol]

Methane production reaction
C + 2H 2 = CH 4 + 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 and some char (unburned cabon)