1. General

The energy from [GLOSS]biomass[/GLOSS] can be released in three different ways:

• Direct [GLOSS]combustion[/GLOSS], utilizing as primary or substitute fuel;
• [GLOSS]Fermentation[/GLOSS]/digestion or;
• Gasification.

Biomass gasification is a thermal process converting dry biomass feedstock into a mixture of gases that can be burnt in internal combustion engines and gas turbines. The gasification process takes place in a sealed container with a restricted supply of air. For this reason the process is sometimes known as “gasification by partial combustion”.

2. Biomass Gasification

The gasification process can be broken down into three phases. The first phase is an
endothermic  process of [GLOSS]Pyrolysis[/GLOSS] during which the biomass is converted by heat into [GLOSS]char[/GLOSS] and [GLOSS]volatile matter[/GLOSS], such as steam, methanol, acetic acids and tars. The second phase is an [GLOSS]exothermic reaction[/GLOSS] in which part of the carbon is oxidised to carbon dioxide. In the third phase, part of the carbon dioxide, the volatile compounds and the steam are reduced to carbon monoxide, hydrogen and methane. This mixture of gases diluted with nitrogen from the air and unreduced carbon dioxide is known as producer gas. If the original feedstock is charcoal, then the gasification process becomes two-phased, and the amount of tar produced is cut down.

The conversion process from solid biomass to producer gas resolve around the initial combustion reaction (a), which occurs in the reactor, nearest the air inlet.

(a) C + O2 + 3.79N2 —> 3.79N2 + CO2 (exothermic reaction)

The carbon dioxide produced in this first reaction is then, in the presence of glowing carbon, reduced to carbon monoxide (b). When the gasifier is first lit, the level of carbon dioxide is at its highest. The quality of the producer gas increases with the increase in reactor temperature and size. The percentage of the carbon dioxide reduced to carbon monoxide will depend on the temperature in the reactor, a factor that will affect the design of a gasifier.

(b) C + CO2 + 3.79N2 —> 3.79N2 + 2CO ([GLOSS]endothermic reaction[/GLOSS])

The ideal reaction that should occur in the reactor bed is (c):

(c) 2C + O2 + 3.79N2 —> 3.79N2 + 2CO (exothermic reaction)

The calorific value of the producer gas formed in this reaction is 4.359 MJ/m3 at standard temperature and pressure ([GLOSS]STP[/GLOSS]). This chemical reaction only occurs with carbon, and with wood as the feedstock there are other elements involved. The main constituents of dry wood are cellulose, hemicellulose, lignin, resin and trace mineral matter. The chemical composition of wood, by mass on a dry and ash free basis is 50% carbon, 6% hydrogen and 44% oxygen, the exact composition varying between species. The energy content of dry wood is remarkably constant at around 20MJ/kg. Typical pyrolysis of wood will produce by mass 30% solids in the form of ash and char, 50% liquids in the form of tar, pyroligneos acid and water vapour and 20% gases in the form of carbon monoxide, carbon dioxide, hydrogen, hydrocarbons and nitrogen.

The combustible content of producer gas is mainly carbon monoxide, with varying fractions of hydrogen and hydrocarbon gases depending on the primary feedstock. The combination of carbon monoxide and nitrogen, gives the gas a relatively low Calorific value of between 4 and 5 MJ/m3. However, the stoichiometric mixture of fuel and gas required by an engine run on producer gas is of the order 1:1 compared with 1:9 for natural gas. Thus the relative power produced by an engine run on producer gas is far more comparable. [1]

Keywords:

Biomass, gasification, pyrolysis.

Source:

[1] De Montfort University, http://www.dmu.ac.uk/ln/itc/gastheo.htm