Biomass is a cheap recycled fuel and which is also sustainable. Biomass probably would have been the original fuel used by human beings ever. The efficiency would have been very low from the very inception but it was never an issue then. Over the years even the renewable biomass thermal energy technologies have improved to obtain higher efficiencies in leaps and bounds due to the dwindling resources and the need for conservation. One such improvement is the development of gasification boilers burning wood gas for space and water heating needs.
The composition of Wood gas
Originally designed to be used in boilers with gas generated from coal the non renewable fossil fuel (during the infancy of the industrial revolution when coal burning was not considered a problem) it was much efficient than wood burning.
What is wood gas?
Wood gas is a mixture of several combustible gases which would release thermal energy at high temperatures when burnt. It is also known as Producer Gas and contains a few other gaseous impurities as well.
A typical sample obtained from wood gas would contain;
• Hydrogen –molecular (14%)
• Carbon monoxide (27%)
• Methane (3%)
• Nitrogen (50.9%)
• CO2 (4.5%)
• Oxygen (0.6%)
• Polycyclic Aromatic Hydrocarbons (PAH)
• Tar
Production of Wood Gas
Producer gas is generated by the Thermal Gasification of Biomass or other carbonaceous material such as coal in a gasifier (wood gas generator or synthesis gas generator).
The process involves two chemical processes at high temperatures (over 700 0C).The first is an endothermic reaction which burns carbon to CO2 but is reduced with an endothermic reaction to produce CO gas.
The second reaction (which is endothermic) is where Carbon is made to react with Steam producing CO, molecular Hydrogen, and CO2. In several gasifiers the actual gasification is preceded by Pyrolysis (transformation due to heat) of carbonaceous biomass matter (wood, Pellets etc) to char, releasing Methane and Tar. In others the gasifier is fed with pre charred biomass.
Wood Gas Boilers (Gasification boilers)
A gasification boiler would use wood gas to produce Heat energy. The technique is known as “updraft” method. The process consists of 4 major stages.
• Drying and release of wood gases in the loading chamber in a slow glowing process
• Burning of Gas mixture with secondary air in the lower chamber at a temperature of 1204 oC
• Flame reheating and Heat exchanging
• Combustion gases exhausting through chimney flue
Today you will find gasification boilers’ efficiency has improved tremendously.
The gasification process converts approximately 75% of the fuel energy content of the biomass to combustible gas. Up to 3 fold increases in the conversion of energy contained in the fuel over that of traditional wood boilers have been noted in gasification boilers.
The efficiency is affected by moisture contend of the fuel (which could be any biomass including wood, pellets, wood shavings, saw dust, grain husk etc.).The longer the storage the lesser will be the humidity of Biomass fuel. A humidity of 20% is ideal for best energy efficiency for low emission of CO, smoke etc. A wood pellet boiler with a centralized hydronic (i.e. Water circulating) system on the other hand could be as efficient as 92%.
Thanks the Techstore Team
