Gasification boilers also known as wood gas boilers are designed for using primarily biomass as fuel. It is an excellent renewable source of alternative energy. The conversion of fuels to heat energy via gasification is an age old process and the industrial revolution itself is partially indebted to its discovery.
What is biomass?
In general Biomass refers to living or recently dead material which could serve as a fuel for the generation of heat energy. This grouping specifically excludes the fuels of plant origin such as Petroleum based fuels and the solid fuels such as coal because these materials have been out of the cycle of energy renewal for millions of years.
Examples of biomass used as fuel include;
Corncobs and kernels
Poplar, willow, eucalyptus and thousands of types of wood
Sorghum, Paddy and Wheat (stalk)
Fire Wood, Wood Shavings and saw dust
Algae, Hemp etc.
and even municipal waste!
It is possible to use this material directly as fuel depending on their moisture content and the constituency but
invariably the combustion process is very low in efficiency.
What is Fire?
When simplified the process of Fire is a chemical reaction involving Carbon, Oxygen and Hydrogen that end in the formation of CO2 and Water all the while emitting heat energy. (called an exothermic reaction).In the presence of excess atmospheric air the efficiency is only approximately 40% meaning that the major portion of your fuel (which in the end really means hard earned cash) is wasted up in the fire!
How can the Process be made efficient?
To answer this you have to get an idea what the process of a fire is. A fire or burning should undergo 3 sub processes of Heat. Viz.
Pyrolysis – Cracking of the biomass at about 450 0C to Charcoal, Gas and Oils
Gasification – Conversion of a solid to a combustible Gas and
Combustion – Burning of the Gas producing CO2 and H2O
While all good fires should exhibit these 3 stages of heat, the hitch is when the burning takes place in an environment with plenty of air. The excess air causes the fuel to burn with less energy and incomplete combustion bypassing the gasification stage.
The answer to this question therefore lies in the technique of Gasification. While in an open fire this is not an easy process, in an enclosed piece of equipment like a gasification boiler or a gasification cooker it can be easily controlled to produce the desired end results.
What Happens to the Biomass fuel in a Gasification Boiler?
In Gasification Boiler the Solid Biomass Fuel is fed in to a hopper in the upper section which is carried to a combustion chamber in an auger (which carries the fuel to the combustion chamber through screw action).The combustion chamber is supplied with controlled Air and the fuel moves downward where in the process meet the Flaming Pyrolysis flame front (which is a by product of the fuel burnt few seconds ago while moving down themselves) rising from below.
The solid fuel is converted to gas and charcoal. The gas which rises up as a flame front is consumed again to produce more gas. The gas is meanwhile also sucked out by mechanical ventilators along with a quantity of products of combustion (char-ash). The high heat generated in the process is utilized to heat up water in heat exchangers in the Boiler (in which heated water is circulated).
What is the Gas produced from Biomass?
The gasification of Biomass generates Producer Gas. A typical sample of Producer gas would show up like this;
Carbon Monoxide Gas (CO) – 22%
Hydrogen Gas (H2) – 18%
Methane Gas (CH4) – 03%
Carbon Dioxide (CO2) – 06%
Nitrogen Gas (N2) – 51%
The gas is also commonly known as wood gas, a reference to the currently widely used biomass fuel which is Wood, Wood Shavings, Saw Dust, Wood Pellets etc. In the above gas analysis you would note Hydrogen is present along with two other gases (CO and CH4) all of which are good fuels. Naturally the mixture will burn with unprecedented efficiency.
Wood gas or Producer gas therefore is an excellent source of energy in heating applications and the Gasification Boiler is a good example.