What do we mean by Gasification?

Wood biomass can either be used directly as a fuel to heat a conventional type of boiler 

and produce steam for power production or a completely separate process can be 

supported where the wood is used to produce a gas. Gasifier technology has undergone 

many improvements in the last 50 years because of increased fuel prices and 

environmental concerns. Gasification has become more modern and is now a quite 

sophisticated technology. The advantage of this technology is the operation of a 

decentralized energy conversion system which can prove to be economic on a small 

scale. 

The main component of this conversion process is the “Gasifier”, in essence a simple 

device consisting of a cylindrical container, which is made out of stainless steel. The 

simplest process – downdraft gasification, can be used to produce electricity on a small 

scale. In such a process, the gasifier is called a “Downdraft Gasifier” and the process that 

occurs inside the gasifier is explained below. The process of gasification can be 

considered as a thermo chemical process, which converts biomass materials into gaseous 

components, which is referred to as “Producer Gas”, containing carbon monoxide, 

hydrogen, methane and some other inert gases. A simple system mixes the producer gas 

with air and the resulting mix can be used to run a diesel engine with only relatively 

minor modifications. The diesel engine can be coupled with a generator in the normal 

way to produce electricity as the final out put. 

Downdraft Gasifier 

Downdraft gasification generally produces a low particulate and low tar gas so it is 

suited for power generation in small scale applications. 

The gasifier must have a chemical reactor where the above process is taken place. The 

fuel wood gets dried, heated, pyrolysed, partially oxidized and reduced inside this reactor. 

The four basic processes of gasification are noted below. 

1. Drying of the fuel 

2. Pyrolysis 

3. Oxidation (Combustion) 

4. Reduction 

3⁄4 Drying of Fuel 

The fuel wood pellets are heated and dried at the top of the gasifier unit. Moisture 

contained in the wood pellets is removed in this region to a level below 20%. 

3⁄4 Pyrolysis 

The dried wood pellets enter the second zone called the “Pyrolysis” zone. The gaseous 

products from devolatilization are partially burnt with the existing air. This process is 

termed “Pyrolysis”.
Both Pyrolysis and gasification turn waste into an energy rich fuel by heating the waste 

under controlled conditions. In contrast to incineration, which fully converts the input 

waste into energy and ash, these processes deliberately limit the conversion so that 

combustion does not take place directly. Instead, the waste is converted into valuable 

intermediates that can be further processed for materials recycling or energy recovery i.e. 

its convert to Pyrolysis gas and charcoal. 

Wood Pellets Pyrolysis Gas Charcoal 

Pyrolysis Process 

3⁄4 Combustion 

 In the combustion zone the outputs from the above zone, react with the remaining char in 

the absence of oxygen at a temperature of around 800-900 °C. 

Basic Reactions 

C + O2 CO2 

4 H + O2 2 H2O 

CnHm + (n/2+m/4) O2 n CO2 + m/4 H2O 

3⁄4 Reduction 

In this region the hot gases formed in the above process is converted in to “Producer 

Gas” by the following two endothermic reactions. 

Boudouard reaction: 

 C + CO2 . 2CO HR = + 160.9 kJ/mol 

Water gas reaction:

C + H2O H2 + CO HR = + 118.4 kJ/mol 

As these reactions proceed the temperature sinks progressively until it becomes so low 

(700 °C) that the reaction rates are insignificant. This means that the extent of the char 

reduction zone is dependent on the amount of energy entering the reduction zone and 

consequently also on the heat losses from the reactor. Although there is a considerable 

overlap, each process can be considered to be occupying a separate zone, in which 

fundamentally different chemical and thermal reactions take place. The fuel must pass 

through all of these zones to be completely converted. 
Composition of Producer Gas 

Constituent of wood gas at 650o

C 

Composition H2 CO CH4 CO2 N2 H20 

(% by 

weight) 18.0 18.0 1.8 10.8 41.4 10.0 

Composition of producer gas of fuelwood with 10% moisture level Source: VIT Research 

Notes 1648-Feasibility of Electricity Production from Biomass by Gasification Systems 

Overall efficiency of plant 

Unit Efficiency (%) 

Gasifier unit 75 

Engine 33 

Generator 92 

Overall 22.5% 

Overall efficiency (Source: VIT Research Notes 1648-Feasibility of Electricity 

Production from Biomass by Gasification Systems)

Basically this efficiency will vary around 16% to 24% in present systems. This will be 

the overall efficiency in production of electricity, but using flue gas to dry the fuel wood 

we can increase the usage of energy in fuel wood.