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The main component of this conversion process is the "Gasifier", in essence a simple device consisting of a cylindrical container, which is often 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 an engine with only relatively minor modifications. The engine can be coupled with a generator in the normal way to produce electricity as the final out put.
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
3. Oxidation (Combustion)
Drying of Fuel
The fuel wood is 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%.
The dried wood enters 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.
The Pyrolysis Process
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.
C + O2 CO2
4 H + O2 2 H2O
CnHm + (n/2+m/4) O2 n CO2 + m/4 H2O
In this region the hot gases formed in the above process is converted in to "Producer Gas" by the following two endothermic reactions.
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.
The most popular gasifier design for personal use is the downdraft gasifier. There are many good reasons for this:
In a downdraft gasifier the fuel is loaded at the top and a fire is lit in the bottom. A suction blower draws in air either through an air jacket like in the Imbert design or down through the top like the FEMA design.
The incoming air allows partial combustion to take place in the lower hearth area. The heat from that combustion produces pyrolysis above and reduction below.