is a process that converts carbon based materials, such as
into a biogas composed mainly of carbon monoxide, carbon
and hydrogen. The gasification process is achieved by reacting the
material at high temperatures, without combustion and with a controlled
amount of oxygen and/or steam.
There are several
different stages to the gasification process.
1. In the Drying
water within the fuel is vaporised. In the Burkhardt reactor
(see right), this occurs at approximately 25 - 275°C.
2. In the Pyrolysis
the fuel decomposes into gaseous volatiles and solid char. In
Burkhardt reactor, this occurs at approximately 350 -
3. In the Oxidation
a limited amount of combustion takes place. The gaseous volatiles and
some of the char react with oxygen to primarily form carbon dioxide and
small amounts of carbon monoxide. These combustion reactions
provide heat for the subsequent gasification
the Burkhardt reactor, this occurs at approximately 700 -
4. In the Reduction
the final gasification reactions take place as the char reacts with the
water vapour generated in the drying zone to produce carbon monoxide
and hydrogen. In the Burkhardt reactor, this occurs at
approximately 850 - 1100°C.
The biogas produced by gasification can be
combusted and, if it is
clean enough, used as fuel for a gas engine. Thus, heat, mechanical or
electrical power or, a combination of these, can be generated.
Burkhardt system, the fuel and oxygen are fed into the
bottom of the reactor and form a fluidized bed within which the
gasification process occurs. The outlet for the biogas product is at
the top of the reactor. The composition of the biogas generated is
shown below. The biogas is fed from the gasifier, via a gas line,
through to a gas engine CHP unit which uses it to generate further heat
The advantage of
gasification is that using the biogas is potentially more efficient
than direct combustion of the original fuel because it can be combusted
at higher temperatures. As a result, the thermodynamic upper limit to
the efficiency of the system defined by Carnot's
rule is higher.
not a new technology, records of gasification experiments using coal
and wood as the feedstock date back to the 17th
century. During World War II, the shortage of petroleum led to an
upsurge of interest in gasification and by 1945 many different types of
vehicles had been adapted to run on biogas generated by gasification
gasification has a long history, it is also a chequered one. Many
biomass systems have failed including several large heat network
schemes in the UK. Many of the problems that have been encountered are
due to problems with the feedstock.
Biomass-based feedstocks are often naturally variable and a variable
feedstock causes problems in the gasification process. Feedstock
consistency and standard is essential if the process is to be well
It is for this
reason that the Burkhardt gasifiers use high-quality wood pellets as
feedstock. By only using EN
Plus A1 wood pellets they ensure a consistent quality of
feedstock for the gasification process and have thus overcome the
traditional problems associated with biomass gasification.