Biomass gasification
DESI Power's analysis at the start of its programme was that biomass
gasification was the most promising technology for providing affordable
and competitive electricity supply and energy services to rural
areas where agricultural and plantations wastes were available.
A large parts of the wastes are, of course, being used in various
forms for cooking, drying, fodder and building materials, with the
largest part being burnt at very low efficiencies. In addition to
such a waste of a valuable raw material, there are other serious
adverse effects resulting from the traditional ways of burning biomass:
exposure of women to smoke and carbon monoxide which causes respiratory
and asthmatic disease, indoor and outdoor air pollution and smog,
and the traditional servitude of girls and women for the collection
and processing of cooking fuel.
DESI Power's aim was to look at modern new technologies which utilise
the available agro and plantation residues at much higher efficiencies
than in the traditional usage to generate electricity in a reliable
manner. The goal was to utilise the selected and proven energy technology
in a socially fair commercialisation system which enabled the villagers
to establish and own local value addition chains for the production
of goods and services which also generated local jobs. Energy services
for cooking, lighting, water supply and social services was planned
to be provided as an integral part of the village project.
Principles of Gasification
Biomass is a natural substance, which accumulates solar energy as
chemical energy by the process of photosynthesis in the presence
of sunlight. Biomass chiefly contains cellulose, hemi-cellulose
and lignin, having an average composition of C6H10O5, with slight
variations. For the complete combustion of biomass the theoretically
amount of air required (defined as the stoichiometric quantity)
is 6 to 6.5 kg of air per kg of biomass and the end products are
CO2 and H2O. In gasification, biomass is subjected to partial pyrolysis
under sub-stoichiometric conditions with the air quantity being
limited to 1.5 - 1.8 kg of air per kg of biomass. The resultant
mixture of gases generated during the gasification process is called
producer gas, contains CO and H2 and is combustible. The raw producer
gas also contains tar and particulate matter which have to be removed
as they are harmful to the engine.
A typical gasifier plant based on IISc technology consists of a
reactor, which receives air and solid fuel and converts them into
gas, followed by a cooling and washing train where the impurities
are removed. The clean combustible gas at a nearly ambient temperature
is available for running diesel-generator sets in dual fuel mode
or gas engine generator sets suitable for running on producer gas
alone. In thermal applications, the cooling and cleaning of the
raw gas is limited to the requirements of the thermal process.
In the down draft reactor, biomass feedstock undergoes drying and
de-volatilisation in the upper zones and produces char. The volatile
matters undergo oxidation in the combustion zone, with air being
partially drawn from the open top and partially supplied by air
nozzles located after the de-volatilisation zone. The gas then flow
through a hot charcoal bed in which the tar produced earlier is
burnt. This is the special feature of the IISc gasification process
which enables the tar in the raw producer gas to be maintained at
relatively low levels. Special washing and cleaning systems developed
as an integral part of the gasification system further reduce the
levels of tar and particles in the cold producer gas to very low
levels. These are the features which are responsible for enabling
the engines to run for long operating hours with maintenance requirements
which are similar to those specified for pure diesel or pure gas
firing.
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CO: 20 + 2%; CH4 : 3 + 1%; H2 : 20 + 2%; CO2: 12 + 1% ; Rest:
N2
The lower calorific value is about 4.5- 5.0 MJ/kg (1000 - 1200
kcal/ m3). |
Applications
Obtained by the process of gasification, can be employed in thermal
application or for mechanical / electrical power generation. Like
any other gaseous fuel, producer gas affords much better control
over power levels when compared to solid fuel. This also paves the
way for more efficient and cleaner operation.
For thermal applications, gasifiers are a good option as a gasifier
can be retrofitted with existing devices such as ovens, furnaces,
boilers, etc. Thermal energy of the order of 4.5 to 5.0 MJ is released
by burning 1 m3 of producer gas in the burner. Flame temperatures
as high as 1200° C can be obtained by optimal air preheating
and pre-mixing of air with gas. Producer gas can thus replace fossil
fuels in a wide range of devices. A few of the devices which could
be retrofitted with gasifiers are furnaces for melting non-ferrous
metals and for heat treatment, tea dryers, ceramic kilns, boilers
for process steam and thermal fluid heaters.
A diesel engine can be operated on dual fuel mode using producer
gas. Diesel substitution of over 80% at high loads and 70 - 80%
under normal load variations can be achieved. The mechanical energy
thus derived can be used either for driving water pumps for irrigation
or for coupling with an alternator for electrical power generation.
Alternatively, a gas engine can be operated with producer gas on
100% gas mode with suitably modified air / fuel mixing and control
system.
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