ENERGY ALTERNATIVES: RENEWABLE
ENERGY AND ENERGY CONSERVATION
TECHNOLOGIES
Wood Gsification :
Biomass gasification is basically conversion of solid biomass
(wood, wood waste , agricultural residues , etc. ) into
combustible gas mixture normally called producer or low Btu
gas. The process is typically used for woody biomass and
involves partial combustion of such biomass . Partial
combustion occurs when air supply is less than adequate for
complete combustion of biomass. It produces carbon monoxide
(CO) as well as hydrogen (H2), both are combustible gases.
Conversion to gas results in loss of up to 25% energy. Use of
gas can be highly efficient and hence overall efficiency
could be very high. Gas can be fed directly into internal
combustion engines (I.C. engines) thereby saving commercial
fuels. Also, it can be employed at any scale (above few kilo
watts ), and hence is ideally suited for decentralised
applications -shaft power, electricity or thermal energy. In
case of shaft power / electricity, the gas is basically burnt
inside an engine (diesel based - compression ignition engine)
with pilot diesel injection to start combustion.
Biomass gasification provides a valuable fuel for both mobile
and non mobile uses. Producer gas can replace natural gas,
gasoline or fuel oils used to make steam for generating
electricity and fire boilers, and produce heat for industries
and homes.It can also be used as fuel for internal combustion
engines for a wide array of purposes. Gasification is an
efficient way to extract heat from biomass. It is estimated
that for each 100 kcal of potential energy in solid fuels,
gasification can extract about 80 kcal in hot raw gas. This
is more efficient than many devices that burn biomass
directly in a hearth or fire box. Producer gas can be piped
short distances and used for industrial purposes like fuel
kilns: brick kilns, ceramics, glass pottery, etc.,for boilers
in rice mills, saw mills, cashew industry or for power
generation. In any case, while using producer gas for heating,
the burner must be designed for operation on low energy gas.
The function of a gasifier is to convert solid carbonaceous
fuel, such as wood into combustible gas by a combination of
oxidation, pyrolysis and reduction process (Talib, et al.,
1987). The major chemical reaction taking place are both
exothermic ( C+O2 -> CO2, C+0.5O2 -> CO ) and endothermic
(C+H2O-> CO+H2, CO2+C-> 2CO). In order to effectively carry
out these reactions, the solid fuel passes through the
following zones: drying, pyrolysis, combustion and reduction.
In drying zone the temperature is around 150°C and moisture
in the biomass is driven off. In pyrolysis zone, at 400°C
thermal break down of wood / bio residues takes place in the
absence of air, resulting in the formation of methanol,
acetic acid and heavy hydro carbon including tar. The solid
material left after pyrolysis is primarily fixed carbon in
the form of charcoal. The pyrolysis material along with the
gases and the organic vapors produced passes through the
combustion zone. In combustion zone, exothermic reaction
takes place and the heat released is used for sustaining both
the pyrolysis and the reduction reactions. The temperature in
this zone ranges from 1000-1500°C. Combustion zone has
controlled introduction of air.
By forcing the vapors through a narrow area directly
underneath the combustion zone, organic liquids and tar
formed in the pyrolysis zone are cracked. In the cracking
process, heat intensity increases. The gases thus formed are
drawn to reduction zone where endothermic reaction takes
place. The mixture of final product gases (producer gas)
consisting of 18-25% CO, 13-15% H2, 3-5% methane (CH4),
0.2-0.4% heavy hydro carbon, 5-10% CO2, 45-54% N2, 10-15% H2O
and particulate matter is drawn into the clean up system at
about 250°C. In clean up system, the gas is cooled and
cleaned to remove particulates and later introduced into the
engine. Commercially available biomass gasification system
for power generation covers the range of 3-500 kW. The major
areas where this system is relevant for power generation in
a decentralised manner are:
(a) Village electrification in remote areas which have
adequate biomass resources,
(b) Energisation of a number of pumpsets located in a
cluster,
(c) Captive power for industrial units located in rural
areas having extensive land area which can be devoted
for energy plantations, and
(d) Captive power for industries that have biomass waste
processes such as paper mills, saw mills,rice mills,etc