|Biogas Based CHP Unit :
|Bio-gas as a fuel for cogeneration units
Cogeneration units are equipment for the combined
production of heat and electricity. Small units mainly use rotary
gas-burning motors, adapted for running on gas fuels. The dominant fuel
is natural gas, but alternative fuels are becoming more common,
especially the various forms of biogas. Biogas can be obtained
from biogas stations constructed mainly around sewage
treatment plants, communal waste dumps or agricultural etc. Instead of
producing only heat by burning biogas in boilers, cogeneration offers the
chance of also producing electricity that can be used for your own consumption
or can be sold to the distribution network. In the case of own consumption, it
is possible to get much cheaper electricity than by buying it from the
network; if it is for sale, it is possible to use the advantageous
purchase rates for electricity generated from a renewable energy source.
Since biogas regularly originates as a by-product during the treatment of
organic waste, the costs for running a
cogeneration unit are mainly for writing-off of the equipment and service costs.
Revenues come from cost savings on heat and electricity, or
from the sale of electricity to the network
|Preconditions for using bio-gas for the operation of cogeneration units
In order to use bio-gas for
the operation of cogeneration units and gain some economic benefits, it is
necessary first of all to be clear about the following:
- Milk Farms and Dairy
- Boiler Room
- Sewage Treatement Plant
- Waste Dump
|What are the properties of bio-gas?
The properties of bio-gas
decide its usability from the point of view of damaging admixtures and energy
substantiality (calorific capacity). Important information is:
- Methane content (best is the whole composition of the gas)
- Firmness of quality of the gas
- Content of damaging admixtures
|What are the possibilities of production and collection of bio-gas?
The amount of bio-gas
produced will influence the decision on the type of cogeneration unit chosen.
if sulpher content is more in bio-gas?
Sulpher Scrubbing System is the best solution for removal of
sulpher in bio-gas|
Sulpher Scrubbing System:
The biogas cleaning plant is based on a
biological process, which utilizes naturally occurring bacteria to
remove H2S by oxidizing the same in the presence of
H2S oxidation phenomenon is encountered in sewage
application. H2S gas is generated in sewage lines due
to action of sulphur reducing bacteria present in the sewage. The
H2S generated in turn is oxidized to H2SO4.
This results in corrosion of sewage lines. This principle is
utilized in treating biogas for removal of H2S.
The biogas stored in the raw gasholder is compressed in a
biogas blower and sent to the gas scrubber column. A Roots type
blower is provided for compression of biogas up to 3000 to 5000 mm
of water gauge pressure. The gas-scrubbing column is packed with
plastic media. The biogas passes through this packed media. The
plastic media is initially seeded with bacterial culture.
Microorganisms capable of oxidizing H2S grow on plastic
media. H2S present in the biogas is oxidized to H2SO4.
A dosing tank containing solution of nutrients is provided. This
tank is provided with an agitator and a nutrient-dosing pump,
which periodically pumps nutrient solution on the plastic media.
This helps in the growth of microorganisms. The biogas passing
over the plastic media contains methane, carbon dioxide and H2S.
A small quantity of carbon dioxide is broken down due to action of
microorganisms into carbon and oxygen. The carbon is utilized for
the growth of microorganisms and the oxygen in oxidizing H2S
to H2SO4. If the concentration of H2S
is high in the biogas, an additional quantity of air is mixed with
biogas, which meets the additional requirement of oxygen. A
careful control over injection of air is maintained by monitoring
flow of biogas as well as air.
Due to generation of H2SO4, the pH of
nutrient solution drops down. The nutrient solution is
periodically replaced with a fresh solution. The acidified
nutrient solution is neutralized with lime and disposed off.
Advantages over the other processes, which are commercially available
- The process is biological and does not use any
chemical like compounds of iron used in other commercial processes. This
avoids use of filter press or centrifuge required to remove the
precipitates generated in removal of H2S. This simplifies the
operation of the plant. This also avoids the recurring costs of the
- The end product is H2SO4 rather
than precipitated sulphur, which is formed, in other commercially
available biological processes. Precipitated sulphur clogs the plastic
media resulting in stoppage of the plant. The H2SO4
generated in the proposed process is disposed off regularly and hence does
not require any maintenance stoppages.
|Are natural gas distributors accessible?
If natural gas distribution
is available, it is possible to use two-fuel cogeneration units for combined
operation on natural gas and bio-gas (fuel switching). It is advantageous
mainly when the production of bio-gas is not constant. In case of low-quality
bio-gas, it is possible to use a mix of bio-gas and natural gas.
|What are the requirements for functioning of cogeneration units?
Will it be enough to have
parallel operation with the network or to use cogeneration units even as an
emergency source of electrical energy, or to operate them in an
|What is the existing consumption and price of energy in the building?
These data are important
for suggesting the suitable type of cogeneration unit and its way of use
|Requirements of the properties of bio-gas?
The properties of the biogas are one of the basic
parameters, which affect the possibility of using the
operational motor in cogeneration units. Some properties may seriously raise
costs, or rule out its use altogether. In evaluating the properties of bio-gas
it is necessary to approach the question in a responsible manner. The following
characteristics must be known:
Methane CH4 content
55 to 65%. The lowest limit is a concentration of 50%.
- for burning bio-gas in cogeneration units the usual pressure value is in the
range of 1.5 to 10kPa.
Gas quality firmness (=stability of content and pressure of bio-gas)
- affects the stability of the running and damaging emissions
Damaging admixture content (mainly sulphur compound, fluoride and chloride)
- these compounds may cause corrosion of the chimney shaft parts and internal motor parts, coming into
contact with greasing oils. With high sulphur content it is advisable to use
|Typical Schematic of CHP Unit
It is generally true that cogeneration is always good to use when it is
suitable. It is possible to use
in all buildings with a demand for the consumption
of electricity and heat, or cooling. These are above all health centres, schools, baths and spas, communal heating, hotels and guest houses, industrial plants, sewage treatment plants, dumps and agricultural establishments.
Water Treatement Plant