• Bio-gas as a fuel for cogeneration units
• Preconditions for using bio-gas for the operation of cogeneration units
1. Installation Possibilities
2. What are the properties of bio-gas?
3. What are the possibilities of production and collection of bio-gas
4. What if sulpher content is more in Biogas?
5. Are natural gas distributors accessible?
6. What are the requirements for functioning of cogeneration units??
7. What is the existing consumption and price of energy in the building?
8. Requirements of the properties of bio-gas?
• Typical Applications

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

1. Milk Farms and Dairy
2. Boiler Room
3. Hospital
4. Hotel
5. Industry
6. Sewage Treatement Plant
7. Waste Dump

The properties of bio-gas decide its usability from the point of view of damaging admixtures and energy substantiality (calorific capacity). Important information is:

1. Methane content (best is the whole composition of the gas)
2. Firmness of quality of the gas
3. Content of damaging admixtures

The amount of bio-gas produced will influence the decision on the type of cogeneration unit chosen.

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 oxygen.

The 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

1. 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 chemicals.

1. 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.

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.

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 "island" regime?

These data are important for suggesting the suitable type of cogeneration unit and its way of use

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
- Commonly 55 to 65%. The lowest limit is a concentration of 50%.

Bio-gas Pressure
- 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 de-sulphurizing equipment.