Biogas in Pakistan – some facts and figures
By Engr. Naeem ul Zaman & Engr. Asim bin Aziz
BIOGAS IS produced by anaerobic (absence of air) decomposition of organic matter by a bacteria present in the stomach of animals including buffalos, cows etc. Bio Gas is also known as Gobar Gas or Marsh Gas. Chemically it consists of 60-65% Methane (CH4), 35-40% Carbon Dioxide (CO2), and traces of Hydrogen Sulfide, Water Vapors, Nitrogen and other gases.
Because of its Methane content, it burns just like natural gas but has a lower heating value of nearly 600 BTUs/cft as compared to 925 BTUs/cft of natural gas (commonly known as Sui gas) which is more than 98% Methane.
Theoretically, it is very simple and easy to produce biogas by mixing equal amounts of fresh buffalo dung and water and put this in a vessel where there is no air. In summer, the gas will appear within few days time and in winter or in cold climate, it may take a little longer. The bacteria; present in the dung does all the work.
Despite the fact that it looks so simple, the production of biogas is not so popular in Pakistan. Large scale efforts were made to popularize biogas production during 1980s and lot of money was spent by the government, but after some time the efforts died down and only a few biogas plants installed at that time are still in production. At present Pakistan Council of Renewable Energy Technology (PCRET) is trying to propagate biogas production but on low key basis. A number of NGOs as well as commercial organizations are also working on biogas production.
Gas produced in a small bio-digester which contains about 20 kg of dung should be enough to meet the fuel requirement of a small family. Based on these calculations, a bio-digester for any number of animals can be designed. The thumb rule is one cubic meter (35.5 cft) digester volume for 10 Kg of cow dung (one cow).
Large size digesters use different type of biomass such as potato waste, chicken droppings, tannery waste, green silage (leaves, corn etc), food scrapes, bakery waste, molasses, slaughter house waste (poultry and other) fats and greases to produce biogas at industrial scale, both for electrical power generation as well as for heating purposes.
A biogas production facility is basically a gas tight vessel with possibility of adding fresh slurry and removal of gas and waste slurry. For intermittent use of gas such as domestic needs, gas storage is necessary. In case of industrial use, either a separate storage tank is used or excess gas is flared (burned). Storage may be an integral part of the digester or it may be a separate facility.
For domestic purposes, biogas is produced at very low pressure that is a few inches of water column. It can safely be transported through flexible plastic pipes recommended for natural gas. For transferring gas for a couple of small size burners over a short distance of few feet, heavy duty plastic tube of 6 mm bore can be used. Since biogas is produced in water slurry in relatively warm condition, it is saturated with water vapors and may also contain traces of foul smelling vapors.
Biogas is 60% methane and 40% Carbon Dioxide with traces of hydrogen sulfide, water vapors and sometimes foul smelling substances. For use of biogas as fuel on domestic level, no treatment is required. When large quantity of gas is consumed as fuel, condensate removal and flame arresters may be required for trouble free use.
In case biogas is used as fuel for internal combustion engines for power production, the impurities present in biogas may be undesirable. Some engine manufactures offer engines that accept raw gas; otherwise the advice of engine manufacture may be sought.
In case it is desired to store and use biogas for automobile use (as it is being done elsewhere) the gas should be purified before compression. There are a number of methods available for purification of bio gas (chemical treatment, membrane separation etc)
Nearly 10,000 BTUs of thermal energy is required to generate one KWt of electricity by internal combustion engines. Since the heating value of bio gas is close to 600 BTUs per cft, to generate one KWt of electricity, 17 cft or 0.5 M3 of biogas is required. Thus to run a 5 KWt generator for one hour, 85 cft (2.5 M3) of biogas is required. To operate a 5 KWt water pump for five hours a day, 425 cft or 12.5 M3 of gas is required. This quantity of gas can be generated from the cow dung of twenty cows. In case of dairy farms with high milk yield cows, the power production would be much higher.
Bio gas can run a petrol engine without any major modification. Even a diesel engine (Peter Engine) can run on a mixture of 50:50 diesel and biogas. There is a lot of potential of power generation from biogas in our rural areas and the economics of farmers can be improved.
Refrigeration units running on gas are available in the market. These units are ideal for chilling milk and preserving agro products before these are sent to the market.
The cow dung contains 16 % dry matter and nearly half of this is converted into gas. Since the slurry to the digester is fed with 8% solids the slurry leaving the digester contains only 4% solid organic matter. While planning a biogas plant, the disposal of this slurry must be accounted for. Totally composted and free from pathogens, it is an ideal crop food.
The biogas digester produces slurry daily; however, the requirement in the fields is intermittent, therefore, the storage of slurry is a must. In case of small bio digesters, the slurry is sun dried and stored in the open before it is used. In case of lager installations, the slurry may be stored in large size ponds. In case of large size dairy farms, where the slurry cannot be consumed in house, it may be dewatered, blended with chemically produced N P K, pelletized and sold as biofertiliser.