Methane Digesters
Anaerobic fermentation or digestion is the most promising process for converting organic materials to methane and other gases. A simple apparatus can be constructed to produce bio-gas. Bio-gas usually contains about 60 to 70 percent methane, 30 to 40 percent carbon dioxide, and other gases. The heat value of raw bio-gas is approximately half that of natural gas under typical Colorado conditions. Take precautions when processing and handling the gas. It is highly explosive and difficult to detect.
Kinetic Studies of Biogas Evolved from Water hyacinth
ABSTRACT
Water Hyacinth - a native of South America is abundantly found in India, Bangladesh, South East Asia and in the Philippines Islands. Under favorable conditions a growth
rate as high as 17.5 metric tons of wet Water hyacinth per hectare per day has been reported. Due to vegetative reproduction it spreads rapidly clogging drainage, ditches,
shedding out other vegetations and interfering with shipping and recreation.
The concept of using aquatic plants for conversion to energy (methane) is gaining attention in tropical and sub tropical regions of the world where warm climate is
conductive to the plant growth through out the year. Anaerobic digestion of organic matter is the oldest method for disposing the waste. The anaerobic digestion of
animal, agricultural and industrial wastes has been widely studied. However, very little work has been done using aquatic plants particularly Water Hyacinth.
The present paper deals with the kinetics of gas produced from Water Hyacinth. The study was done in a batch fed digester. Attempts have been made to reach at an optimum condition for the production of maximum amount of gas by the addition of lower volatile fatty acids, Cow dung and inoculums etc. The important and useful results that was drawn from the study is that we can run the biogas plants even in the cold winter nights by using certain additives. After digestion of Water Hyacinth inoculums can be used as good manure for soil fertility, which is free from harmful chemicals, which is a boon for sustainable agriculture practices.
INTRODUCTION TO METHANE GAS PRODUCTION - AL RUTAN
Methane recovery from organic waste is appealing for several reasons. One is that the "raw" material is just laying around waiting for someone to do something with it. Unlike alcohol which has to be grown first as a starch grain and then processed, animal manure is "coming at you" if you have animals - whether you want it or not. It only makes sense to convert this "difficult resource" into something that is not only "socially acceptable" but also very profitable.
For instance, Fairgrove Farms in Sturgis, Michigan, milks 720 head. The manure from this herd is conveyed to a digester. The biogas produced fuels a caterpillar generator. The generator produces for the power grid in excess of $140.00 worth of electricity every 24 hours. So the farm receives from the local electrical utility a check in excess of $4,000.00 each month. Actually, it is probably more at the present time because this was the figure four years ago
Producing biogas from animal manure is like making bread, beer or wine. All are examples of the process offermentation. And it is obvious to anyone who has experience that these skills are an art as well as a science. Anyonecan bake bread or make wine. But to be really good at these skills requires common sense and sensitivity to theparameters of nature working to succeed. Working with manure is similar. One cannottreat it like "shit" and expect the best results. It takes an abundance of common sense and sensitivity to what themethagenic organisms require to achieve the best results.
Methane, Biogas, or Gobar Gas (Gobar is the Nepali term for manure) is made by the anaerobic (in the absence of oxygen) digestion of manure and plant life. The purpose is to convert this manure into methane to use as cooking fuel.
One method is to use a circular pit made of concrete, that is sealed and manure is added over time. Pipes lead from this container into the house, where gas is emitted at the cooking location. Residue from the combustion process comes out at a different location in a concentrated form and is used for fertilizer. Human waste may also be used as the bacteria is killed in the combustion process.
Methane is a gas made up of one molecule of carbon and four molecules of hydrogen. It is the major component of the"natural" gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 BritishThermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Naturalgas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.
The same types of anaerobic bacteria that produced natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or "digest" organic material in the absence of oxygen and produce "biogas" as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.) Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managedin a "digester" (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primarybenefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.
Biogas produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).
Electricity from cow dung
By G.S. Dhillon
IN a welcome move PEDA, the nodal agency of the Punjab Government for undertaking development of non-conventional sources of power, has embarked on the utilisation of cow dung or the cattle waste originating from the dairy units in the Haebowal complex in Ludhiana, for the production of electricity, in addition to organic manure.
A modern plant is proposed to be put up which with an initial investment of around Rs 10 crore will supply about 1 MW of power, in addition to yielding organic manure.
In the Haebowal complex there are about 200 private dairy units meeting the milk needs of the city’s population. The cattle waste or cow dung resulting from these dairy units are causing pollution problems. Most of the cow dung is “washed down” into the Budha Nullah on whose bank these dairy units are located.
The proposed scheme of PEDA will be putting up a state-of-the-art plant which has not been adopted in the country before. It is proposed to discuss the “improved mode” of anaerobic digestion of cattle waste (cow dung) in this write-up.
Gobar gas plants:
In the presently sponsored models of biogas units, both for “family scale” and the “community scale” units, the mode used is “mesophilic” which works at temperatures around 30°C to 40°C and the consortium bacteria gives better efficiency in the above temperature range. If the temperature drops below, as is the case in winter months, these units stop producing biogas. In this mode the detention period is very lary, so vessel or digestor has to have a large capacity.
In the current models used, as both acidic and methagenoic stages which require alkalineenvironment takes place in one vessel, the whole process is very sensitive and the acid boilin conditions occur most often. The biogas produced is also stored in the same vessel, so problems result and the conversion efficiency of solids is low and so is the calorific value of the gas produced which requires a special burning unit.
Thermophilic mode:
In this mode the consortium of bacteria deployed for bio-degradation of solids is different from that deployed in the mesophilic mode. This bacteria work at higher temperatures of 50°C to 60°C and operate at a much faster rate and with better efficiency of solid digestion. The control provided in this mode is better than the other mode working at a lower temperature range i.e. mesophilic mode.
The detention period required is also smaller, so the size of reactor is smaller. Usually separate vessels are used to accomplish both acidic and alkaline stages in different vessels and the gas produced from each stage is collected and stored separately. The gas obtained is mainly carbon dioxide and separation of this gas improves the quality and calorific value of the gas.
To improve the quality of produced gas, it is scrubbed to remove hydrogen sulphide gas present which imparts obnoxious odour (rotten eggs). The improved gas has comparable characteristics to that of the LPG fuel and the same burners can be used for the so-improved biogas.
From the collected cow dung, the first operation involved is removal of dirt, sand, stones, bricks, etc. before loading the digestor, which is suitably insulated to preserve the temperature of the process. For this removal a rotary drum sieve is utilised. The slurry comprises carbohydrates, proteins and fats, which are worked upon by the consortium of bacteria to produce biogas comprising carbon dioxide, nitrogen, methane and traces of hydrogen sulphide, etc. The ambient range of pH required to prevail in the mix is between 6.2 to 7.8.
At the first stage, the digestion of solids into fatty acids takes place which are later worked upon by the methagenic bacteria into the methane gas. If the latter process proceeds at a slower rate, then there is a gradual accumulation of the fatty acids and drop of pH value occurs leading to acidic conditions prevailing which, in turn, needing buffering action to restore the alkaline environment.
The effluent from the process contains a large amount of water, in addition to particles which would produce manure. So the effluents need to be dewatered for obtaining manure and this is accomplished in the screw press and solids are recovered from the effluent. Thereafter, the liquid is aerobically stabilised before disposal. The effluent flowing out conforms to the standards set by the pollution control agency. The manure obtained is aerobically stabilised and provides an ideal soil conditioner and organic fertiliser.
Power generation is carried out by using the biogas obtained as fuel in compression ignited internalcombustion engines which operate entirely on the biogas. The engines are coupled to the AC generators to produce electricity.
Conclusion:
This mode is regarded as eco-friendly because if the methane resulting from anaeorobic digestion of cow dung while composting is allowed to escape to atmosphere, it would add to the accumulative green house gases but when burnt it produces carbon dioxide which results in the abatement of climatic changes. The unit being set up at Ludhiana, if successful, would lead to setting up many similar units in other towns of Punjab. The experiment being carried out at Ludhiana is thus a trend-setter.
History of Biogas Development
The pioneer of biogas in Nepal was father B.R Sauboll, a Belgian teacher at Godavari St. Xavier's school. He built a demonstration plant in 1955. In 1968 Khadi and Village Industries Commission (KVIC) built a plant for an exhibition in Kathmandu. The Department of Agriculture installed 250 biogas plants during the fiscal year of 1975/76. The Agriculture Development Bank of Nepal (ADB/N) offered interest free loans for the installation of biagas plants. In 1974, Development and Consulting Service (DCS) built four biogas plants according to KVIC design. Gobar Gas and Agriculture Equipment Development Company Pvt. Ltd. was formed in 1977 with joint investment of the United Mission to Nepal (UMN), ADB/N and Nepal Fuel Corporation (which later on merged into the Timber Corporation of Nepal) based on DCS biogas extension organization. As it was difficult to introduce new technology, biogas in rural areas programme of the company was not encouraging in comparison to national potentialities. However, research on various design of biogas plant such as floating steel drum design, concrete fixed dome design, pre-cased tunnel design, plastic bag biodigester, ferrocement gas holders, brick mortar dome, mud dome were tested and experimented. However, fixed dome design is the only one recognized design and became more popular in Nepal.
COMMERCIALISATION OF BIOGAS IN NEPAL
Biogas technology is becoming one of the reliable
alternative energy sources in Nepal. As a result more
than 48500 biogas plants have been installed in the
country. However, it is only about 3.7% of its technical
potentials. Biogas technology has been commercially
introduced since the establishment of Gobar Gas Tatha
Krishi Yantra Vikash (P) Ltd. in the year 1977. Various
research have been carried out in designing and
developing a biogas plant, biogas appliances, alternative
feedstocks, maximising gas production especially in
winter months and end use applications of gas and slurry.
Even though these technologies have not came into
practice and are limited only in papers, due to which the
progress were not attractive as the planners planned. For
commercialisation of biogas plants in Nepal, this paper
has analysed potentiality of biogas plants in the
country, sources and mechanisms of funding, construction
capacity of the companies and users buying capacity with
cost calculations. It has also highlighted some biogas
promotional activities such as development and
distribution of extension and promotion materials,
marketing and slurry extension programme. It has focused
on research and development, training, quality control
and monitoring and evaluation of the programme. It has
also highlighted the importance of co-ordination between
its partners such as Biogas Companies, Nepal Biogas
Promotion Group and other active NGOs, Biogas Appliances
Manufacturers, Banks, BSP and AEPC with their clear
responsibilities for the success of the programme.
Ultimately, emphasis has been given for introducing a
community trust fund concept, diversified end use
applications of gas as well as slurry and integrated
approach of biogas system for commercialisation of the
technology in the country. In this way conclusions and
recommendations are presented.
Basic Digester Process
Methane is produced by bacteria. The bacteria are anaerobes and operate only in anaerobic environments (no free oxygen). Constant temperature, pH and fresh organic matter promote maximum methane production. Temperatures usually are maintained at approximately 95 degrees F. Other temperatures can be used if held constant. For each 20 degrees F decrease, gas production will be cut approximately one half or will take twice as long. A constant temperature is critical.
Temperature variations of as little as 5 degrees F can inhibit the methane-formers enough to cause acid accumulation and possible digester failure.
Anaerobic digestion is a two-part process and each part is performed by a specific group of organisms. The first part is the breakdown of complex organic matter (manure) into simple organic compounds by acid-forming bacteria. The second group of microorganisms, the methane-formers, break down the acids into methane and carbon dioxide. In a properly functioning digester, the two groups of bacteria must balance so that the methane-formers use just the acids produced by the acid-formers.
A simple apparatus can produce bio-gas. The amount of the gas and the reliability desired have a great influence on the cost and complexity of the system. A simple batch-loaded digester requires an oxygen-free container, relatively constant temperature, a means of collecting gas, and some mixing. Because methane gas is explosive, appropriate safety precautions are needed.
Tank size is controlled by the number, size and type of animals served, dilution water added, and detention time. The factor that can be most easily changed with regard to tank size is detention time. Ten days is the minimum, but a longer period can be used. The longer the detention time, the larger the tank must be. Longer detention times allow more complete decomposition of the wastes. Fifteen days is a frequently used detention time. Table 1 shows some recommended sizes, dilution ratios and loading rates for different types of animals.
Little volume reduction occurs in an anaerobic digester. Waste fed into the digester will be more than 90 to 95 percent water. The only part that can be reduced is a portion of the solids (about 50 to 60 percent).