Comprehensive Utilization
Figure 7. Comprehensive utilization of biogas technology
A. Application of Biogas
1. Cooking A cooker is more that just a burner. It must satisfy certain aesthetic and utility requirements, which can vary widely from region to region. Thus, there is no such thing as an allround biogas burner. Most households prefer two-flame burners. The burners should be set initially and then fixed. Efficiency will then remain at a high practical level. Single-flame and lightweight cook-stoves tend to be regarded as stop-gap solutions until more suitable alternatives can be afforded. Biogas cookers require purposeful installation with adequate protection grim the wind. Before any cooker is used, the burner must be carefully adjusted. Test measurements should be performed to optimize the burner setting and minimize consumption. Biogas burner or stoves work satisfactorily for domestic cooking under water pressure of
75 to 85 mm. The stoves may be single or double burners varying in capacity from 0.22 to 1.10 cu.m. of gas consumption per hour. Generally, stoves of 0.22 and 0.44 cu.m. (8 and 16 cu.ft.) capacity are more popular. A 1.10 m. (40 cu.ft.) burner is recommended for a bigger family with larger pant size. The gas demand can be defined on the basis of energy consumed previously. 2. Lighting Biogas can be used for lighting in non-electrified rural areas. Special types of gauze mantle lamps consuming around 0.07 to 0.14 cu.m. of gas per hour are used for household lighting. Several companies in India manufacture a great variety of lamps which have single or double mantles. Generally, mantle lamp is used for indoor purposes and 2-mantle lamps for outdoors. Such lamps emit clear and bright light equivalent to 40 to 100 candle powers. These are generally strong, well built, bright, efficient and easy to adjust. Compared to stoves. Lamps are more difficult to operate and maintain. Different types of lamps are in use in China. They are simple in operation and easy to manufacture and are low priced. In remote places, clay lamps that do not need much skill to manufacture are still being used by Chinese farmers. There is a big demand for biogas in non-electrified rural areas. Luminosity is equivalent to 60 watt if 0.11 – 0.15 m3 per hour of biogas is consumed by common gas lamp, which is relatively less efficient for the use of biogas. In this case, more that 90 percent of the energy is lost as heat. In the tropical areas, this heat may make the room uncomfortably hot. Gas lamp consists of gas inlet hole, an air inlet hole, an air inlet adjustment valve, a mixing tube, a fire resistant clay head and gauze mantle. 3. Fuel for engine Biogas is a combustible gas and can be used as fuel for engine as well as cooking and lighting. The ignition point of biogas is about 800ºC. Its combustion is smokeless and pollution free, with anti-detonation which is an important feature, especially when the biogas contains about 30% CO2. Besides, the mixture of CH4 and air has a wide combustible range and favors condition for forming a mixed gas. Internal combustion engine run with biogas can provide the same power as that operated with conventional fuel, which gives a lot of convenience to fitting system. The materials for combustion engine are locally available and the production of biogas fermentation can save a lot of fossil fuels with consequent economic benefits. The internal combustion engine has generally a four-stroke engine, i.e. intake, compression, power stroke and exhaust. Biogas is conveyed into the cylinder in the intake stage instead of original fuel. 1. Gasoline Engine Only a biogas-air mixer is required to be installed just preceding the carburetor for a four stroke engine. But biogas can not directly be used for a two-stroke gasoline engine. This type of internal combustion engine needs, lubrication supplied by engine oil and fuel. If biogas is used for petrol, this lubrication does not work which could damage
the engine. Because of proliferation of automotive in the world, it seems to be relatively easy to get second hand gasoline car engine so it is quit suitable to modify for biogas. Modification of intake. The main modification of the intake is to provide the biogas after the air filter in the inlet pipe. The procedure to convert the intake system for biogas is mainly to install an extra mixing device at the rear of the air filter. This mixer consists of a manual valve and controlling gas intake and Tshape pipe. For small engines less than 10 hp. it is sometimes advantageous to provide plastic bag near the gas inlet so that the engine can siphon in the gas easily. For starting purposes it is important that the supplied amount of gas must be greater than that of the normal operation. After starting and warming up, the flow of gas must be reduced and controlled by means of a gas tap. It is often easier to start the engine with gasoline and then gradually change to biogas. Maintenance tends to be lower when gas is used as a fuel. Valves, plugs, etc. remain clear and the sump oil needs less frequent changing. The engine tends to run a bit hotter and the cooling system must be in good condition 2. Diesel Engine Because of high thermal efficiency and economy, diesel engines are popular and this engine can also be modified to biogas engine. This engine is usually compression ignition type and does not have spark ignition part. Temperature in a cylinder under pressure stroke is 600 – 700ºC and this temperature is not enough to ignite biogas naturally. Therefore diesel is inevitable for ignition even after modification to biogas engine. The way to convert the intake system for biogas is the same as the case of gasoline engine. 3. Gas on Electric Power Generating electricity is a mush more efficient use of gas than using gas for lighting. To generate 1 kwh. of electricity, 0.7 cu.m. of biogas is required. This would be sufficient for 10 electric bulbs. 60 watts each for one hour. he same 0.7 cu.m. of gas would only be sufficient for 5 gas lamps, each of 100 foot-candle power which is equivalent to 60 watts of electric bulb. Fittings of electric bulb costs less and is cheaper to maintain than biogas lamps. The disadvantage is the high cost of the engine and generator and the distribution wires.
4. Electricity generation Generating electricity is much more efficient use of biogas than using it for gas light. From energy utilization point of view, it is more economical to use biogas to generate electricity for lighting. In this process, the gas consumption is about 0.75 cu.m. per kW hour with which 25x40-watt lamps can be lighted for one hour, whereas the same volume of biogas can serve only seven lamps for one hour (BRTC 1983). Generating electricity is a much more efficient use of gas than using gas for lighting. To generate 1 kwh of electricity 0.7 cu.m. of biogas is required. This would be sufficient for 10 electric bulbs, 60 watts each for one hour. The same 0.7 cu.m. of gas would only be sufficient for 5 gas lamps, each of 100 footcandle power which is equivalent to 60 watts of electric bulb. A fitting of electric bulb cost less and is cheaper to maintain than biogas lamps. The disadvantage is the high cost of engine and generator and the distribution wires.
B. Application of Sludge
1. Granular fertilizer The granular biofertilizer is composed mainly of humus or organic matter and the pant nutrients. It has a carbon to nitrogen ratio of around 13:1 which is desirable because of its closeness to the C/N constancy that exists in many arable soils. This means that the sludge can be used directly for fertilization of crops without the adverse effect of serious competition by decomposition organisms in the soil and the plant for its nitrogen supply. The biogas process presents an improved treatment for both crop wastes and animal manures. In the aerobic decomposition of organic matter, ammonia and carbon dioxide are lost in the air. There is also considerable leaching of soluble nutrients. Thus, the resulting compost has less nutritive elements that are originally present in the raw materials. In the biogas way, when organic materials are decomposed in the waterproof, air-tight chamber, only carbon, hydrogen and oxygen in the form of methane and carbon dioxide are lost. Practically all the other essential elements are retained in the sludge. Organic substances such as human, animal and agricultural wastes can produce not only high quality gas but also a large quantity of digested sludge and effluent (these constitute the biogas fertilizer) which are excellent organic fertilizer. Raw materials such as animal dung and agricultural residues are fermented in biogas plant. The value of the effluent can be even to greater benefit than the value of the gas. Although, this is not usually emphasized enough, it also controls some pests and diseases, and contains high nutrient. Appropriate utilization of biogas effluent can promote the production of crops, improve the environmental condition and speed up ecological balance. During the fermentation process, some solids are broken down by bacteria into water and gas. About 70 percent of the total solid fed into the digester can be expected to come out as
effluent. Digestion process does not increase or decrease any fertilizer nutrients like nitrogen, phosphorous or potash, but it only changes the form of the essential elements. On the other hand, traditional composting method will create great amount of heat and high temperature resulting to the loss of about half of nitrogen and destruction of useful organic substances. 2. Liquid biofertilizer The liquid biofertilizer contains only small amounts of nitrogen, phosphorous and potassium, but considering the fact that a large amount of water is needed for irrigation, these nutrients can build up to excessive quantities. Moreover, the liquid biofertilizer promotes a profuse growth of nitrogen-fixing algae wherever it is applied. Since algae have a short life cycle, the decaying algae supplement the available nitrogen. Like the solid biofertilizer, the liquid sludge also carries trace elements like zinc, iron manganese, copper and others. With continuous use of liquid biofertilizer for irrigation, the trace of elements increased to high levels in soil. 3. Soil conditioner As an organic fertilizer, it plays a very important role in plant nutrition as well as in soil conservation. It is not only a carrier ad source of nutrients, but it also serves as an excellent soil conditioner. It improves the physical condition of the soil by improving texture, moisture-holding capacity, and aeration. It increases the buffering capacity of the soil. It combines with inorganic soil constituents to prevent their loss by leaching but releases them for the use of the plants. It stimulates the growth of the microorganisms. It retards the irreversible fixation of nutrients, prevents soil erosion and smoothes out temperature fluctuations. 4. Feeds for pigs The use of fresh manure directly as feed is not commonly practiced. Many poultry and livestock raisers are understandably reluctant to feed it to their animals. Either they are afraid of spreading diseases among their herd, or they may find the manure too messy to handle, or the idea of feeding it to their animals may grate against their sensibilities. The objectionable characteristics of manure can be eliminated by processing it in a biogas plant. The aerobic pathogens or disease-carrying germs are killed in the anaerobic process. The sludge is not messy. Try sludge looks and handles like humus. It no longer has the offensive smell of manure, and unlike manure, it does not attract flies. Moreover, the biogas process not only retains the nutrients but it also enriches the sludge with B-complex vitamins, particularly vitamin B12, which are synthesized during the biogas digestion. The solids are recovered from the sludge by allowing them to settle out in settling tanks and by draining the liquid. They are then dried, preferably under the sun. The dried lumpy solids are then ground and detoxified before mixing with the other feed materials. In small operations where wet feeding is practiced, the settled sludge may be fed with the slops without drying.
5. Seed Soaking Utilization of biogas effluent to soak rice or wheat seed shows a remarkable effect on initial growth of the plants. The rice or wheat seeds are set in digester effluent for 48 hours for early rice, 36 hours for middle rice, 24 hours for late rice seeds and 8 hours for wheat seeds. Dry the soaked seeds in shade. Soaking quantity is about 5 percent of total weight. The advantage of seed soaking of rice or wheat with effluent are:
• high sprouting rate, with strong and uniform budding • good root development and white branches • growing seedlings were green and with strong stem • the seedlings are resistant to diseases and pests • the survival rate is high.
Compare to soaking with ordinary water, germination and survival rate is higher by as much as 10 to 20 percent respectively. 6. Fishery Poultry and livestock manures are used directly in fishponds to grow plankton but the odor has a tendency to have an adverse effect on the palatability of the fish. This is particularly evident n the case of bangus (milk fish). A better way is to pass the manure through a biogas plant and use the resulting sludge in the fishpond. Several advantages are gained in this manner: The sludge is not odorous and will not adversely affect the palatability of the fish. The sludge promotes the growth of the plankton better than fresh manure does. The sludge has a lower BOD than the fresh manure slurry, thus leaving more oxygen available for the fish in the fishpond. Biogas and biofeed are produced. The sludge from a biogas plant is sufficient to support the growth of plankton in the fishpond and the undigested solid nutrients can serve as direct feed supplement for the fish. The liquid portion of the sludge can be used for fishponds and the solid are recovered as feed material.
Wastewater Treatment System
Figure 8. Biogas technology in wastewater treatment system Sources: 1. 2. Biogas and Water Recycling by: Felix D. Maramba, Sr.(1978) Proceedings of The National Training Course on Biogas Technology - Don Severino Agricultural College (1993) by: C.A.Polinga, J.Q.Dilidili, E.E.Polinga, R.S.Sangalang