ADVERTISEMENTS:
In this article we will discuss about:- 1. Overview of Select Biogas Plant Designs in the World 2. Biogas System Developed by the HEL, Dorset (UK) 3. Selection of Biogas Plant Sites 4. Overview of Solar-Assisted Biogas Systems 5. Biogas Plant Operation 6. Protection of Plant Metallic Parts from Corrosion.
An Overview of Select Biogas Plant Designs in the World:
Several plant designs are popular in western world. A schematic view of a typical on-farm biogas plant used in western countries is shown in Fig. 4.21.
In Philippines one of the early plants used is schematically shown in Fig. 4.22. Being a two stage process it ensures more efficient fermentation of fibrous agricultural waste materials like crop-residues and vegetable wastes.
Schematic view of an oil drum biogas plant commonly used in Indonesia is given in Fig. 4.23.
In several countries including Belgium flexible bag type biogas plants (Fig. 4.24) are popular. These are made of strong plastics resistant to ultraviolet rays. Hypalon-Neoprene coated fabric is commonly used in their construction. These plants are easy to build at low costs, are portable and enable constant gas pressure to be built-up.
H.A. Hamad et al., give an account of a viable model proposed for application of biogas technology in new desert community areas in Egypt. The model involves a novel architectural design and house planning to promote biomethanation for the entire community for meeting its energy, fertiliser and sanitation needs.
ADVERTISEMENTS:
A novel continuous-fed biogas plant made of ceramic precast has been designed by the Energy Research Section of the Development Research and Pilot Centre (DRPC), Ho-Chi-Minh City, Vietnam. The plant consists of two crossed and horizontal iron bars in digester which serve as scum breakers. Scum breaking is affected by movement of slurry resulting from pressure variations.
Besides outlet valve digested sludge can be removed through equalisation tank. By 1983, DRPC had designed two sizes of this plant. The small-scale plant had a 300 litre digester and 150 litre equalisation tank. Loading capacity of this plant was 70 kg piggery waste. Second model was meant for household use and had a 135 litre digester and 65 litre equalisation tank. Loading capacity of family-size plant is 40 to 50 kg of piggery waste. Retention period in both the plants is 40 days.
ADVERTISEMENTS:
Recently, animal industry in Japan has expanded rapidly with the increasing demand for animal products following rapid growth in Japanese living standards. This expansion is characterised by a rapid increase in number of animals and significant decrease in number of farmers.
Anaerobic fermentation of wastes with a view to obtain both fuel and fertiliser has emerged as an attractive option. Initial studies on biogas production from animal wastes were carried out by the National Institute of Animal Industry using la cylindrical digester made of fibre-glass reinforced plastic which is shown in Fig. 4.25.
The size of the digester (200 litres) was adequate for the waste from one pig and it contained about 150 litres of waste slurry. The temperature was kept at 35°C by circulating hot water through a coil in the slurry. The digester was insulated by a 5 cm layer of glass fibre.
Methane fermentation followed from a mixture consisting of 44 kg of fresh pig faeces and 88 litres of tap water with 30 litres of anaerobically-fermented cow dung as seed for the fermentation. The daily loading was 7.5 litres of slurry consisting of 2.5 kg of pig faeces and 5 kg of tap water.
In steady state it produced 170-200 litres of biogas per day. Kagawa Prefecture Livestock Experiment Station way back in 1978 used a much bigger digester of 3 m3 capacity. The digester is equipped with a submersible pump (0.4 kw) and is operated under mesophilic condition (35°C).
Biogas generated from pig wastes is stored in a gasholder (1.8 m3) and used in the experiment involving running a modified petrol engine (5 hp with 2000 rpm) which consumes 24.6 litres of biogas in 1 hour. The capacity of the digester at Nagano Prefecture Livestock Experiment Station is 200 m3 which is constructed under a roof for 30 head of dairy cattle.
Wastes from the dairy cattle are introduced by gravity into the digester with two submersible pumps for stirring and heating. The pumps are surrounded by screens which prevent clogging caused by coarse organic material. Biogas produced is used to generate steam and provide hot water. Systematic experiments with a larger installation are being carried out at the Hiroshima Prefecture Livestock Experiment Station.
Wastes from 200 head of pigs are mixed with 2-3 volumes of tap water and charged into two digesters daily. The digesters with polyurethane insulation are kept under mesophilic conditions by submersible pumps, and produce enough gas to run air-conditioning facilities for about 8 hours each day.
Biogas System Developed by the HEL, Dorset (UK):
ADVERTISEMENTS:
An advanced design of a biogas was in 1980 developed by the Hamworthy Engineering Limited, Dorset. A demonstration unit of this system is working at the Walnut Tree Farm, Chediston, Suffolk (UK) to produce biogas and fertiliser containing high proportions of phosphates and nitrogen.
This system developed was the result of over six years of collaborative endeavours put up by the Hamworthy Engineering Limited and the Department of Microbiology at University College, Galway (Cardiff) which were responsible for the scientific aspect of the project; and the Polytechnic of Wales where pilot plant evaluation studies were carried out.
The biogas plant at the Walnut Tree Farm was designed to treat 18 tons of pig wastes per day with 5 to 6 per cent total solids and 6 tons of cattle waste per day with 8 to 9 per cent total solids and 15 day retention time. The digester has a volume of 350 m3 and a diameter of 9.15 m3 with a conical top.
Biogas is collected from the digester top and held in a small 10 m3 gasholder. The effluent from the plant is passed through a vibratory screen separator to remove any undigested material and the filtrate containing about 1 per cent total solids is pumped to a storage lagoon used for irrigation purposes. The process operates on a continuous cycle with minimum retention time. Wastes are fed into the digester by a pump. There is a provision to warm up and agitate the slurry with a mixing device.
Selection of Biogas Plant Sites:
Biogas plants can be built either close to the source of waste generation or away from it. In case plants are needed to be constructed afar, this may not be cost-effective in many situations. Normally for family-size plaints it is economical to construct plants on site close to households where wastes are generated. For community plants, however, these can be set up at off-site provided economics of plant operation supports such a course.
In family units wastes are usually collected from available sources, mixed to form slurry and finally fed to plants. As per the practice followed in China, pigsty, household latrines, poultry sheds etc. are also connected to plants either directly or through mixing chamber.
For the Chinese practice to succeed in India, social barriers associated with human faeces and pig-wastes need to be removed. If animal sheds and household toilets are to be connected to plants, it is necessary to provide slanting floors for easy wastes-flow.
If animal urine is to be added to feed, animals need to be properly stabled particularly during nights to help store dung and urine. For obtaining higher gas yield, the plant should be so located that it receives direct sunshine for most part of the day. For preventing water contamination, drinking water wells should be at least 15 meters away from biogas plants. Demonstration plants should be located at convenient sites to attract maximum promotional benefit.
In several situations organic wastes need to be kept in storage sheds before use. These sheds can be made of local materials such as bamboo, tree limbs, palm thatch, bricks, tiles, mortar etc. From sheds wastes can be carried to sites manually in shoulder-buckets or animal-drawn carts in close-containers depending upon distances involved.
If water-weeds have to be used as feed then these, on small scale, can be collected by two or more persons pulling a net stretched between two bamboo poles across the water surface. Algae and water-hyacinth need be dried before use. Dried matter can also be used as bedding in animal sheds to soak urine and then used.
An Overview of Solar-Assisted Biogas Systems:
Solar energy is plentiful in most regions of the country. It can be gainfully utilised to keep the temperature of digester contents close to 35°C. Plant slurry can be warmed up in a number of ways. Active and passive methods can be employed to heat up the digester contents using solar energy as external heat source.
Active systems involve heating the feedstock or directly heating the digester contents. Pre-heating of the slurry is only marginally beneficial considering that daily input of slurry is only about 2 to 4 per cent of the total volume. Direct heating of the digester contents runs the risk of killing the bacteria and hence altogether eroding the process efficiency. In passive techniques which involves no moving parts, a greenhouse (solar canopy) is usually built over a biogas digester to arrest the solar radiation.
Solar flat plate collectors integrated with the biogas plant can also be used to warm the digester contents. Breadbox and solar pond are the main types of solar collectors used for slurry heating. A breadbox solar collector consists of a radiation-absorbing tank within a box fitted with a glass cover.
A solar pond consists of a large container kept horizontally on the ground or on insulation, covered with a glazing and filled with liquid. A solar water heating system can also be alternatively used to produce hot water at 50°C which can be circulated into the bed of the digester with the help of heat exchanger to maintain higher temperatures.
M.S. Sodha, et al., carried out studies concerning evaluation of a new concept of greenhouse-coupled biogas plant for increasing biogas yield especially during winter months. Installation of polyvinyl chloride greenhouse type structure over a biogas plant allows solar heating of the content from 18 to about 37° which is optimal temperature requirement for anaerobic fermentation.
Experiments with the two biogas plants of 8 m3 and 85 m3 capacity set up at Masoodpur village near New Delhi indicated that plastic enclosures helped to raise gas yield by 15 to 20 per cent. A mathematical model was also developed which helped to predict plant performance in presence and absence of greenhouse under varying climatic conditions.
Based on calculations made in respect of a biogas plant installed in the vicinity of Delhi, G.N. Tiwari, et al., showed that it was possible to obtain substantial increase in gas yield on a typical winter day by covering the gasholder with a transparent polyethylene sheet during sunshine hours and using a movable insulating material during the off-sunshine hours.
Shallow Solar Pond (SSP) is an inexpensive solar collector which generates hot water. The hot water thus produced can be gainfully utilised in slurry preparation which helps to improve biogas yield particularly during winters. A diagram of the SSP is shown in Fig. 5.6.
Its important components are:
(i) Shallow brick walled container,
(ii) A black rexin liner to act as an absorber of solar radiation,
(iii) Water filled to a depth of 5-10 cm in the container,
(iv) A thin transparent PVC sheet floating on water to prevent evaporation of water,
(v) A second thin transparent PVC cover above the first one providing an air gap between the two PVC covers, and
(vi) Inlet and outlet pipes for admitting and discharging cold and hot water respectively.
Water is generally filled in morning around 8 AM which gets warmed up to the maximum by 3-4 PM. On a sunny day in winter, water temperatures of 40-45 °C can be obtained for a water depth of about 5 cm which corresponds to hot water production rate of 50 litres/m2 of pond area.
It is thus possible to estimate the area of solar pond for a given capacity of digester. As per rates prevailing in 1987, a solar pond costs about Rs 300/m2 and it becomes less expensive if it is constructed along with the digester. Efficiency of the pond can be further improved if insulation is provided below the layer of black rexin.
C.K. Desai, et al., studied aspects relating to the design of solar ponds for storing and supplying heat to a biogas plant digester for obtaining added gas yield. It was found that if ‘the temperature of digester content can be maintained at 40°C, retention period for the slurry can be reduced by over 40 per cent.
Solar ponds can be helpful in preventing heat losses during night and in maintaining digester temperature at desired level. A small solar pond of 0.65 m depth and 670 litre capacity was used to supply heat to a plant of 0.25 m3 capacity. Experimental results confirmed that it was possible to reduce retention period by over 45 per cent and obtain much higher and steady gas yield.
R.A. Gupta, et al., provided a detailed account of solar-assisted biogas plant which incorporates water heater concept for achieving higher gas yield particularly during winter months. Further improvement in the performance of the system was investigated by using insulation on the walls and base of the digester.
They also developed a mathematical relationship which expressed slurry temperature as a function of time. G.N. Tiwari and A. Chandra studied the effectiveness of solar-assisted biogas system particularly during winter months for obtaining higher gas yield. For this purpose biogas unit is expressed to solar radiation to increase the slurry temperature and covered with movable insulation to avoid heat losses from the top and sides of the dome.
A. Beba carried out the performance analysis of a simulated hybrid system incorporating biogas fermentor, waste heat recovery system and solar collectors. It was reported that total annual contribution of heat recovery unit and solar systems were approximately 29 and 37 per cent respectively of maximum attainable limit.
Schematic views of insulated and solar-heated digesters involving solar flat plate collector are shown in Figs. 5.7 and 5.8. Their functioning is self-explanatory.
Biogas Plant Operation:
Some Observations and Recent Developments:
Production of gas normally starts within a week, the gas accumulates inside the holder causing it to float (in movable drum type) and rise. The first gas after start-up may not burn due to high content of carbon dioxide. In such a case accumulated gas needs to be released while observing all safety precautions. The process is to be re-started with drum settling down and again rising with fresh gas which may be similarly tested.
After the test is positive the clear gas can be used. In summer the slurry on top of the digester is prone to become so thick that the drum does not easily sink when released for use. The flame obtained is O.K. to begin with but later becomes low. For alleviating this difficulty, a few buckets of water need to be put into the digester and stirred with a bamboo poles until the slurry becomes sufficiently thin. The drum can be shaken few times a day and flame tested.
The drum (gasholder) should be taken out and given a protective coating of paint at least once a year. Fibrous materials such as straw, grass etc., form a floating scum and can cause blockage of the plant. They should therefore be chopped up to a length of 1-3 cm before being fed to the plant. Bacterial activity should not be overloaded by adding too much material to the pit. Biogas plants should be loaded regularly and uniformly, and the fermentation slurry well stirred to ensure high gas production.
Biogas plant performance can be controlled by studying and monitoring the variation in factors like pH of digester contents, volatile solid concentration, temperature, and nutrient availability, concentration of toxic material, feed composition, retention period, loading rate, agitation, heating and heat balance. A digester which works with minimum of control and operates under equilibrium conditions is referred as to ‘balanced digester’.
However, if any one of the controllable parameters is suddenly changed or if some toxic materials are introduced, the equilibrium is disturbed and an ‘unbalanced digester’ result which functions at less than the normal efficiency. When there occurs a drop in gas production, it can be due to digester imbalance although other factors could have also contributed to it. For example, it can be due to fall in temperature or the rate at which the feed material is being added.
Hence, for identifying the true cause, it is necessary that uniform loading of the digester is maintained and digester temperature closely monitored. If there occurs decline in pH and/or associated increase in volatile acid concentration, it could be due to digester imbalance which needs to be corrected.
However, this requires laboratory facilities, equipment and trained personnel to monitor most of the control parameters affecting the anaerobic process. This creates difficulties to farm owners who do not have access to technology or testing facilities for performance evaluation of their plants.
As a short-cut villagers can adopt simple commonsense rules. If based on their ‘feel’ there is no drop in ambient temperature and they are uniform and regular in loading, it is safe to assume that decline could be due to drop in pH. This can be corrected by adding lime to the digester slurry and if the gas yield improves, the course is justified. However, by adding lime if the yields does not improve, then surely some toxic substances might have contributed towards decline.
In such a situation as a practical remedy it is advised to dilute the normal feed material with other digestible wastes like straw, hay, gas clippings, urine, night-soil during the period of reduced gas production which in the routine course may not be used. Finally, if all else fails and digester remains non-producing, the only course of action left is to empty the digester and start all over again.
While starting up a digester it is a common practice to seed it with an adequate population of both acid forming and methane forming bacteria. S.K. Sharma on the basis of experimental studies carried out at the University of Roorkee found that it is beneficial to start the plant by first filling it with effluent slurry taken from another biogas plant operating on the same or different feedstock.
It has been found experimentally that if the wet cattle dung is kept for anaerobic digestion after mixing with water in 1: 1 ratio (v/v) at 37°C, the usual gas production starts on 7th day but if the influent is mixed with effluent slurry obtained from another biogas plant operating on same feedstock then the gas production starts earlier.
The gas production rate increases with increase in proportion of effluent slurry mixed with influent slurry. If the plant is to be fed with crop/agro-residues only, then mixing of effluent from other plant becomes necessary. It was found that biogas with more than 50 per cent methane could be obtained within 2 days of starting the digester when the influent was mixed with effluent slurry in 1 : 1 ratio (v/v) obtained from a cattle dung fed biogas plant.
According to L. Sasse long retention period is one of the effective ways of increasing gas yield. This is backed by experimental results that a ‘second push’ in gas production occurs after a prolonged retention time. For small farmers with few cattle, use a local material in plant construction, prolonged retention time achievable with somewhat large-size digesters are advisable.
Ferro-cement gasholders are preferred to steel types considering that in the case of former construction materials can be transported to remote areas by cycle. Plastic gas pipes have proven to be cheap and long-lasting as compared to iron-pipes. S. Pal, et al., studied the effect of loading rates on the performance of three types of biogas plants, i.e. KVIC, Janata and poly- ethylene-type (flexi) under shallow water-table conditions.
The performance was evaluated on the basis of daily gas production, volatile solid reduction, temperature profile of slurry in the digester and NPK concentration in effluent slurry. While gas production was found to be least in the Janata plant at highest of the feeding rates, its performance was found to be most efficient in terms of some of the above parameters.
G.H. Dar and S.M. Tandon studied the effect of pre-treatment of crop residues on biogas yield. Addition of pre-treated (1 per cent NaOH for 7 days) wheat straw, lantantana residue, apple and peach leaf litter to cattle dung was found to improve microbial digestibility and biodegradability during anaerobic fermentation at ambient temperature (28-31 °C).
Pre-treatment was found to nearly double the gas output including improvement in methane content in biogas. Z. Pechan, et al., studied the impact of high ammonium nitrogen concentration on biogas yield from anaerobic digestion of poultry manure.
Based on laboratory fermentation of poultry manure conducted at mesophilic temperature, corresponding to average total solid concentration of 11-14 per cent in influent and retention period of 27-58 days, no adverse impact on biogas yield was seen when mean ammonia nitrogen concentration in the effluent rose to 4.07-5.85 g/litre. Biogas yield was however found to be directly related to volatile solid loading rate.
M. A. Hamad, et al., studied the effect of digester pressure on yield and composition of biogas. A fixed dome type plant fed by buffalo dung with retention period as 40 days was used in carrying out the study. Increase in digester pressure was accompanied by decrease in quantity of gas produced and some increase in methane content.
T. Kate and S. Karanjekar evaluated comparative performance of floating and fixed-dome biogas plants in India. Based on evaluation of one year performance, it was concluded that fixed-dome Janata biogas plants are more efficient in gas production, are cheaper to construct and maintain than floating- dome type plants.
P.H. Liao, et al., studied the effect of liquid-solid separation on biogas yield during mesophilic anaerobic digestion of dairy-cattle manure at 35°C in a laboratory-scale digester. Screening of coarse solids from the manure before digestion had a significant effect on biogas production. Biogas yield and methane content in biogas from screened manure were consistently higher than from unscreened manure.
A liquid-solid separation pre-treatment step reduced digester volume for a dairy farm without decreasing biogas yield. K.V. Lo and P.H. Liao studied anaerobic digestion of screened dairy manure in anaerobic rotating biological contact reactors at 35°C using rotating discs made of cedar wood and acrylic plastic. Digester using cedar wood discs had a shorter start-up period than acrylic plastic disc.
Within an operational period of 3 months, the highest methane yield of 1.67 litre per day per litre was obtained at 1 day hydraulic retention time for the reactor with wood media. On the other hand in case of anaerobic rotating biological reactors using plastic media the highest methane yield of 1.89 litre per day per litre was obtained within an operational period of 10 months at 1 day hydraulic retention time.
Based on their experimental studies, T.H. Chen, et al. examined batch digestion of fresh and dry dairy manure and showed that methane yields expressed as nr methane/kg volatile solids fed from fresh and dry manure were statistically the same. However fresh manure produced greater total volume of methane than the dry manure following loss in organic materials during, drying.
Protection of Plant Metallic Parts from Corrosion:
Corrosion can be viewed as an interactive process with environment which tends to destroy desired properties of the material susceptible to it. Apart from environmental impact, corrosion can also be caused due to direct contact with liquids such as electrolyte. Biogas plants of movable drum type large number of which exist in India are sufficiently prone to corrosion as several of its parts such as gasholder are made of steel.
Walls of gasholder are generally found to be more corrosion prone as compared to its top portion. Corrosion depends on several factors such as nature and purity of metal used, level of humidity and oxygen in environment, ambient temperature and type of electrolyte used in situations where electrolyte is the main contributing factor.
A mild steel gasholder acts as an anode which forms Fe++ and Fe+++ ions and releases electrons (2e–). Iron and water are ionised in the following manner:
When hydrogen is liberated, OH– ions tend to move towards anode, Fe++ and Fe+++ ions move towards cathode after passing through water film. It is during these journeys in opposite directions that iron and OH– ions react forming iron hydroxides [Fe (OH)2 and Fe (OH)3], Carbon dioxide of biogas reacts with these hydroxides to form hydroxyl carbonate.
This hydroxyl carbonate finally degenerates into ferric oxide along with a water molecule [Fe2O3 . H2O] signaling the onset of corrosion process. Lack of adequate oxygen also contributes to corrosion by forming certain galvanic cells on metallic surface. In a movable drum type plant, amount of oxygen varies with the position of gasholder.
Degree of corrosion ran be minimised in a number of ways which include correct choice of plant materials, removal of corrosive agents and application of an electrical field at phase boundary like cathodic or anodic protection, formation of new interfaces by applying metallic inorganic and organic coatings. Surface coating among them is largely practiced and is one of the earliest methods of surface protection.
A specific coating well-formulated and applied protects surface either by one, two or even three mechanisms concurrently. Barrier coatings provide protection against the penetrating agents like water and oxygen. By providing sufficiently high thickness of the film, the ingress is arrested. Asphalt and bitumen type of coatings belong to this class. Inhibitive coatings normally contain corrosion inhibitive heavy metal pigments like zinc chromates, and phosphates.
They control corrosion by changing the electrode potential at the anodes so that iron does not oxidise. However, this method has limitation that when inhibitor is consumed by combining with other ions, protection breaks down. The third category of coatings, namely, sacrificial coatings contain zinc metal powder or duct particles as the principal component which makes the film conductive. Protective feature depends on sacrificial nature of zinc which protects the surface from corrosion.
Corrosion in biogas plants is usually prevented by applying protective coatings comprising corrosion-resistant materials and making improvements in plant design. While selecting protective coating, it should be ensured that chosen paint is non-porous, adherent, quick-drying and anodic. Synthetic enamel or coal tar are one of the most commonly used paints for this purpose.
Coating is done by first cleaning the surface, applying two coats of red oxide or red lead primer, applying a phosphatising agent and a zinc-chromate coat, and finally a synthetic enamel or coal tar coat.
It is also sometimes suggested to protect the surface by coating it with chlorinated rubber, epoxy and polyester resins which are usually applied after cleaning the surface. Applying phosphate coat is another effective way for containing corrosion. After coating the surface with zinc chromate, a coat of phosphoric acid is applied to form a protective tenacious film of ferric phosphate which protects the surface.
Apart from using protective coating it should be endeavoured to use corrosion-resistant materials in plant-construction wherever feasible. Digesters made of fiberglass, concrete, masonry brick, stone etc. generally have longer service life than those made of steel. Use of dissimilar metals in plant construction should generally be avoided to minimise corrosion due to electrolysis.
Unless economically prohibitive, pipes of stainless steel or similar corrosion- resistant materials should be used for gas supply. All corrosion-prone surfaces should be periodically inspected to minimise chances of corrosion. Feedstock should be freed from non-biodegradable components like sand by installing screening devices, which unnecessarily cause abrasive action on metallic surfaces.