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The following article will guide you about how to determine the size of a biogas plant.
Checklist for Determining the Size of Biogas Plant:
For selecting plant parameters, it is necessary to first assess gas demand. For determining gas requirement biogas consumption norms for cooking, lighting, driving engines etc. need to be known.
Common norms for biogas consumption for different applications are as follows:
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Gas Consumption Rates for Different Applications:
Cooking – 0.25-0.42 m3 (8-15 ft3) per person per day
Lighting – 0.11-0.15 m3 (4-5.5 ft3) per hour per lamp
Driving Engines – 0.45 m3 (15 ft3) per HP per hour
Generating Electricity – 0.6 m3 per kwh
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i. 1 kg of fresh animal wastes generates 0.05 m3 of gas.
ii. 1 m3 of biogas is equivalent to:
a. 2 kg of fuelwood
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b. 0.6.1itres of kerosene
c. 0.5 litres of petrol
d. 0.4 litres of diesel
e. Cooking of 3 meals for 3 persons
f. Running of 1 HP IC engine for 2 hours
g. Running of 300 litre refrigerator for 3 hours
h. 1.25 kw of electricity
i. Lighting a gas lamp for six hours or six lamps for one hour
j. Illuminating twenty five 40 w electric bulbs for one hour, equivalent to a 60 w electric light burning for 6 to 7 hours
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k. Driving a 3-tonne lorry for 2.8 hours
iii. The weight of methane is roughly half that of air.
iv. Approximately 1 m3 of digester space is occupied by 1000 kg of animal manure and water.
v. 1 m3 of waste materials yield 0.15 to 0.30 m3 of gas per day depending upon the climatic conditions and the type of material used.
vi. Digester volume should be at least 30 times the volume of daily feed rate. Considering that feed remains inside the digester for more than six weeks or so, it is generally preferred to keep the digester volume roughly 42 times the volume or daily feed input.
vii. Volume of the pit can be determined by estimating how much gas will be needed and how it will be utilised. In rural areas a family of five requires 1 m3 per day for cooking and lighting. In summer each m3 of digester space produces 0.15-0.2 m3 gas per day and in winter 0.1-0.15 m3 per day.
While building digester one can go by the norm that 1.5-2 m3 of volume should be allowed per head. Digester volume can also be calculated by following the norm that for two or less people there should be no more than 3 m3 digester volume per head; for three to five people, no more than 2 m3 per head, and for five or more people there should be no more than 1.5 m3 digester volume per head.
Following simple relationship can be used for determining the total gas requirement:
After assessing gas needs, it is necessary to estimate the amount of digested sludge and biogas obtainable from available wastes. While estimating this, it is necessary to know norms of wastes and gas yield from different animals (Table 5.1).
Following simple relationships can be used for estimating biogas yield:
i. Generation of Animal Manure:
Number of animals or other sources of wastes x rate of waste yield per animal per unit time
ii. Assessment of Biogas Yield:
Number of animals or other sources of wastes x rate of gas generation per animal per unit time = Biogas yield per unit time.
Following assessment of biogas demand and supply at a prospective site, rationality of having a biogas plant at that site can be evaluated by systematically proceeding as in Fig. 5.1.
Selection of Parameters Affecting Biogas Plant Size:
Digester Temperature and Detention Period:
There are several factors that affect size of a biogas plant. Temperature and detention period which are interrelated are two important parameters. Detention period is defined as the time taken by the feed to flow from inlet to outlet. A set of recommended norms for detention times in different world regions are given in Table 5.2.
Biogas is formed following anaerobic fermentation of organic wastes due to bacterial action. Bacteria are most active when digester temperature is kept uniform and maintained between 25 and 35°C. If it is lower or fluctuates, biogas yield is suboptimal. At temperatures below 15°C, biogas yield almost ceases.
Due to this reason, biogas plants are less effective in cold climatic regions. For estimating biogas yield more realistically in such regions, temperature is taken as 5°C less than the actual temperature. Fig. 5.2 shows the relationship between detention period, digester temperature and gas yield.
Higher temperature tends to intensify microbial activity leading to increased gas output. The higher is the temperature, the shorter can be the detention period. With shorter detention period, throughput becomes faster, with the result that digester can be made of smaller size. Effect of detention period on gas yield for different feeds is shown in Fig. 5.3.
From the knowledge of climatic conditions at the prospective site, and with the help of information such as given in Figs. 5.2 and 5.3, detention period to suit particular climatic conditions can be selected.
Quantity of Feed to be used:
Plant slurry to be fed to a biogas plant should have organic wastes and water in proper mix. A set of typical mixing ratios are given in Table 5.3.
Estimation of Quantity of Waste Material and Water Added Per Day:
Estimation of Size of Digester and Gasholder:
Volume of digester can be determined by multiplying the volume of slurry added per day with the detention period. Digester volume is essentially the product of feed rate and detention period.
By referring to Table 5.1 or other similar source of information, it is possible to estimate at biogas yield per day in appropriate units such as m3/day.
Volume of gas production per day from plant = ___________ m3/day.
As a rough rule, volume of gasholder is usually taken as half of daily gas production rate.
Volume of gasholder =__________ m3.