- Digester before being discharged and is normally equal to the volume of the digester divided by the daily inputs of substrate25 26. It is important to optimize the retention time in order to ensure, proper digestion of the slurry and extraction of as much biogas as possible before discharge of the slurry.
- If reports are to be believed, perhaps 20 million digesters— almost all of them underground masonry digesters— have been built in China, and many millions have been built in India and other countries as well. Depending primarily on the size of the digester and local prices for bricks (etc.), such a unit may cost from US$350 to US$500 and up, which is a great deal of money for.
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To size a digester, take the daily manure production in your operation and multiply by 20. Research has shown that an above ground mixed reactor similar to tanks designed by Engineered Storage Products Company (ESPC), is going to be more efficient than a plug flow system. Scaling of the digester The size of the digester i.e. The digester volume is determined by the length of the retention time and by the amount of fermentation slurry supplied daily. The amount of fermentation slurry consists of the feed material (e.g., cattle dung) and the mixing water.
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
How To Size A Digester System
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. Full version of halo combat evolved.
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.
How To Size A Biogas Digester
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.
1. Introduction
A biogas plant is an anaerobic digester of organic material for the purposes of treating waste and concurrently generating biogas fuel. The treated waste is a nutrient-rich, nitrogen-rich fertilizer while the biogas is mostly methane gas with inert gases including carbon dioxide and nitrogen. Biogas plants are a preferred alternative to burning dried animal dung as a fuel and can be used for the treatment of human waste. Other feedstock which can be used includes plant material, non-meat or grease food-wastes, and most types of animal dung. Over a million biogas plants have been constructed in the developing world for treatment of organic wastes, alternative energy supply to direct burning in the home, and overall improvement of human health and the environment. Many factors for selection of feedstock and site location must be researched before deciding to install a biogas plant. Successful construction of the biogas plant requires a proper design and adherence to follow correct construction methods. The success or failure of any biogas plant primarily depends upon the quality of construction work. The following instructions are based on the step-by-step instructions from the Government of Nepal Biogas Support Program Gobar Gas and Agricultural Equipment Development Company of Nepal has developed the design for model 2047 biogas plant. This biogas plant model has become prolific across Asia and is known as a fixed-dome plant. The advantages of the fixed dome plant include the simplicity of design, few moving parts, low cost to construct and low maintenance. The disadvantages when compared to a floating-dome digester are primarily the inability to store gas for use on demand; gas from the fixed dome digester must be used as generated or expelled to avoid damaging the digester.
2. Determining Plant Size
How To Size A Digester Tank
This manual includes design and construction material quantities for the Gobar biogas plant models of 4, 6, 8, 10, 15 and 20 cubic meters capacity. Design and size of a plant other than mentioned above is feasible and a skilled engineer should be consulted for deviations from the provided designs. The biogas plant size is dependent on the average daily feed stock and
expected hydraulic retention time of the material in the biogas system. Generally, 24 kilograms of feedstock complimented with 24 liters of water per day with a hydraulic retention time of 35 days will require a 4-cubic meter plant. Table 2.1 below gives some relevant data about the six different sizes of biogas plants presented in this manual.
Size Distortion
The biogas plant detailed in this manual consists of five main structures or components: 1. InletTank; 2. Digester Vessel; 3. Dome; 4. Outlet Chamber; and 5. Compost Pits. The required quantity of feedstock and water is mixed in the inlet tank and the slurry is discharged to the digester vessel for digestion. The gas produced through methanogenesis in the digester is collected in the dome. The digested slurry flows to the outlet tank through the manhole. The slurry then flows through the overflow opening in the outlet tank to the compost pit. The gas is supplied from the dome to the point of application through a pipeline.When a biogas plant is underfed the gas production will be low; in this case, the pressure of the gas might not be sufficient to fully displace the slurry in the outlet chamber. It is important to design the plant keeping hydrostatic pressure higher at the inlet tank than the outlet tank. The hydrostatic pressure from slurry in the inlet and outlet tanks will pressurize the biogas accumulated in the dome. If too much material is fed into the digester and the volume of gas is consumed, the slurry may enter the gas pipe and to the appliances.