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The following points highlight the three main parameters used for selecting biogas plant design. The parameters are: 1. Selection of Design Parameters for JBPs 2. Selecting Design Parameters Using a Cost Minimisation Model 3. Estimation of Heating Needs for Maintaining the Digester at Desired Temperature.
1. Selection of Design Parameters for JBPs:
A biogas digester is essentially a chemical reactor which is linked to both inflow and outflow channels. Rate of gas production is dependent not only on the microbial growth rate but also on the effectiveness of dispersion of the solid, liquid and gas phases and on the residence time distribution within the reactor. Dispersion or mixing is achieved by stirring which leads to higher gas yield.
The fact that all the feed material may not reside in the digester for the same time underscores the need to study residence time distribution. If feed is divided into N smaller portions, some parts might stay longer in the digester than others. Residence time dispersion depends upon such factors like geometry of digester or degree of dispersion.
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All these factors lead to following different design approaches one of which is as follows:
The main parameter which governs the cost of digester is its volume (V) which is made up of two parts, namely, active digester volume Vs and gas storage space. Vs represents the volume occupied by the slurry in the digester which can be computed by the relationship-
Vs (m3) = Slurry feed rate (m3/day) x HRT (days)
Volume of input slurry for a given quantity of organic wastes depends upon the dilution. Slurry is diluted to optimum level as with higher dilution bigger capacity digester is required. A dilution of cow dung with water in the ratio of 1 : 1 is generally preferred.
Composition of cattle dung may vary from place to place and thus it is advisable to analyse the TS fraction so that the amount of water to be added to bring the final TS content within 8 to 10 per cent can be determined. The fraction of TS can be measured by weighing a small quantity (5-10 g) of fresh dung, keeping it in an oven at about 100°C till all the water has evaporated, and then weighing the residue.
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For operational ease, biogas plants are fed on semi-continuous basis which is generally done daily. Volume of gas storage space is so provided that it is sufficient to accommodate gas generated in 24 hours under normal operating conditions.
In movable drum type plants as gas accumulates inside top of the gasholder it rises providing a variable volume for gas storage with pressure remaining more or less constant. On the contrary in fixed dome type plant gas produced pushes the slurry down displacing an equal amount of slurry into the inlet and outlet tanks. This process compresses the gas and raises its pressure.
Fixed dome plants can be made in different shapes- rectangular, cylindrical, spherical or ellipsoidal. However cylindrical shapes are generally more common particularly in India which again can be divided into two types, namely, flat and curved bottom digesters. Digester consists of a cylindrical brick walled pit with inlet and outlet tanks.
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When gas pressure is equal to atmospheric pressure, no gas can be withdrawn for use under this case as slurry level in the digester is equal to that in the inlet and outlet tanks. Resulting gas from the surface of the slurry accumulates in the space above and gets compressed slightly. As the compressed gas pushes down the slurry level in the digester, the slurry level in the inlet and outlet tanks rises to a point such that the pressure of the gas is balanced by the pressure of the water column in the inlet and outlet tanks.
When the storage space is completely filled with gas, slurry level inside digester is pushed to the lowest position and the slurry level in the inlet and outlet tanks rises to the highest position.
Volume of slurry in excess of active digester volume is pushed out through an opening in the outlet tank. Thus under ideal operating conditions, volume of slurry flowing out of the digester in a day equals that flowing in. Design of a biogas plant starts from the information concerning availability of wastes per day.
Following is the description of various parameters used by TERI for this purpose:
W = Weight of animal wastes fed per day (kg/day)
G = Gas production rate (m3/day)
Vs = Active slurry volume in the digester (m3)
Vsd = Slurry displacement volume (m3)
Vd = Dome volume (m3)
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H = Height of the cylindrical portion of the digester upto the top edge of the inlet/outlet opening (lintel), for flat bottom digesters (m)
H’ = Same as H, but for curved bottom digesters (m)
D = Diameter of the digester (m)
d = Slurry displacement inside the digester (m)
dh = Height of the dome (m)
r = Radius of the dome (m)
h = Slurry displacement in inlet and outlet tanks (m)
lb = Length and breadth of the inlet and outlet tanks (m)
Various design parameters can be estimated with the help of the following relationships:
Computation of Slurry Displacements in Inlet and Outlet Tanks:
Maximum pressure exerted on gas is equal to the pressure of the water (slurry) column above the lowest slurry level in the inlet and outlet tanks. This pressure is normally selected to be 0.85 m3 water gauge as a safe limit for brick/concrete domes.
Thus,
h + d = 0.85
i.e., with the knowledge of d, h can be determined from the above relationship.
Length (I) and Breadth (b) of the Inlet and Outlet Tanks:
There is no specific criterion with regard to the choice of the shape of cross-section of inlet and outlet tanks, but generally a rectangular shape with l = 1.5 b is preferred. For a case when inlet and outlet tanks have identical shapes, volume of slurry displaced downwards inside the digester is equal to the total volume of slurry displaced upwards in the inlet and outlet tanks.
Thus,
By using the values of h and G obtained earlier, b can be estimated from the above relationship and l can be selected as 1.5 times of b.
In some cases, an inlet pipe of 6 to 8 inch diameter is provided instead of inlet tank. In this situation, area of the outlet should be doubled to accommodate the slurry displacement. For this-
Computation of the Dome Height:
Volume of dome which is a part of sphere is given by the following relationship-
Recalling that the total volume of gas space is taken as equal to G, Vsd as 0.4 G, the remaining gas space volume which is the volume of the dome becomes (G – 0.4 G) or 0.6 G. Substituting this in the above equation, we get-
Value of dh can be obtained by solving the above equation which is cubic. This, among others, can be solved by iteration.
An algebraic solution also exists which is obtained by computing series of parameters as follows:
Estimation of the Radius of Dome:
Dome radius is determined with the help of the following relationship-
Determination of W for Curved Bottom Digesters:
For digesters with curved bottom, the lower base is equal to the upper dome. H’ is thus computed by the following relationship-
Determination of Other Parameters:
The size of the openings in inlet and outlet tanks are normally kept of size 0.5 m3 x 0.6 m3 regardless of digester capacity. This particular size is selected so that a man can go inside the digester during the construction period.
Digester walls 9 inch thick (full brick) and the walls of the inlet and outlet boxes (below the lintel) are 4.5 inch thick (half brick). For curved bottoms, two brick layers are provided, the lower layer being 4.5 inch and the upper layer 3 inch thick. Concreting whether plain or reinforced should be 4 inch thick.
Selection of Design Parameters for 2 m3 Capacity Plant:
For G = 2 m3, W = 50 kg/day, different parameters can be computed in the following manner:
By performing the calculations in the above manner, parameters and dimensions of large capacity plants can similarly be worked out complete summary of which is summarised in Table 5.5. In India it is customary to express the size or capacity of the plant in terms of designed gas production rate per day whereas in China it is expressed in terms of the volume of the digester.
Thus if the capacity is mentioned as 2 m3 or 70 ft3, it implies that the plant is designed for producing 2 m3 (70 ft3) of gas per day. It is to be emphasised that design parameters are by no means ‘optimum’ ones. For instance, if it is possible to maintain temperature of digester contents at 35 °C throughout the year, HRT can be significantly reduced and other parameters will accordingly change. After fixing the design parameters it is also necessary to work out the estimates of items like cement, sand, brick and steel which are used in plant construction.
2. Selecting Design Parameters Using a Cost Minimisation Model:
D.K. Subramanian, P. Rajabapaiah and A.K.N. Reddy evolved a cost optimisation model for optimising design parameters of movable drum type biogas plants. Volume V’ of a digester is product of volume vsl of slurry which is charged daily and retention time td of slurry.
Thus,
V” – Vsl . td
Necessary experiments for developing a theoretical approach for optimising design parameters were carried out at ASTRA, IISc., Bangalore on a KVIC type conventional plant having a rated capacity of 200 ft3 of biogas/day (5.66 m3 /day). This plant has the following dimensions; diameter and height of gasholder 6 ft (1.85 m) and 4 ft (1.22 m) respectively; and diameter and depth of digester 6.5 ft (1.98 m) and 16 ft (4.88 m) respectively. In KVIC design, mild steel floating gasholder accounts for nearly 40 per cent of the total cost of biogas plant. If r, Rs and Rr are capital costs of gasholder, its sides and roof respectively, then-
Where D is diameter of gasholder, h its height, t its thickness, ρ its density and u its unit cost in Rs/kg (comprising cost of steel, transport, fabrication, welding and painting). Height h of gasholder can be estimated by-
Where v is the volume of gasholder and γ is the maximum fraction of daily gas production, i.e. actual plant capacity C which is intended to be stored in gasholder.
From the equation for total plant costs, it is to be seen that plant costs do not increase linearly with plant capacity. In other words there are definite economies of scale in constructing several plants at a place. As a first approximation excluding excavation costs, the total costs of N plants each of capacity (C/N) is N1/3 times the cost of one plant of capacity C.
Based upon the cost optimisation approach for selecting design parameters, plants were constructed as per the modified dimensions and their performances were compared with the corresponding size of the KVIC model. D.K. Subramanian, et al., carried out cost optimisation studies for two cases- (i) when cost of gasholder alone is to be minimised, and (ii) when cost of gasholder plus digester is to be minimised.
Minimisation of the cost of gasholder alone leads to narrow and deep digesters of conventional plants. It was found that if total capital cost of gasholder plus digester is to be minimised, the optimisation leads to wide and shallow digesters which are less expensive. For evaluating this alternative two prototype plants were designed, constructed and operated which were both less expensive and efficient in performance.
It was found that if the cost of the digester and gasholder is considered, the least expensive plant is one in which the diameter of the digester equals its depth. Field tests of modified experimental models indicated that gas output in warm weather exceeded that from movable drum type KVIC plants of seventies although these are still not as suitable for colder areas due to larger surface heat losses.
G J. Mohanrao carried out experimental studies to analyse the effect of digester diameter to depth ratio on biogas yield. He measured the biogas yield in litres per m3 of digester volume and in litres per kg of volatile solids added. He reported that gas yield is maximum when digester diameter to depth ratio is 1 and remains reasonably high till the ratio is 0.66. Hence, if his experimental results are to form the basis, digester diameter to depth ratio should be kept within the desirable range while designing the digester and it should not be allowed to be less than half in any case.
3. Estimation of Heating Needs for Maintaining the Digester at Desired Temperature:
Temperature affects microbial activity of the two types of bacterias that cause anaerobic fermentation. Mesophiles perform best in the temperature range of 29 to 38°C and the thermophiles are most active between 49 and 57°, C. Gas yield substantially slows down at 15°C and ceases altogether below this temperature. The rate of gas generation nearly doubles for every 10°C rise in temperature variation between 15 and 35°C.
A biogas plant supposed to operate at a temperature of 15°C needs to have digester volume four times greater than one at 35°C. Comparative biogas yield from cattle dung in different states of India based on mean ambient temperatures is summarised in Table 5.9. As expected gas yield in climatically cold states like Jammu and Kashmir is substantially low as compared to the other states.
In view of particular climatic conditions mesophilic digestion is more convenient and economical for most parts of India. For mesophilic fermentation a single temperature range of 30-35°C is considered most convenient and economical to work with.
As gas yield depends on digester temperature, choice of detention period is governed by ambient temperatures. Length of detention period is inversely related to temperature. If temperature is low, a longer detention period is necessary which is met by designing larger digester and hence more expensive biogas systems.
As for the fixed dome Janata biogas plants, temperatures in underground plants are mainly influenced by earth temperatures. The mean gas yield from these digesters is reported to be 0.15 m3/day cubic meter of the digester volume though the rate of gas production varies substantially from north to south and from summer to winter.
The total heat requirement (QT) for maintaining digester slurry at a desired temperature can be estimated as a sum of the heat quantities in the following manner:
QT = QL + QW + Qg + QI – Qr
Where,
QL = heat loss through digester walls, floor and its top
QW = heat loss following evaporation
Qg = heat loss due to the gas leaving the digester
QI = heat required raise temperature of influent slurry the desired level and,
Qr = heat of reaction resulting from anaerobic fermentation.
The heat loss through the digester walls (QL) is the sum of heat loss through top, side walls, and its bottom which can be calculated from the overall heat transfer coefficients of top, side walls and bottom of the digester, respectively. It is reasonable to assume that all losses from digester take place through conduction which can be estimated with the help of the following fundamental equation-
Qf = rate of heat loss through the flow of digester contents (energy/time)
Af = area of flow (distance2)
Total digester heat loss (QL) is, therefore.
QL = QR + Qw1 + Qw2 + Qf
The heat loss due to evaporated water (Qw) is the sum of the sensible heat loss of the steam and the heat of evaporation of water. The sensible heat loss with the dry biogas leaving the digester is the sum of the sensible heat in methane and carbon dioxide.
The heat required to raise the influent slurry to the digester operating temperature (T1) can be calculated from:
Q1 = W . CP (T1 – TI) . EF
Where T1 is the digester operating temperature, TI is the influent slurry temperature, W is the total weight of slurry to be added to the digester per day, CP is the specific heat of the influent slurry and EF is the efficiency factor.
Q = U.A. (T2 – T1)
Where,
Q = rate of heat loss (energy/time)
U = overall coefficient of thermal conductivity (energy/time-area- temperature)
A = area normal to the direction of heat flow
T2 = air temperature outside the digester
T1 = digester operating temperature
Overall coefficient of thermal conductivity U is a function of the unit surface conduction inside and outside plus the thermal resistance of each of the materials as governed by the following relationship-
Where,
x = material thickness (length)
k = coefficient of thermal conductivity of the material (energy/time length-temperature)
hj = inside unit surface conductance (energy/time-area-temperature)
h0 = outside unit surface conductance (energy/time-area-temperature)
Ka = coefficient of conductivity of air and gas (energy/time-area-temperature)
With the help of above relationship, different heat loss quantities can be estimated in the following manner-
Heat loss from digester roof (QR) given by-
The above equation is written with a view to facilitate conceptual under-standing of the heating process.
C.R. Prasad and S.R.C. Sathyanarayan developed thermal model to study heat-transfer from slurry and gasholder to the surrounding earth and air respectively for the case of a movable-drum type biogas plant. Computations were made for two conditions- slurry at an ambient temperature of 20°C and slurry at 35°C, the latter being ambient temperature of mesophilic fermentation. Gasholder is a major source of heat-loss under both conditions.
It was endeavoured to estimate quantity of heat which must be supplied externally to compensate for net heat loss which occurs if the slurry is maintained at 35°C. For a case when part of biogas produced is used for heating the digester contents, it was found that there occurs a net increase in gas output following heating and therefore a net benefit by operating the plant at 35°C. The results of the thermal analysis can be used to develop a strategy for operating the plant at optimum temperature or at temperature higher than the ambient.
J. Kishor, et al., described approaches for thermal balance of fixed dome biogas plants and enhancement of gas yield during winters. They presented a thermal model of a fixed dome biogas plant with or without hot charging and having the ground surface above the plant blackened and glazed. The rate of heat transfer between the ground and plant was determined by electrical simulation by using laboratory scale model of the plant made of copper and suspended in copper sulphate solution.
Studies were carried out keeping in view monthly variation in temperatures at Madras, New Delhi and Srinagar, and suggest strategies to alleviate problems of low temperatures corresponding to 1, 4, 8, 14 and 30 m3 capacity biogas plants. It was reported to be possible to maintain the slurry temperature between 28 and 35°C in a place like Delhi by blackening and single glazing of the ground above the plant.
In places of colder climates even double glazing may not be enough. Hot charging from a shallow solar pond alone was not found sufficient to maintain desired temperature of digester contents during winters in Delhi and Srinagar.
In places of hot climates where temperature is sufficiently high, hot charging from a shallow point or blackening/glazing of the ground was found to be sufficient to meet the objective. In colder places like Srinagar using part of the gas produced to heat digester contents and blackening and double glazing of the ground above dome was suggested to counter the effect of low temperatures.
G.P. Devkota studied the effect of composting on biogas yield which is based on the fact that yield changes with temperature of the digester contents. In regions where the temperature drops significantly at night and for certain seasons of the year, composting is a useful method for generating heat for smaller dome-type biogas digesters.
Tests were carried out in Butwal (Nepal) throughout the cold season with the result that temperature of digester contents insulated by compost was found to be 2°C higher than for uninsulated digesters. Sixty kg of buffalo dung was daily fed into each digester, and it was found that digesters insulated with compost produced 10 to 37 per cent more gas than without.
Example Showing Steps for Selecting Plant Parameters Including Heating Needs:
This example illustrates the steps for computing design parameters of a 100 ft3 capacity movable drum biogas plant. Based on a set of assumptions it is endeavoured to calculate biogas requirement, manure requirement, digester, capacity, gasholder size, heat requirement, heat losses, and digested slurry withdrawn every day from plant. This exercise is based on a similar study reported in NAS publication but with an altered design parameters.
Following is the summary of various assumptions and conversion factors used:
i. Biogas is used to run engine-generator set to produce electricity.
ii. Thermal efficiency of the engine is 40 per cent.
iii. Electrical mechanical efficiency of generator is 75 per cent.
iv. One cubic foot of biogas containing 55 per cent methane provides 550 BTU of heat.
v. Forty per cent of fuel energy is transferable to cooling water.
vi. 1 kwh = 3411 BTU.
vii. Ten pounds of dung plus water is needed to prepare slurry which occupies a volume of 2/7 ft3. This is equivalent to 1.8 litre of slurry per kg of dung.
viii. Digester is expected to operate at 95°F (35°C).
ix. For meeting village needs 125 kwh of electricity is needed to be generated daily.
Estimation of Biogas Requirement:
Heat needed to generate 125 kwh of power per day-
Estimation of Manure Requirement:
With an assumed norm that 1 kg (2.20462 lb) of cattle manure yields 1.3 ft3 of biogas (Table 5.1) or 1 lb of cattle manure produces 0.60 ft3 of gas-
Selection of Digester Capacity:
It is assumed that 4310 lb (1955 kg) or dung is fed to the digester daily which should remain inside for 50 days (Table 5.2) for undergoing anaerobic fermentation.
With the assumed norm that 10 lb of dung occupies 2/7 ft3 of space and 4310 lb of dung is fed to the digester, daily volume needed becomes-
Estimation of Gasholder Size:
Based on presumption that gas is to be used regularly and withdrawn more or less at constant rate, gasholder needs to have only half the volume of estimated daily gas production.
Thus for daily gas production of 2585 ft3/day (73.2 m3/day), gasholder needs to have a capacity of only 1292.5 ft3 (36.6 m3). For a proper alignment in a movable drum type plant, it is an appropriate design practice to keep gasholder diameter smaller by 6 inches (15.24 cm) than that of digester. Diameter of gasholder should therefore be 19.3 ft (5.9 m).
The height of gasholder is thus given by:
It is to be noted that on account of its weight, gasholder tends to compress biogas accumulated with the result that actual volume of gas collected above the slurry surface is less than the gasholder capacity.
Estimation of Heat Requirement:
Estimates of heat requirement for warming up the plant slurry should be based keeping coldest month of the year in view. For tropical and subtropical regions the minimum average temperature for the country as a whole is assumed as 20°C (68°F) and for extremely cold regions like Laddakh the minimum average ambient temperature is assumed as 1°C (35°F).
As the available digester temperature is less than the optimum desired temperature, for obtaining higher gas yield it is desirable to raise the digester temperature from 20°C (68°F) to 35°C (95°F).
As already estimated, 4310 lb (1955 kg) along with equal quantity of water is to be added daily which means that a total of 8620 lb (3911 kg) of slurry is to be fed, every day. If water and dung are assumed to have same specific heat, requisite heat to raise the temperature of input slurry from 68°F to 95°F is-
Estimation of Heat Losses:
Amount of heat losses from digester to the environment depends upon the ambient temperature. For estimating heat losses following temperatures are assumed.
Mean ambient temperature (coldest) = 1°C (35 °F)
Mean ambient temperature close to the digester wall = 20°C (68 °F)
For estimating heat losses, following area calculations are made-
The actual amount of heat produced by the biogas run diesel engine coupled generator which is not converted to electricity based on an overall efficiency of 30 per cent (75 per cent x 40 per cent) is 1,421,250 – 4,26,375 = 9,94,875 BTU (10,49,593 kJ) per day.
This is the same as 70 per cent of 1,41,250 BTU per day. With a total heat requirement of 4,48,670 BTU/hr (4,73,347 kJ/day), the amount of heat available for transfer to cooling water and that which will somehow go to the environment will be 9,94,875 – 4,48,670 = 5,46,205 BTU/day (5,76,246 kJ/day).
Fifty per cent of this heat quantity is assumed to be available for transfer to the cooling water. This implies that 2,73,102.5 BTU (2,88,123 kJ) will be available daily to heat the cooling water.
As this quantity of heat is less than the total amount of heat needed to maintain the slurry at operating temperature 4,48,670 BTU/day, (4,73,347 kJ/day), heat available from the engine generator set is not entirely sufficient to warm the digesters content and some kind of supplementary heating from solar energy or other sources shall be necessary.
Estimation of Quantity of Slurry Withdrawn from Digester Daily:
Quantity of slurry withdrawn per day from digester is usually less than added each day following certain amount of volatilisation that keeps occurring during the interval. As already estimated total amount of slurry added per day to the digester is 8620 lbs (3911 kg) comprising 4310 lbs of manure and 4310 lbs of water. It is commonly observed that 10 per cent of slurry weight is generally volatilised. Thus the weight of slurry withdrawn per day becomes 8620 x 0.90 = 7758 lbs (3519 kg).