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Minimisation of energy input requirements of an anaerobic digester
MINIMISATION OF ENERGY INPUT REQUIREMENTS OF AN ANAEROBIC DIGESTER
P. J. MILLS
North of Scotland College of Agriculture, Aberdeen, Scotland
ABSTRACT
The theoretical maintenance energy requirements for a mesophilic anaerobic digester operating with a ten-day detention time are presented. Methods of applying this energy to the digester are suggested. The design and theoretical performance of a unit to heat the input by means of the output are discussed. Mechanical and gas mixing systems and their power requirements are considered. The net energy output of a theoretical unit is then derived. Theoretical considerations are next compared with practical results obtained from the operation of two field scale digesters, one of which is fitted with a heat recovery unit of the design previously discussed. Differences between theory and practice are shown and reasons for these considered.
INTRODUCTION
Both laboratory and field scale results indicate that predictable levels of gas can be produced by anaerobic digestion of animal wastes (Hobson et al., 1975; Lapp et al., 1975; Fischer et al., 1977). It is apparent that, even if all the gas produced were available for uses other than digester maintenance, the value of the gas would not service the capital cost of a fully automated small (200 pig) farm scale unit. However, at larger (2000 pig) scales, provided continuous, year round use can be made of the gas, the economics of anaerobic digestion being used solely as an energy-producing process look promising. These calculations have generally been made dismissing the benefits of digestion as a treatment process, where, at larger throughputs (about 8 m 3 day 1) it compares very favourably with a separation/aeration process (Mills, 1977) owing to its low running costs, However, in order to optimise the economics of anaerobic digestion, the minimum maintenance energy input is required. 57 Agricultural Wastes (1) (1979)-- :i(:? Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
58
P.J.
MILLS
This paper discusses the theoretical minimisation of maintenance energy and then compares this with field-scale operation of the existing Aberdeen digester (Robertson et al., 1975) and a new unit specially designed to investigate more efficient systems of operation. The work is part of a joint programme. Pilot plant data obtained at the Rowett Institute, Aberdeen, Scotland are used in the operation of these large-scale digesters.
THEORETICAL CONSIDERATIONS
The calculations below are based on the following data. Using a ten-day residence time at 35 °C gas production is 0.4 m 3 gas per kilogramme Volatile Solids when the digester is loaded with pig slurry, and the gas is about 65-70 ~ methane with an average energy content of 6.7 kW h m - 3. Outputs are shown in Table 1. The energy
TABLE 1 GAS AND ENERGY OUTPUTS FROM DIGESTED PIG SLURRY
Total solids, input (%)
Gas output (volume per digester volume per day)
Energy (k W m 3 digester volume)
5 6
1-6 1 "9
0-45 0.54
Volatile Solids = 0-8 T o t a l solids.
inputs to a digester are required for: (a) heating, (b) mixing and (c) control and recording. Item (c) is minimal compared with (a) and (b) unless compressed air is used for control and automatic valve operation.
(a) Heating Heat is required to: (i) heat the input to 35 °C and (ii) compensate for heat losses from the vessel. (i) If the input slurry is stored in an external tank before loading its temperature may be assumed to be ambient (TA). If the input is taken from a tank inside the pig house a temperature of 10°C above ambient may easily be achieved. It is worth noting that the material leaving the animal is at exactly the right temperature for digestion. (ii) If sufficient insulation is put on the vessel it may be assumed that the insulation limits the heat transfer and that the effects of wind on external film coefficients can be ignored. Thus heat loss through the vessel = UoAo(35 - TA). Where Uo is the overall heat transfer coefficient (htc) of the vessel and A D is the surface area of the
ENERGY BALANCES OF DIGESTERS
59
vessel. The least cost practical vessel and that with the smallest surface area per unit volume is a cylinder with a 1 : 1 aspect ratio. Table 2 shows the heat requirements for losses and input for various sizes of vessel, with U = 0-35 W m 2 °C 1 and T A = 0 °C. It can be seen that the heat required for the input greatly exceeds vessel losses and that this tendency increases with scale. The overflow f r o m the digester is at 35 °C. If this could be used to heat up the input at 100"/; efficiency the overall heat requirements of the system could be reduced to less than 10 ~i, of the gas energy. In a system with continuous loading and overflow, a countercurrent heat exchanger could achieve an efficiency of 75 o;. It would be necessary to have another p u m p to overcome the pressure drop in the heat exchanger but this would only require 1 k W to recover 50 kW.
TABLE 2 HEAT REQUIREMENTSOF VARIOUS SIZES OF DIGESTER
Throughput (m 3 day 1) Volume (m 3) Surface area (m2) lnput heat (kW) Loss heat (kW) Total heat (kW) Potential output (kW)
1 10 25 1-7 0.3 2.0 5.0
2"5 25 48 4.3 0.6 4.9 12.5
5 50 75 8.6 0.9 9.5 25
10 100 116 17 1.4 18.4 50
25 250 222
43 2.7 45.7 125
50 500 348 86 4.3 90.3 250
The digester is assumed to be loaded with slurry of 5.5 g?, TS and running at 35~C with ambient temperature 0°C and detention time 10 days.
F o r a small digester loading has to be intermittent. The smallest reliable commercial p u m p for handling slurries o f 5-8 0,'~, dry matter will p u m p about 1 . 2 m 3 h 1. This means that loading~ and consequently o v e r f l o w - - m u s t be intermittent. The most likely alternatives are manual daily loading or automatic hourly (or less) loading. As the loading p u m p has to start before any hot liquid can be displaced from the vessel, it is necessary to retain the w a r m overflow in an insulated vessel and use it to preheat the next batch to be loaded. If the recovery unit consists o f a coil, which contains the whole o f the next input batch, submerged in an insulated tank into which the output runs, the input and o u t p u t will reach the same temperature. This is provided the area of the coil is large e n o u g h for the volume contained and that the insulation on the tank is relatively g o o d in relation to the output residence time. The input is put in the coil because the loading p u m p is capable o f overcoming the coil pressure drop and gravity can be used for the tank side. It would be more desirable to put the output through the coil because o f heat loss; also, owing to its higher temperature and lower solids level, and consequently lower viscosity, a higher heat transfer coefficient would be expected. It will later be seen that the coil-side htc is the limiting factor in the design o f this type of recovery unit (Fig. 1).
60
v.J. MILLS
,rom,ges,er,5°C tOdigester ~
~
Overflow
!
.
Coldinput Fig. 1.
Diagram of heat recovery unit.
For a coil-in-tank, intermittent flow heat recovery unit the temperature of liquor being pumped to the digester is given by:
Ti. = Tar 1 - e
_ UAt "~ O'~-MJ
where: Tin is the temperature of liquor being loaded into the digester ( °C); Tav is the mean of the digester operating temperature and the raw input temperature ( °C); Uis the overall heat transfer coefficient from liquor outside the coil to inside (W m-2 C - 1); A is the coil surface area (m2); M is the mass of liquid per batch (kg) and t is the interval between batches (h). This assumes that the tank is well insulated. For Ti. to exceed 9 0 ~ of the difference between Tav and the raw input temperature, i.e. 45 ~ of the heat required to raise the input to operating temperature is being recovered, UAt/M must be greater than 1-98. If the density of the input is assumed to be 1 kg litre- 1 A I M is fixed for any coil pipe diameter. Table 3 shows values of Ut required to meet this condition for different pipe sizes. The value of t is varied according to the value of U found in practice. TABLE 3 VALUES OF U t
(UAt/M)
REQUIRED TO MAKE PIPE SIZES
Coil pipe diameter (mm) A/M (m s kg- 1) Ut (W h m -2 °C -1)
25 0.16 12.4
> 1 . 9 8 FOR DIFFERENT
40 0.10 19.8
50 0-08 24.8
ENERGY BALANCES OF DIGESTERS
61
Overall U values of 75-100 would be expected with water inside and outside a coil but the value might well drop to 25 with cold, relatively viscous slurry inside and warm digested slurry outside the coil. If U is 25 only a 25 m m or smaller pipe will provide sufficient area if t = ½. The overall htc will be limited by the internal film coefficient for the stationary liquid in the coil. Operation of a mixer to ensure uniform overflow from the unit will improve the coil external film coefficient but have no measurable effect on the overall htc. Transfer of heat to vesselcontents: The most direct method of transferring the heat energy in the gas to the vessel contents would be to direct a gas flame on to the surface of the vessel. However, the high temperature involved would cause heat shock to the contents and scaling on the inside of the vessel. Direct heating of externally circulated contents can be dismissed for the same reasons. The most reasonable alternatives are: (i) use of hot water coils inside the vessel; (ii) use of a water-heated, jacketed screw p u m p inside the vessel: (iii) use of an external water heat exchanger. System (i) has a low capital cost and only requires a low power water p u m p to circulate water. It runs the risk of fouling on the outside of the coils if the water temperature is too high or if there is not sufficient mixing of the vessel contents. The workers using this technique have reported few problems. It has been suggested that PVC pipe could be used for coils as it has a smooth exterior on to which scaling would be unlikely. The low external film coefficient would not be made significantly worse by the poor conductivity of plastics. System (ii) overcomes the scaling problem and also provides mixing. The equipment consists of a water heated jacket with an internal Archimedean screw. The screw lifts liquid over the heated jacket and spreads it on to the top of the digester surface. It suffers the disadvantage of being a piece of apparatus inside the vessel and requires a rotating-shaft seal. System (iii) is relatively expensive from both capital and running aspects but is fail-safe. A more expensive and higher power p u m p is required to p u m p digester liquor than is required to p u m p water and the cost of the heat exchanger is high. However, as the liquid velocity through the tubes is high, high water temperatures and heat transfer rates can be obtained. The system can also be used to mix the vessel contents.
(b) Mixing In an industrial aerobic fermentation system, agitator power inputs may exceed 2 kW m 3 tank capacity. This is about three times the m a x i m u m gas energy output from a digester. Clearly, a much lower energy mixing system must be adopted for anaerobic digestion if it is to have an energy surplus. However, in an anaerobic system mixing in of one of the microbial substrates, oxygen, is not required as it is in an aerobic system. There are three main reasons for requiring mixing in a digester: (i) to maintain a
62
1". J. MILLS
uniform temperature; (ii) to provide intimate mixing of bacteria and substrates for optimum yield (there is some doubt as to whether, as the process is multi-stage with solid substrates, a fully mixed system is desirable); (iii) to prevent the formation of scum or crust, as gas bubbles raise solid particles to the surface where they bind together. Condition (i) can be satisfied by circulating with an external pump with a digester volume turnover time of about 2 h. The power requirement of this is very low (0.1 kW for 10m3). Condition (ii) requires a system which lifts settled material from the bottom and distributes it throughout the vessel. A high volume, low shear mixer (e.g. crossbladed paddle) or gas recirculation, can be used. Condition (iii) requires a system which either causes turbulence in the liquid just below the surface or pushes the scum down into the liquid. Gas mixing or a high shear agitator just below the surface are of the first type. Revolving-rake mechanisms or liquid jetted out over the surface have the second effect. Mechanical agitators suffer the disadvantage of requiring gas seals and bearings. Methane, as a member of the paraffin family, breaks down greases in conventional seals and bearings even though the gas pressure is low. Gas mixing, by compressing gas from the top of the vessel and releasing it at the bottom, requires low power inputs (Table 4). The rate at which gas is recirculated is approximately proportional to the digester area and the pressure required is proportional to the depth at which the pressurised gas is released.
TABLE 4 POWER REQUIREMENTSFOR MIXINGDOMESTIC DIGESTERS (FOR A LIQUIDRING PUMP--COURTESY OF NASH ENGINEERING) Digester volume (m 3) Gas velocity (m m i n - ~) Theoretical power (kW) Motor size (kW)
22 0.032 0-12
76 0-027 0-28 0-75
180 0.026 0-61 1.5
350 0-024 1.01 3.0
600 0-021 1-5 4-8
Domestic digesters use gas flow rates in the range 0-02-0-03 m 3 min 1 (per square metre of digester cross-section area). This figure can be taken as a gas velocity through the vessel if there is no liquid in the vessel. At these low throughputs the power required for continuous mixing would be approximately 3 ~o of the output from a high-rate digester. It is suspected that gas velocities in the region of 0.1 0.15mmin I will be required to prevent crust formation in digesters which have both higher solids levels and a wider range of particle sizes. This level of mixing would require 15 % of the output and would have to be made intermittent if the mixing power is to be insignificant.
E N E R G Y B A L A N C E S O F DIGESTERS
63
Overall energy balance for a theoretical digester For a 50 m 3 digester producing an energy of 25 kW the input requirements are: Vessel loss, 0.9 kW; input, 8.6 kW; agitation 1-0 kW as electric power. lnput heat can be reduced to 4.4 by heat recovery. If the total heat of 5.3 kW is supplied by a 20 kW boiler operating for 6-4 h day - 1 at 75 ~o efficiency, the boiler will require an input heat of 7.1 kW mean. This leaves a net gas energy of 18 kW. To run the circulating p u m p while the boiler is on will require 0.25 kW mean. Thus electrical demand is 1.25 kW. At current electrical costs this is 2.5 pence an hour. The value of the gas at current domestic gas costs is 12.5 pence an hour. So the monetary surplus is 10 pence an hour; £876 per annum. If the gas is burnt in an engine attached to a generator providing 8 kW electricity, 12 kW heat and 5 kW waste, the heat will exceed requirements and a mean of 6.75 kW electricity will remain of value 13.5 pence an hour.
P R A C T I C A L RESULTS F R O M FIELD S C A L E D I G E S T I O N
(a) Heating The first field scale (13 m 3) digester at Aberdeen (Robertson et al., 1975) was designed to have an overall htc of 0.7 W i n - 2 oC - 1. The aspect ratio of both verticalcylinder digester vessels is 1.5:1 and the insulation on the first, which is 50 mm of glassfibre, became wet owing to leaks in the outer skin. As a result the vessel cools by 4~5 °C at an ambient temperature of 0 °C. This is approximately four times the rate at which it should cool. With the poor insulation the heat loss has become greater than the heat required to raise the input to 35 °C. There is only 55 ',~oefficiency of heat transfer from the boiler to the digester using an external heat exchange system. This is partly caused by long underground pipes on which the insulation has deteriorated owing to access to clear blockages. As a result, the digester, which is loaded with slurry from an external tank at ambient temperature, has a heat deficit of 1-2 kW during the winter months. The second digester was designed for 0-34 W m 2 °C ~. Insulation is 100 m m of isocyanate foam pumped between the main vessel and a welded outer steel skin. The pipe work is shorter and of larger bore, as shown in Fig. 2, and heat transfer from boiler to digester is 70 }f, efficient. The rate of cooling is still greater than 0-6 °C per day, at 2.5 °C per day. Part of this loss is caused by the weir. The weir has an insulated lid but this has to have a vent to prevent siphoning, allowing a continuous exit of warm, moist air. The second digester is fitted with an experimental heat recovery unit shown on the right-hand side of Fig. 2. Heating trials on the vessel filled with water gave a heat recovery of 45 48 ~,, with a half-hour batch interval and a 40 m m diameter coil pipe. When operating with slurry heat recovery fell to 35-50 '~;,, indicating a U value of about 24. The recovery unit has a stirrer which operates intermittently, with the
64
P . J . MILLS
Fig. 2. The second Aberdeen field-scale digester. Background, digester tank partly built into site hut. Left, heat exchanger and sludge circulating pump for digester heating. Right, heat exchange unit for recovery of heat from output to heat input, digester feed pump below.
loading pump, to ensure mixed overflow. When this stirrer operates continuously there is no appreciable improvement in heat recovery. With operating conditions the same as those of the first digester and ambient 0 °C, the second digester has a net output of 1-2 kW during the winter months.
(b) Mixing The first of the two field scale digesters is mixed by two turbines on a single shaft extending from top to bottom. The foot and steady bearings are machined from 'Tufnol'. The gas seal is provided by a pipe, concentric with the shaft and of 10 mm greater diameter. This pipe is sealed on to the lid of the vessel and the other end is below the liquid level. This means that the only gas lost is that actually formed in the annulus between the pipe and the shaft. The unit is driven by a 4 kW motor reduced to 80 rpm. In order to reduce the electrical demand the stirrer is operated for 5 min per hour by a time clock. An interesting feature of this mixing regime is the fact that up to 70 9/0 of the gas produced is released while the stirrer is on (Fig. 3). It has been suggested that this is caused by breakage of the surface tension as the
65
ENERGY BALANCES OF DIGESTERS Mix
0
100 Time
Fig. 3.
Influence of mixing on gas production/release.
stirrer starts, releasing preformed bubbles. However, as the pressure at the bottom of the vessel is 1.5 bar, the initial formation of the bubbles should cause the liquid level at the t o p - - a n d thus the gas pressure--to rise. This would result in the meter registering at least 66~o of any gas formed at the bottom but not released. Suggestions of the gas release being caused by sudden intimate mixing of bacteria and substrates are not compatible with the normal behaviour of microorganisms. The capital cost of such a unit is very high and it lies idle 95 ~o of the time. The mean power requirement is 0.33 kW. The second digester is mixed partly by recirculation through the heat exchanger and partly by gas mixing. The liquid returning from the heat exchanger passes through a venturi unit situated in the headspace of the digester vessel. Liquid at 2.5 litres sec- 1 draws in four litres of gas per second and the mixture is returned to the centre of the digester. The existing unit does not develop sufficient suction to return the gas to the bottom; it is feared that the degree of restriction at the venturi necessary to create such suction would result in blockages in the circulation pipe. The presence of the bubbles causes a great increase in bulk liquid flow and turbulence at the liquid surface, thus restricting crust formation. The unit is driven by the same pump as circulates liquid through the heat exchanger and no increase in power consumption of the pump has been detected. Mixing by gas recirculation using a separate gas pump is now being examined.
DISCUSSION
A theoretical digester can produce a net output of methane all the year round. The net output should be 0.35 kilowatts per cubic metre of digester capacity. As the heat
66
P . J . MILLS
for heating the input is much greater than heat losses, gas production should not fluctuate much during the year. This should still apply in extreme climates where the input is still likely to be pumped from a livestock building above 0 °C despite ambient temperatures of - 2 0 °C. Under these conditions for a 50 m 3 digester the ratio of input heat to loss heat is still 6:1. In order to maximise net output it is therefore important to recover as much heat from the discharged effluent as possible. This becomes easier as continuous loading is approached. Continuous loading is also more desirable as it stabilises the operating conditions of the microorganisms. With efficient heat recovery the heat requirement of a digester should be such a small proportion of its output that fluctuations between net summer and winter output will be minimal. Gas mixing should prove to be a lower energy and maintenance system than conventional mechanical systems. There is still some doubt as to whether gas mixing can handle very thick materials with large particle sizes, e.g. cattle slurries, and more experiments are needed. In the practical units operated at field scale, energy balances have not been as good as theoretical models. This is caused by heat losses from the vessel becoming as large as the input heat requirements. This may be partly overcome by improving insulation and eliminating losses. At larger scales heat loss becomes relatively less important as volume to surface area increases and the shape of the vessel, for economic reasons, approaches the 1:1 aspect ratio of least surface area. This should make control of these losses easier at the range of scale at which farm scale digestion is likely to take place.
REFERENCES FISCHER,J. R., IANNOTTI,E. L., PORTER,J. H. & GARCIA,A. (1977). Producing methane gas from swine manure in a pilot size digester, ASAE St Joseph, Missouri. HOBSON,P. N., ROBERTSON,A. M. & MILLS,P. J. (1975). Anaerobic digestion of agricultural wastes. ARC Research RetJiew, 1(3), pp. 82-5. LAPP,H. M., SCHULTE,D. D., KROEKER,E. J., SPARLING,A. B. & TOPNIK,B. H. (1975). Start-up of pilot scale swine manure digesters for methane production. 3rd Int. Symposiumon LivestockWastes, Champagne, Urbana, Illinois. MILLS,P. (1977). A comparison of an anaerobic digester and an aeration systemtreating piggerywaste from the same source. Food, fertilizer and agricultural residues, pp. 415-22, Ann Arbor Science, Michigan. ROBERTSON,A. M., BURNETT,G. A., HOBSON,P. N., BOUSFIELD,S. (~SUMMERS,R. (1975). Bioengineering aspects of anaerobic digestion of piggery wastes, Managing Livestock Wastes, ASAE, St Joseph, Michigan.
P. J. MILLS
North of Scotland College of Agriculture, Aberdeen, Scotland
ABSTRACT
The theoretical maintenance energy requirements for a mesophilic anaerobic digester operating with a ten-day detention time are presented. Methods of applying this energy to the digester are suggested. The design and theoretical performance of a unit to heat the input by means of the output are discussed. Mechanical and gas mixing systems and their power requirements are considered. The net energy output of a theoretical unit is then derived. Theoretical considerations are next compared with practical results obtained from the operation of two field scale digesters, one of which is fitted with a heat recovery unit of the design previously discussed. Differences between theory and practice are shown and reasons for these considered.
INTRODUCTION
Both laboratory and field scale results indicate that predictable levels of gas can be produced by anaerobic digestion of animal wastes (Hobson et al., 1975; Lapp et al., 1975; Fischer et al., 1977). It is apparent that, even if all the gas produced were available for uses other than digester maintenance, the value of the gas would not service the capital cost of a fully automated small (200 pig) farm scale unit. However, at larger (2000 pig) scales, provided continuous, year round use can be made of the gas, the economics of anaerobic digestion being used solely as an energy-producing process look promising. These calculations have generally been made dismissing the benefits of digestion as a treatment process, where, at larger throughputs (about 8 m 3 day 1) it compares very favourably with a separation/aeration process (Mills, 1977) owing to its low running costs, However, in order to optimise the economics of anaerobic digestion, the minimum maintenance energy input is required. 57 Agricultural Wastes (1) (1979)-- :i(:? Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
58
P.J.
MILLS
This paper discusses the theoretical minimisation of maintenance energy and then compares this with field-scale operation of the existing Aberdeen digester (Robertson et al., 1975) and a new unit specially designed to investigate more efficient systems of operation. The work is part of a joint programme. Pilot plant data obtained at the Rowett Institute, Aberdeen, Scotland are used in the operation of these large-scale digesters.
THEORETICAL CONSIDERATIONS
The calculations below are based on the following data. Using a ten-day residence time at 35 °C gas production is 0.4 m 3 gas per kilogramme Volatile Solids when the digester is loaded with pig slurry, and the gas is about 65-70 ~ methane with an average energy content of 6.7 kW h m - 3. Outputs are shown in Table 1. The energy
TABLE 1 GAS AND ENERGY OUTPUTS FROM DIGESTED PIG SLURRY
Total solids, input (%)
Gas output (volume per digester volume per day)
Energy (k W m 3 digester volume)
5 6
1-6 1 "9
0-45 0.54
Volatile Solids = 0-8 T o t a l solids.
inputs to a digester are required for: (a) heating, (b) mixing and (c) control and recording. Item (c) is minimal compared with (a) and (b) unless compressed air is used for control and automatic valve operation.
(a) Heating Heat is required to: (i) heat the input to 35 °C and (ii) compensate for heat losses from the vessel. (i) If the input slurry is stored in an external tank before loading its temperature may be assumed to be ambient (TA). If the input is taken from a tank inside the pig house a temperature of 10°C above ambient may easily be achieved. It is worth noting that the material leaving the animal is at exactly the right temperature for digestion. (ii) If sufficient insulation is put on the vessel it may be assumed that the insulation limits the heat transfer and that the effects of wind on external film coefficients can be ignored. Thus heat loss through the vessel = UoAo(35 - TA). Where Uo is the overall heat transfer coefficient (htc) of the vessel and A D is the surface area of the
ENERGY BALANCES OF DIGESTERS
59
vessel. The least cost practical vessel and that with the smallest surface area per unit volume is a cylinder with a 1 : 1 aspect ratio. Table 2 shows the heat requirements for losses and input for various sizes of vessel, with U = 0-35 W m 2 °C 1 and T A = 0 °C. It can be seen that the heat required for the input greatly exceeds vessel losses and that this tendency increases with scale. The overflow f r o m the digester is at 35 °C. If this could be used to heat up the input at 100"/; efficiency the overall heat requirements of the system could be reduced to less than 10 ~i, of the gas energy. In a system with continuous loading and overflow, a countercurrent heat exchanger could achieve an efficiency of 75 o;. It would be necessary to have another p u m p to overcome the pressure drop in the heat exchanger but this would only require 1 k W to recover 50 kW.
TABLE 2 HEAT REQUIREMENTSOF VARIOUS SIZES OF DIGESTER
Throughput (m 3 day 1) Volume (m 3) Surface area (m2) lnput heat (kW) Loss heat (kW) Total heat (kW) Potential output (kW)
1 10 25 1-7 0.3 2.0 5.0
2"5 25 48 4.3 0.6 4.9 12.5
5 50 75 8.6 0.9 9.5 25
10 100 116 17 1.4 18.4 50
25 250 222
43 2.7 45.7 125
50 500 348 86 4.3 90.3 250
The digester is assumed to be loaded with slurry of 5.5 g?, TS and running at 35~C with ambient temperature 0°C and detention time 10 days.
F o r a small digester loading has to be intermittent. The smallest reliable commercial p u m p for handling slurries o f 5-8 0,'~, dry matter will p u m p about 1 . 2 m 3 h 1. This means that loading~ and consequently o v e r f l o w - - m u s t be intermittent. The most likely alternatives are manual daily loading or automatic hourly (or less) loading. As the loading p u m p has to start before any hot liquid can be displaced from the vessel, it is necessary to retain the w a r m overflow in an insulated vessel and use it to preheat the next batch to be loaded. If the recovery unit consists o f a coil, which contains the whole o f the next input batch, submerged in an insulated tank into which the output runs, the input and o u t p u t will reach the same temperature. This is provided the area of the coil is large e n o u g h for the volume contained and that the insulation on the tank is relatively g o o d in relation to the output residence time. The input is put in the coil because the loading p u m p is capable o f overcoming the coil pressure drop and gravity can be used for the tank side. It would be more desirable to put the output through the coil because o f heat loss; also, owing to its higher temperature and lower solids level, and consequently lower viscosity, a higher heat transfer coefficient would be expected. It will later be seen that the coil-side htc is the limiting factor in the design o f this type of recovery unit (Fig. 1).
60
v.J. MILLS
,rom,ges,er,5°C tOdigester ~
~
Overflow
!
.
Coldinput Fig. 1.
Diagram of heat recovery unit.
For a coil-in-tank, intermittent flow heat recovery unit the temperature of liquor being pumped to the digester is given by:
Ti. = Tar 1 - e
_ UAt "~ O'~-MJ
where: Tin is the temperature of liquor being loaded into the digester ( °C); Tav is the mean of the digester operating temperature and the raw input temperature ( °C); Uis the overall heat transfer coefficient from liquor outside the coil to inside (W m-2 C - 1); A is the coil surface area (m2); M is the mass of liquid per batch (kg) and t is the interval between batches (h). This assumes that the tank is well insulated. For Ti. to exceed 9 0 ~ of the difference between Tav and the raw input temperature, i.e. 45 ~ of the heat required to raise the input to operating temperature is being recovered, UAt/M must be greater than 1-98. If the density of the input is assumed to be 1 kg litre- 1 A I M is fixed for any coil pipe diameter. Table 3 shows values of Ut required to meet this condition for different pipe sizes. The value of t is varied according to the value of U found in practice. TABLE 3 VALUES OF U t
(UAt/M)
REQUIRED TO MAKE PIPE SIZES
Coil pipe diameter (mm) A/M (m s kg- 1) Ut (W h m -2 °C -1)
25 0.16 12.4
> 1 . 9 8 FOR DIFFERENT
40 0.10 19.8
50 0-08 24.8
ENERGY BALANCES OF DIGESTERS
61
Overall U values of 75-100 would be expected with water inside and outside a coil but the value might well drop to 25 with cold, relatively viscous slurry inside and warm digested slurry outside the coil. If U is 25 only a 25 m m or smaller pipe will provide sufficient area if t = ½. The overall htc will be limited by the internal film coefficient for the stationary liquid in the coil. Operation of a mixer to ensure uniform overflow from the unit will improve the coil external film coefficient but have no measurable effect on the overall htc. Transfer of heat to vesselcontents: The most direct method of transferring the heat energy in the gas to the vessel contents would be to direct a gas flame on to the surface of the vessel. However, the high temperature involved would cause heat shock to the contents and scaling on the inside of the vessel. Direct heating of externally circulated contents can be dismissed for the same reasons. The most reasonable alternatives are: (i) use of hot water coils inside the vessel; (ii) use of a water-heated, jacketed screw p u m p inside the vessel: (iii) use of an external water heat exchanger. System (i) has a low capital cost and only requires a low power water p u m p to circulate water. It runs the risk of fouling on the outside of the coils if the water temperature is too high or if there is not sufficient mixing of the vessel contents. The workers using this technique have reported few problems. It has been suggested that PVC pipe could be used for coils as it has a smooth exterior on to which scaling would be unlikely. The low external film coefficient would not be made significantly worse by the poor conductivity of plastics. System (ii) overcomes the scaling problem and also provides mixing. The equipment consists of a water heated jacket with an internal Archimedean screw. The screw lifts liquid over the heated jacket and spreads it on to the top of the digester surface. It suffers the disadvantage of being a piece of apparatus inside the vessel and requires a rotating-shaft seal. System (iii) is relatively expensive from both capital and running aspects but is fail-safe. A more expensive and higher power p u m p is required to p u m p digester liquor than is required to p u m p water and the cost of the heat exchanger is high. However, as the liquid velocity through the tubes is high, high water temperatures and heat transfer rates can be obtained. The system can also be used to mix the vessel contents.
(b) Mixing In an industrial aerobic fermentation system, agitator power inputs may exceed 2 kW m 3 tank capacity. This is about three times the m a x i m u m gas energy output from a digester. Clearly, a much lower energy mixing system must be adopted for anaerobic digestion if it is to have an energy surplus. However, in an anaerobic system mixing in of one of the microbial substrates, oxygen, is not required as it is in an aerobic system. There are three main reasons for requiring mixing in a digester: (i) to maintain a
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uniform temperature; (ii) to provide intimate mixing of bacteria and substrates for optimum yield (there is some doubt as to whether, as the process is multi-stage with solid substrates, a fully mixed system is desirable); (iii) to prevent the formation of scum or crust, as gas bubbles raise solid particles to the surface where they bind together. Condition (i) can be satisfied by circulating with an external pump with a digester volume turnover time of about 2 h. The power requirement of this is very low (0.1 kW for 10m3). Condition (ii) requires a system which lifts settled material from the bottom and distributes it throughout the vessel. A high volume, low shear mixer (e.g. crossbladed paddle) or gas recirculation, can be used. Condition (iii) requires a system which either causes turbulence in the liquid just below the surface or pushes the scum down into the liquid. Gas mixing or a high shear agitator just below the surface are of the first type. Revolving-rake mechanisms or liquid jetted out over the surface have the second effect. Mechanical agitators suffer the disadvantage of requiring gas seals and bearings. Methane, as a member of the paraffin family, breaks down greases in conventional seals and bearings even though the gas pressure is low. Gas mixing, by compressing gas from the top of the vessel and releasing it at the bottom, requires low power inputs (Table 4). The rate at which gas is recirculated is approximately proportional to the digester area and the pressure required is proportional to the depth at which the pressurised gas is released.
TABLE 4 POWER REQUIREMENTSFOR MIXINGDOMESTIC DIGESTERS (FOR A LIQUIDRING PUMP--COURTESY OF NASH ENGINEERING) Digester volume (m 3) Gas velocity (m m i n - ~) Theoretical power (kW) Motor size (kW)
22 0.032 0-12
76 0-027 0-28 0-75
180 0.026 0-61 1.5
350 0-024 1.01 3.0
600 0-021 1-5 4-8
Domestic digesters use gas flow rates in the range 0-02-0-03 m 3 min 1 (per square metre of digester cross-section area). This figure can be taken as a gas velocity through the vessel if there is no liquid in the vessel. At these low throughputs the power required for continuous mixing would be approximately 3 ~o of the output from a high-rate digester. It is suspected that gas velocities in the region of 0.1 0.15mmin I will be required to prevent crust formation in digesters which have both higher solids levels and a wider range of particle sizes. This level of mixing would require 15 % of the output and would have to be made intermittent if the mixing power is to be insignificant.
E N E R G Y B A L A N C E S O F DIGESTERS
63
Overall energy balance for a theoretical digester For a 50 m 3 digester producing an energy of 25 kW the input requirements are: Vessel loss, 0.9 kW; input, 8.6 kW; agitation 1-0 kW as electric power. lnput heat can be reduced to 4.4 by heat recovery. If the total heat of 5.3 kW is supplied by a 20 kW boiler operating for 6-4 h day - 1 at 75 ~o efficiency, the boiler will require an input heat of 7.1 kW mean. This leaves a net gas energy of 18 kW. To run the circulating p u m p while the boiler is on will require 0.25 kW mean. Thus electrical demand is 1.25 kW. At current electrical costs this is 2.5 pence an hour. The value of the gas at current domestic gas costs is 12.5 pence an hour. So the monetary surplus is 10 pence an hour; £876 per annum. If the gas is burnt in an engine attached to a generator providing 8 kW electricity, 12 kW heat and 5 kW waste, the heat will exceed requirements and a mean of 6.75 kW electricity will remain of value 13.5 pence an hour.
P R A C T I C A L RESULTS F R O M FIELD S C A L E D I G E S T I O N
(a) Heating The first field scale (13 m 3) digester at Aberdeen (Robertson et al., 1975) was designed to have an overall htc of 0.7 W i n - 2 oC - 1. The aspect ratio of both verticalcylinder digester vessels is 1.5:1 and the insulation on the first, which is 50 mm of glassfibre, became wet owing to leaks in the outer skin. As a result the vessel cools by 4~5 °C at an ambient temperature of 0 °C. This is approximately four times the rate at which it should cool. With the poor insulation the heat loss has become greater than the heat required to raise the input to 35 °C. There is only 55 ',~oefficiency of heat transfer from the boiler to the digester using an external heat exchange system. This is partly caused by long underground pipes on which the insulation has deteriorated owing to access to clear blockages. As a result, the digester, which is loaded with slurry from an external tank at ambient temperature, has a heat deficit of 1-2 kW during the winter months. The second digester was designed for 0-34 W m 2 °C ~. Insulation is 100 m m of isocyanate foam pumped between the main vessel and a welded outer steel skin. The pipe work is shorter and of larger bore, as shown in Fig. 2, and heat transfer from boiler to digester is 70 }f, efficient. The rate of cooling is still greater than 0-6 °C per day, at 2.5 °C per day. Part of this loss is caused by the weir. The weir has an insulated lid but this has to have a vent to prevent siphoning, allowing a continuous exit of warm, moist air. The second digester is fitted with an experimental heat recovery unit shown on the right-hand side of Fig. 2. Heating trials on the vessel filled with water gave a heat recovery of 45 48 ~,, with a half-hour batch interval and a 40 m m diameter coil pipe. When operating with slurry heat recovery fell to 35-50 '~;,, indicating a U value of about 24. The recovery unit has a stirrer which operates intermittently, with the
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P . J . MILLS
Fig. 2. The second Aberdeen field-scale digester. Background, digester tank partly built into site hut. Left, heat exchanger and sludge circulating pump for digester heating. Right, heat exchange unit for recovery of heat from output to heat input, digester feed pump below.
loading pump, to ensure mixed overflow. When this stirrer operates continuously there is no appreciable improvement in heat recovery. With operating conditions the same as those of the first digester and ambient 0 °C, the second digester has a net output of 1-2 kW during the winter months.
(b) Mixing The first of the two field scale digesters is mixed by two turbines on a single shaft extending from top to bottom. The foot and steady bearings are machined from 'Tufnol'. The gas seal is provided by a pipe, concentric with the shaft and of 10 mm greater diameter. This pipe is sealed on to the lid of the vessel and the other end is below the liquid level. This means that the only gas lost is that actually formed in the annulus between the pipe and the shaft. The unit is driven by a 4 kW motor reduced to 80 rpm. In order to reduce the electrical demand the stirrer is operated for 5 min per hour by a time clock. An interesting feature of this mixing regime is the fact that up to 70 9/0 of the gas produced is released while the stirrer is on (Fig. 3). It has been suggested that this is caused by breakage of the surface tension as the
65
ENERGY BALANCES OF DIGESTERS Mix
0
100 Time
Fig. 3.
Influence of mixing on gas production/release.
stirrer starts, releasing preformed bubbles. However, as the pressure at the bottom of the vessel is 1.5 bar, the initial formation of the bubbles should cause the liquid level at the t o p - - a n d thus the gas pressure--to rise. This would result in the meter registering at least 66~o of any gas formed at the bottom but not released. Suggestions of the gas release being caused by sudden intimate mixing of bacteria and substrates are not compatible with the normal behaviour of microorganisms. The capital cost of such a unit is very high and it lies idle 95 ~o of the time. The mean power requirement is 0.33 kW. The second digester is mixed partly by recirculation through the heat exchanger and partly by gas mixing. The liquid returning from the heat exchanger passes through a venturi unit situated in the headspace of the digester vessel. Liquid at 2.5 litres sec- 1 draws in four litres of gas per second and the mixture is returned to the centre of the digester. The existing unit does not develop sufficient suction to return the gas to the bottom; it is feared that the degree of restriction at the venturi necessary to create such suction would result in blockages in the circulation pipe. The presence of the bubbles causes a great increase in bulk liquid flow and turbulence at the liquid surface, thus restricting crust formation. The unit is driven by the same pump as circulates liquid through the heat exchanger and no increase in power consumption of the pump has been detected. Mixing by gas recirculation using a separate gas pump is now being examined.
DISCUSSION
A theoretical digester can produce a net output of methane all the year round. The net output should be 0.35 kilowatts per cubic metre of digester capacity. As the heat
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for heating the input is much greater than heat losses, gas production should not fluctuate much during the year. This should still apply in extreme climates where the input is still likely to be pumped from a livestock building above 0 °C despite ambient temperatures of - 2 0 °C. Under these conditions for a 50 m 3 digester the ratio of input heat to loss heat is still 6:1. In order to maximise net output it is therefore important to recover as much heat from the discharged effluent as possible. This becomes easier as continuous loading is approached. Continuous loading is also more desirable as it stabilises the operating conditions of the microorganisms. With efficient heat recovery the heat requirement of a digester should be such a small proportion of its output that fluctuations between net summer and winter output will be minimal. Gas mixing should prove to be a lower energy and maintenance system than conventional mechanical systems. There is still some doubt as to whether gas mixing can handle very thick materials with large particle sizes, e.g. cattle slurries, and more experiments are needed. In the practical units operated at field scale, energy balances have not been as good as theoretical models. This is caused by heat losses from the vessel becoming as large as the input heat requirements. This may be partly overcome by improving insulation and eliminating losses. At larger scales heat loss becomes relatively less important as volume to surface area increases and the shape of the vessel, for economic reasons, approaches the 1:1 aspect ratio of least surface area. This should make control of these losses easier at the range of scale at which farm scale digestion is likely to take place.
REFERENCES FISCHER,J. R., IANNOTTI,E. L., PORTER,J. H. & GARCIA,A. (1977). Producing methane gas from swine manure in a pilot size digester, ASAE St Joseph, Missouri. HOBSON,P. N., ROBERTSON,A. M. & MILLS,P. J. (1975). Anaerobic digestion of agricultural wastes. ARC Research RetJiew, 1(3), pp. 82-5. LAPP,H. M., SCHULTE,D. D., KROEKER,E. J., SPARLING,A. B. & TOPNIK,B. H. (1975). Start-up of pilot scale swine manure digesters for methane production. 3rd Int. Symposiumon LivestockWastes, Champagne, Urbana, Illinois. MILLS,P. (1977). A comparison of an anaerobic digester and an aeration systemtreating piggerywaste from the same source. Food, fertilizer and agricultural residues, pp. 415-22, Ann Arbor Science, Michigan. ROBERTSON,A. M., BURNETT,G. A., HOBSON,P. N., BOUSFIELD,S. (~SUMMERS,R. (1975). Bioengineering aspects of anaerobic digestion of piggery wastes, Managing Livestock Wastes, ASAE, St Joseph, Michigan.