Route 1: Dedicated incineration Route 2: Anaerobic digestion + incineration Route 3: Anaerobic digestion + landspreading Route 4: Thermal drying to produce refuse-derived fuelChoice of plant
The plant consisted of a number of features from, fluidized bed incinerator, heat recovery unit, electrostatic precipitator, wet scrubber and a stack or chimney. As has been mentioned earlier the reasons for using a fluidized bed incinerator are ones of economic achievement of a high degree of combustion efficiency and the avoidance of odour nuisances. Any new incineration plants built in the future would therefore be of this variety.
With the flue gas leaving the incinerator at a temperature of 816°C there is a method in which we can use this heat to our advantage. Using simple shell and tube heat exchangers we transfer some of this heat to other sources, in our case to air and water. Ideally we would like to transfer this heat from the flue gases to these other sources without heat loss but this would be impossible. Heat exchanger effectiveness is the ratio of heat transfer to the maximum heat transfer. So, ideally we would like our effectiveness to be high, and for heat exchangers this could be as high as 0.8 but for our particular purpose we are assuming an effectiveness of 0.7.
The use of an electrostic precipitator is to clean the gases from the particulates and ash that they contain after incineration. The type of unit that is required for this particular instance is a high voltage, single stage, plate wire type.
Particles are collected on flat, parallel collecting surfaces spaced 8-12in apart with a series of discharge electrodes spaced along the centre line of adjacent plates. It does so because the particles themselves become highly charged after incineration. The two main sources of power consumption are from corona power and gas pressure drop. Total pressure drop for a precipitator is usually of the order of 1kPa (ref.25).
To achieve high collection efficiency of particulates by impaction, a small droplet diameter and high relative velocity between the particle and droplet are required. In a venturi scrubber this is accomplished by introducing the scrubbing liquid at right angles to a high velocity gas flow in the venturi throat. Very small water droplets are formed, and high relative velocities are maintained until the droplets are accelerated to their terminal velocity. The energy expended in the scrubber (except for the small amount used in the sprays and mist eliminator) is accounted for by the gas stream pressure drop through the scrubber (ref.35).
Public concerns
Experience suggests that odour from appropriately designed and operated systems should not present a nuisance. No information exists to validate the assumption, but it is assumed that viruses and pathogenic organisms will be destroyed in the high temperatures prevailing in a furnace and will be absent from the gaseous, solid and liquid outputs from an incineration system.
The dispersion of pollutants borne by the stack gases can be modelled using computers and assuming a gaussian distribution of the pollutants. The predicted resultant concentrations can then be compared with air quality guidelines established by a number of organisations which include the WHO and EC Some disturbance can be expected in the neighbourhood of a plant during construction but this need not be significant.
The public do not like incinerators and any measures to minimise their visual impact are desirable. Such measures can include the avoidance of generating a visible plume and the housing of incineration plant in attractively clad buildings.
Energy balance
| ELECTRICITY | HEAT | FUEL | |
|---|---|---|---|
| Incineration "incinerator" | 70.00 | / | / |
| Incineration "electrostatic precipitator" | 2.74 | / | / |
| Incineration "wet scrubber" | 11.04 | / | / |
| Incineration "fans" | 0.09 | / | / |
| Ashes transport | / | / | 0.02 |
| NET "ENERGY INPUT" | 83.87 | / | 0.02 |
| Incineration "preheated air" | / | 19.27 | / |
| Incineration "hot water" | / | 4.70 | / |
| NET "ENERGY OUTPUT" | / | 23.97 | / |
| COP for energy balance for plant-processing only | 0.28 |
| COP for energy balance including transportation | 0.28 |
Data and assumptions used for energy balance
To perform the energy balance there were a number of calculations that were needed to be undertaken and to do so there were also a number of assumptions that were also taken into account. The final energy balance can be seen at the end of this web page in tabular form along with the assumptions made. As one can imagine, making assumptions can be very sensitive to your final results so what we did was to have a range of values, best case scenarios and worst case scenarios interms of the assumptions made a determined whether the assumptions we made were case sensitive.
Now, in order to carry out our energy balance, a number of calculations had to be carried out to determine the following:
a) Air supply rate
b) Electrical energy supplied to the incinerator
c) Heat recovered from the flue gases
d) Electrical energy supplied to the electrostatic precipitator
e) Electrical energy supplied to the wet scrubber
Air supply rate
We assumed 25% excess air was supplied to the incinerator which is normal practice in this situation. In doing so we can determine the air supply rate using combustion equations and the information given to us by West of Scotland Water with regards to the sewage sludge characteristics.
Supply rate with 25% of excess air = 5.52 kg/s
Incinerator
Stage 1
The first stage is to determine the bed area of the incinerator. With an operating temperature between 800-900°C we could use the following equation (ref.20).
log Sl = 2.7 - 0.0222M
Now that the bed area had been calculated we could then find the electrical energy required to power the incinerator from reading from a graph of bed area against electrical energy (ref.8).
Electrical Energy Requirements for Incinerator = 70 GW-h
Stage 2
Because of the number of assumptions we would have to have made on using the Fluidized Bed Incinerator (FBI), we have based our calculations on a Well Stirred, Mixed Furnace as it has similar characteristics to a FBI
1) combustion efficiency is high and temperatures are uniform
This stage was to determine the exit flue gas temperature after combustion. We could do this following the steps used with ref.21 and the data we already knew below
1) Qf = Ms * Cv = 12.4 MW
2) Mg = Ms + Ma = 6.11 kg/s
3) Tf = ((Qf / (Mg * Cpg)) + To = 2564 K
4) gr = Effective area of the heat sink
5) Qg' = Reduced furnace density
6) Qg = Qg' * Qf * ((Tf / (Tf - To)) = 10.37 MW
7) Tg4 = Qq / (gr *
8) Tge = Tg - (Tg = 1089 K = 816°C
The above are the important equations used but the method of calculation can be seen in greater detail in ref.21.
Preheated air and hot water
We have two heat exchangers in the heat recovery unit.
Therefore, heat recovered using
Heat recovered in the form of preheated air = 9.27GW-h
Electrostatic Precipitator
Unfortunately we were unable to calculate the volumetric flow rate of the gases leaving the heat recovery unit but for the EP that we are using, plate wire, normal flow rates range between 100-300 m3/s.
For Fan Power
Total power requirement for Electrostatic Precipitator = 313 kW = 2.74 GW-h
Wet Scrubber
The assumptions that we are making are that the volumetric flow rate of the gas entering the scrubber is similar to that of the electrostatic precipitator i.e. 200 m3/s and that the collection efficiency is 98%. Using the following equations from ref.34 we could calculate the necessary power requirements for the scrubber.
Nt = In(1 /(1-Ef))
But, Nt = alfa * Ptbeta
where alfa and beta are characteristic parameters for the type particulates being collected and these can be found in ref.34. These parameters will be prewashed gas with caustic soda using a venturi scrubber with the variables being 0.915 and 1.05 respectively.
Nt = 3.91 and Pt = 11.04 GW-h after conversion from h.p.
Sensitivity of energy balance to chosen variables
The energy balance for this route is based on a number of assumptions and variables. Values for the chosen variables which are the most questionable is shown in the table beneath. Also, the potential "worst case" and "best case" values are shown.
The sensitivity of the results was evaluated by calculating how these above mentioned "worst case" and "best case" values would effect the over-all energy balance.
where Sl = Sludge loading rate
M = Moisture content
Area of bed = Feed rate (dry basis) / SI = 355 ft2
2) simple furnace
3) exit flue gas temperatures are apptoximately the same
Equations involved
where Qf = Total Heat input
Ms = Mass flow rate of sludge
Cv = Calorific value of sludge
where Mg = Total mass flow rate of combustion products
Ma = Mass flow rate of air
where Tf = Adiabatic flame temperature, which can be calculated from a heat balance that neglects any heat transfer or losses
To = Temperature of preheated air
Cpg = Mean effective specific heat of combustion
which involves several simple steps that can be seen with ref.21 = 61.23 m2
which can be calculated in a number of simple steps and is in ref.21 = 0.59
where Qg = Total heat transfer
where Tge = Actual exit gas temperature
Using the data below, we could calculate a value for heat recovered from the flue gases
Q = Mg*cpg* (Tig - Tog) = Ma*cpa*eff* (Toa - Tia)
Heat recovered in the form of hot water = 4.36GW-h
We have calculated the corona power using
Wc = Q ((115.8 + (1.17 /Pt)) = 27.84 kW
where Wc = Corona Power
Q = Volumetric flow rate
Pt = Overall Penetration (particulates removed - 95%)
P = (Q*Pd) /Ef = 285.71kW
where Pd = Pressure drop
Ef = Fan efficiency
where Nt = Number of transfer units, dimensionless
Ef = Collection efficiency
Variable Here Worst case Best case
Excess air added 25% 20% 25%
Heat exchanger efficiency 0.7 0.6 0.8
Volumetric flow rate 200 m3/s 300 m3/s 100 m3/s
| Worst case scenario for Route 1: Dedicated incineration. [GW-h]
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| Best case scenario for Route 1: Dedicated incineration. [GW-h]
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Choice of plant
The digestion of the sludge will be a single-stage mesophilic (35°C) digestion with a retention time of 15 days. It will take place in 5 egg-shaped digesters of 3,000 m3 each. The number and the sizing of the digesters is based on the potential consequences if one of the digesters is out of service for maintenance or repair. A sludge treatment centre must be able to maintain sufficient sludge treatment continuously, and having more than one digester increases the flexibility of the system. With the chosen number and size of the digesters, there is 15% excess reactor volume compared to the sludge volume for a 15 days sludge retention time. - If one digester is out of service, the retention time must be decreased to 12 days which is still acceptable for a stable digestion process.
The produced biogas is used to generate electricity and heat. Most of the heat is used within the plant to heat the digesters. Since the digested sludge will be incinerated, there is no reason to ensure that it is pasteurised before incineration.
After digestion the sludge is incinerated in a plant similar to the one described for "Route 1: Dedicated incineration". The potential heat recovery from incineration is lower because the calorific value of the digested sludge is lower. The ashes are disposed of to a landfill.
Public and environmental concerns
The same as for "Route 1: Dedicated incineration". The anaerobic digestion in itself does not pose any problems.
Energy balance
This is the energy balance for route 2. How the values in the table were generated is documented further down - click on the highlighted process steps to go directly to the data and assumptions used to get the shown value.
| ELECTRICITY | HEAT | FUEL | |
|---|---|---|---|
| Anaerobic digestion "mixing" | 0.26 | / | / |
| Anaerobic digestion "digester heating" | / | 16.40 | / |
| Dewatering | 1.83 | / | / |
| Incineration "incinerator" | 70.00 | / | / |
| Incineration "electrostatic precipitator" | 2.74 | / | / |
| Incineration "wet scrubber" | 11.04 | / | / |
| Incineration "fans" | 0.09 | / | / |
| Ashes transport | / | / | 0.02 |
| NET "ENERGY INPUT" | 85.97 | 16.40 | 0.02 |
| Anaerobic digestion "biogas" | 12.27 | 20.46 | / |
| Incineration "preheated air" | / | 13.14 | / |
| NET "ENERGY OUTPUT" | 12.27 | 33.60 | / |
| COP for energy balance for plant-processing only | 0.45 |
| COP for energy balance including transportation | 0.45 |
Data and assumptions used for energy balance
Mixing:
Digester heating:
Biogas:
Preheated air:
The energy balance for this route is based on a number of assumptions and variables. Values for the chosen variables which are the most questionable is shown in the table beneath. Also, the potential "worst case" and "best case" values are shown.
The sensitivity of the results was evaluated by calculating how these above mentioned "worst case" and "best case" values would effect the over-all energy balance.
Choice of plant
Anaerobic digestion will occur in a two-stage process to ensure pasteurisation of the sludge before application to land. Primary digestion takes place at thermophilic temperature (55°C) in a 3,000 m3 egg-shaped digester at a retention time of 3 days. After this, the sludge is transferred to mesophilic (55°C) digestion for 12 days in 4 parallel 3,000 m3 digesters. The number and size of the digesters was chosen so that even with one digester out of service, the digestion should still provide sufficient treatment of the sludge. - If the primary, thermophilic digester is out of service, the treatment in the mesophilic stage will still be acceptable, even though there will be no pasteurisation of the sludge. - If one of the four secondary, mesophilic digesters is out of service, the mesophilic stage must be decreased to a retention time of 10 days which should still be sufficient.
Thermophilic digestion requires a higher energy input to heat the sludge. At the moment, sludge need not be completely pasteurised but public fear of health hazards could lead to future legislation requiring pasterisation of all sludge before application to land.
The sludge applied to agricultural land will be 50% as liquid sludge and 50% as thermally dried sludge. The excess heat from the thermally drying of sludge will be used for heating in the digestion plant. The thermal drying will be carried out in a drying plant similar to the one described for route 4: Thermal drying to produce a refuse-derived fuel.
It is desirable to apply some of the sludge as liquid sludge because liquid sludge contains more available nutrients than dried sludge.
Drying of some of the sludge is desirable because the dry sludge is a product which is easier to handle and store, but drying is energy consuming. The dry sludge pellets can be transported to land further away, and it can be stored on the farmers premisses. The lower content of available nutrients makes it possible to spread sludge pellets at times where the plants are not actively growing, whereas liquid sludge preferably should be applied to growing crops.
Public and environmental concerns
Pathogens is of major public concern when sludge is applied to agricultural land. However, the pasteurisation in this particular plant should eliminate the fear of contamination with bacteria like E. coli 0157 and Salmonella.
Heavy metals has in the past been a problem for sludge from urban areas because of heavy metals in the sewage from many industries. The heavy metal concentrations in the sludge from Shieldhall does not pose a problem to use of the sludge on agricultural land as can be seen from this analysis (compare to the maximum concentrations given in the UK Code of Practice:
Land availability is a major problem for the area surrounding Shieldhall. A study on the land requirements and land availability for sludge application has been carried out by The MacAulay Land Use Research Institute. The study suggests that if all sludge should be applied to land it would be necessary to utilise land in a surrounding area as big as a 30 kilometre band around Shieldhall. The application rate would have to be fairly big (5-8 tonnes dry solids per hectare per year). Even still, it would be necessary to utilise 5-8% of all suitable land annually - percentages of land utilisation which exceeds what can be found in existing recycling schemes in the UK (ref.36). For this reason, it is probably not feasible to base a sludge disposal strategy for Shieldhall on disposal to land.
Energy balance
The energy balance for this route is based on a number of assumptions and variables. Values for the chosen variables which are the most questionable is shown in the table beneath. Also, the potential "worst case" and "best case" values are shown.
Data and assumptions used for energy balance
Mixing:
Digester heating:
Transport to land as liquid sludge or dry pellets:
Biogas:
Fertiliser value of liquid and dry sludge:
Sensitivity of energy balance to chosen variables
The energy balance for this route is based on a number of assumptions and variables. Values for the chosen variables which are the most questionable is shown in the table beneath. Also, the potential "worst case" and "best case" values are shown.
The sensitivity of the results was evaluated by calculating how these above mentioned "worst case" and "best case" values would effect the over-all energy balance.
Choice of plant
For our case study we chose a closed loop two stage indirect dryer system for the reasons stated below.
Non-backmix, closed loop systems have better overall thermal efficiency. Indirect dryers produce less contaminated vapour than other types of dryers.
One of only two sludge drying plants in the whole of the UK uses this type of system, allowing us to use some of their plant date to help calculate the energy balance for two of our sludge disposal routes. This drying plant is shown below:
The two-stage system is built around a Thin-film indirect dryer which acts as the first stage unit. The 25% DS dewatered sludge cake is pumped into the dryer where a thin film of sludge is distributed onto the inner wall of the dryer allowing for better heat transfer. The sludge is transported through the machine by a giant rota and spends a relatively short time in the dryer before dropping into the second stage Rovactor disc dryer.
At this point the dry solids content has increased from 60-65% and so the need for backmixing has been avoided. The residence time in the Rovactor dryer is approximately 50 minutes raising the sludge to a DS content of 90-95%. Both dryers have a drying temperature of 105-115°C. The dried sludge is then fed to a screw dried sludge cooler to lower the temperature to at least 50°C and thus prevent any exothermic reactions that could occur after the sludge has entered a high oxygen environment. The dryers and the cooling screw operate under reduced oxygen content conditions because of the drying temperature, which is produced by an inert water vapour atmosphere. The drying zone is kept separate from other parts of the plant which operate under higher air levels. This necessitates the need for an air-lock at the outlet of the dry sludge cooler. The vapour from both dryers pass to a condenser. The non-condensable trace organics are transferred to the combustion chamber for destruction and the condensate is usually returned to the inlet works via existing drainage.
Due to the handling and storage difficulties characteristic of the low density fibrous material resulting after drying, it is necessary to pelletize the dried sludge. The sludge, in its cooled state, is fed to the pelletising plant adjacent to the drying plant by a screw feeder where it is processed through the die forming pellets normally 3 to 5 mm in diameter. These pellets then fall into a cooler where the heat of pelletisation is removed after which the pellets are either bagged and weighed or transferred directly to lorries for transportation to the appropriate product outlets such as the nearest coal fired power station.
Energy balance
Data and assumptions used for energy balance
Longannet Coal Fired Power Station: Yearly energy output = 7761.5 GW-h
Draft tube mixers are used here since these are recommended for egg-shaped digesters (Dichtl, 1997; Scottish Envirotec, 1995):
The heat demand consists of 1) the heat needed to heat influent sludge from an average outdoor temperature to digestion temperature, plus 2) the heat loss over digester surfaces.
Heating of influent sludge
Heat loss over digester surfaces
Heat exchanger efficiency
Sensitivity of energy balance to chosen variables
Variable Here Worst case Best case
% VS degraded 50 40 50
Biogas to electricity 30% 25% 35%
Biogas to heat 50% 50% 50%
Retention time 15 days 20 days 15days
Heat transfer coefficient,U 2.0 W/(m2 °C) 2.5 W/(m2 °C) 1.5 W/(m2 °C)
Heat exchanger 70% 60% 80%
Excess air added 25% 20% 25%
Heat exchanger efficiency 0.7 0.6 0.8
Volumetric flow rate 200 m3/s 300 m3/s 100 m3/s
Worst case scenario for route 2: Anaerobic digestion + incineration. [GW-h]
ELECTRICITY HEAT FUEL
Net "energy input" 93.04 21.26 0.02
Net "energy output" 8.18 35.31 /
Best case scenario for route 2: Anaerobic digestion + incineration. [GW-h]
ELECTRICITY HEAT FUEL
Net "energy input" 79.09 13.71 0.02
Net "energy output" 15.75 37.48 /
Route 3: Anaerobic digestion + land application
if sludge is applied as 8 tonnes dry solids per hectare annually.Heavy metal Zn Cu Ni Cd Pb Hg Cr Mn As mg/kg d.s. 1157 286 80 2.0 190 1.6 356 392 6.0 kg/(ha yr) 9.3 2.3 0.64 0.02 1.52 0.01 2.85 3.14 0.05
Variable Here Worst case Best case
% VS degraded 55 45 55
Biogas to electricity 30% 25% 35%
Biogas to heat 50% 50% 50%
Retention time 15 days 20 days 15days
Heat transfer coefficient,U 2.0 W/(m2 °C) 2.5 W/(m2 °C) 1.5 W/(m2 °C)
Heat exchanger 70% 60% 80%
ELECTRICITY HEAT FUEL FERTILISER
Anaerobic digestion "mixing" 0.26 / / /
Anaerobic digestion "digester heating" / 27.37 / /
Dewatering 0.93 / / /
Thermal drying 2.52 / 15.56 /
Transport to land "as liquid" / / 1.4 /
Transport to land "as pellets" / / 0.06 /
NET "ENERGY INPUT" 3.71 27.37 17.02 /
Anaerobic digestion "biogas" 13.50 22.50 / /
Thermal drying "vapour" / 10.50 / /
Fertiliser "liquid sludge" / / / 5.33
Fertiliser "dry pellets" / / / 0.86
NET "ENERGY OUTPUT" 13.50 33.0 / 6.19
Link to energy balance for Route 1 / Route 2 / Route 3 / Route 4
COP for energy balance for plant-processing only 1.00 COP for energy balance including transportation 0.97 COP for energy balance including transportation and fertiliser value 1.10
Draft tube mixers are used here since these are recommended for egg-shaped digesters (Dichtl, 1997; Scottish Envirotec, 1995):
The heat demand consists of 1) the heat needed to heat influent sludge from an average outdoor temperature to digestion temperature, plus 2) the heat loss over digester surfaces.
Heating of influent sludge
Heat loss over digester surfaces
Heat exchanger efficiency
10 km radius: 11% of the zone 20 km radius: 33% of the zone 30 km radius: 56% of the zone
Variable Here Worst case Best case
% VS degraded 55 45 55
Biogas to electricity 30% 25% 35%
Biogas to heat 50% 50% 50%
Retention time 15 days 20 days 15days
Heat transfer coefficient,U 2.0 W/(m2 °C) 2.5 W/(m2 °C) 1.5 W/(m2 °C)
Heat exchanger 70% 60% 80%
Worst case scenario for Route 3: Anaerobic digestion + land application. [GW-h]
ELECTRICITY HEAT FUEL
Net "energy input" 4.02 32.16 0.02
Net "energy output" 9.21 28.91 /
Best case scenario for Route 3: Anaerobic digestion + land application. [GW-h]
ELECTRICITY HEAT FUEL
Net "energy input" 3.71 23.10 0.02
Net "energy output" 15.75 33.00 /
Route 4: Thermal drying to produce refuse-derived fuel
ELECTRICITY THERMAL FUEL
Dewatering "belts presses" 1.72 / /
Thermal drying "dryers & other equipment" 5.54 / /
Thermal drying "boilers" / / 47.77
Pelletizing "plant equipment" 2.22 / /
Transport "pellets to Longannet" / / 0.38
NET "ENERGY INPUT" 9.48 / 48.15
Thermal drying "vapour" / 32.36 /
Incineration "pellets" 32.92 / /
NET "ENERGY OUTPUT" 32.92 32.36 /
Link to energy balance for Route 1 / Route 2 / Route 3 / Route 4
COP for energy balance for plant-processing only 1.14 COP for energy balance including transportation 1.13
FUEL YEARLY QUANTITY CALORIFIC VALUE
Coal 3330 Ktonnes 32 MJ/kg
Oil 74 Ktonnes 3.7 MJ/kg