Biomass - Using Anaerobic Digestion |
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Case Study
Within our hotel we have allowed the following criteria approximately 270 rooms, residency is very high from Thursday to Sunday, and variable during Monday-Wednesday depending on the time of year, the golfing events and conference activities. The hotel and golf course employ 600 staff in all. We decided to estimate the overall average human "population" at the hotel as 600. This equates to a number of people effectively living on site 24 hours a day. In reality, there would probably be more people than this coming and going, but most of these people would not be on site for 24 hours a day. Our figure of 600 presents our best estimate of the hotel population from which to estimate waste streams.
Case study 1 involves a small plant which processes waste produced only within the hotel estate. The waste streams are:-
The other relevant factors and assumptions made during the analysis of this case study are:-
Archived inputs and outputs from our model of the case study 1 plant can be viewed at case study 1 demographics and case study 1 results.
The total mixed waste stream for this case study is very small. The expected tonnages vary from 1.44 to 2.66 tonnes/day, with an expected MIDDLE value of 1.88 tonnes/day. Of the expected amount, 0.9 tonnes per day is expected from primary sewage sludge. This has a dry solids content of only 2.5-5% and therefore represents a relatively low energy feedstock per tonne: a biogas yield of 9-20m3/tonne. Horse manure will provide an expected 0.8 tonnes/day and, being dryer and possibly containing straw, will yield a larger biogas yield of 18-40m3/tonne. The food waste is both dry (15-25% dry solids) and of high energy density (70-160m3/tonne). This means that although there will only be between 0.12 and 0.42 tonnes/day of food waste (0.18 tonnes/day expected), it can provide a relatively large proportion of the total biogas production.
The plant is very small, but it is expensive. The 3 pasteurisers are each 0.4m diameter and 0.4m high. The single digester is 3.6m diameter and 3.6 high. The heat exchangers require a surface area of 0.1-0.2m2each. We assume that 100mm of insulation (k=0.04W/m2K) surrounds the pasteurisers, and 50mm surrounds the digester. Pipework heat loss is neglected, assuming adequate lagging. Although small, the plant still requires many pipes, valves, pumps, mixers and a control system. These are required in order to meet the EU regulations for organic waste processing. The plant schematic is no less complex although the plant is small. The required biogas engine will have a capacity of approximately 0.6 litres, and the induction generator will produce an average electrical power output of approximately 4.3kW. Plant cost varies in a non-linear fashion against equipment sizes. The overall plant cost is estimated to be in the region of £358,000-£482,000, with an expected value of £407,000. This overall cost is made up of the following contributions:- Physical plant cost (PPC) is based on PCE but adds proportional amounts for standard extra physical requirements. PPC is estimate at £361,000.
Total capital cost adds proportional amounts for standard extra service requirements. Total capital cost is estimated at £407,000.
On average, only 1.9 tonnes/day of feed needs to be collected and 1.8 tonnes/day of digestate needs to be distributed. This quantity does not require large trucks, so the transport definition was adjusted to something approximating to a small van. The cost per hour is not proportionately small, however, mainly because the driver's wages are a fixed overhead cost per hour. We assumed that waste feed streams and digestate are transported by small vans with 0.4 tonne capacities, £10/hour costs, speed=10km/h, pumping speed=4 tonnes/hour, 8 hours a day on 5 days of the week available for deliveries, vans based 1km from the plant, 1 hour a day driver lunch break, and 10 km/litre diesel consumption. The model predicts that 1 van will be needed for waste collection, and 1 van will be needed for digestate distribution.
The CO2 balance for the plant is negative, indicating that operating the plant will cause a net reduction in greenhouse gas emissions. Mainly, this is due to the prevention of methane release from the naturally occurring decay that the waste would otherwise undergo. There is very little benefit due to displaced power generation from conventional fossil-based sources.
Since the greenhouse gas emissions balance is beneficial for even this relatively ineffective plant, we can deduce that almost any anaerobic digestion operation will result in a net reduction in greenhouse gas emissions. Any plant that traps methane from decaying matter and transforms it to carbon dioxide, even by simply flaring methane, will result in a reduction in greenhouse gas emissions. The energy balance for this plant is expected to be zero, although the HIGH analysis predicts a positive balance and the LOW analysis predicts a negative balance.
When the predicted energy balance for a plant becomes negative (implying that the plant operation consumes more energy than it can supply), interesting things can occur within the plant. There comes a point where the energy available from the biogas yield becomes too small to sustain the process effectively. This manifests itself in an inability of the hot water from the engine (see plant schematic) to sustain the pasteurisation temperature at 70°C or the the digestion temperature at 38°C. We assume that the pasteurisers have "emergency" electric heating which can supplement the engine heat in order to guarantee 70°C. This feature is primarily included to provide to aid plant start-up from cold. If the electric heating is required during normal operation, the plant is unlikely to be working efficiently. Please click on: Plant Design for a detailed image of our design. It is interesting to examing the thermal performance of the LOW yield analysis for this case study. In the pessimistic LOW yield analysis, the pasteuriser electic heating is not required. However, the hot water which emerges from the engine at 85°C, 0.01kg/s, has to pass substantial energy to the pasteuriser and 3rd heat exchanger, such that its temperature at exit from the 3rd heat exchanger is only 64°C. Compare this with a temperature of 70-78°C which is normal for a "healthy" plant. The lower temperature hot water, in combination with relatively small digester which has a relatively large heat loss per unit volume (due to a larger surface area per unit volume than a bigger digester), means that the digester cannot be maintained at 38°C. The digester temperature, according to the LOW analysis, will drop to about 36°C. Clearly, this plant is on the knife-edge of producing a positive or negative energy balance effect. It should be rememered, however, that our greenhouse gas and energy balances take into account only operational activities. The construction of the plants, employee transport etc. are not accounted for. Therefore, a plant should demonstrate a clearly negative greenhouse gas emission balance and positive energy balance effect before being considered for construction. On these grounds, this feedstream/plant scenario is not viable.
This plant is not financially viable, even with the 30% grant funding assumed. The capital cost, at around £400,000 before grant reduction, combined with the high labour costs of transporting the small feed streams, makes the plant a financial disaster. Assuming the 20 year extected plant lifetime, the net present value of the project is of the order of minus £770,000. No realistic modification to the revenues from energy or digestate production will result in a positive net present value.
The digestion plant for case study 1 is not viable. The plant is a financial diasaster, and may not be a net producer of energy. Although it is possible to conceive of a simpler and cheaper plant design for the same feedstream input quantities, the plant would then not be energy efficient and would not comply with EU food processing regulations. For these reasons, we are being fairly strict with the required complexity and quality of our plant construction, and this leads to very expensive plants if an economy of scale cannot be realised. Clearly, a feedstream input of more than 2 tonnes/day will be required for a viable operation. |