Biomass - Using Anaerobic Digestion

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Case Study

Analysis of case study 1 - A Hotel with Golf Course

map: perth and Kinross area

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:-

  1. The hotel operates its own sewage treatment plant. We assume a sewage throughput equivalent to our hotel "population" of 600.
  2. The "population" of 600 produces food waste. We assume this to be at the normal domestic food waste rates (between 0.2 and 0.7 kg/person/day, with an expected value of 0.3 kg/person/day). At the hotel, this food waste must be gathered by source-separating food waste from the kitchens and other staff/guest residences. Depending on how efficiently the kitchens manage their food wastage, the actual food waste quantities might be towards the upper, 0.7 kg/person/day levels.
  3. There are 20 horses in livery stables on the estate. These provide for equestrian activities. Although only 20 horses are present, they account for a relatively high proportion of the waste tonnage as each horse can be expected to produce between 30 and 55 kg/animal/day. The dryness and biogas yield from this manure can vary widely however, depending upon how it is gathered (water used for hosing down etc.) and how much straw bedding/feed is mixed in with the manure.

The other relevant factors and assumptions made during the analysis of this case study are:-

  • The case study area has an effective radius of 1km.
  • The digestate produced is used on site, applied to the golf courses. It produces a benefit (via fertiliser replacement) of 2 per tonne on average for the digestate (liquor and fibre mix). The digestate transport must of course be paid for by the hotel. The liqour is a liquid fertiliser which could applied by spray, or potentially through installed sprinkler systems if liquor particle size is small enough to avoid blockages. The fibre is a much dryer material which would be applied as a soil conditioner or compost.
  • The plant design consists of 1 digester (to minimise capital cost for this small plant), and 3 pasteurisers. The 3 pasteurisers add cost but are required in order to comply with EU regulations for mixed waste processing, and to provide a positive energy balance.
  • A capital grant of 30% from "Transforming Waste Scotland" and "Wrap Organics".
  • A discount rate of 8% per annum.
  • Loan repayment over 10 years, but a plant lifetime of 20 years.
  • A "gate fee" of 45 per tonne, which is applied to the food and sewage input feed streams. Although the hotel would not actually pay a gate fee to themselves, this figure represents money they save by not paying this fee to an outside waste processing facility.
  • An electrical sale price, under a ROC contract, at 0.04/kWh, and a heat sale at 0.003/kWh.

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.

Case study 1 - Analysis of mixed waste stream

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.

graph: Biogas yields produced per day

Case study 1 - Plant sizing and costs

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:-

graph:case study 1 purchase cost estimate.

Physical plant cost (PPC) is based on PCE but adds proportional amounts for standard extra physical requirements. PPC is estimate at 361,000.

graph:case study 1 physical plant cost.

Total capital cost adds proportional amounts for standard extra service requirements. Total capital cost is estimated at 407,000.

graph:case study 1 total capital cost.

Transport Requirements

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.

Energy and CO2 balances

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.

graph:case study 1 CO<sub>2</sub> balance.

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.

graph:case study 1 energy 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 70C or the the digestion temperature at 38C. We assume that the pasteurisers have "emergency" electric heating which can supplement the engine heat in order to guarantee 70C. 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 85C, 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 64C. Compare this with a temperature of 70-78C 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 38C. The digester temperature, according to the LOW analysis, will drop to about 36C.

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.

Financial Viability

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.

graph:net present value.

Conclusion for Case Study 1

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.