Biomass - Using Anaerobic Digestion

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Case Study

Analysis of Case Study 2 - Auchterarder, Blackford and the surrounding area.

map: perth and Kinross area

Our second case study analysis examines the feasability of a medium sized anaerobic digestion plant, located either at the Auchterarder sewage works or a nearby hotel with golf course in Perth & Kinross, Scotland.

The towns of Auchterarder and Blackford have populations of 3945 and 556 according to the Scottish Census results 2001 [Scrol].These populations are tightly bound by the urban areas of the two towns, shown by the yellow lines on the maps below; they do not include the more spread out rural populations outside the town boundaries.

  map:auchterarder town limits     map:Blackford town limits

Case study 2 involves a medium sized plant that processes waste produced within an area of approximately 80km2 surrounding Auchterarder and Blackford. We model this as an inner zone circle of radius 3km, and an outer zone donut with an outer radius of 5km. Fixed populations and waste pickups are entered for the inner zone, while per-hectare populations combined with the calculated outer zone area determine the outer zone waste stream sizes.

The waste streams are:-

  1. A population of 5100 people in the inner zone contributing to primary human sewage sludge, at 3 sewage works: Auchterarder (3945), Blackford (556) and hotel (600). The rural population does not contribute to the sewage feed stream as it is assumed that they operate their own septic tanks.
  2. A population of 5300 people contributing to source-separated organic kitchen waste. This figure is made up of the 5100 people described above in the inner zone, plus a further 200 population in the surrounding rural area (0.04 people/ha) who also have a municipal source-separated kitchen waste collection. We assume that the kitchen waste is picked up by us from 2 municipal depots within the inner zone and one in the outer zone.
  3. According to data which SEERAD provided to us, there are 5,391 cattle, 49,952 sheep, less than 10,000 pigs (disclosive) and 325 poulty within the case study 2 area. These figures are determined on a parish-by-parish basis. We disregard the sheep and poultry here because the waste is deemed unrecoverable. Of the 5391 cattle, we believe that these are all beef cattle in the Auchterarder area (Dairy cattle predominate closer to Stirling). These cattle spend winter months in barns when 100% of their manure is collectable. In summer, a much smaller proportion will be recoverable. We model this as an average of 50% of 5391 which approximates to 2700 cattle (0.54 cattle/ha) available for manure collection. This is a simplification which we must bear in mind when analysing the results. In reality, there will be a glut of manure in winter and a much smaller amount in summer. This will affect plant sizing and usage of plant capacity. We also add 7500 pigs in the outer zone (1.5 pigs/ha)
  4. There are 20 horses in the inner zone, at the hotel, and 20 in the outer zone due to Easterton stables.

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

  • The digestate produced is used by the participating farmers, or at the hotel where it can be applied to the golf courses. It produces a benefit (via fertiliser cost replacement) of 2 per tonne on average for the digestate (liquor and fibre mix). The digestate transport cost is borne by the farmers and the hotel, as they collect any digestate themselves. 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 2 digesters and 3 pasteurisers. The two digesters provide redundancy in case of maintenance, and also the potential to isolate particularly nasty waste streams from less potent ones. The digesters have 50mm thickness of insulation (k=0.04W/m2K) and the pasteurisers have 100mm insulation thickness.
  • 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.
  • An electrical sale price, under a ROC contract, at 0.04/kWh, and a heat sale at 0.003/kWh.
  • We assumed that waste feed streams and digestate are transported by lorries with 20 tonne capacities, 40/hour costs, speed=25km/h, pumping speed=50 tonnes/hour, 8 hours a day on 5 days of the week available for deliveries, lorries based 1km from the plant, 1 hour a day driver lunch break, and 3.27 km/litre diesel consumption.

Archived inputs and outputs from our model of the case study 2 plant can be viewed at case study 2 demographics and case study 2 results.

Case study 2 - Analysis of mixed waste stream

The dryness and biogas yield from the cattle and horse manure can vary widely, 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 expected magnitude of the waste stream is 172 tonnes/day. Of this, cattle manure accounts for 108 tonnes/day, pig slurry for 53 tonnes/day, sewage for 8 tonnes/day tonnage, and food waste for only 1.5 tonnes/day. Because the bulk of the daily feed stream is made up of relatively low energy sources (manure, slurry and sewage), and the manure/slurry does not incur gate fees, we can expect that transport costs will be relatively large and that profitability will be difficult to achieve.

The waste stream dry solids content is predicted to be about 10.1% which is ideally situated within the 6-12% target range, without adding any water or thickener. The expected C:N ratios is about 16, which is on the low side but not disastrous.

The vast majority of the Biogas production arises from the cattle and pig manure. The Biogas contribution from the human sewage and domestic food waste streams is very small. The total biogas production is expected to be about 4600 m3 per day. As the total input waste feed stream is about 172 tonnes/day, the biogas yield average is 27 m3/tonne which is significantly lower than the Holsworthy and Valorga benchmarks at 80 and 40 m3/tonne. The yield per tonne is low because the feedstream contains very small percentages of high yielding feedstocks. Adding significant amounts of domestic food waste, with its higher proportions of volatile solids per unit mass and higher carbon-nitrogen ratio, would increase the biogas yield.

graph:Case study 2 biogas yields

It would also be possible to increase the biogas yield by 46% by adding up to 4 tonnes/day of waste paper, straw or sawdust. Adding 4 tonnes/day raises the dry solids content to 11.9% which is as high as recommended. Adding further dry ingredient would require adding water in proportionate amounts, reducing the beneficial effect. Adding 4 tonnes/day of dry cellulose-based material has two positive impacts:-

  1. it adds large amounts of volatile and digestible cellulose as the material is dry and therefore concentrated.
  2. it increases the carbon:nitrogen ratio of the overall mix from 16 to 20, allowing the entire feedstream to be digested more efficiently.

graph:Case study 2 biogas yields 2

For the remainder of the case study 2 analysis, we assume that this extra dry feedstock is added regularly, bringing the overall daily tonnage to an expected 176 tonnes. The dry paper/wood based feed could be sourced from municipal waste paper sources or from Perthshire forestry wastes.

Case study 2 - plant sizing and costs

The plant is of "medium size". The expected input tonnages are 116 tonnes/day (LOW estimate), 176 tonnes/day (MIDDLE estimate), and 252 tonnes/day (HIGH estimate). Based on the MIDDLE feed stream estimates, the 3 pasteurisers are each 1.8m diameter and 1.8m high. The two parallel digesters are 13.1m diameter and 13.1m high. The heat exchangers require a surface area of 19.8 m2 (exchangers 1 & 2) and 6.6 m2 (exchanger 3). 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.

The required biogas engine will have a capacity of approximately 64 litres, and the induction generator will produce an average electrical power output of approximately 590kW, of which about 50kW is required to operate plant pumps and mixers, leaving 540kW available for export to the grid. The biogas engine provides about 1050kW of recoverable heat energy from the exhaust, cooling water and turbo-charger. Of this, about 120kW process heat is required to run the plant. The remaining 930kW is available for neihbourhood CHP schemes, at a water temperature and flow rate of 76C and 3.26 kg/s.

Plant cost varies in a non-linear fashion against equipment sizes. The overall plant cost is estimated to be in the region of 3.9-6.3 million, with an expected value of 5 million. This overall cost is made up of the following contributions:-

  • Purchase cost estimate (PCE) of 1.4 million. This includes all the primary plant equipment, costed individually based upon the calculated equipment sizes required.The largest individual material costs are incurred in the two digesters and the digestate storage vessel which can store 20 days' output. The digestate store is a single vessel expected to be 16m diameter and 16m high.

graph:Case study 2 purchase cost estimate

  • Physical plant cost (PPC) is based on PCE but adds proportional amounts for standard extra physical requirements. PPC is estimate at 4.5 million.

graph:Case study 2 physical plant cost

  • Total capital cost adds proportional amounts for standard extra service requirements. Total capital cost is estimated at 5 million.

graph:Case study 2 total capital  cost

Transport Requirements

On average, only 176 tonnes/day of feed needs to be collected and 163 tonnes/day of digestate needs to be distributed. The feed and digestate is assumed to be transported in 20-tonne capacity lorries, with each lorry available for 5 out of each 7 days (weekdays). The model predicts that between 2-4 lorries will be needed for waste collection, bringing a total of 8-18 loads to the plant each weekday.

Between 2-4 lorries will be needed for digestate distribution, taking a total of 8-16 loads from the plant each weekday. These digestate transports are funded by the participating farmers and other digestate users. It is not possible to use the same lorries for feed collection and digestate distribution due to the pathogen crossover that would result.

Transport costs, therefore, are relatively high (250,000-340,000 per annum, about 4 per tonne of feed). Also, the total of 16-34 large vehicle movements each day from monday-friday would have to be taken into account in the environmental impact assessment.

Energy and CO2balances

The CO2 balance for the plant is negative, saving the release of 9400 tonnes of CO2e per annum, 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 also a small benefit due to displaced power generation from conventional fossil-based sources.

graph:Co<sub>2</sub> balance

The energy balance for this plant is quite positive, indicating that the plant is energy efficient. Although the transort costs are large and are dominant factors in the financial balance, the actual energy used during transport is not a dominant factor in the energy balance. The energy balance is dominated by the large amounts of electricity and heat energy made available. The transport energy and lost process energy are only small negative offsets. This energy balance does not account for the following factors, however:-

  • energy used in the construction or maintenance of the plant.
  • energy used in the construction or maintenance of the transport lorries
  • energy used in the daily commute of plant employees

graph:Energy balance

Financial Viability

Unfortunately, although the plant in case study 2 has a favourable greenhouse gas effect and energy balance, it is not financially viable. Even including the 30% capital grant funding for the plant construction, the plant returns a negative net present value over even a 20 year lifetime.

graph:Net present value

The lack of profitability is attributable to a combination of factors:-

  • The feedstock is dominated by low energy density feeds such as cattle manure, requiring large bulk tonnages of wet material transport per tonne of dry-solids material.
  • The low energy density feeds require a large volume plant to process them. The large plant is costly, while the biogas yield is relatively small per tonne of wet feed.
  • The feedstock is dominated by waste streams that do not incur gate fees.
  • Transport distances in the rural environment are relatively large.
  • The market for the digestate is unproven and minimal. Only 2 per wet tonne of digestate is assumed collectable as a fertiliser replacement value.

Conclusion for Case Study 2

It is dissappointing that this case study does not appear viable. In terms of plant size and demographics, it would be just under half the size of the Holsworthy plant in Devon. The plant capital cost is just over half the Holsworthy cost, as we would expect given economies of scale. However, the net present value of this case study 2 plant is so negative that it could not be made zero or positive simply by scaling up the plant. The reasons that the case study 2 plant is so un-viable relative to the seemingly similar Holdswothy plant are:-

  1. Holsworthy achived a 50% capital grant funding for the construction, whereas we can only justify a maximum of 30% expected funding under current EU and UK schemes.
  2. Although rurally based and obtaining a large amount of biogas yield from cattle manure, the Holsworthy plant also uses chicken manure and up to 20% of the daily wet feed is food waste. The chicken manure is dryer (and therefore energy dense) and therefore more efficient to transport. The food waste is also relatively dry but also provides a large income stream at 45 or more per tonne. At our case study 2 plant, only 9.2 tonnes (1.6 of food plus 7.6 of human sewage) from a daily total of 176 tonnes of feed is expected to be food waste incurring gate fees. This is only about 5% of the daily tonnage. Since gate fees at 45 or more per tonne are the major source of revenue for a digestion plant, our case study plant is missing out on the main potential source of cash on 95% of the input feedstream.