Biomass - Using Anaerobic Digestion |
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Case Study
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.    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:-
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 2 plant can be viewed at case study 2 demographics and case study 2 results.
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. 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:-
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.
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 76°C 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:-
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.
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. 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:-
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. The lack of profitability is attributable to a combination of factors:-
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:-
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