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Pelletised fuel production from coal tailings and spent mushroom compost – Part II. Economic feasibility based on cost analysis
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w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Pelletised fuel production from coal tailings and spent mushroom compost – Part II. Economic feasibility based on cost analysis Changkook Ryu⁎, Adela Khor, Vida N. Sharifi, Jim Swithenbank Department of Chemical and Process Engineering, Sheffield University, Mappin Street, Sheffield S1 3JD, UK
AR TIC LE I N FO
ABS TR ACT
Article history:
Due to the growing market for sustainable energy, in order to increase the quality of the
Received 9 May 2007
fuels, pellets are being produced from various materials such as wood and other biomass
Accepted 16 November 2007
energy crops, and municipal waste. This paper presents the results from an economic feasibility study for pellet production using blends of two residue materials: coal tailings
Keywords:
from coal cleaning and spent mushroom compost (SMC) from mushroom production.
Biomass
Key variables such as the mixture composition, raw material haulage and plant scale
Coal tailings
were considered and the production costs were compared to coal and biomass energy
Cost analysis
prices. For both wet materials, the moisture content was the critical parameter that
Fuel pellet
influenced the fuel energy costs. The haulage distance of the raw materials was another
Spent mushroom compost (SMC)
factor that can pose a high risk. The results showed that the pellet production from the above two materials can be viable when a less energy-intensive drying process is utilised. Potential market outlets and ways to lower the costs are also discussed in this paper. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Pelletisation or briquetting have been applied to coal, forestry residues, energy crops and municipal waste segregation residues in order to increase the viability of these fuels. This preprocessing increases the homogeneity of the fuels and allows the pellets to be utilised in existing automated feeding systems of various furnaces. The pelletisation of low density materials such as biomass also reduces storage and logistic costs while the calorific value can be increased by drying prior to pelletisation. This study investigated the economic feasibility of pellet production from two residue materials produced by the coal and mushroom industries: i) coal tailings, and ii) spent mushroom compost (SMC).
Pelletisation will allow the materials to be used in chain grate furnaces and industrial gasifiers or to be fed into the mills of conventional pulverised-fuel-based power stations. Part I of this paper [1] presented the results on the properties of the fuel pellets and the identification of key pelletisation parameters such as the moisture content and pressure. Energy production from mixtures of SMC and coal tailings would complement the recent experience of the power industry on co-firing biomass with a fossil fuel which minimises the investment needed in new equipment and ensures rapid exploitation of the energy available. The utilisation of biomass in the UK is not be feasible without economic and political incentives since the costs of biofuel production can be 3–5 times more than that of conventional fuels. The use of biomass is encouraged under the Renewable Obligation Certificates (ROCs) and the Levy Exemption Certificates (LECs). Under the Renewables Obligation, electricity suppliers are required
⁎ Corresponding author. Tel.: +44 114 222 7523; fax: +44 114 222 7501. E-mail address: [email protected] (C. Ryu). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.11.027
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Table 1 – Collieries in England and Wales for coal tailings [3] County
Colliery (Active deep mines)
Nottinghamshire West Midland Northumberland Yorkshire Derbyshire West Glamorgan Wales
7 (Harworth, Thoresby, Welbeck) 1 (Daw Mill) 2 15 (Kellingley, Maltby) 4 1 2 (Tower)
to source a proportion of the annual electricity from renewable sources. The financial incentives provided by the UK government have resulted in many electricity power stations such as Didcot, Cockenzie and Drax developing specific biomass cofiring processes. The SMC in the UK is currently produced by around 38 mushroom growers in large to significantly small scale farms. Some of the SMC is currently used by the horticultural sector (amateur gardeners, professional growers, landscapers and local authority) as soil improver or plant growing medium. In the survey carried out by ADAS UK Ltd and Enviros Consulting Limited [2], it was estimated that at least 500,000 m3 of SMC was available for disposal in 2005 in the UK whilst 351,100 m3 was used in the horticultural sector. This left about 150,000 m3 or 120,000 tonnes of SMC available in 2005. The majority (61%) of the SMC used in the horticultural sector is for landscapers to improve the quality of the soil as the high pH makes it unsuitable for use as growing medium. The use of SMC in the horticultural sector as an alternative to peat is declining due to reduced availability of SMC in some areas and the increased use of green compost instead. The coal tailings from the coal washing process have been widely disposed of in the settling lagoons as this is the simplest and cheapest option. Table 1 gives the locations of some of the UK coal mines, mainly in the areas of Yorkshire and Nottinghamshire, in which only 7 of the major deep mines are currently in production [3]. In this study, the pellet production costs were calculated for the following parameters: i) different mixtures of coal tailings and SMC, ii) the haulage distance of raw materials and
277
iii) the scale of the pelletisation plant. For these sets of scenarios, the costs were calculated for raw material, transportation, drying and pelletisation. The transportation costs were based on the locations of mushroom farms and collieries in England. The feasibility of this process was assessed using coal and biomass energy prices. Then the potential market outlets and the importance of less energy-intensive drying process were investigated.
2.
Methods for cost analysis
2.1.
Description of pelletisation process
The cost analysis for pellet production from SMC and coal tailings was carried out for the process illustrated in Fig. 1. In the proposed process, SMC is delivered to a colliery where coal tailings are collected from the lagoons and dewatered by a screw press. This has several economic benefits. Firstly, it does not require the transportation of coal tailings. Coal tailings are abundant and readily available while SMC needs to be collected from several farms. Secondly, a colliery site usually has unused space where the pelletisation plant can be installed. Thirdly, the existing transportation system for coal can be used for delivery of the pellets to a nearby power plant, which can reduce the costs. In Fig. 1, both materials are dried to about 10–20% moisture content prior to pelletisation in order to produce the strongest pellets [1]. The drying process can incorporate the mixing of the two materials. After pelletisation, the pellets are cooled down and then stored.
2.2.
Cases for raw material blends
Table 2 lists the fuel properties and the net calorific value (NCV) calculated for 8 blends of the two materials including coal-tailings-only (Case 1) and SMC-only cases (Case 8). The moisture content in the raw materials is assumed to be 70% for SMC and 40% for coal tailings. These are dried to 20% and 10% respectively prior to pelletisation. The moisture content of the blends was then calculated correspondingly. After drying, the
Fig. 1 – Proposed pellet production process from SMC/coal tailing for cost analysis.
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Table 2 – Cases for various mixture composition Case SMC:Coal tailing (wet) SMC fraction in raw material (wet) Raw material Moisture (%) NCV-wet (MJ/kg) Raw material required (ton/tonpellet) SMC fraction in the pellets Pellet properties Moisture (%) Volatile matter (%) Fixed carbon (%) Ash (%) NCV-pellets (MJ/kg)
1
2
3
4
5
6
7
8
0:1 0% 40 12.08 2.67 0% 10.0 22.6 27.7 39.7 19.34
1:6 17% 45 10.33 1.5 10% 11.0 24.6 25.9 38.5 18.07
1:4 25% 47.5 9.46 1.62 16% 11.6 25.8 24.9 37.7 17.37
1:2 33% 50 8.59 1.68 22% 12.2 27.0 23.8 37.0 16.63
1:1 50% 55 6.84 1.76 36% 13.6 29.9 21.3 35.2 14.99
2:1 67% 60 5.1 1.92 53% 15.3 33.3 18.3 33.1 13.11
4:1 80% 64 3.7 2.12 69% 16.9 36.6 15.4 31.1 11.39
1:0 100% 70 1.61 2.31 100% 20.0 42.9 9.9 27.2 8.36
mass fraction of SMC in the blended pellets (Cases 2 to 7) became lower than that in the raw materials. The NCV of the pellets ranged from 8.36 to 19.34 MJ/kg depending on the mixture composition.
2.3.
Framework conditions
Table 3 shows the framework conditions for the cost analysis. The reference case was based on a plant scale of about 24,000 tonpellets/yr (3 tonpellets/h with an availability of 90%). The production costs were divided into four main elements: raw material, transportation, drying and pelletisation costs.
2.3.1.
Raw materials
Raw material prices (excluding delivery) for SMC and coal tailings were assumed to be low: £1 and £2 per ton, respectively. About 60% of SMC used for soil improvement is currently traded at £0–5/ton [4]. As shown in Table 2, the amounts of raw materials required for the pellet production varied from
1.5 (Case 1) to 2.67 ton/tonpellet (Case 8) depending on the mixture composition and the moisture content.
2.3.2.
2.3.3. Table 3 – Framework conditions for the cost analysis Parameters Reference plant
Raw material costs Transportation costs
Capital costs
Energy consumption costs Operation (personnel) Other costs
Value Plant scale Production rate Plant availability Price of SMC Price of Coal tailing SMC haulage (5 cases) Transportation price Investment costs Currency rate Interest rate Service and maintenance Heat price Electricity price Simultaneity factor
Unit
23,652 3 90
tonpellet/yr tonpellet/h %
1.0 2.0
£/ton £/ton
0, 50, 100, 150, 200 0.074
miles
Transportation
Fig. 2 shows the distribution of the haulage distance from 55 SMC farms to 3 main collieries; Ellington in Northumberland, and Rossington and Maltby in Yorkshire. The majority of the mushroom farms are located within 150–200 miles from the collieries sites. Therefore, the SMC haulage distances considered in this study was 0, 50, 100, 150 and 200 miles. The transportation price was fixed at £0.074 per ton per mile, as shown in Table 3. This figure was estimated from the annual average costs data for a 40-tonne tri-axle combination lorry [5]. This type of vehicle has annual time-related costs of £64,050 and mileage-related costs of £0.567/mile. It was assumed that the vehicle has an annual mileage of 57,600 miles with 25 tonnes of raw material per delivery and the transportation company adds 10% on top of the actual costs as profit.
Heat and electricity consumption
Drying and pelletisation processes consume electricity and heat. The electricity and heat prices shown in Table 3 are for the third quarter of the year 2006 acquired from the surveys carried out for fuel suppliers [6]. A simultaneity factor of 85% was used to compensate for electrical installation not operating at full load all the time.
£/ton/mile
(see Table 4) 1.48 7 (see Table 4)
Euro/£ %
19.22 53.00 85
£/MWh £/MWh %
167,350
£/yr
0.5
%
Fig. 2 – SMC haulage from 55 mushroom farms to Ellington, Rossington and Maltby collieries.
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Table 4 – Technical data and investment costs for drying and pelletisation processes based on Thek and Obernberger [7] Parameters
Drying
Type Power (kW) Heat demand (MWh) Utilisation (yr) Service and maintenance (%) Investment (£)
2.3.4.
Pelletisation Pellet mill
Cooler
Storage
Peripheral
Ring-die 233 0.315 10 10 128,378
Counter flow cooler 12 0 15 2 8,784
Silo 30 0 20 1.5 196,622
90 0 10 2 337,838
Tube bundle dryer 77.5 1.63–5.43 15 2.5 175,626 ~ 361,672
Drying and pelletisation
Table 4 shows the framework conditions considered for the drying and the pelletisation process, based on the technical data and investment costs for wood pellets in Austria and Sweden [7]. For drying of the raw materials, a tube bundle dryer was considered. The capacity of the dryer was calculated from the moisture content of the raw materials given in Table 2, and corresponding dryer size and investment costs were scaled up. The pelletisation costs included storage (silo) and peripheral equipment as well as a pellet mill and a cooler. The pellet mill considered was a ring die. However, further investigation is required to find an ideal pellet mill for the materials and the optimum use of binders. The total investment costs ranged from £0.84 million to £1.03 million between cases as shown in Table 4, depending on the capacity of the dryer. The drying and pelletisation processes consisted of four cost categories: capital, energy consumption, service/maintenance, operation and miscellaneous costs. From the investment costs for each process shown in Table 4, the annual capital costs were calculated using the capital recovery factor (CRF), taking into account an interest rate (i) of 7% and the utilization years (n) [7]. CRF ¼
Total
ð1 þ iÞn i ð1 þ iÞn 1
The maintenance costs were added to the capital costs as a percentage of the investment costs for each process. The personnel costs were calculated for an hourly rate of £13/h, annual hours 8760, 3 shifts with 1 person per shift and 0.25 person for substitution per shift. Then £25,000/yr was added for administration and marketing personnel. This resulted in the annual operating costs of £167,350/yr. The other costs include insurance rates, overall dues, taxes and administration costs.
3.
Results and discussion
3.1.
Costs breakdown
835,951 ~ 1,019,517
Table 5 lists the breakdown of the costs for drying and pelletisation for Case 1 – coal tailing pellets. The annual raw material costs for this case was £0.072 million (£3.0/tonpellets). The cost elements from pellet mill to personnel inTable 5 do not vary between cases since they are dependant only on the material throughput after drying, i.e., 3 tonpellets/h. The total of these fixed costs elements was £19.31/tonpellets. Due to the high moisture content of the raw material, the consumption of heat and electricity for drying was the most expensive element. This led to £15.03/tonpellets for specific drying costs, which is 40% of the total costs. For the same reason, the specific consumption costs were 58% of the total. The total annual costs (including raw material costs for coal tailing pellets) were £0.89 million and the specific costs £37.34/tonpellets.
3.2.
Specific and annual pellet production costs
Fig. 3 shows the specific pellet production costs considering various SMC haulages for the 8 cases with different mixture compositions. As mentioned previously, the costs for the pelletisation process and personnel were fixed at £19.31/tonpellets in all cases. The raw material costs varied from £2.67 to £3.0/ tonpellets, which is the lowest item in the total costs. As the materials are very wet and drying is energy-intensive, the drying process was the major cost factor which ranged from £15.03/tonpellets (Case 1) to £30.96/tonpellets (Case 8). This represented 40% (Case 1) to 58% (Case 8) of the total costs excluding transportation. The pellet production costs increased dramatically with SMC haulage distances for the cases with a higher fraction of SMC. For example, the specific costs for Case 5 (SMC:Coal
Table 5 – Breakdown of annual pellet production costs for Case 1 (Coal tailing pellets) excluding raw material costs for the reference plant scale Process
Drying
Pellet mill
Cooler
Storage
Peripheral
Capital costs Consumption costs Operating costs Other costs Total (£/yr) Specific (£/tonpellets)
23,673 334,173
31,116 130,488
1140 4262
21,509 10,655
54,857 31,966
878 358,724 15.17
642 162,246 6.86
44 5446 0.23
983 33,147 1.40
1689 88,512 3.74
Personnel
167,350 167,350 7.08
Total (£/yr)
Specific (£/tonpellets)
132,296 511,544 167,350 4236 815,426 34.34
5.58 21.50 7.08 0.18 34.34
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3.3.
Fig. 3 – Specific pellet production costs for the cases with different SMC haulages (P: pelletisation including personnel, R: raw material; D: drying costs).
Sensitivity analysis
The sensitivity of the costs to different input parameters is shown in Fig. 5 for Case 5 (SMC:coal tailings = 1:1) with SMC haulage of 100 miles. Each parameter was varied by ±10%. The specific production costs for this case was £50.97/tonpellets. The most sensitive parameter was the moisture content of the raw materials which greatly influenced the transportation, drying and energy consumption costs. Decreasing the moisture contents to 63% for SMC and 36% for coal tailings lowered the costs by £4.4/tonpellets. The heat price was also an important parameter. The buoyant energy price may directly affect the pellet production costs, noting that the UK experienced a 30% rise in the energy price during the last quarter of 2005, which was caused by an unprecedented increase in the wholesale gas price. The interest rate and raw material price were the least sensitive factors. Fig. 5 also shows the sensitivity of the investment costs, which confirms that the effect of the investment cost model adopted from Thek and Obernberger [7] is not significant in this study.
3.4.
Energy price and break-even points
tailings = 1:1) increased from £43.87/tonpellets without SMC haulage to £58.07/tonpellets for 200 miles of SMC haulage. For pure SMC pellets, the specific costs without SMC haulage were £52.94/tonpellets and the transportation costs were £9.87/ tonpellets per 50 miles. Fig. 4 shows the annual costs for all the cases with 50 miles of SMC haulage. The annual production costs varied from £0.88 million to £1.49 million/yr with the energy consumption costs being dominant mainly due to the drying process. The above results clearly show how significant the transportation as well as drying costs could be for wetter materials. For the case with high SMC fractions including Case 8, the two assumptions in this analysis that SMC is transported to a colliery and dried by an energy-intensive tube bundle dryer are not realistic. Lowering the moisture content by other means and its effect on the production costs are discussed in Section 3.6 of this paper.
The break-even points for the pellet price can be acquired for two scenarios. The first scenario assumes that the pellet price is based on that of coal. The average purchase price of coal is £1.53/GJ in England [6]. The second scenario is based on the assumption that the SMC in the pellets is priced the same as biomass while the priced for the coal tailings is still based on the coal price. The market price of biomass varies widely depending on the source of materials, physical form and calorific value from £12/ton for roundwood with N50% moisture excluding delivery [8] to £135/ton for oil seed rape with 9% moisture content [9]. The purchase price of biomass for co-combustion at Drax power station in England is in the region of £3.5–£5/GJ [10]. In the calculation, the biomass portion of the pellets was assumed to be £4.5/GJ. This value matches well with the wood pellet price of about £80/ton at 11% moisture [8].
Fig. 4 – Annual costs of pellet production for the cases with SMC haulage of 50 miles.
Fig. 5 – Sensitivity of costs parameters for Case 5 (SMC: Coal = 1:1) with a SMC haulage of 100 miles.
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281
work conditions. For Case 5, the costs fell below the breakeven point only when the plant was 10 times larger than the reference plant (excluding the transportation costs). For coal tailing pellets, the production costs were below the break-even
Fig. 6 – Break-even points based on coal and biomass based prices, respectively.
Fig. 6 shows the break-even points based on the above two scenarios. The coal-based price ranged from £29.59 /tonpellets for coal tailing pellets to £19.80/tonpellets for SMC pellets. This scenario is less favourable as the renewable energy from the SMC in the pellets is not accounted for, despite the higher costs for processing when compared to coal tailings. When the pellet price was based on biomass for the SMC in the pellets, the break-even point for Case 8 increased to £37.63/tonpellets. Fig. 6 also shows that the specific pellet production cost without transportation is well above the biomass based breakeven point. Therefore, two ways of lowering the costs by scaling up the plant and using alternative drying processes were investigated as presented in the next sections.
3.5.
Effect of production scale
The scaling up of the plant lowers most of the cost elements from the price of raw materials to the operation costs, although the cost factor for each element is difficult to predict. The following method was used to estimate the capital cost related to the plant capacity:
C2 ¼ C1
n S2 S1
where Ci = capital cost of the project with a plant capacity Si. The cost factor ‘n’ was taken as 0.6, which is the widely used six-tenths rule [11]. The same cost factor was applied to the personnel costs. For heat and electricity prices, the cost factor was 0.95 which was estimated from the average prices for different scales of non-domestic consumers [6]. The scaling up effect for raw material and transportation was not considered in this calculation. Fig. 7 shows the effect of the plant capacity for three cases: (a) pure SMC pellets, (b) coal tailing pellets and (c) the mixture of SMC to coal ratio of 1:1 (Cases 8, 5 and 1, respectively). The biomass based break-even point is also plotted for comparison purposes. The production costs for the SMC pellets did not meet the break-even point for any case for the given frame-
Fig. 7 – Effect of plant scale up. (a) Case 8 (SMC pellets); (b) Case 5 (SMC:Coal tailings = 1:1); (c) Case 1 (Coal tailings pellets).
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point when the plant capacity was 5 times larger than the reference plant.
combustion of low calorific value materials and, more desirably, iii) power production at coal-fired power stations.
3.6.
3.7.1.
Effect of alternative drying processes on the feasibility
The calculation results showed that the pellet production would be very limited within the framework conditions mainly due to high drying and heat consumption costs for the wet materials. Especially, drying the SMC with 70% moisture content by using heat from fossil fuels is not practical. As shown in the sensitivity analysis, lowering the moisture content by other means is essential. One of the solutions is the co-location of the pelletisation plant with a process plant that generates waste heat because drying the raw materials only requires low grade heat. The use of a screw press to lower the moisture content to about 40% can also be considered at the pelletisation plant or even at large mushroom farms before transportation. Table 6 shows how significantly the moisture content influences the production costs when SMC is dried to 40% moisture content at mushroom farms. The specific production costs were down by £19/tonpellets without transportation and by £39/tonpellets for a haulage distance of 200 miles. When the plant scale increased by a factor of 4, the pellet production costs excluding the raw material and transportation, was further reduced to £24.49/tonpellets. This value is well below the biomass based break-even point (£37.63/tonpellets). In this case, the sum of the costs for raw materials and additional drying processes can be up to £10/tonSMC. Therefore, finding an alternative way for less energy-intensive drying is the key to the success of the pellet production from the two residue materials.
3.7.
Potential market outlets
The pellets from SMC and coal tailings are more suitable for use in large-scale plants. The reasons are (a) both SMC and coal tailings have a high ash content (25–40%) to be disposed of after combustion, which is not ideal for small-scale plants; (b) other biomass products are available in the market at lower prices with higher calorific values and lower ash contents than SMC; (c) the production costs depends significantly on the production scale as discussed previously. Therefore, it is important to secure long-term market outlets in industry. The potential uses for the pellets are i) the production of low grade heat for compost producers, ii) use as supplementary fuel for
Table 6 – Comparison of production costs for SMC excluding raw material costs with 70% and 40% of moisture content (plant scale: 23,652 tonpellets/yr) Costs (£/tonpellets)
Drying (D) Pelletisation + Personnel (P) D+P D + P + Transportation (50 miles) D + P + Transportation (100 miles) D + P + Transportation (200 miles)
Moisture content of SMC 70%
40%
30.96 19.31 50.27 60.14 70.00 89.74
11.79 19.31 31.10 36.03 40.96 50.83
Mushroom compost producers
The SMC pellets are suitable for use by mushroom compost producers who require a low grade heat in order to keep the temperature at 50–70 °C during the 3 weeks of the compost production process (phase II compost). The SMC can be collected by the lorries that deliver the fresh compost to the mushroom farms, which can minimise the transportation costs. This option is not available in England as the mushroom compost is mostly being imported from The Netherlands. However, this can be considered in Ireland or The Netherlands where more than several thousand tonnes of the compost are produced every week. Although the calorific value of the SMC pellets is not high, it still can raise steam replacing natural gas or oil. However, the pellet production is not essential. Economic viability suffers because of the proposed market outlets and the drying or pelletisation costs are well above the coal based price (£12.80/tonpellets in Fig. 6). In order to lower the costs, co-combustion of SMC with other biomass without pelletisation can be considered. Furthermore, condensing boilers can be used to increase the energy efficiency for special application.
3.7.2.
Coal tailings as supplementary fuel
One potential market outlet for coal tailings as supplementary fuel is at sewage sludge incinerators using fluidised beds. As sewage sludge is barely autothermal with about 75% or higher moisture content, most sludge incinerators use oil in order to maintain the bed temperature. The fluidised bed configuration is ideal to elutriate the ash from this high ash content (over 40%) fuel. The coal tailings in the pellet form burn within the bed, while releasing the heat in the bed due to the high fixed carbon content. This can help stabilise the furnace operation. Fig. 7(c) shows that this option is economically feasible at large scales.
3.7.3.
Power plant
The most desirable market option for the pellets is the existing coal-fired power stations where the electricity can be produced from the biomass material. Three of Britain's major coal fired power stations (Ferrybridge, Eggborough and Drax) are located in North Yorkshire, each of which is 14 to 43 miles away from Rossington or Maltby collieries. When applying the same transportation costs (£0.074/ton/mile) to the delivery of the pellets, it would only incur an additional £1.0 to £3.2/ton to the projected costs of pellets.
4.
Conclusions
The pellet production costs for SMC and coal tailings in England were investigated for various mixture compositions, raw material haulage distances and plant scales. The main findings are: • Due to low calorific value and high processing costs, the economic feasibility of the two residue materials for energy production by pelletisation is limited. For the reference
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conditions, the pelletisation costs were higher than the current coal or biomass energy prices. However, for a 120,000 tonpellets/yr plant capacity, the coal tailing pellets could meet the break-even point. • The pelletisation costs excluding drying and raw material costs were about £19/tonpellets in the framework condition for England. • The moisture content of the two wet materials was the critical parameter that greatly affected the costs of transportation, drying and energy consumption. Rather than using a heat-intensive dryer, other ways of material drying should be investigated. This includes the use of a screw press and the use of waste heat from a process plant. • The potential market outlets identified for these pellets were the mushroom producers for SMC, sewage sludge incinerators for coal tailings and power plants for the mixed pellets.
Acknowledgements The authors would like to thank Veolia Environmental Trust (grant reference RES/C/6046/TP) for the financial support for this study. Thanks are also due to Leslie Bareham (independent consultant), Dr John L. Burden (Mushroom Analysis and Advice) and Dr Phillip S. Cock for the technical support.
REFERENCES [1] C. Ryu, K. Finney, V.N. Sharifi, J. Swithenbank, Pelletised fuel production from coal tailings and spent mushroom compost – part I. Identification of pelletisation parameters, Fuel Processing Technology 89 (2007) 269–275.
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[2] ADAS UK Ltd and Enviros Consulting Ltd., Monitoring of peat and alternative products for growing media and soil improvers in the UK, Report for DEFRA, Wildlife Habitat and Biodiversity, 2005. [3] DTI, List of major deep mines in operation, 2007 London, UK. available at http://www.dti.gov.uk/energy/sources/coal/ industry/deep-mines/page13381.html, assessed on 16 April, 2007. [4] D. Tompkins, Organic waste treatment using novel composting technologies, Final report for Carymoor Environmental Trust, 2006. [5] DFF International, Goods vehicle operating costs 2007, Road Haulage Association, UK, 2007. [6] DTI (Department of Trade and Industry), Quarterly Energy Prices: December 2006, URN 06/276d, 2006 ISBN number: 0 85605 681 9 / 978-0-85605-681-9, London, UK. [7] G. Thek, I. Obernberger, Wood pellet production costs under Austrian and in comparison to Swedish framework conditions, Biomass and Bioenergy 27 (2004) 671–693. [8] J. Davis, J. Farquhar, Biomass Wood/Small Round-wood Biomass prices in the UK, Memorandum by Forestry & Timber Association for the House of Lords, UK, 2006. [9] I. Shield, Biomass costs in the UK, British Bio-Energy News (SUPERGEN), 3, 2005, pp. 12–13. [10] O. Baybut, Personal Correspondence. Drax Power Station, PO BOX 3, Selby, North Yorkshire, UK, 2006. [11] R.K. Sinnott, Coulson & Richardson's Chemical Engineering. – Vol. 6: Chemical engineering design; R.K. Sinnott, Oxford: Elsevier Butterworth-Heinemann, 2005.
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Pelletised fuel production from coal tailings and spent mushroom compost – Part II. Economic feasibility based on cost analysis Changkook Ryu⁎, Adela Khor, Vida N. Sharifi, Jim Swithenbank Department of Chemical and Process Engineering, Sheffield University, Mappin Street, Sheffield S1 3JD, UK
AR TIC LE I N FO
ABS TR ACT
Article history:
Due to the growing market for sustainable energy, in order to increase the quality of the
Received 9 May 2007
fuels, pellets are being produced from various materials such as wood and other biomass
Accepted 16 November 2007
energy crops, and municipal waste. This paper presents the results from an economic feasibility study for pellet production using blends of two residue materials: coal tailings
Keywords:
from coal cleaning and spent mushroom compost (SMC) from mushroom production.
Biomass
Key variables such as the mixture composition, raw material haulage and plant scale
Coal tailings
were considered and the production costs were compared to coal and biomass energy
Cost analysis
prices. For both wet materials, the moisture content was the critical parameter that
Fuel pellet
influenced the fuel energy costs. The haulage distance of the raw materials was another
Spent mushroom compost (SMC)
factor that can pose a high risk. The results showed that the pellet production from the above two materials can be viable when a less energy-intensive drying process is utilised. Potential market outlets and ways to lower the costs are also discussed in this paper. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Pelletisation or briquetting have been applied to coal, forestry residues, energy crops and municipal waste segregation residues in order to increase the viability of these fuels. This preprocessing increases the homogeneity of the fuels and allows the pellets to be utilised in existing automated feeding systems of various furnaces. The pelletisation of low density materials such as biomass also reduces storage and logistic costs while the calorific value can be increased by drying prior to pelletisation. This study investigated the economic feasibility of pellet production from two residue materials produced by the coal and mushroom industries: i) coal tailings, and ii) spent mushroom compost (SMC).
Pelletisation will allow the materials to be used in chain grate furnaces and industrial gasifiers or to be fed into the mills of conventional pulverised-fuel-based power stations. Part I of this paper [1] presented the results on the properties of the fuel pellets and the identification of key pelletisation parameters such as the moisture content and pressure. Energy production from mixtures of SMC and coal tailings would complement the recent experience of the power industry on co-firing biomass with a fossil fuel which minimises the investment needed in new equipment and ensures rapid exploitation of the energy available. The utilisation of biomass in the UK is not be feasible without economic and political incentives since the costs of biofuel production can be 3–5 times more than that of conventional fuels. The use of biomass is encouraged under the Renewable Obligation Certificates (ROCs) and the Levy Exemption Certificates (LECs). Under the Renewables Obligation, electricity suppliers are required
⁎ Corresponding author. Tel.: +44 114 222 7523; fax: +44 114 222 7501. E-mail address: [email protected] (C. Ryu). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.11.027
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Table 1 – Collieries in England and Wales for coal tailings [3] County
Colliery (Active deep mines)
Nottinghamshire West Midland Northumberland Yorkshire Derbyshire West Glamorgan Wales
7 (Harworth, Thoresby, Welbeck) 1 (Daw Mill) 2 15 (Kellingley, Maltby) 4 1 2 (Tower)
to source a proportion of the annual electricity from renewable sources. The financial incentives provided by the UK government have resulted in many electricity power stations such as Didcot, Cockenzie and Drax developing specific biomass cofiring processes. The SMC in the UK is currently produced by around 38 mushroom growers in large to significantly small scale farms. Some of the SMC is currently used by the horticultural sector (amateur gardeners, professional growers, landscapers and local authority) as soil improver or plant growing medium. In the survey carried out by ADAS UK Ltd and Enviros Consulting Limited [2], it was estimated that at least 500,000 m3 of SMC was available for disposal in 2005 in the UK whilst 351,100 m3 was used in the horticultural sector. This left about 150,000 m3 or 120,000 tonnes of SMC available in 2005. The majority (61%) of the SMC used in the horticultural sector is for landscapers to improve the quality of the soil as the high pH makes it unsuitable for use as growing medium. The use of SMC in the horticultural sector as an alternative to peat is declining due to reduced availability of SMC in some areas and the increased use of green compost instead. The coal tailings from the coal washing process have been widely disposed of in the settling lagoons as this is the simplest and cheapest option. Table 1 gives the locations of some of the UK coal mines, mainly in the areas of Yorkshire and Nottinghamshire, in which only 7 of the major deep mines are currently in production [3]. In this study, the pellet production costs were calculated for the following parameters: i) different mixtures of coal tailings and SMC, ii) the haulage distance of raw materials and
277
iii) the scale of the pelletisation plant. For these sets of scenarios, the costs were calculated for raw material, transportation, drying and pelletisation. The transportation costs were based on the locations of mushroom farms and collieries in England. The feasibility of this process was assessed using coal and biomass energy prices. Then the potential market outlets and the importance of less energy-intensive drying process were investigated.
2.
Methods for cost analysis
2.1.
Description of pelletisation process
The cost analysis for pellet production from SMC and coal tailings was carried out for the process illustrated in Fig. 1. In the proposed process, SMC is delivered to a colliery where coal tailings are collected from the lagoons and dewatered by a screw press. This has several economic benefits. Firstly, it does not require the transportation of coal tailings. Coal tailings are abundant and readily available while SMC needs to be collected from several farms. Secondly, a colliery site usually has unused space where the pelletisation plant can be installed. Thirdly, the existing transportation system for coal can be used for delivery of the pellets to a nearby power plant, which can reduce the costs. In Fig. 1, both materials are dried to about 10–20% moisture content prior to pelletisation in order to produce the strongest pellets [1]. The drying process can incorporate the mixing of the two materials. After pelletisation, the pellets are cooled down and then stored.
2.2.
Cases for raw material blends
Table 2 lists the fuel properties and the net calorific value (NCV) calculated for 8 blends of the two materials including coal-tailings-only (Case 1) and SMC-only cases (Case 8). The moisture content in the raw materials is assumed to be 70% for SMC and 40% for coal tailings. These are dried to 20% and 10% respectively prior to pelletisation. The moisture content of the blends was then calculated correspondingly. After drying, the
Fig. 1 – Proposed pellet production process from SMC/coal tailing for cost analysis.
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Table 2 – Cases for various mixture composition Case SMC:Coal tailing (wet) SMC fraction in raw material (wet) Raw material Moisture (%) NCV-wet (MJ/kg) Raw material required (ton/tonpellet) SMC fraction in the pellets Pellet properties Moisture (%) Volatile matter (%) Fixed carbon (%) Ash (%) NCV-pellets (MJ/kg)
1
2
3
4
5
6
7
8
0:1 0% 40 12.08 2.67 0% 10.0 22.6 27.7 39.7 19.34
1:6 17% 45 10.33 1.5 10% 11.0 24.6 25.9 38.5 18.07
1:4 25% 47.5 9.46 1.62 16% 11.6 25.8 24.9 37.7 17.37
1:2 33% 50 8.59 1.68 22% 12.2 27.0 23.8 37.0 16.63
1:1 50% 55 6.84 1.76 36% 13.6 29.9 21.3 35.2 14.99
2:1 67% 60 5.1 1.92 53% 15.3 33.3 18.3 33.1 13.11
4:1 80% 64 3.7 2.12 69% 16.9 36.6 15.4 31.1 11.39
1:0 100% 70 1.61 2.31 100% 20.0 42.9 9.9 27.2 8.36
mass fraction of SMC in the blended pellets (Cases 2 to 7) became lower than that in the raw materials. The NCV of the pellets ranged from 8.36 to 19.34 MJ/kg depending on the mixture composition.
2.3.
Framework conditions
Table 3 shows the framework conditions for the cost analysis. The reference case was based on a plant scale of about 24,000 tonpellets/yr (3 tonpellets/h with an availability of 90%). The production costs were divided into four main elements: raw material, transportation, drying and pelletisation costs.
2.3.1.
Raw materials
Raw material prices (excluding delivery) for SMC and coal tailings were assumed to be low: £1 and £2 per ton, respectively. About 60% of SMC used for soil improvement is currently traded at £0–5/ton [4]. As shown in Table 2, the amounts of raw materials required for the pellet production varied from
1.5 (Case 1) to 2.67 ton/tonpellet (Case 8) depending on the mixture composition and the moisture content.
2.3.2.
2.3.3. Table 3 – Framework conditions for the cost analysis Parameters Reference plant
Raw material costs Transportation costs
Capital costs
Energy consumption costs Operation (personnel) Other costs
Value Plant scale Production rate Plant availability Price of SMC Price of Coal tailing SMC haulage (5 cases) Transportation price Investment costs Currency rate Interest rate Service and maintenance Heat price Electricity price Simultaneity factor
Unit
23,652 3 90
tonpellet/yr tonpellet/h %
1.0 2.0
£/ton £/ton
0, 50, 100, 150, 200 0.074
miles
Transportation
Fig. 2 shows the distribution of the haulage distance from 55 SMC farms to 3 main collieries; Ellington in Northumberland, and Rossington and Maltby in Yorkshire. The majority of the mushroom farms are located within 150–200 miles from the collieries sites. Therefore, the SMC haulage distances considered in this study was 0, 50, 100, 150 and 200 miles. The transportation price was fixed at £0.074 per ton per mile, as shown in Table 3. This figure was estimated from the annual average costs data for a 40-tonne tri-axle combination lorry [5]. This type of vehicle has annual time-related costs of £64,050 and mileage-related costs of £0.567/mile. It was assumed that the vehicle has an annual mileage of 57,600 miles with 25 tonnes of raw material per delivery and the transportation company adds 10% on top of the actual costs as profit.
Heat and electricity consumption
Drying and pelletisation processes consume electricity and heat. The electricity and heat prices shown in Table 3 are for the third quarter of the year 2006 acquired from the surveys carried out for fuel suppliers [6]. A simultaneity factor of 85% was used to compensate for electrical installation not operating at full load all the time.
£/ton/mile
(see Table 4) 1.48 7 (see Table 4)
Euro/£ %
19.22 53.00 85
£/MWh £/MWh %
167,350
£/yr
0.5
%
Fig. 2 – SMC haulage from 55 mushroom farms to Ellington, Rossington and Maltby collieries.
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Table 4 – Technical data and investment costs for drying and pelletisation processes based on Thek and Obernberger [7] Parameters
Drying
Type Power (kW) Heat demand (MWh) Utilisation (yr) Service and maintenance (%) Investment (£)
2.3.4.
Pelletisation Pellet mill
Cooler
Storage
Peripheral
Ring-die 233 0.315 10 10 128,378
Counter flow cooler 12 0 15 2 8,784
Silo 30 0 20 1.5 196,622
90 0 10 2 337,838
Tube bundle dryer 77.5 1.63–5.43 15 2.5 175,626 ~ 361,672
Drying and pelletisation
Table 4 shows the framework conditions considered for the drying and the pelletisation process, based on the technical data and investment costs for wood pellets in Austria and Sweden [7]. For drying of the raw materials, a tube bundle dryer was considered. The capacity of the dryer was calculated from the moisture content of the raw materials given in Table 2, and corresponding dryer size and investment costs were scaled up. The pelletisation costs included storage (silo) and peripheral equipment as well as a pellet mill and a cooler. The pellet mill considered was a ring die. However, further investigation is required to find an ideal pellet mill for the materials and the optimum use of binders. The total investment costs ranged from £0.84 million to £1.03 million between cases as shown in Table 4, depending on the capacity of the dryer. The drying and pelletisation processes consisted of four cost categories: capital, energy consumption, service/maintenance, operation and miscellaneous costs. From the investment costs for each process shown in Table 4, the annual capital costs were calculated using the capital recovery factor (CRF), taking into account an interest rate (i) of 7% and the utilization years (n) [7]. CRF ¼
Total
ð1 þ iÞn i ð1 þ iÞn 1
The maintenance costs were added to the capital costs as a percentage of the investment costs for each process. The personnel costs were calculated for an hourly rate of £13/h, annual hours 8760, 3 shifts with 1 person per shift and 0.25 person for substitution per shift. Then £25,000/yr was added for administration and marketing personnel. This resulted in the annual operating costs of £167,350/yr. The other costs include insurance rates, overall dues, taxes and administration costs.
3.
Results and discussion
3.1.
Costs breakdown
835,951 ~ 1,019,517
Table 5 lists the breakdown of the costs for drying and pelletisation for Case 1 – coal tailing pellets. The annual raw material costs for this case was £0.072 million (£3.0/tonpellets). The cost elements from pellet mill to personnel inTable 5 do not vary between cases since they are dependant only on the material throughput after drying, i.e., 3 tonpellets/h. The total of these fixed costs elements was £19.31/tonpellets. Due to the high moisture content of the raw material, the consumption of heat and electricity for drying was the most expensive element. This led to £15.03/tonpellets for specific drying costs, which is 40% of the total costs. For the same reason, the specific consumption costs were 58% of the total. The total annual costs (including raw material costs for coal tailing pellets) were £0.89 million and the specific costs £37.34/tonpellets.
3.2.
Specific and annual pellet production costs
Fig. 3 shows the specific pellet production costs considering various SMC haulages for the 8 cases with different mixture compositions. As mentioned previously, the costs for the pelletisation process and personnel were fixed at £19.31/tonpellets in all cases. The raw material costs varied from £2.67 to £3.0/ tonpellets, which is the lowest item in the total costs. As the materials are very wet and drying is energy-intensive, the drying process was the major cost factor which ranged from £15.03/tonpellets (Case 1) to £30.96/tonpellets (Case 8). This represented 40% (Case 1) to 58% (Case 8) of the total costs excluding transportation. The pellet production costs increased dramatically with SMC haulage distances for the cases with a higher fraction of SMC. For example, the specific costs for Case 5 (SMC:Coal
Table 5 – Breakdown of annual pellet production costs for Case 1 (Coal tailing pellets) excluding raw material costs for the reference plant scale Process
Drying
Pellet mill
Cooler
Storage
Peripheral
Capital costs Consumption costs Operating costs Other costs Total (£/yr) Specific (£/tonpellets)
23,673 334,173
31,116 130,488
1140 4262
21,509 10,655
54,857 31,966
878 358,724 15.17
642 162,246 6.86
44 5446 0.23
983 33,147 1.40
1689 88,512 3.74
Personnel
167,350 167,350 7.08
Total (£/yr)
Specific (£/tonpellets)
132,296 511,544 167,350 4236 815,426 34.34
5.58 21.50 7.08 0.18 34.34
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3.3.
Fig. 3 – Specific pellet production costs for the cases with different SMC haulages (P: pelletisation including personnel, R: raw material; D: drying costs).
Sensitivity analysis
The sensitivity of the costs to different input parameters is shown in Fig. 5 for Case 5 (SMC:coal tailings = 1:1) with SMC haulage of 100 miles. Each parameter was varied by ±10%. The specific production costs for this case was £50.97/tonpellets. The most sensitive parameter was the moisture content of the raw materials which greatly influenced the transportation, drying and energy consumption costs. Decreasing the moisture contents to 63% for SMC and 36% for coal tailings lowered the costs by £4.4/tonpellets. The heat price was also an important parameter. The buoyant energy price may directly affect the pellet production costs, noting that the UK experienced a 30% rise in the energy price during the last quarter of 2005, which was caused by an unprecedented increase in the wholesale gas price. The interest rate and raw material price were the least sensitive factors. Fig. 5 also shows the sensitivity of the investment costs, which confirms that the effect of the investment cost model adopted from Thek and Obernberger [7] is not significant in this study.
3.4.
Energy price and break-even points
tailings = 1:1) increased from £43.87/tonpellets without SMC haulage to £58.07/tonpellets for 200 miles of SMC haulage. For pure SMC pellets, the specific costs without SMC haulage were £52.94/tonpellets and the transportation costs were £9.87/ tonpellets per 50 miles. Fig. 4 shows the annual costs for all the cases with 50 miles of SMC haulage. The annual production costs varied from £0.88 million to £1.49 million/yr with the energy consumption costs being dominant mainly due to the drying process. The above results clearly show how significant the transportation as well as drying costs could be for wetter materials. For the case with high SMC fractions including Case 8, the two assumptions in this analysis that SMC is transported to a colliery and dried by an energy-intensive tube bundle dryer are not realistic. Lowering the moisture content by other means and its effect on the production costs are discussed in Section 3.6 of this paper.
The break-even points for the pellet price can be acquired for two scenarios. The first scenario assumes that the pellet price is based on that of coal. The average purchase price of coal is £1.53/GJ in England [6]. The second scenario is based on the assumption that the SMC in the pellets is priced the same as biomass while the priced for the coal tailings is still based on the coal price. The market price of biomass varies widely depending on the source of materials, physical form and calorific value from £12/ton for roundwood with N50% moisture excluding delivery [8] to £135/ton for oil seed rape with 9% moisture content [9]. The purchase price of biomass for co-combustion at Drax power station in England is in the region of £3.5–£5/GJ [10]. In the calculation, the biomass portion of the pellets was assumed to be £4.5/GJ. This value matches well with the wood pellet price of about £80/ton at 11% moisture [8].
Fig. 4 – Annual costs of pellet production for the cases with SMC haulage of 50 miles.
Fig. 5 – Sensitivity of costs parameters for Case 5 (SMC: Coal = 1:1) with a SMC haulage of 100 miles.
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work conditions. For Case 5, the costs fell below the breakeven point only when the plant was 10 times larger than the reference plant (excluding the transportation costs). For coal tailing pellets, the production costs were below the break-even
Fig. 6 – Break-even points based on coal and biomass based prices, respectively.
Fig. 6 shows the break-even points based on the above two scenarios. The coal-based price ranged from £29.59 /tonpellets for coal tailing pellets to £19.80/tonpellets for SMC pellets. This scenario is less favourable as the renewable energy from the SMC in the pellets is not accounted for, despite the higher costs for processing when compared to coal tailings. When the pellet price was based on biomass for the SMC in the pellets, the break-even point for Case 8 increased to £37.63/tonpellets. Fig. 6 also shows that the specific pellet production cost without transportation is well above the biomass based breakeven point. Therefore, two ways of lowering the costs by scaling up the plant and using alternative drying processes were investigated as presented in the next sections.
3.5.
Effect of production scale
The scaling up of the plant lowers most of the cost elements from the price of raw materials to the operation costs, although the cost factor for each element is difficult to predict. The following method was used to estimate the capital cost related to the plant capacity:
C2 ¼ C1
n S2 S1
where Ci = capital cost of the project with a plant capacity Si. The cost factor ‘n’ was taken as 0.6, which is the widely used six-tenths rule [11]. The same cost factor was applied to the personnel costs. For heat and electricity prices, the cost factor was 0.95 which was estimated from the average prices for different scales of non-domestic consumers [6]. The scaling up effect for raw material and transportation was not considered in this calculation. Fig. 7 shows the effect of the plant capacity for three cases: (a) pure SMC pellets, (b) coal tailing pellets and (c) the mixture of SMC to coal ratio of 1:1 (Cases 8, 5 and 1, respectively). The biomass based break-even point is also plotted for comparison purposes. The production costs for the SMC pellets did not meet the break-even point for any case for the given frame-
Fig. 7 – Effect of plant scale up. (a) Case 8 (SMC pellets); (b) Case 5 (SMC:Coal tailings = 1:1); (c) Case 1 (Coal tailings pellets).
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point when the plant capacity was 5 times larger than the reference plant.
combustion of low calorific value materials and, more desirably, iii) power production at coal-fired power stations.
3.6.
3.7.1.
Effect of alternative drying processes on the feasibility
The calculation results showed that the pellet production would be very limited within the framework conditions mainly due to high drying and heat consumption costs for the wet materials. Especially, drying the SMC with 70% moisture content by using heat from fossil fuels is not practical. As shown in the sensitivity analysis, lowering the moisture content by other means is essential. One of the solutions is the co-location of the pelletisation plant with a process plant that generates waste heat because drying the raw materials only requires low grade heat. The use of a screw press to lower the moisture content to about 40% can also be considered at the pelletisation plant or even at large mushroom farms before transportation. Table 6 shows how significantly the moisture content influences the production costs when SMC is dried to 40% moisture content at mushroom farms. The specific production costs were down by £19/tonpellets without transportation and by £39/tonpellets for a haulage distance of 200 miles. When the plant scale increased by a factor of 4, the pellet production costs excluding the raw material and transportation, was further reduced to £24.49/tonpellets. This value is well below the biomass based break-even point (£37.63/tonpellets). In this case, the sum of the costs for raw materials and additional drying processes can be up to £10/tonSMC. Therefore, finding an alternative way for less energy-intensive drying is the key to the success of the pellet production from the two residue materials.
3.7.
Potential market outlets
The pellets from SMC and coal tailings are more suitable for use in large-scale plants. The reasons are (a) both SMC and coal tailings have a high ash content (25–40%) to be disposed of after combustion, which is not ideal for small-scale plants; (b) other biomass products are available in the market at lower prices with higher calorific values and lower ash contents than SMC; (c) the production costs depends significantly on the production scale as discussed previously. Therefore, it is important to secure long-term market outlets in industry. The potential uses for the pellets are i) the production of low grade heat for compost producers, ii) use as supplementary fuel for
Table 6 – Comparison of production costs for SMC excluding raw material costs with 70% and 40% of moisture content (plant scale: 23,652 tonpellets/yr) Costs (£/tonpellets)
Drying (D) Pelletisation + Personnel (P) D+P D + P + Transportation (50 miles) D + P + Transportation (100 miles) D + P + Transportation (200 miles)
Moisture content of SMC 70%
40%
30.96 19.31 50.27 60.14 70.00 89.74
11.79 19.31 31.10 36.03 40.96 50.83
Mushroom compost producers
The SMC pellets are suitable for use by mushroom compost producers who require a low grade heat in order to keep the temperature at 50–70 °C during the 3 weeks of the compost production process (phase II compost). The SMC can be collected by the lorries that deliver the fresh compost to the mushroom farms, which can minimise the transportation costs. This option is not available in England as the mushroom compost is mostly being imported from The Netherlands. However, this can be considered in Ireland or The Netherlands where more than several thousand tonnes of the compost are produced every week. Although the calorific value of the SMC pellets is not high, it still can raise steam replacing natural gas or oil. However, the pellet production is not essential. Economic viability suffers because of the proposed market outlets and the drying or pelletisation costs are well above the coal based price (£12.80/tonpellets in Fig. 6). In order to lower the costs, co-combustion of SMC with other biomass without pelletisation can be considered. Furthermore, condensing boilers can be used to increase the energy efficiency for special application.
3.7.2.
Coal tailings as supplementary fuel
One potential market outlet for coal tailings as supplementary fuel is at sewage sludge incinerators using fluidised beds. As sewage sludge is barely autothermal with about 75% or higher moisture content, most sludge incinerators use oil in order to maintain the bed temperature. The fluidised bed configuration is ideal to elutriate the ash from this high ash content (over 40%) fuel. The coal tailings in the pellet form burn within the bed, while releasing the heat in the bed due to the high fixed carbon content. This can help stabilise the furnace operation. Fig. 7(c) shows that this option is economically feasible at large scales.
3.7.3.
Power plant
The most desirable market option for the pellets is the existing coal-fired power stations where the electricity can be produced from the biomass material. Three of Britain's major coal fired power stations (Ferrybridge, Eggborough and Drax) are located in North Yorkshire, each of which is 14 to 43 miles away from Rossington or Maltby collieries. When applying the same transportation costs (£0.074/ton/mile) to the delivery of the pellets, it would only incur an additional £1.0 to £3.2/ton to the projected costs of pellets.
4.
Conclusions
The pellet production costs for SMC and coal tailings in England were investigated for various mixture compositions, raw material haulage distances and plant scales. The main findings are: • Due to low calorific value and high processing costs, the economic feasibility of the two residue materials for energy production by pelletisation is limited. For the reference
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conditions, the pelletisation costs were higher than the current coal or biomass energy prices. However, for a 120,000 tonpellets/yr plant capacity, the coal tailing pellets could meet the break-even point. • The pelletisation costs excluding drying and raw material costs were about £19/tonpellets in the framework condition for England. • The moisture content of the two wet materials was the critical parameter that greatly affected the costs of transportation, drying and energy consumption. Rather than using a heat-intensive dryer, other ways of material drying should be investigated. This includes the use of a screw press and the use of waste heat from a process plant. • The potential market outlets identified for these pellets were the mushroom producers for SMC, sewage sludge incinerators for coal tailings and power plants for the mixed pellets.
Acknowledgements The authors would like to thank Veolia Environmental Trust (grant reference RES/C/6046/TP) for the financial support for this study. Thanks are also due to Leslie Bareham (independent consultant), Dr John L. Burden (Mushroom Analysis and Advice) and Dr Phillip S. Cock for the technical support.
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