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Fuel Pelletization with a Binder: Part I s Identification of a Suitable Binder for Spent Mushroom Compost-Coal Tailing Pellets Karen N Finney,* Vida N Sharifi, and Jim Swithenbank Department of Chemical and Process Engineering, UniVersity of Sheffield, Sheffield S1 3JD, U.K. ReceiVed January 8, 2009. ReVised Manuscript ReceiVed March 27, 2009
Spent mushroom compost and coal tailings are wastes that could be used as energy sources, through combustion in a fluidized-bed. Our previous pelletization studies produced a denser, yet friable fuel, which produced dust on transport, handling, and feeding. Thus, a suitable binder for these materials was required to enhance pellet properties. Inorganic (caustic soda) and organic (starch) binders were selected for a range of tests. Pelletization at elevated temperatures was also assessed to evaluate the softening of any lignin present in the straw component of the mushroom compost. Small amounts (up to 1 wt %) of both binders increased pellet tensile strength, where starch had more pronounced impacts, doubling the tensile strength to 411 kPa. Pelletization at elevated temperatures (45-75 °C), however, improved overall pellet quality more significantly, increasing the tensile strength to over 600 kPa, although additional temperature increases (up to 125 °C) did not result in further improvements. Whether a binder or elevated temperature is used at the pelletization stage is strongly dependent on the comparative costs of these options, relative to improvements in product quality. The effect of these binders on combustion will also be a crucial factor to consider, which is assessed in the second part of this paper.
1. Introduction 1.1. Pelletization and Binders. Pelletization, the process by which solid particles are consolidated, is employed in many industries to form a more durable substance and to enhance material characteristics. As such, it has a range of purposes: (i) to facilitate storage, transport, and handling; (ii) to combine a number of substances; and (iii) to recycle and/or reclaim materials.1 When this is related to fuel pelletization, specifically biomass, however, the primary reason is to increase the bulk and energy densities of the material to be somewhat more comparable to that of coal.2 If pressure alone does not produce fuel of sufficient quality, binders can be used to improve pellet properties and achieve appropriate fuel standards. Binders have the ability to enhance agglomeration properties and are often added to materials for the purpose of reducing the adversity of pelletization conditions that would otherwise be required. This includes reducing the pressure and/or temperature needed to form good quality pellets. Binding agents are often inherent within fuels, such as lignin in biomass or tar in coal, which are heat-softening. These can be activated with the use of elevated temperatures to improve agglomeration and thus pellet quality, specifically the compressive strength and density. 1.2. Spent Mushroom Compost and Coal Tailings as Fuels. Spent mushroom compost (SMC)san agricultural wastesand coal tailings, the unwanted substances washed off in coal cleaning processes, have the potential to be combined * Corresponding author. Phone: +44-114-222-7528. Fax: +44-114-2227501. E-mail:
[email protected]. (1) Lyne, C. W.; Johnston, H. G. Powder Technol. 1981, 29 (2), 211– 216. (2) Li, Y.; Liu, H. Biomass Bioenergy. 2000, 19, 177–186.
into fuel pellets and used as an energy source, for example, through combustion in a fluidized-bed.3,4 The waste compost from mushroom growing farms is comprised of two layers: a substrate and a casing layer. The substrate is composed primarily of straw, gypsum, horse manure, poultry litter, and lime that are mixed and composted, after which a peat casing layer is added to the surface for mushroom cultivation.5,6 Once the nutrients have been consumed, the SMC is disposed of in landfill sites or is reused as an agricultural fertilizer, which are both unsustainable and harmful to the environment. The general environmental implications of landfilling waste are well-known. Spreading SMC on agricultural land can have potential detrimental effects on local watercourses, as the phosphorus and nitrates that are common in fertilizers leach into local river networks causing eutrophication7sthe process whereby nutrients enrich the water, enabling excessive plant growth, in particular, algae, which reduces the dissolved oxygen content of the water, thereby inhibiting the development of fish and other organisms that requite oxygen. Furthermore, the generation of SMC far exceeds its demand as a fertilizer, where the current generation rate in the UK is 200 000 tons/ annum, approximately 5 kg for every 1 kg of mushrooms.7 Due to the lack of sustainable waste management solution, this is (3) Finney, K. N.; Ryu, C.; Sharifi, V. N.; Swithenbank, J. Bioresour. Technol. 2009, 100 (1), 310–315. (4) Finney, K. N.; Sharifi, V. N.; Swithenbank, J. Renewable Energy 2009, 34 (3), 860–868. (5) DEFRA: Department for Environment, Food and Rural Affairs. Process Guidance Note 6/30(06): Secretary of State’s Guidance for Mushroom Substrate Manufacture [Online]; Available: http://www.defra.gov.uk/environment/airquality/lapc/pgnotes/pdf/pg6-30.pdf, 2006. (6) Iiyama, K.; Stone, B. A.; Macauley, B. J. Appl. EnViron. Microbiol. 1994, 60 (5), 1538–1546. (7) Williams, B. C.; McMullan, J. T.; McCahey, S. Bioresour. Technol. 2001, 79, 227–230.
10.1021/ef900020k CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
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Table 1. Table of the Optimum Conditions Used to Form the Best Quality SMC/Coal Tailing Pellets variable
conditions
initial moisture content drying minimum pressure maximum pressure holding time SMC: coal tailing ratio
10-11% fully air-dried 2500 psi (17.2 MPa) 6000 psi (41.4 MPa) 20 s 50:50
the most significant barrier to the future development of this industry. Reusing the SMC as a fuel, however, could improve the outlook. The second waste productscoal tailingssis formed when coal is separated from its impurities by crushing, screening, and washing. The ponds and lagoons where it is deposited are usually located in close proximity to the mining area. These have many associated risks, such as environmental contamination and lagoon failure, which necessitate their careful management; removing deposits from these locations and reusing them as a fuel will eliminate these hazards. The characterization and pelletization of these wastes have been previously considered.3,8 As the bulk and energy densities of both materials are appreciably lower than for other fuel resources, pelletization is necessary, particularly for use in a fluidized-bed combustor. The materials have significant ash and moisture contents, thus they require drying prior to pelletization and certainly before they are used as fuels, since this has severe impacts on their calorific values (CV). On an industrial scale, the waste heat generated from the use of these fuels can be used for drying the materials; alternatively, a separate drying system could be employed, although this may only be economically viable in larger-scale plants due to economies of scale. The cost of drying fuels, which often constitute a considerable proportion of overall fuel processing costs (particularly those with high moisture contents), depends also on the drying system employed.9 On the basis of the data from Thek and Obernberger,9 the drying costs of the coal tailings alone (∼40% initial moisture content to 10%) would be in the region of £15/t of pellets, derived from energy consumption costs for a heat price of approximately £19/MWh (from 2006 figures); for SMC (65-70% moisture content, reduced to around 20%), these costs would increase to over £30/t of pellets.10 The materials in this case were dried naturally in a well-ventilated laboratory prior to pelletization and then fully air-dried after, again without any forced heating. The pelletization studies of these wastes tested a range of variables, including pressure, moisture, and SMC/coal tailing ratio,8 from which optimal pelletization conditions were established (Table 1). Although these pellets had considerably greater densities than the initial material, they were friable and produced significant quantities of dust during handling. As such, a binder was required to further enhance their properties, to improve their suitability to feeding via a screw mechanism into a fluidizedbed combustor. As straw is contained within the SMC, it was possible that heating the material could activate any lignin present to act as an inherent binder. The identification of a suitable binder, whether applied or inherent, was the primary aim of this project, achieved through a quantification of improvements to pellet quality. Since these pellets are to be used as a fuel, it was vital that the binder chosen did not (8) Ryu, C.; Finney, K.; Sharifi, V. N.; Swithenbank, J. Fuel Process. Technol. 2008, 89, 269–275. (9) Thek, G.; Obernberger, I. Biomass Bioenergy. 2004, 27, 671–693. (10) Ryu, C.; Khor, A.; Sharifi, V. N.; Swithenbank, J. Fuel Process. Technol. 2008, 89, 276–283.
detrimentally impact combustion, thus this was also evaluated and is detailed in the subsequent paper.11 2. Experimental Procedure and Materials 2.1. Inorganic Binder: Caustic Soda. Caustic soda, also known as anhydrous sodium hydroxide or NaOH, is an inorganic alkali binding agent that has been the focus of pelletization tests, although only one example could be found specifically relating to fuel pelletization. Zhang, et al.12 examined differing biomass treatments to form a binder for lignite briquettes, where rice straw was treated with a number of chemicals (sulfuric acid, lime, and sodium hydroxide) and was then used as a binder. They found that the type and concentration of chemical was influential; furthermore, the binding method of different chemicals varied, and the temperature and heating were found to be prominent influences. Of the chemicals tested, lime and sodium hydroxide were superior and supplementary additions of bentonite, coal tar and/or polypropylene amide made the pellets more water resistant. Sodium hydroxide was able to bond in both its solid and liquor forms. Further studies have investigated the pelletization of a range of other materials, including sheep feed13 and flyash.14 The results of these investigations showed that only small amounts of the caustic soda binder was required to improve pellet quality, namely pellet strength and hardness. These properties could be further enhanced by using high temperatures. The caustic soda used for these experiments was in pellet form and required crushing prior to be being mixed with the SMC and coal tailings. The NaOH had a minimum purity of 97.5%, with small amounts of potassium (up to 0.1%) and carbonate (up to 2%) impurities. Care must be taken when handling NaOH, particularly when it comes into contact with moisture, due to it is corrosive nature. It is thus classified as a hazardous chemical.15 2.2. Organic Binder: Starch. The second potential binder was starch. This is an organic material, which is often used as a biological binding agent for a variety of purposes; it has pharmaceutical applications and uses in fuel and animal feed pelletization. Here, only literature in the context of biomass fuel pelletization is considered. Tabil, et al.16 evaluated the performance of different binders, with the aim of improving the quality of alfalfa pellets, to reduce dust production during transportation and handling. A range of binders were compared: lignosulfonate, bentonite, pea starch, collagen protein, and hydrated lime. All enhanced the durability and hardness of low-quality chop, but did not improve the durability of medium- or high-quality alfalfa. Further testing revealed that just 0.5% of pea starch or hydrated lime could increase pellet durability. Obernberger and Thek17 tested various densified biomass fuels (pellets for combustion) from across Europe, to investigate pellet quality and European standards for pellet production in this context. The pellet parameters that were analyzed included dimension, density, ash content, gross and net CV, ultimate analysis, and the determination of heavy metals. Additionally, the starch content was established to provide an indication of the inclusion of biological binding agents during pellet production, which was added to aid particle binding, to obtain higher abrasion resistance and to reduce the cost of pelletization. The average starch content was found to be 0.22 wt % for wood pellets on a dry basis, with the maximum at 1.25 wt %. They concluded that biological binders (11) Finney, K. N.; Sharifi, V. N.; Swithenbank, J. Energy Fuels http:// dx.doi.org/10.1021/ef900021t. (12) Zhang, X.; Xu, D.; Xu, Z.; Cheng, Q. Fuel Process. Technol. 2001, 73 (3), 185–196. (13) McManus, W. R.; Mathews, M. G.; Choung, C. C. Wool Technol. Sheep Breeding 1975, 22 (3), 31–36. (14) Yoo, J. G.; Jo, Y. M. Fuel Process. Technol. 2003, 81, 173–186. (15) Health and Safety Executive. Domestic Production of Biodiesel s Health and Safety Warning [Online]; Available: http://www.hse.gov.uk, 2008. (16) Tabil, L. G.; Sokhansanj, S.; Tyler, R. T. Can. Agric. Eng. 1997, 39 (1), 17–23.
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Energy & Fuels, Vol. 23, 2009 3197 and the pellet compressive strength (CS, in N), which was assessed using the Brazilian test. For this, a pellet was placed between the two plates of the Monsanto tensometer and pressure was applied. The maximum radial force that the pellet can withstand was recorded before the pellet failed, by deformation or fracture (breaking/cracking). The radial forces measured here would be lower than the axial forces that pellets of this shape (cylindrical) could withstand. The following equation was employed to convert the values obtained:
ΤS )
2CS πLd
(1)
where L is the pellet length and d is the pellet diameter, both in mm. The data for compressive strength, diameter, length, and density were then manipulated to calculate the maximum height of a pile, relating it to the pressure and weight the pellets can withstand, by employing the following equations. Initially, the overall bulk density of the pellets (FP, in kg/m3) was required along with d and L; from this, the mean weight per pellet (WP) in kg was calculated:
Figure 1. Schematic of the compression pelletizer used to form the pellets.
like starch need only be used in small quantities (up to 2 wt %) in the production of fuel pellets. A maize-based starch was used for this experimentation. Initially in powder form, it was mixed with a small amount of water (∼5 mL per 1.25 g) to form a paste before being incorporated into the SMC-coal tailing mixture. 2.3. Compression Pelletiser. Pellets were manufactured using a compression pelletizer, based on a hydraulic press (Figure 1). 25 g of these wastes were weighed and placed into the 26.8 mm diameter cylindrical mold at the center of the pelletizer. The pressure inside the stainless steel column could reach a maximum of 9338 psi (64.4 MPa). Two 250 W rope heaters (OMEGA FGR-060) surrounded the vertical shaft and were set to specific temperatures, which were monitored using a K-type mineral insulated thermocouple. 2.4. Pelletization with Binders. The use of both caustic soda and starch were assessed as binders for 50:50 wt % SMC-coal tailing pellets. The literature discussed above shows that only a small percentage of these binders are required to impact pellet quality. Various amounts of each binder were added to the mixture prior to pelletization, ranging from 0.5 to 2 wt %. The pellets were made according to the pelletization parameters outlined in Table 1, using the maximum pressure suggested. 2.5. Effect of Temperature. As considered above, elevating the temperature may facilitate the agglomeration of particles, especially if a heat-softening binder has been applied or is inherent to the substance.18 As SMC is a biomass, composed primarily of straw, lignin and cellulose may be present, which would soften and act as binders if pelletization occurred at elevated temperatures.19 The rope heaters surrounding the vertical shaft were set to a specific temperature ranging from room temperature (RT ) 15 °C) to a maximum of 125 °C, using the control gauge (Figure 1). Pellets of SMC and 50:50 SMC/coal tailing mixtures were made using the same pelletization conditions outlined above. 2.6. Assessment of Pellet Quality. In all cases, pellet quality was determined by assessing pellet density, tensile strength, pellet pile-up studies, and durability, where single factor (one-way) ANOVA tests were performed to assess the statistical significance between results. The tensile strength (TS, in kPa)san indicator of cohesion related to pellet size and shapeswas calculated from the pellet dimensions (17) Obernberger, I.; Thek, G. Biomass Bioenergy. 2004, 27, 653–669. (18) Moore, J. E. Processing Applications for roll-type briquettingcompacting machines. In Elements of Briquetting and Agglomeration; Messman, H. C., Tibbetts, T. E. Eds.; Institute for Briquetting and Agglomeration: Las Cruces, 1977; Ch 4, pp 35-46.
WP )
FPLπd2 4/1 000 000
(2)
By utilizing Newton’s Second Law of Motion, the maximum weight (W, in kg), was computed:
CS 9.8
W)
(3)
The number of pellets (PNo) in a pile of maximum height that those at the bottom can withstand was thus determined by:
ΡNo. )
W WΡ
(4)
Using this value and the pellet diameter, the maximum height of the pellet pile (HP, in m) was calculated:
ΗΡ )
ΡNo.d 100
(5)
The durability of pellets was assessed to determine the most robust and indicated the ability of the pellets to absorb mechanical shocks and withstand abrasion, friction, and vibrations, simulating the conditions experienced during transportation and handling. For each case, five pellets (restricted by drum capacity) were placed in the 100 mm × 200 mm cylinder with a small baffle and mounted into a lathe, which rotated at 40 rpm. The drum was rotated for 20 minsa total of 800 revolutionssand the weight of the pellets that were intact and the larger pieces were measured at 5 min intervals; this was repeated for each case. This is based on the prototype testing method of Temmerman, et al.20,21 This consisted of a rotating, cylindrical drum containing a baffle, with sieving and weighing the mass at regular intervals. Although there are a range of other testing methods to determine pellet durability, based on the constraints concerning both the project scope (limitations on finances and time scale) and the materials (availability), this was the most suitable technique. Temmerman, et al.20 explore some of these other methods, including prototype techniques (upon which the method herein was based) and standards tests. ASAE S 269.4 uses a tumbling rectangular ASAE drum with a baffle, whereas ¨ NORM M 7135 incorporates an air stream to swirl the particles O (19) Asplan Viak; European Biomass Association; FastBra¨nslePannor; Lambelet Heizungssysteme; Technische Universita¨t Mu¨nchen; Umdasch AG; Whitfield GmbH; UMBERA. Woodpellets in Europe [Online]; Available: http://www.energyagency.at/publ/pdf/pellets_net_en.pdf, 2000. (20) Temmerman, M.; Rabier, F.; Jensen, P. D.; Hartmann, H.; Bo¨hm, T.; Rathbauer, J.; Carrasco, J.; Fernandez, M. Durability of Pellets and Briquettes: RTD Results and Status of Standardisation, Standardisation of Solid Biofuels [Online]; Available: http://www.energetik-leipzig.de/BioNorm/conference/4/Temmerman.pdf, 2004.
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Figure 2. Graph showing the impacts on the tensile strength and density of SMC-coal tailing pellets for (a) the caustic soda binder and (b) the starch binder. The 95% confidence limits are shown as error bars.
within a four-sided pyramidal chamber. In both cases, durability is expressed as the average percentage mass remaining of the total sample. This was thus how the results herein are reported.
3. Results and Discussion 3.1. Comparison of Inorganic and Organic Binders. Up to 1 wt % of both binders improved the tensile strength (an assessment of pellet cohesion relative to size), but further additions weakened the pellets (Figure 2). Opposite trends were seen for the density in both cases. Starch increased the tensile strength more so than the caustic soda. The caustic soda is able to bond to the materials to aid agglomeration and improve pellet quality;11 however, as seen this is not as effective as the starch binder. Starch, which appears better able to provide this binding function, is thought to assist particle cohesion by solubilization and crystallization within the pellet, in addition to physicochemical changes through additions of water (∼5 mL addition per 1.25 g of starch was used here) and/or heat (as examined below).22-24 (21) Temmerman, M.; Rabier, F.; Jensen, P. D.; Hartmann, H.; Bo¨hm, T. Biomass Bioenergy 2006, 30 (11), 964–972.
As seen from the 95% confidence limits in Figure 2, there were noteworthy variations in the results, due to the lack of homogeneity in the materials. Consequently, single-factor ANOVA tests were used to assess the significance in the differences between the conditions; 0, 0.5, and 1.0 wt % of the binders were compared. The only significant difference was found to be for tensile strength, between 0 and 1.0 wt % of starch. Using only a small amount of these binders is beneficial, as quality can be dramatically improved with minimal costs, making these binders economical. An estimation of the maximum pellet pile height was established using eqs 2-5 (Table 2). Both binders can be used to improve the pellet properties and increase the heights of the pellet piles. Larger improvements were noted with additions of the organic starch binder, compared to the caustic soda. The height of the pellet pile can be increased (considered further below), almost doubled using 1 wt % of starch, without damaging those at the bottom. Based on these results, the (22) Thomas, M.; van der Poel, A. F. B. Anim. Feed Sci. Technol. 1996, 61, 89–112. (23) Thomas, M.; van Zuilichem, D. J.; van der Poel, A. F. B. Anim. Feed Sci. Technol. 1997, 64, 173–192.
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Figure 3. Scatter-plots of the effect of pelletization temperature on the tensile strength and density of (a) SMC pellets and (b) SMC-coal tailing pellets. The 95% confidence limits are shown as error bars. Table 2. The Maximum Heights of 50:50 SMC-Coal Tailing Pellet Piles Using Caustic Soda and Starch Binders binder (%) none
caustic soda
starch
variables and parameters
0.00
0.50
1.00
2.00
0.50
1.00
2.00
pressure (psi) pellet length (mm) density (kg/m3) compressive strength (N) weight (kg) number of pellets height of pellet pile (m)
6000 33.8 1224 332 33.9 1449 38.85
6000 38.7 1162 531 54.2 2139 57.33
6000 38.6 1172 587 59.9 2347 62.92
6000 39.4 1169 461 47.0 1811 48.55
6000 36.4 1187 540 55.1 2262 60.62
6000 37.6 1148 650 66.3 2725 73.05
6000 36.5 1190 407 41.5 1693 45.39
optimum percentage of each binder (1 wt %) was chosen and pellets were made to assess their durability. Pellets made with caustic soda were the most durable (with 40.4% mass remaining after 20 min), whereas starch slightly decreased the durability of pellets (31.9%) compared to those without a binder (32.8%). 3.2. Effect of Pelletization at Elevated Temperatures. Pellets made at a range of elevated temperatures were compared to those made under the same conditions at room temperature (15-20 °C). As seen in Figure 3, pellet quality was improved in terms of density and tensile strength with temperature elevations. This was true for both pellet compositions, although it was more pronounced for the SMC, as the largest improvements were seen here. As explored above, it is thought that the
lignin and cellulose present in the biomasssin this case, the straw component of the SMC substratessoftens to aid the agglomeration of particles by filling void spaces within the densified pellet, which then cools and solidifies. This is thought to be similar to the effects of tar-binding during the pelletization of coals at elevated temperatures.25 On the basis of these results, the optimum pelletization temperature is around 75 °C, although vast improvements are seen at just 45 °C, particularly for the SMC, as there would be a larger proportion of lignin and cellulose present. (24) Thomas, M.; van Vliet, T.; van der Poel, A. F. B. Anim. Feed Sci. Technol. 1998, 70, 59–78.
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Table 3. Results for the Maximum Heights of Pellet Piles at Different Pelletization Temperatures, Comparing SMC Pellets and SMC-Coal Tailing Pellets pelletization temperature (°C) variables and parameters
RT
pressure (psi) SMC-coal tailings pellet length (mm) density (kg/m3) compressive strength (N) weight (kg) number of pellets height of pellet pile (m)
6000 100:0 38.1 965 675 69.0 3322 89.0
45 6000 100:0 34.9 1195 2440 249.0 10581 283.6
75 6000 100:0 37.7 1200 2960 302.0 11835 317.2
100
125
RT
45
75
100
125
6000 100:0 33.8 1238 2630 268.4 11376 304.9
6000 100:0 38.8 1168 2490 254.1 9948 266.6
6000 50:50 33.8 1224 332 33.9 1449 38.9
6000 50:50 35.9 1232 710 72.5 2905 77.9
6000 50:50 33.8 1243 930 94.9 4000 107.2
6000 50:50 33.5 1308 770 78.6 3174 85.1
6000 50:50 38.2 1204 575 58.7 2261 60.6
The confidence limits in Figure 3 show the variation in the results were large, particularly for the tensile strengths of the SMC pellets, although the averages suggest sizable differences between the temperatures tested; this variation is primarily due to the heterogeneity of the materials. As such, ANOVA tests were completed to assess the statistical differences between the improvements in densities and tensile strengths of pellets made at RT, 45, and 75 °C. Despite the apparent large variation in results, particularly in the tensile strengths, the ANOVA tests indicated there were statistically significant differences between the densities and tensile strengths of SMC pellets made at RT and 45 °C, and also between those made at RT and 75 °C. No difference was found however between those made at 45 and 75 °C, indicating that 45 °C could be used to similar effect, but with incurring fewer costs. The results for SMC-coal tailing pellets showed no statistical differences in the densities or tensile strengths at any temperatures. The maximum heights of the pellet piles were then computed based on these results (Table 3). As suggested by the tensile strengths and densities, pellets made at elevated temperatures (45 and 75 °C) should be able to withstand significantly higher pellet piles without being damaged. SMC pellets performed consistently better than the mixture of these wastes, particularly at 75 °C, where pellets are thought to be able to withstand pile heights in excess of 300 m. Piles of this magnitude would obviously not be practicable, but indicate that storing these pellets in piles would not be detrimental to their quality. This temperature also considerably increased the pile height for SMC-coal tailing pellets, although not to the same extent. As seen from this table, high tensile strengths are required to stack the pellets, although depending on the type of storage, durability may also be a factor, particularly delivery to the storage facility. There are a range of storage options for such pellets; these include: (i) simple open/covered stores, which are cheap and fuel delivery is straightforward; (ii) underground bunkers, which make fuel delivery easy but are only economic for large-scale systems; (iii) hook-lift bins, which are practical if there is restricted storage space but can cause difficulties with delivery; and (iv) hoppers, which also complicate delivery options and require specific machinery, but are often used when there is limited space available.25 The use of any of these options would clearly restrict the maximum pellet pile height and thus the optimum height of the fuel pile would be lower than both the maximum pile heights calculated here and the height limited by the storage facility. Durability experiments compared pellets made at room temperature to those made at the optimum and elevated temperatures, 75 and 125 °C, respectively, for both SMC and SMC-coal tailing mixtures (Table 4). Pellets made at 75 °C were more durable, losing significantly less mass over time, (25) Jones, D. C. R. Char Briquettes: An Outline of the Theory and Practice of Manufacture s A Record of the National Coal Board’s Research and DeVelopment; National Coal Board.: London, 1969.
Table 4. Durability of SMC and SMC-Coal Tailing Pellets Made at a Range of Temperatures pellet composition SMC SMC-coal tailings
temp (°C)
durability: % mass remaining
RT 75 125 RT 75 125
78.57 93.96 76.25 32.54 54.85 22.40
for both compositions compared to those made at room temperature. As above, further heating does not produce additional improvements in pellet quality, thus extreme heating (∼125 °C) of the materials is disadvantageous, both in terms of pellet quality and energy expenditure (and consequently pelletization costs). SMC pellets were more durable than the mixture, the quality of which was also reflected in the results of the tests above. These studies demonstrated that pelletization at elevated temperatures (up to 75 °C) was beneficial to pellet quality in terms of density, tensile strength, pellet pile-up, and durability for these materials. At these temperatures, it was thought that any lignin present in the SMC softened and acted as a binder. Increasing the temperature beyond 75 °C, however, did not improve pellet quality further. As lower temperatures are cheaper to maintain, slight temperature increases are convenient in terms of pellet quality and pelletization costs. 3.3. Effect of Pelletization at Elevated Temperatures with a Binder. On the basis of the optimum temperature (75 °C) and an optimum amount of the binders (1 wt %), determined above, tests were carried out to investigate the combination of these, to see if further improvements could be made to pellet quality. The use of the binders coupled with elevated temperatures produced improvements in pellet quality compared to the use of these binders alone, although pelletization using only elevated temperatures was generally superior (Table 5). Durability and density, however, were seen to improve for both binders at 75 °C. ANOVA tests showed that the improvements seen with the use of temperature and the binders (compared to the binders used at room temperature) was only significant for density, not tensile strength. 3.4. Determining the Optimum Pelletization Conditions. On the basis of the results of the previous investigation into pelletization parameters for these wastes and the data gained from the above experiments, the optimum pelletization conditions for SMC and coal tailings can be determined. The addition of 1 wt % of a binder to the SMC-coal tailing mix can further enhance the quality of this fuel, when pellets are manufactured according to the conditions outlined in Table 1. Starch, in particular, improved the tensile strength and pellet pile-up results, the mechanisms of which were explained above, although neither binder improved pellet density. The use of elevated temperatures, even just 45 °C, dramatically improved both the tensile strength and density of pellets significantly,
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Table 5. Results for Density, Tensile Strength, Pellet Pile-up and Durability of SMC-Coal Tailing Pellets Made with a Binder at Elevated Temperatures binder
amount of binder (%)
temp (°C)
density (kg/m3)
TS (kPa)
max height of pile (m)
durability: % mass remaining
caustic soda
0.0 1.0 0.0 1.0 0.0 1.0 0.0 1.0
RT RT 75 75 RT RT 75 75
1224.1 1171.6 1242.7 1285.7 1224.1 1147.5 1242.7 1296.1
233 360 609 400 233 411 609 463
38.85 62.92 107.21 63.64 38.85 73.05 107.21 72.88
32.79 40.38 54.85 62.17 32.79 31.94 54.85 67.57
starch
particularly for the SMC alonesmore than the use of these binders. Durability was most improved using a binder at elevated temperatures, although this combination did not further improve pellet tensile strength. As considered above, high compressive/tensile strengths are required for storing pellets without damaging them, as they are often stacked in large volumes. When the pellets are stored, it is important that the pile heights are lower than the maxima outlined in Tables 2, 3, and 5, to ensure that pellet quality is not compromised, although many of these heights would be improbable for covered storage. Pellet pile height would also be restricted by the storage facility, as considered above. Storage for a lengthy period of time is not recommended, as it can be detrimental to pellet quality, particularly durability.27 For transportation, handling, and feeding, for example using a screw feeder, durability (tested by resistance to abrasion and mechanical impacts) plays more of an important role. Extensive handling and feeding phases can be detrimental to pellet quality, particularly if pellet durability is not high. This can lead to the formation of dust, which may cause mechanical problems, as well as lead to a reduction in the amount of fuel that can be viably fed into the combustor. It is vital that the conditions used for pelletization produce appropriate results for both quality assessment criteria (tensile strength and durability) to minimize difficulties with all aspects of storing and using these pellets that may be detrimental to pellet quality. When investigating this for SMC-coal tailing pellets, elevated temperatures (45-75 °C) improved fuel quality the most, in terms of both tensile strength and durability. Despite the superior influences of pelletization at elevated temperatures, the process economics, as explored below, are likely to be the driving factor in the decision making. 3.5. Economics. Adding any preprocessing phase to the utilization of fuels is not economically favorable, although it may be a necessity. The costs associated with pelletization increase with the complexity of the process. Although a simple pelletization operationsfor example with no binder or elevated temperaturessis cheap, the best quality pellets are not necessarily formed,17 as seen herein. These binders are thought to be cost-effective, in that they are cheap to procure and are easy to apply (can be simply combined with the two materials during the mixing phase) while improving pellet qualities, such as tensile strength. On a commercial scale, the cost of 50:50 wt % ratio SMC-coal tailing pellet production (based on capital, consumption, operatingsincluding the dryer and pelletization stagessand maintenance costs) is thought to be about £44/t of pellets excluding transportation costs, increasing to £58/t of pellets with transporting the SMC 200 miles.10 The cost of drying these materials was examined above. The energy consumption during the pelletization phase of operations is not usually (26) Hodsman, L.; Smallwood, M. Woodfuel Heating in the North of England: A Practical Guide [Online]; The National Non-Food Crops Centre; Available:http://www.gos.gov.uk/497763/docs/199734/199731/331496/ 493847, 2004.
very high, although this does depend on the pressure and complexity of the operation. The specific electricity consumption can vary widely, dependent on plant capacity; for a small-scale wood pellet production facility, it is thought to exceed 150 kW h/t of pellets.25 What is often the primary factor affecting the overall energy consumption is the degree of drying requiredsthe amount of moisture in the initial materials compared to the most favorable moisture level. Specific heat consumption can be in excess of 1000 kW h/t of pellets, which is predominantly used for drying.28 While drying and pelletization seems to already be a costly operation, as the moisture contents for SMC and coal tailings are quite high (65-70% and ∼40%, respectively), the use of elevated temperatures in addition to this would considerably increase overall production costs, through greater process complexity resulting in higher capital and operating costs. The extra expenditure used for generating such temperatures could be recovered by utilizing lower pressures, which is a common advantage of pelletization at elevated temperatures. The use of either elevated temperatures or a binder at the pelletization stage is strongly dependent on the comparative costs of these options and the degree of improvement required in product quality. 3.6. Legislation and Policy. There is much legislation in place concerning renewable energy in general, and the use of biomass and biomass wastes specifically. As the SMC portion of the fuel pellets considered here would be classified as a “biomass waste”, many of these policies are directly relevant to the use of these fuels. The Renewables Obligation Order, first introduced in 2002, sets targets to licensed energy providers to increase the amount of electricity generated from renewable resources; for 2015-2016, the target is 15.4%. Each MW h of electricity generated from a renewable source results in the issuing of a certificatesa Renewables Obligation Certificate (ROC). Biomass and biomass-derived energy play a significant part, where the incineration and other treatments (including pyrolysis, gasification, and anaerobic digestion) of biomass and biomass wastes, such as the SMC used herein, would be eligible to qualify. Cofiring this with coal tailings, which are fossil-fuel based, would, however, cease to be eligible after 2016. Despite this, the Renewables Obligation coupled with the Climate Change Levysa support mechanism, whereby renewable energy and CHP are exempt from charges enforced on other energy sourcessprovides a monetary support mechanism to the renewable energy industry and its development. Although the SMC alone performed better in the majority of tests, and often exceptionally so, the superior CV of the coal tailings necessitates its use in these pellets. The SMC has a net CV of 13.0 MJ/kg, compared to the coal tailings, which has a (27) Lehtikangas, P. Biomass Bioenergy. 2000, 19, 287–293. (28) Thek, G.; Obernberger, I. Wood pellet production costs under Austrian and in comparison to Swedish framework conditions. In Proceedings of the 1st World Conference on Pellets [Online]; Swedish Energy Association: 2002, pp 123-128; Available: http://www.bios-bioenergy.at/ uploads/media/Paper-Thek-PelletProductionCosts-2002-05-15.pdf
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considerably greater net CV of 19.2 MJ/kg; its inclusion increases the overall energy content of the pellets to 16.11 MJ/ kg. As such, a 50:50 wt % ratio of these is thought to provide a balance of suitable properties, in terms of pellet quality, energy content, and ROC/other renewable energy legislation eligibility. 4. Conclusions This study compared inorganic and organic binding agents, in addition to the use of elevated temperatures for the formation of high-quality SMC-coal tailing fuel pellets. Up to 1 wt % of the starch binder was more suitable in improving the pellet quality (tensile strength and pellet pile-up) of these wastes, compared to caustic soda. Although the use of elevated temperatures (45-75 °C) with these binders further improved pellet quality, specifically durability, temperature on its own produced more noticeable improvements in tensile strength and density, as the softened lignin aided agglomeration. The comparative costs of these options relative to their improvements will ultimately determine which is employed to enhance the quality for the handling and transporting of these fuel pellets. Future work in this area could address the issues (financial and time) concerning the assessment of pellet durability. A more
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in-depth investigation into this aspect of pellet quality could provide a comprehensive knowledge of these binding mechanisms. Acknowledgment. The authors thank the Engineering and Physical Science Research Council (EPSRC) and Veolia Environmental Trust (grant reference RES/C/6046/TP), who have funded this project. Maltby Colliery, Dr. John L. Burden, and Monaghan Mushrooms Ltd. have also made this work possible by providing SMC and coal tailing samples.
Appendix Acronyms and Nomenclature: CS ) compressive strength (N) CV ) calorific value (MJ/kg) d ) pellet diameter (mm) HP ) maximum height of the pellet pile (m) L ) pellet length (mm) PNo. ) number of pellets in the pile RT ) room temperature SMC ) spent mushroom compost TS ) tensile strength (kPa) W ) maximum total pellet weight (kg) WP ) mean weight per pellet (kg) FP ) pellet bulk density (kg/m3) EF900020K
