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The reuse of spent mushroom compost and coal tailings for energy recovery: Comparison of thermal treatment technologies
Bioresource Technology 100 (2009) 310–315
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The reuse of spent mushroom compost and coal tailings for energy recovery: Comparison of thermal treatment technologies Karen N. Finney *, Changkook Ryu, Vida N. Sharifi, Jim Swithenbank Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Article history: Received 7 January 2008 Received in revised form 21 May 2008 Accepted 23 May 2008 Available online 14 July 2008 Keywords: Energy recovery Spent mushroom compost Coal tailings Thermal treatment technologies
a b s t r a c t Thermal treatment technologies were compared to determine an appropriate method of recovering energy from two wastes – spent mushroom compost and coal tailings. The raw compost and pellets of these wastes were combusted in a fluidised-bed and a packed-bed, and contrasted to pyrolysis and gasification. Quantitative combustion parameters were compared to assess the differences in efficiency between the technologies. Fluidised-bed combustion was more efficient than the packed-bed in both instances and pellet combustion was superior to that of the compost alone. Acid gas emissions (NOx, SOx and HCl) were minimal for the fluidised-bed, thus little gas cleaning would be required. The fuels’ high ash content (34%) also suggests fluidised-bed combustion would be preferred. The Alkali Index of the ash indicates the possibility of fouling/slagging within the system, caused by the presence of alkali metal oxides. Pyrolysis produced a range of low-calorific value-products, while gasification was not successful. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To mitigate the impacts of our growing demand for energy and the increase in waste production, it is possible to use many wastes as fuel resources – two of these wastes include spent mushroom compost, hereinafter SMC, and coal tailings. The formation of the mushroom compost substrate takes place in stages: (i) pre-wetting and mixing of straw, gypsum, horse manure, poultry litter, peat, lime and activators; (ii) phase 1 composting in windrows and bunkers; and (iii) phase 2 composting involving pasteurisation and conditioning. After this, a peat casing layer is added to the surface for mushroom cultivation (spawning, casing, pinning and cropping), after which it is cooked out and discarded (DEFRA, 2006; Iiyama et al., 1994). Disposal is problematic, unsustainable and harmful to the environment, since the majority goes to landfill or is used as agricultural fertilisers, where leaching can be a serious issue for local water courses from both sources, as phosphorous and nitrates cause eutrophication. For every 1 kg of mushrooms grown, approximately 5 kg of SMC is produced. The generation rate of SMC is about 200,000 tonnes/annum in UK alone. The lack of sustainable waste management solution for SMC is the most significant barrier to the future development of this industry. Previous studies into the use of SMC as a renewable fuel propose that SMC can be combusted in a bubbling fluidised-bed to generate power with high efficiency (McCahey et al., 2003; Wil* Corresponding author. Tel: +44 114 2227578; fax: +44 114 2227501. E-mail address: cpp06knf@sheffield.ac.uk (K.N. Finney). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.05.054
liams, 2001; Williams et al., 2001; BioMatNet, 2004). The calorific value (CV) is comparable to other fuels, though drying or mixing with drier materials is needed. ‘Auxiliary fuels’, such as natural gas, can be co-fried with SMC to promote drying and enable greater energy recovery. Combustion of SMC leads to the production of ash, usually about 10% of its original volume, in addition to what it already contains. A preliminary investigation has been completed into the reuse of this ash as a chemical activator to enhance the pozzolanic reactivity of pulverised fuel ash (PFA) in the cement industry (Russell et al., 2005). SMC ash and PFA mixtures ensure rapid early improvements in the strength, although it is too soon to investigate the long-term implications of its use. There are also many potential agricultural, horticultural and industrial uses of SMC including: An agricultural fertiliser (Gent et al., 1998; McCahey et al., 2003; Rhoads and Olson, 1995); a ruminant feed for sheep (Fazaeli and Masoodi, 2006); environmental enrichment in intensive pig farms (Beattie et al., 2001); treatment for coal mine drainage (Stark et al., 1994); bioremediation (Lau et al., 2003); enzyme extraction (Ball and Jackson, 1995); and a novel biosorbent (Chen et al., 2005). Coal tailings are formed from coal cleaning processes, where coal is separated from its impurities. It has a significant proportion of moisture, although dewatering takes place in the lagoons where it is deposited, usually located in close proximity to the mining area. Lagoon management is vitally important, as mismanagement of these sites can result in contamination or lagoon failure, thus removing coal tailing deposits will eliminate these risks (Thompson, 1982). There are currently few uses of coal tailings. Noble and Dobrovin-Pennington (2005) investigated how it replace some
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Nomenclature 1 2 AI BR Cb Cl CV FB IFS IR Mp NOx P PB Pc PFA RDF
SMC-coal tailing pellets raw SMC Alkali Index burning rate cardboard chlorides calorific value fluidised-bed ignition front speed ignition rate Miscanthus pellets oxides of nitrogen pyrolysis packed-bed pine cubes pulverised fuel ash refuse-derived fuel
of the peat in the casing layer added to the mushroom substrate, whereas Tiwari et al. (2004) focussed on its reuse for aggregates or the production of concrete blocks for construction. Only a few have examined its potential as a fuel, such as Chugh and Patwardhan (2004) and Radloff et al. (2004). The latter considered the processes of turning 70,000 tonnes/annum of waste from the Wallerawang colliery into fuel for a power station. This could be the greatest potential use of this waste, where 50 mm-diameter pellets were formed with a binding agent, followed by drying and then combustion with coarse waste from the coal mining industry. There is a clear potential to combine these wastes, for energy recovery through use in thermal treatment technologies. Combustion, gasification and pyrolysis have been utilised with both conventional and renewable fuels to produce heat, energy and subsequent fuel products. These three treatments were compared herein, with the aim of determining the most appropriate technology for the reuse of SMC and coal tailings. Consequently, it is hoped that this will provide a renewable fuel for industry, divert SMC from landfill and aid the cleaning and reclamation of land contaminated by coal tailings. Furthermore, this may partially mitigate the impacts of environmental issues associated with present energy production and waste management strategies. 2. Methods
SMC Sp Sr SOx SO24 Ww
spent mushroom compost switchgrass pellets raw switchgrass oxides of sulphur sulphates willow wood
Greek symbols DM mass loss gCE combustion efficiency k equivalence ratio q density Subscripts C char combustion stage I ignition propagation stage
Table 1 Material characterisation of the coal tailings and the two layers of the SMC Analysis
Constituent
Moisture (%)
Basis
Coal tailings
SMC substrate
SMC casing
ar
40
65.70
68.56
Proximate analysis (%)
Ash Volatile Fixed Carbon
dry
41.25 20.51 38.24
26.89 61.80 11.31
28.87 60.18 10.95
Ultimate analysis (%)
Carbon Hydrogen Nitrogen Chlorine Sulphur
dry
47.87 2.90 1.01 – 1.38
35.13 3.59 2.85 0.51 2.95
35.72 3.01 1.11 0.70 2.16
CV (MJ/kg)
GCV GCV NCV NCV
dry ar dry ar
19.85 11.91 19.22 10.55
14.11 4.94 13.33 3.08
12.37 4.33 11.71 2.51
Table 2 shows the full elemental analyses. In addition to the potential pollutants described above, other issues may arise due to the presence of specific elements. Slagging/fouling may occur due to the alkali metals present, specifically potassium and sodium. The large proportion of calcium, particularly in the SMC casing may be beneficial, however, as this could reduce SOx compounds
2.1. Characterisation of spent mushroom compost and coal tailings Table 1 shows the material characterisation. The results for the SMC were comparable to those previously reported in the literature (Williams et al., 2001). The two SMC layers were quite analogous and the volatile and fixed carbon contents were similar to other types of biomass used for energy recovery. The nitrogen and sulphur contents were high, however it has been found from previous literature that these are mainly bound in inorganic forms (nitrates and sulphates), and thus fluidised-bed combustion will minimise the risks of forming NOx and SOx (Williams et al., 2001). All three substances have significant quantities of moisture and ash, which have detrimental implications on the CVs, as shown in Table 1. These were comparable to other wastes currently used as fuels, such as MSW and sewage sludge. Coal tailings have a comparable CV to sub-bituminous C coal and the other coal tailing samples (Radloff et al., 2004).
Table 2 Selected results from the full elemental analyses Element (mg/kg)
Coal tailings
SMC substrate
SMC casing
Al As Ca Cr Cu Fe K Mg Mo Na P Pb Si Zn
4360.0 14.8 2940.0 13.0 19.0 9700.0 1070.0 2420.0 2.3 450.0 74.3 12.9 1380.0 34.6
441.5 <1 40900.0 3.8 46.1 1240.0 18650.0 4620.0 4.1 2095.0 6655.0 2.6 1620.0 194.5
1435.0 <1 118500.0 6.2 11.7 2580.0 3685.0 4265.0 0.3 600.0 4220.0 5.1 1485.0 44.1
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in a similar manner to scrubbing processes – the sulphur content of all three components was quite significant. This sulphur can inhibit de Novo Synthesis, preventing the formation of dioxins and furans, as chlorine was also present in the SMC layers (Fielder, 1998). Iron and phosphorus were found in significant quantities, although heavy metals were not prevalent. 2.2. Spent mushroom compost and coal tailing pelletisation Optimum values were experimentally-determined for a number of key pelletisation variables. The parameters included the moisture content (10–11%) followed by air-drying, compaction pressure (up to 6000 psi) and pellet composition (50:50 SMC:coal tailings wt% ratio). Pellet quality was based on their density, tensile strength, durability and maximum pellet pile heights (Table 3). The methods and results for these are presented in a previous publication (Ryu et al., 2008).
Table 4 Thermal treatment conditions and results for key parameters for the combustion tests Parameter
FB1
FB2
PB1
PB2
Material
Small pellets 4.6 240–610 60–90
SMC
SMC
3.0 190–770 90
Large pellets 6.26 720–1200 480
13.50 4.22 1.22
12.91 4.04 1.38
17.28 0.49 1.85
17.19 2.76 5.22
Combustion efficiency, gCE (%)
91.7
90.3
90.3
76.7
Average Temperature (°C)
816 813
799 797
1139 1117
1133 728
509
443
1055
735
Amount of material (kg) Primary air (kg/m2h) Secondary air (kg/m2h) Average gas Concentration
CO2 (%) O2 (%) CO (%)
Bed Above bed Freeboard
4.34 720 0–480
2.3. Methods for thermal treatments 2.3.1. Fluidised-bed for combustion A small-scale fluidised-bed was used to combust the pellets (FB1) and the raw SMC (FB2). The fuel was placed in the sealed hopper of the calibrated pneumatic screw feeder. The sand on the perforated distributor plate 200 mm from the base of the reactor forms the fluidised-bed within the 2.3 m 0.15 m stainless steel combustion chamber. This was heated with propane until the temperatures were stable, monitored using K-type mineral insulated thermocouples. Once this was achieved, the fuel was gradually fed in and the feedrate increased as the propane flowrate decreased. Table 4 shows the conditions for these and the subsequent tests. The exhaust produced passed through the cyclone to remove and collect particulate matter and the remaining gas was analysed for CO2, CO and O2 with an ADC MGA300 gas analyser, and NOx (NO and NO2) using a Signal Series 4000 NOx analyser before being discharged to the atmosphere. Chlorides (Cl ) and sulphate (SO24 ) species were collected by a wet chemical method, to indicate the presence of HCl and SOx.
the reaction. The emissions passed through a probe to the cooling tower, where the moisture was condensed out before CO2, CO and O2 concentrations were established. 2.3.3. Pyrolyser Pyrolysis (P) was performed in an electrically-heated, insulated 130 mm 320 mm stainless steel chamber, where the SMC was placed in the primary fixed-bed at the top of the cylinder. During the reaction, char was formed, which remained inside the reactor. This was fed with nitrogen at the base to prevent oxidation and to force the volatiles into the subsequent analysing equipment. These volatiles passed through a heated tube and were condensed to collect pyrolytic liquids. The gases were fed through a condensing tower, where samples were taken to assess the changing syngas (gaseous fuel product) composition with temperature. These were analysed with a Varian CP 3000 gas chromatograph. The exhaust then entered the CO2 and CO analyser. The solid, liquid and gaseous fuels were analysed for composition and CV, as appropriate. 2.4. Further data analysis
2.3.2. Packed-bed for combustion and gasification A packed-bed was also used to combust the pellets (PB1) and SMC (PB2), as well as perform the gasification of the SMC. The fuel was placed onto the perforated grate at the bottom of the 1.5 m 0.2 m reactor chamber and ignited using a gas burner. Primary and secondary air was fed from the bottom and top, respectively. Table 4 shows the operating conditions. The reactor was suspended from a weighing beam to monitor weight-loss during Table 3 Results of pellet density, tensile and compressive strengths and pellet pile-up Variables and parameters
Optimum 1
2
3
4
5
Initial moisture content (%) Drying Pressure (psi) Composition (SMC:coal tailings) Pellet diameter (mm) Pellet length (mm) Density (kg/m3) Tensile strength (kPa) Compressive strength (N) Weight (kg) Number of pellets
15
16.3
16.0
10
15
15
Yes 2700 50:50
No 2700 0:100
Yes 2700 0:100
Yes 6000 0:100
Yes Yes 2700 2700 100:0 Substrate
26.8 43.0 1083.7 116.4 134.3 11.9 451.7
26.8 31.8 1483.3 69.4 59.1 7.1 266.1
26.8 32.0 1371.0 108.7 93.2 9.5 384.4
26.8 4.3 1367.9 364.1 421.7 43.0 1290.4
26.8 56.7 661.4 60.8 92.5 9.4 445.9
Height of pellet pile (m)
12.10
7.13
10.30
34.58
11.95 33.73
26.8 5.8 689.7 179.1 277.1 28.3 1258.7
1: wet coal tailings; 2: dried coal tailings; 3: coal tailings at high pressure; 4: SMC and 5: SMC substrate.
Based on the CO and CO2 concentrations, the combustion efficiency (gCE) was calculated for the combustion cases. A number of other quantitative combustion parameters, namely mass loss (DM), ignition front speed (IFS), ignition rate (IR), burning rates for ignition propagation and char combustion (BRI and BRC) and equivalence ratio (k) were determined for the packed-bed cases to evaluate their performance (Ryu et al., 2006, 2007a; Ryu et al., 2007b). The Alkali Index (AI) was computed to assess the potential for slagging/fouling. Values above 0.34 suggest fouling is certain (Jenkins et al., 1998). Full elemental analyses of the ash aided the determination of this parameter. 3. Results and discussion The operating conditions used in the fluidised-bed and packedbed cases and the key results are shown in Table 4. gCE in the fluidised-bed was superior in the both cases and pellet combustion efficiency was also notably higher compared to the SMC alone. SMC pyrolysis produced a variety of low-CV fuels. These results are discussed below, although as SMC gasification was not successful, it is not considered further. 3.1. Combustion in the fluidised-bed The combustion of the SMC-coal tailing pellets in the fluidisedbed (FB1) achieved the highest gCE (91.7%) and was superior to the
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combustion of SMC (FB2). The gas concentrations were similar for both tests, although there was slightly more oxygen for SMC combustion, indicating fuel lean conditions, which resulted in a slightly lower efficiency (90.3%). The average NOx concentration during pellet combustion was low (8.8 ppm ± 0.76), thus little gas cleaning, for example selective catalytic or non-catalytic reduction, would be required. Furthermore, SO24 and Cl concentrations were 0.52 and 0.61 ppm, respectively, indicating the presence of SOx and HCl species were also minimal. The Ca present in the SMC may have reduced some SOx species, via the mechanism described above. Previous thermal treatments of SMC have shown that NOx and SOx will be negligible if it is combusted in a fluidised-bed, due to the inorganic origin of the majority of nitrogen and sulphur (Williams et al., 2001). It was also reported that though some HCl may form, most Cl should remain as chlorides in the ash (Williams et al., 2001; Williams, 2001). Additionally, it has been suggested that NOx and SOx emissions from coal tailing combustion would also be minimal (Chugh and Patwardhan, 2004). These findings were corroborated here. Temperatures for FB2 were somewhat lower than that for FB1, due to the inferior CV of the SMC. Temperatures in and just above the bed were higher than those in the freeboard in both cases. 3.2. Combustion in the packed-bed The temperatures achieved during combustion in the packedbed were higher than those in the fluidised-bed. The temperatures in the bed for PB2 were similar to those for PB1, although the freeboard was appreciably cooler. As the temperatures were significantly lower, there was less sintering off the ash for PB2. The O2 concentration decreased to lower levels than that in the fluidised-bed and consequently, the combustion products (CO and CO2) were more abundant, particularly CO, indicating less efficient combustion. gCE for PB1 and PB2 were 90.3% and 76.7%, respectively, where the latter was severely affected by the high CO. It was found that the air flowrates were crucial in determining whether or not the fuel would burn, particularly for the SMC. If the air flowrates were too high, this caused dramatic cooling of the reaction and combustion ceased, whereas when the flowrates were too low, there was insufficient oxygen for the reaction to continue. A range of quantitative combustion parameters were calculated (Table 5) and compared to those from the literature for miscanthus pellets (Mp), pine cubes (Pc), willow wood (Ww) and refuse-derived fuel pellets (RDF) (Ryu et al., 2006), cardboard (Cb) (Ryu et al., 2007b), switchgrass pellets (Sp) and raw switchgrass (Sr) (Gilbert et al., 2006). In general, these results corroborate well with other biomass combustion tests. DMI was greater for the pellets compared to the SMC, and as such, the BR during this phase was
much higher. By contrast, the BR during char combustion was more rapid for the SMC, although the overall BR was faster for the pellets. The IFS and IR for the pellets were both significantly lower than that for the SMC, due to the differential densities. Variations between the pelletised and non-pelletised fuels in this study have been confirmed by the differences between pelletised and nonpelletised switchgrass, namely DMI, IFS and IR (Gilbert et al., 2006). DMI was significantly lower for these tests, where the majority of mass loss occurred during the char combustion phase, particularly for PB2. Char formation, rather than combustion, was the significant process occurring during the initial stages of these reactions. As previously suggested, BRI was lower than IR in all cases, thus char remained after volatile combustion. The IFS were similar for all the cases shown, except for the raw switchgrass, due to its very low density (Gilbert et al., 2006). Although the range of IR was vast in previous literature, the results gained were within the range shown in Table 5. The BR was also comparable, except the switchgrass, which showed significantly higher overall BR, although the results for Sp and Sr were very similar. The equivalence ratio at the ignition propagation stage (kI) was also calculated. The values for these were significantly lower than those reported in Table 5. The Alkali Index was calculated from the full elemental analyses of the ash samples, assuming that all K and Na present were in oxide form. Probable fouling will occur with the combustion of these pellets. The combustion of the SMC still had the potential to foul, as there were noteworthy amounts of potassium and sodium in the ash. 3.3. Pyrolysis Three hundred grams of non-pelletised SMC was pyrolysed for 1 h at 500 °C with 2 l/min of nitrogen. The temperature profiles and gas concentrations are shown in Fig. 1. Despite the lack of oxygen in the system, CO2 was still produced in significant quantities, due to the oxygen present in the feed material. As expected, CO was much higher than combustion, as pyrolysis is a thermal decomposition, not an oxidation, process. In addition to these, other gases were also analysed (Table 6). Towards the end of the reaction, the proportion of hydrocarbon fuels increased, where methane (CH4), ethane (C2H4) and propane (C3H8) became more abundant as the temperatures became higher. In addition to these gases, solid and liquid fuels were produced (Table 7). The char was similar in appearance, in terms of particle size and shape, to the original material, although once pyrolysed, the material became dark and uniform in colour. The liquids separated out, where the heavier, paler pyrolysis liquid settled towards the bottom, with the darker, aqueous phase on top. CV tests revealed large differences between their energy values. The paler pyrolysis liquid had a reasonable CV of 5.85 MJ/kg, whereas the
Table 5 Comparison of quantitative parameters for the combustion in the packed-bed Test
PB1 PB2 Mpa Pca Wwa RDFa Cbb Spc Src a b c
Parameter
DMI (%)
IFS (m/h)
IR (kg/m 2h)
BRI (kg/m 2h)
BRC (kg/m 2h)
x BR (kg/m2 h)
kI
AI (kg-alkali/GJ)
q (kg/m3)
51.4 19.4 75–81 84–86 68–82 73–75 68–90 90 65.6
0.7 1.3 0.36 0.7–0.8 1.1–1.2 0.2–0.3 – 0.76 8.9
296.4 376.8 – – – – 190–300 280 508
152.5 72.9 – – – – 135–310 312 385
75.9 120.4 – – – – 55–115 – –
109.8 93.7 94–148 123–134 104–141 97–112 – 252 253
1.27 0.45 2.1–2.3 2.1–2.3 2.9–4.1 2.2–2.4 2.0–2.5 2.39 2.40
0.17 0.13 0.34 <0.1 <0.1 >0.2 – – –
442.8 300.3 660 272–295 181 715 76 370 57
Ryu et al. (2006). Ryu et al. (2007a,b). Gilbert et al. (2006).
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600 20 T1, y = 300mm
Temperature (˚C)
15 400
300
10
200 5
Gas Concentration (%)
500
T2, y = 250mm T3, y = 200mm T4, before condenser T5, after condenser Carbon Dioxide
100 Carbon Monoxide
0
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (hrs) Fig. 1. The temperature profiles and gas concentrations during SMC pyrolysis.
Table 6 Analysis of the changing composition of gaseous fuel products with temperature from SMC pyrolysis Gas
CO2 CO CH4 H2 C2H4 C3H10
Gas concentration (%) with temperature 350 °C
400 °C
450 °C
500 °C
13.02 8.96 0.11 0.04 0.14 0.06
16.61 9.40 0.54 0.08 0.08 0.05
14.44 6.67 0.78 0.22 0.13 0.07
8.19 7.69 1.26 1.34 0.18 0.08
Table 7 Analysis of the solid and liquid fuel products from SMC pyrolysis Analysis
Constituent
Char
Liquid
Yield
%g
43.33 130
34.71 104
Ultimate analysis (%)
Carbon Hydrogen Nitrogen Oxygen
34.2 1.2 1.4 63.2
5 11.8 1.5 81.8
CV (MJ/kg)
GCV NCV
7.95 7.68
2.98 –
darker, aqueous phase had a low CV of just 2.16 MJ/kg. The lower CV product was found in greater abundance, however, which was why the overall CV for the liquid phase was low. Separation of these two phases may be beneficial and further processing, as refining may be required.
temperatures could be generated for the production of useful and moreover saleable heat and/or power. Pellet combustion achieved the greatest efficiency, as the bulk and energy densities were increased by drying and pelletisation, thus pre-processing these wastes is integral to their successful use as a fuel. This was seen particularly in the fluidised-bed, which had the highest overall efficiency. Fluidised-bed combustors usually have higher carbonburnout efficiencies due to their superior fluid-particle contacting, clearly shown in the comparison between the two combustion methods. Pelletised fuels also generally have better combustion efficiencies than non-pelletised fuels, especially in fluidised-beds where the densified fuel can burn within the bed. The ash produced from pellet combustion has the potential to be utilised as an activator for PFA, eliminating the need for landfilling, as discussed previously (Russell et al., 2005). The use of SMC-coal tailing pellets clearly has a wide variety of benefits, on the pelletisation of these substances, the combustion process and the overall environmental considerations. The SMC ensures that lower net CO2 emissions are produced than for conventional fuels, mitigating environmental issues, whereas the coal tailings enhance the CV and thus the energy which can be recovered. Improving the overall CV of the fuel increases the combustion efficiency and the greater bulk and energy densities ensure that combustion takes place within the bed at greater efficiency than the SMC alone. The reuse of both of these wastes also means less land is taken up by landfill sites and coal tailing lagoons; even the ash can be used in an environmental manner. Pyrolysis did produce a combination of solid, liquid and gaseous fuels and although the CVs of these products were reasonable, they were not necessarily sufficiently high to necessitate the wide-scale use of this technology. Other biomass materials could be used in this process to greater effect. Gasification of the material in a small-scale fixed-bed gasifier was not successful.
3.4. Discussion of results 4. Conclusions Three technologies were compared, where combustion was most successful and thus deemed the best thermal treatment for SMC and coal tailings, particularly in pellet form. The combustion of both the pellets and SMC were self-sustaining and sufficient
The main conclusions from this comparison of thermal treatments technologies for the reuse of SMC and coal tailings are as follows:
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SMC-coal tailing pellet combustion in a fluidised-bed was most efficient (91.7%), when compared to packed-bed combustion or the use of unpelletised SMC. This produced minimal acid gas emissions (NOx, SOx and HCl), although the alkali metal oxide content of the flyash was sufficient to cause possible slagging/fouling in the system. Using these wastes for energy recovery provides a sustainable management solution to divert SMC from landfill and aid the reclamation of contaminated land, and is therefore both practical and environmentally-sound. Acknowledgements The authors would like to thank the Engineering and Physical Science Research Council (EPSRC) and Veolia Environmental Trust (Grant reference RES/C/6046/TP) for their financial support of this project. Thanks also go to Maltby Colliery, Dr. John Burden and Monaghan Mushrooms Ltd. for providing SMC and coal tailing samples. References Ball, A.S., Jackson, A.M., 1995. The recovery of lignocellulose-degrading enzymes from spent mushroom compost. Bioresource Technology 54 (3), 311–314. Beattie, V.E., Sneddon, I.A., Walker, N., Weatherup, R.N., 2001. Environmental enrichment of intensive pig housing using spent mushroom compost. Animal Science 72 (Part 1), 35–42. BioMatNet, 2004. NNE5-1999-20229 MON – CHP: Optimised Biomass CHP Plant for Monaghan Integrating Condensing Economiser Technology. Available from:. Chen, G.Q., Zeng, G.M., Tu, X., Huang, G.H., Chen, Y.N., 2005. A novel biosorbent: characterisation of the spent mushroom compost and its application for removal of heavy metals. Journal of Environmental Sciences – China 17 (5), 756–760. Chugh, Y.P., Patwardhan, A., 2004. Mine-mouth power and process steam generation using fine coal waste fuel. Resources, Conservation and Recycling 40, 225–243. DEFRA, Department for Environment, Food and Rural Affairs, 2006. Process Guidance Note 6/30(06): Secretary of State’s Guidance for Mushroom Substrate Manufacture. Available from: . Fazaeli, H., Masoodi, A.R.T., 2006. Spent wheat straw compost of Agaricus bisporus mushroom as ruminant feed. Asian-Australasian Journal of Animal Sciences 19 (6), 845–851. Fielder, H., 1998. Thermal formation of PCDD/PCDF: a survey. Environmental Engineering Science 15, 49–58.
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Gent, M.P.N., Elmer, W.H., Stoner, K.A., Ferrandino, F.J., La Mondia, J.A., 1998. Growth, yield and nutrition of potato in fumigated or nonfumigated soil amended with spent mushroom compost and straw mulch. Compost Science and Utilization 6 (4), 45–56. Gilbert, P., Ryu, C., Sharifi, V.N., Swithenbank, J., 2006. Pelletisation of Herbaceous Crops. Departmental Internal Report, Department of Chemical and Process Engineering, University of Sheffield. Iiyama, K., Stone, B.A., Macauley, B.J., 1994. Compositional changes in compost during composting and growth of Agaricus bisporus. Applied and Environmental Microbiology 60 (5), 1538–1546. Jenkins, B.M., Baxter, L.L., Miles Jr., T.R., Miles, T.R., 1998. Combustion properties of biomass. Fuel Processing Technology 54, 17–46. Lau, K.L., Tsang, Y.Y., Chiu, S.W., 2003. Use of spent mushroom compost to bioremediate PAH-contaminated samples. Chemosphere 52 (9), 1539–1546. McCahey, S., McMullan, J.T., Williams, B.C., 2003. Consideration of spent mushroom compost as a source of energy. Developments in Chemical Engineering and Mineral Processing 11 (1–2), 43–53. Noble, R., Dobrovin-Pennington, A., 2005. Partial substitution of peat in mushroom casing with fine particle coal tailings. Scientia Horticulturae 104, 351–367. Radloff, B., Kirsten, M., Anderson, R., 2004. Wallerawang colliery rehabilitation: the coal tailings briquetting process. Minerals Engineering 17, 153–157. Rhoads, F.M., Olson, S.M., 1995. Crop production with mushroom compost. Soil and Crop Science Society of Florida Proceedings 54, 53–57. Russell, M., Basheer, P.A.M., Rao, R.J., 2005. Potential use of spent mushroom compost ash as an activator for pulverised fuel ash. Construction and Building Materials 19, 698–702. Ryu, C., Yang, Y.B., Khor, A., Yates, N.E., Sharifi, V.N., Swithenbank, J., 2006. Effect of fuel properties on biomass combustion: part I. Experiments–fuel type, equivalence ratio and particle size. Fuel 85, 1039–1046. Ryu, C., Phan, A.N., Yang, Y.B., Sharifi, V.N., Swithenbank, J., 2007a. Ignition and burning rates of segregated waste combustion in packed beds. Waste Management 27, 802–810. Ryu, C., Phan, A.N., Yang, Y.B., Sharifi, V.N., Swithenbank, J., 2007b. Co-combustion of textile residues with cardboard and waste wood in a packed bed. Experimental and Thermal Fluid Science 32, 450–458. Ryu, C., Finney, K., Sharifi, V.N., Swithenbank, J., 2008. Pelletised fuel production from coal tailings and spent mushroom compost–part I. Effect of pelletisation parameters. Fuel Processing Technology 89, 269–275. Stark, L.R., Wenerick, W.R., Williams, F.M., Stevens, S.E., Wuest, P.J., 1994. Restoring the capacity of spent mushroom compost to treat coal-mine drainage by reducing the inflow rate – a microcosm experiment. Water, Air and Soil Pollution 75 (3–4), 405–420. Thompson, M.J., 1982. Spoil disposal. Environmental Geochemistry and Health 4 (2– 3), 87–90. Tiwari, K.K., Basu, S.K., Bit, K.C., Banerjee, S., Mishra, K.K., 2004. High-concentration coal-water slurry from Indian coals using newly developed additives. Fuel Processing Technology 85 (1), 31–42. Williams, B.C., 2001. Energy from Spent Mushroom Compost, The Mushroom People, IPP Limited, Wilmslow, UK. 3a (120) 18. Available from:. Williams, B.C., McMullan, J.T., McCahey, S., 2001. An initial assessment of spent mushroom compost as a potential energy feedstock. Bioresource Technology 79, 227–230.
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The reuse of spent mushroom compost and coal tailings for energy recovery: Comparison of thermal treatment technologies Karen N. Finney *, Changkook Ryu, Vida N. Sharifi, Jim Swithenbank Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Article history: Received 7 January 2008 Received in revised form 21 May 2008 Accepted 23 May 2008 Available online 14 July 2008 Keywords: Energy recovery Spent mushroom compost Coal tailings Thermal treatment technologies
a b s t r a c t Thermal treatment technologies were compared to determine an appropriate method of recovering energy from two wastes – spent mushroom compost and coal tailings. The raw compost and pellets of these wastes were combusted in a fluidised-bed and a packed-bed, and contrasted to pyrolysis and gasification. Quantitative combustion parameters were compared to assess the differences in efficiency between the technologies. Fluidised-bed combustion was more efficient than the packed-bed in both instances and pellet combustion was superior to that of the compost alone. Acid gas emissions (NOx, SOx and HCl) were minimal for the fluidised-bed, thus little gas cleaning would be required. The fuels’ high ash content (34%) also suggests fluidised-bed combustion would be preferred. The Alkali Index of the ash indicates the possibility of fouling/slagging within the system, caused by the presence of alkali metal oxides. Pyrolysis produced a range of low-calorific value-products, while gasification was not successful. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction To mitigate the impacts of our growing demand for energy and the increase in waste production, it is possible to use many wastes as fuel resources – two of these wastes include spent mushroom compost, hereinafter SMC, and coal tailings. The formation of the mushroom compost substrate takes place in stages: (i) pre-wetting and mixing of straw, gypsum, horse manure, poultry litter, peat, lime and activators; (ii) phase 1 composting in windrows and bunkers; and (iii) phase 2 composting involving pasteurisation and conditioning. After this, a peat casing layer is added to the surface for mushroom cultivation (spawning, casing, pinning and cropping), after which it is cooked out and discarded (DEFRA, 2006; Iiyama et al., 1994). Disposal is problematic, unsustainable and harmful to the environment, since the majority goes to landfill or is used as agricultural fertilisers, where leaching can be a serious issue for local water courses from both sources, as phosphorous and nitrates cause eutrophication. For every 1 kg of mushrooms grown, approximately 5 kg of SMC is produced. The generation rate of SMC is about 200,000 tonnes/annum in UK alone. The lack of sustainable waste management solution for SMC is the most significant barrier to the future development of this industry. Previous studies into the use of SMC as a renewable fuel propose that SMC can be combusted in a bubbling fluidised-bed to generate power with high efficiency (McCahey et al., 2003; Wil* Corresponding author. Tel: +44 114 2227578; fax: +44 114 2227501. E-mail address: cpp06knf@sheffield.ac.uk (K.N. Finney). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.05.054
liams, 2001; Williams et al., 2001; BioMatNet, 2004). The calorific value (CV) is comparable to other fuels, though drying or mixing with drier materials is needed. ‘Auxiliary fuels’, such as natural gas, can be co-fried with SMC to promote drying and enable greater energy recovery. Combustion of SMC leads to the production of ash, usually about 10% of its original volume, in addition to what it already contains. A preliminary investigation has been completed into the reuse of this ash as a chemical activator to enhance the pozzolanic reactivity of pulverised fuel ash (PFA) in the cement industry (Russell et al., 2005). SMC ash and PFA mixtures ensure rapid early improvements in the strength, although it is too soon to investigate the long-term implications of its use. There are also many potential agricultural, horticultural and industrial uses of SMC including: An agricultural fertiliser (Gent et al., 1998; McCahey et al., 2003; Rhoads and Olson, 1995); a ruminant feed for sheep (Fazaeli and Masoodi, 2006); environmental enrichment in intensive pig farms (Beattie et al., 2001); treatment for coal mine drainage (Stark et al., 1994); bioremediation (Lau et al., 2003); enzyme extraction (Ball and Jackson, 1995); and a novel biosorbent (Chen et al., 2005). Coal tailings are formed from coal cleaning processes, where coal is separated from its impurities. It has a significant proportion of moisture, although dewatering takes place in the lagoons where it is deposited, usually located in close proximity to the mining area. Lagoon management is vitally important, as mismanagement of these sites can result in contamination or lagoon failure, thus removing coal tailing deposits will eliminate these risks (Thompson, 1982). There are currently few uses of coal tailings. Noble and Dobrovin-Pennington (2005) investigated how it replace some
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Nomenclature 1 2 AI BR Cb Cl CV FB IFS IR Mp NOx P PB Pc PFA RDF
SMC-coal tailing pellets raw SMC Alkali Index burning rate cardboard chlorides calorific value fluidised-bed ignition front speed ignition rate Miscanthus pellets oxides of nitrogen pyrolysis packed-bed pine cubes pulverised fuel ash refuse-derived fuel
of the peat in the casing layer added to the mushroom substrate, whereas Tiwari et al. (2004) focussed on its reuse for aggregates or the production of concrete blocks for construction. Only a few have examined its potential as a fuel, such as Chugh and Patwardhan (2004) and Radloff et al. (2004). The latter considered the processes of turning 70,000 tonnes/annum of waste from the Wallerawang colliery into fuel for a power station. This could be the greatest potential use of this waste, where 50 mm-diameter pellets were formed with a binding agent, followed by drying and then combustion with coarse waste from the coal mining industry. There is a clear potential to combine these wastes, for energy recovery through use in thermal treatment technologies. Combustion, gasification and pyrolysis have been utilised with both conventional and renewable fuels to produce heat, energy and subsequent fuel products. These three treatments were compared herein, with the aim of determining the most appropriate technology for the reuse of SMC and coal tailings. Consequently, it is hoped that this will provide a renewable fuel for industry, divert SMC from landfill and aid the cleaning and reclamation of land contaminated by coal tailings. Furthermore, this may partially mitigate the impacts of environmental issues associated with present energy production and waste management strategies. 2. Methods
SMC Sp Sr SOx SO24 Ww
spent mushroom compost switchgrass pellets raw switchgrass oxides of sulphur sulphates willow wood
Greek symbols DM mass loss gCE combustion efficiency k equivalence ratio q density Subscripts C char combustion stage I ignition propagation stage
Table 1 Material characterisation of the coal tailings and the two layers of the SMC Analysis
Constituent
Moisture (%)
Basis
Coal tailings
SMC substrate
SMC casing
ar
40
65.70
68.56
Proximate analysis (%)
Ash Volatile Fixed Carbon
dry
41.25 20.51 38.24
26.89 61.80 11.31
28.87 60.18 10.95
Ultimate analysis (%)
Carbon Hydrogen Nitrogen Chlorine Sulphur
dry
47.87 2.90 1.01 – 1.38
35.13 3.59 2.85 0.51 2.95
35.72 3.01 1.11 0.70 2.16
CV (MJ/kg)
GCV GCV NCV NCV
dry ar dry ar
19.85 11.91 19.22 10.55
14.11 4.94 13.33 3.08
12.37 4.33 11.71 2.51
Table 2 shows the full elemental analyses. In addition to the potential pollutants described above, other issues may arise due to the presence of specific elements. Slagging/fouling may occur due to the alkali metals present, specifically potassium and sodium. The large proportion of calcium, particularly in the SMC casing may be beneficial, however, as this could reduce SOx compounds
2.1. Characterisation of spent mushroom compost and coal tailings Table 1 shows the material characterisation. The results for the SMC were comparable to those previously reported in the literature (Williams et al., 2001). The two SMC layers were quite analogous and the volatile and fixed carbon contents were similar to other types of biomass used for energy recovery. The nitrogen and sulphur contents were high, however it has been found from previous literature that these are mainly bound in inorganic forms (nitrates and sulphates), and thus fluidised-bed combustion will minimise the risks of forming NOx and SOx (Williams et al., 2001). All three substances have significant quantities of moisture and ash, which have detrimental implications on the CVs, as shown in Table 1. These were comparable to other wastes currently used as fuels, such as MSW and sewage sludge. Coal tailings have a comparable CV to sub-bituminous C coal and the other coal tailing samples (Radloff et al., 2004).
Table 2 Selected results from the full elemental analyses Element (mg/kg)
Coal tailings
SMC substrate
SMC casing
Al As Ca Cr Cu Fe K Mg Mo Na P Pb Si Zn
4360.0 14.8 2940.0 13.0 19.0 9700.0 1070.0 2420.0 2.3 450.0 74.3 12.9 1380.0 34.6
441.5 <1 40900.0 3.8 46.1 1240.0 18650.0 4620.0 4.1 2095.0 6655.0 2.6 1620.0 194.5
1435.0 <1 118500.0 6.2 11.7 2580.0 3685.0 4265.0 0.3 600.0 4220.0 5.1 1485.0 44.1
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in a similar manner to scrubbing processes – the sulphur content of all three components was quite significant. This sulphur can inhibit de Novo Synthesis, preventing the formation of dioxins and furans, as chlorine was also present in the SMC layers (Fielder, 1998). Iron and phosphorus were found in significant quantities, although heavy metals were not prevalent. 2.2. Spent mushroom compost and coal tailing pelletisation Optimum values were experimentally-determined for a number of key pelletisation variables. The parameters included the moisture content (10–11%) followed by air-drying, compaction pressure (up to 6000 psi) and pellet composition (50:50 SMC:coal tailings wt% ratio). Pellet quality was based on their density, tensile strength, durability and maximum pellet pile heights (Table 3). The methods and results for these are presented in a previous publication (Ryu et al., 2008).
Table 4 Thermal treatment conditions and results for key parameters for the combustion tests Parameter
FB1
FB2
PB1
PB2
Material
Small pellets 4.6 240–610 60–90
SMC
SMC
3.0 190–770 90
Large pellets 6.26 720–1200 480
13.50 4.22 1.22
12.91 4.04 1.38
17.28 0.49 1.85
17.19 2.76 5.22
Combustion efficiency, gCE (%)
91.7
90.3
90.3
76.7
Average Temperature (°C)
816 813
799 797
1139 1117
1133 728
509
443
1055
735
Amount of material (kg) Primary air (kg/m2h) Secondary air (kg/m2h) Average gas Concentration
CO2 (%) O2 (%) CO (%)
Bed Above bed Freeboard
4.34 720 0–480
2.3. Methods for thermal treatments 2.3.1. Fluidised-bed for combustion A small-scale fluidised-bed was used to combust the pellets (FB1) and the raw SMC (FB2). The fuel was placed in the sealed hopper of the calibrated pneumatic screw feeder. The sand on the perforated distributor plate 200 mm from the base of the reactor forms the fluidised-bed within the 2.3 m 0.15 m stainless steel combustion chamber. This was heated with propane until the temperatures were stable, monitored using K-type mineral insulated thermocouples. Once this was achieved, the fuel was gradually fed in and the feedrate increased as the propane flowrate decreased. Table 4 shows the conditions for these and the subsequent tests. The exhaust produced passed through the cyclone to remove and collect particulate matter and the remaining gas was analysed for CO2, CO and O2 with an ADC MGA300 gas analyser, and NOx (NO and NO2) using a Signal Series 4000 NOx analyser before being discharged to the atmosphere. Chlorides (Cl ) and sulphate (SO24 ) species were collected by a wet chemical method, to indicate the presence of HCl and SOx.
the reaction. The emissions passed through a probe to the cooling tower, where the moisture was condensed out before CO2, CO and O2 concentrations were established. 2.3.3. Pyrolyser Pyrolysis (P) was performed in an electrically-heated, insulated 130 mm 320 mm stainless steel chamber, where the SMC was placed in the primary fixed-bed at the top of the cylinder. During the reaction, char was formed, which remained inside the reactor. This was fed with nitrogen at the base to prevent oxidation and to force the volatiles into the subsequent analysing equipment. These volatiles passed through a heated tube and were condensed to collect pyrolytic liquids. The gases were fed through a condensing tower, where samples were taken to assess the changing syngas (gaseous fuel product) composition with temperature. These were analysed with a Varian CP 3000 gas chromatograph. The exhaust then entered the CO2 and CO analyser. The solid, liquid and gaseous fuels were analysed for composition and CV, as appropriate. 2.4. Further data analysis
2.3.2. Packed-bed for combustion and gasification A packed-bed was also used to combust the pellets (PB1) and SMC (PB2), as well as perform the gasification of the SMC. The fuel was placed onto the perforated grate at the bottom of the 1.5 m 0.2 m reactor chamber and ignited using a gas burner. Primary and secondary air was fed from the bottom and top, respectively. Table 4 shows the operating conditions. The reactor was suspended from a weighing beam to monitor weight-loss during Table 3 Results of pellet density, tensile and compressive strengths and pellet pile-up Variables and parameters
Optimum 1
2
3
4
5
Initial moisture content (%) Drying Pressure (psi) Composition (SMC:coal tailings) Pellet diameter (mm) Pellet length (mm) Density (kg/m3) Tensile strength (kPa) Compressive strength (N) Weight (kg) Number of pellets
15
16.3
16.0
10
15
15
Yes 2700 50:50
No 2700 0:100
Yes 2700 0:100
Yes 6000 0:100
Yes Yes 2700 2700 100:0 Substrate
26.8 43.0 1083.7 116.4 134.3 11.9 451.7
26.8 31.8 1483.3 69.4 59.1 7.1 266.1
26.8 32.0 1371.0 108.7 93.2 9.5 384.4
26.8 4.3 1367.9 364.1 421.7 43.0 1290.4
26.8 56.7 661.4 60.8 92.5 9.4 445.9
Height of pellet pile (m)
12.10
7.13
10.30
34.58
11.95 33.73
26.8 5.8 689.7 179.1 277.1 28.3 1258.7
1: wet coal tailings; 2: dried coal tailings; 3: coal tailings at high pressure; 4: SMC and 5: SMC substrate.
Based on the CO and CO2 concentrations, the combustion efficiency (gCE) was calculated for the combustion cases. A number of other quantitative combustion parameters, namely mass loss (DM), ignition front speed (IFS), ignition rate (IR), burning rates for ignition propagation and char combustion (BRI and BRC) and equivalence ratio (k) were determined for the packed-bed cases to evaluate their performance (Ryu et al., 2006, 2007a; Ryu et al., 2007b). The Alkali Index (AI) was computed to assess the potential for slagging/fouling. Values above 0.34 suggest fouling is certain (Jenkins et al., 1998). Full elemental analyses of the ash aided the determination of this parameter. 3. Results and discussion The operating conditions used in the fluidised-bed and packedbed cases and the key results are shown in Table 4. gCE in the fluidised-bed was superior in the both cases and pellet combustion efficiency was also notably higher compared to the SMC alone. SMC pyrolysis produced a variety of low-CV fuels. These results are discussed below, although as SMC gasification was not successful, it is not considered further. 3.1. Combustion in the fluidised-bed The combustion of the SMC-coal tailing pellets in the fluidisedbed (FB1) achieved the highest gCE (91.7%) and was superior to the
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combustion of SMC (FB2). The gas concentrations were similar for both tests, although there was slightly more oxygen for SMC combustion, indicating fuel lean conditions, which resulted in a slightly lower efficiency (90.3%). The average NOx concentration during pellet combustion was low (8.8 ppm ± 0.76), thus little gas cleaning, for example selective catalytic or non-catalytic reduction, would be required. Furthermore, SO24 and Cl concentrations were 0.52 and 0.61 ppm, respectively, indicating the presence of SOx and HCl species were also minimal. The Ca present in the SMC may have reduced some SOx species, via the mechanism described above. Previous thermal treatments of SMC have shown that NOx and SOx will be negligible if it is combusted in a fluidised-bed, due to the inorganic origin of the majority of nitrogen and sulphur (Williams et al., 2001). It was also reported that though some HCl may form, most Cl should remain as chlorides in the ash (Williams et al., 2001; Williams, 2001). Additionally, it has been suggested that NOx and SOx emissions from coal tailing combustion would also be minimal (Chugh and Patwardhan, 2004). These findings were corroborated here. Temperatures for FB2 were somewhat lower than that for FB1, due to the inferior CV of the SMC. Temperatures in and just above the bed were higher than those in the freeboard in both cases. 3.2. Combustion in the packed-bed The temperatures achieved during combustion in the packedbed were higher than those in the fluidised-bed. The temperatures in the bed for PB2 were similar to those for PB1, although the freeboard was appreciably cooler. As the temperatures were significantly lower, there was less sintering off the ash for PB2. The O2 concentration decreased to lower levels than that in the fluidised-bed and consequently, the combustion products (CO and CO2) were more abundant, particularly CO, indicating less efficient combustion. gCE for PB1 and PB2 were 90.3% and 76.7%, respectively, where the latter was severely affected by the high CO. It was found that the air flowrates were crucial in determining whether or not the fuel would burn, particularly for the SMC. If the air flowrates were too high, this caused dramatic cooling of the reaction and combustion ceased, whereas when the flowrates were too low, there was insufficient oxygen for the reaction to continue. A range of quantitative combustion parameters were calculated (Table 5) and compared to those from the literature for miscanthus pellets (Mp), pine cubes (Pc), willow wood (Ww) and refuse-derived fuel pellets (RDF) (Ryu et al., 2006), cardboard (Cb) (Ryu et al., 2007b), switchgrass pellets (Sp) and raw switchgrass (Sr) (Gilbert et al., 2006). In general, these results corroborate well with other biomass combustion tests. DMI was greater for the pellets compared to the SMC, and as such, the BR during this phase was
much higher. By contrast, the BR during char combustion was more rapid for the SMC, although the overall BR was faster for the pellets. The IFS and IR for the pellets were both significantly lower than that for the SMC, due to the differential densities. Variations between the pelletised and non-pelletised fuels in this study have been confirmed by the differences between pelletised and nonpelletised switchgrass, namely DMI, IFS and IR (Gilbert et al., 2006). DMI was significantly lower for these tests, where the majority of mass loss occurred during the char combustion phase, particularly for PB2. Char formation, rather than combustion, was the significant process occurring during the initial stages of these reactions. As previously suggested, BRI was lower than IR in all cases, thus char remained after volatile combustion. The IFS were similar for all the cases shown, except for the raw switchgrass, due to its very low density (Gilbert et al., 2006). Although the range of IR was vast in previous literature, the results gained were within the range shown in Table 5. The BR was also comparable, except the switchgrass, which showed significantly higher overall BR, although the results for Sp and Sr were very similar. The equivalence ratio at the ignition propagation stage (kI) was also calculated. The values for these were significantly lower than those reported in Table 5. The Alkali Index was calculated from the full elemental analyses of the ash samples, assuming that all K and Na present were in oxide form. Probable fouling will occur with the combustion of these pellets. The combustion of the SMC still had the potential to foul, as there were noteworthy amounts of potassium and sodium in the ash. 3.3. Pyrolysis Three hundred grams of non-pelletised SMC was pyrolysed for 1 h at 500 °C with 2 l/min of nitrogen. The temperature profiles and gas concentrations are shown in Fig. 1. Despite the lack of oxygen in the system, CO2 was still produced in significant quantities, due to the oxygen present in the feed material. As expected, CO was much higher than combustion, as pyrolysis is a thermal decomposition, not an oxidation, process. In addition to these, other gases were also analysed (Table 6). Towards the end of the reaction, the proportion of hydrocarbon fuels increased, where methane (CH4), ethane (C2H4) and propane (C3H8) became more abundant as the temperatures became higher. In addition to these gases, solid and liquid fuels were produced (Table 7). The char was similar in appearance, in terms of particle size and shape, to the original material, although once pyrolysed, the material became dark and uniform in colour. The liquids separated out, where the heavier, paler pyrolysis liquid settled towards the bottom, with the darker, aqueous phase on top. CV tests revealed large differences between their energy values. The paler pyrolysis liquid had a reasonable CV of 5.85 MJ/kg, whereas the
Table 5 Comparison of quantitative parameters for the combustion in the packed-bed Test
PB1 PB2 Mpa Pca Wwa RDFa Cbb Spc Src a b c
Parameter
DMI (%)
IFS (m/h)
IR (kg/m 2h)
BRI (kg/m 2h)
BRC (kg/m 2h)
x BR (kg/m2 h)
kI
AI (kg-alkali/GJ)
q (kg/m3)
51.4 19.4 75–81 84–86 68–82 73–75 68–90 90 65.6
0.7 1.3 0.36 0.7–0.8 1.1–1.2 0.2–0.3 – 0.76 8.9
296.4 376.8 – – – – 190–300 280 508
152.5 72.9 – – – – 135–310 312 385
75.9 120.4 – – – – 55–115 – –
109.8 93.7 94–148 123–134 104–141 97–112 – 252 253
1.27 0.45 2.1–2.3 2.1–2.3 2.9–4.1 2.2–2.4 2.0–2.5 2.39 2.40
0.17 0.13 0.34 <0.1 <0.1 >0.2 – – –
442.8 300.3 660 272–295 181 715 76 370 57
Ryu et al. (2006). Ryu et al. (2007a,b). Gilbert et al. (2006).
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600 20 T1, y = 300mm
Temperature (˚C)
15 400
300
10
200 5
Gas Concentration (%)
500
T2, y = 250mm T3, y = 200mm T4, before condenser T5, after condenser Carbon Dioxide
100 Carbon Monoxide
0
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (hrs) Fig. 1. The temperature profiles and gas concentrations during SMC pyrolysis.
Table 6 Analysis of the changing composition of gaseous fuel products with temperature from SMC pyrolysis Gas
CO2 CO CH4 H2 C2H4 C3H10
Gas concentration (%) with temperature 350 °C
400 °C
450 °C
500 °C
13.02 8.96 0.11 0.04 0.14 0.06
16.61 9.40 0.54 0.08 0.08 0.05
14.44 6.67 0.78 0.22 0.13 0.07
8.19 7.69 1.26 1.34 0.18 0.08
Table 7 Analysis of the solid and liquid fuel products from SMC pyrolysis Analysis
Constituent
Char
Liquid
Yield
%g
43.33 130
34.71 104
Ultimate analysis (%)
Carbon Hydrogen Nitrogen Oxygen
34.2 1.2 1.4 63.2
5 11.8 1.5 81.8
CV (MJ/kg)
GCV NCV
7.95 7.68
2.98 –
darker, aqueous phase had a low CV of just 2.16 MJ/kg. The lower CV product was found in greater abundance, however, which was why the overall CV for the liquid phase was low. Separation of these two phases may be beneficial and further processing, as refining may be required.
temperatures could be generated for the production of useful and moreover saleable heat and/or power. Pellet combustion achieved the greatest efficiency, as the bulk and energy densities were increased by drying and pelletisation, thus pre-processing these wastes is integral to their successful use as a fuel. This was seen particularly in the fluidised-bed, which had the highest overall efficiency. Fluidised-bed combustors usually have higher carbonburnout efficiencies due to their superior fluid-particle contacting, clearly shown in the comparison between the two combustion methods. Pelletised fuels also generally have better combustion efficiencies than non-pelletised fuels, especially in fluidised-beds where the densified fuel can burn within the bed. The ash produced from pellet combustion has the potential to be utilised as an activator for PFA, eliminating the need for landfilling, as discussed previously (Russell et al., 2005). The use of SMC-coal tailing pellets clearly has a wide variety of benefits, on the pelletisation of these substances, the combustion process and the overall environmental considerations. The SMC ensures that lower net CO2 emissions are produced than for conventional fuels, mitigating environmental issues, whereas the coal tailings enhance the CV and thus the energy which can be recovered. Improving the overall CV of the fuel increases the combustion efficiency and the greater bulk and energy densities ensure that combustion takes place within the bed at greater efficiency than the SMC alone. The reuse of both of these wastes also means less land is taken up by landfill sites and coal tailing lagoons; even the ash can be used in an environmental manner. Pyrolysis did produce a combination of solid, liquid and gaseous fuels and although the CVs of these products were reasonable, they were not necessarily sufficiently high to necessitate the wide-scale use of this technology. Other biomass materials could be used in this process to greater effect. Gasification of the material in a small-scale fixed-bed gasifier was not successful.
3.4. Discussion of results 4. Conclusions Three technologies were compared, where combustion was most successful and thus deemed the best thermal treatment for SMC and coal tailings, particularly in pellet form. The combustion of both the pellets and SMC were self-sustaining and sufficient
The main conclusions from this comparison of thermal treatments technologies for the reuse of SMC and coal tailings are as follows:
K.N. Finney et al. / Bioresource Technology 100 (2009) 310–315
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