- Email: [email protected]
peat pellets in a bench scale fluidised bed combustor
Energy 26 (2001) 81–90 www.elsevier.com/locate/energy
Behaviour of meat and bonemeal/peat pellets in a bench scale fluidised bed combustor K. McDonnell a
a,*
, J. Desmond a, J.J. Leahy b, R. Howard-Hildige c, S. Ward
a
Agricultural and Food Engineering Department, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland b Chemical and Environmental Science Department, University of Limerick, Plassey, Limerick, Ireland c Mechanical and Aeronautical Engineering Department, University of Limerick, Plassey, Limerick, Ireland Received 21 May 1999
Abstract As a result of the recent Bovine Spongiform Encephalopathy crisis in the European beef industry, safe animal by-product disposal is currently being addressed. One such disposal option is the combustion of by-product material such as meat and bone meal (MBM) in a fluidised bed combustor (FBC) for the purpose of energy recovery. Two short series of combustion tests were conducted on a FBC (10 cm diameter) at the University of Twente, the Netherlands. In the first series, pellets (10 mm in diameter and approximately 10 mm in length) were made from a mixture of MBM and milled peat, at MBM inclusion rates of 0%, 30%, 50%, 70% and 100%. In the second series of tests, the pellets were commercially made and were 4.8 mm in diameter and between 12 and 15 mm long. These pellets had a weight of about 0.3 g and contained 0%, 25%, 35%, 50% and 100% MBM inclusion with the peat. Both sets of pellets were combusted at 880°C. The residence times in the FBC varied from 300 s (25% MBM inclusion) to 120 s (100% MBM inclusion) for the first series of pellets. Increasing compaction pressure increased the residence time. For the second series of pellets, the residence time varied from about 300 s (25% MBM inclusion) to 100 s (100% MBM inclusion). MBM was found to be a volatile product (about 65%) and co-firing it with milled peat in a pelleted feed format reduces its volatile intensity. Pellets made from 100% bone based meal remained intact within the bed and are thought to have undergone a process of calcination during combustion. A maximum MBM inclusion rate of 35% with milled peat in a pellet is recommended from this work. 2001 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Tel.: +353-1-706-7236; fax: +353-1-706-7481. E-mail address: [email protected] (K. McDonnell).
0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 0 ) 0 0 0 4 8 - 7
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K. McDonnell et al. / Energy 26 (2001) 81–90
1. Introduction Meat and bonemeal (MBM) is produced in rendering plants where animal offal and bones are mixed, crushed and cooked together. During the cooking process, tallow is extracted and the remaining material which is dried and crushed further is known as MBM [1]. One of the main objectives in combustion of meat by-products such as MBM is to ensure that any living organism is totally destroyed during the process. Fluidised bed combustion is considered as it has the ability of being able to combust fuels efficiently and its low operating temperatures, 750–900°C, result in low emissions of NOx [2]. By increasing the residence time of the product within the combustion zone, the risk of uncombusted material escaping is minimised. Independent reports on the co-firing of milled MBM (2.0 mm sieved grade material) with coal in pulverised coal boilers (thermal outputs of 0.5 MW and 1 MW), have shown that complete combustion of the MBM can be achieved [3,4]. But another study [5] showed that dusty type fuels may travel through a fluidised bed without fully combusting. Co-firing trials on a 12 MW circulating fluidised bed boiler were conducted and emissions from mixtures of wood and coal were investigated. As sawdust was introduced into the bed, it travelled quickly through the freeboard and appeared in the cyclone unburned. The temperature difference between the cyclone inlet and outlet increased on average by 50°C, indicating combustion activity within this region. This suggests that dusty type fuels may travel through the combustor without fully burning out, hence the proposal to pelletise the MBM based fuels. MBM has good fuel properties compared with conventional fuels; it has an average calorific value of 16.18 MJ/kg [3], whereas peat has a value in the range 8–14 MJ/kg [6]. However, MBM’s nitrogen content of 9.85% [3] may lead to high NOx emissions, while its high ash content, averaging between 12.8 and 30.7% [7], will require an efficient ash offtake system. National Power plc (UK) has a 500 kW pulverised coal combustion facility which is designed such that combustible material is exposed to the same time/temperature history as in a commercial pulverised power station boiler, where peak temperatures are in excess of 1500°C with exposure to greater than 1000°C for more than 1 s. Trials were conducted on a range of coal/MBM mixtures, as well as tallow/MBM mixtures [4]. The UK Environmental Agency (EA) guidelines state that protein content should be no more than 0.005% in the ash sample [7]. Ash samples taken at the exit end of the furnace, which were exposed to temperatures of between 1200°C and 1500°C for a period of 4 to 5 s (residence time), had protein levels within the EA guidelines. Samples obtained from the ‘slagging end’ of the furnace (where a residence time of 1 s was estimated) had protein levels greater than 0.005%. Samples taken from a point which was exposed to a residence time of 0.25 s had residual protein levels in excess of the EA limit [7]. These results demonstrate the need for adequate residence times to reduce protein levels in the ash. 2. Fluidised bed combustion of solid fuel particles When a fuel pellet is introduced into the fluidised bed combustor it first starts to dry and devolatilise, followed by the ignition and combustion of volatiles and the residual char [8]. The majority of energy within a fuel may be released by devolatilisation, for example as coal enters a fluidised bed combuster via pneumatic injection, it may release up to 50% of its chemical energy
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near the feed input point as a result of initial devolatilisation. The combustion of volatiles is a gas to gas reaction, i.e. the gases released from the pellet react and combust with the oxygen in the combustor system. Volatile combustion is indicated by the presence of smoke and flames. Once the volatiles have evolved, a solid porous particle remains; this is known as a char particle, which contains mainly carbon and mineral matter. Char combustion is a gas to solid reaction and will therefore occur on the contact surface between the solid and gas. For a porous solid fuel this contact surface area can be high, allowing oxygen to quickly diffuse within the particle. It is important that all the char particles are fully combusted quickly because they are usually small and light compared with the bed material, therefore they may eventually escape from the bed by entrainment in the upward gas flow. 3. Methods and materials 3.1. First series of pellets The work on the behaviour of pelleted MBM (including fish and poultry MBM) fuel material was conducted in two test stages. Initially, Irish sourced MBM or pure bone material (bonemeal) were mixed with Irish milled peat and made into pellets using a hydraulic operated press and die rig. These pellets had a diameter of 10 mm and an average length of 10 mm. Both the MBM and bonemeal were crushed to a maximum particle size of 2.0 mm. Milled peat has a particle size between 5 µm to 50 mm [9] with 90% of a given sample less than 10 mm [10]. The peat was crushed to a uniform powder (⬍3 mm) using a mortar and pestle prior to mixing with the MBM or bonemeal products. The materials had the following moisture content: milled Irish peat, 7.83%; fish bonemeal, 2.85%; MBM, 3.8%; pure bonemeal, 1%; poultry MBM, 8.65%. The milled peat was exceptionally dry compared with a typical power station bunker moisture value between 45 and 65%. Four mixtures were made from the different by-products (MBM or bonemeal) and peat on a weight basis (30%, 50%, 70% or 100% by-product). The peat/poultry MBM pellets weighed on average 1.2 g while the peat/fish bonemeal pellets, peat/bonemeal pellets, and peat/MBM pellets all weighed on average 1 g. All the mixtures were pelleted to a compaction pressure of 10 kg/cm2 with the exception of pure fish bonemeal pellets, which were produced at 20 kg/cm2. Three different types of pure peat pellets (average weight 1.2 g) were made up using different compaction pressures (6 kg/cm2, 10 kg/cm2 and 20 kg/cm2) to investigate the influence of compaction pressure. 3.2. Second series of pellets The second series of tests concentrated on peat/MBM pellets only as the results from the first test series showed this mixture to be the most suited to fluidised bed combustion (see Section 4). Irish sourced milled peat and MBM were mixed into batches of varying weight-based ratios, which were then commercially mixed and pelletised. The milled peat originally had a moisture content of between 40 and 50%, which would cause operation difficulties during the pelleting process. Therefore, the peat was dried to within a 20–30% moisture level. Milled peat has an average bulk density of 350 kg/m3 compared with MBM which has an average of 700 kg/m3.
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The MBM (crushed to 2.0 mm) was of Irish origin and processed in accordance with the current rendering regulation 96/449/EC [11]. It had a moisture content of 6–8%. The mixed material was commercially pelletised by CPM, (Amsterdam) using a standard roller and die system which had a process capacity of 50 kg/h. A range of dies with varying compaction ratios were used (Table 1). Die selection was based upon optimal process efficiency (energy consumption factors) and pellet quality, which implies that the pellets do not break up easily when mechanically handled (e.g. screw conveyor). The compaction ratio is the ratio of the pellet diameter to the die extrusion length. A low compaction ratio was used for the pure peat, as this resisted extrusion through the die due to its fibrous/abrasive nature. However, the MBM required a much higher compaction ratio as a result of its fatty properties which naturally lubricated the die, thus allowing the material to travel through the die more freely. As a result, a die with a high compaction ratio (1:13) was used in order to form a strong pellet (Table 1). All the pellets produced appeared damp, soft and warm immediately after processing with die exit temperatures ranging from 50 to 60°C. Immediate post drying of the pellets is important as they are liable to bond together in this warm/damp environment. The 100% MBM pellets proved to be the softest upon die exit. This was attributed to the fact that optimal commercial process conditions were not applied. Commercially MBM is pelleted at a die entry temperature of 90°C and a compaction ratio of 1:18. The pellets for the second series of trials had a diameter of 4.8 mm and an average weight of 0.3 g. 3.3. 10 cm fluidised bed combustor The combustion was undertaken on an atmospheric FBC (10 cm diameter) located at the University of Twente, The Netherlands (Fig. 1). The bed wall was composed of a quartz glass column; pellets were manually dropped into the bed. The bed material consisted of alumina oxide particles, which had diameters of between 0.5 mm and 0.8 mm. The material had a particle density of 2500 kg/m3. The bed was heated electrically via a series of heating coils around the bed and splash zone. The temperature was regulated manually by a rheostat based on the bed thermocouple reading. For the first test the bed temperature was 880°C, it was set at 800°C for the second test and the superficial gas velocity was set at 0.8 m/s for both tests. The residence time of the individual pellets was recorded visually using a stopwatch; the combustion characteristics of the pellets were also noted. Table 1 Compaction ratios used in the second series of tests Peat (%)
MBM (%)
100 75 65 50 0
0 25 35 50 100
a
Number of observations (n)=10.
Compaction ratio
Average pellet length (mm)
1:4 1:5 1:5 1:5 1:13
15 15 15 15 12
SDa 1.2 1.1 1.1 1.1 1.2
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Fig. 1. Atmospheric fluidised bed combustor (10 cm in diameter).
3.4. Error measurements for the data The data presented represent the average of between three and 10 measurements. The standard deviation (SD) was used as the numbers are a sample of the population. The calculation for the standard deviation was: SD⫽
⌺(x−m)2 (n−1)
冪
where x is the measurement point, m is the arithmetic mean and n is the number of measurements. The standard deviation was calculated using the ‘nonbiased’ or ‘n⫺1’ method with n varying from 3 to 10. Error bars were used to graphically express measured error amounts (i.e. minimum and maximum values obtained) relative to each data average in a data series.
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4. Results and discussion Each pellet displayed typical solid fuel combustion behaviour, consisting of an initial period where smoke and flames were emitted which indicated the presence of volatiles. This was followed by the process of char combustion. During volatile combustion popping was heard, which may be caused by pressure build-up inside the pellet due to the liberation of pyrolysis products such as carbon dioxide within the pellet. As the pellets emerged from the bed during combustion a blue flame was observed, indicating the presence of carbon monoxide, thus combustion activity took place in the bed. In the first test the peat/MBM pellets displayed volatile combustion for a mean period of between 40 and 50 s. Compounds with higher bone concentrations (fish bonemeal and bonemeal) had a mean volatile evolution (life) of 30 s. The 100% pure bone based products remained intact after both volatile and char combustion, they therefore had the longest residence time and remained in the bed. The residence time of MBM and poultry MBM increased when mixed with peat (Fig. 2). It was observed that MBM material with high peat inclusion ratios showed less volatile behaviour (demonstrated by a reduction in flame height and intensity) compared with compounds with low or no peat included in the pellet. During this initial test sparks were emitted from each pellet which contained peat. The rate of spark emissions decreased as the volume of peat decreased. No sparks were emitted from compounds containing 100% bone products. Combustion of 100% MBM pellets were accompanied by sparks. During the first test the influence of compaction pressure of pure peat pellets was examined. From Fig. 3 it can be seen that the residence time increased with increasing compaction pressure. The volume of smoke associated with the pure peat pellets decreased as the compaction pressure was increased. The flame life was on average 20 s at the lower compaction pressure while an average flame life of 30 s was observed at the higher compaction pressure. Also the flame height and intensity decreased with increasing compaction pressure. During combustion, pellets with a lower compaction pressure split into multiple pieces while at higher compaction pressures the pellets split into two or three pieces, indicating increased stability. The rate of spark emissions from individual pellets decreased with compaction pressure.
Fig. 2. Average residence time vs. pellet composition (first series of trials, measurement error bars included).
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Fig. 3. Residence time vs. manufacturing compaction pressure (first trial series, measurement error bars included). Table 2 Analysis of pellets used in the second series of testsa
C H N Ash Moisture a
100% peat
25% MBM 75% peat
35% MBM 65% peat
50% MBM 50% peat
100% MBM
52.03 (0.11) 5.31 (0.02) 1.38 (0.01) 3.5 (0.01) 23.0 (0.80)
46.87 (0.12) 5.42 (0.02) 3.27 (0.01) 12.5 (0.20) 20.10 (0.78)
47.34 (0.11) 5.76 (0.14) 4.49 (0.08) 14.8 (0.22) 23.2 (0.95)
45.89 (0.11) 5.79 (0.06) 5.33 (0.03) 17.5 (0.32) 15.0 (0.74)
40.31 (0.15) 5.97 (0.18) 7.76 (0.12) 30.4 (1.24) 6.90 (0.07)
Values in parentheses are the standard deviation (n=3).
In the second test 10 pellets from each batch as per Table 2 were combusted. Fig. 4 shows that the average residence time increases with increasing peat inclusion ratio. From Table 3 all the pellets had similar flame times and the differences between the pelleted compounds were the char combustion time and the average residence time, as both were reduced with increasing vol-
Fig. 4. Residence time vs. pellet composition, second series of trials.
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Table 3 Pellet residence time (s) for the second combustion trial Pellet type
Average flame life (SDa)
Average char combustion (SDa)
100% Peat 25% MBM 75% Peat 35% MBM 65% Peat 50% MBM 50% Peat 100% MBM
33 (3.2) 35 (4.1)
387 (35) 268 (27)
420 303
37 (3.8)
228 (18)
265
38 (2.5)
219 (22)
257
32 (3.2)
64 (11)
96
a
Average residence time
n=10.
umes of MBM. The pure MBM pellets showed the shortest volatile and char combustion times. During combustion, pellets with a lower compaction pressure split into multiple pieces while at higher compaction pressures the pellets split into two or three pieces, indicating increased stability. The rate of spark emissions from individual pellets decreased with compaction pressure. During the volatile combustion phase, the peat and 25% MBM pellets stayed within the splash zone (Fig. 1), but the 35% and 50% MBM pellets showed a tendency to sink down into the bed. The 100% MBM pellets spent the majority of time immersed within the bed during the volatile combustion period. All of the pellets spent periodic intervals immersed 3–10 cm within the bed during char combustion. No sparks were emitted during this second test. In the first series of tests it was observed that the residence time and the applied compaction pressure during manufacture influenced combustion characteristics. As a result of this observation a further test was conducted on the pellets used in the second series of combustion trials. Ten pellets from each batch, with an average length of 10 mm and a diameter of 4.8 mm, were tested for compression resistance using an Instron 4411 test machine. The cylindrical pellets were placed horizontally (lying upon their largest dimension) on a flat surface. A downward vertical load was applied to each individual pellet. The maximum load required to fracture the pellets was recorded. The results shown in Table 4 indicate that the 100% peat pellets were the most structurally sound, Table 4 Destruction values for pellets in second test 100% peat Max. destructive force (N) Min. destructive force (N) Mean destructive force (N) SDa a
n=10.
202 95 140 53.7
25% MBM 75% peat 171 65 126 53.2
35% MBM 65% peat 142 80 105 31.2
50% MBM 50% peat 124 25 61 50.1
100% MBM 39 13 21 13.3
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requiring a mean destruction value of 140 N. The variation in the results is an indication of the heterogeneous make-up of the raw material. The overlap in destructive forces, especially between the peat/MBM batches, indicates a level of consistent pellet structural quality across the various batches. As a result of this consistency between mixture batches, the variation in residence time was not greatly influenced by the individual manufactured quality of the pellets. The 100% MBM pellets required the least destructive force, and therefore the residence time may have been reduced relative to the other batch pellets due to its poorer structural quality. Hence, in the 100% MBM pellets residence time may be affected by pellet quality. 5. Conclusion Both tests showed the residence time of the pellets was decreased by increasing the MBM inclusion rate. For the second test the peat pellets displayed the longest char combustion period (an average of 387 s compared with 64 s for 100% MBM). The low char combustion for MBM may be as a result of its high ash content (30.4%). The 100% MBM pellets showed the most intense volatile activity; this was evident as these pellets showed relativity high flame lengths and intensity compared with the other pellets. The trials showed that the inclusion of peat within a pellet reduces the intensity of the volatile nature of MBM, hence producing a more stable combustion performance. From the first series of tests, pure bone products appeared unsuitable for fluidised combustion as they remained in the bed. At the end of all the tests bone particles were seen in the bed. In a commercial operation bone build-up may lead to bed hydrodynamic problems, such as an increase in pressure drop across the bed. It is believed that the bone material undergoes a process of calcination in the combustion zone, whereby the bone (CaCO3) is converted to calcium oxide (CaO) in the presence of heat. The commercially manufactured pellets appeared to be more structurally stable than those used in the first stage, as they emitted no sparks during combustion. The maximum MBM inclusion rate in the pellets appears to be about 35%. Inclusion rates higher than this produce soft pellets, which have a higher risk of break-up and disintegration during the combustion phase in a FBC. References [1] Bradley R. Sub-acute spongiform encephalopathies, an overview of the rendering structure and procedures in the European community. Eur J Epidemiol 1991;7:532–44. [2] Geldart, D. Gas fluidisation technology. Chichester, UK: Wiley, 1986. [3] Power Gen Power Technology report. The co-firing of bovine waste in PF-fired boilers—results of combustion test rig. Project No. PT/96/EA 1160/R, 1997. [4] National Power Technology report. Combustion of MBM and tallow at Dicot combustion test facility. Project No. TECH/BGC/017/97, 1997. [5] Lecker B, Karlsson M. Gaseous emissions from circulating fluidized bed combustion of wood. Biomass Bioenergy 1993;4(5):379–89. [6] Hourtai J, Flykatman M. Fossil and renewable fuels for power plant use. Nordic IFRF course: Solid fuels utilisation and enviroment, 1992.
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[7] IPC guidance note S2 1.05, Amplification note No. 1. Combustion of meat and bonemeal. UK Environmental Agency, 1998. [8] Van Der Honing G. Volatile and char combustion in large scale fluidized bed coal combustion. PhD Thesis, University of Twente, 1991. [9] Mooney SJ, Holden NM, Ward SM, Collins JF. The rapid preparation of structural images from undisturbed, noncohesive material (milled peat). Geoderma 1998;86:159–68. [10] Holden NM, Ward SM. The physical properties of stockpiled milled peat from midland production bogs. Ir J Agric Food Res 1997;36:205–18. [11] 96/449/EC. Commission decision on the approval of alternative heat treatment systems for processing animal waste with a view to the inactivation of spongiform encephalopathy agents. Off. J. Eur. Communities, 1996;L184:43–6.
Behaviour of meat and bonemeal/peat pellets in a bench scale fluidised bed combustor K. McDonnell a
a,*
, J. Desmond a, J.J. Leahy b, R. Howard-Hildige c, S. Ward
a
Agricultural and Food Engineering Department, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland b Chemical and Environmental Science Department, University of Limerick, Plassey, Limerick, Ireland c Mechanical and Aeronautical Engineering Department, University of Limerick, Plassey, Limerick, Ireland Received 21 May 1999
Abstract As a result of the recent Bovine Spongiform Encephalopathy crisis in the European beef industry, safe animal by-product disposal is currently being addressed. One such disposal option is the combustion of by-product material such as meat and bone meal (MBM) in a fluidised bed combustor (FBC) for the purpose of energy recovery. Two short series of combustion tests were conducted on a FBC (10 cm diameter) at the University of Twente, the Netherlands. In the first series, pellets (10 mm in diameter and approximately 10 mm in length) were made from a mixture of MBM and milled peat, at MBM inclusion rates of 0%, 30%, 50%, 70% and 100%. In the second series of tests, the pellets were commercially made and were 4.8 mm in diameter and between 12 and 15 mm long. These pellets had a weight of about 0.3 g and contained 0%, 25%, 35%, 50% and 100% MBM inclusion with the peat. Both sets of pellets were combusted at 880°C. The residence times in the FBC varied from 300 s (25% MBM inclusion) to 120 s (100% MBM inclusion) for the first series of pellets. Increasing compaction pressure increased the residence time. For the second series of pellets, the residence time varied from about 300 s (25% MBM inclusion) to 100 s (100% MBM inclusion). MBM was found to be a volatile product (about 65%) and co-firing it with milled peat in a pelleted feed format reduces its volatile intensity. Pellets made from 100% bone based meal remained intact within the bed and are thought to have undergone a process of calcination during combustion. A maximum MBM inclusion rate of 35% with milled peat in a pellet is recommended from this work. 2001 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Tel.: +353-1-706-7236; fax: +353-1-706-7481. E-mail address: [email protected] (K. McDonnell).
0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 0 ) 0 0 0 4 8 - 7
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K. McDonnell et al. / Energy 26 (2001) 81–90
1. Introduction Meat and bonemeal (MBM) is produced in rendering plants where animal offal and bones are mixed, crushed and cooked together. During the cooking process, tallow is extracted and the remaining material which is dried and crushed further is known as MBM [1]. One of the main objectives in combustion of meat by-products such as MBM is to ensure that any living organism is totally destroyed during the process. Fluidised bed combustion is considered as it has the ability of being able to combust fuels efficiently and its low operating temperatures, 750–900°C, result in low emissions of NOx [2]. By increasing the residence time of the product within the combustion zone, the risk of uncombusted material escaping is minimised. Independent reports on the co-firing of milled MBM (2.0 mm sieved grade material) with coal in pulverised coal boilers (thermal outputs of 0.5 MW and 1 MW), have shown that complete combustion of the MBM can be achieved [3,4]. But another study [5] showed that dusty type fuels may travel through a fluidised bed without fully combusting. Co-firing trials on a 12 MW circulating fluidised bed boiler were conducted and emissions from mixtures of wood and coal were investigated. As sawdust was introduced into the bed, it travelled quickly through the freeboard and appeared in the cyclone unburned. The temperature difference between the cyclone inlet and outlet increased on average by 50°C, indicating combustion activity within this region. This suggests that dusty type fuels may travel through the combustor without fully burning out, hence the proposal to pelletise the MBM based fuels. MBM has good fuel properties compared with conventional fuels; it has an average calorific value of 16.18 MJ/kg [3], whereas peat has a value in the range 8–14 MJ/kg [6]. However, MBM’s nitrogen content of 9.85% [3] may lead to high NOx emissions, while its high ash content, averaging between 12.8 and 30.7% [7], will require an efficient ash offtake system. National Power plc (UK) has a 500 kW pulverised coal combustion facility which is designed such that combustible material is exposed to the same time/temperature history as in a commercial pulverised power station boiler, where peak temperatures are in excess of 1500°C with exposure to greater than 1000°C for more than 1 s. Trials were conducted on a range of coal/MBM mixtures, as well as tallow/MBM mixtures [4]. The UK Environmental Agency (EA) guidelines state that protein content should be no more than 0.005% in the ash sample [7]. Ash samples taken at the exit end of the furnace, which were exposed to temperatures of between 1200°C and 1500°C for a period of 4 to 5 s (residence time), had protein levels within the EA guidelines. Samples obtained from the ‘slagging end’ of the furnace (where a residence time of 1 s was estimated) had protein levels greater than 0.005%. Samples taken from a point which was exposed to a residence time of 0.25 s had residual protein levels in excess of the EA limit [7]. These results demonstrate the need for adequate residence times to reduce protein levels in the ash. 2. Fluidised bed combustion of solid fuel particles When a fuel pellet is introduced into the fluidised bed combustor it first starts to dry and devolatilise, followed by the ignition and combustion of volatiles and the residual char [8]. The majority of energy within a fuel may be released by devolatilisation, for example as coal enters a fluidised bed combuster via pneumatic injection, it may release up to 50% of its chemical energy
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near the feed input point as a result of initial devolatilisation. The combustion of volatiles is a gas to gas reaction, i.e. the gases released from the pellet react and combust with the oxygen in the combustor system. Volatile combustion is indicated by the presence of smoke and flames. Once the volatiles have evolved, a solid porous particle remains; this is known as a char particle, which contains mainly carbon and mineral matter. Char combustion is a gas to solid reaction and will therefore occur on the contact surface between the solid and gas. For a porous solid fuel this contact surface area can be high, allowing oxygen to quickly diffuse within the particle. It is important that all the char particles are fully combusted quickly because they are usually small and light compared with the bed material, therefore they may eventually escape from the bed by entrainment in the upward gas flow. 3. Methods and materials 3.1. First series of pellets The work on the behaviour of pelleted MBM (including fish and poultry MBM) fuel material was conducted in two test stages. Initially, Irish sourced MBM or pure bone material (bonemeal) were mixed with Irish milled peat and made into pellets using a hydraulic operated press and die rig. These pellets had a diameter of 10 mm and an average length of 10 mm. Both the MBM and bonemeal were crushed to a maximum particle size of 2.0 mm. Milled peat has a particle size between 5 µm to 50 mm [9] with 90% of a given sample less than 10 mm [10]. The peat was crushed to a uniform powder (⬍3 mm) using a mortar and pestle prior to mixing with the MBM or bonemeal products. The materials had the following moisture content: milled Irish peat, 7.83%; fish bonemeal, 2.85%; MBM, 3.8%; pure bonemeal, 1%; poultry MBM, 8.65%. The milled peat was exceptionally dry compared with a typical power station bunker moisture value between 45 and 65%. Four mixtures were made from the different by-products (MBM or bonemeal) and peat on a weight basis (30%, 50%, 70% or 100% by-product). The peat/poultry MBM pellets weighed on average 1.2 g while the peat/fish bonemeal pellets, peat/bonemeal pellets, and peat/MBM pellets all weighed on average 1 g. All the mixtures were pelleted to a compaction pressure of 10 kg/cm2 with the exception of pure fish bonemeal pellets, which were produced at 20 kg/cm2. Three different types of pure peat pellets (average weight 1.2 g) were made up using different compaction pressures (6 kg/cm2, 10 kg/cm2 and 20 kg/cm2) to investigate the influence of compaction pressure. 3.2. Second series of pellets The second series of tests concentrated on peat/MBM pellets only as the results from the first test series showed this mixture to be the most suited to fluidised bed combustion (see Section 4). Irish sourced milled peat and MBM were mixed into batches of varying weight-based ratios, which were then commercially mixed and pelletised. The milled peat originally had a moisture content of between 40 and 50%, which would cause operation difficulties during the pelleting process. Therefore, the peat was dried to within a 20–30% moisture level. Milled peat has an average bulk density of 350 kg/m3 compared with MBM which has an average of 700 kg/m3.
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The MBM (crushed to 2.0 mm) was of Irish origin and processed in accordance with the current rendering regulation 96/449/EC [11]. It had a moisture content of 6–8%. The mixed material was commercially pelletised by CPM, (Amsterdam) using a standard roller and die system which had a process capacity of 50 kg/h. A range of dies with varying compaction ratios were used (Table 1). Die selection was based upon optimal process efficiency (energy consumption factors) and pellet quality, which implies that the pellets do not break up easily when mechanically handled (e.g. screw conveyor). The compaction ratio is the ratio of the pellet diameter to the die extrusion length. A low compaction ratio was used for the pure peat, as this resisted extrusion through the die due to its fibrous/abrasive nature. However, the MBM required a much higher compaction ratio as a result of its fatty properties which naturally lubricated the die, thus allowing the material to travel through the die more freely. As a result, a die with a high compaction ratio (1:13) was used in order to form a strong pellet (Table 1). All the pellets produced appeared damp, soft and warm immediately after processing with die exit temperatures ranging from 50 to 60°C. Immediate post drying of the pellets is important as they are liable to bond together in this warm/damp environment. The 100% MBM pellets proved to be the softest upon die exit. This was attributed to the fact that optimal commercial process conditions were not applied. Commercially MBM is pelleted at a die entry temperature of 90°C and a compaction ratio of 1:18. The pellets for the second series of trials had a diameter of 4.8 mm and an average weight of 0.3 g. 3.3. 10 cm fluidised bed combustor The combustion was undertaken on an atmospheric FBC (10 cm diameter) located at the University of Twente, The Netherlands (Fig. 1). The bed wall was composed of a quartz glass column; pellets were manually dropped into the bed. The bed material consisted of alumina oxide particles, which had diameters of between 0.5 mm and 0.8 mm. The material had a particle density of 2500 kg/m3. The bed was heated electrically via a series of heating coils around the bed and splash zone. The temperature was regulated manually by a rheostat based on the bed thermocouple reading. For the first test the bed temperature was 880°C, it was set at 800°C for the second test and the superficial gas velocity was set at 0.8 m/s for both tests. The residence time of the individual pellets was recorded visually using a stopwatch; the combustion characteristics of the pellets were also noted. Table 1 Compaction ratios used in the second series of tests Peat (%)
MBM (%)
100 75 65 50 0
0 25 35 50 100
a
Number of observations (n)=10.
Compaction ratio
Average pellet length (mm)
1:4 1:5 1:5 1:5 1:13
15 15 15 15 12
SDa 1.2 1.1 1.1 1.1 1.2
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Fig. 1. Atmospheric fluidised bed combustor (10 cm in diameter).
3.4. Error measurements for the data The data presented represent the average of between three and 10 measurements. The standard deviation (SD) was used as the numbers are a sample of the population. The calculation for the standard deviation was: SD⫽
⌺(x−m)2 (n−1)
冪
where x is the measurement point, m is the arithmetic mean and n is the number of measurements. The standard deviation was calculated using the ‘nonbiased’ or ‘n⫺1’ method with n varying from 3 to 10. Error bars were used to graphically express measured error amounts (i.e. minimum and maximum values obtained) relative to each data average in a data series.
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4. Results and discussion Each pellet displayed typical solid fuel combustion behaviour, consisting of an initial period where smoke and flames were emitted which indicated the presence of volatiles. This was followed by the process of char combustion. During volatile combustion popping was heard, which may be caused by pressure build-up inside the pellet due to the liberation of pyrolysis products such as carbon dioxide within the pellet. As the pellets emerged from the bed during combustion a blue flame was observed, indicating the presence of carbon monoxide, thus combustion activity took place in the bed. In the first test the peat/MBM pellets displayed volatile combustion for a mean period of between 40 and 50 s. Compounds with higher bone concentrations (fish bonemeal and bonemeal) had a mean volatile evolution (life) of 30 s. The 100% pure bone based products remained intact after both volatile and char combustion, they therefore had the longest residence time and remained in the bed. The residence time of MBM and poultry MBM increased when mixed with peat (Fig. 2). It was observed that MBM material with high peat inclusion ratios showed less volatile behaviour (demonstrated by a reduction in flame height and intensity) compared with compounds with low or no peat included in the pellet. During this initial test sparks were emitted from each pellet which contained peat. The rate of spark emissions decreased as the volume of peat decreased. No sparks were emitted from compounds containing 100% bone products. Combustion of 100% MBM pellets were accompanied by sparks. During the first test the influence of compaction pressure of pure peat pellets was examined. From Fig. 3 it can be seen that the residence time increased with increasing compaction pressure. The volume of smoke associated with the pure peat pellets decreased as the compaction pressure was increased. The flame life was on average 20 s at the lower compaction pressure while an average flame life of 30 s was observed at the higher compaction pressure. Also the flame height and intensity decreased with increasing compaction pressure. During combustion, pellets with a lower compaction pressure split into multiple pieces while at higher compaction pressures the pellets split into two or three pieces, indicating increased stability. The rate of spark emissions from individual pellets decreased with compaction pressure.
Fig. 2. Average residence time vs. pellet composition (first series of trials, measurement error bars included).
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Fig. 3. Residence time vs. manufacturing compaction pressure (first trial series, measurement error bars included). Table 2 Analysis of pellets used in the second series of testsa
C H N Ash Moisture a
100% peat
25% MBM 75% peat
35% MBM 65% peat
50% MBM 50% peat
100% MBM
52.03 (0.11) 5.31 (0.02) 1.38 (0.01) 3.5 (0.01) 23.0 (0.80)
46.87 (0.12) 5.42 (0.02) 3.27 (0.01) 12.5 (0.20) 20.10 (0.78)
47.34 (0.11) 5.76 (0.14) 4.49 (0.08) 14.8 (0.22) 23.2 (0.95)
45.89 (0.11) 5.79 (0.06) 5.33 (0.03) 17.5 (0.32) 15.0 (0.74)
40.31 (0.15) 5.97 (0.18) 7.76 (0.12) 30.4 (1.24) 6.90 (0.07)
Values in parentheses are the standard deviation (n=3).
In the second test 10 pellets from each batch as per Table 2 were combusted. Fig. 4 shows that the average residence time increases with increasing peat inclusion ratio. From Table 3 all the pellets had similar flame times and the differences between the pelleted compounds were the char combustion time and the average residence time, as both were reduced with increasing vol-
Fig. 4. Residence time vs. pellet composition, second series of trials.
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Table 3 Pellet residence time (s) for the second combustion trial Pellet type
Average flame life (SDa)
Average char combustion (SDa)
100% Peat 25% MBM 75% Peat 35% MBM 65% Peat 50% MBM 50% Peat 100% MBM
33 (3.2) 35 (4.1)
387 (35) 268 (27)
420 303
37 (3.8)
228 (18)
265
38 (2.5)
219 (22)
257
32 (3.2)
64 (11)
96
a
Average residence time
n=10.
umes of MBM. The pure MBM pellets showed the shortest volatile and char combustion times. During combustion, pellets with a lower compaction pressure split into multiple pieces while at higher compaction pressures the pellets split into two or three pieces, indicating increased stability. The rate of spark emissions from individual pellets decreased with compaction pressure. During the volatile combustion phase, the peat and 25% MBM pellets stayed within the splash zone (Fig. 1), but the 35% and 50% MBM pellets showed a tendency to sink down into the bed. The 100% MBM pellets spent the majority of time immersed within the bed during the volatile combustion period. All of the pellets spent periodic intervals immersed 3–10 cm within the bed during char combustion. No sparks were emitted during this second test. In the first series of tests it was observed that the residence time and the applied compaction pressure during manufacture influenced combustion characteristics. As a result of this observation a further test was conducted on the pellets used in the second series of combustion trials. Ten pellets from each batch, with an average length of 10 mm and a diameter of 4.8 mm, were tested for compression resistance using an Instron 4411 test machine. The cylindrical pellets were placed horizontally (lying upon their largest dimension) on a flat surface. A downward vertical load was applied to each individual pellet. The maximum load required to fracture the pellets was recorded. The results shown in Table 4 indicate that the 100% peat pellets were the most structurally sound, Table 4 Destruction values for pellets in second test 100% peat Max. destructive force (N) Min. destructive force (N) Mean destructive force (N) SDa a
n=10.
202 95 140 53.7
25% MBM 75% peat 171 65 126 53.2
35% MBM 65% peat 142 80 105 31.2
50% MBM 50% peat 124 25 61 50.1
100% MBM 39 13 21 13.3
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requiring a mean destruction value of 140 N. The variation in the results is an indication of the heterogeneous make-up of the raw material. The overlap in destructive forces, especially between the peat/MBM batches, indicates a level of consistent pellet structural quality across the various batches. As a result of this consistency between mixture batches, the variation in residence time was not greatly influenced by the individual manufactured quality of the pellets. The 100% MBM pellets required the least destructive force, and therefore the residence time may have been reduced relative to the other batch pellets due to its poorer structural quality. Hence, in the 100% MBM pellets residence time may be affected by pellet quality. 5. Conclusion Both tests showed the residence time of the pellets was decreased by increasing the MBM inclusion rate. For the second test the peat pellets displayed the longest char combustion period (an average of 387 s compared with 64 s for 100% MBM). The low char combustion for MBM may be as a result of its high ash content (30.4%). The 100% MBM pellets showed the most intense volatile activity; this was evident as these pellets showed relativity high flame lengths and intensity compared with the other pellets. The trials showed that the inclusion of peat within a pellet reduces the intensity of the volatile nature of MBM, hence producing a more stable combustion performance. From the first series of tests, pure bone products appeared unsuitable for fluidised combustion as they remained in the bed. At the end of all the tests bone particles were seen in the bed. In a commercial operation bone build-up may lead to bed hydrodynamic problems, such as an increase in pressure drop across the bed. It is believed that the bone material undergoes a process of calcination in the combustion zone, whereby the bone (CaCO3) is converted to calcium oxide (CaO) in the presence of heat. The commercially manufactured pellets appeared to be more structurally stable than those used in the first stage, as they emitted no sparks during combustion. The maximum MBM inclusion rate in the pellets appears to be about 35%. Inclusion rates higher than this produce soft pellets, which have a higher risk of break-up and disintegration during the combustion phase in a FBC. References [1] Bradley R. Sub-acute spongiform encephalopathies, an overview of the rendering structure and procedures in the European community. Eur J Epidemiol 1991;7:532–44. [2] Geldart, D. Gas fluidisation technology. Chichester, UK: Wiley, 1986. [3] Power Gen Power Technology report. The co-firing of bovine waste in PF-fired boilers—results of combustion test rig. Project No. PT/96/EA 1160/R, 1997. [4] National Power Technology report. Combustion of MBM and tallow at Dicot combustion test facility. Project No. TECH/BGC/017/97, 1997. [5] Lecker B, Karlsson M. Gaseous emissions from circulating fluidized bed combustion of wood. Biomass Bioenergy 1993;4(5):379–89. [6] Hourtai J, Flykatman M. Fossil and renewable fuels for power plant use. Nordic IFRF course: Solid fuels utilisation and enviroment, 1992.
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[7] IPC guidance note S2 1.05, Amplification note No. 1. Combustion of meat and bonemeal. UK Environmental Agency, 1998. [8] Van Der Honing G. Volatile and char combustion in large scale fluidized bed coal combustion. PhD Thesis, University of Twente, 1991. [9] Mooney SJ, Holden NM, Ward SM, Collins JF. The rapid preparation of structural images from undisturbed, noncohesive material (milled peat). Geoderma 1998;86:159–68. [10] Holden NM, Ward SM. The physical properties of stockpiled milled peat from midland production bogs. Ir J Agric Food Res 1997;36:205–18. [11] 96/449/EC. Commission decision on the approval of alternative heat treatment systems for processing animal waste with a view to the inactivation of spongiform encephalopathy agents. Off. J. Eur. Communities, 1996;L184:43–6.