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Curing time effect on mechanical strength of smokeless fuel briquettes
Fuel Processing Technology 80 (2003) 155 – 167 www.elsevier.com/locate/fuproc
Curing time effect on mechanical strength of smokeless fuel briquettes M.J. Blesa *, J.L. Miranda, M.T. Izquierdo, R. Moliner Instituto de Carboquı´mica (CSIC), P.O. Box 589, 50080 Saragossa, Spain Received 4 June 2002; received in revised form 9 September 2002; accepted 25 September 2002
Abstract This work shows the effect of curing time on the mechanical properties of smokeless fuel briquettes which were prepared with olive stone and a low-rank coal previously co-carbonised at 600 jC. Humates and molasses were used as binders which acts with different roles, as a film or matrix depending on the curing. Additives like H3PO4 were also tested. Green briquettes were cured at different times and the molecular changes were followed by both Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decomposition (TPD) followed by mass spectrometry (MS). This last study helps to predict the final properties of the briquettes more clearly than infrared spectroscopy does. This technique appreciates changes when the impact-resistance test does not show differences. The effect of the H3PO4 on briquettes prepared with molasses produce a stabilisation of these materials. Moreover, the mechanical resistance of the studied briquettes is improved by the effect of the curing time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Smokeless fuel briquettes; Binders; Curing; Infrared spectroscopy; Temperature programmed decomposition
1. Introduction The briquetting of coal has been largely empirical relying on simple physical tests methods and experiences [1]. A greater understanding of the physics and chemistry of coal briquetting could lead to better briquette performances and cost-effectiveness and widen the range of coals that can be briquetted successfully making these fuels more attractive to
* Corresponding author. Tel.: +34-976-733-977; fax: +34-976-733-318. E-mail address: [email protected] (M.J. Blesa). 0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 2 4 3 - 6
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consumers [2]. The curing process is one of the final steps of the briquetting that produces transformations –interactions – changes between the carbonised materials themselves as well as with the binder. As a result of this process, the briquettes improve the mechanical resistance and the behaviour of the briquettes in water [3]. The curing should be fixed to both technical and economic aspects so that the global process provides adequate final properties for briquettes [4]. Although there are a lot of studies carried out about agglomeration, most of them are about the empirical subjects and they lack scientific matters that are especially relevant with the curing process [1,5]. In this work, the feasibility to produce environmentally acceptable smokeless fuel briquettes from low-rank coals and biomasses and a better understanding of the physics and chemistry of the curing have been studied to know the role of the binders and the process of curing. Feedstocks for smokeless fuel briquette manufacture include several materials, such as a low-rank coal and olive stones that require to be previously carbonised to reduce the volatile matter and the sulphur content of the coal [6]. Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decomposition (TPD) have been used to acquire information on the main structural changes produced in these binders with the curing [7] and how the presence of different oxygenated functional groups and polymerisations have influence on the final properties of the prepared briquettes. TPD studies followed by mass spectrometry (MS) will help to know the different thermal stability of the structures that constitute briquettes [8].
2. Experimental 2.1. Characterisation of the raw materials The proximate analysis, the sulphur content and the high calorific value of Maria coal (M2), olive stones (O), humates and molasses have been shown in previous papers [6,9]. 2.2. Carbonisation of the selected raw materials The coal used is a low-rank coal named Maria (M2) from Teruel (Spain). This coal was carbonised at 500, 550, 600, 650 and 700 jC to choose the best temperature of desulphurisation [6]. Low temperature carbonised materials were prepared by copyrolysis of M2 coal and olive stone (O), at the optimum temperature, 600 jC, in order to reduce the sulphur content and to increase the high calorific value of the materials [10]. Therefore, the co-carbonised material named (M2 + O)6,50, was prepared at 600 jC with a mixture of M2/O (1:2) to get approximately 50% of coal in the co-carbonised material according to the yield of the carbonisation at 600 jC of M2 and O, separately. 2.3. Briquetting of the co-carbonised materials The co-carbonised material, (M2 + O)6,50, with a size < 1 mm, was mixed with a binder. The amount of binder was optimised between 5.0% and 6.5% for briquettes
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prepared with humates and between 10% and 16% for those prepared with molasses because of the different nature of these binders. Adequate mechanical resistance was reached with 6% humates and 16% molasses. The blend was cold-pressed at 125 MPa in a plug and mould press to produce cylindrical briquettes of approximately 10.5 mm in diameter, 13.5 mm in height and 1.2 g in weight. The briquettes prepared with the co-carbonised (M2 + O)6,50 and humates were cured at room temperature ( f 25 jC) in air at different periods of time from 3 to 48 h. When molasses were used as binder, the briquettes were also cured during 72 h. The proximate analyses of the prepared briquettes, moisture, ash and volatile matter, were determined according to the ISO 589/1981, 562/1974 and 1171/1976 procedures, respectively. The ultimate analyses (C, H, N, S) were determined using a CARLO ERBA 1108 elemental analyser. The high calorific values were determined following the ISO 1928/1976 procedure. In addition, all briquettes are characterised by both Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decompositions (TPD) to better understand the curing process. 2.4. FT-IR spectroscopy and temperature programmed decomposition studies FT-IR spectra of briquettes were run on KBr pellets (120 mg, 1 wt.%) and recorded coadding 64 scans at a resolution of 2 cm 1 in a Nicolet Magna 550 spectrometer. Spectra were scaled to 1 mg sample. The TPD studies were carried out in a vertical reactor (i.d. = 15 mm, L = 380 mm) with one briquette. The temperature was increased up to 600 jC at the rate of 10 jC/ min; after 30 min at 600 jC, it was heated up to 850 jC and it was maintained for 60 min. A flow of 50 cm3/min of the mixture 21% O2 and 79% Ar was passed through the reactor. The gases released went out through a capilar heated at 110 jC to avoid condensations. The evolution of the fragment m/z 15, 22 and 31 was followed by mass spectrometry (MS). These peaks were normalised with the maximum intensity of each run and the results show high repeatability [11]. 2.5. Impact resistance index (IRI) Each briquette was repeatedly dropped from a stationary start at a height of 2 m on to a steel floor until it fractures. The number of drops and the number of pieces the briquette broke into were recorded. Then, these data were used to calculate the impact resistance index (IRI) from the equation:
IRI ¼ ðð100 average number of dropsÞ=average number of piecesÞ
ð1Þ
For laboratory work, an IRI value of 50 was adopted as the lowest acceptable value for fuel briquettes developed for industrial or domestic applications [12].
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3. Results and discussion The production of smokeless fuel briquettes requires the carbonisation of the raw materials used, the low-rank coal (M2) and olive stones (O), to decrease their volatile matter content. As these briquettes have a lower amount of volatile matter than the original materials [4,6], the combustion of this new fuel goes by with a very low production of both smoke and tars, and the reduction in the amount of highly condensed aromatic hydrocarbons produced [13]. Preliminary studies showed that the amount of binder required for briquettes prepared with molasses was higher than that required with humates, consequently, their function is different [14]. Matrix type binders, as molasses, require rather substantial quantities because the strength depends upon the presence of a continuous phase of binder that encloses the solid particles. Table 1 depicts the proximate and ultimate analyses, the high calorific values, as well as the ratio sulphur/thermie (% S/th) of the briquettes prepared. The moisture content of those prepared with molasses and H3PO4 is lower than that of briquettes prepared just with molasses due to the presence of the acid which favours the dehydration of the briquettes. The briquettes prepared with molasses have higher amounts of ash because of the nature of this binder. Smokeless briquettes have been prepared according to Kukrety et al. [15] who considers that these materials are environmentally acceptable because the volatile matter amount is lower than 14% and the high calorific value is considerably higher than 15 MJ/kg. 3.1. Briquettes prepared with humates Fuel briquettes need to be able to withstand the crushing loads they receive during handling, transport, storage and firing. The impact resistance index is considered to be the best general diagnostic of the briquette strength. Moreover, this test suits our needs; it is an easy operation and gives comparable results. The IRI was used as a criteria to check the
Table 1 Proximate and ultimate analysis, high calorific value and % S/th content of the briquettes prepared with (M2 + O)6,50
Moisture Ash VM FC Ca Ha Na Sa a Oby diff HCVa (MJ/kg) S/th
Humates (%)
Molasses (%)
Molasses + H3PO4 (%)
3.9 8.9 13.4 73.8 81.6 2.1 1.4 1.4 4.2 30.8 0.19
3.7 12.9 11.9 71.5 73.6 2.0 2.0 2.0 7.0 27.3 0.31
2.5 12.1 13.4 72.0 75.3 1.6 1.8 1.8 7.1 27.6 0.27
VM: volatile matter; FC: fixed carbon; HCV: high calorific value. a Dry basis.
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mechanical resistance of the briquettes [12]. The evolution of IRI of the green briquettes and those cured at room temperature for 3, 5, 24 and 48 h is shown in Fig. 1. The impact resistance increases continuously from 0 to 48 h and the most important change occurred after 24 h of curing. To find a relationship between the resistance of the briquettes and the chemical structures formed with the curing, infrared spectroscopy was used as an instrumental technique. The goal of this study is to find a relationship between the resistance of the briquettes and the structural changes produced with the curing. FT-IR spectra of green humates and molasses were studied independently [9]. Changes in the carbonised materials were not expected because the temperature of carbonisation was 600 jC and the briquettes were cured at room temperature; the interactions are produced due to the presence of the green binder during the time of curing. The FT-IR spectra depicted in Fig. 2 reflect the materials that constitute the briquettes, co-carbonised materials [4] and humates [9]. The stretching vibrations attributed to the aliphatic C – H bonds, 2950 –2850 cm 1, increase slightly with a curing time of 48 h. This change was calculated by the subtraction of the FT-IR spectra. The 1573 cm 1 band is attributed to the vibration of the carbonyl group of carboxylates. With respect to the 1100– 1000 cm 1 vibration zone, it is not possible to give an unequivocal interpretation because of the overlapping of the C – O – C bonds with the mineral matter (1042 and 483 cm 1). Several TPD followed by MS have been carried out to complete the study of these materials. The first part of the TPD shows the evolution of the gases followed on-line until the same temperature of carbonisation of the raw materials; this removal belongs to the binder of the briquettes, to the effect of the curing and, in general, to the briquetting process. The second part, carried out at temperatures higher than 600 jC, shows the evolution of the gases that mainly proceeds from the carbonised materials and also from the binder but in a minor proportion. Fig. 3 shows the time effect on the curing taking into account the removal of aliphatic structures and methoxy groups. The evolution of the m/z 15 fragment attributed to CH +3 is
Fig. 1. IRI of the briquettes prepared with humates cured at room temperature varying the time of curing.
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Fig. 2. FT-IR spectra of the briquettes prepared with humates cured at room temperature varying the time of curing.
studied. This figure depicts a slight increase of the evolution of CH +3 for briquettes cured at room temperature during 48 h with respect to both briquettes, cured at room temperature during 5 h and green briquettes. This change was calculated by the subtraction of the FTIR spectra shown in Fig. 2. Therefore, most of the CH +3 fragment comes from the binder. Burchill [16] obtained the same result in an analogous study carried out by nuclear magnetic resonance. Fig. 3 also shows that there are two types of CH +3, whose relative maxima intensity appears at 370 and 500 jC. The first type of CH +3 can be attributed to the rupture of OCH +3 in the MS and the second type could be attributed to the cleavage of aryl – aryl ethers or methylene, to arrangements or to intra- or intermolecular recombinations as it was proposed by Van Heek [8].
Fig. 3. Evolution of the fragment m/z 15 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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As the IRI value increases due to the curing time effect, taking into account the evolution of the m/z 15 fragment, the slight difference produced with this curing can be attributed to the effect of curing time on CH +3. When the temperature is higher than 600 jC, the evolution of these fragments is mainly due to the loss of volatile matter belonging to the co-carbonised materials and to the cocarbonised binder interactions that have a minor quantitative importance. Fig. 4 depicts the evolution of the m/z 22 fragment attributed to the loss of CO2 that comes from the decomposition of carboxylic groups. This fragment is studied instead of m/z 44 because, due to the experimental conditions, aliphatic fragments such as C3H +8 can be released. The m/z 22 fragment is specific to follow the CO2 molecule with no overlapping. This figure shows that the formation of this fragment is larger in briquettes cured at room temperature during 48 h than that observed in briquettes cured at room temperature during 5 h and green briquettes. This explains that the formation of oxygenated structures is favoured with curing time in air at room temperature. Two maxima, at 170 and 400 jC of temperature, have been observed which have been attributed to similar structures with different electronic neighbourhood. The CO2 is mainly formed from carboxylic groups at a low temperature. The most stable oxygenated structures decompose at higher temperatures. As the IRI improves with the curing time, the formation of these oxygenated structures could contribute to increase the resistance of the briquettes. When the temperature is higher than 600 jC, the evolution of the m/z 22 fragment (CO+2 +) reveals that there are structures susceptible to decompose whose formations are favoured with the curing time. The amount of CO+2 + is lower than that of CH +3 which points out that there were more aliphatic than oxygenated structures in the carbonised materials at 600 jC.
Fig. 4. Evolution of the fragment m/z 22 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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The evolution of the m/z 31 fragment, which is attributed to the decomposition of methyl ethers (OCH +3) and primary alcohols, is shown in Fig. 5. The main amount of this fragment is released at 350 jC of temperature. There is a clear difference between the evolution of the m/z 31 fragment from the green and cured briquette. Therefore, interactions between the carbonised materials and the binder have been produced. This result is more reliable to compare different samples than the evolution of the band at 1042 cm 1 observed in Fig. 2 because of the overlapping of the C – O – C structures with the mineral matter and the difficulty to select a representative sample which allows comparing the spectrum of the briquettes with different curings. As the amount of sample used to carry out the TPD study is considered representative, the mineral matter effect is avoided. When the temperature is higher than 600 jC, it is not relevant the amount of OCH +3 detected. Therefore, this structure does not belong to the carbonised material and the methoxy structures formed with these mild treatments in air at room temperature have been released at 600 jC. Therefore, FT-IR spectroscopy and TPD studies followed by MS give information to know the path to cure briquettes with humates at room temperature during 48 h which explains the empirical results of the highest impact resistance index (IRI) with this time of curing. 3.2. Briquettes prepared with cane molasses The cane molasses are an active binder due to their composition [17] which makes both agglomeration and combustion of briquettes easier. The briquettes prepared with molasses cured for 0, 5, 24, 48, 72 h have an IRI value of 1000 which reflects an excellent agglomeration. Therefore, this test does not give information to distinguish between the
Fig. 5. Evolution of the fragment m/z 31 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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resistance of these specific briquettes prepared with molasses and those cured at room temperature. Therefore, the mass loss was chosen as criteria of resistance. Fig. 6 depicts the curing time effect on the resistance of the briquettes prepared with (M2 + O)6,50 with and without H3PO4. This acid, which favours the polymerisation of the binder, at 25 jC in air, provides resistance to briquettes as well as the effect of the time of curing. Fig. 7 shows the infrared spectra of these briquettes and the effect of the curing time and the addition of H3PO4 on its molecular structure. The spectra of these briquettes have a broad band at 3400 cm 1 which points out the presence of hydroxyl groups associated. The 2923 and 1408 cm 1 bands tend to increase with the time of curing and are attributed to the stretching and bending vibrations of aliphatic bonds, respectively. The band at 1630 cm 1 appears with the curing time in the spectra of the briquettes prepared with H3PO4. This band could be attributed to the CMO group in ketones, such as enolised h-diketones or cyclic h-ketones. In addition, the band at 1406 cm 1 assigned to aliphatic groups would be due to methylene groups adjacent to carbonyl groups ( –CH2 – CO –). This type of methylene group presents a lower wave number than that of –CH2 – hydrocarbons (1480 – 1440 cm 1). Moreover, it is observed at 1052 cm 1 that the stretching vibration attributed to the typical C –O –C bond, which is present on the sugars that constitute molasses. When H3PO4 is added, the vibration of the O –H that appears at 3400 cm 1 is stronger which could be due to the formation of hydrogen bonds; consequently, a stabilisation of these briquettes occurs. Analogously to the study of the evolution of the fragments m/z 15, 22 and 31 from briquettes prepared with humates, the evolution of these fragments from briquettes prepared with molasses has been also studied by TPD-MS. Fig. 8 shows the evolution of the fragment m/z 15 attributed to CH +3 structures. The amount of this fragment is larger for briquettes cured in air at room temperature, with a longer time of curing than for green briquettes. The rearrangements or the formation of methylene and ethylene bridges with the longest time of curing are favoured. The decomposition temperature of the CH +3 fragment of the briquettes cured at room
Fig. 6. Mass loss of the briquettes prepared with molasses (—) without and (.) with H3PO4 cured in the air with different times of curing.
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Fig. 7. FT-IR spectra of the briquettes prepared with molasses cured at room temperature varying the time of curing (a) without H3PO4 (b) with H3PO4.
temperature during 72 h favours molecular interactions which are more labile because the band is stronger, however, the temperature of the maximum intensity is lower than those cured during a shorter time. The curing at room temperature of the briquettes prepared with molasses is more effective for the briquettes prepared with this binder than for those briquettes prepared with humates because of the increase of the intensity for the evolution of CH +3 which reflects a better stabilisation of these materials. When the temperature is higher than 600 jC, the removal of CH +3 is not so important as it was observed for briquettes prepared with humates, which points out the different behaviour of these binders. The best stabilisation of these briquettes agrees with a larger amount of CH +3 obtained with a curing for 72 h. The evolution of the fragment m/z 22 is depicted in Fig. 9 and is attributed to the structures CO+2 + that come from decarboxylations. When briquettes are cured for 72 h in
Fig. 8. Evolution of the fragment m/z 15 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
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Fig. 9. Evolution of the fragment m/z 22 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
resistance of these specific briquettes cured at room temperature varying the time of curing. In this case, the longer the curing is, the more labile these structures are because they decompose at lower temperature. Moreover, these types of structures (CO+2 +) are removed or stabilised during the curing process because the amount of the fragment m/z 22 is lower for briquettes cured during 72 h than for those cured during shorter times. When the temperature is higher than 600 jC, the evolution of CO+2 + from briquettes prepared with molasses is lower than those prepared with humates and this amount is not relevant. As the carbonised material is the same for every briquette, the lower amount of CO+2 + points out that the final briquettes are more stable which agrees with the empirical results shown in Figs. 1 and 6. The evolution of the fragment m/z 31, attributed to methoxy groups which comes from methyl ethers and/or primary alcohols, is shown in Fig. 10. The evolution of the OCH +3 from green briquettes points out the methoxy structures released from molasses. The interactions produced with the curing between the binder and the carbonised materials are deduced from the difference between the evolution of the green and cured briquettes at room temperature. Green briquettes and those cured at room temperature during 5 and 48 h produce OCH +3 groups at 240 jC. However, the briquettes cured at 72 h release a higher amount of OCH +3 before 200 jC of briquettes. Thus, there are more interactions with this curing but these are more labile. These structures (OCH +3) formed during the curing provide the mechanical strength to the briquettes. When the temperature is higher than 600 jC, there is no evolution of the OCH +3 fragment. Thus, the constituents of the briquettes with a tendency to decompose to OCH +3 have been released at lower temperatures than 600 jC. As a result of the FT-IR and TPD studies, it can be concluded that new structures have been formed at room temperature during 72 h. However, the main changes observed, as resistance is concerned, have been already produced before 48 h, which means that this
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Fig. 10. Evolution of the fragment m/z 31 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
resistance test does not reflect the possible stabilisation reached with the curing at room temperature from 48 to 72 h. An analogous study was carried out with briquettes prepared with (M2 + O)6,50, molasses and H3PO4, there was no relevant changes between the cured and green briquettes [4]. As it can be seen in Fig. 6, it is not necessary to cure at room temperature longer than 5 h when briquettes prepared with molasses contain H3PO4.
4. Conclusions Acceptable smokeless fuel briquettes, as mechanical resistance and combustion behaviour is concerned, have been prepared from a low-rank coal, olive stone as biomass and humates or molasses as binder. As humates used as a solution is a film type binder, the strength of the green or wet briquettes is low but this strength increases with the time of curing at room temperature. FT-IR is an adequate technique to follow the hydroxyl (3400 cm 1) aliphatic (2950 – 2850 cm 1) and carbonyl structures (1500 –1800 cm 1). TPD studies help to complete this characterisation with the evolution of m/z 31, which points out the methyl ether and primary alcohols without interferences. Moreover, this technique is also used to quantify the amount of the studied structures of the briquettes with different curings. However, the technique, which allows the identification of the relative proportions of the structures, is FT-IR. Both techniques, FT-IR and mass spectrometry, are adequate to characterise the briquettes to achieve a better understanding of the physics and chemistry of the briquetting process. Sometimes, they corroborate results and at other times, they are complementary.
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TPD studies help to predict the final properties of the briquettes clearer than infrared spectroscopy. This technique appreciates changes even when the resistance tests do not show differences. The presence of H3PO4 on briquettes prepared with molasses produces a relevant stabilisation.
Acknowledgements The authors wish to thank the ECSC (contract No. 7220-EA/133) and the CICYT (contract No. AMB97-1901-CE) for the financial support to carry out this research.
References [1] B.C. Young, G.W. Kalb, Karbo-Energochem.-Ekol. 41 (1996) 406. [2] British Coal Corporation, Cheltenham, Gloucestershire, The physics and chemistry of briquetting, Technical Report, 1994. [3] M.C. Mayoral, M.T. Izquierdo, M.J. Blesa, J.M. Andre´s, B. Rubio, J.L. Miranda, Thermochim. Acta 371 (2001) 41. [4] M.J. Blesa, Briqueteado de lignitos con aditivos, Seguimiento fı´sico-quı´mico del proceso, PhD Thesis, Universidad de Zaragoza, 2002. [5] W. Pietsch, Size enlargement by agglomeration, Editorial, Wiley, USA, 1991. [6] M.J. Blesa, V. Fierro, J.L. Miranda, R. Moliner, J.M. Palacios, Fuel Process. Technol. 74 (2001) 1. [7] V.S. Beletsky, T.G. Shendrik, A. Kkhelufi, Solid Fuel Chem. 33 (1999) 42. [8] K.H. Van Heek, Structure and pyrolysis behaviour of different coals and relevant model substances, Fuel 73 (1994) 886. [9] M.J. Blesa, J.L. Miranda, M.T. Izquierdo, R. Moliner, Vibr. Spectrosc. 948 (2002) 1 – 7. [10] J.L. Miranda, M.J. Blesa, Briquetting of lignites with additives, Targeted project: Improved Coal Preparation, Transport and Handling, ECSC 7220-EA/133, Final Report, Zaragoza, 2000. [11] A. Arenillas, Influencia del proceso de desvolatilizacio´n sobre la reduccio´n de emisiones de o´xidos de nitro´geno en la combustio´n del carbo´n, PhD Thesis, Universidad de Oviedo, 1998. [12] S.R. Richards, Fuel Process. Technol. 25 (1990) 89. [13] M.A. Musich, Proc.-Inst. Briquet. Agglom., Bienn. Conf. (1997) 11. [14] C.A. Holley, Proc.-Inst. Briquet. Agglom., Bienn. Conf. (1983) 41. [15] N. Kukrety, F.M. Pattarkine, S.M. Saksena, V. Joshi, P.V. Sridharan, Fuel Sci. Technol. 12 (1993) 71 – 82. [16] P. Burchill, G.D. Hallam, A.J. Lowe, N. Moon, Fuel Process. Technol. 41 (1994) 63. [17] H. Schiweek, M. Clarke, Ullmann’s encyclopaedia of industrial chemistry, in: B. Elvers, S. Hawkins, W. Russey, G. Schulz (Eds.), Starch and other Polysaccharides to Surfactants, vol. A-25, 22.2. Molasses, 1993.
Curing time effect on mechanical strength of smokeless fuel briquettes M.J. Blesa *, J.L. Miranda, M.T. Izquierdo, R. Moliner Instituto de Carboquı´mica (CSIC), P.O. Box 589, 50080 Saragossa, Spain Received 4 June 2002; received in revised form 9 September 2002; accepted 25 September 2002
Abstract This work shows the effect of curing time on the mechanical properties of smokeless fuel briquettes which were prepared with olive stone and a low-rank coal previously co-carbonised at 600 jC. Humates and molasses were used as binders which acts with different roles, as a film or matrix depending on the curing. Additives like H3PO4 were also tested. Green briquettes were cured at different times and the molecular changes were followed by both Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decomposition (TPD) followed by mass spectrometry (MS). This last study helps to predict the final properties of the briquettes more clearly than infrared spectroscopy does. This technique appreciates changes when the impact-resistance test does not show differences. The effect of the H3PO4 on briquettes prepared with molasses produce a stabilisation of these materials. Moreover, the mechanical resistance of the studied briquettes is improved by the effect of the curing time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Smokeless fuel briquettes; Binders; Curing; Infrared spectroscopy; Temperature programmed decomposition
1. Introduction The briquetting of coal has been largely empirical relying on simple physical tests methods and experiences [1]. A greater understanding of the physics and chemistry of coal briquetting could lead to better briquette performances and cost-effectiveness and widen the range of coals that can be briquetted successfully making these fuels more attractive to
* Corresponding author. Tel.: +34-976-733-977; fax: +34-976-733-318. E-mail address: [email protected] (M.J. Blesa). 0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 2 4 3 - 6
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consumers [2]. The curing process is one of the final steps of the briquetting that produces transformations –interactions – changes between the carbonised materials themselves as well as with the binder. As a result of this process, the briquettes improve the mechanical resistance and the behaviour of the briquettes in water [3]. The curing should be fixed to both technical and economic aspects so that the global process provides adequate final properties for briquettes [4]. Although there are a lot of studies carried out about agglomeration, most of them are about the empirical subjects and they lack scientific matters that are especially relevant with the curing process [1,5]. In this work, the feasibility to produce environmentally acceptable smokeless fuel briquettes from low-rank coals and biomasses and a better understanding of the physics and chemistry of the curing have been studied to know the role of the binders and the process of curing. Feedstocks for smokeless fuel briquette manufacture include several materials, such as a low-rank coal and olive stones that require to be previously carbonised to reduce the volatile matter and the sulphur content of the coal [6]. Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decomposition (TPD) have been used to acquire information on the main structural changes produced in these binders with the curing [7] and how the presence of different oxygenated functional groups and polymerisations have influence on the final properties of the prepared briquettes. TPD studies followed by mass spectrometry (MS) will help to know the different thermal stability of the structures that constitute briquettes [8].
2. Experimental 2.1. Characterisation of the raw materials The proximate analysis, the sulphur content and the high calorific value of Maria coal (M2), olive stones (O), humates and molasses have been shown in previous papers [6,9]. 2.2. Carbonisation of the selected raw materials The coal used is a low-rank coal named Maria (M2) from Teruel (Spain). This coal was carbonised at 500, 550, 600, 650 and 700 jC to choose the best temperature of desulphurisation [6]. Low temperature carbonised materials were prepared by copyrolysis of M2 coal and olive stone (O), at the optimum temperature, 600 jC, in order to reduce the sulphur content and to increase the high calorific value of the materials [10]. Therefore, the co-carbonised material named (M2 + O)6,50, was prepared at 600 jC with a mixture of M2/O (1:2) to get approximately 50% of coal in the co-carbonised material according to the yield of the carbonisation at 600 jC of M2 and O, separately. 2.3. Briquetting of the co-carbonised materials The co-carbonised material, (M2 + O)6,50, with a size < 1 mm, was mixed with a binder. The amount of binder was optimised between 5.0% and 6.5% for briquettes
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prepared with humates and between 10% and 16% for those prepared with molasses because of the different nature of these binders. Adequate mechanical resistance was reached with 6% humates and 16% molasses. The blend was cold-pressed at 125 MPa in a plug and mould press to produce cylindrical briquettes of approximately 10.5 mm in diameter, 13.5 mm in height and 1.2 g in weight. The briquettes prepared with the co-carbonised (M2 + O)6,50 and humates were cured at room temperature ( f 25 jC) in air at different periods of time from 3 to 48 h. When molasses were used as binder, the briquettes were also cured during 72 h. The proximate analyses of the prepared briquettes, moisture, ash and volatile matter, were determined according to the ISO 589/1981, 562/1974 and 1171/1976 procedures, respectively. The ultimate analyses (C, H, N, S) were determined using a CARLO ERBA 1108 elemental analyser. The high calorific values were determined following the ISO 1928/1976 procedure. In addition, all briquettes are characterised by both Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decompositions (TPD) to better understand the curing process. 2.4. FT-IR spectroscopy and temperature programmed decomposition studies FT-IR spectra of briquettes were run on KBr pellets (120 mg, 1 wt.%) and recorded coadding 64 scans at a resolution of 2 cm 1 in a Nicolet Magna 550 spectrometer. Spectra were scaled to 1 mg sample. The TPD studies were carried out in a vertical reactor (i.d. = 15 mm, L = 380 mm) with one briquette. The temperature was increased up to 600 jC at the rate of 10 jC/ min; after 30 min at 600 jC, it was heated up to 850 jC and it was maintained for 60 min. A flow of 50 cm3/min of the mixture 21% O2 and 79% Ar was passed through the reactor. The gases released went out through a capilar heated at 110 jC to avoid condensations. The evolution of the fragment m/z 15, 22 and 31 was followed by mass spectrometry (MS). These peaks were normalised with the maximum intensity of each run and the results show high repeatability [11]. 2.5. Impact resistance index (IRI) Each briquette was repeatedly dropped from a stationary start at a height of 2 m on to a steel floor until it fractures. The number of drops and the number of pieces the briquette broke into were recorded. Then, these data were used to calculate the impact resistance index (IRI) from the equation:
IRI ¼ ðð100 average number of dropsÞ=average number of piecesÞ
ð1Þ
For laboratory work, an IRI value of 50 was adopted as the lowest acceptable value for fuel briquettes developed for industrial or domestic applications [12].
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3. Results and discussion The production of smokeless fuel briquettes requires the carbonisation of the raw materials used, the low-rank coal (M2) and olive stones (O), to decrease their volatile matter content. As these briquettes have a lower amount of volatile matter than the original materials [4,6], the combustion of this new fuel goes by with a very low production of both smoke and tars, and the reduction in the amount of highly condensed aromatic hydrocarbons produced [13]. Preliminary studies showed that the amount of binder required for briquettes prepared with molasses was higher than that required with humates, consequently, their function is different [14]. Matrix type binders, as molasses, require rather substantial quantities because the strength depends upon the presence of a continuous phase of binder that encloses the solid particles. Table 1 depicts the proximate and ultimate analyses, the high calorific values, as well as the ratio sulphur/thermie (% S/th) of the briquettes prepared. The moisture content of those prepared with molasses and H3PO4 is lower than that of briquettes prepared just with molasses due to the presence of the acid which favours the dehydration of the briquettes. The briquettes prepared with molasses have higher amounts of ash because of the nature of this binder. Smokeless briquettes have been prepared according to Kukrety et al. [15] who considers that these materials are environmentally acceptable because the volatile matter amount is lower than 14% and the high calorific value is considerably higher than 15 MJ/kg. 3.1. Briquettes prepared with humates Fuel briquettes need to be able to withstand the crushing loads they receive during handling, transport, storage and firing. The impact resistance index is considered to be the best general diagnostic of the briquette strength. Moreover, this test suits our needs; it is an easy operation and gives comparable results. The IRI was used as a criteria to check the
Table 1 Proximate and ultimate analysis, high calorific value and % S/th content of the briquettes prepared with (M2 + O)6,50
Moisture Ash VM FC Ca Ha Na Sa a Oby diff HCVa (MJ/kg) S/th
Humates (%)
Molasses (%)
Molasses + H3PO4 (%)
3.9 8.9 13.4 73.8 81.6 2.1 1.4 1.4 4.2 30.8 0.19
3.7 12.9 11.9 71.5 73.6 2.0 2.0 2.0 7.0 27.3 0.31
2.5 12.1 13.4 72.0 75.3 1.6 1.8 1.8 7.1 27.6 0.27
VM: volatile matter; FC: fixed carbon; HCV: high calorific value. a Dry basis.
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mechanical resistance of the briquettes [12]. The evolution of IRI of the green briquettes and those cured at room temperature for 3, 5, 24 and 48 h is shown in Fig. 1. The impact resistance increases continuously from 0 to 48 h and the most important change occurred after 24 h of curing. To find a relationship between the resistance of the briquettes and the chemical structures formed with the curing, infrared spectroscopy was used as an instrumental technique. The goal of this study is to find a relationship between the resistance of the briquettes and the structural changes produced with the curing. FT-IR spectra of green humates and molasses were studied independently [9]. Changes in the carbonised materials were not expected because the temperature of carbonisation was 600 jC and the briquettes were cured at room temperature; the interactions are produced due to the presence of the green binder during the time of curing. The FT-IR spectra depicted in Fig. 2 reflect the materials that constitute the briquettes, co-carbonised materials [4] and humates [9]. The stretching vibrations attributed to the aliphatic C – H bonds, 2950 –2850 cm 1, increase slightly with a curing time of 48 h. This change was calculated by the subtraction of the FT-IR spectra. The 1573 cm 1 band is attributed to the vibration of the carbonyl group of carboxylates. With respect to the 1100– 1000 cm 1 vibration zone, it is not possible to give an unequivocal interpretation because of the overlapping of the C – O – C bonds with the mineral matter (1042 and 483 cm 1). Several TPD followed by MS have been carried out to complete the study of these materials. The first part of the TPD shows the evolution of the gases followed on-line until the same temperature of carbonisation of the raw materials; this removal belongs to the binder of the briquettes, to the effect of the curing and, in general, to the briquetting process. The second part, carried out at temperatures higher than 600 jC, shows the evolution of the gases that mainly proceeds from the carbonised materials and also from the binder but in a minor proportion. Fig. 3 shows the time effect on the curing taking into account the removal of aliphatic structures and methoxy groups. The evolution of the m/z 15 fragment attributed to CH +3 is
Fig. 1. IRI of the briquettes prepared with humates cured at room temperature varying the time of curing.
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Fig. 2. FT-IR spectra of the briquettes prepared with humates cured at room temperature varying the time of curing.
studied. This figure depicts a slight increase of the evolution of CH +3 for briquettes cured at room temperature during 48 h with respect to both briquettes, cured at room temperature during 5 h and green briquettes. This change was calculated by the subtraction of the FTIR spectra shown in Fig. 2. Therefore, most of the CH +3 fragment comes from the binder. Burchill [16] obtained the same result in an analogous study carried out by nuclear magnetic resonance. Fig. 3 also shows that there are two types of CH +3, whose relative maxima intensity appears at 370 and 500 jC. The first type of CH +3 can be attributed to the rupture of OCH +3 in the MS and the second type could be attributed to the cleavage of aryl – aryl ethers or methylene, to arrangements or to intra- or intermolecular recombinations as it was proposed by Van Heek [8].
Fig. 3. Evolution of the fragment m/z 15 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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As the IRI value increases due to the curing time effect, taking into account the evolution of the m/z 15 fragment, the slight difference produced with this curing can be attributed to the effect of curing time on CH +3. When the temperature is higher than 600 jC, the evolution of these fragments is mainly due to the loss of volatile matter belonging to the co-carbonised materials and to the cocarbonised binder interactions that have a minor quantitative importance. Fig. 4 depicts the evolution of the m/z 22 fragment attributed to the loss of CO2 that comes from the decomposition of carboxylic groups. This fragment is studied instead of m/z 44 because, due to the experimental conditions, aliphatic fragments such as C3H +8 can be released. The m/z 22 fragment is specific to follow the CO2 molecule with no overlapping. This figure shows that the formation of this fragment is larger in briquettes cured at room temperature during 48 h than that observed in briquettes cured at room temperature during 5 h and green briquettes. This explains that the formation of oxygenated structures is favoured with curing time in air at room temperature. Two maxima, at 170 and 400 jC of temperature, have been observed which have been attributed to similar structures with different electronic neighbourhood. The CO2 is mainly formed from carboxylic groups at a low temperature. The most stable oxygenated structures decompose at higher temperatures. As the IRI improves with the curing time, the formation of these oxygenated structures could contribute to increase the resistance of the briquettes. When the temperature is higher than 600 jC, the evolution of the m/z 22 fragment (CO+2 +) reveals that there are structures susceptible to decompose whose formations are favoured with the curing time. The amount of CO+2 + is lower than that of CH +3 which points out that there were more aliphatic than oxygenated structures in the carbonised materials at 600 jC.
Fig. 4. Evolution of the fragment m/z 22 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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The evolution of the m/z 31 fragment, which is attributed to the decomposition of methyl ethers (OCH +3) and primary alcohols, is shown in Fig. 5. The main amount of this fragment is released at 350 jC of temperature. There is a clear difference between the evolution of the m/z 31 fragment from the green and cured briquette. Therefore, interactions between the carbonised materials and the binder have been produced. This result is more reliable to compare different samples than the evolution of the band at 1042 cm 1 observed in Fig. 2 because of the overlapping of the C – O – C structures with the mineral matter and the difficulty to select a representative sample which allows comparing the spectrum of the briquettes with different curings. As the amount of sample used to carry out the TPD study is considered representative, the mineral matter effect is avoided. When the temperature is higher than 600 jC, it is not relevant the amount of OCH +3 detected. Therefore, this structure does not belong to the carbonised material and the methoxy structures formed with these mild treatments in air at room temperature have been released at 600 jC. Therefore, FT-IR spectroscopy and TPD studies followed by MS give information to know the path to cure briquettes with humates at room temperature during 48 h which explains the empirical results of the highest impact resistance index (IRI) with this time of curing. 3.2. Briquettes prepared with cane molasses The cane molasses are an active binder due to their composition [17] which makes both agglomeration and combustion of briquettes easier. The briquettes prepared with molasses cured for 0, 5, 24, 48, 72 h have an IRI value of 1000 which reflects an excellent agglomeration. Therefore, this test does not give information to distinguish between the
Fig. 5. Evolution of the fragment m/z 31 of the briquette prepared with the co-carbonised (M2 + O)6,50 and humates.
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resistance of these specific briquettes prepared with molasses and those cured at room temperature. Therefore, the mass loss was chosen as criteria of resistance. Fig. 6 depicts the curing time effect on the resistance of the briquettes prepared with (M2 + O)6,50 with and without H3PO4. This acid, which favours the polymerisation of the binder, at 25 jC in air, provides resistance to briquettes as well as the effect of the time of curing. Fig. 7 shows the infrared spectra of these briquettes and the effect of the curing time and the addition of H3PO4 on its molecular structure. The spectra of these briquettes have a broad band at 3400 cm 1 which points out the presence of hydroxyl groups associated. The 2923 and 1408 cm 1 bands tend to increase with the time of curing and are attributed to the stretching and bending vibrations of aliphatic bonds, respectively. The band at 1630 cm 1 appears with the curing time in the spectra of the briquettes prepared with H3PO4. This band could be attributed to the CMO group in ketones, such as enolised h-diketones or cyclic h-ketones. In addition, the band at 1406 cm 1 assigned to aliphatic groups would be due to methylene groups adjacent to carbonyl groups ( –CH2 – CO –). This type of methylene group presents a lower wave number than that of –CH2 – hydrocarbons (1480 – 1440 cm 1). Moreover, it is observed at 1052 cm 1 that the stretching vibration attributed to the typical C –O –C bond, which is present on the sugars that constitute molasses. When H3PO4 is added, the vibration of the O –H that appears at 3400 cm 1 is stronger which could be due to the formation of hydrogen bonds; consequently, a stabilisation of these briquettes occurs. Analogously to the study of the evolution of the fragments m/z 15, 22 and 31 from briquettes prepared with humates, the evolution of these fragments from briquettes prepared with molasses has been also studied by TPD-MS. Fig. 8 shows the evolution of the fragment m/z 15 attributed to CH +3 structures. The amount of this fragment is larger for briquettes cured in air at room temperature, with a longer time of curing than for green briquettes. The rearrangements or the formation of methylene and ethylene bridges with the longest time of curing are favoured. The decomposition temperature of the CH +3 fragment of the briquettes cured at room
Fig. 6. Mass loss of the briquettes prepared with molasses (—) without and (.) with H3PO4 cured in the air with different times of curing.
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Fig. 7. FT-IR spectra of the briquettes prepared with molasses cured at room temperature varying the time of curing (a) without H3PO4 (b) with H3PO4.
temperature during 72 h favours molecular interactions which are more labile because the band is stronger, however, the temperature of the maximum intensity is lower than those cured during a shorter time. The curing at room temperature of the briquettes prepared with molasses is more effective for the briquettes prepared with this binder than for those briquettes prepared with humates because of the increase of the intensity for the evolution of CH +3 which reflects a better stabilisation of these materials. When the temperature is higher than 600 jC, the removal of CH +3 is not so important as it was observed for briquettes prepared with humates, which points out the different behaviour of these binders. The best stabilisation of these briquettes agrees with a larger amount of CH +3 obtained with a curing for 72 h. The evolution of the fragment m/z 22 is depicted in Fig. 9 and is attributed to the structures CO+2 + that come from decarboxylations. When briquettes are cured for 72 h in
Fig. 8. Evolution of the fragment m/z 15 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
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Fig. 9. Evolution of the fragment m/z 22 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
resistance of these specific briquettes cured at room temperature varying the time of curing. In this case, the longer the curing is, the more labile these structures are because they decompose at lower temperature. Moreover, these types of structures (CO+2 +) are removed or stabilised during the curing process because the amount of the fragment m/z 22 is lower for briquettes cured during 72 h than for those cured during shorter times. When the temperature is higher than 600 jC, the evolution of CO+2 + from briquettes prepared with molasses is lower than those prepared with humates and this amount is not relevant. As the carbonised material is the same for every briquette, the lower amount of CO+2 + points out that the final briquettes are more stable which agrees with the empirical results shown in Figs. 1 and 6. The evolution of the fragment m/z 31, attributed to methoxy groups which comes from methyl ethers and/or primary alcohols, is shown in Fig. 10. The evolution of the OCH +3 from green briquettes points out the methoxy structures released from molasses. The interactions produced with the curing between the binder and the carbonised materials are deduced from the difference between the evolution of the green and cured briquettes at room temperature. Green briquettes and those cured at room temperature during 5 and 48 h produce OCH +3 groups at 240 jC. However, the briquettes cured at 72 h release a higher amount of OCH +3 before 200 jC of briquettes. Thus, there are more interactions with this curing but these are more labile. These structures (OCH +3) formed during the curing provide the mechanical strength to the briquettes. When the temperature is higher than 600 jC, there is no evolution of the OCH +3 fragment. Thus, the constituents of the briquettes with a tendency to decompose to OCH +3 have been released at lower temperatures than 600 jC. As a result of the FT-IR and TPD studies, it can be concluded that new structures have been formed at room temperature during 72 h. However, the main changes observed, as resistance is concerned, have been already produced before 48 h, which means that this
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Fig. 10. Evolution of the fragment m/z 31 of the briquette prepared with the co-carbonised (M2 + O)6,50 and molasses.
resistance test does not reflect the possible stabilisation reached with the curing at room temperature from 48 to 72 h. An analogous study was carried out with briquettes prepared with (M2 + O)6,50, molasses and H3PO4, there was no relevant changes between the cured and green briquettes [4]. As it can be seen in Fig. 6, it is not necessary to cure at room temperature longer than 5 h when briquettes prepared with molasses contain H3PO4.
4. Conclusions Acceptable smokeless fuel briquettes, as mechanical resistance and combustion behaviour is concerned, have been prepared from a low-rank coal, olive stone as biomass and humates or molasses as binder. As humates used as a solution is a film type binder, the strength of the green or wet briquettes is low but this strength increases with the time of curing at room temperature. FT-IR is an adequate technique to follow the hydroxyl (3400 cm 1) aliphatic (2950 – 2850 cm 1) and carbonyl structures (1500 –1800 cm 1). TPD studies help to complete this characterisation with the evolution of m/z 31, which points out the methyl ether and primary alcohols without interferences. Moreover, this technique is also used to quantify the amount of the studied structures of the briquettes with different curings. However, the technique, which allows the identification of the relative proportions of the structures, is FT-IR. Both techniques, FT-IR and mass spectrometry, are adequate to characterise the briquettes to achieve a better understanding of the physics and chemistry of the briquetting process. Sometimes, they corroborate results and at other times, they are complementary.
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TPD studies help to predict the final properties of the briquettes clearer than infrared spectroscopy. This technique appreciates changes even when the resistance tests do not show differences. The presence of H3PO4 on briquettes prepared with molasses produces a relevant stabilisation.
Acknowledgements The authors wish to thank the ECSC (contract No. 7220-EA/133) and the CICYT (contract No. AMB97-1901-CE) for the financial support to carry out this research.
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