Curing Temperature Effect on Mechanical Strength of Smokeless Fuel

Energy & Fuels 2003, 17, 419-423

419

Curing Temperature Effect on Mechanical Strength of Smokeless Fuel Briquettes Prepared with Humates M. J. Blesa,* J. L. Miranda, M. T. Izquierdo, and R. Moliner Instituto de Carboquı´mica (CSIC), P.O. Box 589, 50080 Zaragoza, Spain

A. Arenillas and F. Rubiera Instituto Nacional del Carbo´ n (CSIC), P.O. Box 73, 33080 Oviedo, Spain Received July 19, 2002. Revised Manuscript Received January 6, 2003

The effect of curing temperature on smokeless fuel briquettes has been studied by Fourier transform infrared spectroscopy (FT-IR), mass spectrometry (MS), and temperature programmed decomposition (TPD). These techniques help to predict the final properties of these briquettes which were prepared with a low-rank coal, sawdust, and olive stone as biomasses and humates as binder. The best mechanical properties are reached with both the mildest thermal curing at 95 °C and the cocarbonized at 600 °C of Maria coal (M2) and sawdust (S) due to the fibrous texture of sawdust. The temperature of curing causes the release of a certain amount of oxygenate structures and the decrease of the mechanical resistance.

Introduction The briquetting of coal has been largely empirical relying on simply physical tests methods and experiences.1,2 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 which can be briquetted successfully making these fuels more attractive to consumers.3 Although there are many studies about agglomeration, most of the references are about the empirical subjects,1 and they are lacking in scientific matters specially relevant to the curing procees. This process is one of the final steps of the briquetting which produces interactions between the carbonized materials themselves and also with the binder. The curing should be fixed from the point of view of both technique and economic so that the global process provides adequate final properties for briquettes.4 Although it is not easy the knowledge of the structures that stabilize briquettes, an adequate choice of the fragments studied by temperature programmed decomposition followed by mass spectrometry (TPD-MS) could lead to know different thermal stability of certain structures that constitute briquettes.5 This work studies the feasibility to produce environmental acceptable smokeless fuel briquettes from low* To whom correspondence should be addressed. Phone: (34) 976 733977. Fax: (34) 976 733318. [email protected]. (1) Young, B. C.; Kalb, G. W. Karbo-Energochem-Ekol 1996, 41, 406. (2) Pietsch, W. Size Enlargement by Agglomeration. John Wiley and Sons: New York, 1991. (3) The Physics and Chemistry of Briquetting; Technical Report. British Coal Corporation: Cheltenham, 1994. (4) Blesa, M. J. Briqueteado de lignitos con aditivos. Seguimiento fı´sico-quı´mico del proceso. Ph.D. Thesis, Universidad de Zaragoza, 2002. (5) Van Heek, K. H. Fuel 1994, 73, 886.

rank coals and biomasses and a better understanding of the physics and chemistry of the curing to elucidate 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, Maria (M2), and sawdust (S) or olive stones (O), as biomasses which require carbonization to reduce their volatile matter and the sulfur content of the coal. Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decompositions (TPD) have been used to acquire information on the main structural changes produced with the curing and how the presence of different oxygenate functional groups influences the properties of the prepared briquettes. Experimental Section 2.1. Pyrolysis of Selected Raw Materials. The low-rank coal, Maria (M2), was pyrolyzed at 500, 550, 600, 650, and 700 °C with the aim to decrease the amount of sulfur and volatile matter and to increase the high calorific value.6 Once the temperature of coal desulfurization had been optimized, low-temperature cocarbonized materials were prepared by copyrolysis at 600 °C (6) of a low-rank coal, Maria (M2), and sawdust (S) or olive stone (O) as biomasses. These cocarbonized materials, (M2+S)6,50 and (M2+O)6,50, were prepared with an adequate mixture of Maria coal and sawdust or olive stone to get 50% of the cocarbonized coal in the cocarbonized material to become properly constituted as smokeless fuel briquettes.7 2.2. Briquetting of the Cocarbonized Materials. The cocarbonized materials, (M2+S)6,50 and (M2+O)6,50, were mixed with humates and calcium hydroxide with the aim to retain the sulfur.8 This blend was pressed at 125 MPa at room (6) Blesa, M. J.; Fierro, V.; Miranda, J. L.; Moliner, R.; Palacios, J. M. Fuel Process. Technol. 2001, 74, 1. (7) Kukrety, N.; Pattarkine, V. M.; Saksena, S.; Joshi, V.; Sridharan, P. V. Fuel Sci. Technol. 1993, 12, 71.

10.1021/ef020156f CCC: $25.00 © 2003 American Chemical Society Published on Web 02/05/2003

420

Energy & Fuels, Vol. 17, No. 2, 2003

temperature 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 optimization of the amount of humates for briquettes prepared with and without calcium hydroxide was studied, and the addition of 6% humates was required to reach the highest impact resistance according to Richards.9 The prepared briquettes were cured in an atmosphere containing 21% O2/Ar and a heating rate of 2 °C/min in a vertical reactor (φi ) 15 mm; L ) 380 mm) up to 95, 135, 150, and 200 °C; these temperatures were maintained for 4 h. This curing process was followed by MS and the fragments m/e 18 and 44 were used to know the evolution of H2O+ and CO2+, respectively, because of the importance of the evolution of oxygenate structures during this process. As curing was carried out with O2/Ar, the fragment m/e 44 was followed because it is expected that there were not aliphatic structures. 2.3. FT-IR Spectroscopy. FT-IR spectra of briquettes were run on KBr pellets (120 mg, 1 wt %) and recorded co-adding 64 scans at a resolution of 2 cm-1 in a Nicolet Magna 550. Spectra were scaled to 1 mg sample. As changes in the carbonized materials were not expected due to the mild thermal treatment, those produced can be attributed to humates or to the curing process. Spectra of green humates have been already studied independently10 to distinguish the contribution of this binder. 2.4. Temperature Programmed Decomposition Studies (TPD). The TPD runs followed by MS were carried out in the device described above to discover the structural changes produced due to the curing process. The temperature was up to 600 °C at the rate of 10 °C/min; after 30 min at 600 °C, it was heated to 850 °C and it was maintained for 60 min. A flow of 50 mL/min of Ar was passed through the reactor. The gases released went out through a capillary tube heated at 110 °C to avoid condensations. The evolution of the fragments m/e 15, 22, and 31 was followed to study the influence of the curing temperature on the aliphatic, carboxylic and methoxy structures. As temperature programmed decompositions were carried out in inert atmosphere, aliphatic structures can be removed; therefore, the fragment m/e 22 reflects the evolution of CO2 without the overlapping that can be produced following the fragment m/e 44 due to the possible evolution of C3H8+. The first part of each run shows the evolution of the gases followed on-line up to 600 °C, the same temperature of carbonization of the raw materials. Therefore, this removal belongs to the binder of the briquettes and to the effect of the curing. The second part carried out at temperature higher than 600 °C shows the evolution of the gases which mainly proceeds from the carbonized materials and also from the binder but in a minor proportion. With the aim to compare the results that come from different samples, the evolution of these fragments have been normalized11 with the maximum intensity of each run and the mass of the briquette. These results show high repeatability. 2.5. Textural Characterization. N2 and CO2 adsorption isotherms were obtained using an ASAP 2000 apparatus from Micromeritics. The samples were outgassed until 10-3 mmHg pressure which was reached and maintained. The time required to outgas varied depending on the sample. Values for monolayer volumes adsorbed were determined with the BET equation to N2 adsorption data at 77 K. The CO2 volume intrusion data were determined at 273 K, and the Dubinin(8) Lu, G.; Wang Q.; Sakamoto, K.; Kim, H.; Naruse, I.; Yuan, J.; Maruyama, T.; Kamide, M.; Sakadata, M. Energy Fuels 2000, 14, 1133. (9) Richards, S. R. Fuel Process. Technol. 1990, 25, 89. (10) Blesa, M. J.; Moliner, R.; Izquierdo, M. T.; Moliner, R. Vib. Spectrosc. 2003, 31, 81. (11) Arenillas, A. 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. Ph.D. Thesis, Universidad de Oviedo, 1998.

Blesa et al. Raduskevich (DR) equation was used to fit the CO2 adsorption data.12 The value for the surface occupied by each adsorbate molecule was taken from the literature.13 It was also measured true (helium) and apparent (mercury) densities as well as pore size distribution. The true density, based on the determination of the true volume, was first measured because it is not destructive. Afterward, the apparent density and mercury porosimetry were determined. Previously, the samples were outgassed and it was not used the conventional method due to the size of the briquette which does not allow to introduce it in the device commonly used. The true density was determined twice and calculated the average value by the device used (Accupyc 1330). The operation conditions to determine the apparent density were evacuation time 1 h and maximum pressure of 200 MPa. 2.6. Mechanical Resistance Tests. Fuel briquettes need to be able to withstand the crushing loads they receive during handling, transport, storage, and firing. The impact resistance index (IRI) is considered to be the best general diagnostic of the briquette strength. Moreover, this test suits our needs: it is easy to operate and gives comparable results. Each briquette is 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 breaks into are recorded. These data are then used to calculate the impact resistance index (IRI) from the equation:

IRI ) [100(average number of drops)]/ average number of pieces For laboratory work, an IRI value of 50 has been adopted as the lowest acceptable value for fuel briquettes developed for industrial or domestic applications.9 The compression strength test was carried out on a hydraulic tensile testing machine operated in the compression mode (10 kN Karl Frank apparatus). The results are reported as the maximum crushing load that a briquette can withstand before cracking or breaking.9 The laboratory runs were carried out in a little drum tumble taking into account the ratio sample to voids. This drum tumble have internal dimensions of 42 mm in height × 58 mm in diameter. The sample (17 g) is put in the drum tumble and is turned round up 40 rpm during 1 h. The sample is passed through a screen of 0.8 mm and the percentage that remains above is the abrasion resistance with an acceptable index when it is higher than 95%.9

Results and Discussion 3.1. Characterization and IRI of Briquettes. The briquettes prepared with the cocarbonized (M2+S)6,50 and (M2+O)6,50 with humates with and without calcium hydroxide passed the reference value described in the literature7 taking into account volatile matter content and high calorific value as it is shown in Table 1. An adequate drying of the agglomerates is one of the parameters that should be carefully selected to use successfully humates as binder. Therefore, the evolution of IRI was studied with briquettes prepared with the above-mentioned briquettes cured at 95, 135, 150, and 200 °C. Figure 1 shows the results. The impact resistance of these briquettes decreases with the temperature because the hydrogen bonds formed as well as the van der Waals forces disappear with the molecular movements produced by the effect of the temperature. (12) Dubinin, M. M. Carbon 1989, 27, 457. (13) Lowell, S.; Shields, J. E. Powder Surface Area and Porosity; Chapman & Hall: New York, 1991.

Mechanical Strength of Smokeless Fuel Briquettes

Energy & Fuels, Vol. 17, No. 2, 2003 421 Table 1. Immediate Analysis and High Calorific Value of Briquettes Prepared with Humates

Figure 1. IRI of the briquettes prepared with humates cured at room-temperature varying the temperature. 0: (M2+S)6,50. 9: (M2+S)6,50,Ca. O: (M2+O)6,50. b: (M2+O)6,50,Ca.

It is possible that both the formation of carboxylic groups and the release of CO2 cause the decrease of IRI. Briquettes prepared with (M2+S)6,50 have a higher impact resistance index than those prepared with (M2+O)6,50 due to better interlocking between coal and sawdust than olive stone.4 3.2. Study of the Curing. The molecular changes that take place during the curing were followed by FTIR spectroscopy and mass spectrometry. Figure 2 shows the FT-IR spectra of briquettes cured at 95, 135, 150, and 200 °C of temperature. The soft band at 3418 cm-1 is attributed to the tension vibration of associated hydroxyl groups due to the hydration water of the sample. The strongest vibration band that appears at 1570 cm-1 is attributed to carboxylate groups. There is a shoulder at 1699 cm-1 due to carboxylic acids which increases with the temperature of curing. It is also observed the increase of the 1120 cm-1 band, one of the fundamental frequencies attributed to sulfates.14 It is formed as a consequence of the oxidation of these briquettes produced with the temperature. The curing process was followed by TPD-MS. It was studied the evolution of the m/e 18 and 44 attributed to the loss of moisture and to the decomposition of structures detected as CO2+, respectively. Figure 3 shows that the evolution of the fragment m/e 18, increases from 26 to 193 °C of temperature. This evolution can be observed by Infrared Spectroscopy in Figure 2 with the 3418 cm-1 band attributed to associated hydroxyl groups. Taking into account both FT-IR

carbonized

Caadded/S

moisture (%)

ash (%)

VM (%)

FC (%)

HCVa (MJ/kg)

(M2+S)6,50 (M2+O)6,50 (M2+S)6,50 (M2+O)6,50

0 0 1 1

5.1 3.9 6.8 4.2

12.3 8.9 18.3 17.0

13.1 13.4 15.9 17.2

69.5 73.8 59.0 61.6

29.3 30.8 26.7 27.0

a Dry basis. (M2+S)6,50: cocarbonized at 600 °C of Maria coal (M2) and sawdust (S) with 50% of Maria coal carbonized in cocarbonized material. (M2+O)6,50: cocarbonized at 600 °C of Maria coal (M2) and olive stone (O) with 50% of Maria coal carbonized in cocarbonized material.

and MS results, it can be said that there are more stable hydroxyl groups that take part of crystalline structures of clays, minerals and organic compounds that require higher temperature to decompose. The relative intensity of m/e 44, CO2+, was studied in Figure 4 because this signal is analogous to the evolution of m/e 22 fragment, attributed to CO22+ which comes exclusively from CO2. Decarboxylations can follow these reactions 1 and 2.

It is observed that an increase which starts at 175 °C is attributable to the evolution of CO2+, which comes from carboxylic groups. When the temperature is 200 °C, a considerable decrease of the intensity occurs, which means a lower decomposition of CO2+ structures. Other functional groups, such as carboxylic groups stabilized by different electronic neighborhood, can be released at higher temperatures than 200 °C.15 The reactions 1 and 2 that appears above point out two possible decarboxylations: the first is a consequence of a free radical mechanism,15 which gives a simultaneous decarboxylation and dehydratation, and the second is just a decarboxylation with the formation of a double bond. The evolution of H2O+ and CO2+ structures could be due to the loss of hydrogen bonds and decarboxylations,

Figure 2. FT-IR spectra of the briquettes prepared with humates cured varying the temperature.

422

Energy & Fuels, Vol. 17, No. 2, 2003

Figure 3. Evolution of the fragment m/e 18 to follow the curing of briquettes prepared with (M2+O)6, 50 and humates.

Figure 4. Evolution of the fragment m/e 44 to follow the curing of briquettes prepared with (M2+O)6,50 and humates.

Figure 5. Evolution of the fragment m/e 15 of the briquette prepared with the cocarbonized (M2+O)6, 50 and humates and cured with the temperature.

respectively, which cause the decrease of the IRI. Therefore, it is advisable a mild curing for briquettes prepared with this kind of materials and humates as binder. The curing process followed by TPD-MS, showed the structural changes that take place during the curing with the evolution of the fragments m/e 15, 22, and 31 that come from the decomposition of briquettes. They were normalized with respect to the maximum intensity the mass of the sample. Figure 5 depicts the evolution of the fragment m/e 15. A maximum intensity is produced at 450 °C and shows the loss of methyl structures which can come from the decomposition of long aliphatic structures of the binder. (14) Gadsen, J. A. Infrared Spectra of Minerals and Related Inorganic Compounds; Butterworth: Sussex, 1975. (15) March, J. AdvAnce Organic Chemistry. Reactions, Mechanisms and Structure, 4th ed.; Wiley-Interscience: New York, 1992.

Blesa et al.

Figure 6. Evolution of the fragment m/e 22 of the briquette prepared with the cocarbonized (M2+O)6, 50 and humates and cured with the temperature.

Figure 7. Evolution of the fragment m/e 31 of the briquette prepared with the cocarbonized (M2+O)6, 50 and humates and cured with the temperature.

Simultaneously, the formation of aliphatic/aromatic cycles could be favored due to intramolecular mechanisms. Figure 6 shows the evolution of the fragment m/e 22 associated to the evolution of the CO22+ structure which reflects unequivocally the evolution of CO2 which is due to decarboxilations. This figure shows that the maximum intensity for this fragment appears at 400 °C. The formation of carboxylic groups are favored with the curing temperature. At the same time, the amount of these groups in briquettes cured at higher curing temperature is higher, as was observed in Figure 2 by FT-IR spectroscopy. The fragment m/e 31, attributed to the evolution of OCH3+, is shown in Figure 7, which is higher for briquettes cured at 200 °C than for those cured at 95, 135, or 150 °C. This means that the release of metoxy structures comes from reactions that occur with materials that contain ether and ester functional groups. 3.3. Textural Characterization and Mechanical Resistance of Briquettes Cured at 95 °C. Table 2 shows the results obtained from CO2 adsorption isotherms of briquettes prepared with humates and different cocarbonized materials with and without calcium hydroxide. The addition of calcium hydroxide causes the decrease of the microporosity because of the entrance of CO2 in the micropores is difficult. In general the values of the surface areas of CO2 from briquettes prepared with cocarbonized materials are lower than those reached from the cocarbonized used. This fact could be explained in terms of the collapse of pores caused by the binder.4 The pore size distribution and the volume of briquettes prepared with (M2+S)6,50 with and without

Mechanical Strength of Smokeless Fuel Briquettes

Energy & Fuels, Vol. 17, No. 2, 2003 423

Table 2. Textural Characterization of the Prepared Briquettes binder:

humates

additive:

a

Ca(OH)2

Ca(OH)2

carbonized:

(M2+S)6,50a

(M2+S)6,50a

(M2+O)6,50a

(M2+O)6,50a

SCO2 (m2 g-1) VCO2 (cm3 g-1) Eo (J mol-1) Freal (gcm-3) Fapparent (g cm-3) Vtot (cm3 g-1) porosity (%)

423.9 0.16 23075 1.90 1.15 0.35 39.57

360.7 0.14 22121 1.90 1.09 0.39 42.52

576.5 0.22 19364 1.97 1.24 0.30 36.92

345.6 0.13 23374 2.00 1.22 0.33 39.14

(M2+S)6,50 and (M2+O)6,50 as explained in Table 1.

Table 3. Resistance Values of Briquettes Prepared with Humates carbonized (M2+S)6,50 (M2+O)6,50

compression compression Caadded/S stressa (MPa) stressb (MPa) abrasion 1 1

4.43 2.33

0.54 0.30

prepared with (M2+O)6,50. This fact could be explained due to a better interlocking produced with (M2+S)6,50 because of the fibrous texture of the sawdust that remains in the cocarbonized materials.

97 85

a Vertical position. b Horizontal position; (M2+S)6,50 and (M2+O)6,50 as explained in Table 1.

Ca(OH)2 and (M2+O)6,50 with and without Ca(OH)2 are analogous. The pore size distribution observed for briquettes prepared with cocarbonized of (M2+S)6,50 has a bell shape when the diameter of the pore is high. However, the pore size distribution observed for briquettes prepared with (M2+O)6, 50 followed an ascendant line toward higher pore size. However, neither microporosity nor macroporosity are factors as important as the presence of voids and cracks to explain the resistance results. Table 3 depicts the compressive resistance of the briquettes prepared and cured at 150 °C. This table shows that the stress was higher for briquettes prepared with (M2+S)6,50 than for briquettes prepared with (M2+O)6,50. Therefore, the compression resistance of briquettes prepared with both (M2+S)6,50 and (M2+O)6,50 was measured in both vertical and horizontal position. The test carried out in vertical passed the target value according to Richards.9 The second one is less significant than the first one due to the tensions that are not homogeneous when the briquette is in this position. The abrasion values as well as impact resistance indexes for briquettes prepared with (M2+S)6,50 are higher than the values of resistance of briquettes

4. Conclusions Environmental acceptable briquettes have been prepared taking into account volatile matter and high calorific value. The mildest conditions of curing at 95 °C give the best mechanical properties and specially when the briquettes contain sawdust due to its fibrous texture. The evolution of H2O+ is due to the loss of moisture and the evolution of CO2+ is due to decarboxylations that can come from both carboxylic groups of the materials used in the briquetting process and formed during the curing. The cleavage of hydrogen bonds can contribute to the decrease of IRI when the temperature of curing is increased. The agglomeration process causes the decrease of the microporosity. The presence or absence of Ca(OH)2 has no influence on the distribution of pore size determined by mercury porosimetry. The presence of voids and cracks, as well as the granular or fibrous texture, have a greater influence than the micro- and macroporosity on the resistance of these briquettes. Acknowledgment. The authors wish to thank the ECSC (Contract 7220-EA/133) and the CICYT (Contract AMB97-1901-CE) for the financial support to carry out this research. EF020156F