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Production of fuel briquettes from esparto partially pyrolyzed
Energy Conversion and Management 46 (2005) 1877–1884 www.elsevier.com/locate/enconman
Production of fuel briquettes from esparto partially pyrolyzed A. Debdoubi
a,*
, A. El amarti a, E. Colacio
b
a
b
Unite´ Calorime´trie et Mate´riaux, Universite´ Abdelmalek Essaadi, Faculte´s des Sciences, P.O. Box 2121, Te´touan 93002, Morocco Laboratorio de Quimica Inorganica, Universidad de Granada, Facultad de Ciencias, Granada 18071, Spain Received 22 January 2004; received in revised form 29 April 2004; accepted 19 September 2004 Available online 5 November 2004
Abstract The aim of this study is to prepare a solid fuel from the most abundant biomass of Morocco that can be used by the local population, and in particular by the rural ones, instead of the wood of forest, which is less available and its exploitation is negative for the environment. Esparto is the most available biomass, with a production of 560 000 tons annually of dry matter approximately. However, briquettes of esparto have low calorific power and their mechanical properties are bad. To improve the quality of the briquettes and to have an economically competitive product at the same time, the esparto was partially pyrolyzed at temperatures between 160 C and 400 C, and the pressure of densification has been examined. The combustion profile of the samples has been studied by applying the derivative thermogravimetry technique, and the mechanical properties of the briquettes were tested to evaluate the impact resistance and water resistance. This study showed that strong briquettes can be obtained with a higher calorific value when the esparto is partially pyrolyzed, and a relatively elevated densification pressure is applied. 2004 Elsevier Ltd. All rights reserved. Keywords: Esparto; Briquettes; Partial pyrolysis; Thermogravimetry
*
Corresponding author. Tel.: +212 39 32 03 22; fax: +212 39 99 45 00. E-mail address: [email protected] (A. Debdoubi).
0196-8904/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2004.09.005
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1. Introduction In the last two decades, biomass has acquired considerable importance for a variety of energy and other uses: as bio fuels for domestic cooking, industrial process heating, electrical power generation etc. The advantage of these alternative energy sources is that they are renewable and have low cost production. Therefore, it is an opportunity for developing countries that, for them, have considerable energy deficit and their biomass resources are very abundant. The processes used to convert the biomass to an appropriate form to take advantage of its energy content are pyrolysis, gasification or densification. The last one, known as briquetting technology, appears to be an attractive solution for biomass material. The resulting product is a briquette that has a greater volumetric energy density and is easy for storage and transportation. Some studies have reported the use of biomass in the production of briquettes from tea waste [1], straws of colza [2], wheat straw and waste paper [3], olive refuse [4], cotton stalks [5], or as a binder for lignite [6–8]. Each country evaluates its own biomass resources. The kinds and amounts of biomass show important differences for different geographical areas depending on its climate, flora and agriculture. In Morocco, one of the most abundant biomasses is esparto with immense land coverage estimated at 3.6 Mha, more than half the forest surface of the country with the biggest part situated in the oriental region with 2.3 Mha [9]. Esparto is a vivacious plant, very resistant to dryness, constituted of two parts: an underground part called rhizome that is very important for its regeneration, and the aerial part reaching about one meter of height constituted by branches carrying the leaves called tuft. Forty years ago, the esparto was very well exploited; a good part being exported to Europe where it was used extensively in the paper industry. Locally, it is used in the production of livestock food and as a fuel without any revalorization in the factories of sugar and cement and by the rural population for cooking and heating. These applications created work for local people during 7 months of the year. Since 20 years, the esparto is completely underexploited for lack of an application; the export has ceased; and the national factories donÕt use esparto anymore. Only the domestic use remained. This situation generated an excess of laborers in the zone and contributed to urban migration. All surfaces of the growth of esparto are, in general, very productive. According to some studies, the production of this biomass is 400 to 1800 kg/ha/year depending on the region. Therefore, a surface of 1.5 Mha, identified as ecologically exploitable without risk of deterioration and impoverishment, gives a production of 560 000 T dry biomass per year [10,11]. This quantity of unutilized matter could be evaluated for energy use. In the present work, the combustion characteristics of esparto and partially pyrolyzed esparto were examined, and the mechanical properties of the briquettes were investigated with respect to the temperature of pyrolysis and the pressure of densification.
2. Experimental The esparto plant has been obtained from the oriental region of Morocco. The power calorific value (PCV) was measured by a Parr bomb calorimeter in a dry basis using samples of 1 g accord-
A. Debdoubi et al. / Energy Conversion and Management 46 (2005) 1877–1884
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ing to the standard method [12]. The moisture content was determined after drying a sample at 105 C according to ASTM D2016-25. To obtain the ash content, a dried sample of 2 g is burned at 800 C during 5 h according to ASTM D-5142. The volatile matter was determined according to ISO 562/1974. It was measured by introducing a dry sample of 1 g into a crucible with a top at 950 C during 5 min. The fixed carbon (FC) was calculated as follows: FC% = 100% ash% VM%. The proximate, ultimate and power calorific value analyses are given in Table 1. The moisture content is low, so the process of densification didnÕt require a previous drying operation. The profile of combustion of the samples were determined by thermogravimetric analysis using a Shimadzu TGA-50H analyzer with a rate of flow of the gas of 100 ml/min and heating rate of 20 C/min from 25 C to 950 C. Pyrolysis of the esparto was conducted in a stainless steel reactor (400 length · 20 mm id) with a perforated distribution plate. The bottom part of the reactor is filled by rings of ceramics. Above, a quantity of biomass is placed with a thermocouple for control of the reaction. The reactor was heated with a rate of 10 C/min up to the desired temperature by an electric furnace thermally controlled by a regulator. The water and tars were collected in a trap as condensable products at 0 C. The esparto was partially pyrolyzed at final temperatures of 160, 250, 300, 350 and 400 C, and the losses of mass during this thermal treatment were, respectively, 16.35%, 23.81%, 31%, 61% and 69.5%. Briquettes were produced using a hydraulic press. The esparto was ground to pass through a sieve having an opening of 1 mm before pyrolysis. Then, a quantity of the sample was pressed in a mold of cylindrical configuration with an inner diameter of 30 mm and a height of 80 mm, under different pressures. Five briquettes were prepared for each experiment. No standard test methods have been established for testing fuel briquettes. In the literature, two mechanical properties are considered to be important and are the most used: the resistance to impact with the aim to know the ability of the briquettes to withstand the crushing loads they receive during handling, transport and storage and the resistance against water absorption and disintegration to stimulate severe weathering conditions, which a fuel might encounter during out door storage. The impact resistance was tested by dropping repeatedly a series of briquettes from a stationary position at a height of 2 m until it fractured. The number of drops and the number of pieces the
Table 1 Analysis data and PCV of the esparto and the EPP Samples Esparto EPP (300 C) EPP (400 C)
Ultimate analysis (dry%)
Proximate analysis (dry%)
PCV (MJ/KG)
a
Hu
A
V
FC
5.20 1.9 1.7
2.2 3.8 6.1
80.5 45.9 22.1
16.8 50.3 71.8
C
H
N
O
46.94 64.79 78.32
6.44 4.87 4.1
0.86 1.32 1.54
43.56 30.34 17.58
a
19.1 21.7 26.8
Hu: humidity; A: ash; V: volatile matter; FC: fixed carbon; PCV: power calorific value; EPP: esparto partially pyrolyzed. a By difference.
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briquette broke into were recorded. This data was then used to calculate what is known as the impact resistance index (IRI) from the equation IRI ¼ 100 average no: of drops=average no: of pieces For the laboratory work, an IRI value of 50 was adopted as the lowest acceptable impact resistance for the fuel briquettes [13]. The water resistance time was arbitrarily determined by putting a briquette in a container filled with cold water, and the time required for dispersion was measured. This method has been fixed as the National Turkey Standard for lignite briquettes [14], and the limit is that the water resistance period must equal or exceed 1 h. The Standard has been used for lignite briquettes [15], for lignite blended by biomass [7] and also for biomass briquettes [4].
3. Results and discussions 3.1. The burning profile of esparto The curve of loss of mass of the sample during the combustion is shown in Fig. 1(a). The profile can be divided into four regions. In the first region, some losses of mass appeared near 100 C, corresponding to the release of the moisture content. As the temperature increased, the important and rapid losses of mass, reaching 20%/min in mass, occurring between 250 and 350 C can be attributed to the release of volatile matter and their ignition. This is the characteristic of biomass combustion, known for their high volatile matter, as shown for esparto in Table 1.
Fig. 1. The burning profile of esparto: (a) under atmosphere and (b) nitrogen atmosphere.
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Table 2 Effect of pyrolysis temperature on PCV, IRI and WRT Temperature of pyrolysis (C)
PCV (MJ/KG)
IRI
WRT(min)
25 160 250 300 350 400
19.1 19.7 20.8 21.7 22.9 25.8
1000 1000 450 266 200 150
30 47 65 72 84 89
Briquetting pressure: 200 MPa.
The third region, showing a slow burning loss of about 7.5%/min in mass, arrives between 400 and 550 C. As we can see, this part of the curve is absent in the burning profile of esparto under a nitrogen atmosphere, Fig. 1(b), whereas it has the same behavior in the other regions. This region corresponds to combustion of the biomass materiel where the fixed carbon is an important element with slow burning and a constant heat flux. In the last region, the burning rate apparently results in a mass loss less than 1%/min. This behavior in combustion still continues as the temperature reaches 1000 C. The main reason is that the combustible matter becomes exhausted and the presence of ashes becomes important. These results show that the biggest part of esparto is burned quickly, and it is an inconvenience for the use of the briquettes for heating and cooking, which requires combustion with a constant flux of energy exchange.
Fig. 2. The burning profile of: (a) esparto partially pyrolyzed (300 C) and (b) coal dust.
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In order to improve the combustion characteristics of the briquettes of esparto, the briquettes can be pyrolyzed. This thermal treatment permits removal of the volatile matter and an increase of the power calorific value. However, this pyrolysis was only partial because, on the one hand, it is necessary to produce economically acceptable briquettes for the local market, and on the other hand, a small quantity of volatile matter must be retained for good ignition of the combustion. So, the choice of the temperature of pyrolysis is optimized according to these requirements and the quality of the briquettes formed (see Table 2). The combustion characteristics of esparto partially pyrolyzed at a temperature of 300 C are shown in Fig. 2(a). This sample possessed again a quantity of volatile matter, and the fixed carbon became the main matter in the material with high reactivity and desirable combustion. The loss in mass reaches the maximum rate of 13%/min at the temperature of 429 C, slightly higher than the non-pyrolyzed esparto mass loss of 7.5%/min at a slightly higher temperature of 436 C. The behavior of combustion of this charred biomass is similar to that of the dust of coal, a classic solid fuel, which is given for comparison in Fig. 2(b). The coal has a loss of mass of 9%/min at a temperature of 569 C. Some experiments showed that a further increase of the temperature of pyrolysis decreased the rate of mass loss. 3.2. Briquetting of the esparto At first, the esparto was partially pyrolyzed at temperatures between 160 and 400 C in the fixed bed reactor. The results are shown in Table 2. As the temperature increases, the power calorific value increases (up to 34.5% at the temperature of 400 C) due to the removal of some volatile matter, the moisture content and an increase in the concentration of fixed carbon, thereby providing a higher energy/volume ratio. A series of briquettes was prepared for each temperature of pyrolysis at an applied pressure of 200 MPa. The prepared briquettes were tested mechanically; the first tests concerning the resistance the impact denoted IRI and the second one concerning the resistance to water denoted WRT. The results are shown in Table 2. The briquettes of esparto without pyrolysis shows good resistance to impact (IRI = 1000) but disintegrates in water after just 30 min. This value is greater than those that have been found for briquettes of paper mill waste and less than those from olive refuse, 19 min and 62 min respectively [4]. The pyrolysis of esparto improves the resistance to water from 30 to 89 min as the temperature of pyrolysis is increased from ambient to 400 C, so the esparto will be pyrolyzed for a temperature greater than 250 C to obtain a WRT = 1 h. However, the resistance to impact decreases from 1000 at ambient to 150 at a temperature of 400 C. The texture of the material should be different after the pyrolysis. The charred particles of esparto become less absorbent to the water, while the compaction becomes difficult with the absence of humidity, which is less than 2% after the thermal treatment. Some samples have been kept for a long time in a humid environment, where their moisture reaches 6%, before being densified. So the briquettes formed have their IRI increased 30%. In order to improve the impact resistance of the briquettes, the applied pressure was varied from 150 to 400 MPa. The observed result was the same for the esparto and the partially pyrolyzed esparto at 300 C as can be seen in Table 3. As the pressure increased, the IRI for the pyrolyzed esparto increased from 133 to 833 or remained constant at the maximum (1000) for the esparto, and the disintegration time of the briquettes when they were immersed in water increased from 63 to 112 min and from 10 to 85 min respectively. So, a strong briquette can be produced by
A. Debdoubi et al. / Energy Conversion and Management 46 (2005) 1877–1884
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Table 3 Effect of briquetting pressure on IRI and WRT for esparto and esparto partially pyrolyzed (EPP) Pressure (MPa) 150 200 300 400
EPP(300 C)
Esparto IRI
WRT (min)
IRI
WRT (min)
1000 1000 1000 1000
10 30 78 85
133 266 750 833
63 72 105 112
applying more pressure, but to obtain economically acceptable briquettes, the choice between pyrolysis temperature and pressure of densification must be taken into account. The partially pyrolyzed esparto at 300 C and a densification pressure of 300 MPa gives a briquette with good mechanical properties (IRI = 750 and WRT = 105 min).
4. Conclusions The esparto has a lot of advantages in addition to its abundance and good concentration in the region. It has just 5% of water, which is less than for the majority of biomass mentioned in the bibliography. Thus, the preparation of briquettes does not require a previous drying operation. The partial pyrolysis of the esparto increases its calorific power at the same time it reduces the volatile matter, and the combustion profile is very close to that of coal. On the other hand, the carbonization of the esparto improved the resistance to water but decreased, at the same time, the resistance to impact. The effect of the elevation of the briquetting pressures improves the mechanical properties of the briquettes distinctly. However, the pressure cannot be too big and also the pyrolysis is only partial in order to reach the goal of producing a briquette that is economically attractive for the consumer as well as the producer.
Acknowledgement The authors wish to thank the Consejeria de la presidencia de la Junta de Andalucia (Projects A8/02) for the financial support to conduct this research.
References [1] Demirbas A, et al. Evaluation of biomass materials as energy sources-upgrading of tea waste by briquetting process. Energy Sources 1999;21(3):216–20. [2] Bartel Y, et al. Energizing valorization of some agriculture products by densification. Notebooks Agricultures 1997;6(3):208–12 (in French). [3] Demirbas A, et al. Evaluation of biomass residue. 1-briquetting waste paper and wheat straw mixtures. Fuel Process Technol 1998;55:175–83. [4] Yaman S, et al. Production of fuel briquettes from olive refuse and paper mill waste. Fuel Process Technol 2000; 68:23–31.
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[5] Abasaeed AE. Briquetting of carbonized cotton stalk. Energy 1992;17(9):877–82. [6] Zhang X, et al. The effect of different treatment conditions on biomass binder preparation for lignite briquettes. Fuel Process Technol 2001;73:185–96. [7] Yaman S, et al. Fuel briquettes from biomass-lignite blends. Fuel Process Technol 2001;72:1–8. [8] Blesa MJ, et al. Curing temperature effect on mechanical strength of smokeless fuel briquettes prepared with molasses. Fuel 2003;82:943–7. [9] Kerrouani H. Annals of the forest research in Morocco 1994;27(1):225–41 (in French). [10] Yessef M, et al. Ecological and socio-economic aspect of Alfa grass (stipa tenacissima) Range lands in Morocco, Society for Range Management. Annual Meeting—Spokane W.A, 1992. [11] Chablou A. Study of biomass (stipa tenacissima and Rosmasinus officinalis) in the domanial forest of Debdou. Thesis, Forest National School of Engineers 1996, Sale-Morocco. (in French). [12] ISO 1976. Determination of gross calorific value by calorimeters bomb method. ISO (1928), International Organization for Standardization. [13] Richards SR. Fuel Process Technol 1990;25:89. [14] Findikgil G, Possibilities and limitations of briquetting of Turkish lignites. In: Proc Int Coal Technol, seminar, Istanbul Technical University 1983. [15] Gurbuz Beker U, Ku´cu´kbayrak S. Briquetting of Istanbul-Kemerburgaz lignite of Turkey. Fuel Process Technol 1996;47:111–8.
Production of fuel briquettes from esparto partially pyrolyzed A. Debdoubi
a,*
, A. El amarti a, E. Colacio
b
a
b
Unite´ Calorime´trie et Mate´riaux, Universite´ Abdelmalek Essaadi, Faculte´s des Sciences, P.O. Box 2121, Te´touan 93002, Morocco Laboratorio de Quimica Inorganica, Universidad de Granada, Facultad de Ciencias, Granada 18071, Spain Received 22 January 2004; received in revised form 29 April 2004; accepted 19 September 2004 Available online 5 November 2004
Abstract The aim of this study is to prepare a solid fuel from the most abundant biomass of Morocco that can be used by the local population, and in particular by the rural ones, instead of the wood of forest, which is less available and its exploitation is negative for the environment. Esparto is the most available biomass, with a production of 560 000 tons annually of dry matter approximately. However, briquettes of esparto have low calorific power and their mechanical properties are bad. To improve the quality of the briquettes and to have an economically competitive product at the same time, the esparto was partially pyrolyzed at temperatures between 160 C and 400 C, and the pressure of densification has been examined. The combustion profile of the samples has been studied by applying the derivative thermogravimetry technique, and the mechanical properties of the briquettes were tested to evaluate the impact resistance and water resistance. This study showed that strong briquettes can be obtained with a higher calorific value when the esparto is partially pyrolyzed, and a relatively elevated densification pressure is applied. 2004 Elsevier Ltd. All rights reserved. Keywords: Esparto; Briquettes; Partial pyrolysis; Thermogravimetry
*
Corresponding author. Tel.: +212 39 32 03 22; fax: +212 39 99 45 00. E-mail address: [email protected] (A. Debdoubi).
0196-8904/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2004.09.005
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1. Introduction In the last two decades, biomass has acquired considerable importance for a variety of energy and other uses: as bio fuels for domestic cooking, industrial process heating, electrical power generation etc. The advantage of these alternative energy sources is that they are renewable and have low cost production. Therefore, it is an opportunity for developing countries that, for them, have considerable energy deficit and their biomass resources are very abundant. The processes used to convert the biomass to an appropriate form to take advantage of its energy content are pyrolysis, gasification or densification. The last one, known as briquetting technology, appears to be an attractive solution for biomass material. The resulting product is a briquette that has a greater volumetric energy density and is easy for storage and transportation. Some studies have reported the use of biomass in the production of briquettes from tea waste [1], straws of colza [2], wheat straw and waste paper [3], olive refuse [4], cotton stalks [5], or as a binder for lignite [6–8]. Each country evaluates its own biomass resources. The kinds and amounts of biomass show important differences for different geographical areas depending on its climate, flora and agriculture. In Morocco, one of the most abundant biomasses is esparto with immense land coverage estimated at 3.6 Mha, more than half the forest surface of the country with the biggest part situated in the oriental region with 2.3 Mha [9]. Esparto is a vivacious plant, very resistant to dryness, constituted of two parts: an underground part called rhizome that is very important for its regeneration, and the aerial part reaching about one meter of height constituted by branches carrying the leaves called tuft. Forty years ago, the esparto was very well exploited; a good part being exported to Europe where it was used extensively in the paper industry. Locally, it is used in the production of livestock food and as a fuel without any revalorization in the factories of sugar and cement and by the rural population for cooking and heating. These applications created work for local people during 7 months of the year. Since 20 years, the esparto is completely underexploited for lack of an application; the export has ceased; and the national factories donÕt use esparto anymore. Only the domestic use remained. This situation generated an excess of laborers in the zone and contributed to urban migration. All surfaces of the growth of esparto are, in general, very productive. According to some studies, the production of this biomass is 400 to 1800 kg/ha/year depending on the region. Therefore, a surface of 1.5 Mha, identified as ecologically exploitable without risk of deterioration and impoverishment, gives a production of 560 000 T dry biomass per year [10,11]. This quantity of unutilized matter could be evaluated for energy use. In the present work, the combustion characteristics of esparto and partially pyrolyzed esparto were examined, and the mechanical properties of the briquettes were investigated with respect to the temperature of pyrolysis and the pressure of densification.
2. Experimental The esparto plant has been obtained from the oriental region of Morocco. The power calorific value (PCV) was measured by a Parr bomb calorimeter in a dry basis using samples of 1 g accord-
A. Debdoubi et al. / Energy Conversion and Management 46 (2005) 1877–1884
1879
ing to the standard method [12]. The moisture content was determined after drying a sample at 105 C according to ASTM D2016-25. To obtain the ash content, a dried sample of 2 g is burned at 800 C during 5 h according to ASTM D-5142. The volatile matter was determined according to ISO 562/1974. It was measured by introducing a dry sample of 1 g into a crucible with a top at 950 C during 5 min. The fixed carbon (FC) was calculated as follows: FC% = 100% ash% VM%. The proximate, ultimate and power calorific value analyses are given in Table 1. The moisture content is low, so the process of densification didnÕt require a previous drying operation. The profile of combustion of the samples were determined by thermogravimetric analysis using a Shimadzu TGA-50H analyzer with a rate of flow of the gas of 100 ml/min and heating rate of 20 C/min from 25 C to 950 C. Pyrolysis of the esparto was conducted in a stainless steel reactor (400 length · 20 mm id) with a perforated distribution plate. The bottom part of the reactor is filled by rings of ceramics. Above, a quantity of biomass is placed with a thermocouple for control of the reaction. The reactor was heated with a rate of 10 C/min up to the desired temperature by an electric furnace thermally controlled by a regulator. The water and tars were collected in a trap as condensable products at 0 C. The esparto was partially pyrolyzed at final temperatures of 160, 250, 300, 350 and 400 C, and the losses of mass during this thermal treatment were, respectively, 16.35%, 23.81%, 31%, 61% and 69.5%. Briquettes were produced using a hydraulic press. The esparto was ground to pass through a sieve having an opening of 1 mm before pyrolysis. Then, a quantity of the sample was pressed in a mold of cylindrical configuration with an inner diameter of 30 mm and a height of 80 mm, under different pressures. Five briquettes were prepared for each experiment. No standard test methods have been established for testing fuel briquettes. In the literature, two mechanical properties are considered to be important and are the most used: the resistance to impact with the aim to know the ability of the briquettes to withstand the crushing loads they receive during handling, transport and storage and the resistance against water absorption and disintegration to stimulate severe weathering conditions, which a fuel might encounter during out door storage. The impact resistance was tested by dropping repeatedly a series of briquettes from a stationary position at a height of 2 m until it fractured. The number of drops and the number of pieces the
Table 1 Analysis data and PCV of the esparto and the EPP Samples Esparto EPP (300 C) EPP (400 C)
Ultimate analysis (dry%)
Proximate analysis (dry%)
PCV (MJ/KG)
a
Hu
A
V
FC
5.20 1.9 1.7
2.2 3.8 6.1
80.5 45.9 22.1
16.8 50.3 71.8
C
H
N
O
46.94 64.79 78.32
6.44 4.87 4.1
0.86 1.32 1.54
43.56 30.34 17.58
a
19.1 21.7 26.8
Hu: humidity; A: ash; V: volatile matter; FC: fixed carbon; PCV: power calorific value; EPP: esparto partially pyrolyzed. a By difference.
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briquette broke into were recorded. This data was then used to calculate what is known as the impact resistance index (IRI) from the equation IRI ¼ 100 average no: of drops=average no: of pieces For the laboratory work, an IRI value of 50 was adopted as the lowest acceptable impact resistance for the fuel briquettes [13]. The water resistance time was arbitrarily determined by putting a briquette in a container filled with cold water, and the time required for dispersion was measured. This method has been fixed as the National Turkey Standard for lignite briquettes [14], and the limit is that the water resistance period must equal or exceed 1 h. The Standard has been used for lignite briquettes [15], for lignite blended by biomass [7] and also for biomass briquettes [4].
3. Results and discussions 3.1. The burning profile of esparto The curve of loss of mass of the sample during the combustion is shown in Fig. 1(a). The profile can be divided into four regions. In the first region, some losses of mass appeared near 100 C, corresponding to the release of the moisture content. As the temperature increased, the important and rapid losses of mass, reaching 20%/min in mass, occurring between 250 and 350 C can be attributed to the release of volatile matter and their ignition. This is the characteristic of biomass combustion, known for their high volatile matter, as shown for esparto in Table 1.
Fig. 1. The burning profile of esparto: (a) under atmosphere and (b) nitrogen atmosphere.
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Table 2 Effect of pyrolysis temperature on PCV, IRI and WRT Temperature of pyrolysis (C)
PCV (MJ/KG)
IRI
WRT(min)
25 160 250 300 350 400
19.1 19.7 20.8 21.7 22.9 25.8
1000 1000 450 266 200 150
30 47 65 72 84 89
Briquetting pressure: 200 MPa.
The third region, showing a slow burning loss of about 7.5%/min in mass, arrives between 400 and 550 C. As we can see, this part of the curve is absent in the burning profile of esparto under a nitrogen atmosphere, Fig. 1(b), whereas it has the same behavior in the other regions. This region corresponds to combustion of the biomass materiel where the fixed carbon is an important element with slow burning and a constant heat flux. In the last region, the burning rate apparently results in a mass loss less than 1%/min. This behavior in combustion still continues as the temperature reaches 1000 C. The main reason is that the combustible matter becomes exhausted and the presence of ashes becomes important. These results show that the biggest part of esparto is burned quickly, and it is an inconvenience for the use of the briquettes for heating and cooking, which requires combustion with a constant flux of energy exchange.
Fig. 2. The burning profile of: (a) esparto partially pyrolyzed (300 C) and (b) coal dust.
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In order to improve the combustion characteristics of the briquettes of esparto, the briquettes can be pyrolyzed. This thermal treatment permits removal of the volatile matter and an increase of the power calorific value. However, this pyrolysis was only partial because, on the one hand, it is necessary to produce economically acceptable briquettes for the local market, and on the other hand, a small quantity of volatile matter must be retained for good ignition of the combustion. So, the choice of the temperature of pyrolysis is optimized according to these requirements and the quality of the briquettes formed (see Table 2). The combustion characteristics of esparto partially pyrolyzed at a temperature of 300 C are shown in Fig. 2(a). This sample possessed again a quantity of volatile matter, and the fixed carbon became the main matter in the material with high reactivity and desirable combustion. The loss in mass reaches the maximum rate of 13%/min at the temperature of 429 C, slightly higher than the non-pyrolyzed esparto mass loss of 7.5%/min at a slightly higher temperature of 436 C. The behavior of combustion of this charred biomass is similar to that of the dust of coal, a classic solid fuel, which is given for comparison in Fig. 2(b). The coal has a loss of mass of 9%/min at a temperature of 569 C. Some experiments showed that a further increase of the temperature of pyrolysis decreased the rate of mass loss. 3.2. Briquetting of the esparto At first, the esparto was partially pyrolyzed at temperatures between 160 and 400 C in the fixed bed reactor. The results are shown in Table 2. As the temperature increases, the power calorific value increases (up to 34.5% at the temperature of 400 C) due to the removal of some volatile matter, the moisture content and an increase in the concentration of fixed carbon, thereby providing a higher energy/volume ratio. A series of briquettes was prepared for each temperature of pyrolysis at an applied pressure of 200 MPa. The prepared briquettes were tested mechanically; the first tests concerning the resistance the impact denoted IRI and the second one concerning the resistance to water denoted WRT. The results are shown in Table 2. The briquettes of esparto without pyrolysis shows good resistance to impact (IRI = 1000) but disintegrates in water after just 30 min. This value is greater than those that have been found for briquettes of paper mill waste and less than those from olive refuse, 19 min and 62 min respectively [4]. The pyrolysis of esparto improves the resistance to water from 30 to 89 min as the temperature of pyrolysis is increased from ambient to 400 C, so the esparto will be pyrolyzed for a temperature greater than 250 C to obtain a WRT = 1 h. However, the resistance to impact decreases from 1000 at ambient to 150 at a temperature of 400 C. The texture of the material should be different after the pyrolysis. The charred particles of esparto become less absorbent to the water, while the compaction becomes difficult with the absence of humidity, which is less than 2% after the thermal treatment. Some samples have been kept for a long time in a humid environment, where their moisture reaches 6%, before being densified. So the briquettes formed have their IRI increased 30%. In order to improve the impact resistance of the briquettes, the applied pressure was varied from 150 to 400 MPa. The observed result was the same for the esparto and the partially pyrolyzed esparto at 300 C as can be seen in Table 3. As the pressure increased, the IRI for the pyrolyzed esparto increased from 133 to 833 or remained constant at the maximum (1000) for the esparto, and the disintegration time of the briquettes when they were immersed in water increased from 63 to 112 min and from 10 to 85 min respectively. So, a strong briquette can be produced by
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Table 3 Effect of briquetting pressure on IRI and WRT for esparto and esparto partially pyrolyzed (EPP) Pressure (MPa) 150 200 300 400
EPP(300 C)
Esparto IRI
WRT (min)
IRI
WRT (min)
1000 1000 1000 1000
10 30 78 85
133 266 750 833
63 72 105 112
applying more pressure, but to obtain economically acceptable briquettes, the choice between pyrolysis temperature and pressure of densification must be taken into account. The partially pyrolyzed esparto at 300 C and a densification pressure of 300 MPa gives a briquette with good mechanical properties (IRI = 750 and WRT = 105 min).
4. Conclusions The esparto has a lot of advantages in addition to its abundance and good concentration in the region. It has just 5% of water, which is less than for the majority of biomass mentioned in the bibliography. Thus, the preparation of briquettes does not require a previous drying operation. The partial pyrolysis of the esparto increases its calorific power at the same time it reduces the volatile matter, and the combustion profile is very close to that of coal. On the other hand, the carbonization of the esparto improved the resistance to water but decreased, at the same time, the resistance to impact. The effect of the elevation of the briquetting pressures improves the mechanical properties of the briquettes distinctly. However, the pressure cannot be too big and also the pyrolysis is only partial in order to reach the goal of producing a briquette that is economically attractive for the consumer as well as the producer.
Acknowledgement The authors wish to thank the Consejeria de la presidencia de la Junta de Andalucia (Projects A8/02) for the financial support to conduct this research.
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