Using cotton plant residue to produce briquettes

Biomass and Bioenergy 18 (2000) 201±208

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Using cotton plant residue to produce briquettes Wayne Coates* Bioresources Research Facility, Oce of Arid Land Studies, The University of Arizona, 250 E. Valencia Road, Tucson, AZ 85706, USA Received 29 March 1999; received in revised form 28 September 1999; accepted 22 October 1999

Abstract In Arizona, cotton (Gossypium ) plant residue left in the ®eld following harvest must be buried to prevent it from serving as an overwintering site for insects such as the pink bollworm. Most tillage operations employed to incorporate the residue into the soil are energy intensive and often degrade soil structure. Trials showed that cotton plant residue could be incorporated with pecan shells to produce commercially acceptable briquettes. Pecan shell briquettes containing cotton residue rather than waste paper were slightly less durable, when made using equivalent weight mixtures and moisture contents. Proximate and ultimate analyses showed the only di€erence among briquette samples to be a higher ash content in those made using cotton plant residue. Briquettes made with paper demonstrated longer ¯ame out time, and lower ash percentage, compared to those made with cotton plant residue. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biomass; Cotton stalks; Briquettes; Durability; Cotton plant residue; Pecan shells

1. Introduction In the warmer regions of the United States, cotton (Gossypium ) plant residue must by law be buried so that it does not provide overwintering sites for pests such as the pink bollworm. Cotton residue has little value as a soil amendment, and tillage operations which bury the residue have high energy requirements and often degrade soil

* Tel.: +1-520-741-0840; fax: +1-520-741-1468.

structure. Thus cotton residue is considered a negative value biomass. It has been estimated that cotton residue production is 2.9 times that of lint production [1]. Total cotton plantings in the US exceed 6.4 million hectares (16 million acres), with an average lint production of 800 kg/ha (704 lb/acres) [2]. Thus total cotton residue produced in the United States exceeds 5.1 million tonnes (5.6 million tons) annually. In Arizona, 152,000 ha (380,000 acres) are planted annually to cotton, with average lint production greater than 1200

0961-9534/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 1 - 9 5 3 4 ( 9 9 ) 0 0 0 8 7 - 2

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W. Coates / Biomass and Bioenergy 18 (2000) 201±208

kg/ha (1060 lb/acres), well above the national average [3]. This translates into an average biomass production of 3.5 t/ha (1.5 tons/acres), or a total of 532,000 tonnes (570,000 tons) each year. Combined with the 100,000 tonnes (110,000 tons) of gin trash produced annually in Arizona [4], a substantial biomass resource exists which could be used for energy production. The speci®c energy of cotton stalks ranges from 17.1 to 18.1 MJ/kg (7339±7768 btu/lb). This compares favorably with wood, which ranges from 17.4 to 18.6 MJ/kg (7468±7983 btu/lb) [5]. 2. Harvesting crop residues Interest in harvesting crop residues for energy has existed for many years [1]. The energy required to collect and process residues has been shown to be a small percentage of the energy content of the residue itself [6]. Still a number of factors have contributed to the underutilization of crop residues as a source of energy. Minimizing costs for collection, handling, storage and transportation are most easily accomplished by densi®cation. Typical densities provided by various crop residue collection methods range from 160 to 350 kg/m3 (9.5± 20.8 lb/ft3) [7]. This is much less dense than a cord of wood, which is approximately 638 kg/m3 (37.9 lb/ft3) [8]. A number of studies have investigated cotton stalk harvesting. Sumner et al. [9] used a large round baler to collect cotton stalks, corn stover and soybean residues. The cotton stalks provided the highest burning eciency and longest burn time of the three residues. Sumner et al. [10] investigated cotton plant residue from the standpoint of potential yield, drying time, and type of harvesting system. They found cotton plant roots to comprise 23% of the total yield, and drying time to increase dramatically when the roots were harvested, as compared to harvesting only stalks. Because a commercially viable system for harvesting cotton plant residue was not available, a project to develop one began in 1994 at the University of Arizona. At the conclusion of the pro-

ject, two systems had been developed [11]. Stalks are ®rst pulled with an implement developed for the purpose and then baled using a large round baler, or chopped with a forage harvester and transferred to a cotton module builder. Density of the packages varied, with round bales ranging from 93 to 138 kg/m3 (6±9 lb/ft3), and modules from 168 to 232 kg/m3 (10±15 lb/ft3). Energy required to harvest the stalks averaged 9.2 kWh/t (12.1 hp-hr/ton) for the baling system, and 8.6 kWh/t (11.4 hp-hr/ton) for chopping and moduling. Speci®c energy of the harvested stalks averaged 18.6 MJ/kg (7983 btu/lb) [11]. Once a harvesting system had been developed, the next phase in the research program was to investigate alternative uses for the harvested material. Gomes et al. [12] conducted an economic assessment of the harvesting systems and transport distances to end use facilities. They concluded that the baler system yields a lower market price for residue compared to coal on an energy basis, but that supply constraints and switching costs probably would deter adoption by local electric power generating facilities. Given this scenario, alternative uses were sought. One of the largest pecan groves in the world is located south of Tucson. Associated with the farming operation is a processing plant, with its waste product being pecan shells. In 1994, Pecan Shell Products Inc. began commercially producing briquettes from the shells. The advantages over traditional charcoal briquettes are that pecan briquettes utilize a renewable resource that otherwise would be land®lled, and they are cleaner since charcoal dust is not produced during handling, nor are chemical fumes emitted during combustion. Furthermore, the pecan shell briquetting process is far less energy intensive than that used to produce charcoal briquettes. Waste paper is one of the ingredients used in the pecan briquetting process, added to increase briquette durability. The cost of waste paper has increased signi®cantly with the rise in recycling. As a consequence the company was looking for a substitute material to replace paper. Necessary characteristics were that the replacement not compromise either briquette durability, nor burn qualities.

W. Coates / Biomass and Bioenergy 18 (2000) 201±208

3. Objectives 1. Produce test specimens using various mixtures of pecan shells and cotton plant residue under a range of pressures and moisture contents to establish a test range for the commercial equipment. 2. Conduct semi-commercial runs at the factory with the most promising mixtures. 3. Evaluate durability of the briquettes made using cotton residue, and compare them to production runs. 4. Conduct burn tests, and proximate and ultimate analyses on the briquettes.

4. Procedure Test specimens were produced in a hand operated press, designed for the purpose. The mold was comprised of a stainless steel cylinder and piston, 8.8 mm (2.25 in.) in diameter and 152 mm (6 in.) long. Measurements in the factory had shown briquetting pressure to be approximately 9650 kPa (1400 psi). To provide a range for evaluation, three pressures were chosen for the trials, 4776 (700 psi), 9650 (1400 psi) and 14,500 kPa (2100 psi). Moisture contents of 10, 15, 20 and 25% (wet basis) and cotton percentages of 10, 15, 20 and 25% (by weight) were evaluated, with the test specimens weighing 35 g (0.08 lb), dry weight. This size was chosen to re¯ect the mean weight of pecan/paper briquettes produced at the factory. Twenty ®ve test samples of each combination of the 48 possible combinations of variables were made. Following pressing, the samples were placed in a forced air oven at 1008C (2128F), and dried to constant weight. To assess durability, ASAE Standard S269.4, entitled Cubes, Pellets, and Crumbles Ð De®nitions and Methods for Determining Density, Durability and Moisture Content, was adopted [13]. This was chosen as the test methodology since no recognized standard exists for measuring briquette durability.

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After fabricating the tumbling device described in the standard, trials with the test specimens commenced. Twenty of the most uniform samples, determined by individually weighing each test specimen, were chosen from the 25 samples made with each combination of variables. The selected specimens were divided into two groups, with ®rst one and then the other tumbled following the procedure described in the standard. Results from the durability trials were used to identify the most likely candidate mixtures and moisture contents for the semi-commercial runs at the briquette factory. In consultation with factory sta€ it was decided that a 4.5 kg (10 lb) sample was the smallest that could be processed, would produce sucient briquettes for test purposes, and would also load the dies for a sucient duration to approximate the commercial operation. Briquettes were made using cotton residue following two procedures. In one case the cotton residue was ground separately from the pecan shells, as had been the case when making the test specimens. In the other case the materials were weighed out, and then ground together. This latter procedure more closely resembled the existing factory procedure in which the waste paper and pecan shells are ground together. In addition to producing briquettes using cotton residue, some were also made using paper. This was done to ensure that operating di€erences between the semi-commercial runs and commercial practice would not in¯uence any of the subsequent analyses. Cotton percentages of 10, 15 and 20% were examined in combination with moisture contents of 20, 25, and 30% when the materials were ground separately. When ground together, cotton percentages of 10 and 15%, in combination with 20 and 25% moisture contents, were examined. The range was reduced because the ®rst series of trials had shown higher cotton percentages to produce briquettes with questionable durability. Only 10% paper content was evaluated, in combination with 20 and 25% moisture contents. This series of tests was limited, since the normal factory mixture is 10% paper and 20% moisture

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W. Coates / Biomass and Bioenergy 18 (2000) 201±208

Table 1 Results of the durability trials conducted on the test specimens Moisture (%)

Pressure (MPa)

Cotton (%)

Mean size distribution

Mean size durability rating

10 10 10 10 10 10 10 10 10 10 10 10 15 15 15 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 20 20 20 20 25 25 25 25 25 25 25 25 25 25 25 25

5 5 5 5 10 10 10 10 15 15 15 15 5 5 5 5 10 10 10 10 15 15 15 15 5 5 5 5 10 10 10 10 15 15 15 15 5 5 5 5 10 10 10 10 15 15 15 15

10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25 10 15 20 25

0.0 0.0 0.0 0.0 10.9 44.6 19.8 32.3 197.5 200.0 138.9 106.3 57.1 22.7 11.2 1.2 79.9 68.2 53.9 64.2 305.0 348.4 322.7 342.3 319.3 245.3 264.9 114.7 367.2 363.0 235.8 344.9 357.6 362.9 374.3 389.1 368.0 357.8 358.7 339.2 361.5 357.0 350.9 311.6 373.4 375.4 365.3 368.9

0.0 0.0 0.0 0.0 8.6 26.2 13.4 20.3 66.2 65.3 52.4 45.1 26.5 11.1 9.1 1.2 28.7 35.7 32.2 38.9 84.6 87.1 87.3 87.7 86.9 76.9 73.6 65.6 91.8 91.9 78.6 86.2 95.4 96.0 100.0 100.0 92.0 90.8 89.7 90.1 90.4 89.2 87.7 84.9 93.4 93.9 91.3 92.2

Size distribution by pressure

Durability rating by pressure

0.0

0.0

26.9

17.1

160.7

57.3

23.1

12.0

66.6

33.9

329.6

86.7

236.1

75.8

327.7

87.1

371.0

97.9

355.9

90.6

345.3

88.1

370.8

92.7

Size distribution by cotton %

Durability rating by cotton%

69.5 81.5 52.9 46.2

24.9 30.5 21.9 21.8

147.3 146.4 129.3 135.9

46.6 44.6 42.9 42.6

348.0 323.7 291.7 282.9

91.4 88.3 84.1 84.0

367.7 363.4 358.3 339.9

91.9 91.3 89.6 89.1

W. Coates / Biomass and Bioenergy 18 (2000) 201±208

content. Four runs of each combination of variables were completed. Following briquetting, the samples were transported to the Bioresources Research Facility and then dried using the same procedure as for the test specimens. As with the test specimens, 20 of the most uniform samples from each run were selected for the durability trials. In addition to assessing durability, burn tests and proximate and ultimate analyses were conducted. The latter two tests were performed by a commercial laboratory, with 12 briquettes from a run sent for analysis. To evaluate burn performance, the procedure developed by the Kingsford Company to evaluate charcoal briquettes was followed. Thirty briquettes were arranged in a random pile, approximately 75±100 mm (3±4 in.) deep. One hundred and twenty milliliters (4 oz) of lighter ¯uid was poured slowly over the pile, uniformly wetting the exposed briquette surfaces. After a 1 min interval, the briquettes around the bottom of the pile were lit. Percent of the total surface area covered in ash was recorded every 15 min, over a 1 h time interval. In addition, the time at which the ¯ames self extinguished was recorded. To ensure that wind did not in¯uence the results the trials were conducted indoors in a well ventilated facility.

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5. Results 5.1. Test specimens The size distribution and durability rating of each sample group was calculated following the procedure shown in the ASAE standard. Size distribution is a measure of the mean size of the portions that remain following tumbling, while durability rating is the percentage of briquettes that do not degrade more than 10% from the initial weight. Higher numbers in both cases indicate less degradation during tumbling. The results are summarized in Table 1. For a given moisture content, as pressure increased, so did sample durability except at the highest moisture content. In this case, durability remained essentially constant throughout the pressure range. As moisture content increased, for a given cotton percentage, briquette durability increased. As cotton percentage increased within a given moisture content, briquette durability decreased, with the decline being somewhat less at higher moisture contents. The tests indicated that in general, cotton percentage should not exceed 15% by weight. To permit comparison, a number of briquettes produced commercially by the factory were also

Table 2 Results of the durability trials conducted on briquettes produced at the factory using cotton stalks and pecan shells ground separately and together, and those made with paper Treatment

Cotton (%)

Moisture (%)

Size distribution

Standard deviation

Durability rating

Standard deviation

Grind separate

10 10 15 15 15 20 20 20 10 10 15 15 10 10

20 25 20 25 30 25 30 35 20 25 20 25 20 25

182.9 293.3 58.6 159.9 165.6 81.4 43.9 49.7 265.7 312.7 117.7 98.2 345 358.8

45.4 37.7 36.2 34.6 42.9 34.2 23.9 16.9 27.4 22.5 42.3 16.8 24.9 28.9

63.7 81.9 27.4 61.7 61.3 36 21.6 27.6 75.4 85.1 47.8 45.4 88.3 91.4

7.4 3.9 12.5 5.9 11.8 9.0 9.1 6.7 5.1 2.2 11.0 6.4 3.7 4.2

Grind together

Paper

a

10 10 10 10 10 15 10 20 15 15 15 10 20 10 20 10 15

20 20 25 25 20 20 25 35 20 25 25 20 30 20 25 20 30

11.01 10.37 11.07 8.02 8.06 7.86 5.75 10.39 5.69 6.65 7.77 6.41 9.49 8.10 5.45 6.70 6.54

6.71 6.90 7.01 7.25 7.50 7.81 8.00 8.24 8.40 8.50 8.52 8.53 8.61 8.74 8.91 8.95 9.29

52.77 39.00 38.79 38.86 39.02 38.29 52.97 53.01 52.99 52.87 38.45 53.00 53.01 39.62 53.09 38.33 53.03

40.52 54.10 52.40 53.89 53.48 53.90 39.03 38.75 38.61 38.63 53.03 38.47 38.38 51.64 38.00 52.72 37.68

18.37 18.54 18.64 18.91 18.56 18.61 18.17 18.16 18.05 18.17 18.45 18.13 17.60 18.50 19.14 18.23 17.71

19.69 19.91 20.05 20.39 20.07 20.19 19.76 19.79 19.71 19.86 20.17 19.82 19.25 20.27 21.01 20.03 19.52

56.57 41.89 41.71 41.90 42.18 41.53 57.58 57.77 57.85 57.78 42.03 57.94 58.00 43.41 58.28 42.10 58.46

43.43 58.11 58.29 58.10 57.82 58.47 42.42 42.23 42.15 42.22 57.97 42.06 42.00 56.59 41.72 57.90 41.54

Treatments are: paper, grind materials together, grind materials separately and then combine.

Paper Paper Paper Together Together Together Separate Separate Separate Separate Together Separate Separate Together Separate Separate Separate

0.16 0.15 0.14 0.17 0.16 0.16 0.14 0.18 0.14 0.19 0.15 0.14 0.16 0.15 0.14 0.14 0.18

50.14 50.09 49.81 50.40 49.41 48.72 49.88 47.55 48.18 49.40 48.46 48.53 47.67 48.31 48.16 47.75 47.94

5.20 5.23 5.23 5.43 5.41 5.39 5.40 5.18 5.23 5.19 5.32 5.24 5.24 5.31 5.21 5.11 5.11

0.42 0.41 0.43 0.49 0.52 0.53 0.39 0.59 0.52 0.53 0.63 0.47 0.57 0.64 0.45 0.52 0.59

0.13 0.13 0.12 0.15 0.14 0.14 0.12 0.16 0.12 0.16 0.13 0.12 0.13 0.13 0.13 0.12 0.15

6.71 6.90 7.01 7.25 7.50 7.81 8.00 8.24 8.40 8.50 8.52 8.53 8.61 8.74 8.91 8.95 9.29

37.40 37.24 37.40 36.28 37.02 37.41 36.21 38.28 37.55 36.22 36.94 37.11 37.78 36.87 37.14 37.55 36.92

0.08 0.08 0.07 0.09 0.08 0.08 0.07 0.09 0.07 0.10 0.08 0.07 0.08 0.08 0.07 0.07 0.09

Treatmenta Cotton Water Moisture Ash Volatile % Carbon Hydrogen Nitrogen % % % kg MJ/kg MAF % % kg (%) (%) Sulfur Ash Oxygen Sulfur/ (%) (%) (%) (%) (%) Fixed (MJ/kg) MAF MAF SO2/ (%) (di€) GJ carbon volatile F. Carb. (GJ)

Table 3 Results of the proximate and ultimate analyses conducted on factory made briquettes

206 W. Coates / Biomass and Bioenergy 18 (2000) 201±208

W. Coates / Biomass and Bioenergy 18 (2000) 201±208

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Table 4 Results of the burn tests conducted on factory made briquettes Ash percentage @ Treatment

Cotton (%)

Moisture (%)

15 min

30 min

45 min

60 min

Flame out (min)

Grind separate

10 10 15 15 15 20 20 20 10 10 15 15 10 10 10

20 25 20 25 30 25 30 35 20 25 20 25 20 25 30

7.5 10 12.5 15 15 15 17.5 20 5 10 10 10 5 5 10

35 45 60 65 67.5 62.5 72.5 70 50 60 60 70 60 50 50

67.5 72.5 82.5 82.5 80 80 82.5 80 75 75 80 80 70 60 60

87.5 85 95 92.5 90 90 95 92.5 90 90 95 95 80 80 75

15.05 13.73 14.29 13.32 13.29 14.50 13.08 13.12 15.13 14.87 14.50 14.63 19.87 19.30 17.63

Grind together

Paper

tested using the same procedure. The factory briquettes had a mean size distribution of 270, and a mean durability rating of 76. For equivalent values with the test specimens, moisture contents had to be 15% or greater, with the lower moisture content requiring the highest pressure to produce samples with equivalent durability. 5.2. Semi-commercial runs Table 2 presents the results of the tumbling trials conducted on the test briquettes produced at the factory. Considering ®rst the trials in which the materials were ground separately, it is evident that cotton percentages greater than 10 produce less durable briquettes. As moisture contents increased with the 10 and 15% cotton mixtures, more durable briquettes were produced. For the 20% cotton mixture, however, increased moisture content produced less durable briquettes. Comparing the briquettes made when the materials were ground separately, to those made when the materials were ground together, shows that the latter procedure produced more durable briquettes, at the same moisture content, except for the 15% cotton and 25% moisture combi-

nation. Increasing moisture content produced more durable briquettes for the 10% cotton samples, as it did when the materials were ground separately. The 15% cotton samples, however, did not show a positive e€ect with increased moisture content, as it did when the materials were ground separately. The briquettes containing paper (Table 2) were more durable than those made from cotton stalks. As moisture contents increased, briquette durability increased as it had with the briquettes made using cotton plant residue. Results from the proximate and ultimate analyses are presented in Table 3, sorted by increasing ash content. Briquettes made with paper had a lower ash content than did those made with cotton plant residue. This was expected, since some soil accompanied the harvested plants, and this was re¯ected in higher ash content. Sorting the results by other parameters such as kJ/kg (btu/lb), sulfur content, etc. did not indicate that cotton percentage, the use of paper, or grinding together or separately, had any e€ect on the results. Based on these tests it can be concluded that cotton percentage is of little importance in terms of proximate or ultimate analyses, except

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W. Coates / Biomass and Bioenergy 18 (2000) 201±208

that slightly higher ash contents can be expected at higher cotton contents. The results from the burn tests are presented in Table 4. This table has been divided into three sections. The top section presents the results for the briquettes made when grinding the pecan shells and cotton stalks separately. The middle portion of the table presents the results when the materials were ground together, while the bottom presents the results for the briquettes made with paper. The major di€erence that was observed among the three groups of samples is that when paper was used to produce briquettes, it took more time for the ¯ames to extinguish, and less ash was evident at the conclusion of the test. Grinding the pecan shells and cotton stalks together, or separately, had no apparent in¯uence on the results, nor did cotton percentage or moisture content have an e€ect. 6. Summary and conclusions Cotton plant residue can replace the waste paper now used by the factory as an ingredient when producing pecan shell briquettes, with minimal decrease in briquette quality. Given that cotton plant residue harvesting systems exist, and that an abundance of this biomass is available in Arizona, this material could readily replace the relatively costly paper component now being mixed with the pecan shells. Changeover of the front end of the factory to facilitate utilization of cotton stalks would require an initial infusion of capital, but lower raw material costs should compensate for this expenditure in a reasonable period of time. Such a changeover will eventually reduce production costs. Acknowledgements The author wishes to thank the US Depart-

ment of Energy's Western Regional Biomass program and Pecan Shell Products Inc. for supporting this research program.

References [1] Jenkins BM, Sumner HR. Harvesting and handling agricultural residues for energy. Trans ASAE 1986;29(3):824±36. [2] Arizona Crops, January 19 [issue]. Phoenix: [USDA] Arizona Agricultural Statistics Service, Jan 1996. [3] Arizona Agricultural Statistics. Phoenix: [USDA] Arizona Agricultural Statistics Service 1995. [4] White DH, Coates WE, Wolf D. Conversion of cotton plant and cotton gin residues to fuels by the extruder-feeder liquefaction process. In: Proceedings of the Liquid Fuels, Lubricants and Additives from Biomass Conference. Kansas City (MO), 1994. p. 134±42. [5] AR Energy Systems [Personal communication]. Hickory, (NC) August 25 1994. [6] Coxworth E, Coates W, Zuk B, Craig W, Boerma H SRC Technical Report 115. Saskatoon (Saskatchewan, Canada): Saskatchewan Research Council, 1981. [7] Cheremisino€ NP, Cheremisino€ PN, Ellerbusch F. Biomass applications: technology and production. New York: Marcel Dekker, 1980. [8] Haygreen JG, Bower JL. Forest products and wood science, 2nd ed. Ames (IA): Iowa State University Press, 1989. [9] Sumner HR, Sumner PE, Hammond CW, Monroe GE 1981 Energy available from biomass for grain drying. ASAE Paper 81-3014. St. Joseph (MI):ASAE1981. [10] Sumner HR, Hellwig RE, Monroe GE. Harvesting cotton plant residue for fuel. Trans ASAE 1984;27(3):968± 72. [11] Coates WE. Harvesting systems for cotton plant residue. ASAE Applied Engineering in Agriculture 1994;12(6):639±44. [12] Gomes RS, Wilson PN, Coates WE, Fox RW. Cotton (Gossypium) plant residue for industrial fuel: an economic assessment. Industrial Crops and Products 1997 1997;7:1±8. [13] Cubes, pellets, and crumbles Ð de®nitions and methods for determining density, durability and moisture content. St. Joseph, (MI):ASAE 1996.