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Influence of die pressure on relaxation characteristics of briquetted biomass
Energy Conversion and Management 43 (2002) 2157–2161 www.elsevier.com/locate/enconman
Influence of die pressure on relaxation characteristics of briquetted biomass C.K.W. Ndiema *, P.N. Manga, C.R. Ruttoh Department of Industrial and Energy Engineering, Egerton University, P.O. Box 536, Njoro, Kenya Received 4 May 2001; accepted 7 September 2001
Abstract Briquetting of biomass has been found to be a viable technology for upgrading biomass materials, including agricultural residues, particularly in developing countries where there is abundant biowaste resources. The technology converts the biowaste into forms which are combustible in typical burners. The physical (elongation and voidage) characteristics and, hence, combustion characteristics of the briquettes formed depend on several factors among which the die pressure is prominent. This was confirmed by experimental investigations during which the samples were densified under die pressure ranges of 20–140 MPa. Ó 2002 Published by Elsevier Science Ltd. Keywords: Biomass; Briquettes; Die pressure; Relaxation
1. Introduction Some biomass wastes from sawmills and agro-processing industries can be readily used as fuel. These include wood offcuts and left over slabs from sawmills, coconut shells, corncobs, cotton stalks, coffee husks and wheat and rice straws. The majority of other residues comprise smaller particles in loose form, which makes them unsuitable for direct combustion. Furthermore, their bulk density or heating value per unit volume is much lower, thus making it technically unfeasible for direct use due to combustion and handling problems. Briquetting (densification) of loose and smaller biomass waste is an attractive option for fuel utilization in industrial stoker furnace combustion systems. It is known that in most developing countries, the demand for wood fuels is increasing faster than the sustainable supply, the *
Corresponding author. E-mail address: [email protected] (C.K.W. Ndiema).
0196-8904/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 1 9 6 - 8 9 0 4 ( 0 1 ) 0 0 1 6 5 - 0
2158
C.K.W. Ndiema et al. / Energy Conversion and Management 43 (2002) 2157–2161
consequence of which is the deforestation of vast woodlands [2,8,9]. Application of briquetting technology for biomass waste conversion appears to be an attractive solution, especially in areas where the biowaste resources are substantial and unutilized. The advantages of briquetted fuel include: the ease of charging the furnace, increase in calorific value, improvement of combustion characteristics, reduction of entrained particulate emissions, uniformity in size and shape and good substitution for natural fuelwood. The types of furnaces that can utilize briquetted fuel are those designed for solid fuels having common characteristics, as wood and coal. Since industrial units operate at relatively high temperature, ash slagging becomes a problem when using briquetted fuel derived from agricultural residues due their alkali content. According to a study in India, rice straw briquettes have found a ready market in coal-fired industries [1]. Despite the known improvement in combustion characteristics as compared to loose biowaste, briquetted fuels of the same material made under different conditions have been reported to have different combustion characteristics. In general, briquetted fuel made from different processes or materials differ in handling and combustion behavior, as do woods of different types. It has been established that the relaxation characteristics, which have a bearing on the combustion characteristics of briquetted fuel (e.g. the ability to ignite and the emission of smoke), are influenced by the briquetting process [1,3,4]. The final relaxed density of briquetted fuel and the relaxation behavior following removal from the die depend on many factors related to die geometry, the magnitude and mode of compression, the type and properties of the feed material and storage conditions. Understanding these factors is essential in study of the densification and relaxation of briquetted biomass. This will enable the prediction of the performance of the fuel in a given furnace. Most studies on high pressure compaction of biomass materials have shown that on removal from the die, the density of the compacted material decreases with time to a final relaxed density. For most feed materials, the rate of expansion is highest just after the removal of pressure and decreases with time until the particle attains constant volume [3,5]. The relaxation characteristics, which are mainly measured by the percentage elongation and increase in voidage, depend on many factors related to the feed material and storage conditions, such as relative humidity [9]. Shrivastava (1990) used statistical analysis for the results obtained for rice husks to establish a multiple correlation equation of the form [7] Y ¼ a0 þ a1 P þ a2 T
ð1Þ
where Y is percent volume expansion, T (°C) and P (kg/m2 ) are die temperature and pressure, respectively. a0 , a1 and a2 are constants.
2. Experimental procedure The experimental rig shown in Fig. 1 was designed on the basis of that designed by Zohn (1986) [10]. After setting up the apparatus, a pre-weighed amount of feed material (rice straw) was loaded into the feed hopper and levelled across the hopper before closing the hopper door. The plungers were then advanced into the die to compress the material at a selected die pressure and residence
C.K.W. Ndiema et al. / Energy Conversion and Management 43 (2002) 2157–2161
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Fig. 1. Main features of the densification appartus used for the experiments.
time. The backup plunger was then retracted and the briquette pushed out of the die by further advancement of the densification plunger. After ejection from the die, the length of the briquette was measured while stored under laboratory conditions of 50–60% relative humidity. Measurements of length were taken at 10 s and 24 h after ejection using a vernier caliper (0.005 mm). The same procedure was repeated for various die pressures. The fraction void volume was also determined. 3. Results and discussions For each die pressure, the lengths and fraction void volume of the briquette were replicated three times. 3.1. Percentage elongation The percentage elongation is expressed as Lt LD 100 ð2Þ %wt ¼ LD where LD is the length of briquette in the die and Lt is the length at time t after ejection. Fig. 2 shows the percentage elongation of the briquettes for different die pressures. From the graphs, it can be seen that by the time the first measurement was recorded at 10 s after ejection, more than half of the total expansion recorded after 24 h had already taken place. The expansion is shown to decrease with an increase in die pressure until a minimum occurs at about 80 MPa. The decrease can be attributed to increasing cohesion (bonding) of particles, which apparently reaches a maximum at a die pressure of about 80 MPa. Beyond this pressure, the relaxation of the briquette does not cause a significant change in the percentage elongation. Other factors that come into play at this pressure are the thermal stresses that may induce plastic deformation of the feed material [4,6]. 3.2. Fraction void volume The fraction void volume which is expressed in percentage is determined by the following expression:
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Fig. 2. Percent elongation vs die pressure at t ¼ 10 s and 24 h.
%et ¼
Vt Vs 100 Vt
ð3Þ
where Vs is measured by means of a multivolume pycnometer and Vt is the volume of the sample at time t after ejection, calculated for a right cylinder of length Lt and diameter dt as p Vt ¼ dt2 Lt ð4Þ 4 The influence of the die pressure on fraction void volume, as shown in Fig. 3, decreases continuously with die pressure. However, the decrease is faster in the 35–80 MPa die pressure range. Beyond 80 MPa, the decrease is insignificant. This probably is due to the fact that the interparticle void spaces of the original particles are reduced to a near minimum at about 80 MPa, when bonding is maximum. The remaining spaces are in the form of voids within the plant cell (intra-particle), which can only be reduced if the structure of the original feed material is altered by crushing [4,5,7].
Fig. 3. Percent fraction void volume vs die pressure.
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4. Conclusion The experimental results show that there is considerable influence of the die pressure on the size and form of the briquettes. Specifically, it can be concluded that for a given die size and storage conditions, there is a maximum die pressure beyond which no significant gain in cohesion (bonding) of the briquette can be achieved. This fact is confirmed by the pattern of percentage elongation and fraction void volume of the relaxed briquettes.
References [1] Bhattacharya SC et al. Densification of biomass residue in Asia. In: Energy from Biomass and Waste, vol III, 1980. p. 564–71. [2] Barnard G, Kristoferson L. Agricultural residues for fuel in the third world, Technical Report # 4. Energy Information Programme, London, 1985. [3] Carre J et al. Briquetting agricultural and wood residues: experience gained with a heated die cylindrical screw press, Proceedings of 1st FAO/CNRE Workshop on Handling and Processing of Biomass for Energy, Hamburg, FRG, 1987. [4] Lindley JA, Vissougni M. Physical properties of biomass briquettes. American Society of Agricultural Engineers 1989;32(2):361–6. [5] Miles TR, Miles TR. Densification systems for agricultural residues. In: Thermal conversion of solid waste and biomass. Washington, DC: American Chemical Society; 1980. p. 179–93. [6] Mobarak F, Augustin H. Binderless lignocellulosic composite baggage and mechanism for self bonding. Holzforschung 1982;38:131–5. [7] Shrivastava M et al. Briquetting of rice husks under hot compression, Proceedings of the International Agricultural Engineering Conference and Exhibition, Bangkok, Thailand, 1990. p. 666–72. [8] Svenningson PJ. Fuel briquettes in developing countries: present knowledge. Asset 1990;12(2):21–4. [9] Wamukonya L, Jenkins B. Durability and relaxation of sawdust and wheat-straw briquettes as possible fuels for Kenya. Biomass and Bioenergy 1995;8(3):175–9. [10] Zohn MA, Jenkins BM. An automatic laboratory for densification of particulate materials, ASAE paper no. 266578, 1986.
Influence of die pressure on relaxation characteristics of briquetted biomass C.K.W. Ndiema *, P.N. Manga, C.R. Ruttoh Department of Industrial and Energy Engineering, Egerton University, P.O. Box 536, Njoro, Kenya Received 4 May 2001; accepted 7 September 2001
Abstract Briquetting of biomass has been found to be a viable technology for upgrading biomass materials, including agricultural residues, particularly in developing countries where there is abundant biowaste resources. The technology converts the biowaste into forms which are combustible in typical burners. The physical (elongation and voidage) characteristics and, hence, combustion characteristics of the briquettes formed depend on several factors among which the die pressure is prominent. This was confirmed by experimental investigations during which the samples were densified under die pressure ranges of 20–140 MPa. Ó 2002 Published by Elsevier Science Ltd. Keywords: Biomass; Briquettes; Die pressure; Relaxation
1. Introduction Some biomass wastes from sawmills and agro-processing industries can be readily used as fuel. These include wood offcuts and left over slabs from sawmills, coconut shells, corncobs, cotton stalks, coffee husks and wheat and rice straws. The majority of other residues comprise smaller particles in loose form, which makes them unsuitable for direct combustion. Furthermore, their bulk density or heating value per unit volume is much lower, thus making it technically unfeasible for direct use due to combustion and handling problems. Briquetting (densification) of loose and smaller biomass waste is an attractive option for fuel utilization in industrial stoker furnace combustion systems. It is known that in most developing countries, the demand for wood fuels is increasing faster than the sustainable supply, the *
Corresponding author. E-mail address: [email protected] (C.K.W. Ndiema).
0196-8904/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 1 9 6 - 8 9 0 4 ( 0 1 ) 0 0 1 6 5 - 0
2158
C.K.W. Ndiema et al. / Energy Conversion and Management 43 (2002) 2157–2161
consequence of which is the deforestation of vast woodlands [2,8,9]. Application of briquetting technology for biomass waste conversion appears to be an attractive solution, especially in areas where the biowaste resources are substantial and unutilized. The advantages of briquetted fuel include: the ease of charging the furnace, increase in calorific value, improvement of combustion characteristics, reduction of entrained particulate emissions, uniformity in size and shape and good substitution for natural fuelwood. The types of furnaces that can utilize briquetted fuel are those designed for solid fuels having common characteristics, as wood and coal. Since industrial units operate at relatively high temperature, ash slagging becomes a problem when using briquetted fuel derived from agricultural residues due their alkali content. According to a study in India, rice straw briquettes have found a ready market in coal-fired industries [1]. Despite the known improvement in combustion characteristics as compared to loose biowaste, briquetted fuels of the same material made under different conditions have been reported to have different combustion characteristics. In general, briquetted fuel made from different processes or materials differ in handling and combustion behavior, as do woods of different types. It has been established that the relaxation characteristics, which have a bearing on the combustion characteristics of briquetted fuel (e.g. the ability to ignite and the emission of smoke), are influenced by the briquetting process [1,3,4]. The final relaxed density of briquetted fuel and the relaxation behavior following removal from the die depend on many factors related to die geometry, the magnitude and mode of compression, the type and properties of the feed material and storage conditions. Understanding these factors is essential in study of the densification and relaxation of briquetted biomass. This will enable the prediction of the performance of the fuel in a given furnace. Most studies on high pressure compaction of biomass materials have shown that on removal from the die, the density of the compacted material decreases with time to a final relaxed density. For most feed materials, the rate of expansion is highest just after the removal of pressure and decreases with time until the particle attains constant volume [3,5]. The relaxation characteristics, which are mainly measured by the percentage elongation and increase in voidage, depend on many factors related to the feed material and storage conditions, such as relative humidity [9]. Shrivastava (1990) used statistical analysis for the results obtained for rice husks to establish a multiple correlation equation of the form [7] Y ¼ a0 þ a1 P þ a2 T
ð1Þ
where Y is percent volume expansion, T (°C) and P (kg/m2 ) are die temperature and pressure, respectively. a0 , a1 and a2 are constants.
2. Experimental procedure The experimental rig shown in Fig. 1 was designed on the basis of that designed by Zohn (1986) [10]. After setting up the apparatus, a pre-weighed amount of feed material (rice straw) was loaded into the feed hopper and levelled across the hopper before closing the hopper door. The plungers were then advanced into the die to compress the material at a selected die pressure and residence
C.K.W. Ndiema et al. / Energy Conversion and Management 43 (2002) 2157–2161
2159
Fig. 1. Main features of the densification appartus used for the experiments.
time. The backup plunger was then retracted and the briquette pushed out of the die by further advancement of the densification plunger. After ejection from the die, the length of the briquette was measured while stored under laboratory conditions of 50–60% relative humidity. Measurements of length were taken at 10 s and 24 h after ejection using a vernier caliper (0.005 mm). The same procedure was repeated for various die pressures. The fraction void volume was also determined. 3. Results and discussions For each die pressure, the lengths and fraction void volume of the briquette were replicated three times. 3.1. Percentage elongation The percentage elongation is expressed as Lt LD 100 ð2Þ %wt ¼ LD where LD is the length of briquette in the die and Lt is the length at time t after ejection. Fig. 2 shows the percentage elongation of the briquettes for different die pressures. From the graphs, it can be seen that by the time the first measurement was recorded at 10 s after ejection, more than half of the total expansion recorded after 24 h had already taken place. The expansion is shown to decrease with an increase in die pressure until a minimum occurs at about 80 MPa. The decrease can be attributed to increasing cohesion (bonding) of particles, which apparently reaches a maximum at a die pressure of about 80 MPa. Beyond this pressure, the relaxation of the briquette does not cause a significant change in the percentage elongation. Other factors that come into play at this pressure are the thermal stresses that may induce plastic deformation of the feed material [4,6]. 3.2. Fraction void volume The fraction void volume which is expressed in percentage is determined by the following expression:
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Fig. 2. Percent elongation vs die pressure at t ¼ 10 s and 24 h.
%et ¼
Vt Vs 100 Vt
ð3Þ
where Vs is measured by means of a multivolume pycnometer and Vt is the volume of the sample at time t after ejection, calculated for a right cylinder of length Lt and diameter dt as p Vt ¼ dt2 Lt ð4Þ 4 The influence of the die pressure on fraction void volume, as shown in Fig. 3, decreases continuously with die pressure. However, the decrease is faster in the 35–80 MPa die pressure range. Beyond 80 MPa, the decrease is insignificant. This probably is due to the fact that the interparticle void spaces of the original particles are reduced to a near minimum at about 80 MPa, when bonding is maximum. The remaining spaces are in the form of voids within the plant cell (intra-particle), which can only be reduced if the structure of the original feed material is altered by crushing [4,5,7].
Fig. 3. Percent fraction void volume vs die pressure.
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4. Conclusion The experimental results show that there is considerable influence of the die pressure on the size and form of the briquettes. Specifically, it can be concluded that for a given die size and storage conditions, there is a maximum die pressure beyond which no significant gain in cohesion (bonding) of the briquette can be achieved. This fact is confirmed by the pattern of percentage elongation and fraction void volume of the relaxed briquettes.
References [1] Bhattacharya SC et al. Densification of biomass residue in Asia. In: Energy from Biomass and Waste, vol III, 1980. p. 564–71. [2] Barnard G, Kristoferson L. Agricultural residues for fuel in the third world, Technical Report # 4. Energy Information Programme, London, 1985. [3] Carre J et al. Briquetting agricultural and wood residues: experience gained with a heated die cylindrical screw press, Proceedings of 1st FAO/CNRE Workshop on Handling and Processing of Biomass for Energy, Hamburg, FRG, 1987. [4] Lindley JA, Vissougni M. Physical properties of biomass briquettes. American Society of Agricultural Engineers 1989;32(2):361–6. [5] Miles TR, Miles TR. Densification systems for agricultural residues. In: Thermal conversion of solid waste and biomass. Washington, DC: American Chemical Society; 1980. p. 179–93. [6] Mobarak F, Augustin H. Binderless lignocellulosic composite baggage and mechanism for self bonding. Holzforschung 1982;38:131–5. [7] Shrivastava M et al. Briquetting of rice husks under hot compression, Proceedings of the International Agricultural Engineering Conference and Exhibition, Bangkok, Thailand, 1990. p. 666–72. [8] Svenningson PJ. Fuel briquettes in developing countries: present knowledge. Asset 1990;12(2):21–4. [9] Wamukonya L, Jenkins B. Durability and relaxation of sawdust and wheat-straw briquettes as possible fuels for Kenya. Biomass and Bioenergy 1995;8(3):175–9. [10] Zohn MA, Jenkins BM. An automatic laboratory for densification of particulate materials, ASAE paper no. 266578, 1986.