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Upgrading of biomass by means of torrefaction
EnergyVol. 15, No. 12, pp. 1175-1179,1990 Printedin GreatBritain. All rightsreserved
UPGRADING
03&l-5442/90 $3.00+ 0.00
Copyright0 1990PergamonPressplc
OF BIOMASS BY MEANS OF TORREFACTION
RANU PENTANANUNT, A. N. M. MIZANUR RAHMAN, and S. C. BHA-rrAcHARvAt Asian Institute of Technology. P.O. Box 2754. Bangkok 10501, Thailand (Received
7 November 1989; received for publication
18 May 1990)
Abstract-We present results on upgrading of wood and briquettes by means of torrefaction. The torrefied products showed significantly less smoking during combustion
and a relatively faster rate of combustion. The weight and energy yeilds of torrefied wood are 66.7-83.3 and 76589.6%, respectively; the corresponding values for sawdust briquettes are 76.3-93.8 and 83.1-95.3%) respectively.
INTRODUCTION
Fuelwood is often difficult to use because of its poor combustion characteristics, e.g., low heating value, variable moisture content which is often high, hygroscopic nature, smoking during combustion, etc. For domestic and a number of other applications, wood is often upgraded to charcoal. The charcoal-making process is, however, grossly inefficient, with the product containing only about 55% of the energy of the original raw material in well-managed, commercial operations and as little as 20% in traditional processes. Torrefaction or roasting appears to be an attractive option of upgrading wood to a product which retains about 90% of its energy and can be substituted for charcoal in a variety of applications.’ The important advantages of torrefied wood include high energy yield and hydrophobicity so that it does not regain moisture during storage. We present comparisons of wood and torrefied wood in terms of proximate analysis, ultimate analysis, density, and combustion behaviour. Some results on torrefaction of sawdust briquettes are also presented.
MATERIALS
AND METHODS
The torrefaction unit Small-scale torrefaction was carried out in an experimental unit (see Fig. 1). The unit consisted of a mild steel chamber (61 x 61 x 30.5 cm) placed within an outer chamber (76 x 76 x 46 cm); fiberglass insulation was installed around the sides of the outer chamber. The inner chamber had a perforated bottom and contained the wood, which was torrefied. The desired temperature of the torrefaction chamber was maintained by circulating hot gases obtained from the heat exchanger. When the temperature of the circulating gases fell below the desired range, the control unit switched on the main blower. Consequently, charcoal burned inside the combustion chamber, and the flue gas passed through the heat exchanger and heated up the circulating gases. The main blower operation and charcoal combustion stopped when the temperature of the circulating gases exceeded the desired range. Smoke tests The purpose of the test was to evaluate the smoke density in the exhaust from the combustion of wood, torrefied wood and charcoal. The fuels were burnt in a bucket stove. Three pieces of the fuel saturated with kerosene were first placed on the grate and lit. After the fuel caught fire, more fuel was added. The total weight of fuel added was 200 g in all tests. A To whom all correspondence should be addressed. 1175
RANUPENTANANUN? et al
1176
A exhaust
Flue gas
Pebble Bed Heat Exchanger
Combustion Chomber
Q Blower -.\ %
Hot torrefoction
gos
Control Unit
Blower Torrefoction
Box
Fig. 1. Schematic diagram of the torrefaction process.
metallic hood was placed over the stove. Sample gas was drawn from the discharge pipe of the hood by using the piston-pump sampling device of a Bacharach smoke-tester. The test was performed in accord with ASTM Standard D 215880. The procedure consists essentially of drawing the flue gas through a filter paper and visually matching the shade of the smoke spot with a standard smoke scale. Combustion test The purpose of the combustion test was to compare the combustion characteristics of wood and torrefied wood. The test was performed in an open hearth consisting of a mild steel box of 50 x 50 cm in cross section and 70 cm in height, as shown in Fig. 2. There were two grates. The bottom one was covered with a pebble bed about 10 cm deep to provide some pressure drop so that air would be distributed uniformly across the cross-section of the bed. The samples had equal weights and were placed on the top grate. A cup containing 30 ml of methanol was hung underneath each sample. The methanol was lit and visual observations were made of the ease of lighting, smoking, flame, and incandescence. After one of the samples was almost completely burnt, the fire was put out by spraying water on the samples. Ash analyses were then performed on both samples to determine to amounts of combustible left. [
i_
briquette somple
~-+!!IZL~rnethonolcup
50 75a I
dimeneion in mm
Fig. 2. Combustion test for briquettes.
air
swl~
Upgrading of biomass by means of torrefaction RESULTS
AND
1177
DISCUSSION
The proximate analyses of air-dried wood, wood torrefied by holding at different temperatures for different lengths of time, and commercial charcoal are summarized in Table 1. The moisture content of torrefied wood is seen to be lower than that of either wood or charcoal. The fixed carbon and volatile matter of torrefied wood, which depend on the torrefaction temperature and holding time, fall between those of charcoal and wood. During torrefaction, wood undergoes changes in chemical composition. As is shown in Table 2, the carbon contents of torrefied wood (59.98~64.37%) are greater than that of wood (51.91%) but lower than that of charcoal. Carbon increases at the expense of oxygen and hydrogen, thus leading to decreases in both the H/C and O/C ratios. The weight and energy yields for torrefaction of sawdust briquettes and wood chips on a dry basis are shown in Table 3. Torrefaction of sawdust briquettes yielded weights of 93.8-70.3%, Table 1. Proximate analyses (wet basis). Torrefied
Mood Temp. (‘Cl
2 2 3 3
250-260 260-270 250-260 260-270
Wood
x
Volatile Matter x
4.00
66.47
3.80 3.50 3.20
62.85 65.71 65.71
28.55 32.36
0.98 0.98
30.26 30.26
0.54
11.25
69.80
18.20
0.74
5.8
31.90
59.9
2.40
Moisture
Holding Time (hr)
Charcoal
Fixed Carbon x
Ash
x
0.65
Table 2. Ultimate analyses (dry basis). Composition
(X)
Fuel C
H
0
N
H/C
O/C
TW (2 hrs,250-260'C)
59.98
5.4
33.35
0.25
1.08
O.bl
TU (2 hrs,260-270'C)
60.98
5.1
32.69
0.21
1.00
0.40
TW (3 hrs,250-260'C)
63.44
5.1
30.72
0.18
0.96
0.39
TW (3 hrs.260-270'C)
64.37
5.0
29.80
0.18
0.94
0.38
Wood
51.91
6.1
41.00
0.10
I.40
0.60
Charcoal
82.16
3.3
11.90
0.10
0.48
0.11
Table 3. Energy and weight yields (dry basis).
1178
R.4NuPEwr.4NANuNTetal
0
Smoke
PlTl
Flame
m
Incandescence
Torrcfied
Wood
Wood
( 0
5
IO
15
20
25
30
Time Fig. 3. Combustion
35
40
45
50
55
60
(mins) characteristics
vs time.
with energy yields of 95.4-79.0%, for residence times of 2-4 h. The weight yield decreased with increasing temperature and duration of torrefaction. Similar results were obtained for torrefied wood. The weight yield was 66.7-83.3% and the energy yield 76.5-89.6% for residence times of 2-3 h. Thus, the percentage yields of energy were greater than the percentage yields of weight. The combustion characteristics of wood and of torrefied wood are shown in Fig. 3. When subjected to alcohol flames, both fuels initially smoked but the smoking period was much less for torrefied wood than for ordinary wood. Torrefied wood ceased to smoke within 10 min, while wood continued to smoke with a gradual decrease in smoke intensity to incandescence. From the 15th min, torrefied wood burned incandescently. At the 50th min the torrefied wood was almost completely burnt. Wood became incandescent at the 27th min. There were 15 and 33% of the original combustibles left in the residues of torrefied wood and wood, respectively, at the 50th min when burning of both fuels was terminated. This result indicates that torrefied wood has a significantly higher combustion rate than wood. Similar results were reported earlier for sawdust briquettes produced in a heated die screw press.* Also, torrefied briquettes were found to be practically water-resistent. Untorrefied briquettes disintegrate quickly on exposure to water, are normally difficult to ignite and bum slowly with a high level of smoke. 3 Thus, the technique of torrefaction appears to be particularly useful for upgrading briquettes. Since the briquettes emerge from a briquetting Table 4. Relative smoking of different fuels. Fuel Charcoal Torrcfied wood
Wood
Burner
Perforaance
1
Little
sooting
3
Slight
soothing
6
Severe
sootiAg
Smoke Spot
No.
Upgradingof biomassby means of torrefaction
1179
machine at high temperatures, only small amounts of additional energy inputs are needed for torrefaction. The density of smoke from the combustion of wood, torrefied wood and charcoal in a bucket stove, as well as the degree of sooting for each smoke-spot number based on the Bacharach Smoke Tester, are shown in Table 4. Charcoal shows the least smoke density and sooting, following by torrefied wood and wood, respectively. Sooting affects the use of fuels adversely. Our tests indicate that torrefied wood and charcoal may be suitable for household use since they provide relatively clean combustion.
REFERENCES 1. J. P. Bourgeois and J. Doat, “Torrefied Wood from Temperate and Tropical Species, Advantages and Prospects,” in Bioenergy 84, Vol. 3, pp. 153-159, H. Egnens and A. Ellegard eds., Elsevier Applied Science, London (1985). 2. S. C. Bhattacharya, R. M. Shrestha, and S. Sett “State of the Art of Biocoal Technology,” AIT-GTZ, Biocoal Project Report, AIT, Bangkok (1989). 3. S. C. Bhattacharya, R. M. Shrestha, S. Ngamkajomvivat, and P. Wongvicha, “Biocoal: A Report on Market Opportunities and Requirements in Thailand,” AIT-GTZ Biocoal Project, AIT, Bangkok (1989).
UPGRADING
03&l-5442/90 $3.00+ 0.00
Copyright0 1990PergamonPressplc
OF BIOMASS BY MEANS OF TORREFACTION
RANU PENTANANUNT, A. N. M. MIZANUR RAHMAN, and S. C. BHA-rrAcHARvAt Asian Institute of Technology. P.O. Box 2754. Bangkok 10501, Thailand (Received
7 November 1989; received for publication
18 May 1990)
Abstract-We present results on upgrading of wood and briquettes by means of torrefaction. The torrefied products showed significantly less smoking during combustion
and a relatively faster rate of combustion. The weight and energy yeilds of torrefied wood are 66.7-83.3 and 76589.6%, respectively; the corresponding values for sawdust briquettes are 76.3-93.8 and 83.1-95.3%) respectively.
INTRODUCTION
Fuelwood is often difficult to use because of its poor combustion characteristics, e.g., low heating value, variable moisture content which is often high, hygroscopic nature, smoking during combustion, etc. For domestic and a number of other applications, wood is often upgraded to charcoal. The charcoal-making process is, however, grossly inefficient, with the product containing only about 55% of the energy of the original raw material in well-managed, commercial operations and as little as 20% in traditional processes. Torrefaction or roasting appears to be an attractive option of upgrading wood to a product which retains about 90% of its energy and can be substituted for charcoal in a variety of applications.’ The important advantages of torrefied wood include high energy yield and hydrophobicity so that it does not regain moisture during storage. We present comparisons of wood and torrefied wood in terms of proximate analysis, ultimate analysis, density, and combustion behaviour. Some results on torrefaction of sawdust briquettes are also presented.
MATERIALS
AND METHODS
The torrefaction unit Small-scale torrefaction was carried out in an experimental unit (see Fig. 1). The unit consisted of a mild steel chamber (61 x 61 x 30.5 cm) placed within an outer chamber (76 x 76 x 46 cm); fiberglass insulation was installed around the sides of the outer chamber. The inner chamber had a perforated bottom and contained the wood, which was torrefied. The desired temperature of the torrefaction chamber was maintained by circulating hot gases obtained from the heat exchanger. When the temperature of the circulating gases fell below the desired range, the control unit switched on the main blower. Consequently, charcoal burned inside the combustion chamber, and the flue gas passed through the heat exchanger and heated up the circulating gases. The main blower operation and charcoal combustion stopped when the temperature of the circulating gases exceeded the desired range. Smoke tests The purpose of the test was to evaluate the smoke density in the exhaust from the combustion of wood, torrefied wood and charcoal. The fuels were burnt in a bucket stove. Three pieces of the fuel saturated with kerosene were first placed on the grate and lit. After the fuel caught fire, more fuel was added. The total weight of fuel added was 200 g in all tests. A To whom all correspondence should be addressed. 1175
RANUPENTANANUN? et al
1176
A exhaust
Flue gas
Pebble Bed Heat Exchanger
Combustion Chomber
Q Blower -.\ %
Hot torrefoction
gos
Control Unit
Blower Torrefoction
Box
Fig. 1. Schematic diagram of the torrefaction process.
metallic hood was placed over the stove. Sample gas was drawn from the discharge pipe of the hood by using the piston-pump sampling device of a Bacharach smoke-tester. The test was performed in accord with ASTM Standard D 215880. The procedure consists essentially of drawing the flue gas through a filter paper and visually matching the shade of the smoke spot with a standard smoke scale. Combustion test The purpose of the combustion test was to compare the combustion characteristics of wood and torrefied wood. The test was performed in an open hearth consisting of a mild steel box of 50 x 50 cm in cross section and 70 cm in height, as shown in Fig. 2. There were two grates. The bottom one was covered with a pebble bed about 10 cm deep to provide some pressure drop so that air would be distributed uniformly across the cross-section of the bed. The samples had equal weights and were placed on the top grate. A cup containing 30 ml of methanol was hung underneath each sample. The methanol was lit and visual observations were made of the ease of lighting, smoking, flame, and incandescence. After one of the samples was almost completely burnt, the fire was put out by spraying water on the samples. Ash analyses were then performed on both samples to determine to amounts of combustible left. [
i_
briquette somple
~-+!!IZL~rnethonolcup
50 75a I
dimeneion in mm
Fig. 2. Combustion test for briquettes.
air
swl~
Upgrading of biomass by means of torrefaction RESULTS
AND
1177
DISCUSSION
The proximate analyses of air-dried wood, wood torrefied by holding at different temperatures for different lengths of time, and commercial charcoal are summarized in Table 1. The moisture content of torrefied wood is seen to be lower than that of either wood or charcoal. The fixed carbon and volatile matter of torrefied wood, which depend on the torrefaction temperature and holding time, fall between those of charcoal and wood. During torrefaction, wood undergoes changes in chemical composition. As is shown in Table 2, the carbon contents of torrefied wood (59.98~64.37%) are greater than that of wood (51.91%) but lower than that of charcoal. Carbon increases at the expense of oxygen and hydrogen, thus leading to decreases in both the H/C and O/C ratios. The weight and energy yields for torrefaction of sawdust briquettes and wood chips on a dry basis are shown in Table 3. Torrefaction of sawdust briquettes yielded weights of 93.8-70.3%, Table 1. Proximate analyses (wet basis). Torrefied
Mood Temp. (‘Cl
2 2 3 3
250-260 260-270 250-260 260-270
Wood
x
Volatile Matter x
4.00
66.47
3.80 3.50 3.20
62.85 65.71 65.71
28.55 32.36
0.98 0.98
30.26 30.26
0.54
11.25
69.80
18.20
0.74
5.8
31.90
59.9
2.40
Moisture
Holding Time (hr)
Charcoal
Fixed Carbon x
Ash
x
0.65
Table 2. Ultimate analyses (dry basis). Composition
(X)
Fuel C
H
0
N
H/C
O/C
TW (2 hrs,250-260'C)
59.98
5.4
33.35
0.25
1.08
O.bl
TU (2 hrs,260-270'C)
60.98
5.1
32.69
0.21
1.00
0.40
TW (3 hrs,250-260'C)
63.44
5.1
30.72
0.18
0.96
0.39
TW (3 hrs.260-270'C)
64.37
5.0
29.80
0.18
0.94
0.38
Wood
51.91
6.1
41.00
0.10
I.40
0.60
Charcoal
82.16
3.3
11.90
0.10
0.48
0.11
Table 3. Energy and weight yields (dry basis).
1178
R.4NuPEwr.4NANuNTetal
0
Smoke
PlTl
Flame
m
Incandescence
Torrcfied
Wood
Wood
( 0
5
IO
15
20
25
30
Time Fig. 3. Combustion
35
40
45
50
55
60
(mins) characteristics
vs time.
with energy yields of 95.4-79.0%, for residence times of 2-4 h. The weight yield decreased with increasing temperature and duration of torrefaction. Similar results were obtained for torrefied wood. The weight yield was 66.7-83.3% and the energy yield 76.5-89.6% for residence times of 2-3 h. Thus, the percentage yields of energy were greater than the percentage yields of weight. The combustion characteristics of wood and of torrefied wood are shown in Fig. 3. When subjected to alcohol flames, both fuels initially smoked but the smoking period was much less for torrefied wood than for ordinary wood. Torrefied wood ceased to smoke within 10 min, while wood continued to smoke with a gradual decrease in smoke intensity to incandescence. From the 15th min, torrefied wood burned incandescently. At the 50th min the torrefied wood was almost completely burnt. Wood became incandescent at the 27th min. There were 15 and 33% of the original combustibles left in the residues of torrefied wood and wood, respectively, at the 50th min when burning of both fuels was terminated. This result indicates that torrefied wood has a significantly higher combustion rate than wood. Similar results were reported earlier for sawdust briquettes produced in a heated die screw press.* Also, torrefied briquettes were found to be practically water-resistent. Untorrefied briquettes disintegrate quickly on exposure to water, are normally difficult to ignite and bum slowly with a high level of smoke. 3 Thus, the technique of torrefaction appears to be particularly useful for upgrading briquettes. Since the briquettes emerge from a briquetting Table 4. Relative smoking of different fuels. Fuel Charcoal Torrcfied wood
Wood
Burner
Perforaance
1
Little
sooting
3
Slight
soothing
6
Severe
sootiAg
Smoke Spot
No.
Upgradingof biomassby means of torrefaction
1179
machine at high temperatures, only small amounts of additional energy inputs are needed for torrefaction. The density of smoke from the combustion of wood, torrefied wood and charcoal in a bucket stove, as well as the degree of sooting for each smoke-spot number based on the Bacharach Smoke Tester, are shown in Table 4. Charcoal shows the least smoke density and sooting, following by torrefied wood and wood, respectively. Sooting affects the use of fuels adversely. Our tests indicate that torrefied wood and charcoal may be suitable for household use since they provide relatively clean combustion.
REFERENCES 1. J. P. Bourgeois and J. Doat, “Torrefied Wood from Temperate and Tropical Species, Advantages and Prospects,” in Bioenergy 84, Vol. 3, pp. 153-159, H. Egnens and A. Ellegard eds., Elsevier Applied Science, London (1985). 2. S. C. Bhattacharya, R. M. Shrestha, and S. Sett “State of the Art of Biocoal Technology,” AIT-GTZ, Biocoal Project Report, AIT, Bangkok (1989). 3. S. C. Bhattacharya, R. M. Shrestha, S. Ngamkajomvivat, and P. Wongvicha, “Biocoal: A Report on Market Opportunities and Requirements in Thailand,” AIT-GTZ Biocoal Project, AIT, Bangkok (1989).