Carbonization of some fast-growing species in Sudan

Applied Energy 45 (1993) 347-354

Carbonization of Some Fast-Growing Species in Sudan P. K h r i s t o v a & A. W. Khalifa Forestry Department, Universityof Khartoum, People's Hall POB 6272, Khartoum, Sudan

ABSTRA CT Four wood species, indigenous Acacia seyal ( talh ) and exotic fast-growing Conocarpus lancifolius (damas), Eucalyptus microtheca (kafur) and Prosopis chilensis (mesquite) grown in Sudan, were assessed and compared as raw materials for charcoal making. The effects of production method (traditional earth mound and improved metal kiln) and the physical and chemical properties of the wood and bark on the yield and quality of charcoal produced were assessed. Regression analyses of wood properties and heat value data indicated high negative correlations of the wood heat value with holocellulose and ash, and high positive correlations with wood density, lignin, and alcohol-benzene and hot-water solubles. Carbonization with the Tropical Products Institute metal kiln produced higher yields (33%) than the traditional earth mound (27%), although the difference in energy transformation yields was found to be insignificant both between appliances and species. Charcoal produced by the earth mound had a slightly higher density and was more resistant to shatter, but no significant differences were recorded with respect to the water boiling test or the gross heat value. The exotic species studied gave equal or better charcoal, in terms of yield and quality, compared with the traditionally preferred talh.

INTRODUCTION Charcoal in Sudan has been and will continue to be one of the main sources of fuel for the major part of the population. The population growth imbalance, natural disasters and the exploitation of the natural forest resources have brought about deforestation and desertification. If the trend of agricultural expansion continues at the current rate, for 347 Applied Energy 0306-2619/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

348

P. Khristova, A. W. Khalifa

instance in the Blue Nile Province, by the end of 1990s all the land which is now the source of charcoal production would be under crop plantations) It has been also predicted that a 10-15% regression of the forest area will take place in the West African Sudano-Sahelian Zone between 1980 and 1995 which will be reflected in the Sudan as well.2 Therefore, the economy of wood fuels has become a priority for survival, and new sources and technologies together with proper forest inventory and management are sought to stop the present forest regression. The depletion of forest resources emphasizes the need for afforestation and special plantation programmes using the most appropriate wood species, preferably the fast-growing species, some of which have been introduced successfully in Sudan. 3 The evaluation and comparison of some of them with the traditionally preferred indigeneous species for charcoal making is one of the aims of the present work, together with a comparison of the efficiency of the traditional earth mound with that of the improved metal kiln.

MATERIALS A N D METHODS Three exotic fast-growing wood species (Prosopis chilensis (mesquite), Conocarpus lancifolius (damas) and Eucalyptus microtheca (kafur)) aged 8-10 years were compared with 12-year-old indigeneous Acacia seyal (talh), all collected from the area around Khartoum. The felled and logged trees were air-seasoned for about 3-4 weeks before carbonization to bring them all to similar moisture contents (19-27%). The carbonization appliances used were a Sudanese earth mound and an improved TPI (Tropical Products Institute, London) metal kiln. 4 Representative wood samples from each species were taken from the carbonization stock, sampled and analysed according to the appropriate methods--physically, mechanically and chemically:: The charcoal produced was analysed according to ASTM Standards. 8 The heatcombustion tests were performed using a PARR oxygen bomb calorimeter. The data obtained were statistically analysed. 9

RESULTS A N D DISCUSSION It is well known that both yield and quality of the resulting products depend to a large extent on the appliance and the conditions of the carbonization process, as well as on the physical and chemical properties

Carbonization of some fast-growing species in Sudan

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TABLE 1

Physical Properties of Wood and Bark Properties

Bark-to-wood ratio by volume (%) Density~ of wood (kg cm 3) Density~ of bark (kg cm3) Gross heat value of wood (kJ kg l) Gross heat value of bark (kJ kg 1) Hardness of wood (kgf)

Species Talh

Mesquite

Damas

Kafur

9.8 840 750 18 067 15 547 803

11.0 945 640 20 214 19 09t 888

16.0 779 690 17 742 15 733 580

15-0 850 660 19 096 16 619 956

Oven-dry mass/oven-dry volume. of the raw material used. The bark-to-wood ratios by volume (Table 1) were approximately in the 15% typical for hardwoods, t° However, as bark charcoal is usually friable and with higher ash content, the high damas and kafur bark volume values were expected to influence negatively the charcoal quality. On the other hand, the wood of the medium heavy damas and heavy mesquite, talh and kafur should be favourable for the production of good yield, dense and less friable charcoal. The heat values measured (Table 1) for both wood (17-7-20.2 MJ kg ~) and bark (15.5-19-1 MJ k g t) depended upon the species but in the ranges reported for hardwoods. ~ The species studied had a typical chemical composition for tropical hardwoods (Table 2), with average Kurschner-Hoffer cellulose content (42-45%), lower than average lignin content (19-23%), high extractives, and inorganic matter in the normal range of 1-3°/0. ~2 The higher lignin content of mesquite and kafur suggested that better charcoal yields from these species could be expected. The bark's chemical composition (Table 2) was characterized as usual with lower carbohydrates and much higher extractives, ash and lignin contents compared with wood. The high hot-water and alcohol-benzene (AB) extractives indicated the presence of gums, tannins, resins, phlobaphenes, esters of hydroxy acids and suberin. A regression data analysis of the chemical composition and the heat values measured for wood and bark of the species studied and a number of other wood species available gave a model equation (eqn (1)) for predicting the wood's gross heat value (GHV, in kJ kg 1), on the basis of wood's chemical composition alone, as G H V = 288 898 + 147-19 AB - 155.58 hot H20 + 148-8 lignin - 231-53 holocellulose

(1)

P. Khristova, A. W. Khalifa

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TABLE 2 Chemical C o m p o s i t i o n o f W o o d and Bark (All values expressed as percentages o f solubles or components o f oven-dry unextracted wood)

Component

Species Talh

Ash Total silica Solubles in: Hot water Alcohol-benzene (1:2) I% NaOH Cellulose, Kurschner-Hoffer Holocellulose Alpha-cellulose Pentosans Lignin

Mesquite

Damas

Wood Bark

Wood Bark

Wood

Bark

1.80 0.46

11.78 --

2.12 0.71

6.16 --

4.44 1.99 19.49 44.92 78.83 48.03 14.62 19.13

22.22 6.77 45-95 42-95 59.81 44-19 9.92 8.75

11-02 7.81 24.38 42.78 69-20 42-32 14-86 22-64

15.95 10-17 42-88 35.02 51.66 34.51 8.48 19-28

2-67 0-43

8-60

8-96 11.47 2-39 5.12 2 0 . 2 7 46.24 41.88 32-36 76.94 58-59 47.31 3 0 - 5 5 17.64 1 1 . 6 5 19.83 2 1 . 2 7

Kafur W o o d Bark 0.56 0.70

6-89 --

5.42 3.80 13.23 43.66 78.11 49.60 14.12 20.03

7.34 2.60 30.72 37.56 71.59 36.65 11.59 24-63

R 2 = 0.96. Positive correlations were found for the G H V with the lignin and extractives, and negatives with the holocellulose and ash (Table 3). Thus the variation in heat values between species studied was confirmed to be due to the different contents of their chemical components. Positive correlations were also found between the density and heat value, but in the case of wood only. There were no significant correlations between bark's heat value and the bark's density, as well as between the heat values of the bark and wood. The chemical composition of the bark, in particular the lignin and resin contents, influence significantly the heat value, but not necessarily in the same manner as for the wood in the various species. Accordingly, as underbarked wood was used, it was expected that the heat value of charcoal obtained as well as its energetic yield would be affected. The carbonization cycle in the two kilns had different durations (20 h for the metal kiln and 2-3 days for the earth mound). Thus the slower rate for the earth kiln should give time for the products formed to interact with each other in secondary reactions. As a result, some differences were observed in the yield and properties of the charcoal obtained from the same species by the two appliances. For all the species studied, the metal kiln yield by mass was about 5-6% higher than that of the earth mound, and on average amounted to almost 33%, which was actually equal to the theoretical yield for the with

Carbonization o f some fast-growing species in Sudan

351

TABLE 3 C o r r e l a t i o n M a t r i x for the Chemical C o m p o n e n t s a n d the G r o s s H e a t Value of W o o d

Ash Ash Hot H20 solubles Alcohol benzene solubles Lignin Holocellulose GHV

1.00 0.24 0.37 0.30 0.83 -0.37

HotH20 Alcohol-benzene Lignin Holocellulose GH V

1.00 0.72 0-61 0.62 0.41

1.00 0.89 -0.70 0-80

IO0 -0,77 0,89

1.00 0.88

1-00

corresponding temperature. 13 The average yield of about 27°/,, for the traditional kiln was much higher than the 10-15% reported in the literature for other regions. 14-~6 It was less, however, than the 30% yields reported for the Sudanese earth pit. iv Still 27% is very good, confirming once again that the skilled labour and favourable environmental conditions exist for such kilns in Sudan. The lower yield of the earth kiln could be explained by the difficulties encountered with its fire control so that the charcoal burns to ashes in some parts of the pile before the remainder has been carbonized properly, and the poor and erratic gas circulation leads to some losses of the charcoal produced in the ground, and that which is recovered is often contaminated with earth and stones. In the metal kiln the raw material and product are in a sealed container, giving m a x i m u m control of air supply and gas flows during the carbonization process, and all the charcoal produced can be recovered. Thus it seems that the metal kiln, with its higher yield and especially its lower demand for the raw material quality (smaller logs and forest residues can be used) must become inseparable from the earth kiln for the fuller utilization of the raw material available on the site. The statistical analysis of the charcoal's energetic yield and GHV did not indicate any significant differences between the two kilns as well as amongst the species studied. The heat values obtained for the charcoal from both kilns were in the narrow range of 29.5-29-9 MJ kg ~; the values for the metal kiln charcoal were slightly higher, as also applies for the percentage of the charcoal volatile matter. It seemed that the faster carbonization cycle at the comparatively low temperature (470-480°C) in the metal kiln favoured the production of charcoal of lower density and fixed carbon content (except for mesquite), but with higher volatiles and mass yield (Table 4). The density of charcoal obtained (Table 4)-~tespite the presence of bark--was proportional to that of the wood. The shatter resistance test (Table 4) indicated that the charcoal produced in the metal kiln was more brittle. There was no indication of any connection between the physical and mechanical properties of wood

0.76 0.60 36.7 0.31 3.75 76-74 17.54 5.72

37.0 0.30 3.75 76.84 17.14 6.02

520 29 678

32.76 53.39

Metal

0.75 0.50

530 29 610

27.45 43.50

Earth

Talh

37.4 0.30 3-25 76.92 15.88 7.20

0.55 0.33

590 29 715

27.64 40.63

Earth

37.0 0.30 3.75 77.63 15.32 7.05

0.65 0.49

560 29 890

34.98 50.52

Metal

Mesquite

33-5 0.32 3.65 77.35 15.91 6.76

0.80 0.75

510 29 470

27.34 44.42

32.73 53.02

Metal

33.8 0-33 3.75 75.43 17.83 6.74

0.99 0-80

480 29 580

Damas Earth

Species

34.4 0'33 3.70 74.98 20.10 4.90

1.02 0.88

550 29 460

26.02 40.00

Earth

31.04 49.10

Metal

34.2 0-33 4.50 74.23 22.13 3.64

1.17 0.99

520 29 520

Kafur

ASI 64 mm is the mass percentage of particles with size between 64 and 32 mm remaining after the drop-test, with particles size 254 and 127 mm. b ASI 32 mm is the mass percentage of particles smaller than 32 mm remaining after the drop-test.

Density (kg m 3) Gross heat value (kJ kg t) Shatter resistance test ASI 64 mm a (%) ASI 32 mm b (%) Boiling-water test Heat utilized (%) Specific heat consumption (kJ kg -1) Uniform firebed time (min) Fixed carbon (%) Volatile matter (%) Ash (%)

Charcoal evaluation

Yield by mass on oven-dry wood (%) Energetic yield on oven-dry wood (%)

Charcoal yield

Component

TABLE 4 Charcoal Yield and Evaluation, for the Two Kilns

g~

:x

t~

L~

Carbonization of some fast-growing species in Sudan

353

(Table 1) and the hardness of the charcoal, although a proportionality was observed to exist between the wood and charcoal densities. The earth-mound charcoal, as expected, had a higher ash content, as a result of the earth contamination, but not to a very high extent. However, the generally higher ash content of charcoal from both kilns compared with that of the corresponding wood is due to the bark. Its presence might also be the reason for the lack of proportionality, except for mesquite, between the wood and charcoal heat values. Generally, the chemical properties for the quality of the charcoal obtained from the two kilns were in the range of the British Standard specifications for industrial charcoal, namely, >75% fixed carbon, <20% volatile matter and <7% ash. 17 Having in mind that, in Sudan, the charcoal is actually produced mainly for domestic use, the slight deviation of the eucalypt charcoal from the standard could be neglected and the charcoal obtained from all species studied could be considered as very good for such uses, in the following order: mesquite, damas, talh and kafur.

CONCLUSIONS The carbonization trials of the studied exotic fast-growing wood species showed that good yields and high-quality charcoal could be produced, especially from mesquite and damas, which gave a superior product to that of the traditionally used talh. Thus for a shorter growth cycle and additional benefits from those multipurpose tree species, their plantations could increase the raw material supply for charcoal making in Sudan. The chemical composition of wood and bark affects the yield and quality of the charcoal produced and, with these data, the tentative yield and quality of charcoal could be predicted. The high efficiency (about 27%) of the Sudanese earth mound was confirmed. Nevertheless, the use of the metal kiln gave about a 6% higher efficiency for all the species studied. The qualities of the charcoal from both kilns were almost identical. Hence the combined utilization of both kilns will allow the utmost use of different raw materials available at the site of carbonization, especially the poor-quality forest residues in the metal kiln, thus considerably intensifying and improving charcoal making in Sudan. Growing planned energy plantations of suitable exotic fast-growing species will give a sustained supply of raw material for the production of uniform-quality charcoal in Sudan in a relatively short time. Therefore, an evaluation of the rest of the fast-growing species is very important.

354

P. Khristova, A. IV. Khalifa

ACKNOWLEDGEMENTS The authors wish to express their gratitude to the Energy Council of the National Council for Research and U S A I D for the financial support of the present study.

REFERENCES 1. Earl, D., Sudan Renewable Energy Project. Report on Charcoal Production. Energy Research Council/USAID, Khartoum, 1984. 2. World Bank, Le Dkveloppement accOl~rO en Afrique au Sud du Sahara. World Bank, Washington, 1982. 3. Badi, H. K., Houri, A. & Bayoumi, S. M., The Forests of the Sudan. NCR Press, Khartoum, 1989. 4. Paddon, A. & Harker, A., Charcoal production using transportable metal kiln. Trop. Sci., 22 (1980) 363-8. 5. ASTM, Standard Method for Gross Calorific- Value of Solid-Fuel IsothermalJacket Bomb-Calorimeter. Designation 3286-77. American Society for Testing and Materials, Philadelphia, PA, 1979. 6. British Standards Institution, The British Standard Methods of Testing Small Clear Specimens of Timber, British Standard 373. British Standards Institution, London, 1957. 7. TAPPI, Standards and Suggested Methods. Technical Association of Pulp and Paper Industry, Atlanta, GA, 1982. 8. ASTM, Standard Method for Chemical Analysis of Wood Charcoal. Designation D 176244. American Society for Testing and Materials, Philadelphia, PA, 1966. 9. Little, T. M. & Hillis, F. J., Agricultural Experimentation, Design and Analysis. John Wiley, New York, 1978. 10. Rydholm, S., Pulping Processes. Interscience, New York, 1965. 11. Harker, A., Sandels, A. & Burley, J., Calorific Values for Wood and Bark and a Bibliography for Fuelwood. TPI Report G 162. Tropical Products Research Institute, London, 1982. 12. Hartzmann, L., Features of the Chemical Composition of the Tropical Wood. Teaching Notes, Department of Forestry, University of Khartoum, Khartoum, 1979. 13. FAO, Simple Technologies for Charcoalmaking. FAO Forestry Paper 41. FAO, Rome, 1983. 14. Earl, D., A renewable source of fuel. Unasylva, 27(110) (1975) 21-6. 15. Booth, H., Realities of making charcoal. Unasylva, 33(131) (1981) 37-8. 16. Doat, J., The pyrolysis of tropical woods: theory and results obtained with charcoal and torrefied wood. Proc. IUFRO Congress Manaus (Brasil), 12-23 November 1984. 17. Paddon, A & Satti, K., Evaluation of Non-Traditional Methods of Charcoal Production in Sudan. Field Document 24. Fuelwood Development Project in Sudan, Khartoum, 1987.