Briquetting of carbonized cotton stalk

036C-5442/92 $5.00+0.00

Energy Vol. 17, No. 9, pp. 817-882, 1992 Printed in Great Britain. All rights reserved

BRIQUEnING

Copyright @ 1992 Pergamon Press Ltd

OF CARBONIZED

COTTON STALK

A. E. ABASAJZED College of Engineering, P.O. Box 800, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia (Received 14 August 1991; received for publication 31 January 1992)

Abstract-Woody

biomass, in the form of firewood and charcoal, is the main energy source in the Sudan. Agricultural residues in this country are assessed and their use as energy sources is investigated. Cotton stalks are produced in two large agricultural processes and have been used for this purpose. The conversion process of cotton stalks to usable energy is discussed. We also present the results of surveys which were conducted to assess user acceptance of the new fuel. Encouraging results were obtained. Briquetting of carbonized cotton stalks was found to provide an environmentally sound and socially accepted fuel and also solve a waste-disposal problem.

INTRODUCTION

Biomass (mainly firewood and charcoal) and imported oil supply about 90% of the total energy consumed in the Sudan. While imported oil contributes only lo-15% of the total, scarce hard currency makes this portion a heavy burden on the economy. Greater dependence on biomass is therefore to be expected. Heavy dependence on biomass has resulted in rapid depletion of forests and desertification. This change has led to scarcity, long transport distances, and price escalation of firewood and charcoal.’ Recently, the price of one sack of charcoal has risen from 23 Sudanese pounds to 150 pounds (1 U.S. dollar = 4.5 Sudanese pounds, officially) in a single month. Majzoub* estimates that if the current practice of felling trees continues, the forests in the Sudan will be depleted in 15 yr. Other sources of energy must be found. Agricultural residues are available in abundant quantities but are usually wasted. The purpose of this paper is to assess the available agricultural residues with emphasis on cotton stalk and to describe its use as an energy source. A brief account of the economics of commercial briquetting and the results of various acceptability surveys will also be given.

AGRICULTURAL

RESIDUES

About 80% of the total energy consumed in the Sudan is derived from biomass in the form of firewood and charcoal. Over 75% of this energy is consumed in the household sector.’ Fath and Hashim report that the overall energy efficiency is less than 24%, with 16% efficiency in the household sector, where the efficiency is defined as the ratio of useful to total energy. Thus, over 50% of the total energy consumed is wasted in the household sector, and all of this wasted energy is derived from wood. Table 1 shows the quantities of various crop residues in the Sudan.3 The available fraction in the table indicates the portions which are usually wasted and could be used for energy purposes. Comparing the potential energy which could be harvested from residues, namely, 3.4 x lo6 tons of oil equivalent (TOE), with that derived from wood (8.2 x 106 TOE), we find that over 40% savings in wood could be realized if the agricultural residues are properly utilized.394 Considering collection, transportation, and technology problems, at least 25% savings are possible. Table 1 also indicates that cotton stalk, sorghum stalk and groundnut shells constitute about 80% of the total energy which can be derived from agricultural residues. 877

A. E. ABASAEED

878

Table1.Agricultural crop residues in the Sudan; 1 fed=4.2hectare. Crop

Total Available amount fraction (106 t)

Residue

Sorghum

1 15.12

Millet

I

3.00

Wheat

1

1.16

Rice

I

0.03

Maire

I

aroundnut Sesame

Total

Net Calorific Potential amount value (:xL) (10' t) caJ/t1

1

0.25

I

750

1.508

I

0.251

1

0.35

0.60

I

150

11.6

2.76

0.60

1

1660

23.3

0.906

1

1.40

0.60

1

840

0.281

1

____

I

9060

3.381

1

1 24.82

Cotton stalk is a field residue and there are problems associated with its collection. However, it is available as the result of two major agricultural projects in Gezira and Rahad. The stalks are uprooted, pulled, collected, piled up, and burned in the field every year to comply with a phytosanitary law. This requirement is intended to prevent the spread of cotton diseases to the next year. Farmers pay every year for this activity. Close monitoring and supervision is imposed by management. Farmers failing to perform this duty in a 2-month period (mid-April-mid-June) receive fines.

CARBONIZATION

Depending on end-use, direct briquetting or carbonization followed by briquetting might be advantageous. Direct briquetting produces a product suitable for substituting firewood. However, for household use, briquettes produced directly from residues might prove to be a nuisance. Smoke, unpleasant odors, and need to develop suitable stoves are some of the problems related to the use of uncarbonized briquettes in households. This problem, together with the requirement to comply with the phytosanitary law, have dictated the choice of carbonization followed by briquetting over direct briquetting. The carbonization technology chosen is simple, easy to construct and operate, durable, and locally made. The carbonization kilns are made of empty barrels. Three to four barrels are flattened, welded, and then folded back to make a larger diameter vessel. The stalks are charged into the vessel from the top. Dry grass is used for initial firing. After the fire catches on, the lid is placed and carbonization proceeds. The area underneath the vessel is used to control the flow of air and thus carbonization. The whole operation takes 5-7 h and the production capacity of each kiln is 70-90 kg/cycle with efficiencies of 27-30%. The efficiency is defined as the ratio of the amount of stalk charcoal formed to the stalks charged initially. After 3 yr of operation, the carbonization kilns are almost intact.

BRIQUETTING

After carbonization, the stalks are ground in a hammer mill to the required size. Denser and more compact briquettes are produced with finer particles. However, this advantage is offset by increasing the energy requirement. The ground, carbonized stalks are then mixed with the binding agents. Starch, cellulose fiber, and molasse have been tested as binders. The effects of the various binders on the quality of the briquettes are shown in Table 2. Molasse is available in large quantities as a by-product of the many sugar plants in the Sudan. Therefore, it is

Briquetting of carbonized cottonstalk

879

Table2. Proximate analyses of cotton-stalk briquettes and wood ( Barcoal. Material (Bindor)

Density (Q/-3)

Calorifia Uoieture aontent value (%I (~/ltg)

Ash (%I

Cotton stalk

_____

20.9

Cotton aharaoal

0.198

Briquettes (aellulosie)

0.300

Briquettes (molassa)

0.500

Briquottas (starch)

0.330

25.6

3.2

9.5

17.3

Wood charcoal

0.500

26.9

3.2

4.9

21.4

9.2

4.3

26.9

1.4

10.9

26.2

2.1

1.3

22.0

3.6

16.3

I

chosen as the binding agent. The briquetting into two systems.

technologies

26.5

I

investigated can be generally divided

Press machines In these systems, the ground and carbonized stalks are mixed with the molasses solution and then fed to the hopper of the press machine (essentially a piston and cylinder arrangement). The product of this machine is cylindrically shaped, and densities ranging from 400-500 kg/m3, depending on the applied pressure. Agglomeration machine This machine was designed by the Biomass Technology Group (BTG) of Twente University, The Netherlands. 6*7The major camp onents of this machine are a rotating pan, a hopper, and a binder spray nozzle. The size of the briquette increases as it collects the loose fibers from the rotating pan. The briquettes produced by using this machine are spherical and have a density of 500 kg/m’. The machine has been proven to be robust and reliable after 3 yr of testing. Figure 1 shows a schematic diagram of the operatons involved in the briquetting of

RAW STALKS

SHREDDER

HAMMER

CARBONIZATION KILNS

MILL

HOPPER

SUN DRYING

MOLASSE

ROTATING

AGGLOMERATION PACKING Fig. 1. Schematic

of the briquetting

of carbonized

cotton stalk.

PAN MACHINE

880

A. E. ABA~AEED Table 3. Comparison of stalk briquettes, imported briquettes and wood charcoal. Material Stalks

briquettes

IImported lWood

briquettes

charcoal

c i

(%I

48.0

B2

(%)

N2

(%I

2.7

1 0.143

I 55. o I

2. o

I 0.880 I

I

3.2

I 0.620

71.5

1

I

I

carbonized cotton stalk. The uprooted cotton stalk is first passed through a shredding machine. The sizes of the stalks were reduced to lengths of about 12-15 cm. The shredded stalks are then transferred into the carbonization kilns. The carbonization takes place according to the procedure described previously. The carbonized stalks are then ground in a hammer mill, which was equipped with a 2 mm screen. The ground and carbonized stalks are then fed into the hopper of the agglomeration machine. The feed rates of the stalks from the hopper and of the binder through the spray nozzle to the tilted rotating pan of the agglomeration machine were adjusted in such a way that an approximate ratio of 5: 1 was maintained. The briquettes are then sun-dried and packed into sacks. Each sack contains 35-40 kg of briquetted and carbonized cotton stalk. In Table 3, a comparison between stalk briquettes, imported coal briquettes, and wood charcoal is presented. The high carbon content of the traditional charcoal is due to incomplete carbonization, which is profitable to charcoal producers since they may sell more wood at the price of charcoal.

ACCEPTABILITY

SURVEYS

Surveys were conducted to assess consumer responses to the new fuel in comparison to traditionally used wood charcoal. The results of these surveys are summarized in Table 4. Cotton stalk briquettes are comparable to wood charcoal in terms of burning characteristics and fuel consumption. However, with regard to convenience aspects such as smoke, smell, and ash formation, differences do exist between the two fuels. The smoke occurs only during the initial ignition period. This is probably due to the burning of molasses rather than due to incomplete carbonization. The smell of the briquettes, which was viewed as a negative factor, could be viewed positive when looked at in terms of smell/odor behavior. The ash formation around the regularly-shaped briquettes is quite desirable since the cooking practice of most Table 4. Results of performance tests of briquettes.

Briquetting of carbonized cotton stalk

typical Sudanese food involves an initial period of high heat requirement relatively longer cooking period at lower temperatures.

ECONOMICS

OF BRIQUETTING

881

followed

by a

PLANT

A plant, owned by Rahad corporation and a farmer’s union, was designed to produce 800 tons of briquettes annually. It will use 10 agglomeration machines, each of which has a production capacity of 50 kg/h. As shown in Table 5, the production cost of briquettes is about 90 U.S. dollars per ton.* The raw stalks are taken to have zero cost since the stalks currently have negative value. The current selling price of wood charcoal is 170-229 $/ton. Therefore, if briquettes are sold to retailers at 120 $/ton, the retailers should be left with a comfortable profit margin.

DISCUSSION

AND

CONCLUSIONS

The use of agricultural residues as energy sources will reduce the heavy dependence on woody biomass. While their use can not replace the forests, it will provide the country with the needed time to take remedial actions and to set the right policy for forest utilization. Cotton stalks have been briquetted after carbonization. The carbonization step is believed to be the bottle neck in the overall conversion process. This step is rather slow and laborious. It takes on average, 6 h to carbonize 80 kg of stalks. At best efficiency and two cycles per kiln per day, the number of kilns required to carbonize 800,000 tons of raw stalks will be around 25,000 kilns. This number is very high and impractical given the economic situation in the Sudan. The agglomeration machines are quite satisfactory and suitable for use. They are easy to operate, do not require any special skills, and are robust. Molasses as binders gave good quality briquettes and there is no immediate danger of short supply. The initial surveys revealed that users (usually women) of the fuel would prefer a fuel that improves the convenience aspects rather than the economic aspects. The following questionnaires, therefore, concentrated on convenience aspects because if the product fails the women’s (users’) tests, it will have no chance with the men (buyers). Also, an initially bad image is very hard to erase but, fortunately, the briquettes succeeded in gaining the needed acceptance. Therefore, it is believed that research should be directed to satisfy both users’ and buyers’ needs. The economics of briquetting are favorable. Furthermore, the social acceptance of the briquettes exceeds the potential economic gains. It is important to realize the hidden subsidy that the government is contributing to charcoal since charges for wood do not really reflect its Table 5. Production cost of briquettes.*

cost

Annualiaad cost (102 $1

item

I capital

I

loperation co&l ILabour (Binder Backs Energy Maintenance Total Production coat/t

16.04 10.85

I I

11.98 14.00 12.00 2.00 4.54 71.41 89.26 8

882

A. E.

ABASAEED

replacement cost. Should this be considered, the actual production cost and subsequent selling price of charcoal will be prohibitively high, which will further tilt the economic leverage in favor of briquetting. Acknowledgements-The author acknowledges significant help from many staff members of the Energy Research Council (ERC) who worked on this project since 1984. Support was provided by the USAID through the Sudan Renewable Energy Project @REP), GTZ through the Special Energy Program (SEP) and The Netherlands government through a joint project.

REFERENCES

1. A. E. Abasaeed, “Cotton Stalks: A Useful Waste,” VITA News (April 1988). 2. M. Majzoub, “The Role of NGOs in Stove Commercialisation, CARE International in Sudan,” Proc. of Regional Workshop on the Commercialisation of Improved Cook Stoves, Sudan (1988). 3. H. E. S. Fath and H. H. Hashim, Energy-The International Journal W, 861 (1988). 4. DECON, “Utilization of Biomass” Phase I, Literature Survey and Project Proposals,” German Agency for Technical Cooperation (GTZ), Special Energy Program, Sudan (1985). (NEA) 5. A. Shuli, “The Development of A Sudanese Energy Policy,” National Energy Administration Reports, Khartoum, Sudan (1988). 6. B. R. Schuring, “Cotton Coal Molasses Briquetting, the Design of a Granulator and the Running of the Plant,” M.Sc. Thesis, Twente University, The Netherlands (1987). 7. A. E. Abasaeed, A. H. Hood, and G. E. Ali, “Assessment of ERC Experience in Briquetting Carbonized Agricultural Residues. Parts I and II,” Proc. of the International Workshop on Biomass Fuel Briquetting in Developing Countries, Sudan (October 1988). 8. R. V. Siemons and A. H. Hood, “Cotton Stalk Charcoal Agglomeration International Workshop on Biomass Fuel Briquetting in Developing

in the Sudan,” Proc. of the Countries, Sudan (October 1988).