Cover Image
close this book Boiling Point No. 01 - Special Edition 1989
View the document Briquettes - Briquetting - Briquette Stoves
View the document The Future of Fuel Briquetting
View the document Briquettes- Potential Impact On Urban Poor
View the document Cotton Stalks a Useful Waste
View the document Sudan Briquetting Workshop
View the document Cotton Stalk Charcoal Agglomeration In The Sudan
View the document Densification of Biomass
View the document Utilisation of Agricultural
View the document Briquetting With Partial Pyrolysis
View the document Marketing Of Briquettes
View the document Wastes as Fuel - Heat Content Guide

Cotton Stalk Charcoal Agglomeration In The Sudan

Roland V Siemons, Dr. Ahmed (Biomass Technology Group BV University of Twente, P O Box 217, Enschede, The Netherlands) H. Hood (Energy Research Council, P O Box 4032, Khartoum, Sudan)

Introduction

Biomass is the source for about 80% of the energy consumption in the Sudan, whereas about 90% of the fuelwood (wood and charcoal) is consumed by the household sector. Lack of forest resources and a recent period of drought has lead to a fast rate of deforestation, especially in the Northern provinces. Solutions to this problem are to be found in conservation measures, the development of new resources and reafforestation. In this paper the feasibility of alternative fuel production based on cotton stalks is discussed.

Cotton stalks are an agricultural waste product which have the potential of replacing 5% of the Sudanese wood consumption. If replacing wood charcoal this potential saving amounts to 11% of the domestic charcoal consumption. Carbonisation with subsequent briquetting was identified as a process by which the cotton stalks could be made available as a wood charcoal substitute for the low bulk density, carbonized cotton stalks.

In 1987 a field test was executed in the Rahad region, in order to test production methods, consider production organisation patterns and to perform a market survey. Charcoal was produced in the field, employing metal kilns made from oil drums and sheet metal. Prototype briquetting machines, based on the agglomeration principle were installed in a briquetting plant situated in Village 10.

Since November 1987 on the basis of the technical results obtained in the field, the agglomeration technology was further developed. The agglomerator was provided with a mechanical charcoal feeding device, the binder dosing system was adapted to the specific characteristics of molasses and the production capacity of the agglomerator was increased.

In this paper, the principles of charcoal agglomeration and the economies of cotton coal briquette production with this technology as well as some marketing aspects are discussed.

Charcoal Agglomeration Technology

Agglomeration is a method of size enlargement by glueing powder particles together. This technology is applied for a variety of powders like: hydrated lime, pulverised coal, iron ores, fly ash, cement and many others. There is little or no experience with charcoal (Beaudequin 1984, Reynieix 1987). The process discussed here is often referred to as "tumbling agglomeration". The equipment basically consists of a rotating volume which is filled with balls of varying size and fed with powder and often with a binder. The rotation of the agglomerator results in centrifugal, gravitic and frictional forces, which cause a smooth rolling of the balls. These forces, together with inertial forces, press the balls strongly against the powder which, due to this pressure, sticks to them. In this way the balls grow layer-wise in diameter. In practice there occurs a certain segregation of ball sizes. Large balls tend to "float" on the surface, whereas small balls are mainly located at the bottom. Since the balls grow during the process, and the bulk volume size is limited, the large balls are pushed out of the agglomerator. Figure 1 shows how balls pass through the volume of a pan shaped agglomerator.


Figure 1 - Ball stream lines through agglomeration pan. Shaded area acting volume

There exist a lot of agglomerator designs. They can be drum shaped, pan shaped, conical or plate shaped. For a rotating pan agglomerator the design parameters are the following:

- pan tilt angle

- rotation speed

- powder: binder feed ratio

- absolute powder feed rate

- number and location of feeder points rim height

- scraper position

Unfortunately there exist no design rules for this equipment: 'the final choice of a balling device rests on a careful consideration of the particular application by individuals experienced in the field" (Snow 1984). This is not to say that we need "Guru's" to tell us the truth about agglomeration but to stress the necessity of research which aims at providing equipment designs for specific applications. For a particular agglomerator, the main process parameters are the ball residence time (depending on the powder feed rate and the acting volume, which in turn can be adapted by changing the pan tilt angle) and proper rolling action (depending on scraper position, binder premixing and pan tilt angle). These parameters are closely inter-related and process settings depend very much on the powder characteristics. It has been observed that the properties of charcoal differ substantially for varying charcoal types (charcoal made of hard wood, soft wood or agricultural residues like cotton stalks).

Some powders can be agglomerated without using binders to increase cohesive forces, whereas other powders need such additives. Although other opinions are reported (Beaudequin 1984), it is the BTG's experience that charcoal needs a binder.

In principle, the charcoal agglomeration process consists of the following steps: charcoal grinding, agglomeration of the powder with a binder and briquette drying. Some refinements to this principle exist; the powder can be mixed with a hinder before it is fed into the agglomerator, or the hinder can be fed separately into the agglomerator. It is also possible to premix the powder with a portion of the binder, still feeding some of the binder separately.

In the BTG charcoal agglomeration system, charcoal is premixed with water (up to 15% of water is added to the dry charcoal) in order to improve the agglomeration properties of the powder. To this end, water is added to the charcoal before it is ground in a hammer mill. This is also held to be necessary in order to improve the safety of the grinding process. A second binder mixture is then fed as a separate material flow into the agglomerator. In this way the production capacity of the agglomerator is optimised.

Correct operation of an agglomerator controls both the production level and the quality specifications (ball size, strength, etc). In order to understand the rules for operation, it is necessary to look more closely at the details of the agglomeration process. A simple consideration reveals the complexity of the growth mechanism occurring in the agglomerator. Suppose that a rotating pan is filled with small uniformly sized balls and that the volume is fed with powder and binder. Suppose further that the powder is entirely consumed for diameter growth of the balls. Gradually, as the pan is fed with powder, the balls as well as their bulk volume grow. For this reason the balls will gradually flow out and the number of balls held in the pan will decrease. Since the remaining balls are supposedly capable of consuming all the powder, the size of these balls will grow before they are pushed out of the pan. Finally only one single very large ball is left over. The imaginary process is clearly unstable. Stable operation, therefore, requires that the number of balls contained by the pan remains constant or, in other words, that powder which is fed into the pan must be used, in part, for the formation of nuclei. These nuclei, in turn, will gradually grow and it is thus clear that, in an agglomerator which is operated in an entirely stable manner, a uniform size distribution must develop.

In an agglomerator it is extremely difficult to maintain a uniform ball size distribution and hence stable operation. The reason is that large balls do not only grow by powder layering but also by consuming considerably smaller balls. This can easily be understood from the fact that the largest balls in an agglomerator producing 30mm balls are 1,000 times heavier than the 3mm balls also contained in the same machine. Large ball diameter differences occurring in one agglomerator are a reason for operation instabilities. Automatic agglomeration machines are therefore designed for step-wise diameter increase: The product balls from one agglomeration step serve as the nuclei for the next one, and so on, until the desired ball diameter is reached. These large systems combine size enlargement steps in a complex of agglomerator sections which are often optimized for one single powder type.

The same possibilities can be realised with the BTG designed agglomerator. The machine has a small capacity. In the planned cotton charcoal briquetting plant, ten machines are combined in one unit. If production stability difficulties occur, it is possible to divide the desired size enlargement into a number of steps, say from 0 to 5mm, from 5 to 15mm and from 15 to 30mm diameter balls, each performed in a different machine.

The charcoal agglomerator designed by BTG is very simple to manufacture. It consists of a single 600mm diameter rotating pan. The simplicity of design results in low investment costs which are, however, reflected in higher operational costs. The process is not automatic. It is designed such that the number of counter-actions against process disturbances is very limited. The rotation speed is fixed. The charcoal feed rate as well as the pan tilt angle, scraper position and average binder feed rate can only be adjusted by the plant supervisor.

The following options exist for the directly responsible machine operator:

- stop or start the charcoal feed

- stop or start the binder feed

- temporarily increase the binder feed rate

- remove or add balls.

Nuclei formation can be stimulated by a sudden increase of the binder flow. The remedy against disturbances of the size distribution of the balls in the agglomerator is to manually add or remove (using a shovel) balls of the necessary diameter range. Balls which are removed can be recycled later.

The entire unit for a production capacity of 400 to 500 kg/hr consists of ten agglomeration machines. This number bears a big advantage for production control. Naturally, these machines can be combined in smaller units.

The Economics of Cotton Coal Briquetting

In this paragraph the economics of a 800 t/yr charcoal briquetting plant are examined in more detail. With an annual production of BOO t/yr, the consumption of charcoal, molasses, water and electricity is as follows: - charcoal: 640 t/yr molasses: 210 t/yr - water :550 m3 /yr - electricity: 22,800 kWh/yr.

Annually, the plant processes charcoal made from 2,600 t of cotton stalks. This quantity of stalks becomes available from an area of 1,280 ha under cotton. The infrastructural requirements for successful application of this type of unit are a dry climate (otherwise extra drying equipment is necessary) and the availability of water and electric power. If the latter is not available a generator must be installed. The peak load of this plant is 16.5 kVa, the average load is 14 kVa.

The organisation of the briquetting is then as follows: Charcoal is stored for one season (one yr) at the plant site, the binder (cane sugar molasses) is stored in second hand barrels (a supply for one month). Eighteen men are permanently employed to operate the plant:

- two men operate the charcoal grinder,

- three men are responsible for charcoal powder supply to the agglomerators and for the binder preparation,

- five men operate the agglomerators,

- four men collect the freshly produced briquettes and take care for their drying

- four men are responsible for the sacking and storage of the dry briquettes.

Production of charcoal briquettes requires first of ail a carbonisation step. Carbonisation of an agricultural residue which becomes available over such a large land area is extremely difficult, both technically and organizationally. In this paper, for the economic assessment, it is simply assumed that the product is available against a certain cost.

The cost components are:

- investment in kilns

- labour for stalk collection, preparation and carbonisation

- supervision

- transportation to the briquetting plant

The costs of labour, supervision and transport are estimated to amount to 17 US$/t of charcoal. Costs of investment made for the carbonization of the stalks are dealt with separately (See Table 1). A breakdown of production costs is presented in Tables 1 and 2. The figures given hold for a factory which is in operation for 34 weeks/yr. The reason is that, unless an additional investment in drying equipment is made, no briquettes can be produced during the rainy season. It is shown that the production costs (including capital costs) amount to about 90 US$ per tonne. In 1987 the average charcoal price to private consumers varied between 175 and 220 US$/t. This price difference can be considered large enough to absorb marketing costs. An indication of a pay-out time can be given by estimating an annual revenue. Assuming that the briquettes are sold to retailers for 129 US$/t, which is reasonably below the mean effective purchase price for wood charcoal (see Table 4), the annual revenue would be an amount equivalent to 92,800 US$. This results in a pay-out time of 4.0 yrs.

TABLE 1. Investment cost for a 800 t/yr briquetting plant.

Cost item

Investment (USS)

Lifetime (yr)

Annualised cost (USS/yr)

Charcoal grinder

10,000

10

1,627

10 agglomerators

16.000

10

2,604

Binder mixer and pump system

4,300

10

700

Drying tables

1,200 (3)

5

317

Molasses storage tanks

1,200 (3)

2

691

Carbonisation kilns

19,000 (3)

5

5,012

Building

30.000 (3)

20

3,524

Tools

1,000 (3)

5

264

Freight

8,000

10

1,302

Total

90,700

 

16,041

TABLE 2. Analysis of production cost for a 800 t/yr charcoal briquetting plant.

Cost item

Annual cost (US$/yr)

% of annual cost

Capital cost

16,041

23

Operation cost Charcoal

10,850

15

Briquetting

Supervision (1 man)

2,000

3

Labour (18 men)

9,800

14

Binder (molasses)

14,000

20

Briquette sacks

12,000

17

Energy

2,000

1

Maintenance (5 % of

4,535

6

investment per yr)

   

Total

71,226

100

Production cost per tonne (US$/t)

89.0

Foreign component of production cost (%)

9

TABLE 3. Technical assessment of wood charcoal and cotton coal briquette quality.

Property

Cotton coal briquette

Wood charcoal

Lower heating value(kJ/kg)

22,000 - 24,000

28,000 - 30.000

Ash content (weight %, dry basis)

12

3 -5

Morphology

Spherically shaped

Randomly shaped

 

25 - 35 mm dia.

0.5 - 100 mm

 

Almost no fines,

20 % fines and

 

no brands

brands

Ease of ignition

difficult

easy

Burn-out time

long

short

Smoke formation

more (during . ignition only)

less

Marketing

The marketing of a new product for which a potential demand is proven requires an identification of both a market niche and a marketing strategy. The marketing process for the briquettes has only just begun and still has a long way to go.

In 1987 the first steps were made in assessing the market niche. The market for cotton coal briquettes is determined by several factors. One of them is the product quality. In Table 3 a technical comparison is made with wood charcoal.

It is the consumer's rather than the technical, evaluation which will determine whether or not the briquettes become a success. His appreciation can be completely unexpected. During 1987 several appreciation tests were made amongst people in rural areas and in Khartoum. On the basis of these surveys, a first and tentative product characterization was made:

Positive

- the briquettes are attractive

- easy in handling

- ignition is easy

- they contain less ash

- they produce more heat than wood charcoal

Negative

- The briquettes are made of a waste product (cheap)

- they produce more smoke

- burn-out time is shorter than for wood charcoal

- the smell of the smoke is worse than for wood charcoal

- they contain more ash

It is remarkable that this product image not only contradicts the technical analysis, but is also, inconsistent.

Two of the negative images of the cotton coal briquettes (ie. the smell and the smoke) were considerably improved by adapting the production method. A marketing strategy must aim at turning those negative properties which cannot be improved into positive ones. Some opportunities are:

"The use of a waste product is good for the Sudan" "Ash is a burn-rate controller"

At first sight, although the cotton coal briquettes appear to have some disadvantages, the new fuel seems to be appropriate for the poor man's market. However, given the briquette's immediate positive properties (good inital visual impact, lack of fines and brands) there may also develop a demand for the briquettes as a high quality product.

The existing charcoal demand must be characterized further. Principally two types of customers can be distinguished: i) those who buy charcoal in 35 kg sacks (each containing 20% fines and brands) and ii) those who buy charcoal in small, 2.4 litre volumes (per "Malwa"). Naturally, the first group of customers, who have the money for large purchases, pay the lower price, whereas the latter group pays the higher price for its charcoal. Based on June 1987 price data for Wad Medani, those who buy their charcoal per sack effectively pay 0.174 US$/kg (20% of the charcoal sack's contents consists of useless fines and brands), whereas whose who buy their charcoal per Malwa pay 0.214 US$/kg.

Traditionally, consumers are provided with wood charcoal by retailers. Supposing that the existing dealer network can or should be used for the marketing of the cotton coal briquettes, the retailer group is as important as the consumers, and should be studied closely. In Table 4 retailer revenues and costs are reviewed (Prices for Wad Medani, June 1987). It is shown that the largest profits are made when selling per Malwa. For the retailer, when selling per Malwa, an important difference between cotton coal briquettes and wood charcoal is the absence of fines and brands, for which he receives a relatively small amount of money. At an attractive purchase price, this may result in a somewhat larger retailer appreciation.

The next step is to find out the actual profits by assessing the consumer market volumes of "sacked" charcoal and per "Malwa" sold charcoal.

Table 4 - Retailer revenues and costs in Wad Medani (June 1987)

Method of sale

Purchase cost (LS/sack)

Sale revenue (LS/sack)

Profit

Per Malwa

20

27

9

     

2 (7kg fines)

Per sack

20

22

2

Progress and Prospects Since August 1987

Cotton stalk carbonization briquetting by employing the agglomeration technology seems to be a commercially attractive activity. The Sudanese Islamic Bank has expressed its interest in future participation in new production companies. In March 1988 the Rahad Corporation together with the Tenant Union decided to establish a company to develop the cotton stalk carbonization and briquetting into a commercial enterprise (Rahad Energy Products Corporation Ltd). The Energy Research Council and the Biomass Technology Group provide technical, management and marketing assistance. The first results are expected in June 1989.

The BTG agglomeration unit is applicable for production capacities ranging from 30 to 500 kg/hr (dry briquettes). The equipment is capable of processing a variety of charcoal types ranging from soft wood to hard wood charcoal and charcoal made of agricultural residues. Climatic conditions, in countries other than the Sudan, may lead to the need for additional processes, such as forced drying and post-carbonisation. These technical options are presently being studied.

D fl - Dutch Guilder

LS - Sudanese Pound

References

Beaudequin, F. et al, Agglomeration of Charcoal, in: Egneus, H. et al, Bioenergy 84, Barking, 1985.

Reynieix, M, et al, Pelletization of Vegetable Charcoal Smalls, in: Grassi, G. et al, Biomass for Energy and Industry, 4th E. C. Conference, New York, 1987.

Siemons, R. V., Hood, A. H., Botterweg T., Feasibility Study on Semi-Centralised Charcoal Briquette Production from Cotton Stalks in the Republic of The Sudan, Eschede, 1987.

Snow, R. H., et al, Size Reduction and Size Enlargement in: Green, D. W., et al, Perry's Chemical Engineers Handbook, 6th ed, Singapore, 1984.