As shown by the consumption values presented in the previous chapter, wood fuels are used to a great extent in most rural industries under consideration. However, the technical level and efficiencies of the energy conversion systems in use generally seem to be very low and many improvement possibilities exist. By improving the efficiencies of heat generating equipment and the energy conversion system as a whole, wood fuel consumption can be reduced, the product quality can be improved and, in most cases, the profitability of the processing activity also improved. Without high capital investments in new equipment or in sophisticated technical changes of the existing equipment, small and fairly simple improvements can be achieved by proper management, operation and control of the system in use.
Deficiencies in the energy conversion systems identified are, for example:
use of moist and oversized fuel,
poor firing practice,
no control of combustion air,
wrongly shaped kilns and dryers,
high temperatures of exhaust gases,
fouling of heat transfer surfaces,
heat losses from open flames, cracks, etc.,
no temperature control,
lack of technical knowhow.
By predrying the fuelwood, either by storing, air drying or with the aid of hot exhaust gases, significant amounts of heat energy could be saved. Bhattacharya (17), for example, states that 10 % of energy could theoretically be saved by reducing the moisture content of wood from 45 % to 15 %, but in practice the saving is even higher. In most cases, wood fuel is fed into the kiln intermittently and the combustion temperature fluctuates widely. A lot of excess combustion air is provided to prevent problems with sustaining the combustion flame from occurring. The amount can vary from 150 % up to 700 % as reported in some case studies. In some furnaces there are no combustion grates either and unburnt wood or charcoal can be found among the ash. In many cases, the firebox is also open to the front side and heat losses are high because open flames blaze out of the kiln.
In very few cases, the latent heat of the hot exhaust gases is utilized by recirculating. The gases could, in many cases, be utilized for predrying the raw material or the fuelwood. The shape of the furnace and the dryer could also be redesigned so that the heat of the fuel gases could be fully utilized. Where indirectly heated air dryers are used, rather severe fouling of heat exchange surfaces has been observed. The efficiency could be improved by fitting baffles to the surfaces. Insufficient insulation of pipes, doors and furnace walls is also quite common.
Thermal efficiencies estimated by different persons for a number of rural processing activities are summarized in Figure 4.1. The figure shows that the estimated efficiencies are, in most cases, very low and could probably be raised to 5060 % by improving the wood energy conversion system used.
Lack of capital is, of course, a big obstacle in many cases, and a reason for reluctance to improvements in efficiency. As prices of wood fuels are steadily increasing, the operator of the plant has, for example, to rely on lower and lower fuel grades to afford the fuel purchase.
Neither does he have any extra money for equipment or system improvements, although the payback time of the expenditure would be very short, nor does he have the possibility to increase the product price, which is dictated by the larger industries or the government.
To recommend more specific improvement measures in rural processing activities, the different categories of industrial activity and the applied wood energy conversion systems should be analyzed in detail for each individual case.
FIG. 4.1 - WOOD - BASED ENERGY IN RURAL INDUSTRIES. ESTIMATED EFFICIENCIES OF SELECTED WOOD - BASED ENERGY CONVERSION SYSTEMS
The state-of-the-art of the wood energy conversion systems used at village level has been described quite well in the report by BEST (29), and the findings are summarized briefly in the following chapters (4.1 to 4.5).
There is a wide range of furnaces in use for preparing food and beverages at village level. In restaurants, simple mud or metal stoves, being larger versions of household stoves and accommodating pots up to 50 60 cm in diameter, are generally used. Both wood and charcoal are used as fuel. The charcoal stoves have a grate and are made from sheet metal. Many mud stoves and open fires used in restaurants have large hoods fitted over them to remove smoke and fumes. Some restaurants use cast iron stoves introduced in colonial times and still manufactured in many countries. Brick stoves with chimneys are used in many hotels. One important aspect of restaurant fuel consumption is that the larger stoves used in restaurants and by street food makers are more fuel-efficient than domestic stoves used for preparing family food or food for sale.
There is considerable potential for the introduction of more fuel-efficient stoves. Larger metal and brick stoves with around 40 % thermal efficiencies can be easily designed. The stoves may have up to three pot holes, each hole having a specially fabricated pot that fits in the gas stream.
There are many designs of village baking ovens. The most common is a directly fired dome oven. Wood is placed in the oven and burnt completely, before the bread is pushed inside. Its dimensions vary widely: for small ovens baking up to 100 loaves at a time, the diameter may be 0.7 1.5 m and the height 0.4 1.0 m. A door, whose diameter is usually less than 2/3 of the internal diameter, is built on one side of the oven, usually on the down wind side. The most common material is mud, whilst bricks and cement are also used.
In Sudan, larger directly fired brick ovens have a separate combustion chamber, built underneath the baking oven. The oven is about 2.5 m long, 1.8 m wide and 1 m high. The hot gases from the wood burners in the fire box pass through holes in the floor of the oven and escape through a chimney. These ovens bake over 200 loaves in one firing, may bake well over 1 000 loaves a day, and use about 0.8 kg of wood per kg of flour, as reported by Ahmed et al., 1985 (19).
Some village bakeries use indirectly fired ovens. A metal oven is usually 0.7 1.0 m3 and it can contain up to five shelves holding up to 200 loaves of bread in one firing. The fire is placed underneath the metal oven either on the ground or on a simple metal grate. Some ovens have a simple air control mechanism to assist the heat output. 0.8 1.5 kg of wood are required for baking 1 kg of bread. The actual amount of wood used depends on the combustion chamber configuration, the moisture content of wood, and the degree of attention given to the fire by the baker. In many of these ovens the residual heat can also be used for baking cakes and biscuits, thus maximizing the utilization of fuelwood.
There is considerable room for improvement in these ovens, although, for the present, very little testing and design work has been done. The efficiency of existing ovens varies from 5 to 20 %. Much of the heat losses are due to poor combustion, a high amount of excess air, poor distribution of heat around the oven, and poor circulation of heat in the oven. It is likely that the fuelwood consumption of improved ovens can be 80 % lower, but the capital cost will be considerably higher and the bread will not have a smoky flavour, and this may be unaccountable to consumers.
Some simple modifications have been carried out by field workers. Bricklaid bread ovens can be improved by putting in shelves so that twice as many loaves can be baked for each fuel charge. This has been realized in Ghana, the ovens being made by local masons. The traditional heat oven in Northern Pakistan has been modified by changing its shape, adding insulation and improving gas circulation. Tests have indicated that fuel consumption is reduced to half.
In Western Africa, a common method for preparing the brew is to use 50 l earthenware or aluminium pots over a simple shielded fire or a four-pot mud stove, usually made by the entrepreneurs themselves. An average batch of beer is 700 litres, and one brewing enterprise produces on average, 1 400 liters per week. The batch is boiled for three consecutive days by a group of up to 5 women, before the beer is considered to be ready for fermentation. The average thermal efficiency of the system has been estimated to be 1618 %, and the specific consumption of wood to be 0.2 kg/1 of brewed beer.
By changing the shape and pot seating arrangement of the existing four-pot mud stoves, the fuel consumption can be reduced. Similar savings can be made by enclosing open fires with a mud or brick shield and by adding a metal grate. The cost of modifications if relatively small, but there is still a little incentive to purchase a new stove as the profit from the brewing is considerable. Thus, any new stove should have other advantages, such as reductions in smoke emissions and cooking time, an increase in pot life, and/or easier operational methods.
In Africa and Asia, the fish quantities larger than those processed at the domestic level are usually smoked and/or dried in mud ovens holding 50600 kg of fish. They can be round or square and have a volume of 0.32.0 m3 and a wall thickness of about 1520 cm. The top is usually open and there are small air and wood inlet holes at the bottom. The fish are usually placed on a wire net fixed about 2/3 of the way up the side of the oven. The smoking time can vary tremendously depending on the size and type of fish and on the quality and desired flavour of the end-product. Smoking that involves drying takes, on average, 1020 hours. In some ovens, wood is placed between the fish layers; in others, underneath the fish. The amount of fuelwood used per kg of fish can vary from 2 to 12 kg.
Various attempts have been made to introduce improved smoking and drying units. In Ghana, a simple mud and wooden dryer (chokar) was introduced to womens' cooperatives. This smoker reduced wood consumption by over 50 % and improved the quality of the fish. By fitting a separate sawdust drier separated from the smoker by an integrated tunnel, the fuelwood consumption can be lowered to 20 % compared to the open fire smokers. No attempts have, as yet, been made to introduce these on a larger scale. Similar attempts to introduce improved solar dryers have not proven successful, due to high capital cost per unit of material dried and the low utilization degree of the units.
There is a wide range of kilns used by village potteries. The simplest one is an open pit used mainly in Africa, and the most sophisticated ones are climbing kilns used in parts of Asia. Methods of lying the fire and pots and controlling the output depend on the fuel and clay types available, and on the type and number of pots being prepared.
The simplest, open pit method is used when only a few pots are fired and the clay is of refractory nature. The pots are placed in a pile, interspersed with wood. The wood is lit and the fire allowed to burn at its own pace. For larger firings, the pots are stacked with the fuel and then a layer of dirt or ash is placed over the pots. Holes in the base and at the top of the pile allow air to enter and combustion gases to escape, the rate of firing being controlled by opening or closing these holes.
The most common kiln in Asia is a rectangular or circular updraft kiln without a chimney. Made from ordinary household bricks, this kiln has a series of firing ports at its base, and in some cases a grate. Its volume is 3 10 m3 and its diameter-to-height ratio varies from 6:1 to 1:3, with up to 500 items being fired at one load. Some potteries place fuel inside the kiln, while others fire from air ports at the bottom. Such kilns normally take a few days to build and have a life-time of approximately 10 years. The bricks around the firebox need replacing each year, but otherwise, little maintenance is required.
More sophisticated updraft kilns with chimneys are used by larger potteries in Asia, and in some instances, also downdraft and climbing kilns. They are fired to higher temperatures to produce stoneware and porcelain materials. Most downdraft and climbing kilns are large, with volumes of 5 15 m3. The dimensions of the firebox, internal passageways, firing chamber and chimney vary. Made from refractory bricks, their construction and operation require considerable skill. They can take weeks to build and must be maintained regularly if they are to produce the desired temperatures. If maintained properly, these kilns can last over 20 years.
In Thailand, for example, the pottery kilns located in rural areas are mostly fired by wood, while some are fired by rice husk, whilst those located near Bangkok often use LPG or fuel oil. Use of rice husk is restricted to earthenware kilns, while LPG is generally used for high quality products. Wood-fired kilns are all of intermittent type, of which a so-called snake kiln is the traditional one. This is essentially a long tunnel, inside of which the pottery is located and fired. No standard size for such kilns appears to exist, as reported by Bhattacharya (17). The length of the kiln can be up to 50 m, while the inside dimension of the tunnel is such as to allow manual loading and unloading. The kiln consists essentially of three main parts - the tunnel itself, a fire chamber at one end of the tunnel and a chimney at the other. The firing of the kiln has two stages. During the first preheating stage, which lasts about 12 hours, the natural draft in the kiln is low. To ensure air flow, electrical blowers are generally used during the start-up of the kiln, and all the fuel points located along the side of the tunnel are kept closed by placing bricks over the openings and using clay for sealing. After the first stage, the fire-place room is partially closed by creating a temporary brick setting, and firing of wood is continued by feeding fuelwood through side fuel ports called eyes. Only one pair of fuel ports is used for firing at one time. Firing through a pair of fuel ports continues for about 40 minutes, after which the second pair of ports is opened for firing. Firing thus progresses towards the chimney end of the tunnel. The combustion air enters through the fire-place room and gets preheated before reaching the combustion zone by picking up heat from the articles that have already been fired, and the hot walls of the kiln. The flue gas, on its way from the combustion zone to the chimney preheats the articles awaiting firing.
Bhattacharya measured the amount of excess air used in one pottery to 95250 %, on average 170 %, and concluded that approximately 32 % of input energy could be saved by controlling the amount of combustion air.
Sawmill waste is generally used as fuel in the snake kilns. Round wood does not appear to be quite suitable for firing through the fuel ports. Firewood in the form of small sticks is used as fuel in some places. The wood is quite often dried by spreading it on the surface of the tunnel before firing.
Another type of pottery kiln used in Thailand is the Turiang kiln. This is a semioval furnace provided with a combustion chamber in its front and a chimney at the back. The articles to be fired are stacked inside the chamber. The Turiang kiln is fired in three stages. The first stage is a slow drying/preheating process. In the second stage, the firing temperature is raised to 1 100 °C, during which a glowing exhaust is maintained at the chimney outlet.
The desired temperature of the kiln, at this stage, is controlled only visually. Scrap rubber tyres are sometimes fired along with wood during this stage. Along with large and long firewood, bamboo is also used as fuel, and it gives the articles a pinkish grey touch.
One feature of the Turiang kiln is a poor air/fuel mixture, with the result that the exhaust almost always lets out a lot of smoke - addition of rubber makes the pollution problem worse and results in black and white thick smoke.
In the small-scale brick and tile industries of Indonesia updraft kilns are used, these being the simplest and cheapest technology available, in contrast to the bull's trench and tunnel kilns used by the larger-scale sector. The fuel consumption of the former is 2 5006 000 kJ/kg brick, and of the latter 1 4502 300 kJ/kg brick. The great difference between the efficiencies of the bull's trench and the clamp suggests that there is scope for improvements to the clamp, which would reduce the amount of fuel required per brick. These may be both in the design of the clamp and in its operation.
Brickmaking is an industry that can be found in almost all provinces of Thailand (17). The kilns using fuelwood are normally permanent constructions and firing is intermittent. Two types of kiln construction are usually applied, e.g. simple rectangular chambers (Scotch kiln) and round kilns with conical roof (circular downdraft kiln). The kilns may be of different sizes, a capacity of about 150 000 bricks/charge appears to be quite common. Compared to the rectangular kilns, the round kilns require higher initial capital investment, as well as more maintenance. Especially the top conical roof requires regular maintenance after about four years of operation. The furnace of the kiln is located in front of the door used for loading and unloading the kiln, and it is assembled after the kiln is loaded and ready for firing.
Bhattacharya (17) estimates that the firewood requirement of round kilns is about 0.46 m3/1 000 bricks and that of rectangular kilns 0.59 m3/1 000 bricks. This corresponds to a specific fuelwood consumption of 0.330.47 kg/kg. Due to differences in firing practice, the properties and quality of the bricks are, without doubt, different. In addition to fuelwood, diesel oil is used for the extrusion of the bricks.
The diesel requirement is estimated to 11.25 1/1 000 bricks 5 x 7 x 16 cm3 in size.
Koopmans (9) estimates the average energy consumption of the updraft smaller brick kilns in Indonesia to 3 140 kJ/kg of brick and of the larger continuous kilns to about 1 850 kJ/kg. The firing temperature is higher in the continuous kilns and hence, the quality of bricks is also superior. The fuels used by the smaller brick industry are wood, agricultural wastes, such as rice husk, coconut husk, oil palm nut shells, sugarcane leaves, etc., and in some cases, coal and diesel oil, whilst the continuous kilns are mainly fired with residual oil and to a lesser extent with diesel oil or coal. The energy consumption of the tile kilns is estimated to be slightly higher than that of the brick kilns, i.e. 1 8003 000 kJ/kg for continuous kilns and 3 000 8 500 kJ/kg for intermittent kilns.
Reasons for the low efficiencies of the kilns are open fire boxes, which means that the amount of combustion air cannot be regulated and the kiln is fired with a lot of excess air, radiation losses from the kiln and hot exhaust gases are great, and poor firing practice is general. The operator usually has to push the firewood into the furnace, and due to exposure to radiant heat from the flame, he, of course, tries to stay away. The result is that he pushes in the firewood at relatively long intervals and the firewood logs often burn partly outside the combustion chamber.
In the bull's trench kiln the hot exhaust gases are used for preheating the bricks awaiting burning. The combustion air is also preheated by transferring heat from bricks fired earlier. These kilns are already widely used, for example, in India, Pakistan and Nepal, but are only suitable for large and medium-scale continuous operations. The larger kilns could also possibly be fired with gasifier units that run on agricultural residues.
Most lime production at village level is carried out in very simple shaft kilns. There are two types of shaft kilns: continuously and intermittently fired. On the other hand, there are many different designs of continuously fired kilns, all having grates at the bottom of the kiln, where wood and/or coal is placed. The kilns are usually round, although double cone, square prisms and octagonal shaped kilns have also been built. Their height is usually 23 times the diameter. The kilns are often built on a hillside to facilitate loading and to provide greater structural strength. Various materials are used, such as refractory bricks, household bricks and stone. The lifetime of the kilns is probably 510 years, although the inside of the kiln has to be relined at least twice a year.
The intermittently fired kilns have no grate, but a draught hole at the bottom. The kiln is loaded with alternate layers of wood and limestone. The fire is started at the bottom of the kiln and allowed to burn through, adding either more wood or lime until the volume does not decrease any further, or without further attention. These kilns are normally not more than 3 m high and 2 m wide. They have a capacity of less than 10 m3 or 10 t of limestone. Firing time can vary from 36 to 70 hours, and the kiln is usually fired 2 to 3 times a month. After each 810 firing, the inside wall of the kiln must be repaired.
The batch kilns used in Indonesia are called Cubluk kilns. Their cross sections range from 1 x 1 m to 2.5 x 2.5 m and their heights from 2 to 3 m. Sometimes two or three kilns are built together to form one unit. Wood is invariably used as fuel, sometimes mixed with coconut husk or other materials. The fuel to limestone ratio varies from place to place and ranges from 2 to 3.5 m3 wood per m3 of quick lime.
During the firing process burnt lime is raked out of the kiln through the fire holes in the bottom, while all the other burnt lime is removed after the kiln has cooled down. Unburnt and overburnt stones, charcoal, etc., are removed before the lime is sold. Koopmans (8) states that the firing process is inefficient as the kiln is very low, which implies that almost neither preheating of the combustion air by the cooling lime nor heating up by the combustion products of the limestone occurs. A large amount of unburnt volatiles from the top layers of wood escapes through the top and no control of combustion air is undertaken. Koopmans estimates the thermal efficiency of the batch Cubluk kilns to roughly 17 %. For the larger lime kilns of continuous type, the Padalarang kilns, he estimates a slightly higher efficiency, on average 26 %.
A well designed lime kiln was constructed as a demonstration unit and gave an output efficiency of 56 %. However, the investment cost was about three times as high as for the traditional kiln, and therefore, this design was not accepted by the industry.
Chomcharn (10) calculated the thermal efficiency of a larger lime kiln in Thailand to only 17 % and notes that the efficiencies of the smaller kilns are obviously even lower.
Joseph (19) reports that the efficiencies of the small lime kilns in Malawi are 1112 %, and that there is evidence that this can be at least doubled by a new design of the kilns.
In the rubber industry wood fuels are used for drying. In the drying of crepe rubber, wood is used in boiler radiator systems for producing hot air. The production of sheet rubber involves drying in wood-fuelled smokehouses. Estimated wood consumption per kg of crepe rubber is 0.20 0.80 kg per kg of sheet rubber (4).
The heating systems used by the rubber industry in Sri Lanka are old and their efficiencies have not been improved with the years. The combustion efficiency of the boilers is estimated to 3040 % and most of the energy is reportedly lost through the chimney. The overall thermal efficiency of the drying towers after circulation of the hot water through the radiators is found to be around 15 to 20 %.
With the use of direct air heating systems, where the flue gases are passed through a set of cast iron or steel pipes, the fuelwood consumption can be lowered to 0.1 kg/kg crepe rubber. Fire risks and need of electricity in forcing the air into the drying loft are drawbacks of this method.
The smokehouses commonly used in Malaysia for drying sheet rubber are called Subur smokehouses. Generally, they consist of four smoking compartments, each with varying temperature ranges. Hence, a more or less continuous production chain can be maintained. Heat is supplied to the compartments by a single furnace located in front of the building. The walls of each compartment are generally built from bricks lined with cement mortar to provide good insulation against heat losses during the smoking process. The roof is made from asbestos with a lining of wood. The internal ceiling is not flat and this is deliberately designed to enhance the circulation of air, to ensure an even and uniform heating. The doors of each compartment are also well insulated and the temperatures inside the compartments are checked through inspection doors at the rear wall of each chamber. Smoke outlets are located between the trolley rails, and each outlet is covered with spark arresters and has a flat sliding damper of steel sheet placed over it. The damper controls the amount of smoke and heat entering the compartments. It is a common practice to connect these sliding dampers with steel rods passing through the smokehouse walls. This enables the operator to assist the dampers without having to open the rooms.
The furnace is usually enclosed by an outer casing, so that heat dissipating from the furnace walls can be redirected into the smoke compartments by streams of air through inlet openings. The casing enclosure of the furnace is built from thinly mortared bricks with a roof of reinforced concrete. The furnace door is usually made of cast iron plate and can only be opened by sliding it up vertically. The furnace room also has its own adjustable ventilators, so that the rate of combustion can be controlled. The common practice of preventing cracking is to mix salt with the cement mortar.
Kong & Keng (5) report that most of the more well-established sheet rubber producers of Malaysia today employ trained watchmen to supervise the smokehouses. A temperature difference of 8 °C is sufficient to affect the drying time by as much as two days.
By predrying the wet rubber sheets in the smokehouse for one day, the firewood consumption at the actual smokedrying stage can reportedly be lowered by almost 70 %. Other efforts have been recycling of the smoke and the use of the smoke and the use of solar panels for drying. By reducing the thickness of the rubber sheet by 10 % it should be possible to shorten the drying period and hence energy consumption by about 20 %.
In older conventional types of smokehouses in use in Thailand the fuelwood requirement has been estimated to 1.62.0 kg/kg of dried rubber (17).
Curing of tobacco leaves takes 90 to 110 hours and is carried out in four stages:
Pichetpinyo (40) has estimated the fuelwood consumption to 8.7 kg/kg of dry leaves for the curing barns of Thailand, and Whitelock (13) correspondingly the specific fuelwood consumption to 9.2 kg/kg for the smallholder tobacco industry of Malawi.
The UNDP/WB tobacco industry energy efficiency project in Malawi has shown that the national average of 42 m3 of fuelwood per ton of tobacco is possible to lower to around 15 m3/ton with quite simple technical solutions.
The curing barn is a building with brick walls. The barns are of various sizes, although the most common size appears to be 6 x 6 x 9 m (in Thailand), with a capacity of approximately 4 000 kg of fresh tobacco leaves. The barn has ventilation holes located along the side walls and a top ventilator. Hot flue gases are passed through five pipes located inside the barns.
The barn is usually provided with two furnaces - one on either side of the front door. Generally, the amount of combustion air is not controlled. The flue gases pass through the flue pipe system under the action of normal draft and are exhausted through the chimney. Heat is transferred through flue pipes from the gas to the air inside the barn. The hot air in turn heats and dries the tobacco leaves. The important resistances to the process of flue gas to air heat transfer are the gas to flue pipe convective resistance, flue pipe to air convective resistance, and fouling of the inside surface of the flue pipe due to ash deposits, carbon particles and tar. The inside and outside surfaces of the flue pipes are not provided with fins, which are normally needed to enhance solid surface to gas heat transfer. The chimneys are also located wholly outside the barn so that heat transfer through the wall of the chimney is lost to the ambient. The heating rate is regulated by controlling the fuelwood combustion rate, while the drying rate is regulated by controlling the ventilation openings. Bhattacharya (17) has calculated the heat loss carried away by the flue gas to be 38.5 %, heat loss through ventilation to be 10.8 % and heat loss through structures to be 7 %. The overall thermal efficiency of the curing system is estimated to be 14 %.
By redesigning the furnace and the insulation of the barn, optimizing the ventilation rate, and improving the heat exchange system, essential savings in fuelwood consumption could be achieved. The utilization of wood-fuelled gasifiers also seems to have great application potentials, but extra capital investments are required.
In the coconut processing industries, copra curing, coconut desiccation and oil extraction are to a certain degree made with the aid of wood fuels.
Because smoke tends to discolour the copra and may add a different flavour to the product, coconut shells are preferred as fuel in the copra curing process.
The final stage of desiccated coconut manufacture is drying. This process reduces the moisture content of coconut meat from 5060 % to 3.52.5 %. Two types of dryers are used in Sri Lanka (1). One of the dryers was initially developed for the country's tea industry, and the drying time has been cut to half with this tray dryer. At present, almost all the desiccated coconut mills of Sri Lanka are using wood fuels for the drying operation.
Almost the entire coconut oil industry in Sri Lanka is based on the full press extraction method, where heat is required for drying and cooking the nuts before pressing. The heat required for the semi-automatic dryers is provided from a furnace/heat exchanger unit fuelled with firewood. About 75 kg of firewood is reportedly used to produce one tonne of oil. The thermal efficiency of the tubular furnace/heat exchanger system is estimated to be reasonably high, about 5060%.
In the production of desiccated coconut the sterilization in boiling water consumes about 0.3 kg fuelwood per kg of dry meat. The thermal efficiency of sterilization is estimated to 45 %. The temperature of the fuel gas escaping through the chimney is about 400 °C. In total, the production of desiccated coconut is estimated to consume 1 kg of fuelwood per kg of product.
Although the thermal efficiency of the furnace/heat exchanger units used in the coconut industry of Sri Lanka is fairly high, 55 %, Liyanage (1) estimates that this could be raised to 75 % by the following measures:
providing the heat exchanger surfaces with baffles,
increasing the heat exchange surface with other methods, and
insulating the hot surfaces of the heat exchanger and the dryer.
An interesting new waste heat recovery unit is being tested in Sri Lanka. In this process the pyrolysis gases from carbonization of coconut shells are burnt and the hot flue gases are led to the dryer. The proper application of this new technology to coconut desiccation, copra curing and oil industries will result in achieving self-sufficiency in thermal energy.
In tea processing, heat is required for withering and drying the leaves. Most of heat energy is required for the final drying stage, where the moisture content of the tea leaves is brought down from roughly 7055% to 33.5% (2). Hot air heated by steam radiators is mostly used as drying media. The dryers used by the tea industry are either vibrating tray dryers or fluid-bed dryers, and thus consume electricity in addition to heat energy.
New designs of tubular heat exchangers and fuelwood-fired fluid-bed dryers are being tested in Sri Lanka. A need for a separate type of furnace/heat exchanger system arose, as the older models were unable to sustain the hot air temperature of 125 °C required by the fluid-bed dryers (2). The principle of the new heaters is that the radiant heat is absorbed by a cast iron arch, which is heavily finned on the outside and has a bank of tubes for convective heat exchange, where flue gases flow through the tubes.
A new type of wood-fuelled gasifier/combustor system is also being tested in Sri Lanka. In this two-chamber furnace the wood is gasified in the first chamber and combusted in the secondary chamber. The flue gases can, after dilution, be used as direct heating media in the fluid-bed dryers. Potential savings of fuelwood seem obvious. However, the usefulness of this directly fired gasifier/combustion system has to be tested by the tea industry before implementation on a larger scale.
A new type of forced draught booster fan fitted to the ash pit door of the firewood furnace is also under development in Sri Lanka. This method enables the operator to control the amount of excess combustion air, and the thermal efficiency of the system can still be improved.
Energy in some form is required for most of the stages of coffee and cocoa processing. The amount and type, as well as the energy conversion system, depend on the level of activity. The production at the village level is actually not very energy-intensive, as the berries are almost exclusively dried in the sun. If engine-driven hulling equipment is used, the fuel consumption is estimated to be about 0.010.02 1 oil/kg dry beans and if machine-driven pulpers are used, the energy consumption is roughly 510 1 oil/t dry beans, as reported by Koopmans (12) from Indonesia.
On a larger industrial production scale the energy consumption is much higher as thermal dryers are used for processing. With old multifloor drying houses, still quite frequently used in Indonesia, the fuel consumption is estimated to 6 10 m3 of wood per ton of dry cocoa beans and with a drying system that uses a kind of heat exchanger coupled to a large fan, the fuel consumption is said to be only 2 m3/1 500 kg dry beans, plus the electricity consumed by the 56 kW fan (11).
In the coffee industry different dryer types are used, both mechanical and non-mechanical. In the old type indirectly heated drying houses, where the coffee is spread out in a thin layer on the floor made of perforated plates, the fuelwood requirement is said to be 7501 000 kg of wood per ton of dry coffee beans. In addition to wood, coffee husk (which is obtained 160 kg/ton of dry beans) is also used as fuel.
Newer dryer types are the Mason and the ADS dryers. The former is a rotating drum through which hot air is drawn and the ADS-dryer a directly heated vertical tower, fitted with a mechanical bucket conveyor. Koopmans (12) estimates the fuel consumption to 400500 kg of wood or 150 l of oil per ton of dry coffee for the former, and 170200 1 of oil per ton for the latter, in which only liquid fuels can be used. In addition, both dryers require electricity for driving the 12 kW fan and the 12 kW conveyor motor, respectively.
The roasting of coffee is usually carried out by large industries in industrialized countries and is not dealt with here. Where coffee is roasted for local domestic needs, very simple low-efficiency furnaces are used. A rough estimate of the fuelwood consumption for these is 0.30.7 kg of wood bec kg of roasted coffee.