* Extract from FO DAP/ARC/70/536, Documento de Trabajo No. 15, "Leña, Carbón y Carbonización" por Laércio Osse, Salta, Argentina, 1974.
Discharging of the kiln must start only when it is sufficiently cool.
Surface furnaces (kilns) are rarely constructed single. They are most commonly constructed in groups or batteries including various units.
The site of construction is chosen after careful study covering many aspects, principally the availability of wood fuel at a convenient distance; access, with particular account being taken of entry of fuelwood and exists for the charcoal; water supply for the furnaces and the personnel; the vicinity of villages or houses or the possibility of constructing them; the nature of the terrain (preferably flat) and the quality of the earth (preferably impermeable and firm).
When the site has been chosen construction follows the routine: (see Figs. 15, 16).
1) Clean and carefully level the spot where each kiln will be constructed.
2) Mark the centre of the kiln and, at this point, drive in a round stake P.
3) Mark the line L bearing in mind loading and unloading doors of the kiln will be marked above it. For this reason line L must be oriented with that of the loading and unloading entrances (BC and BD).
4) Mark line D perpendicularly (at a right angle) to line L.
5) Having as centre the stake P, draw circles I and E; the length of the line D will be equal to the diameter of the kilns; the internal circle I will be drawn exactly at this diameter; the external circle E will be traced with this diameter, plus 40 cm.
6) Taking half the distance between lines L and D in the four marked quadrants, trace lines L2 and L3.
7) Where line L cuts the circles mark the openings for charging BC and discharging BD, each with a width of 1 m, and, at the same time, mark the shoulders of each. Where lines L2 and L3 cut the circles, mark the chimneys C.
8) Thickness of the wall: 1 brick. Door columns reinforced with one additional brick. Walls of the chimney: half a brick. Cross section of the chimneys: space equivalent to half a brick.
9) The plan of the kiln is now visible on the surface of the ground.
10) Dig out the foundation trenches in accordance with the plan. Normally a depth of 20-30 cm is sufficient.
11) Make the footings, not forgetting the door openings (BC and BD). The footings should be one brick wider than the body of the kiln. The surface of the footings must be perfectly level.
12) Attach a wooden pole to the centre post P by nailing a loop at a convenient height. (The nail will prevent the loop from falling). This pole will serve as a guide for building the kiln wall. The cord can be moved around the centre post P like a large compass.
13) Start building the wall leaving openings for doorways and making the door columns and chimneys; more than one cord can be used so that two or more brick layers can work at the same time; use mortar made from one part of soft clay (sandy soil is not suitable) and one part of pulverised charcoal, plus sufficient water to give the correct consistency; test the wall frequently with a plumbline and level.
14) When ground level is reached, leave three equally spaced openings between each pair of chimneys T. These openings are called "tatus" and have a cross section equal to that of one brick.
15) When the wall is at a quarter of its height (approximately 45 cm above the footing) leave two openings between each pair of chimneys in line with the two outer "tatus" S, and when the wall reaches three-quarters of its total height (around 1.35 m above the base) leave another two openings between each pair of chimneys in vertical alignment with the outer tatus; these openings are the safety openings and have a cross section of 7 x 7 cm.
16) When the wall reaches 1.6 to 1.7 m height, place across the door columns, in line with the walls, a steel plate or pieces of bamboo and continue the laying of the bricks of the walls over these. The openings for the doorways 1 m wide and 1.6 to 1.7 m high are now completed.
17) When the wall reaches a height of 1.80 m it is complete and must be perfectly plumb and its top surface perfectly levelled.
18) On top of the wall lay a row of bricks on their edge (E Fig. 16); use very little mortar between these bricks; around the outside of this ring of bricks place a steel ring or several rounds of fencing wire with or without barbs and adjust it without tightening too much. (Final adjustment is made after the dome is complete CI)
19) Cut bricks to form the base of the dome and set them inside the ring of bricks facing each other.
20) Remove the guide post used for the construction of the wall and substitute it by a small peg, to the top of which is attached a cord which is used to guide the construction of the dome of the kiln.
21) The dome is constructed with the thickness of half a brick and it is extremely important that a minimum of mortar is placed between the bricks. The bricks are laid and kept in position more by lateral pressure than by mortar.
22) At a quarter of the distance from the base of the dome to the top leave 10 small equidistant openings (B Fig. 15); half way up the dome leave another 10 openings and three-quarters of the distance from the base leave six more openings also equidistant. These apertures are called "bainas" and they have a cross section of 7 x 7 cm. At the top of the dome leave an opening 20 x 20 cm for the lighting hole (A Fig. 15).
23) The flue of the chimneys must start from the inside of the kiln at an angle of 45° and then ascend vertically (Fig. 16). A chimney starting from the inside of the kiln at a right angle with the vertical portion does not function well and is more difficult to clean.
When the brick work of the kiln is finished, the walls can be rendered and the dome covered with mud, or as is more usual, both the walls and the dome are plastered with mud.
The plaster is a fluid mud prepared as follows: clay soil is mixed in plenty of water to the consistency of thick soup which is left until the sandy portions of the earth sink to the bottom of the mixing vessel. The mud free of sand is poured over the surface of the kiln and closes perfectly all cracks left during construction, especially in the dome where the quantity of mortar used is minimum.
The rendering or mud treatment carried out after the kiln is finished and before the first burning is invariably repeated during each cooling cycle. It prevents entry of air and fires. It also helps considerably in cooling the kiln.
The charging of the kiln must follow a routine which is, in general, as follows:
1) Close the discharge doorway using only mud to join the bricks together; do not use mortar which makes opening up difficult and results in almost total loss of the bricks.
2) Stack the long fuelwood on its end carefully distributing first smaller fuelwood around the walls of the kiln and then towards the centre fuelwood or increasing thickness. The wood should preferably have its ends cut at an angle to facilitate circulation of the gases. It is particularly important that the tops of the thicker wood are cut at an angle.
3) Carefully fill all the spaces between the lengths to eliminate open spaces, holes and any fault in the way the wood is piled. This operation is called refilling or smoothing. The piling of the fuelwood and its filling is called stacking.
4) Do not mix fine fuelwood with thick fuelwood. Carefully follow the recommendations of point 2: fine wood around the outside near the walls and, progressively thicker wood towards the centre, the thickest trunks occupying the centre or core of the pile.
5) On top of fuelwood stacked on its end, fuelwood should be stacked on its side in a horizontal position under the dome of the kiln, again starting with fine wood near the walls and thick wood towards the centre. Fill up all open spaces carefully.
6) Near the lighting hole at the top of the dome stack wood which is easy to ignite to facilitate the start of carbonization.
7) Close the charging doorway of the kiln using only ordinary mud to set the bricks, as was done for the discharge door. When the kiln is charged, that is to say full of wood with all the apertures open ("bainas"), safety openings, "tatus" and lighting holes carbonization is started. This operation is called cooking the wood fuel and its phases are briefly as follows:i) Light the kiln by adding a good shovel full of burning coal through the lighting hole to ignite the fuelwood, which should take fire immediately. Burning coals may be substituted by gasoline, kerosene, diesel, oil, etc.
ii) Do not use any other opening to light the kiln, except the lighting hole at the top.
iii) When combustion is started, smoke comes out through the lighting hole; first white in colour and getting darker a few minutes later. Then, when the smoke has taken on a dark colour - a signal that the fire has "taken" close the lighting hole with brick and mud.
iv) When the lighting hole is closed, the smoke will start to come out from the "bainas" (openings in the dome) initially white, then turning blue after some time. When blue smoke comes out, it is a sign that the carbonization zone is expanding and one must start to close "bainas".
v) When the "bainas" are closed, the chimney starts to function (which also can happen before all the "bainas" are closed) and when the four chimneys are functioning normally, close all the safety openings; leave open only the "tatus".
vi) The kiln will continue to operate in this manner until the end of the process.
vii) The charcoal burner takes note of the colour of the smoke; when the smoke is white, grey-white, or somewhat dirty, it signifies that the process is developing; when the smoke from the chimney commences to turn blue or blueish, one must close the "tatus" on each side of the chimneys.
viii) The smoke does not turn blueish in all the chimneys simultaneously. Thus it is necessary to close the "tatus" in turn as the smoke takes on a blueish colour.
ix) The chimneys may continue giving off smoke for many hours after the closing of the "tatus". In this case closing the chimneys must not be allowed as it would result in the production of great quantities of brands (semi-carbonised material) near the chimneys.
x) The chimneys must be closed only when they stop giving off smoke.
xi) When all the openings and the chimneys are closed and emission of smoke has completely stopped, carbonization is finished; the kiln may be covered with mud slurry and allowed to cool. Any parts of the mud coating which fall off must be immediately and carefully re-applied as any entry of air will provoke fire in the charcoal.
xii) The cooling of the kiln may be accelerated by means of a water spray across the lighting hole.
xiii) Wind may interfere with the functioning of the kilns. To protect against wind, openings oriented in the direction of the wind should be partially or, in some cases, totally closed. The chimneys also need protection to deflect the wind away from their tops as it interferes with the discharge of smoke and consequently carbonization.
Theoretically, the ideal temperature at which to open the kiln and start discharging is 60°C (140°F); in practice, the charcoal burner estimates the temperature, feeling the bricks blocking the loading and unloading openings.
A kiln which is not sufficiently cool must never be opened: if this rule is not followed, a fire in the charcoal will certainly start up. If put out immediately with water the result is powdered charcoal - waste.
Before opening the kiln an indispensable element of precaution is to have plenty of water near the discharging door for use in case of emergency, for example, a hose connected to a water pipe, a drum of water, etc.
The spot where the charcoal is going to be stored must be cleaned immediately.
The discharge doorway should be opened quickly. The charcoal burner checks by the smell of the gases given off if there is a fire in the charcoal and, if there is he quickly opens the door so that the fire can be extinguished with water.
Once the discharge door is completely opened and the bricks are carefully stacked to one side the charcoal is taken out of the kiln and stacked. During discharge good charcoal should be separated from all impurities such as stone, bricks utilized as wedges during loading, brands, ashes, etc. The remaining charcoal is marketable. Charcoal unloaded from the kiln absorbs air and, as a result, heats up which may provoke spontaneous combustion. It is, therefore, necessary to leave the charcoal in the open air for at least 24 hours so that it absorbs all the air it can before transfer to silos and stores.
The surface furnace (kiln), as any other installation or plant, needs maintenance to ensure its efficiency.
1. When the coating of mud becomes thick due to many applications, the kiln must be cleaned off and given a fresh coat of mud. A mud coating which is thick increases the cooling time and reduces productivity of the kiln.
2, Although fairly solid and stable the kiln can be damaged on impact and it must therefore be protected.
3. Bricks frequently become loose and must be replaced immediately.
4. The safety openings and the "bainas" should be closed with bricks cut in the form of wedges and sealed with mud; mortar or thick clay should never be used for this purpose as the plugs must blow out easily when the kiln "puffs". The kiln could explode or collapse if it cannot puff freely.
5. When the kiln is charged with small size and very dry wood and the chimneys start to operate, an increase in pressure of the gases in the kiln is common. Their escape channels are the safety openings which are progressively closed as carbonization proceeds. When expulsion of gases occurs the kiln puffs or breathes and, if no way free for exit of gas is allowed, it can provoke an explosion and destroy the kiln.