Home >  Resources >  Charcoal Guide Contents

• Basic Principles
• Instructions for a Small Metal Kiln
• Common Misconceptions
• Effect of Kiln Size on Operation
• Scientific Background
• Bibliography

Common Misconceptions

"Wood must be well seasoned to make charcoal."

- It is preferable to use seasoned wood but it is still possible to make charcoal from freshly cut wood. The much greater moisture content takes longer to be driven off and takes more heat requiring more wood to be fully burnt at the bottom of the kiln, and so the yield will be lower. It is also more difficult to achieve the temperature for carbonization to take place properly.

"Cutting the wood into short pieces will help the carbonization process."

- Cutting the wood into shorter pieces speeds up the seasoning or air-drying of wood. This can result in a lower moisture content which in turn will help the carbonization process.

Water moves through wood 10-15× faster along the grain than across it. If bark is left on, evaporation of moisture radially is half that of a debarked log. The speed of drying is therefore largely determined by the length when the length is less than 10-15× the diameter.

If wood is to be made into charcoal a few weeks to a few months after felling cutting into shorter pieces immediately after felling will speed up air drying leading to lower moisture content by the time the kiln is loaded. Priority should be given to cutting the larger diameter logs into shorter pieces since these inherently take longer to dry. Moisture content can be reduced to the fibre saturation point in this time frame.

If the wood still has substantial moisture content when loaded into the kiln, then cutting into shorter pieces may speed up the removal of this moisture content during the initial phase of the burn. This may reduce the risk of partially carbonized pieces but won’t improve the overall yield.

Once the wood is dry, however, pyrolysis proceeds at a similar rate in any direction, so unless the logs are very short, e.g. similar in length to their diameter, the pyrolysis time is a function of the diameter. Hence, pieces of larger than average diameter should be cut so they can be placed in the hotter central part of the kiln.

"Packing the kiln as densely as possible reduces the amount of air in the kiln, so reducing the amount of charcoal burning away to nothing."

- The main thing that packing a kiln densely does is to maximize the amount of wood in the kiln and thus the yield for a given run. Packing a kiln too densely, though, especially a small one tends to restrict the early flow of exhaust gases through the charge, making it more difficult to reach and maintain the necessary carbonization temperature. The volume of air in the kiln initially is insignificant compared to the volumes of air feeding in and exhaust gases vented during the whole process. Irrespective of initial packing the charge in the kiln tends to collapse during carbonization to a similar consistency.

"The lid must be in place at the start to stop the whole wood charge from catching fire which would unnecessarily reduce the amount of charcoal produced."

- The lid is best left off a small kiln so as to speed up the initial heating up of the kiln. Once hot enough the wood at the top of the charge may light; this will further speed up the heating of the kiln. Only once the kiln has reached the sufficient temperature for carbonization should the lid be put on.

"Once an initial burn has taken place to heat and dry out the wood all air supply must be blocked, otherwise the charcoal will burn away to nothing."

- At the temperatures required for carbonization a kiln loses heat through radiation and convection at a considerable rate, and so the air supply must be kept open for a limited burn to maintain the kiln at these temperatures. Only a fraction of the wood charge is sacrificed by burning.

"Steam, which is from the moisture content of the wood, is only vented from the kiln during the early part of the carbonization process."

- The removal of moisture, although predominantly occurring at the beginning, continues throughout the carbonization process as moisture is removed from the middle of the largest diameter pieces of wood. Water in the form of steam is also one of the volatile products of the pyrolysis reactions.

"Restricting the release vents will help keep the heat in."

- Actually restricting the release vents will tend to slow down the carbonization process and slow the burn at the bottom. This in turn will reduce the temperature leading to the carbonization process slowing or even stopping. Opening up the bottom to allow more air can restore the burn and the overall temperature, but the kiln will tend to be too hot at the bottom resulting in a lot of small charcoal pieces and not hot enough at the top resulting in un-carbonized wood.

"Wind blowing on the kiln will cool it down on that side."

- Actually wind will increase the flow of air through the air inlets on the side facing the wind increasing the burn rate and thus increasing the temperature on that side — this should be compensated for by restricting the air inlets on the windward side.

"If I make sure the air inlets are sealed up, I can make the kiln start to cool down, even if some of the outlets are still producing smoke."

- Towards the end of the process sealing up the air inlets alone may be insufficient to shut down the carbonization, partly because exhaust will tend to draw air in through the smallest crack, and partly because as the wood fully dries out the process becomes increasingly exothermic maintaining the temperature for the pyrolysis reactions without the need for burning with air.

"The process of carbonization will continue for a while until the kiln cools down after shutting off all the outlets to the kiln."

- Carbonization depends on the release of the volatile products of pyrolysis reactions; if these are prevented from being vented from the kiln the concentrations of them will build up rapidly in the kiln and inhibit further reaction.

It is a general fact that processes in nature and engineering work differently at different scales, and this is true for charcoal kiln operation. The standard metal kiln design has a diameter of 2.1-2.4m. The kiln at Parndon Wood is a scaled down version with a diameter of only 1.2m. There are significant differences in operation, in particular the typical time for carbonization. Charcoal can also be made in a much smaller container still such as a 200 litre oil drum, which is different again in operation. The size and carbonization time for various kilns are shown in Table 1 below.

Rate of Carbonization

The differences are accounted for by how heat loss from the kiln and heat generated in the burn vary with size. The rate of heat loss through convection and radiation for the same kiln temperature is proportional to the surface area of the kiln. There is further heat loss due to exhaust gases being vented. This can be assumed to be roughly a constant fraction of the heat generated, although a small kiln is probably less efficient than a larger one.

The potential chemical reaction rate and consequent rate of heat generation for similar material is proportional to the volume of wood. Actually it is proportional to the total surface area of the wood, so the potential rate of heat generation per unit volume is approximately inversely proportional to average log diameter. Generally the aim would be to use smaller diameter logs in a smaller kiln.

The rate of heat generated can, however, be varied: it can be reduced by reducing the pyrolysis rate by restricting the exhaust flow; and it can be increased by increasing the amount of material undergoing complete combustion by increasing the air supply.

For a constant kiln temperature the rate of heat loss is matched by the overall rate of heat generation. As kiln dimensions are increased the surface area and hence heat losses for a given shape increase with the square of linear dimensions. However, the volume and hence for similar material the heat generation potential increases with the cube of linear dimensions. Consequently for a larger kiln the rate of pyrolysis and/or combustion needs to be restricted.

The total heat energy produced during the whole carbonization process for a given ratio of pyrolysis to combustion will be proportional to the mass of the wood charge. For similar material and filling strategy this mass is proportional to the volume of the kiln. The total time for carbonization is equal to the total heat energy generated (≈∝ volume) divided by the rate of overall heat generation equal to the rate of heat loss (≈∝ area). As a result the total carbonization time is very approximately proportional to volume/area and hence increases with linear dimension.

If the rate of the pyrolysis is reduced and the rate of combustion increased to compensate, i.e. the ratio of combustion to pyrolysis is increased, the total heat energy released and the carbonization time can be increased. This might be desirable to ensure larger diameter logs can be carbonized right through. Slower pyrolysis may also produce a higher yield. If larger diameter logs are used the net density of the wood charge may be greater, leading to a greater carbonization time.

Cooling Rate

For a given kiln temperature the heat energy contained in the finished hot carbonized wood is proportional to the mass of charcoal in the kiln. For the same yield efficiency this mass is a fraction of the mass of the wood charge which is approximately proportional to the kiln volume.

The rate of heat loss at any temperature during the cooling phase is proportional to the surface area of the kiln.

The time taken for the kiln to cool down is equal to the integral over the temperature drop of the heat energy capacity of the hot charcoal divided by the instantaneous rate of heat loss. Cooling time is therefore approximately proportional to the volume divided by the surface area of the kiln and hence for the same shape varies with linear dimension.