The material you wish to turn to charcoal is placed in a chamber above a firebox. This chamber is initially sealed except for two vents to atmosphere. The outer chamber is then sealed except for access to the firebox via two small firebox doors, one at each end of the machine. A fire is lit below the inner chamber and the temperature of the inner chamber monitored. As the temperature rises, water in the wood is allowed to escape through the open vents in the inner chamber, along with some non-combustible gases. There comes a point (around 375-400 deg C) when nearly all the water in the wood has been driven off and now the machine is venting gas. At this time the vents are capped, and this action forces the gas being produced by the hot wood in the inner chamber down two pipes and into the firebox.
Once in the firebox this highly flammable gas burns and raises the temperature further. After this point has been reached the machine is self-fuelling and requires only small adjustments to the firebox door and a bleed valve on a retort chamber vent to control gas supply to the firebox, and therefore temperature. As the wood inside the inner chamber turns to charcoal it produces less and less gas and eventually none at all. All the operator has to do then is wait until the machine has cooled sufficiently to allow it to be opened. This usually happens overnight. Because the wood that you are converting has not been in contact with direct flame all that is left in the inner chamber is charcoal of high purity. There is no dirt and no ash. Whilst the machine is operating on gas from the inner chamber it is running in a very clean manner.
All the gas created by the wood being converted is burnt in the firebox and not released to atmosphere. This reduces emissions of greenhouse gases by 75%-90%.
This method of production also allows unusual loads to be converted to charcoal. We have proved the Exeter Retort with some odd materials. We have converted reed, bamboo and even bone with our machine. None of these feedstocks can be processed in the traditional way. The ability to process such materials as well as brash from woodland opens up another market for the product of our retort. The biochar market is becoming established slowly across the world and means that biochar from wetland growth and waste from gardens has a slowly increasing value. Some of our char analysis makes very surprising reading, charred bone in particular lending itself to soil amendment. There have been other surprises too. Bamboo, for example, makes a rather good fuel/BBQ charcoal