- Charcoal


Charcoal production is basically the transformation of virgin (often wet) biomass into charcoal by heating in the absence of air. Charcoal production (or carbonisation) Is one of various pyrolysis processes. Essential for all these processes is a confined surrounding with little or no air present and a heat source to reach the necessary temperature.

The first step in the carbonisation process is drying of the biomass which starts at low temperature (ca. 100 °C). This is followed by thermal breakdown of the biomass at increased temperature starting at 200-250 °C up to the final carbonisation temperature. During this dissociation the structure of the biomass changes, large molecules are broken into smaller ones and evolved in the form of gas. The gas mixture contains CO, CO2, H2O but also larger hydrocarbons and tars. In traditional charcoal processes these hydrocarbons and tars cause the black smoke, typical smell and are hazardous to both environment and health.

In the course of the process the carbon content of the remaining solid residue slowly increases (carbonisation) as preferentially non-carbon components are emitted through the gases. Due to the high temperature chemical bonds within the complex biomass molecules are broken. The weakest chemical bonds i.e. O-O, C-O and N-C bonds are broken more easily than C-C bonds, leading to a carbon rich solid residue, the char. At increasing temperatures the carbon content of the char get higher as more and more non-carbon components are released.

The charcoal production process is similar to the extremely slow natural process of coal formation from plants in the soil.

The energy needed for heating the biomass can be derived from combustion of part of the biomass or by adding heat from an external source. In the latter case the process can be controlled better. Once the carbonisation process has started the produced gases (off-gases) contain sufficient caloric value to supply at least part of the necessary heat for the carbonisation.

Key issue for an efficient carbonisation process and good quality end products is the combination of the heating rate, final temperature and residence time.

In the figure the principle of the CharcoTec process is depicted. For heat generation during start-up a wood stove is used in which wood or other biomass is combusted. The hot flue gases from the wood stove enter the CharcoTec furnace and heat the dried biomass in the carbonisation unit. During heating of the biomass off-gases are produced containing lighter and heavier hydrocarbons. This gas stream is directed back to the wood stove and combusted. Hereby emissions to the atmosphere of these hydrocarbons are prevented and the caloric value of the gases is used for the carbonisation process. After start-up the caloric value of the off-gases is sufficient for maintaining the process temperature at the desired level. Utilising the caloric value of the off-gases increases the overall process efficiency.

The hot flue gases passes to the second reactor containing the container with wet biomass (drying unit). The water vapour from the drying unit and the flue gases leave the process through the chimney.

The CharcoTec technology aims at the removal of the disadvantages of traditional charcoaling without introducing “new” disadvantages for the local economies on terms of less employment, too high capital need and dependency on imported materials. At the same time however it does bring the advantages of the modern technology in terms of higher efficiencies (more charcoal per kg biomass input), in terms of potential use of other than woody materials as a source and thus less deforestation. It also will prevent environmental and health damage by capturing the nasty vapours, combusting them and so reusing the otherwise lost energy content in it, thus reducing fuel cost of the car coal process.

The CharcoTec kiln most probably is the only kiln on the market which can be almost completely produced locally, can be produced for a broad range of capacities and thus cost, is easy to operate – much alike the traditional kilns – and does hardly require any maintenance as has been the case with traditional kilns.

It however always requires two separate reactor vessels (which is to be understood as sophisticated definition of the process space, and looks like a simple cabinet or lorrey) where the firewood is placed in, dried and subsequently carbonised. By separating those two sub-processes the energy re-use is optimised and leads to the highest efficiencies possible with the technology, still keeping the operational concept simple.

The economic advantages of the CharcoTec kiln are such that entrepreneurs in the field of making charcoal over the world can afford the technology as invested money can be recovered within one operational year and the outlook is such that financing can be found easily.

CharcoTec has developed the technology by testing the first prototype in praxis on a site in Bosnia and thereafter in The Netherlands for the improved and in fact demonstration version, which still can be visited today.

Based on the CharcoTec process principles the first pilot-plant has been designed and constructed back in 2014 with the aim to verify the operational functionalities and product quality and to come with an improved design for the commercial plant.

The pilot-plant consisted of a propane burner and two small reactor vessels placed in an internally insulated sea container. The biomass was placed in two cage-like cylindrical containers which are placed from the top in the reactors ; subsequently the reactors are closed with a lid with a sand seal. The reactors are started by passing the hot flue gas from the propane burner alongside the wall of the reactors. The off-gases from both reactors are directed to the propane burner through a gas pipe in order to combust all hydrocarbons (no emissions of hazardous components) and to utilise the caloric value of the off-gas. At several points in the reactors and containers the temperatures have measured.

Tests have been performed with wood logs, wood chips and peat briquettes. The practical tests have been supported by lab-scale thermos-gravimetric analysis to determine the drying and carbonisation behaviour of the used biomass. Samples have been analysed on a TGA instrument from 30 °C to 900 °C. The results are shown below

The results were as expected, first the biomass is dried around 100 oC followed by weight loss due to carbonisation starting at approx. 250 oC The behaviour of the pilot kiln during the tests was found to be similar.

The results were as expected, first the biomass is dried around 100 oC followed by weight loss due to carbonisation starting at approx. 250 oC The behaviour of the pilot kiln during the tests was found to be similar.

The temperatures in the graphs were measured at different positions in the reactor. From T2 it can be seen that the temperature in centre the reactor remains at ca. 100oC up to the point that the wood logs are dried. After that the temperature in the reactor increases relatively fast to the temperature in the container. From these tests it becomes obvious that a large part of the total process time is taken for drying the biomass; depending on particle size up to several hours. Once the carbonisation process starts the caloric value of the evolved gases is sufficient to maintain the process temperature in the reactor; the propane supply was stopped. The efficiency (dry biomass to charcoal) is ca. 35%. The quality of the charcoal was found to be within the standards for BBQ charcoal.

Based on these test results and supported by the theory CharcoTec constructed a second test kiln or better a first demonstration kiln, which is operated on a site near by the city of Groningen in The Netherlands. The installation consists of two gas-tight containers, two furnaces and a wood-fired stove. During the batch-wise operation one of the two containers is utilised for drying fresh wood, whereas the other container is used for carbonisation of the dried wood.

The operation of the kiln is comparable to traditional char coal work. Fresh, wet wood is manually loaded into a container, which is subsequently closed and place In the free kiln, which is connected to a gas-line to subtract the gases coming off the char coal process while operating: in the beginning water vapour during drying and combustible and health endangering off-gases during carbonisation. The off-gas is used to convert the energy of the combustion into drying and/or carbonising before being vented at relatively low temperatures to the environment through the chimney or the wood stove.

The process is started as a traditional kiln by burning some biomass in the wood stove. As the efficiency of the kiln is higher there will be less firewood burned than in a conventional kiln. After the process has ended, the carbonisation container is disconnected from the off-gas line and moved outside for cooling. A new container filled with wet wood while the first batch was processed is than placed into the kiln. By switching a number of valves the two separate kilns reverse their duty, the drier becomes the carboniser and vice versa, and the production of char will get on, thus almost making a continuous process by adding one extra vessel or kiln to the process.

In September 2015 the demo-plant was taken into operation on the site of Stainkoeln in the city of Groningen. The plant consists of two furnaces (insulated 10 ft containers), a wood stove, four reactors of each 3,5 m3 and a chimney.

During operation one furnace is used for drying fresh wood, whereas the other furnace functions as carbonisation furnace. Hot flue gases from the wood stove are used to heat up the carbonisation furnace. The reactors in the carbonisation furnace are indirectly heated by passing hot flue gases alongside the closed wood cabinets containing the fresh dried wood. Subsequently, the flue gases are passed through the drying furnace and the chimney.

The volatiles from the carbonisation reactors are led to the wood stove and combusted. In this way no emissions of hazardous gases take places and at the same time the energy content of the volatiles is used for maintaining the carbonisation process at the desired temperature.

Off-gases (water vapour) from the drying process are vented. The plant is equipped with a number of thermocouples for monitoring the process.

The demo-plant is used to establish and optimize the operational conditions for the production of high quality charcoal from different types and sizes of biomass. Biomasses of different sources (such as wood with varying origin and particle size distribution, agricultural residues like straw, reed and hay as well as peat) are already taken into the program and testing is on-going and can be extended with other materials as a source at any time suitable to the client.

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