Slow pyrolysis is the slow heating of organic material in absence of oxygen. Instead of combusting, the volatiles from the organic material evaporate partly, and a product (charcoal) remains, consisting for a large part (normally 80%) of carbon. Slow pyrolysis is also called carbonisation, and emphasises the solid charcoal as main product, instead of fast pyrolysis which emphasises the liquid product.
Carbonisation takes place in three phases:
- Drying phase – Above 100°C the water trapped in the pores of the biomass is evaporating and driven off as steam.
- Thermal decomposition – The thermal decomposition starts between 260 - 300°C. The cellulose, hemicellulose and lignin components of the wood break down, and volatiles are released. The volatiles consist of CO, CO2, CH4, H2 and “tars”, Tars is a generic name for a number of condensable components, which include higher carbohydrates, acetic acid and methanol. In Figure 1.1 the process of thermal degradation is graphically depicted.
- Cooling – After the thermal decomposition is complete, the charcoal needs to cool in absence of air, or combustion takes place. The cooling phase takes in most industrial processes twice as long as the carbonisation phase, except if some sort of forced cooling is used (such as quenching in water).
As temperatures are low, the mass fraction is still 100% (no moisture loss or carbonisation). When the temperature rises, first the moisture is removed, after which the volatiles are gradually driven off, and the char and ash remain. Factors affecting charcoal yield are a.o. heating rate, peak temperature, feedstock, gas environment, and pressure.
Charcoal production can be carried out in various types of equipment. One convenient way to categorise these is by mode of heating. There are three ways:
- internal heating means that the heat for the carbonisation is supplied by combusting part of the material itself. Simple earth-pit, Beehive and Missouri kilns operate this way. Heat transfer is simple and direct, but efficiencies are generally low and emissions may be high.
- Indirect heating implies that the heat is supplied by an external source. Usually this exteral source is the combustion of the pyrolysis gases that are formed during carbonisation. "Retort" type units fall in this category. This method leads normally to better process control and lower emissions; the indirect way of heating leads however to an energy loss
- heating with recirculating gases this method utilises the hot combustion gases which are directed through the carboniser. The gases should be free of oxygen for this method. A problem here could be the premature removal of pyrolysis gases, which could lower overall efficiency. Large systems such as those by Lurgi and Lambiotte use this method.
In-depth coverage of carbonisation and charcoal production is given in the review by Antal and Grønli (2003)
A process related to carbonisation is torrefaction. Torrifaction can be viewed as incomplete carbonisation. This operation is also called “roasting”, and is applied commercially to f.e. coffee beans. Torrefaction involves the heating of wood or other biomass types to a temperature of 280 – 300°C only, leaving a lot of the volatiles in the “charcoal”. The heating value increases somewhat, the volume decreases with 30%, and the friability of the material increases. The obvious advantage of torrefaction is the high energy efficiency which can in theory be attained (80 – 90%). A drawback is the lower yield in volatiles, which may imply that the process cannot run authothermally.
- Antal, M.J. Jr., Grønli, M.,"The Art, Science, and Technology of Charcoal Production", Ind. Eng. Chem. Res. 2003, 42, 1619-1640,