Torrefaction or carbonation?
The process of carbonization of wood raw materials at temperatures from 200 to 270 C opens up new opportunities in obtaining a product of unique quality compared to ordinary wood. The parameters of the obtained product depend on the parameters of the process, the set temperature and the duration of the process. The scope of its further application, from use in the woodworking industry and the construction of houses to heating premises, depends precisely on the characteristics of the process itself.
Let’s dwell on the main advantages:
- Increased energy consumption in comparison with traditional raw materials – due to a decrease in the total mass after evaporation of moisture and other non-combustible substances.
- Resistant to moisture absorption, the moisture content of raw materials remains at the level of 2%. In everyday life, such wood is not subject to deterioration and decay, which means the loss of its characteristics;
- Does not require coating with paints, varnishes and various chemical compositions, which draws the attention of people who care about their health and prefer natural materials;
- An increase in the energy consumption of wood (22,500 kJ / kg) leads to a decrease in the cost of heating;
- Optimal strength / fragility ratio;
- In the process of combustion and heating, the emission of smoke is practically absent;
- The raw material becomes practically homogeneous, both in moisture content and in size, which is very convenient for the manufacture of pellets. Pellets from this raw material are dark in color.
In English, such processing of raw materials is called torrefacation, in Russian the closest meaning is “roasting”, in other words – drying of raw materials. The process is similar to the process of obtaining charcoal by low-temperature pyrolysis without oxygen. For these purposes, various fuel reactors are used. Equipment for the production of charcoal is ideal. The carbonization process in “GreenPower” charcoal kilns is fully automated and allows you to adjust both the process itself and set the desired temperature to get the product with the specified parameters. Charcoal kilns “EKKO” and “BIO-KILN” do not pollute the environment with emissions, which is the main concept of the enterprise and a big plus when choosing equipment.
When heated to a temperature of 160 0 C degrees, wood loses moisture, when the temperature rises, acetic acid and phenol are released, wood begins to change its physical and chemical properties, including wood becoming more fragile and more hydrophobic.
Torrefaction is initially a soft pyrolysis at a heating temperature of no more than 270 C. While the usual pyrolysis process for producing charcoal or recycling polymers, various household waste occurs at temperatures from 400 – 800 C. With torrefaction, the heating rate is no more than 50 C / min. , the duration of the process is 10-30 minutes. Sometimes it can reach several hours, depending on the parameters of the given product.
Under the conditions of the torrefaction process, the biomass structure changes, which leads to an improvement in the technological parameters of raw materials as fuel.
In the process of torrefaction, excess moisture leaves the biomass, the chains of polymers – cellulose and lignin – are partially decomposed, which, respectively, is accompanied by the release of volatile substances. The release of moisture leads to a decrease in the mass of raw materials by 20-30%, the energy content decreases by 10% due to the evaporation of volatile energy compounds.
Thus, the ratio of higher weight loss and lower energy loss increases the specific heat of combustion of the final product in comparison with the raw materials used. As a result, increasing the energy density in the fried raw material increases the energy yield of the resulting fuel.
The characteristic balance of the torrefaction process
- by mass: biomass (1) = gas (0.3) + biocoal (0.7);
- by energy: biomass (1) = gas (0.1) + biocoal (0.9).
At high torrefaction temperatures, the resulting brown or dark brown dry residue is called “bio coal” (bio coal, torrefied biomass, torrefied wood). Not to be confused with biochar (see previous article).The biocoal is subsequently pulverized into a powder, which is pelletized to produce fuel pellets.
The following can be used as raw materials for torrefaction:
- wood waste and logging waste;
- plant biomass;
- whole parts of burnt trees after a fire;
- energy fractions of waste.
Biocoal is an intermediate between wood and charcoal and has the following characteristics:
- has a better indicator of energy consumption in comparison with pellets: the lowest calorific value is 20-25 MJ / kg, while for pellets it is 17.5-19.5 MJ / kg;
- to a much lesser extent, compared to pellets, it is subject to biological processes of decay;
- hydrophobic, which allows it to be stored outdoors;
- reduces the cost of pellet production due to its fragility.
Thus, the following advantages of the torrefaction process can be highlighted:
- the duration of the process is lower than with pyrolysis, which significantly reduces production costs and service;
- allows the use of wood waste, including burnt wood, wood after fires, that is, processing of burnt wood is possible;
- higher thermal capacity of the resulting product, which reduces heating costs;
- high characteristics of the obtained wood when using it in the woodworking industry and construction;
- the minimum moisture content of the resulting product, which increases the operational capability and does not require special storage conditions.