Torrefaction is the process of the soft pyrolysis of biomass, i.e., the process of heating without air access at temperatures of 220–300°C and atmospheric air pressure under which moisture and volatile substances that are formed as a result of the partial decomposition of cellulose and lignin are extracted from wood. This decreases the raw material weight by 20– 30% and increases the energy density by 10–15% which leads to a significant increase in the specific combustion heat of the final product.
Torrefaction makes it possible to improve the consumer properties of wood fuel:
(1) Depending on temperature and the duration of the process, the combustion heat increases from 14–17 to 19–24 MJ/kg.
(2) Torrefied pellets acquire a high hydrophobicity and resistance to atmospheric and biological effects, which makes it possible to solve problems that are faced during the transportation and storage of conventional pellets.
(3) During the use of torrefied pellets, the efficiency of energy generation is always higher than that in the case of non-torrefied pellets. This is achieved on the basis of an increase in the abiotic temperature of torrefact combustion and a decrease in the enthalpy of exhaust gases, in which the concentration of water vapors is almost two times lower than that during the combustion of conventional pellets. With respect to the heating boiler, the change from conventional wood pellets for torrefied pellets leads to an increase in the efficiency by several percent.
(4) During torrefaction, the mechanical strength of the final product increases by 1.5–2.0 times.
(5) The electric power consumption for the grinding of torrefied wood raw material decreases during the use of torrefact as a fuel (or an additive to the main fuel) in conventional pulverized-fuel boilers.
IMPROVEMENT OF THE GRINDABILITY OF TORREFIED FUEL
Coal boilers that are equipped with a standard coal pulverization system can receive wood pellets as a fuel; however, electric power consumption for their break increases by several times in this case.
A significant decrease in energy consumption (by 7.5–15.0 times) for the grinding of conventional and torrefied wood has been recorded during the various analysis.
One of the main stimuli of the development of torrefaction processes is the combined combustion of coal and wood raw material with hardly any additional costs by power producers.