The disadvantage of the combustion process is in smoke produced. This is because the moisture content of biomass is relatively high comparing to other solid fuels such as coal. It influences to ignite in combustion reactor. Another disadvantage of biomass is that the hygroscopic ability. If biomass is stored for a couple of days, the quality will decrease. Researcher shows that the torrefaction is a path to treat biomass before transportation or thermochemical conversion.
Research in torrefaction to increase the biomass quality is used through several further processes. The pretreatment method is called torrefaction historically resulted from roasting of coffee. In fact that torrefaction has similarities with pyrolysis, but the temperature is lower pyrolysis around 200 – 300 ºC. It is also called mild pyrolysis or low-temperature carbonization. torrefaction will reduce biomass weight around 70% of the initial weight and increase energy yield. This method is beneficial to improve biomass for further thermochemical treatments such as Gasification or combustion applications.
Most researchers use temperature and residence time as research variables in torrefaction research.
The frequently occurring analysis in thermochemical research is analysis proximate, which identify compounds in a mixture to discover the moisture, volatile matter and ash, as well as fixed carbon. In this analysis, increasing temperature increases the heating rate. It is because the fixed carbon content inside of biomass will increase and decrease the moisture and volatile matter.
Biomass is composed by carbon, hydrogen, and oxygen. Ultimate analysis is required to discover this component. Researcher shows that increasing temperature increases carbon and reduces oxygen content. The ratio oxygen/carbon will also decrease as well as the ratio of hydrogen and carbon. This result indicates that torrefaction decreases smoke and increases the efficiency of biomass material.
In a study which is conducted by several researchers in mass and energy yield, suggests that increasing temperature decreases mass and increases in energy yield and energy density. Concerning to grind ability effect Patric Bergmen and Jacob Kiel has observed torrefaction temperature and residence time in wood, willow, and demolition wood. Their experiments show that toffefaction decrease power consumption at ranges 70% – 90%. It can be concluded that increasing of temperature and residence time increase grind ability.
Residence time is life-time material inside of the reactor. Similar to temperature variable, this variable is inversely correlated to mass and energy yield. Increasing of residence time will reduce the mass and energy yield. Meanwhile, energy density will increase. Increasing of residence time increases the heating rate.