The heating rate also plays an important role on the quality of bio-oil and char structure obtained. Bio-oils can be obtained with lower moisture content at higher heating rates, primarily due to inhibition of secondary reactions such as volatile dehydrationand cracking [74] and due to water-soluble fractions (formic acid, methanol, acetic acid) and heavy fractions (rich in phenol and its derivatives). The CO and CO2also increase at high heating rates [32]. Ketones content, levoglucosan, phenol, and toluene levels also go up with the heating rate for the fast pyrolysis of pine wood and grass when the heating rate exceeds 50 -1000°C/s. Higherheating rates do not show a marked effect on the performance of each sub product [70].
With high heating rates, a biochar is obtained with smaller volume pores and a specific surface area. This heating rate can generate significant pressure gradients between the inside and outside of the particle because the volatiles are produced quickly and do not have the ability to instantly be evacuated. This causes some internal structures to crack, increasing the end proportion of macropores in connection with the micropores. Besides this, the biomass passes through a melting phase (metaplastic) which warps the internal structures and blocks the pores. [54-55].