Segmented heating carbonization of biomass: Yields, property and estimation of heating value of chars

To provide preferable operating conditions for biomass carbonization, the carbonization properties of maize straw, cotton stalk and poplar wood were investigated in nitrogen atmosphere by segmented heating process at different final temperatures of 300-800 °C for 1 h. Segmented heating carbonization contributes to higher ratio of mass and energy yields than a constant temperature/linear heating process with final temperature of around 300 °C. The fuel properties and characteristics of raw and carbonized biomass were analysed systematically. Increasing carbonization temperature, the fuel ratios of the obtained chars rise exponentially, and the profile of atomic H/C ratio versus atomic O/C ratio approximately decreases linearly. There are small changes of sulphur and nitrogen content in the chars. To provide an estimation method for higher heating value for torrefied/carbonized biomass, a new correlation is proposed based on ultimate analysis. The correlation is HHV=19.9579+0.1284C+0.3355H−0.1209O−1.3836N−5.4680S, which is validated through experimental data. The correlation has a low level of errors, that the correlation efficient, mean absolute error, average absolute error and average bias error are 0.971, −6.75×10−6MJ, 1.2% and 0.02% respectively. This work will make a contribution to the biomass pretreatment process.