Methods for determining lignocellulosic biochar wettability

Abstract

It was shown that many various biochar types are hydrophobic in low temperature (< 500 °C), but high temperature biochar (> 500 °C) has a tendency of increasing hydrophilicity. Hydrophobicity of low temperature biochar is the most often referred to the effect of pores clogging with tars, since aliphatic compounds vapour only at high temperatures. This research used various methods for the evaluation of biochar wettability, such as the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) test and optical determination of a contact angle (CA). Selection of the best method is difficult because of the fact that many studies used different methods, they have been compared with each other, they refer to different theoretical assumptions and they differ technically. The aim of this article is to compare the three methods used for the evaluation of biochar wettability. The chosen techniques include WDPT, MED and CA measurement. Two different temperatures (450 °C and 700 °C) and two different holding times (1 h and 2 h) were used for the production of different wettability biochar types from five different lignocellulosic feedstocks (pine bark, birch bark, pine wood, birch wood and hemps). Results of wettability and all used techniques overlapped; and this means that it had the same tendency in characterising biochar wettability. Mechanical and practical arguments have been made for the selection of the most suitable method for determination of biochar wettability.