Comparison of single particle combustion behaviours of raw and torrefied biomass with Turkish lignites

This study investigated the combustion behaviour of single pulverized biomass and lignite coal particles under high temperature-high heating rate conditions. Selected fuels included three important agricultural residues in Turkey (olive residue, almond shell, and hazelnut shell), and two lignite coals from the regions of Tuncbilek and Soma in Turkey. Biomass fuels were either raw or torrefied at 275 degrees C for 30 min in nitrogen. The biomass fuels were sieved to a size cut of 212-300 mu m, and the coals were sieved to 106-125 mu m. An optically-accessible drop tube furnace, operated at a wall temperature of 1400 K, was used to burn single fuel particles in air. High-speed cinematography and three-colour pyrometry were used to characterise the combustion behaviour of the fuel particles. All biomass particles ignited homogeneously forming large and circular volatile matter envelope flames, followed by distinct char combustion phases. The Tuncbilek lignite also ignited homogeneously and burned in two combustion stages, first forming bright sooty and elongated flames with contrails, upon extinction of which char combustion ensued. The Soma lignite exhibited extensive fragmentation which resulted in surface ignition of the fragments and gas-phase ignition of the main non-fragmented particle. The cumulative burnout times of all raw and torrefied biomass particles of the selected size cut were shorter or equal to those of the Tuncbilek lignite but longer than those of the Soma lignite. This result signifies the appropriateness for co-firing such biomass fuel particles in furnaces designed for the former coal, rather than those designed for the latter.