Characterization of Ashes from Co-Firing Biochar with Coal under Pulverized-Fuel Conditions

Abstract

This contribution presents results on the systematic characterization of the ashes from the co-combustion of biochar or its corresponding raw biomass and coal under pulverized-fuel conditions. A mallee bark (75–90 μm) was subjected to fast pyrolysis at 500 °C to prepare a biochar. The bark and the biochar were then co-fired with a Collie coal of identical size fraction in a laboratory-scale drop-tube furnace at 1400 °C in air, with biomass/biochar shares of 5, 20, and 40% expressed based on lower heating values. The produced ashes were collected using a cyclone and systematically characterized. The results demonstrate that the morphology of the ashes from the bark and the biochar is of irregular shape, whereas the coal ash particles are round. The ash particles follow a unimodal distribution, with an area-equivalent mode diameter of ∼5–12 μm, except for the ash from the bark combustion that also shows two larger peaks at ∼65 and ∼95 μm. The compositions of the ashes from the bark and the biochar are similar, both rich in Ca and Mg, whereas the coal ash contains dominantly Si, Al, Fe, and Ca. Under identical co-firing ratios, replacing the bark with the biochar results in higher contents of Mg and Ca in the ashes because of the enrichment of these elements in the biochar. The major minerals identified in the coal ash include mullite, quartz, and hematite, and those in the bark ash and the biochar ash are portlandite, magnesite, calcite, and lime. Up to ∼56% of Na, ∼41% of K, ∼56% of Mg, and ∼69% of Ca in the ashes can be recycled via water leaching, with negligible environmental concerns. These data are important in developing suitable strategies for the utilization and management of ashes derived from the co-combustion of biochar (or biomass) and coal.