Pairing combustion experiments and thermogravimetric analysis to uncover timescales controlling cellulose ignition and burnout in a Hencken burner

Abstract

Solid biomass fuels are potential components of several industrial and power-generation decarbonization pathways. Despite considerable literature that documents the fuel properties of lignocellulosic biomass, the fundamental combustion characteristics of biomass constituents are not well understood. We tackle this knowledge gap by analyzing the combustion properties of cellulose (a key biomass component) in a Hencken burner across various temperature and oxygen mole fractions of the oxidizer gases. Combustion data are compared with results from thermogravimetric analysis (TGA) and scaling analyses to reveal two rate-controlling timescales in the ignition of cellulose: devolatilization time and volatile ignition delay time. TGA shows that the devolatilization time is only a function of temperature and occurs on a considerably shorter timescale than oxidation. Hencken burner experiments show that ignition delay time is highly sensitive to temperature; at very low levels of oxygen in the oxidizer gas (mole fractions of 0.01–0.04), the ignition delay time is sensitive to oxygen mole fraction, but this sensitivity disappears at higher oxygen levels. Burnout time is the least sensitive parameter investigated. The combination of experiments and chemical kinetic simulations begins to explain the ignition behavior of the volatiles from cellulose decomposition and lays groundwork for future research to address uncertainties in quantifying the kinetics of cellulose ignition. Finally, this combined combustion-fuel science study raises serious doubts about the use of combustion indices and TGA-calculated properties to predict the true combustion behavior of biomass.