Environmental Factors Governing Pre-ignition Release of Organic Matter in Combustion of Large Wood Samples

JOHN FLYNN, Tami Bond, Colorado State University

     Abstract Number: 499
     Working Group: Biomass Combustion: Outdoor/Indoor Transport and Indoor Air Quality

Abstract
Biofuel burning emissions are most active early in the pyrolysis phase before ignition occurs. During this pre-ignition stage in large form-factor samples (~ 10² cm³), unburned hydrocarbons are released in the form of organic aerosol and heavy and light molecular weight gases, leaving behind a char layer. Through thermal decomposition, mass is transferred from the solid phase into the gaseous region outside the char layer where combustion can occur. These releases vary with time, so that the occurrence of ignition during cellulosic biomass combustion is difficult to predict. After ignition, flames consume unburned fuel and emissions of this organic matter decrease. This pre-ignition phase can produce half of the particulate mass emissions in poor combustion. Therefore, understanding the system’s behavior prior to ignition can provide insight about the conditions that release large quantities of particulate matter, giving guidance for burning design under these high-emitting conditions.

In order to explore the potential for rapid ignition and stable biomass combustion, a deeper understanding of what forces drive the gas and particle phase ejection from pyrolyzing wood is needed. It has been shown that pyrolysis and the release of combustible fuel is a repeatable and deterministic process. [Fawaz, et al., 2021] Environmental factors influencing the rate of mass loss during the short period of pre-ignition pyrolysis are reported here. We summarize pre-ignition mass loss rate, ratios of CO and CO₂, particle size distributions, and emission factors in the early stages of material ejection during pyrolysis. Critical flux conditions that can lead to ignition are identified and contrasted between the two-dimensional system of biofuel combustion and guidance for the one-dimensional systems in fire safety literature.