Freeboard effects on instabilities in a fixed bed biomass combustor

Document Type

Conference Proceeding

Publication Title

Proceedings of the Australian Combustion Symposium 2021


The Combustion Institute


School of Engineering




Junejo, A., Al-Abdeli, Y., & Porteiro, J. (2021). Freeboard effects on instabilities in a fixed bed biomass combustor. In Proceedings of the Australian Combustion Symposium 2021.



Biomass combustion at the lab scale is mostly performed using fixed bed configurations. In the counter current configuration, combustion air is staged and supplied both from beneath the packed fuel bed (primary air, Qp) and above the fuel bed (secondary air, QS). The location and amount of secondary air significantly influences combustion efficiency through combustion of post-bed volatiles in the freeboard. However, secondary air can also lead to instabilities which affect freeboard temperatures and may lead to incomplete combustion and noise-pressure coupling, both of which should be avoided. This paper into combustion instabilities examines the role of primary and secondary freeboard height along with secondary air staging ratios QS / (QS + QP) = 0.25, 0.50, and 0.75, spanning globally fuel rich (QS + QP = 300 L/min) or stoichiometric combustion (QS + QP = 560 L/min). Results show that introducing secondary air at the later stage of 550mm downstream of the packed bed and QS / (QS + QP) = 0.75 and 0.50 causes instabilities at both rich (300 L/min) and stoichiometric (560 L/min) conditions. Instabilities were also observed at rich conditions (300 L/min) if the secondary air is located closer to the packed bed (300 mm). The variation in post-secondary air freeboard height (secondary freeboard) did not generate any instabilities. Once initiated, the occurrence of these combustion instabilities was irregular (erratic) not periodic.

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