Author Identifier

Akram Elsebaie: http://orcid.org/0000-0002-7725-109X

Date of Award

2026

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Engineering Science

School

School of Engineering

First Supervisor

Yasir Al-Abdeli

Second Supervisor

Mingming Zhu

Third Supervisor

Barun Das

Abstract

Biomass offers significant potential as a sustainable and renewable substitute for fossil fuels in energy generation, playing a critical role in the reduction of global warming. However, despite its environmental benefits, biomass combustion faces persistent operational challenges, particularly fouling, which reduces efficiency and increases maintenance costs. Although fouling mechanisms have been studied in various combustion systems, there is limited research which addresses its formation and behaviour in fixed-bed biomass combustors under different air staging conditions. This research investigates fouling behaviour in a counter-current combustor, focusing on the influence of design (secondary air location), operational (air staging flowrates between the primary and secondary air), and fuel (raw and torrefied biomass blends) on fouling, freeboard (post-fuel bed) temperatures, and the breakdown of flue gas (waste) heat to that which can potentially be recovered.

Experiments were conducted using a batch-type fixed-bed combustor equipped with a fouling module through which all flue gases pass. Combustion experiments used raw hardwood pellets, with some final preliminary tests undertaken also using torrefied biomass blends. Temperature profiles were monitored using thermocouples. Gaseous emissions (CO, CO2, and NOx) were measured using gas analyser with fuel mass consumption also monitored using a load cell to determine the onset of steady state conditions. Deposits in the fouling module were collected using air-cooled probes designed to mimic industrial heat exchange processes in heat and power systems.

Analysis of temperature data helped quantify the impact of various test parameters, establish experimental uncertainty, and determine the onset of steady-state operation after start-up. Peak bed temperatures exhibited low uncertainty (< 5%), validating experimental repeatability. At the same time, the transient start-up phases showed significant time-resolved variability in the fuel bed, implying that the transient phases contribution is negligible to the cumulative fouling deposit in short-duration lab-scale tests.

Fouling behaviour was strongly influenced by air staging, where relatively lower ratios of secondary air (Qs) to primary air (Qt) produced more unburnt hydrocarbons, leading to sticky fouling deposits. In contrast, higher ratios of Qs/Qt enhanced combustion efficiency, generating finer, more dispersed fouling particles. Chemical and morphological analyses confirmed that these operational conditions directly determine fouling characteristics.

The role of both design and operational factors on the amounts of heat transfer recovered or wasted into exhaust (through the fouling module) revealed that increasing Qs/Qt improved overall combustor thermal performance with maximum flue gas heat recovery (through the cooled fouling deposition pipes) at Qs/Qt = 0.75. These conditions however also impacted fouling deposits, with ash content increasing to 83.59%. Elemental analysis showed elevated concentrations of inorganic elements like Ca and Al in deposits at higher Qs/Qt ratios.

The project also included a (final, preliminary) study into the impact of using torrefied biomass blends, compared to raw biomass. This was done due to the absence of research available on fouling from such (thermally treated) biomass fuel stocks in practical or lab-scale combustors. While torrefaction enhances fuel quality by increasing fixed carbon, and lowering the O/C ratio, its combustion dynamics remain underexplored. Torrefied and raw biomass blend ratios of 0%, 15%, and 30%, combined with primary freeboard lengths of LI = 200 mm and 300 mm, showed that blending torrefied biomass improved combustion efficiency, reduced CO emissions, and enhanced thermal stability at LI = 200 mm, but also led to higher NOx and variable ash trends. At LI = 300 mm, improved ash reduction was observed, though with unstable emissions at higher blends.

Overall, the study ensures a targeted investigation into the influence of varying operational and design conditions on fouling formation, deposits composition, heat transfer characteristics, and temperature profiles in fixed-bed biomass combustion systems. It highlights the critical limitation and challenges associated with torrefied biomass combustion and offers practical strategies for minimising fouling, enhancing thermal management, and improving emissions control, while also guiding combustors and heat exchangers design.

DOI

10.25958/w28x-3v57

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