Title

Thermal conversion in biomass: Single particle drying and packed-bed combustion

Date of Award

2016

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Dr Ferdinando Guzzomi

Second Advisor

Professor Guan Yeoh

Abstract

Biomass combustion is widely seen as a key source of sustainable energy. To better understand some of the factors that affect the performance of biomass combustors, both theoretical and experimental studies are undertaken on laboratory scale burners. The present study provides an overview of the methodologies employed to model laboratory-scale fixed-bed combustors. This thesis contributes to Computational Fluid Dynamic (CFD) modelling of evaporation for different single biomass particles and also experimental application of different secondary air flow rates, configurations and positions on the particulate and gaseous emissions of combustors.

The overview covers a wide range of biomass combustion topics ranging from modelling and experimental studies of both single and packed bed combustors particularly for laboratory scale fixed-bed systems. It also includes treatment of the fundamental thermo-physical fuel characteristics that should be considered when undertaking macro-scale (bed-level) modelling. The work concludes with overall observations on the modelling of fixed-bed combustion as well as opportunities for further research to resolve specific challenges.

The evolution of various single biomass particles during the drying process is investigated to achieve a deeper understanding of their thermal behaviour. The two most conventional models; Arrhenius and Heat sink models have been employed for modelling several single biomass particles in simulations using Ansys-Fluent 15 software (Research version). Several sub-models are implemented in a commercial CFD code to simulate two different evaporation models. To predict transient evolution of wood composition (moisture and dry wood); the transport equations (energy and moisture evaporation) are modelled to assess reaction, heat loss, effective thermal conductivity, specific heat capacity and radiative and convective heat transfers between the particle and surrounding environment. The developed models show close agreement with a set of previous works (experimental and numerical).

The results indicate that the geometric shape of the particle and the external boundaries that are exposed to radiation and convection strongly influences the evaporation process. The results of this study will be used to model the drying process in fixed-bed combustion (future studies).

Secondly, an experimental study of the effect of air staging distribution and position on particulate emissions in a laboratory scale biomass combustor is performed. Detailed analyse of the temperature in primary and post combustion zones, burning rate, and PM emissions in a fixed-bed laboratory scale combustor are undertaken. Two different secondary air distribution designs, uniform and nonuniform secondary air distribution system, for two different positions are assembled above the bed. The contribution of temperature, secondary air flow rates, configuration and distance from the bed level into burning rate, outflow heat, CO and PM concentrations are investigated. Uniform distribution of secondary air flow rate results in remarkable decrease in particulate matters (PM) and 50% CO emissions. The air staging strategies have been employed to study the effect of burning rate, temperature in primary and post combustion zones, and formation of NO, CO and PM emissions, taking into account the air to fuel stoichiometric ratio. Approximately 40% NO reduction is achieved due to application of non-uniform air distribution at higher secondary air flow rates.

Access Note

Access to this thesis - the full text is restricted to current ECU staff and students by author's request. Email request to library@ecu.edu.au

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