Experiments and modelling into air staging in a fixed bed biomass combustor
Date of Award
Doctor of Philosophy
School of Engineering
Yasir M. Al-Abdeli
Combustion of biomass to produce heat and electricity has increased recently due to environmental concerns over the use of fossil fuels. Being CO2 neutral, biomass has the potential to replace solid fossil fuels, such as coal, in thermal power plants. Direct combustion of biomass is the most widely used thermochemical process to generate heat and electricity.
In staged biomass combustion, primary air is supplied beneath the fuel bed, with secondary air then provided above in the freeboard region. The freeboard can be divided into primary freeboard length (LI), which is upstream of secondary air, and a secondary freeboard length (LII), measured from the secondary air all the way to the exhaust port. Although much research has been completed on laboratory scale biomass combustion, there is a lack of systematic study into the impact of change in primary freeboard length on gaseous emissions, flow dynamics, burning rate and on temperature distributions in the freeboard region.
This research uses a combination of experimental and numerical techniques to investigate the effect of change in primary freeboard length and various stoichiometric conditions on gaseous emissions, flow dynamics, burning rate temperature distribution, radiation errors and flow dynamics temperatures in a batch-type fixed bed biomass combustor.
To date, data in the literature about combustion characteristics are mainly related to the steady state period. In the absence of a systemic method to ascertain the onset of steady state, experiments were carried out in this study to investigate the onset of steady state using both in-fuel bed (fuel bed temperatures, burning rate) and freeboard (freeboard temperatures and emissions) progress variables. Accordingly, a Matlab code was used to predict the steady state regime. A good correlation between stabilization of in-fuel bed and freeboard progress variables was found, which can be used to predict the onset of steady state in batch-type fixed configurations. Based on the methods developed, the radiation error in the thermocouple data was estimated. The amount of radiation error was found to be significant in both staged and non-staged operations of the combustor.
The effect of primary freeboard length on the combustion of wood pellets was investigated at a constant total and primary air stoichiometry. Three primary freeboard lengths of 200, 300 and 550 mm were investigated at a range of secondary to total air ratios (Qs/Qt). Results showed that at constant total air stoichiometry, primary freeboard lengths at 200 and 300 mm had better combustion characteristics than 550 mm, especially at higher values of Qs/Qt = 0.50 and 0.75.
Based on these results, primary freeboard lengths of 200 and 300 mm were further selected to conduct experiments at a constant primary air stoichiometry. A primary freeboard length of 200 mm was found to have higher temperatures and lower gaseous emissions (CO) than 300 mm at Qs/Qt = 0.71 and 0.75.
Non-reacting and reacting CFD simulations were performed to study the effect of Qs/Qt on velocity field and mixing characteristics. Freeboard temperature distributions and emissions were also examined in reacting simulations. The results for non-reacting simulations suggest that Qs/Qt of 0.50, 0.25 and 0.18 induce recirculation zones in the upstream and downstream of secondary air injection. The strength of recirculation zones and mixing characteristics had a positive correlation with Qs/Qt. Similar flow features of non-reacting cases were also observed in reacting simulation. However, the strength of the recirculation zone was far greater in reacting cases. A decline in freeboard temperatures was observed when Qs/Qt increased or decreased beyond 0.25. Furthermore, CO emissions reduced drastically when Qs/Qt increased from 0.06 to 0.71.
Results from the current work will help in the optimization of operating and design parameters (freeboard length and secondary air staging) of staged biomass combustors. Moreover, outcomes from this work provide valuable data to achieve a higher fuel conversion rate and lower emissions for small-scale domestic boilers as well as large-scale moving grate systems.
Access to this thesis is embargoed until 28th April 2025
Junejo, A. (2023). Experiments and modelling into air staging in a fixed bed biomass combustor. https://doi.org/10.25958/ybaa-e663