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Title: The smouldering of peat
Author: Scott, Kathleen
ISNI:       0000 0004 2737 538X
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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A model examining underground smouldering peat combustion is presented. A one-step chemical reaction is considered where the gas and solid are assumed to be in thermal equilibrium. The full model allows porosity, permeability and gas density to vary and considers a buoyant velocity field determined by Darcy's law. Due to the low bulk thermal conductivity of peat, the diffusion of oxygen through it is characterised by a Lewis number much less than one. This results in thermal-diffusive instabilities. These instabilities can cause flame balls to arise in gaseous combustion and a fingering regime to arise in solid combustion. Analytical solutions to simplified spherically symmetrical equations are derived. These equations assume diffusion to be the dominant transport mechanism as well as taking that the porosity, gas molecular weight and gas density all remain constant. The underlying structure of the combustion region is found to be analogous to that of a flame ball. When studied in cylindrical symmetry a single, stable finger can be modelled propagating against an imposed air flow. The effects of heat losses, velocity magnitude and the Lewis number can be studied and results are compared to existing experimental smouldering combustion data. Although no detailed experiments have studied this phenomenon in peat, predicted results capture key qualitative trends found in both filtration combustion of polyurethane foam and in the fingering combustion of paper. In addition to this, when the imposed air flow is reduced to zero a propagating combustion front is predicted, analogous to a self-travelling flame ball. When the velocity field is determined by Darcy's law the dimensionless permeability of the peat plays a key role in determining the range of values over which fingering combustion can occur. Whilst there is little impact of taking the gas molecular weight to be constant, when porosity is allowed to vary and a relationship between porosity and permeability is included an over-blowing extinction limit is identified. This limit is not found in the constant-porosity model where a low-fuel extinction limit is predicted. Peats of differing ages and locations can possess significantly different characteristics. However, the fingering regime is predicted to occur within the range of parameters in which peat soils lie. Experiments suggest that fingering combustion can take the form of both sparse fingers and a complex fingering regime. The cylindrically symmetrical model can not capture tip-splitting. Hence the model does not explicitly account for the distance between two neighbouring fingers. However, an estimate for this value can be made if peat smouldering were to occur in a regime of multiple fingering. An averaged continuum model describing the spread of an ember storm is also presented. The dominant mechanism determining the spread-rate of the fire is the lofting and landing of embers and individual fires are taken to grow in an elliptical manner under the influence of the wind. When an ember storm is spreading at a steady speed, its spread rate is found to be described by a single similarity solution.
Supervisor: Daou, Joel Sponsor: Peak District National Park ; University of Manchester School of Mathematics
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Combustion ; Smouldering ; Lewis Number ; Flame Ball ; Instability ; Peat