The Smouldering of Peat

Abstract

A model examining underground smouldering peat combustion is presented. Aone-step chemical reaction is considered where the gas and solid are assumed to be inthermal equilibrium. The full model allows porosity, permeability and gas density tovary and considers a buoyant velocity field determined by Darcy's law. Due to the lowbulk thermal conductivity of peat, the diffusion of oxygen through it is characterisedby 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 fingeringregime 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 astaking 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 ofa flame ball.When studied in cylindrical symmetry a single, stable finger can be modelled prop-agating against an imposed air flow. The effects of heat losses, velocity magnitude andthe Lewis number can be studied and results are compared to existing experimentalsmouldering combustion data. Although no detailed experiments have studied thisphenomenon in peat, predicted results capture key qualitative trends found in bothfiltration combustion of polyurethane foam and in the fingering combustion of pa-per. In addition to this, when the imposed air flow is reduced to zero a propagatingcombustion front is predicted, analogous to a self-travelling flame ball.When the velocity field is determined by Darcy's law the dimensionless permeabilityof the peat plays a key role in determining the range of values over which fingeringcombustion can occur. Whilst there is little impact of taking the gas molecular weightto be constant, when porosity is allowed to vary and a relationship between porosityand permeability is included an over-blowing extinction limit is identified. This limit isnot 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 inwhich peat soils lie.Experiments suggest that fingering combustion can take the form of both sparsefingers and a complex fingering regime. The cylindrically symmetrical model can notcapture tip-splitting. Hence the model does not explicitly account for the distancebetween two neighbouring fingers. However, an estimate for this value can be made ifpeat smouldering were to occur in a regime of multiple fingering.An averaged continuum model describing the spread of an ember storm is alsopresented. The dominant mechanism determining the spread-rate of the fire is thelofting and landing of embers and individual fires are taken to grow in an ellipticalmanner under the influence of the wind. When an ember storm is spreading at asteady speed, its spread rate is found to be described by a single similarity solution.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joel Daou (Supervisor)