Monday, 24 October 2011

Smouldering and self-sustaining reactions in solids: an experimental approach

I am delighted to announce that the PhD thesis of my student Rory is now online:

Smouldering and self-sustaining reactions in solids: an experimental approach (clik on title for pdf)
by Rory Hadden, PhD, University of Edinburgh, 2011 

The Ostedijk on 21st February (the 5th day) after
the hold was opened and before specialized firefighting activities had
commenced. Derived from photograph courtesy of Agencia EFE.
Abstract: Smouldering combustion governs the burning of many materials in the built and natural environments. Smouldering is flameless, heterogeneous combustion which occurs when oxygen reacts with the surface of a solid fuel. Understanding the conditions which will result in the ignition and smouldering of a porous fuel is important and the phenomena involved are complex and coupled, involving heat and mass transfer, and chemical kinetics. This thesis reports experimental studies of the ignition, spread, suppression and emissions from reactions in porous media. Similar experimental techniques are shown in this thesis to be applicable when studying a wide range of solids which undergo self-sustaining reactions. This thesis is presented in a manuscript style. Each chapter takes the form of an independent paper which has been prepared for journal publication and as such, each chapter can stand on its own as a piece of research. A final chapter summarizes the findings and conclusions and suggests further areas of research.
The evolution of char and peat fractions throughout the
experiment and the mass loss rate. The fraction of peat decreases and
the fraction of char increases from the beginning of the experiment until
around 20 min. During this period, the pyrolysis front is propagating
through the sample, resulting in the formation of char from the thermal
decomposition of peat. After 20 min, the pyrolysis front has propagated
through the sample and the resulting char is undergoing oxidation to
form ash and gaseous products. Error bars represent the average error
from three repeats.

Chapter 1 presents a study of self-sustaining decomposition (SSD) of NPK ammonium-nitrate-containing inorganic fertilizer. Findings were applied to the events that occurred aboard the Ostedijk in 2007.

Chapter 2 is a study of smoulder in polyurethane foam to study the relationship between sample size, critical heat flux and spread rate. This is important becuase smouldering fires are the leading cause of residential fire deaths in developed countries and polyurethane foam is ubiquitous in the modern world.

Chapter 3 presents an experimental investigation into the ignition of porous fuels by hot particles. This is related to the problem of spotting ember ignition in wildland fires which is a major, but poorly understood, spread mechanism. The process of spotting occurs in wildland fires when fire-lofted embers or hot particles land downwind, leading to ignition of new, discrete fires.

Chapter 4 is an investigation into the suppression of smouldering coal. Subsurface coal fires are a significant global problem with fires in China alone estimated to consume up to 200 million tons of coal per year. As global demand for coal increases, accidental fires are a waste of a useful energy resource as well as a source of pollution and greenhouse gases. The results are the first attempt reported in the literature to study the suppression of these fires under controlled laboratory conditions.

The mass flux of CO (red) and CO2 (blue) for experiments
in which flaming was ignited using a pilot flame (solid) and where
only smoulder was observed (dashed). The shaded region represents
the duration of the flaming. The inset details differences in emissions
during the period of flaming.
Chapter 5 presents an experimental investigation of the smouldering behaviour of peat. This is of particular interest in understanding the impact of smouldering fires on the earth system. The longer burn durations and different combustion dynamics of smouldering compared to flaming means that they have been shown to consume large amounts of biomass in, and contribute significantly to the emissions from, natural fires occurring in peatlands. The dynamics of smouldering peat in shallow, strong fronts was studied in the Fire Propagation Apparatus and a smoulder reaction framework with two burning regimes is presented.

Chapter 6 complements Chapter 5 with an analysis of the CO and CO2 emissions for smouldering and flaming peat. This data can be used with large-scale measurement techniques to improve emission estimates. The emissions are found to be dependent of the burning regime and the type of combustion with flaming resulting in higher fluxes of CO2 and lower fluxes of CO compared to peat smouldering. The large majority of emissions (85% of CO2 and 97% of CO) are released during the smoulder phase of the reaction. This highlights the differences in the chemical processes occurring under these two modes of combustion.

Chapter 7 summarizes the research undertaken in this thesis and presents possible further work.