Fundamental study of smouldering combustion of peat in wildfires

Abstract

Smouldering combustion is the slow, low-temperature, flameless burning of porous fuels and the most persistent type of combustion, different from flaming combustion. Smouldering is the dominant phenomena in fires of coal and natural deposits of peat which are the largest and longest burning fires on Earth. These megafires fires contribute considerably to annual greenhouse gas emissions roughly equivalent to 15% of the man-made emissions, and result in the widespread destruction of global ecosystems and regional haze events. Moreover, the atmospheric release of ancient carbon in soil and the sensitivity of peat ignition to higher temperatures and drier conditions create a positive feedback mechanism to climate change.

Compared to flaming combustion, smouldering combustion can be initiated with a much weaker ignition source, and provide a hazard shortcut to flaming. Once ignited, the persistent smouldering fires can consume a huge amount of earth biomass, and burn for very long periods of time (days, years and centuries) despite extensive firefighting efforts or climate changes. For the past few decades, there have been some experimental studies on smouldering peat fires of different scales. However, very few computational work has been done to systematically study such emerging fire phenomena before the research undertaken in this thesis. This thesis is presented in a manuscript style: each chapter takes the form of an independent paper, which has been published or submitted to a journal publication. A final chapter summarizes the conclusions, and suggests potential areas of future research. Chapter 1 first proposes a comprehensive 5-step kinetic model based on thermogravimetric analysis (TGA) to describe the heterogeneous reactions in smouldering combustion of peat. The corresponding kinetic parameters are inversely modelled using genetic algorithm (GA). This 5-step (including drying) kinetic model successfully explains the TG data of four different peat soils from different geographical locations. The chemical validity of the scheme is also investigated by incorporating it into a one-dimensional (1-D) plug-flow model. The reaction and species distributions of two most common fire spread modes, lateral and in-depth spread, are successfully simulated. Chapter 2 presents a new comprehensive 1-D model of a reactive porous media to solve the conservation equations and the proposed 5-step heterogeneous chemical kinetics. This model is used to simulate several ignition experiments on bench-scale peat samples in the literature. The model first predicts the smouldering thresholds, relating to the critical moisture content (MC) and inert content (IC). The modelling results show a good agreement with experiments for a wide range of peat types and organic soils. The influences of the kinetic parameters, physical properties, and ignition protocol on initiating the peat fire are also investigated. Chapter 3 continues to optimize this 1-D model to investigate the vertical in-depth spread of smouldering fires into peat columns 20-30 cm deep with heterogeneous profiles of MC, IC and density. Modelling results reveal that smouldering combustion can spread over peat layers with a very high MC (~250%) if the layer is thin and located below a thick and drier layer. It is also found that the critical MC for extinction can be much higher than the previously reported critical MC for ignition. Furthermore, depths of burn (DOB) in peat fire is successfully predicted, and shows a good agreement with experiments on 18 field peat samples in the literature. Chapter 4 further looks into the kinetic schemes of different complexities to explain the TGA of two peat soils under various atmospheric oxygen concentration. Their best kinetic parameters are fast searched via Kissinger-genetic algorithm (K-GA) method, and the oxidation model is determined for the first time. Then, the kinetic model is applied into the 1-D model to simulate the peat experiment with fire propagation apparatus (FPA) in the literature. Try peat samples are used to minimize the influence of moisture, and ignited under both sub- and super-atmospheric oxygen concentration. Modelling results show a good agreement with experiment, and the stochastic sensitivity analysis is used to test the model sensitivity to multiple physicochemical properties. Chapter 5 investigates the interactions of atmospheric oxygen and fuel moisture in smouldering wildfires with the proposed 1-D model. Modelling results reveal a nonlinear correlation existing between the critical fuel moisture and atmospheric oxygen as MC increases, a greater increase in oxygen concentration is required for both ignition and fire spread. Smouldering fires on dry fuel can survive at a substantially lower oxygen concentration (~11%) than flaming fires, and fuel type and chemistry may play important roles especially in high MC. The predicted spread rate of smouldering peat fire is on the order of 1 mm/min, much slower than flaming fires. In addition, the rate of fire spread increases in an oxygen-richer atmosphere, while decreases over a wetter fuel. Chapter 6 presents an experimental study on smouldering fires spreading over bench-scale peat samples under various moisture and wind conditions. The periodic “overhang” phenomenon is observed where the smouldering fire spreads beneath the top surface, and the overhang thickness is found to increase with peat MC and the wind speed. Experimental results show that the lateral spread rate decreases with MC, while increases with the wind speed. As peat MC increases, the fire spread behaviour becomes less sensitive to the wind condition and its depth. A simple heat transfer analysis is proposed to explain the influence of moisture and wind on the spread rate profile, and suggests that the overhang phenomena is caused by the spread rate difference between the top and the lower peat layers. Chapter 7 summarizes the research of this thesis, and discuss the possible areas for future research.