Initiation of smouldering combustion in biomass

Abstract

Wildfires are naturally occurring phenomena that result in significant and catastrophic damage. Due to climate change, there has been a significant increase in the frequency, severity, and extent of wildfires. Therefore, there is a growing need to mitigate wildfire risk. In order to help mitigate the risk of wildfires, greater understanding is required. One particular gap in knowledge is the impact of smouldering combustion of potential fuel on wildfires. This thesis focuses on combustion of fuel beds in wildfires. Specifically, the thesis targets smouldering combustion. Smouldering combustion is a common type of combustion regime in wildfires and hazard reduction burning (a wildfire mitigation measure). Smouldering is a slow and low-temperature form of combustion, which shows no flame. Smouldering is a serious hazard because of its low ignition temperature, which makes it particularly relevant to fire initiation and spread. Smouldering plays a vital role in wildfires, as many forest biomass fuels such as grass, leaves and coarse woody debris are prone to smoulder. Most previous studies of smouldering combustion have only been carried out on polyurethane foam, due to its importance for residential fires. However, smouldering has been scarcely investigated from the point of view of wildfires. For example, smouldering combustion of forest fuel is scarcely studied. Hence, the project aims to develop a greater understanding of the initiation of smouldering combustion in biomass under different conditions with an emphasis on wildfire. Locating smouldering combustion in wildfires and hazard reduction burning is difficult and time-consuming, as there is no effective method to identify the initiation of smouldering combustion in biomass fuel beds. It is critical to know when and where smouldering combustion in a biomass fuel bed starts, as smouldering combustion could transition to flaming combustion under certain conditions. Radiation is one of the important heat transfer mechanisms in wildfires; however, there are few studies on smouldering combustion in biomass fuel beds started by external radiant heat flux. Although oxidiser flow rate and oxygen concentration have significant in influences on the propagation of smouldering front, their effects on the initiation of smouldering combustion in biomass fuels are not well understood. Hence, the effects of oxidiser flow rate and oxygen concentration on the initiation of smouldering combustion are investigated. Fuels in a forest are diverse, and it is essential to have a better understanding of what effects forest fuels have on smouldering combustion. Thus, the effects of plant species and plant parts on the initiation of smouldering in biomass fuel beds are also investigated. Within this framework, the work presented in this thesis can be split into two main topics: 1. Conditions required to initiate smouldering combustion in bio- mass fuel beds The required radiant heat flux and air flow rate for the initiation of smouldering and flaming combustion in a biomass fuel bed are investigated in an experimental testing rig. This investigation identifies and quantifies smouldering and flaming combustion in a biomass fuel bed based on the measurements of temperature, product gas concentration and mass change, and the required radiant heat flux and air flow rate for the initiation of smouldering and flaming combustion are determined. The effects of heating time and oxygen concentration on the initiation of radiation-aided and self-sustained smouldering combustion are investigated in the same testing rig. In this experimental study, the differences between radiation-aided and self-sustained smouldering combustion are characterised based on the measurements of temperature, product gas concentration and mass change, and the required heating time and oxygen concentration for radiation-aided and self-sustained smouldering combustion are determined. 2. Factors that influence smouldering combustion in biomass fuel beds The results from the first topic reveal that oxygen availability has significant effects on the initiation of smouldering combustion in a biomass fuel bed. The air permeability of a biomass fuel bed determines oxygen availability in that fuel bed. Hence, the air permeability of natural forest fuel beds is investigated in an air permeability testing rig. In this study, the air permeability of natural forest fuel beds is determined using experimental and theoretical methods. A comparison between the experimental and theoretical methods is made. The effects of Euca- lyptus species and plant parts on smouldering combustion are also investigated. In this study, the different plant parts from different Eucalyptus species are characterised based on the results of the thermogravimetric and ultimate analyses. The results of this study show that the differences among the different plant parts from different Eucalyptus can be characterised and quantified based on the results of the thermogravimetric and ultimate analyses. It is also found that Eucalyptus species and plant parts have significant effects on smouldering combustion. Although this thesis covers a series of experimental studies of the initiation of smouldering combustion in biomass fuel beds. There are still many important factors to be considered. For examples, the thesis focuses on small-scale laboratory experiments to better understand the fundamental studies of smouldering combustion of biomass. However, the real-world conditions could be much more complex. For example, forest fuel beds are composed with fuel particles with various sizes and shapes. These factors also have effects on smouldering combustion.