# Haystack Fires in Australia: Causes and Considerations for Preventative Management
## 4 Phases of Biochemical Spontaneous Combustion
### Phase 2  
The anaerobic decomposition of carbohydrates and anaerobic enzymatic decomposition of proteins and amino acids is initiated during this phase (Figure 5). Various plant materials have good insulating properties due to minimum heat loss. Thus, the temperature rises to 60-90 \({}^{\circ}\)C, which results in heat accumulation. Due to the increased temperature, plants lose moisture rapidly which leads to the pectin decomposition and the subsequent formation of formic acid (CH\({}_{2}\)O\({}_{2}\)), ammonia (NH\({}_{3}\)) and acetic acid (CH\({}_{3}\)COOH) and results in the formation of a foul odour [6]. Animal manure, hay, straw and damp grain are examples of typical plant materials that generally heat up to temperatures of 40-90 \({}^{\circ}\)C and above. Further decomposition of proteins takes place at temperatures between 90 and 100 \({}^{\circ}\)C. Hydrogen sulphide (H\({}_{2}\)S) and Furfurral (C\({}_{5}\)H\({}_{4}\)O\({}_{2}\)) are formed later with the result that plant materials desicate and agglomerate. At this stage, the intensity of chemical reactions increases; however, microbial activity decreases. At this elevated temperature (approximately 100 \({}^{\circ}\)C), microbial development is ceased due to death [47]. Water completely evaporates from the plant mass and the temperature continues to increase rapidly until the plant material becomes desiccated at a temperature of approximately 150 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c} \hline
**Temperature (\({}^{\circ}\)C)** & **Alterations Associated** \\ \hline
46–52 & Moulds and odours develop and decrease palatability when coupled with high moisture. \\ \(>\)49 & Reduced digestibility of protein, fibre and carbohydrate compounds. \\
54–60 & Hay is brown and very palatable because of the caramelization of \\  & sugars; unfortunately, nutritional value is reduced. \\ \(>\)66 & Hay may turn black and spontaneous combustion is possible. \\ \hline \end{tabular}
\end{table} 
Table 3: Possible alterations of hay associated with temperature [46]. This stage may be regarded as the start of the dry distillation of the plant material. Dry distillation may be defined as the decomposition process of lignified plant materials with exposure to high temperature conditions having limited or no access to air [48]. Throughout this process, various compounds and gases (including methane, carbon dioxide, carbon monoxide, ethylene, acetylene, etc.) are formed and released that participate later in various complex reactions (Figure 5). Several factors influence the gas formation process including the critical temperature at which the decomposition takes place and the internal pressure of the stored material (bale or stack) [6]. The rate and intensity of the desiccation and decomposition process depends on the moisture content of the stored plant material while the effect of temperature mainly depends on the thickness and size of the stored plant material. Considering the volume of the stored plant material and poor thermal conductivity, heat accumulates inside the stored plant stack due to poor dissipation. This results in a rapid temperature increase to 180 \({}^{\circ}\)C and above as the decomposition process becomes more intensive and faster. Additionally, the moisture content of the stored plant material influences the thermal process. Further decomposition of the plant material leads the temperature rise to 230-260 \({}^{\circ}\)C, which causes an increase in vapour pressure in the stack. Moreover, the accumulated heat inside the stack gets trapped due to low or minimal cooling and heat dissipation. The combustion of the generated gas take place at 250-260 \({}^{\circ}\)C with the formation of pyrophoric carbon, which has a great capacity to absorb oxygen from air [6]. Thus, the temperature increases further, and the carbon starts to smoulder which leads to the burning of fire channels toward the frontal, lateral or upper surface of the stack. The external surface of the stack sets fire after the burning of the fire channels.