Analysis of Spontaneous Ignition of Grass: Chemical Oxidation and Water Vapor Sorption

Self-heating of biomass by chemical oxidation, which can cause spontaneous ignition, is a safety and management concern. This process can be accelerated by aerobic fermentation and water vapor sorption. The chemical oxidation and water vapor sorption of grass were studied in a laboratory oven, measuring the variations in weight and the internal temperature of a sphere with grass within a flexible polymeric network. Both processes were simulated to prove that the proposed mathematical model could fit the experimental data. It was observed that the water vapor sorption capacity of the grass was high, so the experimental increase in the internal temperature of a spherical body was around 47 K, from 73°C to 120°C. This fact can be very important because the chemical oxidation of grass accelerates at high temperatures. For scaling, simulation programs were used to study the sorption and oxidation processes with an increase in internal temperature in spherical bodies and infinite plane slabs. These results can be used to obtain those of other geometric symmetries by interpolation. It was deduced that at 70°C and with vapor sorption, the ignition time can be around 3 days to 5 days, while without vapor sorption, the ignition times can be around 110 days to 140 days. For 35°C the ignition times with vapor sorption can be around 12 days to 18 days, while without vapor sorption the ignition times can be around 3700 days to 4500 days. These results can be of interest for warehouses of similar biomass and for forestry research and management groups of wildfires. Graphical Abstract

Analysis of Flame-Retardant Performance Based on Wood Density and Flame-Retardant Treatment Method

For this study, four types of wood with different densities (spruce and low-, medium-, and high-density fiberboards) were selected from wood widely used as materials in real life, particularly in architectural interiors. For each wood type, flame-retardant paints (water- and oil-based) and flame-retardant liquids were applied two to five times using three flame-retardant treatment methods (roller, brush, and spray) to test the flame-retardant performance. For the four types of wood specimens with different densities, their flame-retardant performances and ignition times were compared and analyzed according to the applied flame-retardant paint (water- and oil-based) and liquid, treatment method, and frequency at which a specific paint was applied. These results can be used as empirical data on changes in flame-retardant performance based on density of wood and flame-retardant treatment method.

Experimental Study of Oriented Strand Board Ignition by Radiant Heat Fluxes

Wood and composite panel materials represent a substantial part of the fuel in many building fires. The ability of materials to ignite when heated at elevated temperatures depends on many factors, such as the thermal properties of materials, the ignition temperature, critical heat flux and the environment. Oriented strand board (OSB) without any surface treatment in thicknesses of 12, 15 and 18 mm were used as experimental samples. The samples were gradually exposed to a heat flux of 43 to 50 kW.m−2, with an increase of 1 kW.m−2. At heat fluxes of 49 kW.m−2 and 50 kW.m−2, the ignition times are similar in all OSB thicknesses, in contrast to the ignition times at lower heat fluxes. The influence of the selected factors (thickness and distance from the heat source) was analysed based on the experimentally obtained data of ignition time and weight loss. The experimentally determined value of the heat flux density was 43 kW.m−2, which represented the critical heat flux. The results show a statistically significant effect of OSB thickness on ignition time.

Combustion of torrefied pellets of furniture work dusts as blends with lignite

In this study, torrefaction of pellets formed from furniture work dusts collected as a part of space cleaning actions in the industry have been examined. Burning behaviour of torrefied dusts and lignite blends was studied. Torrefaction experiments were done under nitrogen atmosphere for one hour at temperatures of 220, 260 and 300°C which corresponded to light, mild and severe torrefaction, respectively. Combustion of blend pellets prepared by adding lignite in specific ratios to powders of torrefied pellets was carried out at 700°C initial temperature using a vertical furnace system through which air was flowing in natural convection. Ignition times of blend pellets were affected from volatile matter and moisture contents. Volatile matter combustion rates were lower than those of raw waste and accordingly, combustion times were higher. However, no relation between volatile matter combustion rates and times was observed. Blending raw or torrefied furniture work dusts with lignite have significantly influenced volatile matter and carbon combustion periods. There was no relation between carbon combustion rates and times. It was concluded that blend pellets of mild or severe torrefaction products of furniture work dusts and lignite behaved similarly to lignite during combustion.

The effects of oscillating boundary conditions on thermal ignition

We investigate the effect that oscillating ambient temperatures have on the ignition times of supercritical stockpiles. Large stockpiles are exposed to seasonal and diurnal temperature variation. We analyse the effects of seasonal temperature variation. When considering ignition within a year of construction, stockpiles built in spring ignited with a lower critical parameter than those built at other times. Consequently, seasonal temperature variation needs to be accounted for when predicting stockpile ignition times. References P. C. Bowes. Self-heating: evaluating and controlling the hazards. Dept. of the Environment, Building Research Establishment, 1984. R. J. Longbottom, B. J. Monaghan, G. Zhang, D. J. Pinson, and S. J. Chew. Self-sintering of BOS filter cake for improved recyclability. ISIJ Int., 59(3):432–441, 2019. doi:10.2355/isijinternational.ISIJINT-2018-627. V. Novozhilov. Thermal explosion in oscillating ambient conditions. Sci. Rep., 6:29730, 07 2016. doi:10.1038/srep29730. N. C. Roy. Convection characteristics in a closed vessel in the presence of exothermic combustion and ambient temperature oscillations. Int. J. Heat Mass Trans., 116:655–666, 2018. doi:10.1016/j.ijheatmasstransfer.2017.09.058.

Analytical Modeling of Solid Material Ignition Under a Radiant Heat Flux Coming From a Spreading Fire Front

This study aims at characterizing ignition of solid targets exposed to spreading fire fronts. In order to model radiant heat fluxes on targets in a realistic way, polynomial heat fluxes are chosen. Analytical solutions for the solid surface temperature evolution regarding different time-varying heat fluxes are discussed for high thermal inertia solids using a mathematical formalism, which allows for the methodology to be extended to the case of low thermal inertia. This formulation also allows calculation of ignition times for more realistic time-dependent fluxes than previous studies on the topic, providing a more general solution to the problem of solid material ignition. Polynomial coefficients are then obtained fitting heat flux coming from absorbing–emitting flames. A characterization of solid material ignition times regarding fire front rate of spread (ROS) is finally performed, showing the need to accurately model heat flux variations in ignition time calculations.