Ignition temperatures of dust layers and bulk storages in hot environments

In many industrial installations, particulate solids (cereals, agri-food products, coal, plants, etc.) are stored or processed. Self-heating of these products, which can lead to fires and explosions, can occur in a variety of situations. Examples include large storage at room temperature, formation of a layer on a hot surface, layer deposited on a surface – insulating or conductive – in a hot environment or even storage of product exposed to heating on one side.The main parameters that determine the occurrence of self-heating are the size of the container, the temperature, the residence time and the characteristics of the product. Depending on the type of situation encountered and these implementation conditions, the analysis of self-heating risks must be based on specific models and/or parameters.This paper presents the different variants and combinations of the theoretical model from the theory of thermal runaway to represent self-heating, taking into account in particular the symmetry or asymmetry of heating, reagent consumption and boundary conditions. It also discusses their adaptation to the previous identified industrial situations.Nine products were chosen to be representative of those used in the different considered industrial situations. They were subjected to self-heating basket tests in isothermal ovens in order to determine the parameters for applying the described theoretical models. These results were compared with the results of self-heating tests in layers of different thicknesses in a hot environment, on an insulating or conductive plate, using a specially developed test protocol, as well as with the results of standardized tests of minimum ignition temperature in 5 mm layers.This led to the proposal of the most appropriate theoretical model to represent the self-heating phenomenon for each of the four identified industrial situations.This analysis can promote better design of industrial equipment and production conditions (temperatures, volumes or product flows …) in order to prevent fires and explosions.     

Investigation of the propensity of sulfide concentrates to spontaneous combustion in storage

Spontaneous combustion of sulfide concentrates can cause many serious problems during storage. In this study, two representative samples (sulfur-rich sulfide concentrate and iron-rich sulfide concentrate) were investigated for the self-heating behavior using the crossing-point temperature (CPT) method. The corresponding apparent activation energies were determined, 13.7366 kJ/mol for iron-rich sulfide concentrate and 21.3817 kJ/mol for sulfur-rich sulfide concentrate. Furthermore, the surface temperatures of sulfide concentrate stockpiles in different seasons were also measured by a thermal image apparatus. It was found the temperature for iron-rich sulfide concentrate stockpile was higher than that for rich-sulfur sulfide concentrate stockpile. These research results show iron-rich sulfide concentrate exhibits stronger propensity to spontaneous combustion than sulfur-rich sulfide concentrate. Particular attention should be paid to the fire hazard of iron-rich sulfide concentrate in daily safety check work. The combination of the CPT method with the infrared thermal image measurement provides a new approach for the risk assessment on the propensity of sulfide concentrates to spontaneous combustion during the production, storage and transportation. The useful data and experience achieved in this investigation can be also referenced in similar cases later.     

The influence of oxygen concentration on the composition of gaseous products occurring during the self-heating of coal and wood sawdust

This article deals with an assessment of the influence of oxygen concentration on the composition and amount of combustion products generated in the course of heating coal particles and wood sawdust at 150 °C. This was done both with normal air and at 15% oxygen in the air in an isothermal furnace. The generated gases were analyzed by a Fourier Transform infrared spectrometer. Results show that under both conditions, the same substances are formed: water, carbon dioxide, carbon monoxide and aliphatic hydrocarbons. However, the quantities changed. At 21% oxygen, the concentrations of carbon monoxide and methane were higher than at 15% oxygen both in coal and wood. The oxygen concentration was also found to affect the rates of release of CO and CO₂. The rate of release of CO was higher at 21% oxygen, but that of CO₂ was higher at 15%, indicating two different mechanisms. In all cases, the concentrations of these gases were higher for coal than for wood. The results have implications for the specification of safe conditions of storage of coal and wood substances and the selection of safety measures.     

On the understanding and control of the spontaneous heating of dried tannery wastewater sludge

We studied the spontaneous heating of dried sludge produced by treating wastewater mainly originating from tanneries. Heating up to burning has been observed in the presence of air and moisture, starting at ambient temperature. To understand and prevent the process we combined chemical and morphological analyses (ESEM) with thermal activity monitoring in insulated vessels. Selective additions of chemicals, either to amplify or depress the reactivity, have been used to investigate and identify both the chemical mechanism causing the sludge self-heating, and a prevention or a mitigation strategy. FeS additions accelerate the onset of reactivity, while S sustains it over time. On the contrary, Ca(OH)₂, Na₂CO₃, NaHCO₃, FeCl₂, EDTA, NaClO can limit, up to completely preventing, the exothermic activity. All the experimental evidences show that the reactions supporting the dried sludge self-heating involve the Fe/S/O system. The total suppression of the reactivity requires amounts of additives that are industrially incompatible with waste reduction and economics. The best prevention requires reduction or removal of S and Fe from the dried solid matrix.     

Prediction of the self-ignition temperature in lagging fires by means of isothermal calorimetry

Under certain circumstances, contamination of a porous insulation material by a combustible liquid may result in a lagging/insulation fire. In the current study, a method based on isothermal calorimetry and modelling to estimate the risk of a lagging fire, or a maximum insulation thickness for a certain system temperature, is presented. The studied system was a combination of mineral wool and rapeseed oil. Full-scale tests were performed to determine suitable ignition criteria and to validate the results from the isothermal calorimetry tests and modelling. We contaminated the lagging using two methods – a direct method and a solvent method. These methods were evaluated in the full-scale tests. The solvent method resulted in more repeatable results than the direct method, where the contaminant was poured on the insulation. Using the calorimetric measurements, we estimated the parameters for the kinetic equation. This result was used to estimate the self-ignition temperature of contaminated lagging installed on a pipe. We found that a temperature increase of 40 °C was a reasonable ignition criterion when modelling.