Water up-take in fuel pellets studied by Dynamic Vapour Sorption (DVS) analysis and its potential role in self-heating during storage

Abstract

The self-heating of wood fuel pellets is a well-recognised problem causing fire incidents in the storage of the pellets as well as severe intoxication of workers by elevated carbon monoxide and carbon dioxide levels and oxygen depletion. Possible factors contributing to the self-heating are considered to be autoxidation and microbiological activity, while the role and contribution to the temperature rise caused by the heat of condensation from water vapour condensing during fast changes in the relative air humidity is less investigated. Using Dynamic Vapour Sorption, the water uptake was measured at 25 °C when increasing the RH from 40 to 80% using 35 fuel pellet samples covering a broad variation in pellet raw materials and process equipment found in Europe (both pilot and industrial scale). The equilibrium total water uptake and speed of the uptake were determined. Total water uptake was 4.56% (range 3.69-6.86%) with no systematic difference found related to the scale of production (industry as compared to pilot plant). In addition, the variations within larger groups of raw material (pine, spruce and pine/spruce mixtures) were relatively small, and the mean water uptake did not differ significantly between these groups. An estimation of the overall potential heat release (when raising the RH% from 40 to 80%) made from the experimental results, taking the early fast water uptake process into consideration (2 h counting for half the total uptake), showed that a heat release of 47 kJ/kg of pellets (range 12-63 kJ/kg) and a potential temperature increase of 45 °C is possible. This estimation clearly demonstrates that the heat of condensation released during water condensation in a pellets silo or in a pellets pile should be expected to be a major contributing factor to initiating temperature rise incidents. In addition, such a temperature increase is expected to assist the initiation of, and to increase the speed of autoxidation of fatty acids and other compounds in the material that will further contribute to a temperature rise. Thus, the results in this study have the potential to improve the basis for modelling the self-heating process in pellet silos/storage and to predict the status of a certain pellet batch by presenting a broad basis for expected variation in the important parameters (specific heat capacity C P and thermal conductivity λ eff) influencing the process, and thus aid in taking preventive actions like venting or shifting the pellets to another silo/pile to reduce risk for self-heating and possible fire.

Review on Self-Heating of Biomass Materials: Understanding and Description

“…It was estimated that the heat from an increase in the moisture content of lignocellulosic material from 5% to 9.5% at a relative humidity of 70% at 20 °C could adiabatically raise the material temperature up to 100 °C, sufficient to induce the carbonization and consequent autoignition . A similar estimation based on measured water uptake of fuel pellets showed that as the relative humidity increased from 40% to 80% at 25 °C, the heat evolution of the first 2 h of water uptake potentially raises the temperature by 45 °C on average and that of the equilibrium water uptake may cause a rise of 87 °C on average . Considering the simultaneous increase of the material temperature during adsorption and thereupon the increase of equilibrium vapor pressure, a theoretical calculation indicated that, however, the water adsorption only possibly raises the temperature to 30–40 °C and just close to 50 °C if the initial material is nearly bone dry .…”

“…Heat from physical processes associated with water is released mainly by adsorption and condensation of water vapor from a humid atmosphere, ,, by wetting due to addition of liquid water, or both . It is occasionally observed in the field that a sudden rise in ambient humidity or a rainfall event causes a sharp increase in the pile temperature and even spontaneous combustion. ,, This leads to the suggestion that the heat of wetting may act as a trigger mechanism to increase the pile temperature above a point where other exothermic reactions, especially chemical oxidation, take place and accelerate , and even raise the temperature directly to spontaneous ignition. , However, the reverse processes, that is, water evaporation and desorption, are strongly endothermic. These processes, particularly self-heating driven water vaporization and transport, cause vapor release and heat loss from the pile and therefore have drying and cooling effects .…”

“…The heat of water sorption (exothermic adsorption or endothermic desorption) is the sum of the latent heat of phase change (condensation or vaporization) and bonding energy between water molecules and the substrate, as shown in Figure . The latter is called the differential or net heat of sorption (DHS); its integral with respect to the moisture content is called the heat of wetting. , Therefore, the adsorption and condensation of water vapor especially on relatively dry material such as wood pellets can release a large quantity of heat. ,, …”

“…31 It is occasionally observed in the field that a sudden rise in ambient humidity or a rainfall event causes a sharp increase in the pile temperature and even spontaneous combustion. 21,31,34 This leads to the suggestion that the heat of wetting may act as a trigger mechanism to increase the pile temperature above a point where other exothermic reactions, especially chemical oxidation, take place and accelerate 30,73 and even raise the temperature directly to spontaneous ignition. 31,72 However, the reverse processes, that is, water evaporation and desorption, are strongly endothermic.…”

“…76,77 Therefore, the adsorption and condensation of water vapor especially on relatively dry material such as wood pellets can release a large quantity of heat. 30,72,73 A few works evaluated the potential contribution to selfheating from the heat of moisture adsorption due to the change in relative humidity. It was estimated that the heat from an increase in the moisture content of lignocellulosic material from 5% to 9.5% at a relative humidity of 70% at 20 °C could adiabatically raise the material temperature up to 100 °C, sufficient to induce the carbonization and consequent autoignition.…”