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Basic Chemical Hazards to Human Health and Safety — I
Published in Jack Daugherty, Assessment of Chemical Exposures, 2020
We discussed Dalton’s law and vapor pressures above. Obviously, the greater the vapor pressure of a liquid the more of it will evaporate in a given time period. This is important because liquids and solids do not burn, vapors do. What appears to be the log burning in your fireplace is actually the vapors generated by the superheated woody substance. This is an oversimplification, a complex series and amount of chemical reactions are taking place, which we summarize by labeling the whole process combustion. Charring begins with dehydration caused by radiant heat from a flame or hot surface. As steam is released, it also damages the partially charred material. Some hydrogen is generated in the process and burning begins at the surface of the material, which not shows visible signs of charring, causing structural changes within the material, in this case, the skin. The hydrogen that is being released, fatty material that vaporizes, and other combustibles are releasing more heat as they burn, and fracturing appears on the char, allowing deeper levels to participate in the process. Again, this is a gross oversimplification, but covers what we need to know in a nutshell.
Fire Hazards and Associated Terminology
Published in Asim Kumar Roy Choudhury, Flame Retardants for Textile Materials, 2020
Charring is a chemical process of incomplete combustion of certain solids when subjected to high heat. The resulting residue matter is called char. By the action of heat, charring removes hydrogen and oxygen from the solid, so that the remaining substance is composed primarily of carbon. Polymers such as thermoset, as well as most solid organic compounds such as wood or biological tissue, exhibit charring behavior (Chylek et al., 2015).
Thermally-based Modification Processes
Published in Dick Sandberg, Andreja Kutnar, Olov Karlsson, Dennis Jones, Wood Modification Technologies, 2021
Dick Sandberg, Andreja Kutnar, Olov Karlsson, Dennis Jones
The surface charring of wood is a process that bums and modifies the wood surface in a controlled manner. The charring process mimics the combustion of wood, but the char layer is an efficient insulator and a poor conductor of heat and it effectively inhibits heat transfer to the active pyrolysing zone, thus retarding further combustion.
Smouldering combustion in cellulose and hemicellulose mixtures: Examining the roles of density, fuel composition, oxygen concentration, and moisture content
Published in Combustion Theory and Modelling, 2022
W. Jayani Jayasuriya, Tejas Chandrashekhar Mulky, Kyle E. Niemeyer
Our findings show that the physical parameters of condensed-phase species control the observed variations in peak temperature, both as density and fuel composition change. Richter et al. [43] also discussed the larger role that physical properties play in wood charring, compared with reaction kinetics. Charring, which occurs through pyrolysis and heterogeneous oxidation, controls burning behaviour and relates to temperature profile (including peak temperature). Figures 5 and 7 also show that the location of peak temperature does not shift significantly even as its value increases. In Figures 5, the shift is 0.6% and 2.3% for 2 and 3 cm profiles, and in Figures 7, the shift is 3.1% and 2.3% for 2 and 3 cm profiles, respectively. This indicates that the change in peak temperature does not notably affect propagation speed.
Application of Levenberg–Marquardt Method for Estimation of the Thermophysical Properties and Thermal Boundary Conditions of Decomposing Materials
Published in Heat Transfer Engineering, 2020
The direct heat conduction problem is defined as determining the consequence (temperature distribution in the medium) from the given cause (i.e. surface heat flux, thermal conductivity, or specific heat), and it is categorized as a well-posed problem. Charring ablative materials are characterized by the irreversible decomposition reactions occurring in-depth of the composite which leads to the chemical conversion of virgin (plastic) material into char. CMA [44] is one of the most comprehensive tools for solving the direct heat conduction problem, and it is used to both design and analysis of the charring materials that are selected for the current study. As shown in Figure 1, the physical problem consists of a planar slab of thickness L initially at the temperature T0. y-coordinate system is fixed on the initial location of ablative surface while x-coordinate system is attached to receding surface. The front surface of specimen is exposed to a transient heat flux of and back surface has been assumed to be adiabatic. Charring materials (e.g. carbon-phenolic composite) discharges energy through both self-immolation (surface recession) and decomposition by which virgin composite chemically converts into char. Decomposition causes the losing mass and generating pyrolysis gas. The gas mass accumulation within the medium leads to gas transporting toward the front surface. Due to the connection of back surface to the specimen holder, this surface is assumed to be impermeable. Governing equations consist of conservation of solid material mass and mixture energy, as following [45]: where where , , , and represent the specific heat, enthalpies of the char, and virgin, char density, and virgin density, respectively. is the specific heat of composite, is the density of the solid, is the thermal conductivity of solid, is the mass flux of the pyrolysis gas, is time, T is the temperature, is the surface recession rate, and is the enthalpy of the pyrolysis gas. The second term on the right-hand side of the equation represents the consumption rate of the chemical energy. In addition, the third and fourth terms represent the convection of energy caused by the transmission of pyrolysis gases and fifth term indicates the convection of energy caused by the moving coordinate system.