Combustion smoldering

S. D. Tse, R. A. Anthenien, A. C. Fer-nandez-Pello 1999, An application on ultrasonic tomographic imaging to study smoldering combustion. Combustion and Flame 116 (1-2), 120-135. 

The conditions used for char preparation in the present chemisorption studies (i.e., progressive slow charring of wood) are intended to be relevant to "real life" smoldering combustion situations. Most previous studies of chemisorption have used chars from cellulose (i.e., avoiding hemicellulose and lignin... 

Lignocellulosics reach temperatures around 80 C in many ways besides. For example, planer shavings and peat insulation around hot pipes or in walls of dry kilns easily attain 100 C, and forest product industries stack still hot wood-base panels at temperatures around 80 C. In many cases the temperatures of the hot materials later on rose above the initial 80 or 100"C, first to levels of smoldering combustion, and finally to those of open flames. In air-exposed and ventilated materials oxidation could cause the heating above 80 or 100 C, but inside tight packs of panels pyrolysis must have been the heat source. 

Borates are widely used in fire retardant applications. For example, cellulose insulation products used in homes and cotton batting used in mattresses and other furnishings are typically treated with boric acid to inhibit smoldering combustion. Borates are also used as fire retardants or fire retardant synergists in plastics, rubber products, and paints, where specialized borates such as zinc borate may be used. 

Yokelson, R. J., R. Susott, D. E. Ward, J. Reardon, and D. W. T. Griffith, Emissions from Smoldering Combustion of Biomass Measured by Open-Path Fourier Transform Infrared Spectroscopy, . /. Geophys. Res., 102, 18865-18877 (1997). 

Ohlemiller, T.J., Modeling of smoldering combustion propagation, Progress in Energy and Combustion Science, 11, 277-310, 1986. 

Rein, G., Lautenberger, C., Fernandez-Pello, A.C., Torero, J.L., and Urban, D.L., Smoldering combustion of polyurethane foam Using genetic algorithms to derive its kinetics, Combustion and Flame, 146 (1-2), 95-108, 2006. 

Borax pentahydrate is an effective flame retardant for wood/cellulosic materials in terms of surface flammability. However, due to the Na20 moiety, it can promote smoldering combustion in cellulose. Thus, in cellulosic material and wood products, it is commonly used in combination with boric acid, which is an effective smoldering inhibitor. For example, the treatment of wood fibers with a partially dissolved boric acid and borax pentahydrate slurry (-1.75% by wt. of boron) results in Medium Density Fiberboard (MDF) that is claimed to pass the ASTM E-84 Class 1 surface flammability standard.12 The additional examples of using borax pentahydrate and boric acid combination are presented in Section 9.2.2.1. 

Figure 1. Graphic presentation of the flaming and smoldering combustion showing the respective roles of combustible volatiles and active char produced by pyrolysis under heat flux at different conditions.

The surface areas of chars prepared from cellulose samples at different HTTs were determined by application of the Dubinin-Po-lany equation to CO2 adsorption at room temperature and compared with the area occupied by surface oxides calculated from oxygen chemisorption at 230 C. The results shown in Figure 25 indicate that cellulosic chars have large surface areas that vary according to the HTT, and peak at about 550 °C. The surface oxides formed by chemisorption occupy only a portion of the total surface area, and the chemisorption also shows a peak for chars formed at about 550 °C, corresponding to the temperature of smoldering combustion. 

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