Charring during combustion

They find that the rate of attrition is proportional to the energy dissipation rate in the bed, which is proportional to uQ -i f, and that it is much higher during oxidation than pyrolysis. They postulate that asperities are produced on the surface of the char during combustion and that these are abraded by collisions with the bed solids. The rate of the carbon surface regression due to attrition is approximately given by... 

Some of the plastics yield a huge amount of carbonaceous residue (char) during combustion while others are not at all carbonized. Hilado explained this difference by the interrelation between the rates of gas evolution and oxygen diffusion. 

AlOOH and AI2O3 have a catalytic effect to form a char during combustion (black color for the residues). The char formation was previously reported when other metallic oxides [30, 31] and other nanofillers such as organomodified clay were used [32]. 

Table VI. Residual Char Yields and Extents of Elemental Volatilisation from Brominated Polyester Resin Samples during Combustion in Air... 

FIGURE 9.21 Sample oxygen profiles through the char particle boundary layer and the char particle itself during combustion proceeding according to the characteristic burning zones. 

Monomers such as Structures 5.16 and 5.17 copolymerize, free radically, as readily with styrene as they do with MMA and give rise to products that are also significantly flame retarded. Flame retardance here too involves both the vapor- and condensed-phase action, with the condensed-phase activity leading to a significant char. In this case, char production is probably initiated by the Friedel-Crafts type condensation of phenyl groups, catalyzed by phosphorus acids liberated during combustion.43 48 49... 

SCHEME 12.1 Possible reaction mechanism of char formation during combustion of PVA/APP/LDH systems. (From Zhao, C.X. et al., Polym. Degrad. Stabil., 93, 1323, 2008. With permission.)... 

Most of the previous studies on flame retardation of polymer nanocomposites are focused on the relationship between macroscopic morphologies of chars and the flammability properties. Fang et al. studied the relationship between evolution of the microstructure, viscoelasticity and graphitization degree of chars and the flammability of polymers during combustion (68). The flame retar-dancy of ABS/clay /MWNTs nanocomposites was strongly affected by the formation of a network structure. Flammability properties... 

In the Arrhenius diagram in Fig. 10 the straw char reactivity is compared to some other chars. Compared to the other chars the combustion reactivity of the straw char is relatively high, but this fact is not reproduced during gasification in CO2. This is astonishing. The straw char contains the intrinsic ash with much potassium, which is reported to be a very efficient gasification catalyst. 

Fletcher, T.H. Swelling properties of coal chars during rapid pyrolysis and combustion. Fuel, 1993, 72, 1485. 

Quartz. The inherent silica retained in the char as quartz or silica released from kaolinite and illite at low temperatures (i.e., 950°C) is partially reduced to silica monoxide. Unlike silica, which boils at 2230°C, silica monoxide melts at 1420°C and boils at 2600°C [19]. The vapor pressure of silicon is low—in the range of temperatures experienced during combustion. Honig reports values ranging from... 

Thermal Stability and Conductivity. Thermal degradation temperature of PMMA, PS, and PVA (poly(vinyl alcohol)) nanocomposites shifts up by 10-100°C. During combustion [179], nanoparticles form a network of char layers that retards the transport of decomposition products. The thermal conductivity of epoxy composites is four times higher than that of the neat epoxy resin with 5 wt% loads. 

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