Production during char oxidation

Factors Affecting NO Production During Char Oxidation... 

Some activation is due to the dehydrating power of the phosphoric acid and, to this extent, the method is analogous to the zinc chloride process. Frequently, the charred product is subsequently ignited for 2 to 8 hours in retorts at 800° to 1,000° C. During this latter stage, the carbon is eroded by being partly oxidized by the phosphoric acid which is reduced to phosphorous and hydrides. These vaporized products are subsequently oxidized to phosphoric acid to repeat the cycle with a fresh batch of raw material. 

Coal has been proposed to have a macromolecular network structure to which concepts of cross-linked polymers can be applied. These concepts have been employed to understand and model such properties of coal as (1) the insolubility, (2) the equilibrium swelling and penetration of solvents, (3) the viscoelastic properties, (4) similarities between the parent coal and products of hydroge-nolysis or mild oxidation, (5) cross-linking during char formation, and (6) the formation of coal tar in pyrolysis. 

As is shown in the burning cycle, the burning of a polymer occurs by the combustion of the volatile products emitted during the oxidative thermal degradation of the polymer. If the degradation mechanism could be altered to produce more char and less volatiles the flame retardancy of the formulation should be decreased. Many flame retardants act in this way, promoting low energy, solid-state reactions which lead to the carbonization of the polymer and a carbonaceous char on the surface. 

Thermal analysis experiments have clearly shown that tin-based fire retardants markedly alter both the initial pyrolysis and the oxidative burn off stages that occur during polymer breakdown These changes have been interpreted as being indicative of an extensive condensed phase action for the tin additive, in which the thermal breakdown of the polymer is altered to give increased formation of a thermally stable carbonaceous char at the expense of volatile, flammable products. The consequent reduction in the amount of fuel supplied to the flame largely accounts for the beneficial smoke-suppressant properties associated with zinc stannates and other tin-based fire retardants. 

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