Condensed-phase pyrolysis solid fuels

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

The mechanism of burning for polymers is believed to take place through thermal pyrolysis of the solid plastic to produce gases that act as fuel for the fire (45). Fire retardants work in both the condensed and the vapor phase to interrupt melting of the polymer and burning of the gases. Triaryl phosphates function well in the vapor phase. Alkyl aryl phosphates are believed to decompose in the flame front to form polyphosphoric acid, which stays in the condensed phase to form char, which reduces flammability and smoke evolution (46. 47). 

Mechanism. No single mechanism explains the action of all fire retardants, so they probably work through a combination of several mechanisms. The mechanisms of fire retardants in wood involve a complex series of simultaneous reactions whose products may affect subsequent reactions. Pyrolysis of cellulose involves dehydration, depolymerization, decarbonylation, decomposition of smaller compounds, condensation, and other reactions. These pyrolysis reactions occur both in the solid phase and vapor phase. Addition of fire retardants will alter the reactions however, this alteration will depend on the additives, the material, and the thermal-physical environment. The presence of oxygen adds subsequent and competitive oxidation reactions to the above series. These oxidative reactions can take place in both the solid and vapor phases. Evidence indicates that most fire retardants reduce combustible volatiles production and limit combustion to the solid phase. The best retardants also inhibit solid-phase oxidation to effectively remove the fuel from the fire. 

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