Basic Research on the Pyrolysis and Combustion of Wood

So many chemical fire retardants for wood are now available that empirical testing of them all is impractical; fundamental research is needed to determine the nature of pyrolysis and combustion of wood and how fire retardants affect them. Some of the prominent theories for the mechanism of fire retardancy include the hypotheses that the chemicals form glassy layers or foams which keeps air from the wood or insulates it, that they increase conductivity so that heat is quickly dissipated from the surface; and that, in undergoing decomposition during a fire, the chemicals absorb heat or release nonflammable gases which dilute the flammable gases. Five methods of analysis were used in these studies: static and dynamic thermogravimetry, differential thermal analysis, measurement of heat of combustion, and analysis of chemical products of combustion. All methods appear promising for further research. These initial studies indicate that in a nitrogen atmosphere, wood, lignin, and cellulose pyrolyze actively during an exothermic trend in the decomposition process, lignin and wood at near 220?C and alpha cellulose at near 275?C. Once the pyrolysis is started, cellulose is nearly completely volatilized–endothermically for the most part–before the temperature reaches 400?C. Lignin volatilizes more slowly–almost entirely in an exothermic reaction– losing one-half of its weight when the pyrolysis is completed at 800?C. Some salts which are good flame retardants lower the threshold decomposition temperature for the active pyrolysis of wood, increase the volatilization below 250?C, exert an endothermic effect at the temperature of greatest volatilization, and minimize the amount of wood volatilized when the pyrolysis is complete. On the other hand, some salts which are also good flame retardants do not change the pyrolysis threshold temperature of volatilization below 250?C, and exert an exothermic effect at the temperature of greatest volatilization, but do minimize the amount of wood volatilized when the pyrolysis is completed. All of the salts investigated, whether fire retardant or not, lowered the activation energy for pyrolysis of wood; borax and potassium carbonate were the most effective in lowering the activation energy.

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