Modelling of soot production in fires

In this study the formation of CO from soot in the hot gas layer has been examined.

The investigation has been made predominantly for rich mixtures, with equivalence ratios from 1.0 to 3.0. To study the soot particle growth as a function of equivalence ratio and residence time, equivalence ratios from 0.5 to 4.0 and residence times from 0.25 s to 10 s, respectively, have been used. The gas phase chemistry has been calculated using the Sandia CHEMKIN code [1]. The perfectly stirred reactor (PSR) concept has been used to model the hot gas layer. The input gas temperature, equivalence ratio and residence have been varied to investigate the thermochemical environment at a given location in the gas layer. Ethene, C2H4, was chosen as the fuel, as it has the necessary carbon/hydrogen ratio to form soot precursors.

The chemical kinetic model used consists of gas phase chemistry including reactions for aromatic chemistry, soot particle coagulation, soot particle aggregation and soot surface growth.

The gas phase chemistry

The gas phase chemistry has been calculated using the GRI-Mech 1.2 scheme [2] for smaller hydrocarbon reactions. The reaction scheme was modified to take higher hydrocarbon reactions (aromatic chemistry) into account. Benzene and phenyl formation are modelled by reactions of C4Hx with acetylene and by cyclization reactions of C6Hx species and recombination of propargyl radicals [3]. Reactions up to four aromatic rings, i.e. pyrene are included in the gas phase aromatic chemistry [4]. The pyrene formation is started from benzene following the HACA (hydrogen abstraction - carbon addition) reaction sequence via formation of biphenyl (i.e. "ring-ring condensation"). The total number of reaction steps is 542 in the gas phase scheme.

The soot particle coagulation dynamics is described by the method of moments [5] having altogether 6 moments. The soot surface growth mechanism is described using a 6-step reaction mechanism, which resembles the HACA mechanism for PAH [3, 6].

Soot formation

Soot formation starts at an equivalence ratio of 1.0 and increases linearly with equivalence ratio. The particle size increases with increased residence time. Reaction between soot and CO2, resulting in CO, occurs at temperatures higher than 950 °C. The higher the temperature and equivalence ratio the higher is the CO formation via this mechanism. Addition of extra CO2 into the mixture increases the formation of CO and decreases the soot volume fraction. This effect increases with increasing temperature and equivalence ratio.

Identification this "new" source of CO increases our knowledge of the production of CO in hot gas layers. The results can be used to predict CO formation in vitiated room fires.

Full detail of the project results are available in SP Report 2002:08.

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Raúl Ochoterena

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RISE Research Institutes of Sweden, Phone 010-516 50 00, E-mail

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