Polycyclic aromatic hydrocarbons from diffusion flames and diesel engine combustion

Abstract

A study has been made of the effects of air flow rate and of several additives to the fuel of a laminar diffusion flame on the formation of ( i) soot, ( ii) carbonaceous residue (i.e. chloroform-insoluble material), ( iii) pentane-soluble material (shown to be essentially polycyclic aromatic hydrocarbons), and ( iv) 3,4-benzpyrene contained in the latter. Increasing air flow rate leads to a decrease in both soot and carbonaceous residue which reach limiting values. Pentane-soluble material and 3,4-benzpyrene decline continuously with increase in air flow rate but at a diminishing rate. Dichloromethane as additive increases the formation of soot and carbonaceous residue to a considerable extent without having any appreciable effect on the formation of pentane-soluble material and 3,4-benzpyrene. Nitroethane, 1-nitropropane, ethyl nitrate and t-butylhydroperoxide all bring about significant reductions in the formation of chloroform-soluble material, including pentane-soluble material and 3,4-benzpyrene. Total soot formation is reduced slightly but the formation of carbonaceous residue is virtually unaffected. Similar results are obtained in a study of the particulate matter in diesel exhaust when the additives are blended with a diesel fuel, and conditions of inefficient operation are simulated. t-Butylhydroperoxide is found to be most effective in reducing the formation of 3,4-benzpyrene. Both the diffusion flame and diesel engine results are in conflict with the hypothesis that ‘carbon formation’ occurs through polycyclic aromatic hydrocarbons as intermediates; these may be the end products of side-reactions, probably free-radical in nature.