Phenolic Antioxidants in Wood Smoke
Doctoral thesis, 2002
Smoke from residential wood burning constitutes a complex mixture of organic compounds with varying environmental and health effects. This thesis focuses on antioxidants and aromatic hydrocarbons emitted from small-scale burning. Smoke samples were collected from laboratory experiments and in chimney outlets from stoves, using gas-tight syringes or adsorbent cartridges. Specific components were assessed by gas chromatography and mass spectrometry.
Primary thermal decomposition products from cellulose and lignin constituted a large part of the organic fraction from incomplete combustion of wood. The lignin-related methoxyphenols are, due to their varying chemical structures, more or less effective antioxidants. Smoke from hardwood burning contained the more efficient 2,6-dimethoxyphenols, whereas softwood smoke almost only contained 2-methoxyphenols. An alkenyl or alkyl side-chain renders the antioxidant stronger, while a carbonyl group reduces the effect. The presence of phenolic antioxidants should be considered when the health hazards of wood smoke are estimated.
Antioxidative 2,6-dimethoxyphenols were the predominant aromatic compounds in smoke from birchwood burning as well as from low-temperature pyrolysis of alder chips, used for meat curing. They improve both the taste and the quality of the foods. Incomplete combustion of newsprint released coniferyl alcohol as the major methoxyphenol. Newsprint is often used to set wood on fire.
Methoxyphenols as well as 1,6-anhydroglucose from cellulose were released at inefficient burning. These compounds appear mainly condensed on particles at ambient temperatures. On more efficient burning, the total amount of organic compounds decreased in the smoke. However, the proportion of the hazardous polycyclic aromatic compounds increased. The carcinogenic benzene was assessed as a prominent aromatic compound in smoke from all studied types of biomass burning, although the ratio to other organic compounds increased with the combustion temperature.
A comparison was made between smoke from a tiled stove and a conventional wood boiler. Stove burning released methoxyphenols and 1,6-anhydroglucose, while the boiler emitted large proportions of benzene and polycyclic aromatic compounds. Oxidative pyrolysis of wood pellets released methoxyphenol antioxidants from the flaming combustion phase. Benzene was the predominant aromatic compound from glowing pellets. New available technology for residential wood burning, including wood pellets and eco-labelled boilers, drastically decreases the emissions of antioxidants as well as hazardous aromatic hydrocarbons through highly efficient combustion.