How is Wood Vinegar Produced?

Wood vinegar or pyroligneous acid is an aqueous liquid produced from the pyrolysis of lignocellulose waste and biomass. Biomass pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen with a temperature ranging from 200 to 600 °C, which produces three major products: solid char (biochar), condensable liquids (tar or bio-oil), and non-condensable gases. In order to prepare wood vinegar, this liquid is kept in a closed container and stands at ambient temperature for about 3 months. The liquid solution then will be separated into 2 phases; a clear upper phase that is wood vinegar is drained off, while the lower phase is called tar sediment.
Wood vinegar has a special smoky odor and is in light yellow to brown color, depending on the feedstock properties and pyrolysis type used for preparing it. The wood vinegar is composed mainly of water (80–90%) and more than 200 organic compounds, among which acids, alcohols, phenols, aldehydes and esters can be found. Due to its complex chemical composition and depending on its water concentration, wood vinegar has a wide range of applications. It can be used as antifungal, sterilizing agent, insect repellent, odor remover, wood preservative, fertilizer, plant growth promoter or inhibitor, animal feed additive, coagulant in the preparation process of natural rubber (NR) sheets. Agriculture is one of the most important application fields of wood vinegar. Recently, the studies on the application of wood vinegar in agriculture were quite intensive, mainly focusing on the antimicrobial properties, pesticidal effects, and the regulatory effects of wood vinegar on seed germination and plant growth.
The yield of wood vinegar is over 20% among the pyrolysis products; this signifies that when 1000 kg of biomass feedstock is pyrolyzed, more than 200 kg of wood vinegar will be produced. In general, the production, composition, and characteristics of wood vinegar are significantly dependent upon the type of feedstock (different biomass resources), the pyrolysis conditions (e.g., heating rate, and temperature), and subsequent preservation time. Several researchers have investigated the effect of the temperature and heating rate on the yield of wood vinegar from pyrolysis. Commonly, higher pyrolysis temperatures result in greater concentration of phenolic compounds and acids in the chemical composition of wood vinegar. Those researchers found the highest yield of wood vinegar in the range of 350-450 °C.
In another study, the influence of different heating rates on the chemical composition of wood vinegar from two wood species was investigated. Chemical compounds identified in the wood vinegar were divided into the following groups: alcohols, ketones, furans and pyrans, phenolic compounds, and other compounds. Among these groups, the phenolic group is the most important because of its higher proportion and principally its noteworthy biological effects. Results showed that these compounds accounted for approximately 60-70 wt% of the whole chemical composition of wood vinegar. In general, higher heating rates led to greater contents of furans and pyrans and lower concentrations of phenolic compounds.
The compositional analysis of wood vinegar indicates that the stability of phenol, acids, and other compounds undergo dramatic changes with the storage time. Some reports showed that the storage times have a significant effect on the changing rate of the characteristics such as the molecular weight and acid content. In addition, the quantity of precipitate during the storage process promotes the changes in the activity of wood vinegar. The results of a study on the influence of the storage time on the wood vinegar components showed that the compounds such as 2-methoxytetrahydrofuran, 1,2-cyclopentanedione and 1,2-butanediol were only discovered in the fresh wood vinegar and not found in the wood vinegar after storage for 11 or 19 months. On the other hand, new compounds were also detected in the wood vinegar (such as butanediol and propanoic acid). The changes of compounds in the wood vinegar are due to the oxidation and degradation during the storage process. It was also observed that the ingredient of carbonyl compounds declined and water-insoluble compounds increased in storage process. Therefore, the chemical composition and characteristics of the collected wood vinegar are affected by the pyrolysis temperature and storage time.