Influence of the Chemical Properties of Wood on the Odor Produced by the Kraft Pulping Process

Abstract

Chemical reactions occur between inorganic compounds and wood chips during the cooking step of the Kraft process. White liquor, which in this process is composed of sodium hydroxide (NaOH) and sodium sulfide (Na2S), is mixed with wood chips in the digester in order to dissolve the lignin and produce wood pulp. However, other compounds are formed during the course of the chemical reactions. These compounds are volatile and malodorous due to the combination of the sulfur in white liquor and the carbon chains in lignin. This combination creates total reduced sulfur (TRS) compounds, which are malodorous gases responsible for the distinctive odor of the Kraft pulping process. This paper discusses the chemical composition of the wood species that contribute to these gas emissions. Gases are also released in other stages of the process, e.g., from the woodchips stored in the woodyard for factories that use only long pine fiber, from the digester, pulp washing filters, and from white, black and green liquor storage tanks. One way to reduce TRS emissions is through continuous attention to and correction of the pH levels of liquors containing sulfide ions.