Proof of concept of black liquor valorisation by partial wet oxidation

In today's pulp and paper industries, large quantities of black liquor are produced when wood or non-wood lignocellulosic feedstocks are digested with cooking chemicals to separate cellulose fibres from lignin, hemicellulose and other degraded components. Although black liquor is considered to be a waste product, it is rich in both dissolved organic components from wood and added cooking chemicals. In order to recover these cooking chemicals and generate energy from the dissolved organic compounds, it is common practice to burn black liquor in a recovery boiler. However, this is not always the case; for example in developing countries, where small-scale non-wood pulping mills tend to lack spent liquor treatment facilities, the resultant spent liquor is discharged into the environment without effective treatment. The aim of this thesis is to prove the concept of using partial wet oxidation (PWO) for the treatment of lignocellulosic-rich waste streams in order to produce chemicals on laboratory scale. In particular, the effluent from wood and non-wood pulping mills - i.e. black liquor - is investigated as a raw material for the PWO process in order to produce valuable chemical feedstocks and thus improve the profitability of pulping mills. Moreover, the data produced by this research will contribute by providing a much-needed estimate of the commercial feasibility of the PWO concept. The study consists of an investigation into the process concept, optimization of operating conditions, and the effect of numerous variables on the process yield in PWO. In addition to the PWO research, the concept of catalytic partial wet oxidation (CPWO) is studied. Fruthermore, the effect on lignin is studied by treating concentrated black liquors in a two-stage process comprising carbonation before PWO of the liquid phase. The experiments were carried out in a 100ml AISI 316 SS bench-scale high-pressure reactor. Feedstocks studied in this study included the following: wheat straw black liquor (WSBL), Kraft black liquor (KBL), hydrolysate from pulp bleach (HLP), and hydrolysate from wood (HLW). Additionally, spruce sawdust with a particle size of less than 1.5 mm was also used as feedstock for the PWO process. Analysis of the products from PWO was performed using the following methods: COD, TOC, pH, solid concentration, high performance liquid chromatography (HPLC), capillary electrophoresis and lignin properties. In this study, typically formic acid, acetic acid and lactic acid were determined quantitatively. Nevertheless, for some cases, more detailed analysis for some other carboxylic acids were also performed.