Evaluation of combined fungal-wet oxidation pre-treatment of radiata pine wood chips for energy conservation

Abstract

Biofuel production from lignocellulose has recently been gaining attention. One major problem of using lignocellulosic materials for the production of biofuel is the low accessibility of cellulose to enzymes and microorganisms. Therefore, pre-treatment of lignocellulose is a critical step in biofuel production from such materials. Of the pre-treatments, fungal treatment has become an important process due to its low-energy demands and selective degradation of lignin and hemicellulose. This capability comes from the unique enzymatic systems, cellulolytic and ligninolytic enzymes, especially in white rot fungi. The low-energy demand of fungal pre-treatment has generated interest in studying the applicability of fungal pre-treatment for biofuel production from woody materials. The most significant drawback of fungal pre-treatment is the lengthy time required for the process. Combining fungal pre-treatment with other pre-treatment methods might reduce the time necessary for the whole process to operate. It can also introduce cost-effectiveness. Thus, combining fungal pre-treatment with other physical and chemical methods has been recently contemplated. The applicability of the combination of fungal with other pre-treatment methods has been considered in a number of recent publications. To be commercially attractive, both energy demand and processing time should be reduced. In terms of energy demand reduction, combined fungal physico-chemical pre-treatment has been effective. However, the lengthy time taken for the whole process has not been significantly improved upon. Wet oxidation is a physico-chemical pre-treatment method which uses pressurised subcooled water and pure oxygen or air to provide an oxidative media. This method has the potential to fractionate insoluble organic contents to simple molecules like water and carbon dioxide. However, the high energy demand for the process makes this method inefficient for biofuel production from lignocellulosic biomasses. The aim of this work was to study the effect of prior fungal pre-treatment on wet oxidation pretreatment in terms of required energy reduction. The study started with an investigation on sole fungal pre-treatment of radiata pine wood chips by two New Zealand white rot fungi. The effect of this kind of pre-treatment on the chemical and physical properties of radiata pine wood chips was investigated. This led to a better understanding of the effectiveness of each fungal strain on selective lignin degradation and cellulose loss. To have a perspective on the combined fungal-wet oxidation pre-treatment of the woody biomass, single wet oxidation pre-treatment of radiata pine wood chips under different operating conditions was also studied. A simplified kinetic model for delignification of radiata pine wood chips pretreated by wet oxidation was investigated. Based on the results on fungal pre-treatment, one of the fungal strains, which has a better performance, was chosen to pre-treat the biomass required for the combined method. Fungally treated wood chips were placed into wet oxidation reactor to find out the effect of the combined method on energy reduction for the whole process as well as its impact on the physical and chemical properties. A simple energy estimation showed that combination of the fungal pretreatment with wet oxidation was able to reduce energy demand up to 70 % while the cellulose loss increased up to 40 % when compared to sole wet oxidation pre-treatment.