Methane potentials and organic matter characterization of wood fibres from pulp and paper mills: The influence of raw material, pulping process and bleaching technique

During the process of pulp- and papermaking, large volumes of fibre-rich primary sludge are generated. Anaerobic digestion of primary sludge offers a substantial potential for methane production as an alternative approach to the inefficient energy recoveries by commonly used incineration techniques. However, a systematic study of the importance of upstream process techniques for the methane potential of pulp fibres is lacking. Therefore, biochemical methane potentials were determined at mesophilic conditions for 20 types of fibres processed by a variety of pulping and bleaching techniques and from different raw materials. This included fibres from kraft, sulphite, semi-chemical, chemical thermo-mechanical (CTMP) and thermo-mechanical pulping plants and milled raw wood. The pulping technique was clearly important for the methane potential, with the highest potential achieved for kraft and sulphite fibres (390–400 Nml CH4 g VS−1). For raw wood and CTMP, hardwood fibres gave substantially more methane than the corresponding softwood fibres (240 compared to 50 Nml CH4 g VS−1 and 300 compared to 160 Nml CH4 g VS−1, respectively). Nuclear magnetic resonance characterization of the organic content demonstrated that the relative lignin content of the fibres was an important factor for methane production, and that an observed positive effect of bleaching on the methane potential of softwood CTMP fibres was likely related to a higher degree of deacetylation and improved accessibility of the hemicellulose. In conclusion, fibres from kraft and sulphite pulping are promising substrates for methane production irrespective of raw material or bleaching, as well as fibres from CTMP pulping of hardwood. [Display omitted] •Kraft and sulphite fibres had high and stable CH4 potentials (390–400 Nml CH4 g VS-1).•Shifts in raw material gave large variations in CH4 potential for CTMP fibres.•Removal of lignin was the most important factor for high CH4 potential.•Bleaching of CTMP softwood improved CH4 potential, likely by deacetylation.•Unbleached TMP fibres were inhibitory to AD, while bleaching alleviated this effect.