Hydrothermal Carbonization of Chemical and Biological Pulp Mill Sludges

Abstract

A modern pulp mill generates a variety of different by-products and waste streams, some of these can be recycled, refined, sold, or used on-site for energy production. However, some, such as chemical and biological sludges produced in wastewater treatment cannot be reused or disposed of easily, mainly due to their high moisture content and poor drying characteristics. Tightening legislation regarding waste disposal as well as the growing need to increase the process efficiencies of pulp mills act as driving forces to find environmentally friendly and energy-efficient techniques for pulp mill sludge treatment. This study summarizes the current methods for pulp mill sludge handling and evaluates the potential of hydrothermal carbonization (HTC), a conversion process through which wet organic substrates can be transformed into a carbonaceous material (hydrochar). Depending on the process parameters, the material’s structure is modified, enabling hydrochar use in energy, soil conditioning and adsorption applications. The sludges were hydrothermally carbonized at 180, 200, 220 and 240 °C for 3 h. The hydrochar and liquid products’ main properties were analyzed. Their potential applications were also evaluated. The effective treatment of sludges from the pulp industry with HTC could transform energy-demanding waste into a value-added source of materials.

Fertiliser Properties of Wastewater Sludge and Sludge Ash - A Case Study from the Finnish Forest Industry

“…non-crystalline) matter. This together with the fact that the detection limit of XRD spectrometry is normally between 1 % -2 % [23] and only the crystalline forms of cellulose fibres are able to be identified by XRD were reasons why the presence of cellulose was not observed in our sludge, although it is a common material in pulp and paper mill sludges [24].…”

“…The poor drying characteristic of the sludge is due to its the gel-like structure consisting largely of extra-cellular polymeric substances and high interstitial water, i.e. water stored within the sludge flocs by capillary forces and chemically bound intracellular water [24]. This indicates that the incineration of this residue as such will lead to an uneconomical energy balance when combusted in biomass boilers.…”

Solar Drying of Sludge from a Steel-Wire-Drawing Industry

“…The main disposal challenge of sludge is directly related to humidity (around 85-90% (wb)) [10]. Thus, several studies have concluded that a drying stage for sludge material is essential before final disposal [17][18][19][20], for transportation, handling, and landfilling cost reduction. Thermal drying is a commonly used method for phase separation treatment in the sludge management and reuse process when it is a co-product.…”

Preparation and Characterization of Pulp and Paper Mill Sludge-Activated Biochars Using Alkaline Activation: A Box–Behnken Design Approach

“…Pulp and paper mill sludge has been utilized as a carbon precursor to produce activated biochar. − This industrial sector generates large amounts of sludge residues commonly used as fuel or sent to landfills, but it could have a more sustainable use . Only in Sweden, 257 kilo tonnes of dry sludge solids are generated annually by paper and pulp mills .…”