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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.

Upgrading biomass waste streams can improve economics in wood industries by adding value to the process. This work considers use of a hydrothermal carbonization (HTC) process for the residual feedstock after lignin and hemicelluloses extraction. Batch experiments were performed at 200&ndash, 240 &deg, C temperatures and three hours residence time with an 8:1 biomass to water ratio for two feedstocks: Raw spruce and spruce after lignin extraction. The proximate analysis and heating value showed similar results for both feedstocks, indicating that the thermochemical conversion is not impacted by the removal of lignin and hemicelluloses, the pretreatment processing slightly increases the heating value of the treated feedstock, but the HTC conversion process produces a consistent upgrading trend for both the treated and untreated feedstocks. The energy yield was 9.7 percentage points higher for the treated wood on average across the range temperatures due to the higher mass yield in the treated experiments. The energy densification ratio and the mass yield were strongly correlated with reaction temperature, while the energy yield was not. Lignocellulosic composition of the solid HTC product is mainly affected by HTC treatment, the effect of lignin extraction is negligible.

The composition, heating value, and reactivity of different municipal and industrial solid waste materials were studied and compared with biomass and coal samples. Reactivity characterization was performed in a vertical tube reactor by dropping a pelletized sample on a porous grid and monitoring the conversion of the sample by Fourier-transform infrared spetroscopy. Samples were tested in ambient oxygen atmosphere at three temperatures typical to incinerator applications, and in reduced oxygen atmospheres, that better represents conditions near the fuel feeding points in incinerators. Thermogravimetrical analysis was used to analyze selected samples and reactivity information was compared with the vertical tube reactor results. This research provides information on the material composition and reactivity for utilization of munical solid waste and sewage sludge in incineration applications. Comparison of two characterization methods illustrates that the characterization method has a significant effect on the reaction rate results, with the implication that the reaction characterization method should be selected to represent the actual conditions of the application. Post-print / Final draft

Biomass torrefaction is a pre-treatment technology with high potential to convert biomass into a valuable commodity. The heat integration of torrefaction and combined heat and power (CHP) plant was investigated in previous work (Sermyagina et al., 2015). The aim of the present study is to assess possible economic benefits from integration. Three most promising integration concepts from the previous work were studied in terms of seasonal operational changes of district heating demand and varying ambient conditions. The performance of two integration concepts were evaluated together with stand-alone and co-located plants. The integration leads to a higher utilization of the CHP boiler capacity during part-load operation, possible increase of the operation time and growth of electricity generation as a result. The total efficiencies of the integrated cases (around 72% in higher heating value terms) are slightly higher than the stand-alone CHP plant (69%) or the co-located option (71%). The integration requires 40% more capital investments than the stand-alone CHP. On the other hand, the total capital investments of the integration cases are 20% lower than in co-located plants, and a profitability evaluation shows that lower investment costs may make integration schemes advantageous over the non-integrated plants. Feedstock price and investment costs are the main economic drivers affecting the profitability of the integrated options. An integration case which uses back pressure steam to account for the torrefaction heat demand showed the highest profitability due to a longer annual operating time, resulting in a growth of electricity and DH production over the stand-alone CHP plant. Post-print / Final draft

An important factor for industrial-scale implementation of torrefaction – a thermal pre-treatment technology that enhances the fuel characteristics of biomass – is the requirement for a low-cost source of heat energy. Significant benefits can be achieved if torrefaction is integrated with a large heat producer. To study this possibility, a new model for the energy and mass balances of the torrefaction process was developed on the basis of available experimental data. The torrefaction model was then integrated into steam power plant simulations, and the performance of different integration schemes was evaluated. To investigate the effect of plant size and operating mode, the same configurations were studied for both large and small combined heat and power (CHP) plants. Large plant operates at full boiler load, and capturing a portion of heat from the plant reduces the power output for all integration cases. At the same time, higher trigeneration efficiencies in comparison with non-integrated case together with fuel consumption decrease due to torrefaction gas co-firing indicate that integration of torrefaction and CHP plant at full load could be beneficial. In case of small plant, free boiler capacity at reduced-load conditions allows to cover the torrefaction heat demand and simultaneously increase the electricity generation for almost all integration cases. Trigeneration efficiency is changing within a relatively narrow range for most of the cases, while the solid fuel consumption is higher than in design case. This work shows that integration method together with process temperature and plant operation mode are the important factors that could have an effect on CHP plant integration with torrefaction process. Post-print / Final draft

Hydrothermal carbonization of biomass presents a promising way to improve fuel characteristics of biomass without preliminary drying. In this process, feedstock is subjected to heating with water at temperatures between 180 and 250 °C during a certain period of time. This paper investigates the effect of process conditions (temperature, time, and the ratio between biomass and water) on the hydrothermal carbonization of coniferous biomass. Three sets of experiments were carried out with coniferous wood chips. Mass and energy yields together with proximate analysis measurements were used for the carbonization process evaluation. Dependencies of hydrochar mass and energy yields from the operation parameters could be described with mathematical correlations. Post-print / Final draft