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Mitigation of global warming, energy security and industrial competitiveness urge the energy-intensive pulp and paper industry (PPI) to transform energy use practices. This study investigates how the PPI has responded to the need for the energy transition in the 2000s. Finland and Sweden as forerunners of energy-efficient operation and decarbonization of the PPI are used as target countries. Understanding of changes in energy consumption is complemented using decomposition analysis (Logarithmic Mean Divisia Index Method) and the energy efficiency index approach. Analysis of companies’ investments in energy technologies is used for explaining changes in energy production. Evidence of significant development towards the more sustainable operation of the PPI was found. Energy consumption per produced unit has decreased, i.e., energy efficiency has improved. Fossil fuels have been partially replaced with bio-based alternatives. Thus, the CO2 intensity has decreased substantially. The generation of renewable electricity has increased in both countries. Examples of Finland and Sweden indicate that the PPI has great potential to contribute to CO2 emission reduction worldwide in the future as energy efficiency can be further improved, and the share of fossil fuels can be decreased increasing the use of biofuels and self-generated green electricity at least in kraft pulp mills.

Of the various waste and side streams created in a kraft pulp mill, the biological sludges from the wastewater treatment plant are some of the most problematic to handle. Incineration is becoming a common solution as landfilling is no longer permitted by legislation in many countries, but this is also problematic due to the high moisture content, poor drying characteristics, and high ash content in the solids. This study evaluates the technical potential of mild hydrothermal carbonization (HTC) at 160 °C for 3 h to improve the energy efficiency of on-site incineration as a biosludge handling method. HTC treatment transforms wet organic substrates into a hydrophobic carbonaceous material (hydrochar). The heating value and elemental composition of both the sludge and the hydrochar product were analyzed. Based on this, a hydrothermal carbonization model developed earlier was adjusted for the feedstock, and process integration modelling performed to evaluate the performance impact on the power and heat generation at the mill. The results indicate that if the alternative is combustion in the power boiler, HTC pre-treatment could allow a significant increase in power generation. If the sludge is combusted in the recovery boiler, a practice often avoided in order to not introduce non-process elements to the chemical recovery cycle but sometimes necessary due to, e.g., absence of a power boiler, a much smaller increase is obtained. The increase is smallest if the freed evaporator plant capacity cannot be utilized for increasing the firing liquor dry solids content.

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.

In this work, the production viability, physical, chemical and mechanical properties of briquettes produced from mixtures of coffee shrub residues and pinewood, were evaluated. The densification was carried out under constant operating conditions (temperature of 120 °C, pressure of 8.27 MPa) in a piston-press type laboratory-scale briquetting machine. Coffee shrub residues were mixed with pinewood in ratios of 25%, 50% and 75%. In addition, reference briquettes of pure pinewood and of each type of coffee shrub residue were produced. To characterize the raw material, ash content, volatile matter, fixed carbon together with the calorific value of produced samples, were measured. To characterize the suitability of the briquettes produced: apparent density, energy density, tensile strength, and equilibrium moisture content were determined. The highest values of energy density (19133–19899 MJ m−3), tensile strength (415–569 kgf), apparent density (1107–1163 kg m−3) and favorable values of equilibrium moisture content (9–11 wt %) were obtained from a mixing ratio of 75% of pinewood. The novel contribution of this research was to develop briquettes with appropriate physical and mechanical parameters from new raw materials that could serve as sustainable fuel sources for local firing systems.

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.

Spent coffee grounds (SCGs) and tea leaf residues are investigated as a potential source of bioenergy and biochemicals and compared to several woody biomasses. The effect of initial processing (coffee roasting and tea oxidizing) on feedstock composition and thermal behavior is assessed experimentally and analytically. In addition to proximate and ultimate analyses, the materials are examined by combustion tests coupled with Fourier-transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectrometry in oxidizing and inert atmospheres. The biggest differences in composition of the samples are found in the high elemental N and S contents of SCGs (2.9 wt% of N and 0.1 wt% of S on average) and especially tea leaf residues (4.5 wt% of N and 0.2 wt% of S on average). The elevated N and S values lead to NO and SO2 release during combustion. Overall, the obtained levels of reactivity and heating values (with a maximum higher heating value of 24 MJ kg−1 for the Arabica-Robusta coffee blend) confirm the significant energy potential of both the coffee and tea samples. Mass spectrometry analysis of the studied materials reveals higher heterogeneity of macromolecular composition of coffee and tea samples compared to woody biomasses. Pyridine, pyrazine, phenols and acetic acid are the most abundant among detected species and have demonstrated beneficial characteristics for a broad spectrum of human activities. The composition characteristics and reactivity test results presented in this work promote more effective valorization and usage of these currently little used biomass residues.

To increase the understanding of hydrothermal carbonization (HTC) of lignocellulosic biomass residues, four feedstocks: giant bamboo, coffee wood, eucalyptus, and coffee parchment, were studied. The effect of operating conditions on the products in terms of yield, composition and energy densification were quantified. Each feedstock was treated for 3 h at temperatures of 180, 200, 220 and 240 °C. For all samples, the higher heating value (HHV), fixed carbon content and energy density increased with increasing reaction severity, while volatile matter content and mass yield decreased. The HHV of hydrochar samples obtained at temperatures ≥220 °C were in the range of 24.6–29.2 MJ kg−1 and indicated the high potential of these materials for fuel applications. The mass yields varied in the range of 46.5–56.9%, with the exception for coffee parchment, where the lower values of 34.4–46.0% were obtained. The fixed carbon varied from 33.8% to 53.0%. The HTC liquor had pH values of 2.9–4.4 due to organic acids. The results were used to model and evaluate different industrial-scale HTC simulation cases. The overall efficiency was similar within all studied biomasses. The integration with a bio-fired power plant allows simplification of the process while also bringing efficiency gains. All studied biomasses appear to be suitable for energy and value-added products generation through HTC treatment. Coffee residues, which have received little research consideration previously, responded well.

Sustainability and energy efficiency have become important factors for many industrial processes, including chemical pulping. Recently complex back-end heat recovery solutions have been applied to biomass-fired boilers, lowering stack temperatures and recovering some of the latent heat of the moisture by condensation. Modern kraft recovery boiler flue gas offers still unutilized heat recovery possibilities. Scrubbers have been used, but the focus has been on gas cleaning, heat recovery implementations remain simple. The goal of this study is to evaluate the potential to increase the power generation and efficiency of chemical pulping by improved back-end heat recovery from the recovery boiler. Different configurations of heat recovery schemes and different heat sink options are considered, including heat pumps. IPSEpro simulation software is used to model the boiler and steam cycle of a modern Nordic pulp mill. When heat pumps are used to upgrade some of the recovered low-grade heat, up to +23 MW gross and +16.7 MW net power generation increase was observed when the whole pulp mill in addition to the boiler and steam cycle is considered as heat consumer. Combustion air humidification proved to yield a benefit only when assuming the largest heat sink scenario for the pulp mill.