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Journal articles
Optimization of energy efficiency and condensate production in evaporation plants for a modern softwood pulp mill, TAPPI Journal April 2026
ABSTRACT: To meet the need to further improve thermal efficiency and environmental performance of kraft pulp mills, new systems and techniques have been developed within the evaporation plant. This paper describes these novel approaches and how they were implemented in a project completed in 2018 for a new evaporator and condensate treatment system supplied by Valmet at the SCA Östrand market pulp mill in Sweden. This project was part of a stepwise upgrade of the complete mill to increase the production capacity of the mill from 430,000 to 900,000 air-dried metric tons/year (ADt/y). As part of this upgrade, the mill had the objectives to increase the energy efficiency of the pulp mill and to minimize the air emissions as much as possible, the effluent volume, and the water usage in the mill. The mill also wanted to have the disposal of the biosludge in the black liquor, and the production of tall oil from black liquor soap, liquid methanol, and turpentine. This required that the new evaporation and condensate treatment system be very closely integrated into the other process departments of the mill, including integration of the hot weak black liquor flash vapor from the digester directly into the evaporator train and the production of multiple streams of clean evaporation plant condensate at the correct temperature for the bleach plant. Heat and mass balance calculation values, which were found to do very well in predicting the effect on actual mill operation, are also presented in this paper.
Journal articles
A systems approach for process debottlenecking towards a sustainable pulp and paper industry, TAPPI Journal April 2026
ABSTRACT: Increasing the competitiveness of the pulp and paper industry requires an effective optimization of its existing assets in line with a long-term vision for process transformation, production upgrade, and product diversification. Currently, pulp production increase is one of the main sources of additional revenue for the kraft industry. Likewise, energy efficiency is often employed as a cost-effective approach to reduce operating costs, enhancing the possibilities to lower fossil fuel consumption and contributing to a low-carbon economy. On the other hand, reaching higher production targets and facilitating process transformation, such as biorefinery implementation, heavily depend on the status and performance of a mill’s current infrastructure; therefore, a system analysis is needed to assess the new production requirements, the bottlenecks, and the interactions across departments. In order to obtain practical improvement solutions, direct and indirect impacts on process performance and resource utilization should be considered. This work provides an overview of the key challenges that need to be addressed for production increase and energy efficiency improvement. The methodology starts by a scope analysis for debottlenecking and screening capacity limitations vs. mill targets, followed by their ranking (bottleneck ranking diagram). Benchmarking, gap analysis, and root-cause techniques are applied to diagnose system inefficiencies. This mill-wide debottlenecking assessment is then used to guide the selection of a long-term sustainable operation and design a portfolio of improvement projects by avoiding cross effects of the short-term projects on the long term. A case study of a kraft pulp mill is used to illustrate the proposed methodology.
Journal articles
Biphasic 1-butanol/water delignification of miscanthus for manufacturing papermaking pulp, TAPPI Journal August 2026
ABSTRACT: Miscanthus has emerged as a promising nonwoody fiber source for papermaking and fiber molding as the world has sought alternatives to wood-based products. A diverse range of chemistries have been explored at the laboratory scale to turn nonwoody biomass into fibers; however, few have been successful at pilot or commercial scales. An emerging organosolv delignification chemistry that has demonstrated previous technical success is butanol/water, having been shown to produce papers with similar properties to other, more conventional chemistries, but offering the benefit of a built-in sugar and lignin coproduct separation. While a range of fiber lengths were achieved (0.45•0.95 mm), addition of sodium hydroxide (NaOH) to the butanol/water delignification chemistry resulted in fiber lengths comparable to commercially available hardwood pulps (0.7•1.0 mm). Lignin concentration was reduced from 19.1 wt% in the raw miscanthus to under 3.5 wt% for most trials. The lowest lignin concentration (2.5 wt%) was achieved at 180°C for 180 min with 50:50 butanol:water and 2 wt% acetic acid. Yet, the high severity of this condition (and similar ones) resulted in a significant truncation of fiber length, which is detrimental for papermaking applications. The highest quality pulp for papermaking was generated at 180°C for 180 min with 50:50 butanol:water and 10 wt% NaOH. This pulp had one of the longest fiber lengths, the highest brightness, and the highest hemicellulose concentration. Thus, butanol/water delignification, especially with NaOH addition, resulted in the successful production of cellulose-rich pulps that have potential in commercial papermaking and fiber molding applications.
Journal articles
Alternative disposal methods for pulp mill biosludge
ABSTRACT: The chemical pulp production process produces notable amounts of biosludge, mostly in the wastewater treatment system. Biosludge is a problematic waste stream due to its properties, and recently, its amounts have been on the increase. The most used disposal processes are incineration in a bark boiler or a recovery boiler now that landfilling is no longer permitted by legislation in many countries. Incineration of wet sludges with poor drying characteristics decreases the energy efficiency of the process. Biosludge is typically mixed with black liquor in the evaporation plant before feeding to the recovery boiler. Although the recovery boiler is designed to handle challenging fuels, biosludge disposal requires consideration of its behavior during evaporation and accumulation of non-process elements in the recovery cycle. This study discusses the currently used sludge handling methods and presents two novel methods, hydrothermal carbonization and anaerobic digestion, to convert sludge into a usable form. Production of hydrochar or biogas offers new usage possibilities for biosludge and improves the energy efficiency of the mill when the new methods replace incineration. The processes are still developing, and thus their economic feasibility is uncertain; however, depending on the case details, they can be considered promising alternatives to be implemented in the future.
Journal articles
Biomass gasification for green ammonia production in kraft mills, TAPPI Journal September 2026
ABSTRACT: As the global energy sector transitions toward net-zero emissions, kraft pulp mills present a compelling opportunity to produce green fuels and achieve net-negative emissions by capturing biogenic carbon, helping offset hard-to-abate sectors. This study explores the techno-economics of integrating biomass gasification for green ammonia production within kraft mills, using KraftSIM modeling to evaluate ammonia production, steam balance, emissions, and utility impacts. The proposed design enables energy and water system integration with the existing mill and allows for a gradual scale-up without disrupting the chemistry and operation of the recovery cycle. Across five scenarios representing different biomass gasification rates ranging from 100 to 1000 bone-dry metric tons per day (BDMT/d), the process yields ammonia at approximately 0.51 metric tons per bone-dry metric ton (t/ BDMT), while generating significant low- and medium-pressure steam and hot water that offset mill utility demands. The incineration of pressure swing adsorption tail gas and ammonia plant purge gas in the power boiler reduces biomass combustion and enhances power boiler efficiency. Economic analysis suggests a net revenue potential of approximately CA$380/BDMT, which could increase with the implementation of biogenic carbon pricing. Overall profitability is strongly influenced by biomass procurement costs, electricity prices, and prevailing green fuel and chemical market premium. This work outlines a practical, near-term decarbonization pathway for kraft mills by coupling green fuel production with potential opportunities for carbon removal.
Biphasic 1-butanol/water delignification of miscanthus for manufacturing papermaking pulp, TAPPI Journal August 2026
ABSTRACT: Miscanthus has emerged as a promising nonwoody fiber source for papermaking and fiber molding as the world has sought alternatives to wood-based products. A diverse range of chemistries have been explored at the laboratory scale to turn nonwoody biomass into fibers; however, few have been successful at pilot or commercial scales. An emerging organosolv delignification chemistry that has demonstrated previous technical success is butanol/water, having been shown to produce papers with similar properties to other, more conventional chemistries, but offering the benefit of a built-in sugar and lignin coproduct separation. While a range of fiber lengths were achieved (0.45–0.95 mm), addition of sodium hydroxide (NaOH) to the butanol/water delignification chemistry resulted in fiber lengths comparable to commercially available hardwood pulps (0.7–1.0 mm). Lignin concentration was reduced from 19.1 wt% in the raw miscanthus to under 3.5 wt% for most trials. The lowest lignin concentration (2.5 wt%) was achieved at 180°C for 180 min with 50:50 butanol:water and 2 wt% acetic acid. Yet, the high severity of this condition (and similar ones) resulted in a significant truncation of fiber length, which is detrimental for papermaking applications. The highest quality pulp for papermaking was generated at 180°C for 180 min with 50:50 butanol:water and 10 wt% NaOH. This pulp had one of the longest fiber lengths, the highest brightness, and the highest hemicellulose concentration. Thus, butanol/water delignification, especially with NaOH addition, resulted in the successful production of cellulose-rich pulps that have potential in commercial papermaking and fiber molding applications.
Controlling sulfidity by recycling sulfur in CNCG, TAPPI Journal July 2026
ABSTRACT: Pulp and paper mills have faced a major dilemma. On one hand, mills should not dispose of the side streams by mixing them with effluent water. On the other hand, there has been no other way to dispose of excess sulfur than getting rid of recovery boiler electrostatic precipitator (ESP) ash and buying sodium hydroxide to replace lost sodium. A new, yet old, process has been introduced to alleviate the problem. An alternative is the onsite production of sulfuric acid (H2SO4) to significantly reduce, or to even eliminate, the need for recovery boiler fly ash dumping. A sulfuric acid plant produces concentrated commercial quality H2SO4 from concentrated non-condensable gases (CNCGs). Up to 99.9% of the sulfur in CNCGs can be converted into concentrated sulfuric acid with this wet-gas sulfuric acid (WSA) process. There are already references on pulp mills highlighting that the process is straightforward to use. Mills can increase the amount of sulfur in CNCGs by controlling the heavy back liquor storage time and temperature. This opens up a new way to control the mill sulfidity by recycling sulfur. The effect of recycling sulfur is highlighted with reference mills. Due to integrated paper production, the Northern Mill is a large energy consumer, producing mostly softwood-based end products. The Eucalyptus Mill is a large standalone hardwood (eucalyptus) pulp mill where electricity is produced substantially in excess of its own use. As a sample case, lignin production with and without sulfur recycling is presented.
Journal articles
Effects of calcium on sodium salt scaling with the presence of resin acids and fatty acids, TAPPI Journal June 2026
ABSTRACT: Reintroducing tall oil soap or its related products into high dry solids black liquor has been found to reduce sodium salt scaling in falling film evaporators. Aside from resin acids and fatty acids, which are the likely scale inhibitors, calcium is reintroduced into black liquor because of the relatively high calcium content of tall oil soap. One concern is that this increase in calcium content might lead to the formation of additional calcium and sodium scales in evaporators. In this work, we investigated the relationship among trace amounts of calcium, sodium salts, resin acids, and fatty acids in a controlled system using a model salt solution and a benchtop setup. We studied the effects of the calcium carbonate addition and calcium carbonate scales on sodium salt scaling in the presence of resin acids and fatty acids. We found that some calcium carbonate is incorporated in the precipitated sodium crystals, and the suspended sodium crystals become larger and more compact with increasing calcium carbonate concentration. Experiments in the benchtop setup show that precipitating calcium carbonate scale on the heat exchanger does not lead to a higher rate of sodium salt scaling. The solubility of calcium carbonate is not affected by the addition of resin acids and fatty acids. These findings indicate that the reduction in sodium salt scaling through the addition of tall oil soap is primarily related to resin acids and fatty acids, rather than to calcium or to interactions between calcium and mixtures of resin and fatty acids.
Journal articles
Preparation of a vegan leather from mycelium with papermaking method, TAPPI Journal June 2026
ABSTRACT: With growing concern over the environmental impacts of both natural leather and conventional synthetic leathers, the development of sustainable and eco-friendly leather alternatives has become an urgent research priority. In this study, an innovative wet papermaking strategy is proposed to fabricate continuous, homogeneous mycelial sheets from chitin-rich fungal mycelium, mimicking the matrix structure of genuine leather. These mycelial sheets were chemically modified to meet the performance requirements of leather foam layers. Subsequently, the modified mycelial sheets were combined with a substrate fabric to produce a novel mycelium-based leather composite (myco-paper leather) that achieves an excellent balance of mechanical properties and tactile qualities. Experimental results demonstrate that the mycelium-sheet-based leather substitute exhibits remarkable mechanical performance, with a tensile strength of approximately 45 MPa and an elongation at break of about 10.5%. After lamination with the substrate fabric, the composite material shows a tear strength of approximately 16 N, along with a desirable hand feel and surface texture. Following tannic acid tanning, the mycelial sheets also exhibit significant antimicrobial and antifungal properties, forming an inhibition zone of approximately 2 mm against Staphylococcus aureus. Through reinforcement with plant fibers, the developed material attains both strength and flexibility, indicating that the wet papermaking of mycelial sheets is a feasible and scalable approach for producing sustainable leather alternatives. This work not only employs a simple and mature papermaking process to process fungal mycelium but also provides a new conceptual and practical foundation for the large-scale production of bio-based leather substitutes. The findings have significant implications for advancing a low-carbon, sustainable leather industry.
Journal articles
Investigation of the factors contributing to malodorous gases emission during secondary fiber reuse, TAPPI Journal June 2026
ABSTRACT: Malodorous gases are commonly produced during secondary fiber reuse, which is harmful to human health and causes environmental pollution. This paper investigated the influence of fiber type and concentration, temperature, and whitewater concentration on the malodorous gases. The results indicated that, in pulp prepared with fresh water, bleached hardwood kraft pulp (LBKP) did not produce malodorous gases after standing for five days. In contrast, the secondary fiber began to release substantial amounts of total volatile organic compounds (TVOC) on the third day and hydrogen sulfide (H2S) and ammonia (NH3) on the sixth day, and black substances began to appear in the pulp, which proved that the microorganisms began to proliferate. With the increase of the secondary fiber concentration, the release amounts of TVOC, H2S, and NH3 gradually rose, along with the black substances in the pulp. With increased temperature, the release of TVOC increased steadily, while the release of H2S and NH3 reached the maximum at about 45°C, and then began to decline. The decrease of the pulp freeness accelerated the generation of the malodorous gases, but the total release amounts of TVOC, H2S, and NH3 were basically the same. With the increase of white water concentration, the release of TVOC, H2S, and NH3 increased rapidly. When the white water/fresh water was 20 mL/80 mL, the slurry changed from pale yellow to aterrimus on the sixth day. Therefore, microorganisms in the secondary fiber caused pulp deterioration, while white water was the main reason for generating a large amount of malodorous gases.