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As of March 2022, TAPPI Journal’s (TJ) publishing model is 100% Open Access (OA) to improve the accessibility of its published articles, increase researcher engagement and make research more visible. This new format helps researchers meet their funding and grant application requirements and potentially increase the number of citations. As in the past, the copyright remains with the author, and unlike other technical journals, TJ does not require a publication fee. Read more.

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Editorial: New research sheds light on critical processes in the recovery cycle, TAPPI Journal July 2026

August 03, 2026

The chemical recovery cycle has been called the backbone of a kraft pulp mill, making modern paper production both economically and environmentally sustainable. It regenerates pulping chemicals (primarily sodium hydroxide and sodium sulfide) used to separate wood fibers, allowing them to be reused rather than continuously replaced, while also recovering energy by burning the organic components of black liquor to produce steam and electricity that supply much of the mill’s power needs. In addition, the recovery cycle minimizes waste, reduces emissions, and recovers valuable by-products, creating a highly efficient closedloop system. Without this process, chemical costs, energy consumption, and environmental impacts would increase, making large-scale kraft pulping significantly less economical and sustainable.

Non-process elements in the lime cycle: New findings regarding NPE distribution and lime mud filterability, TAPPI Journal July 2026

August 03, 2026

ABSTRACT: Proper functioning of the recausticizing cycle is crucial for securing a stable white liquor supply for a pulp mill. Common runnability problems in the modern lime cycle, such as low filterability and poor dewatering of lime mud, high dead load, or ring formation in the lime kiln, are often caused by accumulation of non-process elements (NPEs). This project attempted to verify the traditional rules of thumb regarding the chemical behavior of NPEs: magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), barium (Ba), cadmium (Cd), and lead (Pb) in the causticizing reaction, and to investigate the reasons behind poor lime mud filterability. First, a standardized method for measuring filterability of lime mud was developed. Next, a series of recausticizing experiments, followed by immediate filtration trials, was conducted using mill green liquors, laboratory-made synthetic green liquor, reburned lime/makeup mix, and extra NPE additions. It was found that sodium (Na), potassium (K), and sulfur (S) followed the liquid phase, as expected, while Mg, P, and Ba, but also Pb, Cu, and Cd, showed low solubility and accumulated in the lime mud. The Si and Al accumulated in the lime mud even though the soluble content in liquor was high, while the behavior of Mn, Fe, and Zn was more complex. Based on these results, tentative solubility limits for NPEs were suggested as a first estimate for the maximum possible concentrations of NPEs in mill white liquor. The causticization-filtration experiments revealed that overliming and high content of Mg in green liquor clearly led to lower filterability, but our addition of Si did not have such an effect. As differences in particle size and size distribution have a certain impact on the filtration resistance, the lime mud sedimentation behavior, combined with other characterization methods, might be used as an indication for the filterability. Finally, a list of recommendations for pulp mills was formulated that starts with establishing a comprehensive standard for a lime mud with good filterability. This standard can be used as a benchmark for comparison any time a lime cycle runnability problem is identified.

Controlling sulfidity by recycling sulfur in CNCG, TAPPI Journal July 2026

August 03, 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.

Alternative disposal methods for pulp mill biosludge, TAPPI Journal July 2026

August 03, 2026

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.

Black liquor spray properties in-furnace conditions for swirl cone nozzles TAPPI Journal July 2026

August 03, 2026

ABSTRACT: Recovery boiler furnace operation is controlled for high chemical recovery, high steam production, and low emissions. Good control allows long runs with minimum cleaning without expensive shutdowns. An essential variable affecting boiler operation is black liquor spraying into the furnace. Poorly controlled black liquor spraying causes difficulties, e.g., in chemical recovery, fouling of heat transfer surfaces, and decreased energy production. A challenging measurement strategy with three different swirl cone nozzles . types A, C, and T . was carried out at a mill in the United States. Black liquor spray properties were measured under in-furnace conditions for flow rates of 2.0 L/s to 3.0 L/s. Three spraying temperatures of 119™‹C, 122™‹C, and 126™‹C, which were 2™‹C, 5™‹C, and 9™‹C above the boiling point (ÄTe), were tested. These flow rates and temperatures are typical for industrial nozzle sizes with exit orifice sizes of 17 mm and 22 mm. An imaging method to measure droplet size in a highly challenging furnace environment was developed. Droplet volume median diameter (Dv50) for nozzles A, C, and T varied between 7 mm to 4 mm, 7 mm to 5 mm, and 8 mm to 6 mm, depending on the flow rate and temperature. The high number of droplets detected by the analysis program enabled data fitting into statistical correlations. The size and width parameters for the Rosin™] Rammer droplet size distribution function could be defined. The droplet size distribution was found to be surprisingly wide, probably because of swollen droplets. Spray velocity was measured in two locations, 5 cm from the nozzle and 1.8 m from the nozzle. Near the nozzle, the average velocity increased from 7.3 m/s to 13 m/s when the flow rate was increased from 2 L/s to 3 L/s. Increase of black liquor temperature increased spray velocity in most of the cases. At the distance of 1.8 m from the nozzle, average spray velocities varied between 4.0 m/s to 11 m/s, depending on nozzle geometry, flow rate, and temperature. The opening angles of the spray for nozzles A and C varied between 57™‹ to 78™‹. The opening angle of nozzle T with flow guide varied between 48™‹ to 56™‹. The highest penetration of the spray was estimated for nozzle T because of the narrow spray and large droplets.

Image classification of the furnace bed height in recovery boilers, TAPPI Journal July 2026

August 03, 2026

ABSTRACT: The goal of this study is to develop an approach for classifying char bed height in a recovery boiler during operation. The resulting classified images form a dataset that can be used by the mill to support operational improvements. The study integrates data from the digester, evaporation plant, recovery boiler, and recausticizing plant, along with the classified image data. A convolutional neural network (CNN) was trained on bed camera images that were manually annotated to classify furnace bed height. The model was then used to generate a binary bedheight parameter over a 16-month period, and an extreme gradient boosting (XGBoost) model was applied to identify relationships with operating parameters across the recovery cycle. The outcomes of this research may offer deeper insights into the processes underlying bed dynamics. Ultimately, the developed system may serve as a tool for optimizing bed conditions and supporting stable combustion.

Modern NCG practice, TAPPI Journal July 2026

August 03, 2026

ABSTRACT: New innovations have been put in operation for collection and handling systems of noncondensable gases (NCGs) in modern kraft pulp mills. This work investigates technologies in new pulp mills that aim to achieve the benefits of odor-free processes while enhancing chemical circulation within the mill. It describes and discusses both past and current systems for collecting and handling NCGs. The mills studied have designed and implemented their NCG systems within the past 10 to 15 years. New innovations have emerged, offering promising methods to further reduce occasional sulfurous odor emissions from the kraft process. These methods explore pretreatment and sulfuric gas stream reuse options in process units where these gases are generated or during the transportation of these side streams to their final destruction site. Using modern practices outlined in this paper, malodorous gases can be collected to the extent that a kraft pulp mill is essentially odor-free pending operator errors or major equipment malfunctions. One of the difficult aspects is that NCG systems in kraft mills are often designed case-by-case, as well as by various equipment vendors and with varying numbers of destruction sites. The actual detailed design depends on the chosen layout, process configuration depending on individual equipment purchased, and even on the design practices of individual suppliers.


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