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Journal articles
Prediction of residual calcium carbonate (CaCO3) in the lime
ABSTRACT: In a kraft pulp mill, the rotary lime kiln is responsible for converting calcium carbonate (CaCO3) into calcium oxide (CaO) to be reused in the causticizing plant. An important parameter of the calcination efficiency and the product quality is the amount of CaCO3 not converted to CaO, commonly defined as residual carbonate. This parameter is usually determined through laboratory analysis, which introduces delays in process control and limits the ability to make timely operational adjustments. This work presents a predictive model that functions as a soft sensor for the residual carbonate in a lime kiln of a Brazilian kraft pulp mill. This model was able to estimate residual CaCO3 every 30 min using routinely measured online process variables, which is a considerable reduction from the 6-h average laboratory measurement. The correlation coefficient (r) between the model predictions and the mill values was 0.83, with a mean absolute error (MAE) of 0.33%. Considering the effect of the residence time throughout the lime kiln proved to be decisive in obtaining a satisfactory prediction result. After model validation, a variable importance analysis showed that the carbon monoxide (CO) concentration in the stack flue gas (in ppm), secondary air temperature (in oC), and fan speed (in %) were the most influential variables in predicting the residual carbonate content. Combined with commonly used operation expertise, this subset of variables can serve as additional information to achieve smoother lime kiln operation.
Journal articles
Black liquor spray properties in-furnace conditions for swirl cone nozzles, TAPPI Journal July 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 (ƒ¢Te), 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.
Journal articles
Non-process elements in the lime cycle: New findings regarding NPE distribution and lime mud filterability, TAPPI Journal July 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.
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NCSU Strategic and Financial Analysis for the Forest Product Industry
NCSU Strategic and Financial Analysis for the Forest Product Industry Register Now OVERVIEW INSTRUCTORS REGISTRATION Raleigh, NC TAPPI, The Paper Science and Engineering School, and The Business
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2021 Herman L. Joachim Distinguished Service Award Winner
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TAPPI Journal Awards Best Research Papers for 2025
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