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Recovery boiler back-end heat recovery, TAPPI Journal March 2023
ABSTRACT: Sustainability and efficient use of resources are becoming increasingly important aspects in the operation of all industries. Recently, some biomass-fired boilers have been equipped with increasingly complex condensing back-end heat recovery solutions, sometimes also using heat pumps to upgrade the low-grade heat. In kraft recovery boilers, however, scrubbers are still mainly for gas cleaning, with only simple heat recovery solutions. In this paper, we use process simulation software to study the potential to improve the power generation and energy efficiency by applying condensing back-end heat recovery on a recovery boiler. Different configurations are considered, including heat pumps. Potential streams to serve as heat sinks are considered and evaluated. Lowering the recovery boiler flue gas temperature to approximately 65°C significantly decreases the flue gas losses. The heat can be recovered as hot water, which is used to partially replace low-pressure (LP) steam, making more steam available for the condensing steam turbine portion for increased power generation. The results indicate that in a simple condensing plant, some 1%•4% additional electricity could be generated. In a Nordic mill that provides district heating, even more additional electricity generation, up to 6%, could be achieved. Provided the availability of sufficient low-temperature heat sinks to use the recovered heat, as well as sufficient condensing turbine swallowing capacity to utilize the LP steam, the use of scrubbing and possibly upgrading the heat using heat pumps appears potentially useful.
Journal articles
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Non-process elements in the recovery cycle of six Finnish kraft pulp mills, TAPPI Journal March 2023
ABSTRACT: In this work, the aim was to study the distribution and accumulation of the non-process elements (NPEs) in the recovery cycle of Finnish pulp mills and look at whether the geographical location (North vs. South) correlates with the current Finnish NPE levels. In addition, a comparison to older similar Finnish measurements was made with an attempt to analyze the reasons behind differences in the most typical non-process elements, aluminum (Al), silicon (Si), calcium (Ca), phosphorus (P), magnesium (Mg), manganese (Mn), chlorine (Cl), and potassium (K), taking into account the main elements in the white liquor, sodium (Na) and sulfur (S). The extensive laboratory results gained in this study are from seven sampling points at six pulp mills and present analytical data of metal concentrations. The data obtained presents an update to previous NPE studies. The levels found did not statistically differ between North and South Finland. The NPE levels, apart from phosphorus, found in Finnish pulp mills today have not changed considerably compared to the levels in earlier investigations in the 1990s. In the newest data, the phosphorus concentration was consistently higher in the as fired black liquor, electrostatic precipitator (ESP) ash, lime mud, and green liquor than in the previous results. In addition, the levels of Al, Si, Ca, P, and Mg in recovery boiler ESP ash were consistently higher compared to the older results. As the mills start to close their systems more, a stronger accumulation of NPEs can be expected, increasing the likelihood of more operational problems in the process. Further understanding of where the NPEs accumulate and how they can be most effectively removed will be valuable knowledge in the future.
Journal articles
Magazine articles
Using novel DNA methods to achieve higher process efficiency and performance, TAPPI Journal January 2023
ABSTRACT: Uncontrolled microbiological activity is a challenge for recycled fiber (RCF) mills as it can have negative effects on production and end-product quality. The microbes that exist in these systems have been largely unknown, and the strategies employed to control microbiology have been non-specific. Understanding the specific microbial groups present in RCF mills, their properties, and where they exist, as well as having the ability to accurately measure the true troublemakers, are key to targeted control of the bad actors. In this study, we present the results of a global survey of over 40 RCF paper machines. The same RCF-specific problem-causing bacterial groups were found on different continents, including large densities of newly identified bacteria in paper processes. Those can degrade cellulose and starch, produce acids and odorous substances, and have a significant impact on fiber strength and additive consumption. We also demonstrate how modern DNA tools can quantify the impact of biocidal countermeasures against the actual troublemakers, including bacteria found to degrade cellulose during RCF pulp storage, which may be linked to a negative impact on end-product strength. These novel DNA tools give producers updated biocide program key performance indicators (KPIs) and actionable information to more effectively design and adjust microbiological control to achieve higher process efficiency and performance.
Journal articles
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Advanced real-time digital microscopy of foaming processes, TAPPI Journal January 2023
ABSTRACT: The properties of aqueous foams play a major role in foam forming and foam coating. Inline real-time foam measurements provide highly desired opportunities for optimization and control of foaming processes. This paper presents inline digital microscopy measurements of aqueous foams in foaming processes. It presents methods for providing detailed information on foam quality parameters, such as foam density and foam homogeneity in real time from the process. In addition, this study evaluates the performance of transillumination and front-light illumination in imaging of foams. The tests show very good results for the transillumination approach. Limitations of the image-based optical technique are discussed, and the precision of bubble size distribution measurement is assessed with a certificated reference substance. The measured foam densities are compared against the reference foam densities in the range 100•300 g/L, providing a linear correlation with R2 value of 0.99. In the case of heterogenous foams with a wide bubble size distribution, the bubble size-dependent dimensionless depth of field must be taken into account to obtain accurate estimates of foam density. Bubble-scale foam homogeneity is described by the standard deviation of bubble size distribution in foam.
Journal articles
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Modeling the influence of rheology on smooth rod coating systems, TAPPI Journal November 2022
ABSTRACT: Rod coating methods are of interest for the application of barrier coatings, especially at off-line facilities that may run at moderate speeds and narrow web widths. At lower line speeds and lower coating solids, it is difficult to achieve good coat weight control because of poor loading of the rod. While there is extensive literature available about blade and roll coating, there seems to be less reported on the rod loading of smooth rods to obtain various coat weights. Much of the work is around metering rods working on applicator rolls at high speeds that are associated with the metered size press, with a focus on ribbing instabilities. This work employs a simplified model, neglecting some complex features of rubber deformation and film split, to estimate the influence of the process parameters such as speed, rod diameter, viscosity, and rod loading on the coat weight obtained. As found in practice, at low speed and low viscosities, the coat weight-load curve is steep, leading to poor control of the coat weight and coat weight uniformity. If the viscosity is increased, the curve is modified, and control is possible with rod loading in a normal range. For shear thinning fluids described by the Carreau model, the power law index and other parameters need to be in the correct range to obtain the desired effect. Modeling predictions show a steeper dependence of coat weight with rod pressure when compared to pilot coater data. This may be caused by missing details in the mechanical loading of the rod related to tube pressure or from neglecting the impact of filter cake formation of the applied coating in the model.
Journal articles
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Determining operating variables that impact internal fiber bonding using Wedge statistical analysis
ABSTRACT: In this study, Wedge statistical analysis tools were used to collect, collate, clean up, plot, and analyze several years of operational data from a commercial paper machine. The z-direction tensile (ZDT) and Scott Bond tests were chosen as representative of fiber bond strength. After analyzing thousands of operational parameters, the ones with the most significant impact upon ZDT involved starch application method, starch penetration, and the amount of starch applied. Scott bond was found to be significantly impacted by formation and refining. Final calendering of the paper web has also shown an impact on internal fiber bonding.
Journal articles
Magazine articles
Can carbon capture be a new revenue opportunity for the pulp and paper sector?, TAPPI Journal August 2021
ABSTRACT: Transition towards carbon neutrality will require application of negative carbon emission technologies (NETs). This creates a new opportunity for the industry in the near future. The pulp and paper industry already utilizes vast amounts of biomass and produces large amounts of biogenic carbon dioxide. The industry is well poised for the use of bioenergy with carbon capture and storage (BECCS), which is considered as one of the key NETs. If the captured carbon dioxide can be used to manufacture green fuels to replace fossil ones, then this will generate a huge additional market where pulp and paper mills are on the front line. The objective of this study is to evaluate future trends and policies affecting the pulp and paper industry and to describe how a carbon neutral or carbon negative pulp and paper production process can be viable. Such policies include, as examples, price of carbon dioxide allowances or support for green fuel production and BECCS implementation. It is known that profitability differs depending on mill type, performance, energy efficiency, or carbon dioxide intensity. The results give fresh understanding on the potential for investing in negative emission technologies. Carbon capture or green fuel production can be economical with an emission trade system, depending on electricity price, green fuel price, negative emission credit, and a mill’s emission profile. However, feasibility does not seem to evidently correlate with the performance, technical age, or the measured efficiency of the mill.
Journal articles
Magazine articles
Black liquor evaporator upgrades— life cycle cost analysis, TAPPI Journal March 2021
ABSTRACT: Black liquor evaporation is generally the most energy intensive unit operation in a pulp and paper manufacturing facility. The black liquor evaporators can represent a third or more of the total mill steam usage, followed by the paper machine and digester. Evaporator steam economy is defined as the unit mass of steam required to evaporate a unit mass of water from black liquor (i.e., lb/lb or kg/kg.) The economy is determined by the number of effects in an evaporator train and the system configuration. Older systems use four to six effects, most of which are the long tube vertical rising film type. Newer systems may be designed with seven or even eight effects using falling film and forced circulation crystallization technology for high product solids. The median age of all North American evaporator systems is 44 years. Roughly 25% of the current North American operating systems are 54 years or older. Older systems require more periodic maintenance and have a higher risk of unplanned downtime. Also, older systems have chronic issues with persistent liquor and vapor leaks, shell wall thinning, corrosion, and plugged tubes. Often these issues worsen to the point of requiring rebuild or replacement. When considering the age, technology, and lower efficiency of older systems, a major rebuild or new system may be warranted. The intent of this paper is to review the current state of black liquor evaporator systems in North America and present a basic method for determining whether a major rebuild or new installation is warrant-ed using total life cycle cost analysis (LCCA).
Journal articles
Magazine articles
Modeling and parameter optimization of the papermaking processes by using regression tree model and full factorial design, TAPPI Journal February 2021
ABSTRACT: One of the major challenges in the pulp and paper industry is taking advantage of the large amount of data generated through its processes in order to develop models for optimization purposes, mainly in the papermaking, where the current practice for solving optimization problems is the error-proofing method. First, the multiple linear regression technique is applied to find the variables that affect the output pressure controlling the gap of the paper sheet between the rod sizer and spooner sections, which is the main cause of paper breaks. As a measure to determine the predictive capacity of the adjusted model, the coefficient of determination (R2) and s values for the output pressure were considered, while the variance inflation factor was used to identify and eliminate the collinearity problem. Considering the same amount of data available by using machine learning, the regression tree was the best model based on the root mean square error (RSME) and R2. To find the optimal operating conditions using the regression tree model as source of output pressure measurement, a full factorial design was developed. Using an alpha level of 5%, findings show that linear regression and the regression tree model found only four independent variables as significant; thus, the regression tree model demonstrated a clear advantage over the linear regression model alone by improving operating conditions and demonstrating less variability in output pressure. Furthermore, in the present work, it was demonstrated that the adjusted models with good predictive capacity can be used to design noninvasive experiments and obtain.
Journal articles
Magazine articles
Understanding wet tear strength at varying moisture content in handsheets, TAPPI Journal January 2021
ABSTRACT: A laboratory study was conducted looking at the effects of moisture content on wet tear strength in handsheets. Three different wetting techniques were used to generate the wet tear (Elmendorf-type) data at varying moisture levels, from TAPPI standard conditions (dry) to over 60% moisture content (saturated). Unbleached hardwood and softwood fiber from full-scale kraft pulp production were used. The softwood fiber was refined using a Valley beater to reduce freeness. Handsheets were made with a blend of hardwood and softwood and with refined softwood, without the addition of wet-end chemistry. The resulting grams-force tear data obtained from the test was indexed with basis weight and plotted versus both moisture content and dryness. As moisture content levels in the handsheets increased, the wet tear strength also increased, reaching a critical maximum point. This marked a transition point on the graph where, beyond a critical moisture content level, the tear strength began to decline linearly as moisture increased. This pattern was repeated in handsheets made from a blend of hardwood and softwood and from 100% refined softwood.