The feasibility of utilizing existing process streams in kraft pulp mills as a source of chemicals for lignin extraction

Abstract

Lignin is a sustainable raw material with a high potential for use in the production of renewable products. While the market for lignin is slowly growing, lignin recovery via acid precipitation during the kraft pulping process requires the addition of chemicals that will impact the chemical balance of the pulp mill. This negatively affects both the environmental and business operations. Utilizing existing process streams as a source of chemicals will allow the mill to close the chemical loop and reduce emissions, which will have positive environmental impacts. This study investigated the internal production of sulphuric acid (H2SO4) and carbon dioxide (CO2) for use in lignin separation (also called extraction) at a Swedish kraft pulp mill. The process simulation tool CHEMCAD was used to model and analyze the wet gas H2SO4 (WSA) process to produce H2SO4. The chemical absorption process using monoethanolamine (MEA) to capturing CO2 was also analyzed. The utilization of the sulphur-containing gases to produce H2SO4 can generate an amount that corresponds to a significant lignin extraction rate. The CO2 available in the flue gases from a mill well exceeds the amount required for lignin extraction.

Full Article

The Feasibility of Utilizing Existing Process Streams in Kraft Pulp Mills as a Source of Chemicals for Lignin Extraction

Jonas Kihlman * and Christer Gustavsson

Lignin is a sustainable raw material with a high potential for use in the production of renewable products. While the market for lignin is slowly growing, lignin recovery via acid precipitation during the kraft pulping process requires the addition of chemicals that will impact the chemical balance of the pulp mill. This negatively affects both the environmental and business operations. Utilizing existing process streams as a source of chemicals will allow the mill to close the chemical loop and reduce emissions, which will have positive environmental impacts. This study investigated the internal production of sulphuric acid (H₂SO₄) and carbon dioxide (CO₂) for use in lignin separation (also called extraction) at a Swedish kraft pulp mill. The process simulation tool CHEMCAD was used to model and analyze the wet gas H₂SO₄ (WSA) process to produce H₂SO₄. The chemical absorption process using monoethanolamine (MEA) to capturing CO₂ was also analyzed. The utilization of the sulphur-containing gases to produce H₂SO₄ can generate an amount that corresponds to a significant lignin extraction rate. The CO₂ available in the flue gases from a mill well exceeds the amount required for lignin extraction.

Keywords: Lignin removal; Sulphuric acid; CO₂; Chemical absorption; MEA; WSA; CHEMCAD

Contact information: Karlstad University, Dept. of Engineering and Chemical Sciences, SE-651 88 Karlstad, Sweden; *Corresponding author: jonas.kihlman@kau.se

INTRODUCTION

The forest industry and pulp mills will play an important part in the transition towards a sustainable bioeconomy, as pulp mills have good prerequisites to produce sustainable and renewable products. Lignin is a molecule having a high potential for use in the production of advanced biofuels and renewable chemicals (Norgren and Edlund 2014; Dessbesell et al. 2020). The implementation of a lignin extraction process in an existing pulp mill will increase the consumption of various resources, such as steam, water, and chemicals (Benali et al. 2014).

The conditions for lignin recovery vary among different pulp mills. Each pulp mill is unique and has its own set-up and energy balance. However, one commonality for all pulp mills is that lignin extraction will have an impact on the mass and energy balance of the facility. The influence of lignin extraction in kraft pulp mills has been investigated in several studies (Hamaguchi et al. 2011; Périn-Levasseur et al. 2011; Kannangara et al. 2012; Benali et al. 2014; Kihlman 2016).

Lignin separation via acid precipitation is the most developed and widely implemented process (Hubbe et al. 2019). This process requires the addition of acidifying agents, preferably carbon dioxide (CO₂) and sulphuric acid (H₂SO₄). Depending on their origin, each of these acidifying agents generates emissions of greenhouse gases with varying amounts of global warming potential (GWP) (Wells et al. 2015). The lignin extraction processes are straightforward. Chemicals, in the form of CO₂ and H₂SO₄, are purchased and added to the pulp mill’s chemical balance. The lignin is removed, and the excess chemicals must be extracted to preserve the chemical balance. The cost of the acidifying agents and the make-up chemicals required is one of the main parameters that affect the profitability of the lignin extraction process.

The liquor cycle in kraft pulp mills is becoming increasingly closed because of stricter environmental limits and economic incentives to minimize the need for make-up chemicals. The sodium/sulphur (Na/S) balance in pulp mills is important to enable the stable operation to control the costs of make-up chemicals. To manage the Na/S balance, many mills purge electrostatic precipitated (ESP) ash. Although the ESP ash consists primarily of sodium sulphate (Na₂SO₄), it also contains sodium carbonate (Na₂CO₃). The Na/S balance is often visualized as a vector diagram that shows the input and output of the sodium and the sulphur as kg per ADt (Air-dry metric tons) of pulp. Figure 1 shows the effect of the lignin extraction via acid precipitation using H₂SO₄ schematically. The addition of sulphur disrupts the mill’s Na/S balance, so the sulphur output needs to be increased, e.g. by purging ESP ash. Increasing the sulphur output via such purging will also increase the need for sodium make-up. As illustrated in Fig. 1, the addition of H₂SO₄ in the lignin extraction process has a large impact on the mill’s Na/S balance.

Due to economic and environmental reasons, kraft pulp mills are under increasing pressure to reduce the amount of purged sodium and sulphur they release as effluent (Valeur et al. 2000). The purging of salt into the environment is expected to become an issue in the future, as more stringent environmental regulations could lead to limited rights for the emissions of salt.

Fig. 1. The vector diagram of Na/S, with lignin removal via acid precipitation, corresponding to a lignin extraction rate of approximately 45,000 t lignin/year in the reference mill (Kihlman 2016). The solid lines represent the input and output streams that contain sodium and/or sulphur in the existing mill. The dashed lines represent the input and output streams that contain sodium and/or sulphur linked to the extraction of lignin.

The internal production of H₂SO₄ would reduce the need for bleeding ESP ash to maintain the Na/S balance. The Metsä Group’s bioproduct mill in Äänekoski, Finland, which started up in 2017, is an example of this process. Thanks to the mill’s internal H₂SO₄ production plant, the sulphate emissions (via the purging of ESP ash) into waterways are minimized. Despite a production volume that is almost triple that of the old pulp mill in Äänekoski, the new pulp mill can operate within the same emission limits stated in the old environmental permit and wastewater conditions (Metsä Fibre 2020).

Sulphuric acid is one of the most widely used chemicals in the world. Over 250 million tons of H₂SO₄ is produced every year, primarily for use within the fertilizer industry. The raw material for H₂SO₄ is sulphur dioxide (SO₂) gas, where burning elemental sulphur is the most common way to produce SO₂. Industrial waste gases, such as SO₂, hydrogen sulphide (H₂S), carbonyl sulphide (COS), and carbon disulfide (CS₂), are also sources to produce H₂SO₄ (Kjelstrup and Island 1999; King et al. 2013; Sørensen et al. 2015). Although there are several different processes to produce H₂SO₄, they all include the catalytic reaction of SO₂ with oxygen (O₂) to form sulphur trioxide (SO₃), and the reaction of SO₃ with water (H₂O) to form H₂SO₄.

Climate change and its connection to the greenhouse gas CO₂ has driven the development of effective carbon sequestration. A large amount of the CO₂ emitted is from utility or industrial power systems (Chakravarti et al. 2001). In 2017, 23 of the largest pulp and paper mills in Sweden generated over 22 million tons of CO₂ (Andersson 2019). The capture of CO₂ related to the pulp and paper industry has been investigated in several different publications (Möllersten et al. 2003; Möllersten et al. 2004; Hektor and Berntsson 2007; Hektor and Berntsson 2009; Jönsson and Berntsson 2012; Onarheim et al. 2017). The CO₂ emissions from a modern pulp and paper mill, most of which is biogenic in origin, have three main sources: the recovery boiler, the lime kiln, and the power boiler. There are several different methods for CO₂ capture, based primarily on post-combustion, pre-combustion, and oxy-combustion technologies. All these methods aim to reduce the energy requirements and capital cost, which are costly barriers to entry for commercial implementation on a larger scale (Metz et al. 2005; Teir et al. 2010; Kalatjari et al. 2019).

The aim of this study was to evaluate the feasibility of utilizing existing process streams in a kraft pulp mill to enable and reduce the impact of lignin extraction by moving from a linear approach to a circular approach via the internal production of H₂SO₄ and CO₂. This study investigated the potential internal production rates of H₂SO₄ and CO₂ and the related impact such production makes on the mill’s mass and energy balance.

EXPERIMENTAL

The reference mill in this study was a Scandinavian integrated kraft pulp and paper mill with an annual softwood pulp production capacity of 350,000 ADt and an annual paper production capacity of 165,000 tons. This mill was the same reference mill used by Kihlman (2016).

Concentrated non-condensable gases (CNCG) generated in the reference mill were collected from the fiber line, the evaporation plant, and the methanol recovery plant. These gases are normally burned in a dedicated CNCG boiler, which generates SO₂ and CO₂, but there is also the option to burn them in the auxiliary burner or the lime kiln.

Flue gas rich in CO₂ is generated at four different positions at the reference mill, namely the recovery boiler, the power boiler, the lime kiln, and the CNCG boiler. The lime kiln uses a bio-oil, i.e. tall oil pitch, as a fuel and the power boiler uses biomass as the main fuel. The CNCG boiler produces significantly less flue gas than for the other three sources.

Production of H₂SO₄

The use of sodium sulphide (Na₂S) in the kraft pulping process means that a relatively large amount of sulphur-containing gases are generated and collected, generally as methyl mercaptan (CH₃SH), dimethyl sulphide (CH₃SCH₃), dimethyl disulphide (CH₃SSCH₃), and H₂S. These gases are often referred to as CNCGs. Methyl mercaptan, CH₃SCH₃, and CH₃SSCH₃ are largely formed during the pulping process, whereas H₂S is formed in the recovery boiler (Zhu et al. 2002; Sixta 2006). Methyl mercaptan is formed by the H₂S ions reacting with lignin methoxyl groups, and the CH₃SCH₃ is formed by the mercaptide ions reacting with the lignin methoxyl groups. In the presence of O₂, the CH₃SSCH₃ is formed via the oxidation of CH₃SH (Goheen 1964; McKean et al. 1965; Zhu et al. 2002). At a lower pH level, the Na₂S present in the black liquor starts to convert to H₂S, which generates the H₂S primarily in the evaporation plant. The H₂S is also formed in the recovery boiler and in the molten smelt due to the Na₂S reacting with H₂O vapour and CO₂ (Frederick et al. 1996; Järvensivu et al. 2000). Many mills burn these sulphur-containing gases, followed by chemical absorption of the SO₂ generated using sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH)₂). Although this reduces the sulphur emissions, it requires absorption chemicals that subsequently impact the mill’s operating costs. These sulphur-containing gases can instead be converted into H₂SO₄ and remove the need for absorption chemicals. The internal production of H₂SO₄ is also an efficient way to separate the sodium and sulphur streams, which can benefit the mill’s Na/S balance.

The conventional process for producing H₂SO₄ uses dry SO₂ gas as raw material. By catalytic oxidation, the SO₂ reacts with the O₂ to form SO₃, which reacts with H₂O to generate H₂SO₄. This process is well described by King et al. (2013). In the conventional H₂SO₄ process, the feed gas is dried prior to SO₂ oxidation. The company Haldor Topsoe has developed a process called wet gas sulphuric acid (WSA) (Laursen 2007; Sørensen et al. 2015), whereby wet SO₂ gas is fed directly into the oxidation step. The SO₂ in the feed gas, which also contains H₂O_(g), O₂, N₂, and CO₂, among other chemicals, is oxidized catalytically to form SO₃ (Reaction 2) in a catalytic reactor (SO₂ converter seen in Fig. 2) that contains up to three catalytic beds. The main catalyst reaction is the same as in the conventional process. The reaction is exothermal, so the gas is cooled between the catalytic beds in order to optimize the formation of the SO₃. After the last catalytic bed, the SO₃ cools and reacts with the H₂O_(g) in the feed gas (SO₃ converter seen in Fig. 2) to form H₂SO₄ in gaseous form (Reaction 3). In the conventional process, the SO₃ is absorbed in liquified H₂SO₄. The H₂SO_4(g) is then condensed in the WSA condenser to generate strong H₂SO₄ (Reaction 4). The condensation is carried out at a temperature where very little H₂O_(g) condenses, which produces highly concentrated H₂SO₄. The WSA condenser is a falling film condenser, with tubes made of acid-resistant boron silicate glass, in which the process gas/ H₂SO₄ is cooled by air in a cross-current flow. Part of the hot air generated could be used as combustion air in the incinerator (Laursen 2007; King et al. 2013). A schematic diagram of the process is shown in Fig. 2.

H₂S_(g) + 1.5 O_2(g) ↔ H₂O_(g) + SO_2(g) (518 kJ/mole) [Reaction 1]

SO_2(g) + ½ O_2(g) ↔ SO_3(g) (99 kJ/mole) [Reaction 2]

SO_3(g) + H₂O_(g) ↔ H₂SO_4(g) (101 kJ/mole) [Reaction 3]

H₂SO_4(g) + 0.17H₂O_(g) ↔ H₂SO_4(l) (69 kJ/mole) [Reaction 4]

The heat generated from incineration of the sulphur-containing gases, the SO₂ converter step, and the SO₃ converter step (Reactions 1, 2, and 3), in an industrial application, is used to produce high-pressure (HP) steam. The condensation of the H₂SO₄ (Reaction 4) is carried out in an air cooler, which generates hot air.

The WSA process is mainly used for feed gases containing H₂O_(g) and relatively low amounts of SO₂ (preferably less than 6.5 vol-%). The first WSA plant was installed in 1980 and, by 2012, 110 of such plants were sold throughout the world (King et al. 2013). Within the pulp and paper industry, the WSA process is currently offered by Andritz Oy, who have adapted it to kraft pulp mills. Valmet has developed a similar process, also adapted to kraft mills, that produces 50 wt-% to 70 wt-% H₂SO₄ (Valmet 2017). The first commercial plant of this type was started up in 2017 at the Metsä Bioproduct Mill in Äänekoski, Finland (Valmet 2020). A similar, but smaller, demo plant was also installed at the Smurfit Kappa Piteå mill in Sweden (Valeur et al. 2000, 2001).

Fig. 2. Schematic diagram of the WSA process

The sulphur-containing gases generated in the reference mill are burned in a dedicated gas burner, which generates SO₂ and CO₂. The emissions are reduced by chemical absorption in a scrubber, in which NaOH is used as the absorption chemical and reacts with SO₂, see Reactions 5 and 6. The scrubber liquid generated (mainly NaHSO₃) is returned to the mill’s causticizing plant to maintain the Na/S balance, as seen in Fig. 3. Reaction 5 is favored by lower pH levels, i.e. ≤ 8.5 (Schultes 1998), which is advantageous to reduce the amount of NaOH consumed in the scrubber. The scrubber has two-stages, with a pH control at each, which optimize both the absorption of the SO₂ and the consumption of the NaOH. It is assumed that there is not an excessive amount of NaOH added, so no amount of Na₂SO₃ will be generated according to Reaction 6. Therefore, based on the total consumption of NaOH in the scrubber, the total amount of sulphur in the CNCG generated can be calculated according to Reaction 5. Based on this, the amount of sulphur calculated in the CNCG is approximately 98 kg S/h, which corresponds to 2.4 kg S/ADt. The amount of sulphur released into the CNCG varies between mills, since it depends on the sulphidity and heat treatment in the evaporation and cooking processes. Valmet (2017) estimated the amount of sulphur released into the CNCG to be between 3 kg S/ADt and 7 kg S/ADt. The Smurfit Kappa Piteå, Sweden demo plant released 1.7 kg S/ADt, with the ability to increase it to 4 kg S/ADt by subjecting the black liquor to heat treatment (Valeur et al. 2000).

SO₂ + NaOH ↔ NaHSO₃ [Reaction 5]

NaHSO₃ + NaOH ↔ Na₂SO₃ [Reaction 6]

In this study, the calculated amount of sulphur is converted into a typical CNCG composition based on the information obtained from Bordado and Gomes (1998), as can be seen in Table 1.

Table 1. Estimated Composition of the Sulphur-Containing Gases

In the scenario examined, where H₂SO₄ is produced rather than scrubbing with NaOH, sulphur from the sulphur-containing gases is recycled within the mill’s Na/S balance, as seen in Figs. 3 and 4. The amount of sodium added is reduced since no absorption chemicals are necessary.

The CNCGs generated at the mill have a relatively high H₂O content, and the resulting flue gas contains a relatively low amount of SO₂. Therefore, the WSA process is suitable for use, as the wet SO₂ gas can be fed directly to the oxidation step. A process for utilising the sulphur-containing gases in the production of H₂SO₄ via the WSA process was set-up in CHEMCAD, Version 7.1.4 (Houston, TX). CHEMCAD is a flow sheeting software for process modelling that resolves mass- and energy balances as well as chemical and physical equilibrium.

Fig. 3. Loop of the current CNCG system

Fig. 4. Loop of a future CNCG system retrofitted for internal production of sulphuric acid

Fig. 5. Schematic diagram of the H₂SO₄ plant from the CHEMCAD model

The process was simulated with the main input data according to Table 1. The O₂ excess in both the CNCG combustion and the SO₂ converter was set to 1 wt-%. CHEMCAD GIBS (Gibbs Free Energy Reactor) blocks were used for the incineration, SO₂ conversion, SO₃ conversion, and condenser steps. The Gibbs reactor is based on the principal that at chemical equilibrium, the total Gibbs energy of the system has its minimum value. The NTRL was used as the K-value model, and latent heat was used as the enthalpy model. An overview of the CHEMCAD model is shown in Fig. 5.

The model was validated by comparing the equilibrium constants calculated for Reactions 2 and 3 to data available in the literature (Kjelstrup and Island 1999; King et al. 2013). The model validation shows reasonably consistent values, indicating that the gas composition from the simulation model shows good compliance with data from literature.

CO₂ Capture

The post-combustion method refers to the capture of CO₂ from flue gases generated via the combustion of fossil fuels or biomass in air. Rather than releasing flue gases directly into the atmosphere, CO₂ is separated, and the remaining CO₂-lean flue gas is discharged instead (Metz et al. 2005). Chemical absorption is the most common method to capture CO₂ and it is one of a few techniques that are practical for removing large amounts of CO₂. Compared to other post-combustion processes, chemical absorption ensures high absorption efficiency and selectivity while using a relatively low amount of energy (Metz et al. 2005; Akanksha et al. 2007; Cormos et al. 2009; Rochelle 2009).

Fig. 6. Schematic diagram of CO₂ capture via chemical absorption

Chemical absorption is suitable to recover CO₂ from flue gases due to its low concentration of CO₂. This process does not require design modifications to the boiler. There are several different alkanolamines that are used as absorbents in commercial use, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), triethanolamine (TEA), diisopropanolamine (DIPA), and diglycolamine (DGA). MEA is a well-known and used chemical for chemical absorption and thus used in this work. Prior to the capture of CO₂, the flue gas normally requires cooling and treatment to reduce the number of particulates and other impurities that may be present. Allowing flue gas to encounter an absorbent in an absorber column enables the CO₂ to be captured. The absorbent loaded with CO₂ is then transported to a stripper, where it is heated to release the CO₂ and regenerate the absorbent. The regenerated absorbent is then recycled for further CO₂ capture (Chakravarti et al. 2001; Metz et al. 2005; Edwards 2009), as can be seen in Fig. 6.

The overall reaction between the CO₂ and the MEA is shown in Reaction 7. The reaction involves a weak base (MEA) and a weak acid (CO₂). The equilibrium occurs in the liquid phase, where the CO₂ is absorbed in the aqueous solution of the MEA (Akanksha et al. 2007; Cormos et al. 2009).

CO₂+2HCOH₂CH₂NH₂↔HCOH₂CH₂NH₃⁺+HCOH₂CH₂NHCOO^– [Reaction 7]

Proven chemical absorption processes are commercially available today. Existing production plants have a production capacity that vary between 6 tCO₂/day and 800 tCO₂/day (Metz et al. 2005).

The potential amount of CO₂ available in a kraft pulp mill well exceeds the amount required in the lignin extraction process. Although the recovery boiler generates the largest quantities of flue gas and CO₂, it has a lower fraction of CO₂ compared to the flue gas that exits the lime kiln. The CNCG boiler generates a relatively small amount of flue gas. Therefore, the flue gas from the lime kiln is the most appropriate source of CO₂ for this application because it has a reasonable amount of CO₂ at a relatively high concentration. Flue gas with a higher concentration of CO₂ causes a lower specific steam consumption (Freguia and Rochelle 2003; Garðarsdóttir et al. 2018).

The flow and composition of the flue gas from the lime kiln were calculated based on the amount of tall oil pitch that was consumed as fuel in the lime kiln and the CO₂ that was generated during the calcining process. The conversion of calcium carbonate (CaCO₃) into calcium oxide (CaO) in the lime kiln can be seen in Reaction 8.

Calcining: CaCO₃ + heat CaO + CO₂ [Reaction 8]

The composition of the flue gas from the lime kiln was calculated and is reported in Table 2. Approximately 20% and 80% of the CO₂ generated was from the lime kiln fuel and the calcining process, respectively. In theory, the total amount of CO₂ available in the lime kiln flue gas should exceed the amount of CO₂ required for the extraction of 45,000 tons of lignin per year by a factor of approximately 30 (Kihlman 2016).

Table 2. Calculated Composition of the Flue Gas from the Lime Kiln

Chemical absorption using MEA as the absorbent is an appropriate method to recover CO₂ from flue gases, so it was chosen for the CO₂ capture at the reference mill. A simulation model in CHEMCAD was established to investigate the impact of a chemical absorption process related to the pulp mill. The input data, along with the variables in the simulation model, are compiled in Table 3. The CHEMCAD block and Simultaneous Correction Distillation System (SCDS) were used for the absorber and the stripper. The Amine Model, a built-in package in CHEMCAD, was used for the thermodynamic calculations. The Amine Model uses the Kent-Eisenberg method, which is a simplified way of modelling reactions in a gas sweetening system. An overview of the CHEMCAD model is shown in Fig. 7.

Table 3. Input Data and Variables Used in the Chemical Absorption Process Using MEA as the Solvent

Fig. 7. Schematic diagram of the CO₂ capture plant from the CHEMCAD model

RESULTS AND DISCUSSION

The combustion of the CNCG flow generated a flue gas that contained approximately 5 vol-% SO₂, 13 vol-% H₂O, 7 vol-% CO₂, 1 vol-% O₂, and 74 vol-% N₂. The SO₂ content of the flue gas was within the preferred limits for the WSA process. The H₂SO₄ simulation model was optimized to produce as much H₂SO₄ as possible. The results and detailed input data are presented in Table 4, which is based on the overall input data in Table 1 and the process characteristics, as illustrated in Figs. 8, 9, 10, and 11.

The gas flows of SO₂, SO₃, and H₂SO₄ were calculated based on the theoretical equilibrium conditions, and the results are plotted in Fig. 8 as a function of the temperature in the incinerator. Figure 8 shows that a low amount of SO₂ is generated at lower temperatures, and the SO₂ production reached a maximum at approximately 1,000 °C. This temperature will also achieve a good burn-out of all the combustibles in the incinerator (Wallenius 2020).

Table 4. Input Values Used in the H₂SO₄ WSA-Process Simulation Model and the Results Obtained

The catalytic oxidation of the SO₂ to SO₃ is exothermic, so the catalytic beds in the SO₂ converter are cooled to optimize the formation of SO₃. The maximum formation was reached at a temperature of approximately 430 °C, as shown in Fig. 9. However, the total amount of the H₂SO₄ produced in the WSA process did not change until the temperature reached 560 °C. This indicates that the remaining SO₂ was converted into SO₃ and, eventually, H₂SO₄ later in the process.

Fig. 8. Gas flow based on the theoretical equilibrium conditions at different temperatures in the GIBS incinerator block

Fig. 9. The quantities of SO₂, SO₃, and H₂SO₄ generated in the SO₂ converter at different temperatures. The total amount of H₂SO₄ produced in the WSA process is also shown.

The SO₃ is hydrated into H₂SO_4(g) in the SO₃ converter, according to Reaction 3. Although this reaction is promoted by a low temperature (King et al. 2013), as seen in Fig. 10, corrosion issues dictate the importance of the H₂SO₄ being kept in gaseous form before entering the WSA condenser. The acid dew point, which depends on the concentrations of the H₂SO₄ and H₂O in the gas, is typically between 220 °C to 265 °C (King et al. 2013). Figure 10 shows that the maximum yield of the H₂SO₄ after the SO₃ converter, without any condensation, is achieved at a temperature of approximately 230 °C. The literature reports the normal operating temperature in this SO₃ converter stage to be 250 °C to 290 °C (King et al. 2013; Sørensen et al. 2015). This operating temperature facilitates the energy from the hydration to be available at a higher temperature level, but it also provides a margin that hinders the condensation of the H₂SO₄ and potential corrosion problems. The SO₃ that remains after the SO₃ converter is hydrated in the WSA condenser to produce H₂SO₄.

Fig. 10. The amount of the H₂SO₄ generated in the SO₃ converter at different temperatures. The vapor fraction and the amounts of the H₂O and the SO₃ exiting the SO₃ converter are also shown.

After it is processed in the SO₃ converter, the process gas is cooled further in the WSA condenser, which hydrates the remaining SO₃ and condenses the H₂SO₄. The WSA condenser is a counter-current heat exchanger in which the bottom temperature was found to be approximately 243 °C at the assumed pressure, which produces H₂SO₄ with a high level of consistency. The temperature in the upper section was set to 100 °C to minimize the escape of the H₂SO_4(g) into the stack gas (Fig. 11).

Based on the calculated CNCG flow and the optimized input data, 2,600 tons of H₂SO₄ can be produced annually, as seen in Table 4. A lignin extraction rate of 45,000 tons per annum (TPA) means that the H₂SO₄ that is produced should cover 32% of the amount required (Kihlman 2016). As can be seen in Fig. 12, the amount of H₂SO₄ that is produced corresponds to a lignin extraction rate of 15,250 TPA. Even if the amount of H₂SO₄ produced is insufficient for the desired lignin extraction rate, it can be very important for the mill’s Na/S balance. The amount of fresh H₂SO₄ required will be greatly reduced, which will also reduce the higher removal rate of the ESP ash, and thereby maintain the Na/S balance. As described by Kihlman (2016), the lignin extraction will have a large impact on the removal of the ESP ash. The annual economic value of the H₂SO₄ produced is approximately 180,000€. However, the total economic value is much higher considering that less NaOH make-up chemical is required, compared to the addition of only fresh H₂SO₄. Compared to H₂SO₄, NaOH is a much more expensive chemical, at approximately 9× the cost compared to H₂SO₄.

Fig. 11. The mass flow of the H₂SO₄ in the stack gas exiting the WSA condenser at different temperatures in the top section of the condenser

Fig. 12. The consumption of H₂SO₄ at the different lignin extraction rates

Figure 12 illustrates the fact that the H₂SO₄ that is produced will only cover a limited lignin extraction rate (Lake et al. 2015; Kihlman 2016). There are nevertheless various potential methods and processes available that can increase the amount of sulphur-containing gases available in a kraft pulp mill for the production of H₂SO₄, such as steam-stripping (Nilsson 2017), black liquor oxidation, heat treatment, and the acidification of green liquor (Valeur et al. 2000; Välimäki et al. 2015; Valmet 2017). Acidifying green liquor by stripping it with CO₂ is probably the most effective way to increase the internal production of H₂SO₄.

The capture of CO₂ via chemical absorption consumes a relatively high amount of energy in order to regenerate the absorbent. Unlike most studies of carbon capture and storage (CCS), there are no incentives in this case to capture as much CO₂ as possible because the amount of CO₂ available in the flue gas greatly exceeds the demand for lignin extraction. Therefore, it was investigated whether there are energy benefits to be gained by running the process at a lower yield. Figures 13 and 14 show that at high CO₂ yields, the energy consumption is expected to be very high, but the energy consumption decreases as the CO₂ yield decreases. However, the energy consumption levels out at a CO₂ yield of approximately 85%, so it is unrealistic to attain a lower CO₂ yield. Therefore, a reasonable CO₂ yield should be approximately 85%, which would ensure that the energy consumption remains at a low level and keep the equipment dimensioning flow of the flue gases at a reasonable level.

Fig. 13. The CO₂ yield versus the stripper reboiler duty

Fig. 14. The energy consumption per kg captured CO₂ versus stripper reboiler duty

The energy consumption at a CO₂ yield of 85% is 3,420 kJ/kg CO₂, as can be seen in Table 5. The CO₂ consumption for the lignin extraction is approximately 0.15 to 0.25 per ton of lignin (Kihlman 2016). Therefore, the annual consumption of CO₂ at a lignin extraction rate of 45,000 TPA is approximately 8,100 tons. The capture of this CO₂ corresponds to an annual energy consumption of 8 GWh. The relatively small amount of captured CO₂ means that the energy consumption necessary will only have a minor impact on the mill’s energy balance in terms of both heat and electricity. A lignin extraction rate of 45,000 TPA will decrease the quantity of fuel (i.e. the amount of lignin in the black liquor) fed into the recovery boiler by approximately 335 GWh per year, which will generate significantly less steam.

The chemical absorption process requires low pressure (LP) steam to regenerate the absorbent, cooling water for the condensers, flue gas, amine cooling, and electricity for pumping and CO₂ compression (Table 5).

Table 5. Utility Consumption per kg of Captured CO₂ (at a capture rate of 85%)

Based on a CO₂ yield of 85%, the maximum amount of CO₂ captured from the lime kiln flue gas is approximately 190,000 TPA, which well exceeds the requirement for lignin extraction. There are other possible applications for CO₂ at such a mill, but the potential for using other methods that are currently available is relatively low. Kuparinen et al. (2019) described how CO₂ can be used for acidulation in the production of tall oil and to produce precipitated calcium carbonate. The acidulation of raw soap, which is used to produce tall oil, is normally performed by adding H₂SO₄. Sometimes, spent acid from the chlorine dioxide plant can also be added. However, CO₂ is a weaker acid than H₂SO₄ and only some of the H₂SO₄ that is required can be replaced with CO₂, which corresponds to 4 to 6 kg CO₂/ADt (Kuparinen et al. 2019). Although the production of CaCO₃ varies depending on the local demand, the consumption of CO₂ can be 20 kg CO₂/ADt (Kuparinen et al. 2019). These alternative applications and uses for CO₂, combined with lignin extraction methods, can consume up to 17,200 TPA of CO₂. This is a small volume of the total amount of CO₂ available of the flue gas generated in the lime kiln, but nevertheless a step in the right direction to capture and utilize CO₂.

CONCLUSIONS

1. The internal production of H₂SO₄ is an effective way to close a kraft pulp mill chemical loop and reduce emissions. The potential production of H₂SO₄ from existing sulphur-containing gases at the mill that was studied is sufficient for lignin extraction rate of 15,250 TPA, which is slightly less than the volume of existing full-scale industrial plants. Utilizing existing process streams to produce H₂SO₄ and moving towards a more circular approach may be crucial to implement a lignin extraction process. All efforts that are made to minimize the impact on the mill’s balance will be beneficial.
2. The total amount of CO₂ available at such a mill well exceeds the requirements of lignin extraction. Chemical absorption is an energy-intensive process. Compared to the mill’s total energy balance, the energy required to capture the amount of CO₂ necessary for the lignin extraction process is minor. This also applies to other utilities that are required, such as cooling water.
3. In this study, only a small amount of the total CO₂ present in the flue gas generated in the lime kiln was captured. As interest in bioenergy carbon capture and storage (BECCS) grows, CO₂ can be utilized for lignin extraction. In combination with lignin extraction, BECCS will increase the mill’s consumption of steam significantly. The mill’s capacity to produce steam will be a limiting factor in capturing the large amount of CO₂ present in the flue gas unless investments are made. Therefore, it may be relevant to design a BECCS system to capture a limited amount of CO₂ so that the mill’s energy balance can be fulfilled without making extensive changes and/or investing in auxiliary equipment.
4. Applying this circular approach to lignin extraction is an innovative process. It could be vital, both environmentally and economically, for the future development and expansion of the kraft lignin market.
5. For further studies it would be of interest to examine the possibilities to increase the amount of sulphur-containing gases and thereby enabling even higher production of sulphuric acid. One wants to find out which methods would be most suitable, how much sulphuric would be possible to produce, and in what way would this affect the mill´s Na/S balance.

ACKNOWLEDGMENTS

This study was performed within the Industrial Graduate School VIPP (Values Created in Fibre Based Processes and Products) with financial support from the Knowledge Foundation (Sweden) and AFRY.

The authors would like to thank their supervisor, Lars Nilsson, for his help and guidance throughout this work, Magnus Eriksson and Ola Nilsson at the BillerudKorsnäs Karlsborg Mill for supplying data and answering questions, and Maureen Sondell for her linguistic revision of the original manuscript.

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Article submitted: August 27, 2020; Peer review completed: October 18, 2020; Revised version received and accepted: December 9, 2020; Published: December 16, 2020.

DOI: 10.15376/biores.16.1.1009-1028