Integrated Forest Biorefinery

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Integrated Forest Biorefinery

A Proposed Pulp Mill of 2040

Caroline Wilke, Magnus Lestelius, Ulf Germgård

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RESEARCH REPORT | Karlstad University Studies | 2021:4

Integrated Forest Biorefinery

A Proposed Pulp Mill of 2040

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Distribution:

Karlstad University

Faculty of Health, Science and Technology

Department of Engineering and Chemical Sciences SE-651 88 Karlstad, Sweden

+46 54 700 10 00

ISSN 1403-8099

urn:nbn:se:kau:diva-82467

Karlstad University Studies | 2021:4 RESEARCH REPORT

Caroline Wilke, Magnus Lestelius, Ulf Germgård

Integrated Forest Biorefinery - A Proposed Pulp Mill of 2040

ISBN 978-91-7867-181-6 (pdf) ISBN 978-91-7867-180-9 (print)

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Preface

This report is part of the examination for the doctoral degree of Dr. Caroline Wilke. During the process of writing, the report has been up-dated with state-of-the-art knowledge and the sustainability perspec-tive has been widened.

That authors wish to express their sincere gratitude to the late Techn. Lic. Mr. Hans Magnusson for allowing us to tap into his profound knowledge on biorefinery processes. Without his contribution, the re-port at hand would have been much less complete. Techn. Lic. Magnus-son’s sense for usefulness and viability of methods was also vital. His never-ending support of our projects will not be forgotten.

This study was performed within the Industrial Graduate School VIPP (Values Created in Fiber Based Processes and Products) at Karlstad University, Karlstad, Sweden, with financial support from (i) the Knowledge Foundation, Stockholm, Sweden, (ii) BTG Instruments AB, Säffle, Sweden, and (iii) Karlstad University, Karlstad, Sweden.

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Abstract

The objective of a wood-based biorefinery is to upgrade biomass to val-uable products such as pulp, biofuels, and chemicals. A conventional kraft pulp mill could be transformed into an integrated forest biorefin-ery by implementing a number of new processes. Using byproducts from the pulp mill for other purposes than fuel will ultimately affect the energy balance in the pulp mill since the amount of organic material that is incinerated in the recovery boiler is significantly reduced. Fur-thermore, the important sodium/sulfur balance will be affected since e.g. sulfuric acid is required for some of the new processes.

The following processes are discussed in the report:

▪ Black liquor gasification to produce electric power and heat to the pulp mill, and/or biofuel for the transport sector.

▪ Lignin extraction with the LignoBoost process to produce biofuel or other chemicals and products based on lignin.

▪ On-site production of sulfuric acid from non-condensable gases. ▪ CleanFlow membrane filter, resulting in cleaner green liquor. ▪ Tall oil diesel production from fatty acids.

▪ Extraction of hemicellulose from wood chips to produce ethanol. Black liquor gasification is not currently implemented in pulp mills de-spite its positive effects, e.g. high production of electric power or pro-duction of biofuels. Therefore, the proposed best available technology kraft pulp mill of 2040 still has a conventional recovery boiler but does contain the other new processes. By using the byproducts in these new processes, the load to the recovery boiler is reduced, thus potentially leading to an increased production rate. Furthermore, on-site produc-tion of sulfuric acid results in an internal recycling of sulfur, thus re-ducing the effect on the sodium/sulfur balance and the need for sodium make-up.

Abstract

The objective of a wood-based biorefinery is to upgrade biomass to val-uable products such as pulp, biofuels, and chemicals. A conventional kraft pulp mill could be transformed into an integrated forest biorefin-ery by implementing a number of new processes. Using byproducts from the pulp mill for other purposes than fuel will ultimately affect the energy balance in the pulp mill since the amount of organic material that is incinerated in the recovery boiler is significantly reduced. Fur-thermore, the important sodium/sulfur balance will be affected since e.g. sulfuric acid is required for some of the new processes.

The following processes are discussed in the report:

▪ Black liquor gasification to produce electric power and heat to the pulp mill, and/or biofuel for the transport sector.

▪ Lignin extraction with the LignoBoost process to produce biofuel or other chemicals and products based on lignin.

▪ On-site production of sulfuric acid from non-condensable gases. ▪ CleanFlow membrane filter, resulting in cleaner green liquor. ▪ Tall oil diesel production from fatty acids.

▪ Extraction of hemicellulose from wood chips to produce ethanol. Black liquor gasification is not currently implemented in pulp mills de-spite its positive effects, e.g. high production of electric power or pro-duction of biofuels. Therefore, the proposed best available technology kraft pulp mill of 2040 still has a conventional recovery boiler but does contain the other new processes. By using the byproducts in these new processes, the load to the recovery boiler is reduced, thus potentially leading to an increased production rate. Furthermore, on-site produc-tion of sulfuric acid results in an internal recycling of sulfur, thus re-ducing the effect on the sodium/sulfur balance and the need for sodium make-up.

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Abbreviations

BAT Best available technology BLG Black liquor gasification

BLGCC Black liquor gasification with combined cycle power generation

BLGMF Black liquor gasification with motor fuel production CaO Calcium oxide

ClO2 Chlorine dioxide

CO Carbon monoxide CO2 Carbon dioxide

CTO Crude tall oil

ESP Electrostatic precipitator H2 Hydrogen gas

H2S Hydrogen sulfide

H2SO4 Sulfuric acid

IFBR Integrated forest biorefinery

Na Sodium

Na2CO3 Sodium carbonate

NaOH Sodium hydroxide Na2S Sodium sulfide

NCG Non-condensable gases NOx Nitrogen oxides

O2 Oxygen

PS Polysulfide

S Sulfur

SO2 Sulfur dioxide

SO3 Sulfur trioxide

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1 Pulp mill biorefinery

In the modern society, particularly including the last 100 years, fossil oil has been an important ingredient and reason to an outstanding de-velopment in material wealth and new products. However, this pros-perous time is now coming to an end due to several reasons. One reason is that available crude oil resources are being depleted due to continued and ever-increasing use. Another reason is that our high standard of living negatively affect the global environment in a number of ways, and the increased emissions of greenhouse gases are a big concern. It is thus clear that the negative influence on the global environment is due to direct and indirect activities of humans. The environmental awareness is growing among citizens all around the globe, which has been very clear from e.g. the success story of Swedish Greta Thunberg’s activity for a more environmentally acceptable way of living [1]. In the future, it will thus be important to handle our consumption of natural resources like oil, metals, fresh water, etc. in a sustainable manner, e.g. recycle and reuse, and to find other raw materials that will not affect the environment negatively.

The pulp and paper industry is very fortunate since the raw material used is clearly sustainable, i.e. in most cases wood, non-wood plants like bamboo, or agriculture waste like bagasse and straw from rice and wheat. The energy needed for heating purposes is also based on the same type of material although of lower quality like bark, branches, sawmill dust, etc. However, even if the pulp and paper industry is, by large and potentially, a green industry it is still important to do more and this report is focusing on a number of new processes that could further improve the situation if they are implemented in the pulp mills. These new processes could, for example, be a more efficient use of raw material and energy, but they could also present a smaller risk for the process operators. The processes that are discussed in the report have been considered by the authors to have a good chance to be included in a new wood-based pulp mill built using the best available technology (BAT) of 2040 based on their proven merits. The reference used in each case corresponds to the current technology standard, which in reality is the BAT of 1990-2000 for a wood-based pulp mill.

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1.1 Overview of the kraft pulp mill processes

Wood is both the most important raw material and the most important energy source in a pulp mill. Wood is a heterogeneous material and it has five valuable classes of components, namely cellulose, hemicellu-lose, lignin, extractives, and bark. Cellulose is the main component in the cell wall of the individual wood fibers with about 40-45% and it serves as a skeleton, i.e. cellulose provides support to the cell structure. It is a linear and unbranched polymer of glucose units and the degree of polymerization of cellulose in wood is up to 10 000. Cellulose is sur-rounded by hemicellulose and lignin in a matrix in the cell wall [2-6]. In the kraft pulping process, debarked wood chips are treated with so-dium hydroxide (NaOH) and soso-dium sulfide (Na2S), i.e. white liquor,

in a digester at 140-170°C, thus converting the wood into pulp. Lignin, hemicellulose, and extractives are highly degraded and dissolved in the delignification liquor, which becomes dark and is therefore called black liquor. The remaining components are cellulose and some hemicellu-lose. After the cooking stage, the pulp is washed, screened, and bleached in several stages to remove the residual traces of lignin. The final pulp is then either sent to the pulp drier and the following baling, or to the paper machine for further treatment [7-10].

The black liquor, with the dissolved organics, inorganics and spent cooking chemicals, is evaporated and incinerated in the recovery boiler. A smelt with the spent cooking chemicals is formed and this is causti-cized in the so-called lime cycle to generate new white liquor. During the combustion of the organics in the recovery boiler, heat is generated and is used for steam production. The steam can then be turned into electric energy in a steam turbine [7-10]. A schematic illustration of the kraft pulping process is shown in Figure 1.

1.1 Overview of the kraft pulp mill processes

Wood is both the most important raw material and the most important energy source in a pulp mill. Wood is a heterogeneous material and it has five valuable classes of components, namely cellulose, hemicellu-lose, lignin, extractives, and bark. Cellulose is the main component in the cell wall of the individual wood fibers with about 40-45% and it serves as a skeleton, i.e. cellulose provides support to the cell structure. It is a linear and unbranched polymer of glucose units and the degree of polymerization of cellulose in wood is up to 10 000. Cellulose is sur-rounded by hemicellulose and lignin in a matrix in the cell wall [2-6]. In the kraft pulping process, debarked wood chips are treated with so-dium hydroxide (NaOH) and soso-dium sulfide (Na2S), i.e. white liquor,

in a digester at 140-170°C, thus converting the wood into pulp. Lignin, hemicellulose, and extractives are highly degraded and dissolved in the delignification liquor, which becomes dark and is therefore called black liquor. The remaining components are cellulose and some hemicellu-lose. After the cooking stage, the pulp is washed, screened, and bleached in several stages to remove the residual traces of lignin. The final pulp is then either sent to the pulp drier and the following baling, or to the paper machine for further treatment [7-10].

The black liquor, with the dissolved organics, inorganics and spent cooking chemicals, is evaporated and incinerated in the recovery boiler. A smelt with the spent cooking chemicals is formed and this is causti-cized in the so-called lime cycle to generate new white liquor. During the combustion of the organics in the recovery boiler, heat is generated and is used for steam production. The steam can then be turned into electric energy in a steam turbine [7-10]. A schematic illustration of the kraft pulping process is shown in Figure 1.

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Figure 1: Simplified schematic illustration of the most important processes and products in a modern kraft pulp mill.

Negative environmental impact from e.g. greenhouse gas emissions and a fast dwindling oil supply have resulted in an interest in biorefin-eries based on renewable resources. The biorefinery concept is similar to the petroleum refinery, but it is based on biomass feedstock rather than crude oil. The objective of a biorefinery is to upgrade the biomass to more valuable products such as biofuels, electricity, materials, and chemicals. Wood biomass is a suitable raw material for a biorefinery since it is the most abundant organic material in the world, it is renew-able, and it can be harvested all year round. On national and global lev-els, sustainable land management is being researched and debated since area for both food and material production is needed for growing populations. However, areas in the tempered taiga forest of Nordic countries, North America and Russia are considered suited for the cul-tivation of forest for a multitude of uses [11-14].

The pulp and paper industry is suffering from a decline in demand of traditional paper products and thus has to find new ways to increase its revenues. In the kraft pulping process, only half of the wood biomass is converted into pulp. The remaining half, consisting of mainly lignin

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and hemicellulose, is incinerated in the recovery boiler. Most probably, there is great economic potential if this biomass could be converted into products that have a higher value than energy. A kraft pulp mill can be transformed into an integrated forest biorefinery (IFBR), thus producing e.g. biofuels and chemicals, in addition to the traditional pulp and paper products. An IFBR can use the existing technology and infrastructure of pulp and paper mills, and thus take advantage of the large volumes of biomass feedstock and large production units [11, 15, 16].

Cellulose is considered to be highly valuable when used for pulp and paper products, so new products should not be based on cellulose but instead based on the other wood components or waste. Lignin has a high heating value and is thus important for the energy production in the recovery boiler. Extracting lignin from the black liquor will thus af-fect the energy production in a pulp mill. Most modern kraft pulp mills are today self-sufficient in terms of energy for heating purposes, but the new processes discussed in this report require additional steam and more energy must be generated. A pulp mill also requires a significant amount of water and if the pulp mill is turned into a biorefinery, this demand will likely rise, thus increasing either the amount of water that has to be evaporated before being sent to the recovery boiler or the amount of wastewater that is sent to the wastewater treatment plant. The biggest challenge with an IFBR is thus to fulfill the environmental, energy and water demands without becoming dependent on fossil fuels [11, 15, 16].

The possibility that some new pulp mill processes are included in a kraft pulp mill built in 2040 according to the assumed BAT of 2040 are discussed in this paper. The starting point for the proposed pulp mill is depicted in Figure 2, and the new pulp mill set up is discussed and com-pared to the pulp mill in Figure 1. The discussion is based on open in-formation in the literature and on discussions with industry experts. The proposed pulp mill contains the following new processes and prod-ucts compared to a kraft pulp mill of 2000:

and hemicellulose, is incinerated in the recovery boiler. Most probably, there is great economic potential if this biomass could be converted into products that have a higher value than energy. A kraft pulp mill can be transformed into an integrated forest biorefinery (IFBR), thus producing e.g. biofuels and chemicals, in addition to the traditional pulp and paper products. An IFBR can use the existing technology and infrastructure of pulp and paper mills, and thus take advantage of the large volumes of biomass feedstock and large production units [11, 15, 16].

Cellulose is considered to be highly valuable when used for pulp and paper products, so new products should not be based on cellulose but instead based on the other wood components or waste. Lignin has a high heating value and is thus important for the energy production in the recovery boiler. Extracting lignin from the black liquor will thus af-fect the energy production in a pulp mill. Most modern kraft pulp mills are today self-sufficient in terms of energy for heating purposes, but the new processes discussed in this report require additional steam and more energy must be generated. A pulp mill also requires a significant amount of water and if the pulp mill is turned into a biorefinery, this demand will likely rise, thus increasing either the amount of water that has to be evaporated before being sent to the recovery boiler or the amount of wastewater that is sent to the wastewater treatment plant. The biggest challenge with an IFBR is thus to fulfill the environmental, energy and water demands without becoming dependent on fossil fuels [11, 15, 16].

The possibility that some new pulp mill processes are included in a kraft pulp mill built in 2040 according to the assumed BAT of 2040 are discussed in this paper. The starting point for the proposed pulp mill is depicted in Figure 2, and the new pulp mill set up is discussed and com-pared to the pulp mill in Figure 1. The discussion is based on open in-formation in the literature and on discussions with industry experts. The proposed pulp mill contains the following new processes and prod-ucts compared to a kraft pulp mill of 2000:

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▪ Black liquor gasification: The recovery boiler is replaced by a black liquor gasifier. It produces electric power and heat to the pulp mill, and/or biofuel for the transport sector.

▪ Lignin extraction: Lignin is removed from the black liquor by pre-cipitation using the LignoBoost process. The extracted lignin can be used as biofuel or as raw material for more valuable chemicals and products.

▪ Sulfuric acid: Incineration of non-condensable gases generates sul-fur dioxide, which can be sent to a sulsul-furic acid plant on the pulp mill site. The internally produced acid could be used in the Ligno-Boost process and for the production of tall oil diesel and ethanol. ▪ Green liquor filter: The traditional green liquor filter is replaced by

a CleanFlow membrane filter, which results in a cleaner green liq-uor. This enables an increase in production and a decrease in the consumption of energy and chemicals.

▪ Tall oil diesel: Fatty acids are separated from crude tall oil and re-fined into tall oil diesel, which can be utilized as fuel for the transport sector.

▪ Ethanol: Hydrolysis of the wood chips is done prior to the impreg-nating vessel, resulting in extraction of hemicellulose. The hydrol-ysis generates sugars, which can be fermented into ethanol for e.g. the transport sector.

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6 Fi g ur e 2: P ro po se d k ra ft pul p mil l o f 2 0 40 , w her e the n ew p ro cess es and p ro duc ts ar e m a rk ed i n r ed. 6 Fi g ur e 2: P ro po se d k ra ft pul p mil l o f 2 0 40 , w her e the n ew p ro cess es and p ro duc ts ar e m a rk ed i n r ed.

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2 Black liquor gasification

An essential part of a traditional kraft pulp mill is the recovery cycle, where both spent energy and chemicals are recovered. Black liquor, i.e. the liquor phase after the cook which contains the spent cooking chem-icals and the dissolved organic and inorganic material, is first evapo-rated to increase the dry solids content to 70-80%. The black liquor then enters a Tomlinson recovery boiler, where the black liquor is burnt completely, and only gases and a smelt from the inorganic material re-main. Heat is released during this process and is converted to steam, which is then used in a steam turbine to produce electricity, and the residual steam is used for heating the different reactors in the fiber line. The smelt is dissolved in water, i.e. in filtrate from the lime kiln filter, and forms green liquor, which is then slaked with calcium oxide to pro-duce new white liquor [17-23]. A schematic illustration of the main en-ergy and material streams for a conventional recovery boiler is shown in Figure 3.

Figure 3: Schematic illustration of the main energy and material streams at a con-ventional kraft pulp mill with a Tomlinson recovery boiler (redrawn from [18]).

The traditional Tomlinson recovery boiler has some disadvantages: low electric power generation, risk of explosions and malodorous gas emis-sions [22, 24, 25].The recovery boiler is the most expensive reactor in a pulp mill, and many of the recovery boilers worldwide are near their end of life, meaning that there is a strong need in the industry to im-plement new chemical recovery processes that are affordable, safe and without any odor problems. One option is black liquor gasification

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(BLG). Development of black liquor gasifiers first started in the 1960s and the main driving forces were improved safety, i.e. reduced risk of explosions, and increased environmental friendliness, i.e. reduced malodorous gas emissions. The same driving forces existed in the 1970s, but in the 1980s and 1990s, economical driving forces were the most important ones, where potential energy and cost savings were rec-ognized through increased electrical production efficiency. Another driving force was the separation of sodium (Na) and sulfur (S), which could improve the control of the Na/S balance in the pulp mill. Nowa-days, decreasing the carbon dioxide emissions, especially the fossil fuel-based ones, is the most important driving force. One way of doing this is to produce transportation fuel from wood biomass and thus in-directly reduce the consumption of fossil fuel [26].

2.1 BLG processes

There are two types of BLG processes: low temperature and high tem-perature. The low temperature BLG process operates at 600-700°C, i.e. below the melting points of the inorganics. Due to the low temperature, the reaction rate is slow, thus requiring bigger sized equipment. It could also lead to incomplete carbon conversion or sulfate reduction. The high temperature BLG process operates at 900-1000°C and the in-organics therefore produce a molten smelt. The higher temperature means that the equipment size could be smaller than in the case of low temperature BLG. Due to the molten smelt, corrosion is however an issue. While the low temperature BLG process is air-blown, the high temperature BLG process can either be air-blown or oxygen-blown. The pressurized (25-30 bars) oxygen-blown process is more efficient than the air-blown process at atmospheric pressure [17, 18, 22, 25, 27]. In BLG, black liquor is gasified in a reactor, thus producing a gas, which is referred to as synthesis gas or syngas. The syngas mainly consists of hydrogen gas (H2) and carbon monoxide (CO), but also hydrogen

sul-fide (H2S), carbon dioxide (CO2), methane (CH4), and water [19, 21,

27]. The syngas thus contains sulfur compounds, and H2S needs to be

removed prior to further usage since it is malodorous, toxic and corro-sive [28]. The clean syngas is subsequently incinerated in a gas turbine.

(BLG). Development of black liquor gasifiers first started in the 1960s and the main driving forces were improved safety, i.e. reduced risk of explosions, and increased environmental friendliness, i.e. reduced malodorous gas emissions. The same driving forces existed in the 1970s, but in the 1980s and 1990s, economical driving forces were the most important ones, where potential energy and cost savings were rec-ognized through increased electrical production efficiency. Another driving force was the separation of sodium (Na) and sulfur (S), which could improve the control of the Na/S balance in the pulp mill. Nowa-days, decreasing the carbon dioxide emissions, especially the fossil fuel-based ones, is the most important driving force. One way of doing this is to produce transportation fuel from wood biomass and thus in-directly reduce the consumption of fossil fuel [26].

2.1 BLG processes

There are two types of BLG processes: low temperature and high tem-perature. The low temperature BLG process operates at 600-700°C, i.e. below the melting points of the inorganics. Due to the low temperature, the reaction rate is slow, thus requiring bigger sized equipment. It could also lead to incomplete carbon conversion or sulfate reduction. The high temperature BLG process operates at 900-1000°C and the in-organics therefore produce a molten smelt. The higher temperature means that the equipment size could be smaller than in the case of low temperature BLG. Due to the molten smelt, corrosion is however an issue. While the low temperature BLG process is air-blown, the high temperature BLG process can either be air-blown or oxygen-blown. The pressurized (25-30 bars) oxygen-blown process is more efficient than the air-blown process at atmospheric pressure [17, 18, 22, 25, 27]. In BLG, black liquor is gasified in a reactor, thus producing a gas, which is referred to as synthesis gas or syngas. The syngas mainly consists of hydrogen gas (H2) and carbon monoxide (CO), but also hydrogen

sul-fide (H2S), carbon dioxide (CO2), methane (CH4), and water [19, 21,

27]. The syngas thus contains sulfur compounds, and H2S needs to be

removed prior to further usage since it is malodorous, toxic and corro-sive [28]. The clean syngas is subsequently incinerated in a gas turbine.

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The hot flue gases are used to generate steam, which is then used in a steam turbine [18, 21, 22, 27]. The inorganic material leaves the reactor as either a solid or smelt depending on the operating temperature. The inorganics are slaked and recausticized to produce white liquor in the same manner as for the conventional recovery boiler [21, 22]. A sche-matic illustration of the main energy and material streams for black liquor gasification is shown in Figure 4.

Figure 4: Schematic illustration of the main energy and material streams at a kraft pulp mill with black liquor gasification (redrawn from [18]).

Several different technologies have been tested throughout the history of BLG. Three of them are briefly described below [22, 26]:

▪ SCA-Billerud process: This process was developed by SCA-Billerud in Sweden in 1958. It is based on a low temperature pyrolysis pro-cess. The first commercial plant was installed in Ortviken, Sweden in 1968 and was in operation for 16 years. Although more commer-cial plants were built worldwide, technical challenges and low en-ergy efficiency resulted in the termination of further development of the technology in 1980.

▪ DARS process: The Direct Alkali Recovery System was patented by Toyo Pulp in Japan in 1976 [29] and Associated Pulp and Paper Mills in Australia further developed the process. The DARS process can only be used for soda pulping and it involves direct causticiza-tion of black liquor using iron oxide. The iron oxide reacts with so-dium carbonate, thus generating soso-dium ferrite. The ferrite is then

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hydrolyzed, thus generating sodium hydroxide. The first commer-cial unit was installed in 1986 in Australia but was shut down in 1998 due to problems with sodium ferrite and iron oxide dust. ▪ Direct causticization: ABB in Sweden developed a circulating

fluid-ized bed BLG process in the early 1990’s that could be used for kraft pulping. This process involves direct causticization with titanium oxide instead of iron oxide. A pilot plant was constructed in Väs-terås, Sweden and the test campaigns were successful. The process was however never commercialized and due to reduced interest from the industry, the process was not further optimized and was discontinued in 1997.

There are two more processes that warrant further description: Steam reformer and Chemrec.

2.1.1 Steam reformer

A low temperature black liquor gasifier using steam reforming was de-veloped by Manufacturing and Technology Conversion International (MTCI) in USA in the mid-1980s. The technology is now owned by ThermoChem Recovery International (TRI) in USA. In the mid-1990s, a steam reformer was built in New Bern, North Carolina, USA. It oper-ated for a couple of years before it was dismantled. In 2003, a steam reformer was installed in Ontario, Canada, and in 2004, two parallel reformers were installed in Virginia, USA. Unfortunately, problems with the fluidized beds, the pulsed heater combustors and the gas cleaning system were encountered at the latter installation and it was terminated in 2006 [21, 22, 26].

Steam reforming is an endothermic process operating at atmospheric pressure and at about 600°C. The organic material (C) reacts with steam in a fluidized bed in the absence of air or oxygen, thus generating CO and H2 (Eq.1). The produced CO then reacts with steam to generate

H2 and CO2 (Eq.2). The resulting syngas is thus rich in hydrogen: about

65-73% [21, 22, 26, 27].

hydrolyzed, thus generating sodium hydroxide. The first commer-cial unit was installed in 1986 in Australia but was shut down in 1998 due to problems with sodium ferrite and iron oxide dust. ▪ Direct causticization: ABB in Sweden developed a circulating

fluid-ized bed BLG process in the early 1990’s that could be used for kraft pulping. This process involves direct causticization with titanium oxide instead of iron oxide. A pilot plant was constructed in Väs-terås, Sweden and the test campaigns were successful. The process was however never commercialized and due to reduced interest from the industry, the process was not further optimized and was discontinued in 1997.

There are two more processes that warrant further description: Steam reformer and Chemrec.

2.1.1 Steam reformer

A low temperature black liquor gasifier using steam reforming was de-veloped by Manufacturing and Technology Conversion International (MTCI) in USA in the mid-1980s. The technology is now owned by ThermoChem Recovery International (TRI) in USA. In the mid-1990s, a steam reformer was built in New Bern, North Carolina, USA. It oper-ated for a couple of years before it was dismantled. In 2003, a steam reformer was installed in Ontario, Canada, and in 2004, two parallel reformers were installed in Virginia, USA. Unfortunately, problems with the fluidized beds, the pulsed heater combustors and the gas cleaning system were encountered at the latter installation and it was terminated in 2006 [21, 22, 26].

Steam reforming is an endothermic process operating at atmospheric pressure and at about 600°C. The organic material (C) reacts with steam in a fluidized bed in the absence of air or oxygen, thus generating CO and H2 (Eq.1). The produced CO then reacts with steam to generate

H2 and CO2 (Eq.2). The resulting syngas is thus rich in hydrogen: about

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𝐶 (𝑠) + 𝐻2𝑂 (𝑔) + 𝐻𝑒𝑎𝑡 → 𝐻2 (𝑔) + 𝐶𝑂 (𝑔) (Eq.1)

𝐶𝑂 (𝑔) + 𝐻2𝑂 (𝑔) → 𝐻2 (𝑔) + 𝐶𝑂2 (𝑔) (Eq.2)

The steam reformer consists of a fluidized bed of sodium carbonate (Na2CO3) crystals. The bed temperature is sufficiently low to avoid

smelt formation and thus the risk of explosions. The steam reformer has three inputs; black liquor, steam and heat, and three outputs; hy-drogen rich syngas, flue gas and bed solids. The reactor is indirectly heated by several resonance tubes consisting of pulse combustion mod-ules. Black liquor is injected in the bed reactor and the organic material is fluidized by steam. Sulfur and sodium are completely separated due to the low temperature gasification, and the sulfur ends up in the gas stream while the sodium remains in a solid form. Both chemicals are recovered and later used as cooking liquor in the pulp mill [21, 22, 25-27]. An illustration of the steam reformer is shown in Figure 5.

Figure 5: Illustration of a MTCI/TRI steam reformer (redrawn from [21, 25, 27]).

2.2 Chemrec

A high temperature black liquor gasifier was patented in 1987 by SKF in Sweden. The Swedish company Chemrec further developed the BLG and the technology was later bought by Kamyr AB in Sweden. The tech-nology is now provided by Schmidtsche Schack [26, 27]. Chemrec first developed an air-blown BLG system at atmospheric pressure and a pi-lot plant was installed in 1987 in Hofors, Sweden. A larger commercial plant was installed in 1991 at the Frövi pulp mill, Sweden, and a signif-icantly larger one was installed in 1996 at the New Bern pulp mill,

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North Carolina, USA. The New Bern installation was a booster version, i.e. designed to increase the recovery capacity in the pulp mill and thus increase the pulp mill capacity, and it was operating until 2008. The first pressurized system was installed in 1994 in Skoghall, Sweden. It was an air-blown pilot plant that was rebuilt to an oxygen-blown one in 1997. In 2005, a pressurized oxygen-blown pilot plant was con-structed in Piteå, Sweden. The generated syngas was converted to a bio-fuel and used in trucks from 2010. The BLG process developed further, but the pilot plant was closed down in 2016 [21, 22, 26, 27].

The pressurized Chemrec BLG process utilizes oxygen (O2) as the

gasi-fying agent at about 25-35 bars and operates at 900-1000°C. The or-ganic material first reacts with O2 to produce CO2 (Eq.3). This reaction

is exothermic, thus producing the heat required for the other endother-mic reactions. The organic material also reacts with the produced CO2

to generate CO (Eq.4). Furthermore, the organic material (C) reacts with steam to generate H2 and CO (Eq.5). The generated syngas

con-tains mainly CO, CO2 and H2 along with some CH4 and H2S. The high

temperature means that the inorganic material forms a molten smelt [19, 21, 22, 24-27].

𝐶 (𝑠) + 𝑂₂ (𝑔) → 𝐶𝑂₂ (𝑔) (Eq.3)

𝐶 (𝑠) + 𝐶𝑂₂ (𝑔) → 2 𝐶𝑂 (𝑔) (Eq.4)

𝐶 (𝑠) + 𝐻₂𝑂 (𝑔) → 𝐻₂ (𝑔) + 𝐶𝑂 (𝑔) (Eq.5)

The reactor used in the Chemrec BLG is an entrained-flow reactor with refractory lining. In the beginning of the development, the lifetime of the refractory lining was only 6-8 months and the replacement resulted in a shutdown of the BLG for several weeks. Research has however re-sulted in improved refractory materials and the lining now lasts for more than two years [21, 26, 27]. Black liquor is injected into the reac-tor and is atomized due to the high pressure. Less than half of the re-quired O2 needed for complete combustion is added, thus resulting in

a reducing (gasifying) atmosphere. The temperature of the black liquor at the inlet is about 120°C, but it is quickly heated to 600°C due to the small size of the liquor droplets. During the temperature increase,

mor-North Carolina, USA. The New Bern installation was a booster version, i.e. designed to increase the recovery capacity in the pulp mill and thus increase the pulp mill capacity, and it was operating until 2008. The first pressurized system was installed in 1994 in Skoghall, Sweden. It was an air-blown pilot plant that was rebuilt to an oxygen-blown one in 1997. In 2005, a pressurized oxygen-blown pilot plant was con-structed in Piteå, Sweden. The generated syngas was converted to a bio-fuel and used in trucks from 2010. The BLG process developed further, but the pilot plant was closed down in 2016 [21, 22, 26, 27].

The pressurized Chemrec BLG process utilizes oxygen (O2) as the

gasi-fying agent at about 25-35 bars and operates at 900-1000°C. The or-ganic material first reacts with O2 to produce CO2 (Eq.3). This reaction

is exothermic, thus producing the heat required for the other endother-mic reactions. The organic material also reacts with the produced CO2

to generate CO (Eq.4). Furthermore, the organic material (C) reacts with steam to generate H2 and CO (Eq.5). The generated syngas

con-tains mainly CO, CO2 and H2 along with some CH4 and H2S. The high

temperature means that the inorganic material forms a molten smelt [19, 21, 22, 24-27].

𝐶 (𝑠) + 𝑂₂ (𝑔) → 𝐶𝑂₂ (𝑔) (Eq.3)

𝐶 (𝑠) + 𝐶𝑂₂ (𝑔) → 2 𝐶𝑂 (𝑔) (Eq.4)

𝐶 (𝑠) + 𝐻₂𝑂 (𝑔) → 𝐻₂ (𝑔) + 𝐶𝑂 (𝑔) (Eq.5)

The reactor used in the Chemrec BLG is an entrained-flow reactor with refractory lining. In the beginning of the development, the lifetime of the refractory lining was only 6-8 months and the replacement resulted in a shutdown of the BLG for several weeks. Research has however re-sulted in improved refractory materials and the lining now lasts for more than two years [21, 26, 27]. Black liquor is injected into the reac-tor and is atomized due to the high pressure. Less than half of the re-quired O2 needed for complete combustion is added, thus resulting in

a reducing (gasifying) atmosphere. The temperature of the black liquor at the inlet is about 120°C, but it is quickly heated to 600°C due to the small size of the liquor droplets. During the temperature increase,

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phological changes of the droplets occur and there are three conversa-tion stages: drying, pyrolysis and char gasificaconversa-tion (Figure 6). The end result is a combustible gas and a smelt, which ideally consists of only inorganic material [19, 21, 25-27].

Figure 6: Stages of the liquor droplets during black liquor gasification (redrawn from [19]).

An illustration of the Chemrec BLG is shown in Figure 7. It contains a bed of Na2CO3 and Na2S particles, where the latter is formed during the

gasification. The smelt and the gas that were generated in the gasifier are cooled and separated in a quench cooler. The smelt is dissolved in a quench bath, thus forming green liquor. The green liquor is cooled further and then sent to the white liquor preparation. Na2CO3 and Na2S

are separated during the black liquor gasification, thus enabling two white liquor streams with different sulfidities, i.e. split sulfidity [19, 21, 22, 25-27, 30].

The gases are cooled in a counter-current condenser (gas cooler), i.e. the gas flows upwards while the condensate flows downward. In the lower part of the gas cooler, medium pressure (MP) steam (10-12 bars) is generated, and in the middle part, low pressure (LP) steam (4-5 bars) is produced. In the upper part, the gas can be used to preheat make-up boiler feed water (BFW). Final cooling of the gas is achieved by adding cooling water (CW). The cold raw syngas is finally sent to a cleaner, where H2S is removed, thus generating nearly sulfur-free syngas [19,

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Figure 7: Illustration of Chemrec pressurized entrained-flow black liquor gasifier. The counter-current gas cooler is also shown, where CW is cooling water and BFW is boiler feed water (courtesy of representatives of former Chemrec and of Schmidtsche Schack, who are now supplying the technology [23]).

2.3 Benefits and challenges

So what are the benefits of black liquor gasification compared to the process in a conventional Tomlinson recovery boiler? The most obvi-ous one is the increased electric energy production compared to a tra-ditional recovery boiler. Large amounts of the latent heat in the syngas are recovered, thus increasing the electricity production and reducing the heat surplus of the mill. The electricity production could be twice that of the recovery boiler per ton of black liquor solids according to some studies, but additional biomass is however required to achieve that [19, 22, 25, 30].

When BLG is used for electricity production, it is referred to as black liquor gasification with combined cycle power generation (BLGCC).

Figure 7: Illustration of Chemrec pressurized entrained-flow black liquor gasifier. The counter-current gas cooler is also shown, where CW is cooling water and BFW is boiler feed water (courtesy of representatives of former Chemrec and of Schmidtsche Schack, who are now supplying the technology [23]).

So what are the benefits of black liquor gasification compared to the process in a conventional Tomlinson recovery boiler? The most obvi-ous one is the increased electric energy production compared to a tra-ditional recovery boiler. Large amounts of the latent heat in the syngas are recovered, thus increasing the electricity production and reducing the heat surplus of the mill. The electricity production could be twice that of the recovery boiler per ton of black liquor solids according to some studies, but additional biomass is however required to achieve that [19, 22, 25, 30].

When BLG is used for electricity production, it is referred to as black liquor gasification with combined cycle power generation (BLGCC).

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The cleaned, sulfur-free syngas is combusted in a combined cycle, i.e. a gas turbine followed by a steam turbine. Electricity is first produced in the gas turbine, and the flue gas from the gas turbine is then cooled in a heat recovery steam generator (HRSG) to produce steam. Due to the high production of electric power in the BLG, the remaining heat pro-duction is too low for the pulp mill and additional heat must be gener-ated. Additional biomass is burnt in a bark boiler, thus producing the required steam. The steam from the HRSG and the bark boiler is sub-sequently used in the steam turbine to produce additional electricity [21-23, 25, 30]. The large production of electricity can result in the mill becoming an exporter of electricity, thus generating another source of income, but additional biomass is required to compensate for the addi-tional electric power generation [22, 23, 30]. A schematic illustration of the electricity production in a pulp mill with BLGCC is shown in Fig-ure 8.

Figure 8: Schematic illustration of electricity production in a BLGCC (redrawn from [22]).

Instead of generating electricity from the syngas, it could be used to generate biofuels for e.g. the transport sector. This concept is referred to as black liquor gasification with motor fuel production (BLGMF). The possibility to reduce the fossil fuel dependency is very attractive since it would reduce the emissions of greenhouse gases to the environ-ment. However, production of biofuels would lead to an energy deficit for the pulp mill since electricity and steam generation will be signifi-cantly reduced compared to both BLGCC and conventional recovery

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boiler. It may be required to import biomass or purge the gas from the BLGMF plant to produce enough steam. For all cases, a mill would have to purchase external electricity to cover its needs [22, 23, 30].

There are several different types of biofuels that could potentially be generated from the syngas: methanol, dimethyl ether (DME), Fischer-Tropsch diesel (FTD), hydrogen, and synthetic natural gas (SNG). The syngas from BLG contains H2 and CO, and they can be catalytically

con-verted to methanol, DME, and FTD. This catalytic synthesis requires a very clean syngas, which is achieved through sulfur removal and two CO2 removal units [22, 23]. A schematic illustration of the potential

generation of different biofuels in a kraft pulp mill with BLGMF is shown in Figure 9.

Figure 9: Schematic illustration of the potential generation of different biofuels in a BLGMF (redrawn from [22]).

Another benefit is the separation of Na and S, which not only makes it possible to better control the Na/S balance but also enables modified pulping techniques such as split sulfidity pulping, polysulfide pulping, and green liquor pretreatment. These pulping techniques can improve pulp yield and pulp quality, and reduce wood demand and chemical usage [21, 25-27, 30].

boiler. It may be required to import biomass or purge the gas from the BLGMF plant to produce enough steam. For all cases, a mill would have to purchase external electricity to cover its needs [22, 23, 30].

There are several different types of biofuels that could potentially be generated from the syngas: methanol, dimethyl ether (DME), Fischer-Tropsch diesel (FTD), hydrogen, and synthetic natural gas (SNG). The syngas from BLG contains H2 and CO, and they can be catalytically

con-verted to methanol, DME, and FTD. This catalytic synthesis requires a very clean syngas, which is achieved through sulfur removal and two CO2 removal units [22, 23]. A schematic illustration of the potential

generation of different biofuels in a kraft pulp mill with BLGMF is shown in Figure 9.

Figure 9: Schematic illustration of the potential generation of different biofuels in a BLGMF (redrawn from [22]).

Another benefit is the separation of Na and S, which not only makes it possible to better control the Na/S balance but also enables modified pulping techniques such as split sulfidity pulping, polysulfide pulping, and green liquor pretreatment. These pulping techniques can improve pulp yield and pulp quality, and reduce wood demand and chemical usage [21, 25-27, 30].

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Due to the major separation of Na and S in the BLG process, it is pos-sible to generate two streams of white liquor: lean and sulfur-rich. The sulfur-rich white liquor would be used in the beginning of the cook, i.e. the initial and bulk phases, while the sulfur-lean white liquor would be added at the end of the cook, i.e. the residual phase [31-33]. Split sulfidity can reduce the kappa number by up to 4 units compared to a conventional kraft cook at similar H-factors. Moreover, the pulp yield could increase by 1-2%, and both tensile and burst strength could be improved. Studies have also shown that at similar kappa numbers, the viscosity could be increased by 5-10 cps, equivalent of about 100 dm3_{/kg [21, 25, 27, 34]. Split sulfidity would however increase the}

caus-ticization load since NaHCO3 (sodium bicarbonate) is produced instead

of Na2CO3 during the scrubbing in the sulfur recovery, and NaHCO3

requires twice as much lime as Na2CO3 to form NaOH [25, 27, 30]. A

schematic illustration of split sulfidity pulping is shown in Figure 10.

Figure 10: Schematic illustration of split sulfidity pulping, where GL stands for green liquor and WL for white liquor (redrawn from [21, 27]).

The separation of Na and S could also be used for polysulfide pulping, where part of the sulfur is oxidized to elemental sulfur. The elemental sulfur is then mixed with the sulfur-lean white liquor, thus generating polysulfide (PS) liquor. Polysulfide pulping increases the pulp yield by about 1% per percent of charged PS. The use of BLG will enable higher PS charges, thus increasing the yield even further; up to 3% [21, 25, 27].

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A procedure combining polysulfide pulping with alkali profiling, re-ferred to as hyperalkaline polysulfide pulping, employs two pretreat-ment stages, where the second is the actual hyperalkaline polysulfide stage, followed by a cooking stage. With this process, the yield was im-proved by 1.5% per percent of charged PS, mainly due to more cellulose, compared to modified pulping without polysulfide. Furthermore, the delignification rate was increased, i.e. lower H-factors were required to reach a certain kappa number. Moreover, pulps with high viscosity, good bleachability, and good strength properties were produced[35]. The increase in the requirements of causticization poses a problem for both split sulfidity and polysulfide pulping. One way to avoid this would be through green liquor pretreatment of the wood chips. Sulfur from the H2S stripper in Figure 10 is mixed with part of the sulfur-lean green

liquor stream, thus generating sulfur-rich green liquor, which is added in the impregnation stage. This impregnation leads to reduced deligni-fication rate, but similar viscosity and yield compared to a conventional kraft cook at similar H-factors. The reduced delignification rate could not be overcome by increasing the charged TTA. Both tensile and burst strengths were improved by green liquor treatment, but the biggest benefit is however the reduced load on the lime kiln, which could lead to cost savings in terms of reduced fuel demand [21, 27, 34].

A procedure combining polysulfide pulping with alkali profiling, re-ferred to as hyperalkaline polysulfide pulping, employs two pretreat-ment stages, where the second is the actual hyperalkaline polysulfide stage, followed by a cooking stage. With this process, the yield was im-proved by 1.5% per percent of charged PS, mainly due to more cellulose, compared to modified pulping without polysulfide. Furthermore, the delignification rate was increased, i.e. lower H-factors were required to reach a certain kappa number. Moreover, pulps with high viscosity, good bleachability, and good strength properties were produced[35]. The increase in the requirements of causticization poses a problem for both split sulfidity and polysulfide pulping. One way to avoid this would be through green liquor pretreatment of the wood chips. Sulfur from the H2S stripper in Figure 10 is mixed with part of the sulfur-lean green

liquor stream, thus generating sulfur-rich green liquor, which is added in the impregnation stage. This impregnation leads to reduced deligni-fication rate, but similar viscosity and yield compared to a conventional kraft cook at similar H-factors. The reduced delignification rate could not be overcome by increasing the charged TTA. Both tensile and burst strengths were improved by green liquor treatment, but the biggest benefit is however the reduced load on the lime kiln, which could lead to cost savings in terms of reduced fuel demand [21, 27, 34].

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3 Lignin extraction

Lignin is one of the main constituents of wood with about 20-35% for softwood and 20-25% for hardwood. Lignin is one of the most abun-dant biopolymers and has a very complex structure that is neither lin-ear nor branched, but rather three-dimensional. The large polymer consists of different building blocks called monolignols, e.g. coniferyl, sinapyl and p-coumaryl alcohols. Lignin is hydrophobic and prevents the wood cell from swelling in water, thus making the cell waterproof. It also serves as a barrier and protects the wood against microbial deg-radation. Lignin binds cellulose and hemicellulose together by filling up the space between them, thus providing stiffness to the cell wall. Most of the lignin is found between the hemicelluloses glucomannan and xylan, but so-called lignin-carbohydrate complexes (LCC) may also occur between lignin and cellulose [5, 36-40].

During kraft pulping, lignin is dissolved in the black liquor and it is thus generally considered as a byproduct. Most modern pulp mills are self-sufficient in energy, produced via incineration of black liquor, bark and biomaterial. Many mills are operating at their maximum capacity and the recovery boiler is often a bottleneck. Pulp production is thus limited by the amount of black liquor that can be processed in the recovery boiler. Lignin has a higher heating value than the other components in black liquor, and by removing part of the lignin from the black liquor prior to the recovery boiler, the heat load would decrease. Offloading the recovery boiler can therefore lead to increased pulp capacity in the pulp mill without the need to expand the recovery boiler reactor. Lignin removal may also remove some of the non-process elements that are undesirable, i.e. act as a kidney [41-45].

A method for lignin extraction in pulp mills was patented as early as 1946 [46]. One industrial application for such precipitated lignin is spe-cialty chemicals and the extracted lignin is sufficiently pure for that ap-plication. However, the precipitated lignin cannot be used as fuel since this application requires a very high cleanliness and a low ash content. The method described in the patent has been evolved and now includes the following steps [41-43, 47, 48]:

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1. The black liquor is first evaporated, and a fraction is then removed for lignin extraction.

2. The black liquor fraction is acidified to precipitate the lignin. 3._{The lignin is finally filtered and washed.}

This method is hereafter referred to as the traditional method and it leads to problematic plugging with low filterability. Plugging leads to impurities, mainly ash and sodium, in the lignin due to the uneven washing. Furthermore, plugging will lead to low flow of washing liquor, which results in the need for very large filter areas. The problem with plugging was thought to be due changes in lignin solubility caused by large gradients in pH and ionic strength in the filter cake during wash-ing [41-43, 47, 48].

3.1 LignoBoost

A lignin separation process denoted LignoBoost was introduced on the market in 2006. The technology is today owned by Valmet and the Lig-noBoost process produces a highly purified lignin with a low ash con-tent. A demonstration plant was built in 2007 in Nordic Paper’s pulp mill in Bäckhammar, Sweden. The first commercial plant was installed in 2013 in Domtar’s pulp mill in Plymouth, North Carolina, USA and a second one was started in 2015 in Stora Enso’s pulp mill in Sunila, Fin-land [49, 50]. A schematic illustration of the LignoBoost process is shown in Figure 11. The main difference from the traditional method is a re-suspension stage prior to the washing stage, where the pH is de-creased from pH 9-10 to pH 2-4 [41-43, 47]. The LignoBoost process contains the following operations [41-44, 47, 48, 50-52]:

1. Black liquor with a dry solids content of 30-45% is taken from the evaporation plant. The black liquor is acidified to a pH of 9-10 by injecting CO2. The lignin is precipitated due to the lowered pH.

2. The solid lignin is dewatered, and the remaining black liquor is fil-tered off and returned to the evaporation plant.

3. The lignin cake is then re-suspended and mixed with wash liquor from the following washing stage. Sulfuric acid (H2SO4) is added,

thus rapidly lowering the pH to about 2-4.

1. The black liquor is first evaporated, and a fraction is then removed for lignin extraction.

2. The black liquor fraction is acidified to precipitate the lignin. 3._{The lignin is finally filtered and washed.}

This method is hereafter referred to as the traditional method and it leads to problematic plugging with low filterability. Plugging leads to impurities, mainly ash and sodium, in the lignin due to the uneven washing. Furthermore, plugging will lead to low flow of washing liquor, which results in the need for very large filter areas. The problem with plugging was thought to be due changes in lignin solubility caused by large gradients in pH and ionic strength in the filter cake during wash-ing [41-43, 47, 48].

3.1 LignoBoost

A lignin separation process denoted LignoBoost was introduced on the market in 2006. The technology is today owned by Valmet and the Lig-noBoost process produces a highly purified lignin with a low ash con-tent. A demonstration plant was built in 2007 in Nordic Paper’s pulp mill in Bäckhammar, Sweden. The first commercial plant was installed in 2013 in Domtar’s pulp mill in Plymouth, North Carolina, USA and a second one was started in 2015 in Stora Enso’s pulp mill in Sunila, Fin-land [49, 50]. A schematic illustration of the LignoBoost process is shown in Figure 11. The main difference from the traditional method is a re-suspension stage prior to the washing stage, where the pH is de-creased from pH 9-10 to pH 2-4 [41-43, 47]. The LignoBoost process contains the following operations [41-44, 47, 48, 50-52]:

1. Black liquor with a dry solids content of 30-45% is taken from the evaporation plant. The black liquor is acidified to a pH of 9-10 by injecting CO2. The lignin is precipitated due to the lowered pH.

2. The solid lignin is dewatered, and the remaining black liquor is fil-tered off and returned to the evaporation plant.

3. The lignin cake is then re-suspended and mixed with wash liquor from the following washing stage. Sulfuric acid (H2SO4) is added,

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4. The lignin is dewatered again and a second filter cake is formed. The cake is then washed through displacement washing with wash water and H2SO4 at pH 2-4.

5._{Finally, the lignin is dried and pulverized or pelletized.}

Figure 11: Schematic illustration of the LignoBoost process (redrawn from [16, 41]).

Re-suspending the precipitated lignin instead of washing it directly is the key to the improved lignin extraction with LignoBoost compared to the traditional method. The main changes in pH and ionic strength gra-dients then occur in the re-suspension tank instead of in the washing stage. Hence, the changes in lignin solubility will happen prior to the washing stage, thus preventing plugging of the filter. The size of the filter area can then be smaller compared to the traditional method, thus reducing investments costs. Furthermore, the yield of extracted lignin is increased compared to the traditional method, thus increasing the efficiency [41-44, 47, 48, 50-52].

The use of wash water from the displacement washing stage in the re-suspension stage ensures that the pH of the precipitated lignin entering the washing stage is similar to the wash water of said stage. The pH gradients in the washing stage will thus be small. Furthermore, the re-use of wash water also reduces the consumption of both water and H2SO4 in the washing stage compared to the traditional method.

Fil-trate from both the first and second filtration stages are recycled to the evaporation plant, thus making the LignoBoost practically an effluent-free process [41-44, 47, 48, 50-52]. Some advantages of the LignoBoost process compared to the traditional method are [42, 43, 48, 51]: ▪ The lignin yield is higher; thus a smaller amount of black liquor is

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▪ The smaller stream of required black liquor means that a smaller amount of CO2 is needed for acidification per unit extracted lignin.

▪ Since the problems with plugging is virtually eliminated, the filter area can be reduced.

▪ The re-suspension stage does not require additional water since the wash water from the following washing stage is reused.

▪ The extracted lignin has a high purity: 95-99% with an ash content of 0.3-1.2%.

▪ The residual sodium content is reduced to 0.1-0.4% without plug-ging problems.

H2SO4 is added during lignin extraction and this will affect the Na/S

balance in the mill. The surplus sulfur would then need to be removed through e.g. purging of ESP ash. This would however result in a sodium deficit, i.e. Na2CO3 or NaOH needs to be added to maintain the Na/S

balance. Black liquor carryover will also affect the demand of H2SO4 in

the last part of the LignoBoost process, thus further impacting the Na/S balance. One way to reduce the effect on the Na/S balance would be to use spent acid from the chlorine dioxide (ClO2) plant instead of fresh

H2SO4. However, the spent acid may already be used and recycled to

the closed part of the pulp mill, and other ways to influence the Na/S balance are therefore needed. Since lignin contains about 2-3% sulfur, removing the lignin from the pulp mill provides an opportunity to maintain the Na/S balance despite the added H2SO4. However, if the

extracted lignin is used in-house, e.g. as biofuel in the lime kiln, the sulfur will remain in the loop [43, 44].

The investment cost of a LignoBoost process is higher than that of a traditional one due to more equipment; a re-suspension tank and a sec-ond dewatering equipment are required. Furthermore, H2SO4 is

re-quired in the re-suspension stage. Thus, the consumption of H2SO4 is

higher in the LignoBoost process than in the traditional method. The operating costs depend mainly on the need for CO2 and H2SO4. The

costs could be lowered if spent acid is used instead of fresh H2SO4, and

if CO2 from the flue gases in the lime kiln is used instead of purchasing