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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 51 , No. 5

[ Article ]
Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 51, No. 5, pp. 3-15
Abbreviation: J. Korea TAPPI
ISSN: 0253-3200 (Print)
Print publication date 30 Oct 2019
Received 11 Jul 2019 Revised 20 Aug 2019 Accepted 02 Sep 2019

Global Trends and Prospects of Black Liquor as Bioenergy
Chul-Hwan Kim1 ; Ji-Young Lee1, ; See-Han Park2 ; Sun-Ok Moon1
1Major of Environmental Material Science, IALS, Gyeongsang National University, Jinju, 52828, Republic of Korea
2Major of Pulp and Paper Chemical Engineering, Gyeongsang National University, Jinju, 52828, Republic of Korea

Correspondence to : † E-mail:


Black liquor from kraft pulp mills is a valuable bioresource that can be used to generate renewable energy and to recover cooking chemicals. Consequently, black liquor must not be regarded as a nonrecyclable waste. Kraft pulp mills are known to generate 1.4 to 1.5 tons of black liquor (measured as dry content) per ton of pulp, representing a potential energy source ranging between 250 MW and 500 MW. The effective heating value of black liquor is relatively low at 12 MJ/kg to 13 MJ/kg, compared to fossil fuels. The combustion of black liquor is well controlled according to strict regulations, in order to meet RPS and to obtain renewable energy credit (REC) that equal the percentage requirement for electricity sold. To date, however, Korea has seen no REC benefits in relation to black liquor bioenergy generation, though, in most other countries that produce kraft pulp, surplus renewable energy transactions benefit from black liquor combustion. Only one company produces kraft pulp in Korea, contributing to the consistent growth of the domestic paper industry. As such, it is strongly suggested that the relevant ministries in Korea consider black liquor a valuable resource for generating renewable energy.

Keywords: Black liquor, bioenergy, recovery boiler, renewable energy, REC, RPS

1. Introduction

Approximately 400 million tons of paper and paperboard are produced every year worldwide. Virgin pulp and wastepaper are used to produce paper and paperboard. In particular, virgin pulp produced for paper and paperboard make up roughly 180 million tons per year, of which approximately 76% (roughly 137 million tons) are produced through chemical pulping processes.1) Of these, the most commonly used is the kraft pulping process, representing 89% of chemical pulping and 75% of all pulp produced.1-3)

Unlike conventional pulping methods, the kraft pulping process has several advantages. Firstly, it can produce particularly strong pulp, tolerating various wood species, and it is possible to recover 96% to 98% of once-used alkaline chemicals. Steam generated during the chemical recovery process can also be used for both electricity production and drying of pulp and paper in the process.4) The recovery of alkaline chemicals employs black liquor as a process byproduct after cooking in a digester which serves as an indispensable biomass in the kraft pulping process. Thus, the kraft pulping process uses wood chips as its main raw material and alkaline chemicals as supplementary raw materials to remove lignin, through an eco-friendly process that contributes to greenhouse gas reduction.

However, in Korea, whether or not black liquor can be categorized as a renewable energy resource remains controversial, as some ministries consider it as industrial waste generated during chemical pulping processes. In contrast to this domestic perspective, developed countries recognize black liquor as the largest, easiest-to-obtain, and most concentrated biomass available on the planet.5-12) Through the analysis and study of various advanced cases, this paper aims to qualify grounded data to certify that black liquor should be regarded as a renewable energy resource in Korea.

2. Generating Black Liquor and Its Composition

In order to manufacture kraft pulp, wood chips from coniferous or deciduous timber are used. The wood chips comprise white cellulose and hemicellulose, as well as lignin, which strongly binds wood fibers. Lignin is one of the earth’s most abundant natural organic polymers and is replaced only by cellulose.13,14)

Pulping is a process that separates lignin and strongly binds white wood fibers. It employs alkaline chemicals, called “white liquor,” at high temperatures and under high pressure to affect this process inside a digester. Lignin, such as natural adhesive, is separated and eluted; meanwhile, white liquor is converted into black liquor, consisting of water, hemicellulose, pulping chemicals, alkaline lignin, and other elements.4)

When 1 ton of air-dried pulp is produced, 1.4 to 1.5 tons of black liquor is generally collected; therefore, a mill producing 1,000 ton/day of kraft pulp will obtain roughly 1,400 to 1,500 tons of black liquor (based on dry solids). Based on weak black liquor, 8,000 to 10,000 tons of black liquor can be produced as a byproduct.15,16) If the oxygen delignification process is included in the posttreatment process, 5% to 10% more black liquor can be obtained.4,6)

Globally, more than 1.3 billion tons of weak black liquor is annually produced, of which 200 million tons are used as combustion fuel. In this process, roughly 50 million tons of cooking chemicals in the form of Na2O are recovered, and approximately 700 million tons of high-pressure steam is produced in the process.6) This makes black liquor the fifth most important fuel in the world, following coal, oil, natural gas, and gasoline. Particularly in Sweden and Finland, both large producers of pulp and paper, black liquor is currently recognized as the most important renewable biofuel.6,17)

Black liquor consists of water and solids, which include organic ingredients, such as lignin and carbohydrates, inorganic materials, sodium compounds, sulfur compounds, and other inert materials.18-21) Organic components account for roughly 60% of total black liquor solids. Table 1 provides an example of the elemental composition of black liquor.21,22) Due to the elution of lignin and carbohydrates from wood chips, carbon and oxygen components occupy 77.7%, followed by sodium components derived from alkaline chemicals at 19.7%. Among these components, sodium, sulfate, carbonate, calcium, aluminum, and silica contribute to the formation of crystallization fouling and scale in boiler pipes.18-22) As a result, it is necessary to monitor the hazardous components carefully. Organic matter, in particular, accelerates the rate of scale and fouling in a recovery boiler.

Table 1. 
Example of the elemental composition of kraft black liquor21,22)
Carbon (C) 35.0 Calcium (Ca) 600 ppm
Hydrogen (H) 3.3 Aluminum (Al) 50 ppm
Oxygen (O) 35.7 Silica (Si) 700 ppm
Sodium (Na) 19.7 Iron (Fe) 150 ppm
Potassium (K) 1.6 Carbonate (CO32-) 8%
Sulfur (S) 4.0 Sulfate (SO42-) 3%

Fig. 1. compares the rate of organic compound included in black liquor, generated in the process of manufacturing softwood and hardwood pulp.20) Most carbohydrates contained in black liquor show relatively low calorific value, because they are decomposed into a mixture of nonvolatile hydroxy acid and simple monocarboxylic or dicarboxylic acid.19,23) In general, lignin represents roughly 47% of black liquor generated during softwood kraft pulping processes; on the other hand, hydroxy acid and acetic or formic acid account for approximately 43% of black liquor generated during hardwood kraft pulping processes. Organic matter in black liquor is easily volatilizable; more than 80% of organic matter can be released as gas in pyrolysis. The remaining 20% is fixed carbons.19,23) On the other hand, bituminous coal is roughly 30% volatile matter and 70% fixed carbon.

Fig. 1. 
Comparison of organic components included in black liquor from softwood and hardwood kraft pulping.20)

3. Recovery Cycle of Cooking Liquor

In weak black liquor blown out from the digester, solid content matter is low, which roughly accounts to 15% to 18%. Therefore, in order to recover the cooking liquor through the combustion process in the recovery boiler, and thereafter to affect the causticizing process, excess water contained in the weak black liquor must be removed. To concentrate weak black liquor, it is passed through a multiple-efficiency evaporator, and, once excess water has been removed, the solid content of the black liquor is increased so that it becomes strong black liquor. This is then sent to the recovery boiler, along with solid content of roughly 65% to 85%, via the thickener, or direct contact evaporator.

The makeup chemical Na2SO4 is then added, and the strong black liquor is sprayed into and combusted in the recovery boiler. At this point, both Na and S in the black liquor are reduced with Na2SO4 and then discharged into smelt ash. Adding water to the smelt results in a green liquor comprising Na2S and Na2CO3. The Na2CO3 included in smelt derives from the oxidation reaction and pyrolysis of Na and C in the black liquor. The oxidation reaction, pyrolysis, and the reduction reaction occurring inside the recovery boiler can be summarized as follows:24,25)







After green liquor is regenerated from the reduction of smelt, it is sent to the causticizing area for transformation into white liquor (NaOH and Na2S). Na2S in the causticizing process is regenerated through the reaction between Na2CO3 and Ca(OH)2, as shown in Fig. 2.26)

Fig. 2. 
Liquor recovery cycle during kraft pulping.26)

4. Structure of Recovery Boiler and Combustion of Black Liquor

The purpose of the recovery boiler operation is to recover the consumed chemicals via the oxidation reaction of organic materials included in the black liquor, to reduce the reaction of Na2SO4, and to obtain the heat generated from this process. The general structure of the recovery boiler used in this process is shown in Fig. 3. Steam is obtained through the moving process of a flue gas, generated during the combustion of black liquor. To obtain steam from the recovery boiler, heat generated from the boiler heats the supplied water after passing through three components: the economizer, the steam drum, and the superheater.

Fig. 3. 
Schematic diagram of a recovery boiler.25,26)

All modern recovery boilers, except for those used in small-sized factories, tend to use a single-drum recovery boiler without a mud drum. The single-drum system, shown in Fig. 3(a), is designed to endure higher pressure. It also has a larger processing capacity compared to the two-drum system, shown in Fig. 3(b). The single-drum system is also known to be more advantageous in terms of stability and availability.

The temperature of the feed water rises as it passes through the economizer, due to the high temperature supplied by the flue gas. The feed water then flows to the steam drum, a large circular tube that serves as a feed water tank for boiler banks. In the two-drum recovery boiler, the steam drum is connected through downcomer tubes (boiler banks) to a mud drum. The heated, saturated feed water flows from the steam drum to the mud drum and then moves to the waterwall tube. The high temperature steam and water generated at this point are moved to the steam drum, where steam and water are separated; the separated water is then again circulated to the mud drum or to the waterwall tube. The term “mud drum” derives from the fact that some of the impurities in the feed water will settle, and this “mud” can then be collected and sent to a sewage treatment plant. The water-cooling waterwall tube is used to absorb any excess heat in the boiler and also prevents the inner walls of the boiler from overheating.

The separated steam in the steam drum continuously flows to the superheater; meanwhile, the separated water returns to the boiler (generating) bank tubes. As the steam passes through the superheater, it is heated beyond its saturation point, with a lowered moisture content. The steam at high temperature passing through the superheater is supplied to the turbine to either generate electricity or to assist in pulp and papermaking processes.

The objectives of a recovery boiler air system are as follows: complete combustion of black liquor, good chemical conversion of char and smelt, minimizing carryover of black liquor spray, minimizing emissions of CO and NOx, reducing total sulfur and SO2, and effecting the uniform entrance of gas flow and temperature distribution to the superheater.27)

As shown in Fig. 4, the primary air level is situated close to the bottom of the furnace and plays a crucial role in lowering the height of the combustion area to that of the char bed. The secondary air level is roughly 4-5 m above the primary air level and contributes to the burning of volatile matter contained in the black liquor while also helping to control the height of the char bed. The tertiary air level is approximately 3-5 m above the secondary air level and plays a crucial role in minimizing air pollution sources by completely burning black liquor.27-29) As shown in Fig. 5, the height of the char bed layer formed on the lower side of the boiler is low or high; this should therefore be adjusted to the appropriate height, using the primary and secondary air system.30)

Fig. 4. 
Air system in a recovery boiler’s lower furnace.26)

Fig. 5. 
Char bed shapes.30)

The combusted black liquor forms a char bed while being stacked on the bottom of the boiler; molten smelt containing Na2S and Na2CO3 then accumulates on this char bed. The smelt on the furnace bottom flows out to the dissolving tank, where it is dissolved in a weak wash to form green liquor, as shown in Fig. 6.31)

Fig. 6. 
Smelt spout and shattering.31)

Black liquor contains more inorganic components, and roughly 45% of black liquor is fed into the dissolving tank in the form of smelt. Therefore, despite the high lignin content in black liquor, the calorific value per ton of black liquor solid is relatively low. For comparison, the calorific value of lignin is 25 MJ/kg to 26 MJ/kg, which is higher than 16 MJ/kg to 18 MJ/kg of cellulose and hemicellulose. The higher lignin content of softwoods will result in a higher total heating value. The heating values generated during the combustion process of black liquor is approximately 14.5 MJ/kg in terms of higher heating value and 12.3 MJ/kg in terms of lower heating value for 80% of black liquor solid content.32-34)

5. Biofuels Regenerated from Black Liquor

As noted above, pulp mills that produce 1 ton of kraft pulp can obtain 1.4 to 1.5 tons of black liquor as a byproduct, which is measured as dry content. Thus, black liquor represents a potential energy source of 250-500 MW per mill through black liquor combustion.16) Kraft pulp mills equipped with modern facilities may therefore serve as key sources of renewable energy in future energy systems, because they create surplus energy from black liquor. In countries like Sweden and Finland, which have large pulp and paper industries, black liquor is recognized as the most important energy source among energy derived from biomass.6,16)

According to the International Energy Agency (IEA), the pulp and paper industry currently processes roughly 170 million tons of black liquor (measured as dry solids) per year worldwide, with a total energy content of approximately 2 EJ, making black liquor an extremely important biomass source.16) As shown in Fig. 7., the Food and Agriculture Organization (FAO) expects that black liquor will be increased continuously worldwide and that, by 2025, roughly 270 million tons of black liquor (measured as dry solids) will be generated, which is equivalent to approximately 3.2 EJ in total energy content.16,35)

Fig. 7. 
Estimation of the global production of black liquor (FAO, 2001).

Recently, the carbon and air pollutant emissions in pulp mills have been directly related to specific types of pulping technologies applied for pulp production and energy consumption. Biomass is one of the potential alternative resources that could replace fossil fuels and petroleum-derived materials, as it is carbon neutral and sustainable.36) In pulp and paper mills that use fewer fossil fuels, or do not use them at all, byproducts such as black liquor are combusted in a recovery boiler and converted into renewable energy as an alternative to fossil fuels.

The Intergovernmental Panel on Climate Change (IPCC) definition of biofuels, aimed at the reduction of greenhouse gases, includes black liquor as an example.37) On the other hand, the IPCC defines the term “black liquor” as a lignin-rich byproduct that is removed from wood chips, used as an energy source, and with a solid content of 65% to 70%.38)

The IEA defines black liquor as a biomass-based carbon-neutral byproduct generated during chemical pulping, which can be combusted as a fuel for generating steam and electricity in pulp mills or can be upgraded to produce synthetic gas through gasification processes.39)

In the Renewable Energy Policy Network for the 21st Century (REN21), black liquor is classified as a solid biomass.40) The FAO of the United Nations classifies black liquor as an indirect woodfuel; here, the same definition is applied to black liquor as is employed by the IPCC.41)

Meanwhile, the Climate Technology Center and Network (CTCN) defines black liquor as a waste byproduct generated by the kraft process to remove lignin, hemicellulose, and other extracts from wood, in order to obtain cellulose fibers, and considers it as a renewable energy fuel.42)

In the United States, which produces 617 million TJ of energy annually, RECs are granted to each state based on RPS. In particular, Washington D.C. plans to increase its proportion of renewable energy generation to 50% by 2032. Furthermore, the largest portion of RECs submitted for compliance with RPS in 2015 involved biomass generation, including both wood waste and black liquor, and represented 39% and 27% of the overall requirement, respectively.12)

Japan classifies black liquor as a biomass and annually produces 139,000 TJ of black liquor energy, using 70 million tons of it. As a result, the Japanese government has lowered taxes imposed on the kraft pulp mills representing benefits of ¥17 /kWh. The amount of biomass generated through black liquor is second only to that generated through livestock waste in Japan, and the country aims to produce 4% of its biomass energy from renewable energy by 2030.43)

Moreover, Sweden and Finland produce 167,000 TJ and 146,000 TJ, respectively, of black liquor energy each year, and their governments accordingly impose considerable benefits, via established renewable energy support systems. Sweden provides incentives for black liquor energy production by allocating a market price of 83.5 /MWh, while Finland issues electricity certificates (EC/MWh) to allow free trade in the market.44,45)

In addition, Canada, Brazil, the United Kingdom, Australia, India, Spain, Italy, and other countries that produce kraft pulp classify black liquor as a biomass, providing benefits for energy obtained by black liquor combustion.7)

As noted above, the United Nations and other international organizations recognize black liquor as a biomass or biofuel, not as industrial waste. Accordingly, several countries that produce pulp and paper provide a range of incentives for electricity generation using black liquor. However, even though a kraft pulp mill produces roughly 96 million TJ (1 TJ=23.9 TOE) of black liquor energy per year in Korea, the Korean government does not grant any benefits based on the RPSs.

In Korea, the quality standard of black liquor was only established in November 2018 as being in accordance with the Forest Resources on the Creation and Management Act, enabling the country’s black liquor to be classified as a forest biomass energy source.46) On the other hand, according to the criteria of the Act on the Promotion of the Development, Use and Diffusion of New and Renewable Energy,47) black liquor can be classified as a type of bioenergy in the form of a gas, liquid, or solid fuel, obtained by modifying biological organisms; however, the scope of the detailed items listing the types of bioenergy does not specify “black liquor.”49) Therefore, despite the fact that bioenergy is produced through the combustion of black liquor during the kraft pulping process, RECs have not been created alongside progress toward and in compliance with RPS. This may stem from the misunderstanding that black liquor is not a byproduct generated during the pulp manufacturing process but is an industrial waste derived from the process. This Korean domestic situation is not in line with global renewable energy policy flows, which currently provide several benefits for markets in which energy is derived from black liquor.

The combustion of 1 ton of black liquor is equivalent to the reduction of 0.84 tons of CO2. The annual carbon emissions reduced worldwide through the use of black liquor are estimated at roughly 150 million tons. Currently, Moorim P&P Co., Ltd. In Korea contributes 113,500 ton/year of CO2 reduction through the combustion of black liquor (30.8 MWh). However, if a high-efficiency recovery boiler with a power generation of 54 MWh is introduced, 85,500 ton/year of CO2 can be additionally reduced. As noted above, even though energy production through black liquor meets the requirements noted in the Act on the Promotion of the Development, Use and Diffusion of New and Renewable Energy, it is not specified in the scope of bioenergy. If power generation through the combustion of black liquor is expanded through a Korea power generation support system, a virtuous cycle structure within the forest industry can be expected. Unlike renewable energy sources such as solar power, wind power, and wood pellets, black liquor is an eco-friendly energy source that can produce electric power without causing environmental damage.

If the Korea government were to certify black liquor as complying with RPSs, RECs could be granted. For example, one REC could be granted for producing 1 MW of power in a renewable energy power plant that was certified as complying with RPSs. At that point, the government could determine the weight of the generated power by balancing this weight against the necessity for new and renewable energy supplies, based on their environmental impact. To date, however, black liquor has not been certified as RPS compliant in Korea.

The reason for the variations in the application of the classification criteria of black liquor, according to different standards in different Korean government departments, is that chemical pulp processing is not fully understood by these bodies, particularly the recovery process of spent cooking liquors. Moorim P&P Co., Ltd. is currently the only chemical pulp manufacturing company in Korea, contributing significantly to the development of the domestic pulp paper industry. Therefore, weighting of RECs on black liquor energy is essential for effecting market competitiveness among the Korean chemical pulp company and to support its continued development.

6. Reusing Black Liquor

Through the kraft pulping process, lignin can be extracted in the form of black liquor. Black liquor has been employed for power generation, chemical recovery, and for processing steam in pulp and paper mills. Furthermore, black liquor can be gasified to create a small number of value-added products and to generate power at a large scale in pulp mills.16) Newly applied technology has led to a strong driving force for evolving pulp mills to bio-refineries that convert biomass into a wide range of value-added products. Black liquor is no longer only a fuel but a green fuel. It serves as an interesting alternative to synthetic gas, which can subsequently be converted into a variety of motor fuels such as Fischer-Tropsch, DME, methanol, and hydrogen.16,48-49) Currently, there are 236 recovery boilers worldwide that have not been upgraded in the past 20 years, which may be suitable for replacement with gasification technology. No kraft pulp mills anywhere have introduced the gasification process of black liquor or the biorefinery process. However, two sulfite pulp mills in Sweden and Norway have introduced biorefinery processes to produce vanillin, fine chemicals, ethanol, and biogas with specialty cellulose.

However, there are currently no cost-effective purification strategies for lignin; these processes are required, because black liquor is contaminated with pulping chemicals such as NaOH and Na2S. Thus, only a small quantity of valuable lignin products can be isolated. When lignin extraction is targeted, Wallmo et al.50) and Wallberg et al.51) suggest that a lower content of hemicellulose in black liquor will facilitate the isolation of lignin while at the same time also increasing the purity of the final lignin. For recovering lignin from black liquor, a current state-of-the-art solution involves a combination of precipitation and membrane filtration, such as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF).52,53) The extracted lignin can be used in various areas, such as fine chemical products, polymers, drug manufacturing, adhesives, plasticizers, composite materials, antioxidants, and animal feed, among others.55-59)

One problem that presents following lignin removal from black liquor is that heating value decreases alongside increased lignin removal. This means that auxiliary fuel needs to be added to create full superheating in a recovery boiler.54) Therefore, it is necessary to consider the economic aspects involved in producing value-added products through lignin removal and reducing the calorific value of black liquor.

7. Conclusions

Black liquor is not an industrial waste generated by the kraft pulping process but is a useful byproduct of this process. The lignin and carbohydrates contained in black liquor produce steam during the combustion process, and the inorganic compounds are converted to smelt and are used to recover NaOH and Na2S. Air pollutants generated during the combustion process of the recovery boiler are thoroughly controlled to meet environmental standards, contributing to the minimization of fine dust generation and the reduction of greenhouse gases. International organizations, such as the IPCC, and countries that produce kraft pulp classify black liquor as biomass energy or biofuels and support them with various incentives based on RPSs. In Korea, however, different standards are applied to the classification of black liquor and the granting of REC weights by government departments. Black liquor obtained by the alkali cooking reaction of wood chips is not discharged externally in the form of industrial waste but is used as an important energy resource for steam generation and chemical recovery, using a recovery boiler and a causticizing process. As a result, the United States, Canada, Japan, Sweden, and Brazil classify it as a biofuel, granting several economic benefits based on the use of this green fuel. It is urgent that the perception of black liquor be enhanced in Korea, by accurately reflecting advanced foreign cases. Doing so can serve as an opportunity for driving competitive enhancement and consistent growth of the chemical pulp industry in Korea.

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