NC State
BioResources
Hubbe, M. A. (2021). "Energy efficiency challenges in pulp and paper manufacturing: A tutorial review," BioResources 16(4), 8567-8639.

Abstract

The pulp and paper industry is highly energy-intensive. In mills that use chemical pulping, roughly half of the higher heating value of the cellulosic material used to manufacture the product typically is incinerated to generate steam and electricity that is needed to run the processes. Additional energy, much of it non-renewable, needs to be purchased. This review considers publications describing steps that pulp and paper facilities can take to operate more efficiently. Savings can be achieved, for instance, by minimizing unnecessary losses in exergy, which can be defined as the energy content relative to a standard ambient condition. Throughout the long series of unit operations comprising the conversion of wood material to sheets of paper, there are large opportunities to more closely approach a hypothetical ideal performance by following established best-practices.


Download PDF

Full Article

Energy Efficiency Challenges in Pulp and Paper Manufacturing: A Tutorial Review

Martin A. Hubbe

The pulp and paper industry is highly energy-intensive. In mills that use chemical pulping, roughly half of the higher heating value of the cellulosic material used to manufacture the product typically is incinerated to generate steam and electricity that is needed to run the processes. Additional energy, much of it non-renewable, needs to be purchased. This review considers publications describing steps that pulp and paper facilities can take to operate more efficiently. Savings can be achieved, for instance, by minimizing unnecessary losses in exergy, which can be defined as the energy content relative to a standard ambient condition. Throughout the long series of unit operations comprising the conversion of wood material to sheets of paper, there are large opportunities to more closely approach a hypothetical ideal performance by following established best-practices.

Keywords: Exergy; Pinch analysis; Process integration; Underutilized resources; Heat exchangers; Efficiency; Sustainability

Contact information: Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005 USA; *Corresponding author: hubbe@ncsu.edu

INTRODUCTION

Energy is a dominant factor that affects the economics and environmental performance of the pulp and paper industry (Bajpai 2016; Lawrence et al. 2018). In broad terms, one can envision the pulp and paper industry as being in the bioenergy business. When viewed from this perspective, there are opportunities to improve efficiency and to lower environmental impacts. In this tutorial review, the goal is to cover a broad range of concepts, from elementary to more advanced, that can be important in a variety of different pulp and paper mill facilities. Principles and mechanisms will be emphasized. Other publications are available that go more deeply into certain issues, such as process audits (Gilbreath 2019; Reese et al. 2020; TAPPI TIP 0404-63, 2021) and systematic approaches to such methods as pinch analysis (Atkins et al. 2012). For example, a combined study of water and energy analysis for kraft pulp mills appeared recently (Ahmetović et al. 2021).

Pulp and paper mills not only are huge consumers of energy, in the form of power and/or fuels, but they often are huge suppliers of energy as well. In the case of kraft pulp mills, a major source of energy, the lignin removed from the cellulosic material during pulping, is mostly converted to steam during the chemical recovery process (Gong 2005; Naqvi et al. 2010; Verma et al. 2019). The generated steam can be used in running the processes. By improving the overall efficiency of operations, the industry has come somewhat closer to energy self-sufficiency (Farla et al. 1997; Holmberg and Gustavson 2007; Lundberg et al. 2014). In addition, pulp and paper processing facilities are often well situated to be able to increase their reliance on incineration of underutilized lignocellulosic materials, such as bark, knots, branches, etc. By such usage of bioenergy, pulp and paper mills can decrease their usage of purchased energy, which is often in the form of fossil fuels or electricity (Holmberg and Gustavson 2007; Costa et al. 2019).

The goal of the present work is to review the available published literature regarding ways in which energy usage within the pulp and paper can be reduced, both in terms of the current state of the art and prospects for the future. Some important reductions in energy usage will fall under the category of efficiency. Further gains continue to be achieved by application of thermodynamic principles, using such concepts as exergy (Gong 2005; Utlu and Kincay 2013; Luis and Van der Bruggen 2014; Dincer and Rosen 2021) and pinch analysis (Browne et al. 2001; Atkins et al. 2012). Such practices can avoid some of the unnecessary destruction of useful energy, as in the case where a hot stream is combined directly with a colder stream of fluid. Unnecessary destruction of exergy also happens when steam is vented directly to the atmosphere, giving up its latent heat.

Another theme that will be explored in the sections that follow will be opportunities to obtain energy or at least decrease energy expenditures related to under-utilized resources, including bark, sludge, and wastepaper. Emphasis will be placed, in this article, on approaches that have demonstrated practical success under conditions resembling an industrial process.

Suggested Readings

At the outset, it is important to recognize some important review articles and monographs, to which the reader is referred. These are described briefly in Table 1, along with citations. The existence of these publications takes away some of the burden from the present article to describe full details in the covered areas.

Table 1. Review Articles and Monographs Dealing with Aspects of Energy Reduction in the Pulp and Paper Industry

Energy Audits of Pulp and Paper Processes

Readers of this article are urged to also study articles about energy audits at pulp and paper mills (Sweet 1991; Kong et al. 2016; Reese 2018; Reese and Deodar 2018; Gilbreath 2019; Reese et al. 2020). The largest and quickest savings in energy in pulp and paper mills usually are achieved as a result of system audits. During such audits, emphasis is placed on finding deviations from the intended functioning of operations. Independent measurements are made of flows and other parameters, often using portable flow meters (Reese et al. 2020). Such independent checking routinely reveals problems with meters, valves, pumps, and even with improper settings of devices. Audits of pulp and paper mills, which often involve a team of specialists, can reduce energy usage by about 20%. In typical cases, about a quarter to a half of the identified energy savings can be achieved without capital expenditures (Reese et al. 2020).

Organization of this Article

The organization of sections of the article, after this introduction, will start with consideration of general principles, then application of these principles to the pulp and paper industry, then strategies for implementation, and finally some closing comments. A “process order” arrangement will be followed when discussing application of energy principles within a typical pulp and paper mill. In view of the many technical terms employed in this article, the Appendix contains a glossary.

GENERAL PRINCIPLES OF ENERGY MANAGEMENT

Before considering issues specific to the processing of materials by the pulp and paper industry, background is provided here regarding some fundamental and general principles. Readers not needing this background are urged to skip ahead after scanning the headings. Three main areas are discussed here: simple losses, energy and exergy, and converting various process wastes or byproducts into energy and exergy.

To begin, it is important to focus upon the level of carbon dioxide in the environment, which has a close connection with the consumption of fossil fuels by humans. As illustrated in Fig. 1, the growth of plants in the course of photosynthesis naturally converts carbon dioxide to oxygen and leads to the storage of cellulose and other biomolecules. The radiant energy of the incident sunlight is thus captured in the form of chemical energy. The term “(CH2O)n” that is included in Fig. 1 formally represents polysaccharides (cellulose and hemicelluloses), which are the largest component in the biomass.

Fig. 1. Illustration of the photosynthesis process, by which carbon dioxide from the air is naturally converted to oxygen and biomaterials. Figures within this article are originals drawn by the author, unless otherwise noted.

The process of photosynthesis is essentially reversed when chemical energy stored in the organic materials, including both freshly grown biomaterials and fossil fuels such as coal, petroleum, and natural gas, are burnt. As illustrated in Fig. 2, increased levels of carbon dioxide and certain other gases in the atmosphere have the tendency to reflect back heat energy in the form of infrared radiation. Those rays initially had been reflected from the Earth’s surface, thus causing a net temperature increase in the world’s environments (Black and Weisel 2010).

Fig. 2. Action of greenhouse gases such as carbon dioxide, methane, and nitrogen oxides in trapping some of the energy resulting from initial solar heating of the Earth’s surface, giving rise to increased average temperatures. A: Increased reflection of infrared rays (heat) by greenhouse gases. B. Relative global warming potential (GWP) of CO2, methane, and nitrogen oxides on a molecular basis and also the warming influence of the gases at present levels in the atmosphere

As indicated in part B of the figure, the effects in terms of global warming depend both on the tendency of each type of gas to reflect heat and their present levels in the atmosphere. For instance, although the tendency of carbon dioxide to reflect heat is only about 1/28 of the same quantity for methane (when integrating the effect over time), the much higher level of CO2 in today’s atmosphere than methane implies a greater effect. Another difference is that the half-life of atmospheric methane is about 8 to 12.4 years (Lelieveld et al. 1998; Balcombe et al. 2018), whereas CO2 in the atmosphere is very persistent (Ocko et al. 2018).

Simple Losses

When it comes to saving energy in an industrial facility, many of the cheapest and quickest steps are also the simplest, such as insulating hot pipes (Zaki and Al-Turki 2000; Hong et al. 2011; TAPPI TIP 0404-63 2016) and fixing leaks (Walker 1977; Sherlaw 1980; Chow 1982; Abdelaziz et al. 2011; Hong et al. 2011; Bhutani et al. 2012; TAPPI TIP 0404-63 2016). In an analysis of energy loss in the Taiwanese pulp and paper industry, Hong et al. (2011) estimated that in a typical manufacturing facility, equipment inefficiency accounted for an energy loss of 40%, boiler and electricity generation summed up for an energy loss of 32%, and distribution almost 28% energy loss. Connecting increasing energy efficiency to reducing in greenhouse gas emissions, there are opportunities to reduce emissions by 10 to 30% with “little or no investment,” just by paying attention to simple losses (IPCC 1996).

Figure 3 illustrates the concept of insulation, considering the case of pipes containing a hot fluid that are immersed in stirred water media. In such cases the net flow of heat will be proportional to the gradient of temperature within the pipe walls, which are assumed to have a constant thickness.

Fig. 3. Illustrative example of effect of insulation on the temperature gradients, and thus the flow of heat, in the case of pipes carrying a hot fluid through a hypothetical stirred water bath

Routine replacement of inefficient, common items is another promising place to look for potential savings. A quick pay-back, due to reduced energy usage, often can be obtained by replacing old, inefficient electrical motors with new, efficient ones (Browne et al. 2001; Abdelaziz et al. 2011; TAPPI TIP 0404-63 2016). For example, electrical energy can be saved by adding inverters to blowers (Hong et al. 2011).

Frictional consumption of energy during the running of manufacturing equipment often can be reduced by better lubrication. Figure 4 provides a schematic illustration of how friction affects the amount of energy required to run a continuous mechanical process, such as pulling a fabric adjacent to a surface against which it is being pressed. Among the terms in the expression that can be used to estimate frictional energy losses, it is the coefficient of friction that offers the greatest potential for savings. Savings often can be achieved by correct application of lubricants and maintenance of bearing systems.

Fig. 4. Illustration of a typical situation in the running of a paper machine, where motion of a continuous moving fabric is resisted due to friction as it passes over a surface, to which it is being pressed

Holmberg et al. (2013) estimated that about 15% to 25% of the energy used in a typical paper mill is due to friction. New technology for friction reduction was estimated to result in savings of about 11% in a time-frame of about 10 years and about 23.6% in the time-frame of 20 to 25 years. The reason for these relatively long-time horizons is that processing equipment in a typical paper mill can be used for multiple decades.

Large amounts of heat energy are used in pulp and paper manufacturing systems for heating and evaporation of water. Such heating requirements often can be decreased by finding ways to utilize less fresh water. Figure 5 illustrates a way to greatly decrease the amounts of water that are needed for various washing operations by applying a counter-current washing system. As shown, the cleanest water is utilized for the final washing of the cleanest pulp. The filtrate of that operation is used for cleaning in the preceding stage of washing, and so forth.

Fig. 5. Schematic view of three pulp washing deckers arranged with a counter-current flow pattern of wash solutions and filtrates

An opportunity that is often overlooked by scholars as an energy concern, but that often is a major focus of attention for pulp and paper mill operating teams, is to increase the “up-time” of a process (TAPPI TIP 0404-63 2016). That is because various fixed and semi-variable costs, including labor, insurance, and depreciation, continue inexorably, regardless of whether or not a product is coming off the end of a process line (Hubbe and King 2009). Eliminating such process interruptions also saves energy, since most of the energy consumption in a papermaking process will be happening continuously, whether or not saleable paper is arriving at the reel of the paper machine at every minute.

Energy and Exergy

Definitions

The term exergy can be defined as “shaft energy” or as “usable energy”. Exergy represents the maximum work potential of a system or component at a given state in a specified environment. The reason that such energy is important is that although every material contains thermal energy, the laws of thermodynamics allow only part of that heat energy to be converted to, say, mechanical or electrical energy. The computation of the quantity of exergy always requires the definition of a reference temperature, which might be the average temperature of ambient air adjacent to the facility (Dincer and Rosen 2021). Calculations of changes in exergy make it possible to quantify any unnecessary destruction of useful energy, for instance when directly mixing hot water with cold water in an industrial process (Luis and Van der Bruggen 2014; Costa et al. 2019). Such mixing represents a lost opportunity to save exergy. Exergy analysis also makes it possible to calculate the maximum possible conservation of energy that might be achieved by future advances in technology (Gong 2005).

Pulp and paper mills are an especially appropriate focus of exergy analysis due to the fact that the processes employ large amounts of energy in multiple forms. These can include thermal energy (latent heat of evaporation and sensible heat due to the heat capacity and changes of temperature of materials), electrical energy, and mechanical energy. It is therefore important to analyze the energy and exergy flow to identify the potential areas where the energy efficiency can be improved.

The importance of latent heat is illustrated in Fig. 6, which considers the heating of water at atmospheric pressure from ambient temperature to 200 °C, well above the boiling point. It is notable that in the case considered, the net energy can be estimated based on just three terms.

Fig. 6. A: Illustration of the relative amounts of heat required to raise the temperature of water to the boiling point (sensible heat), to evaporate the water (latent heat), and to further heat the water vapor (sensible heat) at atmospheric pressure; B: Heat losses due to steam leakage

A first component of heat (sensible heat) needs to be applied to heat the water up to the boiling point. Next, latent heat is involved during the phase change from liquid to steam vapor. The third component of applied energy raises the steam temperature to the defined final state.

The leakage of energy is illustrated in part B of Fig. 6, which considers the possibility of escape of steam to the environment at a defective junction in piping. In the case considered, the loss of exergy includes both the latent heat and that portion of the sensible heat associated with temperatures above the reference temperature that has been assigned to the environment. Sensible heat must be included here because the leaked substances are all starting at a temperature well above the reference temperature (e.g. 25 °C).

Heat exchangers and recovery of heat

Within a pulp and papermaking operation, there are simultaneously many process streams that need to be heated up and many other streams that need to be cooled down. If energy conservation were of no concern at all, then all such changes in temperature could be achieved by direct heating and by refrigeration. But exergy is unnecessarily destroyed whenever separated substances having different temperatures are allowed to form an ideal mixture (Gong 2005). Such wastage can be minimized by the use of heat-exchangers. In particular, an economizer is a heat-exchange device designed to extract heat from flue gases (i.e. smoke) so that the energy can be used for the heating of entering air and water streams (Adbdelaziz et al. 2011). Part A of Fig. 7 shows a generic heat exchanger of the tube-shell variety, whereas part B represents an economizer, which transfers heat from flue gas. The numbers shown in part B are intended to suggest a possible use of warm flue gas (for instance starting at 200 °C) to preheat incoming air, before its usage for combustion in a boiler. Within a heat exchanger, the stream to be heated flows through a series of stainless steel tubes. The tubes can be bare or have attached fins (Wejkowski 2016), which increase the heat transfer. As a specific example, blow-line heat from a pulp digester can be used to heat incoming water, as well as preheating of the combustion air to be fed into the boiler (Browne et al. 2001). It has been estimated that the amount of heat in the moist exhaust air leaving from paper machine hoods is about six times greater than the ability of incoming dry air to absorb heat (TAPPI TIP 0404-63 2021).

Fig. 7. A: Typical design of a tube and shell heat exchanger; B: schematic of an economizer, which transfers heat from flue gas to incoming streams of air or water to a process

Another commonly observed wastage that occurs within pulp and paper mill systems consists of the direct venting of steam to the atmosphere. Such venting, which can either be purposeful or due to leakage of seals, should be avoided as much as possible. An exception is routinely made in the case of steam injected into dryer cans (TAPPI TIP 0404-63 2021). Many such systems are routinely operated with about 10% excess of steam that is “blow-through steam”. The purpose of such venting of steam is to continuously displace non-condensable air and keep it from accumulating within the dryer cans. There are major savings in exergy that can be achieved by good control and minimization of blow-through steam (Walker 1977; Sherlaw 1980; TAPPI TIP 0404-63 2021).

Often within a pulp and paper mill system one encounters situations in which a certain source of steam does not have a sufficient combination of temperature and pressure to serve a particular function. Such a problem can be solved by use of a thermocompressor, which is a device that allows controlled blending of steam having different (but ideally not too different) levels of temperature and pressure (Walker 1977; Sherlaw 1980; TAPPI TIP 0404-63 2021). Though thermocompressors consume exergy, their use sometimes results in less loss of exergy (i.e. higher energy efficiency) when compared with other sources of steam having the needed pressure.

Pinch analysis

Pinch analysis is a technique that can be applied to avoid unnecessary exergy losses in an industrial process (Browne et al. 2001; Koufos and Retsina 2001; Wising et al. 2005; Atkins et al. 2012). Because pinch analysis typically considers all unit operations within the entire facility simultaneously, the term “process integration” is often applied. The term is especially used when discussing proposed process changes that come about due to pinch analysis. The general approach of pinch analysis is shown in a simplified form in Fig. 8, which is based on graphics shown by Atkins et al. (2012).

Fig. 8. Simplified diagram for a hypothetical pinch analysis, showing cumulative curves for streams being cooled (source streams) and streams being heated (sink streams). The graphic has been redrawn based on an original by Atkins et al. (2012).

The most notable feature in a typical graphic for a pinch analysis is a pair of composite curves, which together represent all of the streams being cooled and all of the streams being heated within the process as a whole (Koufos and Retsina 2001). Streams of materials being cooled (such as flue case, foul condensate, and exhaust from the hoods of paper machines, etc.) are called source streams. Materials being heated (such as incoming fresh air and water) are called sink streams.

In Fig. 8, the vertical axis represents temperature and the horizontal axis represents heat energy, i.e. enthalpy. In principle, energy can be saved whenever a change in the process results in the two composite curves coming closer together. Points where the composite curves come close together are called pinch points, from which the analysis method gets its name. Within such a diagram, one can envision heat exchangers functioning as bridges (Bonhivers et al. 2016). The cited work advocates the modification of bridge placements, i.e. the re-engineering of the heat exchanger network, as the most promising way to reduce energy consumption. However, there are also practical constraints to keep in mind, such as the distances between unit operations that might most efficiently exchange heat in an idealized system (Koufos and Retsin 2001). It is possible to carry out optimizations in such a way as to meet specified restrictions on heat exchange, corresponding to practical or theoretical considerations (Becker and Marechal 2012). There needs to be an evaluation of energy balance, identification of the inefficient process parts, and an optimum design of heat exchanger network. By combining pinch analysis with a techno-economic analysis of the costs of different design changes and operating costs, one can then make accurate choices between relatively cheap and convenient options (e.g. non-isothermal stream mixing and direct heat transfer), vs. more expensive retrofits to achieve a more efficient network of heat exchangers in the system (Savulescu and Alva-Argaez 2008). As another example, it has been shown that energy can be saved by changing the way in which a heat exchanger network is run during the summer vs. during the winter in a pulp mill (Persson and Bertsson 2009).

Pinch analysis has been carried out to predict potential energy savings that can be achieved by various changes in processes and equipment. For instance, if the usage of steam energy in a pulp and paper mill can be decreased, then the excess steam either could be used to generate more electrical power or to enable isolation of lignin from black liquor (Axelsson et al. 2006). The lignin isolation is of interest because it can decrease the loading on an existing kraft recovery boiler, thus enabling a higher overall production rate at a pulp mill that is dependent on that recovery boiler (Hubbe et al. 2019). As another example, Marinova et al. (2009) employed pinch analysis as a way to develop strategies to save enough excess energy to enable pre-extraction of hemicellulose from wood chips prior to kraft pulping. Pinch analysis also can be applied in broader consideration of biorefinery options for redesign of existing pulp mills, allowing them to serve as sources of hemicellulose-based products, lignin-based products, and cellulose-based products (Moshkelani et al. 2013).

Beyond pinch analysis, it is important to take into account that steam-based systems comprise multiple forms of energy simultaneously, e.g. radiant energy, pressure-related energy, sensible heat (related to heat capacities), and latent heat (i.e. the heat associated with evaporation or condensation). Studies have indicated high potential for energy savings by focusing on the recovery of latent heat in the steam left over after various unit operations (De Beer et al. 1998). As an example, in paper machine drying systems it is important to efficiently return hot condensate to the powerhouse.

Converting Wastes to Energy Products

In principle, there can be a near-zero net accumulation of carbon dioxide in the atmosphere if energy is generated from burning of plant sources, since it is generally understood that such CO2 will be taken up by the next generation of a crop of biomass (Cherubini and Stromman 2011). Important assumptions upon which such projections depend include a maintenance of constant levels of plant growth in future years. In addition, although future forests can replace trees that are harvested today, the delay between harvesting and regrowth has consequences both on the amount of sequestered carbon and on the climate (Timmons et al. 2016). Future growth of trees and other biomass can depend on climate trends, in addition to forest management practices.

Though the pulp and paper industry sector is a major net user of energy, the industry is exceptional due to the relatively large proportion of the energy coming from consumption of renewable energy, especially biomass-based energy. Such energy comes from the incineration of the lignin present in black liquor (which is broken down and removed from the wood during kraft pulping) and also from the generation of steam energy in hog-fuel boilers, using bark and forest residuals (Blackwell and MacCallum 1983). It was reported that the US pulp and paper industry maintained a steady consumption of energy for many years, despite growth in production, mainly as a result of efficiency improvements (Koleff 1998). Opportunities to increase the proportion of plant-based energy during the production of pulp and paper will be considered later in this article. Life cycle assessment (LCA), which is based on an assumption of sustainable future growth of trees and other biomass, has become well accepted as a means to estimate and compare the expected environmental impacts of planned industrial projects.

It has been proposed that future pulp and paper facilities might achieve such high efficiency and that they will be able to use some of the excess electrical energy to actively remove carbon dioxide from the air (Mollersten et al. 2004). Such a system, if successful, would offer the industry the possibility to become an example of technology with negative CO2 emissions. The proposal is to employ captured CO2 as a raw material for bioproducts, for example in an integrated biomass gasifier system in which the syngas undergoes a CO-shift reaction. Though such a proposal might serve to inspire useful research, most present pulp and paper facilities are very far away from generating all of their needed steam and electricity, and they are clearly not in a position to supply excess energy at present. However, as will be shown in the next section, there are a large number of unit operations that make up a pulp and paper facility, and improvements to most of them can provide opportunities to improve the mill’s energy balance (Szabó et al. 2009).

EXERGY SAVING OPPORTUNITIES IN PULP & PAPER PROCESSES

This section reviews published findings related to unnecessary loss of useful energy (i.e. exergy) in a typical pulp and paper manufacturing facility. Though, as will be seen, a pulp and paper operation can be envisioned as an interconnecting web, it is still possible to trace the main flow of materials in a linear path, and the discussion here will follow the chronological order of a typical process. Thus, the first topic involves the potential more effective usage of such materials as branches and bark during the harvesting of wood in forests. Another “before-pulping” opportunity, which pertains to kraft pulping operations, involves energy is the pre-extraction of hemicellulose from wood chips. Next to be discussed are the main pulping methods, starting with mechanical pulping, and then chemical pulping, where emphasis is placed upon use of a recovery boiler to generate steam with the burning of the lignin released from the wood material during chemical pulping. Next comes a discussion of energy savings in the paper machine system. Thereafter, the energy implications of various wastewater treatment options and sludge handling options are considered. In addition, some discussion points related to “whole system” integration, with a focus on energy, are left to the end of the section.

Before Pulping

Biomass pellets

Even before wood supplies arrive at a typical pulp and paper factory, already there are opportunities to recover heat value from under-utilized resources. In particular, the residual biomass, including small branches, has the potential to be converted into pellets (Miranda et al. 2015; Picchio et al. 2020). Figure 9, part A, describes a continuous process in which biomass is continuously squeezed through cylindrical holes (often about 3 to 6 mm in diameter) and cut to a desired length. By such conversion, the biomass becomes denser and uniform. This can be beneficial for storage, easy flowability, and convenience for metering into furnace equipment, where the materials can be converted into energy, including steam or electricity. When such material is used in place of fossil fuels, there is an expected net favorable effect on environmental impacts (Martin-Gamboa et al. 2020). Though pellet manufacture from biomass is widely practiced, it has not yet become common in the context of wood harvesting operations for pulp and paper manufacturing. Presently it is common practice to leave most branches in the forest, where presumably they can decay and return their mineral content to the soil.

Fig. 9. A: Schematic illustration of pellet-forming process in which compressed biomass is squeezed through a small hole and cut to length using a rotating blade; B: Photo of wood pellets (courtesy of Danial Saloni, North Carolina State University, Dept. of Forest Biomaterials)

If a pelletization plant is set up at a pulp and paper mill, then a very promising option is to prepare pellets from bark, which is often removed from pulpwood at the mill site (Lehtikangas 2001; Mobini et al. 2013; Erixon and Bjorklund 2017). Although it is possible to feed bark and other forest residues directly to a power boiler (see later discussion of hog fuel boilers), the pelletization process renders the fuel suitable for use in a wide range of other boilers, including those used for district heating (Andersson et al. 2006).

Bark and wood residues, as they are collected in woods operations and at mill yards, often contain much more water than would be desirable for feeding to a power boiler. The amount of energy needed to evaporate the water becomes subtracted from the net heating value of the material (Demirbas 2005). The effects of moisture content and ash content on the effective heating value of various biomass types is shown in Fig. 10 (Zhao et al. 2017). The area in the plot below the horizontal dashed line defines conditions in which the net energy production is negative. Note that the highest positive value is shown at the upper left of the plot for bituminous coal, which is generally quite dry and has a low ash content. By contrast, at the lower right, the least desirable feedstocks for recovery of energy by combustion are the wet sludge samples (fermented sludge and sewage sludge). Unless these waste materials could be pre-dried using available waste heat, it would be advisable to seek other ways to recover their energy content. For instance, anaerobic digestion with recovery of methane gas is considered later in this review.

Fig. 10. Effects of different moisture contents and ash contents on the effective heating values of various biomass materials, compared to bituminous coal. Replotted from a version presented by Zhao et al. (2017)

Ash content represents the minerals (e.g. SiO2 or CaCO3) that may be present in the raw material. Ash content is regarded as undesirable, not only because it detracts from potential for energy production, but also due to the residue generated during combustion. Many existing boilers have limited tolerance for ash.

A net boost in energy production can be achieved when some of the low-grade waste heat, e.g. from flue gases, can be utilized for pre-evaporation of water in the biomass. For example, the material could be dried before its conversion to pellets (Wolf et al. 2006; Andersson et al. 2006; Erixon and Bjorklund 2017). Greater improvement can be achieved by steam treatment under pressure, followed by rapid expansion. The resulting steam-exploded “black pellets,” which resemble Masonite, are denser, stronger, and more water-resistant than ordinary pellets, and these are regarded as positive attributes. Another option is to use low-grade steam for a pulp and paper mill system to achieve partial drying of the biomass (Erixon and Bjorklund 2017). A related option to consider is torrefaction (Nosek et al. 2017; Mostafa et al. 2019; Wang et al. 2020), which entails heating up of the material to a range of about 200 to 300 °C (Picchio et al. 2020). Torrefaction, which can be carried out either before or after compressing the material into pellets, results not only in water removal, but also in partial decomposition of hemicellulose, yielding higher density and lower tendency of the material to absorb moisture from the air during storage. These changes render the material more valuable as a fuel. In addition, byproducts from the pulp and paper industry, such as lignosulfonate, can be used as binder for the production of pellets (Kuokkanen et al. 2011). Another potential source of material for pellet production is the sludge from wastewater treatment (Matúš et al. 2018). Additional energy-related options for utilization of such sludge are considered in a later section.

The higher effective heating value of bituminous coal, in comparison to dry wood, can be attributed to its elemental composition. As shown in Fig. 11, biomass in general has a much higher ratio of oxygen to carbon in its molecular structure in comparison to coal, in either its anthracite or bituminous forms. The covalent bond connecting carbon and oxygen is not available for combustion, and this difference means that oxygen-containing compounds are at a disadvantage in terms of their energy contribution, even when dry and having a low ash content.

Fig. 11. A so-called Van Krevelen diagram, showing typical ranges of molecular composition for various solid fossil fuel and biomass resources. Redrawn based on a version reported by Grønli (2021)

The higher heating value (HHV) of a fuel can be defined as the maximum amount of heat released during the complete combustion after complete drying of the material of interest. The HHV of bituminous coal has been reported to be in the range 20 to 33 MJ/kg (Fernandez et al. 1997), whereas that of biomass is generally within the range 15 to 20 MJ/kg (Chen et al. 2015).

Hog fuel boilers

To take advantage of forestry residues present at the mill wood yard, many pulp and paper facilities will run an additional power boiler, which is often called the hog fuel boiler or hogged fuel boiler (MacCallum 1983; Costa et al. 2019). The term has been used traditionally to denote the mixture of biomass that has been chopped up to enable more uniform burning. By running such a system, paper companies can generate electricity that either displaces purchased electricity, or they can sell energy back to the grid (Marshman et al. 2010). Heat can be generated from biomass according to the approximate relationships shown in Eqs. 1 and 2.

CH2O + O2 🡪 CO2 + H2O ΔH = – 9.1 MJ/kg (1)

Lignin + O2 🡪 CO2 + H2O ΔH = – 22 MJ/kg (2)

In these equations, CH2O represents the carbohydrates, which can approximate the elemental composition of cellulosic material.

As was noted earlier, with reference to pellet production, the moisture content of hogged fuel has a major influence on the potential for steam production from the boiler. More energy can be generated if waste heat is used to pre-dry the material before its addition to a power boiler (Blackwell and MacCallum 1983; Sweet 1991; Costa et al. 2019). Boiler efficiency decreases at an increasing rate with increasing percentage moisture content of the hogged fuel (Linderoth 1986). The solid red curve in Fig. 12 shows how heat release at the grate of a power boiler was found to depend on the moisture content of the hog fuel. The dashed blue curve indicates the steam production as a function of moisture content (compared to the reference moisture content of 30%). Incoming material that is drier burns faster, which helps to maintain a high rate of energy production in the boiler. In addition, the temperature of incineration will likely be higher when the incoming biomass has a lower moisture content. The drying process of hogged fuel generally happens in two stages: In an initial rapid stage, the rate of drying is controlled by the rate at which heat can be supplied to the outsides of the particles, which are wet. In the later stage, the rate of drying depends on diffusion of moisture to the surface of the particles (Blackwell and MacCallum 1983).

Fig. 12. Boiler efficiency as a function of moisture content of hogged fuel (replotted from Linderoth 1986)

Figure 13 illustrates a hypothetical concept in which the remaining heat content of flue gas is used to decrease the moisture content of biomass before it is fed to a hog-fuel boiler. The numbered yellow circles shown in the figure correspond to various pyrolysis gas-cleaning operations described in an earlier review article (Nelson et al. 2018). The figure has been modified by addition of a hopper at the upper left. By adding such a device, it would be conceivable to feed a controlled stream of medium-temperature flue gas near the base of the hopper. Thus, by the time the biomass has arrived at the base of the hopper, some of the moisture content has been evaporated.

Whether systems such as that illustrated in Fig. 13 can be used in practice will depend a lot on whether measures are taken to reduce the possibility of fires. Uncontrolled fire is an inherent danger during the drying of biomass, especially if direct contact with flue gas is used. Such danger can be avoided by usage of low-pressure steam rather than flue gas (Bruce and Hulkkonen 1998). Fortunately, the moisture content of hogged fuel can be really measured by means of a calibrated infrared-sensing device (Sayegh et al. 1983). There is a need for process control systems that can enable safe operation of biomass drying equipment. Power boilers suitable for hogged fuel can be grouped into two main types – grate-fired boilers, and fluidized bed boilers. A fluidized bed system has some advantages when handling particles that are relatively wet (McDermott et al. 1981), and it also has more flexibility with regard to fuel characteristics.

Fig. 13. Hypothetical scheme for pre-drying of hog fuel before its combustion by means of spent flue gas, after its use for steam generation and cleaning procedures. The figure is a modified version of what was original presented by Nelson et al. 2018). The numbered gas-cleaning steps are for (1) nitrogen oxides, (2) mercury and dioxin, (3) acid gases, (4) particulates, and (5) pollution control testing.

Hemicellulose extraction before kraft pulping

The next situation to consider pertains to kraft pulping. Hemicellulose is an energy-containing component that can be obtained from woody material before the pulping process. In conventional kraft pulping technology, the understood goal typically is to retain as much of the hemicellulose as practical in the pulp fibers. When present within the fibers, hemicellulose contributes positively to inter-fiber bonding within paper (Bai et al. 2012) and it renders the fibers less susceptible to irreversible loss of inter-fiber bonding ability when the paper is recycled (Oksanen et al. 1997). Though some of the hemicellulose becomes dissolved during delignification during kraft pulping, such hemicellulose and its byproducts contribute to energy generation in the recovery furnace (see later discussion). The pre-extraction of hemicellulose from wood chips prior to kraft pulping has been shown to have a negative effect on paper strength (Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008).

Sometimes there are persuasive reasons to extract at least part of the hemicellulose from the chips before they are added to the kraft digester. For instance, such pre-extraction can lead to savings in costs of bleaching the resulting pulp (Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008). In addition, the hemicellulose that becomes extracted is likely to be of relatively low molecular mass and therefore less effective as an inter-fiber bonding agent. Most often it is proposed to remove the hemicellulose by steam treatment, though a steam explosion process has been shown to be even more effective for its solubilization (Martin-Sampedro et al. 2014). The steam explosion option is not recommended when the fibers are to be used for papermaking, due to adverse effects on paper strength. However, such treatment might make sense if the kraft fibers were being converted to chemical products such as glucose, ethanol, furfural, or carboxylic acids (Xing et al. 2011; Moshkelani et al. 2013).

Another reason in favor of pre-extraction of hemicellulose before kraft pulping is that the amount of energy that can be produced by the combustion of hemicellulose is much lower than that of lignin (Marinova et al. 2009; Hamaguchi et al. 2013). For example, the higher heating value (HHV) that can be obtained by pyrolysis of hemicellulose to bio-oil is only about 60% of what can be obtained from the same mass of lignin (Zhao et al. 2017). This difference is consistent with the earlier discussion related to the oxygen content of the biomolecules. Hemicellulose has an O:C ratio of about 1:1 (from the chemical formula for carbohydrates), whereas that of lignin is about 0.3:1 (Robert et al. 1984). When hemicellulose has been pre-extracted, a promising option then is to use enzymes (e.g. xylanases or other hemicellulases) to convert it to sugars (Dhiman and Mukherjee 2018). Yeast can be used to convert such sugars to ethanol, which is a valuable fuel.

Implementation of hemicellulose pre-extraction in an existing kraft pulp mill can be expected to disrupt the balance of materials, steam, and energy (Marinova et al. 2009; Moshkelani et al. 2013). The extraction decreases the energy content of the wood chips being fed to the digester (Hamaguchi et al. 2013). However, the freed-up capacity of the digester might make it possible to feed a greater quantity of wood chips, depending on compression of the material, thus possibly enabling an increase in production rate at the mill. A higher production rate implies a greater flow of burnable material in the black liquor being sent to the recovery furnace. In a favorable case, such changes were projected to enable the separation of some of the lignin from the black liquor, which also decreases the amount of energy produced in the recovery boiler, putting the system back in balance at a higher production rate (Hamaguchi et al. 2013).

Mechanical Pulping

High amounts of energy are required to separate uncooked wood fibers from each other when using mechanical pulping procedures. The type of equipment utilized to separate the fibers is illustrated schematically in Fig. 14