Biomass Conversion and Biorefinery

https://doi.org/10.1007/s13399-020-01243-6

REVIEW ARTICLE

A review of the traditional pulping methods and the recent

improvements in the pulping processes
Drake Mboowa 1

Received: 22 October 2020 / Revised: 17 December 2020 / Accepted: 22 December 2020

# The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021

Abstract
The demand for paper and pulp-derived products to fulfill consumer needs is increasing considerably globally. This work
provides a critical overview of the various traditional pulping methods and describes the recent improvements in pulping
processes. A comparison of different pulping techniques has shown that the mechanical pulping process produces high pulp
yields per unit volume of wood of poor quality (low strength, bonding, fiber morphology, etc.) as compared to chemical pulping
methods. The use of semi-chemical pulping is reported as an effective way of overcoming the disadvantages of the mechanical
pulping process. Recent modifications of the pulping processes that have happened in the last decade have been discussed and
shown to be driven by the desire to save energy and reduce chemical requirements while maximizing pulp yields and quality.
With the emergence of bio-based nanotechnology, post-pretreatment of Kraft and sulfite pulps for making nanocellulose and
lignin-containing nanocellulose with improved fiber characteristics of fiber size, crystallinity, chemical composition, and fiber
surface functionality has been discussed. Furthermore, challenges and prospects of the improvements in pulping processes are
highlighted.

Keywords Lignin . Mechanical pulping . Chemical pulping . Semi-chemical pulping . Nanocellulose

1 Introduction Amazon, eBay, and Alibaba leading in the market for online
sales, there has been an increase in demand for packaging
The pulp and paper industry has existed since the first century materials of goods [6].
when the Chinese invented the first paper-like writing materi- Paper is produced in a three-stage process involving
al. Paper is a layered carpet consisting of a network of cellu- pulping, bleaching, and papermaking or finishing [7]. The
lose fibers bonded together by intermolecular forces (van der pulp and paper industry mainly utilize wood as the feedstock
Waals; hydrogen bonding) [1]. In the last four decades, the for making pulp. In more than a century, woody biomass
global demand for pulp and paper has increased and likely led accounts for 90–95% of the pulp produced globally [4]. The
to an improvement in the level of civilization and economic types of wood species that are being used for making pulp
development (Fig. 1) [3]. The drastic growth of the use of include softwoods (coniferous) such as pine, spruce, pine,
computers for information and communications was predicted and fir, and hardwoods (deciduous) such as eucalyptus, ma-
to cause a decline in paper use worldwide [4], however, the ple, aspen, and ash. With the global increase in demands and
use of such technologies has increased paper usage especially environmental sustainability issues surrounding forests, non-
for the use of printing, record management, publicity, com- wood raw materials like rice straw and waste paper are also
munication, and writing paper [5]. Additionally, due to the currently being used for making pulp [7, 8]. Pulping involves
global increase in e-commerce with online stores like the use of chemical, mechanical methods, and/or biological
methods to rupture bonds of woody materials and separate the
cellulose fibers from lignin [9]. Mechanical pulping encom-
* Drake Mboowa passes refining, grinding, mechanical shearing, and disintegra-
dmbooowa@mail.ubc.ca tion of wood into individual fibers [10]. The chemical pulping
uses chemical processes and heat energy to soften and dis-
1
Faculty of Forestry, Department of Wood Science, University of solve the lignin thereby enabling fiber separation [10]. The
British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4,
Canada
biological pulping techniques are based on the idea that fiber
 Biomass Conv. Bioref.

Fig. 1 Recent world demand for

paper and paperboard (adapted
from Kuusisto [2])

raw materials are organic and hence degrade in natural envi- the Chinese over the next seven hundred years, during which
ronments. Biological pulping involves the treatment of wood time it was kept a closely guarded secret [16]. Throughout the
chips with biological nutrients (mostly white-rot fungi) to soft- six centuries, the art of papermaking became famous and grew
en and remove the lignin [11, 12]. in all places, from the east (Japan) to the south (Thailand), and
To date, the pulp and papermaking processes have not the west (Arabs). In 751 AD, Arabs came in contact with
changed much, however, the pulp and paper industries are papermaking when they seized a Chinese city containing a
making modifications to traditional pulping methods for mak- paper mill. Papermaking then stretched to Samarkand,
ing novel and high-value products like nanocellulose and Baghdad (793 AD), Damascus, Cairo (900 AD), and Fez
chemicals. Improvements in the pulping process have evolved (1100 AD), Spain (1151 AD), France (1348 AD), and
and have been manifested through lignin genetic modifica- Germany (1390 AD) [17]. By the end of the fourteenth cen-
tions, diversification of products made from pulp, develop- tury, a great number of paper mills had been established in
ment of extended impregnation and pre-hydrolysis techniques Spain, Italy, Germany, and France [17].
to improve the Kraft pulping process, and the modification of In 1719, the idea of using wood as a raw material was
chemical pulps for production of nanocellulose with improved reported by Rem! de Reaumur, a French physicist, who
fiber characteristics of fiber size, crystallinity, chemical com- observed the American wasp erecting its paper-like nest
position, and fiber surface functionality [5, 13–15]. from wood fibers [18]. However, it was not until the early
This work aims to provide an overview of the various tra- 1800s that the first paper was made from wood by Matthias
ditional pulping methods and the recent modifications of the Koops, who reduced wood to a pulp by a maceration pro-
pulping processes. This review is limited to giving an over- cess. In 1843, Friedrich Keller discovered the technique of
view of the traditional pulping techniques and the new cutting- making groundwood pulp when he watched children grind-
edge improvements in the traditional pulping processes that ing cherry pits against a wet stone with a piece of wood and
have cropped up in the last decade. observed that de-fibering of the cherry pits was influenced
by water and pressure [18]. In 1851, chemical pulping tech-
nologies came to light, when Watt and Burgess found out
2 History of pulping that it was possible to remove lignin which binds wood
fibers together from wood at high temperatures using sodi-
The Egyptians are credited for producing the first-ever writing um hydroxide [19, 20]. Tilghman developed the sulfite pro-
material around 3000 BC, where they pasted thin layers of cess in 1866, while the Kraft process was invented by Dabl
fibrous stems together. However, the sheets were not consid- in 1879 [21]. In 1869, Behrend came up with a concept of
ered as paper because complete de-fibering (a vital process in steaming wood before grinding; thus, the foundation for the
papermaking) was absent [3]. There was no substantial sub- development of modem mechanical and thermo-mechanical
stitute for any writing material until 105 AD when Tsái Lun pulping processes was laid. Currently, several modifica-
practiced the production of the first authentic pulped paper in tions of these three processes have resulted in the adaptation
China from hemp (Cannabis sativa) and China grass (Ramie, of paper from pulp for writing, wrapping, packaging, cloth-
Boehmeria nivea). With the increasing demand for paper in ing, and a range of other disposable products using a wide
China, papermaking was developed into a highly skilled art by variety of feedstocks.
Biomass Conv. Bioref.

3 Pulp feedstocks IVII [30]. Native cellulose occurs as cellulose I, consisting of

two crystalline forms Iα and Iβ which are differentiated based
The pulp consists of woody materials that have been defibered on the hydrogen bonding patterns between them [24].
by biological, physical, and/or chemical means [22]. The pulp Hemicellulose is the second most abundant amorphous
and paper industries use mostly wood as a feedstock due to its heterogeneous-polysaccharide in the cell wall consisting of
improved specific properties for making paper [7, 23]. linear and branched side chains of two or more monosaccha-
However, there are diverse non-woody sources which can be rides (arabinose, glucose, mannose, galactose, and xylose) and
used as cellulose sources such as sisal, rice straws, cotton chemical functional groups (acetyl, glucuronic acid) with a
linen, sugarcane bagasse, pineapple, and straw [3, 7, 24]. In lower DP of 70–200 [31]. The type of hemicellulose found
terms of the physical structure and chemical composition, in a given substrate is dependent upon fiber source. For in-
wood and agricultural residues are quite heterogeneous and stance, xylans have 1,4-β-D-xylopyranosyl units which are
differ based on environment, age, geographic location, spe- the most abundant in hardwoods and agricultural biomass
cies, etc. [25]. Wood is classified as softwood or hardwood and makeup to 20–37% of hardwood biomass [32]. In con-
with the former having longer fibers that are the most suitable trast, mannan or galactoglucomannan is mostly prevalent in
feedstock for pulping [26]. softwoods and accounts for 16–27% of softwoods [32].
Lignin is an aromatic, rigid, cross-branched, three-dimen-
sional, poly phenylpropane complex structure of high molec-
ular weight and having a DP that ranges from 50 to 500 [33].
4 Composition of pulp feedstock
Lignin forms a matrix with hemicellulose around the cellulose
making lignocellulose rigid and very recalcitrant to pulping
The main composition of woody and non-woody pulp feed-
[27]. De-polymerization of lignin forms three different
stocks are carbohydrates (cellulose and hemicellulose) and
phenylpropane units that are guaiacyl (coniferyl), syringyl
lignin which are disproportionately distributed in the cell wall
(sinapyl), and p-hydroxyphenyl (p-coumaryl alcohol) [15].
layers. The composition of pulp feedstocks plays a key role in
Softwood lignin is composed of guaiacyl monomers; hard-
the choice of pulping method to be used and subsequently the
wood lignin has both syringyl and guaiacyl monomers while
paper yield and quality. Both wood and non-woody pulp feed-
agricultural biomass lignin is mainly comprised of all three
stocks differ in chemical composition (cellulose, lignin, and
types [34]. Lignin plays a key role in determining the quality
hemicellulose) (Table 1).
of the paper produced. It is still challenging to remove lignin
Cellulose is the most abundant polysaccharide of plant cell
from lignocellulosic biomass and its harsh removal using
wall consisting of linear chained β-1,4 linked D-glucose mol-
chemical means impacts the final pulp yield [35]. Successful
ecules with a degree of polymerization (DP) of up to 10,000
removal of lignin is an expensive process and pulp and paper
[27, 28]. Within cell wall microfibrils, cellulose is found in
industries tend to offset the cost by using the lignin waste
less ordered (amorphous) and well-ordered (crystalline) form
material as a source of energy in the form of biogas, biofuel,
[29]. Based on the arrangement and hydrogen bonds within
or electricity to run pulp mills [7].
and between the cellulose chains, different crystalline
allomorphs occur, namely cellulose I, II, IIII, IIIII, IVI, and

Table 1 Chemical composition of the main constituents of pulp 5 Pulping methods for paper production
feedstocks (adapted from Koch [23])

Fiber source Cellulose Hemicelluloses Lignin

Paper production is a three-stage process in which fibrous
feedstocks are converted into pulp, bleached, and then the
Hardwoods 43–47 25–35 16–24 pulp is converted into paper. Pulping is done using bio-
Softwoods 40–44 25–29 25–31 logical enzymes, mechanical energy, and/or chemicals.
Sugarcane bagasse 40 30 30 The resultant pulp is then bleached and washed to make
Corn cobs 45 35 20 paper [36]. The primary goal of pulping is to remove
Corn stalks 35 25 40 lignin as much as possible while keeping the wood fiber
Cotton 95 2 3 structure intact [9]. The choice of the pulping method is
flax 63 12 16 influenced by the species of the fibrous raw material to be
Hemp 70 22 8 pulped. Pulping methods are classified into four catego-
Jute 71 14 15 ries, namely mechanical, chemical, semi-chemical, and
Sisal 73 14 13 biological. The pulp produced from each of these methods
Wheat straws 30 50 20 will be suitable for different applications as the fiber prop-
erties will be modified differently [37].
 Biomass Conv. Bioref.

5.1 Mechanical pulping 5.1.3 Thermo-mechanical pulp process

Mechanical pulping involves the application of mechanical The thermo-mechanical pulp (TMP) process involves impregna-
energy that converts wood chips to pulp [10]. The mechanical tion and preheating of wood chips with steam under pressure
pulping process comprises both grinding and refiner methods (20–40 psi) and temperature (115–155 °C) followed by refining
which mechanically break the bonds of wood chips resulting [10, 40]. The TMP process is two-staged, with the first stage
in fiber separation. The process retains both lignin and cellu- involving refiners being at an elevated temperature and pressure
lose fibers producing about 90–98% paper yield per unit vol- and the second stage encompassing refiners being at ambient
ume of wood with weaker strength properties, softness, high temperature and atmospheric pressure [11]. The high temperature
bulk, and brightness [36]. For improvement in mechanical of steam softens lignin and enhances fiber separation with min-
strength, mechanical pulps are produced from softwoods due imal cutting and fine fiber generation [11]. Refining is carried out
to their long fiber length which makes more contact with one under the glass transition temperature of lignin (140 °C) where
another [36]. Mechanical pulping is divided into four types, fiber separation occurs in the middle lamellae layer of the cell
namely stone groundwood pulping, refiner mechanical wall. TMP is stronger than RMP and SGWP although suffers
pulping, thermo-mechanical pulping, and chemi-thermo- some loss in yield (yield produces > 93%). Due to its strength,
mechanical pulping. TMP is used for packaging materials, catalog, and magazine, and
used as a furnish in printing papers, paperboards, computer cards,
5.1.1 Stone groundwood pulp process and tissue paper [10].

The stone groundwood pulp (SGWP) process is the oldest 5.1.4 Chemi-thermo-mechanical pulping process
mechanical pulping method in which wood is ground into a
pulp. SGWP involves pressing a debarked block of wood Chemi-thermo-mechanical pulping (CTMP) involves the im-
lengthwise against a rotating grindstone revolving at speeds pregnation of wood chips with a combination of steam and
of 1000 to 1200 rev/min [10, 36]. The abrasion force of the chemicals to soften the lignin before mechanical refining [41].
grindstone tears wood into thin fibers and fiber fragments, The lignin-softening chemicals such as carbonates, hydrox-
which are washed away continuously from the grindstone by ides, sulfite, and peroxides and further pretreatment at higher
a water stream. The friction that develops in the grinding zone temperatures break the wood structure apart and remove lignin
raises the temperature which softens the wood hence easing [42]. The CTMP uses less mechanical energy and chemicals
the fiber separation process [10]. The slurry of the fibers and to soften and fibrillate wood chips [41]. The CTMP process is
fiber fragments is screened to separate them from shavings advantageous compared to TMP as it improves mechanical
and other oversized particles and thickened by removing the strength properties while maintaining high yields of 85–95%
water. The SGWP process produces high pulp yields (about [17]. Due to higher strength, pulp produced by the CTMP
95%) because it retains most of its lignin [10]. Generally, process is used in making high-grade printing papers.
paper produced from the SGWP process has low strength,
discolors easily as it ages or when exposed to light [38]. 5.2 Chemical pulping
This ground pulp is utilized in making low-grade products
such as newsprints, magazines, printing paper, and low-cost- Chemical pulping is the widely used pulping method for pro-
grade packaging paper. ducing the world’s high-quality paper [11]. The main objec-
tive of chemical pulping is to treat wood chips with chemicals
5.1.2 Refiner mechanical pulp process in an aqueous solution at an elevated temperature and pressure
to remove hemicellulose and lignin while leaving cellulose
The refiner mechanical pulp (RMP) process encompasses fibers intact [19]. Removal of lignin and hemicellulose render
shredding and grinding of wood chips between rotating metal the fibers flexible and increases contact between cellulose
discs or plates using a refiner at atmospheric pressure [39]. fibers resulting in the production of higher strength paper
Production of RMP is a two-stage process with the first stage [43]. The paper strength and durability gained are at an ex-
involving defibration of wood into individual fibers and fiber pense of pulp yield with the chemical pulping process produc-
fragments and the second stage involving loosening the fiber ing pulp yields which are nearly half those of mechanical
structure to increase fiber flexibility and fibrillation. The RMP pulping methods (40–55% yield) [43, 44]. There are two ma-
process gives rise to a long-fibered pulp which is stronger, jor chemical pulping processes currently in use, that is, Kraft
freer, bulky, and darker, compared to the conventional (sulfate) pulping and sulfite pulping. Incorporation of a pre-
SGWP [38]. The RMP process has undergone modifications hydrolysis step to hydrolyze the hemicellulose before Kraft or
and improvements to include thermo and chemical pulping sulfite processes gives rise to another type of chemical pulp
pretreatments. called dissolving pulp.
Biomass Conv. Bioref.

5.2.1 Kraft pulping process advantages of sulfite pulp over Kraft pulp are brightness, eas-
ily bleached pulp, easily refined pulp, higher yield, and pro-
The Kraft pulping process is a full chemical pulping method, duction of pulps that forms less porous sheets that hold water
which uses sodium hydroxide (NaOH) and sodium sulfide [53]. The disadvantages include low pulp strength, chemical
(Na2S) at a pH above 12, temperature of 155–180 °C, and recovery is nearly impossible, long cooking cycles, and only
steam pressure of 800 kPa as the main cooking conditions to softwood species can be pulped. Sulfite pulping processes are
breakdown wood chips into pulp [17]. The Kraft process is the distinguished into three categories according to their pH, that
most used pulping method globally [45, 46], because of its is, acid sulfite, bisulfite, and alkaline sulfite.
advantages of producing paper of high strength, ability to The acid sulfite process uses calcium as a base. The acid
recover energy and chemicals during pulping, and the ability sulfite cooking is carried out using a strongly acidic cooking
to utilize both hardwoods and softwoods as raw materials liquor (pH 1.5–2.0) to avoid liming up conditions in the
[47]. Kraft pulp is mainly being used for making high strength cooking digester. The pulp temperature is kept within a range
boards and packaging materials. Chemical recovery is a vital of 125–145 °C and the cooking time is up to 7 h [17]. Due to
step during pulping as it provides an avenue for chemical the acid digestion conditions, the resulting pulp is weaker and
recycling making the process cost-effective [47]. The used has low hemicellulose content, as a result, acid sulfite pulp is
chemicals (black liquor) containing lignin, hemicellulose, used for making dissolving pulp and tissue paper. The acid
and extractives (oils, resins, and terpenes) are channeled to a bisulfite is a full chemical pulping process that encompasses
chemical recovery plant where the dissolved lignin and hemi- the use of soluble bases such as sodium, magnesium, and
cellulose are burned to produce electrical energy and steam ammonium and a higher liquor pH of 3–5 [17]. The bisulfite
needed during pulping. Additionally, Kraft mills use their pulping cooking temperature of between 160 and 180 °C is
waste for making supplementary energy via anaerobic diges- higher than that of the acid sulfite process with a short cooking
tion [48–50]. time (0.25–3 h) [17]. This type of pulp produced is relatively
During Kraft cooking, the wood fibers get swollen and strong and is used in making newsprint and medium grade
lignin gets split into hydroxyl (OH−) and hydrosulfide (SH−) writing papers. The alkaline sulfite is a full chemical pulping
ions, which are dissolved in the liquor as phenolate and car- process that utilizes a liquor consisting of approximately equal
boxylate ions. To some extent, the cellulose and hemicellu- amounts of sodium hydroxide (NaOH) and sodium sulfite
loses are also degraded in the process. Kraft pulp mills have (Na2SO3) at temperatures of 160–180 °C and cooking time
two lines, that is, the fiber line and the chemical recovery line of 3–5 h [54]. The alkaline sulfite process produces pulps of
[10]. On the fiber line, half of the wood chips are turned into similar qualities as those of Kraft pulps [17].
pulp while the other half of the wood chips material is used in
electricity and heat (steam) generation in the chemical recov- 5.2.3 Dissolving pulp
ery line. The recovered chemicals suffer a small loss in vol-
ume and this poses an advantage to Kraft pulp over sulfite Dissolving pulp production involves prehydrolyzing wood
pulp. The recovered chemicals are replenished with sodium chips prior to the regular Kraft and sulfite pulping process
salts before they are returning to the cooking digester for re- [9]. The pre-hydrolysis step is vital in the dissolving pulp
use. Kraft pulping process utilizes both softwood and hard- production process as it helps to remove hemicelluloses,
woods as raw materials. which are not desired in the final pulp [55]. The most com-
monly used pre-hydrolysis method is applying direct steam to
5.2.2 Sulfite pulping process the wood chips. The direct steam functions to liberate the
organic acids from wood, which hydrolyzes hemicellulose to
The sulfite pulping process is a full chemical pulping process soluble sugars at elevated temperatures [56]. In Kraft pulping,
that involves cooking wood chips at high temperature and the pre-hydrolysis step removes the hemicellulose while in
pressure in a digester with an aqueous solution of sulfur diox- sulfite pulping, the acidic pulping conditions remove the
ide and calcium, sodium, magnesium, or ammonium bisulfite hemicellulose. Organosolv pulping is also used in the produc-
[17]. The sulfite process breaks down the wood by dissolving tion of dissolving pulp because it can be applied to different
lignin and cleaving lignin bonds through the formation of wood species and plants [55, 57, 58]. Most pulp producers
sulfonate functionalities [51]. Unlike Kraft pulping, sulfite worldwide use softwoods as raw materials for making dissolv-
pulping is suitable for wood species with low extractives (tan- ing pulp; nonetheless, hardwoods can also be used [10].
nins, polyphenols, pigments, resins, fats, etc.), as extractives
interfere with the pulping process [52]. The sulfite pulping 5.3 Semi-chemical pulping
process tends to produce pulps of different types and qualities
with a wide range of applications since the sulfite cooking Semi-chemical pulping is a two-stage pulping process involv-
liquor can be used within the entire pH range [10]. The main ing the use of both chemical and mechanical pulping
 Biomass Conv. Bioref.

processes. The first stage encompasses partial softening of high-yield Kraft or sulfate pulping is a slight modification of
wood chips with chemicals to weaken inter-fiber bonding the traditional Kraft process where either the chemical load
through the removal of a portion of hemicellulose and lignin, used during Kraft cooking is reduced by about one-half or the
and the second stage involves mechanical refining in which cooking time and temperature is decreased [17]. In compari-
chemically treated biomass is refined into individual fibers son with Kraft pulping, the high-yield Kraft pulping process
and fiber bundles [59]. The pulp quality produced using produces much higher pulp yields (55–80%) [40]. Normally,
semi-chemical methods has a wide range of end uses and is high-yield Kraft pulps are weaker in terms of strength and
dependent on the yield, with a 75% yield showing excellent have low brightness compared with Kraft pulps, and as a
stiffness making it suitable for the manufacture of corrugated result are utilized in making newsprint.
containerboard [60, 61]. The most significant semi-chemical
process is the neutral sulfite semi-chemical process. However, 5.3.3 Cold soda pulping process
other semi-chemical pulping methods like high-yield Kraft
and sulfite and modified mechanical semi-chemical pulping The cold soda pulping process is another semi-chemical meth-
processes like cold soda are practiced. od that involves the treatment of wood chips with a sodium
hydroxide solution at temperatures between 20 and 30 °C be-
5.3.1 Neutral sulfite semi-chemical process fore disk refiner defibration [66]. The cold soda process is
commonly combined with the Kraft process to make sodium
The neutral sulfite semi-chemical process (NSSC) is one in hydroxide recovery achievable. The most vital step in the cold
which wood chips undergo partial chemical pulping followed soda process is the fast impregnation of the wood chips with
by disk refining to complete fiber separation [62, 63]. NSSC the cooking liquor to cause fiber swelling and avoid polyose
pulp production is a three-staged process involving impregna- loss. The impregnation time is usually 15–120 min which can
tion of wood chips with sodium sulfite cooking liquor, follow- be made shorter in a pressurized system [40]. Cold soda pulp
ed by cooking the chips at 160–190 °C and then defibration by yields are relatively high (85–92%) since lignin degradation
disk refining [64]. The sodium sulfite is usually buffered with and polyose dissolution are lower [40]. One of the major set-
sodium carbonate, sodium hydroxide, sodium bicarbonate, or backs of the cold soda pulps is low brightness, which can be
sodium bisulfite to make the cooking liquor [63]. The sulfite increased by bleaching using peroxide-hypochlorite. In terms
removes lignin from wood chips while the carbonate, bisulfite, of strength, cold soda pulps are slightly stronger and are often
bicarbonate, and hydroxide control the pH by neutralizing utilized as unbleached coarse grades for corrugating medium
acids produced by wood [63]. Defibration is carried out by production, and as bleached grades for printing papers and
either one, two, or multistage refining processes at atmospher- newsprint.
ic pressure. Due to the residual lignin (15–20%), NSSC pulp is
utilized in making unbleached products where good strength 5.4 Biological pulping
and stiffness are required as in corrugated boards, grease-
proof papers, and bond papers [10]. Anthraquinone additives The chemical pulping processes use hazardous chemicals that
are usually used to increase the rate of delignification. cause severe harm to aquatic life, disturbs the food chain, and
various health implications [67], and as a result, biological
5.3.2 High-yield chemical pulping pulping has emerged lately as an eco-friendly pulping method
[68]. Biological pulping entails the pretreatment of wood
High-yield chemical pulping processes are used to increase chips with white-rot fungi or lignin degradation enzymes be-
the pulp yields of full chemical pulping. The yields range from fore pulping [11]. The biological pulping concept is premised
55 to 70% or higher [65]. This method is a semi-chemical on the ability of white-rot fungi to degrade lignin in wood
pulping process because of the incorporation of disk refining while leaving the cellulose intact [12]. White-rot fungi have
after chemical treatment. High-yield chemical pulps are pro- been used extensively in biological pulping due to their ability
duced by incorporating a disk refiner step in the traditional to hydrolytically and oxidatively degrade wood components
sulfite and Kraft processes. The high-yield acidic sulfite pulp [69, 70]. Biological pulping is energy-saving and improves
is produced when wood chips are cooked at relatively low paper strength compared to mechanical and chemical pulping
temperatures (120–130 °C) and with low acidic liquors [40]. method effects [70, 71]. The economic assessment of the bi-
The resultant products are then defibered using a low energy ological pulping of wood chips is a viable option for the pulp
disk refiner. The high-yield acidic sulfite pulping process uses and paper industrial future. When the biological treatment
calcium, magnesium, or sodium as bases. The pulp yields process is done, the pretreated wood chips are pulped either
from this process are relatively high (60–70%) and have lower by chemical or mechanical means giving rise to biochemical
strength as compared to the traditional sulfite process [40]. or biomechanical pulp, respectively. Biological pretreatment
High-yield acidic sulfite is used for making newsprints. The of wood chips before mechanical and chemical pulping results
Biomass Conv. Bioref.

in significant energy savings and low chemical requirements aimed at replacing fossil-derived products and to lower costs
respectively during the pulping process [70]. incurred during pulping [78–80]. Research and development
in the pulp and paper industry have largely emphasized im-
proving the traditional pulping processes rather than seeking
6 The pulp bleaching process new pulping technologies to make high-value products and
also improve the rate of lignin removal from feedstocks.
The pulps manufactured from the aforementioned pulping Pulp and paper research has been driven by the need to reduce
processes tend to be brownish dark-colored and therefore energy and chemical cost requirements for pulping. Special
are required to be bleached before using them to make paper. attention has been paid to diversification of the use of a broad
Unbleached pulps are often used for making corrugated feedstock base, maximization of pulp yields through improv-
boards and packaging paper that do not require bright- ing on traditional pulping methods, and lignin genetic
colored pulps [22, 61]. Bleaching involves the treatment of engineering.
pulps with chemicals to increase their brightness. Bleaching of
chemical pulps and mechanical pulps involves different strat-
egies [1]. Hydrogen peroxide, hypochlorite, and sodium 7.1 Diversification in pulp feedstock
hydrosulfite are often used to decolourize the lignin in a pro-
cess referred to as brightening [17]. Since mechanical pulps Recent trends in the diversification of pulp feedstock have
retain all the lignin after pulping, their bleaching involves been focused on the suitability of non-woody materials for
chemically altering the amounts of lignin present in the pulp. pulp production. The pulp and paper industry is reversing
It is challenging to fully brightening mechanical pulps since the pulping trend from wood as a sole raw material for paper
the residual lignin tends to yellow the pulp with time [10]. production to non-wood materials [3]. Concerns of global
Bleaching of chemical pulps involves the removal of any re- climate change and enactment of policies barring foresters
sidual lignin that remains after chemical pulping leading to an from cutting down trees as a way of conserving forests have
increase in cellulose fiber-fiber bonding [1]. The harsh caused a major reduction in wood supply for papermaking [1,
chemicals used for bleaching chemical pulps tend to reduce 81]. Non-woody biomass has been sought as a potential solu-
the degree of polymerization of cellulose resulting in weaker tion to the problem since it is readily available and regenerates
fibers [17, 36]. after a short period [81]. Most of the non-wood feedstocks
Recently, the use of enzymes in pulp bleaching has have similar physiochemical characteristics to those of woody
attracted the attention of the pulp and paper industry and has biomass which makes them efficient and useful for pulp pro-
achieved good results as it reduces environmental pollution duction [82]. Grasses such as wheat straw, rice straw, reed,
associated with the use of chemicals as well as reduces the and sugarcane bagasse account for most non-wood feedstocks
chemical consumption required for effective bleaching [72, used during pulping, because of their stronger breaking length
73]. The use of xylanase enzymes as a pre-bleaching agent of paper hand-sheets as compared to other non-wood raw
has been shown to improve the efficacy of bleaching materials [1]. In the last decade, researchers have yielded pos-
chemicals in removing lignin from pulps [74]. It has been itive results in resorting to non-woody biomass for pulp and
suggested that xylanase enzymes hydrolyze the hemicellulose paper reduction [4, 5, 83].
(xylan) portion in the pulp allowing lignin that is associated Bamboo is also widely used for pulp and paper production,
with the short oligosaccharides hemicellulose chains to be specifically in Asian countries [84]. Bamboo is a long-fibered
easily removed by a subsequent chemical bleaching step [1]. fibrous material whose fiber length is comparable to that of
Full biological pulping have also been reported where softwood fibers (1.5–4.4 mm) [1]. Pulps made from bamboo
hemicellulase enzymes work synergistically with lignin- are often used to make corrugated boards and packaging ma-
degrading enzymes such as the laccases and lignin peroxi- terials due to their high strength as compared to other non-
dases enzymes to remove the hemicellulose and oxidize lignin wood pulps. More recently, another feedstock that is being
leaving a cellulose-rich pulp [75–77]. Bleaching of pulp with used widely for pulp production is Eucalyptus. There has been
enzymes has been found to improve pulp properties of bright- an expansion in pulp and paper production in Asia and South
ness and tear index [77]. America where Eucalyptus species have been grown in the
last 20 years [85]. Eucalyptus has become an important feed-
stock in the pulp and paper industry and has posed as a com-
7 Recent trends in pulp and paper petitor to North America’s softwoods because it is economical
manufacture available whenever needed, and presents a high consistency in
quality. In terms of growth, eucalyptus “grows like a weed”
The current and future opportunities for novel pulping pro- (harvested after 7 years) compared to North American pines
cesses are driven by the desire for new bio-based products which take at least two decades to grow.
 Biomass Conv. Bioref.

7.2 Recent improvement of existing pulping methods crystalline fibrils remaining of 2–10 nm in width and several
nanometers in length [13]. In contrast, CNF is produced by
As mentioned earlier, there is a progressive effort of improv- performing a mechanical pretreatment of bleached chemical
ing the traditional pulping methods to maximize pulp yields pulps with or without enzymatic treatment, leading to the pro-
and quality from biomass. Most of the modifications have duction of fibrillated cellulose with both amorphous and crys-
been focused on improving the Kraft pulping process since talline regions of long and flexible elementary fibrils [13].
it is the most used pulping process worldwide. The modifications have led to an improvement in
nanocellulose end fiber characteristics such as fiber size, crys-
7.2.1 Extended impregnation time during Kraft cooking tallinity, chemical composition, and fiber surface functionality
[24]. Nanocellulose is currently used in several novel applica-
The extended impregnation Kraft cooking method is aimed at tions such as nanocomposites, emulsion stabilizers, surgery
reducing reject (shives) to increase the overall Kraft pulp tissues, pharmaceutical plasters, and bandages, due to high
yields. The role of extended impregnation is to ensure even strength and aspect ratio and low density [89–92]. Currently,
penetration, diffusion, and distribution of the cooking liquor most Kraft and sulfite pulp grades are fully bleached pulps
into the wood chips to achieve a homogeneous delignification containing trace amounts of lignin [24]. By removing the
during Kraft pulping [65]. Inalbon et al. [86] reported that it bleaching step from chemical pulps production, it is apparent
takes 15–20 min for the alkali to reach the wood chip core of that the nanocellulose fibers will contain lignin and hemicel-
4.4 mm thick, another 45–60 min to achieve full deacetylation lulose leading to the production of what is commonly known
at a hydroxide ion concentration of 0.5 M and temperature of as lignin-containing cellulose nanofibrils (LCNF). Due to en-
110 °C. vironmental concerns and the desire for higher nanocellulose
yields, LCNF production has caught the interest of re-
7.2.2 Pre-hydrolysis before Kraft cooking searchers, since the residual lignin removal process by
bleaching is no longer necessary. LCNF have been reported
There is a growing trend in pulping that has shifted focus on to exhibit high hydrophobicity and lower oxygen permeability
incorporating a pre-hydrolysis step before Kraft cooking to [14, 42, 88].
produce a pure cellulose pulp for use in making high-value
biomaterials and chemicals. The current population growth 7.3 Lignin genetic engineering
has led to a reduction in raw materials like cotton used in
making textile fibers; hence, the pulp and paper industry is Lignin plays a fundamental role in the utilization of lignocel-
resorting to making cotton-like pulp material for use in the lulosic biomass for pulp and paper production. The presence
textile industry. Earlier researchers have revealed that the cur- of lignin in wood makes it recalcitrant to pulping, requiring
rent cotton growth rate is not enough to meet consumer de- the need for harsh chemical delignification processes for the
mand [87]. This is attributed to a paradigm shift of using production of high-quality paper [15, 93]. Chemical pretreat-
farming land for food production to alleviate hunger. This ments require a lot of chemicals to degrade lignin and tend to
has led to seeking alternative replacement of cotton and the solubilize some portion of the celluloses as well as releasing
recent production of dissolved pulp has increased [87]. pollutants to the environment [67, 94]. To overcome these
problems, recent research efforts have been dedicated to engi-
7.2.3 Improvement of Kraft and sulfite pulps for making neering lignin amount and composition to reduce biomass
nanocellulose recalcitrance towards pulping and chemical processing
through lignin biosynthesis [93]. Therefore, by reducing and
As previously mentioned, the decline in paper consumption altering lignin content in pulp feedstocks, the quality and ef-
and the global increase in preference for digital information fectiveness of pulping may be enhanced [15]. The realization
have made the pulp and paper industry seek new alternative of lignin modification in plants requires a clear understanding
products from woody biomass with improved properties for of lignin biosynthesis at the biochemical and molecular levels
making value-added products [3]. The emergence of bio- [95]. Lignin biosynthesis, particularly monolignol biosynthe-
based nanotechnology has led to modifications in chemical sis, has revealed that modification of lignin type, content, and
(Kraft and sulfite) pulps for making nanocellulose. Chemical composition can be achieved and presents an economic and
pulping processes have been modified to incorporate mechan- environmental benefit to the pulp and paper industry [15, 95].
ical, and biological pretreatments, or a combination leading to In the last decade, two approaches have been undertaken to
the production of either cellulose nanocrystals (CNC) or cel- manipulate the genetics of lignin in plants. The first approach
lulose nanofibrils (CNF) [42, 88]. The main process used for involves breeding transgenic plants with reduced lignin con-
obtaining CNC from pulps is based on acid hydrolysis where tent and the second approach encompasses alteration of lignin
amorphous regions of cellulose are degraded resulting in only composition (guaiacyl and syringyl lignin units) [15]. In
Biomass Conv. Bioref.

softwoods, a methoxylation process at the C5 position manip- is desired to alleviate the aforementioned issues. There is
ulates lignin by reducing the number of free C5 positions that scanty information on the potential of GLR as an alkaline
may contribute to the creation of resistant C5–C5 lignin bonds sulfonation pulping pretreatment to enhance bioconversion
[15]. On the other hand, hardwood lignin can be manipulated of lignocellulosic biomasses to fermentable sugars.
by increasing the syringyl lignin content, partial substitution Reduction in lignin content through lignin genetic engi-
of methoxy groups with hydroxyl groups, integration of alde- neering is the most effective way of reducing costs associated
hydes groups into lignin, and incorporation of ester groups with the pulping of woody feedstocks. However, it is likely
linkages into lignin by partial substitution of alcohol group that lignin modification reduces biomass productivity and
with coniferyl to improve on alkali extractability [15, 96]. may result in a limited supply of biomass [100, 101].
Lignin genetic engineering has been reported as being effec- Although an increase in high-pulp-quality production could
tive at reducing energy and chemical requirements during bio- compensate for the decreases in biomass productivity, the
mass pretreatments for paper production [96]. high productivity of pulping feedstocks is a basic requirement
for the pulp and paper industry. Future work is required to
draw a correlation between lignin amount reduction and pulp
8 Future prospects and conclusions feedstock productivity. Additionally, the development of new
plant varieties with optimized biomass yield and lignin con-
The demand for pulp and paper products has continued to tent will significantly benefit the pulp and paper industry.
grow globally [3]. The demand has been driven by population
growth and consumer desires to use different paper products Author contributions Drake Mboowa designed and wrote this review
article.
such as newsprints, packaging materials, clothes, and sanitary
materials. The pulping industry has for decades not developed
Funding Canadian Queen Elizabeth II Diamond Jubilee Scholarship.
new pulping techniques but rather improved the traditional
chemical and mechanical pulping processes to enhance pulp
Compliance with ethical standards
yields and quality, as well as saving energy and chemical
loading required for pulping. In this work, various pulping Conflict of interest The author declares that there are no conflicts of
processes have been analyzed as being effective at breaking interest.
down wood and non-wood biomass to pulp. Chemical pulping
methods have been shown to produce high-quality paper
products as compared to mechanical pulping methods due to
increasing fiber-fiber bonding which results when lignin is References
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