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Modern kraft pulp mills generate solid waste of 1–2% of incoming debarked wood. Given the size of these plants, with an annual production capacity of at least 1000,000 tons, each plant generates 20,000–30,000 dry tons of waste per year. The largest current use of these residues is for combustion in biomass boilers for steam and power generation. However, the conversion of biomass into biofuels and chemicals is gaining interest due to increasing demands for energy, limited sources of fossil fuels, and growing concerns about the environmental impact of greenhouse gas emissions. This chapter shows the laboratory-scale results of the use of eucalyptus wood wastes to obtain cellulose pulp by alkali pulping reinforced with hydrogen peroxide to obtain alkaline peroxide mechanical pulp or cellulosic bioethanol. Based on the results, an industrial-scale techno-economic analysis of the processes is presented and compared with current alternatives for energy generation.
*Address all correspondence to: lclavijo@fing.edu.uy
1. Introduction
Although waste prevention is normally the first option to avoid later problems in waste management, forest industrialization generates a large amount of wood residue. In the mechanical transformation of wood (e.g., sawn wood, plywood), 30–50% of the logs are not part of the final product (e.g., bark, sawdust, under-sized logs, knots, chips, etc.). Depending on the size and characteristics of this waste and the integration of the wood industrial sector, it will normally be used to produce pulp, boards, or other value-added products, and it can be used as raw material for steam and power generation [1, 2, 3, 4].
In modern pulp mills that apply the best available technologies, 1–2% of the input wood is discarded as pin chips and fines, which are formed during the chipping process and cannot be used to produce commercial cellulose pulp due to their small size [5]. For older mills, the discarded fraction could reach 5–10% [6]. These residues, as well as waste from mechanical transformation industries, are normally burned for steam and power generation.
Direct combustion of biomass in steam boilers is a mature, proven technology for converting biomass to energy. The technology choice largely depends on the specific size of the boiler. Newer solid-fuel-fired boilers with a capacity over 15 MW are fluidized bed boilers, moving grate boilers, or pulverized fired boilers [7, 8]. Small-scale cogeneration plants (with a capacity lower than 15–20 MW) are globally widespread, and their size is normally determined by the local availability of biomass. However, the high capital cost of electricity generation and the small price gap between biomass logistics costs and electricity prices often make them less competitive [7, 9].
The main objective of this chapter is to analyze an alternative to industrial forest residues, in the understanding that their disposal through the generation of electrical energy is a process that will no longer be profitable in the future since it has higher costs than electricity production from solar or wind energy.
Alternatively, lignocellulosic biomass constitutes a material with many possibilities for the development of bioproducts and/or biofuels. It mainly comprises cellulose, hemicelluloses, lignin, and a minor fraction of extractives and inorganic compounds. Pretreatments are required to separate components to use lignocellulosic materials. Within the different chemical pretreatments, alkali is used because it has several desirable characteristics. Alkaline methods mainly use noncorrosive chemical products, and in many cases, reagent recovery is feasible due to the consequent economic and environmental advantages.
One strategy for sustainable development is to establish an economy based on the use of biomass (a “bioeconomy”)—a renewable resource that should be obtained through a production system that preserves the environment. This requires an increase in products’ availability and their corresponding production technologies to simultaneously substitute fuels and materials derived from fossil sources (“economy decarbonization”). This concept can be developed through a biorefinery approach that comprises sustainable biomass processing into a portfolio of marketable products through the combination and integration of different processes. A biorefinery using lignocellulosic materials of forest origin to obtain biofuels and other products contributes to better meeting sustainable development goals related to energy, industry, innovation, infrastructure, and climate. Bioethanol production from lignocellulosic materials as a substitute for gasoline allows the increased incorporation of renewable energy in transport without too many changes in the current infrastructure and engines, facilitating the transition toward decarbonization of this sector.
Obtaining ethanol requires a biomass pretreatment that improves the accessibility and susceptibility of cellulose for enzymatic hydrolysis (cellulases) to glucose, preventing degradation of carbohydrates and the formation of byproducts’ inhibitors of hydrolysis and fermentation processes. This allows for both the fractionation of hemicellulose components and the recovery of lignin. Alkaline pretreatment uses sodium, potassium, calcium, or ammonium hydroxides as reagents and is typically performed at lower temperatures than acid or hydrothermal pretreatment. Sodium and calcium hydroxide (lime) are the most common reactants (conc. 0.05–0.15 ghidroxide/gbiomass), although recirculating ammonia percolation or fiber explosion/expansion treatments by ammonium are also common. Pretreatment with sodium or calcium hydroxide is normally carried out at temperatures between 20 and 130°C, with reaction times ranging from minutes to hours or days, depending on the working temperature [10, 11, 12]. In materials with a high lignin content, such as wood, this pretreatment is less efficient, so its use is more common in agricultural and agro-industrial waste [12].
In alkaline hydrolysis, fiber solvation occurs, producing swelling. The intermolecular ester bonds that cause the cross-linking of the chains are saponified, opening the structure of the hemicelluloses and lignin, and splitting the glycosidic bonds of the hemicellulose chain. Acetyl and uronic groups also react with alkali, separating them from the fiber. Depending on the applied conditions, the depolymerization of lignin molecules can occur by cleavage of the internal molecular bonds of the α and β-aryl ether type, which contributes to the degradation of lignin. All of these changes improve access to the cellulose chains, which become less crystalline with the treatment by reacting to the hydrogen bonds that link them. Under conditions of greater intensity, “peeling” reactions of the hemicellulose and cellulose chains may occur, causing a decrease in the degree of polymerization [10, 13, 14].
Among alkaline pretreatments, an alternative is the addition of an oxidizing agent (oxygen or hydrogen peroxide [H2O2]) to the alkali. The reaction mechanism is similar to alkaline hydrolysis, but it is aided by an oxidative effect on lignin, increasing its splitting from the lignocellulosic biomass. Alkaline peroxide treatments have faster kinetics at low temperatures, thus presenting technological and economic advantages over conventional alkaline treatments [10, 12].
Alkaline peroxide mechanical pulp (APMP) production is a particular case within alkaline treatments at low temperatures. It consists of treating wood with sodium hydroxide (NaOH), H2O2, and stabilizers at temperatures below 100°C, during which simultaneous pulping and bleaching occur. The pretreated material then goes through the mechanical defibration and refining stages [15, 16]. This type of pulping is widely used in China as a process for obtaining semi-chemical pulps from wood and nonwood materials [17].
In this work, the main destination of pulp is as a filler in the production of paper, particularly tissue. There is evidence that the substitution of kraft pulp for high-yield pulps of up to 30% improves the final product’s properties, providing increases in bulk, tensile strength, bending stiffness, internal bond strength, and elasticity modulus [18].
Likewise, several reports have applied alkaline pretreatment to various types of biomasses [19, 20] to improve subsequent enzymatic hydrolysis for the production of bioethanol [21, 22] or to extract hemicelluloses prior to kraft pulping [23, 24]. More recently, the use of alkaline treatment for the extraction of hemicelluloses to prepare xylan films and the extraction of hemicelluloses from commercial pulps to produce nanocellulose have been reported [25, 26].
Once the lignocellulosic matrix is broken, it is necessary to hydrolyze the cellulose to glucose, which is fermented to produce bioethanol. Cellulose hydrolysis is preferably carried out enzymatically to minimize further inhibitions caused by the coproducts of chemical hydrolysis. The hydrolysis of cellulose into glucose through an enzymatic process involves several cellulases that act synergistically: 2 exo-β-glucanases or cellobiohydrolases (CBH: CBH I and CBH II), endo-β-glucanases (EG: EG I, EG II, EG III, and EG V) and a β-glucosidase (βG)) [27]. EG and CBH break down cellulose into smaller polysaccharides. EG first randomly breaks internal glycosidic bonds, then CBH I cleaves sugars from reducing ends, while CBH II cleaves cellulose toward nonreducing ends. This second stage occurs in the liquid phase and is mainly due to the hydrolysis of cellobiose to glucose by ß-glucosidase [24]. The enzyme dose needed to reach an acceptable percentage of glucose and ethanol yields strongly depends on the raw material and pretreatment used [11]. The kinetic effects on the enzymatic saccharification of lignocellulosic are largely related to the presence of polymers and nonionic surfactants, which are partially associated with the removal of lignin and cellulase stability and activation [28].
The microorganism used in fermentation must have certain conditions: high ethanol yield, high productivity, and tolerance to high concentrations of ethanol and sugars. It is also desirable for them to work in the presence of sugar degradation compounds, which are commonly fermentation inhibitors. Furthermore, it is expected that these microorganisms will have resistance to high temperatures and fermentable sugar. The most used microorganism for bioethanol production on an industrial scale is the yeast Saccharomyces cerevisiae, which efficiently ferments hexoses and disaccharides, such as sucrose and maltose. However, it cannot metabolize pentoses, such as xylose and arabinose, which are components of hemicelluloses [11].
The aim of this work was to compare three alternative uses for small-sized industrial forest residues. Therefore, an alkaline pretreatment reinforced with H2O2 was used for the subsequent production of bioethanol or bleached semichemical APMP. Based on the laboratory-scale results, an industrial-scale techno-economic analysis of the processes is presented (based on a plant located in Uruguay) and compared with current power generation alternatives.
Pin chips from a local Eucalyptus pulp mill were received, dried and stored in airtight plastic boxes. The particle size distribution and chemical composition are presented in Table 1. Material below 0.50 mm was discarded.
Component
Amount (%)
Method
Particle size distribution
Glucan
42.0 ± 1.8
NREL\TP-510-42,618
Above 3.36 mm
7.6%
Xylan
14.1 ± 0.2
3.36–1.40 mm
43.7%
Acid insoluble lignin
20.1 ± 1.2
1.40–1.19 mm
38.0%
Acid soluble lignin
3.2 ± 0.1
1.19–0.50 mm
10.7%
Acetyl groups
2.9 ± 0.2
Below 0.50 mm
4.1%
Arabinan
1.0 ± 0.1
Ash
0.7 ± 0.1
NREL\TP-510-42,622
Extractives in ethanol and water
4.3 ± 0.1
TAPPI T 204 cm-97
Table 1.
Raw material composition and particle size distribution.
2.2 Alkaline pretreatment
The process began with the mechanical impregnation of the pin chips with the pretreatment reagents in a device specially designed for this purpose. They were then transferred to a reactor where pretreatment occurred. The treated pin chips were refined in a disk refiner, and the resulting pulp was divided into two groups. One part was used to produce the APMP. For this, a two-stage bleaching sequence was carried out using (1) ethylenediaminetetraacetic acid (EDTA) and (2) H2O2. Then, the papermaking properties of the obtained pulp were determined. The pulp obtained from refining was subjected to an enzymatic hydrolysis process, and the liquid obtained was fermented to produce bioethanol. This process is outlined in Figure 1.
Figure 1.
Schematic description of the process.
According to Kruzolek [16], pressure is essential to achieving proper impregnation. For this, a device was designed and built to allow the pin chips to be immersed in water and reagents. By applying pressure, the occluded air in the pores was eliminated. The device was then returned to atmospheric pressure, achieving a “sponge effect” by expanding the compressed material and simultaneously absorbing the impregnated liquor. The apparatus is shown in Figure 2. The reagents (NaOH and H2O2) and water were externally heated to 80°C in a container. The amount of water was calculated to achieve a liquid-to-dry solid ratio of 7. The device, with the pin chips inside, was placed in a container within the press. Pressure was applied until it reached 0.5 MPa, held for 10 minutes, and then released.
Figure 2.
Impregnator and press are used to remove the air inside the pin chips.
Once the impregnation process was complete, solids and liquids were transferred to a 3 L Parr reactor (model 4522 M) and instrumented with temperature control and agitation, where the process temperature was reached using a defined heating ramp. The pressure in the reactor was maintained at 0.8 MPa using nitrogen gas.
For pretreatment in the Parr reactor, preliminary trials were carried out following a factorial experimental design (Series A) using the following parameters: extraction temperature (50–100°C), time at that temperature (30–90 min), and doses of soda (8–12%) and peroxide (4–6%) used. In the design, a 2:1 (m:m) ratio of NaOH:H2O2 was applied. A constant level of 0.2% diethylenetriaminepentaacetic acid (DTPA) was used as a chelator. The function of DTPA is to remove heavy metals from the solution, particularly Mn2+, Cu2+, and Fe3+, to prevent the decomposition of H2O2. According to Kruzolek [16], the colored extractives present in eucalyptus also react with this chelating agent.
After the extraction, the material was drained and centrifuged at 1000 rpm, which separated the solid fraction from the liquid. The solid fraction was then extensively washed with deionized water until a colorless filtrate was obtained.
In the solid fraction, the extraction yield was determined by considering the initial dry mass and the dry mass obtained after extraction, which was later refined in a disc refiner to produce semi-chemical pulp or to increase the accessibility to the cellulose chains to favor enzymatic hydrolysis. Refining was performed in a disc refiner (Kumagai Riki Kogyo Co., Ltd., Tokyo, Japan) with 30-cm diameter discs and a gap of 0.1 mm. The net yield after refining was also determined.
Brightness was measured (ISO 2470-1:2016) in the refined pulp. The pulps with the highest net yield and brightness were characterized in terms of carbohydrate and lignin content according to the NREL TP-510-42,618 procedure. The liquid fraction (liquor) obtained after alkaline peroxide treatment was characterized in terms of pH and carbohydrates, lignin, and biomass degradation product content (acetic and formic acids, furfural, and hydroxymethylfurfural (HMF)) (NREL TP-510-42,623). Residual peroxide was measured by iodometry.
This experimental design was evaluated in terms of extraction yield, brightness after refining, and enzymatic hydrolysis efficiency.
With approximately 40% of the design completed, the yields obtained were extremely low, and the enzymatic hydrolysis efficiency was far from the expected value. Therefore, and in accordance with the bibliographic review focused on bioethanol production [29, 30], more exhaustive extraction conditions were used (Series B). For these assays, the NaOH:H2O2 ratio was adjusted to 1:1 (m:m). Initial tests were carried out at 75°C for 90 min in the pretreatment stage, varying the NaOH charge between 5% and 45%. For different soda charges (between 8% and 30%), the processing time was increased while maintaining the temperature at 75°C.
Condition applied
Solid fraction
Liquid fraction
T (°C)
Time (min)
NaOH (%)
H2O2 (%)
Extraction yield (%)
Yield after refining (%)
Brightness (% ISO)
pH
Lignin (%)
XS (%)
Acetyl groups (%)
50
30
10
5
90.5 ± 0.1
82.0 ± 0.1
49.7 ± 0.5
11.9
2.4 ± 0.1
0.7 ± 0.1
3.6 ± 0.1
50
30
8
4
85.1 ± 0.1
80.5 ± 0.1
48.3 ± 0.5
10.9
2.0 ± 0.1
n.d.
n.d.
50
60
8
4
89.5 ± 0.1
85.7 ± 0.1
50.2 ± 0.6
10.8
2.2 ± 0.1
0.5 ± 0.1
3.5 ± 0.1
50
90
12
5
86.6 ± 0.1
78.8 ± 0.1
48.6 ± 1.0
12.2
2.7 ± 0.1
0.9 ± 0.1
3.9 ± 0.1
50
90
8
4
87.8 ± 0.1
76.8 ± 0.1
53.4 ± 0.9
10.6
2.0 ± 0.1
0.5 ± 0.1
3.1 ± 0.1
75
30
10
5
90.2 ± 0.1
81.5 ± 0.1
n.d.
9.4
1.5 ± 0.1
n.d.
n.d.
75
30
8
4
78.4 ± 0.1
82.5 ± 0.1
48.8 ± 0.5
10.0
2.4 ± 0.1
0.5 ± 0.1
3.3 ± 0.1
75
60
8
4
85.1 ± 0.1
82.9 ± 0.1
50.2 ± 0.6
10.0
2.6 ± 0.1
0.7 ± 0.1
3.5 ± 0.1
75
60
10
5
78.5 ± 0.1
77.5 ± 0.1
48.9 ± 0.3
11.1
2.5 ± 0.2
0.2 ± 0.1
4.3 ± 0.1
75
90
8
4
89.1 ± 0.1
79. 6 ± 0.1
49.5 ± 0.6
9.6
1.9 ± 0.1
0.3 ± 0.1
2.3 ± 0.1
75
90
10
5
87.5 ± 0.1
81.3 ± 0.1
47.6 ± 0.3
11.1
2.9 ± 0.2
0.8 ± 0.1
3.8 ± 0.1
100
60
12
6
86.5 ± 0.1
71.2 ± 0.1
50.9 ± 0.3
9.1
2.2 ± 0.1
n.d.
n.d.
100
90
10
5
86.8 ± 0.1
83.0 ± 0.1
46.7 ± 1.2
10.4
2.7 ± 0.2
1.0 ± 0.1
3.9 ± 0.1
Table 2.
Solid and liquid fraction characteristics after alkaline pretreatment and mechanical refining of the Serie A assays. (XS: Xylosaccharides; n.d.: Not determined).
2.3 Pulp bleaching and papermaking properties
To produce cellulose pulp, the solids were bleached using a two-stage sequence: the chelating stage (Q) and the H2O2 stage (P). The purpose of the first stage was to remove metals to avoid the catalyzed decomposition of H2O2 [15]. This was performed at acidic pH, using EDTA as a chelating agent. The aim of the second stage was to oxidize chromophoric compounds derived from lignin fragments into colorless structures. The bleaching power of H2O2 is highest at a pH between 10 and 11. The Q stage was performed at 65°C for 60 min using charges of 0.4% EDTA and 0.8% sulfuric acid (initial pH = 3). The P stage was performed at 80°C for 150 min using charges of 0.3% NaOH, 0.375% H2O2, 0.02% magnesium sulfate, and 0.02% DTPA.
After bleaching, some pulps were selected based on the global yield and the final brightness obtained. The papermaking properties of these pulps were determined.
To determine the papermaking properties, pulps were refined using a PFI refiner at 4000 revolutions (ISO 5264-2). In refined and unrefined pulps, drainability in terms of Canadian standard freeness (TAPPI T227 om-17) was determined. Pulp sheets were then prepared and conditioned according to ISO standards (ISO 5264-2, ISO 5269-2, ISO 187). On those sheets, brightness, opacity (ISO 2471:1998), basis weight, tear and tensile index, and tensile energy absorption (TEA) were measured according to TAPPI T410 om-08, TAPPI T414-om86, and TAPPI T494-om04 standards, respectively.
2.4 Enzymatic hydrolysis
The refined pulps were enzymatic hydrolyzed to obtain a liquid fraction rich in glucose, to be fermented for bioethanol production. All assays were performed under the same enzymatic hydrolysis conditions. A commercial enzymatic complex (Cellic CTec 2 (Novozymes)) was used. Enzyme dosage was 15 FPU/gglucan with a solid loading of 15% (w/w). The enzymatic hydrolysis was performed in Erlenmeyer flasks, pulps were suspended in a 0.05 M citric acid buffer solution (pH = 4.8) at 48°C for 96 h with orbital agitation at 150 rpm. Samples were taken at times 0 and 96 h. Tests were carried out by duplicate.
Figure 3 shows the aspects of the raw material, alkaline-treated solid, alkaline-extracted liquor, and refined pulp.
Figure 3.
a) Eucalyptus pin chips used as raw material; b) solid fraction after alkaline-peroxide treatment; c) liquid fraction obtained after alkaline-peroxide treatment; d) pulp obtained after mechanical refining process.
As expected, the solid obtained after pretreatment was brighter than the original raw material. This demonstrates the effect of H2O2. However, the liquor obtained presented a dark color, similar in visual appearance to that obtained in treatments without peroxide.
Table 2 shows the liquid and solid fraction results after pretreatment and refining assays (Series A).
Conditions applied
Solid fraction
Liquid fraction
T (°C)
Time (min)
NaOH (%)
H2O2 (%)
Extraction yield (%)
Yield after refining (%)
Brightness (% ISO)
pH
Lignin (%)
XS (%)
Acetyl groups (%)
75
90
5
5
88.4 ± 0.1
78.4 ± 0.1
n.d.
8.9
1.5 ± 0.1
0.6 ± 0.1
3.2 ± 0.3
75
90
10
10
83.3 ± 0.1
78.7 ± 0.1
n.d.
11.3
2.4 ± 0.1
1.3 ± 0.1
3.1 ± 0.1
75
90
15
15
80.1 ± 0.1
51.2 ± 0.1
n.d.
12.8
2.5 ± 0.1
2.5 ± 0.1
3.4 ± 0.2
75
90
30
30
72.2 ± 0.1
66.8 ± 0.1
51.6 ± 0.5
12.7
3.2 ± 0.2
4.4 ± 0.2
3.5 ± 0.3
75
90
45
45
71.9 ± 0.1
61.2 ± 0.1
58.9 ± 0.9
12.5
3.9 ± 0.2
5.9 ± 0.2
3.5 ± 0.2
75
90
60
60
65.6 ± 0.1
53.7 ± 0.1
45.2 ± 0.4
12.3
4.3 ± 0.2
n.d
3.5 ± 0.3
100
240
8
8
85.9 ± 0.1
56.3 ± 0.1
n.d.
6.9
2.4 ± 0.1
2.7 ± 0.3
3.2 ± 0.2
100
240
12
12
78.1 ± 0.1
67.0 ± 0.1
n.d.
9.6
2.8 ± 0.1
3.6 ± 0.3
3.6 ± 0.3
75
120
8
8
88.3 ± 0.1
58.8 ± 0.1
n.d.
7.4
1.8 ± 0.1
2.1 ± 0.1
3.5 ± 0.2
75
120
12
12
93.6 ± 0.1
48.6 ± 0.1
n.d.
10
2.6 ± 0.1
1.9 ± 0.1
3.5 ± 0.1
75
240
16
16
82.6 ± 0.1
66.6 ± 0.1
n.d.
12.8
2.6 ± 0.1
0.5 ± 0.1
3.9 ± 0.3
Table 3.
Solid and liquid fraction characteristics after alkaline pretreatment and mechanical refining of the Serie B assays. (XS: Xylosaccharides; n.d.: Not determined).
The extraction yields for the alkaline peroxide pretreatment varied between 78.5% and 90.5%. However, no clear trend was observed within the parameters studied. These yields are comparable to those obtained by Liu et al. [31], who worked with aspen chips under similar conditions (temperature: 50–100°C, time: 10–60 min, soda load 1.5%–6.7%, and NaOH:H2O2 ratio 2:1), obtaining extraction yields between 85% and 94%.
All liquors presented a pH higher than neutrality. The final pH of the liquors arose from the soda charge applied in each condition and the acetyl and/or uronic groups released during extraction, which partially neutralized the hydroxyl groups. Consequently, the highest pH in the liquor was achieved under low-severity pretreatment and a high soda charge.
Pulp brightness was between 46.7% and 53.4% ISO. The low variability detected among these values did not support the determination of a general trend. Kruzolek [16] reported brightness values in the range of 55.4–64.9% ISO when working with a smaller charge of soda but a higher NaOH:H2O2 ratio using Eucalyptus grandis chips. Moreover, Latibari et al. [32], when working with Paulownia wood at 70°C for 100 min and charges of NaOH and H2O2 between 1.5% and 5.0% each, reported an ISO brightness between 67.5%–69.0%. In this case, the highest value reached for brightness is explained by the fact that Paulownia wood has 12.1% of extractives in ethanol: acetone because these extractives are normally colored and easily removed in an alkaline medium during pretreatment before bleaching.
Regarding the liquid fraction, xylosaccharides (from the hemicellulose fraction in the raw material) and lignin content were too low to be used for producing value-added products.
The results obtained from the Series B assays are reported in Table 3.
T (°C)
Time (min)
NaOH (%)
H2O2 (%)
Beating rev. (rpm)
CSF (mL)
Sheet density (kg/m3)
Brightness (% ISO)
Opacity (%)
Tensile Index (Nm/g)
Tear Index (Nm3/g)
TEA (J/m2)
50
60
8
4
0
349
405
64.5
93.6
15.9
1.9
0.08
4000
107
546
63.7
93.2
30.2
1.8
1.1
50
90
8
4
0
200
452
73.3
89.6
23.9
2.18
0.2
4000
76
595
72.5
90
37.5
2.04
0.34
75
30
8
4
0
302
453
75.6
86.3
24
2.32
0.2
4000
72
609
75
86.9
42.7
2.3
0.46
75
90
30
30
0
n.d
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
4000
57
786
74
78.5
42.6
1.44
0.34
75
90
45
45
0
n.d
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
4000
37
777
73.8
77.5
41.6
1.2
0.31
75
120
8
8
0
340
455
73.5
84.1
25.5
2.86
0.21
4000
81
623
72.9
84.5
47.2
2.93
0.72
75
240
16
16
0
339
526
72.6
83
35.6
3.57
0.36
4000
79
681
71.1
82.4
60.0
3.36
0.88
100
60
12
6
0
529
410
76.2
82.7
15.5
2.41
0.08
4000
120
624
75.4
84.6
46.8
2.87
0.55
100
90
10
5
0
300
518
70.4
86.9
35.3
3.23
0.43
4000
91
654
69.2
86.2
52.4
3.26
0.74
Table 4.
Paper properties of selected bleached pulps at different degrees of refining.
When working at 75°C for 90 min, the extraction yield decreased linearly with the soda charge (r2 = 0.97). Variations in the other parameters did not show a clear trend. Despite the large increase in the dose of soda and peroxide, the pulp’s brightness hardly improved compared to the results obtained in Series A. Xylosaccharide content in the liquid fraction increased as much as 5.9%, which implies a removal of almost 40% of the Xylan present in the raw material. Li et al. [33] carried out a treatment using corn cobs with 50% H2O2 (pH 11.5) at 30°C for 24 h and extracted 56% of the initial hemicelluloses and 92% of the initial lignin. Gupta and Lee [34], when working at 60°C for 24 h with a soda and H2O2 charge of 50%, reported extraction of 40% of the xylosaccharides originally present in poplar wood. Hemicellulose extraction yields differ among authors and depend on the materials and conditions used. The latter tend to have longer reaction times, a high soda and peroxide charge, and low temperatures. The extracted xylosaccharides can be recovered by precipitation with ethanol and used for biofilm production [35, 36].
3.2 Pulp bleaching and papermaking properties
The bleaching procedure was applied to all pulps, and the nine pulps with the highest values of brightness and net yield were selected. These pulps were refined, and the papermaking properties of the refined and unrefined pulps were determined.
The mechanical and optical properties of the selected bleached pulps are shown in Table 4.
As can be expected, sheet density increased when the pulp was refined, making fibers more flexible and increasing the bonding capacity and the bonding area. Additionally, external fibrillation collaborates to increase interfiber bonding formation [15, 37]. The sheet density in this work was higher than the density reported in other studies working with eucalyptus chips [15, 16] or chips of other hardwood species [32]. The use of small wood increased the external fibrillation during refining, which increased the sheet density. Notably, the density increased with the chemical charge, producing more fibrillation and bulkier pulp.
Brightness improved after bleaching, but the brightness of the commercial pulps was generally higher. However, it is possible to obtain pulp with a higher final brightness when using a longer residence time in the P stage, carrying out this stage in two steps, or with higher chemical doses. Opacity was high for almost all bleached pulps, which is good for paper applications.
Tensile strength is useful in determining resistance to breakage. It is associated with the strength, length, and bonding of the fibers, which are also dependent on the degree of refinement. The tensile strength of the bleached pulps refined at 4000 rpm was high for all the conditions tested, reaching a tensile index of 60.0 Nm/g. The tear index for the refined pulps was also good, in accordance with the values reported by other authors [38, 39, 40].
Of the nine bleached pulps tested, two showed the best papermaking properties. The conditions for these pulps were as follows: (1) 75°C for 240 min with a dose of 16% soda and 16% peroxide and (2) 100°C for 90 min with a dose of 10% NaOH and 5% peroxide. The first pulp had slightly better properties. However, the increase in chemical loads and the high residence time did not justify the use of these conditions at the industrial level. In addition, the second treatment had a noticeably higher yield (83.3% vs. 66.6%), so a greater amount of pulp was obtained from the same amount of pin chips. Consequently, the conditions selected to scale up the process were 100°C for 90 min and a load of 10% NaOH and 5% peroxide.
3.3 Enzymatic hydrolysis
The chemical composition of the pretreated solids used and their behavior in enzymatic hydrolysis are listed in Table 5.
Conditions applied
Solid composition (%)
Hydrolysis parameters at 96 h
T (°C)
Time (min)
NaOH (%)
H2O2 (%)
Glucan
Xylan
Lignin
Glucose (g/L)
Hydrolysis efficiency (%)
100
60
12
6
55.0 ± 0.8
12.0 ± 0.1
29.4 ± 4.4
14.1 ± 1.0
14.2 ± 1.0
75
90
5
5
51.4 ± 1.4
16.5 ± 0.5
26.5 ± 1.7
6.7 ± 0.3
5.2 ± 0.2
75
90
8
4
54.0 ± 1.3
12.2 ± 0.3
30.0 ± 3.9
9.7 ± 0.5
9.9 ± 0.1
75
90
10
5
55.8 ± 1.4
12.1 ± 0.2
32.5 ± 5.5
22.3 ± 1.0
23.3 ± 3.3
75
90
12
12
48.7 ± 3.6
18.4 ± 2.1
28.1 ± 0.1
21.4 ± 1.5
22.9 ± 2.8
75
90
30
30
51.0 ± 0.1
10.7 ± 0.0
30.2 ± 3.0
22.0 ± 0.5
23.3 ± 1.8
75
90
45
45
53.1 ± 0.4
9.4 ± 0.3
26.4 ± 0.2
15.5 ± 0.9
15.8 ± 0.7
75
90
60
60
54.9 ± 0.1
9.9 ± 4.0
29.5 ± 1.3
9.9 ± 1.3
10.5 ± 1.2
75
240
30
30
54.6 ± 0.3
14.9 ± 0.2
24.5 ± 4.0
14.5 ± 3.4
15.1 ± 3.4
75
480
30
30
47.6 ± 1.3
12.3 ± 0.1
26.5 ± 0.3
12.2 ± 1.2
14.3 ± 2.0
Table 5.
Solids composition before enzymatic hydrolysis, glucose content, and hydrolysis efficiency.
The solid pretreated with NaOH/H2O2 presented a maximum enzymatic hydrolysis efficiency of 23% when 10%, 12%, and 30% soda loads were used, and then its value decayed 1.5 times when 45% soda was used. By adding 6.25% H2O2, the efficiency increased from 30–40% and from 40–49% by the cellular structure alteration due to hemicellulose (49%) and lignin (26%) removal, which left the cellulose more accessible to the enzyme [41]. In this work, only slightly lower hydrolysis efficiencies (23%) were obtained by pretreating eucalyptus with NaOH (10)/H2O2 (5) at 75°C for 90 min, despite the fact that lignin removal was much lower (2.8%). Bagasse lignin is more easily solubilized with alkaline solutions due to the presence of lignin H (which contains p-hydroxyphenyl in its structure), which is not present in the wood [42]. Alvarez-Vasco and Zhang (2017) used different temperatures (25–180°C), reaction times (30–90 min), and H2O2 concentrations (4–10%) with spruce and observed that temperatures below 140°C were not effective in hydrolyzing cellulose (efficiency less than 25%) [43].
An increase in peroxide charge or reaction time worsened the enzymatic hydrolysis of the cellulose process. Several authors have reported low yields of enzymatic hydrolysis in lignocellulosic materials pretreated with the addition of alkalis, which can be attributed to the formation of pseudo-lignin or the solubilization of the amorphous fractions of cellulose, leaving more crystalline zones in the solid. Pseudolignin acts as a barrier, reducing the accessibility of the enzymes or retaining toxic compounds (furfural, HMF, phenolic compounds), making it difficult to remove them during washing [44, 45, 46, 47, 48, 49, 50, 51].
The results obtained for the enzymatic hydrolysis of the alkaline peroxide-treated solids (based on the final concentration of glucose and enzymatic efficiency) were low. The low concentration of glucose obtained made it pointless to proceed with subsequent fermentation. Simultaneous hydrolysis and fermentation tests also had poor results (data not shown).
For the different processes, an economic analysis was carried out considering the annual profitability of each option. The revenues from product sales were equalized to the annual production costs (including the amortization of the investment), and the maximum price at which biomass could be purchased was calculated. This option was preferred to the classic analysis through the internal rate of return and the net present value because the investments required for the options listed below were quite different.
An economic comparison was performed for two options: (1) the installation of a power boiler plant to produce energy to be sold to the national grid and (2) the production of APMP to be sold to papermaking companies. To this end, the best conditions found in the experiment were used as working conditions. Due to the lower enzymatic hydrolysis efficiencies obtained, bioethanol production was not analyzed.
Both analyses were performed considering 50,000 dry tons/year of eucalyptus pin chips, with a water content of 45%, which corresponds with the number of pin chips generated for a modern kraft eucalyptus pulp mill with a capacity of 2.1 ADT1/year. The prices used correspond to a plant located in Uruguay.
4.1 Techno-economic analysis for power generation alternative
To calculate the power boiler size (MW), Eq. (1) was used, considering a boiler efficiency of 85% and an efficiency in the vapor cycle of 25% (to convert steam to electricity) [52].
Economic analysis parameters for power generation.
USD means Dollar USA.
Based on [53] and estimating the cost by scaling using the “six-tenths factor rule” [52].
Considering utilities equal to zero to calculate the maximum price of biomass and considering the energy selling price in the range of 50–70 MWh, different scenarios can be analyzed. The results are shown in Table 7.
Energy sale price (USD/MWh)
Biomass maximum purchase price (USD/dry ton)
50
0.43
60
8.4
70
16.3
Table 7.
Result of economic analysis for power generation.
In the first two cases, the price of biomass was quite low and probably would not cover the cost of biomass handling. However, for a scenario where the energy price reached 70 USD/MWh, which could be possible in Uruguay, the biomass price increased to 16.3 USD/ton, which is an attractive value.
4.2 Techno-economic analysis for an APMP mill
Considering the yields obtained in the experiment for a biomass availability of 50,000 ADT/year, the size of the APMP mill turn should be 41,500 ADT/year. The values applied for the economic analysis are listed in Table 8 [52, 54].
Mechanical pulping is considered an aggressive process.
Amortizable capital costs include yard improvements, machinery, equipment, buildings, utility services, and electrical and piping systems.
Considering utilities equal to zero to calculate the maximum price of biomass and considering the APMP selling price between 350 and 500 USD/ADT, different scenarios could be formulated. For this analysis, the recovery of chemical products or energy that would occur if the plant were related to a kraft mill was not considered. In this case, the profits would be even higher. The results are shown in Table 9.
Pulp sale price (USD/ADT)
Biomass maximum purchase price (USD/dry ton)
350
15.2
400
58.2
500
144
Table 9.
Result of economic analysis for APMP mill.
Pulp production seems to be a more attractive business than energy production, although investments in both processes are quite different. However, if the selling price of the pulp falls to 332 USD/ADT, the cost of biomass at the mill gate becomes zero.
For the final disposal of massive waste generated by the forestry industry, three alternatives were proposed: the generation of energy through burning, the production of filler pulp for paper production, and the production of bioethanol.
According to the experimental results, the production of bioethanol was inhibited in the enzymatic hydrolysis stage, and a more detailed study is required to determine the causes. Thus, APMP pretreatment does not seem to be a viable option for bioethanol manufacturing.
The production of APMP from wood residues is possible and results in good-quality pulp. Therefore, the process can be considered by large kraft pulp production industries for the management of their wood residues.
The production of both power and APMP may be economically feasible. In both cases, the utility of the plant is extremely sensitive to the sale price of energy or pulp. In energy production, sales prices of USD 70/MWh are required, which is normally higher than the cost of wind energy but has been possible in some scenarios. In the production of APMP, higher profits are obtained, but the profits depend strongly on the sale price, and a decrease to less than 332 USD/ADT makes the process unfeasible.
The authors thank the National Agency for Research and Innovation (ANII), which has financed this project through their sectorial energy fund (FSE). They also thank UPM for providing the pin chips with which this work was carried out. The authors particularly thank the Department of Materials and Forest Products of the Technological Laboratory of Uruguay (LATU) for allowing us to execute part of the experimental activity in their facilities.
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Notes
ADT stands for Air Dry Ton and is commonly used in the pulp industry as a unit of measure for the pulp produced. It represents pulp with a moisture content of 10%; therefore, 1 ADT is equivalent to 0.9 ODT (Oven Dry Ton).
Written By
Leonardo Clavijo, Mairan Guigou, Norberto Cassella, Gastón Cortizo, Florencia Risso, Lucía Velazco, Mario Daniel Ferrari, Claudia Lareo and María Noel Cabrera
Submitted: 22 August 2022Reviewed: 31 August 2022Published: 06 October 2022
Open access peer-reviewed chapter
