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Technical Evaluation of Hybrid Clones of Corymbia spp. to Produce Market Pulp

- ORCID：
Marcelo Moreira da Costa ¹
✉
- ORCID：
Ricardo de Carvalho Bittencourt ¹
- ORCID：
Thales Augusto Pinto Coelho Nogueira ¹
- ORCID：
Larissa Soares Silva ¹
- ORCID：
Weslley Henrique Martins da Silva ¹
- ORCID：
Sebastião Renato Valverde ¹
- ORCID：
Gleison Augusto Dos Santos ¹
- ORCID：
Daniel Alexander Fernandes Coelho ²
- ORCID：
Claudilene Aparecida Alves Pena ²

1. Department of Forest Sciences, Federal University of Vicosa, Vicosa, Minas Gerais, 36570900, Brazil； 2. Aperam BioEnergia Co., Capelinha, Minas Gerais, 39680000, Brazil

Updated：2022-07-18

DOI：10.1213/j.issn.2096-2355.2022.03.001

Abstract

This study presents hybrid clones of Corymbia spp. developed by Aperam BioEnergia as potential substitutes for Eucalyptus wood in the pulp industry. The biomass of Eucalyptus spp. was compared with that of Corymbia spp. by performing modified kraft pulping and basic density analyses. Comparisons were made by analyzing their respective mean annual increments of cellulose (MAI_cel) and specific wood consumption (SWC), estimated using a kappa number of (19 ± 1). The results showed that one of the hybrid clones (Corymbia citriodora × Corymbia torelliana, ID 4) had higher basic wood density, higher screened yield, better MAI_cel#k19, and lower SWC than the other samples, reaching values of 0.608 t/m³, 54.1%, 24.6 ADt/ha/year (i.e., air dry ton/hectare/year), and 2.74 m³/ADt, respectively. Consequently, clone ID 4 had the highest MAI_cel/SWC ratio score (8.98). Given its high forest productivity and low industrial cost compared with other samples, as well as its advantages over Eucalyptus spp. wood, we concluded that clone ID 4 has great potential as an alternative biomass for pulp production.

Keywords

Corymbia hybrids; modified kraft pulping; mean annual increments of cellulose (MAI_cel); specific wood consumption (SWC)

1 Introduction

The Brazilian pulp sector has aimed to significantly increase its production scale. In recent years, Brazil has regularly ranked as the largest exporter of bleached kraft pulp in the world, mainly produced from Eucalyptus spp. ^[

1]. In Brazil, short-fiber pulp is used to produce different types of paper (printing and writing, tissue, and packaging), with an annual export volume of 15 million air dry tons (ADt). The trade balance of the planted forest sector is equivalent to US$12.5 million/year, 75.2% of which stems from the pulp and paper industry ^{[Reference 2

Baidu Scholar}2]. Therefore, there is considerable demand for wood with operating costs equal to or lower than those of the Eucalyptus spp., especially of equal or superior quality, to be used as a fibrous source in Brazil.

Wood from Corymbia spp. hybrid clones has rarely been explored in the pulp and paper industry. Interspecific Corymbia hybrids tend to have high biomass production, high density, and rapid growth ^[

3]. Furthermore, they are tolerant to biotic and abiotic stresses such as wind, water deficit, frost, and most pests and diseases that cause economic damage to planted forests ^{[Reference 4

Baidu Scholar}4]. Such superiority is believed to be due to heterosis (hybrid vigor), an important phenomenon for improving forest productivity that allows alleles of interest from different species to be combined ^{[Reference 5

Baidu Scholar}5].

Despite the high performance of Corymbia clones, the forest sector still has strong restrictions and concerns regarding the use of Corymbia wood ^[

6]. Therefore, it is necessary to develop new technologies and processes to utilize Corymbia wood. Parameters, such as the mean annual increment of cellulose (MAI_cel, ADt/ha/year), which relates to forest productivity ^{[Reference 7-8}7-8], and specific wood consumption (SWC, m³/ADt), which contributes to predicting operating costs ^{[Reference 9

Baidu Scholar}9], are important for material assessment. Such data will facilitate the ranking of commercial clones based on their pulp production capacity per unit of planted area throughout the planting period.

In recent decades, environmental pressure and the need to increase pulp delignification to improve efficiency and quality has stimulated the development of new pulping technologies, including modified cooking methods ^[

10]. Modified kraft pulping can produce pulp from Corymbia spp. and increase the selectivity of pulping processes for high-density wood. This method provides adequate woodchip impregnation by dividing the alkali charge and preventing carbohydrate degradation ^{[Reference 11

Baidu Scholar}11]. Modified kraft pulping involves hemicellulose dissolution, terminal depolymerization reactions, and alkaline hydrolysis of glycosidic bonds, increasing yield and leading to a loss of viscosity ^{[Reference 12

Baidu Scholar}12].

Therefore, this study aimed to develop technological alternatives for mitigating the major bottlenecks associated with the use of Corymbia spp. hybrid clones with a high basic wood density. We also aimed to rank commercial clones of Eucalyptus spp. and hybrids of Co‑rymbia spp. based on indicative parameters for forest yield (MAI_cel and SWC) using an industrial modified kraft pulping protocol.

2 Experimental

We analyzed 16 woodchip samples from trees aged 6.5 years, obtained from the industrial unit of Aperam BioEnergia, Vale do Jequitinhonha, Minas Gerais State, Brazil. The samples were identified according to their genetic stocks (Table 1).

Table 1 Genetic stocks used for lab-scale evaluation of wood and pulp properties

ID	Genetic stock (6.5 years of age)
1	Corymbia citriodora × Corymbia torelliana
2	Corymbia citriodora × Corymbia torelliana
3	Corymbia citriodora × Corymbia torelliana
4	Corymbia citriodora × Corymbia torelliana
5	Eucalyptus cloeziana
6	Eucalyptus urophylla × Eucalyptus spp.
7	Eucalyptus urophylla × Eucalyptus spp.
8	Eucalyptus urophylla × Eucalyptus spp.
9	Eucalyptus grandis × Eucalyptus urophylla
10	Eucalyptus urophylla × (Eucalyptus camaldulensis × Eucalyptus grandis)
11	(Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla
12	(Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla
13	(Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla
14	(Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla
15	(Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla
16	Eucalyptus urophylla × Eucalyptus pellita

2.1　Modified kraft pulping

After exploratory tests, we selected a target kappa number of (19 ± 1) for effective alkali charge (EA, %), determined as NaOH consumption on a dry wood basis. Modified kraft pulping was performed according to the following steps:

(1) The woodchips were pre-vaporized under low-pressure steam (0.35 MPa) for 15 min, and the woodchip surface temperature was increased to approximately 105℃.

(2) Woodchips were impregnated by adding 55% of the total EA, followed by increasing the temperature from 105℃ to 135℃ (15 min ramp) and maintaining it at 135℃ for 90 min.

(3) Woodchips were cooked by draining the impregnation liquor followed by injection of the white liquor, reaching 45% of the total EA. The temperature was increased from 135℃ to 165℃ over 10 min and maintained at 165℃ for 90 min with a liquor/wood ratio of 4∶1.

(4) Washing occurred at the end of the process and the black liquor was extracted by displacement. The washing process was performed in two phases. The first consisted of an alkaline wash with 3% EA, followed by extraction. During this phase, the temperature was reduced to 90℃ and maintained for 60 min. In the second phase, hot water (90℃, 9 m³/ADt) was applied for 15 min and the temperature was reduced to 40℃.

2.2　Analytical procedures

For woodchip characterization, the resulting brown pulp was analyzed using the following parameters: basic wood density ^[

13], lignin S/G ratio ^{[Reference 13

Baidu Scholar}13], effective alkali ^{[Reference 14

Baidu Scholar}14], kappa number, and screened yield by gravimetry.

The EA consumption (EA_#k19) and screened yield (SY_#k19) for kappa number (19 ± 1) were estimated from three modified kraft pulping tests, in which the applied EA charge was varied and the other conditions kept constant. MAI_cel was calculated as the product of the mean annual increment, screened yield of pulp, and basic density (BD) of the woodchip samples. The samples were ranked according to their MAI_cel/SWC ratio.

Morphological analyses of the brown pulp fibers resulting from kraft pulping were performed using the Valmet FS5 Fiber Image Analyzer, which uses ultra-high definition (HD) technology and artificial intelligence for fiber analysis. Dry pulp (10 mg) from the samples was weighed and suspended in demineralized water for a total volume of 500 mL according to the conditions required by the equipment for short fiber analysis. For each sample, two replicates were performed, and the average was taken.

3 Results and discussion

3.1　Results

Corymbia citriodora × Corymbia torelliana woodchips (IDs 1 and 4) had the highest basic density (0.608 t/m³) (Table 2). The second-highest basic density (0.582 t/m³) was that of Eucalyptus cloeziana (ID 5). The lowest basic density was clone ID 14 (Eucalyptus camaldulensis × Eucalyptus grandis) × Eucalyptus urophylla.

Table 2 Results for BD, lignin S/G ratio, EA, SY, mean annual increment (MAI), MAI_cel, and SWC of genetic stocks, as estimated using a kappa number of (19 ± 1)

ID	BD/(t·m^-3)	S/G ratio	EA/%^a	SY/%^a	MAI/(m³·ha^-1·year^-1)^b	MAI_cel/(ADt·ha^-1·year^-1)^c	SWC/(m³·ADt^-1)^c	Score^d
1	0.608	2.98	16.1	52.5	29.0	10.3	2.82	3.65
2	0.565	3.57	15.9	51.6	34.8	11.3	3.09	3.66
3	0.507	3.05	15.4	53.5	51.8	15.6	3.32	4.70
4	0.608	2.65	16.0	54.1	67.4	24.6	2.74	8.98
5	0.582	2.42	18.9	49.8	45.9	14.8	3.10	4.77
6	0.518	2.56	16.0	52.7	37.2	11.3	3.30	3.42
7	0.472	2.94	15.8	53.6	46.0	12.9	3.56	3.62
8	0.491	2.61	15.8	53.8	59.6	17.5	3.41	5.13
9	0.470	2.71	15.6	53.2	42.2	11.7	3.60	3.25
10	0.511	2.69	16.6	51.1	39.5	11.5	3.45	3.33
11	0.431	2.52	16.8	51.1	48.2	11.8	4.09	2.89
12	0.470	2.66	16.9	52.5	46.7	12.8	3.65	3.51
13	0.488	2.64	17.3	50.9	53.6	14.8	3.62	4.09
14	0.431	2.81	16.6	51.9	47.6	11.8	4.02	2.94
15	0.519	2.88	17.6	50.8	40.0	11.7	3.41	3.43
16	0.471	2.37	17.6	51.1	55.1	14.7	3.74	3.93

^a Modified kraft pulping at an H-factor of 1.031, as described in Section 2.1. ^b Mean annual increment of wood volume under bark. ^c Values for brown pulp. ^d Ranking was based on the MAI_cel/SWC ratio.

Corymbia citriodora × Corymbia torelliana (ID 3) had the lowest EA_#k19 consumption (15.4%) (Table 2), whereas ID 4 had the highest SY_#k19 (54.1%). Eucalyptus cloeziana (ID 5) had the lowest performance as it required the highest EA charge, resulting in the lowest yield (49.8%).

The highest MAI_cel values were for Corymbia citriodora × Corymbia torelliana (ID 4) and Eucalyptus urophylla × Eucalyptus spp. (ID 8), at 24.6 and 17.5 ADt/ha/year, respectively. The lowest MAI_cel value was for a clone of Corymbia citriodora × Corymbia torelliana (ID 1), with 10.3 ADt/ha/year. Furthermore, ID 4 exhibited the lowest SWC value (2.74 m³/ADt). Thus, ID 4 was ranked first as it obtained the highest MAI_cel and lowest SWC values.

Knowing the morphological characteristics of the fibers is extremely important to establish the most suitable purpose for them. The means of the main characteristics of the brown pulp fibers from the kraft pulping are shown in Table 3. The highly ranked clone ID 4 had the second-longest fiber length, shorter than only ID 1 (also a Corymbia spp. hybrid). Despite the lower fiber lengths reported, the analyzed values of the other Corymbia spp. samples are in accordance with the described values of 0.943-1.140 mm for juvenile wood of Corymbia spp. ^[

15-16].

Table 3 Main characteristics of the brown pulp fibers resulting from the kraft pulping for a kappa number of (19 ± 1)

ID	Length/mm	Width/µm	CWT/µm	Coarseness/(mg·(100 m)^-1)	Fibrous population fibers/mg	Fines A/%¹	Fines B/%¹
1	1.03	18.2	2.04	5.5	17970	18.9	0.07
2	0.92	18.2	2.49	6.5	15981	20.8	0.07
3	0.88	17.8	2.15	5.6	20075	16.6	0.09
4	0.95	18.1	1.92	5.2	19992	17.7	0.09
5	0.88	20.0	2.49	7.3	15004	16.6	0.12
6	0.92	19.0	1.49	4.3	26051	17.8	0.14
7	0.91	18.8	2.00	5.6	20268	15.7	0.28
8	0.90	19.3	2.18	6.2	17732	16.5	0.19
9	0.87	19.1	2.02	5.8	20003	14.4	0.30
10	0.84	18.9	2.26	6.3	18281	18.1	0.21
11	0.81	18.4	1.84	5.1	24391	14.2	0.24
12	0.83	18.3	1.93	5.3	23192	14.0	0.15
13	0.80	18.1	1.82	4.9	25809	13.4	0.20
14	0.81	18.7	1.73	4.9	25342	15.6	0.31
15	0.84	17.2	1.79	4.6	26791	14.6	0.20
16	0.86	18.3	1.70	4.7	25173	13.4	0.28

¹ Fines A and B consider materials with length ≤0.2 mm and >0.2 mm, but width <10 μm, respectively.

Other characteristics such as width, cell wall thickness (CWT), coarseness, fibrous population fibers, and fines have a significant influence on the physical and mechanical properties of the pulp ^[

17]. The fines content favors the bonds between fibers ^{[Reference 18

Baidu Scholar}18] and it is possible to infer the presence of a more consolidated wall for the hybrids of Corymbia spp. in relation to Eucalyptus spp. from the analysis of Fines B. Furthermore, because high coarseness can lead to papers with low cohesion ^{[Reference 19

Baidu Scholar}19], consolidation, and poor bonding between fibers, ID 4 stands out again among the hybrids of Corymbia spp.

3.2　Discussion

The results of this study revealed the potential of Corymbia spp. wood for commercial pulp production and analyzed the relationship between basic wood density, EA consumption, and screened yield. Through the modified kraft pulping protocol, we were able to combine optimal temperature and time conditions to obtain the best yields without requiring a high alkali charge for the sample with the highest basic density (ID 4).

Modified kraft pulping allows for better distribution of the alkali charge, thereby avoiding intense chemical activity at the beginning and end of the procedure ^[

20]. Thus, some of the advantages of the method include good impregnation of woodchips before delignification, uniformity, reduced cooking temperatures in the digester, reduced active alkali consumption, increased screened yield, low reject generation, and enhanced pulp quality and bleaching ^{[Reference 21-23}21-23].

A higher basic wood density had a significant positive impact on MAI_cel and SWC in Corymbia spp. In particular, the hybrid Corymbia citriodora × Corymbia torelliana (ID 4) showed excellent forest productivity and kraft pulping performance, reporting good EA_#k19 and SY_#k19 values. Basic density is used to select genetic stocks according to the intended purpose, and is a relevant parameter for the standardization of raw materials received by the industry ^[

24]. High basic wood density is generally associated with increased yield, reduced logistics costs, low SWC, and low EA ^{[Reference 25

Baidu Scholar}25].

SWC during production is also important, as it is directly related to density and yield and associated with several wood quality parameters ^[

9]. Corymbia hybrids exhibited the lowest SWC values. This can be explained by the high basic density of these samples, as denser wood has more cellulose, favoring a higher yield. In other words, smaller volumes of wood are required to produce a unit weight of cellulose ^{[Reference 26

Baidu Scholar}26].

The particularity of each species and clone was analyzed by considering the MAI_cel values. MAI_cel considers the wood volume, basic density, and yield, which reflects the amount of pulp that can be produced. Leading companies in the pulp and paper industry in Brazil report average MAI_cel values of 10-12 ADt of bleached pulp per hectare per year ^[

7]. Here, ID 4 achieved a MAI_cel of 24.6 ADt/ha/year, indicating that this clone may be a good choice for commercial applications aimed at achieving high performance in the forest and pulp industries.

4 Conclusions

The ranking of commercial clones can be applied to distinguish woods with different behaviors in industrial manufacturing processes. Ideally, ranking should be done following industrial kraft pulping protocols to minimize differences between laboratory and industrial conditions. Corymbia citriodora × Corymbia torelliana hybrids, specifically ID 4, ranked the highest, proving to be excellent high-performance alternatives to Eucalyptus spp. in the forest and pulp industries.

Because of their higher basic wood density, Corymbia spp. hybrids have clear advantages in terms of logistics costs and specific wood consumption. In addition to providing numerous benefits to forested areas, these genetic stocks have lower wood production costs per volume (US$/m³) and lower production costs per ton of pulp (US$/ADt).

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