ASSESSMENT OF INDUSTRIAL PERFORMANCE FOR MARKET PULP PRODUCTION BETWEEN EUCALYPT AND Corymbia HYBRIDS CLONES

The search for novel biomasses for uses as alternative fi ber sources, similar to Eucalyptus spp. biomass, holds great value and potential for commercial-scale application. This study aims to present the hybrid clones of Corymbia spp. developed by Aperam BioEnergia as potential substitutes for Eucalyptus wood in the market pulp industry. By performing modifi ed kraft pulping and chemical characterization analyses, it was possible to compare the biomass of Eucalyptus spp. with that of Corymbia spp. Comparisons were made by analyzing their respective pulp average growth rate (PAGR) and specifi c 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 highest PAGR#k19, and lowest SWC than other samples. Clone ID 4 showed lowest value of SWC since, simultaneously presented a higher value of wood basic density and screened yield. Consequently, in agreement with its best results, clone ID 4 had the highest-ranking score, calculated as the PAGR/SWC ratio. This genetic material also showed one of the lowest total lignin content, consequently the highest screening yield. Besides Clone ID 4 showed signifi cantly highest xylan content, among wood samples assessed in this work. For that reason, the ID 4 was the highest-ranked, proving to be an excellent highperformance alternative for forest-industry interface parameters.


INTRODUCTION
The Brazilian pulp sector has aimed to signifi cantly increase its production scale. The country stands out as the largest exporter of pulp to the global market, with a trade balance of US$ 6,0 billion, produced mainly from Eucalyptus spp. (IBÁ, 2021). In the country, short-fi ber pulp is used for the production of diff erent types of paper (printing and writing, tissue, and packaging), with an annual export volume of 15 million air dry tonnes (ADt). Therefore, there is considerable demand for wood with operating costs equal to or lower than that of Eucalyptus spp., as well as with equal or superior quality, to be used as a fi brous source in Brazil (Sanquetta, 2020).
As Eucalyptus, the genus Corymbia belongs to the Myrtaceae family and has a total of 113 species, which are naturally distributed along the east coast of Australia (Tambarussi et al., 2018). Until the 1990s, the species of these genera were classifi ed as Eucalyptus. However, due to genetic divergences, Corymbia was offi cially considered a new genus by the scientifi c community (Hill and Johnson, 1995).
Wood from Corymbia spp. hybrid clones have been little explored in the pulp and paper industry. According to Loureiro et al. (2019), interspecifi c Corymbia hybrids tend to have high biomass production, high density, and rapid growth. Furthermore, these materials are tolerant to biotic and abiotic stresses, such as wind, water defi cit, frost, and most pests and diseases that cause economic damage to planted forests (Nahrung et al., 2010). It is believed that such superiority is due to heterosis, or hybrid vigor, an important phenomenon for improving forest productivity, as it allows the combination of alleles of interest from diff erent species (Goulet et al., 2017).
Despite the high performance of Corymbia clones, the forest sector still has strong restrictions and concerns regarding the use of this material (Reis et al., 2014). Therefore, it is necessary to study and develop new technologies and processes for this genus. Parameters such as pulp average growth rate (PAGR, ADt ha −1 year −1 ), which relates to forest productivity (Foelkel, 2017), and specifi c wood consumption (SWC, m 3 ADt −1 ), which contributes to predicting operating costs (Gallo et al., 2018), are important for material assessment. With these data, it is possible to rank commercial clones on the basis of their pulp production capacity per unit of planted area throughout the planting period.
In recent decades, environmental pressures and the need to increase pulp delignifi cation to improve process effi ciency and pulp quality have stimulated the development of new pulping technologies, including modifi ed cooking methods (Segura, 2012). Modifi ed kraft pulping can be used to produce pulp from Corymbia spp. and increase the selectivity of pulping processes for high density woods. This method provides adequate wood chips impregnation with division of the alkali charge, preventing carbohydrate degradation (Da Silva et al., 2016).
Modifi ed kraft pulping allows for a better distribution of the alkali charge, thereby avoiding intense chemical activity at the beginning and end of the procedure (Santos et al., 2015). Thus, some of the advantages of the method include good impregnation of wood chips before delignifi cation, uniformity and reduced cooking temperatures in the digester, reduced active alkali consumption, increased screened yield, low reject generation, and enhanced pulp quality and bleaching (Segura et al., 2016;Gomes, 2009;Bassa, 2006).
Given the above, this study aimed to develop technological alternatives for mitigating the major bottlenecks associated with the use of Corymbia spp. hybrid clones with high basic wood density. We also aimed to rank commercial clones of Eucalyptus spp. and hybrids of Corymbia spp. on the basis of indicative parameters for forest yield (PAGR and SWC) using an industrial modifi ed kraft pulping protocol and chemical characterization.

MATERIAL AND METHODS
We analyzed 16 wood chips samples from trees aged 6.5 years, obtained from the industrial unit of Aperam BioEnergia, Vale do Jequitinhonha, Minas Gerais State, Brazil. Samples were identifi ed according to the genetic stock (Table 1).

Modifi ed kraft pulping
Pulping was performed in an MK digester (model 610-2), which comprises two reactors with a capacity of 7 L and an electronic control unit connected to a computer. An aliquot of 400 g of dry wood chips was placed in the MK digester and heated electrically. Temperature data were monitored at 1-min intervals during pulping, allowing determination of temperature and H-factor profi les.
The eff ective alkali charge (EA, %), determined as NaOH consumption on a dry wood basis, was selected after the exploratory tests, aiming at a kappa number of 19 ± 1. Modifi ed kraft pulping was performed according to the following steps: (1) Wood chips were pre-vaporized under low-pressure steam (3.5 bar) for 15 min, whereby the wood chips surface temperature was increased to about 105 °C; (2) Wood chips were impregnated by adding 55% of the total EA, followed by increasing the temperature from 105 to 135 °C (15 min ramp) and maintaining at 135 °C for 90 min; (3) Cooking was achieved 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 °C in 10 min and maintained at 165 °C for 90 min with a liquor/wood ratio of 4:1; and (4) Washing was carried out at the end of the process, and black liquor was extracted by displacement. The washing process was carried out in two phases. The fi rst consisted of an alkaline wash at 3% EA followed by extraction. During the fi rst phase, the temperature was reduced to 90 °C and held for 60 min. In the second phase, hot water (90 °C, 9 m 3 ADt −1 ) was applied for 15 min and the temperature was reduced to 40 °C.

Analytical procedures
For the analysis of wood chips, sawdust and brown pulp, the parameters described in table 2 were  Licor branco (WL); licor negro fraco (WBL); álcali efetivo (EA). used. EA consumption (EA #k19 ) and screened yield (SY #k19 ) for kappa number 19 ± 1 were estimated from three modifi ed kraft pulping tests, in which the applied EA charge was varied and the other conditions were maintained constant. PAGR was calculated as the product of average growth rate, screened yield of pulp, and basic density of wood chips samples. Samples were ranked according to the PAGR/SWC ratio, both estimated using modifi ed pulping to kappa number 19 ± 1. For chemical characterization, the 6 best ranked clones will be selected.
Values with the same letters (A -F) within the columns do not diff er signifi cantly at α = 0.05. Tested using ANOVA and post-hoc Tukey. 1 Modifi ed kraft pulping at an H-factor of 1,031, as described in Section 2. The chemical characterization of the samples is described in Table 4.
The results above are consistent with the wood behavior in pulping process. The lowest total lignin values content was found in clones ID 3 and ID 4 and the highest in clones ID 5, ID 13 and ID 16. The removal of lignin, which is the main objective of the pulping process, releases the portion of wood fi bers. Therefore, a low lignin content in wood favors pulping performance and saves chemicals. In addition, delignifi cation is favored when there is a higher S/G ratio (Vidaurre, 2010). The clone that presented the highest S/G ratio was sample ID 3, which is consistent with the lowest consumption of EA% #K19 . The lowest S/G ratio was obtained for the clone ID 16 sample. Consequently, shown a lower performance in the kraft pulping process compared to the others ranked, despite having a lowest total lignin content.
Extractives had lower values in clones ID 3, ID 4, ID 13 and ID 16. In general, extractives impair pulping performance by causing scaling problems in equipment, pipes and tanks, forming pitch (Mokfi enski et al. 2008). High levels of extractives content could impact negatively in the impregnation process and in the chemical consumption, additionally interfere negatively in the yield. The carbohydrate content has similar values to those reported in the literature (Mokfi enski et al. 2008). Clones ID 5 and ID 13 had the highest glycan value and the lowest xylan content.

DISCUSSION
The results of this study revealed the potential of Corymbia spp. wood in pulp production and the relationship between basic wood density, EA consumption, and screened yield. By applying the modifi ed 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 samples with the highest density (ID 1 and ID 4). Table 4 -Sawdust Chemical Composition of the 6 best ranked clones. Tabela 4 -Composição química da serragem dos 6 clones melhores ranqueados.
Values with the same letters (A -E) within the columns do not diff er signifi cantly at α = 0.05. Tested using ANOVA and post-hoc Tukey. 1 Arabinanas 2 Galactans 3 Glycans 4 Xylans 5 Mananas Valores com as mesmas letras (A -E), dentro das colunas, não diferem signifi cativamente em α = 0,05. Testado usando ANOVA e post-hoc Tukey. There was a positive impact of higher basic density on SWC in Corymbia spp. In particular, C. citriodora × C. torelliana (ID 4) showed excellent performance in forest productivity and kraft pulping, aff ording 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 standardization of raw materials received by the industry (Souza et al., 2017). High wood density is generally associated with increased yield, reduced logistics costs, low SWC, and low EA (Jardim et al., 2017).
Another important aspect to be discussed is the SWC during production, given that, in addition to being directly related to density and yield, it is also associated with several wood quality parameters (Gallo et al., 2018). Corymbia hybrids exhibited the SWC values lower than Eucalyptus. Such a fi nding can be explained by the high density of these samples, as denser woods have more cellulose, favoring yield. In other words, smaller volumes of wood would be required for the production of a unit weight of pulp (Gomide et al., 2010).
The particularity of each species and clone was analyzed considering PAGR values. The results demonstrated the potential of clone ID 4 in the forestry, pulp, and paper industries. PAGR takes into account wood volume, basic density, and yield, refl ecting the amount of pulp that can be produced (Souza et al., 2020). Leading companies in the pulp and paper industry in Brazil report average PAGR values of 10 to 12 ADt of bleached pulp per hectare per year (Foelkel, 2017). Here, ID 4 achieved a PAGR of 24.6 ADt ha −1 year −1 , indicating that this clone may be a good choice for commercial applications aimed at high performance in forest and pulp industries.
After chemical analysis of sawdust from the top 6 ranked clones, it was found that Corymbia spp. clone ID 3 has a higher S/G ratio and clone ID 4 has a higher xylan content compared to Eucalyptus clones. Respectively, these chemical composition could be positive in the pulp bleachability and on the beatability of bleached kraft pulps.

CONCLUSIONS
This work evaluated and compare diff erent samples of Eucalyptus spp. and Corymbia spp. on their performance based on key forestry and industrial parameters. Ideally, commercial clones should be ranking, based at least on PAGR and SWC. Industrial kraft pulping protocols should be the accurately choice, in order to minimize diff erences between laboratory and industrial conditions. Clone ID 4 (C. citriodora × C. torelliana hybrids), were the highest-ranked, proving to be excellent highperformance alternative for forest-industry interface parameters. Higher wood basic density shown a clear advantage with regard to SWC (m 3 ADt -1 ), consequently lower production costs per air dry ton of pulp (US$ ADt -1 ). Clone ID 4 meet higher value of SWC since, simultaneously present higher value of wood basic density and screened yield.
Regardless, Corymbia spp. hybrids wood chemistry composition, Clone ID 3 confi rmed lowest EA #K19 consumption value, much associated to the higher S/G ratio. Clone ID 4 shown one of the lowest total lignin content, consequently highest screening yield. Besides Clone ID 4 shown signifi cantly highest xylan content, among wood sample assessed in this work.
Furthermore, as further work, it will be interesting to evaluate these 6 top clones, regarding ECF bleaching, fi ber morphology characterization and bleached pulp physico-mechanical properties.