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Zanuncio, A. J. V., Monteiro, T. C., Lima, J. T., Andrade, H. B., and Carvalho, A. G. (2013). "Drying biomass for energy use of Eucalyptus urophylla and Corymbia citriodora logs," BioRes. 8(4), 5159-5168.

Abstract

Brazil is the world’s largest producer of charcoal, mainly for the steel industry. Fresh wood has high moisture content, which reduces its use for energy. Thereby, drying is a fundamental step for charcoal production. This work aimed to determine longitudinal variation in stem diameter, wood basic density, moisture content, and calorific value of Eucalyptus urophylla and Corymbia citriodora logs. These logs were taken from different longitudinal positions on the trees and dried for 90 d; the net calorific value was determined based on the gross calorific value and moisture content. Curves and models were generated based on this data for moisture content and net calorific value during the 90-d period. The logs from the base and middle of C. citriodora trees had lower initial moisture content, and, after 90 d of drying, all logs from the top reached the equilibrium moisture. Drying the logs increased the wood calorific value, with an increase of 49.36%, 63.86%, and 85.98% for those of the base, middle, and top, respectively. The models generated had a high coefficient of determination and a low standard error.


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Drying Biomass for Energy Use of Eucalyptus urophylla and Corymbia citriodora Logs

Antônio José Vinha Zanuncio,a,* Thiago Campos Monteiro,b José Tarcisio Lima,Hélder Bolognani Andrade,c and Amélia Guimarães Carvalho a

Brazil is the world’s largest producer of charcoal, mainly for the steel industry. Fresh wood has high moisture content, which reduces its use for energy. Thereby, drying is a fundamental step for charcoal production. This work aimed to determine longitudinal variation in stem diameter, wood basic density, moisture content, and calorific value of Eucalyptus urophylla and Corymbia citriodora logs. These logs were taken from different longitudinal positions on the trees and dried for 90 d; the net calorific value was determined based on the gross calorific value and moisture content. Curves and models were generated based on this data for moisture content and net calorific value during the 90-d period. The logs from the base and middle of C. citriodora trees had lower initial moisture content, and, after 90 d of drying, all logs from the top reached the equilibrium moisture. Drying the logs increased the wood calorific value, with an increase of 49.36%, 63.86%, and 85.98% for those of the base, middle, and top, respectively. The models generated had a high coefficient of determination and a low standard error.

Keywords: Gross and net calorific values; Growth; Moisture content; Wood density

Contact information: a: Department of Forestry Engineering, Federal University of Viçosa, MG 36570-000, Brazil; b: Department of Forest Science, Federal University of Lavras, P.O. Box 3037, 37200-000, Lavras/MG, Brazil; c: Forest Engineer, V&M Florestal LTDA, CAPEF, Itapoã Farm, Cedex P. 04, Paraopeba, MG, Brazil; *Corresponding author: ajvzanuncio@gmail.com

INTRODUCTION

The main source of energy in the world is derived from fossil fuels, and their use increases CO2 emissions, contributing to the greenhouse effect (Zhu et al. 2011). Biomass is an alternative replacement for these sources (Rousset et al. 2013; Protásio et al. 2013); however, its production is not sufficient to meet the demand.

Harvesting, drying, transportation, and distribution methods are the biggest challenges in obtaining energy from biomass (Casal et al. 2010; Gonzalez et al. 2011; Zhu et al. 2011). The bioenergy industry in Brazil uses mainly Eucalyptus camaldulensisEucalyptus globulusEucalyptus grandis, and Eucalyptus urophylla to produce charcoal for the steel mill industry, with a consumption of nearly 17 million m3 of eucalyptus wood in 2011 (ABRAF 2012).

All freshly cut timber has large quantities of water (Skaar 1972), which reduces its use in generating power. The charcoal industry requires raw materials with humidity around 35% (Brand et al. 2011). Outdoor storage is the most commonly used method to reduce wood moisture with low cost, but it requires a long drying period.

The drying rate of logs presents the highest values during the first 15 days due to the loss of free water; thereafter, there is a tendency toward lower values until the equilibrium moisture content is reached (Rezende et al. 2010). Wood factors such as anatomy, density, and dimensions, and environmental factors such as temperature, relative humidity, and wind speed affect wood drying (Engelund et al. 2013; Mugabi et al. 2010; Moya et al. 2012; Berberovic and Milota 2011; Watanabe et al. 2012; Muñoz and Moya 2008).

The chemical composition, moisture content, and environmental conditions affect the energetic properties of wood (Sotelo Montes et al. 2003; Brand et al. 2011; Sotelo Montes et al. 2011; Cetinkol et al. 2012; Sotelo Montes et al. 2012; Wang et al. 2012). Wood with high lignin and extractives contents has higher calorific value (Shebani et al. 2008; Silva et al. 2012). The calorific value is reduced by 2 MJ/kg for each 10% increase in wood moisture content (Swithenbank et al. 2011). However, wood does not need to be completely dry, because increasing the calorific value after a certain value does not compensate for the effort required for the continued drying, depending on the species and the environmental conditions (Brand et al. 2011).

The drying of logs has variable outcomes, and its effects on the energy stored in the wood need further studies. Thus, the objective of this study was to evaluate the variation of moisture content and calorific values of Eucalyptus urophylla and Corymbia citriodora logs during a 90-days drying period.

EXPERIMENTAL

Nine trees, each seven years old, were selected. Three were from each clone VM4 and Mn463 of Eucalyptus urophylla, and three from Corymbia citriodora plants that originated from seed. Logs 1.1 m in height were cut from the base (15 cm above ground) and at 50% and 100% of the commercial height of the tree. Variation longitudinal of the diameter, and height mean value of Eucalyptus urophylla clones and Corymbia citriodora are showed in Table 1.

Table 1. Mean Diameter at the Base, Middle, and Top of Logs, and Mean Height of Eucalyptus urophylla Clones and Corymbia citriodora

The coefficient of variation is given in parentheses.

A 5-cm disk was removed from the transverse surfaces of each log, its moisture content was determined, and the average moisture was inferred as the log moisture. The same sample was used to determine basic wood density (ABNT 2003). E. urophylla and C. citriodora logs were grouped without contact with each other, and their transverse surfaces were waterproofed to ensure that the small size of the samples did not affect the results.

Logs were weighed every alternate day during the first 20 days, every 4 days during the next 30 days, and weekly for the last 40 days of the total 90-days drying period.

The drying rate was calculated using the following equation,

DR = (Ui – Uf)/ days (1)

where DR is the drying rate, U(%) is the initial wood moisture content, Uf (%) is the final wood moisture content, and days = number of drying days.

The gross calorific value of the wood from base, middle, and top of each log was evaluated according to standard NBR8633 (ABNT 1984), and the net calorific value was calculated using the following equation (Tillman et al. 1989),

PCi = PCs − [0.0114 × U × PCs (%)] (2)

where PCis the net calorific value, PCs is the gross calorific value, and (%) is the wood moisture content.

The regression model utilized to predict the moisture content and net calorific value was,

y = b × days + c × days0.5 (3)

where y is the moisture in percent or net calorific value (cal/g), a is the intercept, b is the regression coefficient for days, c is the regression coefficient for days0.5, and days is the number of drying days. The model proposed was based on the coefficient of determina-tion.

RESULTS AND DISCUSSION

Mean values of initial moisture content and wood basic density of Eucalyptus urophylla clones and Corymbia citriodora are shown in Table 2. Corymbia citriodora and Vm4 (E.urophylla) had the highest average initial moisture contents at the base, and Mn463 (E. urophylla) had it at the top. The amount of moisture and its distribution in the longitudinal direction varies among species and individuals of Corymbia citriodora, Eucalyptus cloeziana, Eucalyptus grandis, Eucalyptus paniculata, Eucalyptus pilularis, Eucalyptus urophylla, andEucalyptus tereticornis (Engelund et al. 2013; Oliveira et al. 2005).

The initial moisture content was consistent with previously reported values ​​from 59.6% to 159% for Eucalyptus cloeziana, Eucalyptus globulus, Eucalyptus grandis, Eucalyptus paniculata, Eucalyptus pellita, Eucalyptus regnans, and Eucalyptus urophylla and Pinus dunnii (Oliveira et al. 2005; Hansmann et al. 2008; Studhalter et al. 2009; Rezende et al. 2010; Redman and McGavin 2010; Brand et al. 2011).

C. citriodora had the highest values, ​​and Mn463 (E. urophylla) had the lowest values of wood basic density ​​in all longitudinal positions. The densities of E. urophylla and C. citriodora were similar to the previously reported values of 0.429 to 0.596 g/cm³ for Eucalyptus cloeziana, Eucalyptus grandis, Eucalyptus paniculata, and Eucalyptus urophylla (Rezende et al. 2010; Oliveira et al. 2005; Santos et al. 2012; Sette et al. 2012), but they were lower than the reported values of 0.73 and 0.756 for C. citriodora (Oliveira et al. 2005).

Table 2. Initial Moisture Content and Density of Wood at the Base, Middle, and Top of Logs of Eucalyptus urophylla Clones and Corymbia citriodora

The coefficient of variation is given in parentheses.

Drying rates of E. urophylla and C. citriodora logs are shown in Table 3. Samples from the top, with their smaller diameters, showed high drying rates in the first month. Logs of Mn463 (E. urophylla), with higher initial moisture, dried easily due to the short distance traveled by water and reached a drying rate of 3.52% in the first month. Vm4 (Europhylla) and C. citriodora logs had drying rates of 1.9% and 1.59% in the same period, respectively. These rates were lower in the second and third months, showing that the logs were near the equilibrium conditions of the environment. The moisture content of C. citriodora logs reached 35% within 6 days, while Vm4 (E. urophylla) and Mn463 (E. urophylla) took 14 days (Table 3). The maximum moisture content recommended for carbonization is 35% (Brand et al. 2011).

The middle logs of Mn463 (E. urophylla), VM4 (E. urophylla), and C. citriodora showed average moisture contents after a 90-day drying period lower than the 35% moisture content recommended for carbonization (Brand et al. 2011). However, C. citriodora logs reached 35% moisture within 16 days of drying, while those of Mn463 (E. urophylla) and VM4 (E.urophylla) required 78 days and 65 days, respectively.

Table 3. Drying Rate of Logs in Different Periods and Final Moisture Content after 90-day Drying and Its Coefficient of Variation

Drying of the base log after 90 days was more difficult due to its larger diameter (Table 3). C. citriodora logs showed moisture contents below 35% after 71 days of drying, while the logs of the other species needed longer drying periods to reach this level. Factors such as a smaller diameter and higher density favored the drying of C. citriodora logs.

The gross and net calorific values of the wood are show in Table 4. Gross calorific values were similar to those reported for Eucalyptus dunnii, Eucalyptus grandis, Pinus pinaster, and Pinus taeda, between 4,250 and 4,796 cal/g (Brand et al. 2011; Telmo and Lousada 2011; Musinguzi et al. 2012).

Table 4. Gross and Net Calorific Values of Wood (Mean and Coefficient of Variation) During 90 Days of Drying

The net wood calorific value increased by 31.1%, 45.6%, and 81.1% in 30 days of drying; 6.9%, 6.6%, and 2.4% in the next 30 days; and 6.5%, 3.8%, and 0% in the final 30 days for base, middle, and top section logs, respectively. This increase results in a gain in gravimetric yield and carbonization time (Arruda et al. 2011).

Corymbia citriodora had the lowest gross calorific value, but the lower moisture content of logs from the base and middle during the evaluations resulted in higher net calorific value, indicating the importance of drying for better use of biomass. The logs from the top of all species had similar moisture contents after 90 days of drying. This resulted in higher net calorific value for Mn463 (E. urophylla) and VM4 (E. urophylla) because of their higher gross calorific value.

The curves for moisture showed a higher drying rate in the first 15 days (Fig. 1), which is similar to that reported for Cryptomeria japonica lumber and Betula papyrifera flakes (Bedane et al. 2011; Hermawan et al. 2012). Eucalyptus urophylla logs with similar diameters in the base and middle logs showed average moisture contents of 63% and 43%, respectively, after 80 days of drying (Rezende et al. 2010). Drying logs of Eucalyptus sp. with diameters greater than 12 cm had moisture contents of about 55% after 175 days of drying in open air, and the moisture content was 16% to 27% for logs with diameters of 4 to 12.0 cm after this period (Vital et al. 1985).

The curves of the net calorific value followed a reverse trend from that of drying. Top logs showed high moisture losses in the first month, resulting in a higher gain of net calorific value and stabilization at the consecutive periods. Base and middle logs showed a tendency toward increasing net calorific value during the evaluation period (Fig. 1).

Fig. 1. Moisture and net calorific value of Eucalyptus urophylla and Corymbia citriodora as functions of drying time

The regression models showed a coefficient of determination higher than 0.85, indicating that at least 85% of the variations can be explained by the model. The highest standard error was 5.12 (Table 5).

Table 5. Regression Models for Moisture Contents of Eucalyptus and Corymbia Logs, Coefficient of determination (C.D.), and Standard Error (SE) as Functions of Drying Time

Corymbia citriodora that had been produced via seeds showed higher moisture variation, and it was therefore more difficult to generate models for it, thereby reducing the coefficient of determination and increasing the standard errors values.

The models presented for the net calorific values of wood showed coefficient of determination from 0.8515 to 0.9872 and standard error from 36.07 to 123.92 (Table 6).

Table 6. Regression Models for Net Calorific Value of Eucalyptus and Corymbia Logs, Coefficient of determination (C.D.), and Standard Error (SE) as Functions of Drying Time

The models for the net calorific value followed the same trend as for the drying models. Corymbia citriodora produced via seeds had the highest variation, and gave lower coefficient of determination and higher standard errors.

CONCLUSIONS

  1. Base and middle logs from C. citriodora had lower moisture contents after 90 days of drying than those from E. urophylla. Top logs of all species reached the equilibrium moisture content after this period.
  2. The average calorific value of wood increased during the 90-days drying period to 49.3%, 63.9%, and 86.0% for the base, middle, and top logs, respectively.
  3. Drying and calorific value models showed high coefficient of determination and low standard errors for the drying of C. citriodora and E. urophylla logs.

ACKNOWLEDGMENTS

Thanks to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), V&M Florestal, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) for financial support. Global Edico Services rewrote and edited the English version of this manuscript.

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Article submitted: May 30, 2013; Peer review completed: July 10, 2013; Revised version received and accepted: July 22, 2013; Published: August 21, 2013.