The use of biomass can reduce reliance on fossil fuels, to mitigate greenhouse gas (GHG) emissions and to increase energy resource diversification. Among the different renewable energy sources, biomass is one of the promising choices, which has the potential to be used as the raw material to generate electricity [1]. In Thailand, an agricultural-based country, there are many sources of agricultural residues [2] that in total amount of about 61 million ton per year [3], which are used as the main source of biomass energy for industry. However, there are limitations on biomass utilization in Thailand such as the problems of collection, transportation and storage, and availability. For these reasons, fast-growing tree farming for biomass production is likely to be a solution and the most sustainable method of fuel supply for biomass gasification electricity generation [4]. Leucaena is expected to become one of the biomass energy resources, because it is a fast growing tropical tree [5] and with a high yields of woody material [6]. Moreover, leucaena can be used for many purposes including forage, wood, shade, green manure, live fences and erosion control [7].

For energy utilization, information on the productivity related aspects and relative performance of leucaena is not available for large scale cultivation. In order to produce sufficient and sustainable quantities for electricity generation, plantation management must identify the important factors that have an influence on the long term growth and productivity after harvesting leucaena. Spacing is one of the most important factors influencing the growth of leucaena. High plant populations (10,000 - 20,000 trees∙ha⁻¹) were reported to optimize fuelwood production in the Philippines [8]. Guevarra et al. [9] examined even higher plant populations (44,000 - 133,000 trees∙ha⁻¹) were suitable for forage production. Spacing affects management options and the final product which the leucaena production for bioenergy may be optimized with the correct initial spacing [10]. To maximize potential yield, it is particularly important to establish stand densities per unit (spacings) that ensure the best utilisation of the site in long-term. In case of wood quality, leucaena need to evaluate the wood properties. Not only heating value is the most important parameter to characterize the chemical properties for combustion but the ultimate analysis as well. The ultimate analysis provided weight of percentage of C, H, O, N and S [11]. The ultimate analysis is ralated to thermal utilization of the wood [12] and can be also used to evaluate their heating values. Moreover, elemental analysis (P, K, Ca, Mg, Cl and Ash) can be used as the parameters to determine their impact on the environment that play an important role in sustaining energy production system. In addition, relatively little is known about the quality of leucaena wood at different spacings. Therefore, the present study aimed to assess the influence of spacing on the growth, biomass yield and wood quality of leucaena for fuelwood.

Plant height was relatively higher at the wide spacings and lower at the narrow spacings during the period of study. Differences in plant height due to spacing treatments were significantly different in the 1^st, 3^rd and 4^th years. In the present study, leucaena height was significantly affected by competition which has been due to the extremely high planting densities [14]. The higher densities resulted in reduced plant height which was caused by the failure of sprouts to grow after emergence due to the lack of solar radiation. However, growth height of leucaena is more less affected by spacing than stem diameter.

Spacing had an enormous impact on stem diameter. Leucaena responded to wider spacing and lower competition by producing greater stem diameters with the widest spacing (2 × 1 m) produced the largest trees in every years, while the narrow spacing decreased stem diameter. At narrow spacing (≤1 × 0.5 m), diameter growth was suppressed at an early age. The stems were relatively bigger at the widest spacing, presumably because of the higher availability of soil, moisture and light resources [10]. The small-scale power plant of wood biomass in Thailand is mainly accomplished by using both log wood and wood chip.

The minimum size of log wood required for biomass gasification system is 2.5 cm [15]. Therefore, the stem diameter from the wider spacings of 1 × 1, 1 × 1.5, 2 × 0.5 and 2 × 1 m can be used after 6 months in the 1^st year whereas the narrow spacings of 1 × 0.25 and 1 × 0.5 m required 12 months to gain a 2.5 cm of stem diameter. Similar results were also found in the 2^nd and 3^rd years (Table 2). However, stem diameters of all spacings at 6 months in the 4^th year were smaller than those in the previous years, because plants received only 350 mm of rainfall during the 6 months after cutting in the 4^th year compared to the 2^nd and 3^rd years (573 and 547 mm, respectively). Studies on other species have also reported an increase in density resulted in the decrease in stem diameter of trees such as eucalyptus [16] and poplar [17].

The sprout growing from the different spacings showed variations in their regrowth. The sprout number increased with the reduction in stand density and there were significantly differences in large-sized stumps. With small-sized stumps of narrow spacings could cause a decrease in sprouting from such stumps [10], because small area of stump resulted in low sprout number. The high sprout number of wide spacing resulted in no significantly differences in biomass yield in the 3^rd and 4^th years.

It is clearly that the spacing had an influence on the dry weight of woody stem yield. The spacing had distinctly difference dry weight of woody stem yield in the 1^st year, when compared to the 2^nd, 3^rd and 4^th years. The results showed that better performance contributed to a greater woody stem yield in the narrow spacings of 1 × 0.25 and 1 × 0.5 m as compared to that of the widest spacing of 2 × 1 m in the 1^st year. In accordance with Proe et al. [18] stated that maximum yields are generally achieved early in dense plantations whereas the widest spacing may be suitable for harvesting cycle over one year in order to get high yield. However, compare to the 1^st year, the widest spacing of 2 × 1 m increased the dry weight of woody stem more 60%, 91% and 58% and the woody stem of the narrowest spacing (1 × 0.25 m) increased 25%, 44% and 7% in the 2^nd, 3^rd and 4^th year, respectively. The widest spacing of 2 × 1 m tended to produce the high woody stem yield and biomass yield in the last-three years. Similar results were also found in Mitchell [19] and Proe et al. [18], which reported that the wide spacing exhibited the highest biomass yield in long-term. Most of the spacings produced the higher dry weight of woody stem yield in the last-three years when compared with the 1^st year, though the 4^th year tended to decrease dry weight of woody stem yield. The abnormal distribution of precipitation resulted in lower woody stem yield and high leaf yield in the 4^th year (Table 1).

Even though the maximum dry weight of biomass yield was in the narrowest spacing of 1 × 0.25 m, but the seed and planting costs of narrowest spacing were higher than widest spacing (8 times of widest spacing) in the leucaena plantation whereas the biomass yield was not significantly different between narrowest and widest spacing in the last-three years. For these reasons, the wider spacing would be more desirable, because it reduced a number of seeds and allows easier mechanized weed cultivation in terms of weed control [20]. However, the narrowest spacing is likely to decrease weed invasion by increasing the competitiveness of crop with weeds. Moreover, narrowest spacing resulted in higher leaf yield, but lower individual tree yields. The maximum leaf yield achieved at the narrowest spacing of 1 × 0.25 m throughout the experiment of period. In the present study, the leucaena is grown primarily for woody stem in biomass power plant, but the leucaena farmers can benefit from leaf yield, which could be a good additional income for them.

The results of ultimate analysis showed that the overall levels (as percent dry weight) of the five main elements were suitable for being used as fuel wood when compared to other biomass sources [21], The content of C, H and O are the main components of biomass fuel [22]. Leucaena wood tended to contain higher of oxygen content compared with carbon and hydrogen which reduced the energy value [12]. Jenkins et al. [23] reported that carbon content can also be correlated with heating values, with each 1% increase in carbon elevating the heating value by approximately 0.39 MJ/kg. The NO_x is one of the main environmental impacts of solid biofuels combustion, which NO_x emission increased with increasing N content above 0.6% [24]. Leucaena wood has higher nitrogen content than other plant varieties in many reports [11,12,21,24] because it can fix nitrogen from atmosphere. However, the N content of woody stem of leucaena planted at different spacings did not exceed the guiding value (0.6%) [24]. Generally the main elemental constituents of wood biomass are K, Ca and Mg, with smaller amounts of S and P. High amounts of S can cause problems regarding the emissions of SO_x [24]. The S content of wood biomass forms mainly gaseous SO₂. However, the S content of woody stem of leucaena planted at different spacings did not exceed the critical value (0.2%) according to Obernberger et al. [24]. Furthermore, the emission of SO₂ is usually not significant for wood combustion due to their low concentrations of S content [12]. Moreover, high amounts of S can cause problems regarding deposit formation and corrosion. Together with Cl and K play a role in corrosion mechanisms [24]. An increased K content more than 7% leads to a decreased ash melting point, which can cause slag and hard deposit formation in the furnace and boiler [22]. The K content of the woody stem of leucaena planted at all spacings did not exceed the critical value (above 7%), but tended to increase at the wide spacing. Obernberger et al. [24] reported that PCDD/F (dioxins) emission related problems were to be expected for materials with Cl content above 0.3%. The Cl content of woody stem of leucaena planted at all spacings cannot cause the PCDD/ F (dioxins) emissions because Cl content did not exceed the guiding value of 0.3% according to Obernberger et al. [24]. However, not only Cl content is necessary for PCDD/F formation, but also carbon, oxygen and catalysts (Cu) [24]. In case of ash melting point, Obernberger et al. [24] stated that Ca and Mg usually increase the ash melting point, while K decreases it. The content of Ca increases the ash melting point ranging from 15% to 35% [22], while woody stem of leucaena planted at all spacings contained 0.30% - 0.35% Ca and 0.04% - 0.05% Mg. The P, K, Ca and Mg are identified as the most abundant ash-forming elements in biomass, all of which are essential plant nutrients and can be used in agriculture as fertilizers. None of the spacings produced the ash content in the woody stem more than 5%. McKendry [25] stated that the ash content above 5% resulted in the oxidation temperature to be above the melting point of the biomass ash, leading to the clinkering problems in the hearth and subsequent feed blockages. Thus, the ash content from biomass produced at all spacings should not have any adverse clinkering affect on gasification system. The heating values produced in the present study similar to those reported by other researchers [6,26]. All of the spacings produced higher the heating values than the guiding value (3.35 kcal∙g⁻¹) [27].

The spacing for maximum yield in the first four-year for leucaena plantation should not exceed 1 × 0.5 m. It is clear from the present study that leucaena can be harvested when planted at a narrow spacing (1 × 0.25 and 1 × 0.5 m) in the relatively short period of four years. Moreover, they produced not only high stem yield, but also high leaf yield compared to the wider spacings. The chemical composition and heating value, resulted of the present study were similar to other researches so that biomass harvested from all spacings can be used as fuel crop. Therefore, these results when applied to biomass and energy production will enable leucaena to be evaluated as a renewable energy.

This research was partially supported by PTT Research and Technology Institute, PTT Public Company Limited and National Research Council of Thailand.