Introduction

Malaysia has been one of the largest producers and exporters of palm oil for the last 40 years. In 2010, Malaysia had approximately 4.85 million hectares of land under oil palm plantation [1]. The main products generated from oil palm are palm oils, palm kernel oil and palm kernel cake. The byproducts generated from palm oil milling are empty fruit bunch (EFB) and oil palm trunk (OPT), which have great potential to be used as biomass fuel for energy production. During replantation, 74 tonnes of dry OPT per hectare are generated. On the other hand, approximately 23 % of the EFB byproduct comes from the processing of fresh fruit bunch (FFB) in the palm oil mill. The consistency of the supply of biomass byproduct from the palm oil industry has made it an ideal source of raw materials for biomass fuel production, compared to wood wastes from furniture mills and sawmills. In palm oil mills, EFB can be easily separated out from a processing batch, whereas batches of wood wastes from other mills consist of various types of wood and extraneous material (often including significant amounts of bark, leaves, needles, dirt, rocks and traces of chemicals from adhesives and preservatives), which can cause substantial production inefficiencies and substandard lignocellulosic biomass with different higher heating values and chemical compounds. As a result, the use of wood industry waste as raw material for the production of biomass fuel is limited since strict standards for fuel characteristics exist.

Undoubtedly, oil palm biomass is a better choice compared to wood industry waste as a source for biomass fuel production. However, oil palm biomass is a byproduct whose availability largely depends on the primary production of palm oil industry. Hence, there is a need to look for other sustainable feedstock resources to support biomass fuel production. One of the most promising alternative biomass feedstock sources is fast growing energy crops. Ideally, these would allow us to grow our fuel, thus reducing the dependence on fossil fuel and our vulnerability to disruption in energy supply. These crops are fast growing trees harvested within five to eight years after planting; they generate logs with smaller diameters (<20 cm) compared to trees planted specifically for timber production, which requires a longer growth period. Since debarking these logs would be impractical due to their small size, bark is likely to be at least a minor component of these types of feedstocks. Thus, there is a need to investigate the chemical composition of these fast growing timber species with and without removing the bark.

If biomass fuel is intended to be used as a commercial fuel, the capacity of the biomass fuel crop supply to sustainably meet the demands from consumers and power stations is key. Hence, the industrial production of biomass has to be flexible by including different types of biomass. With the environmental constraints now being imposed, it is essential to achieve maximum utilization of all above-ground portions of each harvested tree, including the bark, as biomass fuel feedstock. Therefore, a detailed analysis of biomass with high feasibility and abundant availability is a crucial step in producing biomass fuel as a commercial fuel. The investigation of the higher heating value (HHV) and chemical elemental characteristics of biomass fuels is beneficial in helping to identify plant constituents that correlate with the biomass fuel properties used in modeling various energy conversion processes for optimum conversion efficiency. Researchers in several countries have carried out extensive research to determine the properties of their own available biomass resources. To our knowledge, little is currently known about the properties of biomass fuel resources available in Malaysia, and especially the effect of the inclusion of bark from the fast growing timber species on the overall chemical characteristics and calorific value of the biomass fuels. In this paper, chemical elemental characteristics of biomass fuels with high feasibility and abundant availability in Malaysia are presented. The categories of biomass analyzed in this study are oil palm biomass and fast growing timber species available in Malaysia. The objectives of this study are to identify the variability in biomass fuel properties of a range of oil palm biomass and fast growing timber species and investigate the chemical characteristics of fast growing timber species inclusive and exclusive of bark. Enumeration of such data will allow evaluation of the utilization potential of oil palm biomass and wood inclusive of bark made from fast growing timber species available in Malaysia.

Materials and Methods

Raw Materials

The oil palm biomass used in this study includes oil palm trunk (OPT) and empty fruit bunch (EFB). The selected fast growing timber species were Albizia falcataria, Acacia spp., Endospermum spp. and Macaranga spp., with trunk diameters ranging between 10 to 15 cm. Basic densities of OPT and the fast growing timber species were determined by the water displacement volume method. For the fast growing timbers, two types of each sample were prepared for further analysis; exclusive of bark (from xylem only) and inclusive of bark (from the mixture of xylem and bark). The wood-bark ratios were obtained by scraping off the bark from the logs, and then oven-drying and weighing the samples (wood and bark) to calculate the ratio in percentage. The different species of fast growing timbers, OPT and EFB were ground and sieved to obtain fine chips in the size of 40 mesh.

Determination of Chemical Composition

Chemical analysis was based on the TAPPI standard T 203 for determination of the main lignocellulosic composition. The extractives content, acid-insoluble materials, holocellulose content and alphacellulose content of the samples were determined. The extractives content was determined by the assessment of products soluble in alcohol and acetone. Acid-insoluble materials consisting of a majority of lignin, were determined by quantitative acid hydrolysis with 72 % H2SO4 according to the T-249 em-85. Holocellulose and alphacellulose contents were determined according to T 429 cm-10, Wise method [2]. The ash content was determined by burning (dry oxidation) the oven-dried sample (2 g) in a muffle furnace model at 575 ± 25 °C. This test method was utilized to determine the amount of ash, expressed as the mass percent of residue remaining after dry oxidation of biomass. All analyses were performed in triplicate and the means were presented.

Determination of Higher Heating Values (HHV)

The calorific value was determined according to BSI standard EN 14918 with a bomb calorimeter, in which about 0.5 g of oven-dried biomass was completely combusted under a pressurized (3000kPa) oxygen atmosphere.

Determination of Major Elements (Ash-Forming Elements) and Trace Elements

The chemical compositions of major elements such as Al, Si, Ca, Fe, K, Mg, Na, and P, and trace elements such as As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, Te, V, and Zn, were analyzed for all samples according to CEN/TS 15290. The digestion was carried out using 500 mg sample, 3 ml H2O2 (30 %), 8 ml HNO3 (65 %) and 1 ml HF (40 %) in a closed vessel at a temperature of 190 °C for 30 mins. After cooling, the sample was neutralized by H3BO3 (4 %) and again reheated for another 15 mins. After cooling, the digest was made up to 25 ml and analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES).

The absorbing solution was determined by ion chromatography. For S, Cl and F determination (according to CEN/TS 15408), one gram of sample was combusted in a 30 bar oxygen. After combustion, the gas containing S, Cl and F was collected in an absorbing solution and was analyzed with ion chromatography. All analyses were performed in triplicate and means were presented.

Results and Discussion

Proportion of Bark on the Stem and Basic Density

The proportion of bark and basic density of fast growing timber species and oil palm trunk are shown in Table 1. The relatively high proportion of bark in Macaranga spp. was almost twice the bark volume of Albizia falcataria. Differences in proportion of bark are attributable to bark thickness.

Table 1 Proportion of bark on the stem and basic density of lignocellulosic biomass
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Wood basic density is defined as the ratio of oven-dry weight to saturated volume. It is one of the most important properties of wood since it gives an indication of wood strength as well as pulp and energy yield. Albizia falcataria and Acacia spp. have the highest density compared to the other fast growing timber species and oil palm trunk.

Chemical Composition of Lignocellulosic Biomass

The chemical compositions of lignocellulosic biomass selected in this study are presented in percentage of wood oven-dry weight. Table 2 shows the average percentage of extractives (alcohol-acetone solubility), lignin, holocellulose and alphacellulose for oil palm biomass (oil palm trunk and empty fruit bunch) and fast growing timber species (Albizia falcataria, Acacia spp., Endospermum spp. and Macaranga spp.) inclusive and exclusive of bark.

Table 2 Average chemical composition of selected lignocellulosic biomass
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The results showed that about 90 % of the total content is holocellulose and lignin, while the remainders are extractives and minerals. The alcohol-acetone soluble content of wood consists of waxes, fats, resins and certain other insoluble components such as wood gums. In addition, the fast growing timber species inclusive of bark had slightly higher quantities of alcohol-acetone soluble content than samples exclusive of bark. Small differences in holocellulose and alphacellulose contents were found among species inclusive and exclusive of bark; samples exclusive of bark generally contained higher holocellulose and alphacellulose contents compared to samples inclusive of bark. Smaller differences also existed in lignin quantities among samples inclusive and exclusive of bark.

On the other hand, oil palm biomass (EFB and OPT) contained lower lignin quantities compared to the fast growing timber species. However, higher alcohol-acetone solubility contents were obtained from EFB (6.67 %) and OPT (6.78 %) compared to all the fast growing timber species inclusive or exclusive of bark (1.44–2.77 %).

The ash content of biomass reflects both the handling and processing costs of the overall biomass energy conversion cost. The results tabulated in Table 2 show that ash content of the biomass ranged from 0.50–5.96 %. Fast growing timber species have much lower ash contents compared with oil palm biomass. EFB has the highest ash content of 5.96 %. Samples of fast growing timber species inclusive of bark, except Albizia falcataria and Endospermum spp., contain higher ash contents compared to samples exclusive of bark. The ash content in fast growing timber species inclusive of bark and exclusive of bark ranged from 1.27–2.10 % and 0.50–1.22 %, respectively.

Higher Heating Values (HHV) of Lignocellulosic Biomass

The heating value is an indication of the amount of energy that a fuel contains. The gross heat of combustion derived from bomb calorimeter tests represents the heat generated by a fuel on complete combustion. The higher heating values (HHV) of the lignocellulosic biomass used in this study are tabulated in Table 3.

Table 3 Percentage contribution of extractives (alcohol-acetone solubility) to higher heating values (HHV)
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The results showed that the heating values of fast growing species inclusive of bark ranges from 4288 cal g-1 to 4383 cal g-1, while exclusive of bark ranges from 4144 cal g-1 to 4343 cal g-1. Among all the fast growing timber species tested, Endospermum spp. and Macaranga spp. have the highest heating values for samples inclusive and exclusive of bark. The low variability in heating values of biomass among the species tested reinforced the assumption of elemental similarity in the woods of different species. This was not the case among timber species inclusive and exclusive of bark for each individual. The inclusive of bark proportion in the biomass sample generally increased the heating value except for Macaranga spp., which demonstrated reduced heating values by 55 cal g-1. This reduction in heating values might be due to the higher ash content in the bark of Macaranga spp., since the ash does not contribute to the heat release of the biomass combustion. Indeed, Jenkins et al. [3] stated that high ash content does lower the HHV by virtue of not being a contributor to the fuel value. On the other hand, the inclusion of bark in Albizia falcataria, Acacia spp. and Endospermum spp. did contribute to additional heating values of 160 cal g-1, 127 cal g-1 and 52 cal g-1, respectively. Compared to the fast growing timber species, the oil palm biomass (EFB and OPT) used in this study had poorer heating values of 4315 cal g-1 and 4104 cal g-1, respectively. The difference in heating values obtained in various biomass fuel feedstocks might be due to the differences in their chemical composition, as shown in Table 3.

Influence of Extractives (Alcohol-acetone Solubility) on Higher Heating Values (HHV)

The average heating value of extractive-free and unextracted material, and the calculated contribution of the presence of extractives to HHV for different lignocellulosic materials (inclusive and exclusive of bark) are shown in Table 3. The contribution of extractives to the HHVs can be observed through the differences between the HHVs of extractive-free and unextracted samples.

Table 3 suggests that the presence of alcohol-acetone solubility content positively contributed to the HHV value of the lignocellulosic biomass. This is elucidated by the significantly lower HHV value obtained from the extracted materials as compared to the unextracted materials. Extractives from Albizia falcataria inclusive of bark contributed to the highest HHV of 6.28 %. Generally, samples inclusive of bark had a higher extractives content compared to samples exclusive of bark. The percentage of contribution to HHV by the extractives shows a similar trend. The alcohol-acetone solubility in the bark may have higher constituents that contributed to HHV than the wood exclusive of bark, and thus, the lignocellulosic samples inclusive of bark generally process at higher HHVs. However, the exception is Macaranga spp., for which the extractives of the sample exclusive of bark (1.17 %) gave a larger contribution to the HHV compared to sample inclusive of bark (0.54 %). These differences could be due to the different compounds in the extractives. The chemical components of biomass, except cellulose, have different chemical structures and compositions; consequently, the heating values of extractives may vary among different biomass species [4]. In general, the presence of extractives contributed to the HHV, but a higher extractives content did not inevitably result in a higher HHV.

Influence of Moisture Content on Heating Values (HHV)

The effect of moisture content from fast growing wood species and oil palm biomass was also investigated in this study. The impacts of moisture contents on heating values in samples of Endospermum spp. (inclusive of bark), Acacia spp. (exclusive of bark), EFB and OPT is illustrated in Figure 1. The results showed that the gradient of slopes for the four different biomass are similar, which indicate that the influence of moisture content on biomass fuels does not depend on species, and suggests that mechanisms of combustion of different biomass materials are similar. As moisture content increases, HHV decreases. The linear effect of increasing moisture content on heating values depicted in Figure 1 stresses the benefits of drying the fuel before combustion. By increasing the moisture content to 20 %, the HHV is reduced by approximately 900 cal g-1. Moisture has reduced the heat available from fuel by: (i) lowering the initial gross calorific value of the biomass; (ii) reducing the combustion efficiency since heat is absorbed through evaporation of water in the initial stages of combustion, which lowers both the flame temperature and the radiant heat transfer; and (iii) by the hydrolysis effect of hot water [5]. The gradient of slopes obtained from the plots in Figure 1 can be used to estimate the HHV of biomass solid fuel at various moisture contents using Equation 1 (Eq 1) if the HHV of biomass on a dry basis and the moisture content of the biomass are identified.

Fig. 1
figure 1

Effect of moisture content on the HHV of lignocellulosic biomass

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$$ {\text{Y}} = {\text{X}}-\left( {{44}.{3}*{\text{M}}} \right) $$
(1)
Y:

HHV of biomass at M moisture content (cal g-1)

X:

HHV of biomass on a dry basis (cal g-1)

M:

Moisture content (%)

Trace Elements

Table 4 shows some of the important trace elements found in biomass fuels. Most of these trace elements are heavy metals, and have a large influence on gas emission and ash composition characteristics [6]. Study results showed that these elements are much higher in fast growing timber species samples inclusive of bark than in sample exclusive of bark. Macaranga spp. inclusive of bark contained higher amounts of Cr, Cu, Mn, and Zn, while Endospermum spp. inclusive of bark contained higher levels of Ba, Ni, Pb, Se, and F. Ni was only found in samples of fast growing timber species inclusive of bark and Sb was detected in the Acacia spp. samples inclusive of bark. Among the fast growing timber species samples exclusive of bark, Endospermum spp. contained the highest levels of Ba, Pb and F, while Albizia falcataria contained the highest levels of Cr and Cu, and Macaranga spp. contained the most Mn and Zn. Compared to other wood biomass samples studied, EFB contained the highest amount of Cu and Zn, while OPT was the only biomass in this study detected with As. The content of Cd, Co, Mo, Sn, Te, V, Hg and Cl were not detected in any of the biomass samples investigated in this study.

Table 4 Characteristics of the biomass solid fuels - trace elements
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The amount of elements in the fast growing timber species ranked from high to low were approximately as follows: F>Mn>Zn>Ba>Cu>Pb>Ni>Cr in samples inclusive of bark, and F>Mn>Zn>Ba>Cu>Pb>Cr in samples exclusive of bark. For oil palm biomass samples, the arrangement of elements from high to low were: F>Mn>Ba>Cu>Zn>Cr in EFB and F>Mn>Zn>Cu>Cr>Ba>As in OPT.

Major Elements (Ash-forming Elements)

The ash-forming elements in biomass, such as Al, Si, Ca, Fe, K, Mg, Na and P, are especially important for any thermochemical conversion process. The ash-forming elements are closely related to operational problems such as slagging, fouling, sintering and corrosion. Elements like K, Si, Mg and Ca result in high alkali indices and a high tendency for fouling [7]. Slagging and fouling reduce heat transfer of combustor surfaces and cause corrosion and erosion problems which reduce the lifetime of the equipment. Table 5 shows some clear differences among the fuel data for oil palm biomass and fast growing timber species inclusive and exclusive of bark. For example, the concentrations of Si, Fe, K, Mg, Na and P in oil palm biomass are generally higher than in the fast growing timber species, accounting for most of the top three highest concentrations per biomass sample for each element. The amount of K in EFB, OPT and Endospermum spp. inclusive and exclusive of bark is much higher than other elements, while the samples of Albizia falcataria, Macaranga spp. and Acacia spp., regardless of whether the sample was inclusive or exclusive of bark, had the highest concentrations of Ca compared to other elements. The levels of Al were the lowest among other ash-forming elements for all the biomass types used in this study. Nevertheless, the oil palm biomass contained much higher amounts of Al than the other biomass samples. For the fast growing timber species, the concentration of the elements from high to low can be presented as the sequence: Ca>K>Mg>P>Fe>Na>Si>Al. The exceptional cases were that K in Endospermum spp. was higher than Ca, Si was higher than Na in Macaranga spp., and Na was higher than P and Fe in Acacia spp. In EFB and OPT, the concentration of elements from high to low can be presented as: K>Mg>Ca>P>Fe>Na>Si>Al and K>Ca>Mg>Na>P>Fe>Si>Al, respectively. The extreme properties of EFB with respect to high K content (12194 mg/kg) are a major obstacle for an efficient utilization of EFB as a fuel for power production.

Table 5 Characteristics of the biomass solid fuels – major elements (ash-forming elements)
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Comparison with Commercial Energy Crops Used in Europe on Fuel Characteristics

In Europe, the largest areas where solid biomass energy crops are grown are found in the UK (mainly willow, miscanthus), Sweden (willow, reed canary grass), Finland (reed canary grass) Germany (miscanthus, willow) Spain, and Italy (miscanthus, poplar) [8]. A comparison with commercial biomass solid fuel in Europe on fuel characteristic is shown in Table 6. The European energy crops generally possessed higher HHVs, ranging from 4491–4773, compared to the Malaysian biomass used in this study which had HHVs from 4104–4383. The European energy crops also contain higher levels of Cl and Si. During combustion, chlorine will form hydrochloric acid, which is mainly responsible for corrosion [6]. On the other hand, higher levels of K, Mg, Na and P were found in the Malaysian biomass samples used in this study compared to European energy crops. Alkali metals, mainly K and Na, are readily released in the gas phase during combustion and form alkali silicates that melt at low temperatures (lower than 700 °C) [9]. Therefore, ashes from oil palm biomass with relatively high K, Na and Si levels will start to sinter and melt at significantly lower temperatures than ashes from the European energy crops and fast growing timber species stated in this study.

Table 6 Fuel characteristics of energy crops in Europe [8, 10]
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Conclusions

The selection of desirable biomass sources for specific fuel applications must take into consideration the differences in fuel properties and their characteristics. The experimental data obtained from this study serves as a reference of biomass solid fuels available in Malaysia consisting of fast growing timber species and oil palm biomass (OPT and EFB). Generally, oil palm biomass contains much higher amounts of ash-forming elements and trace elements than most of the fast growing timber species (inclusive and exclusive of bark). In addition, the solid biofuel characteristics are varied among species and also between wood alone and wood inclusive of bark. The results also showed that there are differences in terms of the higher heating values (HHVs), trace element and ash-forming elements contents when sources inclusive and exclusive of bark are utilized. Fast growing timber species inclusive of bark have higher HHVs to some extent, except in the case of Macaranga spp. On the other hand, the European energy crops show higher HHVs, Cl and Si content but lower K, Mg, Na and P content compared to the local biomass used in this study. The data obtained from this study can serve as the fundamental aspects for the selection of suitable biomass to be applied as solid fuel, or as a reference on the fabrication of conversion systems for the selection of biomass solid fuel.