Leaching Characteristics of Inorganic Constituents from Oil Palm

Jul 2, 2014 - Bijal Gudka , Jenny M. Jones , Amanda R. Lea-Langton , Alan Williams , Abby Saddawi. Journal of the Energy Institute 2016 89 (2), 159-17...
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Leaching Characteristics of Inorganic Constituents from Oil Palm Residues by Water Pak Yiu Lam,† C. Jim Lim,† Shahab Sokhansanj,†,‡ Pak Sui Lam,*,† James D. Stephen,§,∥ Amadeus Pribowo,∥,⊥ and Warren E. Mabee§,∥ †

Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada ‡ Environmental Science Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6422, United States § Queen’s Institute for Energy and Environmental Policy, Queen’s University, 138 Union Street, Robert Sutherland Hall, Kingston, ON K7L 3N6, Canada ⊥ Department of Wood Science, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada ∥ TorchLight Bioresources Inc., 1901-5000 Yonge Street, Toronto, ON M2N 7E9, Canada ABSTRACT: Oil palm residues are not currently suitable as feedstock for thermal energy generation because their high ash content can cause slagging, corrosion, and fouling. A water leaching treatment is a potential strategy to reduce the ash content in these residues. This study evaluates the effects of the duration and temperature of water leaching on two types of oil palm residues, namely, empty fruit bunches (EFBs) and palm kernel shells (PKSs). The optimum process duration for ash removal from EFBs was found to be 5 min, as the effect of convection on scrubbing was observed to remove substantial ash from the substrate during this period. A cross-flow model with estimated kinetic parameters of water leaching for EFB and PKS was developed and showed that three leaching stages of EFB achieved the greatest ash reduction from 5.47% to 2.63%. A low ash content of PKS showed no value for ash removal in any leaching process. Although there was no significance in the total ash reduction due to temperature effects, the leaching treatment was found to be most effective in reducing potassium, from 2.42% to 0.69% and 0.36% at 25 and 55 °C, respectively.

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INTRODUCTION Empty fruit bunchs (EFBs) and palm kernel shells (PKSs), byproducts of palm oil production, are considered to be potential feedstocks for biopower generation. The high ash contents of EFBs and PKSs can create significant challenges during combustion, however, as ash and its inorganic constituents cause corrosion, slagging, and fouling in the furnace, necessitating costly furnace repairs and excessive maintenance.1−3 Fouling occurs when the inorganic constituents form ash deposit on the surfaces of the exhaust ducts of the furnace. This leads to a reduction in the heat exchange efficiency and an increased incidence of furnace process control difficulties (e.g., pressure balance). Slagging is the formation of a melted deposit glass layer rich in Fe2O3 and K2O;4 agglomeration of these compounds leads to an increase in the thickness of the glass layer, and interaction of the deposit layers with the metal surfaces within the furnace can accelerate corrosion. A potassium compound, such as potassium chloride (KCl) in EFBs, is reported to evolve into gas-phase condensate and can deposit on low-temperature surfaces of the heat exchanger, causing slagging.5 This retards the heat transfer between the heat exchanger and also corrodes the surface.6 In addition, the suspension of the fly ash in the flue gas stream reduces the convective heat transfer to the heating surface, thereby decreasing the combustion efficiency.7 The removal of the inorganic constituents of EFB and PKS materials is one potential approach to improving the attractiveness of these feedstocks for combustion and power © 2014 American Chemical Society

generation. Water leaching is an effective pretreatment for reducing the inorganic constituents of biomass.8−10 The effectiveness of the leaching process relies on material characteristics, the particle size distribution, and the treatment conditions, such as biomass-to-water ratio on a mass basis, temperature, and leaching duration.11 Although the leaching kinetics for the removal of soluble ash from EFB and PKS have not previously been reported, a number of kinetic models have been developed that describe the removal of organics and inorganics from biomass. Conner et al.12 developed a parallel first-order kinetic model for xylan removal from hardwood. Goto et al.13 developed a parallel firstorder kinetic model for lignin removal from white fir sapwood using supercritical tert-butyl alcohol. Ho et al.14 employed the same approach in developing a second-order kinetic model for the removal of water-soluble compounds from Tilia sapwood. Liaw and Wu15 showed that first-order leaching kinetics of metal and carbon removal from mallee leaf and wood by water is not applicable to a batch system, as a batch approach requires two steps: a rapid leaching step for an initial short period followed by a slow leaching step for a relatively long period. Overall, second- (or higher-) order kinetic models seem to be Received: Revised: Accepted: Published: 11822

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filtrate and the residue was used to determine the total drymass recovery. The weight measurement was carried on an ALC-80.4 analytical balance (Acculab, Edgewood, NY) with 0.0001-g precision. The moisture content was determined by the weight difference before and after drying of the filter paper containing the wet biomass in a precision oven at 105 °C for 24 h. Ash Content. The ash contents of the samples were determined using the NREL biomass chemical composition analysis.17 Approximately 0.5 g of the fully dried sample was placed in a porcelain crucible inside a muffle furnace (Blue M Electric Company, Blue Island, Il). The temperature of the furnace was increased from the ambient temperature to 250 °C for 30 min and then to 575 °C for 3 h. The samples were cooled to 105 °C for 8 h until the samples were collected. The cooled samples were covered, removed from the furnace, and cooled to room temperature inside a glass desiccator. The mass retained in the crucible was expressed as percent ash content. The ash content measurements were repeated three times for each sample. Ash Compositional Analysis: Inductively Coupled Plasma (ICP) Spectrometry. The metal contents of the untreated and treated biomass materials were determined by inductively coupled plasma spectrometry (Thermo Scientific iCAP 6500 Radial View ICP-OES Spectrometer, Loughborough, Leicestershire, U.K.). For each analysis, 25 g of biomass material or 40 mL of leachate was sent to ALS Laboratory Inc., Vancouver, Canada. Preliminary material treatment required acid digestion of biomass material inside either a hot block or an oven.18 The sample was acidified at the time of collection with nitric acid. After the acid pretreatment, the sample was loaded into an inductively coupled plasmaoptical emission spectrometer.19 The sample was heated with acid and substantially reduced in volume. The digestate was filtered and diluted to the volume required for metal analysis. The results yielded the metal contents of alkali metals, alkaline earth metals, and transition metals based on the mass fractions of the samples. Kinetic Model Fittings. The leaching behaviors of certain materials (e.g., organic or inorganic constituents) from a substrate can be studied using the described methodology. The leaching rate can be obtained by fitting the experimental data with kinetic models. The governing model parameters generally include the initial concentration of leachate, the leaching rate constant (k), and the maximum possible concentration of metal in the solution (Csat). The leaching capacity is also defined as the concentration of water-soluble compounds (i.e., watersoluble organics or water-soluble ash) at saturation. An overall mass balance has been proposed for the leaching of EFB or PKS, as follows

critical for the design of a batch system for the removal of organic and inorganic constituents. The resorption of the soluble ash retained on wet biomass after leaching can lead to inefficient removal of ash in a singlestage industrial batch leaching system, which has led some researchers to investigate multiple-stage leaching. Jenkins et al.8 reported that 90−91% of potassium and chlorine and 45% of ash were removed after bana grass was leached with water in multiple stages. Similar performance was observed with leaching of wheat straw and rice straw. A substantial reduction of soluble metal salts following water leaching of pine barks and switchgrass was also reported.11 At the current time, a mass balance model of multiple-stage leaching has not been developed. Given the positive results reported for multiplestage leaching of other feedstocks, this approach was deemed attractive for the investigation of the ash content reduction of EFB and PKS. Development of a mass balance model was identified as a crucial component of the investigation. The specific objective of this research was to study the effects of leaching conditions, including leaching duration, water temperature, and application of multiple-stage leaching, on ash and metal removal from EFB and PKS. The results of this research could have significant implications for the use of EFB and PKS as biomass feedstocks for heat and electricity generation.

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MATERIALS AND METHODS Materials. The empty fruit bunch (EFB) and palm kernel shell (PKS) used in this research were obtained from the Palm Oil Industrial Cluster (POIC) Sabah Sdn Bhd in Lahad Datu, Malaysia, and received with approximately 25% moisture content (wet basis). The samples were kept in a cold room around 4 °C before being used for experiments. Sample Preparation. The as-received EFB and PKS were conditioned to a moisture content of 15% ± 1% (wet basis) at a drying temperature of 50 °C in a convection oven. The materials were further processed into ground particles with a Retsch SM100 model grinder (Retsch Inc., Newtown, PA) with a 4-mm screen size. The particle size analysis was performed according to standard method ASABE S319.3.16 The experimental setup was a Ro-Tap sieve shaker (Tyler Industrial Products, Mentor, OH). Exactly 20 g of the ground sample was placed on top of the stack of sieves from the smallest to the largest mesh number. The mesh numbers of sieves for the particle size distribution were 7, 10, 14, 18, 25, 35, 45, 60, 80, and 100. The nominal sieve openings corresponding to the mesh numbers were 4, 2, 1.41, 1, 0.707, 0.5, 0.354, 0.25, 0.177, and 0.149 mm, respectively. The sieving duration was 5 min. Water Leaching. The leaching durations investigated in this study were 1, 3, 5, 10, 30, 60, and 120 min, and the water temperatures were 25, 40, and 55 °C. To initiate each experiment, 5.0 g of biomass was loaded into a beaker and mixed uniformly with 100.0 g of distilled water on a hot-plate stirrer (IKA Works, Inc., Wilmington, NC). The stirrer provided constant agitation at 360 rpm to ensure thorough mixing of the soluble ash and metal compositions in the leachate. The ratio of wet biomass to distilled water during leaching was 1:20. Following leaching, the mixture was poured into a filtration unit (i.e., a glass funnel) connected to the vacuum pump. Filter paper was placed on top of the funnel so that the solid material could be recovered from the filter paper. The sum of both the

EFB/PKS (s) + distilled water (l) → water‐soluble organics (aq) + water‐soluble ash (aq) + leached EFB/PKS (s)

(1)

Specifically, the ash balance in ash removal is presented as total ash (s) → water‐soluble ash (aq) + insoluble ash (s) (2)

water‐soluble ash (aq) → water‐soluble ash in wet biomass (aq) + water‐soluble ash in leachate (aq) 11823

(3)

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For the batch experiments, the input mass ratio of the soluble ash in the distilled water was assumed to be yn = 0.0000, as fresh water was used as the input in every stage. The L/Vn ratio varies at different stages of leaching as a result of moisture uptake from the input of the distilled water.

where the total ash is the sum of water-soluble ash and insoluble ash originally existing in the unleached EFB or PKS. The water-soluble ash is the sum of the soluble ash that can be retained in the free water of the wet biomass after leaching and leached into the distilled water. The insoluble ash, a residual ash in the leached biomass, is the amount of ash not soluble in the free water of the wet biomass and the distilled water. The model assumes that the ash removal process is irreversible. The second-order kinetic differential equation of water-soluble ash is given by dCt = k(Csat − Ct )2 dt

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RESULTS AND DISCUSSION Insoluble and Soluble Ash. Figure 1 shows the ash contents of water-leached EFB and PKS from 0 to 120 min.

(4)

where k is the second-order leaching rate constant (L g−1 min−1); Csat is the leaching capacity (g L−1), which is the saturated concentration of water-soluble ash within the leachate; and Ct is the concentration of water-soluble ash (g L−1) within the leachate at any time t (min). The concentration of water-soluble ash was measured by the weight of dissolved dry matter within the volume of leachate. Given that the boundary conditions are from Ct = 0 at t = 0 to Ct = Ct at t = t, integration of the differential equation (eq 4) gives Ct =

Csat 2kt 1 + Csatkt

Figure 1. Ash contents of leached biomass from 0 to 120 min (n = 3).

(5)

which, in turn, is rearranged in a linear form as

t 1 t = + 2 Ct C kCsat sat

The mean particle sizes of EFB and PKS were found to be 0.31 and 0.72 mm, respectively. EFB leaching achieved a significant ash reduction from 5.47% to 3.91% at 1 min. After 1 min, further statistically significant reductions of the ash content were not observed in EFB. In PKS leaching, the reduction of the ash content was statistically insignificant throughout the leaching duration from 0 to 120 min. This indicates that the leaching equilibrium occurs within a very short leaching time of less than 1 min for the small amounts of materials tested in this research. Future work will focus on understanding the leaching kinetics using a continuous-moving-bed reactor with a larger flow of biomass. Compared to the ash reductions of EFB, the negligible reductions and the low ash content in PKS suggest that the leaching process might provide little benefit and not be necessary. Table 1 lists the amounts of insoluble and soluble ash in the wet biomass and the leachate during the first 5 min. The soluble ash in the wet EFB was around 10% of the insoluble ash of the input EFB. An ash reduction of approximately 50% from the input EFB was observed at 5 min, with a decrease of insoluble ash from 0.2423 to 0.1201 g. The insignificant amount of soluble ash in the wet PKS was due to the low water uptake capacity. An ash reduction of approximately 25% from the input PKS was observed at 1 min with a decrease from 0.0606 to 0.0455 g. After 1 min, the amount of insoluble ash did not vary substantially. This suggests that a long leaching duration does not improve ash removal from PKS. Leaching Kinetics. Figure 2 presents the concentration of soluble ash (Ct) as a function of leaching duration for EFB and PKS at 25 °C. The short leaching durations confirmed the assumption that the leaching process is irreversible. The concentrations (Ct) of soluble ash from EFB and PKS were calculated from the given amounts of leachate and soluble ash in Table 1. The concentration of soluble ash in EFB leachate increased steadily during the first 5 min from 0.94 to 1.21 g L−1 before saturation. It was observed that the concentration of

(6)

By taking the reciprocal of eq 6, the leaching rate, Ct/t, can be obtained as Ct = t

1 1 kCsat 2

+

t Csat

(7)

When t approaches zero, the initial leaching rate (g L−1 min−1), h, can be computed as h = lim

t→0

Ct = kCsat 2 t

(8)

Multiple-Stage Design: Cross-Flow Model. A cross-flow model was introduced to design a unit operation process of ash removal from biomass by multiple-stage leaching. Earle20 developed the following mass balance of ash removal for the leaching process yn + 1 =

L (xn − xn + 1) + yn Vn

(9)

where yn is the input mass ratio of the soluble ash in the leachate (g g−1), yn+1 is the output mass ratio of the soluble ash in the leachate (g g−1), xn is the input mass ratio of the soluble and insoluble ash in the dry biomass (g g−1), and xn+1 is the output mass ratio of the soluble and insoluble ash in the dry biomass (g g−1). The xn+1 value was updated to the xn value in the subsequent stage, so that the wet biomass with the ash reduction from the previous stage could be used for leaching in the next stage. L is the flow rate of the dry biomass (g min−1), and Vn is the output flow rate (g min−1) of the leachate. Thus, L/Vn is the ratio of the flow rate of the dry biomass (g min−1) to the output flow rate of the leachate (g min−1). 11824

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Table 1. Insoluble and Water-Soluble Ash in the Wet Biomass and Leachate during the First 5 min duration (min)

leachate (L) insoluble ash in wet biomass (g) water-soluble ash in wet biomass (g) water-soluble ash in leachate (g) leachate (L) insoluble ash in wet biomass (g) water-soluble ash in wet biomass (g) water-soluble ash in leachate (g) a

0

1

3

5

EFB − 0.2423a −

0.0827 0.1477 0.017

0.0791 0.1218 0.026

0.0813 0.1201 0.0238

− PKS − 0.0606a −

0.0776

0.0945

0.0984

0.0965 0.0455 0.0006

0.0962 0.0507 0.0005

0.0962 0.0461 0.0007

−

0.0145

0.0094

0.0138

Figure 3. Second-order leaching kinetics of the concentration of soluble ash of EFB and PKS.

Total ash in biomass.

L−1 min−1, 16.35 L g−1 min−1, and 0.15 g L−1, respectively. The linearization of the leaching models for EFB and PKS were well fitted with the initial 5-min leaching time, with R2 values of 0.999 and 0.861, respectively. Compared to PKS, a substantial amount of ash was exposed on the surface of EFB particles because of the smaller particle size, which explains why the initial leaching rate (h) of EFB was much greater than that of PKS. Because the leaching time of EFB required to remove a substantial amount of ash was longer than that of PKS, the leaching rate constant (k) of EFB was smaller than PKS. PKS showed a lower leaching capacity at saturation (Csat), so PKS was expected to achieve saturation within a short time. Multiple-Stage Leaching. Four stages of leaching, with each stage lasting 5 min, were observed using EFB and PKS at room temperature. Table 2 lists different parameters for ash removal in the cross-flow balance model of EFB and PKS at each stage. The input ash content of EFB decreased from 5.47% to 2.63% through the first three stages of leaching; the input ash content of PKS decreased from 1.35% to 1.15% in a single leaching stage. Significant ash reductions were not observed after stage 3 for EFB and after stage 1 for PKS. The L/Vn ratio decreased after the first stage because the biomass retained some of the leachate and carried it to the subsequent stage for leaching. The yn+1 values of EFB were almost the same in stages 2 and 3, which suggests that the majority of the ash was removed in stage 1. The ash reduction in EFB during stage 1 was the greatest, suggesting that the effect of convection on EFB is dominant in removing ash. For stages 2 and 3 of EFB leaching, the effect of diffusion on EFB is dominant. The success of single-stage leaching with PKS suggested that the effect of convection on PKS is dominant in removing ash. Therefore, it is reasonable to hypothesize that a majority of the ash in PKS is exposed on the particle surfaces for leaching. Multiple-stage leaching completely removed the soluble ash retained on the wet biomass. It should be noted that a substantial amount of the distilled water could be reduced by other water recycling streams, which could reduce the usage of fresh water throughout the process. It is likely that the most efficient leaching method would be multiple-stage leaching of small-sized particles. Liu and Bi11 suggested that leaching smallparticle-size biomass achieves greater ash reductions. A greater surface area allows for more effective ash removal due to scrubbing.

Figure 2. Concentration of soluble ash versus time of EFB and PKS leaching.

soluble ash in PKS leachate saturated in the solution between 0.10 and 0.15 g L−1 after a slight increase from 0 to 1 min. The trends leveled off after 3 and 1 min for EFB and PKS, respectively. Ho et al.14 stated that primarily two phenomena occur during the leaching of sapwood with lime: convection due to scrubbing in the early leaching stage and diffusion relating to the remainder of the solution in the later leaching stage. In reality, convection and diffusion take place at the same time throughout the leaching process. Before leveling off, the effect of convection on ash migration from the surface of the biomass particles to the leachate by scrubbing is dominant. After leveling off, the concentrations increase with lower leaching rates. This indicates that the effect of diffusion on ash migration from biomass to leachate is dominant. At this point, the amount of soluble ash within the leachate directly affects the available amount of the soluble ash to be leached from the wet biomass. The models suggest that the saturation capacities of both EFB and PKS are reached within 5 min. Figure 3 shows the linear curve fittings of the second-order leaching model (eq 6) to the Ct data of EFB and PKS throughout 5 min. The intercepts and slopes of the curves can be used to estimate h, k, and Csat. The h, k, and Csat values for EFB were estimated to be 3.7 g L−1 min−1, 2.17 L g−1 min−1, and 1.31 g L−1, respectively. The h, k, and Csat values for PKS were estimated to be 0.36 g 11825

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Table 2. Parameters for Ash Removal in the Mass Balance Model of EFB and PKS at Four Different Stages input ash contenta (%)

L/Vn

xn

yn

xn+1

yn+1

0.0579 0.0367 0.0317 0.0270

0.0000 0.0000 0.0000 0.0000

0.0367 0.0317 0.0270 −

0.0011 0.0002 0.0002 −

0.0137 0.0116 0.0122 0.0122

0.0000 0.0000 0.0000 0.0000

0.0116 − − −

0.0001 − − −

EFB stage stage stage stage

1 2 3 4

5.47 3.54 3.07 2.63

± ± ± ±

0.08 0.24 0.19 0.33

0.0500 0.0407 0.0407 −

stage stage stage stage

1 2 3 4

1.35 1.15 1.21 1.21

± ± ± ±

0.23 0.34 0.09 0.09

0.0500 − − −

PKS

a

Number of measurements: n = 3.

Temperature Effect. Figure 4 shows the ash contents of EFB leached at temperatures ranging from 25 to 55 °C for 5

Aluminum, iron, and sodium were negligible constituents, at less than 0.1% each. The dry ground PKS material had much lower metal contents than EFB; in PKS, the potassium content was determined to be 0.233% (approximately one-tenth of the EFB level). The contents of other metals in PKS were lower than 0.05%, which supported the earlier results that PKS had a lower ash content. Figures 5 and 6 depict the metal contents in dry EFB and PKS, respectively, after water leaching. At 25 °C, EFB and PKS

Figure 4. Ash contents of leached EFB from 25 to 55 °C for a 5-min leaching duration (n = 3).

min. Given that the ash reductions for PKS were not as significant as those for EFB, the effect of temperature on leaching was studied only for EFB. The ash content of EFB ranged between 3.54% and 3.75%, and it was found that changes in temperature had no effect on total ash reduction within the first 5 min (Figure 4). Table 3 presents the metal contents of dry EFB and PKS before water leaching. The dry ground EFB material was found to have a high potassium content of 2.42%; calcium and magnesium accounted for 0.314% and 0.171%, respectively.

Figure 5. Metal contents of leached EFB particles from 25 to 55 °C for a 5-min leaching duration.

showed reductions in all metal contents. A process temperature rise from 25 to 55 °C did not elicit any significant change in the EFB leaching results for aluminum, iron, or magnesium. However, raising the process temperature from 25 to 40 °C resulted in a significant reduction in EFB leaching efficacy for calcium and potassium content. In addition, sodium in EFB was almost completely removed at higher temperature. A possible reason is that a majority of sodium is exposed on the ground particle surface, which allows sodium to be readily washed away when encountering leaching fluid. PKS showed similar trends in the removal of aluminum, calcium, potassium, and sodium, but not iron and magnesium. Overall, it was observed that the metal contents reached the greatest reductions in EFB and PKS at temperatures greater than 40 °C. Although increasing the leaching temperature did not result in a reduction in overall ash content, it did cause a decrease in metal content. This reduction in metal content is valuable because it would have a positive

Table 3. Metal Contents of EFB and PKS Particles before Water Leaching EFBa

PKSb

metal

mass ratio (mg/kg)

amount (mg)

metal content (%)

mass ratio (mg/kg)

amount (mg)

metal content (%)

Al Ca Fe K Mg Na

88 3140 170 24200 1710 120

0.39 13.91 0.75 107.18 7.57 0.53

0.009 0.314 0.017 2.420 0.171 0.012

50 349 132 2330 341 100

0.22 1.57 0.59 10.47 1.53 0.45

0.005 0.035 0.013 0.233 0.034 0.010

a

Dry biomass weight (dry basis) = 4.427 g. bDry biomass weight (dry basis) = 4.495 g. 11826

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(2) Pronobis, M. Evaluation of the influence of biomass cocombustion on boiler furnace slagging by means of fusibility correlations. Biomass Bioenergy 2005, 28 (4), 375−383. (3) Nutalapati, D.; Gupta, R.; Moghtaderi, B.; Wall, T. F. Assessing slagging and fouling during biomass combustion: A thermodynamic approach allowing for alkali/ash reactions. Fuel Process. Technol. 2007, 88 (11−12), 1044−1052. (4) Kostakis, G. Mineralogical composition of boiler fouling and slagging deposits and their relation to fly ashes: The case of Kardia power plant. J. Hazard. Mater. 2011, 185 (2−3), 1012−1018. (5) Konsomboon, S.; Pipatmanomai, S.; Madhiyanon, T.; Tia, S. Effect of kaolin addition on ash characteristics of palm empty fruit bunch (EFB) upon combustion. Appl. Energy 2011, 88 (1), 298−305. (6) Madhiyanon, T.; Sathitruangsak, P.; Sungworagarn, S.; Pipatmanomai, S.; Tia, S. A pilot-scale investigation of ash and deposition formation during oil-palm empty-fruit-bunch (EFB) combustion. Fuel Process. Technol. 2012, 96, 250−264. (7) Inorganic Transformations and Ash Deposition during Combustion: Proceedings of the Engineering Foundation Conference on Inorganic Transformations and Ash Deposition during Combustion; Benson, S. A., Ed.; American Society of Mechanical Engineers: New York, 1992. (8) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. On the properties of washed straw. Biomass Bioenergy 1996, 10 (4), 177−200. (9) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M. Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching. Biomass Bioenergy 1997, 12 (4), 241−252. (10) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Agglomeration problems during fluidized bed gasification of olive-oil residue: Evaluation of fractionation and leaching as pre-treatments. Fuel 2003, 82 (10), 1261−1270. (11) Liu, X.; Bi, X. T. Removal of inorganic constituents from pine barks and switchgrass. Fuel Process. Technol. 2011, 92 (7), 1273−1279. (12) Conner, A. H.; Libkie, K.; Springer, E. L. Kinetic modeling of hardwood prehydrolysis. Part II. Xylan removal by dilute hydrochloric acid prehydrolysis. Wood Fiber Sci. 1985, 17 (4), 540−548. (13) Goto, M.; Smith, J. M.; McCoy, B. J. Kinetics and mass transfer for supercritical fluid extraction of wood. Ind. Eng. Chem. Res. 1990, 29 (2), 282−289. (14) Ho, Y. S.; Harouna-Oumarou, H. A.; Fauduet, H.; Porte, C. Kinetics and model building of leaching of water-soluble compounds of Tilia sapwood. Sep. Purif. Technol. 2005, 45 (3), 169−173. (15) Liaw, S.b.; Wu, H. Leaching Characteristics of Organic and Inorganic Matter from Biomass by Water: Differences between Batch and Semi-continuous Operations. Ind. Eng. Chem. Res. 2013, 52 (11), 4280−4289. (16) ASABE Standards S319.3: Method of Determining and Expressing Fineness of Feed Materials by Sieving; American Society of Agricultural and Biological Engineers (ASABE): St. Joseph, MI, 2006. (17) Determination of Ash in Biomass; Report NREL/TP-510-42622; National Renewable Energy Laboratory (NREL): Golden, CO, 2008. (18) Acid Digestion of Waters for Total Recoverable or Dissolved Metals for Analysis by FLAA or ICP Spectroscopy; Method 3005A; U.S. Environmental Protection Agency: Research Triangle Park, NC, 1992. (19) Inductively Coupled Plasma-Atomic Emission Spectrometry; Method 6010B; U.S. Environmental Protection Agency: Research Triangle Park, NC, 1996. (20) Unit Operations in Food Processing. Earle, R. L., Eds.; The New Zealand Institute of Food Science & Technology Inc.: Palmerstown North, New Zealand, 1983.

Figure 6. Metal contents of leached PKS particles from 25 to 55 °C for a 5-min leaching duration.

influence on feedstock performance in a combustor, including reduced corrosion, slagging, and fouling.

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CONCLUSIONS It is evident that water leaching is an effective process for reducing the ash and metal contents of EFB. There is little benefit to extending leaching beyond 5 min, and a temperature higher than room temperature is recommended only if metal reduction is a priority. High temperature has no significant effect on total ash removal, but it does have an impact on removing specific metal components of ash in EFB (e.g., potassium). Water leaching is not effective in ash removal from PKS. The combustion performances of the leached EFB and PKS, including efficiency, corrosion, fouling, and slagging, will be evaluated in our future research.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1-604-355-8811. Fax: +1-604-822-6003. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded in part by the Natural Sciences and Engineering Research Council of Canada (NSERC, ENGAGE Grant) and TorchLight Bioresources Inc., Toronto, Ontario. The authors thank the Palm Oil Industrial Cluster (POIC) Sabah Sdn Bhd of Lahad Datu, Malaysia, for providing material samples. The authors also acknowledge support from the Office of Biomass Program of the U.S. Department of Energy.

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ABBREVIATIONS EFB = empty fruit bunch ICP = inductively coupled plasma PKS = palm kernel shell

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REFERENCES

(1) Heinzel, T.; Siegle, V.; Spliethoff, H.; Hein, K. R. G. Investigation of slagging in pulverized fuel co-combustion of biomass and coal at a pilot-scale test facility. Fuel Process. Technol. 1998, 54 (1−3), 109−125. 11827

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