Article pubs.acs.org/EF
Quality Determination of Biomass for Combustion: A New HighThroughput Microwave Digestion Method Prior to Elemental Analysis by Inductively Coupled Plasma−Optical Emission Spectroscopy Na Liu,* Uffe Jørgensen, and Poul Erik Lærke Department of Agroecology, Aarhus University, Blichers Allé 20, Postboks 50, 8830 Tjele, Denmark ABSTRACT: A high content of minerals in biomass feedstock may cause fouling, slagging, and corrosion in the furnace during combustion. Here, a new pressurized microwave digestion method for biomass digestion prior to elemental analysis is presented. This high-throughput method is capable of processing approximately 48 samples per hour with reduced laboratory hazards and waste because only a very small volume of hydrofluoric acid is used. Concentrations of 12 elements (Al, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, P, Si, and Zn) were determined using inductively coupled plasma−optical emission spectroscopy (ICP−OES). National Institute of Standards and Technology (NIST) standard reference materials (SRMs) 1515 apple leaves and 1575a pine needles were used for method validation. Recoveries of the 12 elements in the NIST SRMs ranged from 93 to 115%. A comparison of the high-throughput method with the European Union (EU) standard method showed agreements in the range from 98 to 115% (except for Al) when the elements were quantified in plant material of the energy crop Miscanthus × giganteus.
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Tjele, Denmark. The miscanthus plants were cut at 10 cm from the ground and dried at 60 °C until a constant weight (approximately 72 h). Samples were coarse-ground to pass through a 6 mm sieve using a Retsch SM2000 cutting mill (Retsch GmbH, Haan, Germany) and then powdered through a 0.8 mm sieve using an 8-82-K water-cooled Tekemas Christy mill (Chelmsford, U.K.). Standard reference materials (SRMs), 1515 apple leaves and 1575a pine needles from the National Institute of Standards and Technology (NIST, Gaithersburg, MD), were used to evaluate the reliability and control the accuracy of this high-throughput analytical method. The miscanthus sample and SRMs were digested by the same procedure. Reagents and Wares. Reagents used for digestion were EMPURE ISO-grade 65% HNO3 (Merck KGaA, Darmstadt, Germany), AnalaR Normapur-grade 30% hydrogen peroxide (H2O2) (VWR-Prolabo, Herlev, Denmark), and Suprapur-grade 40% HF (VWR, Herlev, Denmark). Ultrapure deionized water (18.2 mΩ resistivity) from an Ultra Clear ultraviolet (UV) plus water system (SG, Barsbuttel, Germany) was used for dilution and cleaning. Plastic apparatus were used during the entire analysis process. Microwave Digestion. A commercial microwave digestion system Anton Paar Multiwave 3000 (Anton Paar GmbH, Graz, Austria), comprising a microwave oven with and a 48 vessel rotor, including a temperature−pressure combined sensor, was employed to conduct the digestion. Before digestion of samples, 3 mL of H2O2 and 1 mL of HNO3 were added in each vessel to run the cleaning program (setting parameters in Table 1 and multiwave graph in Figure 1a). The vessels were then rinsed by the ultrapure deionized water and dried in the oven. A mass of approximately 0.30−0.35 g of sample was mixed in each vessel with 3 mL of H2O2. The mixtures were sealed and placed overnight for oxidation. At the second day, 6 mL of 65% HNO3 was added and the vessels were immediately sealed with caps. The digestion program was run according to the settings in Table 1 (multiwave graph in Figure 1b). After cooling to room temperature, all vessels were transferred to the fume hood and slowly opened. A
INTRODUCTION Biomass for energy purposes is a potential candidate to mitigate the energy crisis and maintain environmental sustainability.1,2 An efficient way to use biomass as an energy source is combustion. However, biomass with high mineral content may cause serious problems, such as fouling, slagging, and corrosion, during the combustion process.2−5 High concentrations of Na and K usually lower the ash melting point, and Si may form silicates, resulting in ash agglomeration in the boilers.2,5 Heavy metals, such as Cd, should be below certain thresholds to avoid environmental pollution and health hazards.2 Traditional methods used for elemental analysis of plant materials usually require pretreatment with inorganic acids, including nitric acid (HNO3), hydrochloric acid (HCl), hydrofluoric acid (HF), and sulfuric acid (H2SO4), or alkalis, such as NaOH.6−10 Improper digestion procedures may result in element losses or heavy interferences in further analysis.10,11 In addition, traditional analytical procedures are time-consuming and, therefore, not suitable for large sample sets. Thus, there is a need to establish a rapid and reliable analysis for quantification of elements in various biomasses (especially with a high Si content, which usually cause difficulties in complete digestion and precise determination) to evaluate their quality for combustion. The objective of this study was to develop and describe a new highthroughput approach using microwave digestion with microvolumes of HF for pretreatment of biomass samples to quantify 12 elements (Al, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, P, Si, and Zn) by inductively coupled plasma−optical emission spectroscopy (ICP−OES) in the dedicated energy crop Miscanthus × giganteus.
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MATERIALS AND METHODS
Received: August 21, 2013 Revised: November 6, 2013 Published: November 7, 2013
Sample Preparation. The reference miscanthus (M. × giganteus) sample was collected in March 2012 from Foulumgård, AU-Foulum, © 2013 American Chemical Society
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Table 1. Setting Parameters of the Microwave Digestion System for Digestion and Cleaning Programs program digestion step 1 step 2 cleaning step 1 step 2
Table 2. ICP−OES Operating Parameters
setting parameters temperature (°C) 190 30 power (W) 1400 0
pressure (bar)
ramp (min)
hold (min)
20
30:00
45:00 15:00 hold (min)
pressure (bar)
ramp (min)
20
05:00
15:00 15:00
p rate (bar s−1) 0.3 p rate (bar s−1) 0.5
ICP−OES system
Thermo Scientific iCAP 6000 ICP−OES
flush pump rate (rpm) analysis pump rate (rpm) pump stabilization time (s) pump tubing type RF power (W) auxiliary gas flow (L min−1) nebulizer gas flow (L min−1) coolant gas flow (L min−1)
60 60 5 orange/white 1150 0.6 0.5 12
Table 3. Wavelengths Used by ICP−OES for Analysis of the 12 Elements and Their Limits of Detection
volume of 50 μL of 40% HF was then added in each vessel to solubilize the Si complex. The digested solution was then transferred to plastic flasks and diluted to a final volume of 50 mL. Elemental Analysis. The concentrations of 12 elements (Al, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, P, Si, and Zn) that may affect biomass quality for combustion purposes were quantified by Thermo Scientific iCAP 6000 series ICP−OES (Thermo Fisher Scientific, Waltham, MA), with the settings summarized in Table 2 and wavelength selection summarized in Table 3. A CETAC ASX-520 autosampler (CETAC, Omaha, NE) combined with an ENC 500 anticontamination enclosure (CETAC, Omaha, NE) was used for automated sample introduction. ICP standard solutions and spiked standards were prepared from 12 single-element standards (Merck KGaA, Darmstadt, Germany). All results are given as mean concentrations on a dry basis ±95% confidence intervals.
element
wavelength (nm)
viewinga
Al
394.401 396.152 315.887 317.933 226.502 228.802 224.700 324.754 238.204 259.940 766.490 769.896 202.582 285.213 257.610 294.920 588.995 589.592 213.618 214.914 250.690 288.158 206.200 213.856
R A R R A A A R A R R R R R R A R A A A R R A A
Ca Cd Cu Fe
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K
RESULTS AND DISCUSSION The documented values and uncertainties for the NIST 1515 apple leaves and NIST 1575a pine needles, expressed as mg kg−1 on a dry basis compared to the results from the highthroughput method are presented in Table 4. Recoveries of the elements were within a range of 95−101% for NIST apple leaves and 93−115% for NIST pine needles. Because the documented value of Si in these two SRMs is not available, the Si concentration determined from this study (697 ± 101 mg kg−1 for NIST apple leaves and 3069 ± 153 mg kg−1 for NIST pine needles) was compared to previously published data. The Si concentration found by the present method in NIST apple leaves was lower than 909 ± 242 and 944 ± 200 mg kg−1 found by Le Blond et al.12 and Feng et al.6 using inductively coupled
Mg Mn Na P Si Zn
LODb (mg kg−1) 14 19 0.023 0.2 10 33 4 0.4 24 4 581 1.1
a
R, radial viewing; A, axial viewing. bLOD = limit of detection (on a dry basis).
Figure 1. Multiwave graph of the microwave digestion system: (a) cleaning program and (b) digestion program. Temperature data were from a temperature−pressure combined sensor (inside the vessel). T vessel data were from an infrared sensor (outside the vessels). 7486
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Table 4. Documented Values for NIST SRM 1515 Apple Leaves and NIST SRM 1575a Pine Needles and Certified Values for the Bioenergy Crop M. × giganteus on the Concentrations of 12 Elements and Their Uncertainties Compared to Values Measured by the New High-Throughput Method Described in This Study NIST 1515 apple leaves
element Al Ca Cd Cu Fe K Mg Mn Na P Si Zn
documented (mg kg−1) 286 15260 0.013 5.64 83 16100 2710 54 24.4 1590
± ± ± ± ± ± ± ± ± ±
9b 150 0.002 0.24 5 200 80 3 1.2 110
12.5 ± 0.3
measured (n = 8) (mg kg−1)
percent recoveries (%)
285 ± 6 15056 ± 291
100 99
5.69 82 15832 2628 51 23.3 1494 697 11.8
± ± ± ± ± ± ± ± ±
0.11 7 285 35 1 1.9 16 101 0.4
M. × giganteus
NIST 1575a pine needles
101 98 98 97 95 95 94 94
documented (mg kg−1) 580 2500 0.233 2.8 46 4170 1060 488 63 1070
± ± ± ± ± ± ± ± ± ±
30 100 0.004 0.2 2 70 170 12 1 80
38 ± 2
measured (n = 8) (mg kg−1)
percent recoveries (%)
± ± ± ± ± ± ± ± ± ± ± ±
93 101 107 115 102 97 90 97 93 97
541 2519 0.248 3.2 47 4030 954 474 58 1039 3069 37
10 42 0.011 0.2 1 51 9 3.2 4 9 153 1
97
certifieda (mg kg−1)
