Article pubs.acs.org/EF
XRF Analysis of Water Pretreated/Leached Banagrass to Determine the Effect of Temperature, Time, and Particle Size on the Removal of Inorganic Elements Trevor J. Morgan,*,† Lars K. Andersen,‡ Scott Q. Turn,† Hong Cui,† and Dong Li† †
Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States Institute for Energy and Transport, Joint Research Centre, European Commission, 1755 LE Petten, The Netherlands
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S Supporting Information *
ABSTRACT: X-ray fluorescence (XRF) spectroscopy was used to assess the effectiveness of various mild pretreatment methods for improving the fuel quality of banagrass, a tropical grass. Three types of pretreatment were used with increasing levels of severity: (i) S1 involves dewatering (pressing) only, (ii) S2 where the sample is pressed and leached, and (iii) S3 where pressing−leaching−pressing process is used. In addition, the influence of particle size (2 or 60−80 mm), leaching water temperature (25 or 75 °C), and leaching time (1 or 3 min) on the extraction of inorganic elements was examined. The results show that the S3 pretreatment is the most effective and that reducing the particle size has a more significant effect than increasing leaching temperature or time. Using a 2 mm particle size at 75 °C for 3 min produced the greatest effect, removing 50−60 wt % of the Na, ∼65 wt % Mg, ∼90 wt % P, ∼90 wt % K, ∼55 wt % S, and ∼95 wt % Cl. Using dewatering alone (S1) to pretreat the banagrass was the least effective approach. The S2 pretreatment produced results that are midway between the S1 and S3 results. However, using small particles with the S2 method gives similar results to processing large particles using the S3 approach.
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INTRODUCTION In this study the influence of water pretreatment (leaching) on the inorganic composition of banagrass was examined by X-ray fluorescence (XRF). The XRF analysis was performed on the raw biomass, i.e., without ashing the sample or producing a fused glass pellet, which can lead to the loss of volatile components. Instead the biomass is simply size reduced to 1000 ppm (typically Mg, Si, Cl, K, Ca) consistently show a positive bias of 3−18% relative. The relative precision (measured as the relative standard error) is less than ±5% (typically ±1%) for concentrations >25 ppm.1,2 Quantifying elements below 25 ppm (Co, Ni, Cu, Zn, Sr, As) is possible in some cases but requires a more detailed study for each specific element.
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Figure 1. Fuel process steps and product streams. Jeffco grinder can be found elsewhere.20 The resulting pulverized material was estimated to be ∼2 mm in length. Leach Water Temperature. Two water leaching temperatures were used, 25 (ambient) and 75 °C. The lower temperature water is representative of tap water, and the higher temperature water may be available from steam boilers or condensers at a processing plant. In these tests, tap water was used (not deionized water), and analysis of the tap water has previously been reported: electrolytic conductivity 0.48 mS/cm, K+ 2.4−3.1 mg/L, Na+ 46−51 mg/L, Ca2+ 7.5−9.0 mg/ L, Mg2+ 12 mg/L, Cl− 0.6 mg/L.10 Trace elements in the tap water were Al 0.6 mg/L, B < 2 mg/L, P < 10 mg/L, Si < 5 mg/L, see ref 10 for the complete results. Leaching Duration. Two leaching times were employed, 1 and 3 min, with constant pressing (by hand) to further improve contact between the water and the sample. The amount of water used in the leaching step was ∼3.750 kg per 1.000 kg of banagrass S0 (wet basis). The ratio of water to S1 sample (dry basis) is in the range of 11−14, depending on the moisture content of the pulverized and chopped S1 sample. A more complete account of the pretreatment procedure can be found elsewhere.10,20 The results from the analysis of the liquid products (juice and leachate) produced from the pretreatments used herein are reported elsewhere.20 The matrix of experimental variables assessed in this study for the S3 samples, eight conditions in total, are summarized in Table 1. Each set of conditions was used to process three batches of sample, i.e., three repeats at each condition, resulting in 24 tests in total.
EXPERIMENTAL SECTION
Sample. Banagrass (Pennisetum purpureum x Pennisetum glaucum) was harvested in Maui, Hawaii, on November 6th, 2013, and shipped to the University of Hawaii at Manoa campus the following day. After arrival, processing was conducted immediately and completed within 3 days. Pretreatment Process. The processing steps investigated in this study are size reduction, pressing, leaching, and a secondary pressing. The final pressed solid product (S3) is dried to equilibrium moisture content (
