Article pubs.acs.org/EF
A Method for the Quantification of Alkali and Alkaline Earth Metallic Species in Bioslurry Fuels Mingming Zhang, Xiangpeng Gao, and Hongwei Wu* School of Chemical and Petroleum Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: This study reports a method for the quantification of alkali and alkaline earth metallic (AAEM) species in bioslurry fuels. The so-called evaporation−ashing−digestion−ion chromatography (IC) method consists of four steps, including evaporation that converts bioslurry into solid-like residue, ashing that converts the residue into ash, acid digestion that dissolves the ash into a solution, and IC analysis that quantifies the AAEM species in the solution. The novelty of the method is the combination of the evaporation step with the existing ashing−digestion−IC method that is developed for solid fuels but not suitable for bioslurry fuels. The evaporation step consists of multi-steps of slow heating and holding at various segment temperatures corresponding to the boiling points of the major compounds in bio-oil, resulting in progressive evaporation of biooil vapors with little carry-over of biochar particles. The method has been successfully applied for quantifying AAEM species in bioslurry fuels with various biochar loading levels (5−20 wt %), with small relative standard errors (within ±3%) and low limitations of quantification (0.4−3.0 ppm). It also overcomes the biochar incomplete oxidation issue associated with the microwave−digestion-based methods, which considerably underestimate the concentrations of AAEM species in bioslurry fuels.
1. INTRODUCTION Biomass fast pyrolysis converts biomass into high-energydensity fuels, including biochar and bio-oil.1−6 Significant research progress has been made in upgrading and refining of bio-oil for producing liquid transport fuel.7−14 A near-term strategy is to produce bioslurry fuels via mixing bio-oil and biochar. Such a concept was initially attempted by commercial developers, such as Karlsruhe.15 Recent studies16−19 in Australia show that the bioslurry fuels from mallee biomass pyrolysis are of small energy and carbon footprints, can be economically transported over a long distance from distributed regional pyrolysis plants to a central utilization plant, and have potential synergies in co-utilization of biodiesel. Therefore, a supply chain of the production, transport, and utilization of bioslurry fuels may be an important strategy for increasing the uptake of biomass energy in existing vast stationary infrastructure (e.g., coal-fired power stations). As a mixture of biochar and bio-oil, bioslurry contains abundant alkali and alkaline earth metallic (AAEM, mainly Na, K, Mg, and Ca) species. Transformation of these ash-forming species in fuels are important to reactor designs and product qualities20−23 and responsible for various notorious ash-related issues during combustion and gasification, including fire-side corrosion,24,25 ash deposition,26−28 fine inorganic aerosol emission,29,30 and bed agglomeration in fluidized-bed operations.31−35 It is therefore of great significance to accurately quantify AAEM species in bioslurry fuels. One approach for the quantification of ash-forming species in fuels includes fuel oxidation (or ashing) to convert these species to ash, followed by acid digestion of the ash into solution and then quantification using analytical instruments, such as ion chromatography (IC) and/or inductively coupled plasma−atomic emission spectrometry (ICP−AES). Methods based on this approach have been developed for solid fuels, including quantifying AAEM species in brown coal using IC36 and analyzing major ash-forming species (including AAEM © 2013 American Chemical Society
species, Si, Al, Fe, Ti, etc.) in biomass and biochar using a combination of IC and ICP−AES.37 In these methods (hereafter referred to as the “ashing−digestion−IC” or “ashing−digestion−ICP” methods), fuel sample ashing deploys a temperature−time profile using slow heating (up to 10 °C/ min), multi-step holding, and low temperatures (up to 600 °C) to avoid sample ignition, hence, reserve ash-forming species in the ash product. The subsequent ash digestion uses a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). These enable complete oxidation, 100% retention of ash-forming species in ash, and complete ash digestion. Such methods have been proven to be useful in accurate quantification of inorganic (including AAEM) species in solid fuels, such as brown coal, biomass, and biochar.36−39 However, it is still questionable whether such methods can be adopted to bioslurry because of the introduction of bio-oil into the fuel. It is doubtful if 100% of ash-forming species in bio-oil can be retained in the ash product after ashing using the ashing program because bio-oil is prone to ignition and the vapors produced upon bio-oil heating may also carry biochar particles out of the ashing crucible during ashing. On the other hand, because of its simplicity and easy operations, microwave digestion40,41 has been widely used for converting bio-oil into a suitable solution. The solution is then subsequently subjected to ICP−AES analysis for the quantification of ash-forming species in bio-oil. However, such a method (hereafter referred to as the “microwave−ICP” method) may not be applicable to quantify ash-forming species in bioslurry because microwave digestion is probably not able to completely digest the solid biochar particles present in bioslurry. Received: August 15, 2013 Revised: September 16, 2013 Published: September 19, 2013 6823
dx.doi.org/10.1021/ef401632h | Energy Fuels 2013, 27, 6823−6830
Energy & Fuels
Article
and discussed in section 3.2. The third method is the existing microwave−ICP method that combines microwave digestion of a sample to produce a solution and quantification of AAEM species in the solution via ICP−AES. The analyses were conducted by ChemCentre (Perth, Australia). Briefly, ∼0.2 g of sample was mixed with 20 mL of a mixture of HNO3, HCl, and H2O (at a volume ratio of 10:0.5:9.5) and digested following the modified EN13805:2002/U.S. EPA 3052 method.44 The digested solution was filtered (if cloudy) before being injected into ICP−AES for analysis. 2.3. Characterization of Other Properties of Bio-oil and Biochar. The bio-oil and biochar samples were also subjected to ultimate analysis using a CHN/O elemental analyzer (PerkinElmer 2400 series II model). The proximate analysis of biochar was conducted using thermogravimetric analysis (TGA; Mettler TGA/ DSC 1 STAR model), following the procedure described in ASTM E870-82.45 The water content of bio-oil was analyzed by Karl Fischer titration according to ASTM E203-96.46 The titrator (Mettler V30 model) was calibrated, and the reagents were selected according to VTT publication 306.47 The solid content of bio-oil was determined on the basis of a procedure modified from VTT publication 306.47 Briefly, ∼2 g of bio-oil was dissolved in methanol and filtrated using three layers of filter paper (MN615, 4 μm pore size). The mass of solid retained on the filter paper was then used to determine the solid content of the bio-oil. The viscosity of bio-oil was measured at 25 °C using a rheometer (HARR Rheometer II model) with a cone/plate 35/4° sensor. About 0.8 mL of bio-oil is required for one measurement. To quantify the total acid number (TAN) of the biooil samples, ∼1 g of bio-oil was dissolved in 50 mL of acetone and then titrated by 0.1 M sodium hydroxide solution using MEP Oil Titrino plus 848.48−50 The properties of the bio-oil and biochar samples are presented in Table 1.
Therefore, this study aims to examine whether these existing methods for the quantification of ash-forming species in solid fuels can be directly applied to bioslurry fuels. The experimental program for such assessment is focused on the suitability of the ashing−digestion−IC method and microwave−ICP method for quantifying AAEM species in bioslurry fuels. Furthermore, a new method was then developed for quantifying AAEM species in bioslurry fuels via incorporating an additional progressive evaporation step considering the presence of bio-oil in bioslurry.
2. EXPERIMENTAL SECTION 2.1. Bio-oil, Biochar, and Bioslurry Samples. The experimental program used two fast pyrolysis bio-oil samples (hereafter referred to as “bio-oil A” and “bio-oil B”), which were supplied by two commercial suppliers. Both bio-oil samples were produced by fast pyrolysis of pine wood at 500 °C. The two bio-oil samples were stored in a fridge at ∼4 °C prior to use. A biochar was produced from fast pyrolysis of a pine wood biomass (4−6 mm) at 500 °C, using a laboratory-scale fluidizedbed reactor scaled up from the one used previously.42 Briefly, ∼500 g of high-purity silica sand (size of 125−355 μm) was first loaded into the reactor and preheated to the desired pyrolysis temperature (i.e., 500 °C). Argon was used as a carrier gas, and its flow rate was adjusted to maintain proper fluidization of bed materials. The feeding rate of pine wood particles was adjusted at ∼1 g/min. Once feeding was completed, the reactor was lifted out of the furnace and cooled to room temperature, with argon continuously passing through the reactor. The biochar and sand particles were then collected from the reactor and sieved using a 1 mm screen to separate the biochar from the sand. Because of experimental limitations, it is noted that the two bio-oil samples and the biochar sample were not produced from the same pine wood sample. Nevertheless, this fact does not influence the validity of conclusions drawn in this study. Bioslurry was prepared according to a similar procedure described elsewhere.18 Briefly, biochar was ground using a ball mill (Retsch MM400) for 8 min at a frequency of 15 Hz. After grinding, particle size analysis (using Malvern Masterizer 2000) showed that ∼80% of char particles were less than 75 μm, which met the requirement of preparation of slurry fuels.18,43 The char particles were carefully mixed with each of the two bio-oil samples to prepare four bioslurry fuels at different biochar-loading levels. The bioslurry samples included three bioslurry fuels prepared from bio-oil A at biochar-loading levels of 5, 10, and 20 wt % (hereafter referred to as “bioslurry A, 5 wt % biochar”, “bioslurry A, 10 wt % biochar”, and “bioslurry A, 20 wt % biochar”), respectively, and one bioslurry fuel from bio-oil B at a biochar-loading level of 10 wt % (hereafter referred to as “bioslurry B, 10 wt % biochar”). 2.2. Quantification of AAEM Species in Bio-oil, Biochar, and Bioslurry Fuels. Three methods were examined for quantifying the content of AAEM species in biochar, bio-oil, and bioslurry. The first method is the existing ashing−digestion−IC method, which has been extensively applied for the quantification of AAEM species in low-rank solid fuels, such as brown coal, biomass, and biochar.36−39 Briefly, a known amount of a sample was placed in a platinum (Pt) crucible, which was then subjected to a temperature−time program in air. The program includes multiple slow-heating and holding steps from room temperature to a termination temperature of 600 °C, originally designed for biomass and biochar. The resulted ash was then digested in a mixture of HF and HNO3 (at a volume ratio of 1:1) at 120 °C for at least 12 h. The solution after digestion was evaporated to remove residue acids and then dissolved in 20 mM methanesulfonic acid (MSA) for AAEM quantification using IC (model, ICS 3000; column, CS12A; and eluent, 20 mM MSA). The second method is a new evaporation−ashing−digestion−IC method designed and developed in this study based on the first method. It deploys an additional step designed for the progressive evaporation of bio-oil vapors as a mean to avoid the loss of AAEM that otherwise would occur during the ashing step in the first method. The detailed design of this method is given
Table 1. Typical Properties of Bio-oil and Biochar Used in This Study samples
bio-oil A
Proximate Analysis 24.4 water content (wt %, ara) ash (wt %, dbb) ndc volatile matter (wt %, dbb) ndc fixed carbon (wt %, dbb) ndc Ultimate Analysis (wt %, dafd) C 42.64 H 7.55 N 0.22 Oe 49.59 viscosity at 25 °C (mPa·s) 112.5 solid content (wt %, ara) 0.01 TANf (mg of NaOH/g of bio-oil) 49.7
bio-oil B
biochar
27.7 ndc ndc ndc
1.4 9.9 18.9 71.2
40.48 8.28 0.30 50.94 57.3 0.1 46.2
86.21 3.19 0.16 10.44
a
As-received basis. bDry basis. cNot determined. dDry and ash-free basis. eBy difference. fTotal acid number.
3. RESULTS AND DISCUSSION 3.1. Evaluation of the Ashing−Digestion−IC Method for Quantifying AAEM Species in Bioslurry Fuels. Table 2 presents data on the concentrations of AAEM species in bioslurry fuels quantified by the ashing−digestion−IC and microwave−ICP methods, along with the data for the bio-oil and biochar samples using the same methods. Three important findings can be observed. First, the concentrations of AAEM species (particularly K and Ca) in biochar quantified by the microwave−ICP method are considerably lower than those determined by the ashing−digestion−IC method. It was demonstrated that the ashing−digestion−IC method is able to accurately quantify AAEM species in brown coal, biomass, 6824
dx.doi.org/10.1021/ef401632h | Energy Fuels 2013, 27, 6823−6830
Energy & Fuels
Article
Table 2. Concentrations of AAEM Species of Biochar, Bio-oil, and Bioslurry Fuels by Different Quantification Methods fuels concentration (ppm) Na
K
Mg
Ca
method method method method method method method method method method method method
1a 2b 3c 1a 2b 3c 1a 2b 3c 1a 2b 3c
biochar
bio-oil A
80 ± 3 111.0 ± 3.0
