Article pubs.acs.org/EF
Experimental and Thermodynamic Analyses of Trace Element Speciation during the Combustion of Ground Cedar Nut Shells Wojciech Jerzak* AGH University of Science and Technology, 30 Mickiewicza Avenue, 30-059 Kraków, Poland ABSTRACT: The objective of this paper was to identify trace elements present in cedar nut shells and the products of their combustion. In the fuel, the following trace elements (TEs) were identified (mentioned in the order from the highest content): Zn, Cu, Cr, Ni, Ba, Sn, V, Mo, Pb, Sr, Sb, As, Co, and Cd. Among the 14 TEs detected in cedar nut shells, zinc has the highest content of 23 mg/kg in the dry fuel. The influence of the combustion temperature from the range of 800−1200 °C on the change of the molar fraction of certain species containing TEs in the combustion products was determined on the basis of equilibrium calculations carried out in FactSage 6.3 software. It has been demonstrated that the combustion of cedar nut shells in the temperature range of 800−900 °C preserves the following elements in the ashes: Zn, Cu, Ni, Ba, Sn, and As, whereas Sb and Cd are present only in the flue gas. The molar fractions have been determined for the dominating species containing TEs in the flue gas and ashes. The presence of elements in the bottom ash after biomass combustion in the temperature of 830 ± 25 °C has been confirmed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM) hyphenated with energy-dispersion spectroscopy (EDS) methods. From the XRD analyzes, it seems that dominating species containing TEs in the ashes are Ca3(VO4)2 in monoclinic and trigonal crystal systems, SrTiO3 in the regular crystal system, and ZnFe2O4.
1. INTRODUCTION Cedar nut shells, a natural seed coat of the Siberian pine (Pinus sibirica), are a cedar oil production waste. Siberian pine can be found mainly in western and eastern Siberia, the Ural, and in the northern parts of Europe. The annual harvest of cedar nuts are estimated at 10−12 million tons.1 The shell constitutes about 51−59 wt % of the cedar nut.2 The kernel film, protecting the kernel of the cedar nut and located between the nut and the shell, is used in the industry as pillow filling. Currently, there is no uniform system for utilization of large amounts of cedar nut shells. Cedar shells are sold to be used, among others, as decorative mulch, substrate for flowers, substrate for the production of alcoholic beverages, mineral-rich feed additives for animals, or fuel.3−10 Some biomass boilers available on the European market have cedar shells mentioned as one of the types of fuel in their specification, for example, retort boilers with a thermal power of 24−35 kW.10 The content of trace elements (TEs), such as Cd, Cu, Pb, and Zn, in cedar shells depends significantly upon the place of harvest.5 In industrialized areas, where there are anthropogenic sources of air pollution, a significant increase of the concentration of Cd and Pb has been noted in the cedar nut shells. The content of TEs in various types of coal is higher than that in cedar nut shells (with the exception of coals with the content of Cd < 0.314), what has been shown in Table 1. As Table 1 demonstrates, in cedar nut shells, the dominating TE is zinc, the content of which lies in the range between 3.67 and 16.10 mg/kg, whereas the content of copper is 10 times lower. TEs are present in the cedar nut shells, cedar nuts, and kernel film.3−8 The cedar nut has a higher content of Cu, Zn, Pb, Cd, As, and Hg than the cedar nut shell.9 During the combustion process, TEs in the shells go through complex physical and chemical transformations and can be found in the fly and bottom ash and the flue gas. The partitioning of TEs on the emission streams to the combustion © XXXX American Chemical Society
Table 1. Comparison of the Contents of Selected TEs in Cedar Nut Shells and Coal TEs
cedar nut shells (mg/kg)
coal (mg/kg)
Zn Cu Ni Pb Cr Cd Co Hg As
3.67−16.13−8 0.38−1.953−8 0.4 ± 0.057,8 0.18−0.753−7 0.17 ± 0.017,8 0.008−0.3143−8 0.02 ± 0.0057,8 0.00386 0.0026
15.4−84.411−14 5.78−25.812−14 6.51−28.612−14 1.94−33.811−14 11.1−35.811−14 0.09−0.511−14 3.5−7.713,14 0.031−0.1111−14 1.72−4811−14
products in the literature is described as the speciation, migration, distribution, or dispersion of elements. The partitioning of TEs depends upon many factors, such as the temperature of combustion, the element boiling point, the size of the particles of the fly ash, the oxygen content in the flue gas, the diameter of the bed material, the combustion atmosphere, the presence of halides in the fuel, etc. The increase of the fuel combustion temperature leads to an intense volatilization of As, Cd, Hg, Zn, and Pb to the flue gas and the decrease of the TE content in the bottom ash.15,16 The concentration of As, Cd, Cr, Cu, Ni, and Zn in the fly ash is much higher than that in the bottom ash, which is confirmed through a study of the fluidized combustion of forestry residues, mix of eucalyptus, pine bark, cotton stalk, and rice husk.17−19 Ratafia-Brown20 believes that the structure of the combustion chamber influences the speciation of TEs between fly and bottom ash to a higher degree than the working conditions of the furnace. As a result Received: October 27, 2016 Revised: December 27, 2016 Published: January 3, 2017 A
DOI: 10.1021/acs.energyfuels.6b02814 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels of the boiling point, TEs have been divided into three basic classes in accordance with Figure 1.14
present in the biomass, than it can be expected that there will be metal chlorides in the flue gas, which are characterized with a bigger volatility than metal oxides. There are a few reasons that have motivated the author of this paper to study TE speciation in the process of cedar nut combustion. First, the lack of experimental research on the process of cedar nut shell combustion, the composition of ash, and the presence of the TEs in the ashes, although the boilers for cedar nut shells are present on the European market. Second, a lack of information on the influence of the combustion temperature of the cedar shells on the distribution of TEs between flue gas and ash. An equally important reason was assessing the possibility of the occurrence of the TE molten phase in the ash through equilibrium calculations for various combustion temperatures.
Figure 1. Categorization of TEs based on volatility behavior.15
2. MATERIALS AND METHODS 2.1. Fuel. For the experimental research, 5 kg of cedar nut shells from the Altai region (middle Asia) were used, the size of which is comparable to a coffee bean (shown in Figure 2). The cedar nut shells
The least volatile elements that do not react with the volatile halides or sulfides and, therefore, can be almost entirely found in bottom ash and slag are included in class 1. Elements from class 2, with a lower boiling point than those from class 1, can react with other elements, forming compounds in the solid or gaseous stage. The elements that evaporate in the easiest way have been classified to class 3. They can react with other substances in the combustion chamber, producing species with a boiling point exceeding 900 °C, for example, CoF3, TiCl3, CdCl2, and CoCl2, and even 1400 °C, e.g., CaF2, TiCl2, KCl, MgCl2, and CuCl.21 The next parameter that decides the partitionong of TEs is the size of the fly ash particles. The smaller the size of fly ash particles collected from the electrostatic dust collector and cyclones, the higher the concentration of As, Cd, Cu, Pb, Sb, Se, and Zn.17,18,22 The aforementioned regularities have not been found in the tests of microparticles of the dust present in flue gas.22 Additionally, the concentrations of TEs in sub-micrometer (1.0 mm comprised about 60 wt %. The decrease of the particle size of the combusted biomass causes a shorter ignition time, narrower reaction zone, higher combustion speed, and lower temperature.30 Additionally, a larger fragmentation of the fuel particles increases the chance of the reaction between ash containing, among others, CaO, SiO2, and volatile compounds, such as KOH and KCl.31 2.2. Applied Research Methods. The analysis of biomass and ash was performed in the Central Laboratory of Energopomiar in Gliwice (Poland) with the use of the indication methods mentioned as follows, with the number of the European Standard given in parentheses: carbon, hydrogen, and total sulfur, infrared (IR) absorption method (EN 15104); nitrogen, katharometric method (EN 15104); chlorine, ion chromatography method (EN 15289); total moisture (EN 14774-2); ash (EN 14775); volatile matter, gravimetric method (EN 15148); and gross calorific value, calorimetric method (EN 14819). The chemical composition of ash and TEs has been determined with the method of inductively coupled plasma optical emission spectrometry (ICP−OES) with the use of a plasma spectrometer Thermo iCAP 6500 Duo ICP. The identification of TEs in the bottom ash has been made using scanning electron microscopy (SEM) in connection with energydispersion spectroscopy (EDS) and X-ray diffraction (XRD). The first tests have been conducted using a scanning electron microscope ESEM Philips XL 30 equipped with an energy-dispersion X-rays B
DOI: 10.1021/acs.energyfuels.6b02814 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels spectrometer (EDS Genesis 4000). There have been 13 images registered of the microstructure of the sample surface through the use of an accelerating voltage of 20 kV, zoom of 65−3500×, spot size of the electron beam of 4−5.2, and working distance between the focal point of the electron beam and the pole of the object lens of 9.8−10.3 mm. In total, 30 EDS spectra analyses of the microareas have performed. For the study of the XRD, the PW 1710 device from the Philips Company has been used (monochromatic radiation of the Co Kα series; beam optics, standard configuration Bragg−Brentano with a point scintillation detector). The Rietveld analysis was performed in the TOPAS 4.2 program, using the crystallographic data from the PDF4+ 2013 database. The scope of the research encompassed the phase qualitative analysis and the evaluation of the crystallite sizes in two examined ash samples. A large number of phases present in the material of the tested ashes and the overlapping of multiple reflexes have significantly hindered the precise identification of phases. The basis of the phase analysis is the XRDs registered in the scanning type 2θ in the range from 15° to 120°. The volume shares have been determined for the analyzed phases. SEM and XRD analyzes have been conducted at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences in Kraków, Poland. 2.3. Combustion Setup and Conditions. The combustion process of the cedar nut shells in an air atmosphere was performed at a station in the laboratory scale. The station consisted of a quartz tube, an electric resistance furnace, an autotransformer, a ceramic boat, a rotameter, an air fan, a K-type thermocouple, and a flue gas analyzer. In the quartz combustion chamber, the temperature was kept at the level of 830 ± 25 °C. The change of the temperature by about 25 °C indicated that another part of the biomass was added or that the combustion process of the biomass portion was coming to an end. The concentration of carbon dioxide and oxygen in the flue gas was monitored with the Land Lancom series II flue gas analyzer. The addition of biomass to the combustion chamber was a consequence of a momentary drop of the CO2 concentration in the flue gas. The temperature of the air fed to the combustion was 25 °C, at p = 0.1 MPa. Figure 3 shows the ceramic boat taken out of the furnace,
(d) the omission of the temperature change history of the fuel particle during the combustion process; hence, the calculation of the final product in each temperature is made independently, etc. Despite the above-mentioned limitations, the equilibrium calculation is a useful tool for prediction of the influence of the combustion temperature and pressure on the speciation of TEs in the processes of fuel combustion. In this paper, the equilibrium calculations are made with the use of Equilib that is a part of the FactSage 6.3 software. The informational simplified calculation model based on an equilibrium reactor is illustrated in Figure 4. To the FacSage 6.3 software, the masses of the
Figure 4. Simplified diagram calculations. following elements have been introduced, in grams: C, O, H, N, S, and Cl, from the ultimate analysis; K, Mg, Si, Ca, P, Al, Fe, Mn, Na, Ti, Ba, and Sr, from the ash analysis; Zn, Cu, Cr, Ni, Ba, Sn, V, Mo, Pb, Sr, Sb, As, Co, and Cd contents in 1 kg of dry fuel; and N2, O2, Ar, and CO2 as air components. The entry oxidizer corresponded to the equivalence ratio (Φ) of 0.714 proportionally to the air composition expressed in wt %: 75.47% N2, 23.20% O2, 1.28% Ar, and 0.046% CO2. The temperature of combustion substrates was 25 °C, and the pressure in the combustion chamber was 0.1 MPa. The calculations were made for the temperature of product combustion in the range from 800 to 1200 °C with a step of 20 °C. In the calculation, three databases were used at the same time, according to Table 2, through the selection of all reagents from the
Table 2. Databases and Solution Phases Used in Calculations number of compounds
a
a
database
solids , liquids,a and gases
liquid-phase solutions
FactPS SGPS FToxide − BSlag-liq
1361 67 0
0 0 23
Pure solids and pure liquids.
whole range of available species (ideal gases, pure solids, pure liquids, and solutions in the liquid phase). FactPS was selected as the priority base, whereas 67 compounds were selected from the SGPS base that were not present in the FactPS database (formerly FACT53), for example, ZnH2O2(g), As4O7(g), K3CrO4(s), and Na2Mo2O7(s). The first stage of equilibrium calculations was the selection of the model of liquid slag in the FToxide base, because it is known from the literature that the ash from the biomass combustion can contain molten salts and oxides.31,32 Results of calculations for models ASlagliq, BSlag-liq, and CSlag-liq were compared. The differences in the composition of the main components of the liquid slag were slight; hence, BSlag-liq was selected for calculations. The model of liquid slag (BSlag-liq) consisted of oxides of Al, As, Ca, Fe, K, Mg, Mn, Na, Ni, Pb, Si, Ti, Ti, and Zn and sulfates of Ca, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn (
