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Effect of using coke dust as a sorbent for removing mercury from flue gases on the contents of selected ecotoxic elements in fly ash Faustyna Wiero#ska, Piotr Burmistrz, Andrzej Strugala, Dorota Makowska, and Sebastian Lech Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03557 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018
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Effect of using coke dust as a sorbent for removing mercury from flue gases on the contents of selected ecotoxic elements in fly ash Faustyna Wierońska*†, Piotr Burmistrz†, Andrzej Strugała†, Dorota Makowska†, Sebastian Lech‡ †
AGH University of Science and Technology, Faculty of Energy and Fuels, Mickiewicz Avenue 30,
30-059 Krakow, Poland, ‡
AGH University of Science and Technology, International Centre of Electron Microscopy for
Materials Science and Faculty of Metals Engineering and Industrial Computer Science, Mickiewicz Avenue 30, 30-059 Krakow, Poland ABSTRACT: Passive methods, which are used for the purification of flue gases, can often be insufficient in the case of the emission of mercury and other ecotoxic elements into the environment. Therefore, it may often be necessary to introduce additional methods of reducing the emission of these pollutants, for example the injection of powdered activated carbon or coke dust into flue gas ducts. The efficiency of coke dust has been confirmed by tests in laboratory and a demo-plants scale. In accordance with the proposed solution, coke dust is dosed before the electrostatic precipitator and then separated fully along with fly ashes. Prior to injection, coke dust contains negligible amounts of mercury (4.810.5 µgHg/kg, depending of the size fraction), which is a value several times lower than the Hg content in subbituminous coals and lignites. However, it can also be a carrier of other ecotoxic elements such as nickel and chromium. Their contents in coke dust are often much higher than in coal used for power production or in fly ashes. In the article, the influence was determined of coke dust dosage as a sorbent for removing mercury from flue gases, on the contents of arsenic, nickel, chromium, lead, copper and zinc in fly ashes. The examined samples came from the demo-plant for monitoring and abatement of mercury emission from coal combustion in pulverised coal boilers. The contents of particular elements in the tested samples were determined using Atomic Absorption Spectrometry with flame atomization (FAAS; Pb, Cu, Ni, Cr, Zn) and with electrothermal atomization (GFAAS; As).
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1. Introduction Due to the limits for mercury emission into the environment from large combustion plants, which were introduced in 2012 in the USA1 and also in 2017 in Europe2, it is necessary to develop effective methods of flue gases purification from these elements. Moreover, passive methods, applied for flue gas purification (e.g. installations for nitrogen oxides, sulphur oxides and particles removal from flue gases), may be insufficient in the case of the emission of mercury and other ecotoxic elements into the environment. One of the methods for reducing Hg emissions involves the use of dusty sorbents. The properties and effectiveness of mercury removal by various sorbents are widely described in literature3-10. Particularly, the injection of Powdered Activated Carbon (PAC) or the injection of Brominated Powdered Activated Carbon (B-PAC) is applied in the power plants in the USA. Depending on the applied solution, the efficiency of flue gases purification from Hg using activated carbons can range from 50% to over 90%5,11. However, it should be noted that the costs of reducing Hg emission grow with the increase of Hg removal efficiency. The unit cost of Hg reduction from flue gases using PAC can range, according to data provided by literature, from 2300 to 12750 USD/kgHg12 or even from 40000 to 90000 USD/kgHg11. Taking into account the above-mentioned cost, cheaper but equally effective sorbents are required. According to Burmistrz et al.11, one of the competitive ways for Hg removal from flue gases is the injection of coke dust before the electrostatic precipitator (ESP) and, subsequently, a full separation in the ESP along with fly ashes. For comparison, the cost of removing of Hg by this method may be smaller than 3000 USD/kgHg (with efficiency up to 90%)11. Coke dust is one of the by-products of coking process. It is formed during the operations of pushing out of coke from the coking chamber, coke transportation and sorting and also during the dry coke quenching process11. Moreover, taking into account the fact that coke dusts contain a large amount of other ecotoxic elements, i.e. chromium and nickel, it is important to determine the effect of dosing those sorbents on the properties of resulting fly ash. Fly ash is one of the Coal Combustion Products (CCPs). Almost all of it is used by different commercial purposes, among others in reclamation works, as an additive for cement and concrete or for road construction13,14. Furthermore, fly ashes may be a source of such elements as Ge, Ga, Se, Be,
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and Li15,16 as well as Rare Earth Elements (REE)17. Yet another way of fly ash utilization is the production of sorbents for the purification of wastewater contaminated with ecotoxic elements like As18. During the combustion processes, ecotoxic elements contained in coal (such as As, Cr, Cu, Zn, Pb, and Ni), create the gaseous form and, next, mostly adsorb on the surface of the fine fly ash particles. For this reason, fly ashes often contain a significant amount of the above-mentioned elements19-22. In order to classify fly ash in accordance with the ASTM C61823, it is important to determine the contents of major oxides (SiO2, Al2O3, Fe2O3 and SO3), the moisture content and the loss of ignition (LOI). Sorbents with a carbon matrix, which are separated together with fly ash in the ESP can strongly influence the LOI parameter. The aim of this research was to determine the effect of coke dust injection (applied as a sorbent for Hg removal from flue gases), on the content of arsenic, nickel, chromium, copper, lead, and zinc in fly ashes.
2. Experimental
2.1. Demo-plant and sampling procedures Examined samples came from the demo-plant for monitoring and reducing mercury emission from coal combustion in Pulverized Coal Boilers (PCBs) operating in the Łaziska Power Plant (Poland) (Fig. 1). This plant was connected to the 225 MW power unit with the use of a bypass. In these Power Plant subbituminous coal blends are burned. A stream of flue gases in the amount of 5000 m3/h was separated from the flue duct after passing through the Selective Catalytic Reduction (SCR) and the air heater (Fig. 2). Subsequently, the flue gas after passing through the particular units of the demo-plant was introduced back into the main flue duct before the commercial-ESP. Samples were taken from each of the three zones of the demo-ESP prior and after the injection of coke dust (in points Ia and b simultaneously). Additionally, samples of the coal blend which was actually burned in the PCB and samples of fly ash from the commercial scale ESP were taken.
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Coke dust was dosed directly into the flue duct before the demo-ESP at 1 kg/h and then completely excreted in the demo-ESP together with the fly ash. The coke dust came from coking plant equipped with the dry coke quenching system.
2.2. Determination of the microstructure of the samples tested
In order to determine the microstructure and the morphology of the coke dust and fly ash samples the Merlin Gemini II (ZEISS) Scanning Electron Microscope equipped with Field-Emission Gun (FEG) and the X-ray diffraction spectrometer (EDS) were used. The Bruker's Quantax 800 Microanalysis System was used for the chemical composition analysis.
2.3. Grain size distribution of fly ash and loss of ignition parameter
The screening method was used for the determination of grain size distribution for selected samples. The 0.2 mm, 0.16 mm, 0.1 mm, 0.063 mm mesh sieves were used. The loss of ignition (LOI) was determined for the samples from each of the ESP zones and for selected grain size fractions in accordance with the ASTM D7348-13 at 950°C. In addition, the determination of the content of unburned carbon (UC) in the fly ash samples was carried out in accordance with the Polish standard PN-G-04571:1998 using the CHS-580 Automatic Analyzer (Eltra).
2.4. Determination of the contents of ecotoxic elements
The analysis of the ecotoxic elements was performed by means of the Hitachi Z-2000 Atomic Absorption Spectrometer (AAS) using the Zeeman background correction effect with the flame atomization (Cr, Zn, Ni, Cu, Pb) mode and with the electrothermal atomization (arsenic). Basic parameters of the AAS method are given in table 1. In order to conduct the AAS analyses, all samples were digested in the Berghoff SpeedWave4 microwave system. The samples with a mass of nearly 100 mg were mineralized with a mixture of nitric acid (V) and hydrofluoric acid. Subsequently, supersaturated boric acid was used as the complexing reagent. 4 ACS Paragon Plus Environment
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3. Results and discussion 3.1. Characteristics of the applied coke dust
The SEM image of coke dust is presented in Figure 3. In the coke dust two characteristic types of grains can be distinguished: smooth grains with a closed structure (Figure 4b) and spherical grains with a tendency to form agglomerates (Figure 4c). The first one were created entirely by carbon. In the case of the second type, in the structure of agglomerates apart from carbon there were other elements, such as Al, Si, Ca, S, and Mg (Fig. 4a with EDS spectrum for GI, GII and GIII). According to research conducted by Marczak et al.3, coke dust has a relatively small specific surface area SBET of up to 32.0 m2/g (Table 2). The small sorption surface may be associated with an increased number of closed-structure grains occurring in coke dust. Compared to the PACs and BPACs used, among others at US Power Plants, the examined coke dust has a less developed mesoporous structure and a small amount of micropores. Nevertheless, coke dust is able to absorb up to 60-90% of mercury from the flue gas3,11. For comparison, the PAC sorption efficiency, depending on the dosage system, can be as high as 90%5. Coke dust contains ecotoxic elements (table 3). Raw coke dust, prior to injection, contains small amounts of mercury (4.8-10.5 µgHg/kg depending on particle size). However, the tested coke dust also had significant amounts of nickel and chromium, much higher than the analyzed samples of the subbituminous coal blend, and even higher than the fly ash samples collected from the commercial scale ESP in the Łaziska Power Plant. It contains also other ecotoxic elements such as As, Cu, Zn, and Pb. Taking into account the increased contents of ecotoxic elements in coke dust as well as the fact that the sorbent should be introduced to flue gases before the ESP and, subsequently, separated in the ESP, it is important to determine the effect of coke dust on the properties of the separated fly ashes in the context of their commercial usage.
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3.2. Fly ash characteristics
A study of the microstructure of fly ash sampled from the demo-ESP showed that it consisted mainly of aluminum-silicon-oxygen spherical particles (Figure 5). This fact is consistent with the literature data27. Furthermore, calcium as well as magnesium and the unburned carbon particles (UCPs) can be distinguished in the investigated fly ash. The porous structure of UCP allows for the penetration of spherical aluminum-silicon particles with a diameter of less than 4 µm (Figure 6). 3.3. Estimated balance sheet of ecotoxic elements
The average contents of the examined ecotoxic elements in fly ash coming from the particular ESP zones (Ia and b, II, and III) prior and after sorption are compared in Table 4. In most cases, after the coke dust injection, the contents of the examined EEs in fly ash increased as a consequence of introducing them with coke dust. On the other hand, the concentrations of these elements in the samples from the first zone of the demo-ESP (Ia and b) were different. It is important that this samples were taken simultaneously, which indicates instability of results for this zone of the demo-ESP. The estimated balance sheet for As, Ni, Cr, Zn, Pb, and Cu was used to verify the effect of the ecotoxic elements contained in coke dust applied as a sorbent on their contents in fly ash. For the balance sheet, the fly ash concentration in the flue gases was assumed to be 25 g/m3. The contents of the ecotoxic elements in fly ash prior the coke dust injection were estimated as the arithmetic mean of the contents of the examined elements in each of the zones (Ia and b, II, and III). Similarly, the contents of the analyzed elements in fly ash after sorption were estimated. The flue gas flow in the demo-plant was equal to 5000 m3/h, while the amount of the injected coke dust was 1 kg/h. The percentage shares of EEs in fly ash associated with their contents in the coke dust (%EERCD) were calculated according to the formula (1):
% =
∗ 100%
EERCD - mass stream of EEs introduced into ESP with coke dust (g/h) EEFA prior dosage – mass stream of EEs introduced into ESP with fly ash, prior coke dust injection (g/h) 6 ACS Paragon Plus Environment
(1)
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It can be observed that the shares of As, Zn, Cu, and Pb associated with coke dust are negligibly small (
