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Mercury interaction on modified activated carbons under oxyfuel combustion conditions Margarita Quirós-Álvarez, Mercedes Diaz Somoano, Wolfgang Bongartz, and Satrugna Vinjarapu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00468 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018
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Energy & Fuels
Mercury interaction on modified activated carbons under oxyfuel combustion conditions M. Quirós-Álvarez1, M. Díaz Somoano1, W. Bongartz2, S. Vinjarapu2 1. Instituto Nacional del carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain 2. CARBOTECH AC GmbH, Elisenstraße 119, 45139 Essen, Germany
Abstract Mercury pollution is a cause for concern that requires global action. The most demonstrated and commercially available technology for mercury control is pulverised activated carbon injection. During oxy-coal combustion, the elevated concentrations of SOx, the moisture level in the flue gas and the recirculation streams may affect the performance of activated carbons as mercury sorbents. This works evaluates mercury oxidation and capture using impregnatedactivated carbons. In this study a novel aspect is considered by the application of a novel thermal desorption procedure for mercury species identification and the elucidation of the interaction mechanism. The results show oxidation efficiencies ranging from 85 to 96%. The mercury is partially retained in the solid by chemical adsorption. The formation of new mercury species HgS, HgI2 and HgO by the interaction was established. Keywords: mercury; speciation; activated carbon; TPD.
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1. Introduction To reduce the impact of anthropogenic emissions of greenhouse gases on the climate, CO2 capture and storage (CCS) or utilization (CCU) technologies for coal-fired power generation has been developed. Oxy-fuel combustion is a promising technology for concentrating the CO2 produced in coal-fired power plants previously to its storage or utilization. In oxyfuel operation, coal is burned with a mixture of O2 and recirculated flue gas, instead of air. The resulting flue gas consists primarily of CO2 and H2O, impurities such as SOx, NOx, and fly ash and trace levels of other gaseous pollutants such as HF, HCl, Hg etc.1 The CO2-enriched flue gas is further compressed, transported, and sequestered or used. A potential risk associated with oxyfuel combustion and mercury species present in the flue gas is related to environmental and corrosion problems. Mercury has a strong affinity toward aluminium, being able to produce failure of the aluminum-based equipment used in the CO2 processing unit. Consequently, mercury must be removed prior to CO2 purification and compression.2,3 Mercury behaviour and fate under oxyfuel combustion conditions has been scarcely investigated.4-8 It has been pointed that complex homogeneous and heterogeneous reactions between mercury and unburned carbon, halogen species, and SO3 occur.6 Oxygen enrichment and chlorine promotes Hg oxidation while the presence of sulphur tend to inhibit this process. Other factors that affect mercury emissions are the recycle loops in the configuration of the plant and the installation of control devices for NOx, SO2, particulate matter (PM), and CO2 emissions. In addition, a higher amount of carbon in ash would.7 In addition to stricter limits for SOx, NOx and particulate emissions, the regulations, known as the large combustion plant best available technology reference document (LCP BREF),9 will set standards for mercury emissions from power plants. Although levels have not yet been agreed, the mercury emission limits being considered in the draft BREF for plants of over 300 MW range between 0.2 μg/m3 and 10 2 ACS Paragon Plus Environment
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μg/m3. The Minamata Convention10 which was signed in Japan in October 2013 aims to “protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds”. Regarding the coal sector, the most relevant section of the Convention is Article 8 on emissions, which calls for all parties to take measures to control mercury. Many options have been investigated to reduce mercury emissions from air combustion systems. However, removal efficiencies range from 0 to 90% depending on coal type, fly ash properties and existing air pollutant control devices (APCDs).11 For conventional air combustion power plants taking the advantage of APCDs is a effectively and low cost option for mercury removal. However, under oxyfuel conditions, lower mercury capture efficiency (∼11%) was obtained when the flue gas passed through the bag house filter. It is possible that the higher SO2 and SO3 concentrations in the oxy-fuel experiments were responsible for the reduced mercury capture efficiencies.12 Ochoa et al.13 observed that the high content of CO2 may decrease the re-emission of elemental mercury due to the solubility of CO2 in the suspension and the decrease in the pH, increasing mercury removal efficiency. Concerning investigations on the development of specific technologies for mercury removal, the use of solid sorbents has been widely evaluated in air combustion conditions however they were rarely discussed under oxyfuel flue gas. The most demonstrated and commercially available technology for mercury control is pulverised activated carbon injection (PAC), both halogenated (usually bromine) or non-halogenated depending on the concentration and speciation of mercury in the flue gas.14-19 With this technology, the mercury, PAC and fly ash are removed in an electrostatic precipitator or fabric filter. High temperatures favour the kinetics of oxidising elemental mercury. The aim of the present work is to study mercury-activated carbon interactions under oxyfuel combustion conditions. In this study a novel aspect is considered by the application of a novel thermal desorption procedure for mercury species identification and the elucidation of the interaction mechanism. 3 ACS Paragon Plus Environment
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2. Materials and experimental methods 2.1. Activated carbons and characterization Three samples of activated carbons were supplied by Carbotech. These activated carbons are special ones : i) CarbonCatalyst D 52/4 NOx which is produced from bituminous coal by steam activation and is mainly used for the removal of NOx from flue gases. The pore system and surface chemistry is modified to create catalysts with enhanced catalytic activity; ii) DGF 4 S 12 extruded activated carbon which is produced from mineral coal by steam activation. It is impregnated with approx. 12% by weight sulphur. It is worth noting that this activated carbon was specifically developed for the adsorption of mercury and iii) DGF 4 KI 2 BIO which is an extruded activated carbon, produced from mineral coal and subsequently impregnated with approx. 2% of potassium iodide. These materials were characterized by different techniques. The apparent density of each solid was estimated by measuring the volume packed by a free fall from a vibrating feeder into a sized graduated cylinder and determining the mass of the known volume. The ash content was calculated by weighting the remaining mass after combustion of the organic matter at 650 ºC. The BET surface area was determined by volumetric adsorption of nitrogen at 77 K. The distribution of the particle size is determined mechanically with a vibrating sieving machine and standard sieves. For comparison, a commercial reference non-doped activated carbon (ref-AC) used for flue gas treatment has been tested under same conditions. 2.2. Mercury adsorption and oxidation The experimental setup consisted of a fixed-bed reactor heated by a temperature-controlled furnace, a gas blending station equipped with mass flow controllers, an elemental mercury generator and an online elemental mercury analyser (Mercury instruments, VM3000) (Figure 1). The oxidised
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mercury was determined by capturing it in an ion exchange resin (Dowex® 1x8) and then analysed by using an automatic mercury analyser (LECO AMA 254). The desired Hg0 concentration was produced by means of a permeation tube (VICI Metronics) kept at 40 ºC. This device emits elemental mercury through its permeable portion at a constant rate and the emitted gaseous mercury is carried out by a nitrogen flow. An atmosphere to simulate oxyfuel conditions was prepared by introducing flows through mass flow controllers of different components. In each test, a mixture of 0.5 g of carbon catalyst was loaded into a glass tube reactor and heated by a tubular furnace to keep the reaction temperature at 150 ºC. A total flow rate of 0.5 l/min of the desired gas composition (64% CO2, 20% N2, 4% O2, 1000 ppm SO2, 600 ppm NO, 100 ppm NO2 and 25 ppm HCl) was passed through the solid bed. The carbon catalyst samples were evaluated in this system for 120 min. The initial elemental mercury concentration was approx. 32 μg/m3. At the beginning of each test this mercury concentration was checked. Then the complete gas composition was passed through the solid bed (500 ml/min) and the outgoing elemental mercury concentration was measured. The amount of elemental mercury (Hg0) was determined integrating the signal registered by the online mercury analyser while the gaseous oxidised mercury (Hg2+) was determined by analysing the mercuryloaded Dowex resin. The amount of mercury retained in the solid (Hgp) was analysed in order to evaluate mercury adsorption. After the mercury tests, the loaded sorbent was analysed by means of a Hg-TPD procedure developed at INCAR20-21 in order to identify mercury species and to propose the interaction mechanism. Mercury species were identified on the basis of the temperature in which they were released.
3. Results and discussion
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Table 1 shows the main physical characteristic of the activated carbons evaluated in this work. The three samples have very similar physical properties. The BET-surface is around 700 m2/g being slightly higher for the DGF 4 KI 2 BIO activated carbon. During the tests, the elemental mercury concentration in gas phase was continuously monitored. When the solid bed was introduced into the reactor a sharply decrease in the signal was observed. Similar behaviour was observed for the different activated carbons (Figure 2). Reductions on elemental mercury concentration in gas phase ranging from 85 to 96% have been observed. In order to establish differences in the fate of the mercury, the oxidised mercury (Hg2+) remaining in gas phase was collected in the DOWEX resin and then analysed by means of an automatic mercury analyser. The amount of mercury captured by the solid was analysed after each test. The percentage of captured mercury, gaseous oxidised mercury and gaseous elemental mercury using the different samples are shown in Table 2. In addition, the mercury mass balance closure of the tests is shown in the table. It can be observed that the presence of elemental mercury in gas phase is minimal, lower than 3%, after passing through the sorbent bed of carbons D52/4 NOx and DGF 4KI 2BIO. Up to almost 8% of the oxidised mercury was not captured by DGF-4S12. The presence of elemental mercury in gas phase was higher for the reference activated carbon. This fact indicates that mercury oxidation occurs in a large extent however, this oxidised mercury is most efficiency removal in the modified activated carbons than in the reference activated carbon. When the reference samples was tested most of the mercury remains in gas phase as oxidised mercury while for modified activated carbons, oxidised mercury is adsorbed on the solid surface. The low mercury capture observed for the reference sample was attributed to the low gassolid contact time. The differences observed on mercury adsorption in the modified activated carbons are probably due to steric effects. In order to understand gaseous mercury-activated carbon interaction, the identification of the mercury species in the solid was carried out. For this analysis a Hg-TPD (temperature programmed 6 ACS Paragon Plus Environment
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desorption) procedure optimised at INCAR was employed.20-21 The main temperature points used to identify the species were (i) the temperature at which thermal release started, (ii) the maximum temperature of release and (iii) the temperature at which desorption returned to the baseline. The thermograms of the samples were then compared with the reference thermograms of fifteen pure mercury compounds in a similar carbonaceous matrix.22 The formation of new mercury species HgS, HgI2 and HgO in DGF4S12, DGF4KI2BIO and D52/4NOx, respectively, was identified (Figure 3). Although physical adsorption cannot be discarded, these results suggest that chemical reaction involving elemental mercury oxidation is the mechanism of interaction. Previous investigations pointed to the formation of chemical species by the interaction of mercury and sulphur.15 This study demonstrates this interaction in which HgS is the dominant mercury species in the solid. Regarding the formation of HgI2, it has been reported18,23 that KI is an effective reagent for Hg0 removal, due to the formation of I2 vapor as a result of the oxidation of solid KI by the O2 present in the gas phase. This would lead to the formation of the identified HgI2. The identification of HgO when D52/4 NOx catalyst carbon was tested, indicates that chemical interaction with the carbon matrix occurs.24 Moreover, the identification of Hg-OM in these samples indicate that contribution of physical adsorption to mercury capture.
4. Conclusions In conclusion, the results from this study point to the oxidation of mercury by the presence of these activated carbons and the capture of mercury by chemical adsorption. High mercury oxidation efficiencies have been observed. The most remarkable difference was observed in mercury capture on the carbon surface. This capture was notably increased by modification of the properties of the activated carbon. In all cases, chemical adsorption occurs and the formation of new mercury species HgS, HgI2 and HgO in DGF4S12, DGF4KI2BIO and D52/4NOx, respectively, was established. 7 ACS Paragon Plus Environment
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Acknowledgments The financial support for this work was provided by the projects CTQ2014-58110-R and GRUPIN14-031.
Table 1. Physical properties of activated carbons. Property
D 52/4 NOx
DGF 4 S 12
DGF 4 KI 2 BIO
Ref-AC
550±30
550±30
500±30
490
Ash content (%)
