Article pubs.acs.org/EF
Mercury Oxidation by Halogens under Air- and Oxygen-Fired Conditions Ignacio Preciado, Tessora Young, and Geoffrey Silcox* Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290, Salt Lake City, Utah 84112-9203, United States ABSTRACT: Gas-phase mercury oxidation by halogens under air- and oxygen-fired (27% O2 in CO2) conditions was studied in a bench-scale, laminar, methane-fired, 300 W, quartz-lined reactor. The experiments provided data on the extents of elemental mercury oxidation in the presence of various flue gas components, including chlorine, bromine, SO2, and NO. A wet conditioning system and a Tekran 2537A mercury analyzer were used to determine mercury speciation and concentrations. Under the same experimental conditions and for both halogens, oxy-firing produced modest increases in mercury oxidation. Extents of oxidation at chlorine concentrations of 100−500 ppmv (as HCl) ranged from 6 to 21% for oxy-firing and from 4 to 15% for air-firing. Oxidation by bromine at concentrations of 10−50 ppmv (as HBr) ranged from 43 to 69% for oxygen-firing and from 15 to 46% for air-firing. Under both firing conditions, the addition of NO (below 300 ppmv) had little or no effect on mercury oxidation by chlorine or bromine and the addition of SO2 (below 400 ppmv) had no impact on oxidation by chlorine. A possible explanation for higher oxidation under oxy-firing conditions is that the CO2 molecule is a more effective third body than N2; that is, carbon dioxide is more effective than N2 at removing energy from the HgCl and HgBr transition-state complexes.
1. INTRODUCTION Coal is one of the most inexpensive and abundant reserves of energy in the world. Approximately 37% of the total electricity in the United States is generated by coal combustion.1 Because of the large amount burned, emissions from coal are significant and of environmental concern. The major environmental issues surround carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SOx), fine particles (less than 2.5 μm), and mercury. The latter is of great concern because of its high toxicity, difficulty in being controlled, and tendency to bioaccumulate in the environment. Oxy-fuel combustion is being considered as a method for CO2 capture and sequestration for coal-fired power plants because its cost appears to be comparable to that of conventional air-fired combustion with amine-based CO2 capture.2,3 The fate and analysis of mercury in oxy-fired systems have not been fully investigated. The fate of mercury under oxy-combustion conditions is important, because of not only emissions but also downstream corrosion. Trace amounts of mercury lead to embrittlement and cracking of aluminum heat exchangers that are used in the cryogenic separation and compression of CO2.4,5 Limited data on mercury speciation in oxy-combustion systems have been published. Some experimental data have indicated that elevated CO2 concentrations potentially enhance mercury retention on fly ash, while high O2 concentrations may promote mercury vaporization and emission.6 Enhancement of mercury oxidation by chlorine has been observed in oxycombustion environments.7 Suriyawong et al.8 evaluated pulverized sub-bituminous coal using O2−CO2 mixtures (20% O2/80% CO2 and 25% O2/75% CO2) to estimate the effects of O2−CO2 on sub-micrometer particle formation and mercury speciation. They found that 10−20% of the mercury in the coal was oxidized and that the © 2014 American Chemical Society
extent of mercury oxidation was unaffected by changing from air- to oxy-firing. Zhuang et al.9 have evaluated different Hg measurement techniques in a CO2-enriched flue gas. They found that, for flue gas with a high CO2 concentration, modifications to continuous mercury monitors (CMMs) may be needed to eliminate biases caused by mass flow controllers or impinger solutions in the flue gas conditioning system. Varied impinger chemistry under oxy-combustion conditions suggests Hg sampling using wetchemistry methods, such as Ontario Hydro, that may bias measured gas-phase Hg concentrations because of the high concentrations of CO2. Studies in laboratory combustion systems have shown that increasing the halogen (chlorine or bromine) content of combustion exhaust gases results in an increase in the amount of mercury in the exhaust gas in an oxidized form.10,11 The homogeneous oxidation of mercury by halogens has been extensively studied at the University of Utah under air- and oxyfiring conditions.12−15 Experiments were performed in a benchscale, methane-fired, quartz-lined reactor, in which the quench rate and flue gas compositions (which included the halogens chlorine and bromine) were varied. Mercury measurements were performed using a Tekran 2537A analyzer coupled with a wet-chemical, speciating conditioning system. The validity of the analytical method has been evaluated in a related study, in which coal was fired in a pilot-scale furnace under air- and oxyfired conditions with the addition of bromine or activated carbon.16 The Tekran analyzer and wet conditioning system were tested. Good agreement in mercury speciation and Received: October 16, 2013 Revised: January 2, 2014 Published: January 5, 2014 1255
dx.doi.org/10.1021/ef402074p | Energy Fuels 2014, 28, 1255−1261
Energy & Fuels
Article
Figure 1. Schematic of the experimental setup. the oxygen−carbon dioxide mixture and sent to the burner. For oxyfired experiments, carbon dioxide and oxygen were regulated using mass flow controllers and blended before diluting the mercury stream. Calibration gases in air were used as sources of chlorine and SO2 (6000 ppm Cl2 and 6000 ppm SO2), and calibration gases in nitrogen were used as sources of bromine and NO (500 ppm Br2 and 6000 ppm NO). NO and SO2 were fed to the burner in select experiments involving chlorine or bromine. The flue gas composition at the exit of the reactor is presented in Table 1. The total chlorine and bromine are expressed as HCl and HBr.
concentrations was obtained using the wet conditioning system and carbon traps. This work studied the fate of mercury oxidation by halogens in the gas phase under oxy- and air-firing conditions. Experiments were performed in a bench-scale, methane-fired quartz reactor with an optimized mercury sampling system to ensure unbiased measurement of mercury speciation in oxycombustion flue gas.
2. EXPERIMENTAL SECTION
Table 1. Flue Gas Composition in the Reactora
2.1. Quartz Reactor. A schematic of the experimental system is given in Figure 1. The reactor was designed to study the effects of chlorine, bromine, NO, and SO2 on homogeneous gas-phase mercury oxidation in combustion gases.10 The reactor is a quartz cylinder of 47 mm inner diameter and 135 cm length. The first 54 cm of the cylinder is enclosed by a Thermcraft high-temperature heater, and the remaining section of the quartz tube was heated with four, individually controlled heating tapes. The later permitted the quench rate to be adjusted to produce time−temperature profiles representative of industrial boilers. A 300 W quartz burner was used to feed methane and oxidant to the reactor. All reactants were introduced through the flame to create a radical pool representative of industrial combustion systems.14 The stoichiometric ratio and the flue gas flow rate [6 standard liters per minute (SLPM)] were the same for air and oxy-fuel conditions. A mixture of 27% O2 and 73% CO2 was used as the oxidant stream in the oxy-firing combustion tests. The percentage of oxygen after combustion was monitored with an oxygen analyzer and maintained at 2.0%, dry basis, for all experiments. Mercury was fed to the system using a mercury vapor permeation tube that was held at a constant temperature in an oil bath. Air was used as the carrier gas at a rate of 120 standard cubic centimeters per minute (SCCM) and passed through the permeation tube to create a Hg−air stream. The carrier gas was then diluted with additional air or
a
species
concentration
Hg0 (μg/m3) O2 (vol %, dry basis) HCl (ppmv) HBr (ppmv) NO (ppmv) SO2 (ppmv)
25 2 0−500 0−50 30−300 0−400
The total chlorine and bromine are expressed as HCl and HBr.
A Tekran 2537A mercury analyzer, a California Analytical Instruments model 100 P, and a California Analytical Instruments model 300-CL were used to continuously measure the concentrations of Hg0, O2, and NO at the reactor exit; mass balance calculations were used to estimate the concentrations of HCl, HBr, and SO2. The flow rate of NO was adjusted to give the desired concentration. Figure 2 shows the changes in temperature with distance in the reactor measured from the end of the quartz burner for air- and oxyfiring tests. The residence time in the reaction zone (at temperature of
