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2001 Progress Report: Control of Mercury Emissions from Coal-Fired Power Plants

EPA Grant Number: R828170
Title: Control of Mercury Emissions from Coal-Fired Power Plants
Investigators: Helble, Joseph J. , Sarofim, Adel F.
Current Investigators: Helble, Joseph J. , Qiu, Joseph , Sarofim, Adel F. , Sterling, R.
Institution: University of Connecticut , University of Utah
EPA Project Officer: Shapiro, Paul
Project Period: July 1, 2000 through June 30, 2002 (Extended to September 30, 2003)
Project Period Covered by this Report: July 1, 2001 through June 30, 2002
Project Amount: $224,642
RFA: Exploratory Research - Environmental Engineering (1999)
Research Category: Engineering and Environmental Chemistry

Description:

Objective:

The purpose of this research project is to develop a novel, low-cost process for mercury emissions control through manipulation of homogeneous mercury chemistry. The objective is to increase the conversion of mercury to its water-soluble oxidized forms within the combustor, thus facilitating capture of mercury in scrubber systems.

Progress Summary:

During Year 2 of the project, significant progress was made in understanding the homogeneous chemistry of mercury in coal combustion systems. Detailed understanding of the chemistry is essential to identification of the appropriate additives and injection temperatures needed to modify mercury chemistry. Initial computational efforts focused on using an existing eight-step model for homogeneous mercury oxidation, which was developed by Helble under prior funding. Most rate constants in this model were taken from literature estimates, however, and detailed comparison with fundamental mercury oxidation experiments was lacking. Using the tools of quantum chemistry and transition state theory, an effort to develop an updated mercury kinetics model was initiated. The initial effort focused on modification of rate constants for these three reactions:

This now-modified eight-step mercury oxidation mechanism, with rate constants for Cl chemistry taken from the literature, was extended to a wider temperature range with sulfur oxidation chemistry (also taken from the literature), which resulted in the first elementary homogeneous kinetics model that could be used to predict mercury oxidation in the presence of sulfur and chlorine. Calculations of mercury oxidation using this model under simulated coal combustion conditions indicated that SO2 can reduce mercury oxidation by Cl2 through the scavenging of OH and O radicals. A reduction also was observed in the presence of HCl, although the effect was smaller. Comparison of model predictions with the limited data that are reported in the literature on the effects of SO2 on mercury oxidation indicated very good agreement.

Subsequent calculations conducted during the reporting period focused on assessment of gas injection as a means to promote mercury oxidation. Initial calculations confirmed our hypothesis that injection of hydrocarbons would result in a rapid increase in Cl atom concentrations (with Cl atoms being one of the most important species needed for mercury oxidation). Calculations indicated that Cl atom concentrations would decay rapidly, however, and suggested that injection temperatures above 1,300 K (i.e., temperatures higher than originally anticipated) would be required to promote mercury oxidation; in part because of the previously unrecognized effects of sulfur chemistry on mercury chemistry. These effects will be explored in detail using the model and the modified experimental system during the next year of the project.

In the experimental tasks, a continuous mercury emissions monitor is being acquired and will be used for data collection in Year 3 experiments. This will replace the existing impinger-based sampling and analysis system, and is expected to reduce experiment time from several hours per experiment down to several minutes per experiment. This has delayed experimental work at Connecticut, however. As a result, initial experiments were conducted by the Utah team as part of this effort. Experiments were conducted in the Utah U-furnace at a firing rate of approximately 75 kW. Under these conditions, temperatures accessible through furnace sampling ports ranged from 976 K at the near flame port down to 434 K near the exit of the exhaust section. These temperatures correspond to the range at which mercury oxidation chemistry is expected to occur. Initial experiments indicated that without hydrocarbon reaction products, measured mercury oxidation at a sampling temperature of 537 K is 50 percent. At an injection temperature below 1,200 K, mercury oxidation is suppressed, which is consistent with our calculations. At an injection temperature of 1,300 K, however, mercury oxidation is increased to 58 percent. Future experiments will focus on exploring different injection temperatures and additive concentrations to identify optimum conditions. Future experiments also will focus on the novel injection of small quantities of water. Calculations conducted by the Utah group indicated an alternative strategy of increasing the conversion by a rapid decrease in temperature such as might be attained by flue gas recirculation or water injection. Assessment of the impact of increasing the cooling rate by water injection showed a very major increase in conversion with water injection around 1,000 K. Therefore, experiments will be conducted to test the impact of enhanced cooling.

Future Activities:

Project Year 3 efforts will focus on extending the detailed chemical kinetic calculations of mercury chemistry to: (1) understand the inhibitory effects of SO2 and NOx on mercury oxidation in more detail; and (2) identify the optimum conditions for hydrocarbon injection in the presence of varying levels of NOx, SO2, and chlorine species. Experimental efforts will focus on the addition of a continuous mercury monitor to the system, conclusion of the baseline homogeneous mercury oxidation experiments, and initiation and completion of the gas injection experiments. Submission of a paper to the "Clearwater Conference," the International Technical Conference on Coal Utilization and Fuel Systems to be held in Clearwater, FL, in March 2003, is planned.

Journal Articles:

No journal articles submitted with this report: View all 6 publications for this project

Supplemental Keywords:

metals, heavy metals, toxics, engineering, environmental chemistry, air. , Toxics, Air, Scientific Discipline, Waste, RFA, Engineering, Chemistry, & Physics, HAPS, Incineration/Combustion, air toxics, particulate matter, Environmental Chemistry, 33/50, Environmental Monitoring, tropospheric ozone, heavy metals, coal combustion, anthropogenic stresses, hydrocarbons, ambient emissions, combustion byproducts, ambient air, benzene, chemical composition, hydrocarbon, benzene emissions, combustion, mercury & mercury compounds, particulates, methane, flue gas emissions, air pollution, mercury, stratospheric ozone, ion chromatography, mercury speciation, anthropogenic stress, Benzene (including benzene from gasoline), Mercury Compounds, coal fired power plants

Progress and Final Reports:
2000 Progress Report
Original Abstract
Final Report

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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