William J. O'Dowd

Mercury Measurement and Its Control: What We Know, Have Learned, and Need to Further Investigate

The U.S. Department of Energy (DOE), through the Federal Energy Technology Center (FETC), manages the largest funded program in the country for developing (1) an understanding of mercury emissions, (2) measurement of these emissions, and (3) control technology (-ies) for these emissions for the U.S. coal-fired electric generating industry. DOE has initiated, or has collaborated with other government and industrial organizations in, these and other efforts relating to mercury and other hazardous air pollutants (HAPs), also known as air toxics. One of DOEs first reports on trace elements in coal was conducted from 1976 through 1978 by researchers at DOEs Pittsburgh Energy Technology Center (PETC, now FETC) and the Pittsburgh Mining Operations of the Department of the Interiors Bureau of Mines. The report was completed less than two years after DOE was formed, and 13 years before Title III of the 1990 Clean Air Act Amendments was enacted (Cavallaro et al., March 1978).

Control of mercury emissions from coal-fired power plants: a preliminary cost assessment and the next steps for accurately assessing control costs

Abstract Mercury emissions from coal-fired power plants have been extensively evaluated for nearly 10 years to determine possible regulation by the Environmental Protection Agency (EPA). Under a court order, a determination will be made on whether it is appropriate and necessary to regulate toxic air pollutant emissions (focusing on mercury) from coal-fired utility boilers by December 15, 2000. If it is determined that regulations are necessary, then the regulatory process will have a fixed timetable. A proposed regulation will be due no later than December 15, 2003, and promulgated no later than December 15, 2004. The utility industry regulatory compliance must be in place by December 2007 since the Clean Air Act requires that sources come into compliance with Maximum Achievable Control Technology (MACT) 3 years after promulgation of the regulations. While it is recognized that the main driver for regulation is the potential risk to human health and that this risk is currently being examined by a number of health-based organizations, the actual form of any regulation would likely be dependent upon the availability of cost-effective control technologies. Furthermore, the diverse nature of the coal-fired utility industry will likely limit the applicability and cost-effectiveness of any given technology for the current boiler population. In light of a potential regulatory determination, this paper examines a few control options that warrant further consideration. A preliminary assessment of mercury capture technologies and associated costs is conducted for sorbent injection technology. Sorbent-based technologies that may be amenable for mercury control include: (1) sorbent injection with and without spray cooling upstream of existing particulate control devices (i.e., electrostatic precipitators and fabric filters); and (2) sorbent injection with and without spray cooling associated with additional control devices designed to augment particulate collection in a primary particulate control device. Important design criteria for each of the control systems are critically assessed for operability, maintainability, and reliability, with the projected impacts of the control system on power plant operations being evaluated. The sorbent-based technology discussed in this paper focuses on the injection of activated carbon associated with the various particulate control devices used in the utility industry. The paper also addresses the next steps and revisions needed to accurately assess possible cost impacts to the utility industry as the mercury control options mature in their development.

Modeling Sorbent Injection for Mercury Control in Baghouse Filters: I—Model Development and Sensitivity Analysis

Abstract A two-stage mathematical model for Hg removal using powdered activated carbon injection upstream of a bag-house filter was developed, with the first stage accounting for removal in the ductwork and the second stage accounting for additional removal caused by the retention of carbon particles on the filter. The model shows that removal in the ductwork is minimal, and the additional carbon detention time from the entrapment of the carbon particles in the fabric filter enhances the Hg removal from the gas phase. A sensitivity analysis on the model shows that Hg removal is dependent on the isotherm parameters, the carbon pore radius and tortuosity, the C/Hg ratio, and the carbon particle radius.

Pilot-scale air toxics R&D assessment of creosote-treated and PCP-treated wood cofiring for pulverized coal utility boiler applications

Abstract This paper presents air toxics emissions test results from a pilot-scale cofiring study of pentachlorophenol- (PCP) and creosote-treated woods to provide data for pre-permitting requirements for utilities interested in biomass cofiring as a means of increasing renewable energy while reducing greenhouse gases and other emissions for pulverized coal-fired utility boilers. These PCP/creosote-treated wood cofiring tests included a comprehensive assessment of air toxics, including dioxins, furans, polycyclic aromatic hydrocarbons (PAHs), heavy metals (Hg, Sb, As, Cd, Cr, Co, Pb, Ni and Se), formaldehyde and other volatile organic compounds, HCl, and particulates. This pilot-scale testing measured ‘uncontrolled’ emissions from the combustor (upstream of flue gas cleanup devices) and showed that PCP/creosote-treated wood could be successfully cofired at 10% heat input without increases in air toxic emissions as compared to a baseline eastern bituminous coal. Air toxics emissions were typically very low, and often near or below detection limits, largely as a result of the good air/fuel mixing and high furnace temperatures associated with pulverized coal combustion. One expected result was an increase in uncontrolled HCl emissions as a result of the higher chlorine content in the treated woods, although even at 10% cofiring levels, HCl emissions were within the range of other US coals. This paper is presented to provide independent data that industry, environmental groups, and regulators may consider in evaluating the opportunities for treated wood cofiring test burns and commercialization in full-scale coal-fired boilers in an environmentally acceptable manner.

Effects of Dilution Sampling on Fine Particle Emissions from Pulverized Coal Combustion

A dilution sampler was used to examine the effects of dilution ratio and residence time on fine-particle emissions from a pilot-scale pulverized coal combustor. Measurements include the particle size distribution from 0.003 to 2.5 μm, PM2.5 mass, and PM2.5 composition (OC/EC, major ions, and elemental). Heated filter samples were also collected simultaneously at stack temperatures in order to compare the dilution sampler measurements with standard stack sampling methodologies. Measurements were made both before and after the bag house, the particle control device used on the coal combustor, and while firing three different coal types and one coal–biomass blend. The PM2.5 mass emission rates measured using the dilution sampler agreed to within experimental uncertainty with those measured with the hot-filter sampler. Relative to the heated filter sample, dilution did increase the PM2.5 mass fraction of selenium for all fuels tested, as well as ammonium and sulfate for selected fuels. However, the additional particulate mass created by gas-to-particle conversion of these species is within the uncertainty of the gravimetric analysis used to determine the overall mass emission rate. The enrichment of PM2.5 selenium caused by dilution did not vary with dilution ratio and residence time. The enrichment of PM2.5 sulfate and ammonium varied with fuel composition and dilution ratio but not residence time. For example, ammonium was only enriched in diluted acidic aerosol samples. A comparison of the PM2.5 emission profiles for each of the fuels tested underscores how differences in PM2.5 composition are related to the fuel ash composition. When sampling after the bag house, the particle size distribution and total particle number emission rate did not depend on residence time and dilution ratio because of the much lower particle number concentrations in diluted sample and the absence of nucleation. These results provide new insight into the effects of dilution sampling on measurements of fine particle emissions, providing important data for the ongoing effort of the EPA and ASTM to define a standardized dilution sampling methodology for characterizing emissions from stationary combustion sources.

Modeling Sorbent Injection for Mercury Control in Baghouse Filters: II—Pilot-Scale Studies and Model Evaluation

Abstract Activated carbon injection for Hg control in a 500-lb/hr pilot-scale coal-fired furnace equipped with a fabric filter for particulate control was evaluated at different operating conditions. The pilot-scale tests showed that Hg removal was improved at lower temperatures and higher C/Hg ratios. The two-stage mathematical model developed to describe Hg removal using powdered activated carbon injection upstream of a baghouse filter was used to obtain Langmuir isotherm parameters as a function of temperature by fitting the model to a subset of experimental data. The predictive capability of the model was then tested by comparing model calculations with additional experimental data from this system obtained using different operating temperatures and sorbent to Hg ratios. Model predictions were in good agreement with experimentally measured Hg removal efficiency. Based on the model predictions, Hg removal in the duct appears to be limited and higher C/Hg ratio, lower operating temperature, and longer cleaning cycle of the baghouse filter should be utilized to achieve higher Hg removal in this system.