This paper is particularly related to elemental mercury (Hg0) oxidation and divalent mercury (Hg2+) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO2). As a powerful oxidant and chlorinating reagent, Cl2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO2, NO, as well as H2O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO2 and NO on Hg0 oxidation and Hg2+ reduction with the intent of unraveling unrecognized interactions among Cl species, SO2, and NO most importantly in the presence of H2O. The experimental results demonstrated that SO2 and NO had pronounced inhibitory effects on Hg0 oxidation at high temperatures when H2O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg2+ back into its elemental form. Data revealed that SO2 and NO were capable of promoting homogeneous reduction of Hg2+ to Hg0 with H2O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H2O was removed from the gas blend.

On a worldwide basis, the projected increase in coal usage over the next two decades in China, India, and Indonesia will dwarf the current U.S. coal consumption of 1 billion tons/year. Therefore, in the United States, coal will be the dominant source of mercury emissions, and worldwide, coal may be the cause of significantly increased mercury emissions unless an effective control strategy is implemented. However, there is much uncertainty over the most technically sound and cost-effective approach for reducing mercury emissions from coal-fired boilers. Several approaches are suggested for mercury control from coal-fired boilers, including enhancing the ability of wet scrubbers to retain mercury. However, many coal-fired boilers are not equipped with wet scrubbers. On the other hand, since almost all coal-fired boilers are equipped with either an electrostatic precipitator (ESP) or a baghouse, sorbent injection upstream of either an ESP or baghouse appears attractive, because it has the potential to control both Hg{sup 0} and Hg{sup 2+}, would appear to be easy to retrofit, and would be applicable to both industrial and utility boilers. Since mercury in the gas stream from coal combustion is present in only trace quantities, only very small amounts of sorbent may be necessary. If we assume a mercury concentration of 10 {micro}g/m{sup 3} and a sorbent-to-mercury mass ratio of 1000:1, the required sorbent loading is 10 mg/m{sup 3}, which is only 0.1% to 0.2% of a typical dust loading of 5-10 g/m{sup 3} (2.2-4.4 grains/scf). This amount of additional sorbent material in the ash would appear to be negligible and would not be expected to have an impact on control device performance or ash utilization. Accomplishing effective mercury control with sorbent injection upstream of a particulate control device requires several critical steps: (1) Dispersion of the small sorbent particles and mixing with the flue gas must be adequate to ensure that all of the gas is effectively treated in the short residence time (typically a few seconds) between sorbent injection and particle collection. (2) Assuming the sorbent particles can be injected and dispersed adequately, a second critical step is the mass transfer by diffusion of the mercury from the bulk flue gas to the particle surface within the available residence time. The ideal case would be to achieve sufficient mass transfer in the duct and not depend on additional transfer within the collection device. (3) Once the mercury molecules reach the surface of a sorbent particle, they will not be trapped unless sorption can occur at a rate equal to the rate of mass transfer by diffusion to the particle surface. Analysis by Rostam-Abadi and others concluded that only a very small surface area would theoretically be required to trap the mercury. The implication is that reactive surface sites are much more important than the amount of surface area. (4) Assuming the sorbent has the capacity and reactivity to trap the mercury that reaches the sorbent particles, the final critical step is long-term stability of the sorbed mercury.

Coal is now the primary source of anthropogenic mercury emissions in the US, accounting for 46%, or 72 tons/year, of the total US Environmental Protection Agency (EPA) estimated 158 tons/year. However, on a worldwide basis, the projected increase in coal usage over the next two decades in China, India, and Indonesia will dwarf the current US coal consumption of 1 billion tons/year. Development of cost-effective mercury control for coal-fired boilers is a primary research need identified in the EPA Mercury Study Report to Congress. A promising approach for mercury control is the injection of an effective sorbent upstream of the particulate control device. Since the amount of mercury in the gas stream from coal combustion is usually in the range of 5 to 10 {micro}g/m{sup 3} (about 1 ppbv), only very small amounts of a sorbent may be necessary. A requirement is that the mercury be tightly bound in the sorbent, not desorbing upon exposure to ambient air or leaching under wet disposal conditions. Many of the attempts at using sorbents to control mercury from coal combustion have met with limited success for unexplained reasons. Recent results at the EERC identified a major interaction between SO{sub 2} and NO{sub 2}more » that may be responsible for the poor sorbent performance observed in many tests. Results indicate that a combination of SO{sub 2} and NO{sub 2} will lead to rapid breakthrough of oxidized mercury species. These results also suggest that bench-scale sorbent data collected without CO{sub 2} and NO{sub 2} are likely to be misleading if they are generalized to combustion systems where these gases are almost always present. Understanding this mechanism will be critical to the development of better sorbents. This paper presents possible mechanisms that may explain the observed SO{sub 2}-NO{sub 2} effects on sorbent performance and lead to a more effective control approach.« less

The US Environmental Protection Agency (EPA) draft Mercury Study Report to Congress (1) estimated anthropogenic mercury emissions to be 253 tons/yr in the US, with the majority (216 tons/yr) from combustion sources. The three main combustion sources listed were coal (72 tons/yr), medical waste incinerators (65 tons/yr), and municipal waste combustors (64 tons/yr). The emissions from both medical waste incinerators and municipal waste combustors were recently regulated, which, together with the reduction of mercury in consumer products such as batteries and fluorescent lights, has already reduced the emissions from these sources, as stated in the final EPA Mercury Report to Congress (2). EPA now estimates total point-source mercury emissions to be 158 tons/yr, with coal remaining at 72 tons/yr, while medical waste incinerators are down to 16 tons/yr and municipal waste combustors are at 30 tons/yr. Coal is now the primary source of anthropogenic mercury emissions in the US, accounting for 46%. In addition, the use of coal in the US has been increasing every year and passed the 1-billion-ton-per-year mark for the first time in 1997 (3). At the current rate of increase, coal consumption would reach 1.4 billion tons annually by the year 2020. On a worldwide basis, the projected increase in coal usage over the next two decades in China, India, and Indonesia will dwarf the current US coal consumption level. Therefore, in the US coal will be the dominant source of mercury emissions and worldwide coal may be the cause of significantly increased mercury emissions unless an effective control strategy is implemented. However, much uncertainty remains over the most technically sound and cost-effective approach for reducing mercury emissions from coal-fired boilers, and a number of critical research needs will have to be met to develop better control (2).

The Energy and Environmental Research Center (EERC) participated in a project with Owens-Coming Fiberglas Corporation (OCF) to test high-temperature fabric filters coated using a sol-gel process with a proprietary form of a vanadium/titanium (V/Ti) catalyst. The catalyst-coated S-glass fabric filters are targeted for use in a power plant or industrial hot-side baghouse facility for simultaneous NOx and particulate control. Issues pertaining to the successful development and marketing of this technology, such as catalyst and filter durability, NOx and particulate control capabilities, ammonia slip levels, and catalyst deactivation were addressed. As part of this project, bench-scale experiments were performed at the EERC to determine the effects on catalyst reactivity and fabric integrity of 1) suspected selective catalytic reduction (SCR) catalyst poisons typically found in Western coal ash, and 2) exposure to a water or acid dewpoint as a result of an operational upset in a baghouse facility.