VALUE-ADDED SORBENT DEVELOPMENT

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.