All plants are unique
Among a coal's properties, its chlorine (Cl) content and loss-on-ignition (LOI) are most critical to mercury reduction. That's because chlorine is the species that actually oxidizes Hg0, and because the UBC component of LOI both accelerates Hg oxidation and acts as the most effective inherent Hg sorbent in flyash. Most bituminous coals contain much more chlorine and generate much more LOI than low-rank coals, and the expected difficulties of oxidizing the Hg from low-rank coals are well established in the test data.
But something even more important lies beyond that generic trend. A coal's Cl content is a random variable that does not correlate with any other coal property. Chlorine content can routinely vary by a factor of two or three during the course of Hg field testing over several days, and in one recent 30-day test program it changed by a factor of five. These variations in coal-Cl change the Hg chemistry and are partly responsible for the variations in Hg speciation data.
Compounding the problems caused by variability in coal properties, the conditions of flue gas cleanup also directly affect the underlying Hg chemistry and therefore directly determine Hg emissions rates (Table 1). As examples, different flue gas quench rates imposed by different designs of economizers and air preheaters can affect Hg reaction rates. Longer ductwork sustains more chemistry than shorter connections. Utility SCR systems are operated at space velocities from 1,700 to 5,000 hr-1, which changes the residence time available for catalytic Hg oxidation by almost a factor of three. In addition, different SCR units use different flow passages and active ingredients that also affect Hg oxidation rates.
Table 1. Flue gas cleaning conditions that affect Hg chemistry. Source: Niksa Energy Associates LLC
The operating temperature of PCDs is crucial to their efficacy, which is why hot-side electrostatic precipitators (ESPs) collect much less Hg than do cold-side ESPs. The excellent contact between flue gas and flyash in the filter cakes of fabric filters enhances Hg0 oxidation and utilizes the full capacities of UBC or sorbent for Hg capture. Spray dryer-absorbers usually perform as well as or better than wet scrubbers, but the details of a scrubber's design have little bearing on its Hg retention.
These aspects of Hg chemistry help to explain much of the most erratic content in the field testing database. But they do little to help utility planners decide which Hg control strategies would work best at their plants. Utilities that aggressively manage fuel quality by burning and blending subbituminous and bituminous coals from different regions will need to contend with the full spectrum of Hg reaction conditions. But even those utilities that limit their fuels to a certain type of coal from a specific region will still need to manage the impact of gas cleaning conditions, especially if their plants use or plan to use SCR systems and scrubbers. In practice, it is impossible for most utilities to conduct enough Hg field tests to cover the ranges of coal quality, cleaning conditions, and cleaning system configurations that will have to be managed through the second stage of CAMR implementation.
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