(141b) Adsorbents for Reduction of Multiple Toxic Metals in Coal-Based Power Generation

A major challenge for clean power using coal-based fuel is implementing techniques of use for adsorbents that can remove toxic metals existing in flue gas or the gas fuel. Toxic metals such as mercury, arsenic and selenium pose special problems as they exist in multiple valences. Although some adsorbents are claimed to be suitably active towards multiple valence states, e.g., As(3+) and As(5+), the design of an effective (cost and efficiency) method to remove multiple metals with the simplest and least mass of adsorbent may require a technique using more than one adsorbent. In smaller coal-fired plants that do not have an FGD it is advantageous to capture mercury at the electrostatic precipitator using an adsorbent with high capacity for Hg(0) that works at high temperature and can be recovered and reused. Sorbent recovery and reuse provides significant cost advantage to utility operation as it may drop the net cost to the utility proportionately to its number of reuse cycles; and also yields a mercury-free fly ash that can be used in secondary markets rather than being disposed in land fills. In larger and newer systems equipped with an FGD it can be advantageous to remove the mercury prior to the FGD to avoid re-emission of Hg(0). If the adsorbent shows activity towards other metals or speciation states such as Hg(2+), As(3+), Se(3+) or copper it has obvious benefit to the coal-fired utility power generation process. This presentation reviews several newly introduced adsorbents that can remove multi-valent toxic metals. One particular adsorbent can be recovered and reused multiple times dramatically reducing the cost of removing mercury from flue gas. The adsorbents are based on SAMMS? technology (self-assembled monolayers on mesoporous supports), a unique way for the materials scientist to create high capacity adsorbents that can match or exceed the capacity and efficiency of synthetic zeolites and powdered activated carbon, and to design adsorbents with specific metal activity.

We review a magnetic adsorbent functionalized with thiol groups (patent pending) that exhibits over 99% removal of gaseous elemental mercury in low temperature, inert and air gas streams and also matches baseline, halogen-doped powdered activated carbon in actual flue gas streams at 150°C (PRB coal) even though it has lower surface area and larger adsorbent size. This adsorbent also binds Hg(2+). If this collectable adsorbent is used prior to the FGD we expect even higher mercury removal than at the ESP. We discuss also two companion thiol-based SAMMS adsorbents available for fluids in both magnetic (THFL-01) and non-magnetic (THSL-01) form. The latter has achieved single digit ppt levels of mercury in industrial and utility aqueous streams over a broad range of pH (3-12) and in the presence of organic fluids and many anions and cations that compromise most adsorbents. The adsorbents also bind copper, arsenite and selenite. They appear highly suited for the challenges of the coal-fired flue gas stream, including the FGD. Finally we discuss several companion adsorbents under final development that have good activity to adsorb copper, arsenate and selenate, common toxic impurities in the FGD. One form of this adsorbent has high capacity for carbon dioxide and may have a place in the sequestering and fixing this gas.