The iPOG™ is a user-friendly software package that predicts Hg emissions rates from full-scale utility gas cleaning systems fired with any coal or coal blend, given a few coal properties, the gas cleaning configuration, selected firing and gas cleaning conditions, and an assortment of Hg control technologies. It predicts the Hg emissions reductions for the most common inherent Hg controls, including systems with only particle collection devices (PCDs), and with ESP/FGD and SCR/ESP/FGD combinations. It also predicts Hg removals for injection of conventional carbon sorbents, brominated carbon sorbents, and halogenation agents, and estimates the Hg removals for different coal pretreatments. The estimated Hg emissions are based primarily on engineering correlations of the Hg field test database from American utilities, with support from NEA’s detailed Hg transformation mechanisms.
The iPOG™ is also a means to extrapolate from a limited set of test data to the full ranges of coal quality and gas cleaning conditions across utility operations of any size and complexity. It is too expensive for a sizeable company to test all the important combinations of fuel quality and gas cleaning conditions in their current and foreseen operations. And data from one system is hard to directly apply to other systems of similar configuration because, inevitably, some of the important cleaning conditions were different in the test than they will be in the other systems. So the most efficient strategy is to first use a limited amount of test data to ensure that the iPOG™ results are accurate for the gas cleaning conditions of interest, then rely on iPOG™ to estimate Hg emissions rates for all the other fuels and gas cleaning conditions that will come into play among the similar cleaning configurations. Since computerized calculations are so much faster and cheaper than field testing, users can easily evaluate much broader ranges of Hg control conditions than are represented in a field test database.
To support entry-level users, default parameter specifications are available for every required input value in iPOG™, although experienced users will appreciate the opportunities to specify their actual cleaning conditions in the calculations. Properties of the coal or coal blend (Rank, Moisture, Ash, S, HHV, Cl, Hg, Blend Percentages) are required to estimate the flue gas composition because there are no means to accurately estimate Hg- or Cl- contents in coals. Furnace conditions (Rating, Load, Gross Efficiency, Firing Configuration, Bottom Ash, LOI, Economizer O2), including the firing configuration (Wall-, Corner-, or Cyclone-Firing) are also required to estimate a flue gas composition, and also to determine a flue gas flowrate. The partitioning of coal ash into bottom ash and flyash is also important because LOI is expressed as a percentage of the retained flyash only. Users must have a flow diagram from furnace exit to stack that shows all air pollution control devices and Hg controls, supplemented with select SCR conditions (Economizer NO Concentration & NO Reduction Efficiency), particulate removal conditions (an over-all Particulate Collection Efficiency), sulfur scrubbing conditions (SO2 Capture Efficiency), and specifications on carbon sorbents (Conventional or Brominated Sorbent, Injection Position & Concentration) and halogenation agents (Wt. Percentage Halogen, Injection Position & Concentration). Both conventional and brominated carbon sorbents are supported.
Since the POG and, now, iPOG™ were developed for a broad user base, including people with no immediate experience in controlling Hg emissions, NEA definitely did not incorporate state-of-the-art calculation sequences to achieve the tightest quantitative accuracy on the calculation results. Tradeoffs were deliberately made to eliminate all but the most basic input requirements at the expense of quantitative accuracy for any particular utility gas cleaning system. Obviously, these tradeoffs limit how the estimates from the iPOG™ should be used. The most general limitation is that the iPOG™ estimates are, for the most part, based on regressions of field test data, rather than on validated chemical reaction mechanisms. The bulk of the field test data came from the extensive program directed by the National Energy Technology Laboratory of the US DoE. iPOG™ users must realize that the estimates from iPOG™ are certainly no more accurate than the qualified measurement uncertainties, which NEA estimates at 10 – 15 % of the total Hg inventory in each test. Differences among cases that are smaller than these tolerances should be ignored.
Another important limitation on the estimates is due to the need to omit all but the essential process characteristics from the input data requirements. Consequently, unlike MercuRator™, the estimates from iPOG™ cannot possibly depict the distinctive features of particular gas cleaning systems. Three particular instances of these system-specific omissions should be kept in mind. First, users do not specify the temperatures of their particulate control devices. In ACI applications, they also do not account for the variable performance of carbon sorbents from different vendors, due to differences in preparation techniques, loadings, and surface areas. Most important, the estimates for Hg capture on the unburned carbon in LOI and also on carbon sorbents does not account for interference by adsorbed SO3. This interference can cut Hg removals in half on conventional and brominated carbon sorbents under the worst circumstances.
The second limitation from system-specific omissions pertains to the oxidation of elemental Hg vapor (Hg0) along SCR monoliths. The iPOG™ accounts for variations in the halogen concentration in the flue gas but it does not account for variations among the SCR design specifications and in the reactivities of the catalysts from different manufacturers and of different lifetimes in service. Collectively, the variations in the SCR design specifications are at least as important as the variations in the halogen concentrations in the flue gas, but these design specifications had to be omitted from the iPOG™ because they pertain to deeply technical and often proprietary information that many utility companies do not even have. The third limitation from system-specific omissions pertains to the retention of oxidized Hg vapor (Hg2+) in wet FGDs. In most FGD systems, essentially all the Hg2+ in the inlet flue gas is retained in the gypsum product or, occasionally, in the scrubber wastewater. Rarely, however, significant fractions of the dissolved oxidized Hg are re-emitted as Hg0. Consequently, iPOG™ users should realize that the relatively high Hg removals estimated for cleaning systems with WFGDs will represent significant over-predictions for the unusual situations where re-emission comes into play.
Users who reach a point in their analyses with iPOG™ where these limitations are hindering their development work on Hg control strategies can consider more comprehensive simulation tools. NEA’s MercuRator™ is one such tool that requires system-specific input specifications and also uses field-test data to calibrate baseline predictions.
B. Krishnakumar, S. Niksa, L. Sloss, W. Jozewicz, and G. Futsaeter, “Interactive process optimization guidance for mercury emissions control,” Energy Fuels, 26(8):4624-34 (2012).
L. Sloss, S. Niksa, B. Krishnakumar, W. Jozewicz, and G. Futsaeter, “Preparing for the UNEP 2013 Global Mercury Treaty with the Process Optimisation Guidance Document (POG and iPOG™),” PowerGen, Las Vegas, NV, 2011.