Entity

Time filter

Source Type

Belmont, CA, United States

Niksa S.,Niksa Energy Associates | Sibley A.F.,Southern Company
Air and Waste Management Association - 8th Power Plant Air Pollutant Control Mega Symposium 2010 | Year: 2010

This paper expands the validation of the "SCR Catalyst Model" that quantitatively describes simultaneous NO reduction and oxidation of SO 2 and Hg 0 along SCR catalysts in coal-derived flue gas streams. The model relates catalyst material composition and bimodal pore size characteristics in a direct, quantitative way to the reactivities for NO reduction, Hg 0 oxidation, and SO 3 production in commercial, full-scale SCR reactors. SCR monoliths sustain two chemically distinct regions. In the inlet region, strong NH 3 adsorption minimizes the coverage of chlorinated and sulfated surface sites, so NO reduction inhibits Hg 0 and SO 2 oxidation. But once the NH 3 has been consumed, the chlorinated surface coverage surges, and the Hg 0 oxidation rate rapidly accelerates, even while the HCl concentration in the gas phase is uniform. This two-stage sequence is favorable for SO 3 control, because NH 3 inhibition makes the SCR perform like a shorter unit. Conversely, NH 3 inhibition is a serious impediment to Hg 0 oxidation. Ammonia inhibition also eliminates the benefit of the rapid film mass transfer at the SCR inlet from promoting Hg 0 oxidation. In many cases, the Hg 0 oxidation rate becomes limited by film transport soon after the Hg 0 begins to oxidize, so that none of the catalyst internal surface area is utilized. The predictions were validated with pilot-scale data to demonstrate the crucial impact of NH 3 inhibition on SCR performance, and with limited full-scale data for catalysts from a single vendor.


Niksa S.,Niksa Energy Associates | Sibley A.F.,Southern Company
Industrial and Engineering Chemistry Research | Year: 2010

This analysis relates catalyst material composition and bimodal pore size characteristics in a direct, quantitative way to the reactivities for simultaneous NO reduction, Hg° oxidation, and SO 3 production along utility selective catalytic reduction (SCR) reactors. SCR monoliths sustain two chemically distinct regions. In the inlet region, strong NH 3 adsorption minimizes the coverage of chlorinated and sulfated surface sites, so NO reduction inhibits Hg7deg; and SO 2 oxidation. Once the NH 3 has been consumed, however, the chlorinated surface coverage surges by orders of magnitude, and the Hg° oxidation rate rapidly increases, even while the HCl concentration in the gas phase remains uniform. Ammonia inhibition also eliminates the benefit of the rapid film mass transfer at the SCR inlet from promoting Hg° oxidation. In many cases, the Hg° oxidation rate becomes limited by film transport soon after the Hg° begins to oxidize, so that none of the catalyst internal surface area is utilized. Shifting the pore size distribution toward macropores in a final catalyst stage appears to be an effective means for directly enhancing Hg° oxidation. The predictions were validated with pilot-scale data to demonstrate the crucial impact of NH° inhibition on SCR performance and with full-scale data for catalysts from a single vendor to show quantitative consistency across broad ranges of coal Cl content, gas hourly space velocity (GHSV), NH 3/NO ratio, and catalyst specifications. © 2010 American Chemical Society.


Niksa S.,Niksa Energy Associates | Krishnakumar B.,Niksa Energy Associates | Ghoreishi F.,Southern Company
Journal of the Air and Waste Management Association | Year: 2016

ABSTRACT: Selective catalytic reduction (SCR) catalysts are deactivated by several mineral and metallic trace elements at highly variable rates determined by fuel quality and furnace firing conditions. With a loss in activity, NO is reduced over a longer inlet length of the SCR monolith, which leaves a shorter trailing section to sustain the most favorable conditions to oxidize Hg0 and SO2. Since virtually no operating SCR was designed for Hg oxidation and since different monoliths are routinely combined as layers in particular units, the Hg oxidation performance of any SCR fleet is largely unmanaged. The analysis in this paper directly relates a measurement or manufacturer’s forecast on the deterioration in NO reduction with age to corresponding estimates for oxidation of Hg0. It accommodates any number of catalyst layers with grossly different properties, including materials from different manufacturers and different ages. In this paper, the analysis is applied to 16 full-scale SCRs in the Southern Company fleet to demonstrate that catalyst deactivation disrupts even the most prominent connections among the Hg0 oxidation performance of commercial SCRs and the behavior of fresh catalysts at lab, pilot, and even full scale. Implications: Catalyst deactivation confounds even the most prominent connections among the Hg0 oxidation performance of commercial SCRs and the behavior of fresh catalyst at lab, pilot, and even full scale. The halogen dependence has been emphasized throughout the literature on catalytic Hg0 oxidation, based on a large database on fresh catalysts. But for deactivated catalysts in commercial SCRs, the number of layers is much more indicative of the Hg0 oxidation performance, in that SCRs with four layers perform better than those with three layers, and so on. The new qualified conclusion is that Hg0 oxidation is greater for progressively greater HCl concentrations only among SCRs with the same number of layers, even for an assortment of catalyst design specifications and operating conditions. © 2016 A&WMA.


Krishnakumar B.,Niksa Energy Associates | Niksa S.,Niksa Energy Associates | Sloss L.,International Energy Agency | Jozewicz W.,Arcadis | Futsaeter G.,United Nations Environment Programme UNEP
Energy and Fuels | Year: 2012

This paper introduces two new tools to quickly identify and assess a broad range of Hg emissions controls for utility gas cleaning systems worldwide. The Process Optimization Guidance (POG) document summarizes the available options for mercury control at most coal-fired power plants, covering everything from efficiency improvements and fuel switching through co-benefit effects (maximizing Hg capture in existing pollution control systems) to Hg-specific sorbent and oxidation technologies. The POG includes a "decision tree" concept that helps the reader determine the potential compliance strategies for particular coal-fired gas cleaning systems. The Interactive Process Optimization Guidance (iPOG) is a user-friendly software package that formalizes the "decision tree" concept in the POG document. It accurately estimates Hg removals and emissions rates for broad ranges of coal quality, the most common configurations for furnace firing and flue gas cleaning, and Hg controls, both inherent and external. Two case studies presented in this paper show its utility in addressing "What if⋯?" scenarios, in which the impact of adding Hg controls to existing cleaning systems can be quickly and conveniently evaluated. The iPOG supports compliance strategies based on coal cleaning and blending, stronger inherent Hg removal in new air pollution control units for NO x and SO x control, and dedicated external Hg controls, such as activated carbon injection (ACI) and halogen addition. This flexibility is compounded by minimal input data requirements and extremely fast execution times. This makes the iPOG useful for those who are new to the technicalities within the issue of Hg control, such as policy makers or even operators in developed countries or countries with economies in transition. Relative novices can "play" with the iPOG, selecting generic coals and simple plant design options, and then discover just how much simple changes in coal characteristics or plant operation may affect emissions. Being based on statistical regressions of an American Hg field test database and streamlined input data requirements, iPOG cannot possibly resolve differences among different Hg control strategies within the measurement uncertainties or depict the distinctive features of particular gas cleaning systems. Such limitations are especially pronounced whenever SO 3 adsorption interferes with Hg removal via ACI and also when distinctive selective catalytic reduction (SCR) design specifications strongly affect Hg 0 oxidation along a SCR catalyst monolith. Whereas the Hg removals for such situations are accurately predicted by previously reported reaction mechanisms, they are beyond the current scope of iPOG. However, the iPOG is fully capable of estimating Hg emissions from a preferred control scenario ahead of expert analysis. © 2012 American Chemical Society.


Krishnakumar B.,Niksa Energy Associates | Niksa S.,Niksa Energy Associates
Air and Waste Management Association - 8th Power Plant Air Pollutant Control Mega Symposium 2010 | Year: 2010

This paper introduces a combined homogeneous and heterogeneous SO 3 production mechanism to determine whether or not particular flue gas cleaning conditions promote SO3 condensation anywhere upstream of the ESP. This mechanism was validated against measurements at different locations along the gas cleaning systems at fourteen power plants representing the entire range of coal-S, furnace stoichiometry, and gas cleaning conditions found in commercial applications. The SO3 production mechanism was then integrated into previously validated Hg transformation mechanisms to account for inhibition of Hg oxidation and removal due to SO3 condensation on fly ash, UBC and activated carbon. This analysis was subsequently used to interpret Hg removals for about two dozen test measurements at Plant Daniel. These tests included different coal blends, ACI concentrations, conventional and brominated activated carbons, and SO 3 concentrations. The simulations clearly identified the tests affected by SO3 interference and predicted the Hg removal by ACI to within 15 % of the test measurements for 19 of the 22 tests at this site.

Discover hidden collaborations