One of the tradeoffs for the higher efficiency of new advanced class gas turbines (namely the G-, H-, and J-class machines) is increased thermal NOx make, which is caused by higher firing temperatures in the gas turbine combustors. The result is GT exit NOx concentrations in the 25 – 35 ppmvdc range for the advanced class turbines, which is significantly higher than the 9 – 20 ppmvdc range for their F-class predecessors.
Since regulators tend to view all gas turbines as being the same, they assume that the same stack emissions levels can be met regardless of the turbine technology. Because of this, the advanced class turbines are expected to achieve the same stack emissions levels as F-class machines without giving consideration to the differences in combustion dynamics between the different classes of turbines.
Modern day F-class machines are expected to achieve stack limits of 2.0 – 2.5 ppmvdc NOx and 2.0 – 5.0 ppmvdc ammonia slip through the use of selective catalytic reduction (SCR) systems, which use ammonia as the reducing agent to convert NOx across a catalyst. These systems provide 72 – 90% NOx reduction while being allowed to slip 22 – 25% excess ammonia (2 ppm NH3 slip/9 ppm inlet NOx to 5 ppm NH3 slip/20 ppm inlet NOx). By contrast, the advanced class machines must provide 90 – 94% NOx conversion while only being allowed to slip 7 – 8% excess ammonia to achieve the same stack emissions levels.
This is increase in required NOx reduction accompanied by the decrease in allowable excess ammonia results in increased SCR system performance requirements that are by no means trivial. As NOx conversion requirements increase to 90% and above, the systems have much less tolerance for non-ideal performance, particularly with such low levels of allowable excess ammonia. As a result, SCR systems for advanced class turbines require higher SCR catalyst volumes, near-perfect ammonia-to-NOx distribution, and air-tight seals around the SCR catalyst perimeter and all catalyst modules. In order to reliably meet stack emissions requirements, these sites will need pro-active SCR management plans that include proper ammonia injection grid design and tuning, a catalyst testing program that takes into account all plant operating modes, thorough catalyst system maintenance, and proper design and selection of replacement catalyst that adapts to changing SCR system needs.
Additional challenges for advanced class turbine SCR and CO catalyst systems are the requirements for low turndown operation, fast start-ups, and frequent cycling. Historically, gas turbines have only been required to operate down to 50% of baseload. Many of the new, advanced class sites are being asked to operate at loads as low as 20%, start up and achieve emissions compliance within shorter timeframes, and to cycle frequently between low loads and baseload. These requirements put additional stress on catalyst systems, primarily attributed to sub-optimal operating temperatures and elevated gas turbine NOx and CO emissions under these operating conditions.