operating data analysis

Advanced Class Gas Turbine SCR and CO Catalyst System Operating Challenges

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.

For the full CCJ article, please click here.

SCR System Design Requirements Matrix
Posted by eking in advanced gas turbine, industrial catalyst services, NOx, operating data analysis, selective catalytic reduction, 0 comments

Combined Cycle Journal: “Consider the impact of new operating regimes on your SCR”

Incorporating real world operating data analysis into your SCR management plan is a topic that is gaining some well-deserved traction within the power industry. Combined Cycle Journal posted an article on November 8th, 2017 highlighting Environex Senior Engineer Andrew Toback’s presentation at the 2017 Combined Cycle Users Group Conference, “Impact of Real-World Operation on Catalyst Performance”. The article focuses on case studies that show the need to revise performance and lifecycle expectations based on actual operating conditions instead of original design parameters, particularly given the challenges and variability imposed by GT upgrades for higher output and lower turndown operation. Deeper knowledge of actual operating conditions and how they impact catalyst system performance sets the foundation for optimizing plant emissions performance.

With over 25 years of power industry experience, Environex understands the importance of evaluating catalyst performance at real world operating conditions, and that is why Full SCR System Evaluations provide the best value to our clients. Full SCR System Evaluations include a combination of catalyst testing, operating data analysis, and physical system inspection that enables us to distinguish system problems from catalyst problems. Laboratory testing of the catalyst across the range of plant operating conditions in conjunction with the data analysis allows us to identify the limiting operating case for your system and evaluate design and operating adjustments that can improve plant emissions performance.

Click here to read the article on ccj-online.com
Operating Data Analysis: Before and After Upgrade

Posted by eking in industrial catalyst services, NOx, operating data analysis, selective catalytic reduction, 0 comments