NOx

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

What is Selective Catalytic Reduction (SCR) Catalyst?

Selective catalytic reduction is a catalytic reaction which uses ammonia to reduce oxides of nitrogen (NOx) into harmless nitrogen (N2) and water. Because of this function, the SCR catalyst is sometimes referred to as a NOx catalyst in the industry. It has been used in many industrial facilities including power plants and chemical refineries to reduce NOx emissions since the 1980’s. It has been used in automotive applications since the mid 2000’s to reduce NOx emissions from heavy duty and light duty diesel vehicles.

Industrial Selective Catalytic Reduction Catalyst

The active component of the catalyst itself can be one of several different materials; vanadium pentoxide is frequently used in industrial settings while copper and iron zeolites are frequently used in automotive applications. Different materials are used because of differences in operating conditions, temperatures, and resistance to impurities in the fuel and environment.

On industrial installations, SCR catalysts are installed along with an Ammonia Injection Grid, or AIG for short. The AIG sprays ammonia, typically stored as aqueous ammonia, anhydrous ammonia, or a urea solution, into the exhaust stream. The ammonia adsorbs onto the SCR catalyst, where it reacts with NOx and oxygen to form nitrogen and water. In a vehicle, onboard urea, known as Diesel Exhaust Fluid (DEF) or AdBlue, is sprayed into exhaust gas upstream of the catalyst brick and diffused using a mixer to optimize airflow and ensure thorough vaporization and even distribution in the exhaust prior to entering the catalyst.

If an Industrial SCR catalyst NOx removal efficiency declines or ammonia usage increases significantly, it could signal a serious problem with the catalyst and inspection and performance testing by a qualified company should be scheduled. See Environex’s SCR System Evaluation page for more information on our services.

In an industrial facility, the AIG may need to be tuned periodically to ensure proper distribution of ammonia into the SCR catalyst. If there are areas of high and low ammonia flow entering the system, the catalyst will not be used effectively and higher NOx emissions and ammonia may result. See AIG Tuning for more information.

Posted by eking in industrial catalyst services, NOx, selective catalytic reduction, 2 comments

What is NOx?

NOx is short for oxides of Nitrogen. It is a class of pollutants which includes nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). When emitted into the atmosphere, they can cause an array of problems, including acute and chronic health problems and smog. Nitrous oxide is also a potent greenhouse gas (GHG). For these reasons, NOx emissions are regulated in many areas of the world.

NOx Smog in Los Angeles

Nitrogen oxides are formed during combustion as high temperatures cause nitrogen from the atmosphere to oxidize. Because only temperature and heat are needed to form NOx, even lightning strikes form the compounds. NOx emissions from industrial facilities like power plants are regulated, as are the emissions from vehicles.

The level of regulation depends on the regulatory agency and the requirements of the area; more densely populated areas tend to require higher NOx reductions than rural areas. Geographic features also contribute to atmospheric NOx buildup, which makes some areas, such as Mexico City and Los Angeles, more susceptible to smog.

Posted by eking in industrial catalyst services, NOx, 0 comments