Air Pollution Control (APC) for Gas Turbines - Hot Topic Hour September 19, 2013

Although APC systems have been in operation on gas turbine (GTs) for many years and are proven to produce low emission rates (as low as 0.8 ppm NO_x with undetectable levels of carbon monoxide), there are many problems and issues unique to control of air pollution from GTs that are far different than those faced by coal- or oil-fueled steam plants. For example, GTs that are operated as peaking units have rapid ramp rates that result in severe temperature and flow velocity changes. The SCR catalyst must be carefully selected based on the operating temperature. Then the balance between the cost of catalysts for different operating temperatures and the cost of cooling or dilution systems must be evaluated. If a CO catalyst is included in the design, it may have an influence on SCR catalyst performance. If the GT is also operated on oil when gas prices are high then the sulfur content of the oil must be considered (in regard to SO₂ to SO₃ conversion and the resulting ammonium bisulfate after the SCR) as well as vanadium content. Gas turbine exhaust is very turbulent and flow modification devices may be required to ensure good mixing of ammonia with the exhaust gas and uniform flow across the catalyst. Backpressure however leads to a reduction in power and temperature changes in the turbine.

The following speakers addressed the current issues related to GT emissions control.

Bill Gretta, VP for the Power Plant Solutions Division Eneractive Solutions, Inc., provided advice for single cycle SCR purchasers. Eneractive Solutions provides system design and optimization, overall SCR and CO catalyst system design, independent SCR catalyst management, SCR reactor inspections, evaluation and testing, ammonia system design and flow modeling. Bill made the following points:

• Simple cycle SCR poses more design challenges than combined cycle.
• Water injection can be used to reduce engine NO_x to 25 ppm but will increase CO.
• NH₃ slip is difficult to maintain.
• CO limit is difficult to maintain with excessive water injection.
• With inlet of 25 ppm of NO_x and 90 percent removal requirement, ammonia slip will be an issue.
• Tempering air adds to the complexity but the alternative of high temperature catalyst is expensive.
• Flow modeling should always be done.
• Improved seals can greatly reduce ammonia slip.  

Bob McGinty, Senior Manager Business Development, SCR NO_x Control Systems at Mitsubishi Power Systems Americas, Inc., reviewed aspects of system design. Mitsubishi is considered one of the original pioneers of SCR systems and catalyst technology with more than 600 SCR systems operating worldwide.  Mitsubishi catalyst technologies have been licensed globally with focus on the homogeneous honeycomb catalyst technology and joint development of plate catalyst technology.  In 1989 Mitsubishi teamed with Corning to form the joint equity US-based catalyst company, Cormetech, to manufacture and supply extruded homogeneous honeycomb catalyst technology products.

Here are some takeaways from Bob’s speech:

• Modular design can be a money saver.
• Most customers are opting for the standard catalyst rather than high temperature catalyst.
• Marsh Landing includes four single cycle units with tempering air. It is a large project completed in April 2013.  It must cycle up and down frequently due to erratic delivery of solar and wind power to the grid.
• There are maintenance challenges with frequent cycling.
• Dust does build up over time in the catalyst. If the unit is near a landfill for example, the catalyst degradation is likely to be greater.
• HEPA inlet air filters can reduce buildup on the catalyst but have their own issues (cost and pressure drop).
• Offshore gas and oil installations are increasingly using SCR. It has been typical offshore in California and is now being required in the Gulf.
• Brazil and other South American countries are requiring SCR.

Craig Sharp, Key Account Manager SCR/DeNOx Catalyst & Technology, Haldor Topsoe, introduced a new catalyst.

The CO and SCR catalyst can be combined into one module. There are four configurations including separate catalysts with the CO catalyst before or after the SCR catalyst and the combined SCR-CO or separate SCR catalyst followed by the CO catalyst. The combined catalyst offers high performance in less space.

Gas Turbine Air Treatment needs to be viewed as an Integrated System

Two recent developments in gas turbine air treatment are the expanded use of HEPA filters for gas turbine intake air and SCR with both CO and NO_x catalyst for the tail-end gas. Some dust that the inlet filter does not remove is going to deposit on the catalyst. A facility can use a very small catalyst pitch and save lots of money, but the increase in pressure loss along with deposition will increase operating cost and decrease electricity output. Therefore, the choice of inlet filter needs to be viewed initially in terms of catalyst pitch selection and then in terms of catalyst life.

First in California and now in other places, the stack gas emission limits are lower than the ambient air particulate matter concentration. In these cases, the inlet air filter becomes a key element in stack gas compliance. Gas turbine inlet ambient air undergoes a series of treatments.  The initial treatment is to remove large weather-related contaminants e.g., snow, rain, etc. The humidity and temperature of the ambient air are also adjusted to increase the weight and therefore electrical output. This treatment can range from fogging nozzles to a full air conditioning system. Droplets are formed, coalesced and removed. Particulate filtration can be with a series of filters ranging from coarse to HEPA or it can take place with self-cleaning cartridges. 

One alternative for NOx control during combustion is the low NO_x burner. Another alternate is water injection. Once the air has been mixed with the gas and combusted it passes through other systems in the gas path. In a combined cycle process there is likely to be a duct burner to adjust HRSG steam temperature. This can add to the pollutants. CO and NO_x catalysts are also utilized. However the accompanying ammonia injection can create ammonia slip which is regulated and also tends to foul the catalyst.

With single cycle systems many of the air treatment challenges are more complex as was pointed out by Bill Gretta in the webinar. If a low temperature catalyst is used, tempering air is needed. This creates a challenge in providing laminar flow to the catalyst. If high temperature catalyst is used, higher catalyst costs are encountered and higher maintenance is possible.

The air treatment needs are not static. The higher performance turbines are more likely to be compromised by small particles. The use of gas turbines and certainly the use of SCR are expanding to applications that are more challenging. The seawater and salts found in marine applications including floating production, storage and offloading (FPSO) units are examples. 

Application in refineries in South America where inlet air quality may be low and fuel includes less than pristine liquids is another example. The rapid cycling of turbines complementing wind and solar is another newer challenge.

The changes and increasing complexity relative to gas turbine air treatment have demonstrated the need for a system to aid decision makers. McIlvaine will address this need with Gas Turbine Air Treatment Global Decisions Positioning System™ (GDPS). Details on this system will be forthcoming.

Bios, Abstracts and Photos can be seen at BIOS, ABSTRACTS, PHOTOS - 9-19-13.htm

The individual presentations are as follows:

• Air Pollution Control for Gas Turbines Webinar - Hot Topic Hour September 19, 2013  (Webinar Recording  Dated:  9/19/2013)

• Air Pollution Control Catalyst and Technology by Craig Sharp, Haldor Topsoe - Hot Topic Hour September 19, 2013  (Presentation  Dated:  9/19/2013)
• Simple Cycle SCR Designs by Bill Gretta, Eneractive Solutions - Hot Topic Hour September 19, 2013  (Presentation  Dated:  9/19/2013)
• SCR / CO / VOC Control by Bob McGinty, Mitsubishi Power Systems - Hot Topic Hour September 19, 2013  (Presentation  Dated:  9/19/2013)