11-06-09 am

Hot Topic Hour at 10 a.m. November 5 on “Gas Turbine Air Pollution Control” Includes Insights on High Temperature Catalysts, PEMS, and Catalyst Cleaning

There are many problems and issues unique to control of air pollution from gas turbines that are far different than those faced by coal-fired power plants. Five highly qualified speakers provided solutions for many of these issues in the webinar yesterday.

Most gas turbines are operated as peaking units and have rapid ramp rates which result in severe temperature and flow velocity changes. In the last nine years there have been as many SCR units installed on single cycle units as on combined cycle units. Previously, SCR was not required on single cycle units because they were generally operated intermittently and the challenge of dealing with the high temperature required gas cooling or use of an unproven high temperature catalyst.

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 gas turbine 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. Back-pressure however leads to a reduction in power and temperature changes in the turbine.

Robert (Bob) McGinty, Sales Manager, Capital Projects and OEM SCRs Johnson Matthey, Inc. Stationary Emission Control Business Unit, discussed the design, technical considerations, control and operational aspects of SCR systems with examples of actual installations. His presentation consisted of the following.

a. NH₃ tank and forwarding pumps, aqueous and anhydrous reagents. When using aqueous ammonia seal-less mag drive pumps are used to ensure against leakage.

b. NH₃ vaporization skid, electric and hot flue gas system is more compact than the aqueous system and does not need pumps. Heaters are used to create the needed pressure.

c. The NH₃ injection grid - arrangement and design is critical to minimization of ammonia slip.

d. SCR catalyst utilizes narrow pitches and thin walls. Thus the required cubic volume is low compared to coal-fired power plants. Cleaning becomes more difficult because of the fragility of the catalyst structure. However, there are few contaminants and catalyst life is generally eight years or more.

e. CO catalyst should be placed ahead of the SCR catalyst otherwise they would convert ammonia to NO_x.

f. Cold flow modeling has been found to be more cost effective than CFG.

g. Constructability considerations have a major cost impact. Modular construction requires a larger capital purchase but saves money in field erection.

McLeod Stephens, Business Development Manager TDC Filter, discussed life assessment of inlet air filters for gas turbines. The filter is a low cost item, but failure can lead to major turbine damage. Thus, it is important to monitor the performance of the filter and prepare to replace it at the optimum time. TDC has a whole program to help the user in this regard. Increases in pressure drop are only one indicator relative to replacement. Corrosion can weaken filters over time, so the timing on replacement of a weakened filter is important. Measurements using the Mullen test can determine whether the media is maintaining its wet and dry burst strength.

David Haehnle of CMC Solutions LLC, described a robust statistical hybrid model PEMS (Predictive Emissions Monitoring System) that has been proven to be a cost-effective continuous compliance monitoring solution for nitrogen oxides and carbon dioxide emission trading programs for gas turbines. PEMS are extremely cost-effective when compared to conventional continuous emission monitoring (CEMs) equipment. Many regulatory agencies and end users are unfamiliar with PEMS and do not give this option a fair evaluation. However, CMC has installed more than 100 of these systems. A recent order for more than 40 PEMS was placed in Saudi Arabia.

Thomas "Nathan" White, Director Business Development SCR/DeNOx Catalyst & Technology, Haldor Topsoe, discussed the use of corrugated composite SCR catalysts in gas turbine applications. Haldor Topsoe has more than 184 SCR systems installed to clean gas turbine exhaust. Six hundred and forty-five SCR units are installed in stationary source applications. The company has developed both low and high temperature catalyst. It has an installation operating at 300°F at Cedar Sinai and a 2008 installation at VRA operating at 1050°F. Several California installations have been achieving more than 98 percent NO_x reduction since 2002. Installations in New England and California are meeting NO_x and NH₃ levels below 1 ppm.

Mike Cooper of SCR Tech / CoaLogix, explained the considerations for cleaning, rejuvenating, or renovating gas turbine SCR catalyst. Unlike catalyst in coal-fired power plants, the gas turbine catalyst can last for eight years or more with just periodic monitoring and cleaning. However, as the effectiveness of the catalyst is reduced the ammonia consumption increases. Therefore, catalyst renovation or replacement can often be justified based on ammonia cost savings. In one example where the turbine operates 70 percent of the time the optimum replacement point was four years.

The bios, abstracts and photos for “Gas Turbines Air Pollution Control” can be viewed as follows: November 5, 2009 BIOS, PHOTOS, ABSTRACTS.htm

The individual slides for “Gas Turbines Air Pollution Control” recording are located in our NO_x Decision Tree as follows:

Bob McGinty – Johnson Matthey

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