NO_x CONTROL OPTIONS

Regulators, operators and suppliers all have to understand the relationship between specific regulations and the technologies to best meet the regulation requirements. In the case of meeting NO_x emission regulations from gas turbine exhausts there are hundreds of options which must be compared and choices made. McIlvaine is providing decisions systems for both users and suppliers.

There is a chain reaction as NO_x emissions from gas turbines are regulated. The European Union has just proposed substantial reductions in the emissions for medium sized gas turbines in the 28 member countries. In the U.S., new ambient air quality standards and tough NO_x emission limits for new gas turbines result in multiple decisions by operators. They include:

The fact that shale gas will be burned in some newly planned Pennsylvania plants adds a new dimension. Many gas turbines while using natural gas as a primary fuel also use diesel oil as a backup fuel. So there is the potential to emit sulfur compounds and to increase NO_x emissions.

The gas turbine combined cycle system often operates with a duct burner ahead of the HRSG. This can maximize the output of the steam turbine but adds an additional source of air pollutants.

Gas turbines can be fitted with dry low NO_x combustors (DLN) to minimize NO_x emissions. This is adequate to meet air regulations in many countries. However, in the U.S., Japan and certain other countries the emission limits for NO_x are so low that it is necessary to inject ammonia and install selective catalytic reduction (SCR) units.

In the U.S. it is also increasingly necessary to install SCR units on peaking turbine exhausts. This presents problems due to the high exhaust temperatures. If one uses a high temperature catalyst there are concerns about catalyst life. If a low temperature catalyst is used, then tempering air is necessary. This increases NO_x/MWh.

NO_x rises as CO decreases. There is also a tradeoff between NO_x and efficiency. There is a tradeoff between ammonia slip and NO_x. The task of optimizing performance to best meet all the goals requires smart sensors, proper information flow and then optimization decisions followed by rapid implementation. At the heart of this program is the emissions monitoring system.

Only very small turbines can operate with periodic stack testing. Predictive emissions monitors are allowed for larger turbines in some countries. These are software systems which use measured parameters such as temperature to calculate NO_x emissions. Where NO_x limits are low a continuous emissions monitoring system is required. It can be in-situ or extractive. Several different measurement techniques are available.

Ammonia is injected ahead of the SCR catalyst to react with the NO_x. So NO_x measurements before the SCR as well as in the stack are necessary to adjust ammonia input. However, ammonia slip (NH₃) is limited in many locales and this requires the use of ammonia slip monitors. The decisions relative to ammonia slip monitor selection are no greater or less than for the other alternatives shown above. A detailed examination of the ammonia slip measurement options illustrates the myriad of decisions which are necessary in the total NO_x control program.

Ammonia Slip Monitor Selection for Gas Turbine SCR System

The decision maker moves through seven levels of decisions before arriving at the selection of a specific type of NH₃ monitor. Each has advantages and disadvantages. The choice is impacted by the severity of the regulations and also the requirements to measure other pollutants.