Primer on gas turbine NOx abatement

Kevin Carpenter, Siemens Power generation

Kevin Carpenter
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Because nitrogen oxides (NOx) contribute significantly to the ozone problem, some parts of the United States have pushed NOx emissions requirements for gas turbines to below 5ppm. These stringent regulations are forcing power plants to retrofit their gas turbine systems with NOx emissions abatement equipment. This month EL&P went to Kevin Carpenter, an engineer at Siemens, to get more information on the subject.

How is NOx formed?

NOx emissions are by-products of the combustion process. During combustion, nitrogen in the fuel is converted to NOx. A typical gas turbine converts the fuel bound nitrogen to NOx at a rate of about 50 percent. The second way NOx is formed is referred to as “prompt NOx formation.” It occurs when nitrogen molecules in the air are broken down by organic components in the fuel and combine with oxygen to form NOx. The most prevalent way NOx is formed is through the “thermal NOx mechanism.” High flame temperatures cause the nitrogen and excess oxygen molecules in the air to combine to form NOx. Flame temperature, residence time at temperature, the amount of fuel/air mixing, nitrogen content of the fuel and the quantity of excess air used for combustion determine the amount of NOx in the exhaust gas.

What can be done to reduce NOx emissions on a gas turbine?

A prime area where NOx emissions can be reduced is in the combustor. Newer gas turbines employ dry low NOx (DLN) combustion technology to achieve high single digit NOx emissions. With DLN technology, the air and fuel are premixed in order to atomize the fuel in the air to allow combustion at a lower temperature. In older diffusion flame technology, the fuel and air are kept apart until combustion occurs. Even with DLN technology, gas turbines require additional NOx abatement equipment in order to reach the 5 ppm and lower NOx emissions levels required in some parts of the country.

For mature frame gas turbines, there are some combustion upgrades available to reduce NOx emissions. While some frames have a DLN combustion retrofit available, most are retrofit with a water or steam injection system. Water and steam injection systems reduce the flame temperature in the combustor and thus hinder the formation of thermal NOx. In addition to contributing to an increase in output, these types of combustor upgrades can reduce NOx emissions from 150 ppm on an uncontrolled mature frame to 25 ppm in some cases. Even the addition of wet compression, which serves to increase power output, can reduce NOx emissions by up to 35 percent.

Currently there are no combustor technologies available for turbines larger than 1.5 MW to achieve less than 5 ppm NOx emissions. The only proven way to achieve low single digit ppm NOx levels is through the employment of a post-combustion method called selective catalytic reduction (SCR).

What is SCR?

Discovered in 1957, selective catalytic reduction (SCR) is a process in which ammonia is added to the exhaust and passed through a catalyst bed in a reactor. The catalyst facilitates a reaction between the ammonia and the NOx, forming harmless nitrogen and water. The reaction generally occurs in a temperature range between 280 °C and 500 °C, depending on the type of catalyst used. A typical SCR system consists of an ammonia storage facility, ammonia forwarding equipment, an ammonia injection grid and a reactor with catalyst.

Does plant configuration matter when retrofitting with SCR?

For SCR retrofits, combined cycle gas turbine plants are better candidates than simple cycle plants. This is due to the fact that the exhaust gases are already being cooled in the heat recovery steam generator (HRSG) to temperatures acceptable for SCR operation. With an existing HRSG, the challenge is to find space within the HRSG that provides the proper temperature conditions and is large enough to house the SCR reactor.

In older combined cycle plants, work on the HRSG, such as cutting and moving the stack to make room for an SCR reactor, is very difficult. Due to space constraints, performing a combustion upgrade on the gas turbine to reduce NOx emissions is the most practical solution. This allows the owner to construct the smallest possible SCR, thus reducing capital and operating costs.

Costs can be higher in simple cycle plants because a reactor to house the catalyst must be constructed. Also, exhaust temperatures in newer turbines are much higher than those acceptable for SCR operation. This necessitates the addition of cooling air to reduce the exhaust temperature. Again, an owner should first consider a combustion upgrade to lower gas turbine NOx emissions as much as possible and then add a smaller, less costly SCR system.

What are some other considerations with SCR?

When installing an SCR, regulatory requirements, inlet NOx concentration, exhaust temperature, flow conditions, backpressure, controls and overall system cost should be considered. To ensure that the SCR system functions properly, it is best to purchase SCR equipment from vendors who are intimately familiar with the workings of gas turbines. An improperly designed SCR system can have serious negative effects on the performance of the gas turbine.

What’s ahead for gas turbine NOx emissions reduction technology?

Many companies are investigating the possibility of achieving low single digit NOx emissions through the application of catalytic combustion. In catalytic combustion, hot premixed combustible gases are passed through a special catalyst honeycomb in the combustor. Combustion begins due to the catalyst facilitating the reaction between air and fuel. Catalytic combustion promises to allow the combustion of very lean mixtures (such as 3 percent methane in air) at much lower temperatures, thereby lowering the contribution of thermal NOx to the exhaust significantly. At present, catalytic combustion has been applied to gas turbines in the 1.5 MW range.

Carpenter is a senior marketing engineer in the gas turbine project marketing and field service integration group at Siemens Power Generation. Siemens offers turnkey retrofit selective catalyst reduction (SCR) systems for the reduction of NOX emissions on older gas turbines. These SCR systems are targeted to allow customers with older mature frames to achieve single digit NOX emissions.

Carpenter holds a bachelor’s degree in ceramic engineering from the University of Illinois, Urbana and an MBA from Southern Illinois University, Edwardsville, and has more than 12 years experience in the development and application of ceramic materials for industrial applications. He has published several papers focusing on selective catalytic reduction for mature frame gas turbines.

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