NOx 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 NOx 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 NOx 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 NOx emission limits for new gas turbines result in multiple decisions by operators. They include:

Process Selection

NOx Impact

Simple Cycle or Combined Cycle

NOx emissions per MWh are less with combined cycle

HEPA or Medium Efficiency Intake Filters

Impacts efficiency and, therefore, NOx

Inlet Air Conditioning

Increases mass flow and efficiency

Dry Low NOx Burner

Reduces raw NOx

Duct Burner

Adds to NOx but increases steam generation

Selective Catalytic Reduction (SCR)

Amount of catalyst is proportional to NOx reduction

Fuel Choice:  Natural Gas, Shale Gas, Oil

Oil will generate more NOx

Wet or Dry Cooling

Dry cooling increases NOx/MWh by up to 5% but saves water

High Temperature vs. Low Temperature Catalyst

Trade off between NOx/MWh and catalyst life and maintenance cost

Ammonia Slip

The higher the ammonia slip the lower the NOx

Automation and Instrumentation

Process optimization reduces NOx while meeting other criteria

 

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 NOx 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 NOx combustors (DLN) to minimize NOx 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 NOx 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 NOx/MWh.

NOx rises as CO decreases. There is also a tradeoff between NOx and efficiency. There is a tradeoff between ammonia slip and NOx. 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 NOx emissions. Where NOx 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 NOx. So NOx 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 NOx control program.

Ammonia Slip Monitor Selection for Gas Turbine SCR System

Decision Sequence

Decision

Category

Decision

Subject

 

Explanation

Level 1

Orientation

Specifier

Purchaser or A/E making the decisions for bid purposes

Level 2

Application

221112

Fossil Fuel 化石燃料

Gas-fired 燃气

Also applicable for

gas turbines used in the oil and gas industry

Level 3

Process

NOx Reduction

Can be used with both SCR and SNCR

Level 4

Location

SCR Outlet

Measure ammonia slip

Level 5

Pollutant

NH3

Ammonia which escapes SCR

Level 6

Product

CEM

Continuous emission monitor to measure ammonia after reaction with NOx

Level 7a

Type

In-situ

 

Big differences between measuring in stack and taking a small sample and conditioning and treating it

Level 7b

Type

Extractive

Extract sample, condition, and measure

Level 8a

Principle

Laser Spectroscopy

(TDL IR)

Advantages: interference free, in situ or extractive

Disadvantages: moisture interference, limited experience

Level 8b

Principle

Automated Wet Chemistry

Advantages: familiarity, quick set up and good for extractive periodic testing

Disadvantages: labor intensive  reagents

Level 8c

Principle

NOx Differential

Advantages:  tried and proven

Disadvantages: poor sensitivity to high NOx levels

Level 8d

Principle

UV Photometry

Advantages: tried and proven

Disadvantages: strong interference from SO2

Level 8e

Principle

Ion Mobility

Advantages: sensitive and interference free

Disadvantages: not suited for corrosive gases, slow response

Level 8f

Principle

IR-Multi

Component

Advantages: multiple species

Disadvantages: cost

 

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.