Nitrogen Oxides Emission Control

In a simple overview of a steam power plant, heat is used to create steam that is used to spin a turbine generator that creates electricity. Heat required to convert water to steam for the process is gathered from different sources. The most common method is the burning of a carbon-based fuel like coal, natural gas, wood, or oil.

By Jim Scott and Bobby Dennis July 1, 2004
What is the Nernst equation?

In a simple overview of a steam power plant, heat is used to create steam that is used to spin a turbine generator that creates electricity. Heat required to convert water to steam for the process is gathered from different sources. The most common method is the burning of a carbon-based fuel like coal, natural gas, wood, or oil. Choice of fuel used at a particular plant depends on geographic region, boiler type, and fuel price. The efficacy of the combustion process governs the safety, fuel use, and emissions of the heat generation process.

Variations in the burning process require minor changes to the fuel/air mixture to maintain the correct ratio and efficient boiler operation. The process of adjusting the fuel or air is called boiler trim. Methods of boiler trim vary by boiler manufacturer, fuel type, and control scheme. Some methods control the amount of air injected into the system, others control the amount of fuel, and still others control both. However, in any scheme, it is critical, for safety and efficiency, to know the amount of oxygen in the process. Most power plants use a measurement device that continuously measures oxygen content near the combustion zone.

Logically, errors in air measurement have direct effect on the control of the boiler, including process safety, fuel use, and plant emissions, including NOx. As a general rule of thumb, a 10% increase in excess air will increase fuel usage by 1%. Fuel use is greatly increased when the boiler operates in oxygen-deficient conditions. Excess air makes extra nitrogen available and requires additional heat to maintain combustion temperatures, creating favorable conditions for the formation of thermal NOx. Some excess air is necessary for safe and efficient boiler operation; fuel has surface area, and takes time to burn. Keeping the excess air volume near and slightly above stoichiometric values (quantitative relationship among reactants and products in a reaction) for the particular fuel minimizes formation of NOx, and keeps fuel use at a minimum.

A common method of oxygen measurement for power plants is actually a subset of the fuel cell type of measurement. The zirconia oxygen measurement has significant differences that make it acceptable for in-situ operation. It uses a hot electrochemical, solid electrolyte to measure oxygen content. It has two precious metal electrodes on either side of a stabilized zirconia, solid electrolyte. The electrodes and zirconia comprise the measurement cell. Zirconia electrolyte becomes porous to oxygen ions at elevated temperatures. One side of the measurement cell is exposed to the flue gas containing oxygen, while the other is placed under the influence of a reference gas.

For simplicity and cost, clean and dry plant air, around 21% O 2 , is usually the source of the reference gas. Any difference between the oxygen concentrations of the process and reference gas causes a voltage inversely related to the oxygen content of the process gas. Zirconia resists chemical attack and generally has low sensitivity to other flue gases such as CO 2 . The requirement for an elevated temperature for the electrochemical reaction allows the measurement to be inserted directly ( in situ ) into the flue gas without extraction.

Impacts on measurement

An illustration of the relationship between excess O2 injected into a boiler and resulting NOx.

Zirconia oxygen measurement technologies depend on the partial pressure of oxygen, which changes as the water vapor drops out of the sample. It is difficult to know the new moisture content of the gas stream without an accurate gas temperature measurement, but it is likely that there is still some moisture in the sample. Without knowing the amount of moisture lost in the extraction process, it is impossible to compare the measurement data of in situ product with that of extractive product. So, in most extractive systems, the process gas is taken from the flue duct and passed through a drying apparatus, in effect removing all water.

The dried sample is then passed over the electrochemical sensor and oxygen concentration is measured. Definition of a dry sample allows predictable and repeatable conversion between the in situ (wet) sample and the extractive (dry) measurement. Actual differences between the measurements vary with fuel type, ambient moisture content, and boiler heat rates, but for a coal fuel, generally a difference between 0.2 and 0.7% is realistic. The ‘In situ vs. extractive’ graphic shows actual differences between wet and dry measurements for a power plant burning PBR (pebble bed reactor) fuel. Note that the dry O 2 measurement is actually higher than the wet. This is contrary to intuition; loss of water actually increases the partial pressure, and thus concentration, of the oxygen for a given volume.

Comparison of wet and dry O2 measurement shows that the dry sample has more O2.

Many zirconia-based detectors have built-in heaters to maintain the temperature setpoint required for operation. Heaters are generally a thin wire, shunt resistor located close to the zirconia cell. Feedback for closed-loop temperature control is measured by a thermocouple, due to the low cost and simplicity of the measurement.

There are also heater-less detectors that use ambient temperature of the process to provide the heat needed for cell operation. These products are limited to use after the process temperature reaches the operational temperature of zirconia. Oxygen measurement during boiler start up or shut down may not be possible with this type of device.

The zirconia cell generates a millivolt output that follows the Nernst equation (see sidebar). The signal is carried to the detector’s junction box via a single wire. Most manufacturers incorporate the negative return for the cell into the actual metal body of the probe body for simplicity. Thus, it is crucial to the performance of the measurement that the detector body is well grounded. Poor grounding leads to electrically noisy oxygen measurements. The process connection into the boiler is usually a sufficient ground plane. However, weak weld joints in the process connection have been found to cause intermittent grounding problems.

The cell signal is transmitted through an external cable to an oxygen analyzer or transmitter. The analyzer converts the cell millivolt and thermocouple measurement to a usable display of oxygen concentration. It also is the temperature controller for heated type detectors. Most first tier analyzers have analog outputs used for control purposes and contact outputs for alarms or valve operation. Some analyzers even have analog inputs to include other process variables in analyzer calculations. There are other low-end products that have a display and minimal functions.

Graphic depition of issues to be considered when reviewing O2 detector performance.

As the zirconia cell ages, its performance deteriorates due to zirconia contamination or oxidation of the metal electrodes. The cell must be calibrated to ensure accurate readings. Calibration is a comparison of the cell output for a fixed gas concentration against the theoretical output at the same gas concentration. The calibration algorithm corrects differences in the values.

Oxygen measurement performance

Installation location, installation quality, and number of measurement points, perhaps have the largest impact. Oxygen measurement is used to monitor excess air in the combustion process. Logic says that the measurement should be near the location of combustion, if not exactly right where the fuel is being consumed. Physical limitations of detector materials prevent product installation directly in the ‘fire ball.’ Metals melt and sag, while ceramics can be harmed by impact from clinkers and other boiler debris. Further, it is a cumbersome and costly to put in situ product through boiler walls covered with steam tubes. So, the measurement is usually set back from the fireball and secondary processes in the flue duct, post economizer before the air preheater. However, in many plants there are a fair number of access ways, expansion joints, and other ‘air in’ leakage points that can skew oxygen readings. Many plants are moving the measurement location closer to the combustion area, fully utilitizing the high temperature nature of the zirconia technology.

Installation quality is equally important. Items such as reference air quality, seal integrity, and maintenance accessibility all can affect oxygen measurement quality. For example, the quality of the reference air is just as important as the quality of the calibration gas. Reasonable changes in the reference air humidity, for instance, can change the process oxygen reading by 0.75% O 2 . It is a variable error that fluctuates with environmental conditions.

Finally, the number and arrangement of measurement points is the most under-developed area of combustion O 2 measurement mainly because of the apparent cost. Boiler features, such as expansion joints, turning vanes, duct bends, soot blowers, and internal truss work, will cause the flue gas to stratify, eddy, or draft in ways that will change with boiler load. The purpose of the discrete O 2 measurement is to find the best representative sample of what is happening in the combustion fireball.

A single oxygen detector located three feet down in a 40×40 ft duct is unlikely to give the best representative sample throughout the effective range of the boiler load. Conversely, the same flue duct covered in 90 O 2 detectors, while it may be attractive to O 2 detector manufacturers, is uneconomic and a maintenance nightmare. It’s also not required. Few plants have spent the time and effort to understand the actual gas flow characteristics in their flue duct. Many plants depend on the flow model created when the plant was commissioned as the basis for their current measurement location, quantity, and arrangement. Numerous post commission additions for the purpose of NOx reductions or fuel efficiency undoubtedly have changed the gas flow through the duct. Some plants use portable test equipment and special access duct access ports to demonstrate the actual profile of the O 2 in the duct cross-section.

Some even take the effort to construct a matrix showing O 2 and flow velocity at different boiler loads in the duct cross-section. The latter is the best way to choose the location, arrangement, and number of insertion probes for the process. Improving the performance of the sample arrangement can profoundly influence fuel consumption and NOx emissions. The ‘Duct O 2 profile’ graphic shows the simple but realistic view of a duct O 2 profile with some of the considerations for reviewing O 2 detector performance.

Author Information
Jim Scott, product manager, and Bobby Dennis, product specialist, are affiliated with Yokogawa Corp. of America’s analytical gas products;

What is the Nernst equation?

The Nernst equation links actual (measurable) reversible potential of an electrode, E , to the standard reversible potential of the electrode couple, E

E = E RT / zF ) ln ( a (RED)/ a (OX))

where R is the universal gas constant, T is the absolute temperature, z is the charge number of the electrode reaction (which is the number of moles of electrons involved in the reaction as written), and F is the Faraday constant. The notation a (RED) represents the chemical activities of all of the species which appear on the reduced side of the electrode reaction, and the notation a (OX) represents the chemical activities of all of the species which appear on the oxidized side of the electrode reaction.