Comment

1 May 2005

High temperature oxygen sensors

By Gary A. Lang

Pick the right ones to reduce NOx emissions, save money.

With fuel costs skyrocketing and a focus on reducing NOx emissions, fast and accurate excess oxygen measurement is essential. Real-time, in-situ measurement and temperature in the radiant heat zone of a boiler or furnace provides a window for viewing combustion conditions closest to the source. Excess oxygen and temperature measurements, taken at strategic sample points around the firebox, provide timely information for optimal trim control of the combustion process. Commercially available in-situ, high temperature oxygen sensors have the potential to solve combustion control and burner management problems with short payback and high annual return on investment (ROI) in power, petrochemical, refining, and glass applications.

Combustion is the rapid combination of oxygen with a fuel resulting in the release of heat. In most combustion processes, the oxygen comes from air. Unfortunately, only 20.9% of air is oxygen. Nitrogen and other non-combustible gases make up the other 78.1%. (The other 1% is made up of other gases such as argon, carbon dioxide, helium, methane, hydrogen, etc.) These gases are detrimental to the combustion process because they require heating and thus create thermal loss.

Ideal combustion minimizes the thermal loss and the amount of NOx produced. However, the reality is most combustion processes use excess oxygen. With insufficient oxygen, the fire is reducing. Incomplete combustion results in unburned fuel going up the stack and possible CO pocket formation, which can damage the refractory and metal parts. The fire oxidizes when excess oxygen is present. The process produces more NOx and uses nearly 60% of the purchased energy to heat nitrogen. The amount of fuel wasted is twice as high on the fuel-rich (reducing) side of ideal combustion as on the fuel lean (oxidizing) side of idea or stochiometric combustion.

Oxygen measurement

Excess oxygen measurements divide into two major sensor types: low temperature (sampling or extractive sensors) and high temperature (1000-3000º) in-situ sensors. Both work, however extractive units have significant limitations. Sampling systems for extractive units require constant maintenance and attention because the high temperature and process constituents create a harsh environment damaging to components such as pumps, heaters, and sample lines. Periodic cleaning and calibration are mandatory for these types of sensors.

High temperature in-situ sensors are the preferred method for measuring excess oxygen in furnaces and boilers where the radiant heat zone temperature is 1000-3000ºF because they are calibration and maintenance free. The lifetime expectancy of the sensor depends on the application. The sensors have lasted seven years in one refinery operation. High temperature in-situ sensors are crucial to combustion optimization. They provide accurate, real-time excess oxygen analysis of the combusted gas in the radiant heat zone with feedback for oxygen trim control of the air/fuel ratio.

Operation theory

The high temperature sensor principle of measurement is zirconia-based. During combustion operations, an important natural phenomenon occurs. The yttrium-stabilized zirconia will conduct oxygen ions at temperatures above 100ºF. The driving force of the ion migration is the difference in the oxygen concentration of the process gas and the supplied reference air. Partial pressure laws of gases state oxygen molecules will travel from higher to lower concentrations. A platinum electrode, inside and outside the zirconia, detects the voltage the movement of the ions produces. This millivolt signal is a function of the difference in oxygen concentration. Knowing the process temperature, reference gas oxygen concentration (20.9%), and the millivolt signal, you can use the Nernst equation to calculate the percent oxygen:

E = 0.215 x T x 1n (O2 reference/ O2 process)

Where:
E = oxygen millivolt signal
T = temperature in Kelvin
O2 reference = oxygen concentration inside the sensor (Ambient air, 20.9%)
O2 process = oxygen concentration outside the sensor (the process gas)

Sensor installation

Sensor measurements provide information to trim the air-to-fuel ratio to the burners continuously. Choose a sample location where combustion is complete. Proper sensor installation is crucial to successful excess oxygen measurement and sensor longevity.

Here are some typical installations:

Typical sensor installation

Actual glass melt furnace installation

Financial justification

Combustion efficiency plays a major role in the overall performance of the process when energy is a significant operation cost. You can estimate fuel savings and NOx reduction using general rules. Burner manufacturers estimate a fuel savings of 1% for every 1% reduction in excess oxygen at a flue temperature of 1200ºF. At higher temperatures, even greater fuel savings are possible. The North American Manufacturing Handbook supplement provides information to help calculate exact savings. Other documentation reveals a 1% reduction in excess oxygen equates to a 20% reduction in NOx output. This is a linear function, thus a 2.5% excess oxygen reduction would result in a 50% reduction in NOx.

Total cost of ownership

Being able to calculate the cost savings is only half the equation. How does cost savings relate to payback and ROI or internal rate of return (IRR)? To calculate, you'll need the total cost of ownership for measurement of excess oxygen and trim control. Using a life of 15 years for the instrumentation and control equipment and calculating the installation, support, utility, and spare parts costs over those 15 years, you can predict the generated payback, annual ROI, and income. Income is the annual cost savings minus the annual costs. The first year of installation costs will be significantly higher than future years owning to cost of the instrumentation and the actual installation, engineering, startup, and training costs.

Process heater installation

Refining example

Process heaters are primary sites for high temperature in-situ sensors. Proper sample location and sensor installation provides fast, accurate measurements of the excess oxygen concentration in the radiant heat zone (firebox) for optimal combustion trim control. See the "Process heater installation" figure. This requires one sensor per radiant heat zone. A large process heater (200+ MMBTU) may require six to eight sensors, while a small process heater (5MMBTU) may only require one sensor. The result is significant fuel savings. The financial analysis reveals a 145% annual ROI and a payback of about three months. Experts expect the analytical system to generate an income of almost $8 million over 15 years based on the below assumptions.

Assumptions

To calculate cost savings:
1% reduction in excess O2=1% savings in fuel
Average fuel cost=$5.45/MMBTU

Total cost of ownership—18 analyzers for 15 years

Installation $ 144,300
Utilities $ 23,900
Support $ 32,800
Spare Parts $ 162,000
Total cost over 15-year life $ 363,000

ROI: Income and costs

We can see the accumulated income generated from the fuel in the "Accumulated income and costs" figure. The predicted annual ROI is 149% with a three-month payback for the four process heaters in this real-life evaluation.

Utility example

Whether the boiler is T-fired or end-wall-fired, installing high temperature sensors in the firebox rather than after the economizer (as the "Boiler unit section" figure illustrates), provides the quick response time and accurate measurement you need for combustion trim control. The location of the high temperature sensor alleviates inaccuracies due to air leakage permitting tighter process control.

We assume fossil fuels, such as coal, to be too inexpensive to offer a significant ROI. However, when we evaluate a 55 MW utility boiler, the financial analysis shows a 49% annual ROI and a payback of about seven months. We expect the oxygen system to generate an income of nearly $1.6 million over 15 years based on the assumptions that follow.

Boiler unit section

Assumptions

To calculate cost savings:
1% reduction in excess O2=1% savings in fuel
Heat input value for coal=12,860 btu/1b
Average price of coal=$130/ton
Heating output average=45 MW=153.5 MMBTU/hr
Average fuel costs=$5.05/MMBTU

Total cost of ownership—eight analyzers for 15 years

Installation $ 72,100
Utilities $ 10,600
Support $ 32,800
Spare Parts $ 96,000
Total cost over 15-year life $ 211,500

ROI: Income and costs

We can see the accumulated income the fuel savings generates in the second "Accumulated income and costs" figure, where income equals $1.55MM. For the coal fired utility boiler in this real-life evaluation, the predicted annual ROI is 49% with a seven-month payback.

With the cost of energy rising at an alarming rate, high temperature in-situ sensors are an essential tool for minimizing fuel costs and NOx formation in a variety of industries. Whether the fuel is coal, oil, natural gas, or biomass, you can calculate a cost justification to determine the economic viability of the excess oxygen measurement. Intangible benefits, such as longer refractory life and more consistent product quality, have not met the savings equation because they are hard to quantify. But they exist, and they'll only enhance the predicted cost savings.

Behind the Byline

Gary A. Lang is director of combustion products at Marathon Sensors, Inc. in Cincinnati.