Oxygen Control

Also known as O2 Trim

Introduction

All combustion requires the correct measure of oxygen; too much or too little can cause undesirable effects. However, the error is almost always intentionally on the high-side (too much oxygen) because the main effect on the high side is low efficiency. Too little air results in carbon monoxide formation, sooting and even explosion if accumulated soot and other non-combusted suddenly get enough oxygen to rapidly burn.

When boiler burners are manually tuned on a periodic basis, they are typically adjusted to about 3% excess oxygen which is about 15% excess air. This is because there are many ambient and atmospheric conditions that can affect oxygen/air supply. For example, colder air is denser and contains more oxygen than warm air; wind speed affects every chimney/flue/stack differently; and barometric pressure further affects draft. See Air Temp and Pressure. Therefore, an excess oxygen/air setting at the time of tuning assumes there will still be enough oxygen available for complete combustion when conditions worsen.

From an efficiency standpoint, the excess O2 means there is more air in the combustion stream than there needs to be. That air also contains moisture, and it all is heated and then lost up the stack. The amount of excess O2 is about directly proportional to the efficiency lost; that is, 3% excess O2 means 3% efficiency drop.

Although it may be possible to monitor and adjust the burner on a daily basis, it is not practical. Automatic O2 systems continuously monitor the flue gases and adjust the burner air supply. They are generically called ‘O2 Trim Systems’.

Operation

An electronic sensor is inserted into the boiler flue, near the boiler, ahead of any dampers or other sources of air leakage into the boiler or flue. The sensor is connected to a control panel that measures oxygen and sends a signal to a control damper on the burner air supply.

From the Hays Cleveland OXY-MIZER™ data sheet:

The OXY-MIZER™ is delivered as a package including the probe, DPM electronics, interconnecting cable (probe to electronics), flow (calibration) panel, and a single calibration gas. The probe has an accuracy of ± 2% net excess oxygen and a response time within one second. The probe failure reading is above 16%, where other less accurate and stable probes fail to 0%. The OXY-MIZER™ failure reading eliminates confusion between probe failure and boilers operating with low excess air. This means true safety monitoring of fuel-rich conditions in burners.

The probe assembly is fully certified in-situ, with an adjustable insertion length of 18″ to 24″ (46 to 61 cm) for service in flue gas to 1000F (538C). The probe has an accuracy of ± 2% net excess oxygen and a response time within one second. It is field-repairable with standard kits. The probe sensor includes the HAYS CLEVELAND patented inverted zirconium oxide cell with low voltage heater and high temperature RTD. This sensor is controlled at a constant temperature in excess of 815C. Its unique design eliminates drift, reduces the effects of contamination from combustion by-products, and provides high accuracy, reliability, and long life, completely outperforming thermocouple-based sensors. This design needs only a single cylinder of test gas.

The OXY-MIZER™ DPM ELECTRONICS unit is available in either panel or surface mounting enclosures with viewing windows. The modular design incorporates an integral probe sensor temperature controller. The full range digital panel meter (0 to 21% oxygen) is independent of outputs, and also indicates sensor operating temperature. The 4-20 mADC control output (continuously adjustable from 0 to 1% through 0 to 21% oxygen), is standard, as are the fully adjustable alarm outputs: analyzer fault and combustibles warning alarm.

The OXY-MIZER™ FLOW PANEL includes hardware for continuous control of reference air, and for calibration air and test gas. The full initial calibration takes minutes to complete. Annually, calibration should be checked by following the same simple procedure, switching the valves and adjusting the electronics if necessary. Calibration of competing analyzers may require two or three test gas cylinders, a portable voltmeter with supplies, and semi-log graphs of the sensor output,or complex programming!

According to Charles Rowan, V-P Sales at Hays-Cleveland:

There are other advantages of the installation of an O2 Trim package in addition to fuel savings. They include:

• Combustion efficiency computation per fuel to alert the owner when service is required on the burner.
• Flue gas temperature monitoring and alarms, alerts the owner when the boiler tubes are fouled (A 40 degree temperature rise above design results in a 1% fuel increase.) and shut down due to high flue gas temperature.
• O2 monitoring and alarm due to low excess air or combustibles.

There are two types of approaches for O2 trim.

1. Single point (jackshaft) positioning with a trim actuator.
2. Parallel positioning (metering), separate actuators for the fuel valve(s) and FD damper.

The most common method today is parallel positioning. The components include:

• Controller: It accepts inputs from the fuel and air actuators, O2 analyzer, optional flue gas temperature sensor and either a master-loading signal for a plant master or lead lag sequencer or a header pressure or temperature sensor.  The controller will interface with the burner management system for purge, low fire, fuel select and other functions.
• Boiler pressure or temperature sensor, mounted in the header.
• O2 analyzer that includes field repairable in-situ probe and electronics.
• Fuel valve actuator(s) (servomotors). One servomotor per valve. In some cases FGR is controlled and a servomotor is supplied for that function.
• Air damper actuator (servomotor).

The controller should have strategies that will include cross limiting and deviation limiting. The Hays Cleveland parallel positioning, O2 technology is based on the AC Station controller, Model AC20-3000. One of the systems is the DirectLink™. The DirectLink™ has UL 1998 certification and UL 508 listing.

Excess Air Effect on Efficiency

The following table is representative for boilers equipped with economizers and air heaters:

Estimating Savings from O2 Trim

Fuel Savings = 1.0  – (Starting Efficiency / Ending Efficiency)

For Example: 4.5% Excess Oxygen reduced to 2.0%

1.0  – (0.7972 / 0.8308) = 0.04044 = 4.04%

NOTE: Because some boilers operate with a very high percentage of excess oxygen, it is common for the first year savings to be substantially higher than this. However, much of that savings can be attributed to a more reasonable manual tuning of the boiler, and not necessarily from the installation of an automatic O2 control system. Well-tuned boilers can expect savings of 2 – 4% according to Hays Cleveland.

Estimating Payback from the Installation of O2 Trim

The equipment costs for O2 Trim varies only a little with boiler size. Investment Costs will vary mainly due to the torque requirements for the servomotors and types of O2 analyzers.  Installation costs are HIGHLY variable; only general installation costs for a ‘typical’ installation are provided below:

For O2 trim systems, 100 to 600 HP boilers, the investment costs are:

• Equipment: Controller in cabinet, pressure sensor, O2 analyzer, flue gas temperature sensor, servomotors (actuators) for fuel valves (2) and FD damper. Equipment cost range is $ 10,000 to $ 11,000.
• Installation: $ 5,000 to 7,000.
• Startup & Training Services: $ 2,500 to $ 4,000.

For O2 Trim Systems, 600 HP to 100,000 lbs per hour boilers, the investment costs are:

• Equipment: Controller in cabinet, pressure transmitter, O2 analyzer, flue gas temperature sensor, actuators for fuel valves (2) and FD damper.  Equipment cost range is $ 11,000 to $ 17,000.
• Installation costs of $ 7,000 to $ 12,000.
• Startup & Training Services: $ 2,500 to $ 4,000.

Source: Charles Rowan, V-P Sales at Hays-Cleveland 3/2005;

Example: Assume a 500 hp boiler with an input of 20,000,000 BTUs, operating 8,000 hours per year with a 50% load factor could save a very conservative 2% with the installation of an O2 Trim system:

20 MMBTUs x 8,000 hrs x 50% x 2% = 2,560 MMBTUs or 2,560 MCF Natural Gas per Year

If Natural Gas cost is $7.00 per MCF, 2,560 x $7 = $17,920 per Year Savings

Payback would be in the 1 – 2 Year Range, depending on Installation Costs

Note that if savings were 4%, the payback could drop to less than a one year payback.

Hays Cleveland O2 Trim System installed on 2 – Boilers with additional SCADA systems included in the common control cabinet.
Photo courtesy of Hays Cleveland.

More Information

Air Temp and Pressure

Air Supply

Exhaust Draft Control

Vent Dampers

Manufacturers

Hays Cleveland
1903 South Congress Avenue
Boynton Beach FL 33426
Telephone: 561-734-9400
Fax: 561-734-8060

1111 Brookpark Road
Cleveland, OH 44109
Telephone: 216-398-4414
Fax: 216-398-8558

Go to their web site at www.hayscleveland.com

Boiler Automation Systems

Cleaver-Brooks Hawk

Source: Text compiled by Bob Fegan based on information from the manufacturer’s web sites referenced and linked above 2-2005; product  images from Hays Cleveland web site linked above; Installation Photo from Hays Cleveland; Excess Air Table and savings estimate formula from CIBO Energy Efficiency Handbook 1997;