William T. Lang
Plant engineers have more reason than ever to closely monitor the conditions inside plant boilers, furnaces, kilns and incinerators. Operators strive to maintain optimal burning conditions and avoid situations that put equipment, personnel or the environment at risk–thus avoiding expensive repair and retrofit.
Imperfect or incomplete combustion can cost real dollars in downtime-related production delays. It may also produce hazardous emissions that can bring a plant out of compliance with the Clean Air Act, and under the gun for hefty fines–shrinking ever-tightening profit margins.
Quality-conscious operators are continually searching for ways to head off problems before they happen–problems that may impair a plant’s process, like imperfectly incinerated material, kilns with uneven firing or boilers with fuel nozzle melt-down.
To monitor the conditions inside a boiler, there is still no substitute for visual inspection. The earliest system was to look through a porthole at the flame through a blue glass. Inspection was severely limited by heat and glare hazards and bound by safety and certainty limitations. The observer could “fry an eye” in the intense heat.
The newest combustion monitoring systems help operators safely keep an eye on the fire. A reliable, high-temperature remote optical monitoring system permits prolonged and accurate observation–not through a porthole, but from within the furnace wall, right next to the flames, in temperatures that could reach as high as 3,500 F.
Safety first. The initial obstacles design engineers overcame were the safety hazards. Using a periscope with a camera and a monitor, they moved the operator to a remote and safe viewing location. No more faces too close to the fire.
Heating up. Physical obstacles were not as readily overcome. First there was the intense heat. How could a camera survive the blazing inferno inside a furnace? The solution has come in stages.
Keeping cool. The initial–and most common–solution is to use a high temperature, air-cooled housing which continually cools and changes the compressed air in the cavity in which the camera is mounted. The air-cooled system is sufficient for all but the hottest environments.
The most recent advance is a water-cooled solid state system, which withstands the 3,500 F temperatures by circulating water inside a stainless steel housing which contains a closed circuit TV (CCTV) chip camera. The closed-loop water system carries away more heat than the air-cooled system.
Seeing light. Another design challenge has been to create a system not subject to monitor screen “white out” resulting from light overload. The industry answer has been to incorporate (in the lens) a remote-controlled motorized iris and spot filter, which the operator can activate to adjust the amount of light reaching the screen.
A look inside
A typical remote combustion monitoring system today consists of a heat-resistant box mounted in the sidewall of the furnace, kiln or boiler; an optical scope with an objective viewing lens; a series of relay lenses and a video camera which relays the image to a video monitor.
- Wall box: the main mounting mechanism for the complete system. It provides a protective housing and is the primary coolant shroud. It’s typically made of heat-resistant nylon.
- Furnace lens: in a typical system, a heat-resistant quartz lens is mounted at the tip of a periscope-like tube ranging from 12 inches to 12 feet in length. Within the wall box, the tube also encloses a series of achromatic relay lenses, which carry the image back to the CCTV camera. Lenses may be mounted in a variety of fields of view from 15 to 90 degrees. Thus the operator can view directly along the axis of the scope, at a right angle to the axis or forward obliquely from the axis.
- Cooling CCTV housing: cools the compressed air and constantly changes the air in the cavity. Ideally, the housing should disconnect easily for servicing. The housing contains both the light volume control and the compact closed circuit TV camera (akin to surveillance cameras).
- Light volume control: permits the operator to easily adjust the amount of light coming to the cameras–insuring a high quality picture from light-off to maximum output.
- Compressed air filter: supplies clean air to the furnace lens by removing oil, water and particles. Air filters are available which use a self-purging particulate and coalescer filter, a pressure differential indicator and an alarm to let the operator know when the elements need to be changed.
- Automatic retractor: some systems are equipped with these to pull back the unit about two feet in the event of a problem. This prevents damage to the camera.
In waste-to-energy plants, the objective is to agitate the trash to get it to burn at as constant and clean a rate as possible. Because of the variable nature of the waste stream being fed into the burner, a number of situations can arise that an operator might head off with a clear view of the interior furnace area.
If fuel clumps up on one side of the grate, an operator might want to make adjustments to the burn rate by adding fuel and firing auxiliary burners. The operator can also signal to a crane to change the mix of what is fed into the furnace charger. Without this additional and timely control, overheating can lead to excessive wear or premature failure of boiler tubes.
Remote furnace monitoring systems are used in utility plants to monitor the points where fuel is injected into a boiler from a series of nozzles–the actual points at which the fuel exits the piping system and emerges into the main furnace where it ignites.
It is critical to monitor the flame pattern as it exits these nozzles: If the fuel system loses velocity or pressure, the flame can hang back into the burner nozzle, causing nozzle distortion or melting. Spotting such a defect early allows the operator to take the nozzle out of service and make external adjustments before damage occurs, saving hours of downtown.
A visual system is essential for monitoring the way ash fuses to the walls of coal-fired furnaces. The ability of the camera to view different angles allows the operator to get a good view of sections of the boiler walls and see what the slagging conditions are like. This allows the operator to properly adjust the silt-stream used for removing the slag deposits from burner walls.
An operator may also keep videotapes of separate burners to make comparisons between coals, which slag differently–affecting heat transfer.
After industries retrofit low NOx burners to comply with the requirements of the Clean Air Act, engineers can use high
temperature combustion viewing systems to directly observe the performance of burners deep within boilers, allowing more immediate control over the air and the fuel-preventing unnecessary and unsafe emissions, and keeping an eye on the fire.
Additional information is available from Lenox Instrument Co., 265 Andrews Road, Trevose, Penn. 19053-3427; 215-322-9990; 800-356-1104; fax: 215 322-6126;www.lenoxst.com.