Process Sensors “Infra-Boiler” System: Measuring Temperature of Hot CO2 Gas or Flames in Power Boilers
There exists the demand for a high degree of accuracy in measuring and controlling the temperature of flue-gas in power generation plants that must be maintained within a very narrow range as directed by EPA standards, while still assuring the energy efficiency and production of steam within the boiler and safety of the plant. Conventional contact-type thermocouples come up short when compared to the reliability and accuracy displayed by carefully specified non-contact infrared temperature sensors.
Use of thermocouples limits the area being measured to the boiler wall surface areas rather than the centermost points of the boiler where the temperature is significantly higher. Also, coal fired boilers typically tend to accumulate soot and ash deposits, insulating the thermocouples, and falsely indicate lower temperature measurement readings inside the boiler or incinerator. Both circumstances are less than desirable as they indicate lower than actual temperatures and can lead to refractory and boiler tube damage. Ideally, a balance must be struck that allows the most favorable system performance while still safeguarding all the related components against damage or destruction. A remote sensing infrared detection system that operates in the mid four micron wavelength, CO2 Gas absorption band region offers just such a solution.
CO2 Gas Measurement Operation: The infrared radiation emitted from the gas stream is collimated onto the IR detector via the sensor’s lens. The acquired temperature reading is the average from the IR sensor’s cone of vision which penetrates up to 8 to 14 foot into the boiler/furnace. This based on an 8 to 12% CO 2 gas concentration. The heavy duty protective cooling housing with protective sapphire window and air purge insures reliable operation in harsh environments.
Applications include: Garbage Incinerators, Hazardous Waste boilers, Fluidized Bed
Boilers, Utility Boilers, Biomass Boilers, Kilns, Furnaces etc
Boiler Design and the Need for Monitoring
Boiler design involves the energy balance between fireside and steam-side parameters. Typically there tends to be more steam-side instrumentation designed and fitted in a boiler control system than there is fireside instrumentation, but since the fireside generates the heat transmitted to the boiler system it is essential to control its operation as well. The lack of fireside control means that air and fuel mix and combust in the burner system and go unmonitored through the flue gas path until exiting the boiler. Clearly, temperature monitoring is needed at a point between the burner and the boiler exit. The solution is the addition of another control point or furnace exit gas temperature check point between the two. The additional control point will make an enormous difference in the overall performance, endurance and reliability of the boiler as it alerts operating personnel to possible trouble situations well before they occur and cause irreparable damage or unnecessary wear and tear.
While the addition of a furnace exit gas temperature check point is essential, its operation can only be considered successful if it achieves maximum penetration into the gas combustion stream. Generally, high temperatures and greater concentrations of combustion by-products or particulates in the flue gas stream reduce the IR sensing depth of measurement capability into the gas. At some distance, there is an extinguished depth of penetration where a pyrometer cannot “see” any further as the detector becomes saturated. Use of a highly stable non-contact infrared pyrometer that functions in the mid four micron wavelength, CO2 Gas absorption band enables an operator to site through the cooler gases that skirt the boiler perimeter and zero in on the measurement within the gas stream that is more representative of the true actual temperature.
Some Benefits Achieved by CO2 Gas and Flame Temperature Monitoring
- The correct monitoring system will trigger a FEGT (furnace exit gas temperature) alarm when the highest temperature and maximum load condition for efficient operation is reached, thus preventing boiler waterwall, reheat and superheat tube failure.
- During boiler start-up it is essential to confirm that there is steam flow through the boiler tubes before exceeding a lower critical temperature limit. This ability may prevent fracture of temporarily non-flowing boiler tubes if all required operating procedures are met.
- For incinerator applications, in addition to trimming burners, a minimum acceptable operating temperature can be monitored to meet or exceed environmental requirements.
- In coal burning operations, an upper temperature limit can be monitored to reduce fusion related maintenance and down-time.
- When used with an on-line utility boiler, the precise temperature readings can be used to trim burners and balance boiler temperature, thereby reducing excessive fuel consumption as the hour to hour demand for power reduces.
- In Selective Non-catalytic Reduction Systems, the system can sense temperature “window” set points for the injection of low nitrogen oxide enhancers with urea or ammonia.
- The pyrometer system replaces high-maintenance thermocouples with reliable, precision, low-maintenance sensors in rugged protective housings.
- The IR temperature system monitors flue gas temperature ramp for startup preventing over-firing that could damage superheat tubes, or under-firing that could send damaging wet steam to the steam turbine buckets.
- The correct IR sensor system reduces slagging and ash fusion by monitoring the maximum allowable temperature in the boiler superheat/reheat section for ash fusion alarm points.
- Boiler gas temperature can be monitored for comparison to steam outlet temperature readings to determine the need for soot blower activation.
- Flue gas temperatures in separate sections of boilers with division walls can be monitored to achieve temperature balance.
A precision non-contact infrared pyrometer system that addresses all of the previously addressed criteria has been painstakingly developed by Process Sensors Corporation. The pyrometer models MY45 and MY46 with pyro-electric detectors are sensitive for IR-radiation in exactly the narrow spectral bands required for this application. Both are especially suited to precisely measure the temperature of hot CO2 and, at the same time, avoid the absorption band of cold CO2.
The infrared radiation emitted from a gas stream is collimated onto the IR detector via the sensor’s lens. This area must be kept free from any intervening objects. The acquired temperature reading is averaged from the sensor’s cone of vision which penetrates from 8 to 14 feet into a boiler/furnace. This is typically based on an 8 to 12 percent CO2 gas concentration. The sight-thru optics of the “Infra-Boiler” system allow the operator to visually see into the boiler/furnace and enable observation of any debris that may become lodged in the sensor’s sight tube/sight path.
The Model MY45’s standard temperature ranges are 100° to 1400°C and 300° to 1500°C. Temperature ranges for Model MY46 are 500° to 1500°C and 800° to 2000°C. Process Sensors also engineers non-standard custom ranges available upon request. The MY45 and MY46 feature a selection of analog and digital output signals essential for the display, control and archiving of measured process temperatures. Analog outputs are switchable from 0 to 20mA to 4 to 20mA. Operators may choose from two bi-directional digital communication interfaces, RS-232 or RS-485, with a maximum baud rate of 19.2 k.
The PSC “Infra-Boiler” System consists of a precision Metis MY45 or MY46 pyrometer that measures temperature in the critical mid four micron wavelength, CO2 Gas absorption band region, graphical SensorTools Software, along with a heavy duty protective cooling jacket, removable sealed sapphire window, sight tube with strong ½ inch air purge inlet, flange, oil coalescent filter, air actuated ball valve and vortex cooler.
The included PSC software, SensorTools, empowers the operator to set all the sensor parameters including temperature range, emissivity, and speed of response as well as archiving and transferring temperature data to programs such as Microsoft Excel for manipulation. For this application it is strongly recommended to dampen the sensor’s response speed to 10 seconds in order to “smooth out” the wide temperature fluctuations that occur during the combustion process. Repeated experience with these combustion applications indicates that longer response times of as much as 30 to 60 seconds should be applied to users’ receiving systems such as a PLC, HMI or DCS in order to provide additional signal averaging.
While the PSC “Infra-Boiler” System is ideal for use with utility power generation and biomass boilers, it has been shown to repeatedly perform successfully with hazardous waste, recovery and fluidized bed boilers, as well as trash incinerators, kilns and furnaces.
Measuring and controlling flue-gas temperatures in the power generation industry is an exacting application requiring exceptionally rugged and specialized instrumentation that deals with severe environmental conditions while supplying precise and reliable temperature measurement. The problems associated with high temperature combustion are numerous and as mentioned, must be tackled to ensure efficient use of energy and resources while safeguarding the integrity of equipment and personnel, while producing the desired result. The Process Sensors “Infra-Boiler” System addresses this arduous task with unwavering accuracy and dependability.
PSC continues to focus on the development and implementation of unique temperature sensor solutions for this industry and countless other industrial process applications with the same vision and fervor it has displayed since its inception in 1996.