Diagnostic system tracks power-plant combustion
Luciano Garif
Economic and ecological running of a thermoelectric power plant is one of the most important challenges today in the energy field. To reach this goal control of the combustion processes is needed. With this in mind, ENEL SpA (Rome, Italy) launched DIACO (diagnostic of combustion), a program for developing electro-optical instruments for combustion diagnostics for both oil- and coal-fired thermoelectric power plants.
For coal-fired plants, the instruments measure the size distribution of pulverized coal, flame stability, fly ash particle size distribution and concentration before and after an electrostatic precipitator (ESP), the unburnt content in fly ash, and the amount of polycyclic aromatic hydrocarbons (PAHs) in flue gases (see figure on p. 46).
Particle monitoring
Gathering data on particulate concentration and size distribution is important for assessing both the combustion quality and environmental impact of power plants. Because the fly ash particulate concentration in a boiler and flue duct (upstream from the ESP) is very much higher than concentration at the stack, it was been necessary to develo¥two instruments: one, DUST HC, is primarily for diagnostics and the other, DUST LC, is for emission detection.
Particulate concentration data from attenuation measurements of light signals may be subject to large errors because the attenuation depends not only on the total content of solids but also to a large extent on the particle size distribution and optical properties of the particulate. Use of the DUST device significantly reduces these kinds of errors. The instrument is an optical particle sizer developed for use in hostile environments such as flue gas ducts and stacks, with an operating temperature on the order of 200°C.
The angular distribution of the light scattered by the particles in the test volume is sampled by means of an annular array sensor. For particles larger than the wavelength, a mathematical relationshi¥exists (via the Fraunhofer theory) between the particle size and the scattered intensity pattern. Therefore, it is possible to derive the particle size distribution from the measured intensity distribution by using a properly developed iterative algorithm.
DUST HC is capable of measuring particle distributions in the diameter range 1.7 to 165 µm at different particle-concentration regimes (from 1 to 50 g/m3 according to the probe volume configuration. From the data given by DUST HC it is possible to derive information on combustion processes in the boiler. DUST LC is capable of measuring low particle concentration (lower limit 10 mg/m3) in the diameter range 1 to 100 µm. (Italian emission standards limit the concentration of particles in flue gases of a thermoelectric power plant to 50 mg/m3).
Comparing data from the two instruments gives information about the performance of the ESP. A coal particle sizer is now under development for measuring the size distribution of coal after milling. It is based on the same principles as the DUST systems.
Other instruments
The percentage of unburnt carbon in the fly ash is a very important parameter because it supplies information on combustion efficiency. Moreover, ash disposal depends on this parameter. The unburnt carbon laser monitoring system uses a CO2 laser beam that supplies thermal energy to the ash for a given period of time, thereby raising surface temperature to greater than 1000°C.
Air or oxygen introduced into the reaction chamber causes the carbon contained in the ash to burn, releasing mostly CO2, which is measured by a gas detector. A calibration curve allows the collected data to be correlated with the percentage of unburnt carbon.
An instrument based on laser-induced fluorescence is being developed to detect PAHs in combustion flue gases. By analyzing the spectral and temporal behavior of the fluorescence signal it is possible, in principle, to sort out a single species. In practice, however, this is very complicated. One has to take into account, for example, the quenching effect of buffer gases such as CO2, O2, H2O, SO2, and others. A system based on a Nd:YAG laser should be able to detect the global PAH content in vapor phase and to discriminate among families with different number of aromatic rings.
In a power-plant boiler, the accidental extinguishing of a burner flame may produce a dangerous situation if the fuel supply is not quickly stopped. An optical-fiber flame detector has been developed to detect the presence of a flame reliably.The optical radiation emitted from a limited portion of the flame area is collected by two lenses and sent to a remote optoelectronic device via a two-channel optical-fiber cable.
If there is a flame in the observation area the detected signals are correlated and their product gives an average value that indicates the presence of the flame. If there is no flame, the detected signals are not correlated, resulting in an average value of their product of practically zero, which activates the alarm condition. The system works even in the presence of intense background noise.
The final instrument in the DIACO program is a robotic optical-fiber television system that allows observation all around the boiler and gives detailed images of components. This project was performed in cooperation with CISE SpA (Segrate, Milan, Italy), a subsidiary of ENEL SpA. The described instruments have been installed and tested on different power plants in Italy.