As we know that the cost of fuels are increasing day by day ,the high cost of fuels makes an economic necessity to increase the efficiency of the fuel , harness it to fullest and to minimize excess air levels and thermal air stack losses . Efforts toward combustion efficiency optimization, however, must be aimed at reducing total energy loss. This requires achieving minimum unburned combustible, as well as thermal stack losses. More precise control of air/fuel ratio, optimized for minimum total energy loss, can yield significant gains in efficiency and result in substantial savings in reduced fuel consumption.
Flue gas concentration of carbon monoxide is a reliable and accurate indication of burner flame stoichiometry and the completeness of combustion. It is the most sensitive indicator of unburned combustibles losses. Used as a primary combustion efficiency parameter, in conjunction with oxygen analysis, carbon monoxide offers significant advantages in controlling combustion at optimum levels of excess air. Controlling air/fuel ratio to an optimum level of carbon monoxide assures minimum total energy loss, and maximum efficiency, independent of variations in burner load, fuel type and fuel quality. The measurement is relatively unaffected by air in-leakage, and burner maintenance requirements are immediately identified .
The CO analyser utilizes infrared absorption spectroscopy to continuously measure CO concentration in combustion flue gases. The infrared source is mounted directly on the flue gas duct or stack on the side opposite from the receiver. Infrared energy is radiated by the source, through the flue gas, to the receiver. The receiver employs gas filter correlation and narrow band pass optical filtration with a solid state detector to determine the absorption of radiation by CO in the flue gas. These principles are illustrated in block diagram form in below given Figure . Infrared energy, radiated by the source, passes through the flue gas, where a portion of the energy is absorbed by any CO present. The remaining energy passes through the receiver window, focusing lens and, alternately, through two gas cells.
One of the two cells is filled with CO,the other, nitrogen. These are inserted alternately in the optical path at a fixed frequency. Energy at the wavelengths of interest is, effectively, fully absorbed in the CO reference cell; however, energy is transmitted through the nitrogen cell without further absorption. After passing through the narrow bandpass filter, the remaining energy impinges upon the detector.
Two energy levels are sensed alternately by the detector: source radiation reduced by the flue gas and reference cell CO and source radiation reduced by flue gas CO only. The resulting signals are rationed and compared with the rationed signals developed under zero CO calibration conditions. The comparative difference in ratios is used to compute flue gas CO concentration. The calibration source and span calibration cell are inserted into the optical path during automatic zero and span calibration of the instrument.
The infrared source module emits broadband infrared radiation, including the waveband of interest, from 4.5 to 4.9 microns. The source consists of a stainless steel body with a conical surface for uniformity of surface temperature and maximum emissivity. The source is heated to a temperature of 1,112°F (600°C) and is controlled at this temperature to assure constant intensity. The source is fully insulated and enclosed in a carbon steel mounting sleeve designed for welding directly to the duct.
Since the IR source module is installed such that the source surface is flush with the inner wall of the duct, the source is not subject to coating or particulate buildup in most applications. Consequently, there is no purge air requirement to maintain source cleanliness. Due to the large diameter of the source surface, focusing is not required, and the source contains no focusing optics whatsoever.
An added benefit of the large diameter source is insensitivity to duct vibration and elimination of the need for constant realignment otherwise required of focused systems. Maintaining the source temperature at 1,112°F (600°C) requires powering the heater at a nominal 50% duty cycle, extending the heater element life considerably. The operating life of the infrared source is approximately four
times that of conventional infrared sources
INFRARED RECEIVER MODEL
The infrared receiver module is designed to house the optics, detector and necessary electronics and hardware to determine absorption of infrared radiation emitted by the infrared source module. The infrared receiver module is designed to house the optics, detector and necessary electronics and hardware to determine absorption of infrared radiation emitted by the infrared source module.
In situ CO instrumentation employing thermoelectrically cooled, photoconductive detectors, the receiver employs a non-cooled pyro electric detector. Not only does this provide reliable, stable performance at high ambient temperature, it completely eliminates the maintenance associated with thermoelectric cooling systems.
FLUE GAS TEMPERATURE MEASUREMENT
The absorption of infrared radiation by carbon monoxide in combustion flue gases is a function of flue gas temperature. The temperature affects the density of the gas and, therefore, the number of molecules encountered by the radiation. In addition, temperature variations induce variations in the infrared absorption characteristics of carbon monoxide.
To account for these variations, flue gas temperature must be measured continuously. Temperature data is input to the receiver module and communicated to the control module. The control module software is fully characterized to provide accurate temperature compensation over the full flue gas temperature range of 200°F to 600°F (93°C to 316°C)
Rosemount Analytical-Product Data Sheet PDS 106510A