OCR Text |
Show moment the mixture of fuel and air becomes fuel rich, carbon monoxide appears immediately in substantial and measurable quantities. The second was that stratified flue gases did not always appear in the expected locations. As an example, CO gases eminating from burner 17 during the 270 MW test appeared in location 6 with almost no disturbance appearing in location 17. SMALL GAS FIRED INDUSTRIAL BOILER - Figure 11 displays the stratification effect observed recently on a small 25,000 lb./hr. two burner, gas fired boiler at a local industrial site. Measurements were taken every six inches across a 4 x 2 ft. passage at the base of the exhaust stack. The peaks on the carbon monoxide curve could be reduced or increased by adjusting air dampers but they could not be eliminated. Both the oxygen and carbon monoxide levels were incredibly high and ultimately the burners had to be serviced in order to improve combustion efficiency. CYCLONE FIRED UTILITY BOILER - The boiler tested was a three burner, cyclone fired boiler designed and built by Babcock & Wilcox. It was a radiant tube boiler with an approximate capacity of 1,000,000 lbs.lhr. of superheated steam at 1,810 psig and 10050 F. The front wall contained three cyclone furnaces in a triangular array. The two outside fUrnaces were on the lower level, and a center fUrnace was on the next higher level. We projected the exhaust gases, if stratified, would follow the paths shown in Figure 12. Nine probes were installed just above the economizer section to withdraw flue gas for analysis. Based on a number of duct mappings, three probe locations were selected as being representative of the measured gases eminating from each of the three burners. The fact of the matter is that only the probes for the outside burners could be located based on duct mapping. The gases eminating from the center burner defused sufficiently with the gases from the outside burners so as not to be distinguishable from them. As a result, the center probe was located based on a geometrical projection of where the flue gases from the center burner should have been. Continuous reading Bailey oxygen analyzers were installed for these tests. Air flows were increased and decreased on a per cyclone basis, and the changes in oxygen levels were then used to confirm the degree to which stratification existed at this installation. Figure 13 displays the increases in oxygen levels across the duct as the air flow to each of the cyclones was increased. An increase in air flow to the center cyclone caused a uniform increase in oxygen levels across the entire duct. However, increases in air flow to cyclones A and C resulted in a predominate effect in the predicted locations. 233 CONCLUSION Flue gas stratification has been observed and measured at a number of boiler installations, both large and small. It is concluded, however, that automative control systems that make use of this phenomena, in most cases, cannot be justified. Many factors contribute to the alteration of the distribution patterns of flue gas constituents which lead to complex and costly control systems. Certain conditions cannot be fully corrected by burner fuel-air adjustment: recognition and interpretation by operating personnel is needed. On the other hand, manual control systems invol ving regularly scheduled measurements of oxygen and carbon monoxide at various exhaust duct locations of any combustion process should prove beneficial in identifying deteriorating burner performance that might not otherwise be detected using a single point or multipoint averaging system. |