Evaluation of Corrosion Hazard in PF Boilers Applying the Oxygen Content in Flue Gases

摘要

An important indicator of corrosion hazard in PF boilers is flue gas content in the boundary layer near watertubes. In practice, the two flue gas components are measured: O_2 and CO. High content of CO (over 5 000 ppm) and O_2 below 1% indicates reductive atmosphere corresponding to the high rate of low - NO_x corrosion. Usually, the measurements of O_2 and CO content are performed once a year, when a boiler is out of the operation. An idea of a monitoring system development, based on flue gas on-line analysis, was born to control corrosion hazard in continuous manner. The system should operate automatically under supervision of a computer applying sensors able to convert gas concentrations into electric signals directly, like zirconia oxygen analyzer for O_2. Unfortunately, similar devices for CO are unavailable, because carbon monoxide concentration in combustion gases is one of the most difficult foreseeable species in combustion systems. However, it is possible to evaluate the CO concentration on the ground of O_2 measurements in flue gas. Based on a long-term experience, an effort to find this dependence was made and the correlation between the oxygen and carbon monoxide concentration in a boiler furnace was worked out. The obtained relationship made it possible to correlate the rate of corrosion and the content of oxygen, and to estimate the corrosion hazard in a furnace on-line from the O_2 content detected by the monitoring system. In the article the relationship between CO and O_2 is presented in the form of graphs and formulas, and the influence of O_2 content on the corrosion rate is shown as well. The obtained relationships were verified by the measurements of watertubes thickness. All the measurements data used in the paper proceeded from the measurements carried out at the BP-1150 boiler in power plant Elektrownia Opole S.A. during 2001 -2003.