OCR Text |
Show 3/4 • Economizer - Increased overall heat absorption and converting the economizer to steaming, or increasing the steam quality at the economizer outlet The flame-delayed combustion concept is applied for any fuel and boiler option for low N O x control. For all of the fuels, that leads to shifting the higher flame and flue gas temperatures toward the furnace end but, depending on the firebox configuration, the furnace exit flue gas temperature could be higher or lower than initial design and operation. For fuel coal, that would have the following negative impacts: • Increased outlet steam temperatures and tube metal temperatures in the superheater and reheater areas. • Increased slagging in the firebox and secondary superheater deposit formation rate. That requires increased use of sootblowing, which leads to increased tube erosion. • Increased heat flux at the firebox end where the steam quality in the waterwalls is higher. That could cause significant tube internal corrosion, related to a transition to departure of nucleate boiling (DNB). For fuel oil, the negative impact is the same as for fuel coal. For fuel gas, the negative impact is similar to that of fuel coal except that the slagging and fouling do not occur because there is no ash content in the fuel gas. Basically, low NOx control for industrial and utility boilers is causing significant reduction of the expected R U L of steam cooled tubing. That directly increases the total capital and power costs for the low N O x project, which has to be recovered from the ultimate customer. To determine in advance the negative impact and mitigating measures, it is necessary to use different steam boiler codes (Figure 1) for calculations and estimates of the different above described processes. The total capital cost and levelized power costs could be estimated in accordance with the Electric Power Research Institute, "Economic Procedures for Power Generating Plants." AFRC-TST.DOC |