04-09-2012, 12:19 PM
Supercritical Boiler with Low Mass Flux, Vertical Wall Design
Vertical Wall Design.doc (Size: 341 KB / Downloads: 298)
Abstract
Supercritical Boiler technology is gaining acceptance worldwide as Clean Coal Technology due to its significant advantages like higher overall plant efficiency, reduced coal consumption, reduced gaseous emissions like SOx, NOx and CO2 and particulate emission. Large capacity units of size 600 to 1000 MW are now built up with supercritical parameters. In India, the trend is clearly towards large capacity Mega / Ultra Mega Power Projects based on supercritical technology. Evaporators of once through supercritical boilers are designed with either “high mass flux spiral/vertical wall” or “low mass flux vertical wall”. This paper deals with low mass flux vertical wall design developed by Siemens AG, Germany. The authors of this paper feel that the low mass flux vertical wall design has many distinct advantages over high mass flux spiral/vertical wall design.
Introduction
The major difference between a drum type boiler and once through supercritical boiler is the furnace wall design. An once through furnace wall design needs to take care of the temperature difference between tube-to-tube at furnace wall outlet due to the variation in furnace heat absorption. Also the occurrence of boiling crisis like DNB and Dryout and associated metal temperatures need to be critically analyzed and taken care in the furnace wall design. There are basically two types of furnace wall designs used namely helically wound spiral wall and vertical wall. The design aspects of these two types of furnace wall are discussed in this paper.
Furnace wall design requirements
Furnace sizing is decided by the heat input, coal and ash characteristics like fouling & slagging tendency, initial deformation temperature, and furnace heat loadings which are derived from the experience of the boiler manufacturer in firing different types of coal.
In short, furnace sizing philosophy is same for both drum type and once through boilers. However, the furnace wall design (evaporator system) is different in case of once through boilers.
Departure from Nucleate Boiling (DNB)
Under certain conditions of heat flux, mass flux, tube geometry, steam quality and pressure, the steam bubbles formed inside the furnace walls do not collapse but coalesce to form a film of superheated steam over the inner wall of the tube. This condition is known as film boiling. The point at which film boiling occurs is known as departure from nucleate boiling (DNB). Refer fig 1. When DNB occurs, the tube metal temperature shoots up. For high heat flux, DNB may occur at low steam quality.
Conclusions
Once through boiler with supercritical parameters improve the overall plant cycle efficiency and heat rate which results in reduced coal consumption, reduced gaseous emissions and pollutants. The vertical wall low mass flux design developed based on the test results of vast number of experiments conducted by Siemens AG, Germany offers significant advantages like natural flow characteristic, reduced feed water pump power consumption, a furnace wall design simpler to fabricate, erect, operate and maintain ensuring the reliability and availability on par with drum type units. The concept is also validated by the experience gained from some of the operating units.