Material Selection in Ultra-Supercritical Boilers
Supercritical (SC) and ultra-supercritical (USC) boiler technologies have been used in recent decades. This technology has several advantages over subcritical boiler types.
The advantages include:
Higher plant efficiency, thereby reducing operating costs
Lower CO2 emissions from subcritical boilers.
Based on the temperature and vapor pressure of the boiler output, the boiler is distinguished :
Subcritical conventional boiler, pressure and steam temperature around 16-17 Mpa and 547 oC, efficiency 38%.
Supercritical boiler, pressure and steam temperature around 22-24 Mpa and 560 oC, efficiency 45%.
Ultra-supercritical boiler, pressure and steam temperature greater than 26 MPa and 700 oC, efficiency is close to 50%.What is supercritical?
Water (water) includes one supercritical fluid, which if the temperature and pressure are greater than the critical point, there is no phase difference between the water phase and the vapor phase (vapor) at that state.
Figure 1. Thickness-pressure graph .
Figure 1 is a graph of water balance. Critical point water is at pressure and temperature around 22.12 MPa and 374.15 oC. In supercritical conditions changes in water to steam occur spontaneously.
Drum Type and Once-through Boilers
As shown in Figure 2, the steam-water circulation system in the boiler is divided into two, the boiler uses a drum (drum type) and a boiler with circulation ‘once-through ‘.
On a boiler drum type, the separation of water and steam occurs in the steam drum. Water is recirculated into water-cooled walls in the furnace. In the once-through boiler water and steam system it passes water-cooled walls only once, there is no recirculation.
SC or USC boilers operate in subcritical regimes at certain loads, namely in wet mode (circulation mode). In wet mode, 0-20% Load, boiler circulating pump (BCP) circulates water back to water-cooled walls, this condition is like circulation on a drum type. When dry mode, 30-100% load, BCP stops operating. Steam waterwall output under superheated conditions, no more water phase.
The tube boiler can experience overheating due to the phenomenon of “departure from nucleate boiling” (DNB). In this condition, the superheated steam film is formed in the inner wall tube shown in Figure 4. The heat transfer decreases dramatically because steam is a poor conductor of heat resulting in a high temperature wall tube.
Water-cooled walls in SC or USC are designed to prevent DNB phenomena from occurring. There are two designs of water-cooled wall structures at USC, namely:
vertical wall with riffled tube
spiral wall with smooth tube
Material at USC Boilers
USC boilers operate at higher pressures and temperatures than subcritical boilers. In has an impact on the selection of boiler pressurized part materials, namely tubes, pipes and headers. USC boilers use higher grade materials than subcritical boilers.
The basis of material selection at USC boilers is: high creep resistance, good weldability, embrittlement resistance, fracture toughness, good heat conductivity, low coefficient of thermal expansion, thermal fatigue resistance, corrosion and erosion resistance, resistance to oxidation, expoliation and spalling, and good price effectiveness
.Some types of steel used in USC boilers are:
Ferritic steel, withstand temperatures of 565OC
Ferritic-martensitic steel, hold to a temperature of 620OC
Austenitic steel, withstand temperatures of 665OC
High nickel alloy, hold up to 700OC
Some considerations in designing furnace / water-cooled walls
The function of the furnace is to prevent the combustion process out of the combustion zone, absorb heat from the combustion process and “move it” to the working fluid. To prevent the combustion process out of the combustion zone, between tube furnaces are welded using a steel membrane.
To avoid the DNB phenomenon, the structure of water-cooled walls: vertical wall with riffled tube or spiral wall with smooth tube.
At certain loads USC operates in subcritical regimes, especially during wet mode. Consequently a tube for a furnace must be able to operate with water-wetted surface.
The material for the lower membrane wall furnace is ferritic steel: T11, T12, T23, or T24.
The upper membrane wall furnace is made vertical, with a roof tube, in subcritical mode ‘plays’ as a superheater. At relatively low pressures (at low loads), the density of steam is also low so the cooling effect on the upper membrane tube is not very good. In sliding pressure boilers, temperature metal furnace tubes at low loads are higher than when full loads. This can result in overheating, hot spots and tube failures when the load is low. Materials for the upper membrane walls include: T12, T23, HCM12, T91, or T92.
Some considerations in designing Superheater and Reheater
Superheater temperature (SH) and reheater (RH) 700OC to 760O
Superheater and reheater tubes must have properties: high creep resistance (> 100 MPa or 14.5 ksi for 100,000 hours), high thermal fatigue resistance, good weldability, resistance to fire-side corrosion & erosion, and resistance to steam-side oxidation.
For low-temperature parts SH and RH, ferritic steel like T22 is still an option. High temperature SH and RH use SS304 and SS347.
Steamside oxidation and exfoliation scale oxide cause several problems: tube thinning, disruption of heat transfer which can cause failure due to creep. Exfoliation can clog the tube, as shown in Figure 6, so that overheating occurs, and can also cause erosion in the turbine. Therefore the RH and SH tubes must have good expoliation resistance. TP347HFG and internally shot-peened Super 304H have high strength and also have a fine grain structure that improves resistance to fluid-side oxidation which can lead to exfoliation.
Figure 6. Conditions caused by expoliation (a). boiler tube clogging (b). erosion of the steam valve 
Headers and Pipes
Besides the tube, other important components in the boiler are pipes and headers. Generally pipes and headers are made of low alloy steel such as P11 and P22. One of the mechanisms of damage to the header is thermal fatigue which can cause cracks. Crack occurs in the ligament between tube holes. At USC boiler the material selection for the header must be in accordance with the high pressure and operating temperature.
The use of CSEF (Creep-Strength Enhanced Ferritic) steel for example Grade 91 and 92 besides being used as superheater and reheater tube material is also used as header material and pipes. The higher Cr content makes the creep resistance of this type of steel two times higher than 2¼ Cr-Mo. This also causes steam-side oxidation resistance and fire-side corrosion to increase. P91 has been used for headers and pipes in USC boilers with temperatures over 600OC. P92 developed from P91 allowable stress is higher and can be used for steam temperatures up to 620OC. More than 620OC, 9% steel Cr resistance to oxidation is limited, so 12% Cr steel and austenitic stainless steel are used for applications at temperatures over 620OC.