Waterwall tubes

In a ouce-throiigh system, the feedwater entering the unit absorbs heat until it is completely couveided to steam. The total mass flow through the waterwall tubes equals the feedwater flow and, during uorm operation, the total steam flow As only steam leaves the boiler, there is no need for a steam drum. 

The boiler has a water-circulating tube wall (waterwall) construction. A layer of smelt freezes on the fireside of the wah due to coohng from the water in the tubes, and this frozen smelt provides a barrier between the tube and the reducing and/or oxidizing gases. Gas composition has been implicated as a major determinant of the corrosion rate on the fireside surfaces. The lower waterwaU is the most critical component. Unlike conventional boilers, a waterwall tube leak cannot be tolerated in a recovery boiler, since a smelt-water reaction has the potential for a catastrophic explosion. 

There are many different high temperature environments in a recovery boiler [2/0]. On the fireside of the boiler there is the molten smelt and the char bed in the bottom of the boiler. There is an oxidizing environment in the lower furnace where air is injected at up to four elevations for the combustion of the heavy black hquor that is sprayed into the boiler. Waterwall tubes are normally subjected to sulfidation. The use of composite tubes in the lower waterwalls has been successful in resisting sulfidation. The lower waterwall tubes may also have liquor running down them. At primary and secondary air ports, there are local environments that promote preferential corrosion and cracking of composite tubes. 

Progress of corrosion-resistant coatings for waterwall tubes... 

The metal temperatures of the waterwall tubes (WWTs) are relatively low, approximately 230-330 C, depending on the steam pressure (2.9-9.8 MPa). Here CRCs such as metal spray coatings, weld overlays, or claddings with high CRMs on the outer surface are applied onto the base material of carbon steel as shown in Fig. 19.12. Table 19.3 shows examples of the application of various CRCs to WTE boilers. 

Tubes in a WT boiler surrounding the furnace and convective pass sections that are welded together to form a continuous membrane. The waterwall prevents heat-path short-circuiting and provides a cooling mechanism for the boiler. 

Waterwall furnaces were employed by the ancient Greeks and Romans for household services. A water boiler, found in Pompeii, was constmcted of cast bronze and incorporated the water-tube principle (2). The earliest recorded instance of boilers performing mechanical work (130 bc) was Hero s engine... 

Often coal ash deposit effects are inter-related. For example, slagging will restrict waterwall heat absorption changing the temperature distribution in the boiler which in turn influences the nature and quantity of ash deposition in downstream convective sections. Ash deposits accumulated on convection tubes can reduce the cross-sectional flow area increasing fan requirements and also creating higher local gas velocities which accelerate fly ash erosion. In-situ deposit reactions can produce liquid phase components which are instrumental in tube corrosion. 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwall of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Furnace cameras also help prevent hot spots or overheat situations close to the walls of the plant s boiler tubes, providing early warning before these burn through the tubes. Some plants view the fire from a spot close to the superheater in order to keep the fire in the center of the furnace. Failure can be very costly. A 2,200-ton-per-day plant had to install new boiler tubes after trash piled up and burned a hole in the waterwalls [2]. 

Camera approach Visually observe the waterwall or refractory wall directly to be certain flames are not touching the walls, causing potentially catastrophic leaks at sites like crude oil (fired) heaters. Besides spotting flames contacting the external tube surface inside the firebox, the camera can also spot other signs of impingement like tubes with a cherry-red color or bulges in the tube walls [12]. 

The temperature regimes in which this corrosion occurs are summarized in Figure 3.2. The data generally show that K2S2O7 will form from K2SO4 and SO3 at 400°C when SO3 concentration is at least 150 ppm as the temperature increases, the SO3 requirement increases, so that, at 500°C, at least 2000 ppm SO3 will be required to form liquid K2S2O7. Sodium pyrosulfate can form at 390°C with = 2500 ppm SO3, but at 485°C, 2% by volume SO3 will be required. Based on these results and the anticipated maximum level of 3500 ppm SO3 in a pulverized-coal boiler, Reid (1971) concluded that pyrosulfates can contribute to metal loss in the waterwall and economizer tubes but may not be a cause of corrosion in superheaters and reheaters in conventional systems (Natesan 2002). 

Local weistage of the stainless steel cladding from composite tubes at ports in the waterwall (particularly primary and secondary air ports) has been attributed to hydroxide condensation [223,224]. The localized corrosion of stainless steel cladding from composite tubes at air ports of recovery boilers has been shown by deposit analysis to be a cycUcal mechanism. Laboratory tests in molten salt supported the theory that this corrosion is caused by molten NaOH [225]. 

---