Radiant versus Convective Superheaters

Radiant superheaters, which are typically located near the furnace exit region, are often used by several boiler manufacturers. Radiant superheaters have to be designed very carefully because they operate in a much harsher environment compared to a convective superheater. The following issues have to be considered:

• It is difficult to estimate the furnace exit gas temperature accurately. Variations in

fuel analysis, excess air, FGR rates, burner location and design, and flame shape can affect this value and consequently the temperature distribution across the furnace exit plane as discussed in Chapter 2. The temperature predicted can be off from the estimated value by even 50°C–110°C as correlations for furnace exit gas temperature are not accurate and established like the heat transfer correlations inside tubes or outside tubes.

• The turning section where the superheater is often located also adds to nonuniformity in gas velocity and temperature profiles, which can affect the superheater performance. Several boiler suppliers locate the superheater at the turning section, and performance prediction is a challenge due to variation in gas velocity and temperature profile across the cross section.

• If the furnace length and cross section are not properly designed or if the burner

is not properly tuned, the flame may even lick the radiant superheater tubes resulting in overheating or even failure. I have seen plant engineers or the boiler supplier blame the burner supplier for the superheater failures or overheating, while the superheater itself could have been poorly designed with inadequate streams or steam velocity inside the tubes. This issue has to be understood by the plant engineers and hence discussed later.

• If one looks at the slope of the chart of furnace exit gas temperature versus heat release rate (see Chapter 2), at part loads, the furnace exit gas temperature does not decrease much compared to full load. Hence, the direct radiation from the flame is still significant while the steam flow is reduced. Also at part loads, the nonuniformity in gas-side velocity profile will be more due to the lower gas velocity at the superheater. This fact coupled with the low steam-side pressure drop and flow distribution inside the superheater tubes will impact the tube wall temperatures often leading to overheating of tubes. Hence, one should be careful while using radiant superheaters at low loads. As can be seen from Figure 3.28, the steam temperature

122 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

increases with decrease in load for radiant superheater, while for a convective superheater, it decreases as energy transfer is dependent on flow gas mass flow. The combination of radiant and convective superheater may be used to obtain a rather steady steam temperature versus load characteristic in a package boiler.

• With the radiant superheater, one also has to consider the direct radiation con-

tribution from the furnace flame. Depending on the ratio of transverse pitch to tube diameter, the first four to five rows may absorb all of the direct radiation increasing the local heat flux in the tubes closest to the furnace. Hence, a conserva- tive estimation of tube wall temperatures of the radiant superheater must be done considering the fact that the furnace exit gas temperature itself has an error in the estimation of over 70°C.

The radiant superheater also absorbs more direct radiation at part loads from the furnace, increasing the steam temperature as well as the tube wall temperature as illustrated with an example in Chapter 2. At low loads, the flow distribution inside the tubes will also not be good. A few tubes can have lower than average steam flow due to lower steam velocity and pressure drop and higher gravity loss. Hence, radiant superheaters are prone to overheating unless carefully designed.

• The convective superheater, on the other hand, is located beyond several screen tubes, and the gas temperature and velocity profile at the superheater inlet region are more uniform and predictable, and hence, the superheater performance can

be evaluated more accurately. At part loads, the gas temperature is significantly lower and the effect of flow nonuniformity does not cause overheating of tubes as in the case of radiant superheaters. However, due to the lower log-mean temperature difference (LMTD), the surface area required will be more than that of the radiant design. The materials used can be of lower grade as the tube wall temperatures will be lower than that of the tubes in the radiant section.

• In heavy oil–fired package boilers, use of radiant superheater is not encouraged

as slagging of ash components may cause deposits and plugging of gas path and high-temperature corrosion failure of tubes. Wide-spaced screen section followed by a convective superheater is a better design option.

• It will also be shown later that the effect of variations in excess air or FGR does not

impact the steam temperature in a convective superheater much, while a radiant superheater is impacted significantly. This can lead to underperformance of the superheater when FGR is introduced at a later data.

• During boiler startup, the gas temperature entering the superheater is a variable that

limits the firing rate and hence increases the startup time as steam flow is absent through the superheater, and the tubes can easily attain the gas temperature in a short time. With the convective superheater located beyond several rows of screen tubes, the gas temperature would be much cooler, and hence, the firing rate can be higher reducing the startup time and associated fuel costs. Chapter 6 on miscellaneous cal- culations gives an example of the time taken by a superheater to attain the maximum tube wall temperature with a given amount of flue gas flow at a given temperature.

Hence, convective superheaters (Figures 3.26 and 3.27) are preferred in package boilers. They may require a little more heating surface due to lower LMTDs, but they are more reliable, and life of these superheaters is much longer. They may also need lower-grade alloy tubes compared to the radiant design.

Steam Generators 123

Final steam temperature

Superheaters in series

Convection superheater erature range

Radiation superheater % of temp

20 40 60 80 100 (a)

% of ﬁring rate

SH 1 SH 2 Eight arrangements possible with or without desh

(b)

FIGURE 3.28

(a) Typical variation in temperature rise versus load for radiant and convective superheaters and (b) two-stage superheater options.

124 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

How Emissions Impact Radiant and Convective Superheaters Example 3.4

FGR and excess air can impact a boiler with radiant and convective superheaters in dif- ferent ways. Presented in the following text is the performance of a steam generator with radiant and convective superheaters. The boiler capacity is the same. The radiant superheater has no screen section, while the convective superheater is located beyond 18 rows of screen tubes. We are comparing the variation in steam temperature for each type of superheater with excess air and FGR rates. Case 1 is with 10% excess air, and case 2 is with low NO x requirement using 15% excess air with 25% FGR rate. This type of situa- tion is seen in several projects where a plant is asked to lower its emissions of NO x and CO emissions by using a low NO x burner after a few decades of operation. When the boiler was purchased, there were probably no emission regulations. The superheaters were designed without steam temperature control system. The purpose of this example is to show the advantage of convective superheater design (Table 3.8).

It may be seen that the boiler with radiant superheater is impacted by changes in excess air and FGR rates much more than the convective superheater design. The difference in steam temperatures is much more with the radiant superheater compared to the convective superheater design. The boiler exit gas temperature increases as the total mass flow of flue gas increases, and hence, the efficiency is lower in both boilers when FGR is introduced. Hence, plant engineers should be aware of the effect of excess air or FGR rate increase on the steam temperatures and with the type of superheater they have. The convective superheater shows more resilience to changes in excess air and FGR rate, which is another advantage of the convective superheater design over radiant, and is hence preferred.

Radiant versus Convective Superheaters