The furnace is considered the heart of the boiler. Both combustion and heat transfer to the boiling water occur here, so it should be carefully designed. If not, several problems may result, such as lower or higher steam temperature if a

Superheater is used; the heat flux should be such as to avoid from DNB concerns. Circulation inside the tubes should be good. There could be incomplete combustion, which leads to lower efficiency and, coupled with a poor burner design, higher emissions of NOx and CO. Also, the flame should not impinge on the walls of the furnace enclosure. Hence it is always good practice to discuss emission control needs with potential burner suppliers who can model the flame shape and ensure that the furnace dimensions used can avoid flame impingement issues while ensuring the desired emission levels.

In boilers fired with fuels that produce ash, the furnace is sized so that the furnace exit gas temperature is below the ash softening temperature. This is to avoid potential slagging problems at the turnaround section. Slag or molten deposits from various salts and compounds in the ash can cause corrosion damage and also affect heat transfer to the surfaces. The gas pressure drop across the convection section is also increased when the flow path is blocked by slag deposits.

One of the parameters used in furnace sizing is the area heat release rate. This is the net heat input to the boiler divided by the effective projected area. This factor determines the furnace absorption and hence the duty and heat flux inside the tubes. Typically it varies from 100,000 to 200,000 Btu/ft2 h for oil — and gas — fired boilers and from 70,000 to 120,000 Btu/ft2 h for coal-fired units.

The volumetric heat release rate is another parameter, which is obtained by dividing the net heat input by the furnace volume. This is indicative of the residence time of the flue gases in the furnace and varies from 15,000 to

30,0 Btu/ft3 h for coal-fired boilers. For oil and gaseous fuels it is not as significant a parameter as for fuels that are difficult to burn such as solid fuels. However, this parameter ranges from 60,000 to 130,000 Btu/ft3 h for typical packaged oil — and gas-fired boilers.

From the steam side, the circulation of the steam-water mixture in the tubes should be good. As discussed in Q7.30, several variables affect circulation, including static head available, steam pressure, tube size, and steam generation. The circulation is said to be adequate when the heat flux does not cause DNB conditions for the steam quality in consideration. Packaged boilers have a low static head, unlike field-erected industrial boilers, and also have longer furnace tubes. However, packaged boilers operate at low pressures, on the order of 200- 1200psig, unlike large utility boilers, which operate at 2400-2600psig, and circulation is better at lower pressures.

Today’s boilers use completely welded membrane walls for the furnace enclosure (Fig. 3.2). Earlier designs were of tangent tube construction or had refractory behind the tubes (Fig. 3.10). With the refractory-lined casing, it is difficult to maintain a leakproof enclosure between the refractory walls and the water-cooled tubes, as a result flue gases can leak to the atmosphere, leading to corrosion, at the casing interfaces, particularly on oil firing. Balanced draft

— — K ‘■ insulation outer casing

Refractory casing

,000000 inner casing — outer casing

Tangent tube

CXKXX) insulation…. outer casing

Membrane wall

Figure 3.10 Furnace construction—membrane wall, tangent tube, and refractory wall.

Furnace design is used to minimize this concern, where the furnace pressure is maintained near zero by using a combination of forced draft and induced draft fans.

The tangent tube design is an improvement over the refractory-lined casing. However, it has the potential for leakage across the partition wall. During operation the tubes in the partition wall are likely to flex or bend due to thermal expansion, paving the way for leakage of combustion gases from the furnace to the convection bank, resulting in higher CO emissions and also higher exit gas temperature from the evaporator and lower efficiency. Present-day boiler designs use forced draft fans, and the furnace is pressurized to 20-30 in. WC, depending on the backpressure. If SCR and CO catalysts are used, the back-pressure is likely to be even higher. With such a large differential pressure between the furnace and the convection bank, a leakproof combustion chamber is desired to ensure complete combustion. If gas bypassing occurs from the furnace to the convection side, the residence time of the flue gases in the furnace is reduced, thus increasing the formation of CO. Another concern with leakage of hot furnace gases from the furnace to the convection bank is the impact on superheater performance; the steam temperature is likely to be lower.

The present practice is to use membrane walls. These consist of tubes welded to each other by fins as shown in Figs. 3.2 And 3.10. A gastight enclosure is thus formed for the combustion products. The partition wall is also leakproof, hence gas bypassing is avoided between the furnace and convection sections. This ensures complete combustion in the furnace enclosure. Typical designs at low pressures use 2 in. OD tubes at intervals of 3.5-4 in. depending on membrane tip temperature. Three-inch tubes have also been swaged to 2 in. and used at 4 in.

Pitch. This ensures a lower membrane temperature as well as reasonable ligament efficiency in the steam and mud drums. At pressures up to 700-750 psig, membranes using 2 in. tubes on 4 in. pitch have been found to be adequate due to the combination of low heat flux in the furnace and low saturation temperature, as evidenced by the operation of several hundred boilers. The 1 in. long membrane with appropriate thickness does not result in excessive fin tip temperatures or thermal stress concerns. At higher pressures, one may use

0. 5in. x 0.75 in. long membranes. Figure 3.11 shows how fin tip temperatures vary with heat flux and membrane length.

The furnace process is extremely complicated, because today’s burners have to deal with various aspects of burner designs such as staged fuel or staged air combustion, flue gas recirculation, and other NOx control methods; hence furnace performance should be arrived at on the basis of experience, field data, and calculations. The furnace exit gas temperature is the most important variable in this evaluation and is a function of heat input, flue gas recirculation rate, type of fuel used, effective cooling surface available, and excess air used. A gas-fired flame has less luminosity than an oil flame, so the furnace exit temperature is higher, as shown in Fig. 3.12 . A coal-fired flame has an even higher furnace exit gas temperature. An oil flame is more luminous and the furnace absorbs more energy, resulting in higher heat flux in the furnace tubes.