Gross efficiency of boiler = 100 — (% Loss 1 + 2 + 3 + 4) (1.3)

Where

1. Stack loss consists of

A. Dry gas loss (Ldg)

B. Moisture loss (Lm)

C. Humidity loss (Lh)

2. Unburnt loss (Lub)

3. Radiation loss (Lr)

4. Unaccountable loss (Lu)

Stack Losses

These are measures of

1. How well the flue gases are cooled

2. How low the flue gas quantities are kept

There is a practical limit to the cooling of flue gases for each fuel without running the risk of low-temperature corrosion as depicted in the graphs of ECON and AH portions in Chapter 6. The excess air for the fuel is a function of the type of firing and the tightness of setting. A tight boiler setting inhibits the inward leakage of tramp air or outward leak­age of flue gas for balanced and forced draft boilers, respectively. The stack loss typically forms —70-80% of total loss.

The Lm varies from 8 to 20% for most fuels depending on the fuel moisture, fuel H2, excess air, and exit gas temperature. The Ldg forms the rest, as the Lh is rather small at <0.1%.

TOC o "1-5" h z Wg(Tg — Ta) X Cp X 100

(a) Dry gas loss (Ldg)% = —g—g p—- (1.4)

W y 6 v GCV/NCV

/i a /T ■ l i /t n, (9H2 + m) X (Hs — ha) X 100 n

(b) Moisture loss (Lm)% = -—2———— -—-—5—— ——————————— (1.5)

W v ’ GCV

or

(9H2 + m) X (Hs — ha — 578) X 100 NCV

/ntt. i, ,TUW Wh X (Hs — ha) X100

(c) Humidity loss (Lh)% = —h—-—2——— ———— (1.6)

W y > GCV/NCV

Where

Wg = weight of gas leaving the system in kilograms per kilogram of fuel burnt (not fired)

Wh = weight of moisture in air in kilograms per kilogram of fuel burnt (not fired) Tg = temperature of flue gas leaving the system in degrees Celsius Ta = temperature of air entering the system in degrees Celsius and the ambient temperature in most cases, less air heated from external source Cp = mean specific heat of gas between Tg and Ta in degrees Celsius Wa = weight of air in steam AH in kilograms per kilogram H2 = percent hydrogen in fuel by weight M = percentage of moisture in fuel by weight

Hs = enthalpy of steam at Tg and 0.07 bar (1 psia) is equal to 578 kcal/kg ha = enthalpy of water at Ta in kilocalories per kilogram

Unburnt Loss

Unburnt loss is a measure of how well the fuel is burnt in the firing equipment for the excess air chosen. Efficiency of heat release of the firing equipment is measured by the amount of carbon burnup, which is

C = (100 — % carbon in the residue) d 7)

B % total carbon at inlet

Carbon burnup varies a great deal with different types of fuels, firing equipment, excess air, and ash content. For gas and oil, carbon burnup is nearly 100% for coal, 98-99% for CFBC and PF, 90-95% for BFBC, and 80-90% for stokers. The finer the fuel size and greater the turbulence, the lower the excess air requirement and higher the carbon burnup. Carbon burnup is also influenced by volatile matter (VM) in fuel. Unburnt loss is generally —1-2% for PF and CFBC, 4-6% for BFBC, 4-10% for coal, and 2% for bagasse. The correct way to estimate this loss is to calculate from the proprietary data of the firing equipment manufacturer and compare with the previous field results to see if it needs any correction. Unburnt losses for various fuels and firing methods are listed in Table 1.4.

Radiation Loss

Radiation losses are below 1%, and become smaller as the boiler size and water cooling increase. Generally the American Boiler Manufacturers Association (ABMA) graph is fol­lowed (Figure 1.5).

• This is a comprehensive loss caused by radiation and convection conductance for the entire boiler, that is, the main boiler setting, ducting, and milling plant.

• The surface to ambient temperature difference is considered as 50°F (—27°C) and the air velocity 100 ft/m (—0.5 m/s).

• A furnace wall must have at least one-third of its projected surface covered by water-cooled walls before reduction in Lr is permitted.

There are several limitations in this graph, yet it is very popular because of its conve­nience and the low value of Lr. For example, the loss from a compact oil-fired boiler is the same as that from a large PF-fired boiler of the same evaporation, as the graph is based on the boiler output and not on the surface area. Likewise, FBC boilers were uncommon when this graph was evolved. Radiation losses from big high-temperature cyclones are not accounted for here. Suitable additional losses may have to be considered.

Unaccountable Losses (Lu)

Unaccountable losses cannot be exactly quantified and are small enough to be combined and assigned a reasonable value. They comprise, usually,

• Heat loss in ash

• Effects of sulfation and calcination reactions in FBC boilers

• Unstated instrument tolerances and errors

• Any other immeasurable losses

For example, in oil firing, the heat carried away by the atomizing steam falls in the last category. The Lu are considered usually for various fuels as given in Table 1.5.

In FBC the sensible heat loss is high, as the bed material also gets discharged along with ash. The loss is higher if limestone is also added for desulfurization. As it is possible to measure or estimate the bottom discharge, it is customary to adopt sensible heat loss instead of Lu.

Manufacturers’ Margin and Tolerance

Manufacturers’ margin (MM) and tolerance come into picture when the calculated effi­ciency figures are to be converted into guarantees. These are somewhat subjective. Toler­ance figure may or may not be allowed. If not allowed, the calculated figures are reduced suitably and minimum efficiency is offered.

Manufacturer’s margin is a safety figure accounting for any inaccuracies of design, manufacturing, and erection that can take place inadvertently. It is also to account for any unexpected variation in performance.

Tolerances cover errors and omissions as well as instrument tolerances encountered in performance trials. These are usually subject to mutual agreement between the buyer and the seller.

It is best to eliminate MM and tolerance and evaluate on a minimum efficiency basis, as practices differ in different countries. The accepted figures in the industry for Lu, MM, and tolerance are listed in Table 1.5.

Heat Credits

Heat credits are sometimes considered when the power consumed in fans and mills is siz­able and a part of energy is returned to the system in the form of heat. In CFBC boilers, for instance, the FD and PA fan heads are high enough to heat the incoming air by 20-30°C, which merits its consideration for heat credit. Also see Section 12.3.4.

Efficiency on NCV

For arriving at boiler efficiency on the basis of NCV, multiplying the efficiency by the NCV/GCV fraction is a rough method, which is nearly correct and good enough for nor­mal purposes. If a more accurate figure or loss breakup is desired,

• Losses 1a, 1c, 2, 3, and 4 have to be adjusted by the NCV/GCV ratio.

• Adjustment has to be made in loss 1b alone for heat carried away by water vapor as given earlier.

See page 17: The example in Appendix A explains this adequately.