Analysis of Boiler Performance

Boiler performance relates to its ability to transfer heat from fuel to water and steam (or some other fluid) while meeting operating specifications. Boiler performance includes all aspects of the operation. Performance specifications include operating capacity and the factors for adjusting that capacity, steam pressure, boiler water quality, boiler temperatures, boiler drafts and draft losses, flue gas analysis, fuel analysis, and fuel burned. Boiler efficiency, as the most important energy performance indicator, is presented as a percentage ratio of heat absorbed in the boiler water and heat supplied to the boiler by fuel. A boiler is essentially a heat exchanger in which the thermal energy released by combustion process is transferred from the products of combustion to the circulated fluid. An energy balance applied to a boiler within the dotted line shown in Figure 2.4 gives the following equation:

Mf • GCV + Mf • hF + Ma • hA + (1 + x) • m • h1 = m • h2 + MFG • hFG + x • m • hBD + Qc (2.17)

where:

Mf

= flow rate of fuel, [kg/s]

GCV

= gross calorific value, [kJ/kg]

hF

= enthalpy of fuel, [kJ/kg]

hA

= enthalpy of combustion air, [kJ/kg]

Mfg

= mass flow rate of flue gas, [kg/s]

hFG

= enthalpy of flue gas, [kJ/kg]

m

= flow rate of steam, [kg/s]

J Blow Down

J Blow Down

Figure 2.4 Relevant parameters for boiler performance analysis hi = feed water enthalpy, [kJ/kg] h2 = steam enthalpy, [kJ/kg]

x = fraction of the feed water flow which is lost as blow-down, [-] hBD = enthalpy of the water at working pressure of the boiler, [kJ/kg] QC = radiation loss, [kJ/s]

The left-hand side of the equation represents the sum of all incoming energy streams and the right-hand side of the equation represents the sum of leaving energy streams. The increase in the enthalpy of the circulated fluid to and from the process represents the useful output. Of course, when the circulated fluid is hot water or hot oil, instead of steam, the enthalpy of hot water or hot oil is used. There is no blow-down for such boilers (x = 0).

Let us consider now the balance (Eq. 2.17) applied to the boiler from Example 1. The additionally required data from this example are:

O2 = 7 % - oxygen content in flue gas, tFG = 245 °C - temperature of flue gas, tF = 10 °C- temperature of fuel, tA = 14 °C- temperature of combustion air, and it produces the following specific case:

4809.0 + 2.1 + 63.4 + 325.0 = 4279.9 + 974.9 + 53.0 + 65.0 (2.18)

Equation (2.18) refers to the actual operation of the analyzed system (column 4 in Table 2.4) before any improvements. It is evident that the largest single item in the equation is the chemical energy of fuel

(4809.0 kJ/s). The heat energy of fuel (2.1 kJ/s) is really small compared to the chemical energy of fuel and it can be neglected. Combustion air heat energy (63.4) depends strongly on its temperature but it only makes up a small percentage of chemical fuel energy. Feed water heat energy (325.0) also depends strongly on water temperature and its influence on boiler energy efficiency cannot be neglected.

On the left side of the balance equation (Eq. 2.18), the dominant item is the one which refers to the heat energy of steam, i.e., to useful output (4279.9). The second one (974.9) represents the heat energy released into the atmosphere by flue gas. As a rule, this is the largest single loss in boilers. The third item on the left side represents the loss occurring in the blow-down process and may amount to a few per cent of the chemical energy of fuel. The last item on the left side of the equation represents loss occurring due to transfer of heat from boiler surface into the environment.

Example 1 demonstrates the impact of improvements of certain parameters of the system by applying available technical measures aimed at increasing the system's efficiency. If the balance equation is written for these changed conditions, the following is obtained:

4411.7 + 1.9 + 43.5 + 416.4 = 4215.7 + 622.0 + 31.2 + 4.6 (2.19)

The difference between individual items in Equations (2.18) and (2.19) is obvious, and it reflects the improvement in boiler energy performance or its efficiency.

There are two practical methods for calculation of boiler efficiency:

(a) direct or input/output method;

(b) indirect or heat loss method.

Both of these methods can be used, but it is generally accepted that the indirect or heat loss method provides the more accurate results in practice.

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