Exergoeconomic Analysis of a Boiler for a Coal Fired Thermal Power Plant

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2.2. Exergy Analysis

The major streams entering and leaving the boiler are shown in Figure 1. A block diagram for the same is developed as shown in Figure 2.

Figure 2. Block Diagram for the boiler system

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The general energy and exergy balance equations for the boiler may be formulated using the first and second law of thermodynamics. The specific thermo- mechanical exergy (neglecting kinetic and potential exergy) is evaluated from Eq. (1):

(1)

The total rate of exergy with any stream can be estimated as given in Eq. (2)

(2)

The total rate of energy with any stream is given by Eq. (3).

(3)

The exergy flow rate leaving the flue gas is given by Eq. (4).

(4)

The exergy balance equation for boiler is given as in Eq. (5).

(5)

The exergy destruction ratio y_D,_B for the boiler is represented as in Eq. (6).

(6)

The exergy loss ratio for the boiler is given by Eq. (7).

(7)

The exergetic efficiency of a boiler is given by Eq. (8).

(8)

2.3. Exergoeconomic Analysis

The generalized cost balance equation for a boiler is given as in Eq. (9).

(9)

The cost balance equation for a boiler system represented by Figure 2 is given by Eq. (10).

(10)

Average costs per exergy unit of fuel and product for a boiler are given as per Eq. (11) and Eq. (12).

(11)

(12)

2.4. Thermoeconomic Variables
2.4.1. Cost of Exergy Destruction

For determining the cost of exergy destruction two approaches are generally used. In the first approach the product from the boiler is assumed to be fixed and the cost of exergy destruction is given by Eq. (13).

(13)

In this approach the fuel rate must account for the fixed product rate and the rate of exergy destruction. Alternatively assuming that the fuel is fixed the cost of exergy destruction is formulated as in Eq. (14).

(14)

2.4.2. Relative Cost Difference

The relative cost difference for a boiler reveals the real cost associated with it and is given by Eq. (15).

(15)

2.4.3. Exergoeconomic Factor

The exergoeconomic factor gives the relative significance of the two categories of costs associated with a boiler namely, non-exergy-related costs (capital investment & operation & maintenance cost) and exergy related costs (cost of exergy destruction and exergy losses). It is given by Eq. (16).

(16)

3. Illustrative Example

To illustrate the benefits of the exergoeconomic technique a boiler for a 55 MW coal fired thermal power plant is considered for which the following data is available

• Coal received is of 16765 KJ/kg calorific value with ash content of 38% and 26% volatile matter.

• For normal running of plant at 55MW, boiler generates the 72.22 kg/s of superheated steam at 93.17 bars.

Calculations are done with following assumptions

1) Specific exergy of fuel has been calculated as in ^[7].

2) Gross calorific value has been used in calculations.

3) Intensive properties of the environment are not changing due to any process.

Table 1. Pressure, temperature, mass flow rate and exergy flow rate for boiler streams of 55 MW Power Plant

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The analysis of actual boiler system and its components is highly complex and tedious in nature due to huge amount of data associated with large number of system components, different operating conditions and nonlinear behaviour of steam properties.

The various costs associated with the functioning of a boiler are given in Table 2.

Table 2. Initial investment costs, annualized costs and corresponding monetary flow rates of boiler in a 55MW power plant

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The values for exergy, cost rates associated with flows and the unit costs of flows for the boiler are given in Table 3.

Table 3. Exergy, cost rates associated with flows and unit costs of flows for boiler at 55MW

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4. Results & Discussion

The boiler has been analyzed from exergetic and thermoeconomic considerations. Variation of various thermoeconomic factors have been calculated for variations of temperature and pressure.

Table 1 illustrates the values for pressure, temperature, mass flow rate and exergy flow rate for the various boiler streams. Table 3 gives the values for exergy and cost rates associated with flows and unit costs of flows for the boiler. Table 4 illustrates the values for PEC and the different thermoeconomic variables for the boiler.

Figure 3 indicates the effect of pressure variation on the values of the relative cost difference (r_B) for the boiler. It shows that with the drop in pressure there is a drop in the values for the relative cost difference as well and this drop becomes more and more prominent as the pressure values reduce significantly.

Figure 4 indicates the effect of variation in pressure on the exergoeconomic factor (f_B). The variation in the values of the exergoeconomic factor is significant for lower values of pressure.

Figure 5 indicates the effect of variation in temperature on the relative cost difference (r_B). The present study illustrates that there exists an inverse relationship between the values of pressure and the corresponding relative cost difference.

Figure 6 indicates the effect of variation in temperature on the exergoeconomic factor (f_B). The present study illustrates that there exists an inverse relationship between the values of pressure and the corresponding exergoeconomic factor.

Figure 3. Effect of pressure on r_B

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Figure 4. Effect of pressure on f_B

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Figure 5. Effect of temperature on r_B

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Figure 6. Effect of temperature on f_B

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