Efficiency of a Coal Fired Boiler in a Typical Thermal Power Plant

This paper presents briefly on the boiler efficiency evaluation procedures by direct and indirect methods useful in thermal power plants. In the direct method consideration is given to the amount of heat utilized while evaluating the efficiency of the boiler, whereas, indirect method accounts for various heat losses. The boiler efficiency evaluated by direct method is found to be lower than that evaluated by indirect method as per the ASME PTC-4.1 standards. However, the direct method helps the plant personnel to evaluate quickly the boiler efficiency with few parameters and less instrumentation.


Introduction
Energy is the basic need and backbone of human activities in industry, agriculture, transportation, etc. It is one of the major inputs for economic development of a country. The whole world is in the grip of energy crisis and the increased pollution associated with energy use. A power plant is assembly of systems or subsystems to generate and deliver mechanical or electrical energy. The primary units of a coal-fired thermal power plant (Figure-1) are fuel handling system, boiler, turbine and generator and cooling system. The fuel can be in a solid or liquid or gaseous form. Abundantly available coal in India is being used as a solid type of fuel. The pulverized coal in fuel handling system transports to a closed container (boiler), which operates under high pressure and converts chemical energy of the fuel to thermal energy [1][2][3]. The combustion air will be supplied to the burners by the forced-draft fan, and pre-heated the air to dry the pulverized coal. Mixture of the fuel and air will be burned in the furnace. From combustion, the heat recovered by the boiler generates steam at the specified pressure and temperature. The flue gas passes through the boiler, economizer, air pre-heater, environmental control equipment, electrostatic precipitator (to extract fly ash), and flows to the stack through an induced-draft fan. The generated steam in the boiler under pressure flows through a super-heater and rises its temperature above water boiling point. The dry super-heated steam enters the turbine to drive the generator for producing electricity, and flows to the condenser for further use as boiler feed-water. This process completes its cycles from water to steam and then back to water.
Thermal efficiency reflects on the boiler operation & maintenance. Reduction in the boiler efficiency and evaporation ratio with respect to time is reported due to heat transfer fouling, poor combustion, operation & maintenance [4,5]. Deterioration in the quality of fuel and water may also lead to poor boiler efficiency. Peter et al. [6] have estimated the mass rate of generation considering fuel flow to the boiler, fuel ash content and estimated combustion efficiency. Song and Kusiak [7] have applied a data-mining approach to optimize the boiler efficiency. The boiler efficiency loss at exit is due to rise in flue gas temperature, which is controlled by the absorption of heat in the primary and secondary air pre-heaters [8,9]. Kaya and Eyidogan [10] have studied the energy efficiency for a natural gas fuelled boiler. Adhikary et al. [11] have adopted a semi-parametric reliability model in the failure analysis of the boiler.
Efficiency is one of the performance parameters useful for proper maintenance of a boiler due to its continuous variation of working parameters. Direct and indirect methods can be utilized for evaluating the efficiency of the boiler. This paper deals with the evaluation of boiler efficiency by direct method as well as indirect method and presenting their limitations.

Boiler Testing Standards
The boiler efficiency under steady loading conditions will be examined by operating for one hour. It is quoted by the British Standards (BS845-1987) as the percentage of available heat on the basis of its gross calorific value (). The German DIN 1942 standard recommends lower calorific value whereas the ASME PTC-4.1 standard demands higher calorific value. Direct and indirect methods can be employed for boiler efficiency evaluation. In direct method, the efficiency is evaluated by dividing the heat output with the fuel power (input) of the boiler, whereas indirect method considers the ratio of sum of major losses to the fuel power input of the boiler, which will be finally subtracted from unity [5].

Calorific Value of Fuels
The amount of heat (kJ) evolved by the complete combustion of 1 kg of fuel is known as the calorific value of a fuel (kJ/kg) [12]. When the products of combustion are cooled down to the surrounding air temperature, the quantity of heat obtained by the complete combustion of 1 kg of a fuel is referred as the gross or higher calorific value ( f GCV ). When the products of combustion are not sufficiently cooled down to condense the steam formed during combustion, the quantity of heat obtained by the combustion of 1 kg of a fuel is termed as the net or lower calorific value ( f LCV ). From the chemical analysis of a fuel, the gross or higher calorific value can be obtained from the Dulong's formula [13]: 2 2 33800 Here C, H 2 , O 2 and S represent the mass of carbon, hydrogen, oxygen and sulphur in 1 kg of fuel. The numerical values in equation (1)  2466 It should be noted that equations (1) and (2)

Boiler Efficiency by Direct Method
Direct method (also known as input-output method) compares the energy gain of the working fluid (water and steam) to the energy content of the fuel, which requires only the heat output (steam) and heat input (i.e. fuel) to evaluate the efficiency [14,15]. This efficiency is defined as the ratio of heat output to the heat input. It is also defined as the ratio of heat addition to steam to the gross heat in fuel [5]. The heat input measurement for coal fuel requires the calorific value of the fuel and its flow rate in terms of mass. There is a need for setting up of bulky apparatus on the boiler-house floor. During the test, samples kept in sealed bags are sent to a laboratory for analysis and calorific value evaluation. This problem can be simplified by mounting the hoppers over the boilers on calibrated load cells. There are several methods for measuring heat output. The heat output measurement requires the flow meters to record the steam generation rate. The boiler efficiency ( d η ) is defined as [5] ( ) 100 d Steam flow rate steam enthalpy feed water enthalpy Fuel firing rate Gross calorific value Here f GCV is the gross calorific value of the coal (kJ/kg), whose measurement is based on mass of coal for which hoppers are mounted over the boilers on calibrated load cells [15]. f w is the fuel burning rate (kg/hr) taken from the load cell. s w is the steam generation rate (kg/hr) obtained from the flow meter. For the specified temperature and pressure, 1 2 3 , , h h h , and 4 h are the enthalpies at super heater inlet, super heater outlet, re-heater inlet and re-heater outlet respectively. These enthalpies can be evaluated utilizing the steam tables with proper interpolation or using the online software package [16].
The temperature and pressure of the sub-saturated water at the super heater inlet measured are 285°C and 126.31 bar respectively for which the enthalpy, 1 h = 1258.92 kJ/kg. In the same way, enthalpies 2 h , 3 h and 4 h are obtained for the specified temperature and pressure in Table-1  The direct method helps the plant engineers to quickly evaluate the boiler efficiency. It requires few parameters for evaluation and less instrumentation for monitoring. However, it is unable to hint the plant operators why the efficiency of the system is lower. It should be noted that heat losses are not taken into account while evaluating the efficiency ( d η ) by the direct method. Hence, it is not possible to find various losses responsible for various efficiency levels. The steam is highly wet due to water carryover, which may mislead the evaporation rate and efficiency [5].

Boiler Efficiency by Indirect (or Heat Loss) Method
The boiler efficiency in the heat loss method (or indirect method) is evaluated by considering the sum of percentages of various heat losses and subtracting from 100 [15,17]. For evaluation of boiler efficiency, the input parameters required are: Gross calorific value of coal ( Adequate supply of oxygen is essential for complete combustion of a fuel. For C kg of carbon, H 2 kg of hydrogen, O 2 kg of oxygen and S kg of sulphur in 1kg of fuel, the total oxygen required for complete combustion of 1 kg of fuel from chemical equations is: As in Ref. [13], V is considered as the volume of the flue gas produced in m 3 /m 3 of fuel gas (when minimum quantity of air is supplied) for complete combustion. V 1 is assumed as the amount of air supplied in m 3 /m 3 of gas in excess of that required for complete combustion. Air contains 21% of oxygen and 79% of nitrogen on the basis of volume. If O 2 be the quantity of oxygen in m 3 /m 3 of exhaust gas, then the excess quantity of air which will contain this volume of oxygen will Boiler extracts such as flue gases and ash content are the major contribution to the loss of heat energy. To perform coal analysis, flue gas analysis and ash analysis [18] samples are taken to laboratory. The heat utilized in producing steam is found to be less when compared to the heat liberated in the furnace. Their difference will provide the heat lost in the boiler. The Bureau of Energy Efficiency [5] described various heat losses in a boiler due to: Dry flue gas (L 1 ); Evaporation of water formed due to H2 in fuel (L 2 ); moister in fuel (L 3 ) and in air (L 4 ); Incomplete combustion (L 5 ); Radiation and convection (L 6 ); Un-burnt carbon in fly-ash (L 7 ) and in bottom-ash (L 8 ). ASME PTC-4.1 standard suggests the boiler efficiency of boiler ( ) I η by indirect method as the difference of the energy input and the sum of the heat losses:

4.6768
Evaporation of H2O formed due to H2 in fuel, (The numerical value 5744 is the heat loss due to partial combustion of carbon) 0.8800 For large capacity boilers, the heat loss due to radiation and convection, L6 0.2000 Un-burnt carbon in fly ash, 7 100 (ma is the mass (kg) of the total ash generated per kg of fuel.
fly GCV is the gross calorific value of fly ash in kJ/kg).

0.2170
Un-burnt carbon in bottom ash, 8 100 is the gross calorific value of bottom ash in kJ/kg) 0.1600 Table-2 presents the input for the evaluation of heat losses in the boiler, whereas Table-3 gives the evaluated heat losses. The efficiency of boiler by indirect method is evaluated from equation (5) as 91.97%. The efficiency of the coal fired boiler calculated by the indirect method is found to be higher than that computed by the direct method. It should be noted that the direct method considers the heat energy utilized per unit of eat supplied, whereas the indirect method considers various heat losses. Measurement errors make insignificant change in efficiency by indirect method. For 90% boiler efficiency, 5% measurement errors in direct method will make the change in efficiency from 85.5 to 94.5%, whereas in indirect method, the change is from 89.5 to 90.5%.
The boiler efficiency test is to be carried out under steady load operation. The combustion efficiency test does not account for the energy usage by burners, fans and pumps, and does not reveal standby losses in firing intervals. The efficiency test does not account for blow down losses and soot blower steam. It is necessary to have periodical cleaning, draft control and excess air control to achieve better performance.
In addition, periodical checks on percentage loading of boiler, boiler insulation and the quality of fuel.

Conclusion
The efficiency evaluation procedures useful for coal-fired boilers in thermal power plants by direct and indirect methods from the Bureau of Energy Efficiency [5] are briefly described in this article. While evaluating the boiler efficiency, consideration is given to the amount of heat utilized in the direct method and various heat losses in the indirect method. The boiler efficiency by direct method is obtained as 83.94% whereas it is 91.96% by indirect method as per the ASME PTC-4.1 Standards. However, the direct method helps the plant personnel to evaluate quickly the boilers efficiency with few parameters and less instrumentation.