The Influence of Throughput on Thermodynamic Efficiencies of a Thermal Power Plant

This study carried out energy and exergy analyses of a thermal power plant in order to evaluate the energetic and exergetic efficiencies and irreversibilities of units, sections and the overall system. It was also, to determine the optimum fuel-air rat io and optimum combustion temperature at different throughputs. The thermal p lant consisting of 23 units and 4 sections was simulated using HYSYS simulation software and EXCEL spreadsheet. The EXCEL spreadsheet was used for the energy and exergy analyses. It was found that throughput did not influence the energy efficiencies of the units but the exergy efficiencies. Throughput did not influence the energy and exergy efficiencies of the sections. The overall energetic efficiencies of the p lant were 18.17, 19.79, 21.42, and 21.45% and the overall exergetic efficiencies were 10.26, 11.22, 11.58, and 11.61% for throughputs of 50, 75, 100 and 110%, respectively. The overall irreversibilities of the plant increased as the throughput increased. The optimum fuel-to-air ratio which gave the optimum combustion temperature in the furnace was found to be 1:12.6 for all the throughputs which was an improvement over the current practice of 1:19.8. Throughput did not influence the maximum combustion temperature in the fu rnace.


Introduction
Exergy analysis is a thermodynamic analysis technique based on the first and second laws of thermodynamics which provides an alternative and illu minating means of assessing and co mp aring p rocesses and systems ration ally and mean ingfully [7]. Several studies have been carried out by researchers [3,5,17] to evaluate the performance of thermal power plants using exergy analysis. Analyses of energy, exergy and exergoeconomics were performed on a natural g as b ased st eam po wer p lant [12]. Th e co mp arison o f coal-fired and nuclear steam power plants using energy and exerg y an alyses to id en tify areas with p oten tial fo r performance imp rovement has b een invest igated [15]. A thermodynamic analysis of a Rankine cycle reheat steam power plant was carried out to study the energy and exergy efficiencies at d ifferent operating conditions of boiler temperature, boiler pressure, mass fraction ratio and work output from the cycle [4]. Exergy and cost balances have been used to study gas-turbine cogeneration system [10]. Thermoeconomic analysis of a coal fired electricity generati ng station was performed [16]. The exergetic destructions were investigated on a steam generation system [1].
This study undertakes energy and exergy analysis of a natural gas based thermal power p lant, located at Egbin, Ikorodu, Lagos State of Nigeria. The objectives of this study are to determine the energetic and exergetic efficiencies and irreversibilit ies of the 23 plant units, 4 sections and the overall system and to determine the optimu m fuel-air rat io and optimu m combustion temperature in the furnace at different throughputs (50, 75, 100 and 110% power output). The Egbin thermal power p lant burns fossil fuels, wh ich primarily consists of natural gas (NG) and high pour fuel o il (HPFO) as back up, and generates steam which is converted to shaft work in the turbine and to electric power in the generator. The steam generated in the boiler enters the turbine at temperature of about 540 0 C and pressure of 12.5M Pa to spin the turbine blade at a very high speed of 3000 rev/min. There are six turb ines units, each capable of generating 220 MW. The water fro m the lagoon is used as cooling water. The water is passed through the condenser to enhance the condensation of water fro m steam. The condensed water in the condenser enters the condensate polishing plant for treat ment befo re being sent back to the boiler for re-use. The cooling water which gains temperature fro m the condenser exits through a discharge channel into the lagoon. The temperature of the effluent water fro m the powerhouse is reduced in the supplementary cooling system before finally being discharged back into the lagoon [11]. The process flow d iagram of the plant is shown in Figure 1

Theory
Two types of exergy of material streams are taken into account, thermo-mechanical (physical) exergy for all streams and chemical exergy associated with the composition of streams with respect to datum environ mental species.

Physical exerg y of stream
The total rate of exergy in a stream is obtained fro m its specific value as: (1) The specific physical exergy of the stream was evaluated fro m the fo llo wing equation: ( The energy rate of a stream was obtained from its specific value as: where h and s are the molar enthalpy and entropy, respectively, of the flowing matter, expressed in functional relationship as: and

Chemical exergy of streams
For a mixture of gases, the molar chemical exergy can be expressed as: (4) where is its mole fraction in the mixture and the molar chemical exerg ies of the individual gases, are g iven in literature [13,18].
Exergy of chemical reaction: The combustion reaction of methane and oxygen is The specific exergy of chemical reaction (kJ/kg) can be written as: This gives the exergy rate of co mbustion in the plant as; (7) where; is the mass flow rate of chemical substance at the inlet.

Energetic and Exergetic efficiencies; and Irreversibility
The equations used in calcu lating the energetic and exergetic efficiencies of equipment units are given by: The irreversibility of the unit is given by: The overall energetic and exergetic efficiencies by [6,9] are: and (13)   (14) But it must be pointed out that the heat energy or exergy fro m the co mbustion reaction must be accounted as an input in the denomination of equations for overall energy and exergy efficiencies given above.
The overall irreversibility of the thermal p lant is given by:

Methodology
Simulati on HYSYS (2003) process simulator was used to simu late the plant. The two major equip ment in the p lant were the turbines and the heat exchangers. The expander was used to simu late the turbine. The Peng-Robinson equation of state was used for the simulat ion. The thermal plant had 23 units and these were grouped into 4 sections, namely; turbine-generator, condenser, regenerators and boiler-furnace sections as shown in Figure 1. In this analysis, the following assumptions were made: the natural gas burnt in the combustor was assumed to be 87.3% methane, 6.3% ethane, 2.7% propane, 1.5% butane, 0.5% pentane, 0.2% heptanes, 1% CO 2 , 0.7% nitrogen; the comp ressed air used in the combustor was standard air; unaccounted heat loss from the system due to radiation and convection was neglected; fuel undergoes complete combustion.
Process data were obtained fro m the plant and thermodynamic propert ies were obtained fro m the HYSYS environment once the process was fully converged.

Energy and Exergy analyses
The exergy and energy calculat ions were done using EXCEL spreadsheet after extracting the thermodynamics data fro m the HYSYS simu lations environment. These were calculated using the formulae given in the theory section applied to each unit, section and overall process of the thermal plant.

Uni ts Energetic and Exergetic Efficiencies
The units energetic and exergetic efficiencies are shown in Table 1. The units energetic efficiencies are mostly 100% because of the assumption of no heat losses that was made in the simulat ions. The exergetic efficiencies are between 23.23-90.7% for the furnace-boiler section and between 6.63 -86.12% for the turbine-generator, condenser and regenerator sections, for all the throughputs. However, as throughput is changed, the operating parameters also change and unit exergetic efficiencies are sensitive to process parameters. The second-law efficiency helped to identify the units that are operating inefficiently. Units with very low exergetic efficiencies are those where the exergy of sink is very low co mpared to exergy of source such as in the condenser.

Sections Energetic and Exergetic Efficiencies and Irreversi bilities
The sections energetic and exergetic efficiencies at different throughputs are shown in Tables

The Overall System Energetic and Exergetic Efficiencies and Total Irreversibility
Also in Tab le 2, it can easily be observed that the energetic and exergetic efficiencies of the overall system for all throughputs were very low co mpared to the units and sections energetic and exergetic efficiencies. The overall energetic efficiencies of the plant were 18.17, 19.79, 21.42, and 21.45% for throughputs of 50, 75, 100 and 110%, respectively. The overall exergetic efficiencies were 10.26, 11.22, 11.58, and 11.61% fo r throughputs of 50, 75, 100 and 110%, respectively, and showed that overall second-law efficiency of the plant was lower than the energy efficiency as was expected. It was observed that throughput did not significantly improve energy and exergy efficiency of the thermal power p lant. As shown in Figure 3, the overall irreversibilit ies at different throughputs showed that irreversilibit ies increased as the throughputs increased. The results compared to the coal-based thermal p lant showed that the energy and exergy efficiencies were 32% and 28%, respectively. These were higher in values due to differences in methods of calculating the efficiencies [19]. The energy and exergy analyses showed further that thermal p lants must be complimented with economic analysis to optimize the operation of the plant.

The Fuel-Air Rati o
The fuel and air flow rates for the throughputs are shown in Table 3. Fro m Table 3, the fuel to air ratio was about 1:12.6 for all the throughputs compared to the present operating ratio of 1:19.8. Excess air results in inco mplete combustion, increased air pollutants emissions and fuel wastage. The total power output generated in the plant was proportional to the total fuel flow rate and the mass flow rate of the steam produced.
The variations of co mbustion temperature in the furnace with air flo w rates at constant fuel rates for different throughputs are shown in Figure 4. It was observed in Figure  4 that the maximu m co mbustion temperature was about the same for all the throughputs and this was the reason why the overall energetic and exergetic efficiencies do not vary much with throughputs. The air input into the combustor can be used directly in controlling the co mbustion temperature rather than the indirect method of using the combustion temperature to control the air flow rate as was done in the plant, which would have reduced the time to reach steady state operation.

Conclusions
In this work, energetic and exergetic analyses were performed on Egbin thermal power plant. It was found that the units exergetic efficiencies were sensitive to plant throughputs but the sections and overall energetic and exergetic efficiencies were not sensitive to variations in plant throughput. The reason for this behaviour was attributed to the fact that the maximu m co mbustion temperatures in the furnace were about the same for all the throughputs. The overall exergetic efficiency was lower than the overall energetic efficiency as was expected because of proper accounting of different types of process exergies of material, heat and work in the plant.