Efficiency of utilization of heat of moisture from exhaust gases of heat HRSG of CCGT

The paper discusses the technology of utilizing the heat of exhaust gas moisture from heat recovery steam gases (HRSG) of combined-cycle gas turbine (CCGT). Particular attention focused on the influence of the excess air factor on the trapping of the moisture of the exhaust gases, as in the HRSG of the CCGT its value varies over a wider range than in the steam boilers of the TPP. For the research, has been developed a mathematical model that allows to determine the volumes of combustion products and the amount of water vapor produced according to a given composition of the burned gas and determine the amount of moisture that will be obtained as a result of condensation at a given temperature of the flue gases at the outlet of the condensation heat exchanger (CHE). To calculate the efficiency of the HRSG taking into account the heat of condensation of moisture in the CHE an equation is derived.


Introduction
CCGT of the utilization type currently has the highest net efficiency in the generation of electricity, which at the most powerful and perfect CCGT 9.02.02 of GE Power has reached 63.7%.The working medium of the steam turbine unit of this CCGT is water.The ways to further increase the efficiency of the CCGT are given in following work [1].This is an increase in the initial temperature in front of the gas turbine and a decrease in heat losses in the steam turbine cycle and HRSG.At the same time, a decrease in heat loss in HRSG is the simplest way.
The combustion of a hydrocarbon fuel, depending on the type of fuel in the exhaust gases is formed about 10-15% moisture vapor.To prevent condensation of this moisture in the flues and the chimney so that there is no corrosion and destruction of the equipment, the exhaust gases are withdrawn at a temperature above the dew point.In this case, the heat of condensation of water vapor is lost and the atmosphere is contaminated with them.
The technology of cooling off gases in boilers below the dew point using condensation heat recovery units is now recognized as one of the most energy efficient [2][3][4][5][6].
This technology is especially important for HRSG of CCGT, their efficiency is not more than 90% due to the low temperature of the gases entering the boiler.At the same time, HRSG of CCGT operate with an excess air factor α greater than 2, and the articles on this topic consider the operation of boilers with α <2.For cooling of flue gases in CHE, can be used low-boiling substances and can be obtained additional generation of electricity based on the Rankine organic cycle with an air condenser [7][8][9][10].
To evaluate the effect on the amount of moisture captured and on the efficiency of the HRSG fuel characteristics, the operating parameters of the flue gases and the air excess factor, a mathematical model is worked out for the operation of the HRSG with a CCGT.

Mathematical model of calculating the HRSG with CHE
The mathematical model of the HRSG with CHE represents a system of equations for calculating the parameters and indicators of its operation.
The initial data: gas composition; air moisture content d air , kg/kg.dry;temperature t 0 ,qС and pressure P g , Pa exhaust gases at the input of the recovery boiler t 1 and t 2 ; coefficient of excess air in the HRSG α.

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Efficiency The model is implemented by the program in the Excel package with the calculation of the parameters of water, fuel and flue gases according to the functions of the REFPROP base [9].

Results and analysis
With this program, the calculations were carried out with the following input data: Composition of gas fuel, %: СН 4 = 98.9;С 2 Н 6 = 0.94; С 3 Н 8 = 0.08; С 4 Н 10 = 0.08.Moisture of air d air =0.01 kg/kg.dry.The temperature of the gases at the input of the HRSG t 0 = 550qС.The gas pressure in HRSG P g = 10100 Pa.The temperature of flue gas at the input of the CHE t 1 = 100qС.Figure 1 shows the dependence of the increase in the efficiency of the HRSG on D and t 2 ,, and in Fig. 2 -dependence of the decrease in moisture content in the CHE from D and t 2 .The calculations based on the model showed that, depending on the temperature of the gases emerging from the CHE, the increase in the efficiency of the HRSG at the gas temperature at the outlet of the CHE 20 °C is 14.4% and is independent of the air excess ratio α.At a temperature of 30°C, the efficiency gain drops slightly with an increase in the excess air coefficient from 13.3% at α = 1 to 12.8% for α = 4.At a temperature of 40°C, the efficiency gain decreases from 11.8% at α = 1 to 10.8% for α = 4.At a temperature of 50°C, the increase in efficiency gain decreases from 10.2% at α = 1 to 7.6% at α = 4.A strong drop in efficiency at a temperature of 50 °C is due to the fact that for α> 1.9 the

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HMTTSC 2017 temperature of the outgoing gases becomes higher than the dew point, so if the contact point is of the contact type, then there is no condensation of moisture, but evaporation.

Fig. 2 -
Fig. 2 -Dependence of decrease in moisture content in CHE.