Project 3: Analysis of diverse heat recovery Steam Cycles Artoni Alessandro Bortolotti Alberto Cordisco Giuliano

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1 Project 3: Analysis of diverse heat recovery Steam Cycles Artoni Alessandro Bortolotti Alberto Cordisco Giuliano We consider a combined cycle with the same simple cycle gas turbine described in the 2 nd project. With the aid of the software Turbogas we find the following operating conditions: - Air mass flow rate supplied to the compressor: 650 kg/s - TIT turbine inlet temperature: 1277 C - Compressor pressure ratio : 18.6 The aim of the project is the analysis of the different obtainable performances of the following heat recovery steam cycles: a. ONE EVAPORATION LEVEL* b. TWO EVAPORATION LEVELS c. THREE EVAPORATION LEVELS WITH SUPERHEATING OF THE HIGH PRESSURE STEAM We post- process the data given out by Turbogas, set with the appropriate working conditions. In the following table we report the most interesting results for a comparison between the three layouts: η cv η rec,th η rec η 2 overall η 2 sc Pel,sc [MW] Tstack [ C] Case a Case b Case c We clearly see that if the plant has more than one evaporation level, all efficiency terms increase. This is due to the fact that the steam cycle better exploits the heat available from the exhaust gases, because the working fluid better follows the cooling curve of the flue gases, reducing the irreversibilities related to heat exchange between finite ΔT. Hence, thermal recovery in the HRSG increases. As a result, the adoption of more evaporation levels leads to higher steam cycle net power and better performance of the system. In addition, the exhaust gas temperature at the chimney is lower since there is a better thermal recovery in the HSRG. Remember that to obtain the maximum heat recovery, the stack temperature should be equal to the ambient temperature. * P eva = 33 bar optimized value of project 2

2 However, in order to avoid the issue of acid corrosion of the gases and enhance dispersion in the atmosphere, such temperature should not be lower than C. Therefore, the best configuration from the heat exchange perspective is the solution with three evaporation levels, because despite the great ΔT of water in the whole cycle, the ΔT between the Δη HRSG Δη stack Case a Case b Case c two flows of gas and water are still modest. Obviously, the main drawbacks of this configuration may be its technical complexity and cost, so that economic feasibility should be verified. For all the previous reasons, we can easily foresee and justify the trends of the efficiency losses terms (referred to the reversible power of the exhausts) related to heat recovery steam generator and stack, as we adopt more evaporation levels. Next, we are going to discuss the adoption of three evaporation levels (case c), performing a deeper analysis of the plant configuration (HRSG layout especially), the heat exchange between hot gases and water (T- Q plot) and the second- law analysis of the steam cycle. STEAM CYCLE CONFIGURATION In the following table we report the thermodynamic quantities for each stream of the working fluid, highlighting mass flow rate, temperature, pressure, enthalpy and entropy.

3 m T [ C] P [bar] h s [kg/s] [kj/kg] - [kj/kgk] TEMPERATURE - THERMAL POWER CHART FOR THE HRSG Water flows: Pre- eco LP IP HP RH Exhausts

4 This plot divides heat exchange as a function of the temperature step seen by the flue gases. In each section there are all the steam flows running through the same part of the HRSG in parallel with each other. Such representation allows keeping the correct proportions between the slopes on the hot side, where they truly depict the heat capacity of the exhaust gases. Hence, it is useful when we focus on the analysis of heat exchange from the exhausts perspective. Water flows: Pre- eco LP IP HP RH Exhausts The second plot shows the process of heat exchange divided for each streamline within the HRSG. In fact, it keeps the correct proportions on the abscissa, i.e. the fraction of heat exchanged. The slopes of the water side are now the correct ones, but on the hot side the exhausts curve is shattered and does not represent the real trend. Thus, this representation is more useful when we are interested in the heat exchange mechanisms from the working fluid standpoint. OBSERVATIONS - The heat exchangers between exhaust gases and water have a counter flow configuration. - The slopes of the lines are inversely proportional to the heat capacity mc!. - The line at high pressure has a lower slope than the line at mid- pressure because of the higher thermal capacity. - Around 50% of the total heat is exchanged with the fluid at high pressure (70 % of the total mass flow rate); this allows having lower irreversibility in the heat exchange process.

5 ENTROPY ANALYSIS HRSC Losses % Δη inflow heat (HRSG) 9.28 Δη steam turbine 7.85 Δη th/el/org 4.60 Δη condenser 7.25 Δη various 1.01 Δη stack 3.14 HRSC tot losses η 2 sc From these tables it is evident how adopting three evaporation levels greatly lowers entropy generation due to heat exchange and stack especially. This entails a rather high second- law efficiency around 65%.