Calculation On Properties Of Water And Steam In Supercritical Region And Thermodynamic Calculation Of Supercritical Boiler

The thermal efficiency of supercritical thermal power units is higher than sub-critical units of the same capacity level, because of the improvement of the operating parameters. And also, supercritical units have great advantage in operating economy and environmental protection. Most type of supercritical boilers is once-through, and the thermal properties of water would change drastically under supercritical pressure. Thus, it’s very important to accurately calculate properties of water and steam to guarantee the accuracy of thermodynamic calculation of supercritical boiler.Firstly, the calculation program on properties of water and steam which we’ve developed completely covered the range of a series of model released by the International Association for the Properties of Water and Steam (IAPWS) since2004. Hereinafter, we simply call the program as IAPWS-IF97program. The IAPWS-IF97program was designed to suit the needs of supercritical boiler thermodynamic calculation, coding work was done with Fortran language on Visual Studio2008platform. The IAPWS-IF97program can achieve the following functions:(1) Calculate specific enthalpy h, density p. dynamic viscosity μ, thermal conductivity λ, specific heat Cp and Prandtl number Pr according to the (p, T) values;(2) Calculate corresponding temperature value T according to the (p, h) values;(3) Calculate the corresponding saturation temperature Ts according to the pressure value p.In particularly, the backward equation v3(p, T) and Helmholtz free energy equation f(p, T) was adopted to calculate the properties of water and steam in critical area (known as region3). The equations allow the program to directly calculate the property parameters of critical area rather than iterative calculation, and significantly improved the calculation accuracy and speed.Then we replaced the calculation program module of water and steam in an existing Qishuyan subcritical boiler thermodynamic calculation program with IAPWS-1F97program. Comparison between the results before and after refilling the properties calculation module showed that the maximum deviation of calculated heat transfer coefficient value to the original value was0.15%, changes in boiler operating parameters were also around0.1%. The results indicated that the IAPWS-IF97program fulfilled the needs of boiler thermodynamic calculation.The supercritical boiler thermodynamic calculation program for B&WB-1910/25.40-M boiler of Lanxi Power Plant4×600MW supercritical units was developed on basis of the IAPWS-IF97program. As the once-through boiler was equipped without steam drum, the calculation of property parameters of working fluid from the outlet of water wall was proceeded according to energy balance; The heat transfer coefficient of steam-side was also took into consideration when proceeding thermodynamic calculation of the economizer, since part of working fluid of economizer already in gaseous state. The thermodynamic calculation was proceeded under following conditions:(1) Boiler Maximum Continuous Rate (known as BMCR);(2)75%Turbine Heat Acceptance;(3)50%Turbine Heat Acceptance. Finally we explored the optimization of Lanxi supercritical boiler under BMCR condition through adjusting the following parameters separately:(1) Flue gas damper;(2) Furnace excess air ratio:(3) Intermediate superheater effective thermal coefficient;(4) Utilization factor of the air preheater.