Conventional steam power plant model

The chosen conventional steam power plant model in this thesis is based on a developed subcritical FBC power plant model, which is used as host plant for the solar field. The steam boiler of the model is designed to be a natural circulation boiler, which fuel pow-er is approximately 400 MW, and the power output of the turbines is approximately 134 MW. The model consists of six diagrams, which include the modelled flue gas side of the steam boiler, water-side of the steam boiler, turbine island, main control loops of the steam boiler and two miscellaneous diagrams for calculation of the energy and mass flow equations of the steam boiler.

The flue gas side of the boiler island consists of the developed FBC boiler module and the modelled fuel supply, combustion air supply, superheaters, reheaters, economizer, and air preheating (Figure 46). Fuel is combusted in the FBC module with excess pre-heated combustion air, from which 70% is modelled to be primary air and 30% is mod-elled to be secondary air. The set point for oxygen level of flue gas is designed to be 3.5% at 100% load, as the pressure at the furnace is designed to be 1.01 bar. Superheat-ers include primary (SH 1), secondary (SH 2) and tertiary heat surfaces (SH 3), whereas reheaters include primary and secondary heat surfaces (RH 1 and RH 2). The

economiz-er consists of two heat surfaces, and the air preheating consists of two parallel heat transfer surfaces: one for primary air and one for secondary air.

Figure 46. Schematic of the flue gas side of the steam boiler modelled in Apros.

Water-side of the steam boiler consists of the steam drum of the boiler and the heat structures of the water tubes in the furnace wall (Figure 47). The water wall of the fur-nace is modelled with multiple water tubes. One water tube is divided to 10 heat pipe sections, in which heat is transferred from the FBC module. Different heat transfer coef-ficients can be given to the different heat pipe sections, as the heat transfer is not uni-form across the furnace wall.

Figure 47. Schematic of the modelled water side of the steam boiler in Apros.

The turbine island consists of turbine sections, a condenser, LP FWHs, a deaerator and HP FWHs (Figure 48). In the model, there are seven turbine sections, two HP FWHs and three LP FWHs. Thus, six bled off steam lines and reheating section are modelled by dividing the turbines into seven sections. The isentropic efficiency (ηst) of turbine can be calculated with Equation 1 (Raiko et al. 2013, p.27):

η = ∆ℎ

∆ℎ (1) in which

∆h is real enthalpy drop of the expansion of steam in the turbine [J/kg]

∆hst is the isentropic or theoretical enthalpy drop of the expansion of steam [J/kg]

The isentropic efficiency is defined in the model to be different for each turbine sec-tions, as the isentropic efficiency of HP turbine (section 1) is modelled to be the highest, and the efficiency drops in the IP (sections 2 and 3) and LP turbine sections (4 to 7) due to pressure reductions of reheaters and bled off steams (Table 4).

Table 4. Isentropic efficiencies of modelled turbine sections.

Turbine section 1 2 3 4 5 6 7 ηst (%) 85 84 84 83 83 83 81

In the HP FWHs, the superheated bled off steam is firstly cooled to saturated steam. The saturated steam is then condensed and subcooled before it is fed to the drain line enter-ing the deaerator. On the other hand, in the LP FWHs the superheated steam is either cooled to saturated steam and condensed or just condensed before it is fed to the drain line entering the condenser. The amount of bled off steam is controlled by keeping the level in the condensing heat transfer surface constant. Thus, as the feedwater mass flow is increased through the FWHs, more steam is condensed in the heating surface. There-fore, the level in the condensing heat transfer surface is increased, and the valve after the condensing heat transfer surface is opened in order to restore the level of the FWH.

Figure 48. Simplified schematic of the modelled turbine island in Apros.

Master control loops include control loops for fuel and combustion air feeding, amount of feedwater, steam temperatures, and furnace pressure. The control loops consists of PID, PI, cascade and feedforward control loops. The fuel supply and combustion air supply control loops are connected in series. As fuel supply is adjusted by the live steam pressure or by the power output of the turbines, the fuel supply control sends a set point for the combustion air feeding. The amount of feedwater is controlled by three element cascade system, which measures the level of the steam drum, the mass flow of produced

trol. Thus, the steam temperature control loops measure the final temperature of the steam and the spray water mass flow through the attemperator, and controls the spray water mass flow accordingly to the steam temperature. In addition, furnace pressure control loop measures the pressure at furnace and adjusts the rotation speed of the flue gas blower accordingly.

Two miscellaneous diagrams for calculation of the energy and mass flow equations are modelled in order to calculate the fuel power of the steam boiler and the thermal power of all the components related to the steam cycle. In addition, the thermal powers of flue gases leaving from furnace and entering the stack are calculated. Thus, it can be ob-served the quantity of energy transferred into the steam cycle and the quantity of ener-gy, which is lost with the flue gases. Furthermore, the wall masses of heat transfer sur-faces are calculated in order to observe the quantity of energy, which is stored in the wall masses of the heat surfaces.