Modeling of Steam and Power System

Modeling of key equipment in the steam and power system is essential for building steam and power balances, which can represent process steam and power demands and respond to variation in production. 

Energy and Process Optimization for the Process Industries, First Edition. Frank (Xin X.) Zhu. 

Although general and detailed discussions for equipment of steam and power system are widely available in public sources, this chapter focuses on modeling of key equipment to fulfill the needs of the modeling steam and power system. 

A boiler requires blowdown to remove concentrated dissolved solids and control the water quality. The lack of blowdown could result in a higher pH of boiler feed water (BFW) in the boiler, which could potentially lead to corrosion. Insufficient blowdown could also cause impurities to carryover to steam. On the other hand, excessive blowdown wastes energy, water, and chemicals. The optimum blowdown rate is determined by various factors including the boiler type and capacity, operating pressure, water treatment, and makeup water quality. Blowdown rate is 2—4% for relatively large boilers and 4—8% for small boilers. It can be up to 10% if makeup water contains high concentrations of solids. Industrial standards for blowdown are available and can be referenced that indicate the amount of blowdown depending on the type and pressure of the boiler. 

Three operating parameters affect boiler efficiency, namely, excess air, temperatures of air, and BFW temperature. Excess air is controlled to fulfill complete combustion. Too much excess air costs extra fuel to bring cold air to combustion temperature. The hotter the air and BFW enter the boiler, the less fuel is required. That is why air and BFW preheat is commonly adopted. 

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In modeling a gas turbine in the context of steam and power system, the purpose is to know the fuel consumption (gfuei) for a given net power output (Wnet) and fuel efficiency. It is also important to know the exhaust mass flow (mex) and temperature (Tex), which determine the integration opportunity with steam turbine cycle. Gas turbine performance curves are provided by manufatcturer. As an example for ilustration, the key parameters are described by the following equations (Manninen and Zhu, 1999) ... 

The purpose is to develop a steam balance for operational supervision as well as for identification of improvement opportunities in the steam system. Models for boilers, turbines, deaerators (DAs), letdown valves, desuperheaters, and steam flash tanks are discussed in the previous chapter. Historian and distributed control system (DCS) data will be coimected to steam balance so that the steam balance is capable of dynamically balancing the steam and power demands due to process variations, units on or off, and weather change. 

USSR National Standard Reference Data Service (NSRDS). The system was developed in 1976-1980 in the All-Union Research Center of NSRDS (now Russian Research Center on standardization, information and certification of raw materials, materials and substances) in Moscow. It provides specialists with attested databases, formed on the basis of standard and recommended reference data. The data of the lUPAC Commission on Thermodynamics, the International Association for the Properties of Steam, the U.S. National Bureau of Standards and other authenticated foreign data are used in the system as well. The informadon blocks of the system are sets of program modules, being the mathemadcal models of substances, and the blocks of numerical data for each substance. The basis for the model of a substance is a unified equadon of state for gas and liquid in the form of a double power expansion of the compressibility with respect to density and temperature. The principles of the molecular-kinetic theory and the dependence of the excess viscosity and thermal conductivity on density and temperature are used for the calculation of the transport properties. 

The system shown in Figure El 1.4 may be modeled as linear constraints and combined with a linear objective function. The objective is to minimize the operating cost of the system by choice of steam flow rates and power generated or purchased, subject to the demands and restrictions on the system. The following objective function is the cost to operate the system per hour, namely, the sum of steam produced HPS, purchased power required PP, and excess power EP ... 

JAEA conducted an improvement of the RELAP5 MOD3 code (US NRC, 1995), the system analysis code originally developed for LWR systems, to extend its applicability to VHTR systems (Takamatsu, 2004). Also, a chemistry model for the IS process was incorporated into the code to evaluate the dynamic characteristics of process heat exchangers in the IS process (Sato, 2007). The code covers reactor power behaviour, thermal-hydraulics of helium gases, thermal-hydraulics of the two-phase steam-water mixture, chemical reactions in the process heat exchangers and control system characteristics. Field equations consist of mass continuity, momentum conservation and energy conservation with a two-fluid model and reactor power is calculated by point reactor kinetics equations. The code was validated by the experimental data obtained by the HTTR operations and mock-up test facility (Takamatsu, 2004 Ohashi, 2006). 

Heavy fuel deposits were expected in boiling systems, and therefore the initial studies of deposition and activity transport for power reactors concentrated on the CANDU-BLW concept until the fields at Douglas Point became a concern. The deposit thickness was proportional to iron concentration in the coolant and to the square of the heat flux (69) deposition was reversible and quickly reached a steady value set by the local conditions. The corrosion products initially deposit by hydrodynamic and electrostatic effects then boiling accelerates deposition by drawing water and its contained iron into the deposit to replace the steam that leaves. Local alkalinity gradients within the deposit determine whether iron crystallizes to cement the deposit or dissolves to weaken it, and erosion processes then define the equilibrium thickness (70), This model works well in explaining deposition under boiling conditions. 

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