Analysis of reheater size impact on power plant performance

In order to be able to analyze performance of the coal fired thermal power plant, a detailed mathematical model of thermohydraulical processes in the steam generator, steam turbine and feedwater line has been developed. Attention has been focused on the influence of the reheater package size on the thermal efficiency of the plant. History of the plant shows that the reheater size had been decreased due to problems with coal ash slugging, what eventually resulted in lower power output and thermal efficiency decrease. Simulation results showed that change of utilised coal for the type with improved slugging characteristics would not achieve design temperatures unless the reheater size would be restored. In this case the plant thermal efficiency would be increased for 0.4-0.7 % resulting in some 140000 euro/a operational cost savings. INTRODUCTION The pulverized coal-fired Power Plant Plomin 1 (Croatia) experienced a lot of problems burning domestic bituminous coal (from the mine Rasa) with a high content of sulfur (8-12 %) and unfavorable ash characteristics. Due to intense ash slugging and other problems that appeared when burning Rasa coal, dimension of the second (end) reheater had been reduced very soon after entering the plant operation. Namely, due to formation of ash deposits, heat flow rate in the furnace was less than designed and the temperature of flue gases in the reheater region was higher than design value. This had led to lower steam production and to higher temperature of reheated steam, above the design value of 540 °C, even with the maximum cooling water injection flow on the 2 injector (used for temperature control). Therefore, it had been decided to reduce the dimension of the reheater from initial 1138 m to 935 m by the subsequent reconstruction. Later on, when the coal reserves in the Rasa mine were depleted, the idea of using imported coal, which would not provoke intense slugging and would have a slightly higher heating value than the Rasa coal, has been grown. But, it was suspected that it would be difficult to achieve design values of the reheated steam temperature (540 °C) due to smaller size of the reheater. Therefore, it was necessary to carry out the analysis of reheater size effect on the reheated steam temperature and consequently to the plant efficiency. For this purposes, a detailed mathematical model of the Thermal Power Plant Plomin 1 has been developed that would allow simulation and assessment of the behavior of the plant in different operational situations. The mathematical model was tested and then adjusted according to available measured data mainly extracted from operational records. The mathematical model was translated into Visual Basic programming language in the MS Excel environment. The software package is rather user friendly with interface shown in Figure 1. It represents the basic input-output file, which allows almost complete communication of the user with the program. NOMENCLATURE  [kg/s] Fuel mass flow rate B [t/a] Annual fuel savings c [%] Coal carbon content Cc [W/Km] Black body radiation coefficient Cs [-] Coefficient accounting for tube layout Cz [-] Coefficient accounting for less tube rows than 10 dv [m] Outer tube diameter Hd [MJ/kg] Lower heat value k [W/mK] Total heat transfer coefficient MCO2 [t/a] Annual CO2 emissions MCO2 [t/a] Annual reduction in CO2 emissions pc [euro/t] Coal price Pe [MW] Plant electric output Pr [-] Prandtl number pCO2 [euro/t] Price of CO2 emission s [m] Tube wall thickness Sc [euro/a] Annual savings due to fuel coal reduction SCO2 [euro/a] Annual savings on CO2 emissions T [K] Temperature 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics