PARAMETRIC ANALYSIS OF THE DIAGRAM OF THE COMBINED-CYCLE GAS TURBINE WITH A COMBINATION OF THREE CYCLES FOR IMPROVING EFFICIENCY WHEN OPERATING IN NORTHERN GAS PRODUCING AREAS

Abstract

The relevance. Combined-cycle gas turbines are considered as one of the promising directions in development of thermal power plants operating on natural and synthesis gas. The interest in their introduction in Russia is caused by large reserves of natural gas, low capital investment and minimal emissions of harmful substances into the environment. It is known from thermodynamics that in order to achieve high cycle efficiency, it is necessary to have a high heat supply temperature and low heat removal temperature, and to ensure equipment operation with minimal internal losses, and to have a rational thermal scheme of equipment interconnection in a cycle. At the present stage, the maximum temperature of heat supply in the combustion chamber of a gas turbine installation with existing structural materials and methods for cooling turbine elements has reached 1600 °C, and heat removal temperature in the condenser during the operation of the Rankine cycle on water cannot be lower than 15 °C. Under these conditions, the electric efficiency of 63 % is achieved at the most advanced three-loop combined-cycle gas turbine with intermediate superheating of steam. For the Rankine cycle when working on water, the condensation temperature of the steam should be above 0 °C according to the freezing condition. For a combined-cycle plant, when operating in conditions of low average annual ambient temperatures, which is typical for Russia and especially remote northern gas production areas, heat can be removed in the Rankine cycle well below 0 °C, but this can be done reliably only by using air-cooled condensers, if an organic working body is used as a working body in the Rankine cycle. The disadvantage of modern organic working fluid is the low limiting temperature of their thermal decomposition, which is usually below 300...400 °C. Subject of the research is combined-cycle plants with cycles on three working bodies, where the upper cycle of a gas turbine unit operates on gas combustion products, the middle cycle of a steam turbine unit operates on water and steam in the temperature range of 100÷650 °C, and the lower cycle – Organic Rankine cycle operates on organic working fluid in the range of temperatures –30÷200 °С. The aim of the study is the choice of a rational technological diagram of a combined-cycle plants using cycles on three working bodies and an air condenser to enable reliable heat removal from the organic working fluid at a temperature below 0 °C and to determine the optimal parameters of the cycles. Methods. Complex heat and power systems, including steam and gas installations, are characterized by a variety of processes occurring in their elements. Such installations can be effectively investigated only with the help of mathematical modeling and optimization methods. When conducting research the authors have applied a systematic approach, methods of energy balances and the calculation of thermodynamic and thermophysical property of working bodies using modern certified programs. Results. The authors developed the original diagram of a combined-cycle gas turbine unit with cycles on three working bodies. In this diagram, the Brighton upper cycle operates on combustion products of natural gas, the average Rankine cycle operates on water and water vapor, the lower – Organic Rankine cycle operates on organic working fluid with condensation in an air condenser. The mathematical model and a program for calculating the proposed scheme was developed. The authors carried out the parametric analysis of the cycles main parameters influence on the efficiency and power of the combine cycle gas turbine and determined the most effective organic working fluid for the lower Rankine cycle.

Keywords

• combined-cycle gas turbine
• organic working bodies
• organic rankine cycle
• air condenser
• math modeling