Parameters Matching And Analysis Of Bottom Cycle System In Gas-steam Combined Cycle

Under the circumstance of seeking for high efficient energy use and environmental friendly development in today’s day and age, Gas-Steam Combined Cycle (GSCC) draws more and more attention due to its advantages of high efficiency and low pollution, and has witnessed a huge development in recent years. The GSCC is composed of the top cycle system (gas turbine) and the bottom cycle system (heat recovery steam generator and the steam turbine). The integrated matching and research on the off-design of the bottom cycle system of GSCC is significantly based on gas turbine exhaust parameters.The research content of this paper is an important part of a project named "The development of design platform of the GSCC system based on a heavy gas turbine".Based on a certain gas turbine exhaust parameters at design condition, five kinds of bottom system models were established, which are single pressure, dual pressure without reheat, dual pressure with reheat, triple pressure without reheat, triple pressure with reheat. Then according to the First Law of Thermodynamics, detailed research on the matching of the steam parameters of five kinds of processes was carried out by setting the bottom cycle power Pst as the objective function. As a result, the matching rules of steam pressure was obtained. After that, on the basic of the optimal matching results, comprehensive analysis design parameters of the system was obtained by increasing constraint conditions. In addition, comparing and discussing the performance characteristics of the five kinds of processes under the comprehensive analysis design was carried out.Finally, with the triple pressure without reheat bottom cycle comprehensive analysis design parameters as the design condition, the off-design model was established, then carried out the trend analysis of bottom cycle system key parameters at off-design condition. The results showed that as the decline of gas turbine power, high pressure and low pressure approach temperature difference reduced to zero or negative, and caused the evaporation of the economizer exit water. Concerning the vaporization in the economizer working in low load, two solutions were presented:the change of design parameters at design conditions and the improvement of the process structure of the bottom cycle system. Meanwhile, the performance changes of the bottom system with different gas turbine conditions were analyzed under those two solutions. The research results of GSCC at design and off-design conditions provided a significance guidance of bottom cycle system design and parameters matching.