Sub-low Pressure Evaporator for Combined Cycle Heat Recovery
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Solution Overview
Problem
Combined cycle power plants face inefficiencies due to high recirculation flow and auxiliary power demand in flashing steam systems, which require larger economizers and increase overall power consumption, diminishing efficiency gains.
Innovation Solution
A water/steam system with a sub-low pressure evaporator and super-heater integrated into the heat recovery steam generator, allowing for enhanced heat extraction at lower temperatures and generation of super-heated steam for the low-pressure steam turbine, reducing the need for recirculation and auxiliary power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If flashing steam systems are used to increase heat extraction from flue gas, then overall efficiency increases, but recirculation flow and auxiliary power demand increase
Solution Approach 1:
The invention changes the pressure parameter by introducing a sub-atmospheric pressure environment in the flashing vessel (pressure between 0.01 to 0.1 bar absolute). This parameter change allows water to flash evaporate at lower temperatures using waste heat from flue gas, increasing heat extraction efficiency while reducing the energy required for recirculation pumps since the flashing process occurs passively due to the pressure differential
Solution Approach 2:
The invention replaces the mechanical recirculation pump system with a passive flashing process driven by pressure differential. The flashing vessel operates at sub-atmospheric pressure, causing water to automatically flash evaporate without requiring mechanical pumping, thereby eliminating auxiliary power demand while maintaining heat extraction efficiency
2Quantity of substance
If flashing steam systems are used to increase heat extraction, then more steam is generated, but economizer size must increase
Solution Approach 1:
The invention segments the steam generation process into two distinct parts: (1) the flashing vessel operating at sub-atmospheric pressure for steam generation, and (2) the economizer for heat pre-extraction. This segmentation allows the flashing vessel to generate additional steam independently without requiring the economizer to be oversized, as the flashing process occurs in a separate pressure environment
Solution Approach 2:
The flashing vessel acts as an intermediary device between the economizer and the steam turbine. It receives heated water from the economizer and converts it to steam through passive flash evaporation in a sub-atmospheric pressure environment. This intermediary function allows steam generation without increasing economizer size, as the flashing process occurs in a separate pressure zone
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration increases overall efficiency by 0.13% to 0.17% points in combined cycle power plants while avoiding the drawbacks of larger economizers and high auxiliary power consumption, allowing for efficient heat extraction and steam generation without requiring turbine redesign.
Implementation Method 1
heat extraction from flue gas at low temperature levels
Implementation Method 2
sub low pressure super-heater for superheating the sub low pressure steam
Implementation Method 3
low pressure evaporator for generating steam from water
Data Source
Figure 1
AI summary
The invention relates to a water/steam system (1) for a combined cycle power plant, comprising a heat recovery steam generator (4) providing a flue gas stream path (6) for extracting heat from a flue gas stream (9) exhausted from a gas turbine, the heat recovery steam generator (4) having a low pressure section (3) including a low pressure evaporator (13) arranged along the flue gas stream path (6) for generating low pressure steam at a low pressure input level (p10) for a main input (54) of a low pressure steam turbine (51). Furthermore, the invention relates to a method for operating such a water/steam system (1). In order to use heat a low temperatures, the present invention provides that in the low pressure section () additionally sub low pressure steam at a sub low pressure level (p11) below the low pressure level (p10) is generated by means of a sub low pressure evaporator (23) in the flue gas stream path (6).