Two-Stage Fuel Gas Preheating in CCPP
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Solution Overview
Problem
Combined cycle power plants face energy losses due to high-grade heat usage in fuel gas preheating systems, and existing methods are complex and costly, limiting the efficiency gains from increased hot gas temperatures.
Innovation Solution
A two-stage fuel gas preheating system using heat from different pressure levels of the water steam cycle, with a first heat exchanger connected to a low or medium pressure feed water line and a second heat exchanger connected to high pressure feed water, reducing pressure losses and allowing for higher fuel gas temperatures up to 350°C, while reintroducing water into the feed system to minimize pump size and heat exchanger costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If high pressure feed water is used for fuel gas preheating, then preheating efficiency is improved, but pressure losses and energy waste increase
Solution Approach 1:
The fuel gas preheating process is divided into multiple stages using feed water from different pressure levels. The first heat exchanger uses medium pressure feed water for initial preheating, while the second heat exchanger uses high pressure feed water for final preheating. This segmentation allows efficient heat utilization at each stage without excessive pressure losses.
Solution Approach 2:
The system changes the pressure parameter of the heat source by utilizing feed water from different pressure levels (medium and high) rather than using a single high pressure source. This parameter variation optimizes the heat transfer efficiency while minimizing energy losses.
2Temperature
If high pressure feed water is used for fuel gas preheating, then preheating efficiency is improved, but system complexity and cost increase
Solution Approach 1:
The feed water system serves multiple functions: it provides cooling water for the condenser, process water for the HRSG, and heat source water for the fuel gas preheating system. By utilizing the existing multi-pressure feed water infrastructure for preheating, the system avoids additional complexity while achieving efficient fuel gas preheating.
3Productivity
If fuel gas is preheated to higher temperatures, then thermal efficiency is improved, but heat exchanger size and cost increase
Solution Approach 1:
The preheating process is segmented into two stages with two separate heat exchangers. Each heat exchanger handles a portion of the temperature increase, allowing for more compact and cost-effective design compared to a single large heat exchanger that would be required to achieve the same total temperature rise.
Solution Approach 2:
The system changes the temperature parameter incrementally through two stages rather than requiring a single large temperature jump. This staged approach reduces the thermal stress and size requirements for each individual heat exchanger.
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 approach enhances thermal efficiency, reduces power losses, and allows for more economical preheating of fuel gas to higher temperatures with smaller heat exchangers, improving overall plant efficiency and reducing costs.
Implementation Method 1
a first heat exchanger for preheating the fuel gas to a first elevated temperature, wherein the first heat exchanger uses heat extracted from a lower pressure level of the water steam cycle
Implementation Method 2
a second heat exchanger for further preheating the fuel gas to a second elevated temperature, wherein the second heat exchanger uses heat extracted from the highest pressure level of the water steam cycle
Data Source
AI summary
The invention refers to a CCPP comprising a gas turbine, a water steam cycle with a steam turbine and a HRSG with at least two pressure levels, and a fuel preheater for preheating the fuel of the gas turbine. The fuel preheater includes a first heat exchanger for preheating the fuel to a first elevated temperature, which is connected to a feed water line from a pressure level of the HRSG, which is below the highest HRSG pressure level, and a second heat exchanger for further preheating the fuel gas to a second elevated temperature, which is connected to the high pressure feed water with the highest pressure level of the HRSG. The disclosure further refers to a method for operating a CCPP with such a fuel preheater.