Flue Gas Recirculation for Induration Furnace Efficiency
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Solution Overview
Problem
High excess air levels in furnaces, such as induration furnaces, result in significant thermal efficiency losses and increased fuel consumption, as they are often required to maintain convective heat transfer and control humidity and flammable vapors, but existing methods for flue gas recirculation do not address efficiency improvements in high excess air systems.
Innovation Solution
A flue gas recirculation system that selectively recirculates exhaust gases from drying, pre-heating, and combustion zones to reduce the amount of excess air required, using a system with exhaust gas outlets, cooling zone intakes, and control means to manage the recirculation flow, compatible with conventional high excess air systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If high excess air levels are used to maintain convective heat transfer and control humidity, then heat transfer rate and process control are improved, but thermal efficiency is significantly reduced
Solution Approach 1:
The patent recovers thermal energy from flue gas that would otherwise be discarded through heat exchangers. The flue gas, containing valuable thermal energy, is used to preheat combustion air and process materials, thereby recovering energy that would be lost with high excess air operation.
Solution Approach 2:
The system changes operational parameters by introducing flue gas recirculation to modify the thermal field. By controlling the amount and timing of flue gas recirculation, the system maintains convective heat transfer while reducing the net excess air requirement, thus improving thermal efficiency.
2Reliability
If high excess air levels are used to control humidity and flammable vapors, then process safety and humidity control are improved, but fuel consumption increases
Solution Approach 1:
The flue gas recirculation system allows the combustion process to serve itself by using the hot flue gas to preheat combustion air and provide convective heat transfer. This self-service mechanism reduces the need for additional excess air, thereby reducing fuel consumption while maintaining process control.
Solution Approach 2:
The system employs feedback control by monitoring process conditions (humidity, temperature, oxygen levels) and adjusting flue gas recirculation accordingly. This feedback mechanism ensures that excess air levels are optimized to maintain safety and process control while minimizing fuel consumption.
3Loss of energy
If flue gas recirculation is introduced to reduce excess air, then thermal efficiency is improved, but system complexity increases
Solution Approach 1:
The flue gas recirculation system is designed to serve multiple functions simultaneously: it preheats combustion air, provides convective heat transfer, controls oxygen levels, and manages flue gas temperatures. This multi-functionality justifies the added complexity by delivering multiple benefits from a single integrated system.
Solution Approach 2:
Heat exchangers serve as intermediary components that facilitate the transfer of thermal energy from flue gas to combustion air and process materials. These intermediaries enable the recirculation system to operate efficiently by providing controlled heat transfer without direct mixing of flue gas with the process stream.
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
The system substantially reduces excess air requirements while maintaining convective heat transfer, leading to increased efficiency and decreased fuel consumption, with economic benefits and reduced flue gas treatment needs.
Implementation Method 1
Many applications require high convective rates to transfer heat from the flame and flue gas into a product or heat load
Implementation Method 2
The system uses the heat exhausted in the flue gas and re-introduces it into the process to replace heat input from fuel
Data Source
AI summary
A method and system for improving high excess air combustion system efficiency, including induration furnaces, using a re-routing of flue gas within the system by gas recirculation. Flue gas is drawn from hot system zones including zones near the stack, for re-introduction into the process whereby the heat recovery partially replaces fuel input. At least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone exist in these furnaces. At least one exhaust gas outlet is provided to each pre-combustion drying and combustion zone. At least part of the gaseous flow from each system zone exhaust outlet is selectively delivered to an overall system exhaust, the remaining flow being selectively delivered via recirculation to cooling zones. Recirculation flow is adjusted to meet required system temperatures and pressures. The method and system provide efficiency improvements, reducing fuel requirements and greenhouse gas emissions.