Pneumatic Coal Conveying for Faster Load Changes in CFB Boilers
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Solution Overview
Problem
Existing CFB boilers face slow load response due to large coal particle size, low furnace temperatures, and oxygen diffusion limitations, leading to inefficient load-increasing rates.
Innovation Solution
A pneumatic conveying system integrating a compressed air and coal powder mixing system with variable-section and tapered coal nozzles to deliver fine coal powder into the furnace via secondary air pipes, enhancing combustion efficiency and heat release.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If CFB boilers use larger coal particles (0.01-8 mm) and lower furnace temperatures, then resource efficiency is improved, but load response speed deteriorates
Solution Approach 1:
The system separates coal feeding into two independent pathways: traditional CFB coal feeding for base load and pneumatic conveying for load adjustment. This segmentation allows each pathway to optimize for its specific function, resolving the contradiction between resource efficiency and load response speed.
Solution Approach 2:
Coal is pre-ground to fine powder (≤0.1 mm) and stored ready-for-use in silos before being rapidly conveyed to the furnace. This preliminary preparation eliminates the time lag associated with on-demand coal processing, enabling rapid load response while maintaining efficient combustion.
2Reliability
If CFB boilers use larger coal particles, then combustion stability is improved, but heat release rate deteriorates
Solution Approach 1:
The system divides coal combustion into two zones: CFB bed combustion using larger particles for stability, and pulverized coal combustion using fine powder for rapid heat release. This spatial segmentation allows both combustion modes to coexist and complement each other.
Solution Approach 2:
The system merges two coal feeding systems (CFB and pulverized coal) into a unified combustion process. Fine pulverized coal is injected into the furnace to supplement the CFB bed, combining the stability advantages of CFB with the rapid combustion advantages of pulverized coal.
3Stability of the object's composition
If CFB boilers increase bed materials and refractories, then thermal inertia is improved, but load change rate deteriorates
Solution Approach 1:
Fine coal powder is prepared and stored in advance in silos, ready for rapid deployment. This preliminary action bypasses the thermal inertia limitation by having fuel immediately available for quick combustion, enabling fast load changes without requiring reduction of bed materials.
4Ease of operation
If CFB boilers use dense-phase zone inert materials, then combustion control is improved, but oxygen diffusion deteriorates
Solution Approach 1:
Fine pulverized coal acts as an intermediary fuel that burns rapidly in the oxygen-rich zone above the dense-phase bed. This intermediary approach allows the dense-phase zone to maintain its control advantages while the pulverized coal provides rapid heat release where oxygen is readily available.
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 increases load change rates from 1% Pe/min to over 3% Pe/min, reduces ignition restart temperature, and ensures safe, rapid load increases in CFB units.
Implementation Method 1
The air compressor supplies compressed air to the air-powder mixer
Implementation Method 2
The outlet of the air compressor is linked via a pipeline to the inlet of a refrigerated dryer
Implementation Method 3
The air-powder mixer blends the compressed air and coal powder to output an air-powder mixture
Implementation Method 4
A conveying system for the air-powder mixture includes a main conveying pipe and branch conveying pipes
Implementation Method 5
The air-powder mixture velocity at the outlet of the variable-section coal nozzle reaches (24-42) m/s
Implementation Method 6
The air-powder mixture velocity at the outlet of the tapered round coal nozzle is (50-72) m/s
Implementation Method 7
enhancing combustion efficiency and heat release
Data Source
AI summary
The present invention relates to a pneumatic conveying system and a circulating fluidized bed (CFB) boiler, belonging to the technical field of CFB boilers. The system comprises an air compressor that supplies compressed air to an air-powder mixer, a coal powder bin that stores coal powder and feeds it into the air-powder mixer via a powder feeder, and the air-powder mixer, which blends the compressed air and coal powder to output an air-powder mixture. The outlet of the air-powder mixer is connected to a main conveying pipe, which branches via a distributor to multiple conveying pipes. These conveying pipes are connected to secondary air pipes on the front, rear, and side walls of the CFB boiler in a sleeve-type manner, delivering the air-powder mixture into the furnace chamber. The invention enhances the load change rate of CFB units from the current 1% Pe/min to over 3% Pe/min, reduces the ignition restart temperature, extends the ignition hold time, and ensures safe and rapid load increase for CFB units.