Oxyfuel CFB Boiler Sulfur Capture via Low Ca/S Ratio
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Solution Overview
Problem
Oxyfuel combustion circulating fluidized bed boilers face challenges in reducing sulfur oxide emissions, with high SO2 concentrations and the formation of harmful SO3 leading to increased acid dew point and corrosion, especially when conventional calcium carbonate injection methods result in excessive calcium oxide in ash and reduced thermal efficiency.
Innovation Solution
A method involving the controlled feeding of calcium carbonate at a low Ca/S molar ratio (0.02-0.1) to capture SO3 in the furnace or upper flue gas channel, with the majority of SO2 captured in a downstream sulfur reduction stage, utilizing a wet scrubber in the discharge channel to minimize sorbent consumption and ash volume, and fine calcium carbonate particles to enhance selectivity and prevent recarbonation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If conventional calcium carbonate injection methods are used to capture sulfur oxides, then sulfur oxide emissions are reduced, but excessive calcium oxide accumulates in ash and thermal efficiency decreases
Solution Approach 1:
The patent changes the key parameter of calcium carbonate injection rate, reducing it to a low Ca/S molar ratio of 0.02-0.1. This parameter change transforms the sulfur capture mechanism from bulk SO2 absorption to selective SO3 capture, minimizing calcium oxide production while maintaining effective sulfur oxide emission control and preserving thermal efficiency
Solution Approach 2:
The patent applies local quality by creating different functional zones: the furnace performs selective SO3 capture with minimal calcium carbonate, while a downstream sulfur reduction stage handles the majority of SO2 removal. This spatial differentiation allows each zone to optimize its function, reducing overall sorbent consumption and ash volume
2Object-generated harmful factors
If high amounts of calcium carbonate are fed to capture SO2 in the furnace, then sulfur oxide reduction efficiency is improved, but ash volume increases and disposal becomes difficult
Solution Approach 1:
The patent extracts the bulk sulfur reduction function from the furnace to a downstream sulfur reduction stage. By removing this function from the combustion chamber, the system eliminates the need for high calcium carbonate injection rates in the furnace, thereby dramatically reducing ash volume while maintaining effective sulfur oxide control
Solution Approach 2:
The patent applies partial action by using only a small portion (Ca/S ratio of 0.02-0.1) of calcium carbonate in the furnace, sufficient only for SO3 capture. The remaining sulfur reduction is handled partially by a downstream stage, avoiding the excessive calcium carbonate injection that would generate large ash volumes
3Object-generated harmful factors
If calcium carbonate is fed at high rates to achieve 98% sulfur reduction efficiency, then sulfur oxide capture is improved, but equipment and operational costs increase
Solution Approach 1:
The patent changes the calcium carbonate injection parameter to a low Ca/S ratio of 0.02-0.1, which dramatically reduces sorbent consumption and associated costs. The parameter change also shifts the capture mechanism to selective SO3 reaction, achieving effective sulfur oxide control with minimal material input and lower operational expenses
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 effectively reduces sulfur oxide emissions by capturing most SO3 in the furnace and SO2 downstream, maintaining a low acid dew point, minimizing excess calcium oxide, and optimizing thermal efficiency while reducing equipment and operational costs.
Implementation Method 1
feeding a second stream comprising CaCO3 to the furnace at such a rate relative to the first stream that the molar ratio of calcium in the second stream to sulfur in the first stream (the Ca/S molar ratio) is between about 0.02 and about 0.1 so as to capture at least a portion of the formed SO3 in the furnace or in the upper flue gas channel to CaSO4
Implementation Method 2
capturing at least a portion of the formed SO2 in a sulfur reduction stage arranged in the discharge channel
Implementation Method 3
At the high temperatures prevailing in the furnace of a CFB boiler, typically from 750 °C to 900 °C, calcium carbonate (CaCO3) of the limestone is usually calcined to calcium oxide (CaO)
Implementation Method 4
sulfur in the fuel mainly oxidizes to form sulfur dioxide (SO2)
Implementation Method 5
A portion of the SO2 is converted to sulfur trioxide (SO3)
Implementation Method 6
sulfur trioxide (SO3), which is especially harmful when combining with water (H2O) to sulfur acid (H2SO4)
Data Source
Figure 1
AI summary
A method of reducing sulfur dioxide emissions of an oxyfuel combustion circulating fluidized bed boiler, comprising the steps of feeding a first stream comprising sulfur-containing carbonaceous fuel to a furnace of the boiler; feeding a second stream comprising CaCO3 to the furnace;feeding a stream of substantially pure oxygen to the furnace so as to combust the fuel, whereby SO2 and SO3 are formed and flue gas consisting mainly of carbon dioxide and water is produced;discharging the flue gas from the furnace to an upper flue gas channel; recycling a first portion of the flue gas from the upper flue gaschannel via a recycling channel back to the furnace, and discharging a second portion of the flue gas from the upper flue gaschannel via a discharge channel, wherein the second stream is fed at such a rate relative to the first stream that the molar ratio of calcium in the second stream to sulfur in the first stream (the Ca/S molar ratio) is between about 0.02and about 0.5 so as to capture atleast a portion of the formed SO3 in the furnace or in the upper flue gas channel to CaSO4.