CFB Boiler Sulfur Removal via Ash Hydration
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Solution Overview
Problem
Conventional circulating fluidized bed (CFB) boilers require high Ca/S molar ratios for effective sulfur dioxide reduction, leading to inefficient thermal performance and difficult ash disposal due to high calcium oxide content in ash products, and the calcination of limestone decreases thermal efficiency.
Innovation Solution
A method involving the classification and hydration of calcium oxide-containing bottom ash to produce calcium hydroxide, which is then used in a dry circulating fluidized bed scrubber to enhance sulfur dioxide removal, reducing the need for high Ca/S ratios and improving ash usability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high Ca/S ratios are used to achieve effective sulfur dioxide removal, then sulfur reduction efficiency is improved, but thermal efficiency of the boiler deteriorates due to excessive energy consumption for limestone calcination
Solution Approach 1:
The patent applies preliminary action by pre-hydrating calcium oxide-containing bottom ash to form calcium hydroxide before reintroducing it to the furnace. This pre-treatment activates the sorbent material, making it more reactive for sulfur dioxide removal and reducing the total limestone required. The chemical activation occurs before the main desulfurization process, thereby reducing the energy burden during operation.
Solution Approach 2:
The patent implements discarding and recovering by collecting calcium oxide-containing bottom ash that would otherwise be discarded as waste, and reactivating it through hydration and grinding. This recovered material is then reintroduced to the furnace as an active sorbent, reducing the need for fresh limestone and lowering the overall calcination energy requirements while maintaining effective sulfur dioxide removal.
2Object-affected harmful factors
If high Ca/S ratios are used for sulfur dioxide removal, then desulfurization efficiency is improved, but ash disposal difficulty increases due to high calcium oxide content in bottom ash and fly ash
Solution Approach 1:
The patent transforms waste bottom ash containing calcium oxide into a valuable resource by hydrating it to form calcium hydroxide, which is then ground and reintroduced to the furnace. This circular approach reduces the volume of waste ash requiring disposal while maintaining effective sulfur dioxide removal, directly addressing the ash disposal difficulty.
Solution Approach 2:
The patent changes the chemical state of calcium oxide in bottom ash from an inactive waste form to an active calcium hydroxide sorbent through hydration. This parameter change transforms the properties of the ash material, making it useful for desulfurization and reducing disposal requirements while improving the overall efficiency of sulfur dioxide removal.
3Object-affected harmful factors
If excessive limestone is fed into the furnace to achieve desired sulfur dioxide removal, then sulfur reduction efficiency is improved, but the layer of calcium sulfate on limestone particles prevents core reaction with sulfur oxides
Solution Approach 1:
The patent applies preliminary action by pre-hydrating calcium oxide to form calcium hydroxide before reintroduction to the furnace. This pre-activation creates a more reactive sorbent that can achieve effective sulfur dioxide removal with smaller particle sizes and thinner calcium sulfate passivation layers, maintaining core reactivity even after multiple reaction cycles.
Solution Approach 2:
The patent effectively segments the sorbent material utilization by separating fresh limestone from reactivated bottom ash. The reactivated ash consists of smaller, more uniformly sized particles with higher surface area to volume ratios, which resist calcium sulfate passivation more effectively. This segmentation allows for more efficient sulfur dioxide removal without requiring excessive limestone feeding.
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 sulfur dioxide reduction efficiency while reducing thermal energy losses and improving the usability of ash products, allowing for more efficient combustion and easier ash disposal.
Implementation Method 1
calcium carbonate (CaCO3) of the limestone is calcined to form calcium oxide (CaO)
Implementation Method 2
calcium oxide (CaO), which reacts with sulfur oxides to produce calcium sulfate (CaSO4)
Implementation Method 3
hydrating CaO in the ground bottom ash portion with a controlled amount of water or steam to Ca(OH)2
Implementation Method 4
converting SO2 in the exhaust gas to CaSO3 and CaSO4
Data Source
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AI summary
A method of and an apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler (10), the apparatus comprising: a furnace (12), means for feeding sulfurous fuel (14) and CaCO3-containing sorbent (16) to the furnace; a dry circulating fluidized bed scrubber (22) comprising a reactor (28) having means for feeding water and Ca(OH)2 for converting SO2 in the exhaust gas to CaSO3 and CaSO4 and a dust separator (30) in gas flow connection with the reactor, means for removing CaO-containing bottom ash (40) from the furnace, a classifier (42) for classifying a portion of the removed CaO-containing bottom ash into a coarse bottom ash portion and a finer bottom ash portion; a fine ash channel (48) for conveying at least a portion of the finer bottom ash portion from the classifier (42) to a grinder (50); a ground ash channel (52) for conveying at least a portion of the ground bottom ash portion from the grinder (50) to a hydrator (54) so as to hydrate CaO in the ground bottom ash portion to Ca(OH)2, and a hydrated ash channel (58) for conveying at least a portion of the Ca(OH)2 from the hydrator (54) to the dry circulating fluidized bed scrubber (22) as a sorbent.