Recycling CaO Bottom Ash for SO2 Removal in CFB Boilers
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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 grinding of calcium oxide-containing bottom ash to separate coarse and finer portions, followed by hydration to produce hydrated lime, which is then reused in a dry circulating fluidized bed scrubber to enhance sulfur dioxide removal, reducing the need for excessive limestone and improving ash usability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high Ca/S molar ratio is used for sulfur dioxide reduction, then sulfur removal efficiency is improved, but thermal efficiency decreases and ash disposal becomes difficult
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-preparation increases the reactivity of the sorbent, allowing effective sulfur removal at lower Ca/S ratios. The calcium hydroxide is formed in advance through contact with water or steam, transforming the sorbent into a more reactive form that requires less quantity to achieve the same desulfurization effect, thereby preserving thermal efficiency.
Solution Approach 2:
The patent changes the chemical state parameter of the sorbent from calcium oxide to calcium hydroxide through hydration. This parameter change transforms the sorbent into a more reactive form that can achieve effective sulfur dioxide removal at lower Ca/S ratios. The chemical transformation from CaO to Ca(OH)2 increases the surface reactivity and reduces the amount of sorbent needed, thus maintaining thermal efficiency while achieving the desired sulfur removal.
2Object-affected harmful factors
If high Ca/S molar ratio is used for sulfur dioxide reduction, then sulfur removal efficiency is improved, but ash usability deteriorates
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-preparation increases the reactivity of the sorbent, allowing effective sulfur removal at lower Ca/S ratios. The calcium hydroxide is formed in advance through contact with water or steam, transforming the sorbent into a more reactive form that requires less quantity to achieve the same desulfurization effect, thereby preserving thermal efficiency.
Solution Approach 2:
The patent changes the chemical state parameter of the sorbent from calcium oxide to calcium hydroxide through hydration. This parameter change transforms the sorbent into a more reactive form that can achieve effective sulfur dioxide removal at lower Ca/S ratios. The chemical transformation from CaO to Ca(OH)2 increases the surface reactivity and reduces the amount of sorbent needed, thus maintaining thermal efficiency while achieving the desired sulfur removal.
3Object-affected harmful factors
If limestone is fed in excess for sulfur reduction, then sulfur removal efficiency is improved, but calcination energy consumption increases
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-preparation increases the reactivity of the sorbent, allowing effective sulfur removal at lower Ca/S ratios. The calcium hydroxide is formed in advance through contact with water or steam, transforming the sorbent into a more reactive form that requires less quantity to achieve the same desulfurization effect, thereby preserving thermal efficiency.
Solution Approach 2:
The patent changes the chemical state parameter of the sorbent from calcium oxide to calcium hydroxide through hydration. This parameter change transforms the sorbent into a more reactive form that can achieve effective sulfur dioxide removal at lower Ca/S ratios. The chemical transformation from CaO to Ca(OH)2 increases the surface reactivity and reduces the amount of sorbent needed, thus maintaining thermal efficiency while achieving the desired sulfur removal.
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 finer 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
AI summary
Sulfurous fuel and CaCO3-containing sorbent are combusted in a furnace of a circulating fluidized bed boiler. A dry circulating fluidized bed scrubber includes a reactor with water and Ca(OH)2 feeds for converting SO2 in the exhaust gas to CaSO3 and CaSO4 and a dust separator in gas flow connection with the reactor. A discharge removes CaO-containing bottom ash from the furnace. A classifier classifies a portion of the removed CaO-containing bottom ash into coarse and finer portions. A fine ash channel conveys some of the finer bottom ash portion from the classifier to a grinder. A ground ash channel conveys some of the ground bottom ash portion from the grinder to a hydrator to hydrate CaO in the ash to Ca(OH)2. A hydrated ash channel conveys some of the Ca(OH)2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.