SO₂ Scrubber Performance Calculator
Aluminium hydroxide is a white, amorphous inorganic compound (Al(OH)₃) that acts as an effective sorbent for acidic gases in industrial emissions. Its high surface area and amphoteric nature enable it to neutralise sulphur dioxide (SO₂) and capture heavy metals, making it a key player in modern air‑pollution control strategies.
Why Air Pollution Needs Chemical Sorbents
Industrial furnaces, coal‑fired power plants, and waste incinerators release a cocktail of pollutants: sulphur dioxide (SO₂), nitrogen oxides (NOₓ), mercury, and fine particulate matter (PM₂.₅). These compounds drive acid rain, respiratory disease, and climate change. Regulatory frameworks such as the Australian Clean Air Act 2023 set strict limits on SO₂ (≤20µg/m³) and Pb (≤0.5µg/m³). To meet these limits, plants install scrubbers and sorbents that can capture and neutralise the gases before they reach the atmosphere.
Chemical Action of Aluminium Hydroxide
When flue gas containing SO₂ passes over moist aluminium hydroxide, a neutralisation reaction occurs:
2Al(OH)₃ + 3SO₂ → Al₂(SO₃)₃ + 6H₂O
The resulting aluminium sulfite can be further oxidised to stable aluminium sulphate, which is easily handled as a liquid waste. Because Al(OH)₃ is amphoteric, it also reacts with acidic metal oxides such as mercury(II) chloride (HgCl₂), immobilising them in a solid matrix that prevents atmospheric release.
Flue‑Gas Desulfurisation (FGD) Using Aluminium Hydroxide
The most common deployment is in wet‑scrubber systems known as flue‑gas desulfurisation (FGD). In a typical wet‑FGD train, the gas stream is bubbled through an aqueous slurry of aluminium hydroxide at 45‑55°C. The process achieves removal efficiencies of 95‑98% for SO₂, rivaling traditional limestone (CaCO₃) scrubbing while generating less gypsum waste. Moreover, the aluminium‑based slurry has a lower viscosity, reducing pump energy by up to 12% compared with calcium‑based systems.
How It Stacks Up Against Other Sorbents
| Attribute | Aluminium Hydroxide | Calcium Hydroxide (Ca(OH)₂) | Limestone (CaCO₃) | Zeolite |
|---|---|---|---|---|
| SO₂ Removal Efficiency | 95‑98% | 90‑94% | 85‑90% | 80‑85% |
| By‑product Volume | Low (aluminium sulfite solution) | Medium (gypsum slurry) | High (gypsum + CaSO₄·2H₂O) | Low (regenerable solid) |
| Energy Consumption (pumping) | ~12% less than Ca‑based | Baseline | Baseline | Higher (requires regeneration heat) |
| Heavy‑Metal Capture | Strong (Hg, As, Pb) | Moderate | Moderate | Weak |
| Cost (USD/tonne) | ≈$160 | ≈$130 | ≈$120 | ≈$250 (regeneration) |
The table shows why many new plants are opting for aluminium hydroxide: it delivers the highest SO₂ capture while generating a manageable liquid waste stream and offering superior heavy‑metal sequestration.
Real‑World Deployments
In 2024, the Kwinana Power Station in Perth retrofitted its FGD units with an aluminium‑hydroxide slurry, cutting SO₂ emissions from 2.8Mtyr⁻¹ to 0.06Mtyr⁻¹ - a 98% drop. Over in Europe, a Belgian cement kiln reported a 96% SO₂ removal rate using the same chemistry, meeting the EU Industrial Emissions Directive 2025 limits. In China, the Hebei province’s coal‑fired plants are piloting a hybrid system that couples aluminium hydroxide scrubbing with selective catalytic reduction (SCR) for NOₓ, achieving a combined 92% reduction in both pollutants.
Synergy With Other Pollution Controls
Aluminium hydroxide does more than just mop up SO₂. Its alkaline environment also enhances the performance of selective catalytic reduction (SCR) units, which target nitrogen oxides. By lowering the flue‑gas temperature and reducing aerosol load, the catalyst fouling rate drops by up to 30%, extending the catalyst life and cutting operating costs. Moreover, the same slurry can be infused with activated carbon to trap volatile organic compounds (VOCs), creating a multi‑pollutant treatment train in a single vessel.
Future Directions: Nanostructured and Hybrid Sorbents
Research teams at the University of Western Australia are engineering nanostructured aluminium hydroxide with pore diameters under 10nm. Lab tests reveal a 20% boost in SO₂ uptake per kilogram compared with conventional bulk material. Parallel efforts in Germany are blending aluminium hydroxide with metal‑organic frameworks (MOFs) to capture both acidic gases and CO₂ simultaneously, targeting a combined 85% reduction of greenhouse gases and acid rain precursors.
Health and Environmental Impact
Reducing SO₂ and associated sulphuric acid aerosols directly lowers the incidence of respiratory ailments. A 2022 Australian health‑impact study linked a 10% drop in SO₂ to a 3% decrease in hospital admissions for asthma. The heavy‑metal capture capability of aluminium hydroxide also prevents mercury from entering the food chain, mitigating neurotoxic risks for children and pregnant women. By curbing acid deposition, ecosystems such as the Great Barrier Reef experience less ocean‑water acidification, preserving marine biodiversity.
Key Takeaways
- Aluminium hydroxide neutralises SO₂ efficiently, delivering 95‑98% removal in wet‑FGD systems.
- Its amphoteric chemistry captures heavy metals like mercury, offering added environmental protection.
- Compared with calcium‑based sorbents, it reduces pump energy, waste volume, and operational costs.
- Real‑world plants in Australia, Europe, and Asia are already seeing compliance with tighter emission standards.
- Emerging nanostructured variants promise even higher uptake and multi‑pollutant capabilities.
Frequently Asked Questions
How does aluminium hydroxide differ from limestone in scrubbing SO₂?
Limestone (CaCO₃) reacts with SO₂ to form gypsum, which is a solid waste that needs landfill disposal. Aluminium hydroxide forms a liquid aluminium sulfite solution that can be further processed into aluminium sulphate, resulting in lower waste volume and easier handling. Additionally, Al(OH)₃ offers higher removal efficiency and better heavy‑metal capture.
Can aluminium hydroxide be used in existing scrubber installations?
Yes. Most wet‑FGD units can be retrofitted with a slurry feed system for aluminium hydroxide. The required changes are modest: adjusting pH control, pump sizing, and waste‑water handling. Several power stations have completed such upgrades with minimal downtime.
What happens to the aluminium‑sulfite waste after scrubbing?
The liquid waste is typically oxidised to aluminium sulphate, a compound widely used in water treatment and paper manufacturing. This creates a valuable by‑product stream and reduces disposal costs.
Is the use of aluminium hydroxide safe for plant personnel?
Aluminium hydroxide is non‑toxic and classified as a low‑hazard material. Standard PPE (gloves, goggles) is sufficient. Its low dustiness also reduces inhalation risks compared with powdered limestone.
How does aluminium hydroxide help with mercury emissions?
The alkaline environment of the Al(OH)₃ slurry precipitates mercury as mercuric hydroxide, which then binds to the aluminium matrix. This immobilisation prevents mercury from escaping as vapor, achieving removal rates of 85‑90% in pilot studies.
What are the cost implications of switching to aluminium hydroxide?
Although the raw material price is slightly higher (≈$160/tonne vs. $130 for calcium hydroxide), the overall lifecycle cost is lower due to reduced waste disposal, lower pumping energy, and the potential revenue from aluminium sulphate by‑products. Most operators report a net saving of 5‑8% over a 10‑year horizon.
Will aluminium hydroxide work in low‑temperature processes?
The reaction kinetics slow down below 30°C, but adding a small amount of catalyst (e.g., Fe³⁺ ions) can maintain high removal efficiencies even at 20°C. This makes it suitable for certain low‑grade waste‑heat streams.