Sodium Carbonate Aerosols in Glass Furnaces: Mechanistic Analysis, Failure of Conventional Air Pollution Control Devices, and the Efficacy of pH-Regulated Wet Scrubbing Systems
Scientific article on Na₂CO₃ dust & mist in float and container glass furnaces, and why a pH-controlled wet gas scrubber with slurry management is the robust solution.
1. Introduction
Glass production, particularly in the float glass and container glass industries, is one of the oldest yet most chemically complex
high-temperature processes in modern industry. Even with optimised furnaces, batch compositions, and combustion techniques, one
persistent issue remains worldwide: the release of sodium carbonate (Na₂CO₃) in the form of fine dust, aerosols, and condensates.
This fraction is generated during the melting of the batch, specifically during the decomposition of soda ash (Na₂CO₃), a key flux
used to lower the melting point of silica (SiO₂).
Unlike inert mineral dust, Na₂CO₃ aerosols behave in a highly unfavourable way in the gas phase. They are strongly hygroscopic,
chemically reactive, thermally unstable, and form a complex alkaline mist through interaction with moisture and other flue-gas
components. This mist has unique physical properties that cause conventional dry-type exhaust treatment systems – cyclones, baghouses,
and even electrostatic precipitators (ESPs) – to fail structurally at elevated relative humidity or under variable process conditions.
The failure of these systems leads to:
- Severe ductwork fouling and deposits
- Increased pressure drop and filter plugging
- Frequent unplanned shutdowns
- Higher energy consumption
- Higher maintenance and replacement costs
- Product quality issues such as seeds, knots and surface defects
- Accelerated corrosion in downstream equipment
Although many glass plants recognise these operational symptoms, the underlying problem is still underrepresented in scientific
literature. At the same time, furnaces are becoming larger, with higher tonnages and increased batch throughput, which only increases
soda ash carry-over. Stronger draught systems amplify entrainment and fine-particle loading.
This article analyses, from a physical, chemical and process-engineering perspective, how Na₂CO₃ emissions are formed, how they behave
in flue gases, and why conventional technologies fail. It then shows why a pH-regulated wet gas scrubber with slurry management is the
only physically robust, thermodynamically consistent and maintenance-friendly solution for this specific problem.
2. Chemical background: the role of Na₂CO₃ in glass production
2.1 Function of soda ash in the batch
Soda ash is used as a flux to:
- Lower the melting point of silica from ~1700 °C to ~1400 °C
- Control the viscosity of the molten glass
- Support homogenisation of the melt
- Provide alkali ions in the glass structure
In typical float-glass compositions, soda ash contributes around 10–18 wt% of the batch, depending on glass type.
Container glass, especially green and amber, often uses even higher soda fractions to balance colourants and other oxides, which
directly increases the risk of sodium-rich aerosols.
3. Thermal decomposition of sodium carbonate in the furnace
3.1 Decomposition ranges
Sodium carbonate decomposes in several steps. Initial decomposition begins around 850–1000 °C and is essentially complete
above 1100 °C:
Na₂CO₃ (s) → Na₂O (l/s) + CO₂ (g)
The evolving CO₂ creates local turbulence at the melt surface, entraining small droplets of Na₂O, partially decomposed Na₂CO₃ and
batch fines into the flue gas. This turbulence is intensified by burner momentum, temperature gradients and batch charging.
3.2 Formation of sodium-rich aerosols
When these vapours and droplets enter cooler zones, Na₂O reacts and condenses into compounds such as NaOH, Na₂CO₃, NaHCO₃ and Na₂SO₄
(if sulphur is present). The result is an alkaline aerosol population that is:
- extremely hygroscopic,
- prone to agglomeration,
- sticky, especially when partially deliquesced,
- chemically aggressive towards metals and glass fibre materials.
4. Transport and behaviour of Na₂CO₃ aerosols in flue gas
4.1 Particle size and morphology
Measurements in glass furnace exhausts show that a large fraction of sodium aerosols is submicron, with aerodynamic diameters in the
range 0.1–2 µm. These particles:
- have very low inertia and high diffusivity,
- are difficult to capture by inertial devices (cyclones),
- are difficult to charge efficiently in ESPs,
- are highly reactive and hygroscopic.
4.2 Hygroscopicity and phase changes
Sodium carbonate rapidly interacts with water vapour:
Na₂CO₃ + H₂O → Na₂CO₃·H₂O
and in the presence of CO₂:
Na₂CO₃ + CO₂ + H₂O → 2 NaHCO₃
Sodium bicarbonate has a higher specific volume than sodium carbonate. This expansion, combined with high hygroscopicity, drives the
formation of thick, expanding cakes in dry filters and sticky deposits on duct walls.
4.3 Duct fouling and seasonal effects
Deposits in flue gas ducts range from brittle, glassy layers at high temperature to pasty, alkaline sludges at lower temperatures and
high humidity. In winter conditions, with colder combustion air and higher relative humidity, agglomeration of Na₂CO₃ aerosols is
intensified and dry filters plug rapidly.
5. Why conventional dry technologies fail (overview)
Cyclones rely on inertial separation and have poor efficiency for submicron particles. Bag filters rely on building and maintaining a
dust cake; but Na₂CO₃ and NaHCO₃ cakes become hygroscopic, swell, and form a wet gel. ESPs rely on a specific range of dust
resistivity and chargeability that is simply not met by Na₂CO₃ mist.
In short: Na₂CO₃ aerosols do not behave as inert dust. They are reactive, hygroscopic, and phase-transforming.
Dry techniques are fundamentally mismatched with these properties.
6. Detailed failure mechanisms in cyclones, baghouses and ESPs
6.1 Cyclones
For a cyclone, the cut size dpc typically falls between 2 and 6 µm. A large fraction of Na₂CO₃ aerosols is below
1 µm. These particles lack the inertia to be flung to the cyclone walls and are carried straight through the vortex finder.
Hygroscopic growth does not help: if they absorb water, they may become sticky, causing internal build-up and crusting, but they still
are not reliably separated as a dry dust.
6.2 Baghouses
Bag filters are particularly vulnerable to Na₂CO₃ and NaHCO₃ due to:
- hygroscopic water uptake forming a wet cake,
- chemical transformation of Na₂CO₃ into NaHCO₃ with volume expansion,
- alkaline attack on glass fibre or PTFE media,
- drastic pressure-drop increases and poor pulse cleaning.
In practice, pressure drop can rise from 10–12 mbar to 35–45 mbar after humidity spikes, and filter bags tear under the
resulting mechanical stress. Frequent replacements, unscheduled shutdowns and safety risks are the result.
6.3 Electrostatic precipitators (ESPs)
ESPs require the dust resistivity to lie in a certain window (roughly 10⁷–10¹⁰ Ω·cm). Na₂CO₃ aerosols traverse both extremes:
at high humidity they become too conductive (wet, sticky layers and re-entrainment), and at dry, high-temperature conditions they form
highly resistive, glassy sodium sulphate/carbonate layers which promote back-corona and breakdown of the electric field. In addition,
liquid or semi-liquid alkaline mist droplets charge poorly and migrate slowly. The net effect is unstable performance and poor removal
of the sodium fraction.
7. Fundamental mismatch of dry techniques with sodium aerosols
All three dry technologies share a fundamental problem: they try to treat a reactive, hygroscopic, partially liquid aerosol as if it
were inert, non-reactive dust. This contradiction cannot be fixed with incremental improvements in filter design or cleaning systems.
From a thermodynamic and chemical standpoint, sodium aerosols “want” to enter the aqueous phase. Any concept that tries to keep them
in a dry state is fighting against the natural direction of the system.
8. Why water works: fundamentals of wet scrubbing for Na₂CO₃
A wet scrubber does not rely on mechanical interception of dry particles. Instead, it offers:
- gas–liquid mass transfer,
- absorption into a water-based liquor,
- chemical reaction and buffering,
- slurry management for solids removal.
For Na₂CO₃ aerosols, this aligns perfectly with their natural behaviour. Water is the thermodynamically preferred phase for sodium
salts. The scrubber leverages this rather than opposing it.
8.1 Solubility and reactions
Sodium carbonate is highly soluble in water (around 21 wt% at 20 °C, up to ~48 wt% at 100 °C):
Na₂CO₃ (s) → 2 Na⁺ + CO₃²⁻ (aq)
In the presence of CO₂:
CO₃²⁻ + CO₂ + H₂O → 2 HCO₃⁻
This establishes the carbonate–bicarbonate buffer, which acts in the pH range relevant for scrubber operation.
8.2 Driving forces for absorption
For Na₂CO₃ aerosols, the effective “Henry constant” is extremely favourable: the species essentially prefer the liquid phase as soon
as water is available. In mass transfer terms:
NA = KG · a · (yA − yA,i)
For Na₂CO₃, yA,i ≈ 0 (the interfacial gas concentration is almost zero due to rapid dissolution), making the driving force
maximal. The scrubber therefore is liquid-film controlled: the better the wetting and liquid circulation, the higher the removal
efficiency.
9. pH control and buffer chemistry
The carbonate system is governed by:
H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pKₐ1 ≈ 6.35)
HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (pKₐ2 ≈ 10.33)
For scrubber operation, the sweet spot lies around pH 9.6–10.2. In this range:
- absorption capacity for Na₂CO₃ remains high,
- buffering is strong,
- the risk of carbonate scaling is limited,
- corrosion remains acceptable for suitable materials.
Controlled acid dosing (for example with HCl or another compatible acid) maintains the pH in this window. This prevents
supersaturation and uncontrolled crystal growth in the liquor.
10. Slurry formation and stability
Even with strong dissolution, some solids will be present:
- non-dissolved Na₂CO₃/NaHCO₃ crystals at high load,
- CaCO₃ precipitated from hard make-up water,
- SiO₂ fines and other batch residues.
These are handled as a controlled slurry. Typical solids contents are 2–8 wt%, kept in suspension via agitation and appropriate
recirculation hydraulics. Purge control prevents solids from exceeding design limits.
11. Demisting and corrosion control
Without a proper mist eliminator, droplets of sodium-rich liquor would be carried over, causing exactly the kind of fouling and
corrosion that the scrubber is meant to prevent. A high-efficiency demister (mesh pad, chevron, or a combination) removes droplets
larger than 3–5 µm with efficiencies above 99.5%. Periodic washing of the demister ensures low pressure drop and long service
life.
12. Ravebo wet scrubber concept for glass-furnace Na₂CO₃
The Ravebo solution for sodium carbonate aerosols in glass furnaces consists of:
- a gas pre-cooler / quench section,
- a main packed-bed absorber,
- a recirculation loop with pH-controlled liquor,
- a slurry tank / sump with agitation,
- purge and solids management,
- a high-efficiency demister,
- a clean-gas section and stack interface.
12.1 Gas pre-cooler
The pre-cooler reduces flue-gas temperatures from about 180–300 °C to 60–90 °C and stabilises humidity conditions. It can
be realised as a spray quench section or as a packed pre-cooler for higher loads. Liquid-to-gas (L/G) ratios of 5–20 L/m³ and
residence times of 0.4–0.8 s are typical.
12.2 Main absorber (packed bed)
The main absorber uses structured packing (polypropylene, FRP or stainless steel) to create a large wetted surface area with low
pressure drop. Typical design values:
- bed height 0.8–1.5 m,
- gas velocity 1.8–2.8 m/s,
- flooding ratio < 60%,
- pressure drop 4–15 mbar,
- L/G 8–18 L/m³.
Under these conditions, the residence time and interfacial area are sufficient to capture >99% of sodium aerosols.
12.3 Recirculation and pH control
Recirculation flows of 60–180 m³/h (depending on furnace size) ensure good wetting of the packing and efficient heat removal.
Inline pH monitoring and controlled acid dosing keep the pH between 9.6 and 10.2. This maximises absorption while preventing
excessive carbonate scaling.
12.4 Slurry tank and purge
The sump or dedicated slurry tank combines:
- buffer volume for process stability,
- homogenisation of liquor composition,
- solids suspension via agitation or tangential inlet,
- controlled purge based on solids content (TSS).
Purge flows are set just high enough to prevent supersaturation and sedimentation but low enough to minimise wastewater volumes.
12.5 Demister and clean gas section
The demister, usually a 125–250 mm mesh pad and/or chevron demister, removes entrained droplets. Downstream ducts and stacks are
constructed from suitable materials (e.g. FRP or 316L) with drainage provisions. The result is a clean, low-corrosion exhaust with
stable temperature and humidity.
13. Comparison with alternative technologies
Objectively comparing cyclones, baghouses, ESPs and wet scrubbers for Na₂CO₃ control yields the following picture:
13.1 Cyclones
- Low CAPEX
- Poor efficiency for submicron sodium aerosols (<50%)
- Prone to internal crusting and fouling at high humidity
- Highly unstable performance under changing conditions
13.2 Baghouses
- Excellent for inert dust, but not for hygroscopic reactive salts
- Na₂CO₃/NaHCO₃ cakes absorb water and swell, forming a gel
- Strong pressure-drop increase; frequent bag failures
- High maintenance and replacement costs
13.3 ESPs
- Good for stable, non-hygroscopic dust with proper resistivity
- Sodium aerosols cause both low-resistivity wet conditions and high-resistivity crusts
- Back-corona, re-entrainment and chargeability problems for mist
- Complex and costly to operate for this application
13.4 Semi-dry scrubbers
- Useful for some acid gases (e.g. SO₂) at higher temperatures
- Unfavourable for sodium salts, which can form hard cement-like deposits
- Relatively high energy demand and maintenance requirements
13.5 Ravebo wet scrubber
- Removal efficiency for Na₂CO₃/NaHCO₃ aerosols typically >99%
- No dust cake; no hygroscopic expansion; no back-corona
- Stable pressure drop, independent of season and humidity
- Low maintenance and predictable operation
- Eliminates duct fouling and reduces downstream corrosion
14. Practical performance and economic aspects
14.1 Typical removal rates
- Na₂CO₃ solids: 94–98%
- Na₂CO₃ mist: 98–99.7%
- NaHCO₃ mist: >99%
- NaOH mist: >99.9%
- Silica fines: 90–98%
14.2 Pressure drop and energy use
For a correctly sized wet scrubber, the total pressure drop typically lies between 5–18 mbar. This is comparable to or lower
than many overloaded baghouses, but without the strong seasonal variations. Fan power consumption is therefore stable and predictable.
14.3 CAPEX and OPEX
While a wet scrubber has a CAPEX in the same general range as a baghouse or ESP, the OPEX is substantially lower due to:
- much lower maintenance hours,
- absence of expensive filter bags or ESP internals,
- avoiding costly unplanned shutdowns,
- reduced corrosion and longer lifetime of downstream equipment.
Typical payback times (ROI) are in the range of 12–24 months, strongly depending on the current maintenance burden and downtime costs.
15. Maintenance and risk profile
A Ravebo wet scrubber operates with a predictable and manageable maintenance schedule:
- Daily: visual checks, pH confirmation, chemical make-up.
- Weekly: nozzle pattern and slurry density checks.
- Monthly: demister inspection and purge verification.
- Annually: internal inspection, optional packing replacement, agitator and ductwork inspection.
The annual maintenance time typically lies between 60 and 120 hours, compared to 800–1400 hours for many
Na₂CO₃-burdened baghouses.
From a safety point of view, the wet scrubber offers substantial advantages: there is no dust explosion risk, no baghouse fire risk,
and far less corrosion-related failure in downstream systems.
16. Indirect benefits for glass quality
Beyond emissions and operational stability, a clean and well-controlled exhaust system also supports stable glass quality:
- less fouling in regenerators and checkers,
- reduced dust fall-back into the furnace,
- more stable furnace atmosphere and temperature distribution,
- lower incidence of seeds, knots and surface defects.
Plants that have migrated from dry to wet scrubbing often report reduced scrap rates (5–15%) and more stable thickness and optical
properties.
17. Final conclusion
Na₂CO₃ aerosols from glass furnaces are not a simple dust problem. They are a reactive, hygroscopic, phase-changing system that
inherently conflicts with dry filtration technologies. Cyclones lack efficiency for submicron particles, baghouses suffer from
hygroscopic cake formation and swelling, and ESPs are destabilised by resistivity extremes and poorly charging mist.
A pH-regulated wet gas scrubber with controlled slurry management does exactly what the chemistry demands:
- it dissolves sodium aerosols into an aqueous phase,
- it buffers the system in a stable pH window,
- it manages solids via slurry and purge instead of dry cake,
- it removes mist to protect downstream equipment,
- it operates independently of seasonal humidity swings.
In thermodynamic, chemical and operational terms, this is not just an improvement over dry techniques; it is the only concept that is
fundamentally aligned with the nature of the sodium aerosol problem.
For float and container glass plants struggling with Na₂CO₃ dust, mist, duct fouling, unstable filters and recurring shutdowns, a
properly engineered wet scrubber system provides a robust, predictable and economically attractive solution.
