Heat exchangers in calcination plants: lime, calcium carbonate and industrial process minerals
Technical criteria for heat recovery from rotary kiln flue gases with abrasive dust, high temperatures and CO₂ content: typology selection, materials and cleaning strategies.
Plants producing quicklime, hydrated lime, precipitated calcium carbonate and other industrial process minerals operate with rotary kilns that generate considerable volumes of combustion gases at temperatures typically between 300 and 600 °C at the preheater outlet. Recovering this residual thermal energy represents one of the best cost-benefit energy efficiency improvements available in the sector, but the nature of the gases — with high concentrations of abrasive dust, significant CO₂ content and, occasionally, sulphur compounds — demands very specific selection and technical design.
1. Production context: rotary kiln and calcination gases
The calcination of calcite (CaCO₃) to produce quicklime (CaO) is an endothermic reaction requiring process temperatures between 900 and 1 100 °C inside the kiln. The resulting combustion gases — enriched with CO₂ released by the mineral's decarbonation — leave the kiln at temperatures that depend on the kiln type and preheating system used:
| Kiln type / process | Typical flue gas temperature at outlet | Particularities for the heat exchanger |
|---|---|---|
| Long rotary kiln without preheater | 350–600 °C | High fine lime dust load (CaO/CaCO₃). High abrasivity. Large gas flow rate. |
| Rotary kiln with cyclone preheater | 200–350 °C | Dust partially separated in cyclones. More moderate temperature. Risk of condensation if excessively cooled. |
| Shaft kiln | 150–280 °C | Gases with very high CO₂ (up to 30–40 % v/v). Moderate dust. High CO₂ concentration may influence the selection of the receiving fluid. |
| Rotary kiln for dolomite / magnesite | 400–700 °C | Dust containing MgO and CaO components. Very high abrasivity. High gas temperature. |
2. Degradation mechanisms specific to this industry
2.1. Mechanical abrasion by particle impact
CaO, CaCO₃ or dolomite particles in kiln gases present a Mohs hardness of 3 to 5 and a particle size distribution that, despite passage through pre-collection cyclones, includes fractions up to 200–500 µm. When they impact tube surfaces at typical gas velocities (8–15 m/s), they cause erosive wear that is particularly severe at fin edges in finned tubes and at bends in gas direction-change zones.
The erosion rate is proportional to particle concentration, particle hardness, the third or fourth power of the impact velocity and the cosine of the impact angle. Minimising it requires design action: reducing gas velocities in the exchanger ducts (typically below 10 m/s in heavily abrasive applications), avoiding geometries that generate direct impingement on surfaces and selecting high erosion-resistant materials for maximum exposure points.
2.2. Fouling and blockage by dust deposition
CaO particles progressively depositing on tube and fin surfaces form an insulating layer that reduces the overall heat transfer coefficient (U) proportionally to its thickness. Under high dust-loading conditions without active cleaning, accumulation can reduce the economiser's thermal performance by 30–50 % within weeks or months. Dry CaO or CaCO₃ dust deposits are generally relatively soft and friable, and can be removed by mechanical vibration, steam blowing (sootblowing) or rapping, provided the equipment design includes adequate access and cleaning systems.
Under conditions of elevated moisture in the gases or during start-up and shutdown cycles with partially cooled gases, CaO (quicklime) particles can hydrate by reacting with moisture in the gases to form Ca(OH)₂. This exothermic reaction can generate hard and expansive deposits on tube surfaces, significantly more difficult to remove than dry dust deposits. Exchanger design and management of minimum wall temperatures during start-ups and shutdowns must account for this risk, especially in kilns processing quicklime without an efficient dust pre-collection system.
3. Heat exchanger typology selection
| Typology | Advantages for calcination gases | Limitations and risks | Recommended application |
|---|---|---|---|
| Bare tubes (no fins) | Maximum abrasion resistance. No preferential wear point due to fin geometry. Direct mechanical cleaning. Lower tendency for dust retention. | Lower surface density per unit volume than finned tubes. | Gases with high dust loading (>5 g/Nm³) and elevated abrasivity. |
| Helically finned tubes | High surface density. Good U coefficient. Compactness. | Dust accumulation in inter-fin channels. Difficult mechanical cleaning. Risk of irreversible blockage. | Clean gases (<1–2 g/Nm³). Not recommended for calcination gases without efficient post-collection. |
| Continuous (plate) fins | Better cleaning access than helical fins. | Dust accumulation in channels. | Moderate dust loading (1–5 g/Nm³). Post-collection cyclones of good efficiency. |
4. Material selection for abrasive-corrosive environments
| Material | Abrasion resistance | Wall temperature limit | Remarks |
|---|---|---|---|
| Carbon steel S235/P235GH | Moderate | ~450 °C | Suitable for moderate temperature zones with reasonably clean gases after pre-collection. Susceptible to SO₂ near the acid dew point. |
| Cr-Mo low-alloy steel (13CrMo4-5, P91) | Good | ~550 °C | Improved high-temperature oxidation and erosion resistance compared to carbon steel. |
| Stainless steel AISI 310S | Good–very good | ~1 050 °C | Excellent high-temperature oxidation resistance. For the first tube rows exposed to the hottest gases (>500 °C). |
| High wear-resistant cast iron (Ni-Hard, white iron) | Excellent | ~400 °C | Maximum impact abrasion resistance. Used for deflectors and shell liners. Brittleness limits use; not suitable for pressure-bearing tubes. |
| Nickel-base alloy (Inconel 625, Alloy 800H) | Very good | ~1 000 °C | For extreme conditions: very high temperature combined with corrosive gas. High cost; application justified on a case-by-case basis. |
In heat exchangers for calcination gases, it is common practice to apply a material zoning strategy: the first tube rows, exposed to the hottest gases and highest particle velocities, are built with more resistant materials (310S stainless steel or Cr-Mo alloy), whilst the final rows, where the gas temperature has already dropped, are executed in lower-cost carbon steel. This strategy optimises total equipment cost while maintaining the desired service life across all zones.
5. Cleaning systems and maintenance access
5.1. Sootblowers
Sootblowers inject a jet of saturated steam or high-velocity compressed air between the tube rows, dislodging dust deposits from the surfaces. They are the most common in-service cleaning solution for finned or bare tube heat exchangers with moderate fouling levels.
5.2. Mechanical rapping systems (rappers)
In bare tube heat exchangers where the dust is dry and friable, mechanical rapping systems — electric or pneumatic hammers striking the headers or casing at regular intervals — can be sufficient to keep surfaces reasonably clean. These are robust, low-maintenance-cost systems effective with dry CaO dust.
5.3. Manual mechanical cleaning during shutdown
Regardless of in-service cleaning systems, the design must include adequate access ports for visual inspection and manual mechanical cleaning during planned outages. In tall vertical exchangers, the design may incorporate hoppers or traps at the low point to collect and evacuate dust that has fallen by gravity.
6. Parameters for plant maintenance engineers to monitor
- Flue gas temperature at the exchanger outlet under comparable operating conditions. A progressive increase indicates dust accumulation or surface degradation.
- Gas-side pressure drop across the heat exchanger. An unusual increase may indicate partial blockage of some rows or ducts.
- Process fluid temperature at the outlet. A reduction from the design value indicates loss of heat transfer capacity.
- Draft fan energy consumption associated with the gas circuit. An unusual increase per unit of lime production may indicate increased gas-side resistance.