I. Preface

Circulating elements (potassium, sodium, chlorine and sulfur) inside the rotary kiln shall be removed before scaling and blockages form in the preheater and kiln system. Such elements (chlorine, sulfur and alkalis) can be stripped out via dedusting to realize controllable circulation.

Bypass ventilation is adopted to extract part of high-temperature flue gas from the kiln inlet (where internal circulation concentrates harmful substances at the highest level), which prevents cyclone blockage caused by condensation of hazardous components within the preheater. Nevertheless, adding a bypass ventilation system will raise the heat consumption of the entire kiln system.

The bypass unit delivers the optimal removal efficiency for chlorine, while its performance on sulfur and alkali substances is much weaker.

II. Basic Principles

III. Typical Technical Process Routes

3.1 Scheme 1: Simple Air Quenching

Flue gas is extracted at 1050–1150 °C. The extracted gas is cooled down to 200–250 °C by ambient cold air. Bypass ash contained in the gas (hot raw meal and crystalline salts) is captured by a baghouse dust collector. Electrostatic precipitators exhibit poor efficiency under such gas conditions, so baghouses are the mainstream choice in most cases. Collected bypass ash is either landfilled or fed into the cement grinding system. Dedusted bypass gas is sent to the inlet of the high-temperature fan or the clinker cooler fan.

3.2 Scheme 2: Air Quenching with Coarse Powder Recirculation

Flue gas is extracted at 1050–1150 °C. The gas is cooled to 350–380 °C using cold air. Coarse ash separated by the cyclone is recirculated back to the kiln inlet. The gas flows into an air cooler for heat exchange and further cools to approximately 160 °C before entering the bag filter. Filtered bypass ash is delivered to the cement mill. Post-dedusting bypass gas can be introduced to the high-temperature fan, clinker cooler fan, or merged into the SP waste heat power generation system.

3.3 Advantages and Disadvantages of Air Quenching Only

3.3.1 Advantages

• Simplified layout of bypass equipment: only one cooling chamber and a cooling fan are installed between the gas extraction point and the dust collector.
• No extra water injection and distribution equipment required (water storage tank, pumps, nozzles, compressors, etc.).
• The bypass flue gas has a high dew point with low moisture content, greatly lowering corrosion risks for gas pipelines and dust collectors.
• Single-stage gas cooling requires only one control loop for temperature regulation.

3.3.2 Disadvantages

• A large volume of cold air is required to cool flue gas from 1150 °C down to 200 °C, compared with water cooling alternatives.
• Large air volume demands oversized cooling fans, gas ducts and dust collectors, leading to higher capital investment.

3.4 Water Cooling + Denitrification Process

Flue gas is extracted at 1050–1150 °C. Primary cooling with cold air reduces gas temperature to 350 °C. Subsequent cooling in a cooling tower brings the temperature down to 150–250 °C to meet inlet conditions for dust collection. Steps 2 and 3 can be integrated optionally by direct water spraying into the mixing chamber. Gas carrying bypass ash (hot raw meal or crystalline salts) is treated by bag filters or electrostatic precipitators. Recovered bypass ash is landfilled or sent to cement grinding.

3.5 Advantages and Disadvantages of Water Cooling + Denitrification

3.5.1 Advantages

• The volume of bypass flue gas handled by dust collectors is reduced by roughly 50% versus pure air cooling.
• Smaller bypass mixing chamber and cooling fan with lower power consumption.
• Downsized bypass dust collector cuts down project investment.
• If the kiln tail primary dedusting equipment is an electrostatic precipitator, higher flue gas humidity helps optimize dust specific resistance and improves ionization efficiency for better dust removal.

3.5.2 Disadvantages

• Elevated moisture content raises the gas dew point, increasing corrosion risks for pipelines, dust collectors and bypass fans.
• Extra auxiliary equipment is necessary: water tank, feed pumps, cooling tower, pressure regulating units, compressors, dual-flow nozzles, etc.
• Uneven water spray distribution may cause wet bottom accumulation inside the cooling tower.
• Two-stage temperature reduction requires additional automatic control loops.
• Bypass ash tends to absorb moisture and turn sticky, creating difficulties in ash discharge from hoppers, storage silos and conveying systems.

IV. Core Technical Competencies

We determine an economical and optimal bypass ventilation ratio through comprehensive calculation and selection covering raw mix proportioning, harmful element circulation balance, bypass air proportion, free lime prediction and emission compliance assessment.

Extraction Point Layout: The position is finalized based on kiln rotation direction, preheater column quantity, preheater type, alternative fuel / co-disposed hazardous waste types and overall process layout.

Patented Quenching Devices: Custom-developed equipment including double-sleeve probe samplers and primary mixed-air cyclones.

Integrated Overall Design: Self-developed thermal balance calculation programs. Combined with local emission standards and pollutant limits, and coordinated with other on-site construction schedules to deliver customized one-stop solutions.

V. Team Project Experience

Our team has undertaken over 10 bypass ventilation projects covering independent design or full set of design plus equipment supply.

VI. Summary and Outlook

Jacob CEO

An expert in pyroprocess and solidwaste recovery . Responsible for the Marketing and Support of Sequoia Industrial with over 14 years in this field. I am determined that our sustainable products and services will be at the forefront of the solution for the generations to come.

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