How to Diagnose Rising CO in a Cement Kiln

Rising CO in a cement kiln is one of the earliest and most reliable signals that something in the combustion system is deteriorating. CO is produced whenever carbon-bearing fuel is oxidised in oxygen-deficient conditions — and in a kiln, that means the fuel–air balance has shifted.

A sustained CO trend upward — even from 200 ppm to 600 ppm — demands a structured diagnostic response. Ignored, it leads to kiln instability, refractory damage, dangerous CO exceedances, and potential emergency stops. Addressed early, it is usually a 30-minute correction.

What Rising CO Means in Kiln Operation

CO in kiln exit gas (measured at the kiln inlet or preheater riser duct) reflects the quality of combustion inside the burning zone. Normal CO values at steady-state operation are typically 50–300 ppm at the preheater exit. Values above 500–800 ppm sustained over 15–30 minutes indicate a developing problem.

CO does not rise in isolation — you will usually see it accompanied by one or more of these secondary signals:

• BZT (burning zone temperature) dropping or fluctuating
• Kiln drive torque increasing (coating building or ring formation)
• Draught / draft fan running high but without corresponding O₂ improvement
• Clinker exit temperature dropping at the cooler inlet
• Feed rate unchanged but NOx falling — indicating a cooler flame

Common Root Causes

1. Fuel–Air Ratio Imbalance

The most common cause. Either the primary or secondary air volume has decreased, or the fuel feed rate has increased without a matching air adjustment. This is especially common after:

• Primary air fan speed changes (manual or SCADA glitch)
• Coal mill fineness change — coarser coal burns more slowly
• Alternative fuel substitution rate increase without adjusting total air
• Fuel calorific value drop (e.g., coal moisture increase)

2. Draft Imbalance (False Air or Blockage)

Draft problems — too little or too much — destroy combustion. Insufficient draft starves the flame of oxygen. Excessive draft can cool the flame and carry unburnt carbon into the preheater. Check:

• ID fan damper position vs. actual flow (check for damper hunting)
• Sudden drop in draught at preheater exit — indicates a blockage or false air ingress
• Riser duct or cyclone wall build-up — especially with volatile-rich raw materials

3. Volatile Cycles (Sulfur, Chloride, Alkali)

High volatile input — particularly sulfur and chloride from raw materials or fuels — creates internal recirculation loops in the preheater. As volatiles condense and re-enter the burning zone, they deposit and restrict gas flow, causing local oxygen deficiency. Signs:

• High SO₂ at preheater exit alongside CO
• Sticky or blocky clinker
• Cyclone stage blockages (especially stage 1 or 2)
• Kiln inlet temperature elevated above normal (e.g., >1050°C when PG is 950°C)

4. Flame Shape or Burner Issues

A poorly shaped flame — too long, too narrow, or impinging on the charge — creates incomplete combustion zones. This often follows:

• Burner pipe reposition without recalibrating axial/swirl air split
• Coal mill granulometry change
• Primary air pressure drop (e.g., primary air fan bearing issue)

5. Feed Surge or Raw Meal Chemistry Change

A sudden increase in feed rate — or a step-change in raw meal LSF — increases the heat demand in the burning zone faster than the thermal inertia of the kiln can absorb. CO rises because the flame cannot keep pace with the increased calcination and clinkering load.

Reference Values

Note: these are indicative ranges. Always use your plant's performance guarantee (PG) values as the primary reference.

Step-by-Step Diagnostic Approach

1. 01
   Check O₂ at kiln inlet and preheater exit. If O₂ is low (<1.5% at kiln inlet), the primary problem is insufficient air to the burning zone — not a fuel issue. Increase ID fan speed or primary air first.
2. 02
   Review the trend of BZT and kiln torque. A falling BZT with rising CO confirms combustion deterioration. Rising torque alongside CO indicates ring formation or coating build-up — the root cause is likely volatile-driven or feed surge.
3. 03
   Check coal mill fineness and fuel feed rate. Confirm coal residue on 90-micron sieve. If residue has increased above 12–15%, combustion efficiency drops. Check coal moisture (target <1% at burner).
4. 04
   Inspect burner pipe position. Burner end should be 200–400 mm inside the kiln hood (plant-specific). Confirm axial and swirl air split matches the current fuel mix. If AFR substitution is active, verify swirl has been adjusted.
5. 05
   Check for cyclone or riser duct blockage. A pressure drop imbalance across cyclone stages or sudden drop in draught at the kiln exit indicates a partial blockage — a major cause of local oxygen deficiency. Increase kiln bypass damper opening if available.
6. 06
   Review SO₂ trend. If SO₂ is rising alongside CO — especially after a raw meal change — investigate sulfur input from the quarry or alternative fuels. Increased bypass damper opening is the short-term mitigation.
7. 07
   Check feed rate history. A feed surge (even 2–3% increase) 20–30 minutes before CO rise is often the trigger. Reduce feed rate by 3–5% temporarily and monitor BZT response.

Corrective Actions

Match the action to the confirmed root cause. The following are first-response steps while root cause confirmation is underway:

• Immediate: Reduce feed rate by 3–5% and increase ID fan speed to restore O₂ above 2.0% at kiln inlet.
• Fuel–air ratio: Reduce fuel feed rate by 2–3% and confirm CO responds within 5 minutes. If CO does not drop, the issue is air-side, not fuel-side.
• Draft problem: Check and reset ID fan speed/damper. Inspect for false air ingress at kiln hood and seals.
• Volatile cycles: Increase bypass damper opening by 5–10% (if available). Reduce sulfur-bearing AFR substitution temporarily.
• Coal fineness: Reduce coal mill throughput to allow separator to improve fineness. Target R90 <12%.
• Burner: If flame is suspect, retract burner by 100 mm, increase swirl air by 5–8%, and monitor BZT.

When to Escalate

Escalate to senior process engineer or shift in-charge when any of the following occur:

• CO exceeds 1500 ppm for more than 5 minutes with no improving trend
• BZT drops below 1320°C (or 60°C below your PG value)
• Kiln torque exceeds 95% of rated capacity
• ID fan at maximum speed and O₂ still below 1.0% at kiln inlet
• Multiple preheater cyclone stages showing blockage
• CO and SO₂ both spiking simultaneously — volatile cycle may be uncontrollable without feed reduction