How to remove CO from the exhaust gas of industrial boilers?

The most effective technical approach for treating carbon monoxide (CO) in industrial boiler flue gas is catalytic oxidation, which converts CO to CO₂ at a relatively low temperature over a suitable catalyst. Under typical operating conditions, proper catalyst selection can achieve a CO conversion rate above 60%, reducing the outlet CO concentration to below the emission limit (e.g., ≤100 mg/m³). Catalyst selection must take into account key parameters such as temperature window, flue gas moisture content, SO₂ concentration, and space velocity – proper selection ensures long‑term stable operation, while improper selection may lead to rapid catalyst deactivation and soaring operating costs.

Low‑NOx Combustion’s “Side Effect” – Why CO Becomes a New Headache in Boiler Flue Gas

Retrofitting boilers with low‑NOx combustion technology effectively reduces nitrogen oxide (NOx) emissions, but it also intensifies the reducing atmosphere in the furnace and increases incomplete fuel combustion, leading to a significant rise in CO concentration in the flue gas. For example, in iron ore sintering flue gas, the CO volume fraction can reach 0.5%–2%. Measurements from a 35 t/h coal‑fired boiler show SO₂ concentrations up to 800 mg/m³ and particulate matter of 150 mg/m³, with CO levels often rising correspondingly. According to relevant emission standards, the CO emission limit for coal‑fired boilers is generally required to be controlled within 200 mg/m³, and in more stringent cases, ≤100 mg/m³. CO not only pollutes the environment and threatens human health, but its ineffective control also brings direct pressure from environmental compliance.

Physical Adsorption vs. Chemical Oxidation – Why Catalytic Oxidation Becomes the Mainstream Choice

The mainstream industrial methods for CO abatement include physical adsorption and chemical oxidation. Physical adsorption uses porous materials to adsorb and concentrate CO, suitable for separating and recovering medium‑to‑high concentrations of CO, but it suffers from limited adsorption capacity and high regeneration energy consumption. Chemical oxidation, on the other hand, uses a catalyst to lower the activation energy of the CO oxidation reaction, converting CO to CO₂ at temperatures far below those required for direct combustion. The reaction is: 2CO + O₂ → 2CO₂. Catalytic oxidation is the most effective method for CO abatement, with core advantages including complete conversion (the product is harmless CO₂), a wide applicable temperature range, and the possibility of coupling with NH₃‑SCR denitrification technology – utilising the reaction heat released by CO oxidation to supply heat to the denitrification system, achieving cascade energy utilisation.

Precious Metals vs. Non‑Precious Metals – Performance Comparison and Selection Logic of CO Oxidation Catalysts

CO oxidation catalysts are mainly divided into two categories: precious metal and non‑precious metal. Precious metal catalysts (represented by Pt and Pd) exhibit high catalytic efficiency and excellent water‑ and sulfur‑resistance. Studies show that a Pt‑based supported catalyst can achieve a CO conversion of 63.9% at 180°C with a water vapour volume fraction of 11.7%, and maintains good water resistance in the temperature range of 180–300°C. However, the high cost of precious metal catalysts limits their large‑scale application in certain scenarios.

Non‑precious metal catalysts, represented by copper‑manganese mixed oxides (the Hopcalite‑type catalyst), are low‑cost and exhibit excellent low‑temperature activity. The active components of Hopcalite catalysts are copper and manganese oxides, and their catalytic mechanism relies on oxygen vacancies in the copper‑manganese oxide lattice and the reversible redox cycles between Cu²⁺/Cu⁺ and Mn⁴⁺/Mn³⁺. When the Cu:Mn molar ratio is between 1:1 and 1:2, the CO conversion can exceed 90% at 25°C. Doping with rare‑earth elements such as cerium can further enhance performance – a modified catalyst with 7.5% cerium doping increases CO conversion by 50% at room temperature and improves the reaction rate by a factor of 8.

Selection logic: For flue gas with high moisture and sulfur content, precious metal catalysts are preferred; for dry, low‑sulfur, or ambient‑temperature conditions, copper‑manganese non‑precious metal catalysts are the better choice.

Temperature, Space Velocity, and Moisture – Three Key Variables Determining CO Catalytic Oxidation Efficiency

Temperature is the primary factor affecting CO conversion. Over a CeO₂ catalyst, a CO conversion of 77% can be achieved at 300°C with a feed containing 1000 ppm CO. However, the temperature windows of different catalysts vary significantly: Hopcalite catalysts can initiate CO oxidation at room temperature (0–40°C), whereas precious metal catalysts typically require temperatures above 180°C to reach optimal activity. In industrial applications, wide‑temperature catalysts have extended the conventional operating window to 240–400°C.

Moisture significantly inhibits catalytic performance. For example, with a Hopcalite catalyst at 25°C, increasing the relative humidity from 30% to 80% can reduce CO conversion from 96% to 43% within 2 hours. When relative humidity exceeds 60%, conversion can drop by more than 50% within hours. For a Pt‑based supported catalyst, increasing the water vapour volume fraction from 0 to 27.1% reduces the catalytic efficiency at 240°C from 73.9% to 62.3%.

Space velocity (GHSV, Gas Hourly Space Velocity) directly affects the contact time between the flue gas and the catalyst. Hopcalite catalysts with a Cu:Mn molar ratio of 1:1–1:2 can maintain CO conversion above 90% at a space velocity of 30,000 h⁻¹. Higher space velocities shorten the contact time and reduce conversion efficiency, so optimisation is required during engineering design.

SO₂ is another important factor. Over a CeO₂ catalyst, introducing 300 ppm SO₂ decreases CO conversion from 77% to 61%. When SO₂ and moisture are present simultaneously, the risk of catalyst deactivation increases further.

From Laboratory to Stack – Engineering Practice of CO Catalytic Oxidation

Case 1: The sintering flue gas from an iron and steel enterprise, after desulfurisation and denitrification, still contained high CO levels (volume fraction in the range of 0.5%–2%) at a flue gas temperature of 185–195°C. A monolithic catalyst was used for CO catalytic oxidation at a space velocity of 3,000 h⁻¹. After 4 months of continuous operation, the CO conversion efficiency remained stable between 76% and 85%.

Case 2: An industrial boiler equipped with a low‑temperature CO oxidation catalyst achieved a CO removal efficiency of over 60% at a reaction temperature of 180°C.

Case 3: A chemical plant’s tail gas treatment system employed a copper‑manganese Hopcalite catalyst, achieving continuous CO catalytic oxidation at ambient temperature, with the outlet CO concentration consistently below the emission limit. This case demonstrates that for dry, low‑sulfur, ambient‑temperature tail gas, non‑precious metal catalysts can provide a cost‑effective technical solution.

Catalyst Deactivation and Regeneration – Common Issues and Countermeasures in Engineering Operation

Catalyst deactivation during operation is mainly caused by: ① inhibition and competitive adsorption of water vapour on active sites; ② sulfate poisoning due to SO₂; ③ pore blockage by dust; and ④ sintering of active components at high temperatures.

Countermeasures:

1. Flue gas pretreatment: Install upstream units for dedusting, dehumidification, and desulfurisation to create suitable inlet conditions for the catalyst.
2. Catalyst selection: Choose water‑resistant or sulfur‑resistant catalysts according to the actual flue gas conditions.
3. Operation and maintenance: Regularly test catalyst activity and establish a catalyst replacement/regeneration schedule.
4. Temperature management: Ensure that the reaction temperature is maintained within the catalyst’s optimal activity window.

author:kaka

date:2026/6/23