1.What are the pollutants produced by fuel combustion?
Annealing furnaces (especially continuous ones) typically use natural gas, coal gas (such as coke oven gas), or heavy oil as fuel (some smaller units still use coal). The combustion process produces the following pollutants:
Particulate matter (PM₂.₅, PM₁₀): Incomplete combustion or fuel containing impurities (such as coal and heavy oil) will emit dust, carbon black, and other pollutants. Long-term emissions may lead to elevated local atmospheric particulate matter concentrations.
Sulfur oxides (SO₂): Fuels containing sulfur (such as heavy oil and high-sulfur coal) produce SO₂ upon combustion. SO₂ is a major cause of acid rain and corrosive to equipment and surrounding vegetation.
Nitrogen oxides (NOₓ): During high-temperature combustion (especially natural gas, where flame temperatures can reach over 1500°C), nitrogen oxides (N₂) in the air react with oxygen oxides (O₂) to produce nitrogen oxides (NO₂ and NO₂), which are highly irritating gases and contribute to photochemical smog. Greenhouse gases (CO₂): The main product of fossil fuel combustion, directly exacerbating global warming, are an emission source that needs to be focused on under the "dual carbon" goal.
2.What are the pollutants in process exhaust gas?
To prevent oxidation of the steel strip, a nitrogen-hydrogen mixed shielding gas is introduced into the annealing furnace. Furthermore, residual organic matter on the strip surface, such as rolling oil and degreasing agents, will volatilize or decompose at high temperatures, producing:
Volatile organic compounds (VOCs): Mineral oils and fatty acid esters in rolling oils, for example, volatilize at high temperatures to form non-methane hydrocarbons, some of which participate in photochemical reactions and contribute to ozone pollution.
Minor amounts of harmful gases: Improper furnace atmosphere control (e.g., excessively high dew point) may produce trace amounts of H₂S (if the fuel contains sulfur and the reduction reaction is incomplete) or HCl (if chloride ions remain on the strip surface after pickling), potentially corrosive to the atmosphere and equipment.
3.What impact does the cooling process in the annealing stage have on water resources?
Cooling Water Consumption
Rapid cooling (for example, in automotive sheet production, where steel strip is cooled from 800°C to below 300°C via roller cooling) requires large amounts of circulating cooling water. Using an open cooling system (such as a cooling tower) consumes fresh water due to evaporation losses, which can exacerbate water shortages, particularly in water-scarce regions.
Wastewater Discharge Risks
If circulating cooling water is not properly treated, it can be contaminated for the following reasons:
Leakage of corrosion inhibitors (such as chromates) and scale inhibitors (such as phosphates) added to the cooling water can lead to heavy metal or eutrophic contamination of the water body;
Small amounts of pickling wastewater (such as hydrochloric acid) remaining on the surface of the steel strip can enter the cooling water along with the steam, potentially lowering the pH value and forming acidic wastewater.
4.What are the solid wastes in the annealing process?
Waste Refractory Materials
Annealing furnace linings (such as the heating and soaking sections of continuous annealing furnaces) are made of materials such as refractory bricks and refractory fibers. These materials age and flake under prolonged high temperatures, generating waste refractory materials during replacement. If these materials contain asbestos (as seen in some older equipment), they become hazardous waste and require special handling to prevent asbestos fibers from contaminating the air.
Waste Catalysts and Adsorbents
In some annealing processes, the shielding gas (nitrogen-hydrogen mixture) requires purification (e.g., to remove moisture and CO₂). Adsorbents such as molecular sieves and activated carbon may be used, which become solid waste upon decomposition. If hydrogen is produced through ammonia decomposition (to provide an H₂ source), the catalysts used in the decomposition process (e.g., nickel-based catalysts) must also be treated as hazardous waste upon decomposition.
Other Waste Slags
Slags from fuel combustion (e.g., ash from coal and heavy oil combustion) containing heavy metals (e.g., vanadium and nickel from heavy oil) can contaminate soil and groundwater if left unused.
5.What impact does the annealing process have on energy consumption and carbon emissions?
Taking a continuous annealing furnace as an example, producing one ton of galvanized coil consumes approximately 50-80 cubic meters of natural gas (or equivalent other energy sources), resulting in approximately 100-160 kg of CO₂ emissions (calculated based on a natural gas carbon emission factor of 2.0 kgCO₂/m³).
If the annealing furnace relies on thermal power (such as an electrically heated annealing furnace), and the electricity comes from coal-fired units, carbon emissions will increase further (the carbon emission factor for coal-fired power generation is approximately 800 gCO₂/kWh).