For high-sulfur petroleum coke, how should the calcination process be specifically adjusted to meet the dual requirements of environmental protection and quality?

For calcination of high-sulfur petroleum coke, to meet both environmental and quality requirements, systematic adjustments must be made from three dimensions: “source sulfur control, process enhancement, and end-of-pipe treatment,” rather than relying on a single measure.

I. Source and Process Control: Enhancing Calcination Desulfurization Efficiency

Conventional calcination temperatures (typically below 1250°C) have limited desulfurization effectiveness for high-sulfur petroleum coke, with desulfurization rates often only in the range of 19%–34%. Therefore, technical measures are needed to significantly improve the desulfurization rate during calcination while ensuring the quality of the calcined coke.

• Increasing calcination temperature and adding additives: Studies show that raising the calcination temperature to around 1300°C and adding 25% sodium carbonate can achieve a desulfurization rate of 69.73%. Another study achieved a desulfurization rate as high as 78.42% at 800°C with an alkali-to-coke ratio of 1.0:0.9. The use of composite desulfurization additives (such as MO1, MC2, MC1, etc.) can achieve desulfurization rates up to 75%. These provide process pathways for reducing sulfur content at the source.
• Adopting staged or ultra-high-temperature calcination processes: For more demanding scenarios, a two-stage high-temperature calcination can be employed. For example, the petroleum coke is first heated in a shaft calciner at 800–900°C to remove most of the volatile matter, and then transferred to a second-stage high-temperature furnace at 1500–1600°C, where sulfur is volatilized in elemental form and recovered, enabling deep desulfurization. If the end product is graphite electrodes, ultra-high-temperature pretreatment above 2000°C can be used, maintained for 20–50 hours, which can reduce the sulfur content to below 0.5% while simultaneously achieving pre-graphitization.
• Optimizing the calcination atmosphere: Introducing a steam-containing or ammonia (NH₃)-containing atmosphere during calcination can help promote sulfur removal and modification of the coke structure.

II. End-of-Pipe Treatment: Ensuring Flue Gas Compliance

Regardless of process enhancements, the flue gas generated from calcination must be efficiently treated to meet increasingly stringent environmental standards.

• Clarifying emission standards and technology selection: According to the amendment to the “Emission Standard of Pollutants for Aluminum Industry,” the SO₂ emission concentration in flue gas from calcination in key regions must be controlled below 100 mg/Nm³. In actual engineering practice, design targets are often stricter, requiring the reduction of SO₂ from inlet concentrations of approximately 6000 mg/Nm³ down to 35 mg/Nm³, necessitating a desulfurization efficiency of over 99.5%.
• Adopting mainstream high-efficiency desulfurization technologies: Currently, the industry primarily uses limestone-gypsum wet scrubbing, dual-alkali, and ammonia-based methods. Among these, ammonia-based desulfurization not only achieves high desulfurization efficiency (converting SO₂ into ammonium sulfate fertilizer) but also co-treats dust and some organic matter in the flue gas, realizing resource utilization with no wastewater or solid waste discharge, making it a clean process worthy of serious consideration.
• Integrating a flue gas integrated treatment system: It is recommended to adopt an integrated process train consisting of “waste heat boiler + baghouse filter + GGH heat exchanger + desulfurization tower.” In-furnace denitrification via SNCR (Selective Non-Catalytic Reduction), combined with baghouse filtration and wet desulfurization, enables synergistic control of particulate matter, SO₂, and NOx.

III. Control of Key Quality Indicators

The ultimate goal of process adjustments is product quality. While enhancing desulfurization, attention must be paid to the quality of the calcined coke.

• Volatile matter control: If the calcination temperature is in the 600–700°C range, the volatile matter content of the treated material should be controlled at around 6%–8% to ensure the performance of subsequent products.
• Microstructure: After high-temperature calcination desulfurization, the surface of petroleum coke may become porous and show increased fracture structures. However, its crystallinity (characterized by the sharpness of the (002) diffraction peak in XRD patterns) typically improves, which is beneficial for subsequent applications. It should be noted that added desulfurization aids (such as sodium carbonate) may leave sodium residues in the coke, which needs to be evaluated and controlled during the process.

Summary: For calcination of high-sulfur petroleum coke, priority should be given to the “high temperature + additives” process route, achieving efficient desulfurization at around 1300°C with alkaline additives. If extremely low sulfur content is required, two-stage calcination above 1500°C or ultra-high-temperature pretreatment above 2000°C should be adopted. Concurrently, end-of-pipe systems must be equipped with ammonia-based or limestone-gypsum wet desulfurization as the core technology to meet ultra-low emission standards. The entire adjustment process must find the optimal balance between desulfurization efficiency and calcined coke quality.