The change in calcination zone length directly determines the residence time of material in the high-temperature zone, which in turn profoundly affects the final resistivity of the product. Its typical influence can be summarized in three aspects: 1. Calcination zone too long — resistivity drops further, but at a heavy cost The longer the calcination zone, the longer the material stays in the high-temperature section. Volatile matter is expelled more thoroughly, hexagonal carbon crystals develop more completely, graphite microcrystallite size increases continuously, and delocalized π-electron mobility improves. Experimental data show that in the 1050–1300 °C range, as calcination temperature and time increase, petroleum coke resistivity drops linearly from 1×10⁴ μΩ·m to several hundred μΩ·m, reaching approximately 460–540 μΩ·m after calcination at 1300 °C. Therefore, appropriately extending the calcination zone favors continued resistivity reduction. But excess is counterproductive. An overly long calcination zone causes: the material stays in the high-temperature section too long, increasing carbon burn-off and volume shrinkage (delayed coke and anthracite can shrink by 20–30%), generating more microcracks; volatile matter escapes too violently, increasing the crushing action on the coke; meanwhile the preheating zone is compressed, the material heats up too quickly, and temperature becomes difficult to control. These factors in turn destroy the structural homogeneity of the product, making resistivity distribution uneven. 2. Calcination zone too short — resistivity remains high, with serious volatile residue A too-short calcination zone means insufficient residence time in the high-temperature zone, incomplete pyrolysis and polycondensation reactions, and inadequate volatile removal. In the 500–700 °C range, carbonaceous raw materials have the highest resistivity, approaching an insulator; only when the calcination zone is long enough for the material to fully undergo the violent structural rearrangement stage at 1050–1200 °C does resistivity drop sharply. If the calcination zone is too short, the material does not spend enough time above 1200 °C, so the resistivity-dropping trend flattens or even stalls prematurely, and the final product resistivity stays high. Actual measurements show that at 1240 °C, when the calcination zone shortens and shifts forward, the volatile content of calcined coke rises significantly, and resistivity increases accordingly. 3. Optimal range — synergy of temperature, length, and position For petroleum coke, when the calcination zone temperature is 1250–1260 °C, the longer the zone and the further back its position, the lower the volatile content and resistivity of the calcined coke. However, the starting end of the calcination zone should not pass the mid-support roller and must remain at least 3 m ahead of it; otherwise, kiln vibration intensifies and lining life shortens. One-sentence summary: Calcination zone length and product resistivity are negatively correlated — the longer the zone (within a reasonable range), the cleaner the volatile removal, the higher the degree of graphitization, and the lower the resistivity; a too-short zone leads to insufficient structural rearrangement and persistently high resistivity. But an overly long zone brings side effects such as burn-off, fragmentation, and temperature runaway, so the core principle is not “the longer the better” but controlling the length at the sweet spot where temperature, length, and position of the calcination zone achieve optimal balance.