The maximum thickness of the gelatinous layer formed by bituminous coal after being heated to a certain temperature is the maximum difference between the upper and lower layers of the gelatinous body measured by a probe in the determination of the gelatinous layer index of bituminous coal. It is one of the important criteria for coal classification. The thickness of the thermal coal gel layer is large and prone to coking; Smelting clean coal has clear requirements for the thickness of the gelatinous layer