Measures to extend the coking time in coke ovens

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When the coking time of a coke oven needs to be extended, due to natural disasters or other objective reasons that prevent the oven from operating at normal capacity for a certain period, this method is employed to maintain low-load production. I. Extending the coking time: In general, large coke ovens operate at reduced load when the coking time is 22 hours or more; this is referred to as operation under extended coking time conditions. The furnace temperature is maintained at around 1200ºC to ensure that the temperature of the furnace head bricks after coal loading does not drop below the crystal transformation point of silica bricks, thereby preventing damage to those bricks. The main feature of the production process with an extended coking time is that, after the coke is matured, it remains in the carbonization chamber for a while before being removed; the longer the coking time, the longer the period during which the furnace remains at that temperature after maturation. Within a cycle time, the period from 20 to 22 hours is the maturation phase, while the period after 20 to 22 hours is the holding phase. 1. Maximum coking time: The so-called maximum coking time refers to the longest period of coking that can be achieved in the absence of an external heat supply. If an additional gas source is supplied, the extent to which the coking time is extended can be unlimited. With the coking time extended, in order to maintain the minimum temperature threshold of the coke oven itself, this limits the minimum amount of heating gas required; any lower amount would prevent the maintenance of that minimum temperature threshold. At this point, the coking time represents the maximum allowable coking time. Calculations show that when the production capacity of a large coke oven drops to 10% of its designed capacity, the amount of gas generated by the oven is sufficient to meet the heating requirements during periods of minimum temperature. However, since the graphite on the walls of the carbonization chamber has been burned away, the amount of waste gas leaking increases. For safety reasons, it is advisable that large coke ovens operate at no less than 15% of their designed production capacity, medium-sized coke ovens at no less than 20%, and small coke ovens at no less than 25%. Therefore, the maximum coking time for large coke ovens is approximately 100 hours, for medium-sized coke ovens it is about 80 hours, and for small coke ovens it is around 50 hours. In practice in China, individual large-scale coke ovens have reached 15% of their designed capacity when operating at low load; generally, this figure is above 25%, meaning that the coking time is below 70 hours. 2. With poor furnace temperature management, the coking time increases; for every additional hour within the range of 22–25 hours, the standard temperature drops by 10–15ºC. When the coking time exceeds 25 hours, the furnace temperature remains relatively constant, and the standard temperature at this point is maintained around 1200ºC, generally not falling below 1150ºC. When the standard temperature drops, the fluctuation range of the straight-line temperature increases due to reduced heat stored in the silicon bricks in the carbonization chamber, decreased heating intensity, and the effect of holding the furnace at a certain temperature during the later stages of coking. This makes it difficult to control the furnace temperature; it is necessary to analyze the temperature variations throughout the carbonization cycle in order to determine the appropriate temperature differences, rather than adjusting the gas supply arbitrarily. An extended coking time has a significant impact on the temperature distribution across the width. The coking time is between 22 and 24 hours, during which the shape of the temperature curve across the width gradually changes. When the coking time reaches around 30 hours, the temperature in the side channels drops sharply, and the cross-sectional curve takes on a \"steamed bun\" shape. This situation arises due to the following reasons. The amount of heat dissipation from the furnace surface is determined by the average temperature inside the furnace. Since the final maturation temperature of the coke cake is not dependent on the length of the coking time, increasing the coking time results in a slight difference in the average temperature inside the furnace compared to normal coking times, but this change is not proportional. This factor leads to an increase in the heat dissipation ratio of the furnace surface. The heat dissipation from the furnace surface relies primarily on the supply of gas and air to the side flues. Since the amount of gas and air supplied to these side flues (usually 30%–40% more than necessary) is designed based on normal coking times, factors such as cracks in the upper and lower burners as well as heat dissipation in the regenerator area pose challenges to the heating process in these side flues. Therefore, as the coking time increases, the temperature in the side channels continues to drop, thereby disrupting the normal distribution of temperatures across the row; the degree of distortion in these temperatures depends on the extent to which the temperatures in the side channels decrease. At high cross-row temperatures, it is primarily necessary to increase the air supply to the side flues to compensate for the increased heat loss. Under normal circumstances, the temperature of the side flue should be maintained at no less than 1050ºC. Therefore, appropriate measures must be taken to maintain the temperature in the side flues, ensuring that the coke cakes mature evenly. (1(1) Methods for increasing the amount of gas and air in the side flue. When heated with coke oven gas, the coking time for bottom-fed coke ovens is less than 24 hours. The gas supply can be increased by enlarging the diameter of the nozzles in the side flues; however, further extension of the coking time does not occur as a result of this. To increase the gas supply to the side flues, it is necessary to reduce the diameter of the nozzles in the middle section. When the coking time varies frequently but can return to normal levels quite quickly, it is common to use iron wires in the middle burner nozzles to increase the temperature of the side burners.