【Q&A on the Production Operations of Haichuan Coke Ovens】10. Analysis of Causes and Countermeasures for Coke Collapse on the Coke Side

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1 Cause Analysis 1.1 Insufficient experience of operators in compacting coal cakes. The new coke ovens in use employ a coal charging process that differs from the top-charging method; these new ovens are 5.5-meter compacting coke ovens, and the coal is charged from the side rather than from the top. Specifically, 21-hammer micro-movement compacting hammers are used to compress the coal into cakes, which are then pushed into the carbonization chamber for coke production. The operators are not proficient in ramming techniques and cannot grasp the key points of this process in a timely manner. As a result, when ramming the coal cakes, the coal material located at the front part of the coal chamber is rammed fewer times during the process due to the movement of the ramming hammer, compared to the coal cakes in the middle and rear sections. This leads to lower density in the coal cakes at the front, resulting in relatively lower expansion pressure; as a consequence, the coal particles cannot be pressed closer together, and the adhesion between them is weak. The interactions between these coal particles result in weak interfacial bonds, which causes the coal cakes on the side of the coke to collapse when subjected to vibrations during removal. 1.2 There is a significant difference in elevation between the bottom of the carbonization chamber in the coke oven and the coal-supporting bottom plate of the ramming coal feeder. At the beginning of operation of the coke oven, due to failures to follow the specified requirements regarding the cutting, baking, and installation of the iron components of the oven, the elevation of the bottom of each carbonization chamber did not fall within the permitted error range relative to the elevation of the corresponding coal-supporting bottom plate of the ramming coal feeder. Under normal circumstances, as the carbonization chambers expand under external influences, this expansion does not occur simultaneously; in some chambers, the rate of expansion is lower than the standard value. As a result, when coal is loaded into these chambers using the ramming coal feeder, the distance between the coal-supporting bottom plate and the bottom of the carbonization chamber increases. When the coal pile has advanced 2–3 meters inside the chamber, the pressure exerted by the coal pile causes the front part of the coal-supporting bottom plate to sink, resulting in cracks at that point. The width of these cracks varies, with some reaching 70–80 mm. Once the coal pile matures, the coke located near the furnace door at the front end of these cracks collapses when the door is opened. 1.3 The safety distance between the coal-supporting bottom plate and the furnace door on the coke side is relatively large. New equipment was put into use before it had been fully calibrated, which caused frequent failures in the limit encoders of the coal-supporting bottom plate used by the coal-feeding vehicles. As a result, the bottom plate moved beyond its designated range and damaged the furnace door on the coke side, leading to significant losses. To reduce the incidence of such accidents, the safety distance between the front end of the coal-supporting bottom plate and the furnace door on the coke side was increased. This meant that during the coking process, the expansion pressure generated acted on the coal cakes on the coke side; since there was no counterforce, the coal cakes within about 1 meter of that area did not bond properly together. The gas produced rose through the loose coal particles, causing the gaps between these coal cakes to increase. After coking, the coke became loose, and it collapsed when the door was removed. 1.4 Lack of attention to the management of coke oven iron components. After installation, following a period of operation, all coke oven iron components must be adjusted. This includes adjusting the furnace doors and knife-edge springs, regulating and replacing the hook screws on the door frames, as well as adjusting the spring tension on the vertical and horizontal tie rods. If adjustments are not made in a timely manner, uneven expansion of the coke oven structure will occur, and some of the iron components used to protect the oven will fail to perform their intended function. If smoke and fire escape from the edges of the furnace door and its frame, and if the pressure in the rising pipe fluctuates and cannot remain within the normal range, this can result in a negative pressure in the carbonization chamber. Air is drawn in through the gaps between the furnace door and its frame, causing the coke on the side facing the furnace to undergo degradation; this also increases the risk of coke collapse even after the door is removed. 1.5 Control of furnace temperature: When a coke oven is first put into operation, it is often affected by various factors, resulting in uneven temperatures across different sections of the oven; such unevenness does not meet the requirements of the manufacturing process. Especially when the standard temperatures on the machine side and the coke side are the same, the temperature at the machine side’s furnace end drops more rapidly due to the longer time required to load coal with the oven doors open, leading to lower temperatures there. To achieve uniformity in temperature, it is necessary to increase the amount of gas used in order to raise the temperature of the burners. However, this simple method of raising temperature causes the overall temperature to increase, and when the temperatures in the burners on the coke side rise, it can lead to collapse of the coke in certain sections of those burners. 1.6 Impact of blended coal: Changes in the moisture and fineness content of blended coal can affect the compacted coal briquettes, reducing their stability and thereby affecting the stability of the coke briquettes as well. If not controlled properly, this can lead to the collapse of the coke. 2. In light of the reasons analyzed above, targeted measures are proposed: (1) Due to the lack of proficiency among personnel in operating new equipment and new processes, it is necessary to adopt a strategy of sending staff out for training and bringing experts in, so as to master the ramming techniques. The ramming methods should be adjusted according to changes in the moisture and fineness of the coal, in order to improve the stability of the coal briquettes. (2) Record the furnace numbers where cracks in the coal briquettes are large during coal loading, perform elevation measurements for each furnace, and compare these values with the elevation on the track of the coal loading machine; identify the differences between them and make adjustments to ensure that the elevation difference remains within the standard range. (3) Improve the safety protection system of the equipment, add mandatory limit-stop devices, reduce the safe distance between the coal cake and the side furnace door, and at the same time enhance the operational skills of the coal charging vehicle drivers and furnace door operators to ensure safety and reliability during coal charging. (4) Improve the technical skills of the workers who work with furnace iron components, increase the staffing level, enhance technical proficiency, and adjust these components in accordance with technical standards to ensure their effectiveness as well as that of the furnace doors. (5) Improve the uniformity of the temperature across the width of the coke oven, especially the temperature in the fire channels on the coke side; the temperatures from the first to the fourth fire channel on the coke side should be kept 60–100°C below the standard temperature, while the temperature in the fire channels at the oven head should remain above 950°C. (6) The moisture content of the blended coal should generally be controlled between 8-11%. Low moisture makes it difficult to compact the coal cake, while too high a moisture level reduces the strength of the coal cake. The fineness of the blended coal should be maintained at 90-93%; low fineness requires more compaction attempts, whereas high fineness lowers the bulk density and compressive strength of the coal cake, thereby affecting its stability. 3 Conclusions In summary, the phenomenon of collapse on the side of the coke occurs in most coking plants that use ramming methods for coke production. However, there are various reasons behind this phenomenon, and it cannot be generalized; a multi-faceted approach is required. Each coking plant should carefully identify the causes based on its own production characteristics and address them one by one. Only in this way can the rate of collapse on the side of the coke be reduced, the workload on workers decreased, and work efficiency improved.