Co-presenter: Prakhar Mishra, Chief (Coke Plants), Tata Steel
Any quantum leap in blast furnace productivity calls for substantial up grading of coke properties. Quality of coke essentially depends on the coal blend and the practice of carbonization. Although India has large resources of coal, the availability and quality of coal for coke making falls short of the requirements. Indian coals are not amenable to the production of good quality blast furnace coke by just optimizing the blend. The two avenues open for making better coke under these circumstances are:
- Incorporation of good prime coking coals into the blend through imports
- Adoption of a suitable pre-carbonization technology
Stamp charging has been established as a versatile technology which not only improves the coke properties that can be obtained from a given coal blend, but also broadens the coal base for coke making, permitting the use of inferior coals without impairing the coke quality. The first stamp charge battery at Tata Steel has achieved operating life of nearly 30 years and still having some residual life. This has been achieved through the guiding philosophy of deploying agile operational practices to produce desired quality and quantity of coke with minimum stack emission and maximizing asset life. Continuous improvement in standard operating practices, optimization of coal blend composition, well designed oven health monitoring and repair practices are the backbone of this achievements. The typical coal blend specifications are as follows: CSN: 5.5 – 6.5, max fluidity: 200 – 800 ddpm, maximum expansion: 50 %, minimum contraction: 20 % and rank of coal (RO): 1.0 – 1.10.
The preferred mode of battery heating is through lean/ mix gas. Proper charging practice is always ensured which impacts quality and asset life by affecting battery temperature profiles. Continuous focus on eliminating cake breakage is ensured through maintaining coal moisture (10-11%), crushing fineness and stamping time to ensure bulk density (between 1.0 – 1.05). The stamp charge coke oven batteries are subjected to take more load on the oven wall due to higher bulk density of the charge and hence it generates higher swelling pressure and higher thermal load. Therefore, stamp batteries are designed with high wall stability. However, for avoiding excessive wall pressure and damage to the oven wall, close monitoring of the coal blend gas pressure, moisture, crushing fineness and bulk density are extremely essential. This limits the usage of prime hard coking coal generating medium gas pressure in the blend to maximum 15% and usage of low VM high gas pressure generating prime hard coking coal to almost nil.
Tata Steel’s vision of adopting stamp charge coke making technology in the early 80’s has proved to be in perfect alignment in today’s highly globalized and competitive coke market. After passing through various phases of learning, today the level of understanding on various aspect of the process and battery design is in high order. Looking into the global reserve of prime coking coals, pre-carbonisation coke making technology like stamp charging is likely to gain popularity.