Reduction of BF coke cost through effective utilization of Indian LVHR and low rank coking coals in blends-A Petrographic Approach

Abstract

India stands 4th in the world in production of crude steel in 2013-14 and set a target of production 300 million tons by 2025. This will require a annual growth rate of more than 20%. India has large reserves on iron ore but does not have required reserves of coking coals. Indian steel makers are highly dependent on import of coking coals. India’s import of coking coals stood at about 32 million tons in 2013 and likely to grow very fast rate as production of crude steel increases. Moreover, Indian coking coals are of inferior quality and required to be blended with low ash good quality imported coking coals for production coke of required quality. Hard coke is the costliest raw material used in Blast Furnace iron making. To reduce the cost attempts have been made to use inferior coking coals in blends. A joint CMPDI,RDCIS-SAIL CIL R&D funded project has been carried out recently for identification means for using cheaper Low Volatile High Rank (LVHR) and low rank Indian coking coals in blends for BF coke making. This paper describes the properties of the different coals investigated and approaches followed for effective utilization these cheaper coals in the blend. The LVHR coal are of high rank (Rr % > 1.35 %), high inertinite content, low reactive content (30-45%), low volatile matter content (18 to 22 %), high ash content (> 30%) and therefore, have lower coking propensities with difficult cleaning characteristics. However, these coals can be suitably utilized after proper beneficiation for blending and preparation of steel grade coal. These coals generally occur in the lower seams of Jharia Coalfield (combined seam V/VI/VII/VIII and even lower seams (II, III & IV) and Karo group of seams (VI to XI) in the eastern part of East Bokaro Coalfield. It has also been observed that poor coking coals having Mean Maximum Reflectance of about 0.70% and CSN value 1 to 4, having abnormally high liptinite content can be used in coke making. This so called low rank coal or poor coking coals are abundantly available in India. Recently CMPDI and RDCIS, SAIL identified low rank high liptinite coals and evaluated their effect in the blend comprising other indigenous prime (PCC), medium (MCC) and imported coking coals through pilot oven carbonization tests under both top charging and stamp charging conditions. Carbonization tests were carried out for use of LVHR and low rank coals both separately and in combination. The washability, characterization and carbonization study was carried out jointly by CMPDI and RDCIS, SAIL from various sources of the country and study was carried out on clean coals at 17%, 15% and 10% ash level. However two coals one LVHR coal from Muraidih, Jharia Coalfield and one low rank coal from Argada seam of North Urimari, South Karanpura coalfield have been selected for pilot plant washability and pilot oven carbonization study with clean coals at 10%, 15% and 17% ash levels has shown their usability in coke making. High volatile low rank North Urimari coal at ash content of about 10% can be used in BF coke making replacing existing medium coking coal up to 20% with improvement in M10 values. If ash content of clean is increased to 15%, coke quality becomes similar to that of base blend and further increase produces deterioration in M10 values. Stamp charging produced improvement by about 4 to 6 units in M10. Stamp charging could reduce deterioration in M10 values due to increasing ash content of North Urimari coal from 10% ash to 17% ash level.

Use of low ash Muraidih coal in blend produced very good quality coke. More improvement in M10 values achieved when North Urimari and Muraidih coals used in combination. Lower ash concentrates produced better results. Moreover, under such circumstances, use of higher ash concentrates (up to 17%) did not produce deterioration in M10 values. Thus, use of both high volatile up to 10% and low volatile up to 10% can be used even up to 17% ash level in blends without any deterioration in Micum Indices. Use of low-ash Muraidih coal provided scope for higher use of imported soft coking coal in the blend without deterioration in Micum Indices. Clean coals of Gidi-A, Jhanjra and Sonepur Bazari do not have coking properties and may be suitable for Coal Dust Injection (CDI) application. The washing of the LVHR coal was economically not viable earlier as the price of imported coking coal was much lower. But in the present scenario of costly imported coking coal, the washing of these coals would be economical if the second product (middlings) is used as power - grade coal. To reduce import of coking coals, work is also in progress to use stamp charging method for inferior coals and Coal Dust Injection (CDI) technology for non coking indigenous coals. Utilisation of such coals in India after suitable beneficiation can reduce the import of coking coals to the tune of around 6-8 million tones per year thereby, saving foreign exchange. Rejects produced from these washeries are suitable for FBC based power plants which in turn will augment power generation in the country. Washing of LVHR Coal is a need of the day. The decision to install new LVHR Coal washeries of total capacity of 21.1 Mt in various subsidiaries of CIL will definitely generate additional revenue for the Coal Company and save valuable foreign currency. It will catalyze development of steel sector and power sector in the country.