A NEW COAL PREPARATION SCHEME TO IMPROVE COKE QUALITY

Transcription

1 A NEW COAL PREPARATION SCHEME TO IMPROVE COKE QUALITY B. K. Sahoo*, S. K. Das*, Ashutosh Gupta*, P. K. Pankaj* & M. P. Reddy** *RDCIS, SAIL, Ranchi ** Bokaro Steel Limited, SAIL, Bokaro Abstract At BSL, Bokaro, crushing index (% of -3.2 fraction) of coal blend was with a high micro-fine content (-0.5) of 47%. Coarser size fraction (+6) was also high (8-9%). Higher content of +6 fraction in coal charge affects coke quality as this fraction is comparatively rich in total inert content and lower in rank. This fraction generates weak spots in coke matrix and is one of the primary reasons for inferior coke quality. Higher content of micro-fine (>40%) affects bulk density of coal charge and it leads to stand pipe (valve box) jaing and is one of the primary reasons for deterioration in coke oven battery. Differential crushing scheme has been developed and introduced by RDCIS to reduce micro-fines as well as coarser size fraction of coal blend and it has improved M10 index of BF coke by 0.3 point. The paper discusses the development and benefits of differential crushing. Introduction Coke quality is most important parameter for smooth Blast Furnace operation. One point change in M10 index affects BF productivity by 2.5% and coke rate by 2.0% [1]. The bulk density of crushed coal charge is important for BF coke quality and Coke Oven productivity. It mainly depends on percentage of ash, moisture and granulometry. 1% reduction in +6 fraction of coal blend reduced BD by 1.8% [2]. It has been reported that charge preparation by selective crushing with pneumatic separation gives a more favorable distribution of coal substance between the size fractions at an overall degree of crushing for the prepared charge of 75-85% <3. Moreover, the fines content (<0.5) of prepared charge is 3-3.5% lower compared to coal charge prepared by using ordinary blend/differential crushing technique [3]. Higher fines content in coal blend has many adverse effects. It increases dust pollution, reduces bulk density, gets carried over with gas during on main charging leading to tar quality deterioration. Therefore, use of selective crushing makes the 1

2 conditions favorable for reducing the overall degree of crushing of the charge as a means of increasing the coke oven output without losing the strength of coke [4]. At Bokaro Steel Plant, Bokaro (BSL), there are 81 silos for storing incoming coals and all of them are provided with weigh feeders for accurate discharge of coal sources. Crushing index (-3.2) of coal charge was maintained about 82-83%, the micro fines (-0.5) content is about 45%, which is quite high. It leads to stand pipe (valve box) jaing and is one of the primary reasons for deterioration in the life of coke oven battery. Coarser size fraction (+6) in coal charge was also as high as 8-9%, one of the primary reasons for inferior M10 index. In order to improve coke quality and reduce operational problems at BSL, RDCIS developed differential crushing of coals for rational coal preparation. This paper discusses development and benefits of rationalization of coal preparation scheme using differential crushing of coals. Importance of coal crushing Crushing of coal is very important to produce good quality metallurgical coke. The purpose of coal crushing is to obtain a homogenous blend from the different coal components by optimizing the crushed coal size and size distribution. In fact, crushing of coal proves to be a very costly operation. The power consumption per ton of coal blend crushed varied in the range of kw/hr depending on the properties of coal blend. This works out to almost 70% of the cost incurred for the preparation of the coal blend. The crushing of coal is an essential pre-requisite for homogenization of coal charge with respect to reactive and inert content. Its main aim is to ensure minimum difference between different size fractions in terms of ash, caking properties etc. Higher fineness, while improving the reactive particle distribution, reduces bulk density and increases more of micro-fines. These microfines are carried away with the gases and cause problems in gas mains and by - product plants. Higher fineness increases overall surface area and increases demand for more caking component. Higher bulk density of coal charge is desirable to increase throughput of coke oven and to reduce gap between different coal grains in the charge, which helps in better bonding during carbonization and improves coke strength. It has been found that size distribution to achieve highest bulk density of coal charge is: 10-6: 32.4%, 6-3: 14.4%, 3-1: 15%, 1-0.5: 17.6% & - 0.5: 20.6%. However, the optimal size distribution for the best strength properties of coke is: : 16.1%, 2-1: 24.9%, 1-0.5: 24.9% & -0.5: 34.1%, the 2

3 percentage passing through 3 (crushing level) being 100% as against 53.2% for maximum bulk density. The grindability of coals varies considerably with rank and petrographic constituents. On the other hand, the caking properties have been found to vary not only with rank and Petrographic constituents but also with coal particle size, as coal is heterogeneous in nature. Therefore, it is important to have a rational approach for satisfying the above contradictory requirements. When the crushing level of the coal blend is low, the quantity and top size of coarser fractions (6.3) are high. As the coarser size fractions and the rest of the blend are dissimilar in nature, they undergo thermal destruction unevenly, shrinkage behavior is not compatible and the coke quality deteriorates. With the increase in crushing level, excessive generation of the finer size fractions of the coal blend, which predominantly consists of vitrinite grains, takes place. It promotes weathering of coal on prolonged storage. This adversely affects the coke strength. According to Dukanov, the optimal percentage fines (-0.5) in a coal blend should be within a range of 35-40%. The increase of dust content (-0.5) in the coal blend involves accumulation of dust in the gas collector (during on main charging) which is manifested in the increase of ash content of tar, higher amount of sediments and more expenditure for cleaning of tar storage systems etc. According to Stepnov, the increase of dust content in a coal blend from 41.0% to 47.5% increased ash content of tar from 0.04% to 0.05%. In addition, the dust content at the coal handling plant and coke ovens increased by times. As per the studies carried out by Butorin, the decrease of fines by 5% increases the bulk density of coal charge from 751 to 763 Kg/m3. Therefore, it becomes quite clear that a rational approach for preparation of coal charge is necessary to produce a good quality coke. Experimental Findings Conventional Crushing Scheme Imported hard coals are fed from all the three streams, Medium coals are mixed with imported hard coals for crushing. It does not take into consideration different grindability characteristics of coals. Lack of homogeneity in coal blend, Concentration of mineral matter and inert content takes place in coarser size fraction; Percentage of coarser size fraction (+6) is high, resulting in inferior coke quality. 3

4 Rational Coal Preparation using Differential Crushing Scheme Keeping the problems associated with conventional crushing of coals at BSL, RDCIS developed differential crushing of coal. This differential crushing scheme takes into consideration of different grindability characteristics of coals. Coarse crushing of inferior and difficult-to-grind coals leads to accumulation of inert material in the coarser size fraction. These inert materials are infusible and become centers of weakness in the resultant coke matrix. These possess very little shrinkage characteristics. Because of differential shrinkage between the inert material and the remaining coke mass, cracks develop reducing the strength of coke. Hence, from this consideration, inert and ash rich coal particles should be crushed as finely as possible, to disperse the inert material. Under this scheme harder coals (Medium coking coals) which are rich in inert content are to be crushed finely so that coarser fraction (+6) of harder variety of coals in coal blend get reduced and softer imported coal should be crushed mildly so that over all micro-fine content in coal blend may be restricted to 40.0%. As a result there will be improvement in coke quality due to lesser numbers of cracks generated during carbonization and better homogenization in coal blend. Size analysis of all incoming coals and different groups of coals was carried out and has been presented in Table 1 & 2. It can be seen that all the imported coal has already higher -3.2 fraction and on crushing fines content increases sharply, as these coals are softer in nature. At the same time, harder indigenous coals have low -3.2 fraction and on crushing also +6 content is high, as these coals are harder in nature. It confirms the necessity of crushing indigenous coals to higher degree and imported coals to lesser degree. As per logistics, coking properties, fluidity and petrography characterization, incoming coals were divided in the three groups (Table 3). Reallocation of silos was also done to facilitate differential crushing. The harder coals (Group II) were fed from one stream having crushers in excellent condition to ensure crushing of harder variety of coals to 85%. The softer imported hard (Group I) and group III coals were fed through two streams and their crushing was restricted to 80% by crushing in not so good crushers. It is difficult to achieve 85% crushing index for Group II coals and life of haers was found to be low. Hence, frequent changing of haers for 4

5 crushing this Group of coal was necessitated. Considering the distribution of silos, differential crushing scheme was worked out while all the three streams are in operation. Results and discussions Industrial trial was carried out. Based on encouraging results, differential crushing scheme was implemented. Table 4 illustrates the results of industrial scale trials. It can be seen that improvement in M10 was 0.35 points even with reduced usage level of imported coal (75.15% against 77.4%) during trial period. There was improvement in M40 value also where it improved from 76.5 to Even in implementation period, when imported coal usage was further reduced to 72.9%, there was no deterioration in M10 and M40 value was even better (77.6). It can be seen that even when use of imported coal decreased from 77.4% to 72.9 %, M10 index did not deteriorate due to differential crushing. Conclusions Differential crushing of coals, developed by RDCIS, has helped in improving quality of coke produced. The implementation of this crushing scheme has resulted in improvement in coke quality even with less use of imported coking coals. References 1. L. Parthasarathy et. Al; Coking coal evaluation, blending and preparation Seminar on coal for steel, Indian Institute of Coal Management 7-9 Dec 1998 CMPDI, Ranchi. 2. P. Roy Choudhury, P. K. Mitra, R. S. Tiwari and S. N. Nag, Effect of raw materials quality and operating parameters on blast furnace productivity and coke rate Volume 3 No. 1, Feb. 2000, IIM Metal News. 3. Sukorukov et. Al; Coke and Chemistry No. 11, pp. 3-5, 1974 Prospects for introducing selective coal crushing with pneumatic separator. 4. Sukorukov et. Al; Coke and Chemistry No. 4, pp. 8-11,

6 Table 1. Size analysis of coal sources (as received basis) Coal sources Dugda Munidih Jamadoba Nil Kathara Mohuda Rajrappa Nil Aus (H) Nil N Zealand Nil Aus (S) Nil Table 2. Size analysis of different groups of coals after crushing Coal groups MCC MCC+PCC IMP(H)

7 Table 3. Grouping of coals Group Coals HGI Vitrinite content Imported Group I (Australia) USA New Zealand Dugda Bhojudih Group II Kathara Rajrappa Swang Mohuda Group Imp.(S) III Munidih Table 4. Comparison of results of differential crushing and conventional crushing Parameter Base Period Trial period Implementation period I/C in blend, % Blend ash, % Crushing index, % % % Coke ash, % M M