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Application of inferior grade of coal for metallurgical as well as for power generation an different industries- A review

Dr. Ranjit Chowdhury

Due to deterioration of coal quality and shortage of coking coal in the country, inferior grade of coking coal of high ash percentage and non-coking coal having plenty of deposit can be used in steel industry, thermal power generation and other industries by pretreatment process through coal preparation and beneficiation.

INTRODUCTION

From an early age coal is used as an indispensible part in the society. Coal is used as an energy fuel as well as number of valuable products. Coal gas, ammonia, coal tar and coke are producers from carbonisation of coal. Water gas, producer gas used methanol and motor fuel are used by gasification ofcoal. Besides this, graphites artificial abrasives, liquid fuels dry ice, pigments for inks and surface coatings, electrodes, activated carbon, on exchange water softner, resins are very commonly used. Although there is a huge deposit of coal through out the world, the good coking coal in different seams have been consumed in India for steel plant usage. By good coking coal, it is meant that by heating at certain temperature it forms plastic layer and at 600° C this liquid layer forms semicoke and ultimately at 900° C the whole mass resolidify to form coke. This is the main principle of carbonisation though the process itself is very complex. So for production of good quality of iron one has to depend on imported coal with good coking propensity and less percentage of ash from abroad, which is not economical. So we have to adopt different technologies for processing coal of inferior quality without deteriorating coke strength for making metallurgical iron. Non-coking coal is used for this purpose which is scattered in Ranignj, Jaria in West Bengal and Bihar. These types of coal are distributed in Assam. Bargili colliery, in Orissa, Kaonjhar and Talchar, in Ramgarh, Lapanga and Singrially, in Meghalaya, pradesh, Nagpur and so on. The responsibilities have been taken up by the public sectors like coal India, Geological Survey of India, CCL, CMPDIL etc. to locate and estimate collection of non-coking coal for future pretreatment and usage of non-oking coal for- Metallurgical purpose; Different alternative roots for iron and steel making; and For thermal power generation in various industries.

METALLURGICAL PURPOSE

The shortage of coking coal is a worldwide phenomenon. In India, the reserves of prime coking coal are very limited. The ash content of coal in mine seams is going up with continuous consumption of prime coking coal of low ash contents. Here different methods of treatment of non-oking coal as well as coking coal of high ash percentage for metallurgical coke have been tried. The method of improving and maintaining coke strength up to the desired level is blending of coking coal with non-oking coal at certain percentage depending on ash per cent and quality of coal. For this purpose, the following technologies can be tried out.

PREHEATING

By preheating process, the inferior non-coking coal before charging to the oven is heated at 150-250° C where by bulk density as well as heat conduct of the coal in the oven is increased improving the quality of coke after carbonisation. Here -20 mm size coal is fed to the fresh drying section of a preheater by a screw feeder, where coal comes in contact with hot gases into the dispersing section of preheater. Crushed coal is transported along with the hot gases into the cyclone, where the coal is recovered. The hot coal is taken to charge bins for distribution to ovens through pipelines. Super heated steam is introduced into the pipelines through a series of nozzles. With the help of diverting valve the coal is directed o required coke oven for charging.

BRIQUETT BLENDING

This process for improving coke quality is widely used in Japan. It has been found that coke strength and reactivity by this new process has similar properties to conventional coke in spite of blending of non?coking coal. If the non?coking coal is blended simply with the conventional charge coal, the strength of coke decreases sharply with an increase in the blending ratio of non-coking coal. On the other hand if the coal is briquetted with the help of binder pitch, the strength of coke obtained from the coal blend is increased by 1.5 per cent. This is because when the coal including the binder is pressed with a roll press, the space between coal particles are smaller, therefore, the reaction between the caking components and inert materials in the coal is worked effectively in the plastic state during coarbonisation. Here coal is pressurised up to a desired level and feed into the mixer and a definite amount of binder is added. The well-mixed coal and binder are fed to the kneading machine, where they are heated to a suitable temperature for brequetting. Then they are fed into the double role press type briquette machine. A 30 per cent briquette and 70 per cent normal coking blend mixture, after being mixed and passed through a coal tower and a charging car are charging into the coke oven.

FORMED COKE MAKING

It is another process for improving coke quality by using non-coking coal and has been tried by a number of countries like U.S.A., Poland, U.K. and India. The coal is carbonized and the char produced is briquetted at 1-2 tonnes/square inch pressure with tar formed at the same time and the briquette devolatilised. This process produced coke suitable for foundries. At CFRI, formed coke has been prepared in a 1-tonne/hr pilot-plant suitable for foundries. This formed coke can also be used in alloy steel when coal originally used is low in phosphorous. The possibility of using formed coke has also caught attention of research organization. Formed coke is a carbonized coal produce manufactured to a required shape. The major requirement of formed coke is in the metallurgical industries although there are domestic uses. Formed coke as shaped fuel is produced mainly from non-coking by carbonisation, devolatilisation in stages at different steps of briquetting, curing and final carbonisation of coal. Atmosphere pollution is virtually eliminated in form coke process.

SELECTIVE CRUSHING

To obtain a satisfactory coke of uniform cellular structure, it is estimated that the crushing of coal to be done in such a way as to ensure that the distribution of particle size from smallest to largest and the petrographic constituents are spread evenly when a mixture of different coal is used for coking, the lack of homogenity by crushing coal becomes evident due to different heardness of the constituents of coal. By selective crushing, the harder constituents are subjected to more intense grinding to reduce over size. Some investigations on the selective preparation of Indian coal of varying coking characteristics are being done at CFRI. The advantage of selective cursing is that improved quality of coke produced method from coal, which reduces the coke rate by 2.4% and increase blast furnace out put by 2.5%. The techniques helps in utilization of higher percentage of inferior or poor coking coal in coal blend.

TECHNOLOGY OF GROUP WISE CRUSHING OF COALS

In order to improve the quality of coal blend and coke, R& D center and BSL have developed the technology of group-wise crushing of coals which aims at decreasing the hetrogenity in the properties of different size fractions of coal blend and improving the properties of coarser size fractions by crushing them the groups to the required crushing levels as dictated by the technological requirement. The existing coal preparation plant at Bokaro has the necessary facilities to introduce this technology without major modifications. It is possible to separately crush the coals in two streams and then mix them together to get the required coal blend for carbonisation. Detailed laboratory analysis were carried out for grouping the incoming coals into following two groups based on their grindability indices, caking/coking property and dilatation property of individual size fraction.

STAMP CHARGING

The coal is crushed 90& -3mm and compressed by stamping machine where by the coke formed is charged to coke oven maintaining certain gap from the wall. The coal bulk density is improved and the carbonized produce have got higher Micum M40 than the original TISCO has got their pilot plant. DSP blend had been tried and good result had been obtained. Up to 25 % noncoking coal is used by this method.

BENEFICIATION OF COAL

Coal of high ash content is harmful for iron production. Ash percentage of different size fractions of coal is lowered in washing using heavy media, jigs, hydrocyclone and froth flotation where coal is washed removing ash and mineral imparities as rejects. The concentrate having good quality of coal is used for blend in coke oven. Middling of higher ash is sent to thermal power station and tailings and rejects are disposed off.

SELECTIVE AGGLOMERATION OF COAL

One of the most useful process developed for reclamation of coking coal portion from high ash coal or their middling or washing slurry is selective agglomeration using fuel oil or kerosin. The oil that is left behind in the agglomeration is recovered in the tar where the agglomerates are carbonized and a part of its contribute beneficially to the coking properties of the coal.

ADDITION OF PITCH AND SOLVENT EXTRACT OF COAL

In countries such as Japan, coals devoid of coking property are made suitable for coking by addition of solvent extract of coal using a wthracene oil. The extract either contains the hydro-aromatic portion of coal or the hydro-aromacity is produced latter during solvent extraction of coal. Hence on addition of non-coking or poorly coking coal it imparts the blending properly. In CFRI, research has shown that it is possible to produce coking coals from non?coking coal by addition of solvent extract of coal obtained using antheracin oil under hydrogen pressure of about 60 kg/cm2 in a batch autoclave. Pitch addition also can be done to the blend for improving coking property.

COAL DUST INJECTION

This is being adopted in B.S.P. as a method of reducing coke rate in blast furnace. Fuel oil and natural gas are the most widely used injectants. India unfortunately is not enriched with these two fuels. But we have large resources of non-coking coal. Besides these techniques described above, thermal shock, with vibrating bed etc. improve quality. Those techniques aim at optimum usage of poor coking coals in the coal blend in coke oven, improvement of coke quality and increase in productivity by adopting new techniques of coal preparation and carbonisation. Using all these above-mentioned techniques, the coke quality is improved and higher percentage of inferior coking coal can be used in coal blend also resulting reduction of coke rate.

DIFFERENT ALTERNATIVE ROUTES IN STEEL INDUSTRY

Direct reduction (DR) for sponge iron production is tried in our country where inferior non?coking coal, limestone, dolomite and iron oxide used in a rotary kiln giving oxidation-reduction reaction where 90 per cent metallisation of sponge iron is produced and is fed in electric arc furnace for special steel. To meet the requirement and demand of market different alternative roots for iron making has been pointed out. Use of coal agglomerates is a recent development as a charge for blast furnace and direction reduction. After the second gas/oil price escalation, much effort was invested into the development of new coal fired process for the production of DRI and /or hot metal. The intension was to utilise low cost non-coking coal as today's cheapest and most widespread available energy source for metallurgical purpose. The advantage in the modern process would incorporate utilisation of 100 per cent non-coking coal as energy source from the ore to liquid metal using high flexibility of various coal types with regard to ash, moisture, volatile matter, sulphur content and granulometry. The general requirements for good quality agglomerates include sufficient strength for handling, transportation and outside storage and complete reducibility in iron and steel furnace without degradation or excessive swelling. A process used to prepare agglomerates from steel plant fines for recycling must tolerate a variety of materials in various proportions. These must include blast furnace dusts, as well as ore and coke fines and also oil in the mill scale. Such a process should be applicable to the removal of zinc and lead from basic oxygen and open-hearth furnace dust. At present, the quality of coking coal is so low that it is not possible to run blast furnaces with reasonably good productivity. Faced with severe constraints posed by the blast furnace iron making, a number of processes are being developed based on the use of non-coking coal to produce liquid iron. Among these processes, Inred and KR have been established on pilot/demonstration scale whereas the others such as Plasma smelt, Sumitomo, Inmetco, Combismelt, Kawasaka, etc. had been successfully tried out in bench/pilot plant scale.

FOR THERMAL POWER GENERATION AND IN VARIOUS INDUSTRIES

Our country is self sufficient in regard to technology required for upgradation of coal. The suggested standard flow sheet consisted of H.M. bath that will give the best results with minimum capital as well as operating cost. The modern concept of designing the washery based on standard flow sheet minimise the construction cost. All studies on economics of preparing non?coking coals conduced so far seem to have suffered from the defect that they were depending on conventional washing processes that made it needlessly expensive and too elaborate a purpose. We are now told of quite a simple device, which does jigging in water instead of air. The projected cost of this equipment is Rs.5 crore for a 3.3-million tonne/year plant (10,000 tons/day). The operating cost is claimed to be Rs.3.21 per tonne of raw coal feed. This is what one may say is a deshaler, forming a first part of a regular washery and can even be sued for getting red to obvious stones and shales at the preliminary stage itself to reduce the load on the main washery and there-by improve its performance.

DESHALLING OF R.O.M. COAL BY HEAVY MEDIUM DREWBOUY VESSEL

The Drewbouy vessel was originally specifically designed for deshaling ROM coal in H.M.S. It is the only available equipment in the world, which can handle lump of 500 x 500 x 1200 mm size, and subsequently it is the only option to treat true ROM coal. The capacity of single line unit can reach 1000 tph, which is corresponding to a 3000 tph mining produced.

BY JIG

Babcock plant is a good illustration of the possibility of utilisation of jigs in preparation plants dedicated to deshaling operation in front of power plant. This has been demonstrated in difficult condition for coarse coal Jig Washing. Raw coal of size ranging from 150 mm to 100 mm can be treated in this process. The successful first employment of a ROM Jig for large refuse separation of run off mine coal opens on wide field of future applications. In view of the favourable secondary conditions such as low water and power consumption, low expenditure for auxiliary equipment this process stands any economical comparison with other possibilities of large refuse separation.

COAL AND INDUSTRIAL FURNACE

Indian industries use six million tones of coal in furnace alone to produce process heat. It has been found that the scope for saving on coal consumption in furnace is as high as 25 per cent.

COAL AND CEMENT INDUSTRY

In cement kiln, a short flame is required and the burner is designed for turbulent flow. The primary air transports the pulverized coal to the nozzle and a tapered section accelerates the coal-air mixture to the required nozzle velocity. The design of pulversied coal burner is dictated by a wide variety of conditions including kiln type and mode of operation, the coal; characteristic and the firing system employed.

COAL AND TEXTILE INDUSTRY

Coal and furnace oil meet 98 per cent of the heat energy requirements in textile mills. Majority of the textile mills in India use coal as the fuel exception for ones in the Mumbai region, which uses oil.

BENEFICATION OF THERMAL POWER GENERATION

Already there is an industry in the coal producing sectors that in the open cast mines, deshaling/destining should be introducing so as to assure consumers satisfaction in respect of size, quality and consistency of the coal supply. In this connection it is to be that the remembered introduction of beneficiation will not only help the consumers, but it will also go a long way in realisation of higher production and productivity. For highly mechanised mines, which have been developed and which are going to be developed, it is not possible to eliminate the extraneous matter like shale and stone coming from roof and floor of the seams within coalmines. Countries where high degree of mechanization has been introduced and consistency and quality of product have to be guaranteed, beneficiation and deshaling is a must.

CLEANING STEAM COAL

In recent years Australia's steam coal industry has undergone a rapid expansion. This expansion has lead to the application of new equipment and revised ideas on coal treatment. New equipments like pulsated jigs, spirals and high frequency screens have lead to the improvements in the efficiency. The availability of economic ash monitors and the acceptance of modular design method has seen the construction of high throughput plants with automatic and some automatic control system. There are indications that India is now poised to undergo a rapid expansion in non-coking coal industry.

CONCLUSION

In view of the detoriation of coal quality and preservation of coking coal for metallurgical purpose upgradation of coal quality by pretreatment mentioned earlier like preheating, partial briquetting stamp charging, beneficiation, imported coal blending is the remedy for future continuation of coking process. For alternative roots of iron making noncoking coal can largely be used by utilising large deposits of non?coking coal in India. The phosphorous content of Indian non?coking coal is quite low. Most of the coals identified for Inred and KR processes have phosphorous below 0.05 per cent. Since there is a great demand to produce quality casting like SG iron, malleable iron etc., Inred and KR hot metals may be a good substitute for the high phosphorous blast furnace pig iron. Both these processes can produce low phosphorous metals through use of noncoking coal. In view of utilisation of non-coking coal for lower generation the price of the coal has almost doubled since 1979 without any improvement in quality and there are more than units of 200 MW in operation today, than eight years ago. In this situation it would be most sensible to utilise every once of combustible matter in coal judicially i.e. washed low ashed coal going to thermal power station and the rejects produced is burnt in fluidised bed boiler for further generation of power station and the rejects produced is burnt in fluidised bed boiler for further generation of power which will meet requirement of industrial and domestic tonnage. It is gratifying to note that coal India has decided to set up a deshaling plant in all open cast mines and two power plants based on washery rejects utilising fluidised bed technology. Furthermore, it can be pointed out that investigation can be directed for removal of high ash stones/carbonaceous matters at a high sp.gr. by this beneficiation process. It is the washability characteristics, which should direct for respective individual non?coking coals as to whether it should be beneficiated or not. With the present technology of pulverized fuel fired boiler, it will not be correct to state that all coals or no coals should be beneficiated for thermal power generation. Each case should be investigated in its own merits and economic are assessed after detailed investigation of washibility charlatanistic. In future, when boiler designs are required for higher capacity thermal power unit, beneficiated coal with limited ash content 25-30 per cent ash can be transported over long distance by railway system. While the higher ash beneficiated middling fraction can be consumed by pit head power plant. Lastly it is very important to point out beneficiation of prime, medium, blendable coal having high percentage ash is needed for charging to coke oven and also benefication of non-coking coal for sponge iron product in. .