Fundamentals of Design Aspects of Gas-fired Cupola

Transcription

1 Fundamentals of Design Aspects of Gas-fired Cupola Due to environmental regulations and social obligation towards society the conventional coke-fired cupola may be converted into gas-fired cupola. The present paper is devoted to the design aspects of gas-fired cupola. The cupola well depth, tap hole, slag hole, shaft height, ber specification, furnace lining, water cooled grates, refractory balls and their development etc has been discussed. INTRODUCTION the diameter of the lined cupola of any capacity can be easily Around 70% of all cast iron produced throughout the world is calculated as: melted in cupola, in which coke ly serves Metal holding capacity per tapping = (π D2 h/4) x kg simultaneously as source of energy, bed material and a Where, carburizing agent in the melting of the iron charge. The Effective depth of well = h cm economy of the process technology is dependent upon the Lined diameter of cupola = D cm relevant local conditions such as Density of molten pig iron = kg/cm3 Availability of raw materials Metal holding capacity = 2000 Kg/hr,= 500 Kg/ 15 minutes Availability of energy carriers So, (π D2 h/4) x kg = 500 Kg/ 15 minutes Environmental regulations D = cm The combustion of coke in coke-fired cupola generates The diameter of cupola may be taken as 70 cm for 2 ton capacity. harmful oxides of carbon and sulphur. A huge quantity of suspended particulate matter is also ejected in atmosphere by TAP HOLE AND SLAG HOLE coke-fired cupola. These, all together, cause severe pollution Taft [2] has ascribed that a large tap hole is necessary as the problem. Apart from this in Indian foundries the energy ferrostatic pressure will half at the tap hole and smaller tap hole consumption to produce per unit ton of iron is significantly takes much longer time to fill a ladle. It is found that a larger tap high. In such circumstances Indian foundry industries are in hole is not required as the ferrostatic pressure at the tap hole was need of such a melting unit which is not only eco-friendly and more than sufficient and does not take much longer time to fill a energy efficient but also economical to survive in future. ladle. The ferrostatic pressure at the tap hole depends not only DESIGN FUNDAMENTALS OF GAS-FIRED CUPOLA upon the weight of the liquid iron, but it also depends upon the slope of the sand bed towards tap hole and cupola well pressure. A conventional coke-fired cupola can readily be converted into gas-fired or new ones can be designed for specific purpose. The Table 1: Effect of tap hole size on tapping main features of the gas-fired furnace are: shell of the cupola, Sr. Tap Hole Avg. Time Nature of Temperature water-cooled grates, ceramic balls and gas-fired burners. No Size (mm) Taken f of the metal CUPOLA WELL DEPTH AND DIAMETER Like conventional coke-fired cupola, gas-fired cupola is also a vertical shaft-type gas-air blast furnace. In case of coke-less cupola i.e. totally gas-fired cupola, the distance between the sand bed and the er portion of the burner quarrel is considered as Well, which serves to collect the metal and slag melted above the ceramic bed. In the present situation, the (seconds) Jet type High effective depth of the well has been taken into consideration in It also depends upon the critical dimensions and angle of the estimating the holding capacity of the well. The effective depth spout which depends upon the pressures. The time factor for of a well is regarded as the distance between the sand bed and filling up a ladle also depends upon the size of the ladle. In the the slag hole. The slag hole generally located at the rear of the pressure situation, the small ladles of 45kg capacity are being cupola directly opposite to the tap hole. Gorfinkel and used for tapping and direct casting of pipes, break drums, Chernobrovkin [1] have established that temperature loss of valves, wheels, and other small components. Initially, (for each metal is 200 C per meter increase in well depth. The slag hole is tapping of intermittent tapped cupola) a tap hole of 25 mm was found to be appropriate at 20 cm above the sand bed for any found to be optimum for filling up a ladle of 45 kg capacity capacity of cupola. Since, the melting units are designed for an within short period. With this size of the tap hole, nearly 135- intermittent tapping (4 tapping/hr), the metal holding capacity 150 kg/min. liquid metal could tapped out. The tapping rate should be kg/cm-height for a cupola of 2 t/hr capacity. If depends upon the rate of ladle re-circulation and the ladle re- metal holding capacity and the effective well depth are known, circulation depends upon the number of ladles being handled

2 which varies foundry to and the combustion rate of the gaseous fuels. The melting rate of foundry and items to be a cupola of a given diameter operated at its optimum cast. combustion rate will, therefore, depend upon the air: gas ratio Of course, a large slag gives the melting rate at the recommended bing rate of air hole is necessary for a which is a function of the combustion of gaseous fuels. The smooth and faster deand CNG contains about 85% CH4 and 10.1% C2H6. The oxygen slagging because of the corresponding air requirement is given be. density slag as coke CH4 + 2O2 = CO2 + 2H2O ash is not present m3 1.7 m3 Moreover, during de- The corresponding air (with 21% O2) is m3 slagging burners are kept C2H O2 = CO2 + 2H2O on firing to avoid any m3 3.5 x m3 splashing and splitting of the density slag, but The corresponding air (with 21% O2) is m3 on the other hand, the Total air required for 2 ton capacity cupola is m3 to burn burners can not be kept on 1m3 of CNG. To keep the environment reducing air gas ratio is firing for a longer kept 1:9. The air bing rate should not exceed m3 / min, period as it will produce otherwise, metal will get oxidised. Alance for the ser chilling effect to the liquid rate of melting at the beginning of melt, deslagging, off blast metal bath and to the periods and down time has been well taken into account during charge material above the design itself to achieve an average hourly output at the optimum ceramic bed. Therefore, bing rate. based on operational The specific bing rate is based on the consumption of experienced of gas-fired gaseous fuels per hour in actual operation under steady cupola of different conditions. It is mandatory to operate a totally gas-fired cupola capacity, a slag hole 75 slightly under reducing condition to avoid any elemental loss. Figure 1: Schematic diagram mm diameter was found The totally gas-fired cupola can be operated slightly under of gas-fired cupola optimum for a smooth and bing without any serious consequences (in terms of metal faster de-slagging without temperature and pollution). Over bing is not at all losing much heat of the liquid metal and the charge material. appreciated. SHAFT HEIGHT The recommended specification for the ber is in excess of The portion of the furnace from ceramic bed to the charging the optimum. It does not mean it als the cupola to be door is considered as shaft. The main function of the shaft is to operated in excess of the optimum in actual operation. Only accommodate a sufficient volume of charge metal and flux to during preheating i.e. before commencement of the charging, absorb the maximum portion of the heat of the combusted the cupola is aled to be operated in excess of the optimum. ascending gases. The discharge pressure at the outlet of the ber must be A recommended shaft height for a gas-fired cupola is sufficient to deliver the required volume of the air against 2.85 m as shown in Design. While operating 2t/hr gas-fired ceramic balls which is placed in multiple layers above the cupola, with less than 2.85 m shaft height, the gaseous water- cooled grates, stock in the stack and burners. For this temperature at the charging door was measured more than purpose high velocity burners are used which have their own 450 C causing loss of substantial portion of gaseous heat. After increasing the shaft height up to recommended height, the temperature at the charging door dropped down to less than 250 C, presented in Table 2, which gave a worthwhile increase in the thermal efficiency from 53% to 55%. Table 2 : Effect of Shaft height on temperature at Charging Door Sr. No Shaft Height (mm) Gaseous temperature at Charging door (+/- 10 C) BLOWER SPECIFICATION The melting rate of a gas-fired cupola depend on air: gas ratio Figure 2: Back pressure at burners head with lapse of operational time

3 back pressure. On the other hand, the fusion of the ceramic balls in the course of melting reduces the bed permeability which leads to a back pressure at the burners head. Thus, total back pressure at the burner s heads depends upon their size bed permeability, the type of metallic charge, and the diameter of the cupola. A typical trend of the back pressure at the burners head is shown in Figure 2. Therefore, the discharge pressure at the burner head, which depends upon the discharge pressure of the outlet of the ber, must be sufficiently high to encounter the total back pressure at burners head and to deliver the required volume of air for complete combustion of the required gaseous fuels. FURNACE LINING AND LINING MATERIAL Internally, the shaft of a gas-fired cupola, differs in structure from a conventional coke-fired cupola, having a compartment of gas burner combustion and a set of water-cooled grates for supporting the refractory spheres and charged materials. The refractories play very important role and are crucial to the fate of a melting furnace. The quality of refractory, and hence lining, has a significant effect on the extended furnace life. An extended furnace life means that the maintenance cycle is longer and the consumption of the maintenance material and labour cost per ton of liquid metal are reduced. Since the combustion chamber i.e. extended portion of the burner housing is a critical zone and required to correspond to a kind of reverberatory furnace in structure. The burner housing with the castable (90% alumina cement) which has high refractoriness, but every time it get cracked and collapsed after two or three trial runs. The shaped sillimanite bricks are found suitable for lining of the combustion chamber. Above sillimanite lining, a 5cm thick coating of castable is applied. The rest of the structure has much common with a conventional coke-fired cupola and can be divided from top to bottom into a preheat zone, melting zone, grate, whirl zone (the zone in the furnace where the combusted gas fols a circular path due to the burner angle and hence angle of the combustion chamber), and well zone. The parts in the gas-fired cupola corresponding to each of these zones may be described in details as fols: Preheat Zone : In the gas-fired cupola preheated zone is not so critical; therefore, 40-45% alumina bricks are used for lining purpose and to accumulate the heat of the fuel gas ascending from the combustion chamber. Be the charging door, the brick containing 40-45% alumina is capable enough to withstand the material impact and the wear. The cast iron blocks are used in front of the charging door i.e. just opposite to the charging door, where lining is facing direct impact of the charge materials. Melting zone and Whirl zone : The melting zone and the whirl zone are very important and critical because, these zones affect the melting rate and liquid metal temperature and finally the slag chemistry. In the course of development work, three types of bricks namely, carbon blocks, Silicon carbide bricks and High alumina (70% Al2O3) has been tested. Since, the carbon bricks and silicon carbide bricks are good thermal conductive, therefore, water cooling of the shell in this zone were mandatory. Since, the furnace wall is susceptible to cooling, thereby affect the tapping rate and the liquid metal temperature. High alumina bricks were found most suitable lining materials for this zone. The result of more than 50 trials run on gas-fired cupola of 1 ton per hour capacity with high alumina bricks lining in these zone confirmed that the tapping temperature and furnace maintenance cycle compared with carbon and silicon carbide linings were improved despite water cooling. The fuel gas generated by combustion consists almost entirely of CO2, H2O and N2. However, there was erosion of the front portion of the extended burner housing, which was made of castable (90% Al2O3), near the furnace wall. These may be due to either residual oxygen or carbon mono oxide evolved by uncompleted combustion at high temperature and pressure or continuous high pressure impact of combusted gas which is coming out of the burner. This was successfully dealt by applying castable. Well Zone : The well zone i.e. the portion be the burner housing was lined with alumina bricks which is economical and ability to withstand long term use. To lessen the prominence of the erosion effects at the tap hole, high alumina bricks were found most suitable. WATER-COOLED GRATES The shaft of the furnace is partitioned by a set of water-cooled grates which consist of steel tubes coated externally with refractory material for insulation purpose. The water-cooled grates support the ceramic balls which form the melting bed and acts as heat exchanger. The size and number of the grates depend upon the internal diameter of the cupola to be designed. The housing is so designed that can be easily removed as and when required. The rise in outlet water varies between C depending upon the metal temperature and hours of operation. Water cooled grate which is mentioned in the literature [2] for use in gas-fired cupola, consist of a cast steel hol pipe for holding water if desired to keep it cool. The reported design of the water-cooled grates provide for unidirectional f of water. Further, the single wall tubular cast steel grates provided in the furnace have to bear the full impact of the charge material being thrown from the charging door of the furnace which is at a height of 6-8 times the diameter of the cupola from the level where the grates are fixed. Thus the water fing through the inner space of the grates may lead to hazardous explosions in case of the grates give away and the water leaks. The cooling grate is designed using seamless pipes of suitable sizes (depending on the diameter of the shaft of the furnace), in which one seamless pipe of a smaller diameter goes into another similar pipe of bigger diameter. The water is introduced from one end of the bigger diameter pipe and gets out through the smaller diameter pipe. A high refractory material is coated on the outer surface of the outer tube. The material used may preferably have a composition consisting of 90% alumina with graphite and calcium-silicate. The coating may be of a thickness in the range of mm on the outer pipe to safeguard the tubes from dynamic load of the charge pieces as well as from the

4 hot metal. The refractory material may be reinforced with the max L 325 of cm Thus the safety factor is very help of welded stainless anchors to the body on the pipe. The high and the grates will not give away even after doubled the function of refractory material is to protect the pipes from heat load for what it is designed For all practical purposes only and also from mechanical shock of the falling mass to be middle grate which is longest L 70cm and bears maximum melted. load is considered The gap between two grates plays very important role in DEVELOPMENT OF REFRACTORY BALLS maintaining the bed permeability and grate housing As it has already been stated that in gas fired cupola the bed is permeability which provides passage for the combustion prepared by placing a multiple layers of refractory balls above product which heats up the bed material and finally melt the the water cooled grates the refractory balls in the system are metallic charge. Taking into the consideration of the semi permanent and dissolve sly in the course of melting permeability factor, the gap between two grates was fixed up process The refractory balls act as heat exchanger and help in 56mm which provides more than 48% permeability for supporting the material to be melted To lead a more consistent combustion product. A set of grates housing is mounted at a operation the refractory balls are required to have high strength certain height from the burner housing. and refractoriness so as to withstand the mechanical shock due The number of grates depends on the internal diameter of the to the heavy fall of the charge material to be melted as well as cupola and outer diameter of the outer seamless pipe used for thermal shock resulting from the passage of molten metal making the grates. The number of grates for a cupola of through it For the easy escape of the combustion product particular capacity can be calculated using the foling through descending charge the bed should have good relation : permeability which depends upon the size of the balls and D = (n-2)d + (n-1)w overall structure of the bed The size of the balls is varied Where, D = internal diameter of the cupola between 125 to 150mm in diameter n = number of grates As aforementioned the material employed for the refractory d = outer diameter of the outer pipe bed constituents is a most important item of the gas fired melting unit operation and much work has gone into its w = gap between two grates development Hitherto 3 known refractory balls used in such For 2 t cupola, furnaces are mainly made of ceramic and graphite and liquid D = 69 cm, w = 56 mm, d = 65 mm glass has been used as additives In this process mixing of So, n = ( )/141 liquid glass with ceramic and graphite is done in a mixer which = is time consuming and has a distinct and small bench life For a cupola of 2 t/hr capacity, the number of grates will be 7 However the balls so prepared have inferior refractoriness considering 65mm (50NB thick pipe) outer diameter of outer Another known process 4 in which refractory clay ball clay pipe. Out of 7 grates, 2 grates would be inactive. The inactive and calcium aluminate cement has been used for all making grates are fixed up both sides in the cupola wall for improving These materials are blended in such amounts to provide a life of the lining near the combustion zone. refractory or a high temperature cement composition which is The deflection in loaded grates which are clamped at both ends air setting acid resistant and non water soluble The refractory can be calculated as given be : balls made with these ingredients were not suitable for gas Length of longest grate (l) = 70 cm fired cupola because of their er service temperature of 1315 C refractoriness and bulk density etc leading to er Outer diameter of Pipe = 65mm (do) service life Inner diameter of pipe = 50 mm (di) Hitherto 5 known process for the production of refractory Weight of charge material P = 2000 Kg materials such as balls for use in the above said furnace E = 2.1 x 106 Kg/cm2 alumina carbon refractory with a binder having a dense Moment of Inertia = I compact shape has been successfully employed, but these balls I = (p/64)x (do4-di4) are cost prohibitive as the materials used are costly, scarce and = x ( ) the method adopted is energy intensive because the process = involves baking at high temperature (above 1400 C). Pl3 192EI After review on the subject it has been found that when a high cm alumina aggregates (90% Al2O3) in higher proportion such as 90-95% is mixed with er proportion (5-8%) of calcium- max l 625 aluminate cement powder in the presence of an additive in the range of 1-2%, resulted in a refractory material which can be cm economically used for ball making. The grains of high alumina Deflection is found cm at L 2 for a dynamic load of 2 aggregate and additive may have a particle size of 5 to 10 tonnes which is much less than the maximum deflection microns and 0.5 to 5 microns respectively.

5 The above said ingredients are mixed with water (8-10%by wt) furnace shell and the lining of the extended portion of the burner and moulded into steel moulds by hand vibrating compaction of housing. desired shape and size. The diameter of the refractory balls may The angle of orientation of the burner is taken such that the be as desired as per requirements. In the present situation, the flame should not strike directly to the opposite wall, as severe refractory spheres used are of 125 mm and 150mm in diameter. lining erosion will be occurred. A tangential angle between 15 The resulting shaped refractory balls are air dried, and cured in to 18 is found suitable for the purpose. This angle forms a whirl water for at least 24 hours to give high strength, and are directly action around the wall and does not heat directly to cupola wall. used in the furnace. The refractory balls so produced have the With this angle, minimal refractory erosion is observed. The properties like refractoriness ( C), and high extended combustion chamber is lined with shaped sillimanite mechanical strength ( kg/cm2) and high thermal bricks and coated with a layer of castable. spalling resistance. COMBUSTION AND CONTROL SYSTEM DEVELOPMENT AND MODIFICATION OF BURNERS Table 5 Chemical composition of CNG supplied by GAIL The combustion performance is greatly influenced by the Constituents Volume ( % ) Other physical properties [44] burner structure, many structural parameters which include the Methane (Ch4) 85.0 Calorific value :8.500 Kcal/m3 diameter and length of quarl, nozzle structure, the method of Ethane (C2H6 ) 6.8 Specific gravity :0.69 Kg/Nm3 mixing gas and air, the length and diameter of the combustion Propane (C3H8 1.8 Compressibility:0.997 Carbon monoxide (CO2 ) 4.8 chamber, and its angle of orientation with the furnace shell and Others 1.6 wall. The HV (high velocity) burners are suitable for melting cast iron in gas fired cupola. Table 3: Burners characteristics corresponding to the capacity of the cupola Capacity of cupola Number of burners required Model of burner Air volume Max (stoichiometric) (Nm3/hr/burner) Flame length (10%XS Air) mm The HV burner is robust and extremely stable burners which can deliver combustion gas velocities in excess of 150 m/s. This results in rapid and uniform heat circulation inside a furnace as well as improved and controlled heat transfer to the load. The HV burner can be easily ignited by means of direct spark when operating on gas such as LPG and CNG. Corresponding to the capacity of the cupola, the required number and the model number and other characteristic features of the HV burner are given in the Table 3. Table 4: Effect of Angle of Orientation of Burners The length and the diameter of extended burner housing are based on the burner's flame length and diameter. The most important aspects are the angle of orientation of burner with the Flame Diameter (10% XS Air mm HV HV HV HV Sr. No Angle of Orientation of Burners Effect 1 90 with shell axis Burners started heating each other 2 45 with shell axis (inclined Oxidation of metal 3 downward) 30 downward with shell axis Still oxidation of metal 4 18 downward with shell axis Found suitable 5 15 downward with shell axis Found suitable 6 10 downward with shell axis Excessive shell heating The combustion of gaseous fuels is optimized to achieve the required results. Although, theoretically gas air ratio for CNG is 1:9.7 i.e., for complete combustion, 1volume of CNG requires 9.7 volumes of dry air. From the gas analysis, gas air ratio of 1:9 for CNG is found suitable for steady state operation. These ratios provide slightly reducing conditions and lead to the minimum elemental loss during actual melting operations. These ratios were determined from the combustion experiments carried out during burner development. A typical chemical compositions of the gaseous fuels CNG is given in The gas and air inputs may be monitored by using water-filled U-tube pressure reading. However, automation system to control the inputs such as gas and air is highly recommended. CONCLUSIONS : The existing conventional coke fired cupola can be converted into gas fired cupola. The gas fired cupola fulfills all the environmental requirements applicable in the country. It emits level of SOx and SPM which is far be the maximum limit prescribed by central pollution control board New Delhi. REFERENCES : [1] Gorfinkel and Chernobrovkin; Determining Optimum Cupola Operating Conditions by Statistical Techniques, Russian castings Production, No.1, January, pp (1969) [2] Taft, R. T.; The Totally gas fired Cupola, Foundry trade Journal, Vol. 137, pp. 21 November,1974, pp (1974) [3] Refractory Ball for Coke-less cupola; Japan patent No [4] U.S. Patent No dated [5] Robert F. Cole; Modern Casting; Vol. 78, No. 12, pp 28-30, (1996)