Download Coke-making in Blast Furnace Iron-making and more Study notes Sustainability Management in PDF only on Docsity! Lecture 4 Production of secondary fuel Coke‐making Secondary fuels are those which do not occur in nature but they are produced from primary (natural reserves) reserves to meets certain specific requirements. It must be clearly noted that coke does not occur in nature. One of the important secondary fuels is coke. Coke is an important raw material in the blast furnace iron‐making. It is a source of chemical and thermal energy in the blast furnace. Coke also helps maintaining permeability of the burden which is required for flow of gases ascending upwards in the blast furnace. It must be clearly noted that coke does not occur in nature. How coke is produced Coke is produced by heating coal to high temperature (T ≈ 1000‐1100oC) out of contact of air until all volatile matters are removed. The process is called “carbonization” or “Destructive distillation of coal”. Coal consists of complex organic compounds in which C, H, N, O, and S atoms are bonded together. As a result of heating these bonds are broken and new bonds are formed between atoms of elements like CO, H2S, NH3, CO2, H2, CH4 and other complex hydrocarbons like C6H6, C2H4 etc. The mass of coal during heating fuses and becomes plastic. It swells during coking and then resolidifies. The structure of coke depends much on fusion, swelling and resolidification. Difference in behaviour of different types of coal account for the difference in structure of coke. For example non coking coal may decompose without becoming plastic at any stage. Mixing of two different varieties of coal i.e. non‐ coking coal with coking coal result in control over coke properties. By‐Product coke‐oven Metallurgical coke (coke used in blast furnace iron making) is produced in by product coke ovens. By‐ product coke ovens are flexible in treating different types of coal and to control the coke properties. Coke is charged batch wise into silica lined or refractory lined retorts and these retorts are heated externally by burning gaseous fuels. Note that retorts are heated indirectly and coal is heated through heat transferred from the walls of the retort. Coal near the wall of the retort is heated faster than coal near the centre. As a result, coke near the wall swells much earlier than coal at the centre. Therefore, proper distribution of coal in the retort would be desirable. The volatile matter from the coal is collected in the by product recovery plant where by product are separated from each other. It takes around 18 hours to convert one batch of coal into coke. Coke is Docsity.com discharge from the other end of the retort by mechanical hopper into a car, where it is wet quenched. Note that hot coke so produced cannot be used directly into blast furnace hence coke is cooled to room temperature. During wet quenching of coal considerable amount of sensible heat is lost and pollutants are discharged in atmosphere. This aspect of coke making is dealt separately in next lecture with a quantitative illustration. The quality of coke depends both on temperature and rate of heating. Metallurgical grade coke is produced at temperature higher than 1000 oC. Among the by‐products, coke oven gas possesses both sensible heat and potential energy. Coke oven gas is used as a fuel to heat the furnace and also in heating the coke oven. The leakage of atmospheric air into coke oven must be avoided as the air causes oxidation of C and results in decrease in yield. Material balance in coke‐making In coke making coal of certain composition is carbonized in a by‐product coke oven. As a result of carbonization, products and by‐products are produced. Main product is coke, whereas by‐products are coke oven gas and tar. The following block diagram represents material balance: coal of certain composition is charged and coke, coke oven gas and tar of certain composition is produced Coal (wt %) Coke Tar Coke oven C (analysis, wt%) (analysis, wt%) gas(vol%) H C C CO O H H CO2 N O O CH4 S N N NH3 Ash(A2) S S N2 M Ash (A2) Ash (A3) H2O Coke‐Oven 1200oC H2 C6H6 etc. Basis of calculation: one may take 1 Kg coal, 100 Kg coal or 1000 Kg coal. Amount of coke is determined by ash balance, namely If W Kg is amount of coke then ash balance is A1×1000/100 = A2 × W/100 + A3 × wt. of tar/100 By knowing weight of tar, W can be calculated. Amount of coke oven gas can be calculated by carbon balance: C from coal = C in coke + C in tar + C in coke oven gas In the present illustration, if Y Kg mole is the amount of producer gas Docsity.com