Technologies for improvement in Coking process in Byproduct Coke Ovens
Technologies for improvement in Coking process in Byproduct Coke Ovens
Coking coals are converted to coke in large byproduct coke oven batteries. The coking process consists of heating blend of crushed coking coals in the absence of air to drive off the volatile compounds. The resulting coke is a hard, but porous carbon material which is used for the reduction of iron bearing materials in a blast furnace. The byproduct coke oven also recovers volatile chemicals in the form of coke oven gas, ammonium sulphate, tars, and oils. In last three to four decades several technologies have been developed which have not only resulted into vast improvements in the process of coking in the byproduct coke oven batteries but also have improved the quality of produced metallurgical coke. Majors of these technologies are given below.
Selective crushing of coals
This technology is a theoretically sound technology and aims at controlling the degree of crushing of the different constituents of coal. It aims to improve homogeneity of reactive and inert components in coal. The reactive components of coals are primarily vitrinites and are the softest constituents while the mineral matters of coals are the hardest components. In conventional coal crushing units, the vitrinites get crushed to a relatively finer size compared to mineral matter constituents when the entire coal is crushed together. For producing coke of higher quality, it is desirable to crush the mineral matter finer than the vitrinite component of the coal so that during the process of coking, when the coal charge softens, the mineral matter is assimilated better, leading to the improved strength. This is usually carried out by crushing of coals in two stages. This technology is helpful when coals are petrographically heterogeneous.
Coal moisture control
Coal moisture control uses the waste heat from the coke oven gas to dry the coal used for coke making. The moisture content of coal blend for charging in coke ovens usually varies in the range of 8 % to 10 %. Drying of coal blend reduces the moisture content in the coal blend to a level of around 6 %. This in turn reduces fuel consumption in the coke ovens. The coke is dried using the heat content of coke oven gas, low pressure steam or any other waste heat source. The benefits of coal moisture control are
- Fuel saving of around 71700 Kcal/t
- Improvement in the quality of coke by 1.7 %
- Increase in coke production by about 10 %
- Reduction in coking period
- Decrease in water pollution
Schematic flow chart of CMC is given in Fig 1
Fig 1 Schematics of coal moisture control
The technology basically involves formation of a stable coal cake with the finely crushed coal by mechanically stamping outside the oven for carbonization. In stamp charging, the bulk density of the coal charged in the oven is increased by physically stamping the charge into a cake. The cake, which is almost similar in size to the oven, is then inserted in the oven. Stamping is carried out in a stamping cum charging cum pushing machine which uses drop hammers for stamping. Coal moisture is maintained at 8 % to 10 % for providing the binding action. Stamping increases the bulk density of charge by 30 % to 35 % to around 1150 Kg/Cum. Crushing of coals to more than 90 % below -3 mm and 40 % to 50 % below – 0.5 mm is needed for a stable cake. With stamp charging, low rank, weakly caking and high volatile coals can be used to the extent of around 20 %, but since the coal charge is compacted to high bulk density there can be problem of increased wall pressures. In order to ensure that the refractory oven walls are not damaged the coal blend used must be carefully chosen by optimum balancing between high and low volatiles coals.
The advantages of stamp charging are increased throughput of 8 % to 10 % due to higher bulk density, improved strength of coke (micum and CSR value) due to closer packing of the individual coal particles during carbonization, produced coke is denser, smaller and more uniform in size and less pollution during charging as fine coals are not charged from the oven top.
High pressure ammonia liquor aspiration system
The high pressure ammonia liquor aspiration system (HPALA) is effective for controlling charging emissions in coke oven batteries. In this system, the ammoniacal liquor, which is a byproduct, is pressurized to about 35-40 Kg/Sq cm and injected through special nozzles provided in the goose neck at the time of charging. This creates sufficient suction inside the oven thereby retaining the pollutants from being released to the atmosphere. The system consists of high pressure multistage booster pumps, sturdy pipe work, specially designed spray nozzles, suitable valves and control instruments. This system emissions control, results in saving in quantity of process steam and increase in the yield of raw gas.
Coke dry quenching
Coke dry quenching (CDQ) is an alternative to the traditional wet quenching. During wet quenching of run of oven coke, sensible heat of the hot coke is dissipated into the atmosphere and is lost. In addition there are air borne emissions (0.5 ton of steam per ton of coke laden with phenol, cyanide, sulfide and dust) and a large quantity of water (around 0.6 Cu m per ton of coke) is needed for wet quenching. The contaminants in water are also discharged in the environment. In a coke dry quenching plant, red hot coke is cooled in specially designed refractory lined steel cooling chambers by counter currently circulating inert gas media in a closed circuit consisting of a cooling chamber, a dust collecting chamber, a waste heat boiler, dust cyclones, a mill fan, a blowing device and circulating ducts. The heat energy from the red hot coke is recovered in the waste heat boiler for use as steam, resulting in energy conservation as well as a reduction in coke particle emissions. Around 80 % of sensible heat is recovered. Dry quenching also improves the coke strength. Other advantages of coke dry quenching are reduction in green house gas (GHG) emissions and improved water efficiency. The details of coke dry quenching are available at http://ispatguru.com/dry-cooling-of-coke/.
Modern leak proof doors
Leaking doors of a coke oven battery are always a major source of pollution. The design of oven doors has gone through a process of evolution, starting from luted doors to the present generation self regulating zero leak doors. The imported features of the leak proof doors are given below:
- A thin stainless steel diaphragm with a knife edge as a sealing frame built in between the door body and the brick retainer.
- Spring loaded regulation on the knife edge for self sealing
- Provision for air cooling of the door body
- Large size gas canals for easier circulation of gas inside the oven.
The advantage of leak proof doors are minimization of door leakages, regulation free operation, longer life due to less warping of the air cooled door body and reduced maintenance.
Land based pushing emission control system
The fumes generated during the pushing of red hot coke contains a large amount of coke dust (around 11 % of total pollutants in coke ovens
Land based pushing control systems mitigate this pollution. It consists of (i) a large suction hood fixed on the coke guide car and moving with the coke guide, directing the fumes to the coke side dust collecting duct (ii) dust collection duct and (iii) and the equipment cleaning of the fumes. The large amount of paroxysmal high temperature fumes are collected under the hot float fan into the large gas suction hood installed in the coke gude car, and enters the dust collection duct through the other equipment. The air is dissipated into the atmosphere after purification by the pulse duct collector and after being cooled by the accumulator cooling. The system is controlled by PLC.
Automation and process control system
Automation and process control for the coke oven battery heating and machines is achieved using a level 2 control system that conducts various process model caiculations based on the processd data collected from a level 1 automation system. The benefits of the automation and process control system include lower energy consumption through reduction in fuel gas consumption, stablize condition and operstion of coke oven battery, consistent quality of coke, reduced emissions, increase in battery life and ease in reporting and analysis of operational and maintenance data.