Coke Oven Gas generation and usage
Coke Oven Gas generation and usage
During the carbonization of coking coal in a coke oven battery for the production of coke, around 25-30% of the coal charged is driven off as effluent gases rich in volatile matter and moisture. This gas is known as coke oven gas (CO gas). In the non recovery or heat recovery coke ovens this gas is burnt in the oven itself and provides the required heat for the carbonization of coal. In case of by product battery, the evolved gas is removed as raw gas and is treated in a byproduct plant to give a clean fuel gas. In the byproduct plant, condensable, corrosive and economically valuable components are removed. During the cycle of coking, the gas is produced during majority of the coking period. The composition and rate of evolution of the CO gas changes during the period and the evolution of CO gas is normally complete by the time the coal charge in the battery reaches 700 deg C. The final yield of clean coke oven gas after treatment in the byproduct plant is around 300 N Cum per ton of dry coal. The yield of gas is dependent upon i) volatile matter in the charge coal and ii) carbonization condition. The density of CO gas at standard temperature and pressure is 0.545 Kg/Cum.
The raw CO gas may contain hydrogen and methane, ammonia, carbon monoxide, carbon dioxide, ethane, ethylene, benzene, oxygen and nitrogen, hydrogen sulfide, water vapor, cyclopentadiene, toluene, naphthalene, hydrogen cyanide, cyanogen, and nitric oxide. The typical composition of the main components in the raw coke oven gas is in Tab
|Tab 1 Typical analysis of Raw coke oven gas|
|Component||Unit||Dry basis||Actual composition|
|Water saturated at 80 deg. C|
|Carbon mono oxide||%||6||3|
|Carbon di oxide||%||3||2|
Raw Coke oven gas contains many chemical contaminants. They are
- Tar vapours
- Light oil vapours (aromatics) consisting mainly of benzene, toluene and xylene (Generally known as BTX fraction)
- Naphthalene vapour
- Hydrogen sulphide gas
- Hydrogen cyanide gas
After drying the raw gas and separating the above chemical contaminants in a byproduct plant coke oven gas (COG) is obtained. Process flow in a byproduct plant is given in Fig. 1
Fig 1 Typical process flow in a CO gas byproduct plant
Combustion of CO gas
CO gas constitute around 18% of input energy in a coke Oven and Byproduct plant (Fig 2)
Fig 2 Energy balance of a coke oven plant
CO gas has a heating value between 4200 to 4800 Kcal/N Cum. It has a theoretical flame temperature of 1982 deg C. It has a rate of flame propagation which allows its actual flame temperature to be close to its theoretical flame temperature.
Analytical data indicate that volatile HAP (Hazardous Air Pollutants) collectively comprises much less than 1% by volume of CO gas after conventional treatment of Raw CO gas in a byproduct plant. Hence the CO gas combustion in well maintained operated combustion units such as process heaters, and boiler etc, results in very low levels of HAP emissions. The filterable particulate matter (PM) emissions from the combustion of CO gas are typically low. HAP metal emissions from CO gas are not significant.
Uses of coke oven gas
Coke oven gas is normally used in coke oven battery heating, heating in other furnaces and for power generation. Coke oven gas can be used as such or can be mixed with BF gas and/or Converter gas before being used as fuel in the furnace.
Typical utilization of CO gas is shown in Fig.3
Fig 3 Typical utilization of CO gas
According to a 2007 study by International Energy Agency approximately 70% of the COG was used in iron and steel making processes, 15% for coke oven heating, and 15% for electricity production. Further the report states that by using more of the COG for power generation (preferably by more efficient combined cycle power generation technique that can provide efficiencies of around 42% as opposed to use in boiler based power plants working on steam cycles with an average efficiency of around 30%) improvements in energy efficiencies can be achieved.
CO gas injection at tuyere level has been successfully tried in the blast furnace in some plants where there is excess of CO gas availability. An integrated steel plant in USA has reported an annual saving of USD 6.1 million by using CO gas as supplementary fuel in blast furnace. They have reported a payback period of just over one year.