Coke Oven Gas Generation and Usage
Coke Oven Gas Generation and Usage
Coke is an essential input for the ironmaking process. For making coke, coal is heated in the absence of air to drive volatile matter (VM) from it. Conversion of coal to coke is called coal carbonization and the process is carried out in coke ovens. A coke oven battery consists of a number of coke ovens. At present, there are two main methods of producing coke for blast furnace.
The first method consists of a recovery process in which the coal is heated in a completely reducing atmosphere and the volatile products are recovered in an associated by-product plant. The coke ovens used for this method of coal carbonization are called by-product ovens and the coke oven battery is called the by-product coke oven battery. During the carbonization of coking coal in a by-product coke oven battery, the VM consisting of around 25 % to 30 % of the coal charged is driven off as effluent gas which leaves the coke oven chambers as the hot raw coke oven gas. Raw coke oven gas is a flammable gas and has a yellowish brown colour and an organic odour.
In the second method, carbonization of coal is carried out in a non-recovery (also known as heat recovery, or energy recovery) coke oven battery. In the non-recovery process, air is introduced above the top of the coke bed in the coke oven and the volatile products generated during the carbonization are combusted in the oven itself for providing the required heat for the coal carbonization process.
The by-product plant is an integral part of the by-product coke making process. The operation of each oven is cyclic, but the battery contains a sufficiently large number of ovens to produce an essentially continuous flow of the raw coke oven gas. The individual ovens are charged and emptied at approximately equal time intervals during the coking cycle. Coking proceeds for 15 hours to 18 hours to produce BF coke. During this period, VM of coal distills out as raw coke oven gas. The time of coking is determined by the coal blend, moisture content, rate of under firing, and the desired properties of the coke. When demand for coke is low, coking times can be increased to 24 hours. Coking temperatures generally range from 900 deg C to 1100 deg C. Air is prevented from leaking into the coke ovens by maintaining a positive back pressure in the collecting main. The coke ovens are maintained under positive pressure by maintaining high hydraulic main pressure of around 10 mm water column in the batteries. The gases and hydrocarbons which evolve during the thermal distillation are removed through the off take system and sent to the by-product plant for recovery.
The large amount gas generated due to the vapourization of VM in the coal during the production of coke in the by-product coke oven battery is treated in an adjacent by-product plant. 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. This gas is known as raw coke oven gas and is processed in the by-product plant. The functions of the by-product plant are to process the raw gas to recover valuable coal chemicals and to treat the raw coke oven gas sufficiently so that it can be used as a clean, environmentally friendly fuel. Raw coke oven gas after treatment in the by-product plant is called clean coke oven gas or simply CO gas.
In the by-product coke oven battery the evolved coke oven gas leaves the coke oven chambers at high temperatures approaching 1100 deg C. The raw CO gas is cooled by adiabatic evaporation of some of the spray liquor (flushing liquor) to around 80 deg C and is water saturated. The temperature of the gas becomes sufficiently low so that it can be handled in the gas collecting mains. From the gas collecting main, the raw coke oven gas flows into the suction main. The amount of flushing liquor sprayed into the hot gas leaving the oven chambers is far more than is required for cooling, and the remaining flushing liquor which is not evaporated provides a liquid stream in the gas collecting main that serves to flush away condensed tar and other compounds. The stream of flushing liquor flows under gravity into the suction main along with the raw coke oven gas. The raw coke oven gas and the flushing liquor are separated using a drain pot (the down comer) in the suction main. The flushing liquor and the raw coke oven gas then flow separately to the by-product plant for treatment. The typical composition of the main components in the raw coke oven gas is in Tab 1.
Tab 1 Composition of raw coke oven gas
|Sl. No.||Chemical name||% Volume|
Saturated raw gas coming from the coke oven battery contains around 46 % to 48 % water vapour. Other component of raw gas contains hydrogen (H2), methane (CH4), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), high paraffins and unsaturated hydro-carbons (ethane, propane etc.), and oxygen (O2) etc. Raw coke oven gas also contains various contaminants, which give coke oven gas its unique characteristics. These consist of (i) tar components, (ii) tar acid gases (phenolic gases), (iii) tar base gases (pyridine bases), (iv) benzene, toluene and xylene (BTX), light oil and other aromatics, (v) naphthalene, (vi) ammonia gas, (vi) hydrogen sulphide gas, (vii) hydrogen cyanide gas, (viii) ammonium chloride, and (ix) carbon di sulphide.
In order to make raw coke oven gas suitable for use as a clean, environmentally friendly fuel gas, the by-product plant is to carry out certain functions which include (i) cool the raw oven gas for condensing of water vapour and contaminants, (ii) remove tar and naphthalene to prevent gas line / equipment fouling, (iii) remove ammonia (NH3) to prevent gas line corrosion, (iv) remove benzol oil for recovery and sale of benzene, toluene and xylene (BTX), and (v) remove hydrogen sulphide to meet local emissions regulations governing the combustion of coke oven gas. Flow diagram of treatment of raw coke oven gas is given in Fig 1
Fig 1 Flow diagram for treatment of raw coke oven gas
The effects of recovery of by-products from the coke oven gas are (i) reduction in the volume of coke oven gas, (ii) reduction in the calorific value of the gas, (iii) effect on flame temperature and flame volume, and (iv) alteration in density and composition of the gas. Clean coke oven gas is a colourless gas with an odour characteristic of hydrogen sulphide and hydro-carbons.
The raw CO gas may contain hydrogen, methane, nitrogen, carbon monoxide, carbon dioxide, ethane, oxygen, ethylene, and benzene. It can also contain some quantities of ammonia, hydrogen sulphide, water vapour, cyclopentadiene, toluene, naphthalene, hydrogen cyanide, cyanogen, and nitric oxide. A typical composition of clean coke oven gas is given inn Tab 2.
Tab 2 Composition of clean coke oven gas
|Sl. No.||Chemical name||% Volume|
The final yield of clean coke oven gas after treatment in the by-product 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 in the range of 0.45 kg/cum to 0.50 kg/cum. CO gas has a calorific value ranging between 4000 kcal/N cum to 4600 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. CO gas carries around 18 % of input energy in a coke oven and by-product battery (Fig 2).
Fig 2 Typical energy balance of a coke oven and by-product plant
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 by-product 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 forms a major component in the energy balance of the steel plant. It is normally used in coke oven battery heating, heating in other furnaces of the steel plant, and for power generation. Coke oven gas can be used as such or can be mixed with the blast furnace gas before being used as fuel in a furnace.
COG can also be used as a reductant in blast furnace. CO gas injection is a process which involves injecting large volumes of coke oven gas into the raceway of a blast furnace. This provides not only a supplemental carbon source but also speeds up the production of liquid iron besides reducing the need for metallurgical coke for reactions in the blast furnace. CO gas injection technology also cuts down absolute CO2 emissions as well as SO2 emissions from the blast furnace.
The production of DRI (direct reduced iron) in the integrated steel route based on the utilization of available CO gas is a very recent phenomenon. Use of CO gas for the production of DRI has several advantages both from economic as well as environmental perspectives. Use of surplus CO gas as a reducing gas to produce DRI recovers 97 % of the available energy compared to recovering 30 % to 40 % by burning the CO gas to produce electric power. CO gas utilization is shown in Fig 3.
Fig 3 Typical gas flow in steel plant showing CO gas utilization
According to a 2007 study by International Energy Agency around 70 % of the CO gas is being used in iron and steel making processes, 15 % for coke oven heating, and 15 % for electricity production. Further the study states that by using more of the CO gas for power generation (preferably by more efficient combined cycle power generation technique which 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.