Coke Oven Gas, its Characteristics and Safety Requirements


Coke Oven Gas, its Characteristics and Safety Requirements

Coke oven gas (CO gas)  is a byproduct gas produced during the production of metallurgical coke in a byproduct coke oven battery, where metallurgical coal is carbonized by heating it in absence of air. During carbonization the volatile matter in the coal is vaporized and driven off. This volatile matter leaves the coke oven chambers as hot, raw coke oven gas. After leaving the coke oven chambers, the raw coke oven gas is cooled which results in a liquid condensate stream and a gas stream. These two streams are processed  in the byproduct plant to recover byproduct coal chemicals and to condition the raw coke oven gas so that it can be used as a fuel gas. The main emphasis of a modern  byproduct plant is 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 byproduct plant is called clean coke oven gas or simply coke oven gas.

The evolved coke oven gas leaves the coke oven chambers at high temperatures approaching 1100 deg C. This hot gas is immediately quenched by direct contact with a spray of aqueous liquor (flushing liquor). The resulting cooled gas is water saturated and has a temperature of around 80 deg C. This gas is collected in the coke oven battery gas collecting main. 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 unevaporated flushing liquor provides a liquid stream in the gas collecting main that serves to flush away condensed tar and other compounds. This 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 byproduct plant for treatment.

Composition of coke oven gas

The chemical composition of the raw  coke oven gas is given in Tab 1.

 

Tab 1  Composition of raw coke oven gas
Chemical name CAS number EC number % Volume
Hydrogen 1333-74-0 215-605-7 52-59
Methane 74-82-8 200-812-7 26-33
Nitrogen 7727-37-9 231-783-9 1.9-5.7
Carbon monoxide 630-08-0 211-128-3 4.5-7.0
Ethylene 74-85-1 200-815-3 2.0-2.8
Carbon dioxide 124-38-9 204-696-9 1.4-2.1
Hydrogen sulfide 04-06-83 231-977-3 0.4-1.2
Hydrogen cyanide 74-90-8 200-821-6 0-1.2
Ethane 74-84-0 200-814-8 0.7-1.1
Ammonia 7664-41-7 231-635-3 0-1.1
Benzene 71-43-2 200-753-7 0-1.0
Carbon disulfide 75-15-0 200-843-6 0-0.3
Toluene 108-88-3 203-625-9 0.1-0.2
CAS- Chemical Abstract Service, EC- European Community

 

The chemical composition of the clean  coke oven gas is given in Tab 2.

 

Tab 2 Composition of clean coke oven gas
Chemical name CAS number EC number % Volume
Hydrogen 1333-74-0 215-605-7 42-65
Methane 74-82-8 200-812-7 17-34
Nitrogen 7727-37-9 231-783-9 1.2-18
Carbon monoxide 630-08-0 211-128-3 4.6-7.5
Carbon dioxide 124-38-9 204-696-9 0.2-3.5
Ethane 74-84-0 200-814-8 0.1 ? 2.9
Oxygen 7782-44-2 231-956-9 0-2.6
Ethylene 74-85-1 200-815-3 0.1 ? 2.5
Benzene 71-43-2 200-753-7 0 ? 0.4
CAS- Chemical Abstract Service, EC- European Community

 

Typical compositions of the raw CO gas and the clean CO gas is shown in Fig 1.

Typical composition of CO gas

Fig 1 Typical composition of coke oven gas 

Characteristics of coke oven gas

Raw coke oven gas has a yellowish brown colour and an organic odor. It is a flammable gas with lower explosive limit of 4 % and upper flammability limit of 75 %. Its vapour density is 0.39 (air=1) and relative density is 0.589.

Raw coke oven gas is a flammable material with a flash point of less than 60 deg C. The gas contains toxic chemicals as given in Tab 3.

 

Tab 3 Toxic elements in raw CO gas
Chemical name Max % by Weight
Ethylene 2.8
Hydrogen sulfide 1.2
Hydrogen cyanide 1.2
Ammonia 1.1
Benzene 1
Carbon disulfide 0.3
Toluene 0.2

 

Clean coke oven gas is a colourless gas with an odor characteristics of hydrogen sulphide and  hydrocarbons. It has a lower explosive limit of 4.4 % and upper explosive limit of 34 %. Its vapour density is 0.36 (air=1).  The density of CO gas at standard temperature and pressure is in the range of 0.45 to 0.50 Kg/Cum.

The clean coke oven gas contains toxic chemicals as given in Tab 4.

 

Tab 4 Toxic elements in raw CO gas
Chemical name Max % by Weight
Ethylene 2.5
Benzene 0.4

 

CO gas has a calorific value ranging between 4000 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.

When exposed at high concentration, CO gas act as a simple asphyxiant. It displaces oxygen and cause rapid suffocation by showing symptoms of oxygen deprivation. It may cause heart problems with prolonged or repeated exposures. CO gas causes damage to the heart through prolonged or repeated exposures. The gas is harmful if inhaled. In case of inhalement, the affected person is to be removed to fresh air and is to be kept comfortable for breathing. CO gas can also cause eye irritation.

In case of a leaking gas fire (both for raw and clean CO gas), it should not be extinguished unless leak is stopped safely or the fire is immediately impacting the human life. All ignition sources are to be eliminated if safe to do so. The fire is to be extinguished with foam, carbon dioxide, dry powder or water fog, once leak is stopped. A solid stream of water is not to be used since it may scatter and spread the fire.

There are special protective equipment and precautions for fighting the CO gas fires. Self contained respiratory protection and full protective clothing are to be worn when fumes and/or smoke from fire are present. Firefighters are to wear full face piece self contained breathing apparatus and chemical protective clothing with thermal protection. Direct water stream scatters and spreads flames and, therefore, are not to be used. The area is to be evacuated. The pressurized gas cylinders are to be removed from the immediate vicinity. The containers exposed to flames are to be cooled with water until well after the fire is out. The valve is to be closed if no risk is involved. It is necessary not to extinguish a leaking gas fire unless leak can be stopped. If leak cannot be stopped and no danger to surrounding area, then the fire is allowed to burn out. Fighting of the fire is to be carried out from a protected location. The buildup of vapours or gases to explosive concentrations is to be prevented.

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.

Safety requirement for the CO gas

The following are the safety requirements for the CO gas.

  • No person is allowed to work in or go to the area where CO gas is present, if the carbon monoxide content in that area is more than 50 ppm then gas mask is to be used.
  • On line monitoring system with alarm for carbon monoxide concentration is to be provided in the areas around equipment/process handling CO gas. Performance of on-line monitoring system is to checked once in a month for its proper operation and records are to be maintained.
  • Either non sparking tools or grease coated tools are to be used while working on charged CO gas pipelines and gas handling system.
  • No person is allowed to work on charged system (where there is possibility of presence of CO gas) without gas masks.
  • Proper escape route and scaffolding is to be provided while working on charged CO gas system at height.
  • The welding current is not to exceed 100 A while welding on charged CO gas system.
  • Cutting or welding jobs are not to be allowed on isolated system without analysis and written clearance of the competent authority. It is to be done only by trained welders in the presence of a competent gas safety man. A minimum level of 20 % oxygen shall be ensured.
  • Proper electrical jumpers are to be provided between flanges and equipments before a gap is created between them.
  • Platform and adjoining structures are to be covered with asbestos/fire resistant clothes while blanking and de-blanking and the person working is not to be allowed to wear nylon or other synthetic fabric/garments.
  • Fire fighting machinery is to be kept in attendance at the place of work in charged CO gas system.
  • Lime water is to be poured after loosening the bolts of flanges in CO gas lines/equipments at the time of blanking or de-blanking or opening the manhole covers.
  • The deposits in CO gas lines and associated equipments are to be kept wet either by steam or by water, after the system has been isolated and opened to atmosphere.
  • Cutting in CO gas lines and associated equipments is to be carried out after cleaning of the deposits. In case, it is not possible, deposit is to be kept wet and a running steam hose is to be kept in readiness to prevent a fire. Also, ingress of fresh air is to be prevented.
  • All jobs within a radius of 40 m which could be a source of fire/ignition are to be stopped and unauthorized persons are not to be allowed to remain in the area during the shutting down of the gas system.
  • Wherever necessary, lighting in enclosed area is to be done with portable spark proof electric lamp of 24 V or explosion proof fittings.All pipelines/systems are to be checked for leakage after completion of repair job. The leakages are to be detected by soap solution and all leakages shall be rectified before charging the system.
  • Blanking/de-blanking jobs on gas lines are not to be taken up at the time of extreme bad weather conditions when the possibility of thundering/lightening exists.
  • Drain pots and other auxiliaries of gas lines shall be inspected for proper operation at least once a month and records are to be maintained.
  • Purging steam/gas is to be used through a detachable hose. Permanent connections are to be blanked after purging requirements are over.
  • There must not be any discontinuity in blanking/ de-blanking. Once started it shall be completed at a stretch.
  • Testing of leaks of running mains of CO gas is to be done only by soap water.
  • Persons required to work in gaseous atmosphere shall be trained in first aid and methods of giving artificial respiration.
  • Water seal/ valves are to be installed above ground level.