Converter Gas, its Characteristics and Safety Requirements


Converter Gas, its Characteristics and Safety Requirements

During the process of steel making in the basic oxygen furnace (BOF), significant amount of gases, rich in carbon monoxide content, are generated during the blow time at a temperature of 950 deg C. This gas is termed as converter gas or BOF gas. Converter gas is also known as LD gas. It is a byproduct gas produced during the production of liquid steel in a basic oxygen furnace (converter), where impurities of hot metal are oxidized with oxygen gas. The main constituents of converter gas are carbon mono oxide (CO), carbon di oxide (CO2), oxygen (O2) and nitrogen (N2).  Composition wise it is similar to blast furnace gas but with lesser percentage of nitrogen in it.

Converter gas is dust laden at the converter mouth. The dust content is around 100 to 120 g/N cum. The recovered converter gas is cleaned in a venturi scrubber using water, followed by processing in the mist eliminators. The gas is then stored in gas holder for steady supply and cleaned further in the electrostatic precipitators (ESP) and finally fed to the gas distribution system. Wet type of gas cleaning plants have capabilities to reduce the dust content of the gas to a level of 5 mg/N cum.

The composition of the gas varies from start to the end of the blow and this is a function of the blow time. In the oxygen rich phase (air ratio= 1) at the beginning and at the end of the blowing period the primary gas is burned completely and no gas is recovered during this period. During CO rich phase (air ratio less than 1) only partial oxidation takes place and a combustible waste gas is formed containing CO, H2, CO2 and N2. The maximum CO and H2 content in the waste gas corresponding to the minimum air ratio (air ratio= 0.1) is present during the main decarburization period (middle part of the blowing period) and maximum gas is recovered during this phase.

Converter gas typically has a high carbon monoxide content, low combustion speed and is very harmful. It has got a calorific value which varies in the range of 1600 to 2400 kcal/N Cum of the gas. It is the function of the air ratio. Lower is the air ratio higher is the calorific value, since nitrogen percentage of the gas reduces. Lower air ratio also means lower specific yield of the gas. Density of converter gas is 0.865 kg/Cum.

The range of composition of converter gas is given in Tab 1. Fig 1 gives typical analysis of converter gas.

Tab 1 Composition of converter gas

Chemical name Chemical formula CAS number EC number % Volume
Nitrogen N2 7727-37-9 231-783-9 12.0-20.0
Carbon monoxide CO 630-08-0 211-128-3 58.0 to 70.0
Carbon dioxide CO2 124-38-9 204-696-9 15.0-20.0
Hydrogen H2 1333-74-0 215-605-7 0.9-1.0
Oxygen O2 7782-44-2 231-956-9 0.1-0.3
CAS-Chemical Abstract Service, EC- European Community

Typical analysis of converter gas

Fig 1 Typical analysis of converter gas

Converter gas is highly poisonous and explosive and requires high degree of disciplined operation at the time of recovery .The gas is invisible and colourless. It cannot be detected by odor. It can readily form explosive mixtures with air, which are easily ignited by a static charge. Therefore, any leakage from flanges, valves and joints, may lead to severe explosion in the area resulting in fatal accident. Any ingress of external air or oxygen can also cause explosion in the system. So, supreme care is needed to avoid any kinds of leakage in the  recovery, transportation as well as the utilization of the converter gas.

In case of leakage, all personnel from danger area are to be immediately evacuated. Self contained breathing apparatus (SCBA) are to be used where needed. All sources of ignition are to removed if it is safe to do so. Gas concentration is to be reduced with fog or fine water spray. Flow is to be shut off if it is safe to do so. Ventilation of the area is to be improved.

Converter gas is a toxic and hazardous gas. Before entering the area it is necessary to measure the level of carbon monoxide  level with the help of a portable CO monitoring device.

Occupational permissible exposure limit for CO gas is 50 ppm and for CO2 gas is 5000 ppm. Converter gas is normally mixed with the blast furnace gas in various proportion and the mixed gas is used for heating purposes in various furnaces of the steel plant.

Converter gas is a chemical asphyxiant. It displaces oxygen and cause rapid suffocation by showing symptoms of oxygen deprivation. It may cause heart problems with prolonged or repeated exposures.

Exposure to low concentrations for extended periods may result in dizziness or unconsciousness, and may lead to death. If a person inhales converter gas of higher concentration, he should immediately move to fresh air area and if he faces breathing difficulties he should be given oxygen gas. Also he should be given medical attention if needed.

In case of a leaking gas fire 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 converter gas fires. Self contained respiratory protection and full protective clothing should be worn when fumes and/or smoke from fire are present. Firefighters should wear full face piece self contained breathing apparatus and chemical protective clothing with thermal protection. Direct water stream scatters and spreads flames and, therefore, should not 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.

Safety requirement for the converter gas

The following are the safety requirements for the converter gas.

  • No person is allowed to work in or go to the area where converter 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 converter 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 converter gas pipelines and gas handling system.
  • No person is allowed to work on charged system (where there is possibility of presence of converter gas) without gas masks.
  • Proper escape route and scaffolding is to be provided while working on charged converter gas system at height.
  • The welding current is not to exceed 100 A while welding on charged converter 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 converter gas system.
  • 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 shall be done with portable spark proof electric lamp of 24 V or explosion proof fittings.
  • All pipelines/systems shall 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 shall 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 converter 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.