Blast Furnace Gas, its Characteristics and Safety Requirements
Blast Furnace Gas, its Characteristics and Safety Requirements
Blast furnace gas (BF gas) is a byproduct gas produced during the production of hot metal (liquid iron) in a blast furnace, where iron ore is reduced with coke to produce hot metal. The blast furnace gas which comes out from the top of the blast furnace is at a high pressure (usually 1.5 atm to 2.5 atm in modern blast furnaces) and normally at a temperature of around 100 deg C to 150 deg C. When there are abnormalities in the burden descent then this temperature can be higher.
A typical blast furnace produces 1200 – 2000 N cum of BF gas per ton of hot metal. The energy content of this gas may equal to an energy export of around 1.2 M cal/ton (5GJ/ton) of hot metal, or approximately 30 % of the gross energy consumption in the blast furnace. This energy content can be utilized by recovering, cleaning, and storing the BF gas and using it as a fuel gas in the steel plant or in the boilers for the production of steam for the electricity generation. The calorific value of the BF gas varies but is usually low (between 650 kcal/N cum to 900 kcal/N cum and depends on the CO concentration). Therefore, BF gas is often enriched by coke oven gas or natural gas prior to use.
In cases where the blast furnace is fitted with a two bell charging system, approximately 1.5 % of the BF gas is lost every time the furnace is charged. Most of this gas can be recovered by discharging the high pressure gas between the two bells into the low pressure side of the gas collection system just prior to opening the top bell for charging.
The BF gas produced at the top of the blast furnace is water and dust laden. The water vapour content is around 20 grams to 115 grams per cum and the dust content is around 20 grams to 40 grams per cum. The particle size of the dust can vary from a few microns to 6 mm. The dust laden BF gas is cleaned in two stages. In the first stage the coarse particles of the dust are removed in the dust catcher by first reducing the gas velocity and then by reversing the flow of the gas. In the second stage finer dust particles in the BF gas are removed either in wet type gas cleaning plant where the gas is washed with water in a series of scrubbers or in dry type gas cleaning plant where the finer dust particles are removed by bag filters.
The high top pressure of BF gas is utilized to operate a generator known as top gas pressure recovery turbine (TRT). Depending upon the size of the blast furnace and the pressure of the top BF gas , TRT can generate electrical energy (power) up to 35 kWh/ ton of hot metal without burning any fuel.
The range of composition of BF gas is given in Tab 1. Fig 1 gives typical analysis of BF gas.
|Tab 1 Composition of BF gas
Fig 1 Typical representative composition of BF gas
The total amount of CO and CO2 gases by volume in the BF gas at the furnace top is around 40 % to 45 % of the total gas volume. The CO/CO2 ratio can vary in a blast furnace. Higher percentage of CO in the gas makes the BF gas more hazardous. The hydrogen content of the gas can vary depending upon the type and amount of fuel injected in the tuyeres of the blast furnace. The balance component of the BF gas is nitrogen. Methane (CH4) can also be present in the BF gas up to 0.2 %. In blast furnace, some hydro cyanide (HCN) and cyanogen gas (CN2) can also formed due to the reaction of nitrogen in the hot air blast and carbon of the coke. The reaction is catalyzed by the alkali oxides. These gases are highly poisonous. BF gas can contain these cyano compounds in the range of 200 mg to 2000 mg/cum.
Characteristics of BF gas
BF gas has the following characteristics
- BF gas is almost a colourless gas (mild whitish) and normally it is odorless. However it may have a slight sulphur odor, but this should not be relied upon as a warning of its presence.
- BF gas a lean fuel gas with a calorific value (CV) in the range of 650 to 900 kcal/N cum. CV of the BF gas is very much dependent upon the coke rate in the blast furnace.
- BF gas has a high density. It is around 1.250 kg/cum at the standard temperature and pressure (STP) which is 0 deg C of temperature and 1 atm of pressure. This density is highest amongst all the gaseous fuel. Since the density is higher than the density of air, it settles at the bottom in case of a leakage.
- BF gas has low theoretical flame temperature which is around 1455 deg C.
- BF gas has a low rate of flame propagation. It is lower than any other common gaseous fuel.
- BF gas burns with a non luminous flame.
- Auto ignition point of BF gas is around 630 deg C.
- BF gas has lower explosive limit (LEL) of 27 % and upper explosive limit (UEL) of 75 % in an air gas mixture at normal temperature and pressure.
- BF gas is flammable and can form explosive mixtures with air, which can be easily ignited by a static charge.
- BF gas has a stable chemical stability under normal storage and handling conditions.
BF gas is considered a hazardous material according to the criteria specified in REACH [REGULATION (EC) No 1907/2006] (REACH is a significant piece of European legislation. The acronym stands for Registration, Evaluation, Authorization and restriction of Chemicals) and CLP [REGULATION (EC) No 1272/2008] and OSHA 29 CFR 1910.1200 Hazard Communication Standard. Higher concentration of carbon monoxide in BF gas makes the gas hazardous. Presence of BF gas in the environment is measured through CO gas monitor.
BF gas is a toxic gas. Hence in the area of BF gas, the level of CO gas is to be monitored regularly with the help of a CO gas monitor. The CO gas concentration is to be kept within 50 ppm when a job is being done in the area. If a person inhales BF 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.
When exposed at high concentration, BF 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
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 BF 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 BF gas
The following are the safety requirements for the BF gas.
- No person is allowed to work in or go to the area where BF 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 BF 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 BF gas pipelines and gas handling system.
- No person is allowed to work on charged system (where there is possibility of presence of BF gas) without gas masks.
- Proper escape route and scaffolding is to be provided while working on charged BF gas system at height.
- The welding current is not to exceed 100 A while welding on charged BF 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 BF 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 BF 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.