Basic Oxygen Furnace Gas Recovery and Cleaning System
Basic Oxygen Furnace Gas Recovery and Cleaning System
During the process of steel making in the basic oxygen furnace (BOF), oxygen is blown in the charge mix and due to chemical reactions taking place in the vessel, a large amount of gas at high temperature and rich in carbon mono oxide (CO) comes out through the mouth of the converter. At this stage this gas is very hot (Temperature 950 deg C or greater) and dust laden. This gas is known as LD gas or converter gas. The composition of the gas varies from the start to the end of the blow and is a function of the blow time. The main constituents of converter gas are carbon mono oxide, carbon di oxide (CO2), oxygen (O2) and nitrogen (N2). Typical composition of the by volume is CO – 55 to 60 %, CO2 – 12 to 18 % oxygen 0.1 to 0.3 % and rest is N2. The first converters were put into operation in November 1952 (VOEST in Linz) and May 1953 (ÖAMG, Donawitz). During the early years of the LD converter process, the top gas was completely burnt at the converter mouth through the open hood and then cooled in the stack either indirectly with water or by evaporation cooling system. At that time around 300 Kg of steam and 250 Cu m of off gas per ton of crude steel were produced.
Environmental aspects were a serious challenge for the converter process at the time it was industrially implemented in 1950s. The fineness of the dusts in the converter off gas forced the suppliers of the process to develop new dedusting systems. 1 gram of converter dust has a visible surface area of between 300 to 500 Sq m. In order to generally avoid the optical effects of ‘brown fumes’, the dust is to be cleared from the system to a level less than 100 mg per cubic meter. For this both wet type and dry type of de dusting systems were used. The challenge became more and more of an opportunity for the converter process as the number of environmental issues grew. And this opportunity helped in developing the system of the recovery of converter gas with suppressed combustion. Today economic and environment demands that the energy in the converter gas and the iron containing dust is collected and efficiently recycled.
During early sixties processes were developed to recover this high calorific value top gas of the converter so that the same can be used as gaseous fuel inside the plant. This has been achieved through suppressed combustion. The process equipment which is installed above the converter mouth has functions to cool down, to clean up and to recover the converter gas with the help of suppressed combustion. With suppressed combustion of the top converter gas, 70-100 Cu m of converter gas per ton of crude steel with a calorific value ranging from 1600 -2000 Kcal/N Cu m of gas is recovered. Besides 80 Kg/ton of crude steel steam is also made in case evaporative cooling system for top gas is adapted.
During the early days of steel making by the converter process, brown fumes from the chimney indicated that converter is working. Today as a result of converter gas recovery and cleaning system, the operation of the converter is detected only from the flare stack.
A typical schematic of the converter gas recovery and cleaning plant is shown in Fig 1.
Fig 1 Typical schematics of converter gas collection and cleaning plant
During the blowing of the converter for making of the steel, atmospheric air is mixed with the gas at the converter mouth. The amount of the atmospheric air which enters the system at the converter mouth is controlled by the hood pressure and a movable skirt. During the blowing period, the initial phase is the oxygen rich phase. In this phase the air ratio (?) is 1. During this oxygen rich phase the primary gas is burned completely and no gas recovery takes place during this period. After this, CO rich phase starts where ? is less than 1. During this phase only partial oxidation takes place and a combustible waste gas is formed containing CO, CO2 and N2. After this the main phase of decarburization takes place during the middle part of the blowing period. During this phase the air ratio (?) is kept at a minimum value and is around 0.1. During this period maximum gas is recovered. At the end of the blowing the value of ? is again kept at 1 and the generated gas is burnt completely with no recovery of the gas.
Converter gas recovery by the suppressed combustion system has the advantage of system structure which is much more compact than the system structure with full combustion and hence it is more flexible for adjustment as per site requirements. During the process hood gas pressure is controlled for preventing the puffing out of the gas from the converter mouth as well as controlling the air ratio (?). The system control is important since it is handling explosive exhaust gases (mostly CO). The system need to be operated in a safe manner. The system need to have high energy performance and should recover both the latent heat and the sensible heat of the exhaust gases.
The CO rich gas coming out of the converter is first cooled in the converter hood indirectly either by cooling water or by evaporative cooling system (ECS) to bring down its nominal temperature from 1600 -1700 deg C to around 900 deg C. Plants adopting ECS recovers a part of the sensible heat of the exhaust gases in the form of low pressure steam. The cooling of the converter gas to 900 deg C is necessary to avoid formation of water gas (CO + H2) during wet cleaning. It is well known that the water gas is highly explosive.
The system need to have high dust collecting performance. The recovered gas is cleaned either by wet type or dry type of gas cleaning plants. More than 90 % of the present de dusting systems around the world operates on the basis of a wet type of process. These systems have a capacity to meet the requirement of less than 50 mg/N Cu m. In the wet system the recovered LD gas is cleaned in venturi scrubbers followed by processing in the mist eliminators. The cleaned gas is then stored in a gas holder for steady supply to the gas distribution system after cleaning it further in the electro static precipitator or is exhausted by an ID fan through a flare stack after flaring. Slurry generated during wet cleaning is transported to thickener, through dip seal pot, launder and bowl rake classifier for wet treatment. Chemicals are added for coagulation and better separation. Over flow of the thickener is recirculated after cooling and sludge is further processed either in vacuum filter or in press filter for use in sinter plant.
Dry type gas cleaning plants with electrostatic precipitators can achieve a dust content of less than 15 mg/ N Cum. In dry cleaning, coarse dust from the converter gas after cooling in waste heat boiler is separated in evaporation chamber followed by electrostatic precipitator for fine dust removal. The comparison between dry and wet types of gas cleaning plants is shown in Table 1. Dry type gas cleaning plants have good future because of their lower energy consumption, higher degree of effectiveness and better quality of the converter gas and economical way of recycling of the dust.
|Tab1 Comparison of wet and dry type gas cleaning plants|
|Subject||Unit||Wet system||Dry system|
|Clean gas dust content||mg/N Cu m||50||10|
|Off gas cleaning after gas holder||Yes||No|
|Drying cost for dust recycling||Yes||No|