Basic Oxygen Furnace Gas Recovery and Cleaning System
Basic Oxygen Furnace Gas Recovery and Cleaning System
During the process of steelmaking in the basic oxygen furnace (BOF), oxygen (O2) is blown in the charge mix and due to the chemical reactions taking place in the converter 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, the gas is very hot (temperature 950 deg C or higher) and dust laden. This gas is known as LD gas, BOF gas, or converter gas. Converter gas is normally classified as a lean gas in terms of its calorific value and Wobbe index, but belongs to the group of rich gases when considered in terms of its combustion properties (and particularly, its combustion temperature).
During the early days of steelmaking 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.
The composition of the converter gas varies with the process used, the recovery method and, specifically, the O2 volume. The composition of the gas varies from the start to the end of the blowing of the heat in the converter and is a function of the blow time. The main constituents of converter gas are CO, CO2 (carbon di oxide), O2 and N2 (nitrogen). Typical composition of the converter gas by volume is CO – 55 % to 60 %, CO2 – 12 % to 18 %, O2 – 0.1 % to 0.3 % and balance is N2.
The first converters were put into operation in November 1952 (VOEST in Linz) and May 1953 (OAMG, 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 cubic meters (cum) of the flue 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 (g) of the converter dust has a visible surface area ranging between 300 square meters (sqm) to 500 sqm. In order to normally avoid the optical effects of ‘brown fumes’, the dust is to be cleared from the system to a level less than 100 mg/cum. 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.
Generally, two systems can be used to handle the top converter gas and to recover energy from the converter gas. These systems are (i) partial/full combustion, (ii) suppressed combustion.
In the full (or open) combustion system (which is no more being prin use), the process gas from the converter vessel is combusted in the flue-gas duct. An opening between the converter vessel and the primary (or converter gas) ventilation allows the entrance of ambient air and thus allows for a partial or full combustion of the converter gas. In this case, the process gas contains around 15 kg to 20 kg of dust per ton of liquid steel (tLS) and around 7 kg CO gas/tLS. Energy is recovered by using the sensible heat in a waste heat boiler. When BOF gas is combusted in the flue-gas duct, the flue-gas is emitted and is required to meet local emission standards. In the open combustion systems there is a large flow (around 1,000 N cum/tLS to 2,000 N cum/tLS) due to the introduction of air into the BOF gas duct.
As the volume of converters increases, exhaust gas treatment equipment becomes larger. Large converters adopt the non-combustion-type system for several reasons, such as the relatively small size of the system as a whole, ease of maintenance, and stable dedusting efficiency. During early sixties, processes were developed to recover the 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 suppressed combustion systems offer the best opportunity for both heat and fuel recovery. A skirt is lowered over the BOF mouth during O2 blowing to reduce air infiltration and inhibit combustion of the CO gas in the flue gas duct. The resultant CO rich gas is collected, cleaned and stored for subsequent use as a fuel gas within the steel plant. A waste heat boiler, generating high pressure steam, can recover the sensible heat of the gas before it is cleaned and stored. This recovers around 10 % to 30 % (0.1 GJ/tLS –0.3 GJ/tLS) of the total energy output. Another 50 % to 70 % is recovered as chemical energy (CO) from the BOF gas. Total energy recovery when applying suppressed combustion with converter gas recovery and a waste heat boiler can be as high as 70 % to 90 %. Energy savings can amount to 0.35 GJ/tLS to 1.08 GJ/tLS with a leak-free system. With energy savings of 0.92 GJ/t steel, CO2 emissions are reduced by 46 kg/t of steel. Energy recovery lowers CO2 generation from the use of fossil fuel and electricity by around 0.05 t CO2/t of steel. The converter gas is not normally collected during the start and end of the blowing because of its low CV and CO content, and is instead flared. Thus CO2 is inevitably emitted. An advantage with suppressed combustion over open combustion systems is the smaller gas flow since no combustion occurs and no additional air is introduced. The cooling and gas cleaning systems are hence smaller. It also results in higher productivity since the O2 blowing speed can be increased and lower energy consumption of the fans. Installing an expert system to optimize the collection of converter gas can save around 30 MJ/tCS (ton crude steel).
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 converter gas, 70 cum to 100 cum of converter gas per ton of crude steel with a calorific value ranging from 1,600 kcal/N cum to 2,000 kcal/N cum 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. The converter gas recovered is mixed with other by-product gases (coke oven gas, and blast furnace gas) and used in the steel plant as a fuel. Steam is mainly used by the vacuum degassing unit of the secondary steelmaking.
Since the refining of the steel is done in a short period of time, around 35 minutes per heat, the dust concentration is very high. In non-combustion type converters with a gas recovery function, the dust concentration is 70 g/N cum to 80 g/N cum at the inlet of the first dedusting device. Non-combustion-type converters, without combusting CO gas, manage the volume of intake air from the throat, and control the concentration to below the explosion limit, thereby recovering CO gas as fuel. Exhaust gas treatment consists of an exhaust gas cooling system and a cleaning system.
The BOF gas, when is recovered for use as a fuel, the gas is to meet certain requirements. Nowadays, in the majority of the BOF shops, the converter gas is recovered as a fuel by introducing suppressed combustion system. Due to the suppressed combustion system, the volume of the generated conmerter gas is around 50 N cum/tLS to 100 N cum/tLS. This results in significant differences in the dimensions of the primary dedusting facilities. The reduced waste gas flow rate characterizing the suppressed combustion method results in a higher raw gas mass concentration, and hence, the efficiency of the dust recovery system is to be increased for an identical clean gas dust load. From a dust recovery point of view, therefore, the suppressed combustion principle allows the use of dedusting systems designed for smaller volumetric flow rates which is required to achieve higher dust recovery rates. Primary dedusting is normally performed by venturi-type scrubbers (around 60 % of the BOF shops) or dry and wet ESP (Electrostatic precipitator). Prior to the venturi or the ESP, coarse particulates are normally moved by means of a deflector etc. Schematic of gas recovery system in a BOF is shown in Fig 1.
Fig 1 Schematic of gas recovery system in a basic oxygen furnace
Suppressed combustion systems can be largely divided into two types namely (i) the OG-type and (ii) the IC (IRSID-CAFL) type. The OG type system basically has no space between the throat and the hood skirt, and controls pressure at the closed throat. The IC type system has a gap of several hundred millimeters between the throat and the hood skirt (which has a slightly larger diameter than that of the throat), and controls pressure at the throat opening. The non combustion type system keeps gas temperature low and shuts out combustion air. Hence, the cooling device and dedusting device installed in the system are smaller than those installed in the combustion-type system. Since the system handles gas which mainly consists of CO, attention is required to be paid to sealing for the flux and coolant input hole and the lance hole, and leak control at the periphery of devices, as well as purge at the gas retention part.
The OG type system is frequently used because of its operational stability. The OG type cooling system makes it possible not only to recover the sensible heat of exhaust gas as steam, but also to increase the IDF (induced draft fan) efficiency by lowering the temperature of the exhaust gas by use of a cooling device. The OG system is normally designed to recover a high percentage of the latent heat and sensible heat of the top converter gas. A pictorial sketch of OG suppressed combustion system for the converter gas recovery is shown in Fig 2.
Fig 2 Pictorial sketch of OG suppressed combustion system
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 stage is the O2 rich stage. In this stage the air ratio (lambda) is 1. During this O2 rich stage the primary gas is burned completely and no gas recovery takes place during this period. After this, CO rich gas stage starts where lambda is less than 1. During this stage only partial oxidation takes place and a combustible waste gas is formed containing CO, CO2 and N2 gases. After this the main stage of decarburization takes place which is around the middle part of the blowing period. During this stage the air ratio (lambda) 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 lambda 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 the site requirements. During the process, the hood gas pressure is controlled for preventing the puffing out of the gas from the converter mouth as well as controlling the air ratio (lambda). The system control is important since it is handling explosive exhaust gases (mostly CO gas). The system need to be operated in a safe manner. The system need to have high energy performance and is required to 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 1,600 to 1,700 deg C to around 900 deg C. BOF shops 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 operate on the basis of a wet type gas cleaning process. These systems have a capacity to meet the requirement of less than 50 mg/N cum of dust. In the wet system the recovered converter 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 ESP or it is exhausted by an IDF 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 ESPs 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|
|Sl. No.||Subject||Unit||Wet system||Dry system|
|1||Clean gas dust content||mg/N cum||50||10|
|7||Off gas cleaning after gas holder||Yes||No|
|9||Drying cost for dust recycling||Yes||No|