Oxygen furnace gas recovery and cleaning system basics

Basic Oxygen Furnace Gas Recovery and Cleaning System

In the steelmaking process of a Basic Oxygen Furnace (BOF)， oxygen (O2) is blown into the mix and a large amount of hot and carbon monoxide (CO) rich gas comes out of the converter mouth due to the chemical reaction that takes place in the converter vessel. At this stage， the gas is very hot (950 degrees Celsius or higher) and dusty. This gas is called LD gas， BOF gas or converter gas. In terms of calorific value and Wobbe index， converter gas is usually classified as lean gas， but in terms of its combustion characteristics (especially combustion temperature)， it falls into the rich gas category.

In the early days of the converter steelmaking process， brown smoke from the chimney indicated that the converter was working. Today， thanks to converter gas recovery and cleaning systems， converter operation can only be detected from the torch chimney.

The composition of the converter gas varies with the process used， the recovery method and， in particular， the amount of O2. The composition of the gas varies from the beginning to the end of the converter blowing heat and is a function of the blowing heat time. The main components of converter gas are CO， CO2 (carbon dioxide)， O2 and N2 (nitrogen). The typical composition of converter gases by volume is CO - 55% to 60%， CO2 - 12% to 18%， O2 - 0.1% to 0.3% and the rest is N2.

The first converters were put into operation in November 1952 (VOEST in Linz) and in May 1953 (OAMG in Donawitz). In the early years of the LD converter process， the top gas was completely burned at the converter mouth through an open hood and then cooled in the stack either indirectly with water or through an evaporative cooling system. At that time， about 300 kg of steam and 250 cubic meters (Cum) of flue gas were produced per ton of crude steel.

When the converter process was implemented industrially in the 1950s， environmental issues were a serious challenge. The fineness of the dust in the converter exhaust gas forced the suppliers of the process to develop new dust removal systems. 1 gram (g) of converter dust has a visible surface area of between 300 square meters (sqm) and 500 sqm. In order to usually avoid the optical effect of "brown smoke"， dust has to be removed from the system to levels below 100 mg/m3. For this purpose， both wet and dry dust removal systems are used. This challenge is increasingly becoming an opportunity for converter processes as environmental concerns increase. This opportunity has contributed to the development of converter gas recovery systems with suppressed combustion. Today， the economy and the environment require that the energy in converter gas and iron-bearing dust be captured and recovered efficiently.

Generally， two systems are available to process the top converter gas and recover energy from the converter gas. These systems are (i) partial/complete combustion， and (ii) suppressed combustion.

In a complete (or open) combustion system (currently no longer used)， the process gas from the converter vessel is burned in the flue gas duct. The openings between the converter vessel and the primary (or converter gas) vent allow ambient air to enter， thus allowing partial or complete combustion of the converter gas. In this case， the process gas contains about 15 kg to 20 kg of dust per ton of liquid steel (tLS) and about 7 kg of CO gas/tLS. Energy is recovered by using sensible heat in the waste heat boiler. When BOF gas is burned in the flue， the flue gas is emitted and needs to meet local emission standards. In an open combustion system there is a large flow rate (about 1000 N cum/tLS to 2000 N cum/tLS) as air is introduced into the BOF gas duct.

As the size of the converter increases， the exhaust gas treatment equipment becomes larger. There are several reasons for using non-combustion type systems for large converters， such as relatively small overall system size， easy maintenance and stable dust removal efficiency. In the early 1960s， processes were developed to recover the high calorific value top gas of the converter for use as a gaseous fuel in the plant. This was achieved by suppression combustion.

The suppressed combustion system provides the best opportunity for heat and fuel recovery. During oxygen blowing， a skirt is lowered over the BOF port to reduce air infiltration and inhibit the combustion of CO gas in the flue. The resulting CO-rich gas is collected， cleaned and stored for subsequent use as a fuel gas in the steel mill. A waste heat boiler that produces high pressure steam can recover the sensible heat of the gas before it is cleaned and stored. This can recover between 10% and 30% of the total energy output (0.1 GJ/tLS - 0.3 GJ/tLS). Another 50% to 70% is recovered from the BOF gas as chemical energy (CO). When applying suppressed combustion and waste heat boilers with converter gas recovery， the total energy recovery can be as high as 70 to 90 %. With leak-free systems， energy savings can reach 0.35 GJ/tLS to 1.08 GJ/tLS. with energy savings of 0.92 GJ/t steel， CO2 emissions are reduced by 46 kg/t steel. Energy recovery resulted in a reduction of about 0.05 t CO2/t steel from the use of fossil fuels and electricity. Due to the low CV and CO content of converter gases， they are usually not collected at the beginning and end of the blowing process， but are burned. Therefore， CO2 will inevitably be emitted. An advantage of suppressed combustion compared to open combustion systems is that the gas flow is lower， because no combustion takes place and no additional air is introduced. As a result， the cooling and gas cleaning systems are also smaller. This also leads to higher productivity， as the O2 blowing rate can be increased and the energy consumption of the fans can be reduced. The installation of an expert system to optimize the collection of converter gas saves about 30 MJ/tCS (tons of crude steel).

The process equipment installed above the converter mouth has the function of cooling， cleaning and recovering converter gas with the help of suppressed combustion. By suppressed combustion of the converter gas， 70 to 100 kg of converter gas per ton of crude steel is recovered， with calorific values ranging from 1600 kcal/year to 2000 kcal/year. In addition to 80 kg/ton of crude steel， steam can be made if the evaporative cooling system is applied to the top gas. The recovered converter gas is mixed with other by-product gases (coke oven gas and blast furnace gas) and used as fuel in the steel plant. The steam is mainly used by the vacuum degasification unit of the secondary steelmaking.

The dust concentration is very high due to the short time of steelmaking， about 35 minutes per heating. In non-combustion converters with gas recovery， the dust concentration is between 70 g/year product and 80 g/year product at the inlet of the first de-gassing unit. The non-combustion type converter manages the inlet of the throat without burning CO gas and keeps the concentration below the explosion limit， thus recovering CO gas as fuel. The exhaust gas treatment includes an exhaust gas cooling system and a cleaning system.

When converter gas is recovered for use as fuel， the gas must meet certain requirements. Nowadays， in most converter plants， converter gas is recovered as fuel by introducing a pressurized combustion system. Due to the suppressed combustion system， the volume of the converter gas produced is about 50 N cum/tLS to 100 N cum/tLS. this leads to a wide variation in the size of the primary de-dusting facilities. The suppressed combustion method is characterized by a reduced exhaust gas flow rate， which leads to a higher mass concentration of the primary gas and， therefore， a higher efficiency of the dust recovery system for the same clean gas dust load. Therefore， from the point of view of dust recovery， the principle of suppressed combustion allows the use of dust removal systems designed for smaller volume flow rates， which are necessary to achieve higher dust recovery rates. Primary de-dusting is usually performed by venturi scrubbers (about 60% of the BOF plant) or dry and wet ESPs (electrostatic precipitators). Prior to the venturi or ESP， coarse particulate matter is usually moved by means of deflectors， etc. A schematic diagram of the gas recovery system in a refinery is shown in Figure 1.

Figure 1 Schematic diagram of the gas recovery system for a basic oxygen furnace

The combustion suppression systems can be broadly classified into two types， namely (i) OG type and (ii) IC (IRSID-CAFL) type. the OG type system basically has no space between the throat and the hood skirt and controls the pressure in the closed throat. the IC type system has a gap of several hundred millimeters between the throat and the hood skirt (the diameter of the hood skirt is slightly larger than the diameter of the throat) and controls the pressure in the throat opening. The non-combustion type system keeps the gas temperature at a low level and shuts off the combustion air. Therefore， the cooling and de-dusting units installed in this system are smaller than those installed in the combustion type system. Since the gas handled by this system consists mainly of carbon monoxide， attention needs to be paid to the sealing of the flow and coolant input holes and lance holes， as well as to the leakage control at the periphery of the equipment and to the cleaning of the gas retention section.

OG type systems are often used due to their operational stability. og type cooling systems not only recover the sensible heat of the exhaust gas into steam， but also increase the efficiency of the IDF (induced draft fan) by reducing the temperature of the exhaust gas using a cooling unit. og systems are usually designed to recover a high ratio of latent to sensible heat from the top converter gas. Figure 2 shows a pictorial sketch of an OG suppression combustion system for converter gas recovery.

Figure 2 Schematic diagram of the OG pressed combustion system

Process

During the blowing of the steel in the converter， atmospheric air is mixed with the gas at the converter mouth. The amount of atmospheric air entering the system at the converter mouth is controlled by the pressure of the hood and the movable skirt. During blowing， the initial stage is the oxygen-rich stage. In this stage， the air ratio (lambda) is 1. During this oxygen-rich stage， the primary gas is completely burned and no gas recovery occurs during this period. After this， the CO-rich gas phase begins with a lambda less than 1. In this phase， only partial oxidation occurs and a combustible exhaust gas containing CO， CO2 and N2 gases is formed. After this， the main decarbonization phase takes place， approximately in the middle part of the blow-off period. During this phase， the air ratio (λ) is kept at a minimum value of approximately 0.1. During this phase， the maximum gas is recovered. At the end of the blowdown， the lambda value is again kept at 1. The generated gas is completely burned and no gas is recovered.

The advantage of the converter gas recovery in a suppressed combustion system is that the system structure is much more compact than in a complete combustion system， so it can be adjusted more flexibly to the site requirements. In this process， the gas pressure in the hood is controlled to prevent gas from being ejected from the converter port and to control the air ratio (λ). The system control is important because it has to handle explosive exhaust gases (mainly CO gas). The system needs to operate in a safe manner. The system needs to have a high energy performance and needs to recover both latent and sensible heat from the exhaust gases.

The CO-rich gas from the converter is first cooled indirectly in the converter hood by means of cooling water or an evaporative cooling system (ECS)， which reduces its nominal temperature from 1600 to 1700 degrees Celsius to about 900 degrees Celsius. Cooling of the converter gas to 900 degrees Celsius is necessary to avoid the formation of water gas (CO + H2) during wet cleaning. It is known that water gas is highly explosive.

The system needs to have a high dust collection performance. The recovered gas can be cleaned by wet or dry gas cleaning equipment. More than 90% of the world's dust collection systems currently operate on the basis of wet gas cleaning processes. These systems have the capacity to meet dust requirements of less than 50 mg/N. In a wet system， the recovered converter gas is cleaned in a venturi scrubber and then treated in a mist eliminator. The cleaned gas is further cleaned in an electrostatic precipitator and then stored in a gas holder for stable supply to the gas distribution system or discharged by an IDF fan through the flare stack after combustion. The slurry from the wet cleaning process is conveyed to a thickener for wet processing through an impregnation seal tank， scrubber and bowl rake classifier. Chemicals are added for coagulation and better separation. The overflow from the thickener is recirculated after cooling and the sludge is further processed in vacuum filters or filter presses for use in the sintering plant.

Dry gas cleaning plants with electrostatic precipitators can achieve dust levels below 15 mg/N Cum. In dry cleaning， coarse dust from converter gas cooled in a waste heat boiler is separated in an evaporation chamber and then fine dust is removed with an electrostatic precipitator. A comparison of dry and wet gas cleaning plants is shown in Table 1. dry gas cleaning plants have good prospects because of their lower energy consumption， higher effectiveness， better quality of converter gas and economic way of dust recovery.