Blast Furnace gas generation and usage
Blast Furnace Gas Generation and Usage
Blast furnace (BF) process is the leading technology for the production of hot metal (HM) required for steelmaking as well as for the production of the pig iron. HM is the main product of the BF. During the production of the HM, BF gas is produced simultaneously. BF gas is the name given to the by-product which is continuously produced from the upward gaseous rise of blast air through the burden in the BF during its operation.
Though the purpose of partial combustion of carbon in the BF is to remove the oxygen (O2) from the ore burden but the volume of gas generated in the BF makes the BF also a gas producer. BF gas is an important source of chemical energy consumed outside the BF process and has a major impact on the gas balance of an integrated steel plant. First of all, the surplus of BF gas is consumed in different furnaces of the steel plant and also in the power plant boiler together with other by-product gases such as coke oven gas and converter gas. The main parameter which has a decisive say about the usefulness of BF gas is its calorific value.
An illustrative simple view of blast furnace operation, showing the BF gas coming out from the furnace top is shown in Fig 1.
Fig 1 Simple view of blast furnace operation
During the production of hot metal (HM) in the blast furnace, hot air blast is blown into the furnace through the tuyeres. O2 contained within the hot air blast reacts with carbon (in the form of coke) to produce carbon di-0xide (CO2) and carbon mono oxide (CO), as per the equations (i) C + O2 = CO2, and (ii) CO2 + C = 2CO. The gas produced by this reaction moves up the furnace shaft which has been charged with ores, fluxes and coke. After a number of chemical reactions as described below and a travel of around 25 m to 30 m the BF gas comes out of the furnace as a heated, dust laden and lean calorific value (CV) combustible gas.
Both carbon (C) in the coke and CO are reducing agents for the ore burden consisting of hematite (Fe2O3), wustite (FeO), and magnetite (Fe3O4). These oxides are reduced to form Fe and CO2. For example, the reduction mechanisms of hematite are given by the equations (i) Fe2O3 + 2C = 2Fe + CO + CO2, and (ii) Fe2O3 + 3CO = 2Fe + 3CO2.
A further source of gaseous release results from the decomposition of limestone and dolomite used as basic fluxes for the removal of the impurities. These reactions are (i) CaCO3 = CaO + CO2, and (ii) MgCO3 = MgO + CO2.
All of these changes are happening in the reaction zone of the furnace, and importantly from the perspective of BF gas composition, chemical equilibrium for the gases released is governed by the Boudouard reversible reaction ( 2CO = CO2 + C) as a set ratio is reached between CO and CO2 for a given temperature. The operational result is for large quantities of hot CO2, CO, and N2 to ascend through the furnace as fresh burden travels down into the reaction zone.
However, there can be further constituents added to the gaseous composition depending on systematic variables. As an example, additional reductants can be injected in the BF in order to reduce coke requirement in the burden, such as pulverized coal, oil, natural gas, or recycled plastics, and thereby improve furnace efficiency. However, burden integrity is to be maintained, necessitating the injection of steam or O2 alongside any additional reductants. These additions lead to fluctuating levels of H2 and H2O in the hot air blast, and subsequently affect the water-gas shift reversible reactions namely (i) C + H2O = CO + H2, and (ii) CO + H2O = CO2 + H2.
The overall chemical composition of BF gas is hence dynamic and is dependent on the furnace operating parameters. A dry volumetric composition of the BF gas representative of typical operation is given in the Fig 2.
Fig 2 Typical representative composition of blast furnace gas
The specific volume of the BF gas (cum/ton of HM) generated, its chemical composition, and its CV is dependent on the operating parameters of BF, such as (i) characteristics of the burden materials, (ii) amount of fluxes charged in the BF, (iii) distribution of burden materials in the BF stack, (iv) grade of hot metal being made, (v) quantity of auxiliary fuel injected into the BF, (vi) the temperature of hot blast, and (vii) the O2 content in the blast. Hence, the operating parameters are of practical importance from the point of view of energy management of the integrated steel plant. The quantity of BF gas transferred to other consumers depends on the amount of gas produced in the BF and on the amount of BF gas consumed in the hot blast stoves of the BF.
The total quantity of CO + CO2 gases by volume in the BF gas at the furnace top ranges from around 37 % to 53 % of the total gas volume. The CO/CO2 ratio can vary in a blast furnace from 1.25:1 to 2.5:1. Higher percentage of CO in the gas makes the BF gas hazardous. The hydrogen (H2) content of the BF gas can vary from 1 % to 7 % depending upon the type and amount of fuel injected in the tuyeres of the BF. The balance component of the BF gas is nitrogen (N2). Methane (CH4) can also be present in the BF gas upto a level of 0.2 %.
In the BF, some hydro-cyanide (HCN) and cyanogen gas (CN2) can also form due to the reaction of N2 in the hot air blast and C of the coke. This reaction is catalyzed by the alkali oxides. These gases are highly poisonous. BF gas can contain these cyano compounds in the range of 200 milligrams per cubic metre (mg/cum) to 2000 mg/cum.
BF gas leaves the BF top at a temperature of around 120 deg C to 370 deg C and a pressure which can range from around 350 mm to 2,500 mm mercury gauge pressure. It carries at this stage around 20 grams per cubic metre (g/cum) to 115 g/cum of water vapour and 20 g/cum to 40 g/cum of dust commonly known as ‘flue dust’. The particle size of the flue dust can vary from a few microns to 6 mm.
BF gas is almost colourless (mild whitish), and an odourless gas. Other main characteristics of the BF are (i) very low CV usually in the range of around 700 kilo calories per cubic meter (kcal/cum) to 850 kcal/cum, (ii) relatively a high density usually in the range of around 1.250 kilograms per cubic meter (kg/cum) at is 0 deg C and 1 atmosphere pressure which the standard temperature and pressure (STP), (iii) low theoretical flame temperature which is around 1455 deg C, (iv) low rate of flame propagation which is usually lower than any other common gaseous fuel, (v) burns with a non luminous flame, (vi) auto ignition point of around 630 deg C, and (vii) has a lower explosive limit (LEL) of 27 % and an upper explosive limit (UEL) of 75 % in an air-gas mixture at normal temperature and pressure. The density of the BF gas is highest amongst all the gaseous fuel. Since the density is higher than the density of air, it settles in the bottom in case of a leakage. High concentration of CO gas in the BF gas makes the gas hazardous.
The high top pressure of BF gas is utilized to operate a generator (top gas pressure recovery turbine, i.e. TRT in short). TRT can generate electrical energy (power) up to 35 kWh/ ton of hot metal without burning any fuel. Dry type of TRT can produce more power then wet type.
Cleaning of BF gas
BF gas coming out of the furnace top contains 20 g/cum to 40 g/cum of flue dust and cannot be used as such. This dust contains fine particles of coke, burden materials and chemical compounds which are formed due to the reactions taking place inside the BF. This dirty BF gas is cleaned in gas cleaning plant in two stages namely (i) primary gas cleaning stage, and (ii) secondary gas cleaning stage.
Primary gas cleaning consists of dust catchers, cyclones, or a combination of both. The gravity principle is used for the removal of large particles (coarser than 0.8 mm) of the dust. In this stage, the BF gas is normally passed through a dust catcher where all the coarser particles are removed. The dust catcher is a large cylindrical structure normally 20 m to 30 m in diameter and with a height of 20 m to 30 m. It is usually lined to insulate it and prevent the condensation of moisture present in the BF gas so that the dust remains dry and does not ball up and flow freely into the conical portion of the dust catcher at its bottom for its periodical removal.
The BF gas is sent to the dust catchers by a single down comer and enters through the top of the dust catcher by a vertical pipe which carries the gas downward inside the dust catcher. This pipe flares at its lower extremity like an inverted funnel, so that as the gas passes downward its velocity (and thus its dust carrying potential) decreases, and most of the coarser dust (coarser than 0.8 mm) drops out of the gas stream and is deposited in the cone at the bottom of the dust catcher. Because the bottom of the dust catcher is closed, and the gas outlet is near the top, the direction of the travel of the gas gets reversed 180 degrees. This sudden reversal in the direction of flow causes more of the dust to get settle down.
After dust catcher the gas is sent to secondary gas cleaning stage. Here BF gas is cleaned either by dry type gas cleaning system or wet type gas cleaning plants. In dry type gas cleaning plants bag filters are used for removal of fines particles of dust while in the wet type gas cleaning plant BF gas is washed of dust in scrubbers in several stages.
Uses of BF gas
The sensible heat in the BF top gases was first utilized in 1832 to transfer heat to the cold blast. Originally, this heat exchanger was mounted on the top of the furnace. In 1845, the first attempts were made to make use of heat of combustion of BF gas, but the burning of BF gas was not successful till 1857. It is probable that the progress in the utilization of BF gas was delayed due to its high dust content, the problems of cleaning and handling, and the low cost of solid fuel. Increasing cost of other fuels and competition forced its use.
In the past BF gas use was restricted to the heating of hot blast stoves in the blast furnaces and using it in multi fuel boilers. It was not considered to be economical for other uses because of its various characteristics. However in the recent years, several factors have contributed to its enlarged use. The factors which have contributed to the enlarged use of gas are (i) increase in the cost of the purchased fuels, (ii) technical improvement in gas cleaning thus improving the cleanliness of the gas, and (iii) technology development for BF gas preheating.
In integrated steel plants, BF gas is normally being used mixed with either coke oven gas or converter gas or both. The mixed gas is used as a fuel in various furnaces of the integrated steel plant. BF gas without mixing and without preheat can be used in BF stoves, soaking pits, normalizing and annealing furnaces, foundry core ovens, gas engines for blowing, boilers for power generation, gas turbines for power generation. With recent advances in the technology, BF gas is also being directly used in the sinter plant furnace.
The thermal advantage of using BF gas in gas engines for blowing and for power generation has to overcome the heavy investment and maintenance expense required for such equipment. The modern boiler house utilizes high steam pressure and temperature with efficient turbo-blowers and generators. This has sufficiently reduced the thermal advantage of gas engines and hence their use has become difficult to get justified. Some steel plants in Asia and Europe have been successful in the use of direct connected gas turbines for driving generators. Preheated BF gas along with preheated air has been used successfully in coke-oven heating, soaking pits, and reheating furnaces.
When BF gas is preheated, it is to have a minimum cleanliness of 0.023 g/cum and in all cases where this gas is used, extra precautions is needed to prevent the escape of unburned BF gas into the surroundings since it contains a large percentage of toxic CO gas.
In blast furnace operations, where the BF gas has a heating value approaching a low value of 700 kcal/cum, it becomes necessary to mix the BF gas with other fuel gases to obtain very high temperature of the hot air blast from the stove.