Blast Furnace gas generation and usage
Blast Furnace gas generation and usage
Blast furnace (BF) gas is a gaseous by product which is generated while producing hot metal (liquid iron) in a blast furnace. The operation of the blast furnace is controlled to produce hot metal of a specified quality and during this production BF gas comes out from the furnace top.
During production of hot metal in a blast furnace, hot air blast is blown in the furnace through the tuyeres. The oxygen of the blast reacts with the coke. 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 and a travel of around 25-30m the BF gas comes out of the furnace as a heated, dust laden and lean combustible gas. Around 1500-1700 Cu m/ton of hot metal of BF gas is generated during the process. Though the purpose of partial combustion of carbon in a blast furnace is to remove the oxygen from the ore but the volume of gas generated in a blast furnace makes the blast furnace as a gas producer. The percentage of CO and CO2 in BF gas is directly related to the amount of carbon in the charged coke and amount of CO2 in the charged flux (Limestone and dolomite). The coke rate (The rate of carbon consumption) in the blast furnace depends mainly upon the type of the hot metal to be made, the chemical and the physical characteristics of the charged materials, the distribution of the materials in the furnace stack, the temperature and the oxygen enrichment of the hot air blast.
The total amount of CO+CO2 gases by volume in the BF gas at the furnace top is around 40% 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 content of the gas can vary from 2% to 5% 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/Cu m.
BF gas leaves the BF top at a temperature of approximately 120 deg C to 370 deg C and a pressure of 345 mm to 2500 mm mercury gauge pressure. It carries at this stage around 20 to 115 grams per Cu m of water vapour and 20 to 40 grams per Cu m 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 has the following characteristics
- Very low calorific value (CV) in the range of 700 to 850 Kcal/Cu m (2930 to 3556 Kilojoules/Cu m). CV is very much dependent on the coke rate.
- It has a high density. It is around 1.250 Kg/Cu m at the standard temperature and pressure (STP) which is 0 deg C and 1 atm. Pressure. This density 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.
- It has low theoretical flame temperature which is around 1455 deg C.
- It has 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.
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 20-40 grams of flue dust and cannot be used as such. This dust contains fine particles of coke, burden materials and chemical compounds that are formed due to the reactions taking place in the blast furnace. This dirty BF gas is cleaned in gas cleaning plant in two stages namely primary gas cleaning stage and 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 20-30 m in diameter and with a height of 20-30 m. It is usually lined to insulate it and prevent the condensation of moisture in 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 gas is sent to the dust catchers by a single down comer and enters through the top by a vertical pipe that 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 0.8mm) 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 must reverse 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 blast furnace 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 as follows:
- Increase in the cost of the purchased fuels
- Technical improvement in gas cleaning thus improving the cleanliness of the gas
- 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 furnace of the plant. BF gas without mixing and without preheat can be used in BF stoves, soaking pits, normalizing and annealing furnaces, foundary core ovens, gas engines for blowing, boilers for power generation, gas turbines for power generation
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 should have a minimum cleanliness of 0.023 grams per cubic meter 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.
BF gas is used for many applications in a steel plant and, in addition, is used frequently for heating coke ovens and sometimes is mixed with other gases as a fuel. In blast furnace operations, where the blast-furnace gas has a heating value approaching a low value of 700 Kcal per cubic meter, it is necessary to switch the gas with other fuels to obtain very high hot-blast temperature from the stove.