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I. Blast furnace gas: This gas is generated by air compression using high-pressure blowers, and after being heated in hot air stoves, it enters the blast furnace. The hot air, together with coke, acts as a fuel source; the resulting gases are carbon dioxide and carbon monoxide. Carbon dioxide, in combination with the extremely hot coke, produces more carbon monoxide. As this carbon monoxide rises, it reduces the iron elements in the iron ore, converting them into pig iron – this is the chemical process involved in iron production. The molten iron remains temporarily at the bottom of the furnace and is released at regular intervals for direct steelmaking or ingot casting. At this time, there is also a large amount of excess carbon monoxide in the blast furnace gas; this mixed gas is what is known as blast furnace gas. Producing one ton of iron generates 2,100–2,200 cubic meters of blast furnace gas. This gas, which contains flammable carbon monoxide, is a gas fuel with a low calorific value; it can be used as fuel for use within metallurgical enterprises, such as for heating hot-rolled steel ingots and preheating ladles. It can also be supplied for civilian use; if coke oven gas is added to it, it is called \"mixed gas,\" which increases its calorific value. Blast furnace gas is a by-product generated during the iron smelting process. Its main components include CO, CO2, N2, H2, CH4, etc. Among these, the combustible component CO accounts for about 25%; the amounts of H2 and CH4 are very small. CO2 and N2 make up 15% and 55% respectively, and its calorific value is only around 3500 KJ/m3. The composition and calorific value of blast furnace gas are related to the fuel used in the blast furnace, the type of pig iron produced, and the smelting process. Modern iron production generally employs production methods characterized by large volumes, high wind temperatures, high smelting intensity, and high amounts of coal powder injected during the process. These advanced production techniques improve labor productivity and reduce energy consumption, but they result in blast furnace gas with a lower calorific value, which makes it more difficult to utilize. In blast furnace gas, CO2 and N2 do not participate in combustion to generate heat, nor do they assist in combustion; on the contrary, they absorb a large amount of heat produced during combustion, resulting in a lower theoretical combustion temperature for blast furnace gas. The ignition point of blast furnace gas is not high, and it seems there are no obstacles to ignition; however, during actual combustion, due to various factors, the temperature of the mixture must be much higher than the ignition point in order to ensure stable combustion. The theoretical combustion temperature of blast furnace gas is low, and a large amount of this gas is involved in the combustion process; as a result, the mixing gas heats up slowly, its temperature remains low, and the combustion stability is poor. Another condition for a combustion reaction to occur is that effective collisions between gas molecules can take place, that is, collisions between molecules that possess sufficient energy to undergo an oxidation reaction. The presence of large amounts of CO2 and N2 reduces the likelihood of such effective collisions between molecules, which results in a slow combustion rate and unstable combustion on a macroscopic scale. Blast furnace gas contains large amounts of CO2 and N2, which do not participate in chemical reactions during combustion; they are transferred almost in equal amounts to the flue gases produced by combustion. The amount of flue gases generated by burning blast furnace gas is much greater than that produced by burning coal. ----------------------------------------------------- II. Converter gas: During the steelmaking process in converters, the carbon in the molten iron reacts with the oxygen introduced at high temperatures to produce a mixture of carbon monoxide and a small amount of carbon dioxide. The recovered top-blown oxygen converter off-gas contains 60–80% carbon monoxide, 15–20% carbon dioxide, as well as nitrogen, hydrogen, and trace amounts of oxygen. The generation volume of converter gas is not uniform throughout a smelting process, and its composition also changes (see figure). Typically, the gas recovered from multiple smelting processes in the converter is fed into a gas storage tank, mixed there, and then delivered to users. When converter gas is ejected from the furnace mouth, its temperature is as high as 1450–1500°C, and it contains a large amount of iron oxide dust; it must be cooled and dedusted before it can be used. Purification comes in two types: wet and dry. ①The typical process of a wet purification system is as follows: after the gas exits the converter, it is cooled to 800–1000°C in a vaporization cooler. It then passes through a first venturi tube, a first elbow dehydrator, a second venturi tube, and a second elbow dehydrator in sequence. Washing water is sprayed at the throat of the venturi tubes, which reduces the gas temperature to around 35°C and lowers the dust content in the gas to approximately 100 milligrams per cubic meter. Then, the purified gas is sent to the gas storage tank using a exhaust fan. The wet process is widely used around the world; 60–80 cubic meters of gas can be recovered per ton of steel, with an average calorific value of about 2000–2200 kcal/cubic meter. ②Some factories in countries such as the United States and West Germany use dry electrostatic dust removal purification systems. The gas temperature is reduced to 1000°C through cooling flues; it is then cooled further to 200°C using an evaporation cooling tower. After dust removal via a dry electrostatic precipitator, the gas becomes clean with a dust content of less than 50 milligrams per cubic meter, and it is sent to the gas storage tank using a vacuum pump. The investment in dry systems is about 12–15% higher than that in wet systems ； However, there is no need to build sewage treatment facilities, and power consumption is low; nevertheless, appropriate measures must be taken to prevent gas from mixing with air to form explosive gases. Blast furnace gas is a medium-calorific value gaseous fuel used within steel plants. It can be used as fuel for industrial furnaces on its own, or it can be combined with coke oven gas, blast furnace gas, and producer gas to form mixed gases with various calorific values. Blast furnace gas contains large amounts of carbon monoxide, which is highly toxic; therefore, strict measures must be taken to prevent leaks during storage, transportation, and use. --------------------------------------------------- III. Coke oven gas: Coke oven gas is a combustible gas produced as a by-product of coke manufacturing. It is obtained through high-temperature carbonization of coal blends used for coking in coke ovens, alongside coke and tar products. It is mainly used as fuel and a raw material for the chemical industry. Producing one ton of coke generates 300-320 cubic meters of coke oven gas. Coke oven gas is primarily composed of hydrogen and methane, accounting for 56% and 27% respectively, with small amounts of carbon monoxide, carbon dioxide, nitrogen, oxygen, and other hydrocarbons ； Its low calorific value is 18,250 kJ/Nm3, density is 0.4–0.5 kg/Nm3, and kinematic viscosity is 25×10`(-6) m2/s. According to the flow diagrams of the coke oven itself and the drum cooling system, the raw gas coming out of the coke oven has already been condensed into a liquid state to a large extent before it reaches that point. At the same time, the coal dust and coke particles contained in the gas are captured, and the water-soluble components in the gas dissolve into ammonia. Tar, ammonia water, along with dust and tar residues, flow into the mechanical tar-ammonia separation tank. After separation, the ammonia water is reused; the tar is sent for centralized processing, while the tar residue can be reintroduced into the coal mixture used for coking. The coal gas enters a primary cooler where it is cooled directly or indirectly to room temperature, at which point any remaining moisture and tar in the gas are further removed. The gas exiting the primary cooler passes through a mechanical tar catcher, which removes the tar mist suspended in the gas by mechanical means; thereafter, it enters a blower where its pressure is increased to around 19,600 pascals (2,000 millimeters of water column). To avoid affecting subsequent gas purification operations, such as discoloration of ammonium sulfide and aging of the desulfurization solution, the gas is passed through an electrostatic tar catcher to remove residual tar mist. To prevent naphthalene from crystallizing out of the gas at low temperatures, a naphthalene washing tower is installed before the gas enters the desulfurization tower to absorb naphthalene using oil. Inside the desulfurization tower, a desulfurizing agent is used to absorb hydrogen sulfide from the gas; at the same time, hydrogen cyanide in the gas is also absorbed. The ammonia in the gas is absorbed by water or aqueous solutions in the ammonia absorption tower, resulting in liquid ammonia or ammonium sulfate. As the gas passes through the ammonia absorption tower, the reaction of sulfuric acid absorbing ammonia is exothermic, which causes the temperature of the gas to rise. To avoid affecting the operations related to the recovery of crude benzene, the gas is cooled in a final cooling tower before entering the benzene washing tower, where washing oil is used to absorb low-boiling hydrocarbons such as benzene, toluene, xylene, and cyclopentadiene, as well as high-boiling substances like styrene and naphthalene-gumma. At the same time, organic sulfides are also removed. 1 Basic Knowledge of Gas Safety 1.1 Properties of Gas 1.1.1 Components of Gas The gas referred to in common usage is artificial gas, which consists of various gas components and is a flammable mixture. Due to the differences in the raw materials used for gas production and the methods employed for its generation and recovery, the components of various types of gas and their respective percentages also vary. Common types include coke oven gas, producer gas, gas from continuous vertical carbonization furnaces, blast furnace gas, and converter gas. The components of common gases are shown in Table 1-1. 1.1.2 Physical and chemical properties of gas (1) Coke oven gas: The purified coke oven gas is a colorless, foul-smelling, toxic gas that is flammable and explosive. Its specific gravity is 0.3623, its calorific value ranges from 16,800 to 18,900 kJ/m3, its ignition temperature is 550–650°C, its explosion limit is 4.5%–35.8%, and its theoretical combustion temperature is around 2150°C. The CO content in coke oven gas is lower than that in blast furnace gas, but it can still cause poisoning incidents. (2) Blast furnace gas: Blast furnace gas is a colorless, odorless, toxic, flammable and explosive gas with a specific gravity of 0.9–1.1. Its calorific value ranges from 3349 to 4187 kJ/m3; the theoretical combustion temperature is around 1500°C, while the ignition temperature is about 730°C. The explosion limit lies between 30.8% and 89.5%. It contains nearly 70% nitrogen and carbon dioxide, and can cause shortness of breath (due to the low oxygen content) and suffocation. (3) Converter gas: The composition of converter gas varies at different stages during the melting cycle, and it is also related to the recovery equipment and the operating conditions during recovery. Blast furnace gas is a colorless, odorless, toxic, flammable and explosive gas with a calorific value of 6800–10000 kJ/m3, a ignition temperature of 530°C, and an explosion limit of 18.2%–83.2%. The theoretical combustion temperature of converter gas is higher than that of blast furnace gas.