Comparison of Limestone Gypsum Method and Semi dry Sintering Machine Smoke Desulfurization Technology
Publishdate：2018-03-21 Views：20
Beixing Energy Conservation and Environmental Protection Network News: Sintering flue gas is the waste gas generated during the high-temperature sintering process of the sintering mixture after ignition, along with the operation of the trolley. The sintering flue gas contains a certain concentration of SO2. The sintering process is one of the main sources of pollution in the steel industry. Solving the SO2 reduction in the sintering process is to focus on the SO2 reduction work in the steel industry. Steel enterprises need to choose appropriate desulfurization technologies based on their own characteristics, including the effectiveness, stability, and economy of desulfurization device operation.
1. Characteristics of sintering machine flue gas
Sintering raw materials such as iron ore and fuel (such as coal powder) both contain certain elemental sulfur or sulfides, which oxidize to form SO2 during the sintering process. The main characteristics of sintering flue gas are: (1) large flue gas emissions, producing approximately 4000-6000m3 of flue gas per ton of sintered ore; (2) The fluctuation range of flue gas temperature is relatively large, ranging from 90 to 150 ℃ with changes in sintering conditions; (3) The moisture content of flue gas is high, and when calculated by volume ratio, the moisture content is generally around 10%; (4) Due to the relationship between the sulfur content of sintering raw materials, the concentration of SO2 in the discharged flue gas changes significantly with the change of ingredient ratio, generally ranging from 1000 to 3000mg/Nm3; (5) The oxygen content in sintering flue gas is high, accounting for about 10% to 15%; (6) Dust contains iron and its compounds, and may also contain trace heavy metal elements due to the use of different raw materials. (7) Instability, due to fluctuations in sintering conditions, the amount of flue gas, flue gas temperature, and SO2 concentration often change, resulting in strong randomness.
2. Current status of sintering machine flue gas desulfurization technology
Based on the characteristics and characteristics of sintering flue gas, the control methods for SO2 emissions during the sintering process mainly include reducing the sulfur content of raw materials, reducing the consumption of raw materials, and treating sintering flue gas. Smoke desulfurization technology is a commonly used method for controlling S02 emissions in domestic and foreign steel enterprises.
The flue gas desulfurization technology is mainly divided into semi dry and wet flue gas desulfurization. Semi dry flue gas desulfurization technology mainly includes spray rotary drying absorption process (SDA), circulating fluidized bed flue gas desulfurization process (CFB), etc; Wet flue gas desulfurization technology mainly includes limestone gypsum wet process, ammonia flue gas desulfurization process, magnesium oxide wet process, etc. At present, the most widely used wet flue gas desulfurization technology in China is the limestone/gypsum method; The widely used semi dry method is the circulating fluidized bed method (CFB).
2.1 Limestone gypsum wet desulfurization process
The limestone gypsum method uses limestone slurry to absorb SO2 in flue gas, react to generate calcium sulfite, which is further oxidized to calcium sulfate dihydrate (gypsum) in the absorption tower slurry tank. After oxidation, the gypsum slurry is concentrated and dehydrated to produce gypsum with a water content of less than 10%, which is sold as a commodity.
Limestone gypsum flue gas desulfurization technology is a mature and widely used flue gas desulfurization technology worldwide, with a desulfurization efficiency of over 95%. After decades of research and development, the existing technical problems such as scaling, blockage, and wear have been successfully solved. The limestone gypsum method is usually used in large power plants and is currently widely used in flue gas desulfurization of sintering flue gas and industrial boilers/kilns.
2.2 Circulating fluidized bed (CFB) semi dry desulfurization process
The circulating fluidized bed flue gas desulfurization process is based on the principle of circulating fluidized bed, which forms a high solid content flue gas fluidized bed through the internal circulation of materials in the reaction tower and high rate external circulation. This enhances the heat and mass transfer performance between desulfurization absorbent particles, as well as between gases such as SO2, SO3, HCl, HF in the flue gas and desulfurization absorbent. At the same time, the operating temperature is reduced to 15-20 ℃ above the dew point, improving the reaction efficiency between SO2 and desulfurization absorbent and the utilization rate of absorbent. Desulfurizers usually use lime or hydrated lime. When the calcium sulfur ratio is 1.3-1.5, the desulfurization efficiency can reach 80-90%, and the by-product of desulfurization is mainly calcium sulfite desulfurization ash.
3. Comparison of Limestone Gypsum Method and Circulating Semi dry Desulfurization Technology
3.1 Validity
From the effectiveness analysis of flue gas desulfurization technology, the main considerations are desulfurization efficiency and site adaptability.
3.1.1 Desulfurization efficiency
The desulfurization rate of limestone gypsum wet desulfurization process can reach over 95%. The flue gas after desulfurization not only has a low concentration of sulfur dioxide, but also greatly reduces the dust content of the flue gas, Beneficial for implementing total quantity control in regions and sintering plants.
The desulfurization efficiency of the circulating fluidized bed semi dry desulfurization process is generally between 80-90%. To achieve a desulfurization efficiency of over 90%, a high calcium sulfur ratio (Ca/S at least 1.5 or higher, much higher than the wet process's 1.03) is required.
3.1.2 Adaptability of the site
For renovation projects, the adaptability of the site is an important consideration. The limestone gypsum wet desulfurization technology system is complex and occupies a larger area compared to the circulating fluidized bed semi dry desulfurization using hydrated lime as the desulfurizer. If using lime as a desulfurizer for semi dry desulfurization, it is necessary to increase the lime digestion plant required for desulfurizer preparation, which occupies an area equivalent to that of wet desulfurization.
3.2 Stability
Ensuring the safe and stable operation of the flue gas desulfurization system without affecting the operation of the original sintering machine is a principle that must be considered in the selection of desulfurization technology.
Limestone gypsum flue gas desulfurization technology is currently a mature and reliable flue gas desulfurization technology. The operation rate of the desulfurization device can reach over 98%, and it has good adaptability to changes in flue gas flow rate, temperature, and SO2 concentration caused by fluctuations in sintering conditions.
The semi dry desulfurization technology of circulating fluidized bed requires water spraying in the fluidized bed to create optimal reaction conditions for desulfurization reaction. However, from the operation of the sintering machine, the amount of sintering flue gas, temperature, and SO2 concentration fluctuate within a large range, making it difficult to accurately adjust the water spraying amount in the fluidized bed. If the water amount is too small, the reaction efficiency will decrease; Excessive water flow can cause material adhesion in the fluidized bed and water entrainment in the outlet flue gas, leading to corrosion and blockage of the subsequent dust collector, affecting the normal operation of the system. At the same time, fluctuations in flue gas volume can also affect the fluidized state inside the desulfurization tower, affecting desulfurization efficiency and stable system operation.
3.3 Economy
For economy, the main considerations are investment costs, operating costs, and the treatment and utilization of by-products.
3.3.1 Investment expenses
At present, the limestone gypsum method has generally eliminated the flue gas reheating system (GGH). In order to prevent the adverse effects of wet flue gas after desulfurization on chimney corrosion, measures such as desulfurization tower+straight exhaust chimney or anti-corrosion treatment of the original chimney are adopted. At the same time, the main equipment has been domestically produced, which greatly reduces the investment cost of wet desulfurization. The overall investment cost of limestone gypsum method is not significantly different from that of circulating fluidized bed semi dry desulfurization.
3.3.2 Operating costs
(1) Desulfurizer
The limestone gypsum desulfurization agent is limestone, which is easy to obtain and inexpensive. The desulfurizer for the semi dry method of circulating fluidized bed is lime or hydrated lime. If lime is used as the desulfurizer, it needs to be reacted by a dry digester to generate hydrated lime before use. Only high-quality lime activity (T60 ≤ 4 minutes, with a temperature increase of 60 ℃ for no more than 4 minutes after adding water) can meet the requirements of semi dry flue gas desulfurization in circulating fluidized beds. Therefore, the quality requirements for desulfurizers in semi dry methods are much higher than those in wet methods.
In wet desulfurization, the Ca/S ratio is generally around 1.03, while in semi dry desulfurization, the Ca/S ratio must be greater than 1.5 to achieve a desulfurization efficiency of 90%. In addition, the unit price of lime is also higher than that of limestone, resulting in high consumption costs of semi dry desulfurization agents.
(2) Electricity consumption
The concentration of SO2 in the flue gas of the sintering machine is not high, and there is no need to take a high liquid to gas ratio in the value of the indicator that affects the power consumption of wet desulfurization. For the flue gas desulfurization of sintering machines, without considering the booster fan, the power consumption of the limestone gypsum desulfurization system during operation is higher than that of the semi dry method (which is about 60% of the wet method). However, in order to meet the requirements of dust emission in semi dry desulfurization, a bag filter must be installed after desulfurization. In addition, the resistance of the tower itself and the resistance of the flue can cause a total resistance loss of over 3800Pa in the system, which is much higher than the 1500-1800Pa in wet desulfurization. The total energy consumption of wet flue gas desulfurization combined with pre installed dust removal system is not significantly different from that of semi dry flue gas desulfurization system.
(3) Water consumption
The water consumption of limestone gypsum desulfurization system includes evaporated water, crystalline water, free water in gypsum, and wastewater; The water consumption of the semi dry flue gas desulfurization system is mainly used for lime digestion and flue gas humidification to improve reaction rate. In general, the water consumption of a semi dry desulfurization system is 60-70% of that of a wet desulfurization system, but the proportion of water consumption in the overall operating cost is not high.
3.3.3 Treatment of by-products
The by-product of limestone gypsum flue gas desulfurization is gypsum, which has high quality and can be used as a cement retarder and raw material for gypsum boards. The benefits of selling gypsum can offset most of the cost of desulfurization agent limestone. The main components of desulfurization ash in the semi dry method of circulating fluidized bed are calcium sulfite, fly ash, calcium sulfate, and partially reacted hydrated lime powder. There is no reliable method for large-scale utilization, and its economic value is lower than that of gypsum, which cannot offset the cost of desulfurization agent lime/hydrated lime.
Beixing Energy Conservation and Environmental Protection Network News: Sintering flue gas is the waste gas generated during the high-temperature sintering process of the sintering mixture after ignition, along with the operation of the trolley. The sintering flue gas contains a certain concentration of SO2. The sintering process is one of the main sources of pollution in the steel industry. Solving the SO2 reduction in the sintering process is to focus on the SO2 reduction work in the steel industry. Steel enterprises need to choose appropriate desulfurization technologies based on their own characteristics, including the effectiveness, stability, and economy of desulfurization device operation.
1. Characteristics of sintering machine flue gas
Sintering raw materials such as iron ore and fuel (such as coal powder) both contain certain elemental sulfur or sulfides, which oxidize to form SO2 during the sintering process. The main characteristics of sintering flue gas are: (1) large flue gas emissions, producing approximately 4000-6000m3 of flue gas per ton of sintered ore; (2) The fluctuation range of flue gas temperature is relatively large, ranging from 90 to 150 ℃ with changes in sintering conditions; (3) The moisture content of flue gas is high, and when calculated by volume ratio, the moisture content is generally around 10%; (4) Due to the relationship between the sulfur content of sintering raw materials, the concentration of SO2 in the discharged flue gas changes significantly with the change of ingredient ratio, generally ranging from 1000 to 3000mg/Nm3; (5) The oxygen content in sintering flue gas is high, accounting for about 10% to 15%; (6) Dust contains iron and its compounds, and may also contain trace heavy metal elements due to the use of different raw materials. (7) Instability, due to fluctuations in sintering conditions, the amount of flue gas, flue gas temperature, and SO2 concentration often change, resulting in strong randomness.
2. Current status of sintering machine flue gas desulfurization technology
Based on the characteristics and characteristics of sintering flue gas, the control methods for SO2 emissions during the sintering process mainly include reducing the sulfur content of raw materials, reducing the consumption of raw materials, and treating sintering flue gas. Smoke desulfurization technology is a commonly used method for controlling S02 emissions in domestic and foreign steel enterprises.
The flue gas desulfurization technology is mainly divided into semi dry and wet flue gas desulfurization. Semi dry flue gas desulfurization technology mainly includes spray rotary drying absorption process (SDA), circulating fluidized bed flue gas desulfurization process (CFB), etc; Wet flue gas desulfurization technology mainly includes limestone gypsum wet process, ammonia flue gas desulfurization process, magnesium oxide wet process, etc. At present, the most widely used wet flue gas desulfurization technology in China is the limestone/gypsum method; The widely used semi dry method is the circulating fluidized bed method (CFB).
2.1 Limestone gypsum wet desulfurization process
The limestone gypsum method uses limestone slurry to absorb SO2 in flue gas, react to generate calcium sulfite, which is further oxidized to calcium sulfate dihydrate (gypsum) in the absorption tower slurry tank. After oxidation, the gypsum slurry is concentrated and dehydrated to produce gypsum with a water content of less than 10%, which is sold as a commodity.
Limestone gypsum flue gas desulfurization technology is a mature and widely used flue gas desulfurization technology worldwide, with a desulfurization efficiency of over 95%. After decades of research and development, the existing technical problems such as scaling, blockage, and wear have been successfully solved. The limestone gypsum method is usually used in large power plants and is currently widely used in flue gas desulfurization of sintering flue gas and industrial boilers/kilns.
2.2 Circulating fluidized bed (CFB) semi dry desulfurization process
The circulating fluidized bed flue gas desulfurization process is based on the principle of circulating fluidized bed, which forms a high solid content flue gas fluidized bed through the internal circulation of materials in the reaction tower and high rate external circulation. This enhances the heat and mass transfer performance between desulfurization absorbent particles, as well as between gases such as SO2, SO3, HCl, HF in the flue gas and desulfurization absorbent. At the same time, the operating temperature is reduced to 15-20 ℃ above the dew point, improving the reaction efficiency between SO2 and desulfurization absorbent and the utilization rate of absorbent. Desulfurizers usually use lime or hydrated lime. When the calcium sulfur ratio is 1.3-1.5, the desulfurization efficiency can reach 80-90%, and the by-product of desulfurization is mainly calcium sulfite desulfurization ash.
3. Comparison of Limestone Gypsum Method and Circulating Semi dry Desulfurization Technology
3.1 Validity
From the effectiveness analysis of flue gas desulfurization technology, the main considerations are desulfurization efficiency and site adaptability.
3.1.1 Desulfurization efficiency
The desulfurization rate of limestone gypsum wet desulfurization process can reach over 95%. The flue gas after desulfurization not only has a low concentration of sulfur dioxide, but also greatly reduces the dust content of the flue gas, Beneficial for implementing total quantity control in regions and sintering plants.
The desulfurization efficiency of the circulating fluidized bed semi dry desulfurization process is generally between 80-90%. To achieve a desulfurization efficiency of over 90%, a high calcium sulfur ratio (Ca/S at least 1.5 or higher, much higher than the wet process's 1.03) is required.
3.1.2 Adaptability of the site
For renovation projects, the adaptability of the site is an important consideration. The limestone gypsum wet desulfurization technology system is complex and occupies a larger area compared to the circulating fluidized bed semi dry desulfurization using hydrated lime as the desulfurizer. If using lime as a desulfurizer for semi dry desulfurization, it is necessary to increase the lime digestion plant required for desulfurizer preparation, which occupies an area equivalent to that of wet desulfurization.
3.2 Stability
Ensuring the safe and stable operation of the flue gas desulfurization system without affecting the operation of the original sintering machine is a principle that must be considered in the selection of desulfurization technology.
Limestone gypsum flue gas desulfurization technology is currently a mature and reliable flue gas desulfurization technology. The operation rate of the desulfurization device can reach over 98%, and it has good adaptability to changes in flue gas flow rate, temperature, and SO2 concentration caused by fluctuations in sintering conditions.
The semi dry desulfurization technology of circulating fluidized bed requires water spraying in the fluidized bed to create optimal reaction conditions for desulfurization reaction. However, from the operation of the sintering machine, the amount of sintering flue gas, temperature, and SO2 concentration fluctuate within a large range, making it difficult to accurately adjust the water spraying amount in the fluidized bed. If the water amount is too small, the reaction efficiency will decrease; Excessive water flow can cause material adhesion in the fluidized bed and water entrainment in the outlet flue gas, leading to corrosion and blockage of the subsequent dust collector, affecting the normal operation of the system. At the same time, fluctuations in flue gas volume can also affect the fluidized state inside the desulfurization tower, affecting desulfurization efficiency and stable system operation.
3.3 Economy
For economy, the main considerations are investment costs, operating costs, and the treatment and utilization of by-products.
3.3.1 Investment expenses
At present, the limestone gypsum method has generally eliminated the flue gas reheating system (GGH). In order to prevent the adverse effects of wet flue gas after desulfurization on chimney corrosion, measures such as desulfurization tower+straight exhaust chimney or anti-corrosion treatment of the original chimney are adopted. At the same time, the main equipment has been domestically produced, which greatly reduces the investment cost of wet desulfurization. The overall investment cost of limestone gypsum method is not significantly different from that of circulating fluidized bed semi dry desulfurization.
3.3.2 Operating costs
(1) Desulfurizer
The limestone gypsum desulfurization agent is limestone, which is easy to obtain and inexpensive. The desulfurizer for the semi dry method of circulating fluidized bed is lime or hydrated lime. If lime is used as the desulfurizer, it needs to be reacted by a dry digester to generate hydrated lime before use. Only high-quality lime activity (T60 ≤ 4 minutes, with a temperature increase of 60 ℃ for no more than 4 minutes after adding water) can meet the requirements of semi dry flue gas desulfurization in circulating fluidized beds. Therefore, the quality requirements for desulfurizers in semi dry methods are much higher than those in wet methods.
In wet desulfurization, the Ca/S ratio is generally around 1.03, while in semi dry desulfurization, the Ca/S ratio must be greater than 1.5 to achieve a desulfurization efficiency of 90%. In addition, the unit price of lime is also higher than that of limestone, resulting in high consumption costs of semi dry desulfurization agents.
(2) Electricity consumption
The concentration of SO2 in the flue gas of the sintering machine is not high, and there is no need to take a high liquid to gas ratio in the value of the indicator that affects the power consumption of wet desulfurization. For the flue gas desulfurization of sintering machines, without considering the booster fan, the power consumption of the limestone gypsum desulfurization system during operation is higher than that of the semi dry method (which is about 60% of the wet method). However, in order to meet the requirements of dust emission in semi dry desulfurization, a bag filter must be installed after desulfurization. In addition, the resistance of the tower itself and the resistance of the flue can cause a total resistance loss of over 3800Pa in the system, which is much higher than the 1500-1800Pa in wet desulfurization. The total energy consumption of wet flue gas desulfurization combined with pre installed dust removal system is not significantly different from that of semi dry flue gas desulfurization system.
(3) Water consumption
The water consumption of limestone gypsum desulfurization system includes evaporated water, crystalline water, free water in gypsum, and wastewater; The water consumption of the semi dry flue gas desulfurization system is mainly used for lime digestion and flue gas humidification to improve reaction rate. In general, the water consumption of a semi dry desulfurization system is 60-70% of that of a wet desulfurization system, but the proportion of water consumption in the overall operating cost is not high.
3.3.3 Treatment of by-products
The by-product of limestone gypsum flue gas desulfurization is gypsum, which has high quality and can be used as a cement retarder and raw material for gypsum boards. The benefits of selling gypsum can offset most of the cost of desulfurization agent limestone. The main components of desulfurization ash in the semi dry method of circulating fluidized bed are calcium sulfite, fly ash, calcium sulfate, and partially reacted hydrated lime powder. There is no reliable method for large-scale utilization, and its economic value is lower than that of gypsum, which cannot offset the cost of desulfurization agent lime/hydrated lime.
