Comparison of limestone-gypsum and semi-dry sinter flue gas desulfurization technologies

2020-06-24

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Sintering flue gas is the exhaust gas produced during the high temperature sintering and forming process after the ignition of the sintering mix and running with the cart, and the sintering flue gas contains a certain concentration of SO2. sintering process is one of the main sources of pollution in the iron and steel industry, and a good solution to SO2 emission reduction in the sintering process is to seize the focus of SO2 emission reduction in the iron and steel industry. Iron and steel enterprises need to combine their own characteristics to choose the most suitable desulfurization technology from the effectiveness, stability and economy of the operation of desulfurization devices.

1 Characteristics of sintering machine flue gas

The main characteristics of sintering flue gas are: (1) large flue gas emissions, about 4000~6000m3 of flue gas per ton of sintered ore; (2) large fluctuations in flue gas temperature, with the sintering working conditions, fluctuations in the range of 90~150 ℃; (3) flue gas The moisture content is large, generally around 10% by volume; (4) Due to the relationship between the sintering raw material sulphur content, causing a large change in SO2 concentration in the emission flue gas with the change of the batching ratio, generally 1000-3000mg/Nm3; (5) Sintering flue gas contains high oxygen content, about 10% to 15%; (6) The dust contains iron and its compounds, due to the use of different raw materials may also contain trace heavy metals. (6) The dust contains iron and its compounds, and may also contain traces of heavy metal elements due to the use of different raw materials. (7) Unstable, because of the fluctuation of sintering conditions, the flue gas volume, flue gas temperature and SO2 concentration often change, and the randomness is strong.

2 sintering machine flue gas desulfurization technology status

In view of the characteristics and features of sintering flue gas, the control method of SO2 emission in the sintering process is mainly to reduce the sulfur content of raw materials, reduce the consumption of raw materials and the treatment of sintering flue gas. Flue gas desulfurization technology is a common method to control S02 emission in domestic and foreign steel enterprises.

Flue gas desulfurization technology is mainly divided into semi-dry and wet flue gas desulfurization. Semi-dry FGD technology mainly includes spray rotary dry absorption process (SDA), circulating fluidized bed flue gas desulfurization process (CFB), etc.; wet FGD technology mainly includes: limestone - gypsum wet process, ammonia FGD process, magnesium oxide wet process, etc. At present, the most widely used wet method in domestic sintered flue gas desulfurization technology is limestone/gypsum method; the most widely used semi-dry method is circulating fluidized bed method (CFB).

2.1 Limestone-gypsum wet desulfurization process

Limestone-gypsum method is to absorb SO2 in flue gas with limestone slurry, which reacts to produce calcium sulfite and further oxidizes to calcium sulfate dihydrate (gypsum) in the absorption tower slurry tank, and the oxidized gypsum slurry is concentrated and dehydrated to produce gypsum with water content less than 10%, which is sold as commodity.

Limestone-gypsum FGD technology is one of the most maturely developed and widely used FGD technologies worldwide, with FGD efficiency reaching over 95%. After decades of research and optimization, the original technical problems of fouling, clogging and wear have been successfully solved. The limestone-gypsum method is usually used by large power stations and is now also widely used in flue gas desulfurization of sintered flue gas and industrial boilers/kilns.

2.2 Circulating fluidized bed method (CFB) semi-dry desulfurization process

Circulating fluidized bed flue gas desulfurization process is based on the principle of circulating fluidized bed, which forms a fluidized bed of flue gas with high solids content through the internal circulation of materials in the reaction tower and the high rate of external circulation, thus strengthening the heat and mass transfer performance between the particles of desulfurization absorbent and between the gases such as SO2, SO3, HCl, HF and desulfurization absorbent in the flue gas, while lowering the operating temperature to 15～20℃ above the dew point and improving the SO2 and desulfurization absorber reaction efficiency, the utilization rate of the absorber. Desulfurization agent is generally used lime or slaked lime, in the case of calcium-sulfur ratio of 1.3 ~ 1.5, the desulfurization efficiency can reach 80 ~ 90%, desulfurization by-products are mainly calcium sulfite desulfurization ash.

3 Comparison of limestone - gypsum method and circulating semi-dry desulfurization technology

3.1 Effectiveness

From the analysis of the effectiveness of flue gas desulfurization technology, the main considerations are desulfurization efficiency and site adaptability.

3.1.1 Desulfurization efficiency

Limestone - gypsum wet desulfurization process desulfurization rate of up to 95% or more, desulfurization of flue gas is not only very low sulfur dioxide concentration, and the amount of dust in the flue gas is also greatly reduced . It is beneficial to the region and sintering plant to implement total control.

Circulating fluidized bed method semi-dry desulfurization process desulfurization efficiency is generally in the range of 80 to 90%, to achieve more than 90% desulfurization efficiency needs to correspond to a very high calcium-sulfur ratio (Ca / S at least above 1.5, much higher than the wet method of 1.03).

3.1.2 Site adaptability

For the renovation project, site adaptability is an important consideration. The limestone-gypsum wet FGD technology system is complex and covers a larger area compared to the circulating fluidized bed semi-dry FGD with slaked lime as the FGD agent. If the semi-dry FGD with lime as FGD agent is adopted, the lime digestion plant required for FGD agent preparation needs to be increased, and the floor space is comparable to that of wet FGD.

3.2 Stability

To ensure the safe and stable operation of the flue gas desulfurization system, and not to affect the operation of the original sintering machine is the principle that must be considered in the selection of desulfurization technology.

Limestone-gypsum FGD technology is currently the most mature and reliable FGD technology, and the commissioning rate of FGD plant can reach more than 98%, and has good adaptability to changes in flue gas flow, temperature and SO2 concentration caused by fluctuations in sintering working conditions.

Circulating fluidized bed semi-dry desulfurization technology needs to spray water in the fluidized bed to create the best reaction conditions for the desulfurization reaction, but from the operation of the sintering machine, the sintering flue gas volume, temperature and SO2 concentration have been fluctuating in a wide range, making it difficult to accurately adjust the amount of water sprayed in the fluidized bed, too little water, the reaction efficiency is reduced; too much water will lead to material bonding in the fluidized bed and export flue gas with water, and cause Subsequent dust collector corrosion, blockage, affecting the normal operation of the system. At the same time, the fluctuation of flue gas volume will also affect the fluidization state in the desulfurization tower to change, affecting the desulfurization efficiency and stable operation of the system.

3.3 Economical

For the economy, the main consideration is investment cost, operation cost and by-product treatment and utilization.

3.3.1 Investment cost

At present, the limestone - gypsum method has generally eliminated the flue gas reheat system (GGH), and in order to prevent the adverse effects of wet flue gas corrosion on the chimney after desulfurization, the measure of desulfurization tower + direct exhaust chimney or anti-corrosion treatment of the original chimney is adopted. At the same time, the main equipment has been localized, which greatly reduces the investment cost of wet desulfurization. The overall investment cost of limestone - gypsum method is not much different from that of circulating fluidized bed semi-dry desulfurization.

3.3.2 Operating costs

(1)Desulfurization agent

Limestone - gypsum method desulfurization agent for limestone, raw materials are easy to obtain, and the price is cheap. The desulfurization agent of circulating fluidized bed semi-dry method is lime or slaked lime, if lime is used as desulfurization agent, it needs to be reacted by dry digester to generate slaked lime before use. Only high-quality lime activity (T60 ≤ 4min, after adding water temperature rise of 60 ℃ not more than 4 minutes) to meet the requirements of circulating fluidized bed semi-dry method of flue gas desulfurization on the use, so the semi-dry method of desulfurization agent quality requirements are significantly higher than the wet method.

Wet desulfurization in Ca / S is generally around 1.03, while the semi-dry desulfurization to achieve 90% of the desulfurization efficiency Ca / S to be greater than 1.5, coupled with the unit price of lime is also higher than limestone, resulting in high costs of semi-dry desulfurization agent consumption.

(2) Electricity consumption

The concentration of SO2 in the flue gas of the sintering machine is not high, and it is not necessary to take a high liquid to gas ratio in the value of the largest indicator of the wet desulfurization electricity consumption. For sinter flue gas desulfurization, without considering the booster fan, limestone - gypsum method of desulfurization system operating electricity consumption is higher than the semi-dry method of desulfurization (semi-dry method is about 60% of the wet method). But the semi-dry FGD to meet the requirements of dust emissions, FGD must be configured after the bag dust collector, plus the resistance of the tower itself and the resistance of the flue, the total system resistance loss in more than 3800 Pa, much higher than the wet FGD 1500 ~ 1800 Pa. Wet FGD plus the front dust removal system energy consumption, the total energy consumption is not much different from the semi-dry FGD system.

(3) Water consumption

The water consumption of limestone-gypsum FGD system includes evaporated water, crystallized water, free water in gypsum and waste water, etc.; the water consumption of semi-dry FGD system is mainly used for lime digestion and flue gas humidification to improve the reaction rate. In general, the water consumption of the semi-dry FGD system is 60-70% of that of the wet FGD 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 in limestone-gypsum FGD is gypsum, which is of high quality and can be used as a cement retarder and raw material for gypsum sheets, and the benefits generated from the sale of gypsum can offset most of the cost of the FGD agent limestone. The main components of FGD ash in circulating fluidized bed semi-dry method are calcium sulfite, fly ash, calcium sulfate, slaked lime powder which is not completely finished reacting, etc. There is no reliable method for large-scale utilization, and the economic value is lower than gypsum, which is far from offsetting the cost of FGD agent lime/slaked lime.

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