Analysis of deacidification system for industrial sludge incineration flue gas treatment

Release time:

2022-08-09

Industrial sludge has high recovery value and significant ecological and economic benefits. However, the flue gas produced by sludge incineration contains a large number of pollutants and needs compre

1、 Deacidification process type

The flue gas generated after incineration of industrial sludge contains a certain amount of hydrogen chloride (HCl), sulfur oxide (SOx), nitrogen oxide (NOx) and hydrogen fluoride (HF). In order to meet the requirements of environmental protection, a deacidification system needs to be set. Acid gas purification process is divided into dry process, semi dry process and wet process. Each process has many combination forms and has its own advantages and disadvantages.

1. Dry deacidification

Dry deacidification is to inject lime powder or baking soda into the flue or dry deacidification tower to make it contact with acid gas and react. Some of the generated products are discharged through the flue or the bottom of the dry deacidification tower, and some are captured by the dust collector together with the dust, which is collected by the ash discharge and collection system before further treatment. The purified flue gas is discharged from the chimney to the atmosphere through the induced draft fan. Dry deacidification has the characteristics of convenient operation and low investment cost, but the dry process principle is the contact reaction between dry powder and flue gas, the reaction contact surface is small, and the amount of dry powder is relatively high.

2. Semi dry deacidification

The semi dry process is to inject a certain concentration of lime slurry into the absorption tower to react with acid gas to ensure that the lime slurry droplets are fully mixed with the flue gas. Part of the by-products generated after the reaction settle at the bottom of the absorption tower for discharge, while another part of the fine particles and dust are captured by the dust remover and collected for treatment. The purified flue gas is discharged into the atmosphere from the chimney through the induced draft fan. Semi dry process technology is mature and is widely used in waste incineration.

3. Wet deacidification

Limestone gypsum method and sodium alkali method are often used in wet deacidification in industrial waste treatment. The wet process has good removal effect on flue gas pollutants. The flue gas behind the furnace enters the wet deacidification tower after dust removal. The flue gas reacts with alkali liquor to remove flue gas pollutants. The purified flue gas is discharged from the chimney through induced draft fan. The waste water discharged from the deacidification tower can be discharged after treatment. There are many types of wet deacidification tower, and their common point is to increase the area and time of gas-liquid two-phase contact as much as possible to improve efficiency. The actual operation experience shows that the wet process has high removal efficiency of Acid Pollutants and can meet the operation requirements of various working conditions.

At present, the wet process is the best, followed by the dry process. The wet process has high removal rate of pollutants, but it will produce wastewater and the system is complex. The dry process has few secondary products and is easy to treat, but its acid gas removal rate is relatively low.

2、 Selection of deacidification process

The incoming materials of industrial sludge are complex and vary greatly. The flue gas generated after industrial sludge incineration has four characteristics. First, smoke carries more dust. Dust is mainly composed of metals, metal oxides or incomplete combustion substances. Second, the moisture content is large. The water content of industrial sludge is relatively high, so the moisture content of dusty flue gas is large. According to the volume ratio, the moisture content is between 30% - 40%. Third, it contains corrosive gas. The flue gas contains a certain amount of hydrogen chloride (HCl), sulfur oxide (SOx), nitrogen oxide (NOx) and hydrogen fluoride (HF). Fourth, the incoming materials are unstable, resulting in unstable flue gas composition and large changes in corrosive gas composition.

The selection of deacidification process needs to meet the following requirements: first, the water in the flue gas does not condense, otherwise the scale of water treatment will become larger; Second, the flue gas temperature at the inlet of the bag filter is high enough without pasting the bag; Third, deacidification by-products are easy to dispose; Fourth, the flue gas is discharged up to the standard. Ceramic tail gas purifier of industrial filter cartridge generator set with particle catcher with back blowing system

The efficiency of dry deacidification is not high, which is generally maintained at 30% - 60%. The semi dry process is mature and widely used in waste incineration. However, due to the unstable incoming sludge, great changes in flue gas composition and small amount of flue gas, people need to ensure the inlet temperature of bag filter, and the efficiency of semi dry deacidification system is difficult to ensure. Wet deacidification has high efficiency, meets the disposal requirements of large changes in flue gas composition, and the system is stable. It is more suitable for industrial sludge incineration flue gas treatment system. Therefore, this paper recommends the combined process of dry deacidification and wet deacidification for the treatment of industrial sludge incineration flue gas.

3、 Technical comparison between limestone gypsum method and sodium alkali method

1. Limestone gypsum method

After dust removal, the flue gas discharged from the boiler enters the deacidification device through the induced draft fan. The flue gas is in full contact with the gypsum slurry sprayed from the deacidification tower to produce chemical reaction, and the SO2 in the flue gas is removed. The purified flue gas is removed by mist eliminator at the top of deacidification tower, and then discharged into the atmosphere through chimney. Most of other harmful substances (such as fly ash, SO3, HCl and HF) have also been removed. The desulfurization by-product gypsum is extracted from the deacidification tower through the gypsum discharge pump and transported to the dehydration system for filtration and dehydration. After dehydration, the water content of gypsum is reduced to less than 10%.

A complete set of limestone gypsum wet full flue gas desulfurization device is composed of flue gas system, absorption and oxidation system, limestone slurry preparation system, gypsum dehydration system, process water system, slurry discharge and accident slurry system. SO2 removal principle of absorber is as follows:

2. Sodium alkali method

Sodium hydroxide is used as desulfurizer in the sodium alkali process. It is pumped into the deacidification tower to react with SO2 in the flue gas to produce sodium sulfite and sodium sulfate. The pH of the solution is adjusted by adding fresh desulfurizer to maintain a high desulfurization rate. The waste liquid is discharged into the wastewater system for treatment. Sodium alkali process has mature technology, high desulfurization rate and no blockage in the absorption process. It can effectively deacidify the flue gas without secondary pollution. A complete set of sodium alkali deacidification system is composed of flue gas system, absorption system, alkali preparation system and process water system. The main reactions involved are as follows:

3. Comparison of two processes

The comprehensive comparison results of limestone gypsum method and sodium alkali method are shown in Table 1. It can be seen from table 1 that limestone gypsum method has more gypsum dehydration system than sodium alkali method. Its system is complex and its investment cost is high, but its operation cost is low and the amount of wastewater is small.

4、 Operation economy analysis

1. Overview

The following takes an industrial sludge disposal project as an example to analyze and compare its operation economy. The composite process of "SNCR + quench tower + dry deacidification + activated carbon injection + bag filter + wet scrubber" is proposed to be selected for the project. The proposed process mainly includes five parts.

One is SNCR. Under the condition of high temperature (800-1000 ℃), NOx is reduced to N2 by reducing agent. The ammonia injection interface is set at the appropriate position of waste heat boiler, and the denitration efficiency is 45% - 60%. The second is quench tower. The flue gas temperature is rapidly cooled from 500 ℃ to less than 200 ℃, avoiding the secondary synthesis temperature range of dioxin and inhibiting the secondary formation of dioxin. Third, dry deacidification. In the flue gas removed, the sulfur content of some acidic substances is high, and the cost of baking soda is high. Hydrated lime powder is used as desulfurizer. Fourth, bag filter. It can remove smoke and dust, and the removal rate is usually more than 99.9%. Fifth, wet deacidification tower. Wet deacidification tower is an important means for flue gas emission up to standard. NaOH or limestone is used as deacidification agent in the deacidification process, which can ensure flue gas emission up to standard even if the concentration of SO2, HCl and other acid gases in flue gas is high.

2. Operating cost calculation description

Relevant cost data are calculated based on 100% load operation of the equipment; The annual utilization hours are determined as 7500h; Wastewater discharge shall comply with the pipe acceptance standard; Dry deacidification is considered as 30% deacidification efficiency; Only the operation cost of wet process is analyzed; The cost of sodium alkali method is calculated according to the benchmark of 30% sodium hydroxide solution.

The total operation cost of sodium alkali method is 985.84 yuan / h higher than that of limestone gypsum method. If there is a centralized wastewater treatment system in the plant area and the operation cost of wastewater treatment facilities is not considered, the total operation cost of the former is 535.84 yuan / h higher than that of the latter.

5、 Conclusion

The combined process of dry and wet deacidification is recommended for flue gas deacidification of sludge incineration project. If the sulfur content of industrial sludge is high, considering the operation cost, hydrated lime powder is recommended as the dry deacidification desulfurizer; For projects with small amount of flue gas and low acid content, sodium alkali deacidification is recommended for wet process, which is not easy to block the equipment and easy to operate and maintain; For projects with high acid content, limestone gypsum method is recommended for wet method, which has less waste water and low operation cost.

(part of this article is collected on the Internet, such as infringement contact deletion)

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