Research and analysis of integrated technology for flue gas desulfurization and denitrification in thermal power plants

Taishan Boiler

The flue gas emitted from coal combustion contains sulfur oxides and nitrogen oxides. Among them, NO2, NO, and SO2 are the main components of air pollution and the key substances that form acid rain. At present, the integrated technology of flue gas desulfurization and denitrification has become a research and development hotspot in various countries to purify flue gas pollution of thermal power plants. However, large-scale promotion and application are currently restricted due to immature technology. Avoiding secondary pollution of by-products of flue gas treatment technology or considering the comprehensive utilization of by-products is an important aspect of improving the competitiveness of integrated desulfurization and denitrification technologies. It is recommended to strengthen the research of by-products to ensure that the developed desulfurization and denitration technologies can realize the recycling of resources and improve the economic and social benefits of the technology. Strengthen the integration of desulfurization and denitrification technologies with existing desulfurization devices. At present, most power plants are equipped with flue gas desulfurization devices, and consider making full use of existing devices and combining them with existing desulfurization technologies. Realize energy saving and emission reduction, reduce investment and operating costs. This article briefly describes the mechanism of desulfurization and denitrification, and researches and analyzes the integrated desulfurization and denitrification technology.

In the composition of my country's primary energy, coal accounts for as much as 70%. Among them, coal-fired power plants are the main places where my country consumes coal and emits nitrogen oxides and sulfur dioxide. Therefore, the control of nitrogen oxides and sulfur dioxide emitted by coal-fired power plants is the primary task in the field of air pollution control in our country today.

1. Desulfurization and denitration mechanism

Desulfurization and denitrification are the removal or reduction of NOx and SOX in the process of coal burning. How to economically and effectively control the SOX and NOx emissions in coal is an urgent need to solve in the field of energy saving and emission reduction in my country and even the world. important question. So far, the technologies that have achieved industrial application scale integration mainly include carbon-based material method, ozone oxidation method, electron beam irradiation method, pulse corona method, metal oxide catalysis method, etc. The method adopted by these technologies is to convert the sulfur and nitrogen in the gaseous pollutants into a more stable form, such as sulfate and nitrate, for industrial use through a series of chemical reactions.

2. Integrated desulfurization and denitration technology

1. Traditional integrated flue gas desulfurization and denitrification technology. The most widely used integrated desulfurization and denitrification technology at home and abroad is mainly the WET-FGD+SCR/SNCR combined technology, which is a combination of wet flue gas desulfurization and selective non-catalytic reduction or selective catalytic reduction. (WET-FGD: wet flue gas desulfurization; SNCR: selective non-catalytic reduction; SCR: selective catalytic reduction) Wet flue gas desulfurization generally uses lime desulfurization or limestone desulfurization, and its efficiency is above 90%, but its The disadvantage is that the scale of the project is large, the one-time investment and operating costs are relatively high, and it is easy to form secondary pollution.

2. Wet flue gas simultaneous desulfurization and denitration technology. The wet flue gas simultaneous desulfurization and denitrification process mainly uses additives to fully improve the solubility of NO and oxidize NO to NO2 in the gas/liquid section. Simultaneous wet desulfurization and denitrification technology is currently in the research stage, mainly including oxidation and wet complexation.

(1) Oxidation method. The chloric acid oxidation process uses a wet scrubbing system to simultaneously remove sulfur dioxide and nitrogen oxides in a set of equipment. This method uses a two-stage washing process of oxidation absorption tower and basic absorption tower, which can remove sulphur dioxide and nitrogen oxides while also having a good removal effect on toxic heavy metal elements, such as Se, Hg, Pb, Cd, Be and As and many more. Study the process of oxidizing SO2 and NOX to nitric acid and sulfuric acid under acidic conditions to make full use of hydrogen peroxide.

(2) Wet complex absorption process. The wet complex absorption process is to add a complexing agent to the solution, pass the flue gas containing NO and SO2 through the solution containing Fe(Ⅱ)EDTA chelate, and the NO in the coal-burning flue gas reacts with Fe(Ⅱ)EDTA to form Ferrous nitrosyl chelate improves the absorption rate of NO and increases its absorption capacity. The coordinated NO can react with dissolved SO2 and O2 to generate N2, N2O, dithionite, sulfate, various N-S compounds and Fe(Ⅲ) chelate. The process can simultaneously perform desulfurization and denitrification, but the method is still in the experimental stage. The obstacles in its industrial application are mainly the loss of chelate during the reaction, difficulty in regeneration, and low reuse rate. This causes the problem of high operating costs.

(3) WSA-SNQX process. The flue gas first passes through the SCR reactor. Under the action of the catalyst, NOx is reduced to N2 by ammonia, and then the flue gas enters the reformer. SO2 is catalytically oxidized to S03, which is condensed and hydrated into sulfuric acid in the cascade membrane condenser, and is further concentrated into Concentrated sulfuric acid sold. In addition to consuming ammonia, this technology does not consume other chemicals, does not produce second pollution such as waste water, has a high denitration rate and reliability, and has low operation and maintenance requirements. The disadvantage is that the investment cost is high, and the storage and transportation of the by-product concentrated sulfuric acid are difficult.

3. Integrated dry flue gas desulfurization and denitrification technology

(1) Solid adsorption/regeneration method. First, activated carbon fiber flue gas desulfurization technology. Activated carbon fiber flue gas desulfurization technology uses a new material of desulfurization activated carbon fiber catalyst to remove SO2 in the flue gas and recycle sulfur resources. A new type of desulfurization technology. According to relevant information, using this technology, the desulfurization efficiency can reach more than 95%. Because this technology has many advantages such as very simple processing technology, less equipment used and easy operation. Second, the NOXSO method. NOXSO technology is a dry adsorption regeneration technology that uses sodium salt supported on γ-A1203 spheres (φ1.6mm) as the adsorbent, which can simultaneously remove SO2 and NOx in the flue gas. The treatment process includes absorption and regeneration. And other steps. The specific operation process is: the flue gas after dust removal enters the absorber, where SO2 and NOx are removed by the adsorbent at the same time, and the purified flue gas is discharged into the chimney. After the adsorbent reaches a certain absorption saturation, it is moved to the regenerator for regeneration. Third, the CuO adsorption method. The CuO adsorption desulfurization and denitration process uses CuO/Si02 or CuO/A12O3 as adsorbents for desulfurization and denitration. The process can achieve a sulfur dioxide removal rate of more than 90% and a nitrogen oxide removal rate of 75% to 80% rate. This method requires relatively high reaction temperature, requires a heating device, and the manufacturing cost of the adsorbent is relatively high.

(2) High-energy radiation method. High-energy radiation method is divided into electron beam irradiation method and pulse corona plasma method. The electron beam method uses high-energy plasma generated by an electron accelerator to oxidize gaseous pollutants such as SO: and NO in the flue gas. After the SO: and NO in the flue gas are strongly oxidized by high-energy electrons, they react with water vapor to form mist-like sulfuric acid. It reacts with nitric acid and ammonia injected in advance to obtain ammonium sulfate and ammonium nitrate, and the net flue gas is discharged into the air through the chimney. The pulse corona law mainly uses high-voltage pulse power to generate electricity instead of the expensive accelerator electron beam, and the reaction mechanism is consistent with the electron beam method. The electron beam method has reached the level of industrialization. In a demonstration project of a thermal power plant, the desulfurization rate can reach about 90%, and the denitration rate can reach about 18%. During the operation, there is no discharge of waste water and residue, and no secondary pollution will be caused. The by-products can be used as raw materials for agricultural fertilizer processing, which has great comprehensive benefits. The disadvantage is that the energy consumption is high, and X-ray protection must be considered, which may cause pollution transfer in the actual project. In addition, the storage and transportation of liquid ammonia is also more difficult.

(3) Urea method. The urea purification flue gas process was jointly developed by the Mendeleev Institute of Chemical Technology and other units in Russia. It can simultaneously remove S02 and NOx. The removal rate of S02 is nearly 100%, and the removal rate of NOx is> 95%. The pH value of the absorption liquid used in this process is 5-9, which has no corrosive effect on the equipment; the removal rate of SO2 and NOx is not affected by the initial concentration of NOx and S02 in the flue gas; the exhaust gas can be directly discharged; the absorption liquid can be recycled after treatment Ammonium sulfate. However, the flue gas processing volume is too small to meet the requirements of industrial applications, and the process needs to be improved.

Concluding remarks:

Our country has carried out a variety of flue gas desulfurization and de-sales process experiments in the 1970s, and has made some achievements. However, traditional technologies are not very practical for our country. For our country, the main technological development direction should be high and new technology with low investment, low operating cost, high efficiency, and product resource utilization. Therefore, the industrialization and economics of this type of technology should be accelerated. Chemical research. Strongly support the research on the mechanism of the integrated process of desulfurization and denitrification. Through in-depth research and analysis of the mass transfer process and gas-liquid reaction process between the SO2 and NOx in the flue gas and the absorbent, it provides a theoretical basis for the industrial application of desulfurization and denitrification technologies.