Integrated denitration and desulfurization process scheme using ozone + hydrogen peroxide combination

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This post was last edited by yuchenchf on 2017-8-4 20:32. Determination of denitration technology routes 2.1 Mechanism of NOx formation. The nitrogen oxides generated during the combustion process in typical combustion equipment include NO, NO2, N2O, etc.; among these, NO accounts for over 90%, NO2 accounts for 5-10%, and N2O accounts for only about 1%. Therefore, the NOx produced during combustion refer mainly to NO and NO2. In the oxidation and reduction reactions of nitrogen-containing substances, based on the main pathways and sources of NOx formation, they can be classified into thermally generated NOx, rapidly generated NOx, and fuel-generated NOx (see Figure 1). Figure 3-1 Reaction pathways for NOX formation and removal (1) Thermochemical NOx. Thermochemical NOx refers primarily to NOx generated by the oxidation of nitrogen in the air involved in the combustion process; its formation is a non-branched chain reaction. The formation mechanism of thermally generated NOx was proposed in 1946 by the Soviet scientist Zeldovich. The overall reaction equations are as follows: N2 + O2 → 2NO (1) NO + ½O2 → NO2 (2). Rapid-type NOx is formed when, based on experimental results regarding the axial distribution of NO in premixed hydrocarbon flames, hydrocarbon radicals (CHi) collide with N2 molecules in the air during combustion, resulting in intermediate products such as HCN, NH, CN, and N; these intermediate products are then further oxidized to form NOx. Although the nitrogen in rapid-type NOx also comes from nitrogen in the air, its formation mechanism is different from that of thermal-type NOx; the main pathways for its formation are shown in the figure below. The generation of rapid-type NOx has little dependence on temperature, whereas the excess air coefficient has a significant impact on rapid-type NOx. During the combustion process, the amount of rapidly formed NOx is very small, and it is generally not considered as a key factor in w NOx control. (3) Fuel-type NOx: Fuel-type NOx refers to NOx generated when nitrogen compounds in the fuel undergo thermal decomposition followed by oxidation during combustion. Its main generation path is shown in the figure below. Since the N-H and N-C bonds are much shorter than the N≡N bond, the formation of fuel-type NOx is much easier than that of thermally generated NOx, making it the primary source of NOx formation. 2.2 Comparative Analysis of Existing Main Denitration Technologies
The economic comparison of the existing main denitration technologies is shown in the table below:

| Technology Name | SCR | SNCR | Ozonation Method |
|-----------------|-----|-----|------------------|
| Reducing Agent | NH3 mainly | Ammonia water or urea solution | O3 |
| Reaction Temperature | 300–400°C | 850–1100°C | 50–200°C |
| Reactor Required? | Yes | No | No |
| Denitration Efficiency | 80–95% | 15–50% | 70–95% |
| Catalyst Required? | Yes, needs regular replacement; expensive | No | No |
| Location of Reducing Agent Injection | Usually between the economizer and air preheater | Furnace or at the furnace outlet | None |
| SO2/SO3 Conversion | Yes | No | No |
| NH3 Emission | 3–5 ppm | 10–15 ppm | None |
| Impact on Combustion Equipment | NH3 and SO3 can form NH4HSO4, causing blockages or corrosion | Almost no impact | No impact |
| System Pressure Loss | Around 1000 Pa | None | None |
| Need for Soot Blowing? | Yes | No | No |
| Impact on Fuel | High ash content can wear out the catalyst; alkali metal oxides can deactivate the catalyst (catalyst poisoning) | None | None |
| Impact on Combustion Equipment Efficiency | Reduces thermal efficiency | None | None |
| Impact of Coal Tar | Coal tar can cause catalyst blockages and cover the surface active components of the catalyst, leading to catalyst failure | None | None |
| Footprint | Large | Small | Small |
| Investment Cost | High | Low | Moderate |
| Operating Costs | High | Low | Moderate |

2.3 Determination of the Denitration Solution for This Project
The combined oxidation method is an extremely suitable denitration approach for this project. Firstly, the temperature range for ozone oxidation is 50–200°C, which fully meets the requirements of this project. Modifications can be carried out on the smoke duct at the rear of the boiler; the scope of these modifications is small, and they have no impact on the boiler. Secondly, the denitration efficiency of the combined oxidation method reaches 70-95%, fully meeting the design requirements of this project. Denitration using the combined oxidation method results in low pressure resistance, as well as lower investment and operating costs. In summary, the denitration technology route selected for this project is the combined oxidation denitration method. 3. Denitration Scheme Design 3.1 Design Principles The main design principles for this project are as follows: (1) Adhering to the principle of not affecting the operation of the boiler, this project adopts a denitration process that involves hydrogen peroxide + ozone oxidation, along with absorption in a wet desulfurization tower. (2) An ozone dosing unit is installed at the outlet of the bag filter, and hydrogen peroxide is sprayed in the flue at the inlet of the absorption tower for oxidation, in order to keep NOx emissions at ≤40 mg/Nm3. (3) Ozone systems use oxygen as the feed gas, and ozone production equipment converts oxygen into ozone. (4) The layout should be carried out as much as possible according to the current condition of equipment and site conditions, striving to ensure that the process flow and facility arrangement are reasonable and aesthetically pleasing, that operations are safe and easy, and that there is minimal impact on existing equipment and facilities. (5) Ensure the safe, stable, reliable, and compliant operation of the denitration system. (6) Minimize the renovation period while ensuring safety and quality. (7) The treatment of by-products should meet the long-term requirements of environmental protection, striving to avoid secondary pollution from these by-products; the process design should minimize the impact of noise on the environment. (8) The level of automation of the equipment is reasonably designed, facilitating operation and management. (9) Engineering modifications should strive to conserve energy and water, thereby reducing the investment and operating costs of the system. 3.2 Principles and Characteristics of the Combined Oxidation Denitration Process 3.2.1 Reaction Principle The basic principle of selective oxidation denitration technology is as follows: In the combined oxidation method for denitration, the strong oxidizing properties of ozone and hydrogen peroxide are utilized to oxidize insoluble nitrogen oxides in lower oxidation states into soluble nitrogen oxides in higher oxidation states; these nitrogen oxides are then absorbed in a scrubber tower, thereby achieving denitration. Our company conducted research on the oxidation mechanism of NO during the simultaneous desulfurization and denitrification process using ozone. Comprehensive simulation experiments were carried out regarding the injection of ozone into flue gas, gas distribution methods, gas-phase mixing techniques, the effects of temperature control, and the impact of dust. As a result, we summarized and established a detailed chemical reaction mechanism between O3 and NOX; this mechanism is quite complex. In actual experiments, research can be conducted on the key reactions between ozone and NO under low-temperature conditions. Under low-temperature conditions, the key oxidation reactions between O3 and NO are as follows: NO + O3 → NO2 + O2 (1), NO2 + O3 → NO3 + O2 (2), NO3 + NO2 → N2O5 (3), NO + O + M → NO2 + M (4), NO2 + O → NO3 (5). The main reaction principles for the oxidation using activated hydrogen peroxide are as follows: NO + H2O2 → NO2 + H2O (6), NO2 + H2O2 → NO3 + O2 (7), NO2 + NO3 → N2O5 (8). The main reaction principles for denitrification are as follows: NO + NO2 + H2O → 2H+ + 2NO2- (9), 2NO2 + H2O → 2H+ + NO2- + NO3- (10), N2O5 + H2O → 2H+ + 2NO3- (11), NO3- + NO → NO2- + NO2 (12), 2H+ + CO32- → H2O + CO2 (13), H+ + OH- → H2O (14). Compared with other chemicals in the gas phase such as CO and SOx, NOx can be oxidized very quickly, which gives oxidation of NOx a high degree of selectivity. Since the NOx in the gas phase is converted into ionic compounds soluble in aqueous solutions, this leads to a more complete oxidation reaction, thereby removing NOx irreversibly without causing secondary pollution. Through the oxidation reaction, the added ozone and hydrogen peroxide are consumed in the reaction, and any excess ozone and hydrogen peroxide can be decomposed in the spray tower. In addition to NOx, some heavy metals, such as mercury and other heavy metal pollutants, are also oxidized by ozone. High concentrations of dust or solid particulates in the flue gas do not affect the NOx removal efficiency. Combined oxidation-denitrification can be applied to: utility boilers fueled by coal, coke, and lignite ； Industrial boilers fueled by gas, coal, and heavy oil ； Furnaces used in the processing and production of lead, iron ore, zinc/copper, glass, and cement ； Burners used for treating biological waste, tires, and other industrial waste ； Acidic gas streams from pickling and chemical processes ； Catalytic cracking off-gas ； Various municipal and industrial waste incinerators, etc. 3.2.2 Process Characteristics This technology combines gas-phase oxidation, liquid-phase oxidation, and wet absorption in an integrated manner, and is designed and modified based on the existing wet desulfurization process to create an integrated desulfurization and denitrification unit. (1) Deep denitrification with high efficiency; the denitrification rate can reach over 95% ； The equipment design is forward-looking; a space has been reserved for adding an ozone reactor. Should it become necessary to meet stricter emission standards, simply installing an ozone generator next to the existing equipment will suffice to achieve the required removal rate, without the need for any destructive modifications to the original structure. This significantly reduces the user’s equipment investment costs. (2) No catalyst is used, eliminating issues such as catalyst poisoning and reactor clogging. (3) The temperature of the flue gas used for denitration is low; efficient denitration can be achieved within the range of 50–200°C. (4) Low maintenance costs, with no issues such as regular catalyst replacement. (5) It occupies a small area, and modular equipment can be arranged flexibly according to on-site conditions. (6) The ozone denitration patented technology fills the gap in domestic low-temperature oxidation denitration technologies, reaching an internationally advanced level. (7) The use of a single-tower structure featuring ozone gas-phase oxidation and alkali absorption has changed the complicated setup involving two separate systems for desulfurization and denitrification both domestically and internationally. (8) The entire desulfurization and denitrification process takes place after the dust collector, without affecting the normal operation of the boiler or the dust collector. This prevents the dust collector from getting clogged, thereby extending its service life and ensuring the stable operation of the entire system. (9) To address the issues of unstable operating conditions in industrial boilers, a variety of complex fuels, and unstable levels of nitrogen oxides in the flue gas, a linked control module is installed to dynamically regulate the amount of ozone injected, thereby ensuring that the sulfur and nitrogen oxide levels in the emitted flue gas remain within acceptable limits, thus achieving energy savings and safety. (10) It eliminates issues such as ammonia escape and urea-related odors associated with traditional denitrification processes; suitable for industrial boilers located near residential areas. (11) Suitable for new denitrification projects where desulfurization equipment is already in place; it requires little floor space, involves minimal retrofitting work, and has a short construction period. 3.3 Description of the process flow
Flue gas treatment process: The flue gas exiting from the dust collector enters the ozone mixing and reaction unit, where ozone is injected. This allows for thorough mixing of ozone with the flue gas. As a result, water-insoluble NO in the flue gas is oxidized into highly oxidized nitrogen oxides that are soluble in water, such as NO2, N2O3, N2O5, etc. The reaction is completed in a very short period of time. Subsequently, in the flue gas duct at the front section of the absorption tower, an atomized spray of hydrogen peroxide solution that has been catalytically activated is used to oxidize the residual NO in the flue gas into water-soluble nitrogen oxides in higher oxidation states. Then, the flue gas enters the absorption tower, where an alkaline solution is sprayed to simultaneously absorb SOx and oxidized NOx from the flue gas, ensuring that the NOx emission concentration remains ≤40 mg/Nm³. Finally, the compliant flue gas is discharged through the chimney. The process flow is shown in Figure 4-1. The denitration system of this project mainly includes: an ozone oxidation system, a hydrogen peroxide oxidation system, a wet absorption system, an electrical control system, etc. The following sections will provide a detailed explanation of each of these components. 4. Ozone oxidation system 4.1 Introduction to ozone generators (1) Principle of ozone generation process The core of an ozone generator utilizes advanced dielectric barrier dual-gap discharge technology; the raw material gas flow passes through narrow gaps between the insulating medium and the high-voltage electrodes, as well as between the insulating medium layer and the grounding electrode of the ozone generator tank. High-voltage electric fields exist in these two annular gaps, enabling discharge on both sides and thus converting the passing oxygen into ozone, resulting in high ozone production efficiency. Industrially, it is generally produced using the corona discharge method, and its principle is shown in the figure below: (2) Basic introduction to ozone generators. An ozone system integration consists of an ozone generation system, a control system, a cooling water system, and detection instruments. Within the high-frequency high-voltage electric field in the ozone generation chamber, partial oxygen (with a purity of ≥99%) is converted into ozone using micro-gap dielectric barrier discharge technology; the resulting gas, which is an ozonated gas, is discharged through the outlet of the ozone pipeline following the gas outlet control valve. An ozone sampling port is provided on the exhaust pipeline of the ozone generation chamber, equipped with a sampling valve, and the ozone outlet concentration is measured using an ozone concentration meter. The oxygen inlet pressure for the ozone generator is 0.095 Mpa; it is necessary to maintain stable inlet pressure and flow rates at the site in order to ensure a consistent ozone production level ； To ensure that the air supply meets the requirements of the ozone generator, an oxygen filter (with a filtration precision of ≤0.01μ) is installed on the air inlet pipeline to further purify the incoming air ； Pressure sensors and temperature sensors are installed on the air intake pipeline to monitor and display the oxygen pressure and temperature in real time; pressure switches and safety valves are also provided to automatically release pressure when the pressure in the ozone generator becomes too high, and to cut off the gas supply if necessary, thereby ensuring the safe operation of the ozone generator ； At the same time, a pressure gauge is installed in the intake pipeline to display the intake pressure on-site. An ozone control valve is installed on the exhaust pipeline to automatically adjust the ozone concentration and output, ensuring that the output of the ozone generator meets the actual requirements. (3) Technical features of ozone generators: The most important component of an ozone generator is the ozone discharge tube. The equipment is made from high-quality 316L stainless steel resistant to ozone corrosion, as well as PTFE (polytetrafluoroethylene), which enhances the long-term reliable operation of the system. A 10% margin is reserved for the number of discharge tubes during design, to compensate for the efficiency loss caused by unforeseen contamination of the discharge tubes. The ozone generator is installed horizontally; it can be placed directly on a base, which facilitates installation and maintenance. Before leaving the factory, the ozone generator has its pipes, valves, instruments, and cables installed, and the entire system undergoes all necessary technical tests in the factory. The ozone generator is designed to operate continuously for 24 hours. 5. The ozone denitration equipment that makes use of hydrogen peroxide systems has high costs both in terms of installation and operation. To reduce these costs, our company has developed a combined oxidation denitration system that utilizes ozone along with hydrogen peroxide. Activated hydrogen peroxide is used to further oxidize NO, significantly reducing equipment investment and operating costs. The hydrogen peroxide system mainly consists of a hydrogen peroxide storage tank, a hydrogen peroxide activation tank, a buffer tank, a metering pump, a spraying system, etc. To ensure effective mixing of hydrogen peroxide with flue gas, we used high-efficiency atomization nozzles to spray the hydrogen peroxide solution, thereby facilitating a rapid mixing and denitration reaction. The hydrogen peroxide dosing unit is located in the section after the induced draft fan and before the absorption liquid dosing; the catalytically activated hydrogen peroxide oxidizes the remaining NO in the flue gas to NO2, which is then absorbed through spraying in the absorption tower. Serial Number Name Unit Quantity Main Parameters 1 Hydrogen peroxide storage tank 1 100 m3 2 Hydrogen peroxide activation tank 1 13 m3 3 Hydrogen peroxide dosing pump 1 150 m3/h × 20 m, 5.5 Kw 4 Hydrogen peroxide transfer pump 1 23.2 m3/h × 12.5 m, 0.75 Kw 5 Jet pump 1 25 m3/h × 20 m, 1.5 Kw 6 Hydrogen peroxide spraying pump 1 211.7 m3/h × 44 m, 5.5 Kw 7 Hydrogen peroxide spraying system set 1 Note: The table above is a preliminary list; it may undergo slight changes as the project progresses. 6. Automatic Control System 6.1 Control Range and Requirements This system is designed for flue gas denitration in 240 t/h coal powder boilers. The control scope mainly includes the following: the control system for combined oxidation and absorption, as well as the monitoring instrument system. The system features fully automatic control and multiple levels of safety protection. It can carry out coordinated control based on the concentration of pollutants in the exhaust gas, and adjusts the amount of ozone injected automatically to ensure that the NOx concentration in the exhaust gas meets the specified standards. At the same time, it takes into account the actual conditions at the site as well as those related to operation and maintenance, thereby making the operation more reliable, safer, and easier to maintain. The entire system can achieve an intelligent automatic control state, operate smoothly, and boasts extremely high reliability. It indicates that the ozone generator for denitrification uses liquid oxygen as its gas source, resulting in low maintenance costs for the denitrification equipment and no need for catalyst replacement. The equipment maintenance cost is 50,000 yuan per year in the first five years, and it does not exceed 100,000 yuan per year after five years. If ammonia-based denitration methods such as SCR are used, the catalyst lifespan does not exceed 24,000 hours, requiring regular replacement every three years, and the replacement cost is very high.