Doing a good job of flue gas denitrification in the process of domestic waste incineration can not only avoid the occurrence of air and soil pollution, but also improve the effect of resource utilization of waste.

Introduction

Good waste treatment plays an important role in improving the quality of urban environment and enhancing the quality of life of urban residents. The flue gas denitrification link has a certain complexity and systemic nature, and the current research and application of flue gas purification system should be strengthened to improve the effect of flue gas denitrification and improve the image of the city with the help of acid removal system, bag filter system and related injection device, water spray system and electrical automatic control system.

Introduction and comparison of flue gas denitrification processes

The mature flue gas denitrification technologies for thermal power plants currently entering industrial application mainly include SCR, SNCR and combined SNCR/SCR technologies.

1 SNCR denitrification technology

SNCR is a selective non-catalytic reduction denitrification technology, that is, the furnace temperature in the region of 850 ~ 1100 ℃ injection of NH3, urea and other reducing agents, reducing agents quickly thermal decomposition into NH3 and NOx and selective reaction with NOx in the flue gas. NOx reduction to N2 and H2O, the method is based on the furnace or furnace reactor, the process is simple and easy to operate. Reaction principle.

The efficiency of the chemical reaction on which the reduction of NO depends depends on the NO flue gas temperature, residence time at high temperatures, including the type and amount of reducing agent injected, mixing efficiency and the amount of NOx in the ammonia compound.The SNCR method uses urea, CO(NH2)2 as reducing agent. It injects an aqueous solution of urea into the path of combustion products loaded with NOx. The thermal decomposition of urea produces chemicals which react with NOx to form nitrogen, carbon dioxide and water.

2 SCR flue gas denitrification technology

SCR reactor should adopt steel structure, and the steel structure design should conform to the requirements of GB 50017; the overall structure design of SCR reactor should fully consider the influence of deviation of flue gas flow at the first catalyst inlet, deviation of flue gas flow, deviation of flue gas temperature and deviation of NH3/NOx molar ratio, and should conform to the provisions of GB/T 21509-2008; the design flow of SCR reactor should meet the pressure drop The SCR reactor should be able to adapt to various operating conditions of biomass power plant boilers as well as load changes and start/stop requirements; the main body of SCR reactor can be supported or suspended. When using the support method, the temperature difference stress of the internal structure of SCR reactor and the horizontal thrust of the load-bearing steel frame caused by the thermal expansion of the support should be fully considered.

The inlet and outlet of SCR reactor should be equipped with compensator to absorb its own axial displacement, radial displacement, angular displacement and vibration; the catalyst in SCR reactor is generally equipped with one or two layers of initial catalyst, and the spare catalyst layer can be retained; SCR reactor should be equipped with sufficient size and number of access doors and necessary flue gas sampling ports; SCR reactor should be equipped with catalyst unit installation, The SCR reactor shall be equipped with lifting devices and platforms necessary for installation, maintenance and replacement of the catalyst unit.

3 SNCR/SCR Combination Technology

Combined SNCR/SCR technology refers to the installation of SNCR and SCR equipment in the flue gas process. In the SNCR section, liquid ammonia, etc. is injected as reducing agent to partially remove NOx from the SNCR unit; in the SCR section, the ammonia escaping through the SNCR process reduces NOx in the flue gas to N2 under the action of SCR catalyst H2O. the combined SNCR/SCR process system is complex, and the denitrification efficiency is usually only 50% to 70%.

4 Flue Gas Denitrification Technology

Advantages of SCR denitrification: suitable for large-scale NOx reductions of up to 90%; improves fuel economy of diesel engines; can be used in a variety of systems including industrial and municipal plants; can be used in conjunction with a diesel particulate filter (DPF) to reduce particulate emissions from diesel engines; the setup is easy to install. disadvantages of SCR denitrification: uninjected ammonia may leak from the SCR catalyst due to over-injection SCR catalyst (ammonia leakage); catalyst required for reduction is expensive to replace; catalyst is easily contaminated by compounds in the combustion gases.

Advantages of SNCR for NOx removal: the setup is easy to install; relatively low capital and operating costs compared to SCR; savings on expensive catalyst replacement; relatively easy retrofit of components in larger boiler units with little downtime.

Disadvantages of SNCR for NOx removal: requires a narrow temperature range for effective NOx reduction; not suitable for large scale NOx reduction; difficult to retrofit components in smaller boiler units due to space considerations for installation of components; prone to ammonia leakage, resulting in unintended NOx emissions.

Cost: Both SCR and SNCR technologies have relatively high operating and capital costs compared to simple methods of NOx removal, such as low NOx burners (LNBs) and Overfire Air (OFA), but they are highly favored for their efficiency. SCR is more expensive than SNCR due to catalyst specifications, but it makes up for it in terms of efficiency. Efficiency: Both SCR and SNCR technologies require the use of reagents to reduce NOx. however, SNCR requires higher flue gas temperatures when injecting reagents. the amount of NOx removed by SCR pollution control (~90%) is much greater than SNCR (30% to 70%), making it the best choice for large-scale reductions in industrial and municipal plants. Compliance with emission control regulations: While the initial investment to reduce SCR NOx is larger than SNCR, it is more cost-effective in the long run and compatible with the greater reductions expected in the future. SNCR is ideal for small-scale, short-term NOx reductions and may not ensure compliance with future regulations.

As China's environmental protection efforts continue, more and more power plants are installing denitrification units. Currently, the mainstream denitrification technologies in China include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and hybrid SNCR/SCR denitrification technologies. Each technology has its own advantages and disadvantages under the influence of many factors. As economic, technical, and environmental protection, some factors are qualitative and some are quantitative, so it has been a challenge to evaluate denitrification technologies scientifically and rationally. However, in China, scientific optimization of denitrification technology is not only beneficial to the promotion of denitrification devices, but also plays an important role in setting denitrification prices. Rational denitrification technology optimization plays a crucial role in denitrification price setting and power engineering construction. The comparison shows that the optimized combination of denitrification technologies has practical value. The results show that the selective catalytic reduction (SCR) method is superior to the hybrid SNCR/SCR denitrification method, while the hybrid SNCR/SCR denitrification method is superior to the selective non-catalytic reduction (SNCR) method. This is in line with the practical application of denitrification technology today.