Application advantages of baking soda dry deacidification combined with catalytic denitrification technology in flue gas purification of waste incineration power plant

1. Overview

At present, most domestic waste incineration power plants adopt the conventional flue gas purification process as follows:SNCR+ lime milk rotary spray semi-dry deacidification + slaked lime dry deacidification + activated carbon jet + bag dust collector + induced draft fan + chimney discharge.With the increasingly strict environmental protection requirements around the country, the emission standards for the waste incineration and power generation industry continue to improve, especially the NOx emission limit set by some provinces has been far lower than the European Union industrial emission directive（EU2010/75/EC），For example, the limit of nitrogen oxides in the Pollution Control Standard of Hainan Municipal Solid Waste Incineration (DB46/484-2019) is 120mg/m3, and the limit of nitrogen oxide emission concentration of waste incineration flue gas is below 80 ~ 120mg/m3.So more efficient, more applicable flue gas desulfurization and denitrification technology needs more and more prominent.

Comparison of emission standards of domestic waste incineration

2. SCR denitration traditional technology solution

2.1 Process Technology

Because the flue gas of waste incineration has the characteristics of high dust content and strong corrosion, the SCR denitration system must be set after the deacidification and dust removal system to ensure the normal operation of the catalyst.Catalyst is the core of SCR denitrification process, and the reaction temperature of catalyst is mainly divided into high temperature, medium temperature, medium low temperature and low temperature. Table 1 shows the main applications of catalysts at home and abroad:

At present, the domestic waste incineration and power generation industry mainly uses low-temperature catalyst. However, since the flue gas temperature after the rotary spray semi-dry deacidification and dust removal is below 140℃, and the appropriate temperature of catalyst reduction reaction in low-temperature SCR system is 160 ~ 180℃, it is necessary to consider adding the flue gas reheat system to heat the flue gas.

For the flue gas purification system with wet washing device, it is also necessary to set one or two GGH to recover part of the heat and reduce the heating steam consumption. The flue gas temperature needs to be raised is still around 30℃. Because the high pressure steam can be directly used for steam engine power generation, the value is very high, and the equipment and pipeline maintenance requirements are high, the heat loss is very large, so that the economic benefit level of the device is greatly reduced.

2.2 Existing Problems

(1) The flue gas temperature of rotating spray semi-dry deacidification and dust removal is below 140℃, while the optimal temperature of catalyst reduction reaction in low-temperature SCR system is 160 ~ 180℃,Therefore, it is necessary to increase the flue gas reheat system to heat the flue gas. If the flue gas temperature rises by 30℃, it needs to consume steam ~ 130kg/ ton of garbage. If this part of steam enters the steam turbine to generate electricity, according to the steam consumption of 5kg/ (KW·h), this part can generate 26KW·h/t of garbage.As a result of a large amount of steam consumption, resulting in a large increase in operating costs.

(2) The precision of rotary spray semi-dry deacidification and desulfurization is not high, and the efficiency of SO2 removal is generally about 85%. Although it can stably meet the requirements of the national standard, it is very unfavorable if the residual SO2 is oxidized into SO3 in the SCR catalyst bed.Because SO3 reacts with water and NH3 in the flue gas, ammonium sulfate and ammonium bisulfate will be generated. These sulfates, especially ammonium bisulfate, are a kind of sticky substance, which will cause corrosion, blockage, poisoning of the catalyst and shorten the service life of the catalyst.

3. Baking soda dry deacidification technology

3.1 Process Technology

This technology uses about 70 mesh special baking soda for desulfurization, and then grinds it into d90=30μm super fine soda powder,With air as the medium, it is sprayed into the deacidification tower by means of pneumatic conveying and mixed with high temperature flue gas of 180℃ ~ 210℃. At high temperature, sodium bicarbonate is decomposed into sodium carbonate Na2CO3, H2O and CO2,Due to the “popcorn” effect during decomposition, the surface area of the particles increases by more than 4 times, and they become extremely loose and porous highly active particles, which react quickly with various acidic gases such as SO2 and HCl in the flue gas，The removal rate can reach 95 ~ 99%, the reaction products are removed by the bag filter, the purified flue gas can reach: dust content ≤5mg/m3, SO2≤10mg/m3, HCl≤5mg/m3, temperature ≥170℃, and then enter the SCR denitration system.

3.2 Technical application advantages

(1) After using baking soda dry deacidification, the flue gas temperature entering SCR denitration system should not be lower than 170℃, which can meet the optimal temperature of catalyst reduction reaction in low-temperature SCR system and does not need additional steam to heat it up.

(2) Baking soda can not only remove SO2, HCl, HF, HBr and other substances, but also effectively remove the existing SO3 in the flue gas, reducing the content of SO3 into the SCR denitrification system, reducing the production of ammonium sulfate and ammonium bisulfate, ensuring the life of the catalyst and reducing the cost.

(3) Baking soda also has a certain synergistic denitrification effect. Sodium sulfite produced by the reaction of baking soda and SO2 has strong reducibility and can partially REDOX reaction with nitrogen oxides. The synergistic denitrification efficiency of baking soda is about 10 ~ 20%.

(4) With the mainstream configuration of rotary spray semi-dry deacidification, the efficiency of SO2 and HCl removal can reach 85% ~ 95%, which can stably meet the current national standard requirements.When the emission concentration is further reduced, the pursuit of higher deacidification efficiency often brings about a series of technical and economic problems, such as the increase of desulfurizer consumption, the increase of fly ash production, and the risk of clotting bags.At this time, the efficiency of SO2 and HCl removal can reach 95% ~ 99% by using baking soda dry deacidification with higher activity, and the best overall technical and economic results can be obtained.

4. Closing Remarks

Along with the continuous strengthening of our environmental management work, it is believed that in the near future, as itself is the green environmental protection project of household garbage incineration power generation industry, flue gas emission standard will be more strict, baking soda dry acid removal and SCR denitration system will be gradually promoted application, technical and economic advantages will be further developed.