In 2015, the NOx emissions of Chinese steel companies reached 972,000 tons, accounting for 8% of the total NOx emissions. There are many iron and steel production processes and the main pollutants emitted by each process are many and different.
The sintering process is the process with the most serious emission of gaseous pollutants. Pollutants such as PM, SO2, and NOx emitted by this process account for 35%, 70%, and 50% of the total emissions of iron and steel enterprises, respectively.
Sintering flue gas in the iron and steel industry has the following characteristics:
①The amount of flue gas is large, 4 000-6 000m3 of flue gas is produced every 1 ton of sintered ore produced.
②The composition of the flue gas is complex, containing a variety of corrosive gases such as HCl, SO2, NOx and HF, heavy metals such as lead, zinc and mercury, toxic gases such as dioxins, and a large amount of dust (concentration up to 10 g/m3).
③ The concentration of SO2 is high and varies greatly. The concentration of SO2 in the flue gas is generally 1,000 to 1,500 mg/m3, and even up to 3,000 to 5,000 mg/m3.
④ The flue gas temperature is low and the fluctuation range is large. The sintering flue gas temperature is 120-180℃, even as low as 80℃ when using low-temperature sintering technology.
⑤ The moisture content and oxygen content are high, the moisture content is generally 7% to 13%, and the oxygen content is 15% to 18%.
At present, there are mainly three technical solutions applied to the treatment of ultra-low emission of sintering flue gas, namely SCR method, activated coke method and oxidation method.
What is SCR?
Selective Catalytic Reduction (SCR) refers to the use of a reducing agent (such as NH3) to “selectively” react with NOx in the flue gas under the action of a catalyst to generate non-toxic and non-polluting N2 and H2O .
Selectivity means that in the flue gas denitration process, the flue gas denitration catalyst selectively reduces NOx to nitrogen, while the SO2 in the flue gas is rarely oxidized to SO3.
Without adding a catalyst, the chemical reaction temperature of ammonia and nitrogen oxides is 900°C. If ammonia is added, part of the ammonia will decompose at high temperatures.
If a catalyst is added, the reaction temperature can be reduced to 320-400°C. The catalyst is generally a mixture of V2O5 and WoO3 with TiO2 as the matrix; the specific formula is determined according to the flue gas parameters.
1. SCR denitration reaction The SCR denitration system sprays ammonia or other suitable reducing agents into the flue gas upstream of the catalyst, and uses the catalyst to convert the NOX in the flue gas into nitrogen and water. In the usual design, liquid anhydrous ammonia or ammonia water (aqueous ammonia solution) is used. No matter what form of ammonia is used, the ammonia is first evaporated, then the ammonia is mixed with dilution air or flue gas, and finally the ammonia is injected into the grille. Spray into the flue gas upstream of the SCR reactor.
In the flue gas of most boilers, NO2 only accounts for a small part of the total NOX, so the influence of NO2 is not significant. The NOX removal efficiency of the SCR system is usually very high, and the ammonia injected into the flue gas almost completely reacts with the NOX. A small part of the ammonia does not react but escapes from the reactor as ammonia.
Generally speaking, for new catalysts, ammonia slip is very low. However, as the catalyst is deactivated or the surface is covered or blocked by fly ash, the amount of ammonia escape will increase. In order to maintain the required NOX removal rate, the NH3/NOX molar ratio in the reactor must be increased. When the pre-set performance standards for denitration efficiency and/or ammonia escape cannot be guaranteed, a new catalyst must be added or replaced in the reactor to restore the activity of the catalyst and the performance of the reactor. The period from when a new catalyst is used to when it is replaced is called catalyst life.
2. SCR system composition and reactor layout SCR reactor generally has three different installation positions in the boiler flue, namely hot section/high ash layout, hot section/low ash and cold section layout.
(1) Hot section/high ash arrangement: the reactor is arranged at a position where the temperature in front of the air preheater is about 350℃. At this time, all the fly ash and SO2 contained in the flue gas pass through the catalyst reactor, and the reactor works The condition is in the “dirty” high dust fume. Because the flue gas temperature of this arrangement is in the range of 300-400°C, which is suitable for the reaction temperature of most catalysts, it is widely used.
(2) Hot section/low ash arrangement: The reactor is arranged between the electrostatic precipitator and the air preheater. At this time, the flue gas with a temperature of 300 to 400°C first passes through the electrostatic precipitator and then enters the catalyst reactor. It can prevent the fly ash in the flue gas from polluting the catalyst and wearing or blocking the reactor, but the SO3 in the flue gas always exists. The biggest problem with this scheme is that the electrostatic precipitator cannot operate normally at a temperature of 300 to 400 ℃, so it is rarely used.
(3) Cold section arrangement: the reactor is arranged after the flue gas desulfurization device (FGD), so that the catalyst will completely work in the dust-free, SO2-free “clean” flue gas, because there is no clogging and clogging of the reactor by fly ash. There is no corrosion problem, and there is no problem of catalyst pollution and poisoning. Therefore, a highly active catalyst can be used, which reduces the volume of the reactor and makes the reactor layout compact. When the catalyst works in “clean” flue gas, its working life can reach 3 to 5 years (the working life in “unclean” flue gas is 2 to 3 years). The main problem with this arrangement is that when the reactor is placed in the wet FGD desulfurization unit, the exhaust gas temperature is only 50-60 ℃, therefore, in order to make the flue gas reach the required reaction temperature before entering the catalyst reactor , It is necessary to install fuel oil or natural gas burners in the flue, or steam-heated heat exchangers to heat the flue gas, thereby increasing energy consumption and operating costs. For general oil-fired or coal-fired boilers, the SCR reactor is mostly installed between the boiler economizer and the air preheater, because the flue gas temperature in this interval is just suitable for the SCR denitration reduction reaction, and ammonia is injected into the economizer and the air preheater. The proper position in the flue between SCR reactors makes it fully mixed with flue gas and reacts with nitrogen oxides in the reactor. The denitrification efficiency of the commercial operation of the SCR system is about 70% to 90%.
Post time: Aug-25-2021