Coke oven flue gas desulfurization and denitrification waste gas

When coke oven gas is burned to heat the coke oven in the coking industry, a large amount of atmospheric pollutants are generated, including sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter. These pollutants are continuously emitted into the atmosphere through the coke oven chimney as organized elevated point sources, causing serious pollution to the environment. Especially, harmful gases such as SO2 and NOx not only form acid rain and destroy the ozone layer, but are also the main gaseous substances of PM2.5, seriously endangering human health. In view of this, the country issued the "Emission Standards for Pollutants from Coking Chemical Industry" (GB16171-2012) in June 2012, which clearly stipulated the emission limits and special limits for pollutants in coke oven flue gas of existing coking enterprises after January 1, 2015. Some areas even put forward stricter requirements. Taking Linfen City as an example, the "Action Plan for Air Pollution Prevention and Control in Linfen City 2018" clearly requires that the coking industry implement the special emission limit transformation of air pollutants step by step. Before October 1, 2018, 50% of coking enterprises completed the special emission limit transformation of air pollutants, and before October 1, 2019, all coking enterprises in the city completed the special emission limit transformation of air pollutants.

1、 Process flow of desulfurization and denitrification of coke oven flue gas

The schematic diagram of the process flow for desulfurization, denitrification, and waste heat recovery is shown in the figure. The flue gas from coke ovens 2 # and 3 # is led out from the original underground flues and combined for denitrification pretreatment before entering the denitrification system. An ammonia spray grille is installed upstream of the denitrification reactor to fully mix ammonia gas into the flue gas. The flue gas mixed with ammonia gas enters the denitrification reactor and undergoes reduction reaction under the action of the catalyst to generate N2 and H2O. After denitrification, the flue gas continues to enter the heat pipe type waste heat boiler for heat recovery. The generated saturated low-pressure steam is transported to the company's thermal network for residential heating in winter, and the cooled flue gas enters the desulfurization system. The desulfurization system adopts a semi dry desulfurization method, and the desulfurized flue gas is pressurized and discharged to the original chimney through an induced draft fan after dust removal, achieving standard emission of flue gas.

1.1 Flue gas denitrification system

This system adopts medium and low temperature SCR denitrification technology, and NH3 is used as the reducing agent. The principle of its denitrification is that NOx is reduced by ammonia gas to harmless nitrogen and water under certain temperature conditions (medium and low temperature 230 ℃~300 ℃) under the action of a catalyst, without producing secondary pollution. The chemical reaction equations for SCR denitrification are shown in equations (1)~(5):

4NO+4NH3+O2-4N2+6H2O (Main Reaction) (1)

6NO2+8NH3——7N2+12H2O (2)

6NO+4NH3——5N2+6H2O (3)

NO+NO2+2NH3——2N2+3H2O (4)

2NO2+4NH3+O2——3N2+6H2O (5)

The ammonia gas from the liquid ammonia station and the air from the dilution fan are fully mixed in the ammonia/air mixer and enter the SCR denitrification reactor together with the coke oven flue gas. The mixed flue gas flows vertically downwards in the reactor, and the inlet of the reactor is equipped with an airflow distribution device and a rectification device to ensure a uniform airflow field of the mixed flue gas; The reactor is equipped with a dedicated medium to low temperature catalyst, with an active temperature of 230 ℃ to 300 ℃. The catalyst can meet the requirement of achieving a denitrification efficiency of over 87.5% when the maximum amount of flue gas is reached, while the conversion rate of SO2/SO3 is controlled within 1%. In addition, the catalyst adopts a "2+1" arrangement, which has high chemical stability, thermal stability, and mechanical stability, thus ensuring that the ammonia escape at the outlet of the SCR denitrification reactor is not greater than 10 × 10-6. The SCR denitration reactor is suitable for any load operation between 50% and 100% working conditions of the coke oven.

1.2 Waste Heat Recovery System

The waste heat boiler adopts a vertical layout, and the flue gas treated by the self denitrification system enters the boiler evaporator and economizer vertically before entering the subsequent desulfurization system. The deoxygenated water from the gas supply enters the economizer and is preheated before being sent to the drum. The steam and water inside the drum participate in the heat absorption cycle of the evaporator heat exchange surface through the rising and reflux pipelines, generating saturated steam with a pressure of 0.8 MPa. After gas-liquid separation, the saturated steam is output and sent to the steam pipe network. The drum, evaporator, and economizer are equipped with drainage outlets, which can regularly remove residual dirt and scale inside. There are two safety valves installed in the boiler system. When the system overpressure reaches 0.85MPa, the safety valves will automatically trip in sequence to release pressure and ensure the safety of the boiler system. When the system pressure returns to normal, the safety valves will return to their seats.

1.3 Desulfurization and dust removal system

The flue gas enters the desulfurization tower from the bottom and reacts with the recycled ash and added sodium carbonate solution. After removing SO2 and other acidic substances from the flue gas, the flue gas reaches the top of the desulfurization tower. The supplied sodium carbonate is sent to the sodium carbonate powder bin through a vacuum feeder, and the sodium carbonate powder is sent to the sodium carbonate solution tank through a star shaped discharge valve at the bottom of the powder bin. It is stirred with water in the solution tank to make a certain concentration of sodium carbonate solution. The sodium carbonate solution is pumped into the desulfurization reactor through a multi-stage centrifugal pump, and the amount of sodium carbonate solution entering the desulfurization tower is changed by adjusting the regulating valve on the solution delivery pipeline to achieve the best atomization effect. After the reaction, the flue gas leaves the top of the desulfurization tower in the form of a mixture and enters the bag filter. The gas and solid are separated in the bag filter. Most of the separated solids are sent back to the desulfurization tower for further desulfurization through the screw conveyor, and a small part is sent to the ash silo through the sorting valve at the outlet of the screw conveyor. After the material in the ash silo reaches a certain height, it is sent out by the bulk loader through the transport vehicle. The dust content in the flue gas at the outlet of the bag filter is reduced to<15mg/m3, and the flue gas after dust removal is sent into the original chimney through the induced draft fan. The exhaust temperature of purified flue gas is above 140 ℃, which will not produce chimney rain around the chimney and can avoid chimney corrosion caused by flue gas temperature below the acid dew point.

In the desulfurization tower, the sodium carbonate slurry rapidly absorbs SO2 upon contact with the flue gas, and has extremely high SO2 removal efficiency at low temperatures. Due to the fact that the sodium carbonate slurry sprayed into the tower is small droplets, the desulfurization product after completing the desulfurization reaction is also extremely fine particles, and it dries quickly while completing the reaction. The reaction equations for the conversion of sodium carbonate into sodium sulfite and sodium sulfate are shown in equations (6) to (7):

SO2+Na2CO3 →Na2SO3+CO2 (6)

2Na2SO3+O2 →2 Na2SO4 (7)

2、 Characteristics of desulfurization and denitrification technology for coke oven dust

(1) Directly utilizing the original temperature of coke oven flue gas for denitrification ensures that the denitrification temperature is within a relatively high temperature range, while eliminating the energy consumption generated by heating the flue gas. After passing through the SCR reactor, the temperature loss of the flue gas is 5 ℃ to 10 ℃, which does not affect the operation of the subsequent waste heat recovery system and meets the requirements of thermal energy recovery and utilization; (2) The waste heat recovery system can efficiently recover and utilize the sensible heat of coke oven exhaust gas, realizing the cascade utilization of heat according to temperature gradient, which meets the national requirements for environmental protection and energy conservation of enterprises; (3) The desulfurization system has high desulfurization efficiency.