Low Temperature SCR Cost Evaluation in Solid Waste Incineration Power Plant

To meet the increasingly strict emission control requirement, low temperature selective catalytic reduction (SCR) system is adopted in some waste incineration power plant because of the limited denitrification capability of selective non-catalytic reduction (SNCR) system in boiler furnace. The article evaluated the increased cost per ton of waste when SCR system is equipped on a 2×400t/d waste incineration power plant in Henan province. The total cost of SCR system, including construction period and operation period, per ton of solid waste is about 37 yuan.


Introduction
According to GB18485-2014, standard for pollution control on the solid waste incineration, the nitrogen oxides (NOx) control limits at hourly average emission is 300 mg/Nm 3 , which can be met by SCNR system. However, due to the environment and public pressure, some local government has issued more strict regulations to control pollution emission. In February 2020, the Office of the Leading Group for Pollution Prevention and Control in Henan Province issued the "Implementation Plan for the Pollution Prevention and Control Strategy in Henan Province in 2020" [1], requiring the emission concentrations of incinerator flue gas particulate matter, sulfur dioxide (SOx), and NOx (1 hour average) under the condition of standard oxygen content of 11%, not higher than 10, 35, 100mg/Nm 3 respectively. So the NOx emission limits should be below 100 mg/Nm³in Henan Province. Low temperature SCR system [2] is necessary for new-build solid waste incineration power plant. The increased cost per ton of waste is an important value for investors' decision making.

Technical Introduction of the Project
The designed waste treatment capacity of this project is 800t/d with the annual designed waste treatment capacity of 29.2×104t/a. 2×400t/d waste incinerator and 1×15MW medium temperature and medium pressure condensing steam turbine + 1×18MW generator set are equipped.
The flue gas purification system of this project adopts a combination of "SNCR + semi-dry FGD Absorber + Dry Deacidification + Activated Carbon Spray + Bag Filter + SCR" process ( Figure 1).  Figure 1. Flow diagram of flue gas purification process.

Design Input
The original emission of NOx in the incinerator (grate furnace) is around 350mg/Nm 3 and the actual SNCR system efficiency is around 30%~60% [2] due to different operation conditions. The theoretical SCR efficiency is up to 95% while in present operation experience the efficiency is around 85% [3].
Here we define the original NOx as 400mg/Nm³ and after SNCR process decreased to 200 mg/Nm 3 , which is the input NOx concentration of SCR system and the final emission control limit is 100mg/Nm 3 . Refer to Table 1 for basic input.

SCR system main equipment list
The SCR system consists of an SCR reactor, ammonia evaporation system, ammonia injection system, detection and control system, etc. Each incineration line is equipped with 1 SCR reactor. The SCR reactor adopts a fixed bed form where the low temperature catalyst (180~220℃) is placed in modules. The number of catalyst layers in the reactor depends on the required catalyst reaction surface area. The When SCR is in operation, it will consume the steam from steam drum and electricity as well as ammonia and catalyst. Table 3 shows the raw material and source consumption for SCR system.

Cost Evaluation
Considering that the SGH steam source is extracted from steam drum, the flow of super heated steam entering the steam turbine under the SCR operation mode is reduced, of which the power generation decreased by about 5.12×10 6 kWh by year. Besides, SCR system will increase the plant power consumption by 3.95×10 6 kWh annually.