Technical Scheme for Development of Gas Turbine SCR Treatment System — Design of Urea Injection System and Control System (Chapter 1)

This document is the technical development scheme for the gas turbine SCR denitration aftertreatment system, and it shall guide the subsequent detailed design, equipment procurement and control system development.

1. Overview The customer requires SCR denitration aftertreatment for gas turbine flue gas to meet the following emission targets: reduce NOₓ from 25 ppm to 2 ppm (denitration efficiency of 92%), reduce CO from 25 ppm to 2 ppm, and limit NH₃ slip ≤ 3 ppm.

Ammonia injection was adopted in the original proposal. However, considering safety, storage and transportation cost, system complexity and operation & maintenance convenience comprehensively, this project recommends switching to 32.5% urea aqueous solution (AdBlue) for SCR reducing agent supply.

The gas turbine parameters provided by the customer are relatively limited at present, including only exhaust parameters and partial reactor outline dimensions. Key data such as catalyst layer position, NOₓ conversion reference basis and inlet O₂ content are missing. Therefore, this scheme carries out feasibility analysis and conceptual design first, and detailed calculation and verification will be performed after the supplementary data are available.

2. Feasibility Analysis 2.1 Summary of Original Design Parameters 2.1.1 Reactor Geometry (Decomposed according to drawings provided by the manufacturer)

Structural Judgment: This equipment is a reducer transition flue duct, not the main body of the SCR reactor. One end is a DN2000 cylindrical nozzle connected to the gas turbine exhaust, and the other end is a large rectangular cross-section connected to the main body of the SCR reactor or the waste heat boiler. The focus of the modification work is on the DN2000 inlet side.

2.1.2 Flue Gas Operating Conditions

Note: All 5 operating cases are under full-load operation. Case 1 is selected as the design point (worst-case condition: maximum flue gas flow and minimum ambient temperature).

2.1.3 Emission Requirements

Note: The conversion reference basis for NOₓ and CO (dry basis at 15% O₂ or others) shall be confirmed by the manufacturer. This scheme is calculated directly based on the given ppm values for the time being.

2.2 Feasibility Analysis 2.2.1 Temperature Condition Is Fully Satisfied

Conclusion: The flue gas temperature of 510–529 ℃ falls within the optimal temperature window for the SCR + urea solution, fully satisfying the requirements for thermal decomposition, reaction and catalyst safety.

2.2.2 Inlet Flow Velocity Calculation (based on Case 1, the worst-case condition) Inlet cross-sectional area: \(A_\text{in} = \pi/4 \times 2.0077^2 \approx 3.165\ \text{m}^2\) Standard volumetric flow rate: \(Q_\text{N} = 31.6 \times 10^4 / 3600 \approx 87.78\ \text{Nm}^3/\text{s}\) Operating volumetric flow rate (510 ℃, \(\rho \approx 0.485\ \text{kg/m}^3\)): \(Q \approx 87.78 \times (273 + 510)/273 \approx 251.9\ \text{m}^3/\text{s}\) Actual flue gas velocity: \(v = Q / A_\text{in} \approx 251.9 / 3.165 \approx 11.8\ \text{m/s}\)

Conclusion: The inlet flow velocity is approximately 12 m/s, which is within the typical velocity range of SCR flue ducts (8–15 m/s). The flow field is reasonable and the pressure drop is controllable.

2.2.3 Cold Zone Effect Calculation (based on Case 1)

Conclusion: The overall temperature drop is less than 2 ℃, which is far below the allowable temperature fluctuation of the catalyst. The local cold zone temperature drop is controlled within 15 ℃ and can be further reduced by optimizing the atomization and layout of the injectors.

2.2.4 Pressure Drop Assessment

Conclusion: The total system pressure drop is approximately 1.2–2.4 kPa, which falls within the typical allowable range of 3–5 kPa for gas turbine SCR aftertreatment. The pressure drop design is reasonable.

2.3 Overall Conclusion Based on the analysis of temperature, flow velocity, cold zone effect and pressure drop, the urea-injection SCR scheme adopted for this project is fully engineering-feasible. The technical challenge lies in the injector arrangement, atomization and mixing under large cross-section and high flow rate conditions. Reasonable AIG array design and CFD verification are required to guarantee the uniformity of NH₃/NOₓ mixing.

3.Technical Scheme 3.1 Nozzle Type Selection

3.1 Nozzle Type Selection

Recommended: Adopt air-assist nozzles for atomization with 4–6 bar compressed air, controlling the SMD at approximately 30 μm. This solution features fine atomization, stable operation and convenient maintenance, making it the first choice for large-cross-section SCR systems.

3.2 Key Design Parameters of the Spray Nozzle

3.3 Lance Layout Scheme 3.3.1 Layout Principles For the large cross-section of DN2000, a single lance cannot cover the entire flow channel, so an AIG (Ammonia Injection Grid) array must be adopted. The recommended layout is as follows:

3.3.2 Side View of AIG Lance Array

Figure 1 Side View of AIG Lance Array (Z-X Plane, 30° Co-current Inclined Insertion)

3.3.3 Cross-section View of AIG Lance Array

Figure 2 Cross-section View of AIG Lance Array (DN2000 Pipe, Plum Blossom Layout with 1 Central + 5 Outer-ring Lances)

3.3.4 CFD Simulation of Lance Mixing

CFD simulation is adopted to verify the lance atomization, urea pyrolysis and NH₃/NOₓ mixing process, so as to ensure the uniformity of NH₃/NOₓ molar ratio distribution at the catalyst inlet cross-section (CV ≤ 5%). Key simulation focuses:

3.4 Estimation of Atomizing Compressed Air Consumption

Compressed air quality requirement: The air must be oil-free and water-free. It is recommended to use an adsorption dryer to reduce the dew point below -40 °C to prevent nozzle blockage by crystal deposition.

3.5 Injection Flow Control Scheme

It is recommended to adopt 6 sets of independent metering systems (one set for each lance), which facilitates single-lance calibration, fault isolation and on-line maintenance.

4. Design Target Estimation (Based on the most unfavorable Case 1)

Note: The above values are conceptual design estimates. The final design shall be verified by the seven-in-one SCR calculation tool with complete supplementary data provided by the supplier.

5.3 Data to be Supplied by the Supplier

The data currently provided by the supplier is insufficient for detailed design. The following information shall be supplemented in accordance with the Data Sheet Provided by the Gas Turbine SCR Control System Supplier: