Numerical Simulation of Elbow Influence on Coal Powder Concentration-Dilution Separator Performance

Literature Overview

This paper, published in the Proceedings of the CSEE in 2003 (Vol. 23, Issue 1, pp. 132-135) by Zhou Hao, Cen Kefa, Fan Jianren, Chi Zuohuo, and Jiang Xiao, investigates the influence of elbows on the performance of coal powder concentration-dilution separators (also known as coal powder air mixture separators or swirl separators) in coal-fired boilers. The research was conducted at the Key Laboratory of Clean Energy Utilization and Environmental Engineering, Ministry of Education. The study was supported by the National Basic Research Program of China (G1999022204).

Technical Background

Coal powder concentration-dilution combustion technology is widely used in coal-fired boilers for its advantages in low-load stable combustion, reduced pollutant emissions, and improved combustion efficiency. The concentration-dilution separator is a critical component that separates the incoming coal powder-air mixture into a concentrated (rich) stream and a diluted (lean) stream, which are then fed to different burners to create a staged combustion pattern.

In corner-tangential combustion boilers, the coal powder air mixture pipes (quarl pipes) are arranged around the furnace corners, and elbows are often required at the burner inlet due to space constraints and piping layout requirements. These elbows introduce non-uniform velocity and coal powder concentration distributions at the separator inlet, which can significantly degrade the separation performance.

Methodology

The study employs both experimental testing and numerical simulation to analyze the effects of elbow configuration on separator performance. The experimental setup included a full-scale or pilot-scale separator with various elbow configurations upstream, instrumented to measure the coal powder concentration and velocity distributions at the separator inlet and outlet. The numerical simulation used computational fluid dynamics (CFD) with a gas-solid multiphase flow model to predict the flow field and particle distribution within the separator and upstream elbow.

The key performance metrics evaluated include:

Key Findings

The study reveals that the elbow structure and arrangement form have a significant impact on the separator's separation effectiveness and the velocity distribution uniformity on both the concentrated and diluted sides. Specifically:

1. The elbow introduces a non-uniform velocity profile at the separator inlet, characterized by a higher velocity on the outer wall side and a lower velocity on the inner wall side. This non-uniformity disrupts the designed flow pattern within the separator, reducing the separation efficiency.
2. The coal powder concentration distribution at the separator inlet is also affected by the elbow, with particles concentrated on the outer wall due to centrifugal effects. This pre-separation of particles before the separator inlet can either enhance or degrade the separation performance depending on the specific separator design.
3. The study proposes design strategies to achieve high separation efficiency and balanced velocity distributions on both the concentrated and diluted sides when a pre-mounted elbow is present. These strategies include optimizing the elbow curvature radius, incorporating flow straightening devices between the elbow and separator, and adjusting the separator internal geometry to compensate for the inlet non-uniformity.
4. The numerical simulation results show good agreement with the experimental data, validating the CFD model for predicting the coupled elbow-separator system behavior.

Engineering Practice Integration

The findings of this study are directly applicable to the design and retrofitting of coal powder concentration-dilution separators in existing corner-tangential combustion boilers. When the piping layout requires an elbow upstream of the separator, the design strategies proposed in this paper can be implemented to maintain acceptable separation performance.

The recommendation to incorporate flow straightening devices (such as honeycomb panels, perforated plates, or guide vanes) between the elbow and separator is a practical and cost-effective solution. These devices help to restore a more uniform velocity and concentration profile at the separator inlet, compensating for the non-uniformity introduced by the elbow.

The study also highlights the importance of considering the entire upstream piping configuration, not just the separator itself, when evaluating and designing concentration-dilution combustion systems. The interaction between the elbow and the separator is a coupled phenomenon that requires a system-level approach to optimization.

Study Insights and Implications

This paper addresses a practical engineering challenge that is often overlooked in the design of coal powder concentration-dilution combustion systems. The focus on the upstream elbow effect, rather than the separator itself, represents a valuable shift in design thinking toward system-level optimization. The finding that even a simple elbow can significantly degrade separator performance underscores the importance of holistic system design in power plant engineering.

A limitation of the study is the relatively limited scope of elbow configurations examined. The findings may not directly apply to more complex multi-elbow arrangements or to separators with different internal geometries. Additionally, the study does not address the long-term effects of erosion and fouling on the separator performance, which can be significant in coal-fired boiler applications.

In conclusion, this paper provides important insights into the coupled behavior of elbows and coal powder concentration-dilution separators. The proposed design strategies for achieving high separation efficiency and balanced velocity distributions in the presence of upstream elbows offer practical guidance for boiler designers and operators seeking to optimize coal powder combustion systems.