Research On Solid-gas Two-phase Flow Character Inside The Pulverized Coal Exhauster

In the in-direct mill system of a pulverized coal fired boiler, a certain amount of ultra-fine coal particles contains in the tertiary air and is difficult to be separated from the gas-solid flow by using the conventional separation methods. In this study, the enrichment of particulate concentration in the tertiary air is proposed, based on kinds of the coal, such that the tertiary air can be used to form fuel re-burning for NOx emission reduction or to be used as the primary air to increase combustion efficiency. A high speed, centrifugal dust exhaust fan (DEF) with high pressure head was also proposed to achieve the gas-solid separation in the tertiary air.Experimental and numerical studies were conducted on the separation efficiency of solid particles from the fine-particle-containing two-phase flow for a DEF with the model of 9-26. The outlet of the fan was in rectangular shape. During the experiments, it was evenly divided into five rectangular sub-outlets with equal area in the radial direction. Hollow glass beans were used to simulate the coal powder. The glass beans exiting from the sub-outlets were collected individually by the bag house type dust collectors. In the same time, the flow rate of the air stream from every outlet was independently measured. With the solid particle collection rate and the air flow rate, the local particulate concentration at each sub-outlet was calculated, and the distribution of the solid concentration at the entire outlet was obtained. The experimental results showed that the distribution of solid concentration in a cross-section at the DEF outlet was uneven. The closer to the outer side of the exit, the higher the particulate concentration. The maximum local particulate concentration was 2-2.5 times of the averaged one, while the minimum particulate concentration was only 30% of the averaged one. However, near the tongue of the volute, there was a high particulate concentration region.Numerical simulation of the internal two-phase flow inside the DEF was conducted by using the Fluent software. The simulation results agree wellwith the experimental data. Calculation further shows that particulate concentration distribution is uneven along the front-back (axial) direction of the fan exit. The concentration near the back wall of the outlet is remarkably higher than that in the region near the front wall. The maximum concentration of glass bean appears in the region near the sidewall of the rear section of the outlet. The simulation predicts the distribution of solid-phase concentration inside the fan, wearing position and manner of the impeller. The predictions are consistent with the practice.According to the results of experiments and numerical simulation, an advanced enrichment method was proposed, by considering the particulate concentration distribution not only in the radial direction but also in the front-back direction (i.e. axial direction). In general, about 80% of the particles in the incoming gas-solid flow can be separated into the outer half channel. The enrichment effect can be further improved since 50% of the particles can be collected in a small portion of the exit, counting 1/8 of the outlet crossing area. The practicability of the particulate enrichment of the tertiary air using the DEF method was validated.Additionally, a computational fluid dynamic (CFD) platform for the DEF simulation was developed based on the Gambit software, and used to improvement the fan performance. It wad found that placing a small vane in the vane channel can improve the flow characteristics in the vane channel, resulting in a mass flux increment up to 5% and a pressure drop reduction near 10% .