Using Computational Fluid Dynamics (CFD) modelling techniques to model airflow behaviour around the shearer, CSIRO researchers, led by principal mining engineer Dr Rao Balusu, have simulated what happens to dust when different operating options are used.
A leading proponent of advanced simulation methods, Balusu has also used CFD tools in cutting-edge research to design ways to optimise goaf gas drainage.
Longwall dust control methods have evolved over the past three decades, largely in the UK and the USA, but are mostly suited to low to medium coal seam heights up to 3m.
When recent studies indicated that operators in thick seam mines were exposed to higher dust exposure levels than their counterparts in medium seam mines, a research project with Australian Coal Association Research Program (ACARP) funding was initiated to identify options for better control of dust.
The research developed three CFD models to represent longwall faces in thick (4.5m), medium (3m) and thin seams (2.1m), with the latter two models developed for comparison purposes.
Once the thick seam longwall face model was validated against field data, it was used for detailed parametric studies to simulate the effect of various parameters and dust control concepts.
The dust control options investigated include various spray configurations, shearer clearer, modified cutting sequences and shearer scrubbers.
Effect of sprays/venturis
The use of water sprays on the shearer is most commonly used for dust control on longwalls and the CFD model was used to investigate the effect of sprays with changing flow rates and directions. The sprays were mounted on the shearer body and were directed towards the TG drum and the cutting MG drum areas to represent the traditional dust control practice in coal mines. The airflow path lines released from sprays/venturis over the shearer at flow rates of 0.1m3/s and 0.5m3/s.
The modelling showed airflow from the spray directed towards the TG drum would merge into the main ventilation stream nicely and travel downstream toward the coal face, while airflow against the main ventilation stream forced the face ventilation to swing over the cutting MG drum towards the walkway area.
Sprays operating at higher flow rates of about 0.5m3/s seemed to have a significant effect on airflow and the behaviour of respirable dust, causing the majority of the particles to follow the turbulent airflow path and disperse towards the walkway area. The modelling results highlighted the importance of the correct combination of spray positions and flow rates and the need to optimise the sprays system for effectively controlling dust particles from different sources.
A novel shearer spray system, known as the “Shearer Clearer”, was originally developed in the US and is currently being used in some Australian longwalls. The system was designed to take advantage of the air-moving capabilities of water sprays to direct the dust cloud downwind and prevent it from floating into the walkway.
The CFD modelling showed the Shearer Clearer helpsed reduce dust dispersion towards the face operators by inducing the dust cloud towards the face area.
On the downside, the simulations indicated a limited area of influence for the Shearer Clearer in thick seam environments. It had only marginal effect in reducing dust migration towards the walkway area, particularly when dust was generated in the face spall area ahead of the shearer.
Cutting direction modifications
Australian longwall mines typically use uni-directional cutting from tailgate to maingate, but this cutting sequence results in higher dust exposure levels for the TG drum operator and chock operators from dust dispersion at the upwind cutting drum.
CFD simulations studied the effect of changing cutting direction and from TG to MG direction with reversed drum.
Cutting from MG to TG direction
Here the TG drum takes a full cut near the roof during MG to TG pass, while the MG drum cuts a minimal amount of coal near the floor. Changes in cutting direction significantly reduced the air diversion towards the walkway near the upwind MG drum area.
In the standard cutting sequence, dust particles from coal spalling ahead of the cutting MG drum would disperse over the shearer and into the walking zone of the face operators. In the modified cutting mode, the majority of the dust particles flowed over the shearer until the TG drum location and then dispersed slightly towards the walkway area.
Results showed that cutting from MG to TG direction, instead of the standard TG to MG cutting, would significantly reduce air diversion and the subsequent dust migration towards the walkway area.
The downside with this is that chocks have to be advanced on the upwind side of the shearer and this is likely to generate a large amount of dust, particularly with poor and weak roofs. As a result, this cutting direction is not suitable for most Australian longwall mines.
To overcome the operational difficulties of MG to TG cutting, a modified cutting sequence was modelled in which coal was cut from TG to MG direction with reversed drum. The leading upwind MG drum cut the floor or middle section of the face and the downwind TG drum cut the roof section of the face. This cutting sequence resulted in reduced obstructions on upwind side of the shearer.
The modified cutting sequence showed a substantial difference in dust flow behaviour. Respirable dust migration towards the walkway would be reduced significantly with the modified cutting sequence compared with dust migration in standard cutting mode.
Other operational issues, such as coal loading and floor control, may need to be considered for application of this modified cutting sequence.
A scrubber system is designed to capture the maximum amount of dust produced in small spaces. However, in the case of a longwall scrubber system, total dust capture is not feasible given the large cross sectional area and high airflow volumes on the face. Modelling was carried out to study the dust-capture potential of the scrubber system, and to assess the feasibility of modifying the flow patterns around the cutting drum to reduce dust roll-up into the operator’s position in the walkway.
CFD modelling was carried out with different scrubber capacities, ranging from 4m3/s to 10m3/s. Total airflow on the face was around 55m3/s.
The positive effect of the scrubber increases as capacity increases. Scrubbers with flow capacity up to 4m3/s seem to have only a marginal effect on respirable dust-flow patterns near the shearer.
The capture and diversion of the respirable particles would be significantly improved with a shearer scrubber operating between 6-10 m3/s. At these higher capacities, the shearer scrubber would capture a significant proportion of the dust and, more importantly, modify the flow patterns around the shearer, thereby reducing the dust migration towards the walkway area.
In conclusion, the modelling work significantly advanced the understanding of air and respirable dust flow patterns around the longwall shearer through the use of CFD modelling techniques.
Adapted from “Investigations of air and dust flow patterns around the longwall shearer”, presented in July at the Eighth International Mine Ventilation Congress, organised by AusIMM.