Field study of longwall coal mine ventilation and bleeder performance

Longwall coal mine operators in the U.S. are required to ventilate multipanel longwall districts, but may have little or no knowledge about what happens to the ventilation air between the inlet evaluation points (IEPs), bleeder evaluation points (BEPs) and bleeder fans. The effectiveness of bleeder performance can directly influence the ability of a ventilation system to remove and dilute coal bed methane emissions. U.S. coal mining stakeholders have acknowledged their belief that the T-junction split at the longwall face tailgate corner is very important in controlling the distribution of ventilation air. To obtain direct measurements of bleeder performance, a tracer gas, sulfur hexafluoride (SF6), was released into the ventilation air stream on active and inactive longwall panels. Testing was performed on multiple longwall panels that included various phases of longwall development and variable path lengths of ventilation air transport to bleeder fan installations. Changes in the T-junction ventilation air distribution over the life of a longwall panel, and its variable effect on panel ventilation airflow are discussed. These findings will assess the effectiveness of commonly applied ventilation strategies for improving air distribution and ventilation controls to meet statutory requirements. Introduction Longwall coal mines in the U.S. use bleeder systems to dilute methane concentrations where panels and gobs connect to form return ventilation airways. Many modern U.S. longwall mines use bleeder fans to remove methane-laden air from these areas inby the active longwall face. The use of bleeder ventilation networks in U.S. mines was documented by Kingery and Dornenburg (1957) as applied to room-and-pillar mines. Since that time, large-dimension longwall panels have evolved in the U.S. coal mining industry. Improvements in auxiliary methane removal systems, such as directional drilling, with the addition of ventilation-assisting technologies such as bleeder fans, have allowed mine operators to mine large-dimension longwall panels in moderately gassy to highly gassy conditions (Thakur, 2006). As a consequence, methane loading and air movement in bleeder systems have become increasingly important and complex phenomena in underground coal mines. Consequently, the movement of ventilation air through longwall bleeders will interact with many aspects of methane drainage such as gob gas ventholes (GGVs), bleeder fans and sealed district ventilation. Statement of problem Although longwall ventilation technology has addressed deeper and gassier coal beds, increasingly large longwall panels and larger-volume coal bed methane reservoirs remain issues of concern. A series of U.S. National Institute for Occupational Safety and Health (NIOSH) stakeholder meetings were conducted to identify problem areas to be addressed by future research in coal mine ventilation and methane control. Input from industry, regulatory agencies, labor and academia prior to research proposal development identified ventilation air transport in bleeders as a high-priority area for future investigations. Airflows are typically measured weekly at inlet evaluation points (IEPs), bleeder evaluation points (BEPs) and bleeder fans. Unfortunately, these measurements are too infrequent to discern fluctuations in flow ranges that occur as a result of changing ventilation conditions. Even when measured flow data are available from these locations, vast expanses are present in the gob where flow rates and directions (e.g., former gateroad entries between gobs) are unmeasured and poorly defined. Other factors influencing the transport of methane-laden air in bleeder systems include rib spalling, roof-to-floor convergence and changing T-junction air distributions with increasing distance to the bleeder fan during panel retreat. The direct determination of these parameters and their influences on longwall panel airflow and transport in the bleeder ventilation network are often not possible. Also, high methane concentrations in the gob can result in increased methane emissions in the longwall returns and bleeders, which can make conforming to the 2% methane-in-air federal statutes very challenging (Mine Safety and Health Administration, 2008). Research design and goals To acquire data for this study, a method to quantify airflows in inaccessible areas of a longwall mine was needed. Tracer gas testing techniques have been used successfully to describe airflow movements in mine gobs, fractured overburden, around and though mine seals and air movement in face sections (Thimons and Kissell, 1974; Timko and Thimons, 1982; Timko et al., 1986; Vinson and Kissell, 1986; Schatzel et al., 1999; Mucho et al., 2000). In addition to the tracer gas mine applications reported in the literature, the American Society of Testing Materials developed tracer gas testing standards applicable to mine voids (ASTM E 2029-99; ASTM E 741-00). The complexity of mine void space, limited access and the variable flow rates of air and gas mixtures can create substantial challenges for tracer evaluation approaches (Grot and Lagus, 1991). This work used a series of tracer gas releases to describe airflow patterns and movements at the longwall bleeder/gob interface and to define ventilation air pathways, rates and volumes of movement at these interfaces. This work also identified the distribution of ventilation air at the longwall tailgate T-junction. A single tracer gas, sulfur hexafluoride (SF6), was chosen to describe ventilation air movement. Tracer gas sampling was conducted underground using tube bundles attached to permissible vacuum pumps and at the surface from operating GGVs and from a bleeder fan site. This study presents ventilation parameters at early, late and completed phases of panel extraction. The tracer gas measurements were designed to be redundant with the tracer gas slug passing by multiple sampling sites underground and at the surface. An important goal of this study was to describe intake air distributions and airflow rates on active and inactive panels, including bleeder sections and transport pathways to the surface. Description of study site The study site was an underground coal mine operating a single longwall in southwestern Pennsylvania. The mine operates in the Pittsburgh coal bed and utilizes a three-entry gateroad system with weekly production of about 151,000 mt (166,000 t). The nearest intake air fan provides 106 m3/s (225,000 cfm) into the section. The shaft diameter is 2.4 m (8.0 ft). The study site is located on one panel of a four-panel district (Fig. 1), where overburden depth ranged from 240 to 270 m (800 to 900 ft). Figure 1 — Study site for tracer gas testing. Longwall configuration for test 1 is shown. A bleeder fan producing 129 m3/s (273,000 cfm) at 3.86 kPa (15.5 in WG) at the start of the study was located adjacent to longwall panel 1 (LW#1) on a bleeder shaft with diameter of 2.4 m (8.0 ft). Airflow around the back end of the panel is separated into an outer loop (Bleeder 1) and an inner loop (Bleeder 2) on its way to the bleeder fan. Two rows of 76-cm (30-in.) pumpable cribs were installed throughout the length of the tailgate No. 3 entry, closest to the longwall block (numbered left-to-right when facing inby). This configuration was generally successful in keeping the entry open until the longwall face passed. Pumpable cribs were installed in the headgate No. 3 entry adjacent to longwall panel 4. The panel block width is 410 m (1,350 ft), entry width is 4.9 m (16 ft) and entry height is 2.6 m (8.5 ft). Each panel had at least three surface GGV sites removing gob gas via a coal bed methanepowered exhauster using a modified diesel internal combustion motor. Methane contents for the Pittsburgh Coal Bed in southwestern Pennsylvania are given in Diamond et al., 1985. Methodology Tracer gas was released from lecture bottles containing about 34 L (1.2 cu ft) of high-purity SF6. An experimental protocol was developed for each tracer gas release utilizing the manual sampling of ventilation air with an underground tube bundle system. Tube bundles were 1.3 cm (0.5 in.) OD polyethylene tubing attached to SKC Inc. Aircheck vacuum pumps, which are U.S. Mine Safety and Health Administration (MSHA) permissible. The experimental protocol called for concurrent surface gas sampling to identify potential pathways of the tracer gas through the underground ventilation network and to the atmosphere. Tube bundles were installed in the panel 3 headgate No. 2 entry and the tailgate No. 3 entry, adjacent to the No. 3 panel block. Four locations for sample retrieval were chosen in the headgate and tailgate entries, as shown in Fig. 1. Filters were installed on the tubing at the inby, open ends of each sampling line. The tubing outby, open ends were covered with electrical tape until a test was performed. For gas sampling, the SKC pumps were attached to the lines and gas samples were retrieved from the exhaust side of the pumps, with one pump attached to each tubing line. The length of each sampling tube is shown in Table 1. Table 1 — Sample tubing lengths, volumes, and transit