Current Underground Coal Mining Technologies

2.1 Introduction

  This chapter presents an overview of the current methods of mining that are used in southwest Virginia and how these technologies are selected. These methods are all currently producing coal but, by themselves, do not have the flexibility to extend the life of the Virginia coalfields.

  A vital factor in the selection of a mining method is the prevailing geology of the area. The properties of the confining strata and the coal itself, dictate which methods can and cannot be successfully employed in each seam. For example, in a longwall operation, an ideal strata sequence would have coal of a uniform thickness covering a large areal extent and a roof that is strong enough for adequate ground control, but not so competent that it inhibits controlled caving. By contrast, a room and pillar operation benefits from a strong roof, and this method allows greater flexibility for variations in the seam thickness and extent. Consideration must also be given to floor conditions, faulting, water, and depth. Depth is a very important factor, because as depth increases the stress on the support pillars increases, requiring the pillars to be larger. This means that less coal is extracted, causing the extraction ratio to decrease and results in room and pillar operations becoming uneconomical in deeper workings. Longwall operations may be preferred under those conditions.

  Economics play a very important role in almost every decision made in the mining industry. Longwall mining systems are very capital intensive, requiring large initial investments and a long mine life to recover that investment. This limits the use of the longwall system to the larger mining companies that also have significant reserves. On the other hand, room and pillar operations can deal with more variable geology and require less investment, which can be recovered over a shorter mine life. Consequently, this method is amenable to smaller reserve tracts.Environmental conditions in the mine also tilt the scale in favor of one method or the other. A longwall face has more concentrated releases of gas and dust because the rate of coal extraction from a single face is greater. Room and pillar operations have less immediate emissions of pollutants, but the ventilation is more difficult to control.

  Another factor that comes into play is component interdependence of a mining method. In a longwall setup, numerous conveyors, the shearer, and the hydraulic roof supports are all interdependent. If one component goes down, the entire system is off line. With such a high capital investment, downtime causes serious financial losses. In room and pillar mining, the system components are not so interdependent. If an element in the system fails the consequences are less severe.

2.2 Elements of the Longwall System

  In conditions where roof control is difficult, the coal of significant lateral extent, and of sufficient thickness, longwall mining is preferred. Longwall mining offers the benefits of enhanced safety due to its system of face supports that cover the entire working face. This method also allows higher extraction ratios, conserving valuable coal reserves. Some other advantages of this system are its flexibility in dealing with greater mining depths, multiple seams, and a significant reduction in roof bolting. (Bibb, 1992)

  There are also significant disadvantages to longwall mining, such as the high capital cost of the required equipment. It follows that interruptions to production can have a serious economic impact whether they are short term such as starting and stopping the shearer, or longer term as longwall equipment is moved from a depleted section to a new panel. There may be problems with gas well location, seam thickness, and in soft floor and roof. There is also the possibility of this system being impractical beneath thick strong roof beds, due the size and cost of the required roof support as well as difficulties in controlled caving. Because of the areal extent required by a longwall section, variability in the seam thickness, roof and floor conditions, local faulting and the presence of wells can also limit the potential of this method.

  There are two distinct types of longwall mining that are employed, advancing and retreating. While the advancing system is sometimes used in other countries, American companies prefer the retreating system. In the advancing method, the development entries progress slightly ahead of the advancing face and away from the main entries, while in the retreating system the entire section is developed prior to commencement of production from the longwall face at the inby end, and mines back towards the main entries. The focus of this chapter will be on the retreating system of longwall mining.

  The development that is carried out for longwall pillars utilizes a continuous miner and the room and pillar method (Fig. 2.1). These entries are called the headgate and tailgate entries, with the ventilation return being the tailgate entry. When one panel is mined out, the headgate entry of the last adjoining panel often becomes the tailgate entry of the next.The coal is removed from the extraction panel by a machine called a coal shearer that moves back and forth along the working face, riding on the sides of the armored face conveyor, fragmenting coal from the face, and dropping it onto the face conveyor (Fig. 2.2). This conveyor leads down the face and onto a main haulage conveyor in an entry of neutral ventilation. The roof above the working face is supported by automated hydraulic supports that are advanced as the mining progresses. Controlled subsidence is achieved by allowing the top to collapse behind the row of supports. The conveyor is snaked over, after passage of the shearer, by horizontal jacks attached to the self-advancing hydraulic supports.

  Moving a longwall set up is a complicated, time consuming process, that entails dismantling, refurbishment and reconstruction of the computer control system, shearer unit, armored-chain conveyor and the support shields,. This process may involve the majority of the underground workforce for a period of one to two weeks at a cost of up to one million dollars. (Suboleski, 1998)

  A method of longwall mining that was developed, primarily, in Germany was the coal plough (Fig 2.3). The coal plough replaces the shearer in a typical longwall setup. The coal plough can mine coal as thin as 18 inches, but beneath 30 inches currently available powered supports cannot be employed, greatly reducing the productivity of the system. The main mechanism of the plough is the armored face conveyor that has two main functions, to transport coal away from the face and to guide the plough unit on the face during mining. There are many problems with the plough system, such as the plough cutting into softer floors and deviating from the desired horizon. The plough also has difficulties in seams where the coal hardness is not reasonably constant across the face. The feasibility of this system is questionable in seams less than 24 inches in thickness.

Figure 2.1 - Typical Longwall Panel Layout (not to scale)

Figure 2.2 - Longwall face section with Conventional Double-Drum Shearer

Figure 2.3 - Longwall Plough Arrangement
(Clark, Cauldon, and Curth, 1982) reproduced with permissionn