Longwall Mining, 3rd Edition

Syd Peng

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9780429535666
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria civile

In the past 13 years since the publication of Longwall Mining, 2nd edition in 2006, although there have been no major changes in longwall mining technology and operations, many incremental developments in the whole system as well as various subsystems of the existing longwall mining operational technologies as detailed in the 2nd edition have been added to this edition.

Major developments are automation, and health and safety technology, as well as equipment reliability, thereby greatly increasing productivity and cutting cost. In particular, the longwall system can now run automatically cut by cut forever without operators' intervention provided that the geology allows it. Other health and safety features such as LASC, personal proximity detection, color lighting, automatic shield water sprays and remote shearer control are fully operational. There are more than 7000 sensors installed in current longwall mining systems. The big data obtained and fast communication technology have been fully utilized to improve and solve operational problems in real time. Those features are fully documented in the new edition. In pursuit of high productivity and cutting cost, life cycle management that increases equipment reliability has been implemented by OEM. Automation improvement such as tail-end automatic chain tensioner greatly extends AFC chain's service life. Other incremental improvements including dust and methane controls, entry development, panel design and face move are addressed. Additional operational issues such as extension of panel width and compatibility test are also discussed. Since the last plow longwall mine was closed in 2018, the chapter on plow longwalling has been dropped and in its place Automation of Longwall Components and System is added. Also, a new chapter Longwall Top Coal Caving Mining (LTCC) is added due to its successful application in Australia since 2005.

Longwall Mining, 3rd edition will be of interest to professionals and academics in the field of mining engineering specifically, serving both as a reference work and an (under)graduate textbook, but will also interest civil, geomechanical and geological engineers and rock mechanics professionals, as well as coal operators, mining consultants, researchers, equipment manufacturers, and government regulators.

Chapter 1
US longwall mining

1.1 Introduction

The concept of longwall mining in the United States dates back to the late 19th century when wide faces were used for coal extraction in the Pittsburgh seam. But modern, fully mechanized longwall mining employing plows and frame supports was first introduced from West Germany in the early 1950s in southern West Virginia. It was used mainly for thin seam mining. During the 1950s and 1960s, the practice did not expand. In fact, due to its poor performance, only a few mines in southern West Virginia employed this technique. But in the mid-1970s when the shield supports were introduced, again from West Germany, to a northern West Virginian mine, resulting in increasing production and safety records, longwall mining was then recognized by the coal industry as a viable mining technique.

Thereafter the number of longwall mines grew steadily and rapidly (Fig. 1.1.1). A maximum of 118 longwall faces existed in 1982, but the number decreased steadily during the 1980s and 1990s and stabilized to around 50 in the first decade of the 21st century. It was gradually reduced further to the high 40s in the late 2000s and stabilized in recent years in the low 40s. It must, however, be emphasized that although the number of longwalls was decreasing, production increased tremendously during this period due to improvements in face equipment automation and reliability, use of heavy duty face equipment, better panel layout, skilled labor forces, and aggressive management.

The growth of US longwall mining production in the past four decades has been phenomenal. In 1975, there were 76 longwall faces, but their production was a mere 4% of underground coal mining production. In 2017, there were only 43 operating longwall faces, but they accounted for 62% of underground coal production. Proportion of longwall production has been steady even though the number of longwalls has decreased slightly in the past decade.

Throughout the past 40 years the basic elements and fundamental principles of longwall mining have remained the same, except that the panel has become much larger and the equipment has become larger, heavier, more powerful, and more reliable, and that automation has gotten more sophisticated and reliable.

US longwalls are highly productive and have established numerous world records since 1990. Productivity increased steadily. It reached the highest in 2000 at around 5.2 tons per man-hour (mine wide) or around 11,600 tons per man-year (Fig. 1.1.2). It began to dip in 2004 due to the hiring of more new young unskilled miners to replace the experienced retiring miners. It started to increase again in 2011 due to industry-wide layoffs and closure of uneconomic mines resulting from a combination of abundant cheap shale gas and Obama Administration’s policy “War on Coal.” It reached the previous high in 2016 and continued to increase.

Figure 1.1.1 Yearly trends of number of longwalls in the United States since 1976

Figure 1.1.2 Productivity trend of US longwall mining

According to the 2019 US longwall census, there were 37 active longwall faces producing from slightly more than one million to 7.23 million clean tons of coal in 2018. It averaged about 4.56 million clean tons or approximately 7.01 million raw tons per longwall, assuming an average of 65% clean coal recovery.

From a production point of view, today’s longwall mining has radically changed the conventional concept of coal mining in terms of reserve, production, safety, and mine planning.

Today, longwall mining has firmly established its role as the most productive and safe underground coal mining method and, consequently, in all cases there is no dispute that longwall mining is the preferred method whenever geological conditions and reserves are suitable.

1.2 Requirements and constraints for high production longwalls

Most US longwall mines operate one longwall unit in a mine and produce three to seven million clean tons annually, although several mines routinely operate two longwalls units occasionally; a few mines even operated three units in order to either keep up with or increase production. The advantages of one single longwall in a mine include a simpler mine infrastructure and high productivity, which is one of the major reasons why modern longwall is the most productive and cost-effective method.

Several conditions are necessary to ensure productivity of modern longwalls. (1) The equipment must be heavy duty with high installed power, have fast machine speed, be highly reliable, be quick and easy to maintain, and have a minimum life of at least one longwall panel without a major lost production breakdown. (2) In order to utilize the equipment mentioned above, longwall panels must be huge, i.e., wider and longer. (3) Favorable geological conditions must exist including reasonable methane gas content, easy to cave roof strata in the gob, and a firm floor and, in some cases, adequate seam thickness.

The major constraints for modern longwall mining are the following: (1) It requires a very large capital investment as compared to other methods, such as room-and-pillar mining. The industry norm is to fully depreciate the longwall equipment in five to seven years. (2) It requires a fairly large-sized reserve, say a minimum of 30–40 million tons, to recover the capital investment. The reserve is preferably a continuous block, but isolated blocks have been proven feasible. (3) Longwall mining is highly rigid in mine plan. Panels are laid out at least two years in advance and normally are extremely impractical, if not impossible, to change later on.

The reasons that US longwalls have become the most productive in the world within the 10- to 15-year period since its introduction from Europe, and acceptance by US coal industry, are as follows:

1. Favorable geological conditions – Coal seams are flat, fairly uniform and shallower, mostly less than 2100 ft (640 m), and the roof strata are generally easy to cave.
2. Excellent mining plan and equipment layout – US longwalls employ multi-entry gateroad development using continuous miners, which allows rapid gateroad development, and the layout creates minimum abutment pressure interaction between adjacent panels. Specific improvements to equipment and layout unique to US longwalls are addressed in Section 7.2, “Layout of Longwall Face Equipment Using the Shearer.”
3. State-of-the-art equipment – All equipment are heavy duty and highly reliable. Individual subsystem reliability is greater than 90% without, increasing to close to 100%, with industry/OEM partnership services programs. The whole system reliability is more than 85% without a partnership program.
4. Highly efficient method of face move – For a 1200 ft (366 m) wide panel, a complete face move can be done in five to seven days (Note, normally the shearer and AFC and stage loader are pre-installed in a new face.). A complete face move is up to two weeks.
5. Excellent management and skilled work force in a highly competitive world market.
6. Production and safety incentive programs.

Among the six reasons mentioned here, items two through four will be addressed in detail in this book.

1.3 Panel layout

During the 1970s–1980s, US longwall mining has evolved from almost a wholly borrowed system, mainly that of Europe, to a unique system – one that is conducive to high-speed mining and subjected to strict safety and health standards. During that period, demonstration projects on advancing longwall mining, multi-slicing longwall mining, thick-seam longwall mining, and inclined seam longwall mining methods had all been tried in Colorado and Wyoming, while thin-seam longwall mining (using the shearer) was experimented with in Kentucky, Virginia, and West Virginia, mostly under federal government funding. The results were not favorable due to various reasons, and their applications were discontinued, except a thick-seam (14.5 ft or 4.4 m) longwall operation in Wyoming that lasted until 1988, and an inclined-seam longwall mine (up to 25^o), also in Wyoming, which was discontinued in 2000 when its reserve became too expensive to mine. The face equipment setup and panel layout (either parallel to the strike or the dip direction) in the aforementioned thick and inclined seams were similar to those in the flat seams.

A longwall mine with a gently dipping seam, up to 7–9°, continues to operate in Utah.

After continuing improvements in the 1970s–1990s, the panel layout and face equipment and their setup were more or less standardized in the mid-1990s and remain so up until now. The current US longwall mining method has the following unique features (for special features in equipment and its layout, see Section 7.2):

1. The process uses a retreating longwall mining method with natural roof caving in the gob.
2. All longwalls are single slice or single seam and operate in flat or nearly flat coal seams.
3. Panel development uses the room-and-pillar method in which multiple entries (two to four entries, mostly three entries) are developed simultaneously in-seam in a rectan...