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Longwall mining in China is going through a resurgence. To meet the needs of the electric power industry, 40-60 productive and efficient new mines, each with annual output of around 5-10Mt, are expected to be established by 2010. Western OEMs such as Joy and DBT supply equipment to key mines in China but the size of the market has justified the development of China’s own expertise in the development of longwall face equipment.

The research work to get China’s local roof support industry established was undertaken by coal mine bureaus, mining colleges and research institutes, in particular the Beijing Coal Mining Research Institute of CCRI, under the leadership of chief scientist Professor Wang Guofa. CCRI has successfully developed more than 300 types of power support and they have been exported to America, India, Turkey, Russia, Bengal and recently three supports structures were brought into Australia.

Professor Wang said with an annual output of 6,000-10,000 supports China has become the biggest manufacturer of shield supports. One of the biggest challenges has been to design shields for China’s abundant thick and extra thick coal seams. The Yangzhou and Lu’an coalfields, for example, have thick coal seams with stable and simple geological conditions, with thickness of 6-8m. The use of heavy-duty fully mechanized coal mining equipment has facilitated average annual output of 5 million tons for Xinglongzhuang, Dongtan, Baodian and Jining No. 3 collieries.

Professor Wang described three types of thick seam conditions characterised for the purposes of designing supports. These are designated Three-Soft, Two-Hard and dipping thick seams.

“Three-Soft” describes the thick coal seams in Kailuan, Xuzhou, Huaibei, Zhenzhou, Longkou, Fengfeng and Handan coalfields. These seams are characterised by soft coal, with shale or mudstone roofs and floors. Supporting the roof in these conditions is hard. Light supports (1800kN yield pressure and weight of 4-6 tons) have been used successfully in thinner thick seam conditions (around 3.5m in thickness)

Two-Hard thick seams have hard roof and hard coal conditions. These seams do not cave with extraction as cavability is poor and caving coal comes off in large lumps. Periodic weighting at the working face is common.

Professor Wang said in the past seams like this were considered unsuitable for sublevel coal caving but in recent years sublevel coal caving technology has been used successfully at Chenjiashan, Honghui and Bingxian collieries.

Bingxian for example, in the past mined the hard No. 8 coal seam, averaging 16.7m in thickness with a shale mudstone and sandstone roof. Annual output was only 230,000 tons. In 2001 Beijing Coal Mining Research Institute of CCRI introduced fully mechanized sublevel coal caving to this mine which has been able to achieve monthly levels of 150,000 tons.

Another of the geological challenges China researchers have faced is designing shields to cope with dipping thick seams.

A working face with a dip larger than 25 degrees, is difficult to control. Shields for dipping faces have to be made stable with some sort of anti-skid in the front and rear AFC. These are all bottlenecks which restrict productivity.

Longwall mining has been used at Donghuantun, Weijiadi, Zhangshuanglou, Nanshan, etc. collieries with dip of 25 to 45 degrees to test an effective way of mining such thick coal seams.

To meet the requirements of this range of conditions the Beijing Coal Mining Research Institute has developed three types of powered supports: the positive pantograph power support;

The positive pantograph supports are suitable for “Three-Soft” and dipping coal seams, which feature a positive pantograph stabilizing mechanism. These shields are of compact design, with general parameters optimised so that the weight of the supports is reduced.

For hard coal faces the ‘opposite pantograph support’ was developed featuring the wide opposite pantograph stabilizing mechanism, improved mechanical characteristics, decreased linkage force, increased anti-off-center loading and general stability. This shield also includes a big outlet for coal caving that allows loading on the front or rear leg. The rear cantilever is hinged jointly to the canopy resulting in a larger space for coal caving. This type of support is popular in China at the moment with over 5000 individual shields in the field.

Finally, a ‘single rotary light-duty power support’ has been developed featuring a single rotary stabilizing mechanism, a canopy in one piece and a yield load usually 2000KN – 3200KN.

This type of power support can be used in a standard working face with annual output of 1 – 1.5 million tons but also to cut coal pillar or corner lump coal. Some 80 sets of these shields are supporting Chinese faces. The Kailuan coalfield has utilized this type of support in 8.3m thick coal seam, dipping 24 degrees with complicated hydrology. Eight working faces have successfully finished using this type of support.

China is also the world leader in the top coal caving mining method and has been developing shields for this purpose for 20 years. In recent years sophisticated design tools have been used to improve the design of Chinese supports. These include CAD software to optimise the structural parameter of supports, CAE software to analyse the force condition of supports and VP (Virtual Prototype) software to simulate movement.

Chinese powered supports are mainly fitted with hydraulic control systems capable of large flows and fast speeds, some mines have adopted the PM31 electro-hydraulic control system. A joint venture between Tiandi Science &Technology Company Co. Ltd (controlled by the CCRI) and Germany’s Marco has been formed to develop and produce the PM31 electro-hydraulic control system.