Descripción del proceso de codificación de las interacciones

The Ruqigou and Gulaben coalfields (figure 4-2) are situated at the border between the Ningxia Autonomous Region and the Inner Mongolia Autonomous Region, in the central Helan Mountains, at elevations between 1,800 m and 2,500 m above sea level. The Ruqigou coalfield is under administration of Ningxia, while the Gulaben coalfield belongs to Inner Mongolia. The Gulaben coalfield lies approximately 10 km west of Ruqigou coalfield and the area covered by both coalfields stretches between latitudes 39°00’ N to 39°01’ N and longitude 106°03’ E to 106°11’ E and covers about 80 km² in total. The coalfields include the mining towns Ruqigou, Baijigou, Dafeng and Gulaben. Chen (1997) reports an annual precipitation of 238 mm for the Ruqigou and Gulaben area with a maximum annual potential evaporation of 2720 mm. As the geological situation and many mining aspects are identical in Gulaben and Ruqigou, the following subsections cover both coalfields.

a) b)

c)

Figure 4-2: Ruqigou and Gulaben

coalfields. a) Coal town in the Ruqigou coalfield; b) burning mountain in the Ruqigou coalfield; c) outcropping coal layers and small scale mining activities in the Gulaben coalfield.

4 Geographic and geological overview of the Ruqigou, Gulaben and Wuda coalfields

4.2.1 Geological situation

The Ruqigou and Gulaben coalfields are part of the Palaeozoic to Mesozoic Fold Belt of the central Helan Shan. The Fold Belt is gently folded and shows a general strike from north-east to south-west. Both coalfields are part of a wide, synclinorial structure following the general strike of the Fold Belt and including the Chutangou Syncline in the north-west, the Houlugou Anticline (Gulaben coalfield) and the Ruqigou Syncline (Ruqigou coalfield) in the south-east. The flanks of the Ruqigou Syncline have low dip angles (5° to 30°), while the western flank of the Houlugou Anticline is rather steep (35° to 75°) (Gielisch and Kahlen, 2003; DMT, 2001).

The coal seams in both coalfields belong to the Middle Jurassic Ruqigou Formation, which consists of alternating sandstone, siltstone, shale and coal layer. The deposition environment was lacustrine-fluvial including swamps and fluvial channels. Zhang (1998) reports a total formation thickness of 349 m, while the total average coal seam thickness is about 41 m (Chen, 1997). More than ten coal seams are described in the Ruqigou Formation, but only three are regionally distributed in the coalfields (Chen, 1997). Mining operations mainly focus on coal seam No. 2 which has an average cumulative thickness of 19 m and can reach a thickness of up to 40 m in some locations (Chen, 1997).

4.2.2 Mining situation and coal properties

Mining operations have been carried out in both coalfields since the 1960’s. The seven mines in the Gulaben coalfield are mainly operated by the Inner Mongolia Tai Xi Anthracite Group, owned by the local government of the Alasag County. The three major mines in the Ruqigou coalfield all belong to the Ruqigou Mining Company (Ningxia Industry Bureau of Coal). Next to the major mines, about 50 privately owned, small-scale mines are still in operation in both coalfields (figure 4-2), although attempts were carried out in recent years to close these mines. The annual coal production in the year 2000 reported from the major mines is about 4 Mt for the Ruqigou coalfield and about 1 Mt for the Gulaben coalfield (Kuenzer et al., submitted; DMT, 2001).

With the exception of the Dafeng Mine (Ruqigou coalfield) and the small-scale private mines, coal is produced underground in both coalfields. Due to the steeper dip in the Gulaben coalfield the degree of mechanisation of the underground mines is limited. The underground mining in the Ruqigou coalfield is mainly restricted to coal seam No. 2 and is carried out by means of mechanized longwall techniques at various levels (DMT, 2001).

The coal quality is very wide ranging, from low volatile bituminous to anthracite coal. The Federal Institute for Materials Research and Testing (BAM, Germany) has recently carried out laboratory investigations on coal from the Gulaben and Ruqigou coalfields. The laboratory experiments were carried out for only a few coal samples from the Wuda, Gulaben and Ruqigou coalfields and are thus

4 Geographic and geological overview of the Ruqigou, Gulaben and Wuda coalfields

not necessarily representative for natural coal properties. A detailed description of the method and results of this primary analysis is given by Krause (2003). The samples from the Gulaben coalfield, taken in a depth of 45 m, were given a rank of 2.5 to 3.0, while the Ruqigou coal samples, taken in a depth of 70 m, were ranked as 2.0 to 3.5. The results of the element analysis as well as the determined water content, ash content and calorific value are listed in table 4-1 and 4-2.

C in % H in % N in %

Gulaben 85.720 ± 2.414 3.242 ± 0.042 0.683 ± 0.052

Ruqigou 74.972 ± 4.586 2.904 ± 0.045 0.653 ± 0.033

Table 4-1: Content of carbon, hydrogen and nitrogen in coal samples of the Ruqigou and Gulaben

coalfields (elemental analysis performed at BAM; Krause, 2003).

Gulaben Ruqigou

Water content in % of mass 0.97 ± 0.02 2.62 ± 0.11

Ash content in % of mass 5.1 14.9

Calorific value in kJ / kg 33661 29226

Table 4-2: Water content, ash content and calorific value of coal samples of the Ruqigou and Gulaben

coalfields (elemental analysis performed at BAM; Krause, 2003).

Self-ignition experiments were conducted by the BAM (Krause, 2003) for coal samples from the Gulaben coalfield by a so-called hot storage experiment. Here, coal samples are stored in a laboratory oven under isothermal conditions. Due to the fact that the self-ignition temperature depends on the specific internal surface (description in chapter 2), the experiment was carried out for coal samples of different volume and shape. Cubical coal pieces with edge lengths of 1 cm, 4 cm and 6 cm showed self-ignition temperatures of 578 K, 568 K and 562 K, respectively. Self-ignition temperatures measured for crushed coal of 100 ml, 200 ml and 300 ml were 567 K, 562 K and 552 K, respectively.

In addition, coal samples from the Wuda coalfield were tested for their capacity for ignition from an external ignition source. The Wuda coal has a lower coal rank than the Ruqigou and Gulaben coal (see next sections) and is thus considered to be more easily ignitable than the Ruqigou and Gulaben coal. Using a heated coil, with surface temperatures between 973 K and 1173 K , that was kept in contact with the unmodified Wuda coal sample for about 34 hours, it appeared that it was not possible to ignite the Wuda coal. Hence, these primary experiments indicate that an external ignition source capable of igniting solid coal with a high coal rank from the Gulaben and Ruqigou coalfields needs to be very hot and in long direct contact with the coal. Nevertheless, Krause (2003) demonstrated that crushed Wuda

4 Geographic and geological overview of the Ruqigou, Gulaben and Wuda coalfields

coal samples could be externally ignited in a similar experiment. It is important to note here that these are only preliminary results and that a more detailed investigation of coal samples from the Ruqigou and Gulaben coalfields is to be carried out in the coming years within the Sino-German Coal Fire Initiative.

4.2.3 Coal fires

Coal fires in the Gulaben coalfields cover an area of about 960000 m² affecting about 10 Mt of coal reserve. In 2002 and 2003 seven active coal fires were reported by the local mining management, stretching in a row along the strike direction of coal seam No. 2. Single fires extend more than 1 km in the strike direction of the coal seam, with a maximum depth of 200 m in dip direction. Beside sub- surface coal fires, surface coal fires burning directly on the outcropping coal seam can be observed at different locations. According to the Tai Xi Anthracite Group, all coal fires in the Gulaben coalfield are strongly linked to mining activities exclusively related to former private mining operations. Fire fighting activities were undertaken in recent years in the Gulaben coalfield, mainly by means of the injection of water-clay or a special mixture of cement and water clay in open cracks. This method’s

a) b)

c)

Figure 4-3: Fire fighting activities in

the Ruqigou and Gulaben coalfields. Coal fire fighting by a) excavating burning coal seams, b) by blurring water (arrow) directly on surface coal fires, c) by injecting a water / cement mixture in open cracks.

4 Geographic and geological overview of the Ruqigou, Gulaben and Wuda coalfields

primary aim is to isolate coal fires from non fire areas, and was successfully applied to cease a coal fire close to mining operations (DMT, 2001).

Nineteen active coal seam fires (e.g. figure 4-2) were reported from the Ruqigou coalfield, affecting an area of more than 2000000 m². The coal fires are widespread along the Ruqigou syncline and are in most cases caused by mining activities. The Ningxia Administration Bureau for Safety Inspection for Coal has, since the late 1970's, undertaken intensive fire fighting activities in the Ruqigou coalfield. In total, around 80 million CNY (~ 8 million Euro) has already been spent on fire fighting activities, with an additional 30 million CNY allocated for the near future. About 80 % of the active coal fires are currently covered by extinction activities, resulting in a nearly complete absence of surface coal fires. Active fire fighting is commonly undertaken by either covering the fire with a loess blanket, by injecting a water-clay-cement mixture in open cracks or by directly excavating the fire (figure 4-3). The fire excavation is often carried out by private companies which gain the right to sell the excavated 'fire coal' (DMT, 2001).