## 5 The ECOAL Project Implementation
### Field System Installation  
To support the prototype design, its installation, and the continuous measurement of temperature and gaseous emissions, the monitoring of temperature in a region of the surface of the coal waste pile (Figure 7A) was performed monthly since February 2013, using infrared thermography [7]. Figure 7B illustrates the thermographic image acquired in February 2015, where the main focus of the combustion process is identified. The results obtained from these prior monitoring temperature campaigns  
Figure 6: Output of the sensing system versus ammonia (NH\({}_{3}\)), carbon dioxide (CO\({}_{2}\)), and methane (CH\({}_{4}\)) concentrations using the multi-pass cell of 160 cm. supported the prototype design and installation for continuous measurement of temperature and gas and contributed to the validation of the measurements performed with the fiber sensing technology. The geographical layout of the deployed fiber optic sensing system is illustrated in Figure 7C, while the general architecture of this system is shown in Figure 8. For distributed temperature measurement, Brillouin Optical Time-Domain Analysis (BOTDA) is proposed to monitor the waste pile, with a single-mode fiber layout extended over the whole area of the waste pile undergoing combustion. The optical cable for distributed temperature measurement could not be a standard one considering the high temperatures. Therefore, a special optical cable fabricated by Brugg (5 mm HDPE - high-density polyethylene incorporating a stainless steel tube) (Brugg Group, Switzerland) was used, permitting reliable operation up to 150 \({}^{\circ}\)C, which can be extended up to 180-200 \({}^{\circ}\)C if the cable is installed without tension and with small curvature. The fibers were loose inside the cable and it was ensured that the cable thermal expansion did not affect the fibers. A length of \(\sim\)2600 m of cable was buried at 10-20 cm depth in platforms that were prepared in the pile slope with a \(\sim\)1 m width (a deeper burial would be risky considering the temperature resistance of the cable). It runs along an extension of \(\sim\)1300 m of the coal pile in the regions where the combustion process seemed to be more intense (Figures 1C and 7A; views of the installation process are shown in Figure 9A. At the end of such a region, a splice box permitted the cable to turn around, returning along the same physical path (which explains why the cable extension used double the measurement region length). As two fibers are used in the measuring cable, redundant measurements are obtained in each position. The temperature estimation is, therefore, achieved by averaging the results obtained from  
Figure 7: (**A**) General view of the area under combustion where is visible the platforms along the slope and the gaseous emissions; (**B**) thermographic image used for preliminary temperature characterization of the region; (**C**) aerial view of S. Pedro da Cova coal waste pile (red contour), showing: (i) region of distributed temperature measurement cable with approximately 1300 m (yellow line); (ii) gas measurements spots (end of green lines); (iii) optical fiber cable path to the data collection station. the two independent measurements realized in the same position. The BOTDA architecture used in this case is essentially similar to the conventional systems described in the literature [46], except that the proposed system incorporates balanced detection [47]. Balanced detection provides substantial advantages in this case as it eliminates common-mode noise, provides an improved signal-to-noise ratio over the conventional single-detector systems, and offers a method of eliminating polarization noise in the BOTDA signal by employing orthogonal probe sidebands. It also delivers a better use of the dynamic range of the detector as it avoids saturation and allows higher input powers. High spatial resolution measurements are performed using the differential pulse-width pair technique [48].