*2.1. Large-Scale Coal Spontaneous Combustion AE Experiment*

#### 2.1.1. Preparation and Analysis of Coal Samples

In this paper, the main objective is to investigate the characteristic patterns and differences of AE signals generated at different temperature stages during the warming and combustion of coal. In addition, we studied the mechanism of AE generation, which provides a theoretical basis for AE signal inversion of temperature and monitoring of coal spontaneous combustion. Based on this, bituminous coal (DT) sample from the Dongtan coal mine in Shandong, China, was selected for the large-scale coal spontaneous combustion AE experiment, with a weight of 3 tons. In order to ensure that the coal pile has better heat storage conditions, some of the coal blocks were crushed below 30 mm. The volatile fraction (Vdaf) and five elemental contents of C, H, O, N, and S of DT coal samples at room temperature were analyzed by a 5E-MAG6700 fully automatic industrial analyzer, and the volatile fraction and elemental composition of coal samples were determined according to the national standard GB/T 212-2008 as shown in Table 1.


**Table 1.** Volatile and elemental analysis of coal samples.

#### 2.1.2. Experimental System

The large-scale coal spontaneous combustion AE test system is mainly composed of a furnace body, an AE monitoring system, a hot air input system, and a thermocouple temperature measurement system. Among them, the furnace body is cylindrical, the surface is a metal shell, and the interior is composed of a refractory brick layer, polyurethane thermal insulation layer, water sandwich layer, etc. The hot air input system consists of an air pump, a heating tank, a metal hose, and a temperature control box. The thermocouple temperature measurement system is composed of K type thermocouple and a temperature recorder. The AE monitoring system is composed of an AE probe, AE host, preamplifier, and waveguide rod, where the AE sensor operates in the frequency range of 10 KHz–2 MHz. Figure 1 shows a large-scale coal spontaneous combustion AE test system diagram.

**Figure 1.** Large-scale coal spontaneous combustion AE test system diagram.

2.1.3. Experimental Methods and Steps

The large-scale coal spontaneous combustion AE experiment is divided into three steps:


The noise reduction process is performed before the experiment, and the threshold value is set in advance. In order to avoid noise triggering, the threshold value is set relatively high, and the signal below the threshold value is not processed in any way. If the waveform below the threshold value is also found to be valuable, then the relatively weak signal can be analyzed by resetting the threshold value. During this experiment, we try to keep it quiet to avoid generating too much noise, and the threshold is set to 5 mV. When extracting the data, the part of the signal is masked out by the set threshold value. The preamplifier used in this experiment has a multiple of 40 dB and a sampling frequency of 200 kHz. During the heating process, the thermocouple temperature measurement system continuously records the internal temperature of the furnace body and saves the temperature data to the temperature recorder.
