Thermal analysis for obtaining coal ignition characteristics in boilers—Part 1: Specific heat capacity, ignition temperature, and ignition heat

The ignition temperatures of coals/low-temperature pyrolysis chars obtained by existing thermal analysis are 200–300 K lower than those of coals in boilers. It is generally believed that the low heating rate β of the thermal analyzer is the main reason for the above results, but the essential reason should be that the reactivity of the sample selected in the previous study is too high, which leads to the advance of coal/char ignition. Meanwhile, the commonly used TG-DTG tangent method is not accurate enough to calculate the ignition temperature. In order to solve the above problems, the nonisothermal TG-DSC was used to obtain the ignition characteristics of two high-temperature (1 400 ℃) fast pyrolysis Zhundong coal chars under different βs, including specific heat capacity, ignition temperature and ignition heat, and the effect of β on the ignition characteristics of coal chars was investigated. The specific heat capacity of coal char was obtained by David Merrick model, which was not affected by β but related to the change of temperature. The relationship between specific heat capacity and temperature was described by an exponential function. The specific heat capacity values of the two types of coal char are 0.65–1.99 kJ/(kg·K) in the range of 0 to 1 600 ℃. The ignition temperature of coal char was obtained by the DSC or DTG inflection point method based on Semenov thermal explosion theory and the TG-DTG tangent method. The results show that the ignition temperatures obtained by the DSC and DTG inflection point method were equivalent, and always higher than that obtained by the TG-DTG tangent method. The ignition temperature increased with the increase in β, but the level of increase gradually decreased, which can be described by an exponential function. The Limiting ignition temperatures of coal chars were obtained by setting β as the commonly heating rate of coal conversion in boilers, namely 10⁵ K/s. The Limiting ignition temperatures of coal char obtained by DSC inflection point method and TG-DTG tangent method are 673–680 ℃ and 644–659 ℃, respectively. The former is consistent with the ignition temperature of the same type of coal samples obtained in boiler-like environment reactors such as the entrained flow reactor, as in the literature, while the latter will underestimate the ignition temperature. Combined with the fact that the reactivity of high-temperature pyrolysis char is lower than that of coal/low-temperature pyrolysis char, indicating that the preparation of high-temperature (equivalent to the boiler ambient temperature) pyrolysis char with appropriate reactivity and the use of DSC or DTG inflection point method to obtain the limiting ignition temperature of char can realize the prediction of coal ignition temperature in boiler by thermal analysis. According to the Limiting ignition temperature, the ignition heats q_ig of the two coal chars in the boiler are 3 414–3 669 kJ/kg, and the theoretical ignition heats of the two coal chars under the air/coal ratios of 0.9–2.5 were calculated to be 1 826–2 811 kJ/kg. q_ig is larger than the theoretical ignition heat value of the corresponding coal char, indicating that the theoretical ignition heat value may overestimate the actual ignition capacity of coal in boilers. This study provides a comprehensive interpretation of the rationality of thermal analysis to obtain the coal ignition characteristics in boilers. The proposed thermal analysis methodology can provide a reference for obtaining the basic data required for CFD simulation of coal ignition characteristics in boilers.