# Intelligent Measurement of Coal Moisture Based on Microwave Spectrum via Distance-Weighted kNN
## 1 Introduction  
Coal is a significant energy resource. For most countries, the coal-based energy structure will be maintained for a long time [1, 2]. Hence, enhancing the coal utilization efficiency is one of the best solutions to alleviate the contradiction between energy consumption and environmental protection [3]. The moisture content (MC) of coal is closely related to its calorific value. In the process of coal coking and burning, both excessive and low MC will cause many problems, including reduced coal utilization efficiency, environmental pollution, and energy waste [4, 5]. Therefore, rapid and accurate measurement of coal MC is of great significance. The coal MC measurement methods can be mainly divided into direct and indirect methods. The direct measurement method is the standard gravimetric method, which is commonly known as a loss-in-weight method, and it can be mainly divided into nitrogen drying and air drying [6]. The standard gravimetric method evaporates free (surface) moisture by drying coal at a controlled temperature and weighing it regularly to determine the dry weight. This is a laboratory measurement method, which can provide a high measurement accuracy. However, this method is time-consuming and can affect the original properties of a coal sample, which is not conducive to rapid and online measurement of coalMC. Currently, the conventional indirect measurement methods mainly include neutron [7], conductance [8], capacitance [9], and infrared reflection [10]. The principle of coal MC measurement by the neutron method is based on measuring the hydrogen content in a coal sample. However, in addition to water, many other chemical impurities in coal also contain hydrogen [11]. In addition, this method relies on expensive equipment due to the risks of using radioactive neutron sources. The conductance and capacitance methods are susceptible to changes in various factors, such as temperature, density, and electrolyte content, which leads to a poor accuracy without compensation strategies [12; 13]. The infrared reflection method has a good determination effect on the moisture content of mixed liquid samples; however, it is limited to the measurement of surface moisture of small solid samples (penetration depth within microns) [14]. The characteristics of the aforementioned methods are summarized in Table 1. Due to the shortcomings of these methods, there is an urgent need for a measurement method that can rapidly and accurately determine coal MC. The microwave method is a new method for MC determination, and has gradually developed into an independent rapid and non-destructive measurement technology with the development of microwave electronics and microwave measurement technology. The microwave method has the advantages of being safe, stable, non-destructive, and contactless, and it has been successfully applied to many MC measurement applications, including food crops, light industrial products, and building materials. For instance, [PERSON] et al. [15] used the traveling-standing wave attenuation method to predict the grain MC. They established temperature and bulk density calibration models based on the maximum field attenuation of the transmission traveling-standing wave. [PERSON] et al. [16] derived a calibration equation for determining the MC of cotton and polyester blended fabrics and predicted and evaluated the MCs of different fabrics under the condition of a 10-GHz microwave propagating in free space. [PERSON] et al. [17] used a microwave method to characterize the dielectric constants of two types of stones in building structures at different humidity levels. The obtained laboratory results could be used to derive MC-relative dielectric permittivity calibration curves that could be employed for noninvasive on-the-field moisture monitoring of structures made of the two types of stones. The free-space transmission measurement method with the antenna group as a signal transceiver and the air as a signal transmission medium has been the most used method in the microwave MC measurement [18].