## 4 Content Prediction of Free Calcium Oxide (f-CaO) in Cement Clinker Using Machine Learning
### Feature Selection in the Distribute Control System Data  
We invoke a simple iterative permutation approach to evaluate the significance of various features in the DCS system dataset on the performance of the predictive models (other automated feature selection methods, e.g., [40], may also be used). This assessment is conducted using the permutation feature importance metric applied to the obtained prediction results, which involves randomly shuffling the features and accordingly measuring the corresponding changes in the prediction results of the trained models, thus quantifying the importance of each feature and its effects on the predictive model. Based on this evaluation, the following five features, out of many others, demonstrate substantial importance during the process of random shuffling, and are consequently selected for model training:  
1. _Intel temperature of waste heat power generation_. The inlet temperature of waste heat power generation refers to the temperature of the waste heat entering the power generation system, which reflects the cooling efficiency of the grate cooler. A high inlet temperature of waste heat power generation indicates poor cooling efficiency of the grate cooler, resulting in slow cooling of the clinker and inadequate dispersion of internal heat, leading to an unstable and thus lower-quality clinker structure. If the grate speed is stable but the inlet temperature of waste heat power generation is too low, it could suggest that the clinker temperature is insufficient. 2. _Secondary air temperature_. The secondary air refers to the air that is reintroduced into the rotary kiln after undergoing heat exchange at the kiln outlet. Maintaining an appropriate temperature for the secondary air is important in cement production. An optimal secondary air temperature can facilitate the sintering of clinker particles, enhancing the degree of sintering and consequently improving the strength and stability of the cement. Moreover, maintaining an appropriate secondary air temperature can stimulate the hydration reaction of gypsum, leading to the production of an appropriate amount of hydrated calcium sulphoaluminate, which, in turn, enhances the cement's property of resistance to sulfate erosion. 3. _Flame temperature in rotary kiln_. The rotary kiln flame temperature refers to the temperature of the combustion flame inside a rotary kiln used in cement production. This temperature is a critical parameter as it directly affects the efficiency of the combustion process, the quality of product formation, and energy consumption. An appropriate  
Figure 5: Thermal image showing the materials discharge position in the grate cooler and the corresponding class activation map in image classification using CNNs. flame temperature can enhance the sintering degree of clinker, stimulate the formation of mineral phases, and facilitate the growth of crystals within the clinker, contributing to an increase in the strength and stability of the resulting cement clinker. 4. _Clinker temperature._ Clinker temperature refers to the temperature of the solid material that emerges from the rotary kiln in cement production after undergoing various chemical reactions and thermal processes. It is one of the key factors that affect the degree of sintering in clinker formation. Increasing the clinker temperature enhances the fusibility of coal ash, making it more prone to react with the mineral materials inside of the rotary clinker. This improvement in reactivity can lead to enhancements in cement quality. Clinker temperature changes can potentially impact the morphology and structure of clinker particles, subsequently influencing the particle distribution and flow characteristics of the resulting cement clinker. 5. _Grate bed pressure._ Grate bed pressure refers to the pressure exerted on the surface of grate coolers in cement production. The pressure on the grate bed is an important operational parameter in controlling the cooling process.