# Integrating Process Re-Engineering Models in Cement Production to Improve Energy Efficiency
## 1 Introduction  
The cement industry is a significant contributor to global energy consumption and CO\({}_{2}\) emissions [1], responsible for an estimated 7-8% of worldwide CO\({}_{2}\) emissions [2]. With a current global production of 4.4 billion tons [3], both production and demand are expected to grow substantially by 2030 [1, 3]. The immense energy demand of cement production, typically consuming 3.4 GJ per ton [3], highlights the urgency for innovative and sustainable alternatives to traditional cement production methods. Essentially, the conventional production of cement begins with mining, grinding raw materials into fine powders and heating the fine mixtures to form clinker [3, 4]. The clinker formation process, which is the main stage in the production of cement, consumes over 70% of the total energy required [2, 4, 3]. Reducing energy consumption and carbon emissions during cement production is critical as it affects both the cost of production and the impact on greenhouse gas emissions. A large portion of CO\({}_{2}\) emissions in the cement industry comes from the clinker production process, which involves heating limestone raw materials and burning fossil fuels. Addressing both energy consumption and carbon emissions in cement production is essential for safeguarding the environment, ensuring economic sustainability, and improving public health [5]. Clinker formation involves heating to calcinate a raw mix of calcium carbonate (CaCO\({}_{3}\)) and alumina-silicate materials such as clay in a rotating kiln to approximately 1450 \({}^{\circ}\)C [6; 7; 8; 9; 10]. This process causes the thermal breakdown of limestone, releasing carbon dioxide (CO\({}_{2}\)) and producing calcium oxide (CaO) [6]. Concurrently, the remaining oxides in the mixture undergo solid-state sintering reactions [7]. This high-temperature fusion, which inhibits full melting, promotes the formation of the main phases of tricalcium silicate (Ca\({}_{3}\)SiO\({}_{5}\)) and minor phases of dicalcium silicate (2 CaO.SiO\({}_{2}\)) alongside tricalcium aluminate (3 CaO.Al\({}_{2}\)O\({}_{3}\)) [8]. Essentially, the energy used to heat the raw mix to start chemical reactions usually radiates from the kilns through the kiln shell [9], amounting to 15-16% of the total energy input [10]. The hot gases also leave the kiln and carry heat energy in the form of CO\({}_{2}\) and other combustion products. Unfortunately, around 35-39% of the total energy input is wasted during the heating process [11]. Energy consumption during clinker formation is highly dependent on the combustion system of the kiln [12], and optimising the performance of the kiln entails reducing energy losses during clinker formation [11; 13]. The applicability and effectiveness of optimising the performance of the kiln vary between different kilns depending on several factors, such as kiln design and type, technology and equipment, operational practices, availability of alternative fuels, and the cost of modifications relative to the potential savings from reduced energy consumption [13]. One of the effective strategies for reducing energy losses for kilns is predictive maintenance, essentially done by adding extra sealing and insulation to the kilns and using superior refractory materials in the kiln lining [14]. Equally, other strategies for reducing energy losses for real-time kiln operations involve integrating complex control systems, such as implementing waste heat recovery systems to capture and reuse heat from the kiln's exhaust or using advanced process control (APC) systems, which predict the performance of the kiln and make adjustments in real time to optimise fuel consumption and clinker quality [15; 16]. Furthermore, energy improvement strategies for the kilns also involve employing model-based algorithms that can learn and predict changes to processes and materials to enhance energy optimisation [17; 18]. Optimisation of energy consumption for cement production, using a model-based approach, can improve kiln performance and reduce energy losses [19].