## 1 Introduction  
Major worries about a number of causes, most notably climate change, the scarcity of oil and its consequent rise in price, population levels, and energy consumption, are rapidly dominating the world's energy supply and demand landscape. Finding a substitute for fossil fuels, especially petroleum fuels, is therefore crucial from an economic, environmental, and social standpoint [1]. Primary fossil fuels are converted somewhat inefficiently into electricity. The conventional generating plant achieves efficiencies of 50% to 60% only, because most of the heat energy is wasted during the conversion process and discharged into the environment [2]. Cogeneration, also known as combined heat and power (CHP) generation, is an advanced and modern technology that outperforms conventional energy conversion systems and is also environmentally friendly [3]. CHP systems are systems that simultaneously provide consumers with electricity and meet heating demands [4]. A tri-generation system (cooling, heating, and power generation) can be created by integrating thermally activated technologies into the CHP system to fulfill the consumer's cooling requirement [5]. CHP systems can enhance the efficiencies of thermal power plants by over 90% and decrease their environmental effects [6]. CHP systems get more attention because they can enhance the economics and sustainability of electrical generating units [7]. CHP economic dispatch can improve the efficiency of the energy conversion process in thermal power stations and reduce the cost of power generation [8]. CHP units have the capacity to generate electricity from a range of fuels while simultaneously recovering and reusing the heat that would typically be lost during the creation of electricity [9]. Moreover, the use of CHP generation systems decreases pollutant emissions, such asCOx, SOx, and NOx [10; 11; 12; 13; 14; 15]. Because of these factors, researchers have focused increasingly on CHP units in recent years in an effort to fully explore their potential for meeting consumer demands for heat and electricity [16; 17]. The economic dispatch (ED) issue, which may be seen as the researchers' initial attempt to maximize the advantages of power systems, tries to determine the best scheduling of the generation units to reduce the fuel cost of power generation subject to operational and technological restrictions [18; 19]. The combined economic dispatch problem not only offers significant economic power generation advantages but also lessens the negative consequences of polluting gases [20; 21; 22]. During studies in this area of research, it was found that many available articles showed the effectiveness of heuristic optimization for the solution of combined heat and power economic dispatch (CHPED), but no one had demonstrated a complete comparative study between all the proposed heuristic approaches in this research area. Due to this research gap, the authors of this article were motivated to conduct this study. The objective of this research is to investigate the best heuristic optimization techniques used to address the nonconvex and non-smooth CHPED optimization issues. The bulk of the articles that used heuristic optimization techniques to find the optimum solution to the CHPED issue are discussed in this proposed article, to the best knowledge of the authors. To familiarize readers with the heuristic approaches used, a brief explanation of the utilized heuristic methods is given, and the most important contributions of each research work are introduced. Additionally, in order to create a helpful survey on the usage of heuristic approaches for the solution of the CHPED issue, the best solutions found in the articles under consideration are tallied. There are comparisons between the publications that have been examined in terms of objective function, restrictions, minimal operational cost, and computing time. This article will be of great use to scholars looking at the best generation planning for CHP systems.