## 1 Introduction  
Conventional thermal power plants (CTPP) play a key role to deal with one of the biggest challenges of the energy sector: the reliable and efficient supply of electricity. The CTP provides backup energy to the electric power system to balance the variable electricity demand and the intermittent generation of non-conventional renewable energy sources such as wind and solar energy. When coping with normal demand variations or when variable renewable energies cannot meet the demand, CTTP generation has to be adjusted employing lower or greater production of electricity (reduction or increase of load) respectively. This adjustment is also applied by doing start-ups and shutdowns of equipment in the power plants. In this context, rapid startups and shutdowns improve the operational flexibility of the power plant by adding to it competitive advantage [1; 2; 3; 4]. Therefore, the optimal design of operating profiles is a research area with high potential [5; 6; 7; 8].  
A thermal power plant often uses a steam generator to take advantage of the heat obtained from its main electricity generation process. In a steam generator, water is heated and then turned into steam that spins a turbine, which is connected to a generator that produces electricity.  
To create high-temperature, high-pressure steam in the steam generator, fuel energy is converted to heat, which is transferred to the drum boiler.  
Figure 1 shows a typical drum boiler. As explained by [PERSON] and [PERSON] [9] the drum boiler has a reservoir for water and steam with a water inlet and a steam outlet at the top. The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam-water mixture. The difference in densities between hot and cold water along with the gravity contributes to the accumulation of the \"hotter\" water and saturated steam into the drum boiler.  
The drum boiler has the potential to improve the competitiveness of a thermal power plant by reducing its startup time. The rate at which a boiler can be brought up to a normal operating state depends on its size, and the length of time it has been shut down. In general, the larger and colder a boiler, the longer it takes to startup.  
The startup is carried out by operating the control valves in ways that the steam flow rate and pressure increase up to their normal setpoints when the steam pressure and combustion control system can be switched over to automatic.  
Although fast startups improve the competitiveness in an open power market, if a startup is carried out too fast, excessive thermal stress can occur in the drum boiler components [10]. Therefore, feasible operating procedures must consider the physical constraints of the drum boiler that ensure its integrity.  
Operating procedure synthesis (OPS) can be described as a planning problem, where actions and their sequence must be found in order to take the process from an initial state to a goal state, such as in startups and shutdowns [11]. Additionally, transient operations can take advantage of OPS in terms of both safety and cost [12].  
Despite the focus on startup-time reduction, current approaches [9; 13; 14] fail shortly because they can only obtain startup curves of the drum boiler state variables but cannot identify the corresponding control actions (operations) and their sequence.  
This paper is structured as follows: Section 2 contains the literature review. Section 3 the problem description. In Section 4 the proposed approach is presented. Next, the case study is described in Section 5. Then, in Section 6 the experiments and results are presented. Finally, Section 7 presents the conclusions and future work.