Feedwater ESRD Boiler with Intermediate Reheat Steam
The main feature of this boiler is that it is equipped with a heating system using steam. The purpose of global power generation through expansion is to work with dry steam at high pressure. To obtain dry steam, the expansion process involves initial steam heating, which can be done in two stages. In this first phase of superheating, there will be a relatively small increase in temperature. As the system operates at high pressure, the steam temperature before reheating is close to the critical temperature (the temperature limit that steam can reach due to technical and material limitations, etc.). For this reason, when the steam expands, it takes little to enter the hood and become wet steam. To avoid this, once the first expansion is complete, the steam is reheated, increasing its temperature and allowing for a second expansion, further from the hood, preventing it from entering. For expansion in this type of system, two types of turbines are used: a high-pressure turbine for the first steam expansion and a low-pressure turbine for the second. The steam exiting the high-pressure turbine after this expansion has an increased specific volume. For this reason, the second stage of heating, or intermediate heating, is formed by a set of larger tube bundles. This is because the upper manifold is positioned further away from the combustion chamber than in previous boilers.
Circuit Area and Water Vapor (b)
The water enters the boiler through the feed tube. It then passes through the normal economizer in the flue gas (5). After passing through the secondary or bypass economizer (4), it goes down the tubes, reaching the top collector of the vaporizers (6). In the upper manifold, water and vapor separate, and the steam enters the primary heating, which is divided into two phases as stated. After passing through the first phase (1), the steam can follow two directions:
- The first is to go to the outlet duct, which leads to the high-pressure (HP) turbine.
- The second is to go through the attemperator (9) located in the upper collector, hence completing the second phase of the primary superheater (2), mixing the output with the steam that has not gone through the attemperator and thereby controlling the steam temperature.
Once this phase is complete, the steam expands in the HP turbine. The steam leaving the turbine then passes to the secondary or intermediate superheater (3) and finally completes its expansion in the low-pressure (LP) turbine. The heat from the combustion gases produced in area (a) of the boiler goes to area (b) to transfer its heat to the pipes and steam.
Depending on whether the turbines are running ahead, the reheated steam circuit may be dispensable, and the flow of gases inside the boiler is variable.
Thermal Gas Flow with Turbine Operation Ahead
The thermal gases enter zone (b) of the boiler. Once there, there are two possible paths:
- The first is to pass through the area of the first phase of the primary superheater (1) and the bypass economizer (4).
- The second is through the second phase of the primary superheater (2) and the intermediate superheater (3).
The amount of gases passing through one pipe or another is governed by a set of valves (dampers) located at the top (7, 8). For this mode of operation, the air inlet valve (6) must be closed.
Thermal Gas Flow without Turbine Operation Ahead
Thermal gases enter zone (b) of the boiler. Once there, they may follow a path that passes through the area of the first phase of the primary superheater (1) and the bypass economizer. This happens because the air valve (6) is open, allowing air driven by the fan to generate a downdraft, as the upper valve (8) is closed. In this way, the combustion gases are forced through the first of the paths that have previously been explained, avoiding overheating.