Optimization of the Closed Supercritical CO2 Brayton Cycle with the Detailed Simulation of Heat Exchangers

Recently scientists and engineers have turned their attention again to carbon dioxide as a working fluid to increase the efficiency of the Brayton cycle. But why has this become such a focus all of a sudden?

The first reason is the economical benefit. The higher the efficiency of the cycle is, the less fuel must be burned to obtain the same power generation. Additionally, the smaller the amount of fuel burned, the fewer emission. Therefore, the increase in efficiency also positively affects the environmental situation. Also, by lowering the temperature of the discharged gases, it is possible to install additional equipment to clean exhaust gases further reducing pollution.

So how does all of this come together? Figure 1 demonstrates a Supercritical CO2 power cycle with heating by flue gases modeled in AxCYCLE™. This installation is designed to utilize waste heat after some kind of technological process. The thermal potential of the exhaust gases is quite high (temperature 800° C). Therefore, at the exit from the technological installation, a Supercritical CO2 cycle was added to generate electrical energy. It should be noted: if the thermal potential of waste gases is much lower, HRSG can be used. More information on HRSG here: http://blog.softinway.com/en/introduction-to-heat-recovery-steam-generated-hrsg-technology/

Any cycle of a power turbine installation should consist of at least 4 elements : 2 elements for changing the pressure of the working fluid (turbine and compressor) and 2 elements for changing the temperature of the body (heater and cooler). The cycle demonstrated in Figure 1 has an additional regenerator, which makes it possible to use a part of the heat of the stream after the turbine (which should be removed in the cooler) to heat the stream after the compressor. Thus, part of the heat is returned to the cycle. This increases the efficiency of the cycle, but it requires the introduction of an additional heat exchanger.

The heat exchangers used in the sCO2 cycle are of three basic types: heaters, recuperators, and coolers. Typical closed Brayton cycles using sCO2 as the working fluid require a high degree of heat recuperation.

Supercritical CO2 Power Cycle with Heating by Flue Gases
Figure 1 – Supercritical CO2 Power Cycle with Heating by Flue Gases

Having examined this scheme and examined the process in detail, we can draw the following conclusions about the advantages of this cycle which is demonstrated in Figure 2: Read More

Introduction to Heat Recovery Steam Generated (HRSG) Technology

The acronym HRSG (Heat Recovery Steam Generated) is in different sources describing the operation of cogeneration and heating plants, but what does it mean? Heat Recovery Steam Generated (HRSG) technology is a recycling steam generator which uses the heat of exhaust from a gas turbine to generate steam for a steam turbine generating electricity.

The simplest scheme of a Combined Cycle Gas Turbine (CCGT) is presented in Figure 1.

The simplest scheme of CCGT
Figure 1: The simplest scheme of CCGT.

In Figure 1, the exhaust flue gases temperature on the outlet of the turbine is equal to 551.709 ℃. This is a too high a temperature to release the gasses into the environment. The excess heat is able to be disposed of while receiving additional electric power which is approximately equivalent to 30% of the capacity of a gas turbine.

To reach the maximum economical and eco-friendly criteria possible for the installation, many pieces of equipment are used including: a waste heat boiler (HRSG); turbines with a selection for a deaerator (Turbine With Extraction, Deaerator); feed and condensate pumps (PUMP2, PUMP); a condenser (Condenser); and a generator (Generator 2). Exhaust gases entering into the HRSG transfer heat to water which is supplied by the condensate pump from the steam turbine condenser to the deaerator and further by the feed pump to the HRSG. Here boiling of water and overheating of the steam occurs. Moving further, the steam enters the turbine where it performs useful work.

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