Examples of oxyfuel cycles

Im Dokument Thermodynamics and Turbomachinery of the Oxyfuel Naki Cycles (Seite 22-25)

Below some examples of oxyfuel cycles are described. Other oxyfuel cycles (e.g., the Lorentzen-Pettersen cycle or the van Steenderen project) are reproduced in [6]. The COOPERATE demo power cycle, which is similar to the Naki cycle, is presented in [7]. Reviews of different oxyfuel cycles are given in [8].

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1.5.1 Oxyfuel Steam Cycle

Fossil fuel like coal is burned with pure oxygen in a boiler. Recirculated exhaust gas is delivered to the combustion area to decrease the temperature. Hence, the hot gas consists only of CO2 and H2O. The hot gas is cooled down in the boiler gener-ating steam for a steam cycle. After leaving the steam generator, the combustion-generated CO2 and H2O are separated. The remaining mass flow is fed back to the combustion area. The H2O of the removed mass flow is separated by an intercooled compression with water separation. The remaining combustion-generated CO2 can then be liquefied and is ready for transport and storage. [5]

1.5.2 Semiclosed Oxyfuel Combustion - Combined Cycle

The Semiclosed Oxyfuel Combustion - Combined Cycle (SCOC-CC) [13] consists of a high-temperature Brayton cycle (high-temperature turbine, combustion chamber, compressor, and HRSG) with CO2 as working fluid and a conventional bottoming double-pressure steam cycle (high-pressure turbine, low-pressure turbine, condenser, and feeding pumps).

Recycled CO2 is supplied to the combustion chamber and is heated up to 1 400

C by burning fuel. Fuel and a nearly stoichiometric mass flow of oxygen are fed to the combustion chamber, which is operated at a pressure level of 40 bar. The exit gas of the combustion chamber which consists of CO2 and H2O drives a high-temperature turbine. After this it passes through a HRSG to generate steam for a conventional double-pressure steam cycle. In a condenser the H2O condenses and can easily be separated. The remaining CO2 stream is compressed by a compressor and fed to the combustion chamber again, after the combustion-generated CO2 has been removed.

The SCOC-CC provides an efficiency of nearly 50 % [13].

1.5.3 Graz Cycle

The Graz Cycle [13, 16, 26, 27] consists of a high-temperature cycle and a low-temperature cycle. Fossil fuel, together with a nearly stoichiometric mass flow of oxygen, is fed to the combustion chamber, which is operated at a pressure level of 40 bar. The high flame temperature is reduced by circulating working fluid and steam from a high-pressure steam turbine. Hence the exit mass flow of the combus-tion chamber (working fluid) consists only of H2O and CO2. After the combustion chamber the working fluid has a temperature of 1 400 C and drives the high-temperature turbine. Then it passes through the HRSG that produces the steam for the high-pressure turbine. After the HRSG about 55 % of the working fluid is compressed and fed back to the combustion chamber. The remaining mass flow which contains the combustion-generated CO2 and H2O is fed to a condensation process in which the water is removed. The condensed water is fed to the HRSG in which it is vaporized and superheated. It then drives the high-pressure turbine and afterwards is fed to the combustion chamber. The condensation heat is used in a

low-pressure steam cycle for further power generation. At the end of the condensa-tion process the captured CO2 is ready for further use or storage.

The Graz Cycle provides an efficiency of about 53 % [13].

1.5.4 Matiant Cycle

The Matiant Cycle [10] is a power cycle with internal combustion, reheating (also internal combustion), and CO2 as working fluid. CO2 is heated up in a combustion chamber by burning natural gas with a stoichiometric mass flow of oxygen. With a temperature of 1 300 C and a pressure of about 40 bar the working fluid drives a turbine and afterwards it is reheated to 1 300 C. The reheating is also done with natural gas and a stoichiometric mass flow of oxygen. After driving a second turbine the working fluid is fed to a recuperative heat exchanger where it is cooled down. In a staged compression with intercooling and water separation the H2O is removed, and finally the CO2 is condensed at a pressure level of 70 bar. The liquid CO2 is pumped to a pressure level of 300 bar and fed to the recuperative heat exchanger.

But before the recuperative heat exchanger, the combustion-generated CO2 is re-moved. The supercritical CO2 is heated up to about 600 C and then it drives a further turbine. This turbine expands the working fluid to 40 bar. The exit mass flow of this turbine is fed back to the recuperative heat exchanger and heated up, before it is fed to the combustion chamber again.

The Matiant Cycle reaches an efficiency of about 45 % [10].

1.5.5 Chemical Looping Combustion

The Chemical Looping Combustion Cycle [9] gets the oxygen that is needed for the combustion of fossil fuels from a chemical process and therefore no air separation unit is needed. There are two alternatives:

• Chemical Looping Combustion - Combined Cycle [9]

Compressed air is piped into an oxidation reactor where the oxygen of the air reacts with a metal to form a metal oxide. As this reaction is exothermic, the air is heated up. The exhaust air of the oxidation reactor which has a reduced oxygen content drives an air turbine and afterwards passes through an HRSG to generate steam for a multipressure steam cycle, before it leaves via the stack. The metal oxide produced flows to the reduction reactor, where it is reduced to metal by fossil fuel which is also fed into the reduction reactor.

In other words, the fuel is burned with the oxygen that was chemically bound to the metal. As a result of this oxygen transport, there is no nitrogen in the area where the fuel is burned just the combustion-generated gases CO2 and H2O. Next, the metal is transported back to the oxidation reactor. The combustion-generated gases drive a so-called CO2 turbine, and are then used to preheat the fuel. The H2O of the exhaust gas is removed through a two-stage intercooled compression with water separation. The nearly pure CO2 is then compressed to 80 bar and further cooled down to 30 C. At this pressure and temperature level it is liquid and can be pumped to 100 bar by a pump.

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It is then ready for transport and storage.

The Chemical Looping Combustion - Combined Cycle provides an efficiency of about 50 %.

• Chemical Looping Combustion - Steam Cycle [9]

In the Chemical Looping Combustion alternative the oxygen reactor is built as a steam generator. The reactors work at atmospheric conditions (pressure).

The exhaust gas generated in the reduction reactor is used only to preheat the fuel, before the H2O is removed and the CO2 is liquefied.

This alternative has an efficiency of about 40%.

1.6 History of the Naki Cycle, an oxy-fuel cycle originally

Im Dokument Thermodynamics and Turbomachinery of the Oxyfuel Naki Cycles (Seite 22-25)