Impact of exhaust gas

Below are excerpts from a paper, ‘Flue Gas Recirculation in a gas turbine: Impact on performance and operational behavior' by Frank Sander, Richard Carroni, Stefan Rofka and Eribert Benz of Alstom Power, Baden, Switzerland, at the ASME/IGTI Turbo Expo in Vancouver.

For natural-gas-fired power plants, post-combustion CO2 capture is the most mature technology for low emissions power plants. The capture of CO2 is achieved by chemical absorption of CO2 from the exhaust gas of the power plant. Compared to coal fired power plants, an advantage of applying CO2 capture to a natural-gas-fired combined cycle power plant (CCPP) is that the reference cycle (without CO2 capture) achieves a high net efficiency. This far outweighs the drawback of the lower CO2 concentration in the exhaust.

Flue Gas Recirculation (FGR) means that flue gas after the HRSG is partially cooled down and then fed back to the GT intake. In this context, FGR is beneficial because the concentration of CO2 can be significantly increased, the volumetric flow to the CO2 capture unit will be reduced, and the overall performance of the CCPP with CO2 capture is increased. For natural gas fired power plants the most promising capture technology is post-combustion CO2 capture.

In this paper, a combined cycle power plant with CO2 capture is presented which is based on a Alstom’s reheat GT24/GT26 in single shaft arrangement In the case of a CCPP with CO2 capture (without flue gas recirculation), the flue gas leaving the HRSG is further cooled before entering the CO2 capture unit. Cooling is carried out by means of a direct contact cooler (DCC) in which water is sprayed into the gas stream. Due to the water produced in the combustion process, some condensate forms and is drained in the DCC. The flue gases are cooled down to around 40 °C. A blower compensates the additional pressure drop and then propels the flue gases into the CO2 capture unit.

In this work Aqueous Ammonia Process (AAP) is used as CO2 capture technology. Steam extracted from the IP/LP cross-over provides the necessary regeneration energy to the stripper in the AAP unit. The hot condensate is subsequently returned to the water/steam cycle. This configuration is used as reference cycle in this work.

The impact of flue gas recirculation on the overall performance (power output and efficiency) is compared to this reference cycle.

In a conventional combined cycle power plant the energy of the flue gases leaving the gas turbine are utilized in a heat recovery steam generator (HRSG). If flue gas recirculation is applied to such a conventional CCPP, part of the flue gas is recirculated back to the inlet of the GT. After the HRSG the recirculated flue gas is further cooled down, close to ambient temperature, before being mixed with fresh ambient air. An additional blower is located between cooler and mixer to overcome the additional pressure drop in the FGR-path.

CO2 enrichment due to FGR

The desired effect of FGR is the so-called “CO2 enrichment” because the concentration of CO2 increases significantly due to FGR. The fraction of flue gases which are recirculated back to the GT range from 30 to 50 % of the GT exhaust mass flow. For these FGR-ratios the CO2 concentration would be between 6.0 and 8.7 mol-%. Depending on the FGR ratio, the CO2 concentration can be doubled at the exit of the GT (4.0 mol-% without FGR). The flue gas is split into two streams after the DCC. The exhaust gas is fed to a mixer where it is mixed with fresh ambient air before entering the compressor of the GT. The remaining proportion the exhaust gas is treated by the CO2 capture unit.

The major consequence of flue gas recirculation is that the CO2 concentration increases, whereas the O2 concentration decreases. The change in composition is determined by the amount of recirculated flue gases. This is expressed by the FGR-ratio, which is defined as the mass flow entering the mixer in relation to the GT exhaust mass flow. As previously mentioned, a typical range of FGR-ratio is 30 to 50 %.

The main findings of the investigations presented in this report are:

• Reheat GT promotes operational flexibility, both with and without flue gas recirculation.

• The flue gas temperature after DCC is the most influential FGR-parameter (in the flue gas path) affecting performance.

• The high inlet temperature of the SEV combustor is beneficial in terms of flame stabilization, in particular, for high FGR-ratio resulting in low oxygen concentrations.

• Flue gas recirculation reduces regeneration steam requirements of CO2 capture unit.

• For the reference cycle (CCPP with CCS): FGR increases net power and efficiency by 3.6 % and 2.1 % respectively.

• For the reference cycle (CCPP with CCS): FGR reduces CoE by 5%. It also reduces minimal CO2 price needed to make CO2 capture viable. Nevertheless, the minimum feasible CO2 prices are markedly higher than the current market levels.

Furthermore, the effects of flue gas recirculation upon the combined cycle power plant itself (without CO2 capture) results in the main findings presented in this report:

• The temperature of the recirculated flue gas (after the direct contact cooler) determines mostly the impact on the GT and the CCPP.

• FGR leads to both an increased compressor inlet temperature and a higher GT exhaust temperature. These two effects (partly) compensate each other such that the GT is more affected than the CCPP.

• In terms of change in working fluid the CO2 concentration depends on the FGR-ratio, whereas the water content strongly depends on the temperature of the recirculated flue gas (after the direct contact cooler).