Compressed air injection, an efficient power boost to simple cycle

July 13 2014 - TI Staff

Below are excerpts from the paper ‘Performance Improvement of Gas Turbine With Compressed Air Injection For Low Density Operational Conditions’ (by S. Arias Quintero, S. Auerbach, R. Kraft of PowerPHASE LLC) presented at the 2014 ASME Turbo Expo.

“During peak hours, usually in summer afternoons, power requirement is usually fulfilled with peaker units, which must be able to cycle between stop and full load quickly, running for short periods of time. Generally gas turbines and hydro are used for this purpose. Although useful, peaker power plants are usually operated in simple cycle. With efficiencies below 40%, (compared to near to 60% for combined cycle units) they comparatively cost more to operate and produce more pollutants.

Density of the inlet air can be reduced either by increase in ambient temperature, or by a decrease in atmospheric pressure (high site elevation). Lower air density reduces the mass flow through the turbine section, resulting in lower power output. In addition the compression work is related to inlet temperature, and more work is required to achieve a pressure ratio as temperature increases. Therefore less turbine output is available for the generator, as the power requirement from the compressor is higher. As ambient temperatures increase, compressor discharge temperature also increases, with a potential to excessive heat loads on the turbine cooling system and impacted hot gas path components life. In addition, as extra fuel needs to be burned to produce power compared to a cooler day, the overall pollutant emissions are higher. Besides the performance and environmental penalties produced, high temperature also results in commercial burdens for the operators of gas turbine, since they cannot produce (and sell) power at a moment when the power demand, and the prices are higher.

In today’s competitive environment, it becomes a very attractive proposal for the power generation industry to increase power capability without the capital investments related to new capacity addition, and for this reason various power enhancement schemes have been developed and implemented since the introduction of gas turbines, either by the reduction of inlet air temperature, the injection of fluids through the combustion chamber, or the burn of additional fuel (either as reheated turbines, or additional firing at the HRSG).

At 40F, exhaust flow of an “F” class gas turbine is around 4% larger than on ISO conditions (59F), resulting in higher output (4.5% more). In an opposite fashion, gas turbine output decreases, as the ambient air temperature is higher. Compression work is increased and the heat rate grows. As temperature increases to 95F, power output decreases by 14.2% if compared to ISO conditions, and heat rate increases 4.2% above its ISO rating. As ambient temperature increases, gas turbine exhaust temperature also does, resulting in a lower pressure ratio across the turbine, which has a fixed inlet temperature. The higher exhaust gases help to produce higher temperature steam, but the overall reduction in airflow, and the higher compression work make power output well below the ISO conditions.     

Similar performance degradation occurs for simple cycle plants; with 13.7% reduction in power and 3.9% increase in heat rate for a 95F day. Furthermore, this gas turbine performance decrease coincides with the peak power requirement, making necessary the dispatch of additional generation units to supply the demand.

The proposed compressed air injection (CAI) scheme, commercially known as TurboPHASE is a modular package designed to increase plant power output, by restoring a portion of the missing inlet air flow caused by high ambient temperatures or plant location at high altitudes. Each module is designed to deliver between 10 and 12 lb/s (4.5-5.5 kg/s) of compressed air, which is roughly 1% of the airflow of an “F” class gas turbine.

An intercooled compressor capable to delivering such amount of air, at pressures above 250 psi (1.72 MPa) requires a power input in the order of 2 MW. A high efficiency reciprocating engine fueled by natural gas, or diesel fuel, is proposed to drive the compressor. This point marks significant differences with earlier compressed air injection (CAI) initiatives like those made by Nakhamkin et al. and others, who proposed an electric driven compressor.

As noted more than 2 MW of power is required to move each compressor. If 5% air injection is intended, more than 10 MW of the incremental power produced is required only to move the compressors. Electricity delivered to the grid, which is the ultimate goal behind power production is severely reduced. By using a reciprocating engine that uses the same fuel as the gas turbine to drive the compressors, the auxiliary electrical loads are reduced to a minimum, and all the incremental power can be sold to the grid.

By using hot exhaust air to heat up the compressed air in a recuperator, air can be injected at conditions near identical of the gas turbine compressor discharge plenum (650F or 343.3 ºC).

In an F class engine, for example, the compressor takes around half turbine power produced, and the generator takes the other half. Therefore, if the inlet flow increases by 5%, the total turbine power is increased around 10-12%, but as half this power is used for compression work, the additional generator power only increases by ~5%. On the other hand, the air injected with the proposed CAI system does not require compression work by the gas turbine, and all the extra turbine work goes straight to the generator.

Compressed air injection is a novel technology that taps into the unused capacity that exists in the gas turbine at simple or combined cycle plants at moderate to high ambient temperatures and/or elevations. The incremental power it produces is significant, with an incremental efficiency better than that of a gas turbine in simple cycle, and when applied to combined cycle plants providing a steady power increase across a wide operational temperature range (40-110F) and relative humidity, which can not be attained with other alternatives. Furthermore compressed air injection does not change the composition of the combustion air, like steam injection does, and for this reason the heat loads at turbine components remain unchanged, as well as their service life. The use of a fueled compressor virtually eliminates auxiliary power requirements, making most of the additional power available for the grid, and its lower incremental heat rate makes it a relatively lower emissions peak power producer.”