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Gas TurbinesSmart-AHAT (Advanced Humid Air Turbine)

Introduction of Smart-AHAT System

The Smart-AHAT system is a highly efficient simple cycle gas turbine system that uses steam injection for power augmentation and a water recovery system to achieve near zero water make-up. The system is less complex and more flexible than a combined cycle system, while plant output and efficiencies are significantly higher than conventional simple cycle plants and are approaching that of combined cycle plants.
The operational characteristics of the Smart-AHAT system are similar to a simple cycle plant and significantly more flexible than a combined cycle plant. It can start-up quickly, has excellent low load operation capability, and responds very quickly to rapid load changes. Moreover, it has very low NOx emissions, near zero particulate emissions, and requires near zero water make-up. Last but not least, due to the smaller number of system components, the Smart-AHAT system has a smaller footprint, shorter construction period, and lower cost than equivalent combined cycle plants.

Smart AHAT Configuration

Figure 1: Smart-AHAT System Schematic Diagram

Figure 1 shows a schematic diagram of the Smart-AHAT system. The major components of the system are a gas turbine, a heat recovery steam generator (HRSG) and a water recovery system (WRS) with heat rejection system.

The gas turbine is a conventional gas turbine designed for large amounts of steam injection for power augmentation. The steam, produced in a conventional, single-pressure heat recovery steam generator (HRSG) from the energy available in the gas turbine exhaust, is injected downstream of the gas turbine's air compressor. The mixture of compressed air and injection steam, along with the gas turbine fuel, is combusted and the resulting combustion gas drives the gas turbine blades and shaft of the generator, thereby producing electricity. The water contained in the exhaust gas exiting the HRSG is recovered in the WRS, treated and returned to the HRSG for steam production. An air cooled radiator (or alternatively, wet cooling towers) is used as a heat sink for the WRS.

Major Components of the Smart AHAT System

Figure 2: Shows a Rendering of the Major Components of the Smart-AHAT System.

1. Gas Turbine System

The gas turbine applied for the first commercially available Smart-AHAT system is the H-50 gas turbine, a heavy duty, frame-type gas turbine that has been designed by Mitsubishi Hitachi Power Systems, Ltd. (MHPS). This gas turbine is ISO rated at 57MW without steam injection and can achieve 70MW with steam injection. It is a scaled model of the larger and proven H-100 gas turbine. The H-50 is specifically designed to accommodate a large amount of injection steam in order to be suitable for the Smart-AHAT cycle.

2. Heat Recovery Steam Generator System

The HRSG involved in the Smart-AHAT system is a conventional, single-pressure HRSG that produces the steam required for the gas turbine steam injection. In addition, the HRSG can be used to produce process steam for cogeneration applications.

3. Water Recovery System

Without a water recovery system, the water (in the form of steam) injected upstream of the combustor would enter the gas path of the cycle and exit through the HRSG stack. Hence, without water recovery, large amounts of water are lost to the atmosphere and a significant amount of demineralized water make-up is required. The Smart-AHAT overcomes this disadvantage by applying a water recovery system (WRS) at the back end of the HRSG, which is able to recover most of the water from the exhaust gas before it exits the stack.
The WRS is a direct, spray-type heat exchanger that reduces the HRSG exhaust gas temperature below the water dew point of the flue gas, thereby causing condensation of the water vapor. Some of the condensate is recirculated to the spray nozzles of the heat exchanger, while the remainder of the condensate is treated and returned as feed water for the HRSG steam production.

4. Heat Rejections System

In the direct, spray-type WRS, the sensible energy contained in the flue gas is transferred to the spray water. While the flue gas is cooled, the spray water is heated and collects, along with the water condensate from the flue gas, at the bottom of the WRS, from where the spray water can be recycled, by cooling and pumping it back to the spray nozzles. The cooling of the recycled spray water is done via a heat rejection system, such as wet cooling towers or air cooled radiators.

Benefits of the Smart AHAT

1. High power output and efficiency, across the ambient air temperature range.

Due to the added mass flow injected in the form of steam, the turbine output in the Smart-AHAT system can be increased by approximately 23%. Despite this increased output, the fuel flow to the combustor is not significantly increased since the steam injected into the combustor is already superheated. This results in a highly efficient, power augmented, simple cycle that approaches combined cycle efficiencies. For example, an H-50 smart-AHAT plant reaches plant efficiencies of 45% (LHV), which is significantly higher than any equivalent simple cycle gas turbine available.

2. Reduced NOx emission

Steam injection for power augmentation involves combustion with humid air. This reduces the flame temperatures in the combustor, and hence, the NOx emissions.

3. Water Preservation

The WRS is able to recover most of the water contained in the HRSG exhaust gas. This includes the amount of water used to produce injection steam, as well as the amount of water produced in the gas turbine combustion process. With the WRS, the Smart-AHAT system exhibits nearly zero water consumption and, due to the recovery of water produced in the combustion process, the Smart-AHAT is a net water producer at certain ambient temperatures.

4. Near Zero Particulate Emissions

Nearly all of the particulates contained in the HRSG exhaust gas are "washed out" in the spray-type WRS and are collected in the water treatment system. Therefore, the Smart AHAT system has near zero particulate emissions.

5. High Operating Flexibility

The operational characteristics of the Smart-AHAT system are similar to a simple cycle plant and significantly more flexible than a combined cycle plant. It can start-up quickly and, due to steam injection, has a larger load range within emissions compliance than a conventional simple cycle gas turbine. It has excellent low load operation capability and responds very quickly to rapid load changes.

6. Smaller Footprint, Shorter Construction Schedules, and lower Cost than Combined Cycle

The components of the Smart-AHAT are less complex and the total number of components is lower than the number of components in a conventional combined cycle plant. The Smart-AHAT system requires an HRSG with only one pressure level, while a combined cycle system of similar size has at least two to three HRSG pressure levels and higher steam pressures.

Figure 3: Smart-AHAT Plant Layout

Most components of the bottoming cycle of a conventional combined cycle plant, such as steam turbine, generator, steam condenser and steam condenser heat rejection system, as well as associated auxiliary equipment, is not required for the smart AHAT system. This results in a smaller plant footprint, lower capital costs and shorter construction schedules than conventional combined cycle plants.

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