Brad Buecker, Contributing Editor, and Chris Mieckowski, Kiewit Power Engineers
Two of the most common methods currently being employed for combustion turbine inlet air temperature control are evaporative cooling and mechanical chilling. The former requires significant quantities of water, while the latter exhibits a large parasitic power consumption.
This article outlines an emerging technology, Absorption Refrigeration Cycle Turbine Inlet Conditioning (ARCTIC) that mitigates both of these issues, while offering precise control over inlet air temperatures. Such control is quite important during peak load periods, when every megawatt can be of enormous value to a power producer.
Also of significant importance is that operation of combustion turbines at maximum efficiency minimizes the CO2 footprint of the machines. This aspect has positive implications for power producers wishing to be as green as possible. The ARCTIC process is based on straightforward absorption-refrigeration technology, and thus should present the reader with easy-to-understand details.
Combustion Turbine Basics
At the risk of being overly simple, a power generating combustion turbine (CT) operates similarly to a jet engine via the following steps.
Inlet air is compressed and injected into the turbine. The compressor is attached to the turbine shaft, and thus the compressor and turbine rotate in unison.
Fuel, typically natural gas but in some cases fuel oil, is injected and ignited in the compressed air stream.
The expanding gas drives the turbine.
Hot exhaust, at 850oF or higher, exits the turbine.
Much energy escapes with the turbine exhaust, and is a primary reason why combined-cycle plants are very popular. A steam generator operating with CT waste heat can improve overall plant efficiency from 35 percent or so to nearly 60 percent. However, many combustion turbines operate in the simple-cycle mode, where a lot of valuable heat is wasted. Why not use some of the heat to improve the capacity and efficiency of the combustion turbine?
The fundamental thermodynamic cycle upon which combustion turbines operate is the Brayton Cycle. A simple flow diagram of the process is illustrated in Figure 1.
Combustor preheating improves turbine efficiency. On the other hand, high inlet air temperatures to the compressor reduce capacity. Figure 2 illustrates these losses due to uncontrolled compressor inlet air temperature for a common aeroderivative combustion turbine.
Inlet Air Cooling
To this point in time, the two most common methods for turbine inlet air cooling are either mechanical chilling or water-fed evaporative cooling. With the former, parasitic power load may be in the 0.8 to 1.6 MW range per 1,000 tons of chilling. Evaporative coolers, as their name implies, chill inlet air by evaporation of water spread upon an inlet media. Water requirements for these coolers may range from 30 to 80 gpm. Also, evaporative coolers can only provide chilling down to a fixed approach to wet bulb temperature. ARCTIC technology, which utilizes the extremely well-known process of closed-cycle, ammonia-based absorption-refrigeration, with turbine exhaust heat as the energy source, eliminates high parasitic power consumption and water usage. As we shall see, this process potentially offers other valuable capabilities including compressor freeze protection, industrial process chilling, and the potential for flue gas water recovery. The latter issue is attracting growing interest in the power generation industry.
Figure 3 outlines the general flow schematic.
The process relies upon classic thermodynamic refrigeration. The turbine exhaust warms an aqueous ammonia solution in heat exchanger coils (HRVG on the diagram) located within the exhaust gas path. The ammonia is then separated in the rectifier to produce a nearly pure vapor, which is condensed, reduced in pressure, and then allowed to expand within turbine inlet air cooling coils (TIAC) located in the inlet air stream. The pure ammonia discharge from the cooling coils is blended with the aqueous ammonia bottoms product from the rectifier (a process that requires additional heat exchangers due to the exothermic reaction), and is re-pressurized for return to the HRVG. Thus, the process is a closed loop.
A standard skid-mounted ARCTIC unit can provide over 2,000 tons of chilling at 220 kW of auxiliary load. The parasitic power load for a mechanical compressor to provide similar chilling ranges from 1.6 to 3.2 MW. During one summer day in 2011 at a plant in the southwestern U.S., an ARCTIC system maintained an inlet air temperature of 48oF (ideal point for an LM6000, more discussion to follow) when the ambient temperature was 107oF.
The ability of the process to cool the inlet air to any desired temperature underlies the value of the technology. Figures 4 and 5 illustrate this benefit for a GE 7FA combustion turbine.
As is evident, the process can maintain capacity 10 to 20 MW higher than mechanical chilling or evaporative cooling, respectively, on a 100oF summer day. Another striking outcome is the virtual flat-line stability in heat rate for the ARCTIC process.
This has implications for those plants that wish or need to minimize the carbon dioxide footprint while maximizing capacity.
Inlet Air Heating?
Let us return again to Figure 2. As can be seen, the ideal inlet air temperature for the LM-6000 turbine is 48oF.
Unlike frame units, subcooling of aeroderivative inlet air also causes a loss of capacity, albeit not nearly as severe as that induced by high ambient temperatures.
A valuable aspect of the ARCTIC process is that the plumbing is arranged such that warm ammonia vapor may be routed to the TIAC to maintain a steady 48oF inlet air temperature during winter operation.
Of course, the warm refrigerant bypass can be utilized for anti-icing in either aeroderivative or frame units.
What about Combined-Cycle Operation?
Use of ARCTIC on combined-cycle units presents an interesting scenario. An HRSG of course relies on the exhaust heat from the combustion turbine as the energy source. Partial use of the waste heat by ARCTIC reduces capacity and efficiency of the HRSG. Counterbalancing this loss is the improved efficiency of the combustion turbine(s).
Flue Gas Water Recovery
An aspect of steam generator operation that is gaining attention, including from EPRI, is that of flue gas water recovery. ARCTIC cooling coils placed in the exhaust of a simple- or combined-cycle unit could recover a significant portion of the moisture in the gas. As is well known, the combustion process, and particularly that from gas turbines, produces a significant quantity of water due to reaction of hydrogen in the fuel with oxygen. Recovery of flue gas moisture can potentially turn some plants from fresh water consumers to water producers. As an example, consider a combustion turbine fired with natural gas as at a fuel flow rate of 87,000 lb/hr. We will use three simplifying assumptions:
- Natural gas composition is 100 percent methane (CH4)
- Complete combustion is achieved in the turbine
- The process recovers 100 percent of the water produced by combustion
Stoichiometric calculations show a water recovery rate of 390 gpm. Given that natural gas is typically a very clean fuel; the recovered water could easily be returned to the inlet of the plant makeup water system or to other processes. An obviously important issue with flue gas water recovery is the effect that chilling would have on the buoyancy of the flue gas stream, and in turn how this might influence fan design and permitting issues. Nonetheless, the idea is gaining interest.
Natural Gas to the Forefront
It has become quite apparent that natural gas will be the fuel choice for many new power plants. To operate at the highest efficiency and capacity, combustion turbine inlet air cooling will be needed. The ARCTIC process offers a method that offers tight control, requires only a small amount of parasitic power, and uses no water, but perhaps may be utilized to produce water. These are all important factors in the continuing efforts to optimize energy usage in this country and elsewhere.
Brad Buecker is a contributing editor for Power Engineering and also serves as a process specialist with Kiewit Power Engineers in Lenexa, Kan. He has over 30 years of experience in, or affiliated with, the power industry, much of it in chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power in Springfield, Ill., and Kansas City Power & Light Company's La Cygne, Kan., station. He has an A.A. in pre-engineering from Springfield College in Illinois and a B.S. in chemistry from Iowa State University. He has written many articles and three books for PennWell on steam generation topics. He is a member of the ACS, AIChE, ASME, and NACE. He is also a member of the ASME Research Committee on Power Plant & Environmental Chemistry, the program planning committee for the Electric Utility Chemistry Workshop, and the program planning committee for Coal-Gen.
Chris Mieckowski is the ARCTIC Product Line Manager for Kiewit Power Engineers in Lenexa, Kan. He has over five years of experience with combustion turbine OEM's on both the Frame and Aeroderivative sides. He spent nine years in the US Navy starting as a nuclear electrician on submarines and concluding his career as a Tomahawk Missile Officer on a guided missile cruiser. He has B.S. in Engineering from the US Naval Academy.
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