USING COMBUSTION TURBINE EXHAUST HEAT FOR TURBINE INLET AIR CHILLING

Brad Buecker Chris Mieckowski Kiewit Power Engineers Kiewit Power Engineers Lenexa, Kansas USA Lenexa, Kansas USA

ABSTRACT: 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 has a large parasitic power consumption. This article outlines an emerging technology, absorption refrigeration turbine inlet air conditioning (ARCTIC

TM) 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 utilizing combustion turbine waste heat, and thus should present the reader with easy.to.understand details. Combustion Turbine Basics At the risk of being overly simple, a basic combustion turbine generator (CTG) 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 heat recovery steam generator (HRSG) operating from CTG exhaust 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 fundamental flow diagram of the process is illustrated below.

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Proceedings of the 2012 20th International Conference on Nuclear Engineering collocated with the

ASME 2012 Power Conference ICONE20-POWER2012

July 30 - August 3, 2012, Anaheim, California, USA

ICONE20-POWER2012-54007

Fig. 1. The basic combustion turbine process. A regenerative heater may be employed to increase efficiency by warming of the inlet air compressor exhaust, but we will focus upon the compressor inlet. The key concept of this paper is that high inlet air temperatures to the compressor greatly reduce turbine efficiency. There is one caveat to this phenomenon, as we shall observe shortly. Let us first examine the effect of high inlet air temperatures for an aeroderivative combustion turbine. The capacity chart below, for a common turbine with a maximum capacity of slightly greater than 50 MW, illustrates that the ideal inlet temperature is 48

o F. As the ambient

temperature rises above this value, capacity drops off rapidly.

Fig. 2. Capacity loss vs. inlet air temperature for a GE*LM6000 PC SPRINT combustion turbine. Note also the capacity loss due to subcooling. As is evident, if this particular combustion turbine were operated in a desert environment, the capacity loss on a 110

o F

day would be nearly 40 percent of the maximum achievable capacity. Another phenomenon, particular to aeroderivative turbines but not the frame

types, is that inlet air subcooling can cause a loss of capacity, although not nearly as dramatic as with high inlet temperatures. We will look at this issue again. Inlet Air Cooling To this point in history, the two most common methods for turbine inlet air cooling have been mechanical chilling or water*fed evaporative cooling. With the former, parasitic power load may be in the 1 to 3 MW range. 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. The figure below outlines the general flow schematic.

Fuel

C T

Air

Exhaust

HRSG

SCR

Fuel

C T

Air

Exhaust

HRSG

SCR

HRVGTIAC

Fig. 3. Generic flow diagram of the ARCTIC system. 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

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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 0.8 to 1.6 MW per 1,000 tons of chilling. During one summer day in 2011 at a plant in the southwestern U.S., an ARCTIC system maintained an inlet air temperature of 48

oF

(ideal point for an LM6000) when the ambient temperature was 107

oF.

The ability of the process to cool the inlet air to any desired temperature underlies the value of the technology. The figures below illustrate this benefit for a GE 7FA combustion turbine.

75,000

80,000

85,000

90,000

95,000

100,000

75,000

80,000

85,000

90,000

95,000

100,000

30 40 50 60 70 80 90 100

Ne

t P

ow

er

(kW

)

ARCTIC

MECHANICAL CHILLING

EVAP COOLING

UNCONDITIONED

9,800

10,000

10,200

10,400

10,600

10,800

11,000

11,200

9,800

10,000

10,200

10,400

10,600

10,800

11,000

11,200

30 40 50 60 70 80 90 100

Ne

t H

ea

t R

ate

(B

TU

/kW

-hr)

Ambient Dry Bulb (°F)

ARCTIC

MECHANICAL CHILLING

EVAP COOLING

UNCONDITIONED

Fig. 4. Capacity and heat rate curves of various inlet air cooling methods 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 100

oF summer day.

Another striking outcome is the near 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 48

oF. 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 48

oF inlet air temperature during

winter operation. Also, 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). The following charts illustrate this effect for a combined*cycle system with a total generating capacity slightly above 500 MW and ambient conditions of 97

oF, 43%

RH.

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Fig. 5. Capacity and heat rate charts for ARCTIC as applied to combined*cycle units. The charts show the trade*off. Capacity is highest with ARTIC cooling, but at a slight loss of heat rate. Flue Gas Water Recovery An aspect of steam generator operation that is gaining attention, including from the Electric Power Research Institute (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 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

The combustion reaction is outlined below.

CH4 + 2O2 → CO2 + 2H2O

Stoichiometric calculations, where two moles of water are produced for every mole of CH4 combusted, show that the theoretical maximum water recovery rate would be 195,750 lb/hr or 391 gallons*per*minute (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. Some plants could potentially become net water producers. 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 air 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.

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Brad Buecker serves as Process Specialist with Kiewit Power Engineers in Lenexa, Kansas, USA. 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, Illinois, USA, and Kansas City Power & Light Company’s La Cygne, Kansas, USA 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 Publishing on steam generation topics. He is a member of the American Chemical Society, American Institute of Chemical Engineers, American Society of Mechanical Engineers, the Cooling Technology Institute and the National Association of Corrosion Engineers. 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, Kansas, USA. 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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