INTRODUCTION

B URNING V ELOCITY

5.1. INTRODUCTION

In the power generation engine, the exhaust is further expanded through a power turbine which drives a generator or other mechanical device (e.g., pump). Note that in the absence of the state change from 3 to 4, the subsequent expansion through the turbine (Points 4–5–6) would simply return the system to Point 1. Since the work generated by the engine is represented by the area bound by the pathway on the P–v or T–s diagram, traveling from Point 3 back to Point 1 without reaching Point 4 or 6 would result in no work being accomplished. Hence without combustion, no work would result from these engines.

Two other factors are noteworthy which directly relate to the combustion process.

First, higher pressures (i.e., Point 3) will result in potentially more area bound by the pathway on the P–v or T–s diagrams. Also, the higher temperature that can be reached at Point 4 will also result in a potentially larger bound area (i.e., more work output). This is important because it illustrates two other underlying principles related to emissions, namely higher pressure ratios and higher temperatures (for the simple cycles shown in Figure 5.1) should be targeted in order to achieve the highest efficiency systems possible. As a result, one of the critical aspects of the combustion science associated with gas turbines is the ability to achieve low emissions while at the same time operating at higher pressures and temperatures. This also illustrates a key difference in aviation and power generation applications. In power generation, the possibility of more advanced relatives of the simple cycle (e.g., recuperated, inter-cooled, reheat) can be considered as a means to achieve high efficiency. However, because these cycle modifications generally add weight (i.e., they reduce the specific power generation), they are not often viable for aviation applications where any added weight is difficult to tolerate. Hence, for aviation applications, the pathway to higher efficiency generally relies upon temperature and pressure increases. One notable exception is the consider- ation of reheat cycles for aviation gas turbines which is being explored for application to long range military aircraft (Sirignano and Liu, 1999; Sturgess, 2003).

5.1.1.1. Stationary Power

Stationary power gas turbines range in capacity from tens of kilowatts to hundreds of megawatts. They are manufactured by a large number of vendors worldwide. In the past decade, 30–120 GW worth of gas turbines have been ordered each year worldwide (DGTW, 2005). Figure 5.2 shows examples of commercial gas turbines representative

Figure 5.2 Examples of commercial power generation gas turbines: (A) Capstone Turbine 60 kW gas turbine and (B) GE 9H gas turbine (>400 MW in combined cycle).

at the two extremes of the power generation spectrum. Because gas turbines producing tens of kilowatts (order of kW) are contrasted to gigawatt class generation, they are often referred to as“microturbines.”This is an unfortunate convention in the labeling of these devices because the prefix associated with the subject in this contraction generally refers to the physical dimension. In the case of microturbine, the physical scales are not on the order of microns, but rather their output is on the order of six orders of magnitude less than the largest systems.

The larger unit shown in Figure 5.2b is designed for power generation at a central station power plant, typically in a combined cycle. The most advanced of these combined cycle systems can achieve fuel to electricity efficiencies approaching 60%

while producing sub-9 ppm of NOx emissions using a complex combustion system discussed below (Myerset al., 2003; Pritchard, 2003; Feiglet al., 2005).

On the other hand, the type of engine shown in Figure 5.2a are expressly designed for distributed generation (Borbeley and Kreider, 2001). Versions with as much as 40–50 MW are available in“trailerable”configurations that would also be considered distributed generation. Deployment of gas turbines throughout a region can be an effective means of deferring transmission and distribution lines. Furthermore, if the engine is sited at a location that can make use of the heat from the exhaust, overall fuel to heat-and-power efficiencies exceeding 70% can be regularly attained. Of course, dispersing engines throughout a region must also be contemplated relative to air and acoustic emissions. As a result, the pollutant emissions from these devices must be extremely low. In the extreme case (California Air Research Board (CARB), 2000), these devices are being required to emit less than 0.07 lb/MW hr of NOx which is equivalent to about 1 ppm NOx at 15% O2 for a 25% efficient engine. The regulation is based on power plants featuring engines like the one shown in Figure 5.2b, butwith exhaust post treatment via selective catalytic reduction (SCR).

5.1.1.2. Aviation Gas Turbines

In aviation applications, gas turbine engines are used for both propulsion and auxiliary power. The more critical role is played by the main engine, though emissions and efficiency of auxiliary power engines also require consideration. Aviation engines are made in a wide range of thrust ratings which service aircraft from single-seaters to 747s. Unlike stationary engines, aviation engines are started and stopped frequently, and also require much more stringent safety consideration.

For aircraft propulsion, the gas turbine represents the principal source of thrust for military and commercial applications. Examples of engines are shown in Figure 5.3.

Figure 5.3a shows a cross section of a turbo-fan engine. In this case, much of the air compressed by the fan bypasses the combustor altogether which results in overall improvement in cycle efficiency. A typical engine is shown in Figure 5.3b.

In addition to main engines, auxiliary power units (APUs) are needed to provide power to the aircraft when power cannot be obtained from the main engines. Figure 5.4 presents a schematic of an APU. Note that this engine is operated to produce shaft power, not thrust. As a result, it can be considered as a power generation engine, but one that must operate over an extremely wide range of conditions and on Jet-A fuel. These devices will not include air bypass. Compared to the main engines, APUs produce relatively little power; however, when the aircraft is on the ground, the APU can be an important source of acoustic and pollutant emissions. As a result, increased attention is

being given to these devices as aircraft manufacturers continue to reduce their overall emissions signature (which includes gate and taxi time).

5.1.2. D

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The need for lean combustion in gas turbines is driven by a number of factors, including market need, regulatory pressure, performance, and reliability. The relative importance of these factors differs for stationary and aviation gas turbines. In this section, the major drivers for each application are discussed. Although the principal motivation for using lean combustion in gas turbines is generally associated with NOx

emissions reduction, it is helpful to summarize the current emissions issues for gas turbines in general. Table 5.1 highlights the current emissions drivers for aircraft and power generation gas turbines.

5.1.2.1. Stationary Power

A serious need for additional energy resources worldwide is apparent in the future.

The International Energy Agency has indicated that 1750 billion kW h of additional energy will be required in the next 20 years (United States Department of Energy (US DOE), 2006). This is especially influenced by high population countries such as India and China which are rapidly developing infrastructure for energy, but it is also a factor in developed countries. Gas turbines play a preeminent role in the stationary power generation market place and should remain a critical part of the market mix for at least the next several decades. This is the case despite competition from reciprocating

KEVLAR CASING SHIELD

FAN BLADES

Rolls Royce

A B

Trent 800

Figure 5.3 Propulsion gas turbine (Rolls-Royce Trent 800): (A) propulsion engine cross section and (B) Trent 800 main engine.

Figure 5.4 Hamilton Sunstrand lean-staged Pyrospin APU (Chen, 2004).

engines and newer technologies such as fuel cells. Alternative technologies compete with gas turbines in certain size classes, but at power generation levels above 5 MW, gas turbines offer the most attractive option due to their relatively low capital, operating, and maintenance costs. These engines are being looked to by the US DOE and the major Original Equipment Manufacturers (OEMs) (e.g., Siemens, GE) for clean power pro- duction from coal and other feedstocks. The configurations for these systems involve high efficiencies as well. As a result, the market will continue to demand gas turbines.

The more specific demand for lean combustion gas turbines is another consideration.

Of the tens of gigawatts of stationary power engines sold each year, a relatively small number have“low emissions”systems and only a handful are equipped with the lowest emission combustion systems. These systems are required in only a relatively small number of areas, mostly in developed countries in regions with air quality issues such as the United States. Nonetheless, as needs for efficient, clean power generation grow, the number of applications featuring these low emissions systems is increasing. Further- more, the engine OEMs view low emissions systems as a competitive and technological edge in the marketplace, and they also have an increasing interesting in being con- sidered“green,”which places emissions and efficiency at the forefront of their research and development. As a result, the market for lean combustion based gas turbines should increase substantially in the next two decades.

In terms of regulatory pressure, legislation involving criteria pollutants continue to bring additional challenges to the gas turbine industry. While post engine treatment is capable of providing regulated levels of criteria pollutants, some regions continue to lower the emissions levels that must be achieved. Conventional wisdom suggests that mitigating pollutant formation is preferred over post engine exhaust cleanup. Real drivers such as capital cost and operating and maintenance costs support this wisdom. As a result, great interest in further reducing NOxemissions exists. As mentioned above, those OEMs that can offer the lowest emissions systems will have an edge not only in markets with tight regulations, but also in their image as environmentally friendly (i.e.,“green”).

5.1.2.2. Aviation

For main engines in aviation applications, the primary drivers for the combustion system do not include emissions. This is associated with the human safety factor that is

Table 5.1

Primary emissions drivers for gas turbine engines

Aircraft Engine Power Generation

Species LTO Cruise Distillate Gas

Soot x x x

HC (VOC, ROG, NMHC) x x x

CO x x x

NOx(NO, NO2) x x x x

SOx(SO2, SO3, sulfates) x x

CO2 x x x x

H2O x x x x

x An issue.

x Current focus.

involved in aviation. Table 5.2 summarizes the priorities associated with design of aviation gas turbine combustion systems. As shown, safety and operability are the most critical factors, followed by cost factors associated with operation and maintenance (e.g., efficiency and durability). Emissions rank near the bottom of the list. In reality, International Civil Aviation Organization (ICAO) regulations do drive combustion system development and, although emissions are relatively low on the list, they have driven the investment directions of the engine companies for the past few decades. For example, some air space authorities have imposed landing taxes based on engine emission levels. Hence to the extent lean combustion can impact emissions, it is an important option for aviation engines.

Regarding military applications, the concept of emissions reduction drops even further in priority. However, for public relations reasons and to reflect the overall leadership role of the government, some consideration is given to emissions reduction.

A recent review discusses the status of emissions issues with respect to military engines and touches on the role lean combustion has played in this effort (Sturgesset al., 2005).

For auxiliary power, cost is a principal driver. Since passenger safety is not as seriously affected by APU operation, the list shown in Table 5.2 for propulsion engines is somewhat different. Of particular note is the need to reduce noise emissions from the APU. This is a consideration while idling and moving about the airport. In this circum- stance, the APU impacts local air quality, and also can provide a nuisance in terms of acoustic emissions. Table 5.3 summarizes these requirements for APU design. Note that the APU, like the stationary generator, is designed to do work, not produce thrust.

However, it is still subject to aircraft safety requirements and must be operable over the very wide range of ambient conditions associated with sea level to flight altitudes.

5.2. RATIONALE FOR LEAN COMBUSTION