12/3/20

The gas turbine

Post Author : Blue Collar Post Date : 12/3/20 Post Time : December 03, 2020

 The gas turbine is a power plant, which produces a great amount of energy for

its size and weight. The gas turbine has found increasing service in the past

40 years in the power industry both among utilities and merchant plants as well

as the petrochemical industry, and utilities throughout the world. Its compactness,

low weight, and multiple fuel application make it a natural power plant

for offshore platforms. Today there are gas turbines, which run on natural gas,

diesel fuel, naphtha, methane, crude, low-Btu gases, vaporized fuel oils, and

biomass gases.

The last 20 years has seen a large growth in Gas Turbine Technology. The

growth is spearheaded by the growth of materials technology, new coatings,

and new cooling schemes. This, with the conjunction of increase in compressor

pressure ratio, has increased the gas turbine thermal efficiency from about 15%

to over 45%.

Table 1-1 gives an economic comparison of various generation technologies

from the initial cost of such systems to the operating costs of these systems.

Because distributed generation is very site specific the cost will vary and the

justification of installation of these types of systems will also vary. Sites for

distributed generation vary from large metropolitan areas to the slopes of the

Himalayan mountain range. The economics of power generation depend on the

fuel cost, running efficiencies, maintenance cost, and first cost, in that order. Site

selection depends on environmental concerns such as emissions, and noise, fuel

availability, and size and weight.

Gas Turbine Cycle in the Combined Cycle or Cogeneration Mode

The utilization of gas turbine exhaust gases, for steam generation or the heating

of other heat transfer mediums, or in the use of cooling or heating buildings or

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parts of cities, is not a new concept and is currently being exploited to its full

potential.

The Fossil Power Plants of the 1990s and into the early part of the new

millennium will be the Combined Cycle Power Plants, with the gas turbine

being the centerpiece of the plant. It is estimated that between 1997–2006 there

will be an addition of 147.7 GW of power. These plants have replaced the large

Steam Turbine Plants, which were the main fossil power plants through the 1980s.

The Combined Cycle Power Plant is not new in concept, since some have been

in operation since the mid-1950s. These plants came into their own with the new

high capacity and efficiency gas turbines.

The new marketplace of energy conversion will have many new and novel

concepts in combined cycle power plants. Figure 1-1 shows the heat rates of

these plants, present and future, and Figure 1-2 shows the efficiencies of the

same plants. The plants referenced are the Simple Cycle Gas Turbine (SCGT)

with firing temperatures of 2400 ◦F (1315 ◦C), Recuperative Gas Turbine (RGT),

the Steam Turbine Plant (ST), the Combined Cycle Power Plant (CCPP), the

Advanced Combined Cycle Power Plants (ACCP) such as combined cycle

power plants using Advanced Gas Turbine Cycles, and finally the Hybrid Power

Plants (HPP).

Table 1-2 is an analysis of the competitive standing of the various types of

power plants, their capital cost, heat rate, operation and maintenance costs, availability

and reliability, and time for planning. Examining the capital cost and

installation time of these new power plants it is obvious that the gas turbine is

the best choice for peaking power. Steam turbine plants are about 50% higher

in initial costs—$800–$1000/kW—than combined cycle plants, which are about

$400–$900/kW. Nuclear power plants are the most expensive. The high initial

costs and the long time in construction make such a plant unrealistic for a

deregulated utility.

In the area of performance, the steam turbine power plants have an efficiency

of about 35%, as compared to combined cycle power plants, which have an

efficiency of about 55%. Newer Gas Turbine technology will make combined

cycle efficiencies range between 60–65%. As a rule of thumb a 1% increase in

efficiency could mean that 3.3% more capital can be invested. However one must

be careful that the increase in efficiency does not lead to a decrease in availability.

From 1996–2000 we have seen a growth in efficiency of about 10% and a loss in

availability of about 10%. This trend must be turned around since many analyses

show that a 1% drop in the availability needs about a 2–3% increase in efficiency

to offset that loss.

The time taken to install a steam plant from conception to production is about

42–60 months as compared to 22–36 months for combined cycle power plants.

The actual construction time is about 18 months, while environmental permits


in many cases takes 12 months and engineering 6–12 months. The time taken

for bringing the plant online affects the economics of the plant, the longer capital

is employed without return, the plant accumulates interest, insurance, and

taxes.

It is obvious from this that as long as natural gas or diesel fuel is available the

choice of combined cycle power plants is obvious.

Gas Turbine Performance

The aerospace engines have been the leaders in most of the technology in the

gas turbine. The design criteria for these engines was high reliability, high performance,

with many starts and flexible operation throughout the flight envelope.

The engine life of about 3500 hours between major overhauls was considered

good. The aerospace engine performance has always been rated primarily on

its thrust/weight ratio. Increase in engine thrust/weight ratio is achieved by the

development of high-aspect ratio blades in the compressor as well as optimizing

the pressure ratio and firing temperature of the turbine for maximum work output

per unit flow.

The Industrial Gas Turbine has always emphasized long life and this conservative

approach has resulted in the Industrial Gas Turbine in many aspects

giving up high performance for rugged operation. The Industrial Gas Turbine

has been conservative in the pressure ratio and the firing temperatures. This has

all changed in the last 10 years; spurred on by the introduction of the “Aero-

Derivative Gas Turbine” the industrial gas turbine has dramatically improved its

performance in all operational aspects. This has resulted in dramatically reducing

the performance gap between these two types of gas turbines. The gas turbine

to date in the combined cycle mode is fast replacing the steam turbine as the

base load provider of electrical power throughout the world. This is even true

in Europe and the United States where the large steam turbines were the only

type of base load power in the fossil energy sector. The gas turbine from the

1960s to the late 1980s was used only as peaking power in those countries. It

was used as base load mainly in the “developing countries” where the need for

power was increasing rapidly so that the wait of three to six years for a steam

plant was unacceptable.

Figures 1-3 and 1-4 show the growth of the Pressure Ratio and Firing Temperature.

The growth of both the Pressure Ratio and Firing Temperature parallel

each other, as both growths are necessary to achieving the optimum thermal

efficiency.

The increase in pressure ratio increases the gas turbine thermal efficiency when

accompanied with the increase in turbine firing temperature. Figure 1-5 shows


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