Higman Chris Gasification (Газификация угля)

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Gasification

in operation have 43%, and IGCCs 38–43%. Serious assessments project between

46% and 50% as being possible with these technologies in the next 10 years. Look-

ing at these figures, one should bear in mind that a four-point increase in efficiency

represents about 10% less CO

production per unit of power produced.

Already there is a capacity of some 3800 MW in installed IGCCs plus 3500 MW

in projects currently under development or construction. Much of this is built

in association with oil refineries, where synergies in the residue or petroleum coke

disposal as well as in hydrogen production have encouraged their development.

Coal-based projects are following.

7.3.1 Comparison with Combustion

The maximum theoretical efficiency of a machine for the conversion of chemical

combustion energy into power is given by the formula for a reversible Carnot process:

η is the fraction of the power

produced by the cycle over the heat of combustion

added to the cycle.

and

are the lowest and the highest absolute temperatures of

the cycle. The lowest temperature is almost always the ambient (or cooling water)

temperature, and hence the formula immediately shows how important the maximum

temperature is for the cycle efficiency. A graphical representation of the above

formula is given in Figure 7-10.

--- - 1

------–==

–100

–80

–60

–40

–20

100

–200 200 600 1000 1400 1800 2200

Temperature [°C]

Carnot efficiency [%]

Figure 7-10.

Figure 7-10.Figure 7-10.

Figure 7-10. Carnot Efficiency as a Function of Temperature

Applications

261

The Carnot efficiency for various cycles are given in Table 7-10, together with

some real values.

As can be seen, the potential for any particular cycle as represented by its Carnot

efficiency is by no means the only consideration when looking at the merits and limi-

tations of different cycles. The efficiencies offered by gas turbines, which operate

with an upper temperature of between 1200 and 1400°C (compared with 500–650°C

for steam turbines), are restricted by the fact that the gas turbine itself requires a

clean gaseous or liquid fuel, whereas the conventional combustion processes can

handle dirty fuels including solids. The diesel engine cycles benefit from the high

temperatures (2000°C), but precisely this property contributes to the extremely high

emissions connected with this technology.

In summary, the conclusion can be drawn that in the actual processes only 54–73%

of the Carnot efficiency is realized. The best results can be obtained in a so-called

Tophat cycle that is a single cycle based on a gas turbine, as discussed in Section

7.3.3. The Carnot efficiencies as given in Figure 7-10 for cryogenic cycles have

been calculated for a

of 27°C and a

of −100 and −200°C. These data are solely

given here as they illustrate how energy-intensive cryogenic cycles, as applied in

ASUs, are. The negative efficiency of −80% of a cryogenic cycle in an ASU that has

a minimum temperature of about −200°C is in absolute terms about equal to a cycle

with a maximum temperature of 1200°C. A negative efficiency for cryogenic cycles

is caused by the fact that the energy for such cycles constitutes a loss. Before dealing

Table 7-10

Theoretical and Practical Efficiencies of Various Power Plant Cycles

Temperatures,

°C Efficiencies, % LHV

Cycle Fuel T

Low

High

Carnot Actual

Actual as

% Carnot

Conventional steam

power plant Coal 27 540 63 40 63

Ditto Ultra supercritical Coal 27 650 67 45 67

IGCC Coal 27 1350 82 46 56

Open-gas turbine cycle Gas 27 1210 80 43 54

Combined cycle Gas 27 1350 82 58 71

Tophat cycle Gas 27 1350 82 60 73

Low-speed marine

diesel

Heavy

fuel oil 27 2000 87 48 55

Low-speed marine

diesel with

supercharger

Heavy

fuel oil 27 2000 87 53 61

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Gasification

with the pro’s and con’s of various more complex cycles, the basic principles of the

steam and gas turbine cycles will be discussed.

Steam Cycles

The simplest cycle for the conversion of heat, which may be derived from full or

partial combustion or other sources such as nuclear energy, is the open steam cycle

depicted as a flow scheme and T-s diagram in Figures 7-11 and 7-12, respectively.

The corresponding points in the cycle are labelled A-F in both figures. The top of

the curve in the T-s diagram is the critical point. Points on the curve to the right of

the critical point represent saturated steam, and points to the left water.

CONDENSER

PUMP

BOILER

TURBINE

SUPER-

HEATER

ECONOMIZER

EXHAUST

FRESH

WATER

Figure 7-11.

Figure 7-11.Figure 7-11.

Figure 7-11. Simple Steam Cycle

A(B)

F′

A′

Figure 7-12

Figure 7-12Figure 7-12

Figure 7-12. T-s Diagram: Simple Steam Cycle

Applications

263

In the open cycle, corresponding to an atmospheric back-pressure turbine, the pump

raises the pressure of the water from atmospheric pressure at the point A to the working

pressure of the boiler at point B. The water is then heated to boiling temperature (C)

and evaporated at constant temperature (D). Superheating is represented in the T-s

diagram by the line D-E. Expansion to exhaust at atmospheric pressure (the vertical

line E-F) takes place in a steam engine such as a turbine. The work extracted from the

process is represented by the upper shaded area of the T-s diagram. The heat input is

represented by the shaded area plus that of the rectangular area below the line A-F.

In the closed cycle with a condenser, known as the Rankine cycle, the expansion

continues to the point F′ at a subatmospheric pressure, and the steam is condensed,

returning it to the state A′. As can be seen from the T-s diagram, the work extracted

is increased by the area of the lower shaded area.

In the last century many improvements have been made to the Rankine cycle. As

regards equipment the most important improvement has been the use of steam tur-

bines instead of piston expansion machines. Another feature that has increased the

efficiency is the use of low level heat from the turbine for preheating the boiler feed

water, thus using less high level heat in the boiler. High-level heat should preferably

be used only where topping-heat is required. Doing this is making good use of the

exergy present in the fuel. For a good understanding of exergy, the reader is again

referred to treatises on thermodynamics (Shvets et al. 1975). Knowledge about

exergy is of utmost importance for energy efficient designs (Dolinskovo and Brodianski,

1991). For the purpose of understanding the cycles discussed in this book it is

sufficient to know that high-level heat should preferably not be used for low-level

heat requirements, and furthermore, that the pressure energy in media should not be

throttled away unnecessarily. This last point is especially important for gases.

Apart from adding a condenser and boiler feed water preheat, there are three

major other improvements to the steam cycle. These are the application of higher

steam pressures, higher superheat temperatures, and reheat cycles. For details the

reader is referred to the literature (e.g., Shvets et al. 1975).

Supercritical Cycles

The biggest competition for gasification based power plants are advanced so-called

supercritical steam cycles. These are called supercritical as both the maximum tem-

perature and the maximum pressure of the steam are above the critical values of

374°C and 221 bar. By combining the benefits of the above features for improving

the efficiency, the state-of-the-art steam cycles can now reach about 45% LHV basis

for plants that are equipped with DeSOx and DeNOx facilities. The costs of these

plants are very competitive, and gasification-based power stations must have even

higher efficiencies and lower cost in order to be successful.

Examples of modern supercritical units are the pulverized coal (PC) coal-fired

double-reheat unit at Aalborg in Denmark (285 bar/580°C/580°C/580°C) and

the single-reheat unit at Matsuura in Japan (241 bar/593°C/593°C). In Germany and

in Japan units are now under design or construction for single-reheat plants of

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Gasification

260 bar/580°C/600°C and 250 bar/600°C/610°C respectively. The next step will

feature plants at steam conditions of 300 bar/600°C/620°C. A simplified flow

scheme for such a plant is given in Figure 7-13. The new steels that are required are

a major issue for all these developments (Viswanathan, Purgert, and Rao 2002).

As a result of all these improvements, it has been possible to increase the

efficiency of steam cycles to 45% LHV basis, that is 67% of the maximum Carnot

efficiency, as shown in Table 7-10.

ECONOMIZER

DEAERATOR

/BFW TANK

LP TURBINE

CYLINDERS

IP TURBINE

PLATEN S/H

TURBINE

FINAL S/H FINAL R/H

PRIMARY R/H

PRIMARY

SUPERHEATER

CAGE ROOF SIDE

& REAR WALLS

LOWER

FURNACE WALL

FURNACE ROOF

UPPER FURNACE

WALLS

HP BFW

PUMP

CONDENSATE

PUMP

CONDENSER

LP BFW

PREHEAT

HP BFW

PREHEAT

IP BFW

Figure 7-13.

Figure 7-13.Figure 7-13.

Figure 7-13. Ultra-Supercritical Cycle Flowsheet (Source: Welford et al. 2002)

Applications

265

This result is obtained with water as a working fluid, which is in some ways the

worst choice for a Rankine cycle because of the very high heat of evaporation and

the high specific heat of liquid water. Yantovski (1996) has studied extensively

the merits of using cycles in which CO

is used as the main working fluid, which

has the advantage that it has a low heat of evaporation. Such a cycle has advantages

when CO

has to be captured and sequestered. A major disadvantage of CO

is that it

is more difficult to handle, as the vapor pressure of liquid CO

is already 40–50 bar

at ambient temperatures.

The reason for discussing in some detail the steam cycle is twofold. On the one

hand the steam cycle plays an important role in combined cycles (CC), and on the

other hand much can be learned from its development that is also of importance for

the Brayton or Joule cycle as used in gas turbines.

Gas Cycles and Combined Cycles (CC)

The most widespread gas turbine cycle is the aircraft engine, which operates an open

cycle using essentially air as a working fluid. In power applications the open cycle is

mainly used for peak-shaving installations. The flow scheme and the T-s diagram of

the Joule cycle are given in Figures 7-14 and 7-15 respectively below. Air (A) is

compressed in a compressor, and the pressurized air (B) is used for the combustion

of the fuel. The resulting very hot gases (C) then enter the turbine, and the still hot

gases (D) are sent to the stack. Although this simple cycle has at best about the same

efficiency (43%) as a fairly advanced steam cycle, the efficiency as a percentage of

the Carnot potential is about 10% lower (see Table 7-10). This problem can be

remedied by using the hot gases leaving the gas turbine as a heat source for a steam

cycle in a heat recovery steam generator (HRSG). In this case the efficiency

increases to almost 60%, which corresponds to over 70% of the Carnot potential.

The flow scheme and the T-s diagram for this combined cycle (CC) is given in

Figures 7-16 and 7-17, respectively. In the T-s diagram it is clearly shown that part

of the space below the area of the Joule cycle is occupied by a relatively small steam

cycle. The ratio of the shaded areas to the area below the upper line of the cycle,

which is a measure for the overall efficiency, is thus considerably increased.

The open cycle is used in airplanes and for peak shaving in power stations. Larger

and more efficient gas turbines are now entering the marketplace with firing

temperatures of about 1500°C. These advanced turbines claim additional economies

of scale, reduced capital costs, and higher overall net efficiencies of 45–50% (LHV

basis). This illustrates how gas turbine manufacturers have concentrated on higher

turbine inlet temperatures. But the efficiency gain achievable by this means has its

limits. Taking the Carnot criterion (Figure 7-10) illustrates that increasing the inlet

temperature from 1200 to 1500°C will increase the efficiency from 80 to 83%.

Assuming a high 60% thereof can be realized, in actual practice this implies that

the cycle efficiency will increase by a mere 2%. Further, one should keep in mind

that blade cooling and low NO

requirements, which are problems that increase

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Gasification

with temperature, will imply a higher parasitic power consumption, all of which

casts some doubt on too great expectations of further increases in gas turbine inlet

temperatures.

In base load power stations combined cycles (CC) are generally used. Although

the Joule cycle is not very efficient, it has the advantage of a very low capital cost.

From the point of view of the Joule cycle it is advantageous to achieve as low a gas

outlet temperatures as possible. Typically, the gas outlet temperatures of the gas tur-

bine are about 550°C. This is, however, a disadvantage for the steam part of the

combined cycle, since high superheat temperatures cannot be achieved. The steam

cycle is therefore relatively capital intensive (expressed in dollars per installed kW).

The main disadvantage of the use of gas turbines is that only gas and light petroleum

distillates can be used as a fuel. To take full advantage of the high efficiency of the

CC in combination with dirty feedstocks as coal and heavy oils, these fuels must

first be gasified.

AIR FUEL FLUE GAS

COMPRESSOR

COMBUSTOR

TURBINE GENERATOR

Figure 7-14.

Figure 7-14.Figure 7-14.

Figure 7-14. Gas Turbine with Open Cycle

Figure 7-15.

Figure 7-15.Figure 7-15.

Figure 7-15. T-s Diagram for Open-Cycle Gas Turbine

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267

7.3.2 State-of-the-Art IGCC

Flow Schemes

The integrated gasification combined cycle (IGCC) is in its present form basically

a combination of gasification and a CC plant. The advantage of incorporating gasifi-

cation into these plants is to convert solid and residual liquid fuels into a form that

gas turbines can accept. Furthermore, gasification provides a means of desulfurization

AIR

FUEL

FLUE GAS

COMPRESSOR

COMBUSTOR

GAS

TURBINE

GENERATOR

STEAM

TURBINE

CONDENSER

HRSG

Figure 7-16.

Figure 7-16.Figure 7-16.

Figure 7-16. Gas Turbine with Combined Cycle

STEAM

CYCLE

JOULE

CYCLE

Figure 7-17.

Figure 7-17.Figure 7-17.

Figure 7-17. T-s Diagram for Combined Cycle

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Gasification

of the fuel before combusting the gas in the CC plant to levels not achievable at

reasonable cost with current flue gas desulfurization (FGD) technologies. Thus,

with a gasifier and the proper gas treating train behind it, one obtains a fuel that can

be combusted as simply and as environmentally friendly as natural gas. IGCC plants

with coal as feedstock can be found in power stations, whereas IGCC plants with

heavy residual fractions as feedstock are found in refineries.

Although the two component parts of an IGCC (gasification and gas turbines)

are both well developed technologies, the combination is nonetheless relatively

new. As is typical for a technology in such a stage of development, there is consid-

erable variation in the optimization of flowsheets for IGCC. This is particularly

true in the matter of effective integration of the two core technologies involved

(Holt 2002).

The most important integration that is applied in almost all IGCC plants is the

integration of the steam system. The steam cycle of a standard CC has an efficiency

of about 38%. The pinch problem caused by the evaporation of the water is the main

reason why it is so low. On the other hand, a steam system that derives its heat only

from the syngas cooler of a gasifier has also a low efficiency of perhaps 38%

because superheating is difficult in a syngas cooler. Combining both steam sources

alleviates the restrictions, as the pinch in the HRSG is eased by the large evaporating

duty in the syngas cooler and more saturated steam can be superheated in the HRSG,

albeit at a lower superheat temperature. It is therefore not surprising that when

combining the heat sources from HRSG and syngas cooler the efficiency becomes

about 40%. This is represented by the “conventional integration” case shown in

Figure 7-18.

Two other possibilities for integration that are applied in some IGCCs both

involve the air separation unit. One possibility is air integration, which involves

COAL

PREP.

GASIFICATION

GAS

COOLING

ACID GAS

REMOVAL

HRSG

GAS

TURBINE

STEAM

TURBINE

ASU

NITROGEN

AIR

OXYGEN

AIR

BFW

DEMIN.

WATER

CONVENTIONAL

INTEGRATION

ADDED FOR MAXIMUM

INTEGRATION

SULFUR

RECOVERY

SULFUR

STEAM

Figure 7-18.

Figure 7-18.Figure 7-18.

Figure 7-18. Block Flow Diagram of an Integrated IGCC Power Plant

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269

using extraction air from the combustion turbine compressor as feed air for the

ASU. This results in an overall somewhat lower compression duty. The ASU main

column then operates at a higher than the normal atmospheric pressure (about 10

bar instead of 5 bar) that results in the production of oxygen and nitrogen at an

elevated pressure of 5 bar instead of 1 bar. Hence the oxygen for the gasification

requires less compression. Nitrogen integration uses the nitrogen stream from the

ASU as diluent for the clean fuel gas to reduce the flame temperatures and hence

emissions. Also, here it is advantageous that the nitrogen already has an

elevated pressure.

Although the efficiency of the IGCC increases with a high degree of integration,

there is a risk of the availability suffering. A compromise that is applied in new

plants is one where part of the air for the ASU is supplied as extraction air from the

compressor of the gas turbine and part from a dedicated compressor in the ASU.

This adds an additional piece of rotating equipment to the plant, but it facilitates the

start-up and improves the overall availability of the plant.

The extraction air leaving the adiabatic gas turbine compressor has first to be

cooled before it can be used in the ASU. This is a further cycling of the temperature

of a main gas stream, which is one of the reasons why the efficiencies of IGCC

plants are still relatively low. In the next section some ideas are discussed to

improve this situation.

Fuel Gas Expansion

Texaco and others have proposed the use of a quench IGCC design in which the gas-

ification is conducted at a higher pressure of about 70 bar (e.g., Allen and Griffiths

1990). Additional power can then be generated by expansion of the clean fuel gas

from 70 to about 20 bar followed by gas reheat and firing in the gas turbine. The gas

cooling by an expander can also be used to provide much of the refrigeration

demand of a physical absorption desulfurization system (Zwiefelhofer and

Holtmann 1996). Fuel gas expansion is being used in two of the heavy oil IGCC

plants at Falconara and Priolo in Italy, which also increases the power generated in

these plants.

Efficiencies

The efficiency of gasification is at best about 80%, which, assuming 60% for the

CC, implies that the overall efficiency of an IGCC will not be much higher than

80 × 60/100 =48%. However, for virtually all gasifiers an oxygen plant (ASU) is

required and the gas treating also requires energy, and hence the efficiencies of the

best first-generation plants are all below 45%. And even the lower figures that are

reached require a fair amount of integration.

For a further understanding of the cycle, a Sankey diagram of a refinery residue-

based IGCC is shown in Figure 7-19.