Higman Chris Gasification (Газификация угля)

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Gasification

will increase the oxygen consumption of a gasification process and will reduce the

overall process efficiency.

For process control purposes where ratios between fuel, oxygen, and/or steam are

known, the temperature can be calculated. This is an important aspect, as temperatures

in slagging gasifiers can only be measured with great difficulty and are generally not

very trustworthy.

Since most modern gasification processes operate at pressures of 30 bar or higher,

temperatures of above 1300°C are required in order to produce a synthesis gas with a

low methane content. The fact that such a high temperature is required in any case for

thermodynamic reasons is why there is little scope for the use of catalysts in gasifiers.

The use of catalysts is restricted to clean gasification environments that are only encoun-

tered in the partial oxidation of natural gas and in steam methane or naphtha reforming.

This observation leads us on to perform the same exercise of investigating the varia-

tions of gas compositions and yield with temperature as shown in Figures 2-3 and 2-4.

In Figures 2-3 and 2-4 we can see that with increasing temperature the gas

becomes increasingly CO rich. In Figure 2-4 the increased oxygen demand at high

temperature is apparent. The H

+CO yield goes through a mild maximum between

1200°C and 1300°C.

2.3.3 Fuel Footprint

For a good understanding of the gasification of any particular feedstock, whether

coal, heavy oil, waste biomass, or gas, it is important to calculate the gasification

characteristics of the fuel in order to be able to understand what is going on in the

reactor and to optimize the reaction conditions.

Table 2-2

Variations of Syngas Compositions and Yields at 1500°C

1 Bar 30 Bar 60 Bar 100 Bar

, mol% 0.00 0.01 0.21 0.34

CO, mol% 63.42 63.33 62.88 62.88

, mol% 34.37 33.89 33.07 33.07

, mol% 0.01 0.27 0.85 0.85

O, mol% 0.01 0.21 0.67 0.67

Others, mol% 2.19 2.20 2.20 2.19

Total, mol% 100.00 100.00 100.00 100.00

+CO/feed, mol/mol 8.61 8.52 8.45 8.36

LHV dry/feed, MJ/mol 2.31 2.31 2.32 2.32

/(H

+CO), mol/mol 0.27 0.27 0.27 0.27

/LHV dry, mol/MJ 0.99 0.99 0.98 0.97

The Thermodynamics of Gasification

Instead of discussing this topic in the general terms of fuel, oxygen-containing

gas, and moderator, we will do so in terms of carbon, oxygen, and steam. But

remember, it holds for any fuel!

A most instructive representation of the fuel gasification characteristic is the

“footprint,” which comprises a graph where for a specified heat loss from the

gasification reactor and a specific set of reactants—coal, oxygen, and steam, each

with a fixed temperature and composition—the steam consumption per kmole

product gas is plotted along the ordinate, and the oxygen consumption per kmole of

product gas is plotted along the abscissa. Such a graph made for various heat losses

10%

20%

30%

40%

50%

60%

70%

1000 1100 1200 1300 1400 1500 1600

Temperature [°C]

Content [mol %]

Figure 2-3.

Figure 2-3.Figure 2-3.

Figure 2-3. Variation of Syngas Compositions with Pressure at 30 bar

1000 1100 1200 1300 1400 1500

Temperature [°C]

[mol/mol] or [MJ/mol]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[mol/mol] or [mol]/MJ

/Heat

content dry

Heat content

dry/Feed

+CO)/Feed

/(H

+CO)

Figure 2-4.

Figure 2-4.Figure 2-4.

Figure 2-4. Variation of Yields with Pressure at 30 bar

Gasification

forms the fuel footprint. In Figure 2-5 an example is given in which for a fixed heat

loss various isotherms are drawn together with the borderline below which no com-

plete conversion of the carbon is thermodynamically possible.

In the lower left-hand corner of Figure 2-5 the straight line represents the minimum

amount of gasifying agents (oxygen and steam) that are required to gasify the coal,

assuming that no H

O, CO

, and CH

are present in the gas. Thermodynamically this

is not possible; the real line assuming thermodynamic equilibrium, and the addition of

just sufficient oxygen and steam so as to gasify all carbon, is represented by the curved

line. The lower temperature part of this line is typical for fluid-bed gasifiers and, for

example, the hot lower part of a dry ash moving-bed gasifier. The departure from the

straight line implies that part of the steam remains unconverted for thermodynamic

reasons. It also means that the CO content becomes higher. It is observed that at

temperatures of about 1500°C, which are typical for dry coal feed entrained-flow

slagging gasifiers, the straight line and the thermodynamic equilibrium line almost touch

each other.

To generate the isotherms, the steam-to-product gas ratio is fixed above the

minimum required for complete gasification at the given temperature. It is seen that

in order to carry the additional steam ballast to the same high temperatures prevail-

ing in the gasifier, more oxygen is required. This is hence a less efficient operation.

–1

0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75

mol oxygen per mol carbon

mol steam per mol carbon

900 1100 1300 1500 1700

1900°C

Temperature

Heat loss

border line above which no

solid carbon can be present

theoretical border line if all

carbon could be converted into CO

Figure 2-5.

Figure 2-5.Figure 2-5.

Figure 2-5. Gasification Footprint for Pure Carbon

The Thermodynamics of Gasification

The surplus of gasifying agent implies that no carbon can be present. Therefore the

two equilibrium constants of the CO shift and the steam methane reforming

equation will have to be used in the calculations instead of those of the three

heterogeneous equations (Boudouard, water gas, and hydrogasification). Because

the steam-to-product gas ratio is known, the number of equations and variables

remain identical and the algorithm can be solved.

In practice, one will never want to operate at the thermodynamic equilibrium line,

as carbon formation may occur at the slightest operational disturbance. Typically,

for dry coal feed slagging gasifiers it is assumed that less than 1% of the carbon

remains unconverted, hidden in ash particles, and 1–1.5 mol% of CO

is present in

the gas. This gives some room for process control.

2.3.4 Surprises in Calculations

When calculations are carried out for a gasifier assuming heterogeneous reactions,

it may come as a surprise that sometimes the quantity of moderator required is

negative. The reason may be that the fuel and/or the blast contain so much ballast

in the form of ash, water, steam, or nitrogen that the desired temperature can only

be obtained by consuming more oxygen than is required to gasify all carbon in the

fuel. What happens in the calculations is that the algorithm tries to reduce the

amount of moderator, but only when the moderator becomes negative is a mathe-

matically valid solution obtained. In other words, one has reached the area below the

abscissa in Figure 2-5. The logical way out is to set the quantity of moderator at zero,

but then there is one equation too many, as the ratio of moderator to product gas is set

to zero. Again, the solution is found in using the two equilibrium constants of the CO

shift and the steam-methane reforming reactions instead of the three for the heteroge-

neous gasification. Examples where this situation may occur are:

• The use of low-rank coal in a dry coal feed entrained-flow slagging gasifier.

• The use of a coal/water slurry in an entrained-flow slagging gasifier.

• The use of air as blast in entrained-flow gasifiers.

In all cases where no moderator is required, the process control becomes much sim-

pler, just as in a furnace where only two streams have to be controlled instead of three.

2.4 OPTIMIZING PROCESS CONDITIONS

Dry coal feed entrained-flow slagging gasifiers operate at temperatures of typically

1500°C. At this temperature the oxygen consumption is high, but nevertheless there is

still some moderator required. The challenge is to operate with close to the minimum

amount of gasifying agent required, as this reduces the amount of expensive oxygen

per unit product gas. But there is more: the less oxygen that is used, the more steam we

need. This is not so bad, as oxygen is more costly than steam. However, there is a

Gasification

more important advantage that becomes clear when one looks at the two most relevant

gasification reactions for this type of gasifier:

C+½ O

= CO −111 MJ/kmol (2-1)

and

C+H

OCO+H

+131 MJ/kmol (2-5)

The heat balance dictates that most of the gasification will be accomplished via

reaction 2-1, but every carbon that can be gasified should be gasified with steam via

reaction 2-5, as this reaction yields two molecules of synthesis gas per atom of

carbon with cheap steam, but reaction 2-1 yields only one with expensive oxygen.

More about optimization will be discussed in Section 6.8.

In order to optimize the process conditions in a dry coal feed entrained-flow

slagging gasifier, first the equations are solved for the heterogeneous case, as this

gives the minimum amount of oxygen per mole synthesis gas. Then in order to

obtain a realistic operating window, the calculations are repeated for a somewhat

higher CO

content in the gas for the homogeneous case, and that is then made the

set point for the further operation.

Although most entrained-flow gasifiers operate with a surplus of gasifying agent

(blast), there are and have been exceptions. One example is the first slagging stage

of a two-stage gasifier where, also because of a lack of gasifying agent, carbon slips

to a second nonslagging stage where it is further converted with steam. Such a gasifier

is discussed in Chapter 5.

2.4.1 Process Indicators

When calculating the coal footprint it is very useful to draw also the iso-CO

and

iso-CH

lines (Figure 2-6). The fact that the iso-CO

lines run more or less perpen-

dicular to the isotherms and the iso-CH

lines more or less parallel, shows that the CH

content of the product gas is a better indicator for the temperature at which the gas

leaves the gasifier than the CO

content. The latter can only be used in cases where

the oxygen/steam ratio is fixed, or where no steam is required as moderator for the

gasification.

Another good indicator of the gasification temperature is the heat flux through the

reactor wall. Where a tube-wall or a jacket is used to protect the reactor wall, the

steam make in the protection is a very valuable indicator for the reactor temperature

(provided that this steam production is not integrated with other steam systems in

such a way that it can not be measured properly).

For the gasifier performance, the measurement of the CO

and CH

content in the

product gas, together with the heat flux through the reactor wall, are the best indicators.

In practice, data will be used that are gathered from experience. The calculations will

←

→

The Thermodynamics of Gasification

give good leads during start-up and the initial nonoptimal operation of the unit, but for

the fine-tuning the real data are more valuable, as will be explained in Section 6.8.

2.4.2 Optimum Operating Point

Efficiencies

There are a number of different criteria that are frequently quoted for gasification

processes (Reimert 1989). The two most commonly encountered are cold gas effi-

ciency (CGE) and carbon conversion. The definitions are:

whereby it is important always to clarify whether the heating values are on a higher

heating value (HHV) or lower heating value (LHV) basis.

Carbon conversion is defined as:

Figure 2-6.

Figure 2-6.Figure 2-6.

Figure 2-6. Iso-CO

and Iso-CH

Lines

0.5

1.5

2.5

3.5

2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5

Oxygen/coal [kmol/100 kg maf coal]

Steam/coal [kmol/100 kg maf coal]

1100°C

1300°C

1500°C

1700°C

Equilibrium

260 ppm CH

26 ppm CH

2600 ppm CH

1% CO

5% CO

3% CO

Cold gas efficiency [%]

Heating value in product gas [MW]

Heating value in feedstock [MW]

-------------------------------------------------------------------------------------

100×=

Carbon conversion [%] 1

Carbon in gasification residue [kmol/h]

Carbon in feedstock [kmol/h]

---------------------------------------------------------------------------------------------- -

–







100×=

Gasification

Despite the frequency with which these figures are quoted for different gasification

systems, care is required with their interpretation, since both only provide a limited

statement about the process efficiencies and gas quality. A process that produces a

gas with a relatively high methane content and therefore has a high cold gas

efficiency will be good in a power application, but it may not be the optimum choice

for a synthesis gas application, in which case the H

+CO yield will provide a better

guide to process selection.

IGCC Applications

The reason why it can be advantageous to gasify coal in power plants is twofold: for

efficiency reasons and environmental reasons.

The efficiency advantage is attributed to the fact that use can be made of the more

advanced combined cycle where gas is fired in a gas turbine (Brayton or Joule cycle)

and the hot gases leaving the turbine are used to raise steam for a conventional steam

(Rankine) cycle. The alternative is firing coal in a conventional steam plant using only

a Rankine cycle. The efficiency for gas firing in a state-of-the-art combined cycle is

about 58%, a figure that has to be multiplied with the gasification efficiency of, for

example, 80%, resulting in an overall efficiency of 46.4%, whereas the efficiency of

coal firing in a conventional state-of-the-art steam plant is about 45%. A simple

calculation shows that the CGE has to be above 78% in order to make gasification

attractive in terms of overall process efficiency. This is all that we will say at this point

as it is sufficient in the context of optimization. More will follow in Section 7.3.

The environmental advantage of gasification-based power stations has always

been used as an important argument in their favor based mainly on the fact that in

the past the sulfur compounds in the fuel gas could be removed with a higher

efficiency than from flue gases from conventional coal or heavy oil fired power

stations. Moreover, there was much optimism that high-temperature sulfur removal

would be possible, which would enhance the efficiency of the IGCC. As will be

explained in Section 7.3, there is a case to be made for reconsidering flue gas

treating for IGCC, as many more compounds have to be removed apart from sulfur.

Furthermore, the efficiency of flue gas desulphurization has improved considerably

over the last twenty years. However, one has to accept that environmental arguments

are a permanently moving target, since the recent discussion of CO

capture and

sequestration may provide a new advantage for fuel-gas treating. This is, however,

still dependent on politics.

For fuel gas applications the gasification temperature has to be as low as possible,

as this will result in the highest CGE (and in the lowest oxygen consumption). In

Figure 2-7 iso-CGE lines have been drawn that clearly illustrate this point, as the

isotherms run essentially parallel to the iso-CGE lines. Although fluxing may help

to enlarge the operating window for certain coals, as will be discussed in Section

5.3, the minimum temperature will always be determined by the reactivity of the

coal and the ash-melting characteristics in slagging gasifiers.

In general, temperatures of below 1400°C for low-rank coals and below 1500°C

for high-rank coals are impractical. As the data in Figure 2-7 show, the operation

The Thermodynamics of Gasification

Figure 2-7.

Figure 2-7.Figure 2-7.

Figure 2-7. Cold Gas Efficiencies

0.5

1.5

2.5

2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3

Oxygen/coal [kmol/100 kg maf coal]

Steam/coal [kmol/100 kg maf coal]

Equilibrium

all C converted

1100°C

1500°C

1700°C

1300°C

84%

82%

80%

0.5

1.5

2.5

2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3

Oxygen/coal [kmol/100 kg maf coal]

Steam/coal [kmol/100 kg maf coal]

Equilibrium

all C converted

1100°C

1500°C

1700°C

1300°C

10.2

10.1

10.0

Figure 2-8.

Figure 2-8.Figure 2-8.

Figure 2-8. Syngas (H

+ CO) Yields for Coal

Gasification

should be as close to the thermodynamic equilibrium line for the heterogeneous

reaction as is practically possible.

Syngas and Hydrogen Applications

For synthesis-gas application the situation is somewhat different. In this case the

moles of H

+CO per unit feedstock have to be optimized. The effect of the operating

conditions on this maximum is illustrated in Figure 2-8.

The optimum along the abscissa is caused by the effect that at lower temperatures

more carbon is converted into CO

and CH

, whereas the aim is to convert it as

much as possible in CO. There is also a maximum along the ordinate. This is caused

by the fact that by adding somewhat more steam less CH

will be formed and some

additional heat will be generated by the CO shift reaction.

The latter effects are small, but they are there. The result is that for a fixed heat

loss and reactant composition and temperature, there is one point that yields the

maximum amount of synthesis gas. The effect is of more importance to heavy oil

gasification than for coal gasification, as the ash limitations for coal do not allow an

operation at such low temperatures where these effects become relevant.

REFERENCES

Barin, I. Thermo-Chemical Data of Pure Substances. Weinheim: VCH Verlagsgesellschaft, 1989.

Gumz, W. Gas Producers and Blast Furnaces. New York: John Wiley & Sons., 1950.

Kersten, S. R. A. Biomass Gasification in Circulating Fluidized Beds. Enschede: Twente

University Press, 2002.

Lath, E., and Herbert, P. “Make CO from Coke, CO

, and O

.” Hydrocarbon Processing

65(8) (August 1986):55–56.

Reimert, R. “Gas Production.” In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.,

vol. A 12. Weinheim: VCH Verlagsgesellschaft, 1989, pp. 218–220.

van der Burgt, M. J. “Techno-Historical Aspects of Coal Gasification in Relation to IGCC

Plants.” Paper presented at 11th EPRI Conference on Gas-Fired Power Plants, San

Francisco., 1992.

Chapter 3

The Kinetics of

Gasification and

Reactor Theory

The kinetics of gasification is as yet not as developed as is its thermodynamics.

Homogeneous reactions occurring, for example, in the gas phase can often be

described by a simple equation, but heterogeneous reactions are intrinsically more

complicated. This is certainly the case with the gasification of solid particles such as

coal, (pet) coke, or biomass because of their porous structure. The latter complication

causes mass transfer phenomena to play an important role in the gasification of

solids.

3.1 KINETICS

The kinetics of coal gasification has been and still is a subject of intensive investi-

gation. Despite this, the results of such investigations have to date flowed into the

design procedures for commercial gasification reactors to only a limited extent. In

contrast to Chapter 2, therefore, the presentation of kinetic theory in this chapter is

restricted to the basic ideas and an indication of how and where the appropriate

application of kinetic theory could help in the design of future reactors.

A simplified reaction sequence for coal or biomass gasification can be described,

as in Figure 3-1.

For coke gasification, where the volatiles have already been removed in a separate

process step, only the char reactions apply. As is discussed in other parts of this

book, counter-current moving-bed gasifiers such as the Lurgi gasifier are an exception

to this model, since the oxygen reacts with the coke and the devolatilization takes

place using the hot synthesis gas. The volatiles in such a process do not come into

contact with free oxygen.

For oil gasification, where the feedstock consists almost entirely of pure volatiles,

the pattern is also slightly different.