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15.08.2018 Солнце в сеть




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Dry-Coal Feeding with Lock Hoppers

Lock hoppers have been used for over a century in water gas reactors and in blast furnaces for sluicing lump coal, coke, and iron ore into vessels that operated under a slight overpressure. They were developed further in the 1930s for operation at 25 to 30 bar in connection with the Lurgi pressurized moving-bed gasifier. In the Ruhr 100 pilot plant they have been demonstrated at 100 bar (Reimert 1986).

In general, a lock hopper system consists of three vessels that are situated on top of each other and separated from each other by valves (see Figure 6-1).

The top hopper is at atmospheric pressure, and the middle one is the actual lock hopper. The bottom hopper can be a storage vessel that is at an elevated pressure, but it can also be the gasifier itself, as is the case with moving-bed gasifiers. The principle is that of any sluicing system. During loading, the valve between the atmospheric hopper and the lock hopper is open, and the valve between the lock hopper and the bottom hopper is closed. After the lock hopper has been filled, the first valve is closed and the second opened, after which the pressure in the lock hopper increases from atmospheric to the elevated pressure, and the solid material will drop into the bottom hopper. The valve between the lock hopper and the bottom hopper is then depressurizing the gas in the lock hopper, the valve between the top hopper and the lock hopper is opened, and the cycle can be repeated.

Adopting the lock-hopper system for pressurizing pulverized materials such as coal requires major modifications to the lock-hopper system. The solids, for example, have to be kept fluidized during transport and in the hoppers. This requires provid­ing the hoppers with spargers or other means for the introduction of the fluidizing gas. The dusty gas leaving the hoppers on depressurizing has to be cleaned and some­times has to be repressurized, which further complicates the lock-hopper system and makes it a costly piece of equipment. Finally, there is the drawback that lock hoppers are discontinuous. This is not a problem for processes, which have long residence times such as a blast furnace or a moving-bed coal gasifier, but it is more problematic for entrained-flow gasifiers, which have residence times in the order of seconds. In the latter case the pressurized hopper must be sized such that it is filled during the whole lock-hopper cycle so as to ensure a continuous flow of solids to the downstream equipment.

The use of lock hoppers for coal pressurization presents a problem for dry-coal feed entrained-flow slagging gasifiers when pressures higher than 30-40 bar are required. The problem is not limited to more complex equipment such as valves, fluidizing systems, and the compression of fluidizing gases. More important is that the gas consumption for fluidizing the pulverized coal in the pressurized hoppers becomes higher at higher pressures. Furthermore, the amount of gas required for the transport of the coal to the burners increases, creating a burden for the gasifier, as this gas has to be heated to the high gasification temperature.

Transport Gases

Nitrogen. Using nitrogen as transport gas has the drawback that the product gas becomes contaminated, which is particularly relevant when the gas is to be used for chemical synthesis or for the production of hydrogen. The only chemical application where the presence of nitrogen does not pose a problem is ammonia synthesis. In IGCC power stations the presence of nitrogen means that less nitrogen is available for quenching, for example. However, in IGCC applications the presence of some inert material in the gas has hardly any effect on the overall process efficiency.

In IGCC applications nitrogen is therefore the gas that is most commonly used in lock hoppers and for the subsequent dense phase transport to the burners. The nitrogen is available from the air separation unit (ASU), supplying the oxygen required for the gasification. It should be possible to get a loading during dense phase transport of 400kg/actual m3. In practice, the loading is about 300kg/actual m3 as then the coal flows more smoothly. This implies that, when operating at a pressure of 30 bar and a temperature of about 90°C, for every kg of coal 0.09 kg nitrogen is required for transport. At a pressure of 70 bar the latter figure would increase to 0.21 kg. The nitrogen (plus argon) percentages in the product gas correspond to 2.7 and 5.1 mol% for pressures of 30 and 70 bar, respectively (see Table 6-1). The same percentage of 5 mol% nitrogen is obtained at 30 bar when the oxygen purity is reduced from 99 to 95 mol%. Although in IGCC applications the higher nitrogen content in the gas has only a marginal effect on the overall process efficiency, it does slightly increase the duty of the syngas cooler and of the gas treating.

For chemical applications, the higher inert content of the gas will cause a subse­quent synthesis to run under less favorable conditions. In such a situation, if nitrogen is to be used as transport gas it is often more attractive to run the gasifier at a lower pressure and to increase the duty of the syngas or hydrogen compressor, which is in any case required in most such applications. Examples where this applies are methanol synthesis and hydrocrackers.

Syngas. Using syngas for the high-density transport of pulverized coal to the gasifier instead of nitrogen largely reduces the problem of nitrogen contamination. In case a gas quench is used, as is the case currently in the SCGP gasifier, the syngas can best be taken from the discharge of the recycle gas compressor. Nevertheless, the use of syngas for transport of coal is in most cases not an attractive solution, although the nitrogen contamination of the gas is typically reduced from 3-5 mol% to less than 1 mol% (see Table 6-1). The problem with syngas as transport gas is that in the lock hoppers, the gas also has the function of providing a barrier between the oxidizing atmosphere of the atmospheric pressure coal and the reducing atmosphere of the gasifier, a function that syngas cannot fulfill. The obvious choice for the barrier function is nitrogen. It is inevitable, therefore, that the transport syngas will always be contaminated with some nitrogen. All in all, syngas is not an attractive option, and in practice the only practical alternatives are nitrogen and C02.

Table 6-1

Influence of the Coal Transport Medium, Pressure, and Oxygen Purity on the Syngas Purity

Single-Stage

Process

Slurry Feed

Single-Stage Dry Feed

Temp.,°С

1500

1500

1500

1500

Press, bara

70

30

70

30

70

30

70

30

Coal transport

medium

Water

co2

co2

Syngas Syngas

n2

n2

n2

CGE, %

65

82

82

82

82

82

82

82

IGCC eff., %

38

50

50

50

50

50

50

50

02, mol%

99

99

99

99

99

99

99

95

Wet raw product gas, mol%

CO

37.4

64.5

62.9

63.2

63.5

61.9

60.6

60.8

H2

15.4

31.9

30.3

32.8

32.4

32.2

31.0

31.3

co2

6.0

2.0

4.7

1.0

1.0

1.0

1.0

1.0

h2o

40.3

0.2

0.4

1.9

1.9

1.9

1.9

1.9

CH4

0

0.2

0.5

0

0.1

0

0.1

0

H2S

0.2

0.3

0.3

0.3

0.3

0.3

0.3

0.3

n2+a

0.7

0.9

0.9

0.8

0.8

2.7

5.1

4.7

Dry raw product gas, mol%

CO

62.6

64.6

63.2

64.4

64.8

63.2

61.9

62.1

H2

25.8

32.0

30.4

33.5

33.0

32.8

31.6

31.9

co2

10.1

2.0

4.7

1.0

1.0

1.0

1.0

1.0

CH4

0

0.2

0.5

0

0.1

0

0.1

0

H2S

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

n2+a

1.2

0.9

0.9

0.8

0.8

2.7

5.1

4.7

H2/CO

molar ratio

0.41

0.50

0.48

0.52

0.51

0.52

0.51

0.51

co2/h2s

molar ratio

34

7

16

3.3

3.3

3.3

3.3

3.3

Note: The IGCC efficencies are calculated on the basis of the standardized, idealized

conditions of Appendix E.

The effect of syngas on process efficiency and the syngas cooler duty is the same as for nitrogen, provided that the pressures and temperatures are similar (see Table 6-1).

Carbon Dioxide. The use of C02 as transport gas is only a serious option where it is available at no additional cost, that is, where a CO shift and subsequent C02 removal is already part of the downstream gas processing. For many chemical appli­cations, such as hydrogen or methanol production, this is the case. If C02 capture and sequestration becomes a requirement for power production, it would also be the case for IGCC applications. The effect of C02 on process efficiency and the syngas cooler duty is only marginally different from nitrogen, provided the pressures and temperatures are the same (see Table 6-1). The H2/CO ratio of the syngas may decrease slightly, but this generally would have little influence on subsequent gas processing. The effect of the H2S/C02 ratio on the acid-gas removal system will be discussed in Chapter 8.

The major advantage of C02 over nitrogen as transport gas is that it does not dilute the gas with additional inerts. It has the advantage over syngas as transport gas in that it is not toxic and it slightly reduces the process steam requirements.

Although the most complex lock hoppers are required for pulverized coal, they are often also used for the discharge of fly slag that is separated in cyclones and or filters downstream of the gasifier. Lock hoppers in which the continuous phase is a liquid are used in some gasifiers for sluicing the slag out of the gasifier (see also Section 6.2.2).

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