Higman Chris Gasification (Газификация угля)

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Gasification

The coal from the lock hopper is distributed over the area of the reactor by a

mechanical distribution device, and then moves slowly down through the bed under-

going the processes of drying, devolatilization, gasification, and combustion. The

ash from the combustion of ungasified char is removed from the reactor chamber via

a rotating grate and is discharged into an ash lock hopper. In the grate zone the ash is

precooled by the incoming blast (oxygen and steam) to about 300–400°C.

The blast enters the reactor at the bottom and is distributed across the bed by the

grate. Flowing upward it is preheated by the ash before reaching the combustion zone in

which oxygen reacts with the char to CO

. At this point in the reactor the temperatures

reach their highest level (Figure 5-2). The CO

and steam then react with the coal in the

gasification zone to form carbon monoxide, hydrogen, and methane. The gas composi-

tion at the outlet of the gasification zone is governed by the three heterogeneous gasifi-

cation reactions: water gas, Boudouard, and methanation (for details see Section 2.1).

While describing the process in the form of these four zones, it should be stressed

that the transition from one zone to the next is gradual. This is especially noticeable

in the transition from the combustion zone to the gasification zone. The endothermic

gasification reactions already begin before all the oxygen has been consumed in

combustion. Thus the actual peak temperature is lower than that calculated by

assuming a pure zonal model (see Figure 5-2) (Gumz 1950; Rudolf 1984).

This gas leaving the gasification zone then enters the upper zones of the reactor

where the heat of the gas is used to devolatize, preheat, and dry the incoming coal.

In this process the gas is cooled from about 800°C at the outlet of the gasification

zone to about 550°C at the reactor outlet.

A result of the counter-current flow is the relatively high methane content of the

outlet gas. On the other hand, part of the products of devolatilization are contained

unreacted in the synthesis gas, particularly tars, phenols and ammonia, but also a

wide spread of other hydrocarbon species. Bulk removal of this material takes place

immediately at the outlet of the reactor by means of a quench cooler in which most

of the high-boiling hydrocarbons and dust carried over from the reactor are con-

densed and/or washed out with gas liquor from the downstream condensation stage.

GAS

COAL

STEAM,

OXYGEN

OR AIR

ASH /

SLAG

DRYING/

DEVOLATILIZATION

GASIFICATION

COMBUSTION

CUMULATIVE RAW SYNGAS

COMPOSITION [MOL% (DRY)]

UNDECOMPOSED

STEAM

200150100050

ASH

GAS

GASIFIER

BOTTOM

TEMPERATURE [°C]

STEAM,

OXYGEN

COAL

2500 12501000750500

GASIFIER

TOP

Figure 5-2.

Figure 5-2.Figure 5-2.

Figure 5-2. Temperature and Gas Composition Profiles in a Lurgi Dry Ash Gasifier

(Source: Adapted from Rudolf 1984; Supp 1990)

Gasification Processes

Gas Liquor Treatment

The gas liquor from the quench cooler typically contains the elements outlined in

Table 5-3 (Supp 1990).

There are a number of alternate strategies for handling this material. Lurgi has

developed two specific processes for treating it, in particular for allowing the recov-

ery of pure ammonia.

COAL FEED

WASH

COOLER

LOCK PRESSURIZING

GAS

BFW

PREHEATER

SYNGAS

COOLER

GASIFIER

GAS

LIQUOR

LOCK

EXPANSION GAS

LP STEAM BFW

C.W.

RAW SYNGAS

GAS

LIQUOR

BFW

GAS

LIQUOR

ASH

WATER

ASH

ASH LOCK

OXYGEN

STEAM

BFW

Figure 5-3.

Figure 5-3.Figure 5-3.

Figure 5-3. Process Flowsheet of Lurgi Dry Ash Gasification (Source: Supp 1990)

Table 5-3

Gas Liquor Composition

Suspended Matter 1000 mg/l

Sulfur 600 mg/l

Chloride 50 mg/l

and NH

ions 10,000 mg/l

Cyanide 50 mg/l

Phenols 1000–5000 mg/l

COD 10,000 mg O

Source: Supp 1990

Gasification

The process sequence (Figure 5-4) includes mechanical tar separation, followed

by the Phenosolvan extraction process in which a raw phenol fraction is recovered.

Sour gases and ammonia are then selectively and separately stripped from the

dephenolated condensate in the CLL (Chemie Linz-Lurgi) unit. The sour gas stream

is free of ammonia and so suitable for processing in a standard Claus sulfur recovery

unit (see Section 8.4). The stripped condensate contains less than 50 ppm NH

and

less than 1 ppm H

S and can be discharged to a biotreater. The ammonia product

from the CLL unit contains less than 4% water, less than 0.1 wt% CO

, and less than

1 ppm H

S. Further details on these processes can be found in Supp (1990) and

Stönner (1989).

Equipment Issues

Both the Lurgi gasifier and the BGL gasifier described in Section 5.1.2 have moving

parts that are absent in other gasifiers. For caking coals and sometimes for distribu-

tion reasons, a stirrer is installed in the top of the reactor. The distribution function is

required to ensure an even depth of the coal bed over the whole cross-section of the

reactor. With caking coals the distributor avoids pasting-up of the coal bed due to

agglomeration of the coal particles after their temperature becomes higher than the

softening point. The dry-bottom gasifier also has a rotating grate at the bottom for

the ash outlet.

Moving-bed gasifiers require a relatively high amount of maintenance. However,

in virtually all cases many gasifiers operate in parallel, and by proper scheduling the

total unit will have a high availability.

Process Performance

An important feature of the Lurgi dry-bottom gasifier is its low oxygen consumption

and high steam demand. The exact data depend on the feedstock. Table 5-4 provides

data on the basis of three different coals that cover a broad range of coalification or

rank.

GAS LIQUOR

TAR REMOVAL

PHENOSOLVAN

SOUR GAS

CLL

AMMONIA

TAR RAW PHENOL WASTE WATER

Figure 5-4.

Figure 5-4.Figure 5-4.

Figure 5-4. Gas Liquor Treatment for Lurgi Dry Ash Gasification

Gasification Processes

Moving-bed gasifiers need graded coal, whereas the total mine output will always

contain a large percentage of fines (the more modern the mine, the more fines).

Although it is in principle possible to make briquettes with these fines, by binding

them with heavy tar that is co-produced in the gasifier, in practice part of the fines

cannot be processed.

Heavily caking coals cannot be processed in moving-bed gasifiers. Mildly caking

coals require the assistance of the stirrer in order to avoid pasting-up of the bed. Tars

and other oxygenated compounds are also produced as by-products. These products

form about 25% of the total hydrocarbon feed input in terms of energy. Correcting

the oxygen consumption and the CGE (cold gas efficiency) for this fact increases

the former and reduces the latter by about one-third. When you add to this that the

sensible heat in the gases cannot be used very effectively due to the presence of the

tars, the overall efficiencies of the moving-bed processes are not much higher than

that of fluid-bed or entrained-flow gasifiers.

Table 5-4

Typical Performance Data of Lurgi Dry Ash Gasifier

Type of Coal Lignite Bituminous Anthracite

Components (maf)

C, wt% 69.50 77.30 92.10

H, wt% 4.87 5.90 2.60

S, wt% 0.43 4.30 3.90

N, wt% 0.75 1.40 0.30

O, wt% 24.45 11.10 1.10

Raw gas composition (dry)

S, mol% 30.4 32.4 30.8

CO, mol% 19.7 15.2 22.1

, mol% 37.2 42.3 40.7

, mol% 11.8 8.6 5.6

, mol% 0.4 0.8 0.4

, mol% 0.5 0.7 0.4

Feed components

per 1000 Nm

CO + H

Coal maf, kg 950 750 680

Steam, kg 1180 1930 1340

Oxygen, Nm

170 280 300

Source: Supp 1990

Gasification

Applications

The most notable commercial installation forms part of the Sasol synthetic fuels

operation at two sites in South Africa, where in all thirteen MK III, eighty-three MK

IV, and one MK V reactors produce a total of 55 million Nm

/day syngas, the

largest gasification complex in the world. The synthesis gas generated produces

170,000 bbl/day of Fischer-Tropsch liquid fuels as well as forming the basis for a

substantial chemical industry (Erasmus and van Nierop 2002). The reason that Lurgi

dry-ash gasifiers are used in the SASOL complex in South Africa is that at the time

the complex was built it was the only pressurized gasifier available and was very

suitable for the high ash-melting point coals to be processed.

The Lurgi type moving-bed dry ash gasifier is in widespread use around the

world in, apart from South Africa, the United States, Germany, the Czech Repub-

lic, and China. The plants in Germany and the Czech Republic use some of the gas

to fuel gas turbine combined cycle power plants. In the United States Lurgi dry ash

gasifiers are used for the production of substitute natural gas (SNG). In China, the

Lurgi process is used for the production of town gas, ammonia, and hydrogen. In

the SNG, IGCC, and town gas applications, the high methane content (10–15%) of

the product gas is an advantage, since methane is the desired product for SNG,

and for IGCC and in town gas applications methane increases the heating value of

the gas.

5.1.2 British Gas/Lurgi (BGL) Slagging Gasifier

The BGL slagging gasifier is an extension of the Lurgi pressure gasification tech-

nology developed by British Gas and Lurgi with the ash discharge designed for

slagging conditions. Initial work took place in the 1950s and 1960s but ceased

with the discoveries of natural gas in the North Sea. Work resumed in 1974 after

the “oil crisis.” An existing Lurgi gasifier in Westfield, Scotland was modified for

slagging operation and operated for several years, proving itself with a wide range

of coals and other solid feedstocks, such as petroleum coke. The motivation for

the development of a slagging version of the existing Lurgi gasifier included a

desire to:

• Increase CO and H

yields (at the expense of CO

and CH

• Increase specific reactor throughput.

• Have a reactor suitable for coals with a low ash-melting point.

• Have a reactor suitable for accepting fines.

• Reduce the steam consumption and consequent gas condensate production.

The decline in interest for coal gasification generally in the 1980s prevented

commercialization of the technology. In the mid-1990s the first commercial project

was realized at Schwarze Pumpe in Germany, where a mixture of lignite and

Gasification Processes

municipal solid waste (MSW) is gasified within a large complex and the syngas is

used for methanol and power production (Hirschfelder, Buttker, and Steiner 1997).

A number of other projects also for MSW gasification are currently under consideration.

Process Description

The upper portion of the BGL gasifier is generally similar to that of the Lurgi dry ash

gasifier, although for some applications it may be refractory lined and the distributor

and stirrer omitted. The bottom is completely redesigned, however, as can be seen in

Figure 5-5.

In contrast to the Lurgi dry ash gasifier there is no grate. The grate of the Lurgi

dry ash gasifier serves two purposes, distribution of the oxygen-steam mixture and

ash removal. In the BGL gasifier the former function is performed by a system of

COAL

DISTRIBUTOR

COAL

LOCK

TAR RECYCLE

JACKET STEAM

WASH

COOLER

RAW GAS

STEAM &

OXYGEN

COOLING

WATER

SLAG

LOCK

SLAG

COAL

Figure 5-5.

Figure 5-5.Figure 5-5.

Figure 5-5. BGL Gasifier (With permission: Lurgi)

Gasification

tuyères (water-cooled tubes) located just above the level of the molten-ash bath. The

tuyères are also capable of introducing other fuels into the reactor. This includes the

pyrolysis products from the raw gas, as well as a degree of coal fines that cannot be

introduced at the top of the reactor for fear of blockage.

The lower portion of the reactor incorporates a molten slag bath. The molten ash

is drained through a slag tap into the slag quench chamber, where it is quenched

with water and solidified. The solid ash is discharged though a slag lock.

Process Performance

A comparison between the performance of the Lurgi dry ash and the BGL gasifier is

given in Table 5-5.

The much lower steam and somewhat lower oxygen consumption of the slagging

gasifier result in a much higher syngas production per unit of coal intake and a much

lower yield of pyrolysis products compared with the dry bottom unit. Further, the

content of the gas is lower and the methane content in the gas is halved.

Table 5-5

Comparative Performance of Lurgi Dry Ash, BGL, and Ruhr 100 Gasifiers

Raw Gas

Composition (Dry)

Dry Ash

Gasifier

Slagging

Gasifier

Ruhr 100

90 Bar

Carb. Gas

Ruhr 100

90 Bar

Clear Gas

, mol% 30.89 3.46 29.52 35.47

CO, mol% 15.18 54.96 18.15 17.20

, mol% 42.15 31.54 35.11 39.13

, mol% 8.64 4.54 15.78 7.93

, mol% 0.79 0.48 1.02 0.04

, mol% 0.68 3.35 0.35 0.19

S + COS, mol% 1.31 1.31

, mol% 0.36 0.36

Feed components per

1000Nm

CO+ H

Coal maf, kg 750 520

Steam, kg 1930 200

Oxygen, Nm

280 230

kg pyrolysis

products/1000kg coal 81 19

Source: Supp 1990; Lohmann and Langhoff 1982

Gasification Processes

Waste Gasification

As mentioned above, the BGL gasifier has found a commercial application in waste

gasification at the SVZ Schwarze Pumpe plant near Dresden in Germany. A 3.6m

diameter BGL gasifier is fed with a mixed fuel of various wastes and coal. During

test operation the unit has gasified a mixture of 25% hard coal, 45% refuse derived

fuel (RDF) pellets, 10% plastic waste, 10% wood, and 10% tar sludge pellets.

During the performance test the following production and consumption figures

were achieved (Greil et al. 2002):

Gasification pressure is 25 bar. The ash content of the feed is discharged as a glassy

frit. Analytical tests have demonstrated the nonleachability of the slag, and that

disposal requirements, according to TA Siedlungsabfall, class 1 (German waste regu-

lations), have been met.

5.1.3 Ruhr 100

Another development of the Lurgi dry-bottom gasifier was the high-pressure Ruhr

100 gasifier which was designed for operation at 100 bar. Apart from its high-

pressure capability, the reactor contained a number of other new features. It had two coal

lock hoppers, which halves the lock gas losses by half by operating alternately. This

was implemented to counter the rise in such losses, which accompanies the rise in

operating pressure. It had the additional effect of simplifying the drive arrangement

for the stirrer, which could now be mounted centrally. Another new feature was the

addition of a second gas tapping at an intermediate level between the gasification

and devolatilization zones. Not all the gas volume is required to achieve devolatili-

zation of the feed, so the excess is drawn off from the bed as “clear gas.” The meas-

ured compositions of the carbonization gas containing the volatiles and the clear gas

are given in Table 5-5.

A 240 t/d pilot plant was started up in 1979. By September 1981 many of the most

important goals of the tests had already been confirmed. Increasing the pressure

from 25 bar to 95 bar, approximately doubled the throughput of the reactor. The

pressure increase also increased the methane production from about 9 mol% in the

product gas to 16 mol% (Lohmann and Langhoff 1982).

Mixed Fuel (ar) 30 t/h

Specific oxygen

consumption <0.2 Nm

/Nm

dry syngas

Gasification

agent ratio

steam/oxygen <1.0 kg/Nm

dry syngas

Raw syngas (dry) 31,500 Nm

Gasification

5.2 FLUID-BED GASIFIERS

The history and development of coal gasification and fluid-bed technology have

been intimately linked since the development of the Winkler process in the early

1920s. Winkler’s process operated in a fluidization regime, where a clear distinction

exists between the dense phase or bed and the freeboard where the solid particles

disengage from the gas. This regime is the classic or stationary fluid bed. With

increasing gas velocity a point is reached where all the solid particles are carried

with the gas and full pneumatic transport is achieved. At intermediate gas velocities

the differential velocity between gas and solids reaches a maximum, and this regime

of high, so-called “slip velocity” is known as the circulating fluid bed. Over the

years gasification processes have been developed using all three regimes, each pro-

cess exploiting the particular characteristics of a regime to the application targeted

by the process development. These differences are portrayed in Figure 5-6.

In fluid-bed gasification processes the blast has two functions: that of a reactant

and that of the fluidizing medium for the bed. Such solutions, where one variable

has to accomplish more than one function, will tend to complicate or place limitations

on the operation of the gasifier, as in for example, turndown ratio. These problems

are especially severe during start-up and shutdown. In most modern gasification

processes oxygen/steam mixtures are used as blast. However, when the gas is to be

used for power generation, gasification with air may be applied—and in the case of

biomass gasification, it often is.

Some simplified reactor sketches for bubbling fluid beds, circulating fluid beds,

and transport reactors are given in Figures 5-9, 5-10, and 5-11. In Figure 5-7, the

GAS FLOW

SOLIDS FLOW

CIRCULATING FLUID BED

STATIONARY FLUID BED

MEAN GAS VELOCITY

MEAN SOLIDS VELOCITY

SLIP

VELOCITY

INCREASING EXPANSION

INCREASING SOLIDS

DENSITY

TRANSPORT REACTOR

VELOCITY

Figure 5-6.

Figure 5-6.Figure 5-6.

Figure 5-6. Fluid Bed Regimes (Source: Greil and Hirschfelder 1998)

Gasification Processes

temperature profiles for both the coal and gas are given. Temperature profiles for

transport type gasifiers are very complex.

5.2.1 Common Issues

Operating Temperature

Any fluid bed depends on having the solid particles of a size that can be lifted by the

upward flowing gas. A large portion (over 95%) of the solids content of the bed of

a gasifier is ash, which remains in the bed while the carbon leaves the reactor as

syngas. If the ash content of the fuel, be it coal, biomass, or other, should start to

soften sufficiently that the individual particles begin to agglomerate, these newly

formed larger particles will fall to the bottom of the bed, and their removal poses a

considerable problem. For this reason fluid-bed gasifiers all operate at temperatures

below the softening point of the ash, which is typically in the range of 950°C–

1100°C for coal and 800–950°C for biomass.

On the other hand, the lower the temperature of the gasifier operation the more tar

will be produced in the product synthesis gas. This is partly due to the fact that when

the coal particles are heated slowly, more volatiles are produced, as discussed in

Chapter 3. It is also due to the fact that at lower temperatures there is less thermal

cracking of the tars produced.

The best known fluid-bed gasifiers that have no tar problems are regenerators of

catalytic cracking units which operate under reducing—that is, gasification—conditions

and can be found in some refineries. In these units the carbon residue that remains after

gases and volatiles have been extracted from the feed, is gasified in a bubbling fluid-bed.

2500 12501000750500

GAS

COAL

STEAM,

OXYGEN

OR AIR

GASIFIER

BOTTOM

GASIFIER

TOP

COAL

GAS

GAS & COAL

STEAM, OXYGEN

OR AIR

TEMPERATURE [°C]

ASH

Figure 5-7.

Figure 5-7.Figure 5-7.

Figure 5-7. Bubbling Fluid-Bed Gasifier (Source: Simbeck et al. 1993)