Higman Chris Gasification (Газификация угля)

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Gasification

sample under specified conditions and are reported as IDT (initial deformation

temperature), ST (softening temperature), HT (hemispherical temperature), and FT

(fluid temperature). For gasifier applications the ash-melting characteristics should

be determined under reducing conditions, as these data may differ considerably

(generally, but not universally lower) from data for oxidizing conditions.

An additional property required for slagging gasifiers is the slag viscosity-

temperature relationship. It is generally accepted that for reliable, continuous slag-

tapping a viscosity of less than 25 Pa.s (250 Poise) is required. The temperature

required to achieve this viscosity (T

250

) is therefore sometimes used in the literature

(Stultz and Kitto 1992). Some slags are characterized by a typical exponential

relationship between viscosity and temperature over a long temperature range. For others

this relationship is foreshortened at a critical temperature (T

) at which the viscosity

increases very rapidly with decreasing temperature. For a slagging gasifier to operate

at a reasonable temperature, it is necessary for the slag to have a T

<1400°C.

The relationship between ash-melting characteristics and composition is a

complicated one and is dependant largely on the quaternary SiO

-Al

-CaO-FeO

(Patterson, Hurst, and Quintanar 2002). In general, slags that are high in silica and/

or alumina will have high ash-melting points, but this is reduced by the presence of

both iron and calcium—hence the use of limestone as a flux. However, the SiO

/Al

ratio is also important, and where the calcium content is already high (as in some

German browncoals), there can be some advantage to lowering the ash melting point

by adding SiO

. Properties of some typical ashes are given in Table 4-6.

In dry ash moving-bed gasifiers and in fluid-bed gasifiers, coals with a high ash

melting point are preferred, whereas in slagging gasifiers, coals with a low ash melting

point are preferred.

The caking properties of a coal and the melting characteristics of its ash are the rea-

son that there are forbidden temperature ranges that have to be taken into account,

both in design and during operation. In entrained-flow gasifiers only the ash properties

are important.

The ash that is produced in gasifiers always has a lower density than the minerals

from which they originate, due to loss of water, decomposition of carbonates, and

other factors, and the presence of some carbon. The bulk density of the ash in

particular may be low due to the formation of hollow ash particles (cenospheres).

This means that special attention has to be given to the transport of such ashes.

Slag is very different from ash as it has been molten and is in fact a fusion-cast

material similar to glass. Ideally, slag becomes available as an inert, fine, gritty material

with sharp edges due to the sudden temperature drop upon contact with a water bath.

Because lumps of solid slag will form during process upsets, a slag breaker is sometimes

installed between the water bath and the slag depressurizing system.

Chemical Composition of Ash

In Table 4-6 some of the major components of various ashes are given. Apart from

these there are many trace components present that do not contribute much to the

Feedstocks and Feedstock Characteristics

Table 4-6

Ash Fusion Temperatures and Analysis

Coal Ash-Melting Point Analysis of Ash as Oxides

reducing °C oxidizing °C SiO

TiO

CaO MgO Na

O K

OSO

Pocahontas No. 3 West IDT IDT 60.0 30.0 1.6 4.0 0.6 0.6 0.5 1.5 1.1 0.1

Virginia Bituminous >1600 >1600

Ohio No. 9 Bituminous 1440 1470 47.3 23.0 1.0 22.8 1.3 0.9 0.3 2.0 1.2 0.2

Illinois No. 6 Bituminous 1270 1430 47.5 17.9 0.8 20.1 5.8 1.0 0.4 1.8 4.6 0.1

Pittsburgh, WV Bituminous 1300 1390 37.6 20.1 0.8 29.3 4.3 1.3 0.8 1.6 4.0 0.2

Utah Bituminous 1280 1320 61.1 21.6 1.1 4.6 4.6 1.0 1.0 1.2 2.9 0.4

Antelope Wyoming

Sub-bituminous 1270 1260 28.6 11.7 0.9 6.9 27.4 4.5 2.7 0.5 14.2 2.3

Texas Lignite 1230 1250 41.8 13.6 1.5 6.6 17.6 2.5 0.6 0.1 14.6 0.1

Source: Stultz and Kitto 1992

Gasification

melting characteristics of the ash but can have a major effect on the environmental

problems associated with coal use: for example, mercury, arsenic, zinc, lead, cadmium,

chromium, chlorine, and fluorine. Most of the elements present in coal as given in

Table 4-7 can appear at least in part in the ash.

Table 4-7

Detailed Chemical Analysis of

Minerals in Coal

As, ppmv 2.1

B, ppmv 35

Ba, ppmv 130

Be, ppmv 1.2

Br, ppmv 1.5

Cd, ppmv 0.07

Ce, ppmv 24

Co, ppmv 3.5

Cr, ppmv 7.0

Cs, ppmv 0.30

Cu, ppmv 9.2

Eu, ppmv 0.32

F, ppmv 227

Ge, ppmv 0.50

Hf, ppmv 1.8

Hg, ppmv 0.13

La, ppmv 17

Mn, ppmv 84

Mo, ppmv 1.6

Ni, ppmv 10

Pb, ppmv 14

Rb, ppmv 2.1

Sb, ppmv 0.57

Sc, ppmv 2.9

Se, ppmv 3.1

Sm, ppmv 1.4

Sr, ppmv 316

Th, ppmv 8.4

Ti, ppmv 0.43

U, ppmv 2.1

V, ppmv 17

W, ppmv 0.73

Feedstocks and Feedstock Characteristics

4.1.6 Coke

Coke is a material consisting essentially of the fixed carbon and the ash in the coal.

It was in the past a common fuel in water gas plants, but as it is more expensive than

coal, anthracite is now often the preferred fuel. It is virtually never used in gasification

plants. Coke plays a very important role in blast furnaces, which may be considered

to be very large gasifiers (Gumz 1950). One of the main reasons to use coke in blast

furnaces is that it is much stronger than coal.

4.1.7 Petroleum Coke

Petroleum coke, more often named petcoke, is increasingly considered as an attractive

feedstock for gasification, in particular as it becomes more and more difficult to fire

this high-sulfur material as a supplemental fuel in coal-fired power stations. Apart

from the feed systems that are similar to those for pulverized coal, the behavior of

petcoke in gasifiers is very similar to that of heavy oil fractions. When gasified in

certain entrained-flow slagging coal gasifiers, it is essential to add ash, because otherwise

the build-up of a proper slag layer on the membrane wall will not be successful

(Mahagaokar and Hauser 1994). In the Elcogas plant in Puertollano, Spain petcoke

is processed together with (high-ash) coal (Sendin 1994).

4.2 LIQUID AND GASEOUS FEEDSTOCKS

In 2002 some 154 million Nm

/d of synthesis gas was produced by partial oxidation

of liquid or gaseous feeds (Simbeck and Johnson 2001; SFA Pacific 2001). If fed to

state-of-the-art IGCC units, this would have generated some 11,500 MW

. By far

the largest portion of this synthesis gas (about 80%) is generated from refinery

residues, typically visbreaker vacuum bottoms or asphalt. The most important

product from these plants is ammonia. Methanol is also important, but refinery

hydrogen and power applications via IGCC are rapidly increasing (Figure 4-2).

Most plants with gaseous feed are small units for the production of CO-rich synthesis

gases, particularly for the production of oxo-alcohols. The largest single gas-fed

plant is the Shell unit at Bintulu, Malaysia, which serves as the front end for a synfuels

plant using Fischer-Tropsch technology. The number of projects recently announced

indicates that this type of application is likely to gain increasing importance in the

near future. However in the long term one must recognize that neither oil nor natural

gas availability is as great as that of coal. The reserves-to-production ratios are 40

and 62 years, respectively, compared with 216 years for coal.

4.2.1 Refinery Residues

Over 95% of liquid material gasified consists of refinery residues. Being a high

temperature, noncatalytic process, partial oxidation is by and large flexible with

Gasification

regard to feedstock quality and does not make great demands on the specification of

a liquid feed. However, every feedstock needs to be evaluated to ensure that design

aspects are correctly selected for its particular requirements. Generally, any feed-

stock would need to fulfill the following requirements:

• The fuel per se must be single phase at the reactor burner inlet. Small amounts of gas

in a predominantly liquid feed may ignite close to the burner, causing damage at that

point. Droplets of liquid in a predominantly gaseous phase feed can lead to erosion

and premature wear of equipment. This does not pose a limitation for most feed-

Table 4-8

Liquid and Gaseous Gasification Capacities

Number

of Reactors

Syngas Production

(MM Nm

/d)

Approximate

Feed Rate

Liquid feed 142 124 42,000 t/d

Gaseous feed 41 31 10.9 MM Nm

Total 183 155

Source: SFA Pacific 2001

Table 4-9

Applications for Gasification of Liquid and Gaseous Feedstocks

Synthesis Gas from

Total Syngas

(MM Nm

/d)

Approximate

Product Rate

Liquid

Feeds

(MM Nm

/d)

Gaseous

Feeds

(MM Nm

/d)

Ammonia 50.5 0.8 53.7 6.93 MMt/y

Methanol 15.3 6.3 21.7 3.01 MMt/y

Hydrogen 7.1 2.0 9.0 12.50 MMNm

Synfuels – 7.6 7.6

Other

chemicals 9.4 12.4 21.9

Power 34.3 – 34.3 2554 MW

Other 4.7 1.6 6.2

Total 122.1 30.6 154.3

Note: Totals do not tally exactly due to some double counting on multiproduct facilities.

Source: SFA Pacific 2001

Feedstocks and Feedstock Characteristics

stocks, but must be considered in the design of the preheat train. It is important in

this connection, however, to ensure that abrasive solids are kept to a minimum.

• The feedstock must be maintained within certain viscosity limits at critical points

in the plant, such as at the burner. Again, this is generally a matter that can be

addressed in the design of the feed transport and preheat systems and will not

place any limitation on choice of feed.

• Too high a level of certain components in the ash, such as a high sodium content,

limits the use of a syngas cooler, since the sodium salts deposit on the exchanger

surface. Details are discussed under Sodium below.

The term “refinery residue” covers a wide range of material, some of which is solid at

ambient temperatures, and others that are liquid under these conditions. Common to

all is their origin in being the product of the distillation and possible subsequent

treatment of crude oil. The most common residues are obtained by thermally cracking

(visbreaking) vacuum residue or by subjecting it to solvent de-asphalting.

The specifications shown in Table 4-10 show typical gasifier feeds having a high

sulfur content, high metal contents, and high viscosities (Posthuma, Vlaswinkel, and

Zuideveld 1997). The data for tar sands bitumen, which are discussed in Section

4.2.2, are also included.

C/H Ratio. The C/H ratio of heavy refinery residues can vary between about 7 kg/kg

(vacuum residue) and 10 kg/kg (asphalts), depending on the crude source and refining

history. There are no specific limitations on the C/H ratio for gasification, but of

course it does have an effect on syngas quality.

If all other conditions are maintained equal, the feeds with a higher C/H ratio will

produce a synthesis gas with a lower H

/CO ratio. Whether this is an advantage or a

disadvantage will depend on the application.

Figure 4-2.

Figure 4-2.Figure 4-2.

Figure 4-2. Growth of Hydrogen and Power Applications for Gasification of Liquid

and Gaseous Feedstocks (Source: SFA Pacific 2001)

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

Syngas capcity [GW

]

Gasification

Sulfur Content. Typically, the sulfur content of a residue can vary between 1% and

7%. In exceptional cases (e.g., in parts of China) material with as little sulfur as

0.15% is gasified.

A sulfur content over this range has little effect on the design or operation of a

gasifier, but it is an important issue for the design of the subsequent gas treating. A low

sulfur content gives rise to a low H

S/CO

ratio in the sour gas from the acid gas

removal unit, and this can be decisive for the design of the sulfur recovery unit.

Selective physical washes such as Rectisol can be designed to concentrate the H

in the sour gas, and with a low sulfur feed this would be necessary. Equally, low

sulfur feeds can have surprising effects on a raw gas shift catalyst. This catalyst is

required to operate in the sulfided state, and if insufficient sulfur is available to

maintain this, then the catalyst can lose activity (BASF). Again, it is perfectly

possible to design around this by including a sulfur recycle. It is therefore good

practice to consider the low sulfur case carefully when specifying the basis of design

for a new plant.

Table 4-10

Typical Refinery Residue Specifications

Feedstock Type

Visbreaker

Residue

Butane

Asphalt

Tar Sands

Bitumen

Elemental

analysis/Units

C, wt%

85.27 84.37 83.60

H, wt% 10.08 9.67 1.90

S, wt% 4.00 5.01 6.08

N, wt% 0.30 0.52 1.52

O, wt% 0.20 0.35 1.90

Ash, wt% 0.15 0.08 5.00

Total, wt% 100.00 100.00 100.00

C/H, kg/kg 8.3 8.7 44

Vanadium, mg/kg 270–700 300 250

Nickel, mg/kg 120 75 100

Iron, mg/kg 20 20

Sodium, mg/kg 30 30

Calcium, mg/kg 20 20

Viscosity (100°C), cSt 10,000 60,000 110

Density (15°C), kg/m

1,100 1,070 1,030

LHV, MJ/kg 39.04 38.24 34.11

Feedstocks and Feedstock Characteristics

Corrosion effects in a residue gasifier, which can be connected with the presence

of sulfur, are in most cases independent of the actual content of sulfur, and one

needs to look at what other circumstances are contributing to the corrosion. In the

event of, for example, high temperature sulfur corrosion, the solution to the problem

will generally lie in avoiding the high temperatures rather than trying to lower the

sulfur content of the feed.

Nitrogen Content. The refining process concentrates the nitrogen present in the

crude oil into the residue. Nonetheless, there is seldom more than 0.6 wt% nitrogen

in a gasifier feedstock. Much of the nitrogen entering the reactor as part of the feed-

stock is bound in organic complexes, and under the gasifier conditions it reacts with

the hydrogen to form ammonia and hydrogen cyanide. The more nitrogen there is

contained in the feed, the more ammonia and cyanide will be formed. For details see

Section 6.9.2.

Ash Content. The ash content of typical feedstocks are summarized in Table 4-10.

The evaluation of the feed must be done on the basis of both individual ash compon-

ents as well as the total ash content. The most critical and most common components

are discussed in the following sections.

Satisfactory experience with ash contents up to 2000 mg/kg has been achieved.

Individual plants have run with even higher ash contents (Soyez 1988).

Vanadium. Experience of up to 700 mg/kg is available, and in one case of up to

3500 mg/kg at the reactor inlet. With the exception of residues from some

Central and South American crudes, feedstocks with over about 350 mg/kg are

unusual. Many residues from Far East crudes have an order of magnitude less

than this.

Vanadium as a feed component has two undesirable properties:

• In an oxidizing atmosphere vanadium is present as V

, which has a melting

point of 690°C (Bauer et al. 1989). At temperatures higher than this, V

diffuses

into refractory linings, whether of a reactor or a boiler fired with a high vanadium

fuel such as carbon oil (see next bullet), and destroys the refractory binder. In a

reducing atmosphere (i.e., in normal operation for a gasification reactor) vanadium

is present as V

, which has a melting point of 1977°C and is therefore not

critical. For plant operation the lesson is that special care should be taken during

heat up and after shutdown, when the reactor could be subject to an oxidizing

atmosphere at temperatures above 700°C (Collodi 2001).

• When combusted in a conventional boiler, a fuel with a high vanadium content

will cause slagging and fouling on economizer heat-transfer surfaces. Where soot

from the gasifier is admixed to oil (carbon oil) for external firing in an auxiliary

boiler, this sort of fouling can be a particular problem. With 700 mg/kg vanadium

in the boiler feed, cleaning of the surfaces may be required as often as once or

more per year.

Gasification

• Problems with burners and syngas coolers have also been reported when operating

with over 6000 mg/kg vanadium in the reactor feed (Soyez 1988). This high level

of vanadium in the feed is, however, extremely unusual.

Nickel. There is no generally recognized upper limit for nickel in gasifier feedstocks.

Nickel does, however, have an important influence on the gas treatment. In the presence

of carbon monoxide and under pressure, nickel and nickel sulfide both form nickel

carbonyl, a gaseous compound that leaves the gasification as a component of the

synthesis gas. This topic is handled in more detail in Section 6.9.8.

It is also worth noting that upon air contact nickel sulfide can react to form NiSO

in the water. The NiSO

goes into solution in the water and can then be recycled to

the scrubber, where it can increase the amount of carbonyls formed.

Sodium. Sodium can be present in the gasification feedstock either as sodium

chloride or as sodium hydroxide. The sodium leaves the reactor as sodium chloride,

or as sodium carbonate where the origin is sodium hydroxide, which have melting

points of 800°C and 850°C, respectively (Perry and Chilton 1973). In plants where the

synthesis gas is cooled by steam raising in a heat exchanger, these salts deposit on

the heat-exchange surface. The most serious effect is fouling of a large section of the

surface, causing an increase in exchanger outlet temperature. The depositing can,

however, also accumulate locally to the extent that there is a significant increase in

pressure drop. It is therefore desirable to ensure that the sodium content does not

exceed 30 mg/kg, or at the maximum 50 mg/kg. It is our experience that already as

much as 80 mg/kg in a feed that had been contaminated with seawater caused the

outlet temperature of a syngas cooler to rise at more than 1°C per day.

Fortunately, the sodium chloride fouling is a reversible phenomenon, and operation

with a sodium free feed will cause the outlet temperature to reduce again even if not

quite

to the value prevailing before the sodium ingress. Full recovery requires a steam out.

Sodium compounds—and for that matter other alkali metals—have the additional

unpleasant property that they diffuse into the refractory lining of the reactor, where they

effect a change in the crystal structure of the alumina from α-alumina to β-alumina,

which leads in turn to a gradual disintegration and loss of life of the refractory.

For most refinery applications the sodium limitation is not a restriction, since the

desalting process in the refinery effectively maintains the sodium level within

allowable limits. In cases where a high sodium content is expected on a regular and

steady basis, then quench cooling is a preferable choice to a syngas cooler.

Calcium. Typically, there may be some 6–20 mg/kg calcium in a refinery residue.

Calcium can react with the CO

in the synthesis gas to form carbonates. This does

not normally present any problems. Where significantly more calcium is present,

these carbonates can precipitate out of the quench or wash water depositing in the

level indicators, possibly with disastrous results if not recognized in time. A certain

degree of depositing on the surface of syngas coolers is also possible (Soyez 1988).

Feedstocks and Feedstock Characteristics

Iron. The iron content of the feed can be as high as 50 mg/kg, but is generally lower.

The behavior of iron is similar to that of nickel. Carbonyl formation in the synthesis

gas takes place at a lower temperature than for nickel. This is discussed in more

detail in Section 6.9.8.

Silica. Silica can find its way into refinery residues from a number of sources, as

sand, catalyst fines from a fluid catalytic cracker (FCC), or from abraded refractory.

Typically, there may be between 20 and 50 mg/kg in the residue, which can be

tolerated. Larger quantities can cause two types of problem.

On the one hand, silica is an abrasive material that largely passes into the water

system, where it settles out to some extent. In recycle systems, it is partly mixed with

the feedstock, increasing the quantity being fed to the reactor. Abrasion has been

reported on the feedstock charge pumps as a result (Soyez 1988).

Silica entering the system as FCC catalyst fines can also deposit close to the

burner area of the reactor, with the risk of disturbing the flame pattern in the reactor.

Additionally, there is the problem that under the reducing conditions in the

reactor, silica is reduced to volatile SiO according to the reaction

SiO

=SiO ↑+H

The SiO condenses at about 800°C while cooling in the syngas cooler and deposits

on the exchanger surface. This is one reason why all oil gasifiers use a low-silica

high-alumina refractory lining (Crowley 1967).

Chloride. Chloride in the feedstock is mostly present as NaCl. Smaller quantities

may also be present as K-, Fe-, Cr-, Ca-, and Mg- compounds.

Although large quantities of chlorides will damage the plant, the limitation of the main

source, NaCl to 30ppmw Na, is usually sufficient to limit the overall chloride intake.

The potential problems of fouling or corrosion associated with chlorides are

described in Section 4.1.

Naphthenic Acids. Naphthenic acids can be present in residues to a greater (e.g., those

derived from Russian crudes) or lesser extent. While this is not of importance for the

gasification process itself, it can be an important evaluation criterion, since it would need

to be considered in the selection of metallurgy for the feedstock transport system.

Viscosity. Many refinery residues have an extremely high viscosity and are (subject-

ively) solid, having the consistency of street bitumen at ambient temperatures. Since

the viscosity decreases considerably with increased temperature, it is often desirable

to transport and store this material in a heated condition.

The temperature dependency of viscosity takes the form of the Walther equation

(Große et al. 1962, L4):

log(log(ν+

)) = m*(log(

) −log(T)) + log (log(ν

))