Higman Chris Gasification (Газификация угля)
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40
Gasification
of the specialist. Current developments in this field are encouraging, even if the path
ahead is still a long one. Optimization of burners and reactor design, as well as their
influence on one another, is the area most likely to be a fruitful field of application
in the near future.
REFERENCES
Bockelie, M. J., Denison, M. K., Chen, Z., Linjewile, T., Senior, C. L., Sarofim, A. F., Holt,
N. A. “CFD Modeling for Entrained Flow Reactors.” Paper presented at Gasification
Technologies Conference, San Francisco, October 27–30, 2002.
Bürkle, S. “Reaktionskinetische Charakterisierung abfalltypischer Stoffe und deren
Verbrennung in einem Drehrohrofen unter Sauerstoffanreicherung.” Ph.D. diss., Engler-
Bunte-Institut der Universität Karlsruhe (TH), 1998.
Ergun, S. B. “Kinetics of the Reaction of Carbon Dioxide with Carbon.” J. Phys. Chem. 60
(1956):480–485.
Hedden, K., “Die Bedeutung der Reaktionsfähigkeit des Brennstoffs für koksbeheizte
Schachtöfen.” Chemical Engineering Science 14(1961):317–330.
Hofmockel, J., Liebner, W., and Ulber, D. “Lurgi’s Multipurpose Gasification Application
and Further Development.” Paper presented at IChemE Conference, “Gasification for the
Future,” Noordwijk, The Netherlands, April, 2000.
Jüntgen, H., and van Heek, K. H. Kohlevergasung. Munich: Thiemig, 1981.
Reimert, R. and Schaub, G. “Gas Production.” In Ullman’s Encylopedia of Industrial Chem-
istry, 5th ed., vol. A12. Weinheim: VCH Verlagsgesellschaft, 1989, p. 215.
Smoot, L. D., ed. Fundamentals of Coal Combustion. Amsterdam: Elsevier, 1993.
Smoot, L. D., and Smith, P. J. Coal Combustion and Gasification. New York: Plenum, 1985.
Thoenes, D. Chemical Reactor Development. Dordrecht: Kluwer Academic Publishers, 1994.
Ulber, D. “Modellierung der Flugstromdruckvergasung von Öl-Rückständen.” Ph.D. diss.,
Rheinisch-Westfälische Technische Hochschule, Aachen: 2003.
Westerterp, K. R., van Swaaij, W. P. M., and Beenackers, A. A. C. M. Chemical Reactor
Design and Operation. New York: John Wiley & Sons, 1987.
Williams, A., Backreedy, R., Habib, R., Jones, J. M., Pourkashanian, M. “Modeling Coal
Combustion: The Current Position,” Fuel 81 (March 2002):605–618.
Williams, A., Pourkashanian, M., Jones, J. M., Skorupska, N. Combustion and Gasification of
Coal. New York: Taylor and Francis, 2000.
41
Chapter 4
Feedstocks and Feedstock
Characteristics
4.1 COALS AND COKE
Approximately 2250 MMtoe/y of coal is consumed each year (BP 2002). Of this
about 35 MMt/y is gasified to produce 150 million Nm
3
/d of synthesis gas (Simbeck
and Johnson 2001). Roughly half of this gas is generated in the Sasol synfuels plants
in South Africa where the synthesis gas is used for the production of liquid
hydrocarbons and other chemicals. Most of the remainder of this gas is used for
ammonia production and—in China—for the production of town gas.
The total proven reserves of coal amount to 984 ×10
9
t worldwide. Coal consump-
tion has been stable over the last ten years. Nonetheless it plays an important part in
the thinking of many long-term energy strategies, despite its contentious role in the
production of CO
2
as a greenhouse gas. The reason for this can be seen from the
figures in Table 4-1. The ratio of reserves to current production (R/P ratio) is 216. In
other words, at current consumption rates the world’s reserves would last 216 years.
Table 4-1
World Coal Reserves by Region
Region % of Total Reserves R/P Ratio
North America 26.2% 234
South and Central America 2.2% 381
Europe 12.7% 167
Former Soviet Union 23.4% >500
Africa and Middle East 5.8% 246
Asia/Pacific 29.7% 147
World 100% 216
Source: BP 2002
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Gasification
Compare this with 62 years for natural gas and 40 years for oil. Furthermore the
reserves are more evenly distributed than oil (65% of reserves in the Middle East) or
natural gas (72% in the Middle East and Russia).
The composition of coals is very complex, and the types of coal differ considerably.
The detailed petrographic composition of the organic part of coals, often characterized
by a so-called maceral analysis, has little influence on most gasification processes,
and the interested reader is referred to the many treatises on this subject (e.g., Speight
1983; Smoot and Smith 1985; and Kural 1994). Important for gasification are the age
of the coal, its caking properties, its water content, and its ash properties.
4.1.1 Formation of Coal
All coal has been formed from biomass. Over time this biomass has been turned into
peat. When covered under a layer of overburden, the influence of time, pressure, and
temperature convert this material into browncoal or lignite. Subsequently, the latter
material will turn into sub-bituminous coal, then into bituminous coal, and finally
into anthracite. Coal is often classified in terms of its rank, which increases from
browncoal to anthracite. The classification of coal by rank for ash and moisture-free
coal is given in Tables 4-2 and 4-3. Figure 4-1 provides an alternative presentation.
Browncoal, lignite, and sub-bituminous coals are called low-rank coals, whereas
higher-rank coals are often called hard coals. The terms
browncoal
and
lignite
are
essentially synonymous, lignite being used more often in the United States and
browncoal in Europe and Australia.
4.1.2 Coal Analysis
The methods generally used for specifying the analysis of coals has developed along
pragmatic lines and are aimed at providing a useful guide to coal users rather than a
purely chemical approach. The two types of analysis for any coal are the proximate
analysis and the ultimate analysis.
Table 4-2
Classification of Coals
Class Volatile Matter Fixed Carbon Heating Value
wt% wt% MJ/kg
Anthracite <8 >92 36–37
Bituminous 8–22 78–92 32–36
Sub-bituminous 22–27 73–78 28–32
Browncoal (Lignite) 27–35 65–73 26–28
Feedstocks and Feedstock Characteristics
43
Table 4-3
Classification of Coals
Class Fixed Carbon Heating Value Agglomerating Character
wt% BTU/lb
I. Anthracitic 1. Meta-anthracitie ≥98 Nonagglomerating
2 Anthracite 92–98
3. Semianthracite 86–92
II. Bituminous 1. Low-volatile bituminous coal 78–86
2. Medium-volatile bituminous
coal
69–78 Commonly agglomerating
3. High-volatile A bituminous coal <69 ≥14,000
4. High-volatile B bituminous coal 13,000–14,000
5. High-volatile C bituminous coal 11,500–13,000 Agglomerating
III. Sub-bituminous 1. Sub-bituminous A coal 9,500–10,500
2. Sub-bituminous B coal 8,300–9,500 Nonagglomerating
3. Sub-bituminous C coal 8,300–9,500
IV. Lignitic 1. Lignite A 6,300–8,300
2. Lignite B <6,300
Source: ASTM D 388
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Gasification
Proximate Analysis
The proximate analysis determines the moisture, volatile matter, fixed carbon, and
ash in the coal (see Figure 4-1). The analysis is an essentially practical tool
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Moisture
Volatile matter
Fixed carbon
Lignite
Subbituminous C
Subbituminous B
Subbituminous A
High volatile bituminous
High volatile bituminous B
High volatile bituminous A
Medium volatile
Low volatile bituminous
Semianthracitic
Anthracite
Meta-anthracitic
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
HHV [MJ/kg]
Lignite
Subbituminous C
Subbituminous B
Subbituminous A
High volatile bituminous
High volatile bituminous B
High volatile bituminous A
Medium volatile
Low volatile bituminous
Semianthracitic
Anthracite
Meta-anthracitic
Figure 4-1.
Figure 4-1.Figure 4-1.
Figure 4-1. Proximate Analyses of Coals by Rank (Source: Baughman, 1978)
Feedstocks and Feedstock Characteristics
45
providing an initial indication of the coal’s quality and type. The methods for per-
forming these analyses have been standardized by all the major standards institutions
(e.g., ASTM, ISO, DIN, BS, and others). These standards, though similar in nature,
are different from one another in, for example, the temperature specified for determin-
ing the volatiles content, so it is important when providing data to specify the
method used.
Moisture
is determined by drying the coal under standard conditions for 1 h at
104–110°C. The method determines the sum of all moisture; that is, both the surface
moisture caused by rain and so on, and the inherent moisture. The inherent moisture
is the water that is very loosely bound in the coal. It can vary from a few percent in
anthracite to 60–70% in browncoal. (Equilibrium moisture is defined by ASTM
D1412, BS1016 part 21; bed/inherent moisture by ASTM D3173, DIN 51 748).
Volatile matter
is determined by heating the coal in a covered crucible for a
defined time at a defined temperature (e.g., 7 min at 950°C to ASTM). The loss in
mass, minus the mass of the moisture, represents the mass of the gaseous constituents
formed by the pyrolysis under the conditions mentioned (ASTM D3175, BS 1016
part 104, DIN 51 720).
Ash
is the inorganic residue that remains after combustion of the coal. It consists
mainly of silica, alumina, ferric oxide, lime, and of smaller amounts of magnesia,
titanium-oxide, and alkali and sulfur compounds (ASTM D3174, BS 1016 part 104, DIN
51 719).
Fixed carbon
is determined by subtracting from 100 the mass percentages of mois-
ture, volatile matter, and ash (ASTM D3172). It should be remarked that fixed carbon
is an artificial concept and does not mean that this material was present in the coal as
pure C in the beginning.
Although the proximate analysis already tells the expert a lot about the coal, for
gasification it is mandatory to have also the ultimate analysis at our disposal, which
tells us more about the elemental composition of the hydrocarbonaceous part of the
coal.
Ultimate Analysis
For the ultimate analysis the percentages of carbon, hydrogen, oxygen, sulfur, and
nitrogen are determined. In the past, oxygen was sometimes reported as “by
difference.” If at all possible this should not be accepted, as it makes it impossible
to have any control over the quality of the analysis. Proper balances are the basis
for a good process design and a good operation of plants, but a good balance is
equally dependant on a good elemental analysis. (Carbon and hydrogen ASTM
D3178, BS1016 part 106: Nitrogen ASTM D3179, BS1016: Sulfur ASTM D3177,
BS1016 part 106)
Table 4-4 lists proximate and ultimate analyses of various types of coal.
The relevance of
sulfur
in the coal for gasification is the same as for oil-derived
heavy residual feedstocks, which generally contain more sulfur than most coals, and
which are described in Section 4.2. Sulfur contents in coal range from 0.5–6 wt%. It
46
Gasification
Table 4-4
Analyses of Various Coals
Coal Proximate Analysis, % Mass ar
Ultimate Analysis, %
Mass on maf Coal LHV
Country Region Class
Fixed
carbon
Vol.
Mater Water Ash C H O N S
maf coal
MJ/kg
Germany Rhein Browncoal 17.3 20 60 2.7 67.5 5.0 26.5 0.5 0.5 26.2
U.S.A. North Dakota Lignite 27.8 24.9 36.9 10.4 71.0 4.3 23.2 1.1 0.4 26.7
U.S.A. Montana Sub-bituminous 43.6 34.7 10.5 11.2 76.4 5.6 14.9 1.7 1.4 31.8
U.S.A. Illinois Bituminous 39.3 37.0 13.0 10.7 78.4 5.4 9.9 1.4 4.9 33.7
Poland Typical Bituminous 54.9 35.6 5.3 4.2 82.8 5.1 10.1 1.4 0.6 36.1
South Africa Typical Bituminous 51.3 32.7 2.2 13.8 83.8 4.8 8.4 2.0 1.0 34.0
China Datung Bituminous 50.9 28.1 11.9 9.1 84.4 4.4 9.5 0.9 0.8 33.4
India Typical Bituminous 30 23 7 40 75.5 6.4 15.2 1.5 1.4 32.1
Australia Typical Bituminous 44.9 41.1 4.5 9.5 81.3 5.8 10.0 2.0 0.9 33.8
Germany Ruhr Anthracite 81.8 7.7 4.5 6.0 91.8 3.6 2.5 1.4 0.7 36.2
Feedstocks and Feedstock Characteristics
47
may be present in coal in three forms: pyritic sulfur, inorganic sulphates, and as sulfur
in organic compounds. These three forms of sulfur can be determined using ASTM
D 2492. In coals with a high sulfur content, most of the sulfur is generally present in
the form of pyrite. Note that the quantity of pyritic sulfur is an indicator for the potential
abrasiveness of the coal.
The
nitrogen
content in coals ranges from 0.5–2.5wt%. Only part of the nitrogen in
the coal is converted into ammonia and HCN upon gasification, whereas the remain-
der is converted into elemental nitrogen. The presence of the coal-derived nitrogen in
the product gas is one reason why it is not always essential to gasify coal with very
pure oxygen (>99 mol%), even when the gas is used for the production of syngas or
hydrogen. The percentage of the nitrogen in the coal that is converted into elemental
nitrogen upon gasification will depend on the type of nitrogen compounds in the coal.
4.1.3 Other Minerals in Coal
Beyond the elements described above, which are provided with every ultimate analy-
sis of coal, it will be found that a substantial part of the periodic table can be shown
to be present in coals. These other elements can be divided into macro components,
the presence of which is usually given in wt%, and the micro or trace elements that
are only present at ppm levels. Values from a sample coal within the typical range
are given later in Table 4-7.
The
chlorine
content in coal is mostly well below 1 wt%. However, in some coals
it may be as high as 2.5 wt%. In combination with a low nitrogen content in the coal,
this will result in a high caustic consumption in the wash section of a gasifier.
Chlorides have three possible detrimental effects in the plant:
1. Chlorides have a melting point in the range 350–800°C; they deposit in the syngas
cooler and foul the exchanger surface. The first indication of this is an increase in
the syngas cooler outlet temperature.
2. In the reactor chlorides can react with the hydrogen present to form HCl, which
will decrease the pH of the wash water or condensate.
3. Chlorides may also form NH
4
Cl with high nitrogen feeds. With such feedstocks the
chloride deposits as NH
4
Cl in the economizers at temperatures below about
280°C. Further, as an aqueous solution this leads to severe chloride stress corrosion
in stainless steels that are used, for example, in burners and instrument lines.
Coals also contain
phosphorus
, but this has less significance for gasification than,
for instance, for the steel industry.
Basis of Reporting Analyses
It is of paramount importance that the users of the analytical data of coal are able to
convert various types of analysis into each other. Apart from the difference in units
48
Gasification
used that can readily be transferred into each other, there are problems associated with
the basis for the analysis. The proximate analysis and the ultimate analysis may be
given based on the so-called as-received (ar) coal; that is, the coal including moisture
and ash. When doing so it is also important to record whether the sample was taken
direct from storage or air dried before analysis, since the difference between total and
inherent moisture cannot be determined. But it is also possible to get these analyses on
a moisture-and-ash-free (maf) basis, a moisture-free (mf) basis, or an ash-free (af)
basis. Furthermore, the heating value may be the higher heating value (HHV) or the
lower heating value (LHV). In relation to the latter it should be kept in mind that the
HHV of steam and water are positive and zero, respectively, and that the LHV of steam
and water are zero and negative, respectively. A program is included on the companion
website to convert given data for proximate and ultimate analysis as well as for the
heating value on any basis (ar, af, mf, maf, and HHV or LHV) into any other basis.
4.1.4 Other Properties
Heating Value
The heating value is obtained by combustion of the sample in a calorimeter. If not
available, the heating value can be calculated with, for example, the Dulong formula
(Perry and Chilton 1973, p. 9–4) from the ultimate analyses:
HHV in MJ/kg =33.86*C +144.4*(H − O/8) + 9.428*S
where C, H, O, and S are the mass fractions of the elements obtained from the ulti-
mate analysis. There are other formulae for calculating the heating value from the ulti-
mate and/or proximate analyses (e.g., Channiwala and Parikh 2002):
HHV in MJ/kg =34.91*C +117.83*H − 10.34*O − 1.51*N + 10.05*S − 2.11*Ash
It is always useful to calculate the heating value from these analyses, as it is a good
crosscheck on measured values. If the deviation is more than a few percent, all analyses
must be checked.
Caking and Swelling Properties
Another important property of a coal is the swelling index. The swelling index is
determined by heating a defined sample of coal for a specified time and temperature,
and comparing the size and shape taken by the sample with a defined scale. There are a
number of different scales defined in, for example, ASTM D 720-91, BS 1016 (Gray-
King method), or ISO 335 (Roga method). The swelling index is an indicator for the
caking properties of a coal and its expansion on heating. Softening/caking does not
occur at a certain temperature but over a temperature range. It is an important variable
Feedstocks and Feedstock Characteristics
49
for moving-bed and fluid-bed gasifiers. For the gasifiers of entrained-flow systems, the
coal softening point has no relevance. However, the softening point may limit the
amount of preheating of the pulverized coal feedstock used in dry coal feed gasifiers.
Hardness
Physical properties are not very relevant for the operation of a gasifier as such. The
hardness of the coal is, for example, mainly important for the milling and grinding
upstream of the gasifier. The hardness of a coal is usually dependant on the nature
and quantity of its ash content, although some coals, such anthracites, are also hard.
A high ash content or a very high hardness of the ash in the coal can make a feedstock
unattractive for gasification because of the high cost of milling and grinding. Ashes
with high silica and/or alumina contents have a high hardness. The hardness is generally
characterized by the Hardgrove grindability index (ASTM D 409).
Density
The density is primarily of importance for the transport of the coal. In this connection, it
is important to discriminate between the particle density and the bulk density of the
coal. The bulk density is always lower, as is shown in Table 4-5.
4.1.5 Ash Properties
Melting Properties
For all gasifiers the ash-softening and ash-melting or fusion temperatures are
important variables. For fluid-bed gasifiers these properties govern the upper operating
temperature at which agglomeration of the ash is initiated. For entrained-flow gasifiers
it is essential to ensure that the ash flows continuously and that the slag tap does not
freeze up. The method for determining these temperatures is specified in ASTM
D1857, “Fusibility of Coal and Coke Ash,” or similar specifications, such as ISO
540. In these methods the temperatures measured relate to the behavior of an ash
Table 4-5
Comparison between Particle and Bulk Density for Various Coals
Fuel Density (kg/m
3
)
Particle (true) Bulk (apparent)
Anthracite 1450–1700 800–930
Bituminous coal 1250–1450 670–910
Lignite 1100–1250 550–630