Higman Chris Gasification (Газификация угля)
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Chapter 8
Auxiliary Technologies
8.1 OXYGEN SUPPLY
The oxygen supply to a gasifier is one of the most expensive single parts of any
gasification project. Cost estimates for various different IGCC projects put the ASU
with its associated compressors at between 10 and 15% of the total plant cost. It also
makes a significant contribution to overall operating costs, the power requirement
for compression being of the order of magnitude of 5–7% of gross generator output.
The arrangement for oxygen supply is one of the most important early decisions in
any gasification project.
This decision on oxygen supply is not only technical but to a high degree also
commercial. This is because the industrial gas market is dominated by a small
number of highly competitive companies who are able not only to build oxygen
plants for third parties, but are also willing to build and operate their own plants
close to or even on a client’s site and supply oxygen and other gases “over the
fence.” In some parts of the world, these companies have their own extended pipe-
line networks, which can also be integrated into the oxygen supply strategy for a
gasification project.
8.1.1 Technologies
Cryogenic Processes
Since the commercialization of the Linde-Fränkle process in the 1920s, oxygen sup-
ply has been dominated by cryogenic technology.
The principle features of cryogenic air separation are shown in Figure 8-1. Air is
compressed, dried in a prepurification unit, and then cooled to its liquefaction
temperature. The liquid air is then distilled into its two main constituents, oxygen
and nitrogen. These separated products are then heated and vaporized. This basic
flow scheme has formed the basis for all processes, or cycles as they are known, to
this day, although many detail improvements have been made over the years to
decrease costs and improve efficiency. In addition to these improvements, the size
of air separation units has risen dramatically over the last 40 years. The 1400 t/d
(40,000 Nm
3
/h) ASU supporting a gasification plant with 65 t/h visbreaker residue
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Gasification
feed that came on stream in 1972 was at that time the largest in the world (Butzert
1976), but there are several plants now running with a capacity of about
90,000 Nm
3
/h (3200 t/d), including for instance that at Rozenburg, The Netherlands,
which among others provides the oxygen for the Shell Pernis gasifiers.
The operating pressure of modern plants varies with the application. Low-pressure
(LP) cycles, which supply by-product gaseous nitrogen at only atmospheric pres-
sure, operate at about 5–7 bar, depending on oxygen pressure and required energy
efficiency. Where much of the nitrogen is required at higher pressures, it can be
advantageous to operate the ASU at elevated pressure (EP) above this level. This is
particularly appropriate in an IGCC with air and N
2
integration, since the air is in
any case available from the gas turbine compressor at the higher pressure. Operating
at the higher pressure also has the advantage of tending to reduce equipment size
and cost.
The prepurification unit (PPU) has seen substantial changes since the original
Fränkle regenerators. After a period when most plants were equipped with reversing
exchangers, in which water and CO
2
were frozen onto the surfaces of the main heat
exchangers and evaporated off in the regeneration part of the cycle with waste nitrogen,
WASTE
NITROGEN
OXYGEN
NITROGEN
AIR COMPRESSOR
MOLECULAR
SIEVES
COLD BOX
AIR
Figure 8-1. Air Separation Unit
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295
the 1970s saw the introduction of molecular sieves for prepurification. Most modern
PPUs have twin beds of silica gel and molecular sieve. They hold back not only
water and CO
2
but also potentially dangerous hydrocarbons that may be in the
atmosphere.
The partially liquefied air enters the lower, high-pressure column where pure
nitrogen is drawn off as overhead product. The bottoms, an oxygen-rich liquid, is
expanded through a turbine into the upper, low-pressure column, where in the simplest
cycles, pure oxygen is drawn off as bottoms product. The overhead is an impure
nitrogen stream, which like the main products is used for chilling the incoming air
before being used as regeneration gas in the PPU and then being discharged to the
atmosphere. Where large quantities of pure nitrogen (<10 vppm O
2
) are required as
in ammonia plants, a reflux can be added to the top of the LP column and some of
the impure nitrogen also recovered as pure product. Typically, the maximum
amount of pure product obtainable is about 70% of the incoming air.
Production of argon is possible by tapping the middle of the LP column, where
the argon concentration is highest, and adding a further distilling stage. Final purifi-
cation of the argon takes place by the catalytic reduction of final traces of oxygen
with hydrogen. This additional processing stage requires a higher operation pressure
of the ASU at the upper range of LP cycles.
For the production of pressurized oxygen, two cycles are used: the compression
cycle, and the pumped liquid cycle. In the former gaseous oxygen leaves the cold
box at slightly above atmospheric pressure and is compressed in a compressor.
Alternatively, the liquid oxygen can be pumped up to the required pressure and the
vaporized under pressure. This latter cycle, also known as internal compression,
required when introduced about 5–7% more energy, but cycle development has
reached the point where there is little difference between compression and pumped
liquid cycles.
Air separation has a widely recognized reputation for reliability. This is important
for the gasification process, since oxygen production is at the beginning of the flow-
sheet, and loss of oxygen brings the whole downstream facility to a standstill.
Traces of water or CO
2
can slip past the PPU and over a period of time freeze out on
the heat exchangers. If this goes beyond a tolerable limit for the operation, then the
coldbox must be reheated to ambient temperature and derimed. Typically, under
normal operation this may be necessary every two years. The deriming itself may
take about one or two days.
The vulnerability of a gasification plant to interruptions of oxygen supply makes
the consideration of building some liquid storage capacity an important issue for the
planning of any gasification project. The only economic method of oxygen storage
is as a liquid at low temperature. Typically, under such conditions a boil-off of some
0.2–0.5%/d, depending on the size of tank, must be expected. The principle aspects
to be considered are the response time in which oxygen from the storage is available
at the gasifier (seconds), and the size of the storage. For the latter consideration,
some hours worth of storage to cover a compressor trip and restart would be a minimum.
A storage volume to cover a coldbox deriming period is unrealistic.
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Gasification
Alternative Processes for Small Quantities
For small units, other processes are available. They cannot, however, reach the
purity obtainable with a cryogenic unit. Pressure swing adsorption units are available
up to a capacity of about 140 t/d (4000 Nm
3
/h), but they can only reach a purity of
about 95% O
2
. The product purity obtainable with polymer membrane technology is
much less—about 40% O
2
—and such units are available for capacities of 20 t/d
(600 Nm
3
/h). The by-product capability of both these technologies is poor, but they
both have the advantage of quick start-up compared with cryogenic units (Smith and
Klosek 2001). Given that most small gasification facilities are for chemical applica-
tions where even 5% nitrogen in the oxygen is unacceptable, their use in connection
with gasification is likely to be limited. The most probable gasification application
could be with biomass power applications where sizes are also at the lower end of
the scale.
Future Developments
Oxygen production by means of ion transport membranes is the subject of intense
research and development (Allam, Foster, and Stein 2002).
The principle of these devices is based on the use of nonporous ceramic membranes
that have both electronic and oxygen ionic conductivity when operated at high tem-
peratures, typically 800 to 900°C (Figure 8-2).
Oxygen from the feed side of the membrane adsorbs onto the surface of the mem-
brane, where it dissociates and ionizes by electron transfer from the membrane. The
oxygen diffuses through the lattice of the membrane, which is stoichiometrically defi-
cient in oxygen. The driving force is the differential partial pressure of oxygen across
the membrane. Oxygen ions arriving at the product side of the membrane release their
electrons, recombining and desorbing from the surface as molecular oxygen. Electrons
flow in counter current from product to the feed side of the membrane.
MEMBRANE
PRESSURIZED
AIR FEED
OXYGEN DEPLETED
NONPERMEATE
ELECTRONS
O
2−
O
2−
O
2−
LOW PRESSURE HIGH-PURITY
OXYGEN PRODUCT
O
2
+ 2e
−
O
2
+ 2e
−
½
½
→
→
Figure 8-2. Principles of Ion Transport Membrane (Source: Allam et al. 2000)
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297
This ion-transport mechanism is specific to oxygen so that, discounting leakage
through any cracks or seals and so on, the product is 100% oxygen.
Considerable work has been performed in scale-up and demonstration of produc-
tion facilities (Armstrong et al. 2002). Integration with both the gasification and/or
the combustor of the gas turbine is mandatory for its success. Commercialization is
currently expected “in the 2006 to 2008 time frame.”
8.1.2 Pipeline and Other Supply Possibilities
Pipelines
In areas where there is already an established pipeline network, a gas supply
contract can be especially attractive. For small plants close to a pipeline, this may be
a very simple affair, simply drawing on existing capacity. For large plants, purpose-
built capacity will have to be added, but synergies are often possible with a need for
increased capacity to feed the pipeline system.
The following is a list of major pipeline operators and the location of their networks.
Air Products Houston—Port Arthur, Texas
Decatur, Alabama
Rotterdam, The Netherlands
Humberside, England
Mab Ta Phut, Thailand
Onsan/Ulsan, South Korea
Air Liquide Dunkirk – Antwerp – Rotterdam – Liege
Corpus Christi – Lake Charles
Weswego – Geismar/Baton Rouge (Louisiana – Mississippi
River Corridor)
Praxair Northwest Indiana
Houston area
Wilmington, California
Detroit, Michigan (in Ecorse) and in other enclaves
throughout the United States.
BOC Various locations
Linde Ruhr district, Germany
Leuna district, Germany
Messer Griesheim Dortmund-Cologne, Germany
Saarland, Germany
Tarragona, Spain
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Gasification
Over the Fence
Even in locations where no pipelines are available, it is possible to buy oxygen
“over the fence.” In this business model, which has a long tradition in oxygen
supply to the steel industry, a gas supply company will own and operate a dedicated
air separation unit within the gasification complex. By-products such as argon,
which may be of little interest to the gasifier operator, can make such an arrangement
very attractive. The specialist, operating know-how of the gas companies enable
them to achieve excellent plant availability. In some locations where this model is
practiced, it has been extended to cover utility supply.
8.2 SYNTHESIS GAS TREATING
Although the number of gasification processes for any particular feedstock is
limited, the range of choice for acid gas removal (AGR) processes and other gas
treating tasks appears at first sight somewhat bewildering. Their selection requires a
basic knowledge of the different classes of process and a systematic approach to
defining the task in hand. Although the approach developed in this chapter cannot
replace the expertise of the gas purification specialist, it can help to ensure that he or
she has all the facts available to make an optimum selection.
The term
acid gas removal
is a general term that is often used as a synonym for
desulfurization, but strictly speaking in the synthesis gas environment it includes
also CO
2
removal, and it is in this inclusive sense that the term is used here.
A large number of different processes have been and are used for acid gas
removal, but they are all based on one of the following principles:
• Absorption (physical or chemical) in a liquid solvent with a subsequent desorption step.
• Adsorption (again physical or chemical) onto a mass of solid particles.
• Diffusion through a permeable or semipermeable membrane.
• Chemical conversion, generally on a catalyst, often as a preparatory step to one of
the above three methods.
Practically all commercially available processes operate with the syngas “cold,” that
is, at ambient temperatures or lower (chemical adsorption of trace H
2
S on zinc oxide is
the prominent exception). The loss of efficiency in IGCC applications, which is
associated with gas cooling between gasifier and gas turbine, has generated consid-
erable interest in the possibilities of hot desulfurization processes, and these are
treated separately as a topic in its own right.
8.2.1 Selection Criteria
As part of the selection process it is necessary to consider the following criteria:
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299
•
Gas purity
. The demands on syngas purity can vary extremely with application. A
chemical application such as ammonia, methanol, or SNG can require desulfurization
to 100 ppbv or lower. For an IGCC power application with a limit of 5 ppmv
SO
2
in the flue gas, about 40 ppmv at the outlet of the AGR is satisfactory. But
desulfurization is not the only purity criterion. Ammonia syngas requires 10 ppmv
max CO
2
. Other limitations may arise indirectly; in a hydrogen application for
instance, any sulfur entering the PSA unit becomes concentrated in the tail gas,
where emissions regulations may also create a sulfur slip limitation.
•
Raw gas composition
. The washing solution must be able to cope with impurities
in the raw gas. Most gasification processes leave about 5% of the sulfur as COS,
which in many AGR processes is not absorbed as well as H
2
S, if at all, so where
deep sulfur removal is required, it may be necessary to convert it to H
2
S prior to
the main sulfur removal stage. But the raw gas composition issue is not just a
matter of the previous “gas purity criterion”; the action of minor impurities on the
solution itself also have to be considered: for example, HCN in the raw gas reacts,
particularly with primary amines causing solution degradation.
•
Selectivity
. The selectivity of a gas separation process is the ability, for example,
to remove H
2
S while leaving CO
2
in the synthesis gas. There can be a number of
motivations leading to a desire for high selectivity.
䊊
The capital and operating expense of most gas-washing systems correlates
well with the amount of solvent in circulation. A nonselective wash that also
washes out large quantities of CO
2
, may (in the case of a chemical wash will)
inflate the solution rate and therefore costs considerably. A good selectivity
can therefore make a contribution to good economics.
䊊
The acid gas removed from the syngas is usually processed in a Claus sulfur
recovery unit (SRU). It is important for the Claus process that the H
2
S stream
to the SRU not be too dilute. For a typical air-blown SRU, the lower limit for
practical operation is about 30 mol%. In the context of gasification, it is
usually possible to operate the SRU in an oxygen-blown mode, since the SRU
oxygen requirement is generally small compared to that necessary for the
gasifier. An oxygen-blown SRU can operate with as little as 10 mol% H
2
S in
the sour gas. Either way, excess CO
2
in the sour gas will inflate the cost of the
SRU. When looking at the selectivity of an AGR system, it is therefore neces-
sary to consider the effect on the SRU.
䊊
For an IGCC application CO
2
in the synthesis gas contributes to the total mass
flow through the gas turbine and so to the power output. It is therefore in prin-
ciple desirable to leave any CO
2
in the synthesis gas rather than washing it out.
For many gasification processes with a low content of CO
2
(<5%) in the syngas
this is not usually decisive, but with a higher CO
2
content it can be important.
• Other issues that need to be reviewed as part of the selection process are corrosion
(often the practical limit on higher solvent loading), co-absorption of the useful
gas components (in syngas application hydrogen and carbon monoxide, but in
general this would include hydrocarbons), solvent losses through degradation or vapor