Note: Descriptions are shown in the official language in which they were submitted.
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COAL GASIFICATION
BACKGROUND OF THE INVENTION
[0001] This invention relates to the gasification of coal.
[0002] Gasification is a process which is carried out by contacting a
carbonaceous fuel
material and steam under suitable conditions of temperature and pressure in
order to form
syngas, which is a mixture of carbon monoxide, hydrogen and carbon dioxide.
Various types of
gasification processes which have been proposed include, at least, a counter-
current fixed bed
gasification process, a co-current fixed bed gasification process, a fluidized
bed gasification
process, an entrained flow gasification process, an in-situ underground
gasification process,
and a plasma gasification process. These processes are dependent, at least, on
the manner in
which the steam and the carbonaceous material are fed to a gasifier and on the
characteristics
of a burden in the gasifier.
[0003] In a plasma gasifier, constituted with a number of plasma torches, a
high voltage
current is fed to each plasma torch thereby creating a high temperature plasma
gas stream. A
plasma torch can be operated with a number of gases including nitrogen, argon,
steam and
carbon monoxide. The size of a plasma torch is restricted by its power output
and its operating
lifetime.
[0004] When a plasma torch reactor is used for the gasification of coal,
process energy is
supplied by the plasma torches only and no coal combustion is carried out.
Thus the calorific
value of the coal is irrelevant. However the volume and quality of the syngas
as well as the
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economic viability of the process depend on the carbon content of the coal and
on the process
efficiency. The latter parameter is determined by the rate of gasification.
[0005] In a conventional gasification process, a portion of the coal is
combusted with oxygen
to form carbon dioxide and small amounts of carbon monoxide, in order to
produce the
required process energy. In a plasma process the oxygen is replaced by a
plasma gas, such
as nitrogen or argon, which is produced in a gas separation plant which is
similar to an oxygen
plant. Raw syngas diluted with the plasma gas is generated. To the applicant's
knowledge,
commercial plasma torch reactors have been implemented only for the
gasification of
municipal solid wastes.
[0006] An object of the present invention is to implement coal gasification
using plasma arc
techniques derived from a metallurgical direct-current (DC) plasma arc
furnace.
SUMMARY OF THE INVENTION
[0007] The invention provides in the first instance a method of producing
syngas wherein a
carbonaceous feedstock is exposed to a plasma arc generated by a DC supply in
a dry-steam
environment.
[0008] Preferably the carbonaceous feedstock is coal and, for example, is a
low grade or
discard coal.
[0009] The plasma arc may be generated by the use of equipment similar to that
employed in
a DC open arc furnace. The equipment may include multiple graphite electrodes
of solid or
hollow design.
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[0010] The plasma arc may be produced in a reactor and the carbonaceous
feedstock may be
introduced via a plasma arc plume into a lower region of the reactor. In a
variation the
feedstock is introduced into an upper region of the reactor in an entrained
flow section of the
reactor. The carbonaceous feedstock may be fed together with a dry steam feed
into the
.. reactor or it may be fed separately, from a dry steam feed, into the
reactor. If hollow
electrodes are used the feedstock may be introduced into the reactor through
these
electrodes.
[0011] The plasma arc may be an open arc, or the electrodes may be immersed in
slag in a
bottom region of the reactor i.e. immersed-arc heating may be implemented.
Open-arc heating
and immersed-arc plasma heating may be combined.
[0012] The invention further extends to an apparatus for producing syngas from
a
carbonaceous feedstock material which apparatus comprises a DC arc gasifier,
which
processes the feedstock material and which includes a lower vessel within
which a plasma arc
is established and an upper vessel which is operated as an entrained flow
gasifier.
[0013] The plasma arc may be established as an open arc or an immersed arc. In
the latter
instance the immersed arc may be produced in a molten slag layer in a lower
region of the
lower vessel.
[0014] The feedstock material, i.e. the carbonaceous material, may be supplied
to the lower
vessel, or to the entrained flow gasifier which, in turn, feeds into the lower
vessel.
[0015] The apparatus may include one or more electrodes which may be hollow
and the
feedstock may be introduced into the lower vessel through the electrodes.
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[0016] The lower vessel may include a slag containment structure with a slag
tapping facility
and at least one electrode which establishes a DC plasma arc in the lower
vessel.
[0017] The apparatus may be operated at atmospheric pressure, or at a pressure
which is
higher than atmospheric pressure.
BRIEF DESCRIPTION OF THE DRAWING
[0018] The invention is further described by way of example with reference to
the
accompanying drawing which illustrates from one side and in cross section a DC
arc plasma
gasifier according to one form of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
[0019] In implementing the invention use is made of a structure which is
similar to a multiple
electrode DC open arc furnace. With the adoption of this concept a larger
gasification capacity
is possible. As indicated, plasma-based gasification does not require oxygen
and a gas
separation unit is not required. It is anticipated that the exclusion of these
features will lower
the capital cost of the plant.
[0020] During gasification the process temperature is a principal parameter in
acting on the
kinetics and on carbon conversion. The process temperature depends on the
nature of the
gasification structure and on design factors with an object being to achieve a
maximum and
efficient transfer of energy from the plasma arc by means of radiation and
convection.
[0021] The accompanying drawing illustrates a DC arc gasifier 10 according to
the invention
which includes a lower vessel 12 and an upper vessel 14 which is operated as
an entrained
flow column 16.
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[0022] The lower vessel 12 includes a slag containment structure 20 with a
cover 22. Multiple
electrodes 24, 26 etc. pass through respective ports in the cover. Seals 28
are used to seal
respective interfaces between the cover 22 and each electrode 24, 26, etc.
[0023] The structure 20 has slag tap holes 30 at strategic locations.
5 .. [0024] A slurry 36 of pulverised coal 38 and dry-steam 40 is introduced
into the entrained flow
column 16 at an appropriate position on a side of the column. As is described
hereinafter, the
slurry 36 could also be introduced via an arc plume (not shown) below the
cover 22, into the
containment structure 20. The feeding position should be appropriately
implemented to
optimise efficiency of gasification. Both feeding arrangements can be adopted.
[0025] At a lower end 44 the entrained flow column 16 discharges process
residues (not
shown) through the cover 22 into the containment structure 20.
[0026] The upper vessel 14 is operated as an entrained flow gasifier. The
pulverised coal 38
and the dry steam 40 are contacted inside the vessel 14 in co-current flow.
Gasification
reactions take place within a cloud of fine particles. Low grade coal and
discard coal are
suitable for this type of gasification because of the high operating
temperature, in the vessel
14, which is created by the plasma arc. The high temperature, the prevailing
pressure inside
the vessel 14, the residence time, the steam-to-coal ratio, and the steam
characteristics, are
parameters which are selected to achieve a higher throughput of feedstock. Due
to the
aforementioned conditions tar and methane (volatile) are not present in the
product gas.
[0027] Additional heating may be performed in the entrained flow gasifier 16
by partially
combusting a gas stream which is generated in the lower vessel 12 with oxygen.
This helps to
reduce electricity consumption in the entrained flow gasifier 16.
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[0028] The coal/steam slurry 36 is introduced into an upper region inside the
structure 20. It is
possible, though, to introduce the slurry 36 into a lower region inside the
structure 20, or into
the upper region and into the lower region. Nozzles, not shown, are optionally
used at the
lower, discharge end 44 of the entrained flow column 16 to direct the
coal/steam slurry 36 into
the interior of the structure 20. Alternatively or additionally, as is
notionally indicated by dotted
lines 50, the coal/steam slurry 36 is fed to the interior of the structure 20
through the electrodes
24, 26 etc. For this purpose each electrode is formed with an elongate tubular
bore 52 which
is lined with a layer of protective material, e.g. a ceramic sleeve, which is
inert to steam and
which is resistant to thermal shock. In use of the apparatus the slurry 36 is
thus fed, in an open
arc plume, into the interior of the structure 20.
[0029] The electrodes 24,26 are shown in an open arc configuration i.e. with
lower ends
displaced from a slag melt 56 which accumulates in a lower region inside the
structure 20. A
high temperature reducing condition is established by the open arcs in the
vessel 12 above the
slag melt 56. It is possible, though, to immerse lower ends of the electrodes
24, 26 into the
molten slag 56 so that the coal gasifier operates as a slag-based, immersed-
arc, reactor. A
hybrid concept is possible, i.e. a gasifier in which use is made of immersed-
arc, and open-arc
plasma, heating.
[0030] It is important to be able to control various process parameters during
the operation of
the gasifier 10 in order to achieve effective operation.
These parameters include the
magnitudes of the voltages and currents supplied to the electrodes, and the
power
consumption and the stability of the arcs inside the vessel 12 in the presence
of steam or
syngas, or in the presence of steam and syngas in various proportions.
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[0031] During the operation of the gasifier 10 the temperature inside the
upper vessel 14
should be maintained above 600 C to achieve efficient coal gasification.
Higher temperatures
are preferable since they increase the rate of gasification. The energy within
the upper vessel
14 comprises the enthalpy of the gas product flowing through it.
[0032] The operation of the metallurgically-based gasifier 10, in the lower
vessel 12, is similar
to that of an immersed arc smelter. Most of the arc energy is transferred to
the molten slag
bath 56 by convection. The pulverised coal 38 and dry steam 40 are injected
into the arc
plume or plumes above the bath 56. It is desirable to make use of dry-steam to
limit the effects
of electrode corrosion and to help to stabilise the arc. In a molten bath coal
gasification
process, heat transfer to a reaction zone and the extent of the contact
between the coal and
the steam, might limit the kinetics and reactor throughput. In this instance
the upper vessel 14,
which is operated as an entrained flow gasifier 16, provides additional
throughput. Thus the
lower vessel 12 is used to produce syngas which is combusted to provide energy
for the
gasification of coal in the entrained flow gasifier 16.
[0033] The gasification process takes place in a steam plasma. The behaviour
of water
molecules and, particularly of hydrogen molecules, is relevant as such
behaviour can dominate
the plasma composition. Steam plasma requires more energy to dissociate than
either air, or a
carbon monoxide plasma. The net effect is that a steam plasma has a higher
electrical
resistivity than air, or a carbon monoxide plasma. The introduction of steam
into a reaction
zone thus causes a meaningful change in resistivity. This must be done,
though, in a manner
which does not adversely affect arc stability. A longer arc is established,
due to the higher arc
resistivity brought about by the dry-steam environment, and the electrodes
thus require a
higher voltage.
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[0034] The process product i.e. the syngas is drawn from an upper region of
the lower vessel
12 via a port 60.
[0035] The present invention has several advantages over existing incinerator
or plasma torch
gasification reactor designs. A first benefit is that a separate plant to
produce pure oxygen as a
feedstock for the reactor is not required, thereby saving capital and
operating costs. A second
benefit lies in the fact that the plasma arc operates at an extremely high
temperature, at least
of the order of 10000 C, resulting in syngas of an improved quality. Thirdly,
a metallurgical DC
furnace uses proven and efficient technology at a much higher operating power,
up to 80MW,
than can be achieved with a plasma torch reactor. This means that a DC
gasifier based on
techniques associated with a metallurgical DC furnace, are expected to produce
economy-of-
scale benefits.
