U.S. patent application number 12/298501 was filed with the patent office on 2009-12-17 for direct smelting plant with waste heat recovery unit.
Invention is credited to Rodney James Dry.
Application Number | 20090308205 12/298501 |
Document ID | / |
Family ID | 38624471 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090308205 |
Kind Code |
A1 |
Dry; Rodney James |
December 17, 2009 |
DIRECT SMELTING PLANT WITH WASTE HEAT RECOVERY UNIT
Abstract
A direct smelting plant for producing molten metal from
metalliferous feed material in a direct smelting process is
disclosed. The plant includes a process controller for adjusting
the volumetric flow rate of fuel gas supplied to a burner unit of
at least one of the unit operations of the plant so as to at least
meet selected requirements of the plant to operate the direct
smelting process.
Inventors: |
Dry; Rodney James; (West
Australia, AU) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
38624471 |
Appl. No.: |
12/298501 |
Filed: |
April 24, 2007 |
PCT Filed: |
April 24, 2007 |
PCT NO: |
PCT/AU2007/000532 |
371 Date: |
February 18, 2009 |
Current U.S.
Class: |
75/707 ; 266/144;
266/78 |
Current CPC
Class: |
C21B 2100/42 20170501;
F27B 3/26 20130101; F27D 19/00 20130101; C21B 13/0073 20130101;
C21C 2100/02 20130101; Y02P 10/134 20151101; F27D 17/004 20130101;
Y02P 10/136 20151101; C21C 2100/06 20130101; Y02P 10/25 20151101;
F27D 13/00 20130101; C21B 13/0013 20130101; C21B 2100/66 20170501;
C21B 13/143 20130101; F27B 3/225 20130101; C21B 2100/62 20170501;
C21C 5/56 20130101 |
Class at
Publication: |
75/707 ; 266/144;
266/78 |
International
Class: |
C22B 5/00 20060101
C22B005/00; C21C 5/38 20060101 C21C005/38; C21D 11/00 20060101
C21D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 24, 2006 |
AU |
2006902129 |
Claims
1-35. (canceled)
36. A direct smelting plant for producing molten metal from
metalliferous feed material in a direct smelting process that
includes: (a) a direct smelting vessel for producing molten metal,
molten slag, and an off-gas by way of a process for direct smelting
metalliferous feed material in the vessel; (b) a first fuel gas
supply apparatus for supplying off-gas from the direct smelting
vessel for use as a fuel gas in burner units of two or more unit
operations of the plant and/or external to the plant; (c) a second
fuel gas supply apparatus for supplying another fuel gas, such as
natural gas, from another source to the burner unit of at least one
of the unit operations, and (d) a process controller for adjusting
the volumetric flow rate of fuel gas supplied to a burner unit of
at least one of the unit operations so as to at least meet selected
requirements of the plant to operate the direct smelting
process.
37. The plant defined in claim 36 wherein the unit operations
include one or both of (i) a waste heat recovery unit for producing
steam for use in the plant and/or for generating electricity for
use in the plant or externally of the plant and (ii) a plurality of
stoves for producing a hot blast of air or oxygen-enriched air for
use in the direct smelting process.
38. The plant defined in claim 37 wherein the second fuel gas
supply apparatus is adapted to supply fuel gas, such as natural
gas, to one or both of the waste heat recovery unit and the
stoves.
39. The plant defined in claim 37 or claim 38 wherein the process
controller is adapted to adjust the volumetric flow rate of fuel
gas supplied to the burner unit of the waste heat recovery
unit.
40. The plant defined in claim 39 wherein the process controller is
adapted to adjust the volumetric flow rate of fuel gas, such as
natural gas, supplied to the burner unit of the waste heat recovery
unit via the second fuel gas supply apparatus.
41. The plant defined in claim 37 wherein the process controller is
adapted to adjust the volumetric flow rate of fuel gas supplied to
the burner unit of the stoves.
42. The plant defined in claim 41 wherein the process controller is
adapted to adjust the volumetric flow rate of fuel gas, such as
natural gas, supplied to the burner unit of the stoves via the
second fuel gas supply apparatus.
43. The plant defined in claim 37 wherein the process controller is
responsive to the flow rate and/or the calorific value of off-gas
and to the steam requirements of the plant and/or to the hot air or
oxygen-enriched air requirements of the process and determines the
volumetric flow rate of fuel gas, such as natural gas, required for
the burner units of the waste heat recovery unit and/or the stoves
via the second fuel gas supply apparatus having regard to the flow
rate and/or calorific value of off-gas and the requirements of the
plant and/or the process at any point in time.
44. The plant defined in claim 43 wherein the process controller is
responsive to the flame temperature of the burner unit of the waste
heat recovery unit.
45. The plant defined in claim 44 wherein the process controller is
responsive to the flame temperature of the burner unit of the waste
heat recovery unit so as to maintain the flame temperature above a
minimum temperature.
46. The plant defined in claim 43 wherein the process controller is
responsive to the steam requirements of the plant by reference to
required values of steam flow rate or steam pressure during
different operating states of the process, as defined herein.
47. The plant defined in claim 43 wherein the process controller is
adapted to adjust the volumetric flow rate of fuel gas supplied to
the burner unit of the stoves via the second fuel gas supply
apparatus so that the combined fuel gas supplied to the stove
burner unit is a predetermined flow rate and/or calorific
value.
48. The plant defined in claim 43 wherein the process controller is
adapted to adjust the flow rate of fuel gas supplied to the burner
unit of the stoves via the second fuel gas supply apparatus so that
the stove burner unit operates at a constant calorific value at
least at the commencement of a heating phase of the stoves.
49. The plant defined in claim 43 includes an apparatus for
monitoring the calorific value of off-gas at different locations of
the plant.
50. The plant defined in claim 49 wherein the off-gas calorific
value monitoring apparatus is a mass spectrometer.
51. The plant defined in claim 49 or claim 50 wherein the process
controller is responsive to monitored values of calorific
values.
52. The plant defined in claim 36 includes a unit operation in the
form of a pretreatment unit for pretreating metalliferous feed
material.
53. The plant defined in claim 52 includes an apparatus for
supplying off-gas from the direct smelting vessel for use as a
fluidising gas in the pretreatment unit.
54. The plant defined in claim 53 includes an apparatus for
splitting off-gas discharged from the direct smelting vessel into
(i) a first stream for the stoves and the waste heat recovery unit
and (ii) a second stream for the pretreatment unit.
55. The plant defined in claim 54 includes an apparatus for forming
a combined off-gas stream from (i) off-gas in the first stream and
(ii) off-gas in the second stream that is discharged from the
pretreatment unit.
56. The plant defined in claim 55 wherein the off-gas calorific
value monitoring apparatus is adapted to monitor the calorific
value of off-gas in the combined off-gas stream.
57. The plant defined in claim 56 wherein the process controller is
responsive to the monitored calorific value of off-gas in the
combined off-gas stream.
58. The plant defined in claim 57 wherein the process controller is
adapted to adjust the volumetric flow rate of fuel gas supplied to
the waste heat recovery unit via the second fuel gas supply
apparatus in response to the monitored values of calorific value of
the combined off-gas stream.
59. The plant defined in claim 56 wherein the off-gas calorific
monitoring means is adapted to monitor the calorific value of
off-gas in the first and the second streams.
60. A molten bath-based direct smelting process for producing
molten metal from a metalliferous feed material in a direct
smelting plant that includes: (a) direct smelting metalliferous
feed material in a direct smelting vessel containing a molten bath
of metal and slag and producing molten metal, molten slag, and an
off-gas, the process having different process states; (b) supplying
off-gas produced in the vessel during a smelting campaign as a fuel
gas to burner units of two or more unit operations of the plant
and/or external to the plant, and (c) supplying another fuel gas,
such as natural gas, from another source to the burner unit of at
least one of the unit operations, (d) adjusting the volumetric flow
rate of fuel gas supplied to the burner units of the unit
operations so as to at least meet requirements of the plant during
the course of the process.
61. The process defined in claim 60 wherein the unit operations
include one or both of (i) a heat recovery furnace of a waste heat
recovery unit for producing steam for use in the plant and/or for
generating electricity for use in the plant or externally of the
plant and (ii) a plurality of stoves for producing a hot blast of
air or oxygen-enriched air for use in direct smelting metalliferous
feed material in the vessel.
62. The process defined in claim 61 includes adjusting the
volumetric flow rate of the other fuel gas supplied to the burner
units of the waste heat recovery unit and/or the stoves to maintain
a predetermined flow rate and/or calorific value to the burner
units.
63. The process defined in claim 62 includes monitoring the
calorific value of off-gas at different locations of the plant.
64. A direct smelting plant for producing molten metal from
metalliferous feed material in a direct smelting process that
includes: (a) a direct smelting vessel for producing molten metal,
molten slag, and an off-gas by way of a process for direct smelting
metalliferous feed material in the vessel; (b) at least two off-gas
processing units for receiving and combusting the off-gas; (c) a
first fuel gas supply apparatus for supplying off-gas from the
direct smelting vessel for use in burner units of the off-gas
processing units; (d) a second fuel gas supply apparatus for
supplying another fuel gas, such as natural gas, from another
source to burner units of said at least two off-gas processing
units; (e) process controller for controlling: i) volumetric flow
of off-gas to one of the off-gas processing units to meet the
requirenments of the unit with a balance of off-gas being supplied
to a remaining off-gas processing unit or units; ii) volumetric
flow of the other fuel gas to the off-gas processing units.
65. The plant defined in claim 64 wherein the off-gas processing
units comprise stoves for supplying hot blast to the direct
smelting vessel and a waste heat recovery unit for generation of
steam.
66. The plant defined in claim 65 wherein the process controller is
adapted to control supply of off-gas and the other fuel gas to the
stoves such that the combined supply of off-gas and the other fuel
gas to the stoves has a substantially constant calorific value.
67. The plant defined in claim 65 or claim 66 wherein the process
controller is adapted to control supply of the other fuel gas to
the waste heat recovery unit in response to variations in the
volumetric flow of the off-gas to effect combustion of the off-gas
in the waste heat recovery unit.
68. The plant defined in claim 65 comprises one or more than one
off-gas supply valve for controlling the volumetric flow rate of
off-gas to the stoves and for diverting supply of off-gas to the
waste heat recovery unit, and off-gas calorific value sensing
apparatus for sensing calorific value of off-gas, and wherein the
process controller is adapted to monitor off-gas calorific value
and to operate the off-gas supply valve or valves to divert off-gas
to the waste heat recovery unit in response to the calorific value
of the off-gas falling below a pre-determined threshold value.
69. The plant defined in claim 68 wherein the pre-determined
threshold value is a value at which the off-gas no longer makes a
positive contribution to the calorific value of the combined fuel
gas steam of the off-gas and the other fuel gas.
70. The plant defined in claim 68 wherein the pre-determined
threshold value is 1.8 MJ/Nm3 (mega-joules per normal cubic meter).
Description
[0001] The present invention relates to a molten bath-based direct
smelting plant and process for producing molten metal in a direct
smelting vessel.
[0002] In particular, the present invention relates to recovering
energy from off-gas released from a direct smelting vessel.
[0003] The present invention relates particularly, although by no
means exclusively, to molten bath-based direct smelting processes
for producing molten iron from iron-containing metalliferous feed
material, such as iron ores, partly reduced iron ores and
iron-containing waste streams (for example, from steelmaking
plants).
[0004] A known molten bath-based direct smelting process is
generally referred to as the HIsmelt process. In the context of
producing molten iron, the HIsmelt process includes the steps of:
[0005] (a) forming a bath of molten iron and slag in a direct
smelting vessel; [0006] (b) injecting into the bath: (i) a
metalliferous feed material, typically iron ore in the form of
fines; and (ii) a solid carbonaceous material, typically coal,
which acts as a reductant of the metalliferous feed material and a
source of energy; and [0007] (c) smelting metalliferous feed
material to iron in the bath.
[0008] The term "smelting" is herein understood to mean thermal
processing wherein chemical reactions that reduce metal oxides take
place to produce molten metal.
[0009] In the HIsmelt process metalliferous feed material and solid
carbonaceous material are injected into the molten bath through a
number of lances/tuyeres which are inclined to the vertical so as
to extend downwardly and inwardly through the side wall of the
direct smelting vessel and into a lower region of the vessel so as
to deliver at least part of the solids material into the metal
layer in the bottom of the vessel. A blast of hot oxygen-containing
gas, typically air or oxygen-enriched air, is injected into an
upper region of the vessel through a downwardly extending lance to
cause post-combustion of reaction gases released from the molten
bath in the upper region of the vessel. Typically, in the case of
producing molten iron, the hot air or oxygen-enriched air is at a
temperature of the order of 1200.degree. C. and is generated in hot
blast stoves. Off-gases resulting from the post-combustion of
reaction gases in the vessel are taken away from the upper region
of the vessel through an off-gas duct. The vessel includes
refractory-lined water cooled panels in the side wall and the roof
of the vessel, and water is circulated continuously through the
panels in a continuous circuit.
[0010] The HIsmelt process enables large quantities of molten iron,
typically at least 0.5 Mt/a, to be produced by direct smelting in a
single compact vessel.
[0011] However, in order to achieve high molten iron production
rates in the HIsmelt process it is necessary to (a) generate and
transport large quantities of hot air or oxygen-enriched air and
carrier gas (for solids injection) to the direct smelting vessel,
(b) transport large quantities of the metalliferous feed material,
such as iron-containing feed materials, to the vessel, including
generating and transporting large quantities of carrier gas to the
vessel (c) transport large quantities of hot off-gas from the
vessel, (d) transport large quantities of molten iron and slag
produced in the process away from the vessel, and (e) circulate
large quantities of water through the water cooled panels--all
within a relatively confined area.
[0012] In view of the above, high molten iron production rates
require a HIsmelt plant that includes (a) a pressurised direct
smelting vessel and ancillary equipment such as lock hoppers for
supplying solid feed materials to the vessel and pressure control
equipment on the off-gas duct of the vessel, (b) stoves that
produce the high flowrate of hot air or oxygen-enriched air for the
vessel, and (c) off-gas treatment equipment that is capable of
processing large quantities of off-gas discharged from the
vessel.
[0013] It has been proposed to use at least part of the off-gas
from a direct smelting vessel during a smelting campaign as a fuel
gas in off-gas treatment equipment that includes a waste heat
recovery unit and, more particularly, as a fuel gas in a burner
unit of a waste heat recovery furnace of the waste heat recovery
unit that produces steam for (a) generating electricity and (b)
operating the HIsmelt process.
[0014] The term "smelting campaign" is understood herein to mean
operation of a molten bath-based direct smelting process, such as
the HIsmelt process, without a total shutdown of the process
involving end tapping of molten metal and slag from a direct
smelting vessel.
[0015] It has also been proposed to use at least part of the
off-gas from a direct smelting vessel during a smelting campaign as
a fuel gas in burner units of stoves that produce air or
oxygen-enriched air for operating the HIsmelt process.
[0016] The possible uses of off-gas from a direct smelting vessel
during a smelting campaign are not confined to the above unit
operations of waste heat recovery unit and stoves, and the off-gas
can be used as a fuel gas in burner units of other unit operations
that are part of the plant and/or are external to the plant.
[0017] One HIsmelt process flowsheet currently proposed is designed
to operate in a number of "states" that have different operating
conditions during a smelting campaign, including by way of example
the following process states: [0018] (a) start-up; [0019] (b) hot
metal production, i.e. supplying pretreated metalliferous feed
material such as hot ore, solid carbonaceous material such as coal,
and hot blast air; [0020] (c) hold--i.e. no pretreated
metalliferous feed material, supplying solid carbonaceous material
and hot blast air; [0021] (d) idle--i.e. no pretreated
metalliferous feed material and no solid carbonaceous material,
supplying hot blast air; and [0022] (e) off-wind--i.e. no
pretreated metalliferous feed material, no solid carbonaceous
material, and no hot blast air.
[0023] Typically, the volumetric flow rates of off-gas produced in
the direct smelting vessel in the above process states are
different. For example, typically, the flow rate of off-gas is
relatively high during a hot metal production state and relatively
low during an idle state. By way of further example, typically,
there is no off-gas during an off-wind state and typically there is
no calorific value in off-gas during an idle state.
[0024] The volumetric flow rates and calorific values of off-gas
produced in the direct smelting vessel during the course of a given
process state may also be different as a consequence of variations
in operating conditions during the process state. For example,
there may be variations in operating conditions during the metal
production state that would result in different flow rates and
calorific values of off-gas being produced.
[0025] Consequently, the applicant has realised that during at
least some states of the HIsmelt process there are needs for
natural gas (or other fuel gas other than off-gas) to be supplied
to burner units (or other types of combustion units) of different
unit operations that form part of a particular HIsmelt plant and/or
that may be external to the plant in order to meet the operating
requirements of the plant or the external operations.
[0026] For example, in the context of the HIsmelt plant described
above, there is a need for natural gas (or other fuel gas other
than off-gas) to be supplied to a burner unit of a waste heat
recovery unit to meet the steam requirements of the plant during a
smelting campaign.
[0027] In addition, in the context of the HIsmelt plant described
above, the applicant has realised that there is a need to supply
natural gas (or other fuel gas other than off-gas) to burner units
of stoves to compensate for varying flow rates and calorific values
of off-gas from the direct smelting vessel.
[0028] In addition, in the context of the HIsmelt plant described
above, the applicant has realised that there is a need for varying
flow rates of natural gas (or other fuel gas other than off-gas) to
the burner unit of the waste heat recovery unit and the burner
units of stoves during a given process state to compensate for
varying flow rates and calorific values of off-gas from the direct
smelting vessel during the process state to meet the operating
requirements of the plant.
[0029] In addition, the applicant has realised that there is likely
to be considerably greater rates of change of the calorific value
of off-gas produced in a HIsmelt process than occurs in blast
furnaces and, hence, there is a need to closely monitor the
calorific value of off-gas.
[0030] In broad terms the present invention provides a direct
smelting plant for producing molten metal from metalliferous feed
material in a direct smelting process that includes: [0031] (a) a
direct smelting vessel for producing molten metal, molten slag, and
an off-gas by way of a process for direct smelting metalliferous
feed material in the vessel; [0032] (b) a first fuel gas supply
apparatus for supplying off-gas from the direct smelting vessel for
use as a fuel gas in burner units of two or more unit operations of
the plant and/or external to the plant; [0033] (d) a second fuel
gas supply apparatus for supplying another fuel gas, such as
natural gas, from another source to a burner unit of at least one
of the unit operations, and [0034] (e) a process controller for
adjusting the volumetric flow rate of fuel gas supplied to a burner
unit of at least one of the unit operations so as to at least meet
selected requirements of the plant to operate the direct smelting
process.
[0035] Depending on the circumstances, the term "fuel gas" as used
in paragraph (e) above may refer to off-gas and/or another fuel
gas, such as natural gas.
[0036] One, although not the only possible, unit operation may be a
waste heat recovery unit for producing steam for use in the plant
and/or for generating electricity for use in the plant or
externally of the plant.
[0037] Another, although not the only other possible, unit
operation may be a plurality of stoves for producing a hot blast of
air or oxygen-enriched air for use in the direct smelting
process.
[0038] The second fuel gas supply apparatus may be adapted to
supply fuel gas, such as natural gas, to one or both of the waste
heat recovery unit and the stoves.
[0039] The process controller may be adapted to adjust the
volumetric flow rate of fuel gas supplied to the burner unit of the
waste heat recovery unit.
[0040] In particular, the process controller may be adapted to
adjust the volumetric flow rate of fuel gas, such as natural gas,
supplied to the burner unit of the waste heat recovery unit via the
second fuel gas supply apparatus.
[0041] The process controller may be adapted to adjust the
volumetric flow rate of fuel gas supplied to the burner unit of the
stoves.
[0042] In particular, the process controller may be adapted to
adjust the volumetric flow rate of fuel gas, such as natural gas,
supplied to the burner unit of the stoves via the second fuel gas
supply apparatus.
[0043] As described above, the HIsmelt process currently proposed
by the applicant is designed to operate in different "states" and
there will be different volumetric flow rates and calorific values
of off-gas produced in the direct smelting vessel in the states and
during the course of a given state. In addition, the requirements
of the plant for steam (via the waste heat recovery unit) and/or
hot air or oxygen-enriched air (via the stoves) to operate the
HIsmelt process may be different in different states and may vary
during the course of a given state. Consequently, depending on the
available flow rate and calorific value of off-gas at any given
point in time, it may be necessary to supply natural gas (and/or
other fuel gas) to the burner units of (a) the waste heat recovery
unit of the HIsmelt plant to generate sufficient steam for the
process and/or (b) the stoves of the HIsmelt plant to generate
sufficient hot air or oxygen-enriched air for the process.
[0044] Preferably the process controller is responsive to the flow
rate and/or the calorific value of off-gas and to the steam
requirements of the plant and/or to the hot air or oxygen-enriched
air requirements of the process and determines the volumetric flow
rate of fuel gas, such as natural gas, required for the burner
units of the waste heat recovery unit and/or the stoves via the
second fuel gas supply apparatus having regard to the flow rate
and/or calorific value of off-gas and the requirements of the plant
and/or the process at any point in time.
[0045] Preferably the process controller is responsive to the flame
temperature of the burner unit of the waste heat recovery unit.
[0046] More preferably the process controller is responsive to the
flame temperature of the burner unit of the waste heat recovery
unit so as to maintain the flame temperature above a minimum
temperature.
[0047] Preferably the process controller is responsive to the steam
requirements of the plant by reference to required values of steam
flow rate or steam pressure during different operating states of
the process, as described above.
[0048] Preferably the process controller is adapted to adjust the
volumetric flow rate of fuel gas supplied to the burner unit of the
stoves via the second fuel gas supply apparatus so that the
combined fuel gas supplied to the stove burner unit is a
predetermined flow rate and/or calorific value.
[0049] Preferably the process controller is adapted to adjust the
flow rate of fuel gas to the burner unit of the stoves via the
second fuel gas supply apparatus so that the stove burner unit
operates at a constant calorific value at least at the commencement
of a heating phase of the stoves.
[0050] Preferably the plant includes an apparatus for monitoring
the calorific value of off-gas at different locations of the
plant.
[0051] The off-gas calorific value monitoring apparatus may be any
suitable apparatus such as a mass spectrometer.
[0052] Preferably the process controller is responsive to monitored
values of calorific values.
[0053] Preferably the plant includes a unit operation in the form
of a pretreatment unit for pretreating metalliferous feed
material.
[0054] Preferably the plant includes an apparatus for supplying
off-gas from the direct smelting vessel for use as a fluidising gas
in the pretreatment unit.
[0055] Preferably the plant includes an apparatus for splitting
off-gas discharged from the direct smelting vessel into (i) a first
stream for the stoves and the waste heat recovery unit and (ii) a
second stream for the pretreatment unit.
[0056] Preferably the plant includes an apparatus for forming a
combined off-gas stream from (i) off-gas in the first stream and
(ii) off-gas in the second stream that is discharged from the
pretreatment unit.
[0057] Preferably the off-gas calorific value monitoring apparatus
is adapted to monitor the calorific value of off-gas in the
combined off-gas stream.
[0058] Preferably the process controller is responsive to the
monitored calorific value of off-gas in the combined off-gas
stream.
[0059] Preferably the process controller is adapted to adjust the
volumetric flow rate of fuel gas supplied to the waste heat
recovery unit via the second fuel gas supply apparatus in response
to the monitored values of calorific value of the combined off-gas
stream.
[0060] Preferably the off-gas calorific monitoring apparatus is
adapted to monitor the calorific value of off-gas in the first and
the second streams.
[0061] In broad terms, according to the present invention there is
also provided a molten bath-based direct smelting process for
producing molten metal from a metalliferous feed material in a
direct smelting plant that includes: [0062] (a) direct smelting
metalliferous feed material in a direct smelting vessel containing
a molten bath of metal and slag and producing molten metal, molten
slag, and an off-gas, the process having different process states;
[0063] (b) supplying off-gas produced in the vessel during a
smelting campaign as a fuel gas to burner units of two or more unit
operations of the plant and/or external to the plant, and [0064]
(c) supplying another fuel gas, such as natural gas, from another
source to the burner unit of at least one of the unit operations,
[0065] (d) adjusting the volumetric flow rate of fuel gas supplied
to the burner units of the unit operations so as to at least meet
requirements of the plant during the course of the process.
[0066] Preferably the unit operations include a heat recovery
furnace of a waste heat recovery unit for producing steam for use
in the plant and/or for generating electricity for use in the plant
or externally of the plant.
[0067] Preferably the unit operations include a plurality of stoves
for producing a hot blast of air or oxygen-enriched air for use in
direct smelting metalliferous feed material in the vessel.
[0068] Preferably the process also includes adjusting the
volumetric flow rate of the other fuel gas supplied to the burner
units of the waste heat recovery unit and/or the stoves to maintain
a predetermined flow rate and/or calorific value to the burner
units.
[0069] Preferably the process includes monitoring the calorific
value of off-gas at different locations of the plant.
[0070] According to the present invention there is also provided a
direct smelting plant for producing molten metal from metalliferous
feed material in a direct smelting process that includes: [0071]
(a) a direct smelting vessel for producing molten metal, molten
slag, and an off-gas by way of a process for direct smelting
metalliferous feed material in the vessel; [0072] (b) at least two
off-gas processing units for receiving and combusting the off-gas;
[0073] (c) a first fuel gas supply apparatus for supplying off-gas
from the direct smelting vessel for use in burner units of the
off-gas processing units; [0074] (d) a second fuel gas supply
apparatus for supplying another fuel gas, such as natural gas, from
another source to burner units of said at least two off-gas
processing units; [0075] (e) process controller for controlling:
[0076] i) volumetric flow of off-gas to one of the off-gas
processing units to meet the requirenments of the unit with a
balance of off-gas being supplied to a remaining off-gas processing
unit or units; [0077] ii) volumetric flow of the other fuel gas to
the off-gas processing units.
[0078] Preferably the off-gas processing units comprise stoves for
supplying hot blast to the direct smelting vessel and a waste heat
recovery unit for generation of steam.
[0079] Preferably the process controller is adapted to control
supply of off-gas and the other fuel gas to the stoves such that
the combined supply of off-gas and the other fuel gas to the stoves
has a substantially constant calorific value.
[0080] Preferably the process controller is adapted to control
supply of the other fuel gas to the waste heat recovery unit in
response to variations in the volumetric flow of the off-gas to
effect combustion of the off-gas in the waste heat recovery
unit.
[0081] Preferably said plant comprises one or more than one off-gas
supply valve for controlling the volumetric flow rate of off-gas to
the stoves and for diverting supply of off-gas to the waste heat
recovery unit; off-gas calorific value sensing apparatus for
sensing calorific value of off-gas; and the process controller is
adapted to monitor off-gas calorific value and to operate the
off-gas supply valve or valves to divert off-gas to the waste heat
recovery unit in response to the calorific value of the off-gas
falling below a pre-determined threshold value.
[0082] Preferably the pre-determined threshold value is a value at
which the off-gas no longer makes a positive contribution to the
calorific value of the combined fuel gas steam of the off-gas and
the other fuel gas. Preferably the pre-determined threshold value
is 1.8 MJ/Nm3 (mega-joules per normal cubic meter)
[0083] The present invention is described in more detail
hereinafter with reference to the accompanying drawings, of
which:
[0084] FIG. 1 is a diagrammatic view of one embodiment of a direct
smelting plant in accordance with the present invention; and
[0085] FIG. 2 is an enlarged view of the wet cone scrubber and
off-gas cooler in the off-gas stream that supplies off-gas to the
waste heat recovery unit and the stoves shown in FIG. 1.
[0086] The following description of the plant shown in the figures
is in the context of using the plant to smelt iron-containing feed
material to produce molten iron in accordance with the HIsmelt
process as described in International application PCT/AU96/00197 in
the name of the applicant. The disclosure in the patent
specification lodged with the International application is
incorporated herein by cross-reference.
[0087] The process is based on the use of a direct smelting vessel
3.
[0088] The vessel 3 is of the type described in detail in
International applications PCT/AU2004/000472 and PCT/AU2004/000473
in the name of the applicant. The disclosure in the patent
specifications lodged with these applications is incorporated
herein by cross-reference.
[0089] The vessel 3 has a hearth that incudes a base and sides
formed from refractory bricks, side walls which form a generally
cylindrical barrel extending upwardly from the sides of the hearth
and include an upper barrel section and a lower barrel section, a
roof, an off-gas duct 9 in an upper section of the vessel 3, a
forehearth 67 for discharging molten metal continuously from the
vessel 3, and a tap hole for discharging molten slag periodically
from the vessel 3.
[0090] The vessel 3 is fitted with a downwardly extending
water-cooled hot air blast ("HAB") lance 7 extending into a top
space of the vessel 3 and eight water-cooled solids injection
lances 5 extending downwardly and inwardly through a side wall and
into the slag.
[0091] In use, the vessel 3 contains a molten iron bath.
Iron-containing feed material (such as iron ore fines, iron-bearing
steel plant wastes or DRI fines), coal and fluxes (lime and
dolomite) are directly injected into the bath via the solids
injection lances 5.
[0092] Specifically, one set of lances 5 is used for injecting
iron-containing feed material and fluxes and another set of lances
5 is used for injecting coal and fluxes.
[0093] The lances 5 are water cooled to protect them from the high
temperatures inside the vessel 3. The lances 5 are typically lined
with a high wear resistant material in order to protect them from
abrasion by the gas/solids mixture being injected at high
velocity.
[0094] Iron-containing feed material is pretreated by being
preheated to a temperature in the range of 600-700.degree. C. and
prereduced in a fluidised bed preheater 17 before being injected
into the bath.
[0095] Coal and fluxes are stored in a series of lock hoppers 25
before being injected at ambient temperatures into the bath. The
coal is supplied to the lock hoppers 25 via a coal drying and
milling plant 71.
[0096] The injected coal de-volatilises in the bath, thereby
liberating H.sub.2 and CO. These gases act as reductants and
sources of energy. The carbon in the coal is rapidly dissolved in
the bath. The dissolved carbon and the solid carbon also act as
reductants, producing CO as a product of reduction. The injected
iron-containing feed material is smelted to molten iron in the bath
and is discharged continuously via the forehearth 67. Molten slag
produced in the process is discharged periodically via the slag tap
hole (not shown).
[0097] The typical reduction reactions involved in smelting
injected iron-containing feed material to molten iron that occur in
the bath are endothermic. The energy required to sustain the
process and, more particularly these endothermic reactions, is
provided by reacting CO and H.sub.2 released from the bath with
oxygen-enriched air injected at high temperatures, typically
1200.degree. C., into the vessel 3 via the HAB lance 7.
[0098] Energy released from the above-described post combustion
reactions in the vessel top space is transferred to the molten iron
bath via a "transition zone" in the form of highly turbulent
regions above the bath that contain droplets of slag and iron. The
droplets are heated in the transition zone by the heat generated
from post combustion reactions and return to the slag/iron bath
thereby transferring energy to the bath.
[0099] The hot, oxygen-enriched air injected into the vessel 3 via
the HAB lance 7 is generated in hot blast stoves 11 by passing a
stream of oxygen-enriched air (nominally containing 30 to 35% by
volume O.sub.2) through the stoves 11 and heating the air and
thereafter transferring the hot oxygen-enriched air to the HAB
lance 7 via a hot blast main 41.
[0100] The operation of the stoves 11 is coordinated to ensure that
there is a continuous, uninterrupted flow of hot, oxygen-enriched
air at a constant straight line temperature in the main 41 to the
HAB lance 7.
[0101] Each stove 11 operates in accordance with a repeating
sequence of phases that comprises a heating phase, a bottling
phase, and a heat exchange phase that is a longer time period than
the heating phase.
[0102] The stoves 11 are heated during heating phases of the stoves
11 by combusting (a) a fuel gas in the form of cooled and cleaned
off-gas from the vessel 3 and/or (b) optionally another fuel gas
such as natural gas (supplied via a line indicated by the numeral
85 in FIG. 1), and (c) combustion air in burner assemblies (not
shown) of the stoves 11 and thereafter passing the combustion
products through the stoves 11.
[0103] During heat exchange phases of the stoves 11, oxygen from an
oxygen plant 29 is mixed into streams of pressurised air generated
by a blower 31. These oxygen-enriched air streams are passed
through the stoves 11 and are heated in the stoves 11 and thereby
produce the hot, oxygen-enriched pressurised air streams for the
vessel 3. These hot, oxygen-enriched air streams are often referred
to as "hot blast" or "hot air blast".
[0104] The bottling phases of the stoves 11 are phases in which one
of the stoves is essentially closed and is neither heated by
combusted off-gas (and other fuel gas, such as natural gas) nor
cooled by heat exchange with air streams.
[0105] The duration of the bottling phases of a given stove 11 is
at least the amount of time required to open and close the valves
necessary to change-over off-gas and hot air streams so as to
switch over (a) the given stove from a heating phase to a heat
exchange phase and (b) the other stove from a heat exchange phase
to a heating phase.
[0106] Combustion products released from the stoves 11 during
heating phases of the stoves 11 are cleaned in a flue gas
desulphurisation (FGD) system 13. The FGD removes sulphur, which
typically occurs in the form of hydrogen sulphide (H.sub.2S) and
sulphur dioxide (SO.sub.2), from the combustion products. The
off-gas produced in the vessel 3 contains sulphur and the sulphur
is not totally removed in the off-gas cleaning that occurs
downstream of the vessel 3 before the off-gas reaches the stoves
11, as described hereinafter.
[0107] Prior to being passed to the FGD system, combustion products
released from the stoves 11 during heating phases of the stoves 11
may pass through heat exchangers (not shown) and preheat cooled and
cleaned off-gas from the vessel 3 and combustion air before the
heated off-gas and combustion air is supplied as feed materials to
the burners of the stoves 11 during heating phases. The vessel
off-gas and combustion air may be preheated to a temperature of
around 180.degree. C.
[0108] Off-gas is released from the vessel 3 via the off-gas duct 9
in the upper section of the vessel 3 and passes initially through a
radiation cooler, hereinafter referred to as an "off-gas hood", 15.
Typically, the off-gas leaves the vessel and enters the hood at a
temperature of the order of 1450.degree. C.
[0109] The off-gas is cooled as it passes through the off-gas hood
15 and thereby results in the generation of steam which accumulates
in steam drum 35. The off-gas hood may be of a type described in
U.S. Pat. No. 6,585,929 that cools and partially cleans
off-gas.
[0110] The off-gas stream leaving the off-gas hood 15 is at a
temperature of approximately 1000.degree. C. and is split into two
streams.
[0111] With particular reference to FIG. 2, one split off-gas
stream leaving the off-gas hood 15, which comprises between 55-65%
of the off-gas from the vessel 3, passes first through a wet cone
scrubber 21.
[0112] The scrubber 21 quenches and removes particulate material
and soluble gaseous species and metal vapours from off-gas flowing
through the scrubber. The off-gas temperature drop in the scrubber
is from approximately 1000.degree. C. to below 100.degree. C. and
typically between 65.degree. C. and 90.degree. C.
[0113] The scrubber 21 includes an upper chamber 71, a lower
chamber 73, and a vertically extending pipe 75 that interconnects
the chambers 71, 73. The scrubber 21 includes an off-gas control
valve 77 in the lower end of the pipe 75. The control valve 77
includes an hydraulically operated cone element 79 that can move
vertically to open or close the lower end of the pipe 75. The
scrubber 21 includes water sprays 69 in the upper chamber 71 and
further water sprays (not shown) positioned in relation to the pipe
75 and the control element 79. Re-circulating water within the
scrubber and make-up water are supplied to the sprays.
[0114] The control valve 77 controls the flow rate of off-gas
through the scrubber 21. This is the first variable flow rate
constraint on off-gas from the vessel 3. Consequently, the control
valve 77 controls the pressure in the direct smelting vessel 3,
preferably to 0.8 bar gauge while the process is producing molten
iron.
[0115] The off-gas from the scrubber 21 leaves the scrubber 21 via
an outlet 81 in the lower chamber 73 and passes through an off-gas
cooler 23 that further cools the off-gas to below 50.degree. C.,
typically between 30.degree. C. and 45.degree. C., to remove
sufficient moisture from the off-gas for it to be used as a fuel
gas. Typically the off-gas leaving the cooler has 5% or less
H.sub.2O and a mist content of less than 10 mg/Nm.sup.3 and
typically 5.0 mg/Nm.sup.3.
[0116] Under typical metal production conditions, the resulting
off-gas is suitable for use as a fuel gas in (a) the stoves 11 (as
described above) and (b) the WHR system 25. In addition, the
scrubbed and cooled off-gas is suitable for drying coal in the
drying and milling plant 71.
[0117] For the above purposes, the off-gas from the off-gas cooler
23 is split into three streams and supplied to downstream unit
operations, specifically the stoves 11, the WHR system 25, and the
drying and milling plant 71, via an apparatus that is collectively
described as a "first fuel gas supply apparatus". Specifically, one
stream is passed to the stoves 11, another stream is passed to the
WHR system 25, and a third stream being passed to the drying and
milling plant 71. The flow rate of off-gas in the streams is
controlled via off-gas supply valves (not shown).
[0118] The off-gas stream from off-gas cooler 23 is a relatively
rich off-gas. The stream that is passed to the WHR system 25 is
mixed with cooled and cleaned off-gas that has passed through the
preheater 17 as described hereinafter, which is a relatively lean
off-gas, due to some pre-reduction of the iron-containing feed
material in the pre-heater by CO and H.sub.2 in the off-gas.
[0119] As detailed above, under typical metal production
conditions, the combined off-gas stream has a calorific value that
makes it suitable for combustion as a fuel gas.
[0120] The combined off-gas stream, an additional source of fuel
gas in the form of natural gas (supplied via a line indicated by
the numeral 83 in FIG. 1), and air are supplied to and combusted in
the WHR system 25.
[0121] It is noted that the line 83 mentioned in the preceding
paragraph and the line 85 mentioned earlier in the context of
supplying natural gas to the WHR system 25 are part of a fuel gas
supply apparatus that is collectively described as a "second fuel
gas supply apparatus".
[0122] The combined off-gas stream is combusted within the WHR
system 25 in a manner that maximises CO destruction, while
minimising NO.sub.x formation.
[0123] The off-gas released from the WHR system 25 is combined with
off-gas gas from the stoves 11 and then passes to the FGD system
13. SO.sub.2 is removed in the FGD system 23 and the exhaust gas is
released to the atmosphere via a stack 45.
[0124] The other split stream, which contains approximately 35-45%
by volume of the off-gas stream, is passed through the fluidised
bed preheater 17 for iron-containing feed material. The preheater
17 removes moisture from and preheats and prereduces the
iron-containing feed material. The off-gas is a source of energy
and a fluidising gas in the preheater 17.
[0125] A process controller of the plant controls the off-gas flow
to the preheater 17 (a) to be above a minimum flow rate to maintain
fluidising conditions in the preheater 17 and (b) to preheat
iron-containing feed material to a substantially constant
temperature, in the range of 600-700.degree. C. while the process
is producing molten metal.
[0126] The off-gas released from the preheater 17 is passed through
a cyclone 61 and entrained dust is separated from the off-gas.
[0127] The off-gas then passes through a wet cone scrubber 63 that
removes particulate material and soluble gaseous species and metal
vapours from the off-gas and cools the off-gas from between
500.degree. C. and 200.degree. C. to below 100.degree. C. and
typically between 65.degree. C. and 90.degree. C.
[0128] The scrubber 63 is the same basic construction as the wet
cone scrubber 21 described above. In particular, the scrubber 63
quenches and removes particulate material and soluble gaseous
species and metal vapours from off-gas flowing through the
scrubber. Moreover, as is the case with scrubber 21, the scrubber
63 includes an off-gas flow control valve that has an hydraulically
operated cone element that can move vertically to open or close the
valve and thereby control flow of off-gas through the scrubber.
[0129] The off-gas from the scrubber 63 then passes through an
off-gas cooler 65 that further cools the off-gas to below
50.degree. C., typically between 30.degree. C. and 45.degree. C.,
to remove sufficient moisture from the off-gas for it to be used as
a fuel gas. Typically the off-gas leaving the cooler has 5% or less
H.sub.2O and a mist content of less than 10 mg/Nm.sup.3 and
typically 5.0 mg/Nm.sup.3.
[0130] As is described above, the cooled and cleaned off-gas is
then combined with a stream of off-gas from cooler 23 and used as a
fuel gas in a waste heat recovery (WHR) system 25.
[0131] The WHR system 25 includes: [0132] a thermal oxidiser, ie
burner assembly, 37, and associated combustion chamber; [0133] a
WHR unit, ie boiler, 39; [0134] a steam drum; and [0135] heat
exchange equipment, such as superheat coils and a demineralised
water economiser.
[0136] The WHR system 25 produces saturated steam. The saturated
steam is combined with the saturated steam from the steam drum 35
of the off-gas hood 15 and the superheat coils of the WHR system 25
generates superheated steam from the saturated steam.
[0137] The steam raising equipment of the WHR system 25 comprises:
[0138] a radiant screen to protect the downstream coils; [0139] a
two-stage superheater section with desuperheater controls (where
the quantity of superheat is controlled by injecting demineralised
water as required to maintain the superheated steam at a
temperature of 420.degree. C.); [0140] a main evaporator section,
consisting of three modules of convective coils; [0141] an
economiser section; and [0142] a steam drum with three element
demineralised water control.
[0143] The steam raised in the WHR system 25 and the off-gas hood
15 is used to drive the HAB blower 31 and the main air compressor
(not shown) of oxygen plant 29, with the remainder being passed
through a turbo-alternator that generates electrical power required
to operate the plant.
[0144] The turbo-generator system includes a condensing turbine
designed to receive superheated steam. The discharge from the
turbine passes through a surface condenser operating at vacuum with
the resultant condensate being pumped to the de-aerator via
condensate pumps.
[0145] The use of the off-gas as a fuel gas within a plant offsets
an amount of electrical power that would otherwise need to be
obtained from an external electricity supply grid, which makes the
plant generally self sufficient in terms of electrical power.
[0146] Typically, the burner assembly 37 of the WHR system 25 is a
cylindrical carbon steel shell, with internal refractory and
insulation.
[0147] In use, the burner assembly 37 of the WHR system operates
with varying combined off-gas flow rates from the above-described
split streams of off-gas, due to a number of factors including (a)
variations in off-gas that is produced during operation of the
process and therefore discharged from the vessel 3, (b) variations
in the steam requirements of the plant, (c) variations in off-gas
available for the burner assembly 37 of the WHR system 25 because
of competing calls on off-gas for the stoves 11, and (d) variations
in off-gas requirements for the stoves 11. In other words, the
burner assembly 37 of the WHR system is supplied with and
controlled so as to combust the balance of off-gas after competing
calls from the stoves 11 and other units that utilise off-gas have
been satisfied. As detailed further below, these calls may vary
with the calorific value of the off-gas.
[0148] The process is designed to operate in a number of "states"
that have different operating conditions during a smelting
campaign, including by way of example the following process states:
[0149] (a) start-up; [0150] (b) hot metal production--supply hot
ore, coal, fluxes, and hot blast; [0151] (c) hold--ie no hot ore,
supply coal and hot blast; [0152] (d) idle--ie no ore and no coal,
supply hot blast; and [0153] (e) off-wind--ie no ore, no coal, and
no hot blast and in some instances a fuel gas such as natural
gas.
[0154] During a hold state the calorific value of the off-gas can
vary between being relatively lean and being relatively rich. The
calorific value depends on the feed rate of coal into the bath and
the feed rate of hot air blast into the vessel 3. These parameters
affect the level of carbon in the off-gas and the levels of CO and
CO.sub.2 in the off-gas.
[0155] The calorific value of the off-gas during an idle state is
relatively low. Typically only hot air blast is supplied to the
vessel 3 (along with nitrogen purge gas supplied through the solids
injection lances 5) and so the off-gas has a composition similar to
air.
[0156] During an idle state the hot metal temperature is monitored
and, if necessary, a fuel gas, such as natural gas, is supplied
into the top space above the molten bath. This fuel gas is
combusted in the hot air blast. This assists with heating the
vessel 3 and the molten bath.
[0157] Combustion of fuel gas in this manner is typically complete
and so the calorific value of the off-gas does not increase
compared to the situation of an idle state where only hot air blast
is supplied to the vessel 3.
[0158] Prior to combusting fuel gas in the vessel 3 when the
process is in an idle state, the operators of the vessel may either
tap slag to a minimum level or may even perform a slag drain. A
slag tap leaves a certain minimum level of slag in the vessel 3
whereas a slag drain removes substantially all of the slag from the
vessel. Reducing the level of slag in the vessel 3 allows the metal
to be heated directly by the combustion. Slag acts as an insulator
in these circumstances and reduces the amount of heat seen by the
metal.
[0159] The volumetric flow rates and calorific values of off-gas
produced in the vessel 3 in the above process states are different.
For example, the flow rate and calorific value of off-gas are
relatively high during a hot metal production state and relatively
low during an idle state.
[0160] In addition, the volumetric flow rates and calorific values
of off-gas produced in the vessel 3 during the course of a given
process state are also different as a consequence of variations in
operating conditions. For example, there may be variations in
operating conditions during a hot metal production state that would
result in different amounts and calorific values of off-gas being
produced. For example, the off-gas calorific value can fall below
1.8 MJ/Nm3 (mega-joules per normal cubic meter) during a hot metal
production state, particularly during process pertebations.
[0161] In addition, the volumetric flow rate of fuel gas that is
available for the WHR system 25 varies with the phases of the
stoves 11. Specifically, the split off-gas stream to the WHR system
25 has a substantially higher flow rate when the stoves 11 are
operating in the bottling phases of the stoves. As is described
above, substantially lower amounts of off-gas are required by the
stoves 11 during bottling phases of the stoves 11 than is required
during heating phases of the stoves 11.
[0162] In addition, the steam (and electricity) requirements of the
plant, and therefore the required volumetric flow rates and
calorific values of fuel gas for the WHR system 25, are different
in different states of the process. For example, the steam (and
electricity) requirements of the plant are typically of the order
of 40-60% higher during a hot metal production state than during a
start-up state.
[0163] In addition, the fuel gas requirements of the stoves 11 are
different in different states of the process. For example, larger
amounts of fuel gas are required during a hot metal production
state than during an idle state.
[0164] In view of the above, during at least some states of the
process there is a need for an alternative fuel gas, such as
natural gas (or other fuel gas other than off-gas), to be supplied
to the burner assembly 37 of the WHR system 25 to meet the steam
requirements of the plant during a smelting campaign. In addition,
this alternate fuel gas ensures that the WHR system is capable of
combusting off-gas with a low calorific value that was not supplied
to other units of the plant, such as the stoves.
[0165] In addition, in view of the above, there is a need for
varying flow rates of an alternative fuel gas, such as natural gas
(or other fuel gas other than off-gas), to be supplied to the
burner assembly 37 of the WHR system 25 in order to compensate for
varying flow rates and calorific values of off-gas from the vessel
3 during a given state of the smelting campaign to meet the steam
requirements of the plant.
[0166] In addition, in view of the above, during at least some of
the states of the process, there is a need for an alternative fuel
gas, such as natural gas (or other fuel gas), to be supplied to the
burner assemblies of the stoves 11 to compensate for variations in
flow rates and calorific values of off-gas to thereby maintain
target flow rates and target calorific values of fuel gas for the
burner assemblies. For example, during a hot metal production
state, the flow of combined fuel gas to the stoves, being the
combined off-gas and alternative fuel gas, is controlled to have a
constant calorific value. As the calorific value of the off-gas
varies with process conditions, so does the required flow rate of
off-gas to the stoves. If the calorific value of the off-gas falls
below 1.8 MJ/Nm3 all of the off-gas is diverted to the WHR system
because the calorific value of the off-gas is too low to make a
useful contribution to heating the stoves 11, at any flowrate. It
is the calorific value of the off-gas that, at least in part,
determines the call for off-gas made by the stoves. The balance of
off-gas is supplied to and combusted by the WHR system.
[0167] Supply of an alternative fuel gas, such as natural gas, is
particularly necessary when the process is operating in the
off-wind, hold and idle states. During these states, off-gas flow
to the stoves 11 is shut off altogether or at least is
substantially reduced and, therefore, another fuel gas, such as
natural gas is required to maintain operation of the stoves 11 at a
required level during these process states.
[0168] Consequently, the process controller of the plant operates
the burner assembly 37 of the WHR system 25 with varying flow rates
of natural gas as an additional fuel gas to provide required flow
rates and calorific values of fuel gas at any point in the
process.
[0169] In addition, consequently, the process controller of the
plant operates the burner assembly 37 of the WHR system 25 with
varying flow rates of air to combust the varying flow rates of
off-gas and natural gas to ensure optimum combustion.
[0170] In addition, consequently, the process controller of the
plant operates the burner assemblies of the stoves 11 with varying
flow rates of natural gas as an additional fuel gas to provide
required flow rates and calorific values of fuel gas at any point
in the process.
[0171] In addition, consequently, the process controller of the
plant operates the burner assemblies of the stoves 11 with varying
flow rates of air to combust the varying flow rates of off-gas and
natural gas to ensure optimum combustion.
[0172] In addition, the process controller of the plant commences
ramping up the air flow rate to the burner assembly 37 of the WHR
system 25 a predetermined time period, typically 30 seconds, before
there is an increase in off-gas to the burner assembly 37 due to a
decrease in demand for off-gas in the stoves 11.
[0173] Similarly, the process controller of the plant commences
ramping down the air flow rate to the burner assembly 37 of the WHR
system 25 a predetermined time period, typically 30 seconds, before
there is a decrease in off-gas to the burner assembly 37 due to an
increase in demand for off-gas in the stoves 11.
[0174] In one particular example of the operation of the
above-described plant, the process controller controls: [0175] (a)
the volumetric flow of natural gas to burner units of the stoves 11
so that the combined flow rate of off-gas and natural gas has a
substantially constant calorific value during the heating phases of
the stoves; and [0176] (b) the volumetric flow rate of natural gas
to the WHR system 25 in response to variations in the volumetric
flow rate of off-gas to the WHR system to effect combustion of the
off-gas in the WHR system.
[0177] With regard to item (b) above, in context of the particular
example the WHR system 25 requires an amount of fuel gas, which may
be provided by off-gas and/or natural gas to provide at least a
minimum threshold calorific value.
[0178] The calorific value of off-gas at different points in the
plant is an important parameter in determining the flow rates of
natural gas required for the burner assembly 37 of the WHR system
25 and the burner assemblies of the stoves 11 at any point in
time.
[0179] The plant includes mass spectrometers CV1, CV2, and CV3 at
selected locations of the plant to determine the calorific values
of the off-gas at these locations. The measured values of calorific
values are processed by the process control for the plant as part
of the process of determining required flow rates of off-gas and
natural gas.
[0180] The selected locations are in the off-gas hood 15 (CV1),
downstream of the off-gas cooler 23 and upstream of the split of
the off-gas to the stoves 11 and the WHR system 25 (CV2), and
downstream of the pre-heater 61 (CV3).
[0181] Operating the above-described process with a range of
different states also has an impact on pressure control in the
vessel 3 during the different states.
[0182] In addition, the preheater 17 has certain minimum flow gas
requirements in order to maintain the iron-containing feed material
in a fluidised state. Gas flow through the preheater 17 is
controlled by the control valve in the wet cone scrubber 63 that is
downstream of the preheater 17.
[0183] The above description indicates that vessel pressure control
is via the control valve 77 of the wet cone scrubber 21 when the
process is producing molten iron, i.e. when operating in a hot
metal production state.
[0184] More particularly, the plant includes a pressure sensor P1
in the off-gas hood 15 that monitors the pressure in off-gas
flowing through the hood on a continuous basis. The process
controller of the plant is responsive to the monitored pressure and
operates the control valve 77 of the wet cone scrubber 21 to adjust
the pressure as required, preferably to maintain a constant vessel
pressure, when the process is operating in a hot metal production
state. The time constant of the control circuit of the control
valve 77 is considerably faster than the time constant of the
control circuit of the control valve in the scrubber 63 downstream
of the preheater 17. Hence, as between the control of pressure in
the vessel 3 and the control of gas flow rate through the preheater
17, during the metal production state, the control of vessel
pressure dominates.
[0185] It is still necessary to maintain control of pressure in the
vessel 3 during other states, particularly the hold and idle
states, of the process. Such pressure control is achieved during
these states via the above-described control valve in the wet cone
scrubber 63 downstream of the preheater 17 rather than via the
control valve 77 of the wet cone scrubber 21.
[0186] More particularly, when the process is operating in these
states, the control valve 77 of the wet cone scrubber 21 is at
least substantially closed so that there is no or at most a minimal
flow of off-gas through the scrubber 21 and thereafter to the
stoves 11 and the WHR system 25 from this source. Consequently, the
control valve in the wet cone scrubber 63 becomes the dominant
pressure controller during the hold and idle states. This also
ensures gas flow through the pre-heater so that the metalliferous
material is maintained in a fluidised state.
[0187] Additionally, when the process moves into the hold and idle
states, the process controller operates to reduce the flow rate set
point of hot air blast that is supplied to the vessel 3 from the
stove 11. The pressure set point of the vessel may also be reduced.
Typically the set point is reduced from 0.8 bar gauge to 0.4 bar
gauge.
[0188] During hold and idle states, a portion of the off-gas that
has passed through the preheater 17 is recycled and combined with
the off-gas from the vessel 3 so as to assist with maintaining
fluidising conditions within the preheater 17.
[0189] At off-wind state, no hot air blast is supplied to the
vessel. The scrubber 63 downstream of the preheater 17 is closed
and all of the off-gas within the preheater 17 is recycled so as to
operate as fluidising gas.
[0190] During hold and idle states the stoves 11 produce a reduced
amount of hot air blast. In order to ensure that the stoves 11 do
not exceed a maximum temperature, the total energy of the fuel gas
supplied to the stoves 11 is reduced compared with the total energy
of the fuel gas supplied to the stoves during hot metal production.
In this way, the energy input to the stoves 11 is reduced during
hold and idle states so as to match the reduced energy requirements
of the reduced hot air blast flow.
[0191] Many modifications may be made to the embodiment of the
present invention described above without departing from the spirit
and scope of the invention.
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