INTEGRATION OF OXYGEN TRANSPORT MEMBRANE WITH COAL BASED DRI PROCESS

Abstract

A process for the integration of oxygen transport membrane with a coal based DRI process is provided. The process includes introducing a heated, compressed air stream into an oxygen transport membrane separator, thereby producing a permeate stream and a retentate stream, introducing at least a portion of the retentate stream into a fired heater along with a combustion air stream, and introducing at least a portion of the permeate stream into a DRI shaft furnace.

Description

BACKGROUND

Direct reduction of iron ore (DRI) process is commonly used where there is shortage of coking coal required for blast furnace or shortage of scrap for Electric Arc Furnace/Basic Oxygen Furnace. Direct reduction shaft furnace technology as provided by Midrex, HYL etc. uses syngas for iron ore reduction. Syngas can be produced from carbon source such as natural gas, coal or refinery off-gas using many different processes such as CO₂ reforming, steam methane reforming, coal gasification, partial oxidation, auto thermal reforming etc. Natural gas based DRI process is commonly used worldwide. However, coal gasification based DRI process is a potential technology for places where natural gas and coking coal is not easily accessible. Air Separation Unit is commonly used for producing oxygen for coal gasification and DRI process. However, the overall process can be very energy and capital intensive due to combination of cryogenic and warm processes.

U.S. Pat. No. 6,149,859 describes the use of syngas produced from coal gasifier for reducing iron ore in a direct reduction shaft furnace. The syngas from the gasifier is cooled and scrubbed to remove water soluble impurities, dust etc. Syngas is further sent to water gas shift reactor to increase hydrogen in syngas. CO2 is removed from syngas using acid gas removal unit in order to improve reductant to oxidant ratio in syngas. Syngas is further heated to 426-815 C before entering the shaft furnace. Syngas reacts with iron ore to produce DRI as product.

U.S. Pat. No. 6,395,055 from Midrex describes direct reduction of iron ore process where reducing gas comprises of cleaned exhaust gas from DRI shaft furnace and make-up reducing gas arriving from an outside reforming circuit. Oxygen is injected into the reducing gas prior to the shaft furnace in order to increase the temperature of reducing gas between 1000 and 1150 C by partial combustion of H2 and CO in the reducing gas. Methane is also injected in a controlled manner into the reducing gas in order to produce additional reforming gas H2 and CO and increase the productivity of DRI.

U.S. Pat. No. 5,643,354 describes the COREX process where DRI from shaft furnace is sent to melter/gasifier and is melted to produce pig iron. The exhaust gas from DRI shaft furnace is combusted with oxygen enriched air in a fired heater or direct combustor. Oxygen from flue gas is recovered in oxygen transport membrane (OTM) and used in a gasifier. This patent uses oxygen transport membrane with DRI shaft furnace and coal gasifier but the OTM is used on flue gas and not on air.

The above patents do not integrate oxygen production with the DRI process economically.

SUMMARY

A process for the integration of oxygen transport membrane with a coal based DRI process is provided. The process includes introducing a heated, compressed air stream into an oxygen transport membrane separator, thereby producing a permeate stream and a retentate stream, introducing at least a portion of the retentate stream into a fired heater along with a combustion air stream, and introducing at least a portion of the permeate stream into a DRI shaft furnace.

In another embodiment, the process includes introducing a heated, compressed air stream into a first oxygen transport membrane separator, thereby producing a high pressure permeate stream and a high pressure retentate stream, introducing the high temperature retentate stream into a second oxygen transport membrane separator, thereby producing a low pressure, compressing at least a portion of the low pressure retentate stream, thereby producing a second high pressure retentate stream, combining the first high pressure retentate stream and the second high pressure retentate stream, thereby forming a combined retentate stream, introducing at least a portion of the combined retentate stream into a coal gasifier. And introducing at least a portion of the low pressure permeate stream into a DRI shaft furnace.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates one embodiment of the present invention.

FIG. 2 illustrates another embodiment of the present invention.

FIG. 3 illustrates another embodiment of the present invention.

FIG. 4 illustrates another embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

A fixed bed dry bottom coal gasifier, such as those offered by Lurgi GmbH, can accept high ash non coking coal and produce syngas. Fixed bed dry bottom coal gasifier operating at high pressure 20-150 bar more preferably 30-100 bar produces syngas rich in methane 8-30% more preferably 12-25%. Coal gasification requires oxygen for gasifying coal to produce syngas. Syngas is further used as a reducing gas along with recycled exhaust gas from the DRI shaft furnace for iron ore reduction process. Oxygen is injected to the reducing gas in order to partially combust hydrogen and CO and increase the temperature of reducing gas to >1000 C. Oxygen for coal gasification and DRI process is generally produced from the cryogenic air separation unit. Coal gasification and DRI process operates at high temperature while ASU operates at cold temperature. The combination of warm and cold process makes the entire process inefficient and expensive.

The present invention deals with integration of oxygen transport membrane (OTM) with the direct reduction of iron ore (DRI) process. Air compressed to 10-40 bar is optionally pre-heated using hot retentate stream and sent to the OTM at high pressure. Oxygen ions at high temperature ˜700-1000C will permeate to produce oxygen product at 1-10 bar pressure. Lower pressure on the permeate side will improve the recovery of oxygen but increases the compression cost since coal gasification operates at high pressure and oxygen is needed at higher pressure. Steam can be used as sweep stream on the permeate side to increase the pressure of oxygen and hence reduce the cost associated with compression. The mix of Oxygen and Steam can be sent to the coal gasifier. Alternatively oxygen product can be cooled while producing steam and then compressed to the desired pressure for coal gasification. Alternatively oxygen can be recovered at different pressures in stages from the OTM with high pressure from the first stage, lower pressure from the second stage and so on in order to reduce the compression cost. Direct reduction of iron ore process operates at low pressure 1-5 bar and low pressure oxygen can be injected in reducing gas in order to combust H2 and CO to raise the temperature of gas and partially oxidize excess methane to produce additional H2 and CO for the DRI process.

Air for Oxygen transport membrane can heated in the fired heater used for heating the DRI reducing gases. Alternatively, the oxygen transport membrane can be embedded inside the fired heater used for heating DRI reducing gas to improve the performance of OTM and avoid heat losses. The combined heating of reducing gas and OTM will save on capital cost for the furnace and piping etc. The exhaust gas from the DRI shaft furnace along with make-up syngas can be used as fuel for the fired heater.

Retentate side gas is at high temperature ˜900° C. and sensible heat is recovered by pre-heating air or generating steam or pre-heating reducing gas for the DRI process. Retentate stream will contain unrecovered oxygen 3-18% and it can be used for combustion process in the fired heater.

Both DRI exhaust gas and make-up syngas are cleaned to remove CO₂ and other impurities. Gas cleaning can be combined or done separately for the two streams. Coal gasifier operates at much high pressure than the DRI shaft furnace. Make-up syngas is expanded in a turbo expander or any other expansion device. Electricity generation from turbo expander may be used to compress exhaust gas or any other purpose in the DRI process.

Coal gasification is a process that converts coal from a solid to a gaseous fuel through partial oxidation as given in equation (1). The amount of inlet oxygen is controlled carefully to provide heat of reaction and partially oxidize coal to produce hydrogen and CO along with other by-products such as CO₂, CH₄ etc.

2CH_n+O₂=2CO+nH₂  Equation 1

Lurgi fixed bed dry bottom gasifiers (FBDB) are highly energy efficient because of counter-current flow pattern which allows the feed to be pre-heated by cooling the exiting syngas. Coal is heated and dried in the top part followed by devolatilization as it descends through the carbonation zone. Below this area the devolatilized coal is gasified by reaction with steam and CO₂. The highest temperature is reached in the combustion zone near the bottom of the gasifier. The char steam reaction together with the presence of excess steam keeps the temperature in the combustion zone below the ash slagging temperature. Since the temperatures inside the gasifier are below ash melting point, oxygen requirement is low and ash is collected in the solid form. Low temperatures inside the gasifier result in by-product formation such as tar, oil, naphtha, ammonia, phenol. These by-products can sometimes be used as petrochemical plant feed stock or used as fuel in steel plant. Make-up syngas from coal gasifier is cleaned and scrubbed to remove water soluble impurities, ash, phenol, tars, oil, ammonia, sulphur, CO₂ etc

The make-up syngas from FBDB gasifier is at high pressure (˜20-150 bar) depending on the coal gasification operating pressure. This high pressure is not required for the DRI process and the syngas is expanded in a turbo-expander. The higher pressure and low temperature <1000 C operation of FBDB gasifier results in syngas with high methane content 8-30% more preferable 12-25%. Syngas composition is highly dependent on FBDB gasifier operating condition and the feed stock properties.

Syngas from coal gasifier is used as a reducing gas along with exhaust gas from the DRI shaft furnace to reduce iron ore. Reducing gas is generally heated at 800-950° C. in a furnace or fired heater before sending to the DRI shaft reactor in order to better utilize the reductant for iron ore reduction. DRI shaft furnace operates in a counter-current mode with iron ore fed from the charge hopper located in top of the furnace and hot reducing gas entering from the middle of the furnace through the bustle. Iron ore is pre-heated in the top zone of the shaft furnace while cooling the exiting gas. The oxides of iron (Fe₂O₃ and Fe₃O₄) react with hydrogen or Carbon monoxide from the reducing gas to produce FeO. FeO further reacts with H₂ or CO at high temperature (>700° C.) to produce metallic iron.

3Fe₂O₃+CO/H₂=2Fe₃O₄+CO₂/H₂O  Equation 2

Fe₃O₄+CO/H₂=3FeO+CO₂/H₂O  Equation 3

FeO+CO/H₂=Fe+CO₂/H₂O  Equation 4

Oxygen is needed for coal gasification process and sometimes oxygen is injected in the reducing gas to combust hydrogen and CO and increase the temperature of reducing gas in order to improve the reaction kinetics inside the shaft furnace. Oxygen is generally produced from conventional Air Separation Unit which is cryogenic process. Air Separation Unit is energy intensive process involving refrigeration cycles while the coal gasification and DRI process is warm process involving furnace for heating the reducing gas. This combination of cryogenic and warm process makes the overall process highly energy intensive and expensive.

The present invention deals with integration of oxygen transport membrane with coal gasification and DRI process. FIGS. 1, 2, 3 and 4 shows alternative embodiments of the present invention. In order to avoid confusion, identical elements have the same number throughout these figures.

Turning to FIG. 1, inlet air 131 compressed to 10-40 bar 133, in compressor 132, is optionally pre-heated in heat exchanger 134 using hot retentate stream 137 and sent 135 to the OTM 136 at high pressure. The pressure differential across the membrane inside OTM 136 enables oxygen ions to permeate from feed side to permeate side. OTM are made up of ceramic oxide material which is selectively permeable non porous in nature. Oxygen ions at high temperature ˜700-10000 will permeate to produce oxygen product at 1-10 bar pressure. Lower pressure on the permeate side will improve the recovery of oxygen but increases the compression cost since coal gasification operates at high pressure and oxygen is needed at higher pressure. Steam 141 can be used as sweep stream on the permeate side to increase the pressure of oxygen and hence reduce the cost associated with compression. At least a portion 113 of the mix of Oxygen and Steam 111 can be sent to coal gasifier 101. Alternatively oxygen product 113 may be cooled in heat exchanger 114 while producing steam 116, and then compressed 115 to the desired pressure for coal gasification 101.

Alternatively oxygen can be recovered at different pressures in stages from the OTM with high pressure from the first stage 111, lower pressure from the second stage 113 and so on in order to reduce the compression cost as shown in FIG. 4. At least a portion 143 of the lower pressure oxygen 113 may be cooled in heat exchanger 140 while producing seam 141, and then compressed in compressor 142 to approximately the same pressure as high pressure oxygen stream 111, after which these two high pressure streams may be combined. The number of stages could be 2 to 4 (2 stages are shown)

Air for Oxygen transport membrane 136, 144 can be heated in the fired heater 109 used for heating the DRI reducing gases 107, 110 as shown in FIGS. 3 and 4. Alternatively, the oxygen transport membrane 136 can be embedded inside the fired heater 109 as shown in FIGS. 1 and 2 used for heating DRI reducing gas 107, 110 to improve the performance of OTM and avoid heat losses. The combined heating of reducing gas and OTM will save on capital cost for the furnace and piping etc. The exhaust gas from the DRI shaft furnace 119 along with make-up syngas 108 can be used as fuel for the fired heater 109.

Retentate side gas 137, 145 is at high temperature ˜900° C. and sensible heat is recovered by pre-heating air (134 in FIGS. 1, 2 and 3) or generating steam or pre-heating reducing gas (142 in FIG. 2) for the DRI process. Retentate stream 138 will contain unrecovered oxygen 3-18% and it can be used for combustion process (combined with 139 to form 140) in the fired heater as shown in FIG. 1.

Both exhaust gas 119 and make-up syngas 102 are cleaned 103, 126 to remove CO₂ and other impurities. Gas cleaning can be combined (not shown) or done separately (shown) for the two streams. Coal gasifier 101 operates at much high pressure than the DRI shaft furnace 117. Cleaned make-up syngas 104 is expanded 107, 108 in a turbo expander 106 or any other expansion device. Electricity generation from turbo expander may be used to compress exhaust gas or any other purpose in the DRI process (not shown)

Direct reduction of iron ore process 117 operates at low pressure 1-5 bar and low pressure oxygen 111, 113 can be injected in reducing gas 110 in order to combust H2 and CO to raise the temperature of gas >1000 C in order to improve the reaction kinetics for DRI production inside the shaft furnace. Syngas 102 from the fixed bed dry bottom coal gasifier 101 contains high methane content due to low temperature and high pressure operation. Some of the methane can be reformed in-situ in the DRI shaft furnace 117 but excess methane could cause lower bed temperature reducing the kinetics inside the shaft furnace. Excess methane is partially oxidized outside the DRI shaft furnace where oxygen is injected. Make-up syngas 102 is at higher pressure than the DRI shaft furnace 117 and it is expanded 106 before entering the shaft furnace. Make-up syngas expansion can be before or after the fired heater. Syngas expansion can be carried out by turbo expander or any other gas expansion device. Electrical power can be generated by gas expansion and used for compressing exhaust gas or any other equipment in the DRI process.

Exhaust gas 119 from the DRI shaft furnace contains CO, H₂, CO₂, H₂O along with other impurities. Exhaust gas 119 exits the shaft furnace 117 at a temperature 200-800° C. more preferably from 350-550° C. Exhaust gas 119 is cooled and scrubbed 120 to remove dust or other water soluble impurities. Water is condensed from the exhaust gas and removed. Sensible heat from exhaust gas can be used for producing steam or heating any other process stream (not shown). A portion 123 of exhaust gas 121 is used as fuel in the fired heater 109 and remaining portion 122 is recycled 105 as reducing gas. Reducing gas quality is measured by the amount of reductant to oxidant ratio (H₂+CO/H₂O+CO₂) in the reducing gas. The higher reductant to oxidant ratio is preferred for improved reaction kinetics inside the shaft furnace. CO₂ is removed from exhaust gas in gas cleaning means 126 and make-up syngas in gas cleaning means 103 in order to ensure reducing atmosphere inside the shaft furnace. CO₂ can be removed using any know CO₂ removing technology such as adsorption, absorption, cryogenic or membrane. CO₂ removal can be separate for make-up syngas and exhaust gas as shown, or combined together (not shown).

Claims

1. (FIGS. 1, 2, 3) A process for the integration of oxygen transport membrane with a coal based DRI process comprising;

a) introducing a heated, compressed air stream into an oxygen transport membrane separator, thereby producing a permeate stream and a retentate stream,

b) introducing at least a portion of said retentate stream into a fired heater along with a combustion air stream, and

c) introducing at least a portion of said permeate stream into a DRI shaft furnace.

2. The process of claim 1, wherein said oxygen transport membrane separator is located within said fired heater.

3. The process of claim 1, wherein said permeate stream is introduced into said DRI shaft furnace along with a syngas stream.

4. The process of claim 3, wherein at least a portion of said permeate stream is introduced into a coal gasifier, said coal gasifier producing said syngas stream.

5. The process of claim 1, further comprising;

d) providing at least a portion of the heat in said heated compressed air stream by indirect heat exchange with at least a portion of said retentate stream.

6. (FIG. 4) A process for the integration of oxygen transport membrane with a coal based DRI process comprising;

a) introducing a heated, compressed air stream into a first oxygen transport membrane separator, thereby producing a high pressure permeate stream and a high pressure retentate stream,

b) introducing said high temperature retentate stream into a second oxygen transport membrane separator, thereby producing a low pressure

c) compressing at least a portion of said low pressure retentate stream, thereby producing a second high pressure retentate stream,

d) combining said first high pressure retentate stream and said second high pressure retentate stream, thereby forming a combined retentate stream,

e) introducing at least a portion of said combined retentate stream into a coal gasifier and

f) introducing at least a portion of said low pressure permeate stream into a DRI shaft furnace.

7. The process of claim 1, wherein said permeate stream is introduced into said DRI shaft furnace along with a syngas stream generated by said coal gasifier.