WO2012148172A2

WO2012148172A2 - System for producing syngas and reducing tar using air and steam on fluidized bed

Info

Publication number: WO2012148172A2
Application number: PCT/KR2012/003195
Authority: WO
Inventors: Yong Soo Cho; Chang Soo Youn; Gi Chul Myoung; Jae Hun Song; Uen Do Lee; Won Yang; Young Tai Choi; Young Doo Kim; Jeong Woo Lee; Min Woo Park; Dong Yun Lee; Ho Keun Jang
Original assignee: Korea Institute Of Industrial Technology; Seen Tec Co.,Ltd.
Priority date: 2011-04-27
Filing date: 2012-04-25
Publication date: 2012-11-01
Also published as: KR101066187B1; WO2012148172A3

Abstract

Disclosed is a system for producing syngas and reducing tar using air/steam on a fluidized bed, which includes a gasifier (10) in which a solid fuel is gasified to produce a combustible syngas; a first loop seal (20) connected to the gasifier (10) to receive the sand from the gasifier; a CFB combustor (30) which receives the sand from the first loop seal via a loop seal connector (33) and the syngas from the gasifier via a syngas supply unit (34) and combusts the fuel to heat the sand; a cyclone (40) connected to the CFB combustor (30) to separate gas and solid particles discharged from the CFB combustor (30); a downcomer (50) disposed under the cyclone (40) to drop the sand; and a second loop seal (60) in which the sand dropped via the downcomer (50) is collected to form a sand bed (61).

Description

SYSTEM FOR PRODUCING SYNGAS AND REDUCING TAR USING AIR AND STEAM ON FLUIDIZED BED

The present invention relates to a system for producing syngas and reducing tar using air and steam on a fluidized bed, which is an integrated fluidized bed gasifier having a gasification region and a combustion region separated from each other and in which syngas produced in a gasifier is directly introduced into the upper portion of a combustor falling outside a dense bed wherein a main combustion reaction occurs, so that tar present in the syngas is thermally cracked and thus reduced and steam reforming is carried out using steam resulting from heat exchange with steam of the combustion gas, thus producing a syngas suitable for use in gas engines, multi fuel fired boilers, and any conversion processes for high-valued products such as SNG(Synthetic Natural Gas), or synthetic liquid fuels.

Generally, gasification is a process of converting a solid fuel into a syngas composed of H₂, CO, CO₂, CH₄, etc. The syngas produced by gasification may be variously utilized as a material for producing not only electricity and heat but also for chemical synthesis and producing hydrogen gas and SNG (Synthetic Natural Gas).

In the gasification process, gasifier and working conditions are determined depending on the purpose of the reaction and the products, and gasifiers may be classified into an entrained bed gasifier, a fluidized bed gasifier, and a moving/fixed bed gasifier depending on the kind thereof.

An entrained bed gasification reactor has been utilized for many integrated coal gasification combined cycles (IGCC). However, the use of fuel by this reactor is limited, and such a reactor is applied to large capacity gasification compared to other kinds of gasifiers. A moving/fixed bed gasification reactor which is used for comparatively small capacity and it is limited to control tar with the primary measure. Also, a fluidized bed gasifier may be operated using a large variety of fuels and may be applied to a variety of capacities, and may have various possibilities to control tar and the composition of syngas by using a novel reactor configuration or combination of several reactors.

Products resulting from gasification include syngas, char, and tar. Tar is an organic polymer material having a molecular weight higher than that of benzene, and the composition of products may vary depending on the kind of reactor, the reaction temperature, the retention time, etc., and tar may be condensed at or below a dew point (350℃) and thus may exist as a very viscous liquid.

Tar may foul or plug pipes and tubes at or below the dew point, making it difficult to operate the system continuously, and also may cause corrosion of pipes and tubes, and thus must be removed in order for a system to be able to be stably and continuously operated.

To remove tar, physical methods using a wet scrubber, a wet electrostatic precipitator, a barrier filter, a cyclone, etc., and methods using a catalyst and thermal cracking are employed.

As shown in FIG. 1, Korean Patent No. 10-0999470 discloses a two-stage circulating fluidized bed reactor, comprising a riser 111 that is a combustion furnace wherein the combustion of solid fuel occurs, a first cyclone 112 connected to the upper portion of the riser to separate gas generated upon combustion from solid particles of solid fuel, a downcomer 113 connected to a main reactor from the first cyclone, a main reactor 114 which is connected with the downcomer and is operated on a bubbling fluidized bed so that gasification of solid fuel occurs, a loop seal 115 for recirculating a solid circulation material of the main reactor to the riser, and a second cyclone 116 for discharging gas from the main reactor 114 or collecting fine particles.

However, Korean Patent No. 10-0999470 is problematic because the syngas produced in the main reactor is discharged in the state of containing a large amount of tar, undesirably requiring the additional post-treatment system of a tar refinery. Furthermore, the syngas produced in the main reactor may flow backwards to the downcomer 113 connected to the main reactor from the first cyclone, undesirably lowering the efficiency of the cyclone. Also, although the gas resulting from combustion and the gas resulting from gasification may be separated to thus enable the formation of a medium calorific value gas (10 ~ 15 MJ/Nm³) without nitrogen dilution, oxygen or steam should be introduced as a gasification medium, which undesirably requires an additional system.

Accordingly, an object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, in which a syngas produced in a gasifier may be supplied to a combustor at high temperature so that tar present in the syngas is thermally cracked, thus producing a syngas having reduced tar.

Another object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, in which the syngas may be supplied to a circulating fluidized bed (CFB) combustor to reduce the amount of combustion gas necessary to circulate a heat transfer medium, thus decreasing the load of the combustor, and also, a hot gas retention space is disposed downstream of the cyclone to provide an enough retention time required by the reaction, so that the steam of the combustion gas is subjected to steam reforming with the syngas and tar, thus producing a cleaner and more reformed syngas which has higher hydrogen content with water gas shift (WGS) reaction.

A further object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, in which steam obtained via heat exchange may be supplied to a gasifier, thus producing a syngas having an increased hydrogen yield by steam reforming.

Yet another object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, in which the combustion gas may be prevented from flowing backwards from the CFB combustor to the gasifier and the syngas is prevented from flowing backwards from the gasifier to the downcomer, thus increasing the efficiency of a cyclone.

Still another object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, wherein instead of supplying expensive oxygen and steam being fed as a fluidizing medium and a gasification medium, a heat source necessary for the gasifier is obtained via partial oxidation using part of air, and the loss of gas resulting from using air is supplemented by the production of syngas due to the cracking of tar, and steam resulting from heat exchange is supplied to the gasifier, thereby reducing the cost.

Still another object of the present invention is to provide a system for producing syngas and reducing tar using air and steam on a fluidized bed, wherein the loads of the combustor and the gasifier are appropriately controlled, thus making operating conditions adapted to crack tar depending on the properties of fuel used and adjusting the composition of the syngas produced depending on the needs.

In order to accomplish the above objects, the present invention provides a system for producing syngas and reducing tar using air and steam on a fluidized bed, comprising a gasifier 10, which includes sand as a heat transfer medium charged therein and in which a solid fuel is gasified to produce a combustible syngas composed mainly of H₂, CO, CO₂ and CH₄; a first loop seal 20 connected to the gasifier 10 so as to receive the sand and unreacted char from the gasifier; a CFB (Circulating Fluidized Bed) combustor 30, which receives the sand and the unreacted char from the first loop seal via a loop seal connector 33 and receives the syngas produced in the gasifier via a syngas supply unit 34 and heats the sand; a cyclone 40 connected to the upper portion of the CFB combustor 30 so as to separate gas and solid particles discharged from the CFB combustor 30; a downcomer 50 disposed under the cyclone 40 so that the sand which was separated in the cyclone 40 is dropped therethrough; and a second loop seal 60, one end of which is connected to the lower portion of the downcomer 50 and the other end of which is connected to the upper portion of the gasifier 10 and in which the sand is transported to the gasifier from a sand bed 61 formed by being dropped via the downcomer 50.

The gasifier 10 may include a steam supply unit 13 at one side thereof so as to receive hot steam.

The gasifier 10 may include a sand supply unit 15 at one side thereof so as to receive sand from the outside.

One side of the lower portion of the first loop seal 20 may be connected to the gasifier 10 so as to receive the sand and unreacted char, and one side of the upper portion thereof may be connected to the CFB combustor 30 so as to supply the sand and unreacted char to the CFB combustor 30 via the loop seal connector 33.

The syngas supply unit 34 of the CFB combustor may be disposed above the loop seal connector 33.

A connector between the CFB combustor 30 and the cyclone 40 may be formed so as to be inclined at an angle.

The second loop seal 60 may include a second ventilation unit 62 at one side thereof so as to transport the collected sand to the gasifier 10.

The second loop seal 60 may be U-shaped.

The system may further comprise a hot gas retention pipe 70 which receives gas discharged from the cyclone 40 so that steam of the combustion gas or additional steam from heat exchanger is subjected to steam reforming with the syngas and tar for a sufficient retention time, thus producing less tar and more hydrogen containing syngas.

According to the present invention, an integrated fluidized bed gasifier having a gasification region and a combustion region separated from each other is provided, in which a syngas produced in a gasifier is directly fed to a combustor at high temperature falling outside the dense bed wherein main combustion occurs, thereby thermally cracking tar present in the syngas.

Also according to the present invention, olivine or dolomite as heat transfer medium is supplied to the gasifier to reduce tar with its catalytic function during gasification process.

Also according to the present invention, the syngas is supplied to a CFB combustor to reduce the amount of combustion gas necessary to circulate a heat transfer medium (fluid sand or the like), thus decreasing the load of the combustor, and also, a hot gas retention space is disposed downstream of the cyclone to provide a retention time required by the reaction, so that the steam of the combustion gas is subjected to steam reforming with the syngas, thus producing a reformed syngas.

Also according to the present invention, steam obtained via heat exchange can be used in the gasifier, thus increasing the hydrogen yield via steam reforming.

Also according to the present invention, loop seals which may maintain pressure by a sand bed are respectively formed between the CFB combustor and the gasifier and between the gasifier and the downcomer, thus preventing the syngas from flowing backwards to the cyclone and preventing combustion gas from flowing backwards to the gasifier.

Also according to the present invention, combustion load of the CFB combustor and partial combustion load of the gasifier can be appropriately controlled, thus maximizing the cracking of tar and the calorific value of the syngas.

FIG. 1 is a schematic view showing a typical circulating fluidized bed reactor;

FIGS. 2a and 2b are schematic views showing a system for producing syngas and reducing tar using air and steam on a fluidized bed according to the present invention;

FIG. 3 is a detailed cross-sectional view showing a gasifier, a first loop seal, and a CFB combustor according to the present invention; and

FIG. 4 is a detailed cross-sectional view showing a cyclone, a downcomer, and a second loop seal according to the present invention.

According to the present invention, a system for producing syngas and reducing tar using air and steam on a fluidized bed includes a gasifier 10 for producing a combustible syngas, a first loop seal 20, a CFB combustor 30, a cyclone 40, a downcomer 50, a second loop seal 60, a hot gas retention pipe 70, etc. The gasifier 10 should be maintained at a higher pressure than the first loop seal 20 and the CFB combustor 30.

The gasifier 10 includes a first air supply unit 11 for supplying air and a first fuel supply unit 12 for supplying a solid fuel at predetermined positions thereof, and the gasifier receives air and the solid fuel respectively from the first air supply unit 11 and the first fuel supply unit 12 so that the solid fuel is partially oxidized or gasified thus producing a combustible syngas composed mainly of H₂, CO, CO₂, and CH₄. As such, the reaction for converting the solid fuel into syngas is endothermic, and in order to retain the reaction temperature of the gasifier 10, the solid fuel is partially oxidized by air to thus transfer the heat necessary for the gasifier 10. Furthermore, the gasifier 10 includes a steam supply unit 13 at one side thereof so as to receive hot steam, and thus the hot steam may be used as a medium for steam gasification in the gasifier 10. The solid fuel may include biomass, coal, RDF etc.

At the lower portion of the gasifier 10, a sand bed 14 that transfers heat necessary for the endothermic reaction is positioned to a predetermined level, and the sand bed 14 is transported to the CFB combustor 30 via the first loop seal 20 together with part of unreacted char obtained after gasification. A first ventilation unit 21 is provided at the lower portion of the first loop seal 20 so that the introduced sand is efficiently fed to the CFB combustor 30. The pressure of the first loop seal 20 must be maintained higher than that of the CFB combustor 30 so that the combustion gas produced in the CFB combustor 30 is prevented from flowing backwards. Also, provided at one side of the gasifier 10 is a sand supply unit 15 that supplies sand from the outside. In the case where the amount of a fluid material is decreased due to the sand lost via the CFB combustor 30, when the gasifier 10 cannot maintain a higher pressure than the first loop seal 20 and the CFB combustor 30, the sand is fed from the sand supply unit 15.

The CFB combustor 30 is operated at a high rate in order to re-heat and circulate the sand used as the heat transfer medium in the gasifier 10, and includes a second air supply unit 31 for supplying air and a second fuel supply unit 32 for supplying fuel at predetermined positions of the lower portion thereof, so that the combustor receives air and fuel. Also, formed at one side of the CFB combustor 30 is a loop seal connector 33 that receives unreacted char and sand from the first loop seal 20. Furthermore, the CFB combustor 30 includes a syngas supply unit 34 at one side higher than the fuel supply unit thereof so as to receive the syngas from the gasifier 10. As such, when the syngas supply unit 34 is positioned at the bottom of the CFB combustor 30, the introduced syngas is combusted, undesirably decreasing efficiency. Hence, the syngas supply unit is preferably located at a position falling outside the dense bed of the loop seal connector 33.

The CFB combustor 30 heats the sand fed via the loop seal connector 33 while combusting the fuel supplied from the bottom thereof. As such, char contained in the sand is combusted together to thus provide a heat source. Furthermore, while the sand and combustion gas generated at the bottom of the CFB combustor 30 are mixed with the syngas supplied from the gasifier 10 by the pressure of air supplied from the second air supply unit 31, they move upwards. As such, while tar present in the syngas supplied to the CFB combustor 30 passes along with the combustion gas through the inside of the CFB combustor 30, it is thermally cracked by the high temperature of the CFB combustor 30, and the reaction continues for a long time while passing through a hot region after the cyclone 40.

Hence, fouling or plugging in the downstream pipes and tubes caused by the tar may be reduced. The CFB combustor 30 is operated at a high rate to circulate the heat transfer medium. While the syngas is supplied to the CFB combustor 30 from the gasifier 10, it functions as secondary air, thus providing an appropriate operation rate to thereby reduce the load of the combustor. The cross-sectional area of the CFB combustor may be reduced to efficiently circulate the fluid sand which is a bed material under a position where the syngas is supplied, if required.

The cyclone 40 is connected to the upper portion of the CFB combustor 30, and receives the combustion gas, syngas, sand and so on discharged from the CFB combustor 30, so that gas and solid particles (sand + ash) are separated by centrifugal force and collected. A gas discharge unit 41 is formed at the upper portion of the cyclone 40 to discharge the separated gas. The connector between the CFB combustor 30 and the cyclone 40 is formed so as to be inclined at an angle, thus preventing the solid particles discharged from the CFB combustor 30 from accumulating.

The downcomer 50 is located under the cyclone 40, and plays a role in that the hot heat transfer medium, namely, the sand which was separated in the cyclone 40 is dropped and transported to the second loop seal 60 connected to the bottom thereof.

The second loop seal 60 is U-shaped, and one end thereof is connected to the lower portion of the downcomer 50, and the other end thereof is connected to the upper portion of the gasifier 10, so that the sand dropped via the downcomer 50 is collected to form a sand bed 61. Also, the second loop seal 60 includes a second ventilation unit 62 at one side thereof to transport the collected sand to the gasifier 10. The second loop seal 60 has the sand bed 62 formed by the solid circulation rate and may maintain the pressure higher than the gasifier 10, and also may prevent the syngas produced in the gasifier 10 from flowing backwards to the loop seal. The sand of the second loop seal 60 may be fed depending on the amount of sand that is supplied from the gasifier 10 to the CFB combustor 30. In the case where the sand of the second loop seal 60 is equal to or less than a predetermined amount, sand may be fed via the sand supply unit 15 connected to the gasifier 10.

The hot gas retention pipe 70 is connected to the gas discharge unit 41 of the cyclone so that the discharged syngas stays for a sufficiently long period. The hot gas retention pipe 70 provides a retention time long enough for the reaction of the syngas that passes therethrough, thus producing a reformed syngas via steam reforming. A typical heat exchanger (not shown) is disposed at the end of the hot gas retention pipe 70. Also, a heating element for maintaining the temperature of the retention pipe may be formed in the hot gas retention pipe 70 so that a steam reforming reaction efficiently takes place therein.

Below is a description of the operating procedure of the system for producing syngas and reducing tar using air and steam on the fluidized bed according to the present invention.

First, a predetermined level or more of a sand bed is formed in the gasifier 10 via the sand supply unit 15. The sand fed to the gasifier 10 is used as a heat transfer medium that maintains the temperature of the gasifier 10 by heating the sand in the CFB combustor 30 and then recirculating it to the gasifier 10.

Separately, olivine or dolomite as heat transfer medium can be used for tar reduction in the gasifier 10 and it is supplied to the gasifier 10 from the sand supply unit 15.

In the gasifier 10, the solid fuel fed from the first fuel supply unit 12 is partially oxidized by means of air fed from the air supply unit 11 at the lower portion of the gasifier, thus supplying a heat source necessary for gasification and producing a combustible syngas composed mainly of H₂, Co, CO₂ and CH₄. The produced combustible syngas is fed to the CFB combustor 30 via the syngas supply unit 34 from the upper portion of the gasifier 10. Furthermore, char produced upon gasification is continuously gasified in the gasifier 10. As such, hot steam which is supplied via the steam supply unit 13 of the gasifier 10 may be used as a medium necessary for gasification.

When part of the sand bed 14 located at the lower portion of the gasifier 10 is introduced into the first loop seal 20, the sand is fed to the CFB combustor 30 via the loop seal connector 33 positioned at the top of the first loop seal 20 by means of air discharged from the first ventilation unit 21, so that it is heated. As such, part of the unreacted char produced in the gasifier 10 is supplied together with the sand to the CFB combustor 30.

The sand supplied to the CFB combustor 30 is heated by combustion heat resulting from combusting the fuel that is fed to the CFB combustor 30, and part of the char transported together with the sand is combusted along with the fuel. While the heated sand and combustion gas are mixed with the syngas of the gasifier 10 supplied from the syngas supply unit 34 by means of the pressure of the air fed from the second air supply unit 31, they move upwards. As such, while tar present in the syngas supplied to the CFB combustor 30 passes through the CFB combustor 30 together with the syngas, it is thermally cracked due to the high temperature of the CFB combustor 30, thus reducing the amount of tar. The syngas is subjected to steam reforming with steam of the combustion gas, and the reaction continues while the syngas passes through the hot region after the cyclone 40.

The heated sand, syngas, combustion gas and so on, which were transported to the upper portion of the CFB combustor 30, move to the cyclone 40 connected to the upper portion of the CFB combustor 30, and are then separated into gas and solid particles (sand + partial ash) by centrifugal force in the cyclone 40. As such, the connector between the CFB combustor 30 and the cyclone 40 is formed to have a predetermined inclination so that the solid particles are prevented from accumulating on the connector.

The gas components separated in the cyclone 40 are supplied to the hot gas retention pipe 70 via the gas discharge unit 41, and the solid particles such as sand which are a hot heat transfer medium are dropped along the downcomer 50 and transported to the second loop seal 60.

While the combustion gas and the syngas which are contained in the gas components supplied to the hot gas retention pipe 70 pass through the hot gas retention pipe, the steam of the combustion gas or from the heat exchanger is subjected to steam reforming with the syngas thus producing a reformed syngas.

The sand dropped along the downcomer 50 is collected as the sand bed 61 in the U-shaped second loop seal 60, and part of the hot sand thus collected is transported to the gasifier 10 via the second ventilation unit 62 provided at one side of the second loop seal 60. The sand of the second loop seal 60 may be supplied depending on the amount of the sand fed to the CFB combustor 30 from the gasifier 10. If the amount of sand in the second loop seal 60 is equal to or less than a predetermined level, the sand may be further supplied via the sand supply unit 15 connected to the gasifier 10.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

A system for producing syngas and reducing tar using air and steam on a fluidized bed, comprising:

a gasifier, which includes sand as a heat transfer medium charged therein and in which a solid fuel is partially oxidized or gasified to produce a combustible syngas;

a first loop seal connected to the gasifier so as to receive the sand and unreacted char from the gasifier;

a CFB (Circulating Fluidized Bed) combustor, which receives the sand and the unreacted char via a loop seal connector connected to the first loop seal and receives the syngas produced in the gasifier via a syngas supply unit connected to the gasifier and heats the sand;

a cyclone connected to an upper portion of the CFB combustor so as to separate gas and solid particles discharged from the CFB combustor;

a downcomer disposed under the cyclone so that the sand which was separated in the cyclone is dropped therethrough; and

a second loop seal, one end of which is connected to a lower portion of the downcomer and the other end of which is connected to an upper portion of the gasifier and in which the sand is transported to the gasifier from a sand bed formed by being dropped via the downcomer.
The system of claim 1, wherein the gasifier includes a steam supply unit at one side thereof so as to receive hot steam.
The system of claim 1 or 2, wherein the gasifier includes a sand supply unit at one side thereof so as to receive sand from outside.
The system of claim 1, wherein one side of a lower portion of the first loop seal is connected to the gasifier so as to receive the sand, and one side of an upper portion thereof is connected to the CFB combustor so that the sand is supplied to the CFB combustor via a first ventilation unit.
The system of claim 1, wherein the syngas supply unit of the CFB combustor is disposed above the loop seal connector.
The system of claim 1, wherein a connector between the CFB combustor and the cyclone is formed so as to be inclined at an angle.
The system of claim 1, wherein the second loop seal includes a second ventilation unit at one side thereof so as to transport the collected sand to the gasifier.
The system of claim 1, wherein the second loop seal is U-shaped.
The system of claim 1, wherein when a sand bed of the gasifier is equal to or less than a predetermined level, the sand of a sand bed of the second loop seal is supplied, and when the sand bed of the gasifier and the sand bed of the second loop seal are equal to or less than a predetermined level, sand is supplied via the sand supply unit.
The system of claim 1, further comprising a hot gas retention pipe which receives gas discharged from the cyclone so that steam is subjected to steam reforming with the syngas for a sufficient retention time to thus produce a reformed syngas with less tar and more hydrogen.