CN107001090B

CN107001090B - Gasification system with water treatment and reuse

Info

Publication number: CN107001090B
Application number: CN201580067771.5A
Authority: CN
Inventors: A.巴塔查亚; N.沃拉; S.K.科塔; V.沙; U.贾特雷迪
Original assignee: BL Technologies Inc
Current assignee: BL Technology, Inc.
Priority date: 2014-12-12
Filing date: 2015-12-10
Publication date: 2022-02-18
Anticipated expiration: 2035-12-10
Also published as: WO2016094612A1; CN107001090A

Abstract

In gasification systems, desalination units are used to treat the main grey water stream. The desalination unit is preferably an evaporator. The desalination unit removes chlorides and produces a main recycled grey water stream having a total dissolved solids concentration of 100ppm or less. In conventional gasification systems, a desalination unit replaces the grey water settler. The gasification system operates at a water recirculation loop blowdown rate of 10% or less.

Description

Gasification system with water treatment and reuse

Technical Field

The present application relates to water treatment and reuse in gasification systems.

Background

In a typical gasification process, a feedstock, such as coal, is pulverized and fed to a reaction zone of a gasifier. An entrained flow gasification system (entrained flow gasification system) may feed the pulverized feedstock as a dry powder or mixed with water to form a slurry. Other gasification systems, such as fixed bed or fluidized bed systems, typically feed pulverized feedstock as a dry powder. In the reaction zone, the feedstock is heated with a sub-stoichiometric amount of oxygen. The reaction produces syngas (syngas is composed primarily of carbon monoxide and hydrogen), as well as slag, soot, and various contaminants. In a typical gasification reactor, the reaction products enter the quench zone (quenching zone) of the reactor where they are contacted with water. The reactor produces a quench water blowdown stream and a syngas. The syngas flows from the gasifier to the syngas scrubber along with some contaminants. In the syngas scrubber, the syngas is contacted with scrubbing water. The scrubbing water entrains fine solids and volatiles in the syngas. The syngas scrubber produces a product syngas and a syngas scrubber blowdown stream.

Some water may be recovered from the syngas scrubber blowdown in one or more flash vessels. At least a portion of the syngas scrubber blowdown, such as the flash vessel bottoms, is considered black water. In systems using water quenched gasification reactors, some unvaporized feedstock may be extracted along with some water from the quenched water blowdown to carry solids. At least a portion of the quench water blowdown is also considered black water. The black water is typically treated in a settler to produce an effluent, which may be referred to as grey water. A portion of the grey water is returned to the syngas scrubber or gasifier, forming a primary water recirculation loop. Another portion of the grey water is removed from the system as a grey water blowdown stream and replaced with make-up water to prevent excessive concentration of one or more contaminants in the primary water recirculation loop.

The grey water blowdown stream contains various contaminants and must be treated before it is discharged, stored in a brine pond or deep well disposal, or processed to produce a zero liquid discharge. Some typical treatments include ammonia stripping, biological removal of organics, and solid-liquid separation, such as nanofiltration or reverse osmosis. Optionally, one or more of these treatment steps may produce treated water, which may be returned to the gasifier or syngas scrubber in one or more make-up water recycle loops. The settler bottoms can be treated, which can recover more water in the make-up water recycle loop, or the settler bottoms can be returned to the gasifier as a feedstock, or both.

Disclosure of Invention

Gasification systems and methods having a desalination step in a primary water recycle loop are described. The black water is treated to remove chlorides and other dissolved solids, optionally among other contaminants, before recycling the treated water to the syngas scrubber or the gasifier or both. The total dissolved solids of the water for which the desalination step continues in the main water recycle loop is preferably 100ppm or less. The chloride concentration of the water for which the desalination step continues in the main water recycle loop is preferably 10ppm or less. In one example, the desalination step is provided by a thermal evaporator.

a) Substantially all (e.g., at least 80%) of the black water, or b) a majority (at least 50%) of the quench water blowdown, or c) a majority (at least 50%) of the syngas scrubber blowdown, is treated in a primary water recycle loop with a thermal evaporator or other desalination unit. Desalted water, such as condensate from the evaporator, continues in the primary water recycle loop. The water treated in the desalination unit preferably comprises quench water blowdown.

The evaporator or other desalination unit is optionally a primary black water treatment unit for dissolved solids. In this sense, the desalination unit can be considered as an aspect of replacing the black water settler in conventional gasification systems, allowing the settler (or other solid-liquid separation device) to be designed to operate primarily for soot removal. Optionally, the desalination unit may also allow one or more flash vessels in conventional gasification systems to be omitted. Evaporators are more expensive to build and operate than settlers. It may be that for this reason the evaporator has previously only been considered to treat grey water blowdown. Even though some water from the grey water blowdown treatment may be returned to the gasification system, the grey water blowdown treatment system is not typically considered part of the primary water recirculation loop of the gasification system. In one embodiment, the desalination unit processes black water within the gasification system.

In one method, chlorides are removed from the recycle water in the gasification system. For example, chlorides may be removed in a bleed stream from a desalination unit. The chloride removal ratio can be selected to maintain a selected equilibrium chloride level in the water recycled from the gasifier quench and/or scrubber in the gasification system. The selected equilibrium chloride concentration may be 500ppm or less, for example in the range of 150 to 200 or 300 ppm. The equilibrium chloride concentration may be selected in the design of the system or may be selected as a set point or other value in a controller or control process associated with the desalination unit.

When an evaporator is added to the primary water recirculation loop, the primary water recirculation loop blowdown ratio is reduced. Despite the reduced blowdown ratio, the equilibrium chloride concentration in the water circuit is also reduced because the evaporator produces a blowdown brine having a high concentration of dissolved solids. The reduced chloride concentration in the primary water recycle loop may reduce scaling and corrosion in the gasification system and reduce the syngas dew point. Both the need for make-up water and the cost of treating the water recirculation loop blowdown are reduced. In one embodiment, the gasification system makeup water requirement is reduced by more than 25% relative to a comparative system having only a settler.

The gasification system described in this application operates with a primary water recirculation loop blowdown ratio of 10% or less black water. However, in the main black water treatment stream, the Total Dissolved Solids (TDS) concentration in the main water recycle loop is maintained at 100ppm or less by using a desalination unit, such as a thermal evaporator. At least one third, but optionally all (e.g., 80% or more), of the water recycled to the gasification reactor for quench water or the syngas scrubber, or both, has a TDS concentration of 100ppm or less, or a chloride concentration of 10ppm or less, or both.

Drawings

FIG. 1 is a process flow diagram of a gasification system.

Detailed Description

FIG. 1 shows a gasification system 10. The system has an entrained flow gasifier 12 that receives oxygen A and slurry B. Oxygen A is optionally provided as air to the gasifier 12 in sub-stoichiometric amounts. The gasifier 12 may operate, for example, at about 1250 ℃ to 1600 ℃, at a pressure of about 30 to 80 atmospheres, and with a residence time of about 0.4 to 2 seconds. Slurry B in the illustrated example is a coal slurry produced by feeding coal C and water Q to a grinder 14, a grinder discharge tank 16 and a slurry tank 18. However, the gasification system 10 may also be configured to gasify any other hydrocarbonaceous material, such as wood, waste plastic, or petroleum coke. Further, the gasifier 12 may be fed with dry feedstock instead of slurry B. Alternatively, fixed bed or fluidized bed gasifiers may be used, which are typically operated with dry feedstock and at lower temperatures and pressures, but longer residence times, than entrained flow gasifiers. The gasifier 12 produces syngas E, quench water blowdown R, and gasifier bottoms D. The syngas E is treated in a syngas scrubber 20 to produce a product syngas F, which is typically further processed before being used as fuel. The syngas scrubber 20 also produces a scrubber blowdown G. The gasifier bottoms D is processed in a lockhopper 28 (otherwise known as a bucket locker) to separate slag H from bottoms water I. The slag H (which still has some water mixed with it) is further separated into coarse slag J and fine slag K by the scraper conveyor 30. In another option, the gasifier 12 may be combined with a Radiant Syngas Cooler (RSC). Generally, the raw syngas exiting the gasifier 12 may be cooled by radiant and/or convective heat exchangers and/or by a direct quench system, wherein water or cooled recycle gas is injected into the hot raw syngas. The syngas may be passed through one or more flash vessels for heat recovery and cooling of the syngas.

The quench water blowdown R flows through the high pressure flash drum 22 and the low pressure flash drum 24 along with the scrubber blowdown G. The flash tank bottoms S flows through the vacuum flash tank 26 along with the fine slag K (along with some water mixed therewith) to form black water L. In some cases, a second vacuum flash drum may be added. Although some water is removed in the flash drum 22 and the flash drum 24, a majority (at least 50%, typically 80% or more) of the quench water blowdown R and a majority (at least 50%, typically 80% or more) of the scrubber blowdown G are present in the black water L. Most (at least 50%, usually 80% or more) of the water in the slag H is also present in the black water L. In the alternative, no more than half of the water (i.e., no more than half of the combination of quench water blowdown R and scrubber blowdown G) is removed in any single treatment unit upstream of the black water L. Thus, the black water L is considered part of the primary water recirculation loop in the gasification system 10. In the illustrated gasification system 10, the primary water recycle loop is made up of stream G, R, streams S, L, M, N, O and G between flash drums, and

intermediate processing units

20, 12, 2, 24, 26, 30, 34, 38, and 40.

Optionally, black water L is treated in a solid-liquid separator 30 (otherwise known as a soot separator) to produce a solids fraction M and a pretreated black water N, otherwise known as grey water. The solid-liquid separator 30 (if present) is primarily used to recover soot or slag. The solid fraction M contains small particles of slag and soot, which can be added to the slurry B to increase the production of syngas. If the solid-liquid separator 30 is used, the pretreated black water N preferably includes 80% or more, or 90% or more of the black water L. One third or more, or optionally all, of the black water N or L in the main water recycle loop is continuously fed to the desalination unit 34. The desalination device 34 produces an effluent O and a brine P.

The solid-liquid separator 30 is preferably a hydrocyclone or centrifuge, but may also be a filter, a gravity separator (alternatively referred to as a settler or clarifier), or an axial separator or other device that produces a dewatered solids fraction M. Such as between a hydrocyclone and a centrifuge, a hydrocyclone is preferred. Depending on the type of solid-liquid separator 30 used, the solid fraction M may be difficult to pump. In this case, the solid fraction M may be transferred to the soot tank 32 and diluted with some effluent O. The diluted soot Q may then be transported to the grinder 14 using a transfer pump Q to be combined into slurry B. Some effluent O may also be sent to lock hopper 28. A flocculant or coagulant, such as a polymeric flocculant in an amount of 1-10ppm, may be added to the black water L treated in the solid-liquid separator 30.

The majority of the effluent O is recycled to the syngas scrubber 20 or the gasifier 12, optionally after passing through a deaerator 38 and a heat exchanger 40. If all of the effluent O has passed through a desalination device 34 (e.g., an evaporator) that produces a degassed effluent O, then the deaerator 38 is not required. Some of the effluent O may pass directly or through the syngas scrubber 20 to the gasifier 12 for use as gasifier quench water U. In this way, the effluent O replaces at least a portion of the material, for example, one third to all of the conventional recycle of non-desalinated grey water from the settler. For sufficient flow through the desalter 34, the gasification system 10 may only require one vacuum flash drum upstream of the settler.

Optionally, a bypass line V may be present around the desalter 34. The bypass line V may have a blowdown outlet W and various valves not shown. In one option, the bypass line 34 is normally kept closed, but is temporarily opened when needed to service the desalination device 34. In another option, the bypass line V is used to provide a path in the primary water recirculation loop in parallel with one or more desalination units 34. For example, if an existing gasification system is to be retrofitted in a main water recirculation loop with two or three desalination units 34 assembled in parallel, the bypass line V may continue to be used, with only some, i.e., one or two, of the final number of desalination units 34 installed. After all of the desalination units 34 have been installed, the bypass line V may be normally closed, but temporarily opened when needed, in order to service the desalination device 34 instead of providing excess capacity of the desalination units 34. In another option, the gasification system may operate normally with only a portion of the black water L or N passing through the desalination device 34. In this option, at least one third, preferably at least half, of the black water L or N is passed through the desalination device 34. At some point below the one-third threshold, the amount of water flowing through the desalination device 34 will not substantially affect the pH or chloride concentration of the recycled water and is not considered part of the primary water recycle loop. In the case where the black water L or N flows through the one or more desalters 34 and the bypass line V simultaneously or when the black water L or N flows through the one or more desalters 34 and the bypass line V simultaneously, the blowdown outlet W is preferably kept closed so that all blowdown from the primary water recycle loop is extracted as saltwater P. In this way, blowdown from the primary water recycle loop is still recovered at a higher chloride concentration than in the black water L or N, and the volume of makeup water required is correspondingly reduced.

Having at least one desalination device 34 in the primary water recycle loop, with or without a bypass line V, allows for adjustment of the chloride concentration in the effluent O by, for example, changing the ratio of effluent O to brine P or changing the flow in the bypass line V. The desalination device 34, particularly an evaporator, can also help manage the ammonia concentration in the primary water recirculation loop. In one approach, the chloride level in the effluent O flowing into the syngas scrubber 20 is adjusted to provide a syngas dew point (otherwise known as a hydrocarbon dew point) within a desired range, such as below 250 ℃, or within a range of 230 ℃ to 240 ℃. The syngas dew point increases with chloride concentration. Maintaining a reasonably low syngas dew point may, for example, allow for the use of lower cost alloys in the syngas scrubber 20 or other components.

The desalination device 34 is preferably a thermal evaporator. The evaporator may be, for example, a vertical tube falling film evaporator with Mechanical Vapor Compression (MVC), or referred to as Mechanical Vapor Recompression (MVR). Alternatively, the evaporator may be a steam driven evaporator, optionally using exhaust steam from the gasification system 10, such as exhaust steam from a jacket around the gasifier 12. The evaporator may be operated with a seeded slurry process to reduce fouling in the evaporator. In one example of a seeded slurry process, calcium sulfate crystals are promoted to form in the evaporator. The evaporator may be seeded with calcium sulfate seeds or, although the phrase is expressed as "seeded slurry," crystals may form spontaneously from compounds in the evaporator feed water even without the addition of seeds. In some cases, a compound such as sodium sulfate or calcium chloride is added to the water in the evaporator (e.g., by adding the compound to the feed water or to the evaporator sump) to alter the relative concentrations of calcium and sulfate ions to promote crystal formation. The effluent O from the evaporator may also be referred to as condensate or distillate. The effluent O preferably has less than 100ppm Total Dissolved Solids (TDS) and less than 10ppm, or less than 5ppm, chloride. The low chloride concentration in effluent O may allow for the use of less corrosion resistant (and therefore less expensive) materials in gasification system 10 as compared to conventional practice. The high chloride concentration in the brine P may allow for the removal of less blowdown (brine P) from the primary water recycle loop or for the addition of correspondingly less make-up water T, as compared to conventional practice. The low TDS concentration may reduce scaling problems in the gasification system 10. Including overheads from flash drums 22, 24 and 26, preferably all scrubber blowdown water or gasifier quench water is recycled at less than 100ppm TDS and less than 10ppm chlorides, although optional.

The brine P preferably has a flow rate that is 10% or less of the flow rate of one or more of the black water L, the pretreated black water N, or the effluent O. Flash drums 22, 24, and 26 are each optional. However, at least one flash drum may be used to remove non-condensable gases such as H₂And S. The brine P may optionally be further processed in a crystallizer (e.g., with a forced circulation evaporator), spray dryer, brine concentrator, or other reduced or zero liquid discharge system. Alternatively, the brine P may be removed for disposal or further processing off-site. The brine P flow rate or other parameters affecting the removal of chloride from the desalter 34 can be designed or actively controlled to provide a selected chloride concentration in the effluent O or a selected equilibrium chloride concentration in the water recycle loop, for example as measured in the flash drum bottoms S. The selected equilibrium chloride concentration may be in the range of 500ppm or less, for example 150 to 200 or 300 ppm. The selected equilibrium concentration may be achieved during the design of the gasification system 10 and its operation, or the selected equilibrium concentration may be achieved in real time, for example by connecting a chloride concentration sensor to a controller for the desalination device 34 or by otherwise adjusting the operation of the desalination device 34 as needed so that the brine P removes enough chloride to keep the equilibrium chloride concentration below a selected value or within a range that includes the selected value.

In the modeling example of the coal gasification system shown in FIG. 1, the black water has 120m³Flow rate per hour. The black water was pretreated to remove soot, which removed 10m ³10% solids soot mixture per hour. Processing the remaining pre-treated black in a vertical falling film mechanical vapor compression (MVR) evaporatorAnd (3) water. Yield 105m³The distillate per hour is used to return to the syngas scrubber or gasifier. The distillate had a TDS concentration of about 50ppm, a chloride concentration of less than 1ppm and a pH of 8.5. To produce 5m³An hourly blowdown having a TDS concentration of about 75,000ppm and a pH of about 5. The equilibrium chloride concentration in the low pressure flash drum bottoms is less than 150 ppm. The system being modeled is generally as shown in fig. 1.

The soot removal factor of the model was predicted based on processing a sample of black water in a laboratory scale centrifuge. The black water is obtained from a coal gasification plant that supplies a coal slurry to an entrained flow gasifier. The black water contains a mixture of gasifier quench water blowdown and syngas scrubber blowdown. The centrifuge was run at 500rpm with a 5 minute residence time and no chemical additives.

Optionally, coagulants or flocculants may be used to reduce residence time. Polymeric flocculants are preferred, without additional metal salt coagulants or flocculants. For example, in some later experiments, a cationic polyacrylamide compound was added to black water in a centrifuge. Similarly, when treating black water in another solid-liquid separation device, a coagulant or flocculant, preferably a polymeric flocculant, may be added. A preferred flocculant is a cationic polyacrylamide flocculant, such as BetzDearborn CP1153 available from GE Water & Process Technologies. The dosage may be 1 to 10ppm or 3-5ppm of the polymeric flocculant. In the black water test in a laboratory scale centrifuge as described above, the clear effluent had a turbidity of 58NTU after 20 minutes of treatment at 1000rpm in the centrifuge without addition of flocculant. For CP1153 at 3ppm, the clarified effluent had a turbidity of about 9NTU after 3 minutes of treatment at 1000rpm in the centrifuge. For CP1153 at 5ppm, the clarified effluent had a turbidity of about 4NTU after 3 minutes of treatment at 1000rpm in the centrifuge. The Total Suspended Solids (TSS) concentration of the clarified effluent for the black water samples centrifuged with 3ppm CP1153 and 5ppm CP1153 was about 1 mg/L.

The evaporator performance factor for this model was based on distilling the above centrifuge effluent in a bench scale distillation unit to produce a 95% distillate recovery. Sodium sulfate was added to the centrifuge effluent to simulate the addition of sodium sulfate to the feed water, which would be appropriate for a calcium sulfate seeded slurry thermal evaporator. The pH of the centrifuge effluent was 7.4. After 95% recovery, the pH of the brine in the bottom of the distillation flask was about 5.

The expected loss of ammonia gas during distillation has resulted in a reduction of the pH of the brine to less than 3 after 95% recovery, based on the ammonium concentration in the evaporator feed water. This suggests that expensive corrosion resistant materials would be required in the evaporator and that fouling may make the operation of the evaporator unreliable. However, the pH of the brine remained stable at about 5 over the 18 day test with the seeded slurry evaporator, and no unusual fouling was observed.

It is uncertain why the pH of the brine is maintained at about 5. One possibility is that formic acid in the feed water of the evaporator produces ammonium formate (NH) in the brine₄COOH) and thus reduce the amount of volatile ammonia vapor. Another possibility is to combine sodium with formate or other compounds, either initially in the feed water or added to simulate the seeded slurry process, to produce a buffering compound. Formate is produced in the gasifier from the gasification of coal (and possibly other feedstocks) because the reaction products are quenched in the range of about 250 to 300 degrees celsius. Accordingly, the inventors predict that a reduced change in pH during distillation (i.e., observed brine pH of 5 rather than predicted brine pH of 3) is likely to occur at least in any process where gasifier quench water is part of the black water treated in the evaporator.

The brine was further boiled and produced a loose white solid composed primarily of calcium sulfate, suggesting that the brine was suitable for the seeded slurry evaporator process in the evaporator and that the spent brine could be crystallized in a zero liquid discharge system.

In a comparative example, a gasification plant was modeled, wherein the centrifuge and evaporator in the above model were replaced with an additional vacuum flash drum and a conventional settler. The settler produced an effluent with almost 300ppm chloride and a pH of 7. Ash water discharge rate of 27m³In terms of hours. The equilibrium chloride concentration in the low pressure flash drum bottoms is in excess of 250 ppm. The comparison system required about 40% more make-up water.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A method of treating water in a gasification facility, the method comprising the steps of:

desalinating syngas scrubber blowdown or quench water blowdown, or both, in a primary water recycle loop of the gasification plant including a brine concentrator, wherein the primary water recycle loop does not include a grey water blowdown treatment system, and the desalinating occurs in the brine concentrator; and

recycling the desalinated water in the primary water recycle loop to a syngas scrubber or a gasifier quench water stream of the gasification plant.

2. The method of claim 1, wherein at least half of the quench water blowdown is desalted.

3. The method of claim 1, wherein at least half of the syngas scrubber blowdown is desalted.

4. The process of claim 1, wherein the desalinated water recycled to the syngas scrubber or the gasifier quench water stream has a chloride concentration of less than 10 ppm.

5. The method of claim 1, wherein the desalted water recycled to the syngas scrubber or the gasifier has a total dissolved solids concentration of 100ppm or less.

6. The method of claim 1, wherein 80% or more of the black water produced in the gasification plant is desalinated.

7. The method of claim 1, wherein the brine concentrator comprises a thermal evaporator.

8. The method of claim 7, wherein the thermal evaporator is operated with seeded slurry.

9. The method of claim 7, wherein the thermal evaporator is operated with a calcium sulfate slurry.

10. The method of claim 1, further comprising a soot removal step prior to the desalting step, wherein the soot removal step comprises adding a polymer flocculant.

11. The method of claim 1, wherein the feed water to the desalting step is adjusted to promote calcium sulfate crystal formation.

12. The method of claim 1, wherein the feed water to the desalting step comprises gasifier quench water blowdown.

13. The method of claim 1, comprising the step of adding sodium to the desalting step.

14. The method of claim 1, wherein the gasifier is an entrained flow gasifier.

15. The method of claim 1, wherein the gasifier is fed coal.

16. The method of claim 1, wherein the gasifier is fed a coal slurry.

17. The method of claim 1, further comprising forming ammonium formate during the desalting step.

18. A gasification system, comprising:

a primary water recycle loop, wherein water is drawn from a gasifier or a syngas scrubber or both, and returned within the primary water recycle loop to the gasifier quench water inlet or a syngas scrubber or both, wherein the primary water recycle loop does not include a grey water blowdown treatment system; and the number of the first and second groups,

a brine concentrator in the primary water recirculation loop.

19. A gasification system in accordance with claim 18 wherein said brine concentrator comprises a thermal evaporator.

20. A gasification system in accordance with claim 18 wherein said gasifier is an entrained flow gasifier.

21. The gasification system of claim 20 wherein the gasifier receives coal.

22. A gasification system in accordance with claim 20 wherein said gasifier receives a coal slurry.

23. A gasification system in accordance with claim 18 wherein said brine concentrator receives gasifier quench water blowdown.

24. A gasification system in accordance with claim 18 wherein at least a majority of blowdown from said primary water recirculation loop is removed by said brine concentrator.