JP6488364B2 - Supercritical water gasification system - Google Patents

Description

  The present invention relates to a supercritical water gasification system and a supercritical water gasification method for generating fuel gas by decomposing a slurry body prepared by adding water and a catalyst to biomass in a supercritical state.

  In recent years, in the technology of gasifying hydrous biomass (shochu residue, egg-collecting chicken manure, organic sludge, organic sewage, etc.) with supercritical water, the heat of the product obtained when gasifying the biomass with supercritical water is reduced. A supercritical water gasification system including a double-pipe heat exchanger that heats a hydrous biomass gasified with supercritical water or a slurry body of the biomass has been developed. 1 and 2).

  Here, a general biomass gasification system includes a heat exchanger, a heater, a gasification reactor, and the like, and converts organic matter into hydrogen, methane, ethane by hydrolysis reaction with supercritical water with high reactivity. -Gasify to carbon monoxide, carbon dioxide, etc. For example, a heat exchanger is a device that heats a slurry-like slurry body. This slurry body is adjusted by adding and mixing water, activated carbon, etc. with biomass, such as shochu residue, egg-collecting chicken droppings, organic sludge, and organic sewage. In the supercritical water gasification reaction, activated carbon is used as a catalyst. The heater is a device that raises the temperature of the slurry heated by the heat exchanger to a gasification reaction temperature of 600 ° C. The gasification reactor is a device that hydrothermally processes the slurry to gasify the organic matter to produce a supercritical high-temperature fluid. The fluid in the supercritical state is then separated into a gas component and a liquid component in a gas-liquid separator, and the gas component is used as a fuel gas.

JP 2007-271146 A JP 2009-242697 A

  However, in the supercritical water gasification system as described above, a viscous fluid derived from glucose (such as tar) is generated in the temperature range of 250 ° C. to 350 ° C. during gasification, and in the temperature range of around 400 ° C. Produces a lignin-derived viscous fluid (tar, etc.). And when the tar adheres and accumulates on the inner wall surface of the heat exchanger or heater pipe, it causes clogging, clogging due to that, or a problem such as a decrease in heat transfer rate. There is.

Therefore, conventionally, as a technique for suppressing tar production, it is effective to improve the rate of temperature rise for glucose-derived tar, and for lignin-derived tar, a radical scavenger is mixed into the raw material, It is considered effective to perform high-pressure injection during heating.
However, even if these methods are used, tar generation itself cannot be completely eliminated or tar cannot be decomposed, so that the generated tar is completely prevented from attaching to the pipe and increasing its thickness. I can't do it.

  Therefore, the present inventors paid attention to preventing the blockage of the flow path due to the above-described viscous fluid such as tar causing clogging in the piping of a heating device such as a heat exchanger.

  The present invention has been made in view of the above problems, and decomposes and removes the viscous fluid adhering to the inner wall surface of a pipe of a heating device such as a heat exchanger, and the pipe flow path caused by the clogging with the viscous fluid. A supercritical water gasification system and a supercritical water gasification method that can prevent clogging and a decrease in heat transfer rate and increase equipment utilization by reducing continuous operation time and reduce equipment costs. The purpose is to provide.

In order to solve the above problems, a supercritical water gasification system according to the present invention is:
A gasification reactor for gasifying a slurry body made of a raw material containing biomass with supercritical water, and heating for raising the temperature of the slurry body before being gasified with supercritical water in the gasification reactor A supercritical water gasification system comprising:
A piping system for supplying the slurry body to the heating unit and the gasification reactor;
A tar adhering section specifying portion for specifying a section in which tar has adhered in the piping system;
An injection device for injecting an oxidant not containing inorganic substances into the specified section;
Switching means for switching the supply to the piping system from the slurry body to water when a section to which tar has adhered is identified by the tar adhesion section identifying section;
A controller that controls the injection device to supply the oxidant to the specified section by the injection device after the piping system is replaced with water ;
The tar adhesion section specifying part is
A differential pressure measuring unit that measures differential pressures at a plurality of different predetermined positions in the piping system,
When the differential pressure between the adjacent predetermined positions measured by the differential pressure measuring unit is equal to or greater than a reference value, the section between the adjacent predetermined positions is specified as a section where tar is attached,
When the differential pressure decreases to a reference value, the control unit stops supplying the oxidant by the injection device,
The switching means comprises switching the supply to the piping system from water to the slurry after the supply of the oxidant is stopped .

Furthermore, in the supercritical water gasification system according to the present invention,
The controller is
When it is detected that a predetermined time has elapsed since the supply to the piping system is switched to water, it may be determined that the piping system has been replaced with water.

Furthermore, in the supercritical water gasification system according to the present invention,
The controller is
When it is detected that a predetermined amount has been injected after the supply to the piping system is switched to water, it may be determined that the piping system has been replaced with water.

Furthermore, in the supercritical water gasification system according to the present invention,
The oxidizing agent is preferably hydrogen peroxide, hypochlorous acid, nitric acid, or ozone.

Further, the supercritical water gasification method according to the present invention comprises:
A process of gasifying a slurry produced from a raw material containing biomass with supercritical water in a gasification reactor, and heating the slurry before being gasified with supercritical water in the gasification reactor A supercritical water gasification method comprising a step of raising the temperature in a part,
In the piping system for supplying the slurry body to the heating unit and the gasification reactor, the differential pressures at different predetermined positions in the piping system are measured,
In a piping system for supplying the slurry body to the heating unit and the gasification reactor, a section where tar adheres is specified,
When a section to which tar is attached is identified, the supply to the piping system is switched from the slurry body to water,
After the piping system is replaced with water, an oxidant that does not contain inorganic substances is injected into the specified section .
When the differential pressure decreases to a reference value, the supply of the oxidant is stopped,
After the supply of the oxidizing agent is stopped, the supply to the piping system is switched from water to the slurry .

Furthermore, the supercritical water gasification method according to the present invention comprises:
When a section where tar adheres is identified in the piping system, the supply to the piping system is switched from the slurry body to water,
It is preferable that the oxidant is supplied to the specified section after the piping system is replaced with water.

  According to the supercritical water gasification system and supercritical water gasification method according to the present invention, the viscous fluid adhering to the inner wall surface of the pipe of the heating device such as a heat exchanger is decomposed and removed, and clogged by the viscous fluid. It is possible to avoid the blockage of the piping channel and the decrease in the heat passage rate due to this, and it is possible to increase the equipment utilization rate and reduce the equipment cost by increasing the continuous operation time.

It is a figure which shows schematic structure of the supercritical water gasification system which concerns on one Embodiment of this invention. It is a flowchart which shows the tar removal treatment procedure demonstrated as one Embodiment of this invention.

  Hereinafter, embodiments of a supercritical water gasification system according to the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention can be appropriately changed without departing from the spirit or idea of the invention which can be read from the claims and the entire specification, and a supercritical water gasification system with such a change is also applicable to the present invention. It is included in the technical idea.

<Overall configuration of supercritical water gasification system according to the present invention>
FIG. 1 is a diagram showing a schematic configuration of a supercritical water gasification system 10 described as an embodiment of the present invention. In the following description, the “supercritical water gasification system” is simply referred to as “gasification system”.

  As shown in FIG. 1, in the gasification system 10 according to the present invention, a slurry body is prepared from biomass such as shochu residue, egg collection feces, organic sludge, organic sewage and the like as raw materials, and the prepared slurry body is heated. Combustion gas is generated by pressing. As shown in the figure, the gasification system 10 includes a raw material preparation unit 20, a heat treatment unit 30, and a gas processing unit 40.

  The raw material preparation unit 20 is a part that prepares a slurry body from biomass. The raw material preparation unit 20 includes a preparation tank 21, a pulverization pump 22, a supply pump 23, and a heat exchanger introduction pump 24.

  The preparation tank 21 is a container for mixing biomass, water, and activated carbon (a kind of nonmetallic catalyst). In this preparation tank 21, a mixture as a gasification raw material in which biomass, water and activated carbon are mixed is prepared. For example, porous particles having an average particle diameter of 200 μm or less are used as the activated carbon. The mixing ratio of biomass, water, and activated carbon is appropriately adjusted according to the type, amount, moisture content, and the like of biomass.

  The crushing pump 22 is an apparatus for crushing the solid content of the mixture prepared in the preparation tank 21 to obtain a uniform size (preferably an average particle size of 800 μm or less, more preferably an average particle size of 500 μm or less). is there. By being processed by the pulverization pump 22, the mixture becomes a slurry-like slurry body.

  The supply pump 23 supplies the slurry discharged from the pulverization pump 22 to the heat exchanger introduction pump 24. The heat exchanger introduction pump 24 pressurizes the slurry body sent from the supply pump 23 and supplies it to the heat treatment unit 30. The slurry body is pressurized to about 22.1 to 38 MPa by the heat exchanger introduction pump 24.

  Here, the adjustment tank 21 is not shown in the figure, but is formed in a substantially cylindrical shape with an upper opening and a conical lower portion, and a predetermined amount of biomass, water, and nonmetallic catalyst are contained therein. It is preferable that etc. can be stored. In addition, it is preferable that a stirring means for stirring the biomass is detachably provided inside the adjustment tank 21, and the biomass, water, and the nonmetallic catalyst are stirred and mixed in the adjustment tank 21 by the stirring means.

  The mixture mixed in the adjustment tank 21 is fed from the adjustment tank 21 to the crushing pump 22 described later by the head pressure of water or biomass. At this time, the biomass that is so large that it does not deserve to be mixed is difficult to adjust the above-mentioned mixing amount, and is therefore fed to the crushing pump 22 described later without being mixed.

  Due to the rotation of the crushing blades (not shown) of the crushing pump 22, the mixture and biomass supplied from the adjustment tank 21 to the inside of the casing (not shown) of the crushing pump 22 via the suction passage (not shown) have a predetermined size. It is crushed. Then, the mixture and biomass crushed by the crushing pump 22 are transferred to the supply pump 23 via a raw material supply pipe (not shown).

  A crushed raw material tank for storing the gasified raw material crushed to a predetermined size is separately provided, and crushed by the crushing pump 22 in the middle of a raw material supply pipe connecting the crushing pump 22 and the crushed raw material tank. A circulation pipe for returning the mixture and biomass to the adjustment tank 21 is provided, and the mixture and biomass are circulated between the crushing pump 22 and the adjustment tank 21 via the circulation pipe 16 to reduce the particle size of the biomass, and then the crushed raw material tank You may comprise so that it may supply to the supply pump 23 via.

  At this time, a first on-off valve as an outlet valve of the crushing pump 22 is provided in the middle of the circulation pipe, and a second on-off valve as an inlet valve of the crushed raw material tank is provided on the raw material supply pipe on the inlet side of the crushed raw material tank. Providing a first on-off valve in an “open” state and a second on-off valve in a “closed” state to circulate the mixture and biomass between the crushing pump 22 and the adjustment tank 21 via a circulation pipe. Is possible. Further, by setting the first on-off valve in the “closed” state and the second on-off valve in the “open” state, it becomes possible to supply the mixture and biomass with reduced particle size to the crushed raw material tank.

  The heat treatment unit 30 is a part that heats and gasifies the slurry body prepared by the raw material preparation unit 20. The heat treatment unit 30 includes a heat exchanger 31 as a heating device, a heater 32, and a gasification reactor 33.

  The heat exchanger 31 is a device that heats the slurry body sent from the raw material preparation unit 20. The heat exchanger 31 is, for example, a double tube heat exchanger including a double tube, and includes a low temperature channel 36 and a high temperature channel 37. The slurry body sent from the raw material preparation unit 20 circulates in the low temperature flow path 36. On the other hand, a supercritical high-temperature fluid (hereinafter also referred to as a post-treatment fluid) generated in a gasification reactor 33 described later is introduced into the high-temperature channel 37, and this post-treatment fluid flows. Then, heat is exchanged between the treated fluid flowing in the high temperature channel 37 and the slurry body flowing in the low temperature channel 36.

  The heater 32 is a device that heats the slurry body sent from the heat exchanger 31. The heater 32 includes a combustion device 32a. The combustion device 32a introduces and burns the combustion gas sent from the gas processing unit 40 into liquefied petroleum gas such as propane gas or air, and heats the slurry body. Thereby, the temperature of the slurry introduced into the heater 32 is raised to, for example, about 600 ° C. The heated slurry body is sent to the gasification reactor 33.

  The gasification reactor 33 keeps the slurry body sent from the heater 32 in a supercritical state, and keeps it constant at the gasification reaction temperature (temperature at which the slurry body is gasified (gasification is possible)). This is an apparatus for hydrothermally treating an organic substance contained in a slurry body. The gasification reactor 33 includes a combustion device 33a. The combustion device 33a introduces combustion gas sent from the gas processing unit 40 into liquefied petroleum gas, mixes it with air, burns it, and heats the slurry body. Then hydrothermally treat. In this hydrothermal treatment, the slurry body is hydrothermally treated for 1 to 2 minutes under conditions of, for example, 600 ° C. and 25 MPa. The hydrothermally treated slurry body undergoes a gasification reaction and then is sent to the high-temperature channel 37 of the heat exchanger 31 as a post-treatment fluid.

  Gasification of biomass with supercritical water can be carried out under the conditions of a temperature of 374 ° C. or higher and a pressure of 22.1 MPa or higher using the above-mentioned non-metallic catalyst. It is preferable to carry out at a temperature and pressure (in the range of 600 ° C. or higher and 22.1 to 35 MPa) that can suppress and increase the carbon gasification rate. By treating the biomass with supercritical water in this way, the biomass can be decomposed and a fuel gas such as hydrogen gas, methane, ethane, or ethylene can be generated.

  Here, in the conventional gasification system, the tar generated at the time of gasification adheres and accumulates on the inner wall surface of the pipe of the heat exchanger or the heater, resulting in clogging, resulting in clogging, In some cases, troubles such as a decrease in the heat transfer rate may occur.

  Therefore, in the case of the present embodiment, the inlet and outlet of the heat exchanger 31 (specifically, the heat exchanger 31 is divided into three sections, each of which is configured by a U-shaped piping system arranged sideways. In addition, differential pressure measuring units 34a to 34c are provided for measuring the differential pressure between the inlet and outlet of the intermediate section. Further, differential pressure measuring units 35a and 35b for measuring the differential pressures at the inlet and outlet of the heater 32 and the gasification reactor 33 are provided. That is, a differential pressure measuring unit is provided that measures a differential pressure in a section between predetermined locations adjacent to a plurality of predetermined locations in the piping system that supplies the slurry to the heat exchanger 31 and the heater 32. If there is a section where the differential pressure measured by the differential pressure measuring units 34a to 34c, 35a, 35b is higher than a reference value based on the differential pressure when there is no adhesion of tar or the like, the tar is included in the section. It is determined that is attached. In this case, after substituting the slurry body circulating in the gasification system 10 with pure water, the injection device 50 is used until a predetermined time or a differential pressure is reduced to a reference value at a position where the measurement result is increased. A tar removing process is performed by injecting an oxidizing agent (for example, hydrogen peroxide, hypochlorous acid, nitric acid, etc.) that does not contain an inorganic substance through the pump 51.

Specifically, as shown in FIG. 2, in the tar removal processing procedure, the measurement results by the differential pressure measurement units 34a to 34c, 35a, and 35b are observed (SP1: No), and the measurement results rise from the reference value. (SP1: Yes), the slurry body which is the gasification raw material currently distribute | circulating in the gasification system 10 is substituted to a pure water (SP2). Then, the passage of a predetermined time or the flow rate / condition in the gasification system 10 is observed (SP3: No), and when the above-described replacement with pure water is completed (SP3: Yes), the point where the measurement result has increased Then, injection of hydrogen peroxide solution (H ₂ O ₂ ) supplied from the injection device 50 via the pump 51 is started (SP4).

At this time, the passage of a predetermined time or the differential pressure is observed (SP5: No), and when the differential pressure decreases to the reference value (SP5: Yes), hydrogen peroxide supplied from the injection device 50 via the pump 51 is obtained. Stop the injection of water (H ₂ O ₂ ) (SP6). And the pure water currently distribute | circulating in the gasification system 10 is substituted to the slurry body which is a gasification raw material (SP7).

  In the present embodiment, the tar adhesion amount section is specified based on the increase in the differential pressure. However, the present invention is not limited to this, and the tar adhesion amount section may be specified by, for example, a heat passage rate (dirt coefficient). Moreover, although this embodiment describes the gasification system 10 provided with the heat exchanger 31 as a heating apparatus, this invention is not limited to this, and may not have a heat exchanger depending on the gasification system. Needless to say, it is necessary to raise the temperature to the reaction temperature by some kind of heating device before the gasification raw material is gasified.

Furthermore, the above-mentioned tar removal treatment can be performed at a temperature near the gasification operation condition simply by replacing the raw slurry body with pure water while maintaining the state during operation. Therefore, the reactivity of H ₂ O ₂ is high. Thus, the efficiency of tar removal work can be improved. In addition, since it is not necessary to cool and stop the apparatus in order to switch between the raw slurry body and H ₂ O ₂ , the time available for gasification increases and the equipment can be used effectively. Furthermore, by injecting H ₂ O ₂ at a pinpoint into a dirty part, it is possible to effectively clean the trouble occurrence part. In addition, since the chemical solution can be used effectively, the amount of drug to be used can be reduced to a low cost.

  The gas processing unit 40 is a part that extracts fuel gas from the processed fluid sent from the heat exchanger 31. The gas processing unit 40 includes a cooling mechanism 41, a decompression mechanism 42, a gas-liquid separator 43, and a gas tank 44.

  The cooling mechanism 41 is a device that cools the processed fluid sent from the heat exchanger 31. The decompression mechanism 42 is a device that decompresses the processed fluid sent from the cooling mechanism 41. The gas-liquid separator 43 cools the processed fluid cooled by the cooling mechanism 41 and decompressed by the decompression mechanism 42, such as wastewater containing at least one of liquid components (solid such as activated carbon and ash, and viscous fluid such as tar). It is a device that separates into a liquid) and a gas component (a gas such as hydrogen or methane). Among these, the liquid component is treated as drainage, and the gas component is sent to the gas tank 44. The details of the gas-liquid separator 43 will be described later.

  The gas tank 44 is a container for storing the gas (product gas) separated by the gas-liquid separator 43. A part of the product gas stored in the gas tank 44 is supplied to the heater 32 and the gasification reactor 33 and consumed as fuel gas. In addition, this fuel gas can also be used as a power generation or a power source.

  As described above, in the gasification system 10 of the present embodiment, the differential pressure measuring units 34 a to 34 c that measure the differential pressure between the inlet and the outlet across the inlet side and the outlet side of the heat exchanger 31. Are provided between the inlet side and the outlet side of the heater 32 and the gasification reactor 33, and differential pressure measuring units 35a and 35b for measuring the differential pressure between the inlet and the outlet are provided. When the measurement results by the parts 34a to 34c, 35a, and 35b rise from the reference value based on the differential pressure when there is no adhesion of tar or the like, the slurry body circulating in the gasification system 10 is changed to pure water. After the replacement, the hydrogen peroxide solution (oxidant) is injected from the injection device 50 through the pump 51 until the predetermined measurement time or the differential pressure drops to the reference value at the location where the measurement result has increased. So heat exchanger 31 and heating The viscous fluid adhering to the pipe inner wall surface of the gasification reactor 33 and the gasification reactor 33 can be decomposed and removed, so that the blockage of the pipe flow path and the decrease in the heat transfer rate due to clogging by the viscous fluid can be avoided in advance. By increasing the operation time, the equipment utilization rate can be increased and the equipment cost can be reduced.

  Moreover, it is good also as providing the control part (not shown) which controls ON / OFF of the injection apparatus 50 based on the measurement result of the differential pressure | voltage measurement parts 34a-34c, 35a, 35b. Further, a timer for detecting the passage of time is further provided, and the control unit controls the injection device 50 to be turned off when the timer detects that a predetermined time has elapsed since the injection device 50 was turned on by the timer. It is good to do. In this case, the oxidant can be automatically injected and stopped based on the measurement results of the differential pressure measuring units 34a to 34c, 35a, and 35b and the detection of the passage of a predetermined time by the timer.

  Further, the gas-liquid separator 43 may be provided with two gate valves 43a and 43b as pressure reducing mechanisms in its discharge path. Thereby, even if it is a case where the inside of a separator main part is a pressure higher than atmospheric pressure (for example, 0.3-0.5 MPa), without missing an internal pressure and a gas component (product gas), The liquid component can be discharged efficiently.

  Although not shown in the present embodiment, the gasification system 10 may be provided with a power generation device that generates power by using the generated gas (fuel gas) stored in the gas tank 44 as fuel. . In this case, as the power generation device, for example, existing devices such as a gas engine (reciprocating engine, rotary engine), a steam turbine, a Stirling engine, and a fuel cell can be widely applied. And by using the fuel gas obtained by this gasification system 10 and generating electricity with a gas engine, electric power and exhaust heat can be obtained, so resource saving of fossil fuels such as coal and oil is achieved. be able to.

  Moreover, you may make it equip this gasification system 10 with the pre-processing apparatus (not shown) which hydrothermally processes a hydrous biomass previously. In this case, since the biomass can be decomposed from a high molecular weight to a low molecular weight, the contact efficiency between the biomass treated in the gasification reactor 33 and water or a nonmetallic catalyst is increased, and further generation of tar and the like is further suppressed. And fuel gas can be efficiently generated from biomass.

DESCRIPTION OF SYMBOLS 10 ... Gasification system, 20 ... Raw material preparation part, 21 ... Preparation tank, 22 ... Grinding pump, 23 ... Supply pump, 24 ... Heat exchanger introduction pump, 30 ... Heat treatment part, 31 ... Heat exchanger, 32 ... Heater 32a ... Combustion device, 33 ... Gasification reactor, 33a ... Combustion device, 36 ... Low temperature channel, 37 ... High temperature channel, 40 ... Gas treatment unit, 41 ... Cooling mechanism, 42 ... Decompression mechanism, 43 ... Gas-liquid Separator, 43a ... Gate valve, 43b ... Gate valve, 44 ... Gas tank, 50 ... Injection device, 51 ... Pump

Claims (6)

A gasification reactor for gasifying a slurry body made of a raw material containing biomass with supercritical water, and heating for raising the temperature of the slurry body before being gasified with supercritical water in the gasification reactor A supercritical water gasification system comprising:
A piping system for supplying the slurry body to the heating unit and the gasification reactor;
A tar adhering section specifying portion for specifying a section in which tar has adhered in the piping system;
An injection device for injecting an oxidant not containing inorganic substances into the specified section;
Switching means for switching the supply to the piping system from the slurry body to water when a section to which tar has adhered is identified by the tar adhesion section identifying section;
A controller that controls the injection device to supply the oxidant to the specified section by the injection device after the piping system is replaced with water ;
The tar adhesion section specifying part is
A differential pressure measuring unit that measures differential pressures at a plurality of different predetermined positions in the piping system,
When the differential pressure between the adjacent predetermined positions measured by the differential pressure measuring unit is equal to or higher than a reference value, the section between the adjacent predetermined positions is specified as a section where tar is attached ,
When the differential pressure decreases to a reference value, the control unit stops supplying the oxidant by the injection device,
The switching means switches the supply to the piping system from water to the slurry after the supply of the oxidant is stopped . The controller is
When detecting that the feed to the piping system has passed a predetermined time is switched to the water, supercritical water gasification of claim 1, wherein the piping system is characterized in that determines that displaced by water system. The controller is
Supercritical water gas according to claim 1 in which the feed to the piping system is when it is detected that a predetermined amount injected from the switched to water, the piping system is characterized in that determines that displaced by water System. The supercritical water gasification system according to any one of claims 1 to 3 , wherein the oxidizing agent is hydrogen peroxide, hypochlorous acid, nitric acid, or ozone. A process of gasifying a slurry produced from a raw material containing biomass with supercritical water in a gasification reactor, and heating the slurry before being gasified with supercritical water in the gasification reactor A supercritical water gasification method comprising a step of raising the temperature in a part,
In the piping system for supplying the slurry body to the heating unit and the gasification reactor, the differential pressures at different predetermined positions in the piping system are measured,
When the measured differential pressure between the adjacent predetermined positions is equal to or greater than a reference value, the section between the adjacent predetermined positions is specified as a section where tar has adhered,
When a section to which tar is attached is identified, the supply to the piping system is switched from the slurry body to water,
After the piping system is replaced with water, an oxidant that does not contain inorganic substances is injected into the specified section .
When the differential pressure decreases to a reference value, the supply of the oxidant is stopped,
After the supply of the oxidant is stopped, the supercritical water gasification method is characterized in that the supply to the piping system is switched from water to the slurry . When a section where tar adheres is identified in the piping system, the supply to the piping system is switched from the slurry body to water,
The supercritical water gasification method according to claim 5 , wherein the oxidant is supplied to the specified section after the piping system is replaced with water.

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