CN111701436A - Novel dual-alkali desulfurization method - Google Patents
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Abstract
The invention relates to the technical field of flue gas desulfurization, and discloses a novel dual-alkali desulfurization method, which comprises the following steps: s1, inputting the lime slurry into a first-stage spray tower for preliminary desulfurization; s2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, and pumping the mixed liquid in the regeneration tank into a sedimentation tank; s3, inputting the supernatant into a filtrate circulating tank, adding sodium hydroxide and a scale inhibitor into the filtrate circulating tank, and pumping the mixture into a secondary spray tower, wherein the bottom of the secondary spray tower is provided with a water inlet pipeThe discharged liquid is input into a regeneration tank; and S4, inputting the precipitate into an oxidation pond for aeration and oxidation, and performing filter pressing by using a plate and frame filter press. The invention has the following advantages and effects: the secondary spraying can greatly improve the desulfurization effect and the reduction speed of calcium oxide reaction; na (Na)2CO3Calcium ions in the regeneration tank can be fully precipitated, and the calcium ions are prevented from entering a secondary spray tower to generate calcium scale; and the scale inhibitor is added into the filtrate circulating tank, so that scaling in a secondary spray tower and a pipeline can be further avoided.
Description
Technical Field
The invention relates to the technical field of flue gas desulfurization, in particular to a novel dual-alkali desulfurization method.
Background
The double alkali method adopts sodium-based desulfurizer to carry out in-tower desulfurization, and the sodium-based desulfurizer has stronger alkalinity and can fully absorb sulfur dioxide. Meanwhile, the generated reaction product has low solubility, the phenomenon of crystal saturation can not occur, and the problem of scaling and blocking is solved. In addition, after the desulfurization product is discharged into the regeneration tank, the sodium-based sulfur agent can be regenerated by utilizing the reduction regeneration property of calcium hydroxide. The regenerated substances can be recycled after being treated. The dual-alkali desulphurization process has low cost and is widely used in the desulphurization modification of small and medium boilers.
However, in the operation process of the double alkali method system, scaling phenomenon can be generated in the spray tower and in the pipeline because the absorption liquid of the double alkali method contains a large amount of Ca2+、Na+、SO3 2-、SO4 2-Plasma, these ions can form the crystallization in tower wall and pipeline under supersaturated state, and the crystallization thickness can even reach tens of centimetres or tens of centimetres, leads to desulfurization system resistance increase, under-deposit corruption, flow reduction etc. seriously influences the desulfurization efficiency of spray column.
Disclosure of Invention
The invention aims to provide a novel dual-alkali desulfurization method which has the effects of high desulfurization efficiency, difficult scaling and low operation cost.
2. The technical purpose of the invention is realized by the following technical scheme: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, and inputting the lime slurry into a first-stage spray tower to carry out preliminary desulfurization on sulfur-containing flue gas;
s2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant in the step S2 into a filtrate circulating tank, adding 5-20 mg/L of sodium hydroxide and scale inhibitor into the filtrate circulating tank, uniformly mixing, pumping into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, and mixing with the discharge liquid of the primary spray tower in the step S2 for regeneration reaction;
and S4, inputting the sediment obtained in the step S2 into an oxidation pond for aeration oxidation, and pumping the sediment into a plate and frame filter press by a slurry pump for filter pressing.
The invention is further provided with: the scale inhibitor comprises, by mass, 10-30 parts of polyaspartic acid, 20-30 parts of sulfonated lignin and 50 parts of water.
The invention is further provided with: the scale inhibitor further comprises 3-6 parts by mass of sodium hydroxycitrate.
The invention is further provided with: the scale inhibitor further comprises 5-10 parts by mass of potassium xylene sulfonate.
The invention is further provided with: in the step S1, the liquid-gas ratio in the primary spray tower is 10-30L/m3
The invention is further provided with: and a rolling screen is arranged in the digestion tank.
By adopting the technical scheme: the quicklime is dissolved with water as a desulfurizer, and because the quicklime contains impurity particles, the quicklime is slowly dissolved in the water after being directly added, and precipitates are more, and the pipeline is easily blocked.
The invention is further provided with: the secondary spray tower is provided with a plurality of spray layers, each spray layer is evenly provided with 8-10 nozzles, and the nozzles on each spray layer are uniformly distributed in a staggered mode.
The invention is further provided with: the nozzle is a silicon carbide large-caliber nozzle.
By adopting the technical scheme: spray the layer and evenly set up a plurality of nozzles, every layer of nozzle dislocation equipartition can avoid spraying liquid and flue gas blind area appearance, can effectively promote desulfurization efficiency.
The invention has the beneficial effects that:
1. according to the method, the primary spraying in the traditional double-alkali method is changed into the secondary spraying, and the lime slurry in the digestion tank is input into the primary spraying tower by adopting a larger liquid-gas ratio, so that on one hand, the sulfur-containing flue gas can be preliminarily absorbed, the lime slurry is in direct contact with the sulfur-containing flue gas, calcium is fully enabled to absorb sulfur dioxide in the flue gas, dual functions of desulfurization and reduction are achieved, the desulfurization effect and the reaction reduction speed of calcium oxide can be greatly improved, and the process efficiency value of lime consumption as a main desulfurizer is reflected; on the other hand, the larger liquid-gas ratio is adopted in the first-stage spray tower, so that the saturation of calcium sulfate in the first-stage spray tower can be reduced, and the gypsum scaling can be inhibited.
2. The effluent in the first spray tower is mixed with the effluent in the second spray tower, and the first spray tower contains a large amount of Ca (OH)2Can be matched with NaHSO in the secondary spray tower3Reaction to form Na2SO3，Na2SO3Can flow back to the first-stage spray tower to absorb SO2To produce NaHSO3Therefore, the discharged liquid in the first-stage spray tower can be discharged to NaHSO in the second-stage spray tower3Regeneration is carried out, and the spraying amount of the first-stage spraying tower and the second-stage spraying tower is adjusted, so that the consumption of NaOH can be greatly reduced, and the sulfur-containing flue gas treatment efficiency is improved.
3. Adding a certain amount of Na2CO3，Na2CO3Can be mixed with Ca in the regeneration tank2SO3Reaction to form Ca2CO3Precipitation, on the one hand Ca2CO3Solubility lower than Ca2SO3Calcium ions in the regeneration tank can be fully precipitated, and the calcium ions are prevented from entering the secondary spray tower to generate calcium scale; and Na is added into the regeneration tank2CO3The viscosity of the solution in the regeneration tank can be reduced, and the crystallization of the regeneration liquid after entering the tower is avoided; on the other hand, Na2CO3With Ca2SO3Reaction can generate Na2SO3，Na2SO3The sulfur-containing flue gas can enter the secondary spray tower to be absorbed, other ions are not introduced into the desulfurization system, and the desulfurization system is optimized.
4. Adding scale inhibitor into the filtrate circulating tank can further avoid scaling in the secondary spray tower and pipeline, and the scale inhibitor is mainly prepared from polyaspartic acid, sulfonated lignin and sulfurMagnesium, polyaspartic acid and Ca2+Form stable and soluble chelate, which can interfere with Ca2SO4In the crystal forming process, the crystal structure of the calcium scale is changed, so that the calcium scale can form floatable flowing soft scale, and the deposition of the scale in a spray tower and a pipeline is avoided; the polyaspartic acid can also chelate trace Fe in the spray liquid2+、Cu2+Plasma metal ions, these metal ions being SO3 2-And O2Reaction to form SO4 2-Thus avoiding CaSO3The oxidation of the catalyst reduces the generation of calcium scale in the spray tower and simultaneously can avoid Na2SO3Conversion to Na2SO4The consumption of liquid caustic soda is reduced, and the sulfonated lignin as a corrosion inhibitor can be adsorbed on the metal surface to form a film and prevent dissolved oxygen in water from diffusing to the metal surface, thereby playing a role in inhibiting corrosion on one hand, and reducing the oxygen and SO in water on the other hand3 2-The reaction rate is further reduced, and the calcium scale and Na are further reduced2SO4The rate of generation of (d); magnesium sulfate can promote lime dissolution, improve desulfurization rate and lime utilization rate, and reduce Ca in slurry2+The ion concentration is favorable for inhibiting gypsum scaling, and the magnesium sulfate is used as an additive, so that the crystal structure of a solid product can be changed and the solid product can be removed more easily.
5. The sediment in the sedimentation tank is aerated and oxidized in the oxidation tank, and Ca in the sedimentation tank can be oxidized2SO3Conversion to Ca2SO4Due to Ca2SO3Is flaky fine particles in water, is difficult to filter and dewater, and contains Ca2SO3The filter pressing effect can be improved by forcedly oxidizing the precipitate in the oxidation pond, and filter residues obtained by filter pressing are convenient to treat; after the magnesium sulfate in the filtrate circulating tank finally enters the sedimentation tank, Ca is facilitated2SO3The oxidation of (2) is beneficial to solid-liquid separation.
6. The sodium hydroxycitrate and potassium xylenesulfonate can reduce the viscosity of the spray liquid, can make suspended matters in the spray liquid in a dispersed state, so that the suspended matters are not easy to agglomerate, avoid the formation of crystal nuclei, and can effectively reduce Ca2SO3、Ca2SO4And Na2SO4The crystallization of (2) can avoid crystallization in the tower by the synergistic effect of the polyaspartic acid and the sulfonated lignin.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a process flow diagram of the present invention.
Detailed Description
The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. It is to be understood that the described embodiments are merely a few embodiments of the invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without any inventive step, are within the scope of the present invention.
Example 1
A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, arranging a rolling screen in the digestion tank, inputting the lime slurry into a first-stage spray tower to primarily desulfurize sulfur-containing flue gas, wherein the liquid-gas ratio in the first-stage spray tower is 10L/m3；
S2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant obtained in the step S2 into a filtrate circulation tank, adding 20mg/L of sodium hydroxide and a scale inhibitor into the filtrate circulation tank, wherein the scale inhibitor comprises 30 parts by mass of polyaspartic acid, 20 parts by mass of sulfonated lignin, 20 parts by mass of magnesium sulfate, 3 parts by mass of sodium hydroxycitrate, 5 parts by mass of potassium xylene sulfonate and 50 parts by mass of water, uniformly mixing the mixture, pumping the mixture into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, mixing the discharge liquid with the discharge liquid of a primary spray tower obtained in the step S2, carrying out regeneration reaction, arranging a plurality of spray layers in the secondary spray tower, uniformly arranging 8 spray nozzles in each spray layer, and uniformly arranging the spray nozzles in each spray layer at staggered angles, wherein the spray nozzles are silicon carbide large-caliber nozzles;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
Example 2
A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, arranging a rolling screen in the digestion tank, inputting the lime slurry into a first-stage spray tower to primarily desulfurize sulfur-containing flue gas, wherein the liquid-gas ratio in the first-stage spray tower is 30L/m3；
S2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant obtained in the step S2 into a filtrate circulation tank, adding 5mg/L of sodium hydroxide and a scale inhibitor into the filtrate circulation tank, wherein the scale inhibitor comprises 10 parts by mass of polyaspartic acid, 30 parts by mass of sulfonated lignin, 10 parts by mass of magnesium sulfate, 6 parts by mass of sodium hydroxycitrate, 5 parts by mass of potassium xylene sulfonate and 50 parts by mass of water, uniformly mixing the mixture, pumping the mixture into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, mixing the discharge liquid with the discharge liquid of a primary spray tower obtained in the step S2, carrying out regeneration reaction, arranging a plurality of spray layers in the secondary spray tower, uniformly arranging 8 spray nozzles in each spray layer, and uniformly arranging the spray nozzles in each spray layer at staggered angles, wherein the spray nozzles are silicon carbide large-caliber nozzles;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
Example 3
A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, arranging a rolling screen in the digestion tank, inputting the lime slurry into a first-stage spray tower to primarily desulfurize sulfur-containing flue gas, wherein the liquid-gas ratio in the first-stage spray tower is 10L/m3；
S2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, pumping the mixed liquid in the regeneration tank into a precipitation tank, and separating the mixed liquid into a supernatant and a precipitate after precipitation in the precipitation tank;
s3, inputting the supernatant obtained in the step S2 into a filtrate circulation tank, adding 20mg/L of sodium hydroxide and a scale inhibitor into the filtrate circulation tank, wherein the scale inhibitor comprises 30 parts by mass of polyaspartic acid, 20 parts by mass of sulfonated lignin, 20 parts by mass of magnesium sulfate, 3 parts by mass of sodium hydroxycitrate, 5 parts by mass of potassium xylene sulfonate and 50 parts by mass of water, uniformly mixing the mixture, pumping the mixture into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, mixing the discharge liquid with the discharge liquid of a primary spray tower obtained in the step S2, carrying out regeneration reaction, arranging a plurality of spray layers in the secondary spray tower, uniformly arranging 8 spray nozzles in each spray layer, and uniformly arranging the spray nozzles in each spray layer at staggered angles, wherein the spray nozzles are silicon carbide large-caliber nozzles;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
Example 4
A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, arranging a rolling screen in the digestion tank, inputting the lime slurry into a first-stage spray tower to primarily desulfurize sulfur-containing flue gas, wherein the liquid-gas ratio in the first-stage spray tower is 30L/m3；
S2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant obtained in the step S2 into a filtrate circulation tank, adding 5mg/L of sodium hydroxide and a scale inhibitor into the filtrate circulation tank, wherein the scale inhibitor comprises 10 parts by mass of polyaspartic acid, 30 parts by mass of sulfonated lignin, 10 parts by mass of magnesium sulfate and 50 parts by mass of water, uniformly mixing the mixture, pumping the mixture into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, mixing the discharge liquid with the discharge liquid of the primary spray tower in the step S2 for regeneration reaction, arranging a plurality of spray layers in the secondary spray tower, uniformly arranging 8 spray nozzles in each spray layer, and uniformly arranging the spray nozzles in each spray layer in a staggered angle, wherein the spray nozzles are silicon carbide large-diameter nozzles;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
Example 5
A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, arranging a rolling screen in the digestion tank, inputting the lime slurry into a first-stage spray tower to primarily desulfurize sulfur-containing flue gas, wherein the liquid-gas ratio in the first-stage spray tower is 30L/m3；
S2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant in the step S2 into a filtrate circulating tank, adding sodium hydroxide into the filtrate circulating tank, uniformly mixing, pumping into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, mixing with the discharge liquid of the primary spray tower in the step S2, and carrying out regeneration reaction, wherein a plurality of spray layers are arranged in the secondary spray tower, each spray layer is uniformly provided with 8 nozzles, the nozzles of each spray layer are staggered and uniformly distributed at an angle, and the nozzles are silicon carbide large-diameter nozzles;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
The boiler type of the plant is a 20T coal-fired boiler, the number of the boilers is 1, and the smoke gas volume is 60000m3H, sulfur content 0.6%, initial concentration of sulfur dioxide 1800mg/m3The CaO concentration of the quicklime added into the digestion tank is 85 percent, the liquid caustic soda purity is 30 percent, and the calcium-sulfur ratio is as follows: 1.03, CaO consumption of 45kg/h, liquid caustic soda (NaOH) consumption of 20kg/h and Na in the running process2CO3The consumption is 10kg/h, the water consumption is 2T/h, under the condition, the primary spray tower 5d and the secondary spray tower 5d are respectively operated according to the embodiment 1 to the embodiment 5, the discharge liquid and the discharge gas of the secondary spray tower in the double alkali desulphurization system of the 5d are sampled, the samples are sampled once every 30min, 10 samples are totally collected, and the Ca in the samples is analyzed2+Concentration, Na2SO4And calculating the average values of the concentration, the turbidity and the sulfur dioxide emission concentration of the secondary spray tower.
The test method comprises the following steps:
Ca2+the detection method comprises the following steps: GB 7476-87 EDTA titration method for measuring calcium in water.
Na2SO4The detection method comprises the following steps: because the solution to be tested contains a large amount of Na2SO3Firstly, dilute hydrochloric acid solution is adopted to treat SO in solution to be measured3 2+After the removal, the method of GB 29209-2012 sodium sulfate serving as national food safety standard is adopted to treat Na2SO4And (5) detecting the concentration.
Turbidity: and a GDS-3 photoelectric turbidity meter is adopted for detection.
The test results are shown in table 1 below:
TABLE 1 detection results of water and gas outlet of the second stage spray tower
As can be seen from Table 1, Na was added to the regeneration tank2CO3Can fully precipitate calcium ions in the regeneration tank, and prevent the calcium ions from entering a secondary spray tower to generate calcium scale and Na2CO3Can reduce Ca in the regeneration tank2CO3The concentration reduces the viscosity of the solution, and avoids the crystallization of the regenerated liquid after entering the tower; the sodium hydroxycitrate and the potassium xylenesulfonate can reduce the turbidity of the spray liquid, and the sodium hydroxycitrate and the potassium xylenesulfonate can disperse suspended matters in the spray liquid, so that the suspended matters are not easy to agglomerate, the formation of crystal nuclei is avoided, and the Ca can be effectively reduced2SO3、Ca2SO4And Na2SO4Crystallization of (4); the scale inhibitor is added into the filtrate circulating tank, SO that the formation of calcium scale can be reduced, and the polyaspartic acid and the sulfonated lignin can also avoid SO3 2-And O2Reaction to form SO4 2-Reduce gypsum scale and Na2SO4And (4) generation of crystals.
Claims (8)
1. A novel dual-alkali desulfurization method is characterized in that: the method comprises the following steps:
s1, preparing quicklime into lime slurry in a digestion tank, and inputting the lime slurry into a first-stage spray tower to carry out preliminary desulfurization on sulfur-containing flue gas;
s2, delivering the discharge liquid at the bottom of the primary spray tower into a regeneration tank, adding sodium carbonate into the regeneration tank, pumping the mixed liquid in the regeneration tank into a sedimentation tank, and separating the mixed liquid into two parts, namely supernatant and precipitate after the mixed liquid is precipitated in the sedimentation tank;
s3, inputting the supernatant in the step S2 into a filtrate circulating tank, adding 5-20 mg/L of sodium hydroxide and scale inhibitor into the filtrate circulating tank, uniformly mixing, pumping into a secondary spray tower, inputting the discharge liquid at the bottom of the secondary spray tower into a regeneration tank, and mixing with the discharge liquid of the primary spray tower in the step S2 for regeneration reaction;
and S4, inputting the precipitate obtained in the step S2 into an oxidation pond for aeration oxidation, pumping the precipitate into a plate and frame type filter press by using a slurry pump for filter pressing, wherein filter residues obtained by filter pressing are gypsum, and part of filter pressing liquid flows back into a filtrate circulation pond.
2. The novel dual-alkali desulfurization method according to claim 1, characterized in that: the scale inhibitor comprises, by mass, 10-30 parts of polyaspartic acid, 20-30 parts of sulfonated lignin, 10-20 parts of magnesium sulfate and 50 parts of water.
3. The novel dual-alkali desulfurization method according to claim 2, characterized in that: the scale inhibitor further comprises 3-6 parts by mass of sodium hydroxycitrate.
4. The novel dual-alkali desulfurization method according to claim 2, characterized in that: the scale inhibitor further comprises 5-10 parts by mass of potassium xylene sulfonate.
5. The novel dual-alkali desulfurization method according to claim 1, characterized in that: in the step S1, the liquid-gas ratio in the primary spray tower is 10-30L/m3。
6. The novel dual-alkali desulfurization method according to claim 1, characterized in that: and a rolling screen is arranged in the digestion tank.
7. The novel dual-alkali desulfurization method according to claim 1, characterized in that: the secondary spray tower is provided with a plurality of spray layers, each spray layer is evenly provided with 8-10 nozzles, and the nozzles on each spray layer are uniformly distributed in a staggered mode.
8. The novel dual-alkali desulfurization method according to claim 7, characterized in that: the nozzle is a silicon carbide large-caliber nozzle.
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