CN106830479A - Using flue gas and electrodialytic desulfurization wastewater zero-discharge treatment system and method - Google Patents

Abstract

A zero-discharge treatment system and method for desulfurization wastewater using flue gas and electrodialysis, the system includes a chemical precipitation flocculation unit, a lime-flue gas softening unit, a membrane concentration unit and an evaporation crystallization unit; based on the above system, the chemical precipitation flocculation is first used The unit treats the raw water of desulfurization wastewater, and then uses lime milk and _CO2 in the flue gas of the power plant to efficiently remove hardness ions such as calcium and magnesium in the effluent of the chemical precipitation flocculation unit, and at the same time achieve carbon dioxide emission reduction; the softened effluent enters ions after ultrafiltration treatment The selective electrodialysis unit conducts high-efficiency concentration to obtain NaCl concentrated water, and further obtains NaCl industrial salt through evaporation and crystallization; the electrodialysis fresh water passes through the low-pressure roll-type reverse osmosis membrane to obtain reusable fresh water; the reverse osmosis concentrated water re-enters the electrodialysis unit; The present invention uses _CO2 in the flue gas of the power plant to replace expensive soda ash, which not only reduces the cost of softening, but also promotes the reduction of _CO2 emissions; introduces electrodialysis for concentration, and makes electrodialysis and reverse osmosis both in the best treatment conditions , reducing energy consumption.

Description

Zero-discharge treatment system and method for desulfurization wastewater using flue gas and electrodialysis

technical field

The invention relates to the technical field of treating desulfurization wastewater by adopting zero-discharge technology, in particular to a zero-discharge treatment system and method for desulfurization wastewater using flue gas and electrodialysis.

Background technique

At present, most coal-fired power plants in my country use limestone/gypsum wet desulfurization technology (WFGD) for flue gas desulfurization, and a certain amount of desulfurization wastewater needs to be discharged regularly during the desulfurization process. The water quality of desulfurization wastewater is extremely poor, and it has the characteristics of high salt content, high content of suspended solids, high hardness, strong corrosion, and large changes in water quality with time and working conditions. In addition, desulfurization wastewater also contains trace heavy metal elements, such as mercury, chromium, cadmium, lead, etc., many of which are the first type of pollutants that are strictly controlled in the national environmental protection standards. Domestic power plants usually use chemical precipitation flocculation to treat desulfurization wastewater, which can remove most of the heavy metal ions, chemical oxygen demand (COD), suspended solids (SS) and other substances in the wastewater. However, the effluent treated by chemical precipitation and flocculation still has a high salt content.

With the increasingly prominent environmental protection issues, the use of zero discharge technology to treat desulfurization wastewater has received more and more attention. The "China Water Conservation Technology Policy Outline" promulgated in 2005 clearly requires enterprises to develop waste water reuse and zero discharge technologies. The national "Water Pollution Prevention and Control Action Plan" (Water Ten Measures), promulgated in 2015, encourages advanced enterprises in energy conservation and emission reduction, and industrial clusters to achieve higher standards in terms of water efficiency and sewage discharge intensity. Therefore, research on the zero discharge process of desulfurization wastewater is the only way for thermal power plants to achieve sustainable development.

At present, domestic research on the zero-discharge process of desulfurization wastewater is in its infancy. The basic idea is to undergo certain pretreatment of desulfurization wastewater, use steam heating to evaporate and concentrate the desulfurization wastewater, and the generated steam is condensed for reuse, and the concentrated liquid crystallization product is packaged and shipped . However, the investment and operation cost of the existing desulfurization wastewater zero discharge process is relatively high. This is mainly because the calcium and magnesium ions in the desulfurization wastewater are likely to cause scaling and corrosion of the evaporative crystallization system, resulting in increased equipment depreciation costs; and the high energy consumption of the evaporative crystallization process leads to increased operating costs.

In order to reduce equipment damage, desulfurization wastewater needs to be softened to remove calcium and magnesium ions in the water. The conventional softening process is the lime-soda ash method, and the cost of the agent is relatively high. Based on the calculation that the total amount of calcium and magnesium ions in desulfurization wastewater is about 200mM, about 22kg of carbonic acid needs to be added to treat 1 ton of wastewater. Calculated based on the unit price of soda ash of 1500-2000 yuan/t, the cost of soda ash alone is as high as about 40 yuan per ton of water, which is significant. Adds to the cost of the zero-emissions process.

In order to reduce operating energy consumption, it is necessary to reduce and concentrate desulfurization wastewater. The reverse osmosis method has the advantages of simple process and high desalination rate. However, there are some inherent problems in the reverse osmosis process that cannot be solved: the recovery rate of product water is low; Low; high salt content leads to high operating pressure and increased energy consumption. When treating desulfurization wastewater with poor water quality, the operating pressure of the reverse osmosis method often reaches above 6MPa, and the investment and operation costs are further increased. In order to reduce the process cost of zero discharge of desulfurization wastewater, it is necessary to choose a concentration process that is more suitable for desulfurization wastewater treatment.

Contents of the invention

In order to solve the above problems, the object of the present invention is to provide a zero-discharge treatment system and method for desulfurization wastewater using flue gas and electrodialysis, through the coupling and optimization of various process links, reduce the operating cost of the zero-discharge process, and realize resource recovery utilization and environmental protection.

In order to achieve the above object, the present invention adopts the following technical solutions:

A zero-discharge treatment system for desulfurization wastewater using flue gas and electrodialysis, including chemical precipitation flocculation unit 1, lime-flue gas softening unit 2, membrane concentration unit 3 and evaporation crystallization unit 4;

The chemical precipitation flocculation unit 1 is composed of a neutralization tank, a sedimentation tank, a flocculation tank, a clarification tank and a 1# clear water tank connected in sequence through pipelines, and the water inlet of the neutralization tank is connected with the discharge port of the desulfurization wastewater; A lime milk dosing device is set in the mixing tank; an organic sulfur dosing device is set in the sedimentation tank; a flocculant and coagulant dosing device is set in the flocculation tank; a mud discharge port is provided at the lower part of the clarification tank , the sludge outlet is connected to the sludge dewatering equipment through pipelines;

The lime-flue gas softening unit 2 is composed of a causticizing reaction tank, a carbonization reaction tank and a 2# clear water tank connected in sequence through pipelines, and the causticizing reaction tank and the 1# clear water tank in the chemical precipitation flocculation unit 1 pass through the pipeline connected; the causticization reaction tank is provided with lime milk, sodium sulfate, flocculant and coagulant dosing device, and there is a sludge discharge port at the bottom of the causticization reaction tank, and the sludge discharge port is connected with the sludge dewatering equipment through pipelines; A gas distributor is arranged at the bottom of the carbonization reaction tank, and the gas distributor is connected with the flue gas compressor fan. The carbonization reaction tank is provided with a flocculant and coagulant dosing device, and there is a mud discharge port at the bottom of the carbonization reaction tank. The port is connected to the sludge dewatering equipment through pipelines; the 2# clear water tank is provided with a dilute hydrochloric acid dosing device;

The membrane concentration unit 3 is composed of an ultrafiltration device, an electrodialysis device and a reverse osmosis device connected in sequence through pipelines; The outlet of the ultrafiltration device is connected to the water inlet of the electrodialysis device, and the concentrated water return port of the ultrafiltration device is connected to the neutralization tank of the chemical precipitation flocculation unit 1; the concentrated water discharge port of the electrodialysis device is connected to the evaporation crystallization unit 4 connected, the fresh water discharge port is connected with the water inlet of the reverse osmosis device; the concentrated water discharge port of the reverse osmosis device is connected with the water inlet of the electrodialysis device.

The electrodialysis device is a multi-stage countercurrent reverse electrode electrodialysis, using a monovalent ion selective membrane,

The reverse osmosis device adopts a roll-type reverse osmosis membrane.

The evaporative crystallization unit 4 adopts a mechanical recompression evaporator.

A zero-discharge treatment method for desulfurization wastewater using flue gas and electrodialysis, the method is realized by using the system, and the method includes the following steps:

(1) The raw water of desulfurization wastewater is passed into the neutralization tank, sedimentation tank, flocculation tank and clarification tank of the chemical precipitation flocculation unit 1 in sequence for neutralization, precipitation, flocculation and clarification, and the clarified liquid finally enters the 1# clear water tank;

(2) The clarified liquid in the above step (1) is passed into the causticizing reaction tank and the carbonization reaction tank in the lime-flue gas softening unit 2 successively to carry out causticizing and carbonizing treatment, and the clarified liquid enters the 2# clear water tank;

(3) The clarified liquid that above-mentioned step (2) obtains is passed into membrane concentration unit 3, at first this clarified liquid is carried out ultrafiltration treatment, and the product water of ultrafiltration is passed into electrodialysis device and carries out salt separation and concentration; Electrodialysis produces The fresh water is passed into the reverse osmosis device for purification, and concentrated water containing NaCl is produced at the same time; the fresh water generated by reverse osmosis is reused for production, and the generated concentrated water enters the electrodialysis device for treatment;

(4) Pass the concentrated water containing NaCl produced by the electrodialysis device in the above step (3) into the evaporation crystallization unit 4 to obtain industrial-grade NaCl crystalline salt and condensed water, and the condensed water is reused for production;

In the described step (1), milk of lime is added in the neutralization tank, organic sulfur is added in the sedimentation tank, milk of lime or organic sulfur reacts with iron, lead, chromium, copper, mercury heavy metal ions to produce hydroxide or sulfide;

In the step (1), a flocculant is added to the flocculation tank to cause the suspended matter in the waste water to precipitate, and a coagulant aid is added to the clarification tank to accelerate the separation of the produced sediment;

In described step (2), milk of lime and sodium sulfate are added in the causticizing reaction tank, and following reaction takes place:

Ca(OH) ₂ +MgSO ₄ →CaSO ₄ +Mg(OH) ₂ ↓

Ca(OH) ₂ +Na ₂ SO ₄ →CaSO ₄ +2NaOH

Remove Mg ²⁺ ions in wastewater and generate sodium hydroxide, then add certain flocculants and coagulants to the system to fully flocculate and clarify;

In described step (2), pass into flue gas in carbonization reaction pool, following reaction takes place:

2NaOH+CO ₂ →Na ₂ CO ₃ +H ₂ O

Na ₂ CO ₃ +Ca ²⁺ →CaCO ₃ ↓+2Na ⁺

Remove Ca ²⁺ ions in wastewater, and then add certain flocculants and coagulants to the system to fully flocculate and clarify;

In described step (2), the add-on of milk of lime and sodium sulfate should control the amount that produces sodium hydroxide between 0.05-0.2g/L in the causticizing reaction tank;

In the step (2), the flue gas introduced into the carbonization reaction tank is the net flue gas through denitrification, dust removal and desulfurization, which is slowly introduced at normal temperature and pressure, and the amount of flue gas introduced should make the system pH between 9-13;

In the step (3), the purpose of ultrafiltration is to remove suspended solids (SS) in the water, so that the effluent meets the requirements of subsequent membrane treatment and evaporation crystallization treatment, and the ultrafiltration concentrated water is returned to the chemical flocculation precipitation unit 1 for neutralization pool;

In the step (3), the electrodialysis ion-exchange membrane adopts a monovalent ion-selective membrane, so that the Cl ⁱⁿ the wastewater is enriched in the concentrated water, while the SO ₄ ^2- is left on the fresh water side to improve the purity of subsequent crystallization;

In the step (3), the total dissolved solids (TDS) in the fresh water produced by the reverse osmosis device is less than 0.05%, meeting the requirements of industrial reuse water, and the fresh water recovery rate of the membrane concentration unit is not lower than 90%.

In the step (1), the flocculant added in the flocculation tank is one or more of polyferric chloride sulfate, polyaluminum chloride, and polyferric sulfate; the coagulant aid added in the clarification tank is polyacrylamide , sodium polyacrylate, chitosan or one or more.

In the step (2), the flocculant added in the causticizing reaction tank is one or more of polyferric chloride sulfate, polyaluminum chloride, and polyferric sulfate; the coagulant added in the causticizing reaction tank One or more of polyacrylamide, sodium polyacrylate, and chitosan;

In the step (2), the flocculant added in the carbonization reaction tank is one or more of polymeric ferric sulfate chloride, polyaluminum chloride, and polyferric sulfate; the coagulation aid added in the carbonization reaction tank is poly One or more of acrylamide, sodium polyacrylate, and chitosan.

In the step (3), the electrodialysis device adopts pole-reversing operation to avoid scaling on the membrane surface.

Compared with the prior art, the present invention has the following advantages:

1) Use the clean flue gas of coal-fired power plants that has undergone denitrification, dust removal, and desulfurization to soften the effluent water of the chemical precipitation flocculation unit, which can replace expensive soda ash, reduce softening costs, achieve higher softening efficiency, and avoid follow-up equipment. Fouling and corrosion, in turn contribute to _CO2 emission reductions. Calculated based on the total amount of calcium and magnesium ions in desulfurization wastewater being 200mM, about 8.8g of carbon dioxide can be reduced while treating each ton of desulfurization wastewater.

2) Introduce electrodialysis to improve the traditional membrane concentration process, so that both electrodialysis and reverse osmosis are under the best treatment conditions, reducing energy consumption and treatment costs; electrodialysis ion exchange membranes can use monovalent ion-selective membranes, so that The SO ₄ ^2- in the wastewater stays on the fresh water side, and the Cl ^- ions are enriched in the concentrated water, and then the industrial grade NaCl crystalline salt is obtained through evaporation and crystallization.

Description of drawings

Fig. 1 is a flow chart of the system and method of the present invention.

detailed description

The present invention will be described in more detail below in conjunction with the accompanying drawings and specific embodiments.

The main components of desulfurization wastewater raw water are as follows: TDS 20000‐60000mg/L; SS 5000‐10000mg/L; SO ₄ ^{2‐2000‐6000mg} /L; Ca ²⁺ 1500‐5000mg/L; Mg ²⁺ 3000‐6000mg/L. As shown in Figure 1, the desulfurization wastewater is passed into the chemical precipitation flocculation unit 1 for neutralization, precipitation, flocculation, and clarification treatment, and the precipitation flocculation reaction is carried out by adding lime milk, organic sulfur, polyferric sulfate chloride and polyacrylamide to obtain Clarified liquid that meets the water quality requirements of "Water Quality Control Indicators for Limestone-Gypsum Wet Desulfurization Wastewater in Thermal Power Plants" (DL/T 997-2006); pass the obtained clarified liquid into lime-flue gas softening unit 2 for causticization and carbonization and clarification treatment, by adding milk of lime and sodium sulfate, the Mg ²⁺ in the system will settle down, and at the same time, sodium sulfate and milk of lime will react to form calcium sulfate precipitate and sodium hydroxide; add polyferric sulfate chloride and polypropylene to the system Amide, fully flocculated and clarified. The obtained clarified liquid is passed into the carbonization pool, and the clean flue gas that has undergone denitration, dust removal and desulfurization is passed into the carbonization pool. The carbon dioxide in the flue gas is absorbed by the sodium hydroxide in the system to produce carbonate, and the carbonate and calcium ions form Calcium carbonate precipitates. In the lime-flue gas softening unit 2, the accumulation of causticizing reaction sodium hydroxide is 0.1g/L, and the pH of the system becomes 10 after the flue gas is fed. Add polyferric sulfate and sodium polyacrylate to the system for full flocculation and clarification. The total amount of calcium and magnesium ions in the lime-flue gas softening unit 2 shall not exceed 20mg/L. The clarified liquid obtained from the lime-flue gas softening unit 2 is subjected to ultrafiltration treatment, and the ultrafiltration water production rate is 95%, and the concentrated water is returned to the chemical precipitation flocculation unit for re-coagulation and precipitation; the SS in the fresh water produced by ultrafiltration is less than 0.2mg /L, passed into the electrodialysis device for concentration. The electrodialysis device adopts frequent electrode reversal operation to prevent scaling; the ion exchange membrane uses a monovalent ion selective exchange membrane to separate Cl ^- and SO ₄ ² - in the wastewater, so that sodium chloride can be enriched in the concentrated electrodialysis water , to improve the purity of subsequent crystallization. The electrodialysis adopts 5 parallel 2 series connections to produce concentrated water with a TDS of 15%. After evaporating and crystallizing unit 4, industrial grade NaCl crystalline salt is obtained. The TDS of electrodialysis fresh water is 0.5%, which is further purified through the anti-pollution roll-type reverse osmosis membrane. The TDS of reverse osmosis fresh water is below 0.05%, which can be used as industrial reuse water. The TDS of reverse osmosis concentrated water is 2%, which is concentrated to 6% after first-stage electrodialysis, and industrial grade Na ₂ SO ₄ crystalline salt is obtained after evaporation and crystallization, and the electrodialysis fresh water is returned to the reverse osmosis inlet. The water recovery rate of the membrane concentration unit is 80%. The zero-discharge method of desulfurization waste water proposed by the invention can realize waste resource utilization, eliminate waste water and solid waste pollution, and have good economic and environmental benefits.

Claims (9)

1. flue gas and electrodialytic desulfurization wastewater zero-discharge treatment system are utilized, it is characterised in that：The system includes chemistry Precipitation flocculation unit (1), lime-flue gas pliable cell (2), film upgrading unit (3) and evaporative crystallization unit (4)；

The chemical precipitation flocculation unit (1) is by being sequentially connected the neutralization pond for connecing, sedimentation basin, flocculation basin, depositing reservoir by pipeline Constituted with 1# clear water reserviors, the water inlet of neutralization pond is connected with the floss hole of desulfurization wastewater；Milk of lime is provided with the neutralization pond Chemicals dosing plant；Organic sulfur chemicals dosing plant is provided with the sedimentation basin；Flocculant is provided with the flocculation basin and flocculation aid adds Medicine device；Mud discharging mouth is arranged at the depositing reservoir bottom, and mud discharging mouth is connected by pipeline with device for dehydrating sladge waste；

The lime-flue gas pliable cell (2) is by being sequentially connected the causticizing reaction pond for connecing, carburizing reagent pond and 2# by pipeline Clear water reserviors are constituted, and causticizing reaction pond is connected with the 1# clear water reserviors in chemical precipitation flocculation unit (1) by pipeline；The causticization is anti- Ying Chizhong is provided with milk of lime, sodium sulphate, flocculant and flocculation aid chemicals dosing plant, and there are mud discharging mouth, spoil disposal in causticizing reaction bottom of pond portion Mouth is connected by pipeline with device for dehydrating sladge waste；The bottom in the carburizing reagent pond is disposed with gas distributor, gas distributor It is connected with flue gas compression fan, flocculant, flocculation aid chemicals dosing plant is provided with carburizing reagent pond, carburizing reagent bottom of pond portion has Mud discharging mouth, mud discharging mouth is connected by pipeline with device for dehydrating sladge waste；Watery hydrochloric acid chemicals dosing plant is provided with the 2# clear water reserviors；

The film upgrading unit (3) is by being sequentially connected the ultrafiltration apparatus for connecing, electrodialysis plant and reverse osmosis unit group by pipeline Into；The water inlet of the ultrafiltration apparatus is connected with the delivery port of the 2# clear water reserviors of lime-flue gas pliable cell (2), ultrafiltration dress The delivery port put is connected with the water inlet of electrodialysis plant, concentrated water water return outlet and the chemical precipitation flocculation unit (1) of ultrafiltration apparatus Neutralization pond be connected；The concentrated water discharge mouthful of electrodialysis plant is connected with evaporative crystallization unit (4), and fresh water floss hole is filled with counter-infiltration The water inlet put is connected；The concentrated water discharge mouthful of the reverse osmosis unit counter-infiltration is connected with the water inlet of electrodialysis plant.

2. utilization flue gas according to claim 1 and electrodialytic desulfurization wastewater zero-discharge treatment system, its feature exist In：The electrodialysis plant is multi-stage countercurrent pole-reversing electroosmosis, employs monovalent ion selective membrane.

3. utilization flue gas according to claim 1 and electrodialytic desulfurization wastewater zero-discharge treatment system, its feature exist In：The reverse osmosis unit employs coiled reverse osmosis membrane.

4. utilization flue gas according to claim 1 and electrodialytic desulfurization wastewater zero-discharge treatment system, its feature exist In：The evaporative crystallization unit (4) employs mechanical recompression evaporator.

5. flue gas and electrodialytic zero emission treatment of desulfured waste water are utilized, it is characterised in that：The method is to utilize right It is required that what system described in 1 to 4 any one was realized, methods described comprises the following steps：

(1) desulfurization wastewater raw water is passed through neutralization pond, sedimentation basin, flocculation basin and the clarification of chemical precipitation flocculation unit (1) successively Pond is neutralized, is precipitated, being flocculated and clarifying treatment, and clarified solution finally enters 1# clear water reserviors；

(2) by the clarified solution in above-mentioned steps (1) be passed through successively causticizing reaction pond in lime-flue gas pliable cell (2) and Carburizing reagent pond carries out causticization, carbonization treatment, and clarified solution enters 2# clear water reserviors；

(3) clarified solution that above-mentioned steps (2) are obtained is passed through film upgrading unit (3), hyperfiltration treatment is carried out to the clarified solution first, The product water of ultrafiltration is passed through electrodialysis plant and carries out point salt and concentration；Electrodialysis produce fresh water be passed through reverse osmosis unit carry out it is pure Change, while producing the concentrated water containing NaCl；The fresh water that counter-infiltration is produced is back to production, and the concentrated water of generation reenters electrodialysis Device is processed；

(4) concentrated water containing NaCl that above-mentioned steps (3) electrodialysis plant is produced is passed through evaporative crystallization unit (4), is acquired Technical grade NaCl crystal salts and condensed water, recycling condensing water is in production；

In the step (1), milk of lime is added in neutralization pond, add organic sulfur in sedimentation basin, milk of lime or organic sulfur and iron, Lead, chromium, copper, the reaction of mercury heavy metal ion produce hydroxide or sulfide；

In the step (1), flocculant is added in flocculation basin, the suspension in waste water is produced precipitation, added in depositing reservoir and helped Solidifying agent, makes the sediment of generation accelerate to separate；

In the step (2), milk of lime and sodium sulphate are added in causticizing reaction pond, following reactions occur：

Ca(OH)₂+MgSO₄→CaSO₄+Mg(OH)₂↓

Ca(OH)₂+Na₂SO₄→CaSO₄+2NaOH

Mg in removing waste water²⁺Ion simultaneously produces NaOH, then to certain flocculant and flocculation aid is added in system, fills Divide flocculation clarification；

In the step (2), flue gas is passed through in carburizing reagent pond, following reactions occur：

2NaOH+CO₂→Na₂CO₃+H₂O

Na₂CO₃+Ca²⁺→CaCO₃↓+2Na⁺

Ca in removing waste water²⁺Ion, then to certain flocculant and flocculation aid is added in system, fully flocculation clarification；

In the step (2), the addition of milk of lime and sodium sulphate should control to produce the amount of NaOH to exist in causticizing reaction pond Between 0.05-0.2g/L；

In the step (2), the flue gas being passed through in carburizing reagent pond is the neat stress by denitration, dedusting and desulfurization, normal It is slowly introducing under normal temperature and pressure, the intake of flue gas should make the pH of system between 9-13；

In the step (3), the purpose of ultrafiltration is to remove the suspended solid (SS) in water, water outlet is met follow-up film process and steaming Send out the requirement of crystallization treatment, the neutralization pond of ultrafiltration concentrate recirculation to chemical flocculation precipitation unit (1)；

In the step (3), electrodialysis-ion exchange film uses monovalent ion selective membrane, makes the Cl- in waste water in concentrated water Enrichment, and SO₄ ^2-Dilute side is stayed in, the purity of later crystallization is improved；

In the step (3), total dissolved solid TDS meets industry below 0.05% in the fresh water that reverse osmosis unit is produced The requirement of recycle-water, film upgrading unit fresh-water recovery rate is not less than 90%.

6. method according to claim 5, it is characterised in that：In the step (1), the flocculant added in flocculation basin is One or more in poly-ferric sulfate chloride, aluminium polychloride, bodied ferric sulfate；The flocculation aid added in depositing reservoir is poly- third One or more in acrylamide, Sodium Polyacrylate, shitosan.

7. method according to claim 5, it is characterised in that：In the step (2), the flocculation added in causticizing reaction pond Agent is one or more in poly-ferric sulfate chloride, aluminium polychloride, bodied ferric sulfate；What is added in causticizing reaction pond helps solidifying Agent is one or more in polyacrylamide, Sodium Polyacrylate, shitosan.

8. method according to claim 5, it is characterised in that：In the step (2), the flocculation added in carburizing reagent pond Agent is one or more in poly-ferric sulfate chloride, aluminium polychloride, bodied ferric sulfate；What is added in carburizing reagent pond helps solidifying Agent is one or more in polyacrylamide, Sodium Polyacrylate, shitosan.

9. method according to claim 5, it is characterised in that：In the step (3), electrodialysis plant uses the behaviour of falling pole Make, it is to avoid the fouling of face.