Ship Scrubber Unit (1)

Limestone-gypsum FGD technology

Gypsum flue gas?scrubber?method is the main stream?scrubber?technology in the domestic market at present, and its core technology has been successfully applied by most companies in China. This method mainly uses inexpensive limestone, and then polishes the limestone into powder and adds water to make a slurry of?scrubber?absorption lime, which eventually produces gypsum dihydrate after a series of reactions such as dissolution, neutralization, oxidation and crystallization.

The limestone-gypsum method of flue gas?scrubber?technology uses limestone slurry as the desulfurizing agent, spraying and washing the flue gas in the absorption tower to make the sulfur dioxide in the flue gas react to form calcium sulfite, while air is drummed into the slurry of the absorption tower to force the conversion of calcium sulfite into calcium sulfate, and the by-product of the desulfurizing agent is gypsum.

The limestone-gypsum FGD system includes flue gas heat exchange system, absorption tower?scrubber?system, desulfurizer slurry preparation system, slurry evacuation system, process water system, gypsum dewatering and storage and transportation system, and wastewater treatment system.

The traditional limestone - gypsum method of flue gas?scrubber?process uses calcium-based?scrubber?agent to absorb sulfur dioxide after the generation of calcium sulfite, calcium sulfate, due to its small solubility, it is very easy to form scaling and blockage in the?scrubber?tower and pipeline.

At present, limestone-gypsum FGD technology has become the preferred wet FGD process on land due to the cheap price of limestone and its easy transportation and preservation. Although this process is widely used in industrial flue gas?scrubber, the system operation is stable and the efficiency of?scrubber?can be increased to more than 95% by adding some chemicals appropriately. However, for the treatment of ship engine exhaust, the technology still has more serious problems such as large floor space, complex system management, large initial investment, scaling, clogging, wear and corrosion of equipment. Therefore, if the limestone-gypsum method is to be promoted for flue gas?scrubber?on ships, the?scrubber?process needs to be optimized and the cost performance of the process needs to be improved.

Rotary spray drying?scrubber?method

The rotary spray drying method (SDA) belongs to the semi-dry FGD technology, which is characterized by simple equipment, low investment and operating costs, small footprint, etc., and the FGD rate reaches 70% to 95%. The method uses the principle of spray drying to spray the absorber slurry atomized into the absorption tower. In the absorption tower, the absorber is chemically reacted with the sulfur dioxide in the flue gas while absorbing the heat in the flue gas to make the water in the absorber evaporate and dry, and the waste residue is discharged in dry form after completing the?scrubber?reaction.

The reaction system of rotary spray drying flue gas?scrubber?method is mainly composed of limestone slurry preparation system, reaction tower system, dust removal and purification system, fly ash conveying and treatment system, activated carbon injection system and automatic control system. The method mainly includes four in steps: preparation of absorber, atomization of absorber slurry, mixing of mist particles with flue gas, absorption of sulfur dioxide and being dried and?scrubber?waste slag discharge.

Rotary spray drying method generally uses quicklime as absorbent. The quick lime slurry is sprayed into uniform droplets by high speed rotating atomizer of 15000~20000r/min, and the diameter of its fog particles can be less than 100 microns, which has a large surface area. solid waste residue with little water content.

SDA technology has achieved good results in treating exhaust gases from the combustion of?low and medium sulfur fuels; however, ocean-going vessels generally use?poor quality (heavy, high sulfur) fuel oil?as fuel with relatively high sulfur content, so for SDA technology, when treating exhaust gases from the combustion of high sulfur fuels, a high concentration of limestone slurry is required as the?scrubber?absorber, which will cause a series of problems such as clogging and corrosion to the equipment. In addition, the end product of the reaction is C In addition, the end products of the reaction are CaSO4, CaCl2 and CaF2, which are also difficult to handle. Therefore, without the development of a new type of?scrubber?absorbent and the end products of the reaction cannot be recycled, it is difficult for the rotary spray drying flue gas?scrubber?reaction system to be widely applied on international ships.

Compared with dry and semi-dry methods, wet?scrubber?is widely used with its mature technology. The ship?scrubber?index is stricter than the general industrial?scrubber, and is restricted by the specific environment on board, so it is very difficult to develop and apply the technology and process for ship?scrubber. The method of?scrubber?must be based on the specific working conditions and the required?scrubber?index.

The wet FGD process is the most widely used?scrubber?method in the world, featuring high?scrubber?efficiency, and its?scrubber?reaction process is gas-liquid reaction, with fast?scrubber?speed and high efficiency. At present, the wet flue gas?scrubber?technology widely used in ships is mainly magnesium-based - seawater?scrubber?technology and seawater open scrubbing method, alkaline aqueous solution closed-loop scrubbing method and seawater - alkaline aqueous solution mixed scrubbing method, which can ensure that the SO2 content in the exhaust gas after?scrubber?is lower than the exhaust gas index of burning sulfur containing 0.1% m/m fuel oil, and the various water quality indexes of the scrubber water after the exhaust gas is treated meet the IMO regulations.

The core of the wet FGD scrubbing method is the absorption tower, which is a single-stage open spray, integrated absorption of sulfur dioxide device. In the scrubbing zone of the FGD tower, the water pump pumps the seawater/alkaline aqueous solution to the spraying layer, where the seawater/alkaline slurry is atomized under the action of the nozzles and comes into full contact with the engine exhaust upstream and absorbs the hydrogen sulfide H2S, sulfur dioxide SO2 and sulfur trioxide SO3 in the flue gas, while dissolving and scrubbing most of the soot in the flue gas.

Seawater?scrubber

Seawaterscrubber?tower is a more maturescrubber?technology developed in recent decades, through the sea water pump to the weak alkaline natural seawater into thescrubber?tower, in the form of spray and the host, auxiliary and boiler generated exhaust gas mixed in thescrubber?tower, the use of seawater's acid-base buffering capacity and strong neutralization of acidic gases to effectively remove SO2 in the flue gas. Thus, the discharged exhaust gas meets the sulfide emission requirements, and the scrubber water containing sulfate is discharged into the sea after treatment to meet the standards.

The basic components of seawater open-loopscrubber?tower system include: flue gas system, seawater water supply system, seawater discharge system, SO2 absorption system (spray tower), control module, gas sampling and monitoring system, and water quality sampling and testing system.

Washing process: seawater is pumped into the scrubber?tower and sprayed out through the nozzles in the spray tower, reacting with the retrograde flue gas. The desulfurized flue gas continues to go up through the gas monitoring system and is discharged into the atmosphere, while the neutralized sulfide solution goes down through the water quality testing system and then flows into the sea through the seawater discharge system.

Features: Seawater scrubber?devices and processes are relatively simple, efficient and environmentally friendly, reliable and economical, less polluting to the ecosystem, low construction and maintenance costs, no additional reactants required, plus seawater scrubber?is most readily accepted by the crew (inert gas systems that have been used on ships for decades also use seawater for cooling and scrubbing) and is considered one of the more desirable methods of treating ship engine exhaust.

The open ocean FGD method is often used in FGD projects that are more regional (seawater areas), have relatively small flue gas volumes or have simple process requirements. Some waters have alkalinity values too low to be utilized by open FGD towers, such as the Great Lakes region of the United States, the Port of St. Petersburg, and the Mississippi River.

At present, the effect of seawater scrubber?method is not very satisfactory when dealing with the gas emitted from the combustion of high sulfur fuel, the equipment occupies large space, the scrubber?efficiency is low in the low salinity sea, the energy consumption of the pump is large, the sediment is easy to block the spray hole, etc. Once these problems are solved, it will greatly promote the application of this technology in the treatment of ship exhaust.

Closed-loop?scrubber?with alkaline aqueous solution

Closed-loop?scrubber?tower with alkaline aqueous solution is suitable for ships sailing in SECA area, fresh water and harbor waters for a long time. Its working principle is to have a special alkaline solution (NaOH, Mg(OH)2, etc.) diluted with fresh water circulating cabin cabinet on the ship. Scrubbing water mixed with alkaline solution is used as flue gas scrubbing medium, and the scrubbing water is circulated in the closed system. When the closed system is started, the circulating pump draws the scrubber water from the system cabinet, cools it through the heat exchanger and delivers it to the scrubber tower, where it is mixed with the exhaust gas generated by the main engine, auxiliary engine and boiler in the?scrubber?tower in the form of spray, and after the reaction, the sulfide is removed and the qualified flue gas is discharged into the atmosphere.

The basic components of the closed-loop?scrubber?tower system include: seawater cooling system, freshwater circulation system,?scrubber?tower, control module, water treatment unit, gas sampling and monitoring system, water quality sampling and testing system, dosing unit and circulation cabinet, storage cabinet, etc.

Washing process: first add the appropriate amount ofscrubber?chemicals according to the ship's load, and enter thescrubber?tower after mixing thescrubber?chemicals and fresh water in the circulation cabinet during thescrubber?work. The reaction process in thescrubber?tower is similar to the open-loop type, but the destination of the solution after washing is different from the open type which is directly discharged into the sea, and the solution after washing in the closed-loopscrubber?tower is circulated into the circulation cabinet. When the solution concentration and residual debris in the circulation cabinet reach a constant value after a period of operation, the water treatment unit starts to separate the solution: the water treatment unit will continuously extract the long-used washing water containing sulfate from the circulation cabinet for treatment, and the resulting waste sludge is stored on board and will be transferred for disposal when the ship is in port. The lye solution will be continuously added to the wash water as the main reaction medium to participate in thescrubber?reaction.

The fresh water of the closed loop FGD tower system is a closed loop design, and the seawater is not directly used as a scrubbing solution to react with the flue gas, but only as cooling water to cool down the circulating fresh water, so the closed loop FGD tower is truly meaningful zero emission.

Features:?lower energy consumption, low requirements for sea environment (not affected by seawater alkalinity); more system components, occupy large space on ships, and the cost of installation and retrofitting increases; need to bear the cost of reactants (caustic soda neutralizer), storage space for reactants and washing water; washing water is not directly discharged into the sea, generally arriving at the port after treatment, there is no problem of polluted seawater, and can meet the sulfur oxide control zone emission requirements.

Hybrid Scrubber

Hybrid FGD towers, as the name implies, are composite systems that combine the characteristics of both open and closed loop FGD towers, allowing switching between the two modes depending on the requirements of the environment, and are suitable for ships sailing on global routes, including offshore, SECA zones, freshwater zones and in-port waters. However, in some cases, a hybrid FGD tower will also refer to a device that adds additional alkaline (to enhance the effectiveness of sulfur removal) to an open system, depending on how the product is named by the specific supplier.

The hybrid FGD tower is an organic combination of open-loop and closed-loop FGD towers, which can be switched between open and closed modes according to demand during FGD work. The basic components of the hybrid FGD tower system include those of the open and closed-loop FGD tower system, and its control module adds functions such as automatic switching and more detection and monitoring points.

Hybridscrubber?tower washing process: open mode using seawater washing flue gasscrubber, and seawater openscrubber?tower work the same way; closed mode using freshwater added desulfurizer circulating solution washing flue gasscrubber, and alkaline solution closed-loopscrubber?tower work the same way.

Features:The hybridscrubber?system can be flexibly selected and switched between dual modes: open mode can be used to use natural seawater forscrubber?in waters without drainage requirements, reducing the consumption of desulfurizing agent and lowering costs; closed-loop mode can be used in waters with strict control of sulfur emissions to achieve zero emission requirements. The initial installation and retrofitting of hybridscrubber?devices are costly and take up a lot of ship space.

Ammonia flue gasdesulfurisation?technology

Ammonia flue gasdesulfurisation?technology is a flue gas purification technology that uses ammonia as an absorbent to remove SO2 and other pollutants from flue gas. Ammonia and flue gas are mixed in contact in the absorption tower, and the sulfur dioxide in the flue gas reacts with the ammonia to produce ammonia sulfite, which is oxidized to produce ammonia sulfate solution, which is crystallized, dehydrated and dried to produce ammonia sulfate.

Ammonia is a good alkaline absorbent, the absorption of SO2 is an acid-base neutralization reaction, the stronger the alkaline absorbent, the more favorable absorption, ammonia is stronger than calcium-based absorbent; ammonia absorption of SO2 in flue gas is a gas-liquid or gas-gas reaction, fast reaction rate, complete reaction, high utilization rate of absorbent, highscrubber?rate, simple system, small equipment size, low energy consumption, there is no scaling and blockage phenomenon.

Ammonium sulfate, a by-product ofscrubber, is an agricultural fertilizer, and the revenue from the sale of the by-product can reduce the operating costs of the ship.

Ship Super Gravity Scrubber?System

Super gravityscrubber?technology is a wet flue gasscrubber?technology, the basic principle is to use the centrifugal force generated by the rotation of the rotating bed to produce a super gravity environment to strengthen the reaction and transfer process. It breaks through the limitation of gravity field of traditional mass transfer equipment, has wide adaptability to absorbent, replaces the traditional hugescrubber?tower, significantly reduces the size of the device, and greatly improves the efficiency of the reaction and separation process. The speed of super gravity equipment is 900rpm, and the removal rate of sulfur dioxide is more than 98%.

Supergravity technology uses the powerful centrifugal force generated in the rotating packed bed --- supergravity, so that the flow rate of gas and liquid and the specific surface area of the packing is greatly increased, and the liquid is in contact with the gas at a great relative speed in the curved flow channel under high dispersion, high mixing, strong turbulence and rapid renewal of the interface, greatly enhancing the transfer process of a device.

Rotating packed bed and the traditional tower equipment has a major difference, one is the liquid phase flow from the gravity field conditions into a super-gravity field conditions, fluid mechanical properties and gas-liquid mass transfer between the heat transfer law is different; second is the equipment from the traditional static device into a rotating motion device. In the rotating packed bed, the materials between different phases make compulsory contact motion, and the liquid phase is dispersed into thin film or fine droplets, which greatly improves the phase boundary area; violently stirring the velocity, concentration and temperature boundary layer, which strengthens the transfer process.

Rotating packed bed is divided into two types, counter-flow type rotating packed bed and staggered flow type rotating packed bed: counter-flow type rotating packed bed, which is characterized by forced airflow from the outer circular periphery of the packed bed into the rotating packed bed, from the outside to the inside for compulsory flow, and finally out from the middle. And the liquid is ejected from a static distributor located in the center and sprayed into the rotating body, which flows out through the packing from inward to outward under the action of centrifugal force, so that an efficient counter-flow contact occurs between the gas and liquid, and under the high-speed rotation of the circular rotator, the strong centrifugal force is used to make the gas-liquid film thinner and the mass transfer resistance is reduced, which enhances the mass transfer rate and processing capacity of the equipment.

The super gravityscrubber?system uses the redox nature of the alkaline absorber to absorb the SO2 in the exhaust gas, and the sulfur-containing solution is fed into the settling-oxidation regeneration tank (referred to as "settling tank"). In the settling tank, the crystalline singlet sulfur settles by gravity to the bottom of a funnel-type cone, and the separated sulfur slurry (containing about 5%-15% wt. sulfur) is sent to a belt vacuum filter, where the product sulfur (particle size 50-150 μm) is obtained by filtration.

The advantages ofsupergravityscrubber?technology compared to conventionalscrubber?technology are:

1. the absorption and oxidation of SO2 and the regeneration process of the absorber in a super gravity machine with ultra-high mass transfer efficiency using a high sulfur capacity absorber, which greatly reduces the amount of absorber solution;

2. The equipment is small in size and easy to arrange. The system can reduce the volume by more than?40%?compared with the current U-type tower and I-type tower while meeting the SO2 emission reduction efficiency, which greatly reduces the requirement for ship installation space and has a good prospect of promotion and application.

3. high?scrubber?efficiency of more than?98%?and fast?scrubber?rate;

4.modular skid-mountable;

5. wide range of application,?scrubber?efficiency is not easily affected by the nature of the flue gas;

6. wide adaptability of absorbent, which can be ammonia (ammonia), caustic soda, soda ash, sodium citrate, organic amine, ionic liquid, etc., and high utilization rate of absorbent;

7. strong adaptability to changes in unit load, capable of adapting to fast start, cold start, warm start, hot start, etc;

8. Good operational reliability, not easy to scaling.

9. The use of settling tank is conducive to the separation of solid and liquid, and there is no secondary pollution problem because of the direct generation of sulfur.

10.This technology can be used for?denitrification (NOx)and?decarbonization (CO2)?in addition to?desulfurisation.

Shipowners can choose according to the characteristics of the above mentioned?scrubber?towers as needed to meet the requirements of ships sailing in different emission control areas. In addition, shipowners can also choose FGD towers with U-orI-type?tower structure and double-tower structure according to the characteristics of their respective ship types and oil-consuming equipment.

Double tower structure of?scrubber: the?main engine flue gas can go straight to tower 1, and the auxiliary engine flue gas goes to tower 2. There is a gas-tight remote control valve between the two independent towers, which can regulate the opening and closing of the gas-tight valve according to the actual flue gas emission under the ship's working condition, so as to realize the switching of single tower washing work and double tower washing work.

(All pictures come from internet)