Method and Apparatus for Cleaning Combustion Gases

Field of Invention The present invention relates to a method and apparatus for cleaning combustion gases, in particular combustion gases of marine diesel engines and small liquid fuel power plants, The method and apparatus relate especially to emission abatement of sulfur oxides (SO _x =Sθ ₂ +SO ₃ ), unburαed hydrocarbons (UHC) ₃ particulate matter (PM) and a large fraction of water contained in the exhaust gases. The method, by eliminating deposit forming and corrosive constituents of the gases, allows exhaust gas re-circulation (EGR) to control emissions of nitrogen oxides and prevents visible condensation plume formation at the stack without need to reheat the gases

Background of the Invention Reducing atmospheric emissions from ships is a high environmental priority for many regions of the world because the ship engines use the lowest quality fuels and typically are not equipped with emission control devices. Consequently they cause as much air pollution as all land-based sources combined in many densely populated regions of the world. The current approach to alleviate the problem is to force usage of clean fuels, particularly low sulfur contents fuels, in most exposed seas (ex. the Baltic Sea and North Sea/English Channel), coastal and inland waterways. The problem is that the ship engines may be considered to be some sort of incinerators, which burn the otherwise useless residual hydrocarbons and in the waste majority of cases ships have just one fuel tank and are unable to carry fuels of different qualities, Even if in some cases the possibility exists, switching from one fuel to another requires modifications of the fuel and oil systems, mixing compatibility of the fuels to avoid pump seizure, as well as development of special switching routines for the fuel and oil, specific to each engine, not yet available, Other problems are the low availability of high quality fuels particularly in remote regions and the large price differentials between high and low quality fuels typically exceeding the 30 % level. As on average about 65 % of the cost of operation of a ship are fuel costs, the requirement to use low sulfur fuel without the possibility of changing fuels during voyage will have large impact on the economy of shipping in general. It is obvious that rapid development of a low cost, compact, flexible in terms of the range of flow rates, and reliable enough to be

accepted by the classification societies exhaust gas cleaning system to control SO _x and particulate emissions to be used alone or as a part of an EGR system controlling NO _x emissions too, is the top environmental issue For marine engine industry and for small, distributed power generation industry using liquid fuels where elaborate gas cleaning systems developed for large power plants cannot be used.

Seawater is a weak base with considerable acid buffering capacity so the concept of using it to wash the exhaust gases using a scrubber comes immediately to ones mind as the preferred solution of the problem of removing and neutralizing the sulfur oxides which react with water to form sulfuric (H ₂ SO ₄ ) and sulfurous acid (H2SO3), Indeed seawater scrubbers for land-based power plants became popular after the development by ABB Environmental in Norway and Norsk-Hydro of the Flakt-Hydro process in the 1970-ties as described in the well known paper by Stromnen, S, O and Hjelm, F.: "Sulfur in Fuel Gases can Safely be Absorbed by Seawater and Returned Iu Oceans", ICHEME Symposium Series No 131 pp 95-108, 1993. Currently seawater based scrubber facilities for power plants, are offered by many companies. Typically the systems offered use electrostatic precipitators to remove PM before the scrubber, intense reheat of exhaust after scrubber to avoid visible condensation plume formation at the stack and extensive discharge water cleaning facilities. Few data are available regarding the details of the processes but it is known that large amounts of seawater are used for the washing in the range often exceeding 15 kg per Nm ³ of the gases, which makes water cleaning a problem, The first commercial applications of exhaust gas seawater scrubbers in the marine industry were introduced not as an answer to today emissions control problems but. as means to generate inert gas for oil tankers to fill air space in cargo tanks to prevent hydrocarbon vapor explosions. The first such systems appeared in 1930's and their installation became common since 1970's. Many companies supply the systems. One of the most advanced is the Moss Flue Gas System supplied by Hamworthy KSE Moss AS from Norway. The scrubber unit is combining three scrubbing principles, a high energy, large pressure drop Venturi scrubber, a wet filter and a spray section, for high-efficiency cooling and cleaning the gases. Concentric arrangement with a demister section and a mesh type wet filter gives independence of ships pitching and rolling without loss of efficiency. However, soot-blowing devices are also included in the units, which shows that deposit formation is a problem. The system uses about 15 kg of water per Nm ³ of gas and produces gases having a temperature of 5 ⁰ C above seawater temperature and 100 % humidity.

SOz is reduced in the system from typically 3000 ppm to less than 100 ppm and soot extraction equal to 99 % but just for particles larger than 1 micrometer is claimed.

A conceptually identical system is proposed for emission abatement of marine engines in the patent WO9944722, where the scrubber serves also as a silencer. The proposal does not seera to offer a practical solution due to excessive power requirements, lack of gas reheat to avoid condensation plume formation and production of large quantities of polluted water. No performance data of the system were reported.

Much better documented is the performance of a recent development of a combination of a sieve tray scrubber or bubble washer with a silencer for exhaust cleaning of marine engines known under the name "Ecosilencer®", developed during the period of many years by the Canadian company Marine Exhaust Solutions, The first design followed the US Patent 6402816, and the latest the much simpler patent WO 03045524. The patents conclude the long evolution line of engine exhaust gas bubble washer scrubbers described in ex. US Patent 4137715, US Patent 4190629, US Patent 4300924, US Patent 5129926, DE 42200850 and others Although the system is promising it has as other bubble washers low flexibility, requires low average gas flow rates leading to large sizes, produces a large amount of small droplets due to bubble bursting at the surface of the water pool difficult to eliminate, produces saturated exhaust plumes and thus needs intense reheating the gases, requires cleaning of large amount of polluted water and is sensitive to ship movements. It has also the main disadvantage of all seawater scrubbers that they are completely inefficient in low salinity zones. Consequently there is a need for an improved exhaust gas cleaning system for marine engines operating in any waters, much smaller, more flexible, less complicated and requiring less water cleaning, Similarly there is a need of simple gas cleaning facilities for small power plants using liquid fuels, where application of solutions developed for large power systems is not economically viable, In a preparatory theoretical and experimental study of a number of scrubber types for exhaust gas cleaning of diesel engines including condensing scrubbers the author has found however, that condensers working in a film condensation mode of the exhaust vapors can be used to capture in addition to almost all the sulfuric acid, large fraction of the solid particulate matter (soot, ash) due to thermophores! s, diffusiophoresis and Stefan flow, condensing unburned hydrocarbons, vapor phase of sulfurous acid and vapor phase of nitric acid due to parallel condensation as well as after addition of acid neutralizing chemicals also to capture the SOj contained in the exhaust by absorption. The effectiveness of the condensation and absorption

processes was observed to increase substantially when water vapor concentration in the gas increases so that optimally the whole gas cleaning process should be arranged in a system where the most efficient and convenient to use, film evaporation and film condensation follow one after another enhancing the pollutant capture processes. This may seem questionable as the current state of art in building exhaust heat recovery units for engines burning heavy fuels, marine engines in particular, is not to allow the gas temperatures to fall below 160-180 ⁰ C io avoid deposition of high temperature boiling point viscous hydrocarbons and concentrated sulfuric acid on the walls, which capture soot and generate a danger of violent and destructive soot fires. It was then also surprisingly found that when the temperature of the condenser walls is well below the mentioned above temperature and in particular below the diluted sulfuric acid dew point a massive deposition on the walls of the weak sulfuric acid _> water and all the other condensable vapors in form of a film occurs, capturing the solid particles and causing the film to flow easily under gravity without any danger of soot fires and deposit build up for engines burning even the worse heavy and residual fuels containing large amount of sulfur and ash. The films containing large amount of combustion water can then, after injection of alkaline liquid, be used for SO ₂ absorption and re-circulated to increase the vapor contents in the system enhancing further the gas cleaning. In addition the film flow in the condenser separates the condensable liquids from the exhaust gas in such a way that outside the film the gas has much higher temperatures and is dry which prevents condensational plume formation when the gas is released into atmosphere without need of reheating.

Utilization of condensation to recover the latent heat and control emissions is a known technology and was subject to many patents ex. US 5405590; US 5080619; US 5368086; US 5826518; US 6273990 but in none of them the question of assuring stable and well controlled film evaporation and subsequent condensation with recirculation of the condensate in a simple integrated unit is considered and an optimal structure of an evaporator and condenser for that purpose, compact and having low pressure losses, proposed, it is most likely because to recover maximum amount of high quality (high temperature) heat according to current state of the art the gas and wail temperatures should be close to each other whereas liquid film formation requires large temperature differences. Some patents ex, US 2004031480 consider an optimal structure of a condenser suggesting finned "tube in a cross flow" arrangement, For diesel engines either condensation-growth particle scrubbers US 5176723 or very complicated systems including several complex reactors, soot and condensed hydrocarbon capturing devices and

cooling using atmospheric air or eventually a heat pump are proposed as in US 6240725 and JP 2005240692.

Summary of the Invention

Considering the deficiencies of the current art it is an object of the invention to provide a method and apparatus for cleaning combustion gases especially of heavy fuel burning marine diesel engines and small liquid fuel power plants using heat exchanger based evaporation and condensation in the exhaust gases, where both the evaporator and condenser operate in film modes.

Yet another object of the present invention is to provide an integrated evaporator-condenser combination operating over a large flow rate range and in particular allowing high gas flow rates with minimal pressure losses.

λ still further object of the invention is to provide an evaporator-condenser combination for gas cleaning which does not require reheating of the gas to avoid condensation plume formation in the atmosphere. Another object of the invention is to provide an evaporator-condenser combination composed of large number of identical tubes where precise dosage of neutralizing chemicals to the liquid film can be realized

A still further object of the invention is to provide an evaporator-condenser combination based on tested technology, insensitive to ship movements, which can easily be accepted by the classification societies for installation onboard ships.

Finally, it is an object of the invention to provide an evaporator-condenser combination where the evaporation and condensation is realized using seawater for both cooling and heating,

As more precisely specified in the following description and claims the objects of the invention are achieved by a method for cleaning combustion gases, in particular combustion gases of marine diesel engines and small liquid fuel burning power plants comprising the steps of: introducing exhaust gases containing sulfur oxides, unburned hydrocarbons and solid particulate matter into a heat exchanger comprising at least one heat transfer surface, preferably in form of a tube, having an evaporator section and a condenser section integrated with each other;

spraying a neutralizing solution in water onto said at least one heat transfer surface at a rate proportional to the exhaust gas flow rate being introduced into said heat exchanger; evaporating at least portion of the neutralizing solution in said evaporating section to allow at least a portion of sulfur oxides and nitrogen dioxide to react with water vapor to form acidic gases; condensing said acidic gases and unburned hydrocarbon vapor in form of a condensate film, which deposes on said heat transfer surface in said condenser section, reacts further with the neutralizing alkaline components, absorbs sulfur dioxide and captures while deposing the solid particulate matter; removing said condensate film exiting the heat transfer surface separately from purified exhaust gases; discharging purified exhaust gases from the process,

The remaining features of the method according to the invention are apparent from dependent claims 2-9 enclosed herewith, The objects of the invention, are also achieved by an apparatus for cleaning combustion gases, in particular combustion gases of marine diesel engines and small liquid fuel power plants comprising; an exhaust gases inlet and exhaust gases outlet; a heat exchanger located between said exhaust gases inlet and exhaust gases outlet and comprising at least one heat transfer surface having an evaporator section and a condenser section integrated with each other and placed in a housing with a circulating cooling water inlet at the bottom of the heat exchanger and a high flow rate outlet situated in the upper part of the heat exchanger and a low flow rate outlet situated above the later; means for separating the gas and the condensate located at the bottom of the heat exchanger; a spray system for introducing a neutralizing solution in water onto said heat transfer surface at a rate proportional to the exhaust gas flow rate being introduced into said heat exchanger wherein said spray system being connected with said collecting means or with said low flow rate outlet of cooling water.

The remaining features of the apparatus according to the invention are apparent from dependent claims 1 1-16 enclosed herewith,

The apparatus according to the invention is preferably composed of a bundle of open pipes resembling a smoke tube boiler, where the cleaned gases flow downwards and the temperature controlling water upwards and where the condensate collected at the bottom is partially re- circulated and injected at the top after addition of acid neutralizing chemicals, to be partially evaporated to enhance the gas cleaning processes. Both evaporation and condensation occur in a falling film mode and the film is used for absorption of remaining SOj. Absorption of COs is prevented and absorption of SOj maximized by keeping the pH of the condensate at the bottom of the apparatus close to 6-6.5.

The method and apparatus according to the invention, by eliminating condensing, deposit forming and corrosive constituents of the gases, allows exhaust gas re-circulation (EGR) to control emissions of nitrogen oxides and prevents visible condensation plume formation at the stack without reheating the gases.

Brief Description of the Drawings

The invention will be described in the following in greater details by way of example only and with reference to the attached drawings in. which:

Fig, 1 is a schematic partially broken away side view of the exemplary apparatus for cleaning combustion gases with integrated evaporator-condenser sections taken in direction of arrows I-I of Fig 2;

Fig. 2 is a schematic partially broken away top view of the apparatus of Fig, 1 taken in direction of arrows H-Il;

Fig 3 shows schematically an experimental apparatus in which experiments on the method according to the present invention have been carried out, atid

Fig, 4 is a closer sectional view "A" of Fig. 3 showing the condenser section exit on a relatively large scale presenting the gas and liquid film separation step according to the invention,

Best Mode of Carrying Out the Invention

The basis of the present invention is the observation that the key process in cleaning combustion gases, in particular exhaust emission abatement for diesel engines and small power plants using low quality fuels, is to increase water contents (humidity) of the exhaust gases and then it removal by condensation, which moves the pollutants from gas into a liquid film. To minimize pressure losses in the exhaust gas flow and allow high flow rates the process is realized in flow separation free heat exchangers ex. cooled tubes, utilizing two very effective methods; film evaporation and film condensation. The water needed for the gas treatment is obtained from re- circulation of the condensate or filtered water supply ex, seawater preheated in the apparatus. Use of evaporation and condensation enhances all the liquid-gas mass transfer and reaction processes including capture of solid particles and droplets formed by bulk condensation which are deposited in the condensation films by theαnophoresis, diffusiophoresis and Stefan flow generated by the condensation, capture of SO3 and NOj, which after reaction with water vapor to form gaseous acids condense to contribute to the film formation, capture of unburned hydrocarbons by parallel condensation, and finally capture of SO ₂ by absorption in the condensate film. To enhance the SO2 absorption, neutralizing alkaline additives in the form of ex, sodium hydroxide (caustic soda) or sodium carbonate (ash soda) solutions in water are added to the condensate or process water. Thus, all pollutants carried in the exhaust gas of a diesel engine or small liquid fuel power plant, except NO are removed to smaller or larger degree depending on the process parameters in the proposed exhaust gas cleaning apparatus. Removing the acids, particulate matter and UHC allows however reliable and simple low pressure cold exhaust gas re-circulation to control NO emissions without causing corrosion, and deposit formation in the engine or combustion equipment using even the worse fuels, The object, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof.

Referring now to Fig. 1-3, an apparatus according to the invention is shown. The apparatus comprises an exhaust gas inlet 1 through which the exhaust gas enters the apparatus and exhaust gas outlet 2 through which said gas exits the apparatus. λ number of heat transfer surfaces 3 in form of preferably vertical tubes 4 are located between the exhaust gas inlet 1 and the clean exhaust gas outlet 2 to form the heat exchanger of the apparatus. The heat exchanger is located

in a housing 5 through which a cooling water 6 flow is realized. For that purpose the housing 5 is provided wkh water inlet 7 located at the bottom of the heat exchanger connected to a water pump 8 and with a high flow rate outlet 9 placed in the upper part of the housing 5, as well as a low flow rate water outlet 10 located above the high flow rate water outlet 9. Each heat transfer surface 3 is divided into two integrated sections: evaporator section 1 \ and condenser section 12 being parts of the same vertical tube 4. The division of heat transfers surfaces 3 into evaporator section I t and condenser section 12 is realized by the rate of cooling water flow - higher in the condenser section 12 and tower in the evaporator section . 1 , The evaporator section 1 1 is formed between the high flow rate outlet 9 and the low flow rate outlet 10 and the condenser section are placed below the evaporator section 11 ,

Means for separating the purified gas and the condensate are located at the bottom of the heat exchanger. Said means for separating the gas and condensate comprises an exhaust gas tube 22, a condenser section surface 20 for depositing condensate film being situated coaxially with the exhaust gas tube 22. Further, said means for separating comprises an accumulator 13 for collecting the condensate film 19 exiting the condenser section 12 of the heat transfer surfaces 3 being situated below said condenser section surface 20. The diameter of said exhaust gas tube 22 is smaller than the diameter of said deposing surface 20 at least twice the thickness of the condensate film 19 to allow it to flow down to the accumulator 13 (shown the best on Fig. 4). The accumulator 13 is connected with a duct 14 provided with a pump 15 to discharge part of the condensate from the accumulator 13 and circulate it to the spray system of the apparatus. The surplus condensate is removed for cleaning or disposal via the duct 17. Optionally, the duct 14 may be connected with the low flow rate outlet 10 via a water duct 23 to add heated cooling water to the concentrated neutralizing supplement before it is sprayed onto heat transfer surfaces 3. The heated water duct 23 is provided with a valve 24 to control the portion of the water to be added into the duct 14, if it is required, without disturbing the outflow of the heated cooling water from the evaporator section of the apparatus.

The spray system comprises a collection, of nozzles 16 adapted for spraying an alkaline neutralizing solution, such as ex sodium hydroxide or sodium carbonate, onto the heat transfer surface 3 with a rate proportional to the gas flow rate. Additionally, the duct 14 has a condensate outlet 17 via which the surplus condensate is drained for cleaning and disposal, as well as it is provided with a neutralizing solution inlet 18 through which the concentrated supplement is

admitted in response to a signal from a control system when. pH of the condensate drops below a predetermined level.

The method of cleaning combustion gases, according to the invention is the following.

Referring to Fig. 1 and 2, the exhaust gas enters the apparatus via an inlet 1 and then flows over a number of heat transfer surfaces 3 in form of preferably vertical tubes before exiting from the outlet 2. In parallel, diluted alkaline neutralizing solution streams il, i2... containing, ex. sodium hydroxide or sodium carbonate, are sprayed through a collection of nozzles 16 onto the heat transfer surface 3 with a rate proportional to the gas flow rate and are first partially evaporated in the evaporator section 11 and then condensed in the condenser section 12, The acidic gases (SO3 and NO2) react with water vapor and then condense, dissociate and react with the neutralizing basic components and are removed with the condensate. The alkaline condensate absorbs also and removes SO ₂ front the gases, In parallel unburned hydrocarbons condense, are captured by the condensate film and removed from the gas, At the same time thermophoresis, diffusiophorests and Stefan flow induced by the condensation cause deposition of solid particulate matter (soot, ash) carried by the gas and fog droplets of condensable material formed by bulk condensation on the condensation films 19 (see Fig, 4) flowing along the condenser surfaces 20. To minimize the fog formation process and maximize film condensation the temperature of the condenser section 12 of the heat transfer surfaces 3 should be kept well below the dew point temperature of the condensing vapors preferably more than 25K below the dew point,

The condensate exiting the heat transfer surfaces 3 of the condenser is collected in an accumulator 13. The accumulator 13 serves as a buffer for the condensate circulation returning the condensate to the inlet of the apparatus to maintain a constant flow rate of the liquid through the system When it exits the accumulator 13 the condensate passes through a duct 14 to which the concentrated alkaline supplement is admitted through the inlet 18. The pH of the solution in the accumulator 13 is controlled, As the acidic gases enter into the condensate and react with the neutralizing alkaline additives the pH of the condensate drops. When it falls below a predetermined level, typically 6-6,5, a control system admits more of the concentrated alkaline solution to the spray system via the inlet 18, Alternatively instead of re-circulating the condensate, a portion of the cooling water taken from the hot water outlet of the evaporator section 10 may be used for spraying of the neutralizing

additives and enhancement of humidity of the exhaust gases. This can be preferred in marine application where arbitrary quantities of alkaline seawater are available able to reduce the use of neutralizing chemicals.

The division of the heat transfers surfaces 3 into evaporator sections 11 and condenser sections 12 is realized by the rate of cooling water flow- The cooling water 6 is admitted at the bottom of the heat exchanger via the water inlet 7 and it main part is released at the beginning of the condenser section 12 through the high flow rate outlet 9. A much smaller portion is released at the entrance of the evaporator section 11 through the low flow rate outlet 10 When surface seawater is used for cooling, the water entering the apparatus has typically a temperature of about 10-15 ^ϋ C The temperature increases in the condenser section 12 to about 20-25 ⁰ C Temperatures below 71°C should be kept in the evaporator section 11 to avoid scale formation. The evaporator section 1 1, between the outlets 9 and 10, produces surplus heat as the gases entering the apparatus have typically ISO ⁰ C. The heat is of high enough quality to be effectively utilized, Exactly the same functions of all the elements are realized in the test apparatus, Fig. 3. The nozzle 16 in form of swirl or multi-orifice atomizer is used in this case to distribute the alkaline solution on the evaporator surface. Admission of the exhaust gas through a number of flow restricting orifices 21 assures uniform gas flow distribution across the inlet of the apparatus.

At the bottom of the condenser section 12, the concept of the gas-condensate separator according to the invention is illustrated, It is more distinctly outlined in Fig, 4. The separator is preferably in form of a skimmer i.e. an exhaust gas tube 22 that is large enough to accept the gas but not the condensate film 19 flowing along the condenser section surfaces 20, Optionally, other water separator types can be used,

The gas streams j 1 , j2.. , . leaving the condenser section 12 have a much higher temperature, than the liquid condensate film 19, and they are dry because most of the condensable components of the exhaust gases are captured in the film, thus the gas does not require reheating to avoid condensation in the exhaust plume, producing visible exhaust at the stack.

Although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that various changes omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.