Technical field

The present invention relates to a method for removing particles from exhaust gas, in which method an exhaust gas flow from an internal combustion engine is led to an exhaust gas particle cleaning device according to the preamble of claim 1. The present invention also relates to an arrangement for removing particles from exhaust gas according to claim 11.

Background art

Exhaust gases are normally cleaned by catalyst devices, but the presently available catalyst devices do not remove particles. Exhaust gas particles, particularly in diesel engine exhaust gases, are generally smaller than 0.1 μm in size, typically below 0.2 μm. Particles of this size class are extremely difficult to remove because of the negligible mass of the particles. In the vehicle industry, particles of this size are removed by filters, whereby deposited particles are removed by burning them off the filter. However, if particles contain a high fraction of ash, as when burning heavy fuel oil, deposits cannot be removed by burning them off the filter. Particles, much larger than normally found in diesel engine exhaust gases, can be removed by other means than filtration.

Summary of invention

An object of the invention is to avoid the disadvantages of prior art and to achieve a method which provides for a sufficient particle size for removal and for removal of large numbers of particles normally contained in an exhaust gas flow. This object is attained by a method according to claim 1.

The basic idea of the invention is to introduce the exhaust gas flow from an internal combustion engine, e.g. a diesel engine, into an exhaust gas particle cleaning device in which water supersaturation is present, whereby particles are grown to a larger size for removal. The exhaust gas particle cleaning device is provided with a higher temperature section and with a lower temperature section causing particle growth at the lower temperature section. The grown particles can then be removed from the exhaust gas particle cleaning device.

In order to achieve a given particle growth it is advantageous that the residence time of the exhaust gas in the exhaust gas particle cleaning device is kept sufficiently long.

The advantageous features of the method according to the present invention are given in claims 2 - 10. The main and advantageous features of the arrangement according to the present invention are given in claims 12 - 16.

Brief description of drawings

In the following the invention will be described, by way of example only, with reference to the accompanying drawings, in which

Figures 1 -4 illustrates different embodiments of the arrangement according to the present invention.

Detailed description

According to the present invention, as shown in Figures 1 -3, an exhaust gas flow from an internal combustion engine 1 , e.g. a diesel engine, is led to an exhaust gas particle cleaning device 9. Advantageously the exhaust gas flow is firstly introduced into a catalyst device 2, since the catalyst device generally re- quires a higher temperature.

In the exhaust gas particle cleaning device supersaturation is achieved by means of changes in temperature. This is done by providing the exhaust gas particle cleaning device with a higher temperature section and a lower temperature section causing particle growth at the lower temperature section. Difference between lower temperature and higher temperature may in theory be 1 - 5O ⁰ C. In practice the difference of 3 - 7 ⁰ C might be sufficient for desired particle growth at the lower temperature section. Maximum value for higher temperature is about 7O ⁰ C in order to obtain saturation phenomenom in the exhaust gas. Heat for the higher temperature section may be derived from the exhaust gas or from an external source. The lower temperature can be achieved by a cooler 10 or by adiabatic expansion. The particles can be grown to different sizes for efficient removal.

Exhaust gas particles from diesel engines are composed of solid particles and volatile compounds. The solid particles are composed of carbon and ash components of fuel and lube oil. The volatile compounds generally consist of some unburned fuel, lube oil and sulphuric acid. In order to reach high efficiency in particle removal, both the solid particles and the volatile compounds have to be removed.

The exhaust gas particle cleaning device 9 advantageously comprises two parts, a first part 3 for particle growth and a second part 4 for particle removal. In this way the first part 3 for particle growth can be provided with the higher temperature section 3a and the lower temperature section 3b. The second part 4 can advantageously be a centrifugal mist separator. The principle of the centrifugal mist separator is illustrated in figure 4.

The first part 3 for particle growth is arranged before, i.e. upstream of the second part 4 for particle growth in the direction of the exhaust gas flow. The advantage of having a first part 3 for particle growth and separate second part 4 for particle removal allows for adapting the second part 4 for particle removal to prevailing particle size, i.e. the particle size achieved in the first part. The exhaust gas flow is firstly introduced into the first part 3 and then further led to the second part 4.

In such a two-part exhaust gas particle cleaning device, supersaturation is maintained in the first part 3 for particle growth of the exhaust gas cleaning device. Line 8 indicates a water supply or water vapour supply to the first part 3. The water vapour is brought to supersaturation by changes in temperature, whereby particle growth commences in the first part 3.

Preferably all particles are grown to a given desired particle size. Particles can also be grown to different sizes. In order to initiate required particle growth there is sufficiently long residence time of the exhaust gas in the exhaust gas particle cleaning device, i.e. in the first part 3 of a two-part exhaust gas particle cleaning device. Consequently, a desired given particle growth can be achieved by keeping the residence time long enough. Residence time of the exhaust gas in the exhaust gas particle cleaning device 9 can be adjusted by changing the flow rate in the first part 3. This can be achieved by guiding portion of the exhaust gas pass the first part 3 and/or by increasing the number of first part 3 units.

Grown particles can also be mixed with un-grown particles in order to enhance coagulation and consequently to provide for optimal removal. Optimal removal is achieved by matching particle growth in the first part 3 to particle removal efficiency in the second part 4. Particles of a size smaller than optimal or adequate removal capability of the second part 4 are grown in first part 3. Smaller particles are grown faster than larger ones in water supersaturation that is provided in the exhaust gas particle cleaning device, Therefore it is possible to in- crease the amount of particles having the size within the range removal capability of the second part 4. Preferably the amount of particles having the size of 0,1 - 1 ,0 μm is increased in the first part 3 and more preferably the amount of particles having the size of 0,3 - 0,8 μm is increased in the first part 3. This has an advantage in that devices for the second part 4 for removal can be selected more advantageously in view of cost, size, availability etc.

Depending on the degree of particle cleaning requirement the number of exhaust gas particle cleaning devices 9 as well as first parts 3 and second parts 4 may vary. They can e.g. be arranged in parallel, whereby an exhaust gas flow or a part of an exhaust gas flow can be introduced to the/each exhaust gas par- tide cleaning device 9 in order to achieve a required degree of particle cleaning, i.e. in order to achieve an appropriate emission level. The cleaned exhaust gas flow or parts of the cleaned exhaust gas flows can be combined with each other, or even with an un-cleaned exhaust gas flow, depending on the desired emission level.

In the following some examples of the arrangement according to the present invention will be discussed more in detail. Clearly, other combinations of the dif- ferent components, i.e. the internal combustion engines, catalyst devices, exhaust gas particle cleaning devices, first parts and second parts, as well as exhaust gas flow patterns may vary.

In Figure 1 the exhaust gas from an internal combustion engine 1 is firstly led to a catalyst device 2 and then to the first part 3 and subsequently to the second part 4 of the exhaust gas particle cleaning device. Line 5 indicates a by-pass flow, with respect to the exhaust gas particle cleaning device, of the exhaust gas flow. In this embodiment there are two first parts 3 and two second parts 4. Portion of the exhaust gas flow is arranged to by-pass the first part 3. Particles that are grown in the first part 3 are mixed with un-grown particles after the second part 4 or before the second part 4 as indicated with reference numbers 5a and 5b, respectively. Broken line 7 from the second part 4 indicates the removal of water including extracted particles. Line 6 indicates the final exhaust gas flow that has been cleaned, and which also may be combined with the by-pass flow 5.

In Figure 2 the exhaust gas from an internal combustion engine 1 is firstly led to a catalyst device 2 and then to the first part 3 and subsequently to the second part 4 of the exhaust gas particle cleaning device. Line 5 indicates a by-pass flow, with respect to the exhaust gas particle cleaning device, of the exhaust gas flow. In this embodiment there are two first parts 3 and one second part 4, whereby an arrow line indicates that the exhaust gas from the second first part 3 is led to said one second part 4. Portion of the exhaust gas flow is arranged to by-pass the first part 3. Particles that are grown in the first part 3 are mixed with un-grown particles after the second part 4 or before the second part 4 as indi- cated with reference numbers 5a and 5b, respectively. Broken line 7 from the second part 4 indicates the removal of water including extracted particles. Line 6 indicates the final exhaust gas flow that has been cleaned, and which also may be combined with the by-pass flow 5.

In Figure 3 the exhaust gas from an internal combustion engine 1 is firstly led to a catalyst device 2 and then to the first part 3 and subsequently to the second part 4 of the exhaust gas particle cleaning device. Line 5 indicates a by-pass flow, with respect to the exhaust gas particle cleaning device, of the exhaust gas flow. In this embodiment there is one first part 3 and two second parts 4, whereby an arrow line indicates that the exhaust gas from said one first part 3 is also led to the other second part 4. Portion of the exhaust gas flow is arranged to by-pass the first part 3. Particles that are grown in the first part 3 are mixed with un-grown particles after the second part 4 or before the second part 4 as indicated with reference numbers 5a and 5b, respectively. Broken line 7 from the second part 4 indicates the removal of water including extracted particles. Line 6 indicates the final exhaust gas flow that has been cleaned, and which also may be combined with the by-pass flow 5a.

In Figure 4 the principle of the centrifugal mist separator 4 is illustrated. The centrifugal mist separator 4 comprises a motor 42 which is arranged to rotate a disk stack 41 , i.e. rotatable exhaust guiding part. Exhaust gas is guided to centre of the disk stack 41 and exits between disks of the disk stack 41. Speed of the exhaust gas flow is increased by rotating a disk stack 41 in the centrifugal mist separator.

Another alternative is centrifugal mist separator where a motor which is arranged to rotate a rotatable exhaust guiding part comprising two co-axial pipes. Exhaust gas is guided between pipes. At least one of the pipes is rotated such that centrifugal forces are applied by a motor.

Residence time of the exhaust gas in the exhaust gas particle cleaning device 9 can therefore also be adjusted by speed of the motor of the centrifugal mist separator 4. The arrangement further comprises a return line 43 from location after the centrifugal into location between the lower temperature section 3b and the centrifugal mist separator 4. The return line 43 allows recirculation of the exhaust gas from location after the centrifugal into location between the lower temperature section 3b and back to the centrifugal mist separator 4.

The arrangement further comprises device for exhaust gas measurement 45. The speed of the motor 42 of the centrifugal mist separator 4 can be partly or fully controlled based on this exhaust gas measurement 45. Clearly also a one-part exhaust gas particle cleaning device can be used.

The description is intended to clarify the basic idea of the invention. The invention may vary in detail within the scope of the ensuing claims.