WO2012089922A1 - Particle measurement unit - Google Patents

Particle measurement unit Download PDF

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Publication number
WO2012089922A1
WO2012089922A1 PCT/FI2011/051157 FI2011051157W WO2012089922A1 WO 2012089922 A1 WO2012089922 A1 WO 2012089922A1 FI 2011051157 W FI2011051157 W FI 2011051157W WO 2012089922 A1 WO2012089922 A1 WO 2012089922A1
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WO
WIPO (PCT)
Prior art keywords
particle measurement
measurement unit
particle
essentially
clean gas
Prior art date
Application number
PCT/FI2011/051157
Other languages
French (fr)
Inventor
Kauko Janka
Original Assignee
Pegasor Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pegasor Oy filed Critical Pegasor Oy
Publication of WO2012089922A1 publication Critical patent/WO2012089922A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/02—Devices for withdrawing samples
    • G01N1/22—Devices for withdrawing samples in the gaseous state
    • G01N1/2247—Sampling from a flowing stream of gas
    • G01N1/2252—Sampling from a flowing stream of gas in a vehicle exhaust
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B08—CLEANING
    • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
    • B08B5/00—Cleaning by methods involving the use of air flow or gas flow
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
    • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
    • F01N3/023—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters using means for regenerating the filters, e.g. by burning trapped particles
    • F01N3/027—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters using means for regenerating the filters, e.g. by burning trapped particles using electric or magnetic heating means
    • F01N3/0275—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters using means for regenerating the filters, e.g. by burning trapped particles using electric or magnetic heating means using electric discharge means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
    • G01N15/06—Investigating concentration of particle suspensions
    • G01N15/0656—Investigating concentration of particle suspensions using electric, e.g. electrostatic methods or magnetic methods

Definitions

  • Fine particles provide a serious health risk and thus measures to avoid their exhaust to air as well as techniques to be able to measure fine particle concentrations are of great importance. Especially it is important to measure fine particles generated by combustion, e.g. by power plants, small-scale biomass combustion and combustion engines.
  • the current development aims to continuous decrease of fine particle emissions from e.g. diesel engines.
  • Fine particle measurement is, however, also important for the research and development of combustion engines and combustion-engine-based vehicles.
  • Fine particle emissions from combustion are reduced by filtering the particles.
  • particle filters are typically installed to the stacks, chimneys or engine exhaust pipes.
  • the Diesel Particulate Filter (DPF) needs to be frequently regenerated by burning the soot particles accumulated on the filter. This can be realized by increasing the filter temperature and feeding a subsequent amount of air through the filter during the regeneration cycle.
  • Patent application WO 2009/109688 Pegasor Oy, 11.9.2009, describes a process for measuring particle concentrations in a gas using an ejector for producing an essentially constant sample flow and for efficient mixing of the particle-containing sample and an essentially clean, ionized gas.
  • the invention also relates to an apparatus implementing such process.
  • the process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.
  • the method essentially all particles traveling in the exhaust pipe are charged and the particle concentration is determined from the electrical current carried by the particles.
  • the apparatus does not collect the particles.
  • Both apparatus described above are non-collecting and thus should not suffer from particles accumulated on the device.
  • the particle accumulation becomes a problem.
  • the measurement units which are based on particle charging particle accumulation leads to soiling of discharge unit and electrical insulators, change in the mass flow through the sensor, etc.
  • the oxygen concentration of diesel exhaust gas is sufficiently less than the oxygen concentration of air and cannot be freely increased.
  • the closed loop fuel feedback control system is responsible for controlling the air/fuel ratio of the catalytic converter feed gas.
  • the electronic control module keeps the air/fuel ratio adjusted to around the ideal 14.7 to 1 ratio. If the air/fuel ratio deviates from its preprogrammed swings, catalyst efficiency decreases dramatically for NO x reduction. For the reduced oxygen concentration the ozone generation is much reduced as well.
  • the aim of the invented process and apparatus is to overcome the problems of the prior art where the soot accumulating on the particle measurement unit creates a serious problem to the reliable operation of the measurement unit.
  • the accumulation problem is less severe in particle measurement units which do not collect the particles (non-collecting particle measurement units), and the present invention is especially beneficial in removing accumulated particles from such devices.
  • the invented process may beneficially be used in a particle sensor used for on-board monitoring of exhaust particles from a combustion engine and especially in a sensor used to monitor a particle filter in an exhaust pipe of a diesel engine.
  • the invented process may also be used in particle sensors monitoring stack and chimney emissions.
  • the invented process is based on oxidizing the particles accumulated on the walls etc. of the measurement unit.
  • the particles When the particles are accumulated the carbon (C), nitrogen (N), sulphur (S), hydrogen (H) and other non-metallic substances in the particles are converted into gaseous oxides which exit the measurement unit.
  • the invented process is thus beneficially used to monitor particles which mainly comprise non-metallic substances, such as emissions from combustion processes, like combustion engines.
  • Such parts comprise the input of the measurement unit, active measurement parts, such as an ion trap of a measurement unit based on particle charging, insulators of an electrical measurement unit, etc.
  • Particle oxidization can be carried out by using ordinary air for oxidizing the elements in the particle.
  • oxidization with air normally requires quite high oxidization temperatures, typically in excess of 450°C. This creates special requirements for the materials used in the particle measurement unit, especially to the electrical insulators used in electrical measurement units as well as to the construction of the unit. Also the heating requires high power feeding to the system, which is in many case difficult.
  • the oxidation temperature can be lower than with air if more efficient oxidizers than air are used.
  • Such compounds include e.g. ozone (0 3 ) or some nitrogen oxides (N x O y ).
  • the present invention is predominantly beneficial when it is used with particle measurement units which are based on particle charging using a corona discharge unit.
  • the corona discharge also produces ozone (O 3 ). Ozone production is found to be more efficient with negative corona voltages.
  • Such particle measurement units can work continuously with high, negative corona voltages, but usually it is more beneficial to carry out the particle measurements with low corona voltages and thus have low power consumption during the measurement and then carry out the cleaning of the measurement unit with higher corona voltage, preferably with high, negative corona voltage.
  • the corona current produced during the particle removal is preferably more than 1 microampere ( ⁇ ⁇ ) for a single corona discharge unit.
  • the gas composition may vary and the production of ozone may vary as well.
  • the gas flow containing the ozone may then be mixed with the gas flow containing the particles or it may be directly fed towards the surfaces from which the accumulated particles are to be removed.
  • the ozone-containing gas flow may be so high that it passes also through the input of the measurement unit, towards the input flow. The measurement then works in flush mode and the input channel is effectively cleaned from accumulated particles.
  • Figure 1 shows an embodiment of the invented apparatus where ozone is produced in the measurement unit and used for removing the accumulated particles.
  • the invented process is aimed for cleaning a particle measurement unit which used to measure particle concentration from combustion exhaust gases.
  • the oxygen concentration of such exhaust gas is less than the oxygen concentration of air (20.8vol-%).
  • the invented process comprises feeding essentially clean gas, where the oxygen concentration is higher than the oxygen concentration of the exhaust gas, into the particle measurement unit, ionizing the essentially clean gas by using an electrical discharge unit, using the ionized gas to charge at least a fraction of the particles to be measured, using the electrical discharge unit to produce ozone (0 3 ) from the essentially clean gas and using at least a fraction of the produced ozone (O 3 ) to essentially remove particles attached to at least some parts of the measurement unit by oxidation of the attached particles.
  • the essentially clean gas may be ordinary filtered air, where the oxygen concentration naturally is higher than 20 volume- % (vol-%).
  • the essentially clean air may also be a mixture of different gases, such as a mixture of oxygen (0 2 ) and at least another gas, such as nitrogen (N 2 ) or carbon dioxide (C0 2 ).
  • An essential feature of the invention is that the oxygen content in the essentially clean gas is higher than the oxygen concentration of the exhaust gas, so that the essentially clean gas is a more effective ozone (O 3 ) precursor than the exhaust gas.
  • the oxygen concentration of the exhaust gas may vary from less than 1 vol-% to almost 20 vol-% or even higher, if oxygen-enriched air or pure oxygen (0 2 ) is used for combustion.
  • the exact amount of oxygen in the essentially clean gas differs from case to case, but is typically roughly the amount of oxygen (0 2 ) in air, i.e. slightly higher than 20 vol-%.
  • water (H 2 0) vapor may work as an inhibitor for ozone generation.
  • dry, essentially clean gas The dew point, i.e. the temperature to which a given parcel of humid gas must be cooled, at constant barometric pressure, for water vapor to condense into liquid water, of the essentially clean gas should preferably be maximum 7°C, more preferably maximum 3°C and most preferably maximum -20°C.
  • the elevated temperature should be higher than 300°C to ensure proper particle oxidation but less than 450°C to prevent high temperatures in the measurement apparatus, which may cause extra deterioration of the apparatus.
  • the electrical discharge unit is preferably a corona discharge unit which is effective in ozone generation above certain electrical discharge current (ion current).
  • the electrical current should be preferably at least 1 microampere (1 ⁇ ), more preferably at least 2 microamperes (2 ⁇ ) and most preferably at least 3 microamperes (3 ⁇ ).
  • the invented process may be used so that it produces essential amounts of ozone (0 3 ) either constantly or periodically.
  • Essential amount of ozone means in this case that the ozone production rate is so high that it effectively keeps the particle measurement clean, i.e.
  • the advantage of the periodical production of high ozone levels is that the overall power consumption of the particle measurement unit can be kept low.
  • Essential removal of attached particles or prevention of particle accumulation means that the maintenance/cleaning period of the particle measurement unit is sufficiently long, preferably more than 1000 hours, more preferably more than 2000 hours and most preferably more than 4000 hours.
  • the present invention beneficial to prevent particle entry into the particle measurement unit during the cleaning cycle (i.e. period for high ozone production).
  • sample flow inlet can be prevented.
  • the particle measurement unit exit flow is blocked and in still another embodiment of the present invention, the particle measurement unit inlet flow is blocked.
  • Figure 1 shows an embodiment of the apparatus 1 of the present invention for monitoring particle concentration.
  • the apparatus comprises means 12 for feeding an oxidizing agent into at least some parts of the apparatus 1.
  • Apparatus 1 of the embodiment is a non- collecting particle sensor used to monitor particle concentration in the exhaust pipe 2 of a combustion engine.
  • Apparatus 1 is attached to the exhaust pipe via couplings 14 and 15.
  • An essentially continuous flow of exhaust gas is sucked into apparatus 1 from input 3and fed back to the exhaust pipe via output 4.
  • Output 4 may be closed with a cap 5.
  • Essentially clean air is fed into apparatus 1 from clean air input 6.
  • the amount of the clean air is adjusted with controller 7.
  • the clean air flowing through channel 12 is ionized by a corona discharger 10.
  • the ionized air charges the particles in throttle 13 and the current carried by the particles is measured to form a signal relative to the particle concentration.
  • the high voltage required for corona formation is provided from the high voltage generator 8 which is controlled by a control unit 9.
  • the high voltage is electrically insulated from the casing and other parts of apparatus 1 by insulator 11.
  • the apparatus also includes other insulators not shown in the figure.
  • the throttle 13 or the walls of the sensor after the throttle (in the direction of gas flow) preferably includes an ion trap (not shown in the picture) used to remove the ions not attached to particles.
  • the critical parts in the sensor are the input coupling 14, throttle 13, including the ion trap and insulators 11. Particles accumulated on the walls of the apparatus 1 are removed by heating apparatus 1 to an elevated temperature. Heating may be carried out e.g. by external heaters, by hot exhaust gas or by heating the essentially clean air. Apparatus 1 may be either continuously heated or it may be heated only during the particle removal cycle.
  • the high voltage generator 8 is controlled by the control unit 9 so that the output voltage of generator 8 is high enough for efficient ozone generation.
  • the output voltage of generator 8 is negative during the particle removal cycle.
  • the corona discharger 10 may thus be advantageously used as an ozone generator with a suitable corona voltage. This is a valuable feature especially with on-board particle sensors where the low cost of the sensor is a major requirement. As the preferred process of the present invention does not require an additional ozone generator, but the ozone may be generated using the same corona discharge unit 10 which is used for ionizing clean air, the sensor cost is not increased.
  • the output flow of apparatus 1 can be closed with cap 5 which guides the generated ozone through input 4, towards the particle flow.
  • the input 4 of apparatus 1 can be effectively cleaned from accumulated particles.
  • Such flush mode may also be generated by increasing by controller 7 the amount of essentially clean air flow to such a level that the clean air starts to flow through input 4 towards the particle flow.
  • the particle measurement apparatus 1 of the present invention thus comprises means 6, 7 for feeding essentially clean gas into the particle measurement unitl, an electrical discharge unit 8 used to ionize the essentially clean gas, and means 9 for adjusting the electrical current produced by the electrical discharge unit 8 to at least 1 microampere (1 ⁇ ).
  • particle measurement apparatus 1 comprises means 9 for adjusting the electrical current produced by the electrical discharge 8 unit to at least 2 microamperes (2 ⁇ ).
  • Particle measurement apparatus 1 may further comprise means for heating at least some parts of the particle measurement apparatus to 300-450°C.
  • Particle measurement apparatus 1 may further comprise means for adjusting the essentially clean gas flow entering the apparatus through inlet 6, at least to a value which essentially prevents sample flow through inlet 3 into apparatus 1.
  • Particle measurement apparatus 1 may further comprise means for essentially closing inlet 3 of apparatus 1. Essentially means in this case that the inlet flow is restricted to less than 10% from the flow without closing inlet 3.
  • Particle measurement apparatus 1 may further comprise means 5 for essentially closing outlet 4 of apparatus 1. Essentially means in this case that the outlet flow is restricted to less than 10% from the flow without closing outlet 4.
  • Particle measurement apparatus 1 may further comprise means for preventing the use of the apparatus 1 if the dew point of the essentially clean gas is more than 7°C. Such prevention can be realized e.g. by providing an alarm signal to the operator, using controller 7 to set the clean gas flow to either zero or to high value (which prevents sample flow through inlet 3), closing either the inlet 3 or outlet 4, switching the electrical power of apparatus 1 OFF, or by some other suitable means.

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Abstract

A process for cleaning a particle measurement unit used to measure particle concentration from combustion exhaust gases where the oxygen concentration is less than the oxygen concentration of air (20.8vol-%), comprising feeding essentially clean gas, where the oxygen concentration is higher than the oxygen concentration of the exhaust gas, into the particle measurement unit, ionizing the essentially clean gas by using an electrical discharge unit, using the ionized gas to charge at least a fraction of the particles to be measured, using the electrical discharge unit to produce ozone (03) from the essentially clean gas, and using at least a fraction of the produced ozone (03) to essentially remove particles attached to at least some parts of the measurement unit by oxidation of the attached particles is disclosed. A particle measurement apparatus which comprises means for carrying out the process and use of the process for cleaning a particle measurement apparatus are also disclosed.

Description

Particle measurement unit
Background
Fine particles provide a serious health risk and thus measures to avoid their exhaust to air as well as techniques to be able to measure fine particle concentrations are of great importance. Especially it is important to measure fine particles generated by combustion, e.g. by power plants, small-scale biomass combustion and combustion engines. The current development aims to continuous decrease of fine particle emissions from e.g. diesel engines. Fine particle measurement is, however, also important for the research and development of combustion engines and combustion-engine-based vehicles.
Fine particle emissions from combustion are reduced by filtering the particles. Such particle filters are typically installed to the stacks, chimneys or engine exhaust pipes. The Diesel Particulate Filter (DPF) needs to be frequently regenerated by burning the soot particles accumulated on the filter. This can be realized by increasing the filter temperature and feeding a subsequent amount of air through the filter during the regeneration cycle.
It is important to measure the fine particle concentration at least after the filter in the stack, chimney or exhaust pipe to monitor the function of the filter. Patent application WO 2009/109688, Pegasor Oy, 11.9.2009, describes a process for measuring particle concentrations in a gas using an ejector for producing an essentially constant sample flow and for efficient mixing of the particle-containing sample and an essentially clean, ionized gas. The invention also relates to an apparatus implementing such process. The process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.
United States Patent Application Publication US 2006/0156791, Dekati Oy, 20.7.2006, describes a method and apparatus for on-board monitoring of particles from the exhaust gas of a combustion engine. In the method essentially all particles traveling in the exhaust pipe are charged and the particle concentration is determined from the electrical current carried by the particles. The apparatus does not collect the particles.
Both apparatus described above are non-collecting and thus should not suffer from particles accumulated on the device. There are other non-collecting apparatus as well, such as those based on optically monitoring the particle concentration. However, it is impossible to produce a unit which would not collect at least some of the particles. In long-term operation, such as with on-board sensors for combustion engine exhaust gases, the particle accumulation becomes a problem. With the measurement units which are based on particle charging particle accumulation leads to soiling of discharge unit and electrical insulators, change in the mass flow through the sensor, etc.
In particle measurement units which are used for short-term operation the sensor can be cleaned between measurements. However, for those measurement units which are used continuously for a long time, such as particle sensors used to monitor the exhaust of combustion engines there is a need for the measurement unit cleaning during normal operation without disassembling the unit.
Application US 2006/0156791 describes (chapter [0038]) cleaning effect on the corona head based on the effect of oxidizing radicals produced by the electric discharge to burn off accumulating soot in the oxidizing gaseous atmosphere. The oxidizing radicals are generated in discharge near the corona head. Due to their extremely short life time they cannot reach other surfaces than the discharge electrode (corona head). For that reason radicals cannot clean other surfaces .
However, the oxygen concentration of diesel exhaust gas, as any exhaust gas arising from a combustion process, is sufficiently less than the oxygen concentration of air and cannot be freely increased. This is due to the closed loop fuel feedback control system is responsible for controlling the air/fuel ratio of the catalytic converter feed gas. During the closed loop operation, the electronic control module (ECM) keeps the air/fuel ratio adjusted to around the ideal 14.7 to 1 ratio. If the air/fuel ratio deviates from its preprogrammed swings, catalyst efficiency decreases dramatically for NOx reduction. For the reduced oxygen concentration the ozone generation is much reduced as well. Obviously, it would be possible to increase the ozone generation by increasing the voltage of the corona head. However, this would lead to accelerated particle accumulation into the particle sensor, as the increased corona voltage would effectively generate an electrostatic precipitator inside the particle sensor, greatly diminishing the value of the increased ozone generation. Brief description of the invention
The aim of the invented process and apparatus is to overcome the problems of the prior art where the soot accumulating on the particle measurement unit creates a serious problem to the reliable operation of the measurement unit. The accumulation problem is less severe in particle measurement units which do not collect the particles (non-collecting particle measurement units), and the present invention is especially beneficial in removing accumulated particles from such devices.
The invented process may beneficially be used in a particle sensor used for on-board monitoring of exhaust particles from a combustion engine and especially in a sensor used to monitor a particle filter in an exhaust pipe of a diesel engine.
The invented process may also be used in particle sensors monitoring stack and chimney emissions.
The invented process is based on oxidizing the particles accumulated on the walls etc. of the measurement unit. When the particles are accumulated the carbon (C), nitrogen (N), sulphur (S), hydrogen (H) and other non-metallic substances in the particles are converted into gaseous oxides which exit the measurement unit. The invented process is thus beneficially used to monitor particles which mainly comprise non-metallic substances, such as emissions from combustion processes, like combustion engines.
It is of particular importance to remove the accumulated particles from special parts of the measurement unit. Such parts comprise the input of the measurement unit, active measurement parts, such as an ion trap of a measurement unit based on particle charging, insulators of an electrical measurement unit, etc.
Particle oxidization can be carried out by using ordinary air for oxidizing the elements in the particle. However, oxidization with air normally requires quite high oxidization temperatures, typically in excess of 450°C. This creates special requirements for the materials used in the particle measurement unit, especially to the electrical insulators used in electrical measurement units as well as to the construction of the unit. Also the heating requires high power feeding to the system, which is in many case difficult. Thus it is generally beneficial to be able to carry out the oxidation process at elevated temperatures which are lower than the elevated temperatures used with air oxidation.
The oxidation temperature can be lower than with air if more efficient oxidizers than air are used. Such compounds include e.g. ozone (03) or some nitrogen oxides (NxOy). The present invention is predominantly beneficial when it is used with particle measurement units which are based on particle charging using a corona discharge unit.
Particularly when such units are used in a mode with high corona voltage, the corona discharge also produces ozone (O3). Ozone production is found to be more efficient with negative corona voltages. Such particle measurement units can work continuously with high, negative corona voltages, but usually it is more beneficial to carry out the particle measurements with low corona voltages and thus have low power consumption during the measurement and then carry out the cleaning of the measurement unit with higher corona voltage, preferably with high, negative corona voltage. The corona current produced during the particle removal is preferably more than 1 microampere ( Ι Α) for a single corona discharge unit. When the corona discharge is used to produce ozone in the gas flow containing the particles to be measured, the gas composition may vary and the production of ozone may vary as well. Thus it is an essential feature of the present invention to produce the ozone in an essentially constant and clean gas flow. The gas flow containing the ozone may then be mixed with the gas flow containing the particles or it may be directly fed towards the surfaces from which the accumulated particles are to be removed. In one embodiment of the present invention the ozone-containing gas flow may be so high that it passes also through the input of the measurement unit, towards the input flow. The measurement then works in flush mode and the input channel is effectively cleaned from accumulated particles.
List of figures
The invention is in below described in more detail with reference to the figures, where
Figure 1 shows an embodiment of the invented apparatus where ozone is produced in the measurement unit and used for removing the accumulated particles. Detailed description of the invention
The invented process is aimed for cleaning a particle measurement unit which used to measure particle concentration from combustion exhaust gases. The oxygen concentration of such exhaust gas is less than the oxygen concentration of air (20.8vol-%). The invented process comprises feeding essentially clean gas, where the oxygen concentration is higher than the oxygen concentration of the exhaust gas, into the particle measurement unit, ionizing the essentially clean gas by using an electrical discharge unit, using the ionized gas to charge at least a fraction of the particles to be measured, using the electrical discharge unit to produce ozone (03) from the essentially clean gas and using at least a fraction of the produced ozone (O3) to essentially remove particles attached to at least some parts of the measurement unit by oxidation of the attached particles.
The essentially clean gas may be ordinary filtered air, where the oxygen concentration naturally is higher than 20 volume- % (vol-%). The essentially clean air may also be a mixture of different gases, such as a mixture of oxygen (02) and at least another gas, such as nitrogen (N2) or carbon dioxide (C02). An essential feature of the invention is that the oxygen content in the essentially clean gas is higher than the oxygen concentration of the exhaust gas, so that the essentially clean gas is a more effective ozone (O3) precursor than the exhaust gas. The oxygen concentration of the exhaust gas may vary from less than 1 vol-% to almost 20 vol-% or even higher, if oxygen-enriched air or pure oxygen (02) is used for combustion. Thus the exact amount of oxygen in the essentially clean gas differs from case to case, but is typically roughly the amount of oxygen (02) in air, i.e. slightly higher than 20 vol-%.
In some cases water (H20) vapor may work as an inhibitor for ozone generation. Thus it is at least in some cases beneficial to use dry, essentially clean gas in the invented process. The dew point, i.e. the temperature to which a given parcel of humid gas must be cooled, at constant barometric pressure, for water vapor to condense into liquid water, of the essentially clean gas should preferably be maximum 7°C, more preferably maximum 3°C and most preferably maximum -20°C.
For effective removal of the accumulated particles it is beneficial to heat at least some parts of the measurement unit from which the particles are to be removed, to an elevated temperature. Typically the elevated temperature should be higher than 300°C to ensure proper particle oxidation but less than 450°C to prevent high temperatures in the measurement apparatus, which may cause extra deterioration of the apparatus.
In the invented process the electrical discharge unit is preferably a corona discharge unit which is effective in ozone generation above certain electrical discharge current (ion current). The electrical current should be preferably at least 1 microampere (1 μΑ), more preferably at least 2 microamperes (2 μΑ) and most preferably at least 3 microamperes (3 μΑ).
The invented process may be used so that it produces essential amounts of ozone (03) either constantly or periodically. Essential amount of ozone means in this case that the ozone production rate is so high that it effectively keeps the particle measurement clean, i.e.
effectively either prevents particle accumulation on at least some parts of the measurement unit or essentially removes particles attached to at least some parts of the measurement unit by oxidation of the attached particles. The advantage of the periodical production of high ozone levels is that the overall power consumption of the particle measurement unit can be kept low.
Essential removal of attached particles or prevention of particle accumulation means that the maintenance/cleaning period of the particle measurement unit is sufficiently long, preferably more than 1000 hours, more preferably more than 2000 hours and most preferably more than 4000 hours.
When essential amount of ozone is produced periodically, it is in some embodiments of the present invention, beneficial to prevent particle entry into the particle measurement unit during the cleaning cycle (i.e. period for high ozone production). There are various ways how sample flow inlet can be prevented. In one embodiment of the present invention, the feeding rate of the essentially clean gas into the particle measurement unit in so high that the sample flow into the particle measurement unit is prevented. In another embodiment of the present invention, the particle measurement unit exit flow is blocked and in still another embodiment of the present invention, the particle measurement unit inlet flow is blocked.
Figure 1 shows an embodiment of the apparatus 1 of the present invention for monitoring particle concentration. The apparatus comprises means 12 for feeding an oxidizing agent into at least some parts of the apparatus 1. Apparatus 1 of the embodiment is a non- collecting particle sensor used to monitor particle concentration in the exhaust pipe 2 of a combustion engine. Apparatus 1 is attached to the exhaust pipe via couplings 14 and 15. An essentially continuous flow of exhaust gas is sucked into apparatus 1 from input 3and fed back to the exhaust pipe via output 4. Output 4 may be closed with a cap 5. Essentially clean air is fed into apparatus 1 from clean air input 6. The amount of the clean air is adjusted with controller 7. The clean air flowing through channel 12 is ionized by a corona discharger 10. The ionized air charges the particles in throttle 13 and the current carried by the particles is measured to form a signal relative to the particle concentration. The high voltage required for corona formation is provided from the high voltage generator 8 which is controlled by a control unit 9. The high voltage is electrically insulated from the casing and other parts of apparatus 1 by insulator 11. The apparatus also includes other insulators not shown in the figure. The throttle 13 or the walls of the sensor after the throttle (in the direction of gas flow) preferably includes an ion trap (not shown in the picture) used to remove the ions not attached to particles.
The critical parts in the sensor are the input coupling 14, throttle 13, including the ion trap and insulators 11. Particles accumulated on the walls of the apparatus 1 are removed by heating apparatus 1 to an elevated temperature. Heating may be carried out e.g. by external heaters, by hot exhaust gas or by heating the essentially clean air. Apparatus 1 may be either continuously heated or it may be heated only during the particle removal cycle.
During particle removal the high voltage generator 8 is controlled by the control unit 9 so that the output voltage of generator 8 is high enough for efficient ozone generation.
Preferably the output voltage of generator 8 is negative during the particle removal cycle. The corona discharger 10 may thus be advantageously used as an ozone generator with a suitable corona voltage. This is a valuable feature especially with on-board particle sensors where the low cost of the sensor is a major requirement. As the preferred process of the present invention does not require an additional ozone generator, but the ozone may be generated using the same corona discharge unit 10 which is used for ionizing clean air, the sensor cost is not increased.
The output flow of apparatus 1 can be closed with cap 5 which guides the generated ozone through input 4, towards the particle flow. In such flush mode the input 4 of apparatus 1 can be effectively cleaned from accumulated particles. Such flush mode may also be generated by increasing by controller 7 the amount of essentially clean air flow to such a level that the clean air starts to flow through input 4 towards the particle flow. The particle measurement apparatus 1 of the present invention thus comprises means 6, 7 for feeding essentially clean gas into the particle measurement unitl, an electrical discharge unit 8 used to ionize the essentially clean gas, and means 9 for adjusting the electrical current produced by the electrical discharge unit 8 to at least 1 microampere (1 μΑ). In a preferred embodiment of the present invention, particle measurement apparatus 1 comprises means 9 for adjusting the electrical current produced by the electrical discharge 8 unit to at least 2 microamperes (2 μΑ).
Particle measurement apparatus 1 may further comprise means for heating at least some parts of the particle measurement apparatus to 300-450°C.
Particle measurement apparatus 1 may further comprise means for adjusting the essentially clean gas flow entering the apparatus through inlet 6, at least to a value which essentially prevents sample flow through inlet 3 into apparatus 1.
Particle measurement apparatus 1 may further comprise means for essentially closing inlet 3 of apparatus 1. Essentially means in this case that the inlet flow is restricted to less than 10% from the flow without closing inlet 3.
Particle measurement apparatus 1 may further comprise means 5 for essentially closing outlet 4 of apparatus 1. Essentially means in this case that the outlet flow is restricted to less than 10% from the flow without closing outlet 4.
Particle measurement apparatus 1 may further comprise means for preventing the use of the apparatus 1 if the dew point of the essentially clean gas is more than 7°C. Such prevention can be realized e.g. by providing an alarm signal to the operator, using controller 7 to set the clean gas flow to either zero or to high value (which prevents sample flow through inlet 3), closing either the inlet 3 or outlet 4, switching the electrical power of apparatus 1 OFF, or by some other suitable means.

Claims

Claims
1. Process for cleaning a particle measurement unit used to measure particle
concentration from combustion exhaust gases where the oxygen concentration is less than the oxygen concentration of air (20.8vol- ), comprising:
a. feeding essentially clean gas, where the oxygen concentration is higher than the oxygen concentration of the exhaust gas, into the particle measurement unit;
b. ionizing the essentially clean gas by using an electrical discharge unit;
c. using the ionized gas to charge at least a fraction of the particles to be measured; d. using the electrical discharge unit to produce ozone (03) from the essentially clean gas; and
e. using at least a fraction of the produced ozone (03) to essentially remove particles attached to at least some parts of the measurement unit by oxidation of the attached particles.
2. Process of claim 1 comprising feeding essentially clean gas, where the oxygen concentration is higher than 20 vol- , into the particle measurement unit.
3. Process of claim 1 or2comprising heating the said at least some parts of the measurement unit from which the particles are to be removed, to an elevated temperature.
4. Process of claim 3comprising heating the said at least some parts of the measurement unit from which the particles are to be removed, to at least 300°.
5. Process as in any of the claims 1-4 comprising heating the said at least some parts of the measurement unit from which the particles are to be removed, to less than 450°C.
6. Process as in any of the claims 1-5 comprising using the electrical discharge unit which is a corona discharge unit to produce ozone (03).
7. Process as in any of the claims 1-6, comprising producing an electrical current of at least 1 microampere (1 μΑ) by the electrical discharge unit.
8. Process as in any of the claims 1-7, comprising producing essential amounts of ozone (03) periodically.
9. Process as in any of the claims 1-7, comprising producing essential amounts of ozone (03) constantly.
10. Process as in any of the claims 1-9, comprising feeding essentially clean gas into the particle measurement unit in such quantities that the sample flow into the particle measurement unit is prevented.
11. Process as in any of the claims 1-10, comprising feeding into the particle measurement unit essentially clean gas with a dew point of maximum 7°C.
12. Process as in any of the claims 1-11, comprising essentially blocking the particle measurement unit exit flow.
13. Process as in any of the claims 1-12, c ompri s ing essentially blocking the particle measurement unit inlet flow.
14. Particle measurement apparatus (l)comprising:
a. means (6,7) for feeding essentially clean gas into the particle measurement unit (1); b. an electrical discharge unit (8) used to ionize the essentially clean gas;
c. means (9) for adjusting the electrical current produced by the electrical discharge unit
(8) to at least 1 microampere (1 μΑ).
15. Particle measurement apparatus ( 1 ) of claim 14comprising:
a. means (9) for adjusting the electrical current produced by the electrical discharge unit to at least 2 microamperes (2 μΑ).
16. Particle measurement apparatus (1) of claim 14 or 15 c ompri s in g :
a. means for heating at least some parts of the particle measurement apparatus to 300-
450°C.
17. Particle measurement apparatus (1) as in any of the claims 14-16, wherein the apparatus is a non-collecting particle measurement unit.
18. Particle measurement apparatus (1) as in any of the claims 14-17, comprising means 87) for adjusting the essentially clean gas flow at least to a value which essentially prevents sample flow through inlet (3) into apparatus (1).
19. Particle measurement apparatus ( 1 ) as in any of the claims 14-18, comprising means (5) for essentially blocking outlet (4) of apparatus (1).
20. Particle measurement apparatus (1) as in any of the claims 14-19, comprising means for essentially blocking inlet (3) of apparatus (1).
21. Particle measurement apparatus (1) as in any of the claims 14-20, comprising means for preventing the use of the apparatus (1) if the dew point of the essentially clean gas is more than 7°C.
22. Use of a process for cleaning a particle measurement unit used to measure particle concentration from combustion exhaust gases where the oxygen concentration is less than the oxygen concentration of air (20.8vol- ), comprising: feeding essentially clean gas, where the oxygen concentration is higher than the oxygen concentration of the exhaust gas, into the particle measurement unit; ionizing the essentially clean gas by using an electrical discharge unit; using the ionized gas to charge at least a fraction of the particles to be measured; using the electrical discharge unit to produce ozone (03) from the essentially clean gas; and using at least a fraction of the produced ozone (03) to essentially remove particles attached to at least some parts of the measurement unit by oxidation of the attached particles, w h e re i n the process is used in such a way that the cleaning period of the particle measurement unit is longer than 1000 hours.
PCT/FI2011/051157 2010-12-31 2011-12-28 Particle measurement unit WO2012089922A1 (en)

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FI20106396A FI20106396A0 (en) 2010-12-31 2010-12-31 Process, method and apparatus for controlling particles

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