The present invention relates to a method for cleaning moist exhaust gas and more particularly to a method for cleaning moist exhaust gas according to the preamble of the independent claim 1.

The present invention further relates to a cleaning arrangement for cleaning moist exhaust gas and more particularly to a cleaning arrangement for cleaning moist exhaust gas according to the preamble of the independent claim 11.

The present invention relates to cleaning of moist exhaust gas coming from a processing system in which organic material is processed and which the exhaust gas comprises particles and impurities with organic and/or inorganic material from the processing of the organic material.

BACKGROUND OF THE INVENTION

In a paper mill paper dust is the most known particulate matter air contaminant. As paper dries in the drying section of the paper machine, fibres will detach from the paper line and contaminate the air. Most of these fibres will be in the exhaust air from the paper machine hood. Exhaust air that is contaminated by processes, chemicals and smells decreases the air quality and cannot be discharged as such. Therefore, air must be treated with an air treatment technique.

There are many ways to treat air or gas flows containing impurities for reducing harmful emissions. Prior art knows different ways of using water as the main component to treat gas flows containing impurities. Water catches the pollutant whether it is particles or an absorbed compound. One of them is spray absorption in which spray of water droplets is generated and the gas flow is directed through the spray such that interchange of gas molecules from air to liquid is obtained. In the spray absorption gas flows are directed upwards and water falls downwards as droplets in a chamber. At the top of the chamber water droplets are formed by water as it flows through spray nozzles. This type of spray absorption requires a water circulation system to produce water droplets with the spray in the chamber.

Another prior art solution to clean gas with wetted walls. In the wetted- walls technique liquid film is produced to walls such that a smooth surface is generated to the walls. The particles contained in the air flow then collide with the liquid film, stick to it and the air flow will lose the particles to the wetted wall. However, in the wetted-wall technique, the air flow is typically turbulent and only the particles which motion is controlled by the air turbulence and the air motion are present. Therefore, not all airflow contaminants can be removed by this technique.

One of the problems associated with the prior art is that the techniques for cleaning exhaust gases containing impurities are quite complex and require technical preparation and construction as well as facilities.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an efficient and simple way to clean exhaust air generated in a process in which organic material is processed. An object of the present invention is further to provide a way to clean exhaust gas without a need for a water circulation system.

The objects of the invention are achieved by a method for cleaning moist exhaust gas and a cleaning arrangement which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the idea of providing an effective and simple method and arrangement for cleaning exhaust gases without connection to a water system or without water circulation.

A method for cleaning moist exhaust gas discharged from a dryer in a processing system according to the invention comprises the steps of supplying moist exhaust gas from the dryer through an exhaust fan to a mixing chamber, the moist exhaust gas having a first temperature; supplying fresh outside air from outdoors through an outside air fan to the mixing chamber, the fresh outside air having a second temperature, the second temperature being below the first temperature; mixing the moist exhaust gas and the fresh outside air together in the mixing chamber for forming a mist containing water vapor; generating a mist having the relative humidity of 100% by supplying additional fresh air to the mixing chamber and mixing the additional fresh air with the mist in the mixing chamber; creating cleaned exhaust gas and droplets from the mist having the relative humidity of 100% in the mixing chamber; directing the droplets from the mixing chamber to a collection basin; and discharging the cleaned exhaust gas as discarded air from the mixing chamber to outdoors. The cleaned exhaust gas is created through diffusiophoresis and/or condensation effects.

At least part of the cleaned exhaust gas can be recirculated back to the process from which the moist exhaust gas to be cleaned enters for cleaning. The processing system from which the exhaust gas is discharged processes organic material, such as wood, pulp or sludge, for example in a paper mill or in a veneer mill and the exhaust gas discharged from the process therefore may contain turpentine ingredients or glue ingredients. The moist exhaust gas may comprise particles or gaseous pollutants which are organic or inorganic or both. For example, the moist exhaust gas coming from pulp or paper manufacturing process or wood process may comprise emissions of sulfur oxides (SOx) and nitrogen oxides (NOx) The exhaust gas and the fresh outside air are supplied to the mixing chamber such that they are mixed together. The fresh outside air is supplied from outdoors into the process facility in which the moist exhaust gas is cleaned. The exhaust gas and the fresh outside air meet each other in the mixing chamber. The fresh outside air has lower temperature than the exhaust gas such that mist is generated when the exhaust gas and the fresh outside air meet each other. Further fresh outside air is supplied to the mixing chamber so that the relative humidity of 100% is reached in the mixing chamber and the size of the droplets become larger and they fall to a bottom of the mixing chamber. The droplets are then directed from the bottom of the mixing chamber to a collection basin, which the collection basin may be part of the mixing chamber or a totally separate part. The collection basin may alternatively be the bottom of the mixing chamber from which the droplets can be discharged. The exhaust gas contaminants, such as adhesive components, i.e., the glue ingredients or even formaldehyde, are absorbed on the droplets formed, cleaning up the remaining exhaust gas. The droplets containing the contaminants are discharged from the mixing chamber to the collection basin and the cleaned exhaust gas is released from the mixing chamber to the outside air.

In this application the outside air or the outdoors means atmosphere outside the factory facilities. The factory facilities meaning the space in which the processing system or the gas cleaning arrangement is provided.

According to the invention the step of discharging the cleaned exhaust gas from the mixing chamber to outdoors further comprises directing the cleaned exhaust gas discharged from the mixing chamber through a droplet separator prior to discharging to outdoors.

There may be moisture in the cleaned exhaust gas, which may lead to water runoff or small ponds near the outlet of the cleaned exhaust gas. Therefore, the cleaned exhaust gas is directed through a droplet separator in the air exit path from the cleaning arrangement to outdoors.

According to the invention the step of supplying moist exhaust gas from the dryer through an exhaust fan to a mixing chamber comprises the moist exhaust gas having the first temperature between 42°C - 180°C. Alternatively, the step of supplying moist exhaust gas from the dryer through an exhaust fan to a mixing chamber comprises the moist exhaust gas having the first temperature between 50°C - 100°C. Alternatively, the step of supplying moist exhaust gas from the dryer through an exhaust fan to a mixing chamber comprises the moist exhaust gas having the first temperature between 50°C - 85°C.

The temperature of the moist exhaust gas varies according to the process. For example, the moist exhaust gas coming from a dryer of a paper machine is typically between 50°C - 85°C, whereas the moist exhaust gas coming from another process may be warmer.

According to the invention the step of supplying fresh outside air from outdoors through an outside air fan to the mixing chamber comprises the fresh outside air having the second temperature at most 40°C. Alternatively, the step of supplying fresh outside air from outdoors through an outside air fan to the mixing chamber comprises the fresh outside air having the second temperature at most 30°C. Alternatively, the step of supplying fresh outside air from outdoors through an outside air fan to the mixing chamber comprises the fresh outside air having the second temperature at most 20°C.

According to the invention the method further comprises the steps of arranging a first temperature sensor in connection with a discharge of the cleaned exhaust gas from the mixing chamber to outdoors; providing a control system connected to the first temperature sensor and to the outside air fan; providing measurement data from the first temperature sensor to the control system; and regulating operation of the outside air fan for supplying the fresh air to the mixing chamber based on the measurement data processed in the control system.

The supply of the fresh outside air is controlled by regulating the outside air fan. When the first temperature sensor detects that the temperature of the cleaned exhaust gas rises, the control system gives the outside air fan a command to increase the amount of fresh air, i.e., by increasing the fan speed.

According to the invention the method further comprises the steps of arranging a second temperature sensor in connection with a supply of the fresh outside air from outdoors to the mixing chamber; connecting the second temperature sensor to the control system; providing measurement data from the second temperature sensor to the control system; and processing the measurement data from the first temperature sensor and from the second temperature sensor for regulating the operation of the outside air fan.

The control system analyses the ratio between the fresh outside air temperature and the cleaned exhaust gas temperature, and the control system then adjusts the outside air fan speed based on the analysis.

According to the invention the method further comprises the steps of arranging a first moisture sensor for carrying out a moisture measurement in connection with the supply of the moist exhaust gas from the dryer to the mixing chamber; providing a control system connected to the first moisture sensor and to the outside air fan for processing measurement data and regulating operation of the outside air fan; providing the measurement data from the first moisture sensor to the control system; and regulating operation of the outside air fan for supplying the fresh air to the mixing chamber based on the measurement data processed in the control system.

The supply of the fresh outside air is controlled by regulating the outside air fan. When the first moisture sensor detects increase in the humidity of the moist exhaust gas, the control system gives the outside air fan a command to increase the amount of fresh air, i.e., by increasing the fan speed.

According to the invention the method comprises the steps of carrying out a first moisture measurement with the first moisture sensor in connection with the supply of the moist exhaust gas to obtain a first moisture value; carrying out a second moisture measurement with the first moisture sensor after the first moisture measurement and after a time delay in connection with the supply of the moist exhaust gas to obtain a second moisture value; comparing the first moisture value and the second moisture value; and if the second moisture value is greater than the first moisture value: increasing the supply of fresh outdoor air by regulating the operation of the outside air fan.

The humidity of the moist exhaust gas may vary and therefore the moisture measurement is repeated after a time delay. Based on the measurement and moisture values, the speed of the outside air fan is adjusted to either produce more fresh outside air to the mixing chamber, to maintain the supply as it is or to reduce it.

According to the invention the method comprises the steps of determining a moisture threshold value for the moist exhaust gas to be supplied from the dryer to the mixing chamber; carrying out a first moisture measurement with the first moisture sensor in connection with the supply of the moist exhaust gas to obtain a first moisture value; comparing the first moisture value to the moisture threshold value; and if the first moisture value is above the threshold value: increasing the supply of fresh outdoor air by regulating the operation of the outside air fan.

According to the invention the step of supplying moist exhaust gas comprises the moist exhaust gas having the humidity ratio between 50g/kg - 1000 g/kg, or the humidity ration between 50g/kg - 600 g/kg.

A cleaning arrangement for cleaning moist exhaust gas discharged from a dryer in a processing system according to the invention comprises a mixing chamber for mixing gases; an exhaust conduit arranged between the dryer and the mixing chamber for supplying moist exhaust gas from the dryer in the processing system to the mixing chamber; a fresh air conduit arranged between ambient outdoor space and the mixing chamber for supplying fresh outdoor air from outdoors to the mixing chamber; a collection basin arranged underneath the mixing chamber for containing liquid; a fluid connection from the mixing chamber to the collection basin for conveying droplets from the mixing chamber to the collection basin, a discharge conduit arranged between the mixing chamber and ambient outdoor space for discharging cleaned exhaust gas from the mixing chamber to outdoors.

According to the invention the cleaning arrangement further comprises an exhaust fan arranged in connection with the exhaust conduit for regulating the supply of the exhaust gas from the dryer to the mixing chamber; an outside air fan arranged in connection with the fresh air conduit for regulating the supply of the fresh air from the outdoors to the mixing chamber.

The exhaust fan is provided in connection with the exhaust conduit between the dryer and the mixing chamber, and the outside air fan is provided in connection with the fresh air conduit between the ambient outdoor space and the mixing chamber.

According to the invention the cleaning arrangement further comprises a droplet separator arranged in connection with the discharge conduit for separating remaining droplets from the cleaned exhaust gas.

The droplet separator preferably comprises a discharge outlet for water separated from the cleaned exhaust gas.

According to the invention the cleaning arrangement further comprises a first temperature sensor arranged in connection with the discharge conduit for measuring the temperature of the cleaned exhaust gas; and control system having a communication connection to the first temperature sensor and to the outside air fan for regulating the operation of the outside air fan as a response to measurement data from the first temperature sensor.

According to the invention the cleaning arrangement further comprises a first moisture sensor arranged in connection with the exhaust conduit for carrying out a moisture measurement of the moist exhaust gas; and control system having a communication connection to the first moisture sensor and to the outside air fan for regulating the operation of the outside air fan as a response to measurement data from the first moisture sensor.

An advantage of the invention is that the arrangement and method do not require water connections, and therefore the requirements for setting up and maintaining the cleaning arrangement are simple. Another advantage is that there is no need for a wash water circuit and there is no need to think about the recycling or further use of contaminated water, because only a small amount of contaminated water accumulates in the collection basin.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail by means of specific embodiments with reference to the enclosed drawings, in which

Figure 1 shows a cleaning arrangement according to the invention; and Figure 2 shows a further embodiment of a cleaning arrangement according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a cleaning arrangement for cleaning moist exhaust gas discharged from a dryer 1 in a processing system 100. The processing system 100 is illustrated as a box because the processing system 100 may be any kind of processing system that processes organic material, such as a paper machine or a veneer machine, and bind impurities, such as turpentine ingredients or paper dust, to the air. The cleaning arrangement comprises a mixing chamber 10 for mixing gases coming from an exhaust conduit 11 arranged between the dryer 1 and the mixing chamber 10 for supplying moist exhaust gas from the dryer 1 in the processing system 100 to the mixing chamber 10 and a fresh air conduit 12 arranged between ambient outdoor space 0 and the mixing chamber 10 for supplying fresh outdoor air from outdoors to the mixing chamber 10. The ambient outdoor space 0 surrounds the factory facilities F which is shown as a dotted line. The terms outdoor space and outdoors all refer to the ambient outdoor space 0 surrounding the processing system 100 together with any other factory space and which provides fresh outside air to the cleaning arrangement. The exhaust conduit 11 comprises an inlet having a connection with the dryer 1 and an outlet to the mixing chamber 10 for supplying moist exhaust gas from the dryer 1 through an exhaust fan 15 to a mixing chamber 10, the moist exhaust gas having a first temperature. The exhaust fan 15 is arranged in connection with the exhaust conduit 11. The fresh air conduit 12 comprises an inlet from the ambient outdoor space 0 and an outlet to the mixing chamber 10 for supplying fresh outside air from outdoors through an outside air fan 16 to the mixing chamber 10, the fresh outside air having a second temperature, the second temperature being below the first temperature. The outside air fan 16 is preferably provided in connection with the fresh air conduit 12 between the inlet and outlet but alternatively, it may be provided in connection with the inlet of the fresh air conduit 12. The moist exhaust gas and the fresh outside air are mixed in the mixing chamber 10 such that a mist having the relative humidity of 100% is generated by supplying additional fresh air to the mixing chamber 10 and mixing the additional fresh air with the mist in the mixing chamber 10. When a mist having a relative humidity of 100% is generated, the mist droplets grow and the contaminants from the moist exhaust gas are absorbed into the droplets, creating the remaining clean air as the droplets drop to the bottom of the mixing chamber 10.

The droplets are directed from the mixing chamber 10 to a collection basin 13 arranged underneath the mixing chamber 10 through a fluid connection 23 from the mixing chamber 10 to the collection basin 13. The collection basin 13 may be part of the mixing chamber 10 or a separate part connected to the mixing chamber 10 through the fluid connection 23 for conveying the droplets to the collection basin 13. The cleaned exhaust gas is then discharged as discarded air from the mixing chamber 10 through the discharge conduit 14 arranged between the mixing chamber 10 and ambient outdoor space 0 for discharging cleaned exhaust gas from the mixing chamber 10 to outdoors.

The supply of the fresh outside air can be regulated by adjusting the operation of the outside air fan 16. The adjustment of the outside air fan 16 can be based on measured values of the temperature of the cleaned exhaust gas discharged from the mixing chamber, or the temperature of the moist exhaust gas supplied from the dryer to the mixing chamber, or the temperature of the fresh outside air, or the moisture of the moist exhaust gas or any combination of these. The measurement is made by a first temperature sensor 22 arranged in connection with the discharge conduit 14 such that the first temperature sensor 22 is arranged to measure the temperature value of the cleaned exhaust gas discharged through the discharge conduit 14. The measurement of the moist exhaust gas supplied through the exhaust conduit 11 is made by a third temperature sensor 18 arranged in connection with the exhaust conduit 11. The measurement of the moisture value of the moist exhaust gas supplied through the exhaust conduit 11 is provided by arranging a first moisture sensor 21 for carrying out a moisture measurement in connection with the supply of the moist exhaust gas from the dryer to the mixing chamber 10. The measured values are provided to a control system 19 which is connected to the first temperature sensor 18 and to the first moisture sensor 21 as well as to the outside air fan 16. The control system 19 then operates the outside air fan 16 based on the measured values. The cleaning arrangement may further comprise a second temperature sensor 20 arranged in connection with the fresh air conduit 12 for measuring the temperature of the fresh outside air supplied through the fresh air conduit 12 to the mixing chamber 10. The second temperature sensor 20 is also connected to the control system 19.

Figure 2 shows an alternative embodiment of the cleaning arrangement which is otherwise the same as the one presented in figure 1 but the discharge conduit 14 supplying cleaned exhaust gas from the mixing chamber 10 to ambient outdoor space 0 comprise a droplet separator 17 connected to the discharge conduit 14 such that the cleaned exhaust gas is discharged from the mixing chamber 10 through the droplet separator 17 to the ambient outdoor space 0. The droplet separator 17 removes the remaining droplets from the cleaned exhaust gas prior to discharge the cleaned exhaust gas to the ambient outdoor space 0.

The dashed arrows represent gas or air flows in the cleaning arrangement and the plain arrows represent the fluid connections discharging the droplets from the cleaning arrangement. The collection basin 13 as well as the droplet separator 17 may collect the water droplets such that there is no water connection away from the droplet separator or the collection basin 13 but they have water containers that need to be emptied.

The invention has been described above with reference to the examples shown in the figures. However, the invention is in no way restricted to the above examples but may vary within the scope of the claims.