Technical field

The invention relates to chemical recovery boilers. The invention relates to methods for controlling chemical recovery boilers. The invention relates to safety and efficiency of chemical recovery boilers. The invention relates to dissolving smelt in a dissolving tank of a chemical recovery boiler.

Background

Spent liquor, i.e. the black liquor, the brown liquor, or waste liquor created in pulp manufacture is burnt in a chemical recovery boiler, on one hand, in order to recover the energy it includes, and on the other hand, in order to recover chemicals in it and to recycle them back to circulation. A char bed is created at a bottom of the chemical recovery boiler when burning black or brown liquor, which in a high temperature forms into smelt. Smelt is removed from the boiler as a continuous flow via smelt spouts to a dissolving tank.

Typically, the smelt is very hot (for example 750 to 820 °C). When the smelt enters or drops to the dissolving tank, the hot smelt hits an aqueous solution, whereby the liquid boils as the smelt drops to the solution. The possible splashes of smelt cause danger to the personnel working and moving in the surroundings as well as mechanical damages to the system, e.g. to the dissolving tank or a hood. In order to diminish the effects of the splashes, the smelt is typically shattered before it is allowed to enter the aqueous solution. The smelt is typically shattered by using a shattering nozzles through which a shattering fluid is flown. Steam and/or water can be used as the shattering fluid. Because of this, there is typically a protection area near the smelt spouts, moving on which area should be avoided and working on which area requires using special protection equipment.

In order to ensure that the shattering of the smelt is effective, the flow of the shattering fluid is typically fixed to a reasonably high level to ensure sufficient shattering in many typical operating conditions. Having a high flow of shattering liquid typically decreases boiler efficiency and increases wear of a hood of the smelt spout and, occasionally, the dissolving tank or the smelt spouts themselves. Yet, occasionally, the shattering may be insufficient, causing safety problems and noise.

Summary

It has now been found how the problems of prior art can be diminished. It has been found that by detecting the process optically and/or acoustically, information on the flow of smelt in a smelt spout or from the smelt spout can be obtained. However, it is well known that such information is not obtainable by a living operator, since the conditions near smelt spouts are harsh. Thus, the process is observed using an optical or an acoustical detector, which typically is an electronic sensor. The information thus obtained is then used to control a flow of the shattering fluid.

Brief description of the drawings

Fig. 1 a shows a schematic view of a chemical recovery boiler,

Fig. 1 b shows a dissolving tank of a chemical recovery boiler,

Fig. 2a shows in detail shattering of smelt and means for controlling a flow of shattering fluid,

Fig. 2b shows a sensor arranged in a casing,

Fig. 3a shows in detail shattering of smelt, detecting flow of smelt in a smelt spout, and a controllable valve configured to control a flow of the shattering fluid,

Fig. 3b shows in detail shattering of smelt and a controllable valve configured to control a flow of the shattering fluid,

Fig. 3c shows in detail shattering of smelt and a controllable valve configured to control a flow of the shattering fluid,

Fig. 4a shows in detail shattering of smelt and detecting a shattering pattern of the smelt,

Fig. 4b shows in detail detecting a shattering pattern of the smelt and detecting flow of smelt in a smelt spout,

Fig. 5 shows in detail shattering of smelt and acoustically detecting a parameter indicative of a flow of the smelt through the smelt spout, Fig. 6 shows a sensor arrangement for determining a parameter indicative of a flow of the smelt through the smelt spout with multiple sensors, Fig. 7 shows a part of a chemical recovery boiler comprising a equipment for cleaning at least one smelt spout, and a sensor for determining a parameter indicative of a flow of the smelt through the smelt spout.

Detailed description

Figure 1 a is a schematic representation of a chemical recovery boiler 100. The chemical recovery boiler 100 comprises a furnace 105. Combustion air 110 and liquor 120 are fed to the furnace 105 to burn the liquor 120. The liquor may be fed through a liquor gun 125 such that droplets 122 of the liquor are formed into the furnace 105. The droplets oxidize and fall to form a char bed 130 at a bottom of the furnace 105. In this way, the liquor 120 is treated in a reduction process, and chemical are recovered in the chemical recovery boiler 100. From the char bed 130, smelt 140 flows through smelt spouts 145 into a dissolving tank 150. Hereinabove the term “liquor” (120) refers to spent liquor, i.e. liquor that has been used in the process. The liquor 120 may refer to black liquor, which may be concentrated, of a Kraft process (i.e. Sulphate process); or a brown liquor, which may be concentrated, of a Sulphite process; or waste liquor created in pulp manufacture. Hereinbelow, the embodiments are disclosed as being part of the Kraft process. However, the principles are equally applicable to a Sulphite process and for waste liquor.

In the process, some aqueous solution, typically weak white liquor, is fed into the dissolving tank 150, and when the smelt 140 dissolved into the solution, green liquor is formed in the Kraft process. Green liquor is used in the Kraft process as known to a skilled person in the field.

Figure 1 b shows a typical smelt spout area A1 of a chemical recovery boiler. In the smelt spout area, smelt spouts 145, along which the smelt is directed from the furnace 105 to the dissolving tank 150, are arranged. The air 110 in Fig. 1 b is fed at a primary air level, while Fig. 1 a shows also other levels for feeding air. Fig. 1 b also shows a working area A2, wherein an operator 192 is working.

Figure 2a shows details of a smelt spout area of a chemical recovery boiler 100. Along the smelt spout 145, smelt 140 is directed from the furnace 105 to the dissolving tank 150. The dissolving tank 150 is at least partly filled with the aqueous solution 152 (e.g. weak white liquor and/or green liquor), to which the smelt 140 dissolves. Fig. 2a shows a surface 155 of the aqueous solution 152. When the hot smelt 140 falls to the aqueous solution 152 arranged in the dissolving tank 150, it evaporates some of the aqueous solution 152, since the temperature of the smelt 140 is very high as detailed above. If the smelt 140 would fall to the aqueous solution 152 in large pieces, the evaporation of the solution 152 would cause explosions, which pose safety risks and noise, which is inconvenient to the operators of the chemical recovery boiler. For this reason, shattering fluid 170, typically steam, but sometimes water or a combination of water and steam, is injected to the smelt 140 before the smelt enters the aqueous solution 152. Thus, the shattering fluid 170 is injected to the smelt 140 above a surface 155 of the aqueous solution 152. A purpose of the shattering fluid 170 is to shatter the smelt to small droplets 142 of smelt. The shattering fluid 170 is injected using a nozzle 175. The droplets 142 of smelt fall to the aqueous solution 152 subsequently, whereby instead of one large explosion (i.e. evaporation of the solution), a sequence of much smaller explosions is experienced. This diminishes safety risk and noise.

In the past, a chemical recovery boiler has been designed in such a way that the flow of the shattering fluid is at a reasonably proper level. The level is reasonably high in order to provide for sufficient shattering of the smelt. However, the flow is not controlled, because an operator 192 cannot obtain information of the process, because of the harsh environment within a smelt spout area A1 .

It has now been found that a flow of the shattering fluid 170 can be controlled. Thus, when less shattering is needed, the flow can be reduced, which improves boiler efficiency and reduces wear of the parts of the boiler 100, such as the hood, as detailed in the background. Moreover, if more shattering is needed, the flow can be increased, which improves safety and reduces noise.

Having a high flow of shattering liquid typically decreases boiler or plant efficiency, in particular when steam is used as the shattering fluid, and increases wear of the boiler parts. Yet, occasionally, the shattering may be insufficient, causing safety problems and noise. For controlling the flow of the shattering fluid 170, the chemical recovery boiler 100 comprises a controllable valve 210. The controllable valve 210 is configured to control a flow of the shattering fluid 170 through the controllable valve 210. The controllable valve 210 is configured to control a flow of the shattering fluid 170 through the nozzle 175. Herein controlling the flow refers to controlling a mass flow through the nozzle 175. Herein the term “fluid” refers to gaseous or liquid substance, such as water or steam, in particular steam.

For obtaining information on at least one of [A] the flow of smelt 140 through the smelt spout and [B] shattering of the smelt, the chemical recovery boiler 100 comprises sensor arrangement 220. The sensor arrangement is configured to measure optically and/or acoustically a first parameter indicative of [A] a flow of the smelt through 140 the smelt spout 145 and/or [B] shattering of the smelt. Herein the term “parameter” refers to one or more properties that are expressible by one or more numbers. Examples of parameters include a velocity, a pattern, and a combination of a velocity and a pattern, as detailed below. A value of such a parameter is measured using at least one sensor. Thus, the sensor arrangement 220 comprises a first sensor 221. The first sensor 221 is configured to measure optically or acoustically at least a quantity or quantities of the first parameter, i.e. at least a part of the first parameter.

The method has been found to be effective, when the liquor 120 is black liquor. Black liquor refers to the cooking residue of a Kraft process. The black liquor may be concentrated.

As indicated above, water or steam can be used as the shattering fluid 170. Preferably, steam is used as the shattering fluid 170. When steam is used, preferably, a pressure of the steam is from 2 bar to 50 bar, preferably from 10 to 35 bar. The value of the pressure may relate to pressure just downstream from the controllable valve 210, if the shattering fluid flows through the controllable valve 210 to the nozzle 175 (see e.g. Fig. 3a). The term “just downstream” refers to a point at most 10 cm downstream from the valve 210. The pressure may refer to a pressure at a location between the nozzle 175 and a first valve upstream from the nozzle 175; e.g. in Fig. 3c, downstream from the valve 210”; in Fig. 3b downstream from the valve 212; and in Fig. 3a downstream from the valve 210. Moreover, the location may be just downstream from the first valve 210 (Fig. 3a), 212 (Fig. 3b), or 210” (Fig. 3c) upstream from the nozzle 175. What has been said above about the meaning of “just downstream” applies.

Using steam as the shattering fluid has been found to be functional at least when the liquor 120 comprises black liquor. The steam pressures recited above have been found to be functional at least when the liquor 120 comprises black liquor.

In this way, an embodiment of the new method comprises

- treating spent liquor (e.g. black liquor, brown liquor, or water liquor) in a furnace 105 of the chemical recovery boiler 100 to form a char bed 130 into the furnace 105;

- conveying smelt 140 from the char bed 130 of the furnace 105 through a smelt spout 145 to a dissolving tank 150 comprising an aqueous solution 152, and

- shattering the smelt 140 above a surface 155 of the aqueous solution 152 (i.e. upstream from the aqueous solution 152) by shattering fluid 170.

In order to control the amount of the shattering fluid 170 used, the embodiment further comprises

- measuring optically or acoustically a first parameter indicative of at least one of [A] a flow of the smelt 140 through the smelt spout 145 and [B] shattering of the smelt, and

- based on a value of the first parameter of the flow of the smelt, controlling the mass flow of the shattering fluid 170.

Correspondingly, an embodiment of the new chemical recovery boiler 100 comprises

- a furnace 105 for burning spent liquor,

- an injection gun 125 for feeding the spent liquor into the furnace 105,

- a dissolving tank 150 for dissolving smelt 140,

- a smelt spout 145 for withdrawing smelt 140 from the furnace 145 and for conveying the smelt 140 to the dissolving tank 150, and

- a nozzle 175 configured to inject shattering fluid 170 to the smelt 140 to form droplets 142 of the smelt 140. As detailed above, the nozzle 175 is configured such that in use, the shattering fluid 170 shatters the smelt 140 to droplets 142 above a surface 155 of an aqueous solution 152 arranged within the dissolving tank 150.

In order to control the amount of the shattering fluid used, the embodiment further comprises

- a sensor arrangement 220 configured to measure optically and/or acoustically a first parameter indicative of at least one of [A] a flow of the smelt 140 through the smelt spout 145 and [B] shattering of the smelt, and

- a controllable valve 210 configured to control a mass flow of the shattering fluid 170.

The controllable valve 210 is configured to control the flow of the shattering fluid 170 through the nozzle 175.

Controlling the flow of the shattering fluid 170 can be done automatically or by an operator. Preferably, the process control is automated. Thus, an embodiment of the method comprises automatically controlling the flow of the shattering fluid 170 based on a value of the first parameter. Corresponding, and with reference to Figs. 3a to 3c, an embodiment of the recovery boiler 100 comprises a control unit 230 configured to receive a value of the first parameter from the sensor arrangement 220 (e.g. from the first sensor 221 ), and based on the value of the first parameter, to control the controllable valve 210. Moreover, the sensor arrangement 220 is configured to send a signal S to the control unit 230, the signal S being indicative of a value of the first parameter. The signal S may be configured to be transferred through a wire (as in Figs. 3a to 3c) or wirelessly, as in Fig. 7.

As for other parts of the smelt spout area, referring to Figs. 2a and 3a to 3c, the dissolving tank 150 may be provided with a lower hood 157, and the smelt spout 145 may be covered with an upper hood 158. Underneath the hood 157, 158 or hoods 157, 158, hood showers 159 may be provided. The hood showers 159 may spray water or weak white liquor in order to wash the hood. The hood or hoods 157, 158 may be openable. For example, the upper hood 158 may comprise an openable and closable door 162. By opening the door 162, the smelt spout 145 may be maintained and/or observed. For example, the smelt spout 145 may be cleaned with a rodding bar 164. Even if not explicitly shown in Figs. 4a to 6, those embodiments may also comprise hoods 157, 158, hood showers 159, and a door 162.

In the embodiment of Fig. 3a, the shattering fluid 170 flows through the controllable valve 210 to the nozzle 175. In the embodiment of Fig. 3b, a part of the shattering fluid 170 flows through the controllable valve 210 to elsewhere, e.g. to be re-used in the process, and another part flows to the nozzle 175. The mass flow through the nozzle 175 is affected by the controllable valve 210 as shown in Fig. 3b. Preferably, the shattering fluid 170 flows and is configured to flow thorough the controllable valve 210 to the nozzle 175 as indicated in Fig. 3a.

It is also possible to use as the shattering fluid 170 either high pressure steam or low pressure steam, or their mixture. Referring to Fig. 3c, the chemical recovery boiler may comprise a first outlet for steam at a first pressure P1 and a second outlet for steam at another pressure P2. Such outlets may be located e.g. after different parts of a steam turbine arrangement. By using the controllable valve 210 in combination with another controllable valve 210’, either of these steams or their mixture can be used as the shattering fluid, as detailed in Fig. 3c. By opening the controllable valve 210 and closing the valve 210’ only the steam at pressure P2 would be used as shattering fluid. And vice versa. This may be beneficial in terms of energy balance of the chemical recovery boiler 100. If needed, the flow can be further controlled with yet another controllable valve 210”, as shown in Fig. 3c.

In this way, the flow of the shattering fluid can be controlled directly, as in Fig. 3a, or by changing a pressure of the shattering fluid upstream from the nozzle 175, as in Fig. 3b, or by selecting, from at least two sources (indicated by the pressures P1 and P2 in Fig. 3c) a source for the shattering fluid, the selection of the source affecting the mass flow of the shattering fluid through the nozzle 175. The pressure can be changed by a controllable valve 210 (Fig. 3b), and the source for the shattering fluid can be selected by a the controllable valve 210 (Fig. 3c).

As for the magnitude for controlling the controllable valve 210, the chemical recovery boiler 100 may comprise a secondary sensor 218 (see Figs. 3a to 3c) configured to measure a value indicative of at least one of [A] the flow of the shattering fluid 170 and [B] a pressure of the shattering fluid 170 upstream from the nozzle 175. This value may be referred to as a value of a secondary parameter. More precisely, the secondary sensor 218 may be arranged between the nozzle 175 and a first valve upstream from the nozzle 175. The term “first valve” has been discussed above in connection with a typical pressure. Moreover, the secondary sensor 218 may be arranged just downstream the first nozzle. The term “just downstream” has been discussed above in connection with a suitable pressure.

The amount of shattering fluid 170 can be further controlled by using a signal from the secondary sensor 218. The control valve 210 can be further controlled using a signal from the secondary sensor 218. This has the benefit that a temporal delay for the feedback is much shorter than when only a feedback from the sensor arrangement 210 is used for controlling. Moreover, the control of the flow of the shattering fluid 170, e.g. the control of the control valve 210, can be done more accurately, since there is feedback about a value indicative of the flow of the shattering fluid 170 itself, too. As is well known, a pressure (in particular when measured by the secondary sensor 218) is indicative of the flow (in particular the flow of the shattering fluid): the greater the pressure, the higher the flow; unless the nozzle 175 is also controlled to prevent this from happening. The value receivable from the secondary sensor 218 may be used by an operator or, more preferably, in the control unit 230, as detailed in Figs. 3a to 3c.

Such a secondary sensor 218, even if shown only in Figs. 3a to 3c, may be used in connection with other embodiments, too. However, even if shown in the Figs. 3a to 3c, the secondary sensor 218 is not necessary.

In the embodiments, the nozzle 175 configured to form a jet of the shattering fluid 170. The jet shatters the smelt 140 to droplets 142 upstream from the aqueous solution 152 arranged within the dissolving tank 150.

When the flow, i.e. mass flow, of the shattering fluid is controlled (e.g. Figs. 3a to 3c), the controlling is performed preferably in such a way that the flow of the shattering fluid 170 through the valve 210 achieves multiple values during a period of time. One of these values may be zero (i.e. the flow is stopped e.g. during maintenance), but preferably, multiple different non-zero values are obtained. More specifically, in an embodiment, the valve 210 is configured to control the flow of the shattering fluid and the valve 210 is controlled such that

- at a first instance of time, the mass flow of the shattering fluid 170 through the valve 210 has a first value and

- at a second instance of time, the mass flow of the shattering fluid 170 through the valve 210 has a second value, wherein

- the first value is different from the second value,

- the first value is greater than zero, and

- the second value is greater than zero.

In other words, in an embodiment, a nozzle 175 is configured to inject shattering fluid 170 to the smelt 140 to form droplets 142 of the smelt, and the valve 210 is configured to control the flow of the shattering fluid, and the valve 210 is controlled such that

- at a first instance of time, the mass flow of the shattering fluid 170 through the nozzle 175 has a first value and

- at a second instance of time, the mass flow of the shattering fluid 170 through the nozzle 175 has a second value, wherein

- the first value is different from the second value,

- the first value is greater than zero, and

- the second value is greater than zero.

As is clear, the controllable valve 210 may also be continuously controllable. Thus, the flow of the shattering fluid may be controlled in such a way that the mass flow of the shattering fluid (through the valve 210 or the nozzle 175) takes continuously multiple different values during a period of time.

In addition to the controllable valve 210, the chemical recovery boiler 100 may comprise a second valve 212, through which the shattering fluid 170 flows to the nozzle 175, as shown in Figs. 2a, 3a, and 3b. The second valve 212 can be used to fully stop the flow of the shattering fluid e.g. for maintenance. Moreover, the second valve 212 can be arranged to be open during normal operation. When the second valve 212 is open, the controllable valve 210 can be used for controlling the flow, as detailed above and below. The second valve 212 may be a manual valve or an automated valve. Even if shown only in Figs. 2a, 3a, and 3b, such a second valve can be applied in connection with any embodiment, e.g. any one of Figs. 4a, 4b, 5, 6, and 7. Naturally, the second valve 212 is not needed, at least, when the controllable valve 210 can be used to close the flow of the fluid 170 (see Fig. 3a).

In a preferable embodiment, the first parameter indicative of the flow of the smelt is measured optically. Correspondingly, in an embodiment of the system, the sensor arrangement 220 comprises a first sensor 221 comprising a detection unit configured to detect electromagnetic radiation, such a camera. The detection unit may be an optical detection unit (e.g. camera) configured to detect visible light, infrared radiation and/or ultraviolet radiation. The detection unit may comprise a filter or filters for selecting a proper wavelength band to be detected. A camera for the purpose may be a line camera or a matrix camera. A line camera comprises at least two light detection units arranged on a straight line, and thus provides only one dimensional images of the area it is imaging. A matrix camera comprises multiple light detection units arranged in a matrix, and this provides two dimensional images of the area it is imaging.

As detailed above, the environment to which a sensor (221 , 222, 223) of the sensor arrangement 220 is placed, is harsh. Droplets of the smelt and/or the aqueous solution may fly off from the surface 155 of the aqueous solution 152 or directly from the smelt spout or as a result of shattering, and may contaminate the optical detection unit, if not prevented properly. This applies in cases where the first sensor 221 comprises an optical detection unit as well as to cases where it comprises a sound detection unit, such as a microphone. For these reasons and with reference to Fig. 2b, preferably, the first sensor 221 of the sensor arrangement 220 is arranged within a first casing 321 . The first casing 321 is configured to protect the first sensor 221 from the smelt and/or from the aqueous solution. Structural details of the casing 321 may vary.

As an example, the first casing 321 of Fig. 2b limits an aperture 331 for allowing light and/or sound to enter into the first casing 321 for enabling the detection thereof by the first sensor 221. In order to keep the first sensor 221 clean, means 341 for feeding gas, such as air, into the first casing 321 is provided. As the gas is fed to the first casing 321 , the gas exits the first casing 321 from the aperture 331 thus preventing the droplets of smelt and/or the aqueous solution from flying to the first sensor 221 . A flow of the gas is depicted by arrows drawn with dashed lines in Fig. 2b. In general, droplets do not tend to fly counter currently. Even if shown only in Fig. 2b, such a protector (i.e. the casing 321 and the means 341 ) can be used in connection with any embodiment. Moreover, even if such a protector (i.e. the casing 321 and the means 341 ) is shown only as protecting the first sensor 221 , a protector operating on same principles can be used to protect a second sensor 222 and/or a third sensor 223 of the sensor arrangement 210.

When the first parameter or a part thereof is measured optically, the flow of the shattering fluid 170 can be predictively controlled. Thus, by detecting the flow of a first portion of the smelt, the controlling of the valve 210 can be done before the first portion of the smelt enters into the aqueous solution 152. In this way, the process can be predictively controlled. More precisely, in such a case, the first portion of the smelt 140 makes a contact with the aqueous solution 153 at a third instance of time. Moreover, the embodiment comprises, at a fourth instance of time, measuring optically the first parameter indicative of a flow of the first portion of the smelt through the smelt spout. Implicitly in such a case the fourth instance of time is before the third instance of time. Moreover, the embodiment comprises, in between the fourth instance of time and the third instance of time, controlling the flow of the shattering fluid. As detailed above, the flow can be controlled by the valve 210. Such rapid control may be hard manually. However, an automated control process may have the required speed.

It has been found that the flow velocity of the smelt is a parameter that can used to control the flow of the shattering fluid. Flow velocity is indicative of flow of the smelt through the smelt spout. Thus, in an embodiment, the first parameter is indicative of a flow velocity of the smelt 140 in the smelt spout 145. One possibility is to increase the flow of the shattering fluid 170 in a case, where the measurements reveal that a flow velocity of the smelt 140 is increasing. Correspondingly, in such a case where the measurements reveal that a flow velocity of the smelt 140 is decreasing, the flow of the shattering fluid 170 can be reduced. The first sensor 221 of Figs. 3a to 3c, 4b, and 6, is an example of an optical sensor that is suitable for measuring the flow velocity of the smelt in the smelt spout.

The flow velocity can be measured e.g. by observing movement of solid particles with the flow of smelt. Such solid particles are visible from images taken from the smelt within the smelt spout. Moreover, by taking at least two images subsequently, the movement of these particles can be analysed. As the particles have the same velocity as the smelt, the flow velocity can thus be determined.

It has been found that another parameter that can describe the shattering efficiency is a smelt shattering pattern. On one hand, the smelt shattering pattern is indicative of flow of the smelt through the smelt spout. The denser the pattern, the higher the flow. On the other hand, the smelt shattering pattern is indicative of shattering of the smelt. The smaller droplets of smelt, the better shattering. However, e.g. skewness of the patter may be indicative of poor shattering. Referring to Fig. 4a, in an embodiment, the first sensor 221 is suitable for measuring the shattering pattern of the smelt. A number and a characteristic size of the droplets 142 of the smelt can be determined from the shattering pattern. If the number is too small and/or the droplets 142 are too large, the flow of the shattering fluid can be increased. Correspondingly, when the droplets 142 are sufficiently small, or even smaller than required, the flow can be decreased.

Referring to Fig. 4b, two optical sensors (221 , 222) can be used; one (221 ) for measuring the flow velocity of the smelt, and another (222) for measuring the shattering pattern. Referring to Fig. 6, the boiler may comprise, but need not comprise an acoustic sensor. Even if the boiler comprises an acoustic sensor (222 of Fig. 6), the boiler may comprise two optical sensors (221 , 223). One (221 ) for measuring the flow velocity of the smelt, and another (223) for measuring the shattering pattern.

Thus, even if the first parameter is measured optically, a second parameter indicative of the shattering of the smelt can be measured acoustically. Moreover, the flow of the shattering fluid can be controlled based on the value of the first parameter and a value of the second parameter. The second parameter can be, alternatively, be interpreted to form a part of the first parameter. As detailed above, herein the term “parameter” itself refers to one or more properties that are expressible by one or more numbers.

However, the first parameter needs not be measured optically. Fig. 5 shows an embodiment, wherein the first sensor 221 is an acoustic sensor. Thus, an embodiment comprises measuring acoustically the first parameter, wherein the first parameter is indicative of the shattering of the smelt. It has been found that when the droplets 142 of the smelt 140 are large, a lot of noise is caused by the explosions, as a result of the droplets 142 entering the aqueous solution 152. Correspondingly, when the droplets 142 are small, less noise in caused. Therefore, the noise level and/or other characteristics can be used to control the flow of the shattering fluid. One possibility is to increase the flow of the shattering fluid 170, in a case, where the measurements reveal that a noise level is increasing. Correspondingly, in such a case where the measurements reveal that a noise level is decreasing, the flow of the shattering fluid 170 can be reduced.

The first parameter may be measured by using a microphone. Herein the term “microphone” may refer to a sound detector or an arrangement comprising multiple sound detectors, including a line microphone or a matrix microphone. A line microphone comprises at least two sound detectors arranged on a straight line. A matrix microphone comprises multiple sound detectors arranged in a matrix. Line and matrix microphones are capable of detecting also a direction of a source of the sound. Thus, in case the chemical recovery boiler comprises at least two smelt spouts, as it typically does, and acoustic measurements are done, preferably, the acoustic measurements are performed using a line microphone or a matrix microphone. This enables to locate the smelt spout of interest, even if the noise level is determined elsewhere than near a certain smelt spout. In a corresponding embodiment, the first sensor 221 of the sensor arrangement 220 comprises a microphone, such as a line microphone or a matrix microphone.

Since noise is generated only when the smelt enters the aqueous solution 152, it seems that acoustic measurements cannot be done in a predictive manner in the meaning disclosed above for optical measurements. Therefore, preferably, at least optical measurements are performed.

Referring to Fig. 6, in an embodiment, the sensor arrangement 220 comprises - the first sensor 221 , which is configured to measure optically a part of the first parameter indicative of a flow of the smelt through the smelt spout and/or indicative of shattering of the smelt, and - a second sensor 222 configured to measure acoustically, a second parameter indicative of shattering of the smelt.

The embodiment of Fig. 6 further comprises a third sensor 223, which is configured to measure optically another part of the first parameter indicative of a flow of the smelt through the smelt spout and/or shattering of the smelt. However, the third sensor 223 is not needed, even if both acoustic and optical measurements are to be made.

The measurement results of the sensor arrangement 220 (at least the first sensor and optionally only the second 222 sensor or both the second 222 and third 223 sensors), can be used by an operator or automatically. Figs. 3a to 7 relate only to the latter, as they show the control unit 230. In the embodiments of Figs. 4b and 6, the control unit 230 is configured to receive a value of the first parameter from the first sensor 221 and a value of the second parameter from the second sensor 222, and based on the values of the first and second parameters, to control the controllable valve 210. In the embodiment of Figs. 6, the control unit 230 is configured to receive a part of the value of the first parameter from the third sensor 223, and based on the values of the first and second parameters, to control the controllable valve 210. Herein the parts of the value of the first parameter are obtained from both the first 221 and the third 223 sensor.

Preferably, the first sensor 221 comprises a camera, such as a line camera or a matrix camera. The second sensor 222 or the third sensor 223 may comprise a microphone, such as a line microphone or a matrix microphone. The second sensor 222 or the third sensor 223 may comprise a camera, such as a line camera or a matrix camera.

The operator may use the information thus obtainable from the sensor arrangement 220, which comprises a number of acoustic or optical sensors (221 , 222, 223), in a best possible way to control the flow of the shattering fluid. In the alternative, the control unit 230 may be configured to receive information (e.g. the signal S) from all these sensors and to control the flow of the shattering fluid using the information from all these sensors. In addition to controlling the flow of the shattering fluid, a position of the nozzle 175 can be adjusted. Herein the term “position” relates to location and/or direction of the nozzle 175. By adjusting a direction of the nozzle 175 the location wherein the shattering of the smelt occurs can be adjusted. By adjusting a location of the nozzle 175 the location wherein the shattering of the smelt occurs can be adjusted. Thus, it is possible to adjust the position of the nozzle in such a way that the shattering is most efficient. The value of the first parameter, optionally in connection with several other values of the first parameter, can be used to adjust the position. E.g. if a shattering pattern indicates that shattering occurs mainly on a left side of the smelt spout, the nozzle can be moved or turned to right. Thus, an embodiment of the method comprises adjusting a position of the nozzle based on the value of the first parameterof the flow of the smelt. In a corresponding chemical recovery boiler, a position of the nozzle is adjustable.

An operator may perform the adjustment to the position of the nozzle e.g. during maintenance of the recovery boiler. However, also the position of the nozzle can be automatically controlled. Such a recovery boiler comprises an actuator for controlling a position of the nozzle. Moreover, in such a case, the control unit us configured to control the position of the nozzle using the actuator and based on the value of the first parameter, to control the controllable valve. Thus, the control unit is also configured to receive the value of the first parameter. In a corresponding method, the position of the nozzle is adjusted by a control unit through an actuator.

Such an adjustment can be based in particular on a shattering pattern detected as indicated above. E.g. the shattering pattern may be skewed, whereby a position of the nozzle 175 may be adjusted so as to reduce the skewness.

Concerning the placement of the first sensor 221 , there are multiple possibilities. The first sensor 221 may be placed above a part of a hood 157, 158 of the dissolving tank 150, as depicted e.g. in Figs. 2a, 3a, 3b, and 3c. This may be beneficial at least when a flow velocity of the smelt 140 in the smelt spout 145 is detected. In such a case, a part of the hood 157, 158 may protect the first sensor 221 from excessive heat coming from the smelt 140. For example, the door 162 may be opened for measurements (see Fig. 2a). However, a the first sensor 221 (see Fig. 4a, 4b, or 6) can be placed underneath a part of the hood 158. Such a sensor can be protected from excessive heat e.g. by placing the first sensor 221 to the first casing 321 shown in Fig. 2b and by blowing sufficient amount of gas using the means 341 for feeding the gas. In the alternative, other types of protective casing may be used to protect the first sensor 221 .

In case the shattering pattern is detected by one of the sensors (221 , 222, 223) the sensor may be placed e.g. in a upper part of the dissolving tank 150, as depicted in Figs. 4a (see ref 221 ) and 4b (see ref. 222) and 6 (see ref. 223).

Referring to Fig. 7, the chemical recovery boiler 100 may comprise a cleaning apparatus 500 for cleaning a smelt spout 145 or for cleaning several smelt spouts 145 of the recovery boiler 100. The cleaning apparatus 500 comprises a cleaning unit 510. The cleaning apparatus 500 and the cleaning unit 510 are encircled in Fig. 7 by dotted lines to indicate which parts of the figure belong to these entities.

In case the cleaning unit 510 is configured to clean only one smelt spout 145, the cleaning unit 510 need not be movable. However, a cleaning unit 510 may be movable from one smelt spout to another. Figure 7 shows an embodiment, wherein the chemical recovery boiler comprises a movable cleaning unit 510. For moving the cleaning unit 510, the cleaning apparatus 500 comprises a rail 505, along which the cleaning unit 510 is configured to move from one smelt spout to another. In Fig. 7 the rail 505 extends in a direction that is normal to the plane the Fig. 7, whereby only one smelt spout is shown. The cleaning unit 510 comprises a carriage 512 that is connected to the rail 505 in a movable manner so as to move the carriage 512 along the rail 505. The cleaning unit 510 comprises a cleaning member 514. The cleaning member 514 is connected to the carriage 512. The cleaning member 514 can be equipped with a cleaning tool 516 or more than one different cleaning tools 516. The tool(s) 516 can be selected e.g. from the group of a brush, a paddle, a chisel, a water nozzle, a pneumatic nozzle, or a device producing vibration and/or shocks.

Referring to Fig. 7, in an embodiment, the first sensor 221 is fixed to the cleaning unit 510. In this embodiment, the first sensor 221 is movable with the cleaning unit 210 from one smelt spout to another. In an embodiment, not shown, the first sensor may be fixed to the rail 505. In such a case, the sensor arrangement 220 may comprise several sensors, e.g. one for each smelt spout 145. For safety reasons, the recovery boiler 100 may comprise a wall 190 in between the smelt spout area A1 and the working area A2, as shown Figs. 4a to 6. However, the embodiments function also when the chemical recovery boiler 100 does not comprise the wall 190.