The invention relates to a method and installation for conversion of paper residue into a mineral product. Such a method and installation are known from practice and are taught by WO96/06057. The term paper residue as used herein includes both paper sludge resulting from the industrial production of paper, and paper waste such as paper having on average too short fibers. The paper residue may also include deinking sludge. The mineral product obtained with this method and installation can be used, for instance as replacement for cement or as a sorbent for removal of metals from (hot) gas streams.

The known method employs a fluidized bed device that forms part of the installation, whereby below the fluidized bed device there is a distribution plate for securing an even distribution and supply of at least combustion air to the bed mate- rial and to the paper residue that is introduced into the fluid ^¬ ized bed device for conversion to said mineral product.

A wind box is provided below the distribution plate for supplying the combustion air to the bed material and to the pa ^¬ per residue above the distribution plate. The wind box may in certain situations in addition to the combustion air also supply cleansed recirculated flue gases.

Although it is not necessary, it may be preferable from a viewpoint of energy-efficiency that a heat exchanging section is employed which receives in a first part thereof (ambient) air -which may be supplemented with recirculated flue gases-, and in a second part separate from the first part flue gases from the fluidized bed device for exchanging heat between said flue gases and said (ambient) air for heating the latter. The heat exchanging section is then further connected to the wind box for sup- plying the heated (ambient) air to the wind box for use as com ^¬ bustion air.

When employing the method and installation for converting paper residue into said mineral product continuously, the problem occurs that the quantity of bed material and the dimen- sions of its particles vary causing that the duration of unin- terrupted or undisturbed operation of the installation may be limited. It is a known problem in the prior art that the amount of bed material and the diameter of its particles increase to a level that measures have to be taken. At times it is possible to remove during operation of the installation the deteriorated bed material and replace it by bed material having the appropriate properties. At other times this is not possible, in which case it is required to interrupt operation of the installation in or ^¬ der to allow that the bed material is replaced for material hav- ing the required specification.

The proper operation of the installation for conversion of paper residue requires that the bed material and the diameter of its particles are maintained at a specified level. This specified level may vary a little between installations. The op- timum level that applies to a specific installation may there ^¬ fore need to be determined on a moderate trial and error basis. Generally speaking the parameters that are desirable for the bed material and the diameter of its particles that are capable to entertain desirable fluidization conditions in the fluidized bed at fluidization velocities of more than 0.5 meter per second, are that the particles are maintained at a diameter between 0.7 and 4 mm, preferably between 1.2 and 1.8 mm. The height of the fluidized bed should be maintained at a level so that the pres ^¬ sure difference between a location immediately above the distri- bution plate and the freeboard area will be in the range 40-200 cm water column. The bed material is further preferably main ^¬ tained at a level of its spherical shape factor of approximately 0.8.

The just-mentioned spherical shape factor, or spheric- ity, has been introduced in the general literature on fluidized bed combustion to account for deviation from the ideal spherically shaped particle. It is customary to define the sphericity as the ratio of the surface area of a perfect sphere and the surface of the particle under consideration, whilst both parti- cles have identical volumes:

Sphericity = As / Ap and 0 < sphericity < 1, wherein As indicates the surface area of a perfect sphere, and Ap indicates the surface area of the particle that is consid ^¬ ered, and wherein both particles have identical volumes.

Application of basic mathematics regarding the surface area and the volume of an ideal sphere results in:

Sphericity = ( (4*pi* (3/ (4*pi) ) ^Λ (2/3) ) * Vp ^A (2/3) ) / Ap, wherein Vp is the volume of the particle under consideration and Ap is the surface area of this particle.

It goes without saying that replacement of the bed ma ^¬ terial goes at the expense of production quantity, whereas main ^¬ taining the conversion process with the deteriorated bed mate ^¬ rial goes at the expense of production quality. Both have a fi- nancial impact, and the invention is now aimed at maintaining the bed material as used in this fluidized bed within specifica ^¬ tion, so as to avoid the necessity to interrupt or disturb the continuous process of converting paper residue into a mineral product and to keep both production rate and production quality as high as possible.

The prior art, notably GB-A-1 474 711 and JP 58069314, discloses fluidized bed devices in which it is known to control the feed rate of combustion air.

According to the invention the above mentioned problems are addressed by implementing the installation with a control system that satisfies the following limitations:

Al . the control system is arranged to control the tempera ^¬ ture of the combustion air in the wind box within a range of said process parameter that is selected at a predefined tar- get value of said temperature plus and minus 25° Celsius, preferably plus and minus 15° Celsius, wherein

A2. the control system is further embodied such that the target value of the temperature of the combustion air in the wind box expressed in centigrades is defined by the equation Ttarget= -500 * calorific value + 1400, wherein the calorific value expressed in MJ/kg relates to the paper residue including any additional matter that is introduced into the fluidized bed device, and/or Bl . the control system is arranged to control the amount of combustion air supplied to the wind box at a predefined tar ^¬ get value plus and minus 15% for each square meter area of the distribution plate, wherein

B2. the control system is further embodied such that the target value of the amount of the combustion air Qtarget that is supplied to the wind box for each square meter of the distribution plate is defined by the equation Qtarget= 4.35 * organic fraction, wherein the organic fraction relates to the percentage organic material forming part of the paper residue that is in ^¬ troduced into the fluidized bed device,

and/or

B3. the control system is further embodied such that the target value of the amount of combustion air sup ^¬ plied to the wind box for each square meter area of the distribution plate is set at a level of 1.7 m3/s for each square meter area of the distribution plate.

It is remarked that it is possible that other gases are included in the combustion air. Said other gases may for instance be recirculated flue gases escaping from the fluidized bed, preferably cleansed, and advantageously also reheated.

It is further remarked that when in this application reference is made to the term calorific value, this may relate to different definitions. The inventors have found that best re ^¬ sults with the invention are obtained when the calorific value relates to the overall energy release of the paper residue and any other additional matter that is introduced into the fluid ^¬ ized bed device, which takes into account the energy conversion of constituent materials of said paper residue (and other intro ^¬ duced matter) , including mineral transitions during the combus ^¬ tion in the fluidized bed device.

The invention is also embodied in a method for conver ^¬ sion of paper residue into a mineral product, using a fluidized bed device with a wind box and a distribution plate connected to the wind box for an evenly distributed supply of at least com ^¬ bustion air to the bed material of the fluidized bed above the distribution plate, and to paper residue introduced into the fluidized bed device, wherein the amount of bed material and the size of said bed material particles above the distribution plate is controlled by maintaining a process parameter within a prede ^¬ fined range, which process parameter relates to the amount of combustion air. According to the invention this method has the following limitations:

Al . the temperature of the combustion air in the wind box is controlled, and the range of said process parameter is se ^¬ lected at a predefined target value of said temperature plus and minus 25° Celsius, preferably plus and minus 15° Cel- sius, wherein

A2. the target value of the temperature of the combus ^¬ tion air in the wind box expressed in centigrades is defined by the equation Ttarget= -500 * calorific value + 1400, wherein the calorific value expressed in MJ/kg relates to the paper residue including any addi ^¬ tional matter that is introduced into the fluidized bed device,

and/or

Bl . the amount of combustion air supplied to the wind box is controlled at a predefined target value for each square me ^¬ ter area of the distribution plate, and the range of this parameter is selected at said target value plus and minus 15%, wherein

B2. the target value of the amount of the combustion air Qtarget that is supplied to the wind box for each square meter of the distribution plate is defined by the equation Qtarget= 4.35 * organic fraction, wherein the organic fraction relates to the percentage organic material forming part of the paper residue that is in- troduced into the fluidized bed device,

and/or

B3. the target value of the amount of combustion air supplied to the wind box for each square meter area of the distribution plate is set at a level of 1.7 m3/s for each square meter area of the distribution plate.

The invention will hereinafter be further elucidated with reference to an exemplary embodiment of an installation and a method of its operation in accordance with the invention, and with reference to a schematic drawing of this installation. In the drawing:

-figure 1 shows schematically the installation for con ^¬ version of paper residue;

-figure 2 shows schematically the heat exchanging sec- tion forming part of the installation shown in figure 1 ;

-figure 3 shows a graph pertaining to the relation of the rate of change of the amount of bed material depending on the temperature of the combustion air in the wind box of the in ^¬ stallation shown in figure 1 ;

-figure 4 shows a graph pertaining to the relation of the rate of change of the amount of bed material depending on the amount of combustion air and other gases that are introduced into the wind box of the installation shown in figure 1 ;

- figure 5 shows a graph pertaining to the relation of the calorific value of the paper residue and other material that is introduced into the fluidized bed, and the desired target value of the temperature of the combustion air and other gases that are introduced into the wind box; and

-figure 6 shows a graph pertaining to the relation of the percentage of organic material in the paper residue, and the desired target value of the amount of combustion air and other gases that are introduced into the wind box for each square me ^¬ ter of the distribution plate.

Wherever in the figures the same reference numerals are applied, these numerals refer to the same parts.

With reference first to figure 1 reference 1 refers to an example installation for conversion of paper residue into a mineral product. This installation 1 comprises a fluidized bed device 2 with a distribution plate 3 for securing an even dis- tribution and supply of at least combustion air to the bed mate ^¬ rial and to the paper residue that is introduced into the fluid ^¬ ized bed device for conversion to the mineral product. A wind box 4 is provided below said distribution plate 3 for supplying combustion air symbolized by arrow 5 to the wind box 4 and even- tually to the material and paper residue above the distribution plate 3.

In accordance with this example the installation 1 may further comprise a heat exchanging section 6 which receives in separate parts ambient air symbolized by arrow 7 through the op- eration of fan 8. The heat exchanging section 6 receives flue gases escaping from the fluidized bed device 2 through the free ^¬ board 10 and through the connecting ducts 9 for exchanging heat between said flue gases and said ambient air 7 for heating this ambient air which is to be used as combustion air 5. The heat exchanging section 6 is to this end connected to the wind box 4 for supplying the heated ambient air 7 to the wind box 4.

Figure 2 shows in detail the heat exchanger 6, and shows that in this example not all ambient air 11 must be preheated in the heat exchanger 6 in order to convert to combustion air 5 which may be introduced into the wind box 4. It is also possible that a part 12 of the ambient air 11 is bypassing the heat ex ^¬ changer 6 and is mixed with the preheated air 13 that leaves the heat exchanger 6 for providing the flow of combustion air 5 that may be introduced into the wind box 4.

According to the invention the installation 1 is operated in a manner to control the amount of bed material above the distribution plate 3 and the diameter of the bed material parti ^¬ cles. For this purpose a control system is employed which is known per se and which therefore needs no elucidation. The inventive merit of the invention is embodied in the manner how this control system is used, notably to monitor and maintain a process parameter within a predefined range, whereby the process parameter is selected from the group comprising the amount of combustion air 5 and the temperature of the combustion air 5 de ^¬ livered to the wind box 4.

In the method for conversion of paper residue into a mineral product according to the invention, wherein the amount of bed material above the distribution plate 3 and the dimension of the particles of this bed material are controlled by main ^¬ taining the temperature of the combustion air introduced into the wind box within a predefined range, preferably a target value of said process parameter is defined in dependence of at least the calorific value of the paper residue and any other ad- ditional matter that is introduced into the fluidized bed device 2. If any further matter is introduced into the fluidized bed device 2 together with the paper residue, the calorific value of this further matter must be taken into account as well. Further when the process parameter is the temperature of the combustion air 5 introduced into the wind box 4, the range of said process parameter is selected at the target value of said temperature plus and minus 25° Celsius, preferably plus and minus 15° Celsius.

Suitably the target value of the temperature of the com ^¬ bustion air introduced into the wind box 4 is defined by the equation T _target = -500 * calorific value + 1400, wherein the calo ^¬ rific value relates to the paper residue and any other matter that is introduced into the fluidized bed device 2. This is shown in figure 5 wherein the ordinate relates to the mentioned calorific value, and the abscissa relates to the target value of the temperature of the combustion air that is introduced into the wind box 4.

It may also be advantageous that the process parameter is the amount of combustion air 5 introduced into the wind box 4, in which case the range is selected at a target value of said amount plus and minus 15%.

Results of the invention are illustrated by figures 3 and 4 respectively.

Figure 3 shows a graph illustrating the results when the temperature of the combustion air 5 introduced into the wind box 4 is varied. In this figure the ordinate shows said temperature of the combustion air 5 in centigrades, whereas the abscissa shows the rate of change of the amount of bed material per hour as measured by the variation of the bed pressure in millibar per hour. The amount of bed material proves to be virtually constant at around 410°C at the prevailing calorific value of the paper residue that is introduced into the fluidized bed device 2 which amounted in this example approximately 2 MJ/kg.

Figure 4 shows a graph illustrating the results when the amount of the combustion air 5 introduced into the wind box 4 is varied. In this figure the ordinate shows said amount of combus ^¬ tion air 5 in cubic meters per second normalized with respect to the square area of the distribution plate 3, whereas the ab ^¬ scissa shows the rate of change of the amount of bed material per hour. The correlation between the amount of combustion air and the rate of change of the amount of bed material is margin ^¬ ally less than the correlation between the temperature of the combustion air and said rate of change, however there is a defi ^¬ nite relation between the amount of combustion air 5 introduced into the wind box 4, and the rate of change of bed material above the distribution plate 3. An optimum appears to be present when approximately near to 1.7 m ³ per second per m ² of distribu ^¬ tion plate is introduced into the wind box 4. Further investiga ^¬ tion has shown that this optimum relates to a situation in which approximately 40% organic material is present in the paper resi ^¬ due, which in most practical circumstances is the case.

Figure 6 shows the relation between a variable amount of organic material present in the paper residue (this is shown at the ordinate) , and the optimum amount of combustion air for each square meter of the distributional plate, which is shown at the abscissa. The relation between the two is defined by the equa- tion Q _target = 4.35 * organic fraction, wherein the organic fraction relates to the percentage organic material forming part of the paper residue that is introduced into the fluidized bed device, and Qta _r get relates to the target value of the amount of the com ^¬ bustion air that is supplied to the wind box for each square me- ter of the distribution plate. When no information on the organic fraction is available, the value of 1.7 m ³ per second per m ² of distribution plate is preferably taken as the target value of the amount of combustion air that is to be introduced into the wind box.

The above given example must consistent with the law not be considered limiting with respect to the appended claims. The protective scope that the invention merits must solely be deter ^¬ mined by the appended claims in their broadest sense, without being considered to be restricted to the offered example. The example is only offered for the purpose of removing any possible ambiguity that may be present in the appended claims.