WO2018050668A1

WO2018050668A1 - System for the metering and gravity-based injection of powders in dense phase

Info

Publication number: WO2018050668A1
Application number: PCT/EP2017/072970
Authority: WO
Inventors: Muriel MARCHAND; Jean-Marie Seiler
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2016-09-14
Filing date: 2017-09-13
Publication date: 2018-03-22
Also published as: EP3512790A1; FR3055889A1

Abstract

The present invention relates to a dense-phase powder supply system (1), essentially consisting in suitably coupling the metering of a discontinuous flow to an injection device, the geometric properties of which are calculated based on experiential knowledge relating to a given powder and the use of the constituent material thereof.

Description

DOSAGE AND INJECTION SYSTEM BY GRAVITY OF POWDERS IN

PHASE DENSE

Technical area

The present invention relates to the field of powder supply systems, specifically dosing and injection of powders in dense phase, that is to say with little or no carrier gas.

It relates in particular to cohesive powders, such as wood powders, that is to say that they have a natural tendency to spontaneously form arches and vaults.

The invention relates more particularly to an application of food, pressurized containers or not, in powders of carbonaceous material charge, such as biomass, coal or any other type of powders such as a waste grind ... The Containers to be fed can advantageously be gasification reactors, or other thermochemical conversion systems. In this application, the mass flow rates of solid can be very variable, from a few kilograms up to several tons per hour in industrial conditions.

The aim of the invention is to improve such powder supply systems, in particular by a fine, precise, stable and reproducible metering and a dense phase powder injection which retains the characteristics of the metering.

Although described with reference mainly to the advantageous application of dosing and injection of biomass powders, the invention can be applied to any type of powders whose physical and mechanical properties (particle size, density, humidity, avalanche angle ...).

Prior art

In the field of the supply of powders of various and varied containers, two main categories of known devices can be distinguished, those having a powder dosing function, that is to say which make it possible to control the quantity of powder, and those for the injection of powders into a container, continuously or discontinuously.

Among the widespread metering devices, mention may be made of screw devices, rotary locks or pressurization chambers, commonly called "lock hoppers" in English. For example, the patent application WO2012 / 152742A1 discloses a rotary lock for metering a powder which feeds directly from the lock a kneader. These conventional dosing devices apply both to the field of pharmacy or chemistry, which require low flow rates, as for example described in patents US Pat. No. 4,850,259 and US Pat. No. 7,898,435 B2, than in the field of industrial handling of high powders. rates. In particular, patent US9227790B2 relates to a gasification plant for biomass or coal in which the powder is conveyed by a helical screw conveyor.

There are also pressurization airlock, whose more or less important volume defines the fineness of the dosage. One advantage of locks and pressurizing airlocks is that they also have a sealing effect against possible pressure fluctuations upstream and downstream.

The main disadvantage of metering screws, locks and lock-hoppers is the highly discontinuous nature of the generated powder flow, as illustrated in FIG. 1, where it can be seen that for a known rotary lock, if it is still possible to define a constant average flow, the variations of flow over time can be important.

Until now, in order to overcome this drawback, the solution consisted in arranging a large number of metering devices, either in series with a volume, so a flow rate, decreasing, for example rotating locks with an increasing number of buckets of decreasing size going to the injection point, or in parallel, with an alternating dosage of several devices. In addition to the investment and management constraints of alternate operation, this solution has the disadvantage of generating a significant footprint. On the other hand, for cohesive powders, problems of plugging in buckets occur frequently.

Another known solution, particularly in the field of conveying and gasification of coal powder, is to control the solid flow rate by an additional gas flow: see publications [1], [2]. A conveying with gas, also referred to as "aerated injection", has the advantage of not generating additional space. On the other hand, it generates highly diluted phase flow, which may not be compatible with many downstream processes.

This is particularly problematic in a thermochemical conversion plant in which the gas does not participate in the reaction and thereby decreases the efficiency. This can be particularly penalizing at high pressure, because of the increased density of the gas, which increases the ratios between the mass flow rate of the gas and the mass flow rate. solid. For example, implementing a convection gas by diluting the powder particles in a biomass gasification plant would require using a portion of the energy of the gasification reaction to heat the initially cold gas, which would have the effect of undesirable to decrease the overall yield of the reaction.

Proposals have been made for deaeration systems of the solid / fluid mixture, as presented in US patent application 20111/0139257 A1 which proposes a passive system fulfilling this function. This passive system, however, has the disadvantage of generating a flow against the current in the deaeration zone: in fact, the solid descends by gravity, while the gas back by suction to vents. Control of this flow also imposes constraints on the geometry of the de-aeration device, in particular its inlet section and its length. On the other hand, in such a system the solid flow rate is not defined independently of the gas flow.

With regard to the injection devices, mention may be made of the old literature and provided on the discharge silos. These "funnel" type elements in the form of a truncated cone make it possible to inject a granular material into a smaller container.

On the other hand, these funnels, usually called injection cones, do not have the function of dosing the granular material and provide just a discharge by gravity. Thus, in these known injection devices, the value of the flow rate is not adjustable.

Various gas-free injection devices can also be found, described in detail in the publication [2]. Thus, as for the dosing devices, these injection devices find an application in the diluted phase with a solid volume fraction reached which is a few%.

Another penalizing point of this type of aerated injection is that the dosing and powder injection functions are not separated (publication [3]), since it is the two-phase mixture and therefore the gas flow which determines in much of the solid flow, as is the case for US 2011/0139257 Al.

Finally, for very cohesive powders and / or not easily fluidizable, this type of injection is complex to achieve and very dependent on the nature of the powders, as explained by the publication [2]. If in the prior art, there are dense phase powder supply systems that combine a powder metering device and an injection device, the disadvantages specific to each device remain.

There is therefore a need to improve the dense phase powder supply systems, in particular in order to overcome the aforementioned drawbacks of known metering devices and injection devices, in particular flow fluctuations that are incompatible with the process, a discharge without flow control, interdependent solid and gas flows, a gas flow rate incompatible with the efficiency of the process, and without using a dilute phase feed, and / or a mechanical installation generating an investment and / or a large size (s).

In general, there is a need to define a system for supplying dense phase powders, in particular cohesive powders, which makes it possible to obtain an assay having the following properties:

- end: the amplitude of the flow fluctuations around the mean flow must remain low;

- accurate: the flow rate must be in accordance with the setpoint, within the limit of course measurement accuracy flowmeters;

- stable, in the statistical sense of the term: the average flow over a given period of time, must remain the same throughout the duration of the process (absence of slow drift)

- reproducible: the flow must not vary from one day to another.

There is also a need for an injection downstream of the assay which retains the properties defined above for the assay, especially with a constant flow rate even over very short periods, typically of the order of one second, and which functions both at low and high flow rates, at low and high pressure.

The object of the invention is to meet at least part of these needs.

Presentation of the invention

To this end, the invention relates, in one of its aspects, to a dense phase powder supply system comprising:

- A dosing device, adapted to dose the powder at a given average flow rate; a device for injecting the powder by gravity, in the form of a truncated cone, arranged below the metering device so as to receive by gravity the powder dosed at the given average flow rate, the opening angle and the type of material constituting the wall of the injection cone being predetermined from the mechanical properties of the powder, while the diameter of the exit orifice of the truncated cone of injection is predetermined from the so-called correlation of Rose & Tanaka, according to the following equation:

W =

- kp * dp) ^2.5 [Eq. 1] in which:

W: mass flow rate of powder,

k: constant which depends on the shape of the cone,

k _P : constant which depends on the shape of the powder particles,

D: diameter of the exit orifice of the cone,

pb: apparent density of the powder,

dp: average diameter of the particles.

: experimental coefficient of weighting, intrinsic to the powder, determined experimentally for different powders and from several injectors of different diameters.

The dosage can be discontinuous can be achieved by a dosing device selected from a screw, a rotary lock or a lock hopper.

The values of the constant k _P are of the order of 1 to 2.6 depending on the material and more precisely of the order of 1 for wood. The constant k takes into account the angle that the cone makes with respect to the vertical.

The coefficient a, less than 1, has been determined experimentally for different powders and from several injectors of different diameters. For each powder, the link between the delivered powder flow rate and the injector diameter is made. The values of a extend over the range [0.85-0.99] and more preferentially on the beach

[0.86-0.90].

By way of example, the value a = 0.86 was determined for a beech powder with a particle size of approximately 200 μιη. This value less than 1 shows that the correlation of Rose and Tanaka slightly overestimates the flow.

The powder can be delivered directly by gravity into a container, however, according to an advantageous embodiment, the feed system comprises a hollow tube for discharging the powder to be injected at the outlet of the injector. Thus, this tube is the junction between the injector and the receptacle. This discharge tube, which can be of variable length and geometry, advantageously allows the powder to be conveyed downstream of the injector over more or less important distances. In addition, the length of the discharge tube and its geometry (straight or bent tube) can control the pressure drop in order to ensure the sealing of the injection system vis-à-vis pressure fluctuations to the downstream and therefore to guarantee the stability of the powder flow.

According to another advantageous embodiment, the supply system comprises a solid tube arranged coaxially inside the injection cone so as to delimit with the outlet orifice an injection ring whose external diameter corresponds to to that obtained for the diameter D. With this solid tube, thus an annular injection. In the case of a gasification of the powder, the thermal energy can be provided by oxycombustion. In this case, it may be interesting to place the oxygen-fed burner in the center and injecting wood powder around it. It goes without saying that it is ensured that the passage section for the powder is retained relative to the circular case.

According to yet another advantageous embodiment, the supply system comprises a vibration device rigidly connected to the injection cone. This vibration device and coupled to the injector can help the flow of very cohesive powders.

Another subject of the invention is, according to another of its aspects, a thermochemical conversion plant comprising a gasification reactor arranged downstream of the system described above.

The plant is particularly intended to convert biomass powder, the reactor being a driven flow type reactor.

The invention also relates to a method for producing such a system, comprising the following steps:

a powder is selected whose mechanical properties are measured,

the average flow rate of the powder to be injected into a container downstream of the cone is determined, the angle of opening of the injection cone is determined by calculation from the knowledge of the mechanical properties of the powder and the roughness of the wall of the cone, as well as the exit diameter from the equation equ 1].

These properties will determine the powder-powder friction angle and the powder-wall friction angle. The minimum outlet diameter of the cone to obtain a mass flow of the powder is then determined by the equation [equ 1].

In other words, the system according to the invention operates as follows:

a metering device that can be conventional, such as for example a screw or a rotary lock, gives the target average flow rate (coarse dosing, in packets);

the powder dosed in packets then falls by gravity into the injection cone to the parameters determined beforehand as a function of the geometry of the cone, the nature of the powder and the wall (nature of the material constituting the wall of the cone, angle of cone opening and diameter of the cone outlet).

The opening angle of the cone is defined according to the mechanical properties of the powder and the nature of the material constituting the wall of the cone. This takes into account the flowability of the powder, including for highly cohesive powders.

The diameter of the outlet orifice of the cone is optimized to ensure the temporal smoothing of the solid flow and thus creation of a continuous flow at the injector outlet. This diameter must be small enough to eliminate the injection discontinuities and large enough not to limit the flow and thus reach the target value of flow.

Thus, the invention essentially consists of judiciously coupling a discontinuous flow rate measurement with an injection device whose geometric properties are calculated from experimental knowledge of a given powder and the use of its constituent material.

This combination will allow, for a given powder, in particular cohesive powder, to have an injection, without mechanical aid or by addition of gas creating a dilute phase, with flow smoothing which is thus continuous, stable even for very short periods of time , and reproducible.

The invention can be reproduced at will for any installation in which it is necessary to supply a container, regardless of the value of the flow rate and at low or high pressure. The invention is particularly advantageous to implement in a thermo-chemical conversion plant in a driven flow reactor (RFE). Indeed, the safety rules in this type of installation require to guarantee a very high stability of the injected powder flow.

detailed description

Other advantages and characteristics of the invention will emerge more clearly from a reading of the detailed description of exemplary embodiments of the invention, given by way of nonlimiting illustration and with reference to the following figures, among which:

FIG. 1 is a curve illustrating the flow discontinuity of a powder in dense phase, introduced by a metering device constituted by a rotating sluice according to the state of the art;

FIG. 2 is a schematic view of a dense phase powder supply system according to the invention;

FIG. 3 is a curve illustrating the temporal evolution of the mass of a wood powder, measured by a weighing system at the outlet of a feed system according to the invention;

FIGS. 4A and 4B are photographic reproductions respectively of the top and the bottom of an injection cone according to the invention, according to a variant making it possible to inject powder into an annular shaped conduit, which is useful for certain applications. .

It is specified here throughout the present application, the terms "lower", "upper", "above", "below", "inside", "outside", "" internal "" external "are to be understood by reference a truncated cone injection according to the invention with its axis arranged vertically with the entry on the top and the exit on the bottom.

It is also specified that the terms "upstream", "downstream", "input", "output" are to be considered in relation to the direction of flow of the powder in the system according to the invention.

Figure 1 has already been commented on in the preamble. It will not be detailed later. The dense phase powder feed system 1 shown in FIG. 2 firstly comprises a conventional dosing device 2. It may be a helical screw, a rotary lock or a pressurization lock.

A feed hopper 20 may be implanted at the inlet of the metering device.

Below the metering device 2 is arranged an injector 3 in the form of a truncated cone smoothing and injection. The arrangement is such that the powder dosed at the outlet of the metering device 2 falls by gravity directly into the cone 3.

According to the invention, the geometrical parameters that are the opening angle and the exit diameter of the injection cone have been predetermined by calculations from the experimental knowledge of the characteristics of the given powder. The nature of the constituent material of the injection cone 3 was also chosen beforehand and participated in the definition of the value of the opening angle. The exit diameter was calculated from the modified Rose and Tanaka correlation by the addition of a multiplicative coefficient experimentally determined by the inventors for several powders.

It is specified here that in the definition of an injection cone according to the invention, the height of the cone does not intervene. It can be chosen independently of the nature of the powder, provided of course to respect the known rules of the art, namely that the value of the height must be sufficient to allow the filling of the cone to a height equivalent to a few diameters Release.

An example of definition of the parameters of the cone 3 for a powder of beech wood particles, of unit size of the order of about 200 μιη, with a target mass solid flow rate of 50 kg / h, is indicated here.

First, the powder is tested using an experimental shear cell, which quantifies its flowability. In this shear cell, a normal stress is applied to a powder column and the tangential stress (or shear stress) required to move the powder in a horizontal plane is measured. These tests give access to the powder-powder friction angle (friction of the upper part of the cell on the lower part) and to the flow function of the powder, which characterizes its ability to flow. Tests performed according to the same protocol but replacing the bottom wall of the powder column with a plate made of the material provided for the injection cone, used to measure the powder-wall friction angle.

The data established using these shear measurements make it possible to calculate the minimum aperture angle of the convergent, for a regular flow of the powder mass.

By way of example, a half angle of opening of the cone with respect to the vertical, equal to 10 °, is determined here.

The more the material used to make the injection cone is smooth and the more the bulk flow is favored, which results in low cone half-opening angles for rough materials, such as steel. Standard stainless and larger values for smooth materials.

The measurements made by the inventors on beech wood powders show that a half-cone angle of 10 ° for a rough type 304 L stainless steel corresponds to a half angle of 25 ° for the same stainless steel having undergone mirror polish treatment.

After extensive literature review, the inventors have furthermore come to the conclusion that the diameter of the outlet orifice of the injection cone 3 must favor the flow and the temporal smoothing of the solid flow, which is by nature discontinuous at the outlet of the device. dosage 2.

Thus, the diameter of the outlet of the cone must be small enough to eliminate upstream dosing discontinuities but large enough not to limit the flow.

The determination of this diameter is made from the Rose & Tanaka correlation, as explained in the publication [4], which is modified experimentally in the invention by the addition of a multiplicative coefficient.

This modified correlation results in the following equation:

in which :

W: mass flow rate of powder,

k: constant which depends on the shape of the cone,

kp: constant which depends on the shape of the powder particles, D: diameter of the exit orifice of the cone,

pb: apparent density of the powder,

dp: average diameter of the particles.

The values of the constant k _P are of the order of 1 to 2.6 depending on the material and more precisely of the order of 1 for wood. The constant takes into account the angle of the cone relative to the vertical.

The coefficient a, less than 1, has been determined experimentally for different powders and from several injectors of different diameters. For each powder, the link between the delivered powder flow rate and the injector diameter is made. The values of a extend over the range [0.85-0.99] and more preferably over the range [0.86-0.90].

In practice, for the example of beech wood powder, the value of 0.86 was experimentally determined by the inventors from injectors of several variable output orifice diameters, as mentioned previously.

To validate the primordial advantage of the system according to the invention, which is to generate a continuous flow with a stable flow rate at the outlet of the injector 3, the inventors have carried out tests on an experimental installation for conveying and injecting at atmospheric pressure. to achieve a stable output of 50 kg / h output.

This installation comprises a feed hopper leading to a metering screw 2 sold under the name GAC 207 by the company GERICKE, which is driven by a motor coupled to a frequency converter.

This screw 2 discharges by gravity the powder into the standard type 304 stainless steel test cone 3 to be tested, with a 10 ° cone half-angle and an outlet orifice diameter equal to 12 mm. The roughness standard of 304 stainless steel is 0.015 mm.

In addition, a discharge tube of circular section and length equal to 200 mm was added downstream of the injector cone 3. This discharge tube allows in real operating conditions to convey the powder while maintaining the passage section of outlet of the injector 3, to the container in which the powder must be poured. This tube can also act as a buffer against the pressure fluctuations downstream of the injection. The experimental validation of the device was made by weighing the mass of powder at the outlet of the injector over time and determining the mass flow rate.

The weighing is carried out by a set of three load cells or weighing sensors sold under the name F60X100 under the trademark SCAIME. Calibration of these load cells led to an uncertainty of 0.005% of the measurement. Their resolution is given by the manufacturer at 27 grams. Tests in the laboratory showed an effective resolution of about 45 grams.

During an injection test, the powder harvested at the injector outlet 3 is weighed over time.

Figure 3 shows the curve of the mass of powder harvested as a function of time. As visible, this curve is a perfect line. The perfect linearity of mass evolution as a function of time reflects flow stability.

The flow is therefore deduced from the slope of the signal. It should be noted that for a flow rate of 50 Kg / h, the measurement resolution of the load cells limits the frequency response of the weighing system to approximately 0.33 Hz.

FIGS. 4A and 4B show an advantageous alternative embodiment of the injection cone 3. In these figures, a solid tube 5 is shown arranged coaxially inside the injection cone 3.

This defines with the outlet port 30 an annular injection space 6. An annular injection of powder may be advantageous in many applications.

In particular, in the case of a gasification of a wood powder by oxycombustion, the burner is preferably supplied with oxygen at its center and thus the injection of wood powder ring around does not interfere with the operation of the burner .

Other variants and advantages of the invention can be realized without departing from the scope of the invention.

The invention is not limited to the examples which have just been described; it is possible in particular to combine with one another characteristics of the illustrated examples within non-illustrated variants. References cited

[1]: "Pneumatic convection of solids", KLINGZING, G.E., RICE, F., MARCUS, R. & LEUNG, L.S., third edition, 2010.

[2]: "Pneumatic conveying design guide", MILLS, D, Elsevier, 2006.

[3]: "Effects of Aeration on the Discharge Behavior of Powders". FERRARI, G., BELL, T.A. Power Handling and Processing 10, 269-274, 1998.

[4]: ROSE, H. F. and TANAKA, T. The Engineer (London), 208, 465. 1959

Claims

A dense phase powder supply system (1) comprising:

- A dosing device (2), adapted to dose the powder at a given average flow rate;

a gravity injection device (3) for the powder in the form of a truncated cone, arranged below the metering device so as to receive by gravity the dosed powder according to the given average flow rate, the angle of opening and the type of material constituting the wall of the injection cone being predetermined from the mechanical properties of the powder, while the diameter of the outlet orifice (30) of the truncated cone of injection is predetermined according to the so-called Rose & Tanaka correlation, from the following equation:

in which :

W: mass flow rate of powder,

k: constant which depends on the shape of the cone,

k _P : constant which depends on the shape of the powder particles,

D: diameter of the exit orifice of the cone,

pb: apparent density of the powder,

dp: average particle diameter,

a: experimental coefficient of weighting, intrinsic to the powder, determined experimentally for different powders and from several injectors of different diameters.

2. Feeding system (1) according to claim 1, comprising a hollow discharge tube (4) of the powder to be injected at the outlet of the injector.

3. Feeding system (1) according to claim 1 or 2, comprising a solid tube (5) arranged coaxially inside the injection cone so as to delimit with the outlet orifice (30) a injection ring (6) whose section corresponds to that obtained for the diameter D.

4. Feeding system (1) according to one of the preceding claims, comprising a vibration device rigidly connected to the injection cone.

5. Thermochemical conversion plant comprising a gasification reactor arranged downstream of the system according to any one of the preceding claims.

6. Installation according to claim 5, for converting biomass powder, the reactor being a driven flow type reactor.

7. Method of producing the system according to one of claims 1 to 4, comprising the following steps:

a powder is selected whose mechanical properties are measured,

the average flow rate of the powder to be injected into a container downstream of the cone is determined,

the angle of opening of the injection cone is determined by calculation from the knowledge of the mechanical properties of the powder and the roughness of the wall of the cone, as well as the exit diameter from the equation equ 1].