RU2791027C2 - Multi-hearth furnace containing levers carrying mixing teeth of an optimized profile, application for firing biomass - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention primarily relates to a multi-hearth furnace (1) designed for heat treatment of a charge of material M containing at least one stirring lever (5) attached to a rotary shaft above each hearth (3) of the furnace and containing stirring teeth (7) fixed perpendicular to the lever, with each stirring tooth (7) having a leading edge tilted from the bottom up, and the inclined leading edge forms the leading edge (70) for the material M to be mixed, while the leading edge (70) of each mixing tooth is tilted so that the width at the bottom of the tooth is greater than the width at the top. The invention also concerns the use of a multi-hearth furnace.

EFFECT: improvement in the mixing of the charge of the material to be heat treated in the furnace.

10 cl, 10 dwg

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to a multi-hearth oven, also referred to as a deck oven (known by the abbreviation MHF).

The invention is directed to improving the mixing of the material charge to be heat treated in a furnace, in particular to obtain a uniform and controlled material transfer.

In the context of the invention, "mixing" means the act of mixing the material to be heat treated, distributing it by dividing it into parts, and passing it through the oven from hearth to hearth.

A preferred application of the invention is the roasting of biomass, preferably lignocellulosic biomass.

"Biomass" in the context of the invention means any inhomogeneous material of plant or animal origin containing carbon, such as lignocellulosic biomass, forest or agricultural residues (straw), which may be quasi-dry or water-saturated, such as household waste.

In particular, the biomass may be of the lignocellulosic type such as wood and agricultural materials and have any moisture content, preferably between 10 and 60% water, and be introduced into the plant according to the invention in all different forms such as chips, flakes, etc. .

Although the multi-hearth kiln has been described in relation to biomass roasting, the multi-hearth kiln according to the present invention can be used for many other applications, among which are the thermal treatment of mineral ores (coke, chamotte, etc.), coal, sludge in sewage treatment plants .

BACKGROUND OF THE INVENTION

In a context where consumption never stops growing, the possibility of increasing the value of biomass for the diversification of energy resources is being considered. In particular, thermal reforming lines based on gasification and combustion are considered.

The so-called "Biomass to Liquid" plants, abbreviated as BtL, are plants designed for the thermochemical conversion of biomass through gasification to produce liquid fuels. These BtL plants have already been described in detail with a lot of economic data both in terms of investment and operating costs: [1], [2]. BtL plants, more generally plants for the production of gaseous or liquid fuels from biomass, which are currently under development, are demonstration projects that aim to test and validate the entire process chain of the thermochemical conversion process. The first step realized or being carried out consists entirely in drying the wet biomass. In fact, the moisture content of newly harvested biomass is typically between 30 and 50%, with this initial moisture content varying depending on the type of biomass, the collection period, and where the biomass is stored. It has already been proven that the water contained in the biomass significantly reduces the efficiency of the gasification, since this includes the generation of useless steam and the evaporation of the water needed in the gasification reactor. Moreover, it is now generally accepted that the natural drying of biomass cannot really be envisaged in an industrial application, given the time required: [3], [4], [5].

In other words, on an industrial scale, only forced drying of biomass can be envisaged. Similarly, it can be agreed that drying lignocellulosic biomass below a moisture content of about 15% is meaningless, because below this value the wood is very hygroscopic and reabsorbs water on contact with air during its transfer to the gasification reactor and its intermediate storage. In addition, extracting the final moisture content significantly increases energy consumption. Thus, it seems generally accepted that drying biomass to a moisture content of about 15% before gasifying it is a good compromise in terms of conversion efficiency and required energy consumption.

Burning of biomass, preferably lignocellulosic biomass, is a pre-treatment step for biomass to be injected as a powder into a controlled flow reactor (gasification reactor) or into a so-called biomass-coal co-combustion reactor in a coal-fired power plant or to form it into pellets. In fact, the fibrous and elastic structure of the biomass makes its fine grinding energy-intensive and gives the ground product characteristics unsuitable for injection in powder form. Roasting is a mild thermal treatment of biomass at the interface between drying and pyrolysis, usually carried out at temperatures between 200 and 300°C, aimed at removing water and altering some of the organic material of the biomass to break its fibers. In other words, this mild heat treatment affects the fibrous structure of the biomass, thereby facilitating its grinding and introduction into the gasification or co-combustion reactor.

Roasting pre-treatment also improves the storage properties of the biomass, making it particularly hydrophobic and resistant to biodegradation. Granulation is also one of the possible applications after firing the material.

Roasting products are solids, roasting gases and condensables.

In the currently known installation for drying and roasting lignocellulosic biomass, three main blocks can be distinguished:

- a preheating/drying reactor in which the biomass feedstock is heated and part of the moisture can be evaporated, as explained below;

- the roasting reactor, in which the dried biomass is heated to the roasting temperature, the residual moisture is completely removed and the phenomenon of depolymerization occurs;

- a calcined product cooling unit in which the calcined product is cooled in an inert atmosphere.

Existing roasting reactors are of the rotary type, with multiple floors, also called multi-hearth furnaces (multi-hearth furnaces, abbreviated as MHF), with a tunnel saturated with liquid or steam, with a screw or even with a fluidized bed.

Modern multi-hearth furnaces operate at atmospheric pressure, in a temperature range between 250°C and approximately 1050°C and with a possible large reaction surface. They also have the advantage of continuously mixing the product to be processed. In addition to burning biomass, they are used in the field of environmental protection (pyrolysis of solid waste, coal recovery) and raw materials (calcination and recrystallization of industrial mineral ores and pyrometallurgy).

In FIG. 1-4 are schematic representations of a prior art multiple hearth furnace, generally designated 1, with its various components. The furnace 1 comprises a casing 2 within which is fixed a row of hearths or cooking plates 3 of circular cross section arranged in parallel one above the other. Hearths 3 and casing 2 can be made of steel coated with a refractory material. The rotary shaft 4 is located vertically on the central axis Z of the furnace. The rotary shaft 4 can be rotated by mounting a gear motor 40 . The rotary shaft 4 contains levers 5, called stirring arms, which are fixed to it by being located above each hearth 3. For example, four stirring arms 5 are fixed above each hearth 3, evenly spaced 90° relative to each other. The function of each of the stirring arms 5 is to stir the charge M, such as biomass, to feed into the furnace through the upper opening 20 of the shell 2 and move it through each under 3 in a spiral path to the lower opening 21 of the shell 2, through which the material.

As shown in FIG. 2, each agitating arm 5 comprises a profile 6 to which is attached a plurality of identical teeth 7, called agitator teeth, of rectangular cross section, arranged parallel to each other at right angles to the profile, but all inclined at an inclination angle α different from 90° , relative to the longitudinal axis X of profile 6.

In addition, all mixing teeth 7 are centered on the X-axis of the profile 6, that is, the width L of each tooth is evenly distributed on both sides of the X-axis.

The mixing teeth 7, with their support profile 6, form a rake-like tool that makes it possible to transfer the charge M from one hearth 3 to the adjacent hearth below in the most even manner while stirring as many of them as possible so that each charge particle receives the same amount of heat when passing through the furnace. This makes it possible to achieve the most homogeneous thermal transformation, for example, roasting throughout the charge (reducing the amount of lack of fusion and overcooked pieces, etc.).

Thus, as schematically shown in figure 3, the material M is displaced in two adjacent hearths 3 in opposite directions, respectively, towards the center and outside of the oven and vice versa.

To do this, each of the two adjacent hearths 3 is provided with one or more detachable transfer holes 30, 31, respectively, near, preferably around the rotary shaft 4 and on the outer periphery of the hearths 3.

In the example shown in FIG. 1, the arrangement of the various hearths 3 and their breakaway transfer holes 30, 31 results in the final discharge of the material M at the periphery of the casing 3, whereas in the example of FIG. 3, the final extraction of the material M takes place centrally around the rotary shaft 4.

In other words, in the furnace 1, the charge of material M is fed into the upper hearth 3 and mixed to pass through the latter through one or more transfer holes 30, 31 leading to the hearth 3 immediately below, and so on. In this way, the material M passes through each under 3 to the lower opening 21, from where the material is discharged.

As shown in FIG. 1, the hot gases G, such as flue gases, generally circulating in countercurrent to the charge, bring the furnace to the desired temperature and cause the desired heat treatment reaction or reactions (strong drying, calcining, etc.) of the charge. Thus, heat is generated during the combustion of either the components of the charge itself or a special fuel. For example, oil burners 12 may be placed through casing 2 on some hearths 3. Steam may also be injected into the furnace to improve furnace control. The furnace is typically operated in a controlled atmosphere and contains means for controlling the temperature and residence time of the charge in the furnace. The batch feed can be adjusted continuously to maintain a constant thickness of the stirred bed inside the furnace. Such a furnace operating continuously is disclosed, in particular, in a sludge incinerator in a sewage treatment plant: [6].

When the inventors implemented such a lignocellulosic biomass kiln, they were able to identify various problems associated with the shape and arrangement of the teeth as described above.

First of all, the rotational displacement R of the straight rectangular teeth tends to push the material forward instead of separating it and guiding it in a central or outward direction. In other words, the prongs create a sort of cluster of material that forms a wall at the end of the prong, with the latter rotating R along with the movement of the levers. This is indicated in FIG. 4 and 6, where it can be seen that a cluster of material M is formed at the end of the vertical leading edge 70 of the stirring bar 7. Thus, the cluster of material M is pushed by the leading edge 70 in accordance with the rotation R of the stirring arm 7.

This wall of particles tends to be reinforced at the angle α of the inclination of the teeth. Thus, with the current tines, the more the α value is increased to adjust the furnace transfer parameters, the more the space between two adjacent tines is reduced, causing clogging and strengthening of the charge particle wall pushed by the end of the tines and therefore not shifted laterally.

As shown in FIG. 6, identical teeth 7 of the same lever, centered on its axis X and all inclined at the same angle α, form a cluster or wall of material M along the entire length of the lever.

Therefore, there is a need for improvements in multiple hearth furnaces, in particular in order to avoid or at least minimize charge accumulations which are created by the stirring teeth and which interfere with uniform and controlled charge transfer of the circulating material to be heat treated.

The object of the invention is to at least partially satisfy this need.

DISCLOSURE OF THE INVENTION

For this purpose, the object of the invention is a multi-hearth furnace designed for heat treatment of a charge of material M, containing:

- a casing of the central axis, containing:

• upper opening opening designed to feed into it the charge of the material to be processed,

the lower opening opening through which the possibility of discharging the processed material charge is provided,

- a shaft mounted for rotation around a central axis inside the housing,

a plurality of hearths fixed inside the casing parallel to each other, each of the hearths having one or more opening holes suitable for transferring the mixed material M to the hearth immediately below, the transfer hole or holes of one hearth being located near the rotary shaft, while the hole or holes of the adjacent hearth is or are located on the outer periphery of said adjacent hearth,

- at least one stirring arm attached to the rotary shaft above each hearth and containing stirring teeth fixed at right angles to the arm.

In such a furnace, the rotation of the shaft makes it possible to thermally process and transfer the material M, mixed with teeth, along a spiral path along the height of the casing between its upper and lower openings.

According to the invention, each mixing tooth has a leading edge inclined from bottom to top, the inclined leading edge forming a leading edge for the material M to be mixed.

Thus, the invention essentially consists in defining a new tooth profile for mixing the material to be cooked, with a leading edge for the material that is no longer vertical, as in the prior art, but inclined.

By its inclination, the leading edge of the teeth will assist in the separation of the material particles and hence provide a more uniform distribution and transport of the material over the largest surface area of each hearth.

It should be clarified that the leading edge of the tooth is the edge that first contacts the incoming material to be mixed.

Thus, when the material to be mixed reaches the periphery of the hearth, the leading edge of the tooth is the outer side edge, while when the material enters the center of the hearth, the leading edge is the inner side edge.

According to a preferred embodiment, the leading edge of each agitating tooth is inclined such that the width of the lower part of the tooth is greater than the width of the upper part.

Preferably, the leading edge of each agitating tooth is inclined at an angle between 5° and 70° with respect to the longitudinal axis (X) of the arm.

According to another preferred embodiment, each stirring bar is triangular or trapezoidal, preferably rectangular, when viewed from the front.

Preferably, each stirring bar has a height between 100 and 500 mm and a length between 100 and 800 mm.

In accordance with the first preferred configuration, each mixing tooth may be flat.

According to a second preferred configuration, each agitating tooth is formed from a flat portion containing a material leading edge and a deflected portion along the extent of the flat portion, wherein the flat portion is orthogonal to the longitudinal axis of the arm, while the deflected portion is inclined at one or more inclinations other than 90°, relative to the longitudinal axis of the lever.

The deflected portion may be formed from another flat portion forming a non-zero angle with the leading edge flat portion or the curved portion.

According to a preferred embodiment, at least some of the teeth supported by the same arm are laterally offset relative to the longitudinal axis (X) of the arm, while their rear edges, called tail edges, are separated from each other by an equal pitch (P) along lever length.

When the furnace according to the invention is intended for burning biomass, the temperature is preferably between 200 and 300°C. The temperature is more preferably between 250 and 300°C for woody biomass and between 220 and 280°C for agricultural type biomass. In fact, wood responds to higher temperatures than agricultural biomass.

The multiple hearth furnace according to the present invention can be incorporated into a plant for thermochemically converting biomass into synthesis gas, commonly known as syngas, by gasification in a gasification reactor to produce a fuel or fuel, in particular liquid fuel, or another synthesis product. Synthesis can be carried out according to the Fischer-Tropsch method to obtain liquefied gas oil or other fuel.

The invention also relates to the use of the above-described kiln for burning lignocellulosic biomass or for any heat treatment of mineral ores (coke, chamotte, etc.), coal, sewage treatment plant sludge or for the production of substitute fuel for coal of fossil origin, etc.

IMPLEMENTATION OF THE INVENTION

Other advantages and features of the invention will become better understood upon reading the following illustrative and non-limiting detailed description of the invention, given with reference to the drawings, in which:

- fig. 1 is a schematic longitudinal sectional view of an example of a prior art multiple hearth furnace;

- fig. 2 is a schematic perspective view of a mixing arm for the material to be processed in the furnace of FIG. 1;

- fig. 3 is a schematic partial longitudinal sectional view of another example of a prior art multiple hearth furnace;

- fig. 4 is a schematic front view of a prior art rectangular stirring bar supported by a lever in a prior art furnace, FIG. 4 shows a cluster of material M formed at a leading vertical material edge;

- fig. 5 is a plan view of a prior art rectangular agitating tooth, FIG. 5 shows the angle of the tooth with respect to the longitudinal axis of the arm;

- fig. 6 is a plan view of a prior art stirring arm supporting a plurality of rectangular teeth, all centered on the longitudinal axis of the arm and inclined at the same angle;

- fig. 7 is a schematic front view of a rectangular trapezoidal mixing bar according to the invention, supported by a lever in a furnace; in fig. 7 shows the movement of the material M by the tooth;

- fig. 8 is a plan view of a mixing bar according to a preferred embodiment of the invention; figure 8 shows the curved part of the tooth in the extent of its flat part;

- fig. 9 is a plan view of the stirring arm according to the invention of FIG. 7 supporting a plurality of flat teeth, each of which is axially offset relative to the longitudinal axis of the arm;

- fig. 10 is a plan view of the stirring arm according to the invention of FIG. 8 supporting a plurality of teeth with a flat portion and a curved portion each axially offset relative to the longitudinal axis of the arm.

In the following description and throughout the application, the terms "front", "rear" are used with reference to the directions of biomass transfer in the multiple hearth furnace according to the present invention. Thus, the leading edge of the mixing tooth is the edge that first impacts the material to be processed in the oven, while the trailing edge is the edge that contacts the material that is first separated by the leading edge.

Likewise, the terms "top", "bottom", "top", "bottom" refer to the vertical operating configuration of the furnace according to the invention. Thus, the top opening is the inlet for the material to be heat treated, while the bottom opening is the outlet for the treated material.

Fig. 1-6 relating to prior art multi-hearth furnaces with stirring arms supporting stirring teeth have already been discussed above. Therefore, they will not be described below.

For clarity, the same element from the prior art and according to the invention are designated by the same position number.

It should be noted that only the mixing teeth according to the invention and their arrangement on the supporting arm are described below, these elements being able to be mounted as such in the multiple hearth furnace described in connection with FIG. 1 and 3, instead of the prior art arm with its agitating teeth shown in FIGS. 2, 4, 5 and 6.

Figure 7 shows a stirring bar 7 according to the invention.

This tooth 7 of a rectangular trapezoidal shape has a front edge 70 that is inclined from bottom to top, from the largest width to the smallest width of the trapezoid.

This leading edge 70 forms the leading edge for the material M to be heat treated in the furnace. This leading edge for the material M will help to separate the particles of the material and thus avoid the formation of a cluster forming a wall that rotates with the movement of the levers, as with a rectangular tooth from the prior art.

As a guide, the stirring barb 7 with leading edge 70 according to the invention may have a height between 100 and 500 mm and a width between 100 and 800 mm. The teeth according to the invention may be metallic.

According to the embodiment of FIG. 7, the mixing bar 7 according to the invention is flat.

According to another embodiment of FIG. 8, the agitating barb 7 is formed from a flat portion 71 containing a leading edge 70 and a deflected portion 72 along the extent of the flat portion. The flat portion 71 is fixed orthogonally to the longitudinal axis X of the arm that supports the tooth, while the deflected portion is inclined at one or more inclinations other than 90° relative to the longitudinal axis X of the arm. In FIG. 8, deflected portion 72 is a curved portion.

The flat part 71 at right angles to the X-axis will allow the tooth to work at right angles to the swing arm. Thus, the flat portion 71 will separate the material M through the leading edge 70 to create a clean groove of material as indicated by the two arrows in FIG. 8.

On one side of the groove (right arrow in FIG. 8) the separated material can be displaced by the tooth surface, and on the other side of the groove (left arrow in FIG. material. This ensures uniform separation and transfer of material down the surface of the hearth.

A preferred variant of fixing identical flat-shaped teeth 7 with an inclined leading edge 70 according to the invention is shown in FIG. 9. Here, all the teeth are offset laterally relative to the X-axis of the lever.

Thus, the first tooth 7.1 has a width L1 located on one side of the X-axis, which is less than a width L2 placed on the other side of the X-axis.

The second tooth 7.2, adjacent to the first tooth 7.1, in its part has a width L3, located on one side of the X-axis, which is greater than the width L4, placed on the other side of the X-axis.

In order not to create blockages from the material, contributing to the narrowing, the pitch P between the rear ends of the teeth 7.1-7.4 is constant along the entire length of the lever, regardless of the displacement of each of the teeth.

In FIG. 10 also shows an offset configuration of teeth 7.1-7.9 according to the invention, but with teeth 7 with a flat part 71 at right angles to the axis, extended by a curved part 72.

Other variations and improvements may be made that in no way go beyond the scope of the invention.

While the barb of the invention is rectangular in the illustrated embodiments, a rectangular triangular barb and, more generally, any barb shape with a leading edge for material that is slanted along the height of the barb can be created instead.

The stirring teeth are preferably made of metal, but may be made of any material compatible with the conditions encountered in a multiple hearth furnace.

The invention is not limited to the examples described above; in particular, it is possible to combine with each other the characteristics of the illustrated examples in variants which are not illustrated.

BIBLIOGRAPHY

[1]: Tijmensen MJA, Faaij APC, Hamelinck CN, Hardeveld MRM. "Exploring the Possibilities of Fluids and Fischer-Tropsch Energy Through Biomass Gasification". Biomass and bioenergy 2002; 23:129-52.

[2]: Haarlemmer G, Boissonnet G, Imbach J, Setier P-A, Peduzzi E. "Second Generation Biofuels Type BtL - Production Cost Analysis". Energy and Ecology 2012; 7.

[3]: Simpson WT. "Equilibrium moisture content of wood outdoors in the US and worldwide". FPL-RN-0268. Forest Products Lab: Madison, Wisconsin, 1998

[4]: Höldrich A, Hartmann H. "Log storage - "Drying rate and dry matter loss", 14th European Biomass Conference, Paris, France, 2005.

[5]: Scholz V, Idler C, Daries W "Energy loss and mold development during storage of wood chips in non-ventilated stacks", 14th European Biomass Conference, Paris, France, 2005.

[6]: C. Brunner Biosludge Incineration, Waste Management, Volume 11, pp 155-162, 1991.

Claims (16)

1. Multi-hearth furnace (1), designed for heat treatment of the charge of material M, containing: - casing (2) of the central axis (Z), containing: • upper opening opening (20), designed to feed into it the charge of the material to be processed, • lower opening opening (21) through which the processed material charge can be discharged, - a shaft (4) mounted for rotation around a central axis (Z) inside the housing, a plurality of hearths (3) fixed inside the casing parallel to each other, each of the hearths having one or more opening holes (30, 31) suitable for transferring the mixed material M to the hearth immediately below, the transfer hole or holes (30 ) one hearth is located near the rotary shaft (4), while the hole or holes (31) of the adjacent hearth is or are located on the outer periphery of said adjacent hearth, - at least one mixing arm (5) attached to the rotary shaft above each hearth (3) and containing mixing teeth (7) fixed at right angles to the lever, each tooth (7) having a leading edge inclined from bottom to top, wherein the inclined leading edge forms a leading edge (70) for the material M to be mixed, wherein the leading edge (70) of each mixing tooth is inclined such that the width at the bottom of the tooth is greater than the width at the top. 2. A multi-hearth oven according to claim 1, wherein the leading edge (70) of each stirring tooth is inclined at an angle between 5° and 70° relative to the longitudinal axis (X) of the arm. 3. A multi-hearth oven according to any one of claims 1, 2, in which each stirring tooth has, when viewed from the front, a triangular or trapezoidal, preferably rectangular, shape. 4. A multi-hearth oven according to any one of claims 1 to 3, wherein each stirring bar has a height between 100 and 500 mm and a length between 100 and 800 mm. 5. A multi-hearth oven according to any one of claims 1 to 4, wherein each stirring tooth is flat. 6. A multi-hearth furnace according to any one of claims 1 to 4, in which each mixing tooth is formed from a flat part (71) containing a leading edge for material, and a deflected part (72) along the length of the flat part, the flat part being orthogonal to the longitudinal axis ( X) of the lever, while the deflected part is inclined at one or more inclinations other than 90° relative to the longitudinal axis (X) of the lever. 7. The multiple hearth furnace of claim 6, wherein the deflected portion is formed from another flat portion forming a non-zero angle with the leading edge flat portion. 8. The multiple hearth furnace of claim 6, wherein the deflected portion is formed from the curved portion. 9. A multi-hearth oven according to any one of claims 1 to 8, wherein at least some of the teeth supported by the same arm are laterally offset relative to the longitudinal axis (X) of the arm, while their rear edges, called tail edges, are separated from each other by the same pitch (P) along the length of the lever. 10. The use of a multiple hearth furnace according to any one of claims 1 to 9 for burning biomass such as lignocellulosic biomass, or for thermal processing of mineral ores, coal, sewage sludge, or for the production of substitute fuel for coal of fossil origin.

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