PROCESS FOR PREPARING PELLETS FOR FIRING AN INDUSTRIAL FURNACE

Abstract

The invention relates to a method for producing pellets which are capable of providing free flowing powder suitable for firing an industrial furnace from municipal and/or other waste, the process comprising the following steps: (i) providing waste material comprising one or more thermoplastic material(s) of more than 40%, based on the total dry weight of the waste and one or more cellulosic material(s) of more than 30%, based on the total dry weight of the waste, wherein the waste has a particle size distribution with more than 80% larger than 5 mm, more than 95% smaller than 60 mm, (ii) subjecting the waste material through a pelletiser with holes between 4-16 mm and a length ratio of more than 2, and subjecting the pellets through a second pelletiser with holes between 4 and 10 mm, and a length ratio of more than 2 to provide pellets with a diameter between 4 and 10 mm, and a length of between 3 and 50 mm. The invention also relates to pellets obtained and having advantageous properties.

Claims

1. Method for producing pellets which are capable of providing free flowing powder suitable for firing an industrial furnace from municipal and/or other waste, the process comprising the following steps: (i) providing waste material comprising one or more thermoplastic material(s) of more than 40%, based on the total dry weight of the waste and one or more cellulosic material(s) of more than 30%, based on the total dry weight of the waste, wherein the waste has a particle size distribution with more than 80% larger than 5 mm, more than 95% smaller than 60 mm, (ii) subjecting the waste material through a pelletiser with holes between 6-16 mm and a length ratio of more than 2, and subjecting the pellets through a second pelletiser with holes between 4 and 10 mm, and a length ratio of more than 2 to provide pellets with a diameter between 4 and 10 mm, and a length of between 3 and 50 mm.

2. Pellets obtainable with the method of claim 1.

3. Pellets having the following properties: a. a diameter between 4-10 mm diameter b. comprising substantially homogeneously molten plastic, obtained by twice pelletising the waste material, and c. when ground in a hammer mill, the resulting powder has good flow properties.

4. Method according to claim 1, wherein the pellets have a Kahl hardness of between 8-40 kgf.

5. Method according to claim 1, wherein the pellets have a bulk density of 470 g/L or higher.

6. Method according to claim 1, wherein the pellets after being ground in a hammer mill have a bulk density of 220 g/L or higher.

7. Method according to claim 1, wherein the calorific value (LCV) of the pellets is about 19-28 GJ/ton.

8. Method according to claim 1, wherein the hydrogen content of the pellets is in the range of 7 to 8 wt % of the dry weight pellets, wherein the oxygen content of the pellets is in the range of 20 to 30 wt % of the dry weight pellets.

9. (canceled)

10. Method according to claim 1, wherein the pellets comprise: one or more thermoplastic material(s) in an amount of 40-70 weight %, based on the total dry weight of the pellets; and one or more cellulosic material(s) of more than 30-50 weight %, based on the total dry weight of the pellets.

11. Method according to claim 1, wherein the pellets have a diameter of between 6 and 10 mm and a length of between 4 and 40 mm.

12. (canceled)

13. Process of firing an industrial furnace comprising the steps of: providing pellets according to claim 3; grinding the pellets in a mill; feeding the powdery fuel into a flame of the furnace, wherein the fuel is used in an amount to provide more than 70% of the energy requirement of said furnace.

14. Process according to claim 13 wherein the industrial furnace is used in a process to produce electricity.

15. Process according to claim 13 wherein the pellets are ground in a hammer mill.

16. Pellets according to claim 3, wherein the pellets have a Kahl hardness of between 8-40 kgf.

17. Pellets according to claim 3, wherein the pellets have a bulk density of 470 g/L or higher.

18. Pellets according to claim 3, wherein the pellets after being ground in a hammer mill have a bulk density of 220 g/L or higher.

19. Pellets according to claim 3, wherein the calorific value (LCV) of the pellets is about 19-28 GJ/ton.

20. Pellets according to claim 3, wherein the hydrogen content of the pellets is in the range of 7 to 8 wt % of the dry weight pellets and wherein the oxygen content of the pellets is in the range of 20 to 30 wt % of the dry weight pellets.

21. Pellets according to claim 3, wherein the pellets comprise: one or more thermoplastic material(s) in an amount of 40-70 weight %, based on the total dry weight of the pellets; and one or more cellulosic material(s) of more than 30-50 weight %, based on the total dry weight of the pellets.

22. Pellets according to claim 3, wherein the pellets have a diameter of between 6 and 10 mm and a length of between 4 and 40 mm.

Description

DETAILED DESCRIPTION

[0041] In a first aspect, the invention relates to a method to produce pellets which are capable of providing free flowing powder suitable for firing an industrial furnace from municipal and/or other waste, the process comprising the following steps: (i) providing waste material comprising one or more thermoplastic material(s) of more than 40%, based on the total dry weight of the waste and one or more cellulosic material(s) of more than 30%, based on the total dry weight of the waste, wherein the waste has a particle size distribution with more than 80% larger than 5 mm, more than 95% smaller than 60 mm, (ii) subjecting the waste material through a pelletiser with holes between 4-16 mm, preferably 6-16 mm and a length ration of more than 2, and subjecting the pellets through a second pelletiser with holes between 4 and 10 mm, and a length ratio of more than 2 to provide pellets with a diameter between 4 and 10 mm, and a length of between 3 and 50 mm.

[0042] The die of the pelletiser preferably is a cylindrical die, but flat dies are known, and can be used as well. Also, a first flat and second cylindrical die can be used, or first a cylindrical and secondly a flat die.

[0043] By the term “thermoplastic material” is meant thermoplastic polymers. The waste material used in the preparation of the pellets of the present invention comprises at least 40% thermoplastic material, preferably at least 45 weight % or at least 50 weight % thermoplastic material, like for example about 55 weight % or about 60 weight % thermoplastic material.

[0044] Generally, the amount of plastic material in the pellets is about 80% or less, preferably 70% or less. Hence, suitable ranges comprise 40-80 wt % of plastic, or most preferably 50-70 wt % of plastic.

[0045] Examples of thermoplastic polymers used herein are listed in US2010/0116181. Typically, the thermoplastic material or component may be a packing material or any type of plastic waste.

[0046] Preferably, at least 20 weight %, more preferably at least 40 weight %, even more preferably at least 50 weight %, and most preferably at least 60 weight % of the thermoplastic material are polyethylene homo- or copolymers.

[0047] The term “cellulosic material” used in the present invention relates to for example paper, carton, wood, cardboards, textiles such as cotton, rayon and/or viscose. The waste material used in the present invention comprises at least 30 weight % of cellulosic material, preferably more than 35 weight % or more of cellulosic material. Generally, the amount of cellulosic material is about 60 wt % or less, preferably about 50 wt % or less cellulosic material based on the total dry weight of the pellets. Suitable ranges include 30-60 wt % cellulosic material, preferably 30-50 wt % cellulosic material. Cellulosic material can also be denoted as biomass.

[0048] As used herein, the term “pellet” or “pellets” is used when referring to pellets of the present invention comprising one or more thermoplastic material(s) and one or more cellulosic material(s). The pellets are not limited by a degree of inhomogeneity. The pellets the present invention may be the commercially available Subcoal® pellets that can be pelletised a second time.

[0049] Suitable processes to make pellets are described in the art, as for example in U.S. Pat. No. 6,635,093. It is however to be noted that the pellets should be pelletised twice in order to obtain pellets that are sufficiently homogeneous with respect to (molten) plastic and cellulosic material. Nevertheless, knowing the required final properties of the pellets of the invention, it may be possible to obtain such properties by using special dies or other processes.

[0050] Pellets have a uniform size range (diameter) generally within a range of 4-10 mm, preferably 6-10 mm. The length of the pellets generally will be between 3 and 50 mm, preferably 4-40 mm, and even more preferably between 5-30 mm.

[0051] The pellets can be produced by selecting waste plastic and biomass from refuse or paper recycling plants and the like. It is possible to use different selected waste streams in combination in order to achieve a required mix of plastic and cellulosic materials. The raw material preferably is shredded to a size of 5 cm or less for the largest dimension, preferably to a size of 4 cm or less. In a further embodiment, the raw material is shredded to a size of about 3.5 cm or less, preferably about 2.5 cm.

[0052] Preferably, the waste has a particle size distribution with more than 80% larger than 5 mm and more than 95% smaller than 60 mm. Preferably, more than 90% is smaller than 40 mm. In a further preferred embodiment, the waste has a particle size such that about 20 wt % or more has a size of more than 30 mm.

[0053] The material can be dried to a moisture content of about 2-15 wt %, preferably 5-15 wt %, and more preferably less than 10 wt %, and the material is pressed through a die with appropriate holes. Drying is preferably done after shredding.

[0054] The holes of the die can have a diameter of between about 4-20 mm, and an aspect ratio of between 2 and 20, preferably of at least 4. Preferred dimensions are a diameter between 4-16 mm more preferably 6-16 mm for the first pelletizing device, and between 6-10 mm for the second pelletizing device. The aspect ratio or length ratio (which phrases are used interchangeably) is at least 2. This, the thickness of the die (which defines the length of the path that the material travels through the die) is at least twice the diameter of the holes. The aspect ratio (length divided by diameter, or length ratio) preferably is about 4-15, and more preferably about 6-12.

[0055] The holes in the first and second pelletizing device may be the same or different, the holes being of the same diameter or larger in the first pelletizing device. In another embodiment, the holes in the first pelletizer are smaller than in the second pelletizer.

[0056] The two pelletizing steps allow effective melting of plastics that impregnate fibrous or film-like materials, which aids in improved grindability. During the first pelletizing step, the material generally is heated till below or around 100° C., preferably between 70-90° C., which allows shearing and grinding of the raw material. During the second pelletizing step, the temperature of the pellets preferably rises till about 100° C. or higher, preferably about 105° C. or higher, and may rise up to about 120° C. The temperature during the second pelletizing step generally will be higher than in the first pelletizing step with about 5° C. or more, preferably about 10° C. or more like for example up to 20 or 30° C. This relatively high temperature in the second pelletizing step allows further melting of the plastics. The improved molten and impregnated pellets show a substantial higher bulk density, and can relatively easily be distinguished from prior art pellets.

[0057] The heating value or calorific value or calorific heating value of any fuel is the energy released per unit mass or per unit volume of the fuel when the fuel is completely burnt. The quantity is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. With other words, it is the amount of heat released during the complete combustion of a specified amount of it.

[0058] Calorimetry measures the higher heating value (HHV) and uses the following procedure. It fully combusts the sample using pure oxygen and then produces carbon dioxide and water. The water is initially produced as a vapor. However, once the entire sample is combusted (i.e., the test is complete) the water vapor condenses. This condensation process releases additional heat. Technically this additional heat is latent heat from the conversion of water from a vapor to a liquid phase. The combination of the heat released during the combustion of the sample and the subsequent heat released during the conversion of water vapor to liquid provides the maximum heat that can be obtained. This is known as Higher calorific value (HCV) or Higher heating value (HHV).

[0059] If the process maintains the water produced in the vapor state, then the latent heat is not recovered. This is known as the Lower calorific value (LCV) or Lower heating value (LHV). The LHV is only the heat of combustion and does not include the heat released during condensation of the water vapor. LHV is the key measurement for most combustion systems that convert heat to power or energy.

[0060] The HHV and LHV are valid for complete combustion of the fuel to CO.sub.2 and H.sub.2O.

[0061] The calorific value (LCV) of the pellets is generally about 19-28 GJ/ton, which is lower than full plastic material, which generally has a calorific value of 31-35 GJ/ton (on dry weight).

[0062] Preferably, halogen elements like chlorine are present in the pellets in an amount below 1 wt %, more preferably below 0.3 wt %. High input of this elements may lead to corrosion in the dry and/or wet gas cleaning system and in addition to chlorine emission with the drain water of the top gas scrubber.

[0063] The oxygen content of the pellets is preferably in the range of 20 to 30 w % of the dry weight pellets.

[0064] The hydrogen content of the pellets is preferably in the range of 6 to 8 w % of the dry weight pellets.

[0065] Preferably, the pellets may comprise 1 to 10 weight % of moisture, more preferably about 5 wt % or less. The amount of moisture may be below 2 or below 1%.

[0066] Preferably, the strength of the pellets is about 8 kgf or more, more preferably about 10 kgf or more. Generally, the strength is about 40 kgf or less, often about 25 kgf or less. It is however possible to have even harder pellets, for example having a strength of up to 70 kgf or less, for example 60 kgf or less. It may be preferably to have a strength of about 30 kgf or less.

[0067] The hardness can be measured with a Kahl pellet hardness tester, available from Amandus Kahl GmbH&Co KG, Hamburg. A sufficient strength has the advantage that the pellets have a relatively high density, which allows efficient transport, and the strength precludes the formation of large amounts of fines during the transport. The Kahl pellet hardness tester is one of the standard test methods in the industry.

[0068] The pellets obtained or obtainable with the process of the present invention can be milled in a hammer mill such that the powder shows good flow properties, and such that preferably 25-70 wt % of the powder has a particle size between 2 and 3.15 mm.

[0069] These pellets unexpectedly allow to even fully replace powdered coal in high end industrial furnaces. The pellets preferably have one or more of certain properties, obtained by twice pelletising.

[0070] Preferred properties comprise one or more of the following: [0071] a diameter between 4-10 mm diameter [0072] a Kahl hardness of between 8-40 kgf [0073] a substantially homogeneously molten plastic, obtained by twice pelletising the waste material [0074] a bulk density of 470 g/L or higher

[0075] The pellets can be ground in a hammer mill (like for example a California pellet mill; 11.5×28 having a 6.4 mm screen and a tip speed of 108 m/s). The resulting particles have preferably a bulk density (tapped) of 220 g/L or higher

[0076] The pellets, when ground in a hammer mill, contain virtually no pieces of plastic film or fibrous strands, as these would be detrimental for flow properties

[0077] The pellets are ground to relatively small particles of below 3.15 mm. Generally, the weight percentage of particles larger than 3.15 mm is about 15 wt % or less, preferably about 10 wt % or less and even more preferably about 7.5 wt % or less. More preferably, more than 95 wt %, more preferably more than 98 wt % of the ground material is smaller than 3.15 mm. Yet, the particles are not dusty. Preferably, more than 25 wt % is larger than 2 mm, and more preferably more than 30 wt % is larger than 2 mm. In addition, preferably about 75 wt % or more is larger than 1 mm.

[0078] In one embodiment, preferably, the ground material comprises about 30 wt % of material with a size between 2 and 3.15 mm, and even more preferred about 40 wt % or more.

[0079] The (tapped) bulk density is measured as follows: an amount of pellets is poured in a 100 mL cylinder (diameter 2.5 cm), and measuring the amount of pellets present in gram. Tapping is done by placing the beaker on a vibrating surface (0.5 mm vertical vibration; 240 times per minute) for 5 min; and measuring the volume of pellets. The tapped density is the amount in gram amount divided by the volume measured.

[0080] The pellets which are obtained or obtainable by twice pelletising have a higher bulk density. Pellets obtained by pelletizing once generally have a bulk density below 460 g/L. The pellets according to the present invention generally have a bulk density of 470 g/L or higher, preferably 480 g/L or higher. Generally, the density is 700 g/L or lower. The bulk density of the pellets more preferably is about 500 g/L or higher, and even more preferred, about 550 g/L or more.

[0081] The grinding is tested in a hammer mill (California pellet mill; 11.5×28) having a 6.4 mm screen and a tip speed of 108 m/s, used according the manufacturers description.

[0082] The bulk density (tapped) of the ground pellets (the particulate material) generally is 220 g/L or higher, preferably 230 g/L or higher. This is contrasted with standard pelletized material of the prior art, which generally has a bulk density after milling often is below 200 g/L.

[0083] Preferably, the average particle size of the ground particles is less than 2.5 mm, and preferably larger than 1 mm.

[0084] The grinding in an industrial setting generally takes place in a suitable mill, such as a hammer mill, jet mill or the like. Preferably, a hammer mill is used.

[0085] It appears that if singly pelletised pellets are ground to a particle size smaller than 3 mm in such apparatus, a material is obtained with particles and a significant amount of small plastic fluffy material (plastic film parts and fibrous material). This fluffy material strongly influences flow and handling properties negatively. Thereby, both transport and feeding of the particles into a furnace is hampered. Also burning is less stable. Virtually no fluffy material is observed if grinding is performed on the pellets according to the present invention.

[0086] The process of firing an industrial furnace according the invention comprises the steps of: (i) providing pellets, as described above, (ii) milling the pellets in a mill, preferably such that between 25 and 70 wt % has a particle size between 2 and 3.15 mm, and (iii) feeding the powdery fuel into the flame of the furnace, wherein the fuel is used in amount to provide more than 70% of the energy requirement of said furnace.

[0087] Preferably, the fuel is used in amount to provide more than 80%, and more preferably more than 90% of the energy requirement of said furnace.

[0088] In an even more preferred embodiment, said pellets are provided as a complete replacement of fossil fuel in the production of electricity.

[0089] In a further aspect of the invention, the invention relates to the use of pellets as described above, as fuel for an industrial furnace after being ground, preferably such that between 25 and 70 wt % of the particles have a size between 2 and 3.15 mm.

[0090] The particles are blown into the raceway of an industrial furnace at an adiabatic flame temperature in the range of about 1200° C. to about 2500° C. and air volume in the range of 1280-2000 Nm.sup.3/kg*1000. The temperature is generally dependent on the type of industrial furnace.

[0091] Hereinafter, the present invention is described in more detailed and specifically with reference to the examples, which however are not intended to limit the present invention.

EXAMPLES

Examples 1-2 and Comparative Experiment A

[0092] A series of tests have been conducted with RDF comprising about 45% plastic, about 40% biomass, about 5% other materials and about 10% moisture.

[0093] First and second (if applicable) pelletizing was done through a die having 6 mm holes and a length of 70 mm (aspect ratio 11.7). The die speed was about 200 rpm.

[0094] The pellets obtained after the first pelletizing step had a bulk density of 460 g/L, while the twice pelletised pellets had a bulk density of 508 g/L. The following tests have been done:

[0095] Grinding and compare single pelleted 06 mm pellets: [0096] With a Hammer mill screen with a 06.4 mm holes screen at a speed of 96 Hz. Because at this speed difficulties arose (as can be concluded from the high energy consumption), it was not useful to also test at lower tip speed.

[0097] Grinding and compare double pelleted 06 mm pellets: [0098] With a Hammer mill screen with a 06.4 mm holes screen at a speed of 48 Hz [0099] With a Hammer mill screen with a 06.4 mm holes screen at a speed of 96 Hz

[0100] Screen, power consumption and speed are given in the table below, together with the bulk density.

TABLE-US-00001 Screen Tip Power Bulk density Experiment Pelletized in mill speed consumption of product A Once 6.4 mm 108 m/s 23.6 kW 203 g/L 1 Twice 6.4 mm  54 m/s  6.3 kW 286 g/L 2 Twice 6.4 mm 108 m/s 16.9 kW 233 g/L

[0101] The products of examples 1 and 2 have been analysed for particle size distribution according to the methods DIN 18123: 2011-04 and DIN-EN 15149-1&-2: 2011-01. Sieve fractionating over 0.5 mm, 1 mm, 2 mm, 3.15 mm and >3.15 mm yielded the following results:

TABLE-US-00002 Size distribution (wt %) Experiment 1 2-1 2-2 2-3 Fraction < 0.5 mm 2.7 9.8 6.7 12.0 Fraction 0.5-1 mm 10.5 20.1 17.1 24.7 Fraction 1-2 mm 29.7 33.9 33.7 32.4 Fraction 2-3.15 mm 57.1 36.2 42.5 30.9 Fraction > 3.15 mm 0 0 0 0

[0102] From these results it appears that grinding double pelleted pellets on a Hammer mill with a screen with 06.4 mm holes compared to grinding single pelleted pellets (compare experiment A with 2) shows that the energy consumption decreased per amount of pellets (kg). Furthermore, particles instead of fluff were produced, increasing the bulk density from about 200 to 230 (15% increase). Because of the lower energy consumption, a higher capacity is possible when grinding double pelleted pellets

Example 3

[0103] From selected solid refuse with an energy content of 23 GJ/ton, reduced in size below 50 mm, pellets were prepared by twice pelletising over a die with holes of 6 mm diameter, and an aspect ratio of about 11.

[0104] The pellets showed a density of 625 g/L, and were substantially darker than the pellets obtained after the first pelletizing step, showing a more homogeneous texture.

[0105] Milling was performed with a California hammer mill with a tip speed of 108 m/s and a 6.4 mm screen. Results are shown in the next table. Furthermore, ground material was sieved over a 3 mm screen sieve with hand shaking. This allowed fluff to stay on the screen. The amount of fluff was very low.

TABLE-US-00003 Size distribution (wt %) Experiment 3-1 3-2 Fraction < 0.5 mm 5.7 12.5 Fraction 0.5-1 mm 9.3 11.9 Fraction 1-2 mm 23.3 21.1 Fraction 2-3.15 mm 52.8 48.7 Fraction > 3.15 mm 8.9 5.8