NL2022251B1

NL2022251B1 - Process for preparing pellets for firing an industrial furnace

Info

Publication number: NL2022251B1
Application number: NL2022251A
Authority: NL
Inventors: Jennissen Lars
Original assignee: Subcoal Int B V
Priority date: 2018-12-19
Filing date: 2018-12-19
Publication date: 2020-07-03

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 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. The invention also relates to pellets obtained and having advantageous properties.

Description

PROCESS FOR PREPARING PELLETS FOR FIRING AN INDUSTRIAL FURNACE TECHNICAL FIELD

[0001] The invention relates to a process for preparing pellets from waste materials, and to a process for firing an industrial furnace.

BACKGROUND ART

[0002] The process for firing an industrial furnace is a process used for example in the production of electricity. The furnaces for producing electricity are the most demanding and efficient furnaces currently in use. Other industrial furnaces that require high process stability are blast furnaces in steel production and cement- and lime kilns.

[0003] The furnace generally is supplied with powdery (pulverized) coal, oil or gas. The fuel generally is supplied through a number of burners, lances (or tuyeres). In case the furnace is used to produce electricity, the heat of the combustion is used to produce steam, which is used to drive turbines.

[0004] The amount of pulverized coal that can be injected depends on the coal and coke quality, furnace geometry, and operational practices. Furthermore, the pulverized coal has a low bulk density and bad storage characteristics. Coal therefore is pulverized just before use. A main disadvantage of pulverized coal is the fact that it is from a non-renewable source and therefore causing substantial CO2 emission.

[0005] Alternative fuels have been suggested, and to a certain extend also used, to reduce the burden of CO2 emission. Such alternative fuels need to allow its use in seamless processing. The alternative fuels also need to allow injection in the flame.

[0008] Alternative fuels suggested to be used for the high end industrial furnaces are plastic pellets, mixed plastic/biomass pellets, wood pellets, sewage sludge pellets and the like.

[0007] One advantage of using plastic-only waste generally is that plastic waste has low thermal conductivity and high energy content. A disadvantage of using plastics-only waste is that such mixtures originating, for example, from domestic, urban or municipal waste are relatively valuable products that can be used to make (recycled) plastic products. A further disadvantage is, that despite a high calorific value, the waste plastic pellets are difficult to process in such a way that a suitable particle size distribution is obtained. Milling causes temperature increase, rubberybehavior of the plastics to such an extent that cryogenic milling is necessary.

Cryogenic milling is however too expensive.

[0008] The delivery of alternative fuel into a furnace may vary depending upon the nature of the waste material and the type of furnace being supplied. There are several ways to directly use alternative fuel in furnace technologies. Such technologies include direct use by injection of powdered alternative fuel via or at the level of the lances, co- grinding of pellets with coal as described in WO2015/155193, or mixing coal and powdered alternative fuel before injecting the mixture into the furnace.

[0009] The alternative fuels that are used directly in a furnace have issues with processing, like dust formation, transport of powder and the like. Therefore, these alternative fuels are used in practice only to partly replace fossil fuel in high end furnaces. Generally, in actual practice, the amount of alternative fuel is less than 30%, but in any case less than 50% relative to the powdered coal.

[0010] US2010/116181 describes pelletising plastic/cellulosic materials with a relatively low amount of plastic (less than 40 wt%), which, according to WO2008/107042 can be milled to particles largely below 2 mm that can be used as secondary fuel in combination with powdered coal. Secondary fuel with low amount of plastic has a relatively low combustion value, which is disadvantageous if such fuel would have to fully replace coal.

[0011] There is thus a need in the field for a process in which alternative fuel can be produced such that it is suitable to be supplied into an industrial furnace, particularly an industrial furnace for producing electricity, to even fully replace coal.

[0012] Furthermore, there is a need for a process in which alternative fuel can be produced with improved handling properties, such as reliable transport of the powder.

SUMMARY OF THE INVENTION

[0013] The object of the present invention is to provide a process of making pellets comprising plastic and biomass (cellulosic fiber) that can be used to reliably replace fossil fuel completely in high end industrial furnaces used in modern equipment for producing electricity.

[0014] A further object is to provide a process to reliably run a high end industrial furnace with alternative fuel with improved handling properties.

[0015] 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 and more than 95% smaller than 60 mm, (ii) subjecting the waste material through a pelletiser with holes between 6-18 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.

[0018] The pellets obtained with the process of the first aspect can be milled in a hammer mill such that a powder is obtained that shows good flow properties. Preferably, 25 and 70 wt% of the powder has a particle size between 2 and 3.15 mm.

[0017] These pellets unexpectedly allow to completely replace powdered coal in high-end industrial furnaces such as furnaces used in plants to produce electricity. The present invention therefore also relates to pellets having certain properties, obtained by twice pelletising.

[0018] The pellets preferably have one or more of the following properties: - a diameter between 4-10 mm diameter - a Kahl hardness of between 8-40 kgf - comprise substantially homogeneously molten plastic, obtained by twice pelletising the waste material - and/or a bulk density of 470 g/L or higher

[0019] The pellets can be effectively ground in for example a hammer mill {such as a California pellet mill; 11.5x28) having a 6.4mm screen and a tip speed of 108 m/s.

The resulting particles of the twice pelletised material has much better flow properties that particles that result from once pelletised material. Once pelletized material in such conditions require too high energy input to be effectively ground.

[0020] The twice pelletised pellets, when ground in a hammer mill, contain relatively low, and preferably virtually no pieces of plastic film or fibrous material. This is advantageous as this is important to achieve good flow properties

[0021] In a further aspect of the invention, the process of firing an industrial furnace according the invention comprises the steps of: (i) providing pellets, which are prepared by (a) 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 materials) of more than 30%, based on the total dry weight of thewaste, wherein the waste has a particle size distribution with more than 80% larger than 5 mm, more than 95% smaller than 60 mm, (b) subjecting the waste material through a pelletiser with holes between 6-16 mm and a length ration of more than 2, and (c) subjecting the pellets through a second pelletiser with holes between 4 and 10 mm, and a length ratio of more than 2, (ii) milling the pellets in a mill 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.

[0022] The powdery fuel according the invention preferably is used in an amount of about 90% or more of the energy requirement of said furnace, and even more preferably substantially al energy requirement is obtained by the powdery fuel according the invention.

[0023] In a further aspect of the invention, the invention relates to the use of pellets comprising one or more thermoplastic material(s) of more than 40%, based on the total dry weight of the pellets and one or more cellulosic material(s) of more than 30%, based on the total dry weight of the pellets, wherein the pellets are produced by subjecting waste material comprising said plastic and cellulosic material through a pelletiser with holes between 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, as fuel for an industrial furnace after being ground preferably such that between 25 and 70 wt% has a particle size between 2 and 3.15 mm.

[0024] The pellets obtained after the twice pelletizing process have advantageous properties over normal pellets, and it was unexpected that such further processing step led to pellets that can be used in one of the most demanding processes to even fully replace coal.

[0025] The twice pelletised pellets can be converted in powders with very good flowing and running properties by grinding in a standard equipment such as for example a hammer mill. Thereby, improved bulk, transport, and better dosing properties are achieved in comparison with once pelletised material.

[0026] The pellets have very high cold crushing strength resulting in negligible generation of fines in stock house and good resistance to disintegration during transport.

[0027] Further benefits and advantages of the present invention will become apparent in the detailed description with appropriate reference to the accompanying drawings.

5 DETAILED DESCRIPTION

[0028] 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 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.

[0029] The die of the pelletiser preferably is a cylindrical die, but flat dies are known, and can be used as well.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] The term “cellulosic material” used in the present invention relates to for example paper, carton, wood, cardboards, textiles such as cotton, rayon and/orviscose. 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.

[0035] 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 16 inhomogeneity. The pellets the present invention may be the commercially available Subcoal® pellets that can be pelletised a second time.

[0036] Suitable processes to make pellets are described in the art, as for example in US6635093. 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) 15 plastic and cellulosic material.

[0037] Pellets have a uniform size range (diameter) generally within a range of 4 — 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.

[0038] 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.

[0039] 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.

[0040] The material can be dried to a moisture content of about 5-15 wt%, and the material is pressed through a die with appropriate holes. Drying is preferably done after shredding.

[0041] 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 6-16 mm for the first pelletizing device, and between 6-10 mm for the second pelletizing device. The aspect ratio (length divided by diameter) preferably is about 4-15, and more preferably about 6-12.

[0042] 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.

[0043] 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.

[0044] 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).

[0045] 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.

[0046] The HHV and LHV are valid for complete combustion of the fuel to CO; and HO.

[0047] 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).

[0048] 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.

[0049] The oxygen content of the pellets is preferably in the range of 20 to 30 w% of the dry weight pellets.

[0050] The hydrogen content of the pellets is preferably in the range of 6 to 8 w% of the dry weight pellets.

[0051] Preferably, the pellets comprise 1 to 10 weight % of moisture, more preferably about 5 wt% or less.

[0052] 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.

[0053] 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.

[0054] The pellets obtained 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.

[0055] 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.

[0056] Preferred properties comprise one or more of the following: - a diameter between 4-10 mm diameter - a Kahl hardness of between 8-40 kgf - a substantially homogeneously molten plastic, obtained by twice pelletising the waste material - a bulk density of 470 g/L or higher

[0057] The pellets can be ground in a hammer mill (like for example a California pellet mill; 11.5x28 having a 6.4mm screen and a tip speed of 108 m/s). The resulting particles have preferably a bulk density (tapped) of 220 g/L or higher

[0058] 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

[0059] The pellets are ground to relatively small particles of below 3.15 mm. Thus, 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, morethan 25 wt% is larger than 2 mm, and more preferably more than 30wt% is larger than 2 mm. In addition, preferably about 75 wt% or more is larger than 1 mm.

[0060] 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.

[0061] The pellets which are obtained 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/

[0062] The grinding is tested in a hammer mill (California pellet mill; 11.5x28) having a 6.4mm screen and a tip speed of 108 m/s, used according the manufacturers description.

[0063] The bulk density (tapped) of the ground pellets (the particulate material) generally is 220 g/L or higher, preferably 230 g/L or higher.

[0064] Preferably, the average particle size of the ground particles is less than 2.5 mm, and preferably larger than 1 mm.

[0085] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] In an even more preferred embodiment, said pellets are provided as a complete replacement of fossil fuel in the production of electricity.

[0070] 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.

[0071] 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 Nm3kg*1000. The temperature is generally dependent on the type of industrial furnace.

[0072] 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

[0073] A series of tests have been conducted with RDF comprising about 45% plastic, about 40% biomass, about 5% other materials and about 10% moisture.

[0074] 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.

[0075] 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:

[0076] Grinding and compare single pelleted @6mm pellets: - With a Hammer mill screen with a &6,4mm holes screen at a speed of 96Hz.

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.

[0077] Grinding and compare double pelleted @6mm pellets: - With a Hammer mill screen with a @6,4mm holes screen at a speed of 48Hz - With a Hammer mill screen with a &6,4mm holes screen at a speed of 96Hz

[0078] Screen, power consumption and speed are given in the table below, together with the bulk density.

Experiment Pelletized | Screen in Tip speed Power Bulk density a rn | a

[0079] 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 15149-18-2: 2011-01. Sieve fractionating over 0.5 mm, 1 mm, 2 mm, 3.15 mm and >3.15 mm yielded the following results: Experiment / 1 2-1 2-2 2-3 mn

[0080] From these results it appears that grinding double pelleted pellets on a Hammer mill with a screen with 28,4mm 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

Claims

Conclusions

A method for producing granules capable of providing free-flowing powder suitable for firing an industrial furnace from urban and / or other waste, the process comprising the steps of: (i) providing waste material comprising more than 40%, based on the total dry weight of the waste of one or more thermoplastic materials and more than 30%, based on the total dry weight of the waste of one or more cellulose materials, the waste having a particle size distribution of more than 80% larger than 5 mm, more than 95% smaller than 80 mm, (ii) subjecting the waste material through a pelletizer with holes between 6-16 mm and a length ratio of more than 2, and subjecting the pellets through a second pelletizer 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.

Granules obtainable by the method of claim 1, preferably obtained by the method of claim 1.

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

A method or granules according to any one of claims 1 to 3, wherein the granules have a Kahl hardness of between 8-40 kgf.

A method or granules according to any of claims 1 to 4, wherein the granules have a volume density of 470 g / L or higher.

A method or granules according to any one of claims 1 to 5, wherein the granules after being milled in a hammer mill have a volume density of 220 g / L or higher.

A method or granules according to any of claims 1-8, wherein the calorific value (LCV) of the granules is about 19-28 GJ / ton.

A method or granules according to any one of claims 1-7, wherein the hydrogen content of the granules is in the range of 7 to 8 wt®% of the granular dry weight.

A method or granules according to any one of claims 1 to 8, wherein the oxygen content of the granules is in the range of 20 to 30 wt% of the dry weight of the granules.

A method or granules according to any one of claims 1-9, wherein the granules comprise: - one or more thermoplastic materials in an amount of 40-70 mass%, based on the total dry weight of the granules; and - one or more cellulosic materials in an amount of 30-50% by mass, based on the total dry weight of the granules.

A method or granules according to any one of claims 1-10, wherein the granules have a diameter of between 6 and 10 mm and a length of between 4 and 40 mm.

Use of granules according to any one of claims 2-11, as fuel for an industrial furnace after being ground into particles, preferably such that between 25 and 70 wt% has a particle size between 2 and 3.15 mm.

Process of firing an industrial furnace comprising the steps of: providing granules according to any one of claims 2-11; grinding the granules in a mill, preferably to obtain a particle size such that between 25 and 70 wt% has a particle size between 2 and 3.15 mm; feeding the powdery fuel into the flame of the furnace, the fuel being used in an amount to provide more than 70% of the energy requirement of said furnace, preferably more than 90% of the energy requirement of said furnace .

Use or process according to any of claims 12-13, wherein the industrial furnace is used in a process to produce electricity.

Use or process according to any of claims 12-14 wherein the granules are ground in a hammer mill.