WO2000039255A1

WO2000039255A1 - Method for producing fuel from the biomass component of organic wet fraction (owf)

Info

Publication number: WO2000039255A1
Application number: PCT/NL1999/000803
Authority: WO
Inventors: Petrus Adrianus Maria Van Den Broek; Erik Ten Brummeler; Willem Elsinga; Marcus Fredericus Pruijn; Bastianus Christianus Van Winden
Original assignee: Arcadis Heidemij Realisatie B.V.
Priority date: 1998-12-23
Filing date: 1999-12-23
Publication date: 2000-07-06
Also published as: AU2007100A; EP1060233A1

Abstract

The invention relates to a method for producing a fuel from waste, in particular the organic wet fraction (OWF) of waste, comprising of providing an organic wet fraction (OWF); at least partially drying the organic wet fraction (OWF); separating the organic wet fraction (OWF) to size into a first fraction with dimensions above a determined cut-off point and a second fraction with dimensions below a determined cut-off point; separating each of the fractions on the basis of at least diameter, and optionally density, into a first sub-fraction consisting substantially of organic material and a second sub-fraction consisting substantially of inert material; optionally mixing the two first sub-fractions; optionally bringing the optionally mixed sub-fractions into a manageable form in order to obtain a fuel. Suitable techniques for separating each of the fractions on the basis of at least diameter, and optionally density, are for instance air separation and ballistic separations such as the throw-off technique.

Description

METHOD FOR PRODUCING FUEL FROM THE BIOMASS COMPONENT OF ORGANIC WET FRACTION (OWF)

The present invention relates to a method for producing fuel from the biomass component of waste, in particular from organic wet fraction (OWF) .

Dutch households alone already produce 7.7 million tons of waste annually. A large part of the domestic refuse consists of glass, kitchen and garden waste (KGW) and paper. This refuse is collected separately and reused. Of the remaining household waste, 1.4 million tons is disposed of to landfill and 2.3 million tons is incinerated. In order to achieve an incineration process which is as constant and clean as possible, pre- separations take place at different waste incineration plants. This pre-separation usually consists of a metal separator and a drum screen, whereby the material is divided into a fine, so-called "organic wet fraction" (OWF) and a coarse fraction (RDF = Refuse Derived Fuel) . The coarse fraction is incinerated and renewable energy is generated with the released heat output . The organic wet fraction (OWF) amounts to about 40% of the total non- KGW domestic refuse. OWF is non-combustible and at the moment is still disposed of to landfill. The amount of presented OWF will increase sharply in the coming years . OWF already forms a problem at the moment for a number of reasons. Firstly, composting or fermenting is not worthwhile. Diverse studies have shown that the compost from OWF does not comply with legal requirements. The reason for this is that the heavy metal content is too high according to legal standards currently in force. In addition, OWF is too wet and contains too much sand to make direct incineration possible. Disposal of OWF to landfill is often only permitted under exemption, wherein additional charges are also made.

The expectation is that in the coming years several hundred thousand tons of OWF will be produced for which no processing capacity is available. It is therefore the object of the present invention to develop a technically and economically feasible processing technique for OWF which results in saleable end products such as a green fuel and sand.

This is achieved by the invention with a method comprising the steps of : a) providing an organic wet fraction (OWF) ; b) at least partially drying the organic wet fraction (OWF) ; c) separating the organic wet fraction (OWF) to size into a first fraction with dimensions above a determined cut-off point and a second fraction with dimensions below a determined cut-off point; d) separating each of the fractions on the basis of at least diameter, and optionally density, into a first sub-fraction consisting substantially of organic material and a second sub-fraction consisting substantially of inert material; e) optionally mixing the two first sub-fractions ; and f) optionally bringing the optionally mixed sub- fractions into a manageable form in order to obtain a fuel. It has been found that the usefulness of the fuel in incineration plants depends to a significant extent on the calorific value (MJ/kg) , the quality (composition) of the flue gases released during incineration, and the clogging by sinter processes of the ash residue during the incineration. Clogging is enhanced by constituents which already melt and clog at a temperature lower than the incineration temperature. These are for instance sodium salts, potassium salts and glass. Slagging occurs particularly because glass which is present melts. Glass melts at 500°C while incineration in a fixed-bed furnace takes place at 900°C. The melting glass forms a whole with the rest of the ash, whereby the grating of the furnace becomes blocked. The ash normally falls apart into separate small slags.

Sodium and potassium salts are difficult to remove from the OWF with a dry separating process, but according to the invention it has now been found that glass can however be removed quite readily with a separating process based on at least diameter and optionally density. A ballistic separating process is found to be particularly suitable. Screening with a determined mesh width also results in a considerable reduction of glass in the fuel. In the research which resulted in the present invention it was established that it is important for the quality of the fuel to achieve a removal of glass, with the consequence that slagging does not occur, or hardly so.

The organic wet fraction can be obtained in per se known ways, for instance by removing metals, for instance by means of a metal separator, from the waste, particularly domestic waste, and by removing the coarse fraction from said waste by means of a separation to size .

Drying of the organic wet fraction (OWF) advantageously takes place at least partially by means of composting. Moisture is consumed in the composting process, whereby the composted material becomes drier.

The cut-off point used as limit in the separation to size depends on the starting material and the properties thereof. It has been found that in the case of standard domestic refuse a cut-off point of 10 mm is very suitable in finally arriving at an acceptable fuel. The first fraction with dimensions above the cut-off point preferably consists of material with a size between roughly 10 and roughly 40 mm. The second fraction has dimensions below the cut-off point, i.e. of less than about 10 mm.

The separation into fractions on the basis of at least diameter, and optionally density, can take place in different ways. Separation on the basis of diameter alone can be performed by screening when glass is present in particles smaller than the lighter organic particles. The technique of air separation is also suitable. This technique is further elucidated in the example. It is however particularly recommended to apply a ballistic separating process, such as for instance by placing the material for processing on a high speed conveyor belt which is disposed horizontally. The conveyor belt is situated at a minimum height of 4.5 m. The belt has a minimum speed of 8 m/s. The material is thrown off in horizontal direction. Heavier particles of a determined diameter are thrown further than particles with a lower density of the same diameter. By arranging separating compartments at different distances from the belt and separated by intermediate walls, different fractions can be kept separate. It is generally the case that particles with an equal product of diameter and density are thrown equally far. The minimum height is necessary to allow the particles to achieve a vertical speed component. An optimal separation is hereby obtained. A process which makes use of this technique is the so-called ASTER™ technology.

After passing through the different separating steps the remaining organic material can be brought into a manageable form. Pelletizing can be envisaged here. The invention further relates to a fuel to be obtained from waste by performing the method according to one or more of the foregoing claims.

The present invention will be further elucidated with reference to the example below, which should not be deemed as limitative. Reference is made in the example to the following figures: figure 1 shows the distribution of dry and organic matter in fractions of dried OWF; figure 2 is a schematic representation of an air separator with zigzag distribution; figure 3 shows particle size distribution after air separation of the heavy fraction at 10 m/s; figure 4 shows particle size distribution after air separation of the light fraction at 10 m/s; figure 5 shows a comparison of the particle size distribution in the light fraction and the heavy fraction after air separation; figure 6 is a schematic representation of an ASTER installation; figure 7 shows the distribution of dry and organic matter in the 0-10 mm fraction after ASTER separation; figure 8 shows the distribution of dry and organic matter in the 10-40 mm fraction after ASTER separation; figure 9 shows the mass balance of the method according to the invention.

EXAMPLE

Production of a fuel from domestic refuse

1. Introduction

After extraction of an organic wet fraction from a quantity of standard domestic refuse, the OWF is first dried, whereby it loses much weight. The material is then separated in order to remove from the OWF as much inert material as possible, such as stones, glass and sand. After separation the material is pelletized and incinerated. Pelletizing can optionally be omitted.

2. Material and method 2.1 Drying

The OWF can be dried by means of composting. To enable prediction of the possible composting or fermenting behaviour of the OWF, the composition of the organic matter of the OWF was determined. For this purpose the OWF was analysed for the quantity of cellulose, lignin, protein, starch, fats and other carbohydrates (see table 1) . If the organic matter content is higher than 30% of the dry matter and the lignin content is lower than 20% of the organic matter, the waste is theoretically compostable . Table 1

The composition of the organic matter shows that the OWF particularly contains less quickly degradable substances such as crude cellulose, fats and lignin. If the lignin content is lower than 20%, performing of the composting experiment is worthwhile. Table 1 shows that this is the case here.

Two composting experiments were carried out with the unseparated OWF using the CAT set-up. This is a "closed" composting simulator in which temperature and oxygen are measured continuously. The temperature of the compost and oxygen concentration in the air phase are preset and controlled. The temperature and oxygen concentration are held at the set level by introducing more, or less, ambient air into the vessel.

The first composting experiment was carried out with 300 litres of OWF with a weight of 162 kg, although without feedback of the air. The experiment stood for 15 days. The temperature was round 60 °C for two days and thereafter lower.

After ending of the CAT experiment the dry matter content and the organic matter content of the OWF were determined. Table 2 shows the mass balance over the first composting experiment. Table 2

The table shows that 75% of the water has evaporated, whereby the dry matter content has risen by 30%. The total weight has decreased by 35%.

The second composting experiment was carried out with 162.6 kg of OWF and stood for 19 days. In contrast to the first composting experiment, this composting experiment was performed with feedback of the used air. The advantage hereof is that the fed-back air is warmer than fresh air. Warm air absorbs more water, whereby the OWF dries more quickly. The temperature was round 60°C only for the first few days and thereafter the temperature falls and fluctuates round 27°C. This is roughly equal to the temperature in the CAT space. The mass balance of this CAT experiment can be found in table 3.

Table 3

75% of the water also evaporated in this CAT experiment, so that the dry matter content rose to 80%. The organic matter content was reduced by 9%. The total weight of the OWF was reduced by 40% by the drying, so that the number of tons of material to be separated becomes considerably smaller.

The particle size distribution of the dried OWF was then determined using a dry screening. The dry and organic matter content of the different fractions was determined. The results of this experiment are shown in table 4 and figure 1.

Table 4

The analysis results show that the fraction >10 mm has a high organic matter content (68.2%) . This would in principle be high enough to make fuel therefrom without further separation. However, this fraction also contains stones and larger pieces of glass. These can cause problems during pelletizing and must therefore be separated.

2.2 Separation by means of air separation

Air separation is a much used method of separating plastic and paper from domestic refuse. Since air separation also appeared to be a good method to separate the lighter (organic) material from the heavier (inert) material in OWF, an air separation experiment was carried out, wherein use was made of a so-called zigzag separator. This air separator separates particularly on the basis of the falling behaviour, the density and the surface area of a particle. Figure 2 shows a schematic view of the used air separator.

The material is fed in at the top 1 of zigzag part 2. The embodiment shown is a rise separator, i.e. the separator does not blow but sucks. The lighter material is carried by the suction force to cyclone 3. The light fraction 4 comes out at the bottom of cyclone 3. The heavy fraction 5 enters zigzag part 2 and falls downward into a container (not shown) . The zigzag part contains six bends 6 of 120° . In a zigzag channel 7 the classification takes place at the position of each individual bend. The sequence of bends makes repeated classification possible. The separation performance is determined by the separation characteristic of a single bend and the particle exchange between the bends. The run-down of the particle movements at a bend generally depends not only on the process conditions, particle properties and bend geometry, but also on the direction in which the particles approach the bend.

A total of about 70 kg OWF with a dry matter content of 85% is air separated. The air separation experiments were performed at two different air speeds, i.e. 10 m/s and 12.5 m/s. Visually the better separation appeared to take place at the air speed of 12.5 m/s. From the analysis results, however, it was found that the separation was better at the air speed of 10 m/s. The topflow or overflow of the air separator particularly contains much fine material and very light material such as thin plastic. The underflow contains stones and glass, but also the heavier and larger pieces of plastic, wood and pieces of paper. A possible drawback is that parts of the OWF are caked to each other. These lumps are heavy and always move to the underflow. This problem is however quite simple to solve, for instance by transporting the material using a screw. Table 5 shows a mass balance of the air separation experiment.

Table 5

The particle size distribution of both the light and the heavy fraction was determined. For this purpose a quantity of each fraction was screened over screens with a mesh width of 35 mm, 10 mm, 4 mm, 2 mm, 1 mm, 500 μm, 250 μm and 63 μm. The results of these particle size determinations are shown in tables 6 and 7 and figures 3 and 4. Table 6

Table 7

The heavy fraction contains little material smaller than 1 mm. The finer material in the heavy fraction is probably material which was adhered to the larger portions and which has been detached by the vibration of the screens. The lighter material contains almost no material > 35 mm. This is because this material is too heavy to be sucked upward.

In order to determine how the material with a differing particle size is distributed over the light and heavy fraction, it was determined how much of each particle size fraction ended in the heavy and the light fraction. The results hereof can be found in table 8 and figure 5. Table 8

It can be inferred from figure 5 that the air separator separates the OWF particularly to particle size. The large particles end up in the heavy fraction and the small particles in the light fraction.

A flow of light material with a higher organic matter content is indeed obtained by means of air separation. However, this organic flow is only 47% of the dry OWF, whereby 53% is thus left which must be disposed of to landfill or processed in other manner. A more efficient separation is achieved when the dried OWF is first separated into different fractions. During screening the larger chunks present in the OWF also fall apart .

2.3 ASTER technology

It was found from the air separation experiment that the dried OWF can still not be wholly separated according to density. So as to further increase efficiency use was made of another, ballistic technique. The dried OWF is now pre-separated into two fractions, i.e. the fraction >10 mm and the fraction between 0-10 mm.

An example of a ballistic separating method is the so-called ASTER™ technology. Such a technique will be referred to hereinbelow with the general term "ballistic system" . This technique separates according to density, shape and size. The technique is based on the force of gravity and uses about 20 times less energy than an air separator. This method is also cheaper to purchase. The installation for use in this technique (figure 6) operates as follows. Via the infeed 8 the material is carried onto a conveyor belt 9. The conveyor belt has a ribbed profile so that the material remains properly in place, and rotates at high speed (maximum 10 m/s) . The material is hereby flung from the belt at great speed. The material is collected behind the belt in a closed collecting space 10. In the present embodiment this is 7 m high and 5.5 m long, and is divided into five different compartments 12-15. The lightest material thus enters the first container 12, the heaviest material ends up in the last container 15. In order to obtain a good separation a blower 16 is suspended just behind the conveyor belt which blows the light material into a separate first collecting container 11. When the collecting space is long enough, container 11 is unnecessary.

In order to divide the OWF into the two fractions a large quantity of dry OWF was screened over a screen with a mesh width of 10 mm. With this dry screening the OWF is separated into 25% material larger than 10 mm and 75% material smaller than 10 mm.

The fractions 0-10 mm and >10 mm were both individually separated into 5 fractions by means of the ballistic system. Table 9 shows the mass balance of the fraction 0-10 mm, table 10 the mass balance of the fraction >10 mm. Shown in the form of a graph in figures 7 and 8 is the distribution of the dry and organic matter of both experiments. Table 9

* fraction 1 is the lightest, fraction 5 the heaviest

Table 10

* fraction 1 is the lightest, fraction 5 the heaviest

The analysis results show that the OWF is separated into an inert and an organic flow. Light fractions >10 mm in particular have a high organic matter content. The light fractions of 0-10 mm have a somewhat lower organic matter content. This is because the material < 10 mm in the starting material also has a lower organic matter content .

The fractions 1-3 of the fraction 0-10 mm and the fractions 1-4 of the fraction 10-40 mm were mixed and then incinerated. When these fractions are mixed there results a fuel with an organic matter content of 55% . In order to obtain a fuel with a higher organic matter content, the fraction < 2 mm could be separated beforehand. In this manner material with an organic matter content of 66% can be obtained.

The fractions 4 and 5 of the fraction 0-10 mm and the fraction 5 of the fraction 10-40 mm consist largely of stones and glass. These fractions can for instance be marketed as rubble or gravel.

In figure 9 is given the total mass balance of this experiment. The mass balance shows the following process. During drying the mass of the OWF is reduced by 40%. The remaining 60% is screened over a screen with a mesh width of 10 mm. The stones are removed from both the overflow and the underflow using a separating technique. This results in 13% stones and glass and 47% organic material. The organic material is mixed, pelletized and incinerated. The inert material can be marketed as rubble or gravel .

2.4 Incineration

Prior to the dry separation process the contents of nitrogen, chlorine, sodium and potassium are determined in order to predict the incineration behaviour. The results of these analyses can be found in table 11.

Table 11

The concentration of sodium + potassium lies round 8,000 mg/kg. It is known that clogging takes place at a sodium + potassium content of 10,000 mg/kg. The calorific value will be round 18 MJ/kg. The invention therefore provides a method to make a fuel from OWF and analogous waste flows, such as non-KGW domestic refuse, the organic fraction from industrial waste, green waste, grounds maintenance waste, the organic fraction from construction and demolition waste, the organic fraction from "old" excavated waste landfills, by choosing drying and separation such that the fuel to be formed can be processed in different types of incineration plant . For the OWF from household waste analysed in the example this implies that a minimum of 12% of the fine inert fraction (<2 mm consisting particularly of glass) must be separated in order to produce a usable fuel .

On the basis of the content of glass and organic matter it is possible to choose whether and how the techniques of screening and ballistic separation are applied. It is generally the case that if the glass content of dried material is greater than 0 and the organic matter content (of the dry matter) is more than 60%, a ballistic separation is required to remove at least some of the glass. When the glass content is less than 1% and the organic matter content less than 60%, glass can be removed using an additional screening step (e.g. mesh width 4 mm) . The small particle fraction contains inert (non-combustible) parts. If the glass content of dried material is less than 1% of the dry matter and the organic matter content more than 60%, a screening step is then only necessary to separate the fine fraction (e.g. mesh width <4 mm) .

Claims

1. Method for producing a fuel from waste, in particular the organic wet fraction (OWF) of waste, comprising the steps of: a) providing an organic wet fraction (OWF) ; b) at least partially drying the organic wet fraction (OWF) ; c) separating the organic wet fraction (OWF) to size into a first fraction with dimensions above a particular cut-off point and a second fraction with dimensions below a particular cut-off point; d) separating each of the fractions on the basis of at least diameter, and optionally density, into a first sub- fraction consisting substantially of organic material and a second sub-fraction consisting substantially of inert material; e) optionally mixing the two first sub-fractions ; and f) optionally bringing the optionally mixed sub- fractions into a manageable form in order to obtain a fuel.

2. Method as claimed in claim 1, characterized in that the organic wet fraction is obtained by removing metals from the waste, particularly domestic waste, and by removing the coarse fraction by means of a separation to size.

3. Method as claimed in claim 1 or 2 , characterized in that the organic wet fraction (OWF) is at least partially dried by means of composting.

4. Method as claimed in claims 1-3, characterized in that the cut-off point is 10 mm.

5. Method as claimed in claim 4, characterized in that the first fraction with dimensions above the cut-off point consists of material with a size between roughly 10 and roughly 40 mm.

6. Method as claimed in claim 4, characterized in that the second fraction with dimensions below the cutoff point consists of material with a size of less than about 10 mm.

7. Method as claimed in claims 1-6, characterized in that the separation on the basis of at least diameter, and optionally density, takes place by means of screening.

8. Method as claimed in claims 1-6, characterized in that the separation on the basis of at least diameter, and optionally density, takes place by means of air separation.

9. Method as claimed in claims 1-6, characterized in that the separation on the basis of at least diameter, and optionally density, takes place by means of a ballistic separating process.

10. Method as claimed in claim 9, characterized in that the ballistic separating process is the ballistic system.

11. Method as claimed in claims 1-10, characterized in that optionally bringing the optionally mixed sub- fractions into a manageable form to obtain a fuel consists of pelletizing.

12. Fuel to be obtained from waste by performing the method as claimed in one or more of the preceding claims.