WO2007093281A1

WO2007093281A1 - System and method for disinfection of refuse derived fuel

Info

Publication number: WO2007093281A1
Application number: PCT/EP2007/000809
Authority: WO
Inventors: Fabio Coggiola; Gianluca Porto; Giulio Rinaldi
Original assignee: Rivoira S.P.A.; Unieco Societa' Cooperativa
Priority date: 2006-02-16
Filing date: 2007-01-31
Publication date: 2007-08-23
Also published as: ITMI20060290A1

Abstract

A system and a method are described for the disinfection of refuse derived fuel (RDF). The system comprises a disinfection chamber (17) in which the refuse derived fuel to be disinfected is introduced, and an ozone generator (18) able to introduce ozone (O3) into said disinfection chamber (17), so as to disinfect said refuse derived fuel through contact with ozone. This disinfection has the result of reducing the bacterial load (BL) of the refuse derived fuel to below a hazard level, preventing a rapid bacterial growth within the time necessary for the use of said refuse derived fuel, and eliminating the unpleasant odours of the refuse derived fuel.

Description

SYSTEM AND METHOD FOR DISINFECTION OF REFUSE DERIVED FUEL

DESCRIPTION

The present invention refers to a system and to a method for disinfecting the refuse derived fuel (RDF).

The majority of municipal solid waste (MSW) is currently dealt with in tips. However, new European laws require increasingly less use of tips for the management of municipal solid waste. Therefore, this waste is treated in recycling centres.

The organic/wet part of the waste is composted/stabilised (SOF - stabilised organic fraction) so that it can be used as an additive for landfills, road surfaces, etc

The inorganic/dry part of the waste (after separation of metals and inert substances such as glass, ceramics, etc), consisting of paper/cardboard, plastic packaging, textiles and wood, is ground and dried so that it can be used as fuel (RDF refuse derived fuel) in waste to energy processes such as co-combustion in cement furnaces, waste to energy plants, incinerators, etc.

In order for RDF to be considered as fuel, it must meet quite stringent requirements, such as:

- a calorific content above a minimum value,

- a moisture content below a minimum percentage, and - a particle size with a pre-established grain size.

Besides these requirements, the end user needs the RDF to be stable from the point of view of fermentation/putrefaction of the organic residue (which leads to unpleasant smells during the storage alongside the combustion plant) and not to constitute a health hazard for the workers in terms of bacterial or viral infections. Therefore it is necessary to perform a "disinfection" of the RDF.

Systems are currently known to the art for disinfection in general which use disinfectants in the liquid phase, and containing chlorine-based products. However, these systems hold some drawbacks . In fact the liquid disinfectant increases the percentage of moisture in the RDF, with the result that the RDF no longer complies with the required requirements.

Furthermore, after the disinfection, the RDF continues to contain residues of said disinfectant. This is a very serious drawback, especially in the case of chlorine residue, since subsequent combustion of the RDF would lead to the production of dioxins, which are highly harmful for humans and for the environment.

Object of the present invention is to overcome the drawbacks of the prior art by providing a system and a method for disinfecting the refuse derived fuel that are effective, efficient and at the same time reliable and safe for the environment and for humans.

Another object of the present invention is to provide such a system for disinfecting the refuse derived fuel that is cheap and easy to produce.

Another object of the present invention is to provide such a method for disinfecting the refuse derived fuel that is fast and can be integrated into the refuse derived fuel production line, without an excessive impact on the production times and costs.

These objects are achieved in accordance with the invention with the system and the method whose characteristics are listed in appended claims 1 and 13, respectively.

Advantageous embodiments of the invention are apparent from the dependent claims.

The system for disinfecting the refuse derived fuel (RDF) according to the invention comprises:

- a disinfection chamber in which said refuse derived fuel to be disinfected is introduced, and - an ozone generator able to introduce ozone into said disinfection chamber, so as to disinfect said refuse derived fuel through contact with ozone.

Said disinfection system makes it possible:

- to lower the bacterial load (BL) of the refuse derived fuel to below a hazard level, - to prevent a rapid bacterial growth within the time necessary for the use of said refuse derived fuel, and - to eliminate the unpleasant odours of the refuse derived fuel.

Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplifying and therefore non-limiting embodiment thereof, illustrated in the appended drawings, in which:

Figure 1 is a diagrammatic view illustrating the operating principle of an ozone generator;

Figure 2 is a diagrammatic view illustrating a plant for treating waste water with ozone, used for validation tests of the present invention; Figure 3 is a graph showing the course of the bacterial growth of a sample of RDF not treated with the disinfection system according to the invention and of two samples of

RDF treated with the disinfection system according to the invention;

Figure 3A is a histogram showing the bacterial growth values measured in a further test done on three samples of RDF which underwent treatment according to the invention, compared with an untreated sample of RDF; and

Figure 4 is a block diagram illustrating a RDF production line, equipped with the disinfection system according to the invention.

The applicants have surprisingly discovered that a valid solution for disinfecting RDF could consist of a gas and particularly of ozone (O₃).

In fact ozone is characterised by a strong oxidizing power, is emitted in the gaseous phase and after exerting its disinfection function leaves no trace/residue in the RDF, since ozone is converted back to oxygen. Among the oxidizing molecules, ozone comes immediately after the hydrofluoric acid (HF) for the extreme instability of the molecule (O3), which tends to be converted to molecular oxygen (O2).

Ozone is a blue coloured gas whose physico-chemical characteristics are shown in Table 1 below. Table 1

Thanks to these characteristics, ozone has a very high oxidizing capacity, which is exerted through the rupture of complex organic molecules, besides the breakdown of the cell membrane of the microorganisms with which it comes into contact. This direct oxidizing action requires considerably shorter contact periods compared with the traditional liquid disinfectant systems using chlorine-based products.

Besides the great speed of destruction of the bacteria, ozone ensures an effective inactivation of viruses and the elimination of unpleasant odours and tastes, through the oxidation of compounds such as sulphides.

As shown in Figure 1 , ozone O3 is generated starting from the oxygen O2 contained in a flow of pure oxygen or of air, which is fed to an ozone generator The ozone generator has an electrical discharge area, known as the crown effect, formed in a tube of dielectric material, between a low voltage electrode and a high voltage electrode. Oxygen, on passing through the electrical discharge area, undergoes the process of conversion into ozone and thus at the outlet of the ozone generator there will be oxygen O2 and ozone

O₃.

The use of the ozone for disinfecting liquids, such as waste liquids, in which it is readily soluble and gases with which it mixes well, such as for example waste gases from industrial plants, is known to the art.

However, in the prior art there were prejudices in using the ozone for disinfecting solids: a) As stated above, the ozone is a gas that dissolves in the liquids and which mixes with other gases, but its behaviour and its reaction in contact with solids was unknown. b) The ozone is a highly unstable gas that is converted to oxygen after a short time; because of this it was considered unsuitable for treatment of solids. c) The ozone is a highly comburent gas, therefore its use was not thought of for disinfecting something that is in itself combustible.

Initially tests and experimental validation studies were carried out, using an existing system for disinfecting waste water, like that shown in Figure 2, and designated as a whole with reference numeral 100. The validation system 100 comprises an ozone generator 101 connected to the electrical power network. An oxygen tank 102 is connected to the inlet of the ozone generator 101. The outlet of the ozone generator 101 is connected to the base of a RDF treatment column 103, having a capacity of 20 litres, for example. In the upper part of the column 103 an outlet duct 104 is provided for connection to an exhaust duct for discharge of the ozone into the atmosphere.

In the upper part of the column 103 an inlet duct 105 is provided for introduction of the RDF. The RDF is in the form of confetti which falls downward through gravity, being hit by an opposite flow of ozone which tends to rise upwards. In the lower part of the column 103 a tapping duct 106 is provided to withdraw samples of treated RDF.

The data collected during the validation tests are necessary to identify the optimal conditions in terms of concentrations and of exposure time of the RDF to the ozone, necessary to achieve the pre-established objectives. Object of the tests is to verify the doses, the concentration, the contact periods and the efficiency of the ozone treatment, applied to RDF.

The oxidation of ozone therefore has the objective of neutralising/stabilising the bacterial load (due to the organic residue inside the RDF) for a preset time (necessary to complete the subsequent production operations and the final delivery to the waste to energy plants), avoiding the formation of the unpleasant odours characteristic of the putrefaction and the possible growth of infection-carrying bacteria.

Preliminary validation was conducted with a batch modality, with known amounts of RDF and taking samples at set intervals of time, the samples being subjected to precise chemical and biological analyses in an accredited laboratory. Various tests were conducted, varying the operating conditions (flow rate and concentration of ozone, contact period, type of RDF in terms of particle size, amount of organic residue, moisture content etc.).

From these tests it was ascertained that a significant reduction in the bacterial load (BL), well below a contamination hazard level, was obtained both when the RDF was treated with oxygen (O₂) alone, and when the RDF was treated with oxygen (O₂) and ozone (O₃).

However, it was found that the samples treated with oxygen alone underwent an exponential bacterial growth after 48 hours and after 72 hours. This situation is not desirable, since the time that passes between the end of the RDF production process and the use thereof as fuel is about 48-72 hours.

The samples treated with oxygen and ozone, on the other hand, had a linear bacterial growth in the following 48 hours and in the following 72 hours. It should be considered that a linear growth for a bacterial population is entirely negligible.

Given the promising results of the preliminary validation, a pilot system was created which serves for dimensioning the final plant and whose purpose is to simulate the industrial process and to be able to collect more precise process data (O₂/C>₃ flow rate, contact periods, etc.) for a correct final dimensioning.

The pilot system, compared with the validation system 100, has an exposure column with a horizontal axis which is set in rotation around its own axis to allow a greater contact of the RDF with the ozone.

The RDF sample used is lkg in weight, so as to facilitate dimensioning on an industrial scale (several tons/h of material to be treated). Furthermore, the sample of RDF was ground further to reach a particle size of about 20/30 mm. The most significant results among the various tests carried out with the pilot system are reported by way of example.

The test began with a blank RDF sample (not disinfected) which had a bacterial load BL = 2.35 * 106 CFU/g. Two lkg samples (A and B) were taken from this sample. Sample A underwent treatment in the pilot system with a flow rate of 1 litre of O2 a minute for a contact period of 12 minutes. In addition, sample B underwent treatment in the pilot plant with a flow rate of 2 litres of O2 a minute for a contact period of 12 minutes.

The bacterial load of samples A and B was then analysed at three set intervals: 24 hours after treatment, 48 hours after treatment and 72 hours after treatment. The results are summarised in the three tables shown below. Table 2

Table 3

Table 4

In Figure 3 a graph shows the course of the bacterial growth of a blank sample and of samples A and B which have undergone the ozone treatment.

As can be seen from tables 2-4 and from the graph in Figure 3, the result was positive, in that the bacterial load was reduced by about 60% to 90% of the initial load and regrowth thereof at 24h, 48h and 72h is negligible (an activated bacterial load grows exponentially). Furthermore the typical urban waste odour of the RDF was eliminated

Further tests were carried out with the pilot system to provide a greater statistical certainty. The histogram in Figure 3A shows the results of a test carried out on three samples, A, B and C treated with ozone, compared with an untreated (blank) sample. An oxygen flow of 3 L/min was fed to the ozone generator and the three samples A, B and C underwent contact periods of 12 min, 8 min and 4 min, respectively. As shown in the histogram in Figure 3 A, even HO h after treatment, the bacterial load of samples A, B and C treated with ozone remains negligible if compared with the bacterial load of the untreated (blank) sample.

Furthermore, other tests also demonstrated a disinfecting contribution of the oxygen, which contributes to the reduction of the bacterial load, but not to the subsequent "neutralisation":

Therefore, according to the tests carried out with the test system, the following typical process data were set up.

The amount of ozone per kg of material to be disinfected can range from 0.1 g to 4 g, and is preferably 2 g.

Since a standard ozone generator has an oxygen to ozone conversion rate of about 10- 15%, it follows that the oxygen flow to be fed the ozone generator must be up to 40 litres/h, typically 20 litres/h, per kg/h of RDF to be treated. The ozone concentration range in the oxygen flow may be between 0.1% and 15%, typically 10%.

Since air contains about 20% oxygen, it would be necessary to overdimension the ozone generator excessively to make it operate with high air flows, considering that the amounts in question in a RDF production plant are of the order of tons/h of RDF. Furthermore, the ozone produced would be excessively diluted, at the expense of the efficacy and efficiency of the process.

Therefore, it is advantageous to use stocking from pure oxygen. Besides, this supply of oxygen is entirely reasonable since it weighs minimally on RDF production costs.

The contact period of RDF with ozone can range from 1 minute to 60 minutes, and is typically between 10 min and 20 min. These contact periods do not affect the continuous production cycle of RDF. In fact, it must be considered that during the production process, RDF must remain in the grinders for pulverization for times in the order of minutes.

The system and the method for production of RDF according to the invention are described with reference to Figure 4.

The municipal solid waste (MSW)₅ unselected and contained in plastic bags, is loaded, by means of a loading hopper 1 , onto a slatted conveyor belt 2 which feeds it towards a primary grinder 3 which serves to break the containing bag. The MSW coming from the primary grinder 3, by means of a conveyor belt 4, is sent to a primary screen 5 which has 80 mm holes to separate the MSW into an organic fraction (OF) and a dry light fraction (DLF), which are sent to two different conveyor lines 6 and 7.

The OF is passed through an electromagnetic separator 8 to separate residual metal and then through an induction field separator 9 to separate inert materials, such as glass, ceramics and the like. Lastly, the organic fraction, by means of a conveyor 90, is sent to a dioxidation plant 10 for disposal.

The DLF, on the other hand, is sent to an electromagnetic separator 11 to separate residual metals and to a secondary grinder 12, which serves to carry out pulverization until a DLF particle size of about 70 mm is obtained. The ground DLF is sent to an induction field separator 13 to separate the residual inert materials and then to a secondary screen 14.

The secondary screen serves to screen the largest pieces which are considered OF and sent, by means of a conveyor line 91, to the OF conveyor line 90, towards the dioxidation system 10. The less coarse pieces of DLF, on the other hand, pass through the secondary screen 14 and by means of a conveyor 15 are sent to a grinder 16 which carries out pulverization until a particle size suitable for the disinfection treatment according to the invention is obtained, for example a particle size less than 40 mm, preferably between 20 and 30 mm.

The ground DLF with a suitable particle size is sent to a disinfection chamber 17. The disinfection chamber 17 is in the form of a cylindrical container, with a horizontal axis, and is mounted rotatably around its own axis.

The disinfection chamber 17 comprises a gas inlet duct 19 connected to an ozone generator 18 which supplies oxygen (O2) and ozone (O3) into the disinfection chamber 17 and an exhaust duct 19' to discharge into the atmosphere the oxygen (O2) and any ozone (O3) remaining in the chamber 17 after the treatment. The exhaust duct 19' is a catalytic ozone removing duct to avoid introducing ozone into the atmosphere.

In order to maximize the contact between the gas and the DLF, the disinfection chamber 17 is made to rotate around its own axis and the DLF is fed into the disinfection chamber with a flow counter to that of the gas coming from the ozone generator 18.

For a good treatment efficiency, the ozone generator 18 must deliver an amount of ozone between 0.1 g and 4 g, preferably 2g, for 1 kg of material to be disinfected. Since the disinfection chamber 17 has a capacity of about 1 ton, the ozone generator 18 is sized so as to be able to generate up to 4 kg of ozone for each ton of material present in the disinfection chamber 17.

So as not to overdimension the ozone generator 18 excessively, it is connected to a liquid oxygen tank 20, by means of a vaporiser 21 which supplies oxygen vapour to the ozone generator 18. The oxygen tank 20 and the vaporiser 21 must ensure an oxygen flow up to 40 litres/h, typically 20 litres/h, per kg/h of material to be treated. Since the material to be treated has a flow of about 1 ton an hour, the oxygen tank 20 and the vaporizer 21 are sized so as to ensure a flow rate up to 40,000 litres/h (40 m^/h), preferably 20,000 litres/h (20 m³/h)

The contact periods of the DLF with the gas inside the disinfection chamber 17 can be between 1 minute and 60 minutes, and are typically between 10 min and 20 min.

On completion of the disinfection process, the DLF leaving the disinfection chamber 17 has a high moisture content of about 25—45% in weight. Therefore the wet DLF is sent to a drying chamber 22 in which the moisture content is reduced to below 15% in weight, which represents a threshold value to be able to be considered a refuse derived fuel (RDF).

Finally the RDF is compressed and sent to the storage 23. The time in the storage 23 does not generally exceed 72 hours. In fact within this time the RDF must be used as fuel in cement works or in a waste to energy plant.

Numerous changes and modifications of detail within the reach of a person skilled in the art can be made to the present embodiment of the invention, without thereby departing from the scope of the invention as set forth in the appended claims.

Claims

1. A system for the disinfection of refuse derived fuel (RDF) characterised in that it comprises - a disinfection chamber (17) in which said refuse derived fuel to be disinfected is introduced, and

- an ozone generator (18) adapted to introduce ozone (O3) into said disinfection chamber (17), so as to disinfect said refuse derived fuel through contact with the ozone, with the result of - reducing the bacterial load (BL) of the refuse derived fuel to below a hazard level;

- preventing a rapid bacterial growth within the time necessary for the use of said refuse derived fuel, and

- eliminating the unpleasant odours of the refuse derived fuel.

2. A system according to claim 1, characterised in that the amount of ozone introduced into said disinfection chamber (17) is between 0.1 and 4 g, preferably 2 g, for each kg of material to be disinfected.

3. A system according to claim 1 or 2, characterised in that the contact periods of the ozone with the material to be treated inside said disinfection chamber (17) are between 1 min and 60 min, preferably between 10 min and 20 min.

4. A system according to any one of the preceding claims, characterised in that it comprises a tank (20) containing pure oxygen connected to said ozone generator (18) to supply oxygen (O2) to the ozone generator (18).

5. A system according to claim 4, characterised in that said tank (20) contains liquid oxygen and an evaporator (21) is provided to supply vaporised oxygen to said ozone generator (18).

6. A system according to claim 4 or 5, characterised in that said tank (20) supplies the ozone generator (18) with a flow of oxygen below 40 litres/h, preferably 20 litres/h, for a flow of material to be treated of 1 kg/h.

7. A system according to any one of the preceding claims, characterised in that said ozone generator (18) provides a flow of oxygen with an ozone concentration between

0.1% and 15%, preferably 10%.

8. A system according to any one of the preceding claims, characterised in that said disinfection chamber (17) has a substantially cylindrical shape with a horizontal axis and rotates around its own axis to maximise the contact of the material to be disinfected with the ozone.

9. A system according to any one of the preceding claims, characterised in that the ozone is introduced into said disinfection chamber (17) counter to the flow of the material to be treated.

10. A system according to any one of the preceding claims, characterised in that said disinfection chamber comprises an exhaust and removal duct (19') for the discharge of the oxygen and for the removal of any ozone remaining after the treatment.

11. A system according to any one of the preceding claims, characterised in that it comprises a grinder (16) disposed upstream of said disinfection chamber (17) to reduce the particle size of said material to be disinfected to below 40 mm, preferably in the range of 20 mm to 30 mm.

12. A system according to any one of the preceding claims, characterised in that it comprises a drying chamber (22) disposed downstream of said disinfection chamber (17) to reduce the moisture content of said disinfected material to below 15% in weight.

13. A disinfection method for refuse derived fuel (RDF) characterised in that it comprises the following steps:

- generating ozone (O3), and

- putting said generated ozone into contact with said refuse derived fuel to disinfect it, with the result of - reducing the bacterial load (BL) of the refuse derived fuel below a hazard level,

- preventing a rapid bacterial growth within the times necessary for the use of said refuse derived fuel, and

- eliminating the unpleasant odours of refuse derived fuel.

14. A method according to claim 13, characterised in that an amount of ozone between 0.1 g and 4 g, preferably 2 g, for each kg of material to be disinfected is used.

15. A method according to claim 13 or 14, characterised in that the ozone remains in contact with the material to be disinfected for a time ranging from 1 min to 60 min, preferably in the range from 10 min to 20 min.

16. A method according to any one of claims 13 to 15, characterised in that said step of generating ozone comprises supplying pure oxygen to an ozone generator (18).

17. A method according to claim 16, characterised in that said ozone generator (18) is supplied with a flow of oxygen of less than 40 litres/h, preferably 20 litres/h, for a flow of material to be treated of 1 kg/h.

18. A method according to any one of the claims 13 to 17, characterised in that a flow of oxygen with an ozone concentration between 0.1% - 15%, preferably 10%, is fed onto said refuse derived fuel.

19. A method according to any one of claims 13 to 18, characterised in that said material to be disinfected is made to rotate in a disinfection chamber (17) to maximise the contact of the material to be disinfected with the ozone.

20. A method according to any one of claims 13 to 19, characterized in that the ozone flow is counter to the flow of the material to be treated.

21. A method according to any one of claims 13 to 20, characterised in that it comprises a grinding step before said disinfection step to reduce the particle size of said material to be disinfected to below 40 mm, preferably in the range from 20 mm to 30 mm.

22. A method according to any one of claims 13 to 21, characterised in that it comprises a drying step after said disinfection step to reduce the moisture content of said disinfected material to below 15% in weight.