GB2342872A - Method of treating municipal waste to produce fuel - Google Patents

Abstract

A method of treating municipal waste to produce fuel comprises the first step of screening the waste to remove any non-combustible material, and separate it into fractions depending on size. Then the waste is shredded to a first predetermined size and this shredded waste is formed into bales. The bales can be stored until they are required for use. When the bales are required, they are shredded for a second time into particles of a second predetermined size, which is smaller than the first, producing a homogeneous mix of combustible waste. Preferably the size of the waste material after the first shredder is 250mm or less, and after the second shredder it is 25mm or less. By forming the waste into bales it can be stored for use as fuel during periods such as public holidays when no refuse is collected.

Description

TREATMENT OF MUNICIPAL WASTE This invention relates to the treatment of municipal waste.

Current methods of treating municipal solid waste to produce useful fuels suffer from a number of disadvantages, not the least of which is that it is not possible to store the waste for other than short periods without the formation of leechate and without the emission of odours.

It would be desirable to be able to provide a method of treating municipal solid waste to produce useful fuel which enabled the waste to be stored for longer periods than heretofore, typically 5 days or more, and which eliminated the problem associated with leechate and odour heretofore present without the need to dry and pelletise or briquette the waste.

It would also be desirable to produce an end product in the form of a homogeneous mix.

According to the present invention there is provided a method of treating municipal waste to produce a useful fuel comprising the steps of: a) screening the waste into a series of fractions of different sizes and removing non-combustible material therefrom; b) shredding the screened waste to a first predetermined size, typically 250mm or less, preferably at a rate of 25 to 30 tonnes per hour; c) forming the shredded waste into bales for the purpose of storage, and d) shredding the baled waste to a second, smaller predetermined size, typically 0.5 to < 25mm, preferably at a rate of 10 to 12 tonnes per hour, thereby to produce a. homogeneous mix of combustible waste.

For many years now both in USA and in Europe the term "REFUSE DERIVED FUEL"has been commonly understood, but this term covers a wide range of technologies from straight forward incineration or mass burn, to the manufacture of pellets for use in industry.

To design a fuel preparation plant it is necessary to have a knowledge of the mass balance of the incoming material.

For the purposes of the following description, we have assumed a mass balance of: paper & board 25% plastic film 5% dense plastic 6% textiles 6% ferrous metal 7% wood 3% non ferrous metal 1% organic/putrescible 35% glass 8% other (-10mm) 401 Having established the above, it is necessary to establish just what form the fuel which is going to be prepared is to take ie: flock-being a fine material with 600-o > 12mm (max size 25mm) coarse-up to 150-300 mm densified-pellets or briquettes The form of the fuel is of course mainly dependent upon the process chosen ie: flock-gasification or pyrolysis for energy recovery or for use in a cement kiln coarse-fluidised bed waste to energy plant densified-for use in an industrial application, community heating scheme etc.

In the early days of preparation of fuel from municipal waste, the practice, particularly in the USA, was to shred or mill the waste first, then classify it to take out the non-combustible fraction. This was not the correct solution, and had it been looked at more carefully, it would have been clear that all that was happening was that contaminants were being ground and mixed into the resultant "fuel". In the case of fuel pellets, this resulted in a higher ash content, and a higher chlorine content in the boiler. In all cases, energy was being wasted heating non combustible materials.

With the greater emphasis today on recycling, it is necessary to incorporate into the design of any preparation plant an opportunity to remove metals, glass and other materials to enable them to be reused as secondary materials.

The plant for performing the method of the invention is capable of providing prepared fuel at a rate of up to 12 tonnes per hour to the drier, as per the following specification : loose feed-250-300 kgm3 maximum size 25 mm free from non-combustibles availability-7 days-24 hours-340 days (8,160 hours) storage-minimum 5 days-1500 tonnes The operating hours and feedstock delivery amounts would be as follows: Hrs tonnes Feedstock Delivery 0800 nil 0900 nil 1000 nil 1100 63 1200 63 1300 nil 1400 30 1500 63 1600 33 33 Total 252 tonnes Plant requirement is for up to 12 tonnes per hour to gasifier over 24 hours and 7 days per week. This gives a total of prepared fuel of up to 2016 tonnes per week.

Remembering that the glass and metal fraction has been removed along with other contraries to allow ease of operation, the gross figure is 20% higher at up to 2420 tonnes per week.

Fuel preparation is as follows (8 hour shifts): Hrs Used Balance 0800-50 tonnes 0900 30 tonnes 20 tonnes 1000 30 tonnes (10) tonnes 1100 30 tonnes 23 tonnes 1200 15 tonnes 71 tonnes 1300 30 tonnes 41 tonnes 1400 30 tonnes 41 tonnes 1500 30 tonnes 74 tonnes 1600 30 tonnes 77 tonnes 1700 15 tonnes 62 tonnes Total 240 tonnes It is necessary to hold a stock of between 50 and 60 tonnes. When you consider that the refuse collection vehicles (RCV) will commence deliveries to the site at around 1030 hrs, it is essential to have some waste held over from the previous day to allow plant start up, bearing in mind that, if there is going to be a problem with the process plant it will inevitably manifest itself on start up.

The incoming waste which is being delivered by RCV's has a bulk density whilst in the RCV of approx 300 kgm3. When it is discharged onto the concrete apron of the reception area it will expand, and the bulk density will decrease to approx 150 kgm3.

The storage area must be constructed in such a way that RCV's can discharge whilst the process system can still be loaded with feedstock. It must be possible to tip into two areas for this reason, and also to allow each area to be completely emptied and disinfected on a daily basis.

The size of the reception area should be considered carefully with the following considerations in mind: i. The amount of waste which is necessary for the running of the plant bearing in mind the times of feedstock delivery compared with the process plant requirement. ii. Space to enable the tipping of RCV's to take into account the need to spread the material for the identification of contraries not welcome in the process plant ie: carpets, furniture, white goods etc. iii. Storage area which enables the waste to be stored over the most desirable area keeping in mind that a large proportion of the waste is biologically active and therefore can attack the cellular structure of other materials causing it to become wetter (not a desirable occurrence when preparing fuel) which will have to be dried before it is finally processed. iv. Control of heat retention is done by creating the largest surface area, which allows radiation and convection to deal with the heat and release it almost as and when it is generated. v. There will be a certain amount of leechate from the waste depending upon the time of year and the day of the collection, Spring and Autumn for green waste in general, and the weekend cuttings from lawns which have been in plastic bags etc since Sunday but not collected until Friday.

The stored material will emit odours and water vapour, traces of methane, hydrogen sulphide and ammonia and probably bacterial and fungal spores. There is no published evidence that these emissions are particularly harmful to other than sensitive individuals, and most are of such a small nature that they can scarcely be detected under normal circumstances. Workers in this area should be issued with masks and the correct safety clothing.

Even when contained in plastic bags, the waste will also create dust every time it is moved. The more violent the movement, the more obvious the dust will become. It will, if allowed, coat the structural steel work and plant with a thick layer of fibres. These fibres, whilst not highly inflammable, will smoulder if ignited causing considerable volumes of smoke. Good housekeeping is therefore essential.

The process capacity of the plant is 30 tonnes per hour, and the design specification of the plant calls for the removal of all non-combustible materials. This is not an easy task. It means that the incoming material must be screened in such a way as to allow the burden depth of any picking belts to allow the easy identification of materials and their subsequent removal.

There are a number of method of screening. In this case the use of a trommel or the use of a Starscreen (see below) are considered most appropriate. For the purposes of this project, the Starscreen is considered most suitable because of its flexibility and capacity.

The Starscreen for this project has been designed and tested by Lubo of Holland in such a way as to allow four different fractions to be separated to facilitate easypicking: i. Fines, 0-10 mm ii. Cans etc iii. Bottles etc iv. Newspapers etc The greatest benefit for the division of the fractions comes from the Starscreen's ability to be able to be adjusted by speed to allow larger fractions or smaller' fractions to be separated. When the speed of the variable speed motors is increased in any particular section, then the resultant separation is of a smaller fraction, and, conversely, when it is decreased, then the resultant fraction is larger.

With regard to capacity, because of the fact that the material is driven across its length, it is possible to vary the tonnage throughput, again because the six individual motor reducers are capable of being slowed or speeded up at will.

Having divided the waste into four fractions, these fractions are taken from the Starscreen by four conveyors, the first of which is the fines conveyor where no picking is envisaged. The second, with the majority of cans, but also some glass, can be handpicked in the case of the glass, while the metal fraction will be mechanically removed by means of magnets and eddy currents. The third line can be handled similarly, but the fourth line must be handpicked in the case of all metals. The reason for this is that the metal fractions are oversized and therefore, in the case of the magnet, it would be impossible to determine the working height of the magnet. Similarly, with regard to the eddy current, the bulk would be too great for this process.

The equipment for the mechanical separation of the metal content is manufactured by Eriez of the UK.

The ferrous content will be removed by overband magnets, and the non ferrous content will be removed by eddy current, whereby an induction charge is given to the non ferrous material causing it to"jump"and separating it from the other materials.

Both of these metal separation machines have been tried and tested on this application with the co-operation of Eriez Magnetics.

The whole or part glass bottles can, if required, be sorted by a Binder Whole Glass Bottle Sorting Unit into colours ie: clear, green and brown. This unit can process between 3 and 9 tonnes per hour with an efficiency of more than 9901. This would enhance the price received for the glass.

Instead of mixed glass with a limited use and a low price or even nil value, clean sorted glass cullet would be produced which could be sold at the market price. It is envisaged that approx 1.5 tonnes per hour could be recovered ie: 60% of the glass content.

Having removed the contraries, glass and metals from the waste, the waste is baled automatically.

Automatic baling can be achieved using an automatic baler as manufactured by PAAL of Germany and having a capacity of 30 tonnes per hour, producing bales of approx 750 to 1500 Kgs in weight.

As the gasification plant is running 24 hours per day and 7 days per week, the supply of the waste to the gasifier is very important, and must be available at all times including Bank Holidays, and or course in the unlikely event of a breakdown. If we assume that the longest holiday period is the Easter holiday, then we need to store fuel for a minimum period of 5 days or up to 1500 tonnes (approx 1000 bales). We also need to feed the drier system of the gasification plant with prepared fuel on a 24 hour basis. This is the reason for the second Vecoplan Shredder & Debaler.

If we consider that the bulk density of the incoming waste is 150 kgsm3, and let us assume that, after the removal of metals and glass, and after the process through the gyroscopic mill, the bulk density has remained the same.

If we did not bale the material, we would require a storage area of 4800 m3, a large area, but we would also have the problems of storage as previously described. It is also not possible to carry out the second shred at the rate of 30 or even 15 tonnes per hour and achieve a shred size of 25mm. Therefore the baling operation cannot be avoided.

It is also the only way that the waste can be stored for such a length of time without major problems with heat, smell and leechate.

A report from VAROM bv in Holland, where waste was prepared as requested by ourselves follows.

CONTENTS 1. Introduction 2. Objective 3. Set-up experiment 4. Progress experiment 5. Conclusions 1. Introduction This report describes the outcome of an experiment regarding household waste. The objective was to determine the progress in temperature and odour. The waste was pressed in bales. The tests were conducted by VAROM B. V.

2. Objective The main objective was research towards temperature and odour progress.

3. Set-up experiment The following 3 tests have been carried out simultaneously: test 1: bale 100% household waste, shreddered, storage outside; test 2: bale 100% household waste, shreddered, storage inside, wrapped; test 3: bale 100% household waste, shreddered, storage inside.

Through a slowly turning shredder, the waste was reduced to a fraction < 400 mm. This fraction was directly pressed into big bales, without screening. Three bales were wrapped in order to cut outside air off. All bales were equipped with appliances to register the temperature deep within the bale and just below the surface of the bale.

Data Shredder : Doppstadt/Bison slowly turning Pressed wight per m3: 725 Kg waste composition : 42% paper, 13% synthetics (70% solid, 30% foil), 3t textile, 28% organic waste, 16% various Caloric value waste : 17.770 kJ/Kg 4. Progress experiment This temperature in the bale rises within a couple of days up to more than 50 C. The temperature just below the surface of the bale is 3 to 5 C higher. This temperature decreases slowly (about 10 C in 15 days).

The temperature in the wrapped bales increases and decreases quickly (within 24 hours from 52.7 C to 44.9 C).

It is likely that this is caused by the lack of oxygen as a result of the wrapping.

Odour observations were taken around the bales, both in and outside. Though it is hard to attach values to these observations, one could say that the odour diminished simultaneously with the decrease in temperature. After three weeks however, there was still some odour around the bales (temperature bale 42 C).

5. Conclusions a) Shredded household waste can be pressed into bale satisfactory. b) The heating within the bales decreases. Therefore, it is possible to keep bales in stock without risk of fire. c) Keeping a reasonable amount of unwrapped bales in stock will cause minor odour problems.

A shredder and debaler as manufactured by Vecoplan Gmbh of Germany and comprising twin rotors working at a speed of 165 rpm each, could be used. The shredder material is forced through a screen to achieve the desired size of 25 mm. Because of its method of operation, materials as described in the previous section are cut to the correct size. It is necessary to cut the baling wires before shredding to allow"leafing"of the material as it passes from the feed conveyor to the shredder, as the discharge is by means of a conveyor with magnetic drum. This wire does not have to be removed and therefore cutting can take place automatically.

Shredding tests were carried out on the Vecoplan Shredder Model VVZ300 using the material prepared by VAR (Varom) in Holland. a) The material was fed, shrink wrapped, into the shredder. This was found to be more difficult, as the bale was prevented from coming apart b) The resultant shredded material was found to be of a size between 0.5-10mm in 85% of the discharge. The larger shreds were of an acceptable size to the gasifier manufacturer, and were below 25 mm. c) The throughput of the shredder was put at up to 12 tonnes per hour d) The weight of the shredded material was found to be in the region of 300 kgsm3 when prepared for the drier.

Moisture content was 42%.

Conclusions. i) The bales must be fed by conveyor and the wires cut automatically before reaching the shredder. ii) The shred size could be increased giving longer life to the cutting edges of the shredder. iii) The throughput of the shredder being 10-12 tonnes per hour means that two shredders will be necessary (one working, one resting, on an 8 hour rotation). iv) The weight of the material means that no compaction of the material is necessary before the drier.

The fuel preparation plant, first stage incorporates: feed hopper feed conveyor bag opener feed conveyor sorting belt sorting cabin speed up conveyor star screen platform & walkways fines conveyor contraries conveyor glass conveyor glass shopper (optional) glass feed unit (optional) automatic glass colour sorting unit (optional) oversize sorting conveyor small fraction sorting conveyor medium fraction sorting conveyor Fe conveyor overband magnet x2 eddy current separator x 2 al conveyor feed conveyor feed conveyor shredder (primary) feed conveyor automatic baling press The fuel preparation plant-second stage incorporates : bale feed conveyor secondary shredder takeaway conveyor magnetic drum The operation of the plant is as follows.

The waste (feedstock) is loaded into the process plant by means. of a mechanical shovel into an"on floor"hopper.

Because of the design of this hopper, it is possible to clean in and around it on a regular basis, thus avoiding unsightly mess and odours. The design of the tipping floor and the layout of the equipment also lends itself to good housekeeping, whereby it can also be accessed easily and cleaned and disinfected. Lessons having been learned from the old design waste to energy plants where the feed method was by means of an overhead grab lifting waste from a deep pit, which, in honesty, was never cleaned, thereby creating odours, leechate and vermin infestation.

The waste (feedstock), having been loaded into the system, is then checked physically for contraries ie. rope, wire, long textiles (curtains) and white goods, in fact anything which will impair the operation of the process plant.

Material is then fed onto the Starscreen, with its easily adjusted variable speed motors which allows fraction size to be adjusted between a large range of sizes.

The discharge from the Starscreen is in four streams, thus allowing easy identification and therefore removal of the desired fractions, manually and by mechanical means (magnets and eddy currents).

Having removed the desired fractions, the waste (feedstock) or residue now requires to be shredded to conform with Gasifier Manufacturers specification.

All material (feedstock) must now be baled ready for storage. Because of the requirement for stocks to be held, this storage area must be able to hold up to 1500 tonnes of material after baling. The storage requirement is for approx. 1000 bales of lm3 each, ie. 1500 m3. A good discipline is required for the storage of the bales so that they are used on a rotation basis, ensuring that they do not stand for too long a period. Six bays are envisaged, one being filled and five available for stock. Once baled, the material (feedstock) can be stored for far longer than the requirement of the plant.

The bales, having been stored in a rotation system, are now ready to be fed into the gasification feed system. Because of the known weight of the bales, it is possible to have a fairly accurate feed to the system ie. 6-12 tones per hour or approximately 8 bales per hour. It is not possible to obtain a shredder of sufficient efficiency to reduce domestic MSW to the required particle size at the rate of 25-30 tonnes per hours ie. processing the waste (feedstock) within the 8 hour shift of the sorting and baling plant.

We therefore need to have a secondary shredder capable of shredding at the rate of 10-12 tonnes per hour, and also, because of the requirement of the plant, we need to have two machines to ensure non stop production and allow maintenance on a regular basis.

The bales are fed into the shredder where they are cut to the correct particle size and fed to the dryer. An option here is for the waste (feedstock) to bypass the baler on the"day"shift and be fed directly to the shredder for onward transmission to the drier. It is also possible to increase the bulk density by adding an auger feed to the drier, thereby compacting the feed material to the desire kgm3. After the shredder, a magnetic drum is located to collect the pieces of baling wire created by the action of shredding wired bales.

It is estimated that the full complement of staff at the plant would be 13 persons: loaders hovel driver 1 pre-sort cabin 2 sorting cabin 4 baler operator 1 fork lift/shredder 3 supervisor 1 weighbridge/admin 1 TOTAL 13 The total area required is a little less than 1 hectare (approx. 100,000 sq. ft).

Building height would be 7,500mm to eaves.

The site, which should be equipped with a weighbridge, comprises: car park-for staff & visitors vehicle turning area tipping hall process hall-fuel preparation fuel storage & gasification area The tipping hall and fuel storage & gasification area should be designed with a 1 metre ventilation gap around at least three sides, whilst the process hall should be solid walls all round. All buildings should be portal type (free span).

The site should be surrounded by, at least, a strong steel link fence of approx 2.4 metres in height and equipped with lighting to street standards. It should, ideally, be located on or near to a landfill for the following reasons: No increase in vehicular traffic Residents would be less objectionable Residues can be disposed of economically In the case of a major breakdown feedstock can be land filled without fuss.

Until a suitable site is identified, it is extremely difficult to assess the costs associated with land and building works. Clearly the exact location and the land quality will dictate the costs, particularly with regard to availability of utilities and infrastructure.

A portable weighbridge complete with computer and software will be sufficient for the needs of the site.

A mobile plant will consist of one loading shovel, one 360 grab and two forklift trucks complete with hydraulic bale clamps. The most efficient way to obtain these items would be on a contract hire basis. This would ensure swift attendance in the event of a breakdown and replacement where'necessary.

Claims (5)

1. A method of treating municipal waste to produce a useful fuel comprising the steps of: a) screening the waste into a series of fractions of different sizes and removing non-combustible material therefrom; b) shredding the screened waste to a first predetermined size; c) forming the shredded waste into bales for the purpose of storage, and d) shredding the baled waste to a second, smaller predetermined size, thereby to produce a homogeneous mix of combustible waste.

2. A method as claimed in claim 1 in which the first predetermined size is 250mm or less.

3. A method as claimed in claim 1 or claim 2 in which the second predetermined size is < 25mm.

4. A method of treating municipal waste substantially as described herein.

5. A homogeneous mix of combustible waste prepared according to the method of any one of claims 1 to 4.