GB2472599A

GB2472599A - Thermal treatment of waste

Info

Publication number: GB2472599A
Application number: GB0914029A
Authority: GB
Inventors: James Crankshaw; Paul Greenwell
Original assignee: Sterecycle Ltd
Current assignee: Sterecycle Ltd
Priority date: 2009-08-11
Filing date: 2009-08-11
Publication date: 2011-02-16
Also published as: GB0914029D0

Abstract

The invention relates to a process for the thermal treatment of waste or other materials (e.g. municipal solid waste) in a batch process in two or more autoclaves,wherein a first autoclave is filled with waste, and subjected to a cooking cycle at a temperature above 115 °C and a pressure of above 1 bargand wherein during the cooking cycle, gasses with steam is purged to a condenser or scrubber, and steam is injected to the autoclaveand wherein after the cooking cycle, steam is transferred from the first autoclave which is at the end of its cooking cycle to another autoclave which is at least partly filled and which is at or before the start of a cooking cycle.

Description

PROCESS FOR THERMAL TREATMENT OF WASTE OR OTHER MATERIALS IN A

BATCH PROCESS IN TWO OR MORE AUTOCLAVES

The invention relates to an apparatus and process for the thermal treatment of waste or other materials in a batch process in two or more autoclaves.

Waste, in particular municipal solid waste (MSW) and similar types of commercial waste, is an increasing burden on landfills and other forms of waste treatment. Part of the waste stream may be recycled through separated collection. Part of the landfill may be circumvented by incineration; however this is very costly and may have negative environmental impacts. One of the alternatives suggested is treating the waste in a vessel, which may be under pressure, with heat and/or steam while tumbling the content in order to breakdown the organic content and enable easier separation (henceforth the vessel', or autoclave). Several examples of this idea exists, such as disclosed in EP 0771237, US 4844351, US 6306248, US 6397492, US 4974781, WO 03/09299, WO 2004/018767, WO 03/025101, W02008/010970, W02008/081028 and US 2008202993.

The technology has interesting potential: The organic fibres can be used as clean biomass fuel or compost; the glass/grit fraction -if sufficiently clean -can be used either by separating the glass for uses in recycled glass products, or it can be used as recycled aggregates. The municipal waste does not have to be pre-selected or pre-treated, and is reduced to about 30 volume% of its original size. Further, when fibres are separated, and other useful materials are recycled, landfill can be limited to less than 10 volume% of the original waste.

The technology, however, has several challenges. One is that it can be highly energy-intensive. In many countries this adds significantly to the variable operating costs of the process, and to the capital costs as steam-raising plant of a greater capacity is required than is necessary when the following invention is used. Previous methods have been suggested for overcoming this challenge, the best of which is the use of two or more vessels. The operation of these vessels is synchronised but out of phase, so that the steam and heat from one vessel is used to pre-charge and heat a vessel that has just been filled with untreated waste. An example of this is described in U55540391 and W02008/081 028. The drawback to this method is that the energy savings actually are less than expected.

Another problem is the cycle time. Many tonnes of waste have to be treated, and a shorter cycle time per tonne of waste makes the process substantially more economical.

As for example described US5540391 and W02008/081028, inert gas preferably is withdrawn by applying a vacuum, and introducing steam before starting a cooking cycle; this can be done once or twice, depending on the amount of inert gasses allowed.

The present invention provides for a process for the thermal treatment of waste or other materials in a batch process allowing improvement in energy efficiency, and allowing improvement in cycle time.

The present invention provides a process for the thermal treatment of waste or other materials in a batch process in two or more autoclaves, * wherein a first autoclave is filled with waste, and subjected to a cooking cycle at a temperature above 115 °C and a pressure of above 1 barg * and wherein during the cooking cycle, gasses with steam are purged to a condenser or scrubber, and steam is injected to the autoclave * and wherein after the cooking cycle, steam is transferred from the first autoclave which is at the end of its cooking cycle to another autoclave which is at least partly filled and which is at or before the start of a cooking cycle.

The present inventors have found that a substantial amount of (non-steam) gasses is formed during the cooking cycle. These gasses may be methanol, ethanol, other VOCs, carbon-, nitrogen-or sulphur-oxides, and other gasses. Hence, steam swap from one to another vessel in the processes as described in US5540391 and W02008/081028 appeared to result in swapping "inert" gasses in stead of swapping steam. For example, a 90 vol% filled vessel of 60 m3 contains about 5-6 tonnes of steam-energy. In a process according to US5540391, in the first swap, only 1.5 tonne of useful steam is swapped, which is 25% less than what appears possible with applying the current invention. In further swaps the amount of useful steam decreased even further, till after about 5 swaps no useful energy was transferred at all. Hence, the present inventors realized that purging the autoclave during the cooking cycle, and introducing further fresh steam has a number of advantages with respect to steam swap and additionally shortening cycle time. Thus, by purging, the steam present in the autoclave at the end of the cooking cycle is relatively pure steam with a high energy content, and swapping this steam has a substantial benefit with respect to energy saving (more than 25% of the energy present in the autoclave at the end of a cooking cycle can be saved in this way). In a continuously operated plant, this has the further advantage, that each time, relatively pure steam with a known energy content is swapped, which is independent from the treated waste. Relatively pure is here meant to refer to the gas fraction. Of course, the steam that is transferred from one autoclave to another may contain small particles, debris and the like. The solids have little influence on the energy content of the swapped steam.

Furthermore, it appeared that applying vacuum to the autoclave before starting the cooking cycle has relatively low influence on the gasses formed during the cooking cycle. Therefore, it is preferred to take out the inert gasses that are initially present (mainly air) during the cooking cycle at elevated pressure as well, and not to apply a vacuum step after filling of the vessel but before initiating the elevated pressure cycle.

Nevertheless, the present invention also has benefits when a vacuum step is applied before the cooking cycle, because the steam swap is much more economical anyhow.

The present invention has the further advantage that the odour abatement process is much more reliable and the necessary apparatus can be substantially smaller than in the prior art. In the process of US5540391 or W02008/081 028, most VOC's (that cause possibly odour in the environment) are released within a few minutes together with (part of) the steam, either at the beginning, or at the end of a cycle. The gas and steam is first treated in a condenser/scrubber, and the inert gas that exits this condenser/scrubber is further treated to remove VOC's. With the present invention, it appeared that the peak amount of VOC's could be lowered to only 20% or even less. Thus, the use of a number of controlled purges or (semi)continuous purge of inertNOC gasses to the condenser/scrubber allows cleaning of the VOC's, and emitting the "inert" (non-scrubbed or non-condensed) gasses with a relatively low peak emission, allowing smaller apparatus and more secure operation.

Furthermore it appeared that the amount of fibre produced with the process according to the present invention was increased with about 10%. This means that even less waste needs to be landfilled.

Purging of an autoclave in treating MSW is described in US6397492 and US2008/020993. These references do not describe the steam swaps to another autoclave, nor any of the other advantages found by the present inventors.

Preferably, the steam transfer from one to another vessel is aided by one or more thermocompressors that cause the gas/steam stream to be transferred to be faster and more complete.

The present invention furthermore provides a process for the thermal treatment of waste in an autoclave at elevated pressure and humidity, wherein the one or more autoclaves are purged during the cooking cycle, and wherein steam is at least partly transferred from one or more autoclave vessels to an accumulator and wherein the one or more autoclave vessels are charged with steam from the accumulator. In this embodiment, the purged steam is temporarily stored in an accumulator, and thereafter transferred to another autoclave.

The present invention furthermore provides for an apparatus for the thermal treatment of waste in an autoclave at elevated pressure and humidity comprising one or more autoclaves and a steam accumulator in connection with both the steam exhaust pipes and steam inlet pipes of the autoclaves.

The process according to the present invention may be used in a process to treat waste in a variety of ways. An exemplary embodiment is described in more detail hereunder, without being limited thereto.

In a preferred embodiment of the technology according to the present invention, the process to treat waste and other materials comprises the following steps: (1) optionally, pre-treating material to improve its properties (2) feeding material into an autoclave (pressure vessel) (3) agitating the autoclave while creating a steam atmosphere at elevated pressure in the autoclave by applying steam (and optionally heat) to the contents of the autoclave, (cooking cycle) to change the properties of the contents (4) depressu rising the autoclave (5) discharging the autoclave (6) classifying the treated waste to yield several fractions, for example a fibrous fraction, a shattered glass and grit fraction, and a fraction with larger parts such as plastics and metals (7) optionally, further separation and/or improvement of the fractions, wherein (i) during step 3 at least part of the steam and gas atmosphere is purged to a condenser or scrubber, optionally via an accumulator or other steam and gas treating apparatus, and further steam is fed into the autoclave vessel (ii) during step 4, at least part of the steam is transferred to another vessel, which is in an initial phase of stage 3, optionally through a steam accumulator.

The process and apparatus of the invention are applicable to any enclosed [batch] vessel-based heat-or steam-treatment process for waste and other materials where it is desired to process and separate useful fractions of a mixture of substances.

This includes (i) methods using pressure from atmospheric up to steam pressures including and beyond 4 barg (ii) methods using one or more of direct steam, hot gas or water, hot jackets and internal heated features [i.e. flutes/helices] as a method of heating (iii) vessels where the waste is charged and discharged from the same or different ends (iv) methods where the contents are changed by chemical transformation (i.e. by a cooking process) or chemical treatment like adding reducing agents and (v) wastes and other material from municipal, industrial or other sources including but not limited to medical facilities, agricultural and food waste or products, the products of kerbside recycling schemes or biomass from forestry and horticulture.

The autoclave vessel preferably can be tipped along its axis, (in the art usually but not limited to +60° to -20° about the horizontal) and it may be tipped to an incline or decline to an optimum position for this part of the process at any stage, but usually after loading has finished. Such a position may distribute the material uniformly or concentrate it in one place or position it closer or further away from a steam or other inlet, or position it for some other purpose. A man skilled in the art will be able to find the optimal angle, which may be the same as the loading angle, or different.

The material inside the vessel preferably is agitated by rotation of the vessel, intermittently if required, as soon as loading has commenced. It is also possible to mix the content with a stirrer.

The apparatus preferably comprises a steam boiler, and may have a steam accumulator (for clean, fresh steam) which is sized to hold a substantial volume of steam. For instance, if using vessels which can process 20 tonnes of waste per batch an accumulator capacity of between 50 m3 and 150 m3 is preferred, more usually about m3. Hence, the accumulator is preferable about 1 times the volume or more of the autoclave it needs to exchange steam with, preferably about 2 times or more. For economic reasons, generally, the size will be about 8 times or less, preferably about 6 times or less.

The waste may be pretreated, The pretreatment step is optional, but may include selecting waste, such as for example taking out massive blocks of concrete, carpets and the like. Other pretreatment may include mixing several waste streams and/or pretreatment like adding water or chemicals to the waste, or applying size reduction to e.g. 10-40 cm pieces. However, -preferably -heavy commuting of the waste should be avoided, in order to preclude batteries and the like to become crushed, and thereby causing heavy metal contamination.

Feeding of the waste material to an autoclave can be performed as is common in the art, like for example by a conveyer from a hopper. Preferably, the amount of waste is weighed such as to be able to adjust the process to the amount of waste in the vessel.

The vessel can be about horizontal during charging. In case of a horizontal vessel, rotation of the vessel having helical fins or paddles that causes the waste to be transported over the length of the vessel. However, it is preferred to have the vessel tilted above the horizontal, such that the opening of the vessel is at the higher point by for example about 5° or more, preferably about 100 or more, and even more preferably about 15° or more, and even more preferably about 30° or more. Sometimes, in particular with smaller vessels, it may be possible to have the vessel in a vertical position with the opening upwards, but generally, the tilt will be about 700 or lower, and even more often about 60° or lower. An angle of between 45° and 70° is considered optimal, in particular if the vessel can be rotated.

Preferably, the vessel is rotated during loading, which causes the waste to flow more easily and quickly into the vessel, with less chance of sticking or bridging during filling. Bridging could give the premature appearance that the vessel may be fully loaded. The vessel is loaded with the material to be treated to generally about 50% or more, full, preferably about 65% or more full, but preferably about 90% full, and most preferably at least about 95% full. The vessel may be heated at any time during or after loading, via the addition of steam or another method including via steam from the accumulator, and it may also be topped up, to take on more or different materials, at any point in the cycle.

When the vessel is filled, the door of the vessel is closed to allow vacuum or pressure.

The content of the vessel is agitated, preferably by rotating the vessel while applying heat to the contents of vessel in a cooking cycle, meaning treating the waste at elevated temperature and pressure, to change the properties of the contents and the content generally is sterilized and cleaned. Wood and fibrous material are broken down to fibres; plastic is partly softened and balled, adhesives are broken down, such that a treated waste can be obtained that is relatively clean and easy to handle.

In one embodiment of the invention -which is preferred, because of shortened cycle time -, a vacuum is not applied before the cooking cycle, but steam is introduced, while increasing the pressure and venting the air out of the vessel at the same time.

In another embodiment of the invention, a vacuum of about -0.1 barg to up to - 0.9 barg, preferably about -0.4 to -0.8 barg is applied. This has the advantage of withdrawing air and a small amount of VOC's, which can be treated to preclude emission in the environment. Applying vacuum has the advantage that steam that is thereafter introduced is more effective because it is not diluted by air.

The content of the vessel is heated to create a steam atmosphere, and held at a temperature of 115°C or more, preferably 121 °C or more, preferably about 130°C or more, and generally about 180 °C or less, preferably 160 0 or less, and even more preferably 150 °C or less. This cooking step is performed usually while rotation and agitation is progressing, at the approximate temperature for from 20 to 90 minutes, more usually up to 60 minutes. Without venting, it appeared impossible to keep the temperature of the waste at the door side of the autoclave -in case this is opposite to the steam inlet and therefore indicative for the temperature of the content of the vessel -at a temperature of above 115 °C, despite the regular addition of steam at about 150 00.

Preferably dry, saturated steam of a temperature of about 170 °C or more is injected into the vessel, even more preferably of about 184 °C or more. For practical reasons, a temperature of about 240 00 or less is used, preferably a temperature of about 224 00 or less. The pressure of the steam may be about 8 bar (abs) or higher, or about 11 bar (abs) or higher. In practice, a pressure of about 25 bar (abs) or less is used, but if higher pressure steam would be available on a site, like for example 50 bar (abs) steam, this could be used as well. The high pressure steam may be reduced in pressure just before entering the autoclave.

Preferably, the time of high pressure steam injection to reach the vessel operating pressure of about 3-8 bar (abs) is about 30 mm or less, but can be about 20 mm or less, depending on the capacity of the steam boiler, (clean steam) accumulator, and cross section area of the pipes. Generally, the time will be 2 mm or more, and likely about 5 mm or more. Obviously, a shorter time has an advantage of shorter cycle times.

During the cooking cycle, and preferably at least in the initial phase of the cycle, gas and steam are purged towards the condenser/vacuum system. Any steam is condensed, and non-condensable gasses can be treated to make them harmless.

Purging the gas has as an advantage, that the temperature in the vessel remains high, which allows the cooking cycle to be shorter. Furthermore, the resulting energy in the autoclave can be effectively swapped to another autoclave in the form of steam swap; the odour abatement can be more effective and/or simple and the quantity of the fibre is higher.

Purging takes place throughout the cooking cycle and can be performed either in several individual vents or continuously throughout the cook cycle. If performed in discrete vents this is for example performed at 15%, 30%, 45%, 60%, 75% and 90% of the cook cycle but may be performed at any time. At each vent gases amounting to approximately 100% of the free volume of the autoclave are purged from the vessel and this is controlled by monitoring the pressure within the vessel.

Preferably, at least two times is vented, more preferably at least four times.

Generally, venting does not take place more than 10 times, as that amounts virtually to continuous purge. It has shown to be effective to have up to six or seven venting cycles.

If performed on a continuous basis the temperature of the purge stream is maintained at a temperature of approximately 10 degrees less than the inlet steam temperature by means of a modulating control valve located on the purge line outlet from the vessel. The control valve is modulated using a control system which receives inputs from temperature sensors located on both the steam inlet pipe and the purge outlet pipe. For example if the steam inlet temperature was 148 °C the purge outlet pipe would be controlled at between 135 and 148 °C. Control could be exerted by simply opening and shutting the steam outlet pipe, or by gradually opening or closing the valve in the outlet pipe. The latter method is preferred, as it allows continuous smooth operation in an easy manner.

If an amount of gas and steam is purged from the autoclave, it is necessary to supply fresh steam. It is preferred to supply the steam during the purge, but it may be supplied only after the purge.

Generally, the amount of steam used during the purge(s) is about 0.5 tonne or less, and generally between 10-300 kg.

In one embodiment of the invention, the process to treat municipal solid waste comprises the steps of, (3) agitating the vessel while having steam pressure in the vessel at about 2.5 bar abs or higher, and at about 8 bar abs or lower, and a temperature of about 127°C or higherand of about 170°C or lowerforat least about mm; wherein after step (2), steam at a temperature of about 165 °C or more and a pressure of about 5 bar (abs) or more, but wherein the pressure is at least 2 bar more than the required pressure during the cooking cycle, is injected into the vessel after closing to raise the pressure within the vessel to 2.5-8 bar (abs) in condensing atmosphere, and thereafter in step (3) preferably, for a suitable period continue to inject steam at a reduced flow rate to allow heating of the waste material by steam condensation, and thereby maintain the waste treatment conditions.

Preferably, the temperature in step 3 is about 160 °C or lower, preferably, the temperature is about 130 °C or higher. The pressure is preferably about 6 bar (abs) or lower in vessel in steps (3), and preferably about 3 bar (abs) or higher.

After treatment of the waste in the cooking cycle, at least part of the steam is transferred to another autoclave, which is loaded with waste that needs treatment. The transfer generally is effective till about 3 bar (abs), but may be even lower like for example 2 bar (abs), depending on cycle time requirements and the efficacy of the (optional but preferred) thermocompressor.

After treatment of the waste, and after the at least partial steam transfer, and during or after reducing the vessel pressure towards atmospheric pressure i.e. below 2.5 bar (abs), preferably below 2 bar (abs) and most likely below 1.5 bar (abs), a vacuum is applied to the vessel using a vacuum system, and a condenser or scrubber or combined condenser/scrubber to treat the extracted waste gas steam. The steam is condensed, and water soluble organic compounds are scrubbed out of the gas stream.

Remaining gases can be further treated in abatement techniques such as a biofilter, thermal oxidizer or a further scrubber. The vacuum can be applied while using an eductor, an ejector set or -preferably -a vacuum pump. An eductor is suitable for scrubbing the extracted waste stream and for applying vacuum.

It is not necessary that the autoclave is achieving a pressure below atmospheric while applying the vacuum system.

A vacuum system is defined in this invention as a mechanically driven pump, eductor, ejector set or other device that is designed to displace a gas or vapour and in so doing reduces the pressure in any vessel or equipment to which it is connected to atmospheric or below atmospheric pressure.

In one embodiment of the invention, it is preferred to achieve a vacuum of about 0.6 bar (abs) or lower, as better drying of the fibres is achieved, including further expansion and break down. Generally, the vacuum will be about 0.1 bar (abs) or higher, as a lower vacuum will take longer to achieve, and may be less economical.

Preferably, the vacuum applied in step (3) is about 0.5 bar (abs) or lower, and most preferably about 0.4 bar (abs) or lower. Generally, the vacuum can be about 0.2 bar (abs) or higher, and may be about 0.3 bar (abs) or higher.

An important advantage of using a vacuum system is that in depressurizing the vessel, time is gained (each batch will have a shorter cycle time in comparison to simply letting pressure off to atmospheric), and even more importantly, that when opening the vessel after the autoclave treatment, no live steam is emitted and vapours displaced during emptying the autoclave contain only extremely small concentrations of Volatile Organic Compounds. This can be further improved, by letting in fresh air, while still having the vacuum pump working. By cooling the contents by evacuation and air purging, even less steam will be emitted. Letting in fresh air in one embodiment is done after drying at reduced pressure has been effected for some time, but in another embodiment is done as soon as about atmospheric pressure is reached.. The latter process has the advantage that no additional control on below-atmospheric pressure is necessary.

Hence, it is not necessary to apply vacuum because with the purging of the present invention, the content of the autoclave is relatively dry (about 50 wt% water in the fibres even if no vacuum is applied).

The vapours extracted and/or purged can be condensed in a condenser/scrubber, and optionally recirculated in the system. The remaining "inert" gas stream needs to be cleaned as well, as it still contains odorous VOC's. Preferably, cleaning of the gas e.g. air, and/or steam purge is achieved in a scrubber, bio-filter, thermal oxidiser or the like. However any other suitable abatement technique may be used. Most preferred is a bio-filter.

A scrubber may be defined as an apparatus in which a contaminated gas flow is directly contacted with a liquid flow, within which part of the VOC components and other contaminants have solubility and where the liquid stream is at a lower temperature than the gas. Thus the liquid stream is able to dissolve part of the VOCs, condense steam, and capture other contaminants present in the gas stream. The scrubber can also be designed to capture a high proportion of the total contaminants in the gas stream via the use of a range of multi-stage contact internal designs.

Further optimisation with respect to environmental burden is achieved when the scrubbing liquid is recirculated. The gas may comprise steam, volatile organic compounds (VOC's) and dust. In the scrubber, steam is condensed, increasing the extraction rate of steam and/or air from the vessel. The scrubbing liquid can be further treated in a biofilter, thermal oxidizer, AD equipment, either in batch, but preferably as a purge stream.

Hence, in a preferred embodiment of the invention, the fluid to clean the exhaust gas is water. More preferably, the water is process water that is recycled in the system. This is an advantage, as much of the dust and optionally (part of the) volatile components may be kept in the waste treating cycles, and may be broken down in a next cycle. The process water can be used to wet the MSW material, and to produce steam within the vessel. There is no need to use boiler quality feed water for these purposes.

The scrubber of the present invention, depending on autoclave size, only needs a pump capacity of about 200 cubic metres or less per hour, and at a pressure of for example 2 bar gauge to be effective. The scrubber may be, for example, a spray condenser.

Two or more vessels are used in an alternating fashion, while using the heat and steam from the discharge of step (4) to apply heat and/or steam to another vessel in a step (3'). This is also an advantage for efficient use of the vacuum system, as one system comprising for example one pump/eductor or the like or set thereof can be used intermittently for the different vessels. Hence, only one system is needed for processing with two or more vessels. It is particularly preferred to have two or more vacuum pumps in the system as in case the one needs maintenance, it is still possible to operate the plant.

Thermocompressors may be used to enhance the speed at which steam is transferred or be used to increase the quantity of steam transferred from one autoclave to the other, thus enhancing the energy-saving potential of the invention. Other apparatus, such as one or more positive-displacement pumps, may also be used for this purpose.

Typically the thermocompressor is driven by steam at 20 barg and combines with steam from the exhausting vessel at a starting pressure of 3.5 barg (which can fall to as low as -0.7 barg towards the end of the exhaust cycle) to be fed into the other autoclave at between Sand 13 barg.

After any part in the cycle but particularly towards the end, the vessel is depressurised by venting some or all of the steam in the first autoclave into the other autoclave. Thereby, this heat energy is transferred to another autoclave which other autoclave is preferably at the start, or an initial stage of a cooking cycle.

The fully treated waste can be discharged. In case a tiltable vessel with one door is used, the vessel is tilted preferably to about -20° or lower. However, the method is also suitable in autoclaves with two doors and/or with fixed autoclaves.

Generally, the waste is further classified over screens, picking line, eddy current and magnet, destoner and/or other apparatus. Generally, several useful fractions are obtained from the treated waste. Examples of such fractions include, but are not limited to, a fibrous fraction, shattered glass and grit, and larger parts such as plastics (like PET bottles) and metals.

The fibrous fraction may consist for more than 95% of organic materials, and can be used as composting additive; fuel, biogasification or otherwise. The clean glass and grit fraction can be used as filling material for roads or otherwise.

The present invention furthermore provides a process for the thermal treatment of waste in an autoclave at elevated pressure and humidity, wherein the one or more autoclaves are purged during the cooking cycle, and wherein steam is at least partly transferred from one or more autoclave vessels to an accumulator and wherein the one or more autoclave vessels are charged with steam from the accumulator.

The steam transfer from and to the vessels to and from the accumulator preferably is aided by one or more thermocompressors that cause the gas/steam stream to be transferred faster and more completely.

The apparatus and process according to the further invention allows for substantial energy savings to be gained when using even one autoclave, although it is preferably used with two or more autoclave vessels. If used with more vessels, the accumulator negates the need for any particular phase difference to be maintained between vessels and thereby effectively increases the plant output and/or energy savings.

In order to increase flexibility further, preferably the piping is such that the boiler steam can be added directly to the autoclave vessels and/or to the accumulator.

As explained above, it appears that during the cooking cycles of municipal and other waste, wood and other materials, quite some volatile organic compounds develop. These VOC's cause the steam swaps from one vessel to another to be substantially less efficient and to cause a decrease in temperature of the autoclave, which causes the cooking cycle to prolong. The use of an accumulator allows the controlled purge of inertNOC gasses to the condenser, allowing cleaning of the VOC's, and emitting the inerts to effectively increase the effective steam supply to the autoclave(s).

The use of an accumulator makes it is also possible to clean the steam to a sufficient extent from solids, foam, strongly acidic or basic components and the like.

Thus, the accumulator with steam from the autoclave vessels use the energy to the extent as full as possible, with a quality control, and measures to correct.

Thus, a further advantage of using the accumulator is that by increasing the amount of steam that is reused between vessels it also prevents, if required, the release of volatile compounds produced during treatment. This reduces odorous emissions from the process and allows their release into odour abatement apparatus in a controlled manner, thus reducing the size, capacity, capital and operating costs of odour abatement apparatus required.

These benefits are achieved with as little as a single vessel it allows these savings to be achieved with as small a plant as is desired. Further, by allowing these benefits to be achieved without synchronisation of autoclave vessels it allows for natural or planned variation in phase and cycle times between vessels.

The accumulator is a pressure vessel which can withstand working pressures of at least about 10 barg or more, preferably about 15 barg or more, and even more preferable about 20 barg or more. Generally, the working pressure is about 30 barg or less, more preferably about 24 barg or less.

The accumulator is sized to hold a substantial volume of steam and water. For instance, if using vessels which can process 20 tonnes of waste per batch an accumulator capacity of between 50 m3 and 150 m3 is preferred, more usually about m3. Hence, the accumulator is preferable about 2 times the volume or more of the autoclave it needs to exchange steam with, preferably about 4 times or more. For economic reasons, generally, the size will be about 8 times or less, preferably about 6 times or less.

The accumulator is connected, via valves and steam-carrying conduits, to both the steam inputs and steam exhausts of the one or more vessels in the plant in such a way that it can receive fluid in the form of water and/or steam from as many vessels as desired and transmit fluid in the form of water and/or steam to as many vessels as desired. The accumulator is also connected to receive steam and water as required from a steam boiler or other apparatus. The use of an accumulator does not preclude the direct transfer of steam between vessels if desired.

The transfer of fluid between the accumulator and any of the vessels may be aided if required via the use of one or more thermocompressors.

Before discharge to another vessel the accumulator, via the addition of steam from vessels and other heat sources, reaches a pressure of up to 17 barg (207°C). On discharge to an autoclave the accumulator pressure drops rapidly to the desired pressure depending upon downstream equipment, which is typically between 5 and 8 barg.

The accumulator is designed to optimise the efficiency gained, and to function reliably given that much of the steam and fluid it uses contains entrained volatile compounds and particulates. As such the accumulator is fitted internally and externally with conduits, profiles, access hatches and other equipment so that any or all of the following are possible: (i) when steam enters the accumulator it mixes very well with, and transfers as much of its [latent] heat as possible to, the liquid in the accumulator (ii) the temperature of the accumulator and of its contents may be varied independently of the temperature of the inputs (iii) chemicals to treat the contents may be injected (iv) sludge and particulates may be filtered and removed from the accumulator, more usually from the bottom (v) scum and floating particulates may be removed from the surface of the water in the accumulator (vi) foam or scum is not present in the steam charging the vessels unless desired (vii) ready access to the accumulator is possible for cleaning.

The above description has been focussed on the treatment of municipal solid waste, but the process is equally applicable to other processes where compactable material is treated under elevated pressure and temperature. The process is in particular suitable for treatment of medical waste, because medical waste is often delivered in containers with relatively large free space.

Other types of compacting materials are waste from slaughterhouses, supermarkets, and waste from landfills that need to be sanitized.

The invention will be elucidated with the following non limiting example.

Examples 1 and 2, and comparative experiment A Two autoclaves of 60 m3 volume were operated in treating municipal waste which was introduced in the vessels alternating. Steam of 10 barg was reduced to 4 barg just before entrance in the vessel. The steam was at that moment 150 00. In comparative experiment A, no vent was applied, but the steam pressure was kept -by regularly introducing steam -at about 4 barg. During a two hour cooking cycle, it appeared that the temperature measured at the opposite end of the vessel decreased from about 145 00 to as low as about 110 00 within half to three quarter of an hour. The amount of fibre was about 55% of the waste.

The steam-containing gas present in the autoclave at the end of the cooking cycle could be transferred to another vessel, but the energy content was about 25% less than expected for full steam swap. After 5 steam swaps, virtually no useful energy was transferred.

In contrast, when regularly purging (like 6 times, between 15-90% of the cycle) the free-volume of the vessel (about 30 m3) the temperature of the steam at the opposite end of the vessel remained above 130 00 Furthermore, the steam released during the second half of the cooking cycle contained a relatively high energy content, and upon releasing the steam at the end of the cooking cycle, the steam could be effectively be transferred to the other autoclave for (pre)heating the content of the vessel. The amount of fibre obtained from the content of the waste was 60%. The peak-VOC in the odour abatement apparatus was about 115th of the amount of comparative experiment A. If a further example, "continuous" purge was executed. The temperature at the end of the vessel was measured, and if the temperature was less than 10 00 below the set temperature of 145 00, gas was purged, and fresh steam was introduced till the temperature was at the set point. The peak amount of VOC's in the odour abatement apparatus was less than 1110th of the amount of comparative experiment A. The transfer of steam was in particularly effective when this was enhanced with the use of a thermocompressor. In an analogous way, it was easily possible to achieve an effective treatment temperature of 140 to 148 00 while introducing steam of about 15500

Claims

CLAIMS1. Process for the thermal treatment of waste or other materials in a batch process in two or more autoclaves, * wherein a first autoclave is filled with waste, and subjected to a cooking cycle at a temperature above 115 °C and a pressure of above 1 barg * and wherein during the cooking cycle, gasses with steam is purged to a condenser or scrubber, and steam is injected to the autoclave * and wherein after the cooking cycle, steam is transferred from the first autoclave which is at the end of its cooking cycle to another autoclave which is at least partly filled and which is at or before the start of a cooking cycle.
2. Process according to claim 1, wherein steam transfer from one autoclave to another autoclave is aided by one or more thermocompressors.
3. Process according to any one of claims 1-2, wherein the temperature during the cooking cycle is about 121 °C or higher, preferably 130 °C or higher, preferably 140°C or higher.
4. Process according to any one of claims 1-3, wherein the autoclave is initially filled for about 75 vol% or more, preferably about 90 vol% or more
5. Process according to any one of claims 1-4, wherein about 25% or more of the energy content is swapped from the first to the second vessel.
6. Process according to any one of claims 1-5, wherein the process comprises the following steps: (1) optionally, pre-treating material to improve its properties (2) feeding material into an autoclave (pressure vessel) (3) agitating the autoclave while creating a steam atmosphere at elevated pressure in the autoclave by applying steam (and optionally heat) to the contents of the autoclave, (cooking cycle) to change the properties of the contents (4) cooling and/or depressurising the autoclave (5) discharging the autoclave (6) classifying the treated waste to yield several fractions, for example a fibrous fraction, a shattered glass and grit fraction, and a fraction with larger parts such as plastics and metals (7) optionally, further separation and/or improvement of the fractions, wherein (i) during step 3 at least part of the steam and gas atmosphere is purged to a condenser, and further steam is fed into an autoclave vessel (ii) during step 4, at least part of the steam is transferred to another vessel, which is in an initial phase of stage 3.
7. Process according to any one of claims 1-6, wherein purging is effected for at least four times during a cooking cycle.
8. Process according to any one of claims 1-6, wherein purging is effected (semi) continuously.
9. Process according to any one of claims 1-8, wherein gasses from the autoclave are purged to a scrubber or condenser to lower the content of VOC's and/or inerts in the gaseous phase.
10. Process according to claim 9, wherein the gasses emitted from the condenser/scrubber are further treated in a biofilter.
11. Process to treat municipal solid waste according to any one of claims 5-10, which comprises the steps of, (3) agitating the vessel while having steam pressure in the vessel at about 2.5 bar abs or higher, and at about 8 bar abs or lower, and a temperature of about 127 °C or higher and of about 170°C or lower for at least about 15 mm; wherein after step (2), steam at a temperature of about 165 °C or more and a pressure of about 5 bar (abs) or more, but wherein the pressure is at least 2 bar or more than the required pressure during the cooking cycle, is injected into the vessel after closing to raise the pressure within the vessel to 2.5-8 bar (abs) in condensing atmosphere, and thereafter in step (3) preferably, for a suitable period continue to inject steam at a reduced flow rate to allow heating of the waste material by steam condensation, and thereby maintain the waste treatment conditions.
12. Process for the thermal treatment of waste in an autoclave at elevated pressure and humidity, wherein steam is transferred from one or more autoclave vessels to an accumulator and wherein the one or more autoclave vessels are charged with steam from the accumulator.
13. Process according to claim 1, wherein steam transfer from the autoclave vessels to the accumulator is aided by one or more thermocompressors.
14. Process to treat municipal solid waste according to any one of claims 1-11, wherein an accumulator is used in accordance with any one of claims 12-13.