WO2012172329A2

WO2012172329A2 - Apparatus and process for treating waste

Info

Publication number: WO2012172329A2
Application number: PCT/GB2012/051335
Authority: WO
Inventors: Ian Cecil Toll
Original assignee: Aerothermal Group Limited
Priority date: 2011-06-17
Filing date: 2012-06-13
Publication date: 2012-12-20
Also published as: GB201519318D0; GB2492070B; GB2528611B; GB2528611A; WO2012172329A3; GB201110259D0; GB2492070A

Abstract

Municipal solid or other waste is treated in apparatus comprising an autoclave treatment plant for steam treating the solid waste and a supercritical oxidation reactor downstream of the autoclave treatment apparatus for converting a product stream to heat, water and carbon dioxide. The product stream may be an organic-rich stream form the autoclaved material or may be sludge from an anaerobic digestate of the organic- rich stream. In either case, disposal of sludge in landfill or by incineration is avoided.

Description

APPARATUS AND PROCESS FOR TREATING WASTE

FIELD OF THE INVENTION

This invention relates to apparatus for the treatment of solid waste and to the treatment of solid waste in the said apparatus.

BACKGROUND TO THE INVENTION

Our International application PCT/GB2011/050145 (the contents of which are incorporated herein by reference) discloses a method for treating solid waste, comprising steam autoclaving the waste, anaerobically digesting an organic-rich fraction of the autoclaved waste, recovering methane-containing gas from anaerobic digestion, internally combusting the methane-containing gas to generate power and exhaust gas, and generating steam for autoclaving using the exhaust gas. It further discloses a plant for treating solid waste, comprising at least one autoclave for steam treating the waste, at least one anaerobic digestion tank for digesting an organic-rich fraction of the autoclaved waste, a recovery system for recovering methane-containing gas from the or each digestion tank, at least one internal combustion engine for combusting the methane-containing gas and generating power, and a steam generator fed with combustion gas from the internal combustion engine for generating and accumulating steam for supply to said at least one autoclave.

Disposal of the results of autoclaving and/or anaerobic digestion is a problem. The most straightforward way of disposing of the sludge from anaerobic digestion is to spread it over the ground, but digestate of this kind cannot be spread over agricultural ground and disposal in landfill is expensive. It could alternatively be dewatered and incinerated, but again incineration is not readily accepted and the resulting ash is treated as hazardous landfill. There is presently no easy and economical solution to the digestate disposal problem, and the technical director of a major waste disposal company has said that the biggest problem related to the autoclaving/anaerobic digestion approach is what to do with the resulting digestate.

AEROTHERMAL, 008 -PCT SUMMARY OF THE INVENTION

In some embodiments the invention provides apparatus for the treatment of solid waste comprising: autoclave treatment apparatus for steam treating the solid waste; and a supercritical water oxidation reactor downstream of the autoclave treatment apparatus for converting a product stream to water and carbon dioxide.

In embodiments the above apparatus may comprise screening apparatus for separating a product stream from the autoclave treatment apparatus into a stream of organic-rich aqueous material and a reject stream of mechanically separable solids and the supercritical water oxidation reactor is arranged to receive the organic-rich aqueous stream and oxidise the organic material of said stream to water and carbon dioxide.

In other embodiments the above apparatus comprises screening apparatus for separating a product stream from the autoclave treatment apparatus into a stream of organic-rich aqueous material and a reject stream of mechanically separable solids and at least one anaerobic digestion tank for digesting the organic-rich aqueous material, wherein the supercritical water oxidation reactor is arranged to receive and oxidise sludge from the anaerobic digestion to water and carbon dioxide.

The invention further comprises a process for treating solid waste which comprises processing the waste in a plant as described above. BRIEF DESCRIPTION OF THE DRAWINGS

How the invention may be put into effect will now be described, by way of example only, with reference to the accompanying drawings, in which like parts are denoted by like reference numbers, and:

Fig 1 shows apparatus for the treatment of solid waste including autoclave treatment apparatus and a supercritical water oxidation reactor downstream of the autoclave treatment apparatus for converting an autoclave product stream to carbon dioxide and water;

Fig 2 shows apparatus for the treatment of solid waste including an autoclave, an anaerobic digestion plant for digesting a product stream of autoclaved material and a supercritical water oxidation reactor downstream of the autoclave treatment apparatus for converting sludge from the anaerobic digester to carbon dioxide and water; Fig. 3 is a simplified oblique view from a lower end thereof of an autoclave and support structure, upper and lower doors being shown in their closed positions;

Fig. 4 is an oblique view of the autoclave of Fig 3 from its upper end, an upper door being shown in its open position;

Fig. 5 is a slightly oblique side view of the autoclave showing the lower door in its open position; and

Fig 6 is a further side view of the autoclave with both upper and lower doors open and with the autoclave viewed in longitudinal vertical section to reveal its internal flights.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Solid waste Wastes which may be treated by the method and apparatus of the invention include including but are not limited to municipal solid waste (MSW). Suitable waste will be normally classified as non-hazardous and non-toxic and may be at least in part biodegradable or may be wholly biodegradable. Its composition may depend on the extent of pre-sorting demanded by a municipality. It may include household waste or sorted fractions of household waste, catering waste (including waste from restaurants or other catering facilities), biodegradable supermarket waste, paper and biodegradable plastics waste, partly or wholly biodegradable commercial waste or mixtures thereof. It may include food and kitchen waste and paper or other organic materials waste and as a component non-biodegradable recyclable waste e.g. plastics, glass or a mixture thereof. It may also include specialised wastes such as animal and fish-based waste e.g. slaughterhouse waste, shellfish waste, poultry product waste and supermarket food waste.

The systems of Figs 1 and 2.

Fig. 1 shows incoming waste 10 arriving at 12 at reception hall 14 leading to an autoclave and screening plant 24. A recyclables stream 16 passes together with a stream recyclable discharge 20. Potable water 26 is provided for autoclave treatment of the waste as described below. A reject stream 28 from the autoclaves passes to landfill 22.

A screened organic fraction 32 from the autoclave plant 24 passes via slurry mixing tank 32 as stream 34 to dewatering plant 36 where it is mixed with flocculating agent 38. An aqueous stream 40 from the dewatering plant passes to water treatment plant 42, with aqueous stream 44 being recycled to the autoclave and screening plant 24 and surplus effluent passing to supercritical water oxidation reactor 54. Dewatered solids 48 pass to cake storage tank 50 and thence as stream 52 to the supercritical water oxidation reactor 54, where it is reacted with oxygen and converted to excess water stream 56, carbon dioxide stream 50 (the carbon dioxide being of value and optionally recovered) and steam stream 60 which is fed to turbine 62. Electrical power from turbine 62 passes at 64 to the grid, and spent steam is recycled as stream 66 to the autoclave and screening plant 24.

The apparatus of Fig 2 is similar except that organic-rich stream 80 passes from the plant 24 to anaerobic digesters 82. The resulting methane-containing gas passes as stream 84 to generators 86, steam boiler 90 and as exhaust stream 92 to atmosphere. Process steam from boiler 90 passes as stream 92 to the autoclave and screening plant 24. Digestate from the anaerobic digesters passes as stream 96 to digestate storage tank(s) 98 and thence as stream 100 to dewatering apparatus 23. Power to the grid comes both from generators 86 as 102 and from turbine 62 as 64.

The supercritical oxidation plant 54 may use air or oxygen as oxidant and additionally produces an inorganic solids-rich stream which may be centrifuged for removal of any residual inorganic products of the destruction of the organic material from the autoclaves or form anaerobic digestion. It is not excluded that the supercritical oxidiser may be configured to operate just below supercritical temperature e.g. intermittently.

Autoclaves It has long been proposed to treat solid waste including municipal solid waste by means of an autoclave, although commercial embodiments have been few. US-A-4540495 (Holloway, 1985, the disclosure of which is incorporated herein by reference) is concerned with a process for the treatment of municipal solid waste (MSW). It discloses that the waste comprises inorganic, organic and synthetic fractions. The major portion of the inorganic fraction is said to be metal and glass containers, ceramics, masonry, building materials and the like. The organic fraction which is stated to comprise 80 wt% of MSW consists of lignocellulose e.g. paper products together with yard (garden) waste and food waste. The synthetic fraction comprises plastics containers, plastics film and other synthetic plastics products. The organic fraction is said to represent the industrial world's largest economically accessible source of lignocellulose feedstock for conversion into alcohol and other industrial chemicals. It is further explained that MSW is an environmental concern owing to the dwindling availability of landfill sites. A treatment process is disclosed in which MSW is fed into a pressure vessel, subjected to heat at 132-160°C (270-320°F) under a pressure of from 276-517 kPa (40 to 75 psi) for 30-90 minutes with introduction of steam to give a residual moisture content of 60-70%, discharged and classified to give an organic fraction as fines with moisture content 60-70%.

US-A-4884351 (Holloway) discloses an autoclave for the handling of municipal solid waste which is in the form of a cylindrical vessel inclined at about 15° to the horizontal and having frustoconical ends each closed by a hinged hatch. The hatch at the higher end serves as inlet for the waste to be processed and that at the lower end serves as an outlet for processed waste. The autoclave is supported for rotation about its longitudinal axis and has internal flighting angled at about 30° to its axis by which in a forward rotation mode the fighting directs material to the lower end of the autoclave during filling and/or discharge and in a reverse rotation mode material being processed is conveyed upwardly and axially towards the higher end and is mixed and agitated, reverse rotation being during processing of the material. Heating is by introduction of saturated steam via an inlet on the axis of the vessel and at the upper end thereof, the processing temperature being 48-108°C (120-228°F) preferably 88-102°C (190-215°F) to rupture bags of plastics film but to leave low density plastics materials substantially intact so that they are easily identifiable and separable from other components of the waste. US-A-4974781 (Placzek) is similar and has as its object the re-pulping of re- pulpable waste material, the water content of the waste typically being 50 wt%. Waste and water is added to a rotary autoclave or so-called "trommel" to give a moisture content of at least 30% of the moisture absorptive components of the waste, 65-75% moisture content being considered an optimum. A working temperature of 100 - 115°C (212-240°F) is considered best for plastics recovery and 115-149°C is considered best for re-pulping. The autoclave which in use is downwardly inclined at an angle of 4° is provided with lifting blades and directional flighting, a waste inlet at its upper end and a waste outlet at its lower end. The inlet and outlet each have a closure device in the form of a sliding gate valve which is movable axially towards or away from the inlet or the outlet. Steam and water can pass into the autoclave from its lower end via injection piping that extends into and rotates with the autoclave, the piping being connected to a rotary seal on the axis of rotation of the autoclave adjacent the discharge end.

In embodiments of the invention disclosed in International application PCT/GB2011/050145 (the contents of which are incorporated herein by reference), running reliability of a rotary autoclave for MSW can be improved and the range of materials that can be effectively treated is improved by employing an autoclave having a fixed downwardly facing attitude and injecting steam through a port in a bottom discharge door of the autoclave. In particular a fixed attitude facilitates making the autoclave body or tunnel of material of adequate thickness not only to resist internal steam pressure but also to continue to do so if there is corrosion or erosion as a result of processing wet loads of MSW. For example in a commercial-scale autoclave of diameter e.g. 3-5 metres and length 10-20 metres the autoclave body or tunnel may be formed of steel plate of significantly greater than the 9 mm steel plate as in other proposals e.g. 12-25 mm, the precise thickness depending e.g. on the dimensions of the autoclave or autoclaves proposed to be used. The autoclave may face forwardly and downwardly at an angle of 5-20°, e.g. 10-15°, conveniently about 15°.

The door may be hinged to a support frame of said autoclave for rotational movement between one position in which a discharge opening of the autoclave is revealed and another position in which the discharge opening is closed. Advantageously the door carries a rotary coupling for receiving steam from a supply pipe as the autoclave is rotated. A plenum chamber for steam in may be provided said door. Steam may be injected into the interior of the autoclave through a plurality of one-way devices providing parallel paths from the plenum chamber into the interior of the autoclave, thereby facilitating steam injection without undue pressure drop across the devices. For that purpose the cross-sectional area of the path or paths from the plenum chamber into the autoclave defined by said at least one one-way device may be equal to or greater than the area of an inlet for injected steam into the plenum chamber. Injecting the steam into the autoclave may be through at least one porous sintered metal disc leading from the plenum chamber into the autoclave or it may be through at least one mushroom or poppet valve or other one-way valve leading from the plenum chamber into the autoclave. The autoclave may also have an inlet door for waste at its upper end, and an axially located inlet in said door for water to be sprayed into the autoclave to condense steam therein. Water and steam leaving the plenum chamber pass directly into the internal space of the autoclave, and not through distribution pipes extending along that space. The door may be supported for hinged movement between open positions and a position spaced from and axially aligned with the discharge opening and is supported for translational movement between the spaced axially aligned position and the position in which the discharge opening is covered.

The method of treatment of the solid waste may include injecting steam from a steam accumulator having a capacity for a body of steam at a temperature and pressure effective to heat and fully penetrate the load and may also include injecting recycled steam from a second autoclave which has substantially completed its treatment cycle.

In an embodiment the autoclave has generally helical internal flights, and it is rotated during steam injection in a direction such that the flights lift the waste from the discharge end into the body of the autoclave. Process control may include monitoring load at upper and lower ends of the autoclave while the flights are lifting the waste from the lower end, equalization of the load at the upper and lower ends compared to the loads at the end of waste introduction indicating that lifting is taking place. Process control may further include monitoring pressure at upper and lower ends of the autoclave, substantial equality of pressure indicating that the steam has fully penetrated the load. In embodiments of the present process the processing time is considered to have started when the load has become fully penetrated by the steam/.In a further feature liquid water is introduced into the autoclave as the load is introduced, the water advantageously being near boiling and introduced in an amount of 25-100% based on the weight of the introduced load, e.g. 25-50 wt% based on the weight of the introduced load. A yet further feature comprises spraying water into the autoclave after steam injection and completion of the processing cycle in order to bring about steam condensation, the amount of water sprayed into the autoclave typically being 25-50wt% of the weight of the waste at the start of processing.

The present system in some embodiments uses an inclined tunnel-shaped rotating-drum autoclave that has an internal Archimedes screw welded to the vessel. This is rotated in one direction during loading to facilitate the loading of the autoclave, and rotated in the other direction during operation to break up the waste and ensure that the load is evenly processed. Once the vessel is fully loaded, all the air is extracted to create a vacuum. This vacuum bursts open any packaging or unopened containers and also helps to ensure that, when the steam is let into the vessel, it completely penetrates the load. When the chamber has reached its optimal operating conditions (160°C and several atmospheres pressure), the mixture is allowed to cook for about 40 min.

In embodiments three types of autoclave (all scaled from the same basic design) may be supplied in pairs to allow the steam to be recycled from one autoclave to the other to save energy. A relatively small autoclave has in an embodiment a seven-tonne capacity and is primarily aimed at processing food waste. 15-Tonne and 30-tonne vessels are suitable for local-authorities and large scale treatment of municipal standard waste. A pair of the 30-tonne autoclaves can process around 600 tonnes a day (200,000 tonnes a year), which equates to the waste disposal needs of about 400,000 people. Based on a lOOktpa plant and recognised prices for the components of a standard tonne of waste from the borough of Tower Hamlets in London, this will produce annually over £3.5 million worth of fibrous floe, plus over £1 million worth of recyclable material, and generate £6 million of gate fees for a commercial operator (or save the same amount for a local authority). If the organic matter, including the cellulose floe, is instead processed in anaerobic digesters and used to produce electricity, this will generate an additional £2.5 million worth of 'green' electricity and cover all the heat and energy needs of a plant.

A pilot-scale autoclave for demonstrating the construction and operation of the autoclaves 10, 12 is shown in Fig 3. The autoclave 10 has a cylindrical body sloping downwards as shown at about 15° and having a central cylindrical region 210 bounded at its upper and lower ends by welded-on lower and upper support rings having cylindrical side surfaces 212, 216 and lower side surfaces 214, 218. On the further sides of the support rings the body has lower and upper tapered e.g. frustoconical or dished regions 220, 222 which are removably closed by the lower and upper doors 14, 16. The autoclave is supported in a fixed attitude relative to the horizontal in a framework having first and second sides 224, 226 joined by cross-members e.g. 228, 234. At its lower the autoclave body is supported for rotation in the framework by support wheels 230 carried by cross-members 228 which run on the side surface 212 of the lower support ring and by thrust rollers 232 which run on the lower side surface 214 of the lower support ring and provide a reaction for the sideways component of the load of the autoclave body and its contents (i.e. load in a direction longitudinally of the autoclave body). At its upper end the autoclave body is supported for rotation by support wheels 234 which run on the side surface 216 of the upper support ring. Drive motor 238 carried by the frame is operable to rotate the autoclave body in either direction via drive chain or belt 240 and driven wheel 242.

The pivot mechanism for lower door 14 is as follows. At a location spaced upwards from the axis of the autoclave the support frame has fixing brackets 244, 246 for hinge pin 246 which carries hinge sleeve 248. The door 14 is attached to the sleeve 248 by arm 250 and is balanced by counterweights 252, 254. Fluid delivery line 256 passes along arm 250 to pressure-tight rotary pipe coupling 258 where the radially incoming steam or water is supplied to the door 14 through which it passes axially inwards and upwards into the autoclave. Flow through line 256 is controlled by valve 260, and there is an end coupling for steam and water supply pipes. The upper door 16 is similarly supported by brackets 262, 264 on the frame that support hinge pin 266 and hinge sleeve 268. Similarly to the door 14, the door 16 is mounted to the hinge sleeve by arm 270 and is counter-weighted by weights 272, 274, a steam and water supply line 277 leading to control valve 276 and then to connector 278 which is visible in this view and which provides a connection to steam and water supply lines.

Fig. 4 is an oblique view of the autoclave from its upper end with the door 16 in its open position to reveal waste inlet 243. Drive wheel 239 on the shaft of motor 238 is also apparent. A safety plate 245 of metal or plastic covers the motor and drive belt 240 to reduce the risk of injury to operators of the autoclave. In Fig 5 the lower door 14 is shown in its open position for discharge of treated waste. In Fig. 6 the autoclave is shown in side view in longitudinal vertical section to reveal single start or two start internal helical flights 280 thereof defining an Archimedean screw, the doors 14, 16 being shown in their open positions.

At the start of processing upper door 16 is opened and a conveyor for MSW material is introduced into upper opening 243, this occupying some 2 minutes of processing time. The autoclave is then rotated in a forward direction to permit loading to take place, the internal flights 280 forwarding the load towards the lower door 14. At the same time any water needed at the start of processing may be introduced through the upper door 16. If the load comprises food waste only, it may not be necessary to add water since the putrescible content of food waste already has a water content of circa 80%. If paper or card is present in the load, then water is desirably added to prevent undue densification of the load that would interfere with subsequent processing. The amount of water added will depend on cellulosic content and should be in an amount that is effective to maintain mobility of the load during subsequent processing and to soften the lignin content of the load. It may comprise 25wt% based on the weight of the MSW, more usually about 50 wt% and if the cellulosic content is high 100% or above, the 50 wt% figure being typical. The load volume at initial filling should be <75% of the internal volume of the autoclave.

On completion of loading, door 16 is closed and the pressure in the autoclave is reduced using a vacuum pump to remove air and volatiles from the autoclave, the discharged gas being filtered/deodorised by means of individual filters/chemi-scrubbers and/or carbon filters before being vented to atmosphere. During venting the autoclave is rotated in the reverse direction so that the load is continuously circulated towards the upper door 16 and then returns under gravity. Support rollers 230 and 236 include strain-gauge based load cells by which the load in the autoclave at various stages can be checked. These load cells, in particular are employed during this stage and during subsequent hot processing of the load to check for a relatively even load distribution between upper and lower parts of the autoclave, showing that the load has not remained compacted at the lower end of the autoclave. On completion of the vacuum pre-treatment stage which may last about 15 minutes, steam and optionally further water are introduced through door 14 to raise the internal temperature of the autoclave e.g. to about 160° and the pressure to about 6 bar. Pressurization of the autoclave may take some minutes, substantial quantities of the introduced steam condensing in the initially cold load as indicated above to increase the water content thereof. Circulation of the load through the autoclave by reverse rotation is continued, and even load distribution continues to be monitored to check that the load has not compacted and remains at the bottom of the autoclave. Penetration of the steam into and through the load is gradual, and pressure is monitored at both ends of the autoclave, rise of pressure at the upper end of the autoclave to or close to the rated processing temperature ~160°C indicating that the pressurization step is complete. By introducing steam from the lower end and monitoring pressure (or temperature) at the upper end of the autoclave, it is possible to ensure that the whole of the load has been penetrated by the steam. Processing at the working temperature and pressure is then carried out for a period of time effective to break down the load and in particular any paper and cellulosic content of the load and water being added from below or above the load via door 14 and/or 16 as desired. It will be appreciated that the load volume material shrinks substantially during processing as plastics items are softened and board structures collapse but the mass density is increased.

On completion of the processing step the autoclave is abruptly de-pressurised and water is injected through the upper door 16 and sprayed into the interior of the autoclave to collapse the steam in the load and avoid a steam plume. Abrupt de- pressurising is advantageous since it disrupts any residual cell structure in the load material and makes the load contents more accessible to the microbes in the subsequent anaerobic digestion step. As previously noted, a considerable volume of water may need to be added for this purpose, this being possible because of the load shrinkage during the thermal processing step, and the volume of added water typically being -50 wt% of the mass of the waste being treated. De-pressurisation may take 10 minutes. In a dual autoclave installation, the steam from the working autoclave will, of course, be recycled to the start-up autoclave as previously described. The autoclave is again subjected to vacuum treatment, this stage lasting for some minutes. The direction of rotation of the autoclave is then again reversed, the lower door 14 is opened and the load is discharged, some minutes being allowed for this operation. It will be appreciated that the load has now been diluted with large amounts of water so that at the end of processing the combined collapsed load and added water approximately 50% fills the autoclave, but this is not a problem because the feedstock for the subsequent AD digestion stage is desirably a dilute aqueous slurry.

Thermocouples and load cells for the autoclave may provide inputs for a microcontroller or computer with appropriate stored instructions e.g. to execute the following start up logic for one of a pair of autoclaves with steam recycling:

1. Record load cell readings and measure differential.

2. Inject set amount of water into the autoclave through the open door.

3. Record load cell readings and measure differential.

4. Add known weight of waste with slow forward rotation.

5. Record load cell readings and measure differential.

6. Stop rotation, close door and confirm closed condition.

7. Record load cell readings and measure differential.

8. Start rotation in reverse direction and start vacuum pump.

9. Record load cell readings and measure differential.

10. When pressure has fallen to a preset level (PI) stop the vacuum pump and start steam recycling via the top door. After pressure has stabilised, start fresh steam injection via the lower door.

11. When pressure at the top door has risen to a preset level (P2) stop steam injection.

12. With rotation on, record the upper and lower load cell readings.

13. Turn rotation off and leave for a set time before taking a further set of load cell readings.

14. Calculate the average change in weight for both load cell positions.

15. Restart rotation in reverse direction and, after a set time, take a further set of load cell readings.

16. Calculate the average change in weight for both load cell positions.

17. Calculate an average of the averages calculated in 10 and 13. This is the weight movement induced by rotation. This will be compared to a set value which is the criterion for successful movement. 18. IF the average change exceeds the set value then the steam supply is turned on again and the pressure allowed to rise to the main set point (P3).

19. IF the average change is less than the set value then a set amount of water will be injected through the bottom door and the process returns to step 7.

20. If this is still unsuccessful in mobilising the load, this loop can be repeated.

21. If it is unsuccessful after a specified number of loops, the process will be put on hold and operator intervention will be requested.

In an embodiment alternately operating autoclaves are mounted in support frames for rotation about their longitudinal axes, slope downwardly at about 15° and are provided at opposed ends with lower and upper doors. The autoclaves may, for example, each process a 15 tonne load, and be of length typically 13m and diameter 3.33m. Water which is preferably heated to near boiling e.g. 90°C can be pumped from a dilution tank via a lower end door of each autoclave. For each processing cycle, 7.5 tonnes of water may be added at the start of the cycle through the lower door in this way. Steam from an accumulator can pass through the lower end door into one or other autoclave. Typically about 3.25 tonnes of steam is injected via the bottom connection and turns into condensate. At any given time either water or steam may be introduced and when required, the pressure within either autoclave can be reduced by respective vacuum pumps. The autoclaves may work at 110-170°C, a temperature of 160°C and a pressure of about 6 bar being considered optimum as a feedstock to AD.

At the end of a process cycle, steam can be recycled from one of the autoclaves which is ending its processing cycle to the other autoclave which is beginning its processing cycle. Recycled steam can enter through the top door. During depressurisation within an autoclave condensate is re-evaporated and transferred to the other autoclave, the other autoclave then having already been loaded and evacuated by the vacuum pumps. The recycled steam preheats the second autoclave before fresh steam is admitted from the steam accumulator and this minimises the quantity of fresh steam required. The remaining steam in the autoclave at the end of its cycle can then be condensed by adding cold water. About 15 tonnes of water may be added at the end of the processing cycle, condensing residual steam and cooling the waste to about 70°C.

On completion of the processing cycle, a waste stream from the or each autoclave passes via a conveyor to a separator e.g. a star screen for separation into an organic-rich waste stream and an mechanically separable reject stream. Recyclables pass from the separator and it is expected that about 3.5 tonnes per cycle of recyclables will be removed in this way. For direct treatment the organic fraction may be passed direct or with water dilution to the supercritical water oxidiser. For preliminary anaerobic digestion the digestible organic fraction passes to wet sorting station where it may be combined with cold water for cleaning and cooling, about 12 tonnes of water being added to cool the waste to about 50°C. The waste may then be passed via a gravity conveyor to a stirred day tank which can accommodate material from several autoclave batches each amounting including condensate and added water to about 50 tonnes. In order to accommodate four autoclave loads, the holding tank will need to be of size about 250m³, and its contents may be stirred to maintain the organic materials in suspension.

Effects of autoclaving MSW

Breakdown of the organic components in an autoclave of the general kind described above can result in a product from which an organic-rich stream can be separated, that stream being either directly subjected to supercritical water oxidation or being subjected to anaerobic digestion, resulting sludge then being subjected to supercritical water oxidation.

With reference to anaerobic digestion, it is explained in PCT/GB201 1/050145

(the contents of which are incorporated herein by reference) that hydrolysis is the controlling step in the anaerobic digestion (AD) of organic solids. The process of hydrolysis requires weeks to complete in a traditional AD process. A major disadvantage for AD of solid wastes is that the process requires large reactor capacities. Through an autoclave pre-treatment, the majority of organic solids with an appropriate combination of contact, processing temperature and processing time can be thermally hydrolysed and liquidised. Hence, the retention time for the following AD process can be significantly shortened and the digester tank size can be significantly reduced. Furthermore, the combination of thermal and mechanical degradation induced by the autoclave has the effect of vastly increasing the amount of organic material that can be digested by AD. It is further explained in the above application that another major drawback for traditional AD is the ammonia toxicity to the anaerobic micro-organisms associated with treating high protein content wastes. Thermal denaturation and/or hydrolysis of protein in an autoclave alleviate the inhibition of bacterial activity by ammonia build- up. High protein waste includes slaughterhouse waste and animal by-product wastes as well as food waste e.g. from supermarkets and catering establishments. A major problem in slaughterhouse waste is the treatment of blood, and it is believed that slaughterhouse blood waste can be treated in an autoclave of the present kind and then passed on for anaerobic fermentation without unacceptable ammonia build-up. A further major weakness for AD is that the process has limited tolerance to shock loadings mainly caused by uneven qualities of feedstock. Autoclaving produces a thoroughly homogenised feedstock for the AD which significantly reduces the risks from shock loadings.

There is therefore a benefit in putting MSW through an autoclave, as the resulting material of high organic fraction and high water content can be subjected to anaerobic digestion which breaks down organic matter to produce methane gas, which can be used to drive a generator to produce 'green' electricity. Because the electricity is produced from a renewable source, in UK it currently attracts extra allowances under the Renewable Obligation Credits (ROCs) scheme as of Dec '09, making it worth around 15p per kWhr, and most of this electricity can be supplied to the National Grid. The process of generating electricity also generates waste heat, which is used to produce the steam for the autoclaves via waste heat recovery boilers. In addition, surplus heat can be used for other purposes. After removal of metals and plastics, cellulose floe can either be removed or as in the disclosed embodiment left in the mixture that goes into the anaerobic digester.

The bio-gas that comes off the digester is used to generate electricity. The generator is only about 35% efficient, and the rest of the energy is released as heat, of which part is used to generate steam for the autoclave. The resulting sludge from the digester can be burnt as bio-mass, put into a gasifier to produce 'syngas', composted or even formed into a building material.

Processing the organic materials in the autoclave results in them breaking down much more quickly in the anaerobic digester; the lignin (a complex chemical compound) in the organic matter starts to break down, so more gas is produced more quickly. The gas yield can be double that form non-autoclaved waste; furthermore, the peak gas flow rate can be produced in four days rather than four weeks.

The EU landfill directive calls for the amount of organic waste sent to be halved by 2013, and this requirement is backed up by an escalating tax regime. EU Landfill Tax is rising at a rate of £8 per tonne per year (it is currently at £40 per tonne) and is expected to reach £70 per tonne within 5 years. Including tax, the cost of disposing of waste to landfill is currently around £60 a tonne. The social climate is also in favour of sustainable waste solutions; there is a general desire to show more concern for the environment, but at the same time, people do not like the idea of being fined for putting out to much rubbish or mixing up recyclable products. Embodiments of the present process and apparatus not only remove the need to separate out different types of waste; they can also offer local authorities the chance to profit from their waste, rather than paying to get rid of it.

Autoclaving at an appropriate temperature and for an appropriate time can help to avoid excessive concentrations of volatile fatty acid (VFA) building up, which is an indication that anaerobic digestion is failing. Anaerobic microorganisms used in anaerobic digestion are a mixed culture. They mainly contain three groups of bacteria: hydro lytic enzyme bacteria, acidogenic and acetogenic bacteria, and methanogenic bacteria. The hydrolytic enzyme group is responsible for hydrolysing long chain organic compounds into soluble small molecular substrates which can then be converted to VFA's by the acidogenic bacteria and eventually to acetic acid by the acetogenic bacteria. Finally the methanogenic bacteria will convert acetic acid to biogas, which mainly contains methane and carbon dioxide. When an anaerobic digester is reasonably loaded, these groups of bacteria are working in harmony. Once the loading increases, each group of bacteria will develop to reach a new balance to cope with the change of feeding rate. When the digester is overloaded, however, the metabolic balance of the different groups of anaerobic bacteria will be destroyed. The enzyme group becomes overdeveloped and development of the methanogenic bacteria will become reduced. However, the acidogenic/acetogenic bacteria are a very strong group and can carry on fast metabolism under tough circumstances as long as the temperature is maintained at a suitable level. Under these conditions a build-up of VFA's in the digester can be observed and the process failure becomes inevitable.

Autoclave pre-treatment can bring about cellular disruption which can facilitate subsequent anaerobic digestion or direct supercritical water oxidation. It can hydro lyse the majority of the cellulosic material in the waste which can reduce the need for bacterial enzyme hydrolysis in a downstream anaerobic digestion process. When the digester is fed with autoclaved waste, the mechanism of the metabolism of the anaerobic bacteria will be automatically emphasised on the development of methanogen. Therefore more biogas will be produced by the autoclaved materials than non-autoclaved at the same loading rates. In other words, to reach the same gas production rate, higher loading rates can be applied on the autoclaved waste than on the non-autoclaved waste. This means for treating waste streams with the same solids concentrations shorter retention time can be used on the autoclaved waste. Hence the digester volume can be reduced.

Super-critical oxidation

As explained above either the organic fraction from autoclaving and screening or sludge from subsequent digestion of that fraction may be subjected to supercritical anaerobic oxidation.

Supercritical oxidation is described in US-A-4338199 (Mondar, the contents of which are incorporated herein by reference). That specification explains that organic materials can be oxidized in an oxidizer by forming a reaction mixture of the organic materials, water and oxygen with the reaction mixture at supercritical conditions. Substantially complete oxidation of organics using supercritical water can be carried out at high speed in relatively uncomplicated equipment. At supercritical water conditions, oxygen and nitrogen should be completely miscible with water in all proportions. Thus two-phase flow of gases and water are eliminated and single fluid phase flow results which allows simplification of the reactor construction often without the need for mechanical mixing. For example, if the feed is at 374° C. prior to the onset of oxidation, the heat released by oxidation elevates the temperature of the water-organic-oxygen stream appreciably and it can easily reach 450°.-700°C. If the mean temperature in the oxidizer is 400°C or above then the residence time in the oxidizer can be less than 5 minutes. Since the oxidation occurs within a water phase, dirty feeds can be used without the need for off gas scrubbing. For example sulphur in the fuels can be oxidized to solid sulphate which would be readily recovered from the effluent stream from the oxidizer. Inorganics precipitate as a waste slurry, since the solubility of inorganic salts in supercritical water drops to very low levels above 450-500°C. The effluent from the oxidizer can easily be designed to be above those temperatures thus causing inorganics in the stream to precipitate and be readily removed as by cyclones, settling columns or filters. Thus the water output from the system is purified of inorganic salts. The heat of oxidation of the organics in the feed is recovered directly in the form of high temperature, high pressure steam. Although the Mondar process has been proposed for the treatment of sewage sludge it has not hitherto been proposed for the treatment of an organic fraction of MSW or for sludge from the anaerobic digestion thereof.

A recycle reactor for carrying out processes of this type is disclosed in US 6017460 (Chematur). A reactor design said to reduce clogging and corrosion is disclosed in US 2008/0073292 (Stenmark). It comprises an essentially vertical reactor section and an essentially non-vertical reactor section connected together, wherein said essentially vertical reactor section has a cross-sectional area which is substantially larger than the cross-sectional area of said essentially non-vertical reactor section, wherein: said essentially vertical reactor section has an inlet in an upper portion of said essentially vertical reactor section provided for receiving a flow comprising organic material and water; said essentially vertical reactor section is configured to receive oxidant and to oxidize organic material of said flow through supercritical water oxidation while said flow is flowed through said essentially vertical reactor section; said essentially vertical reactor section has an outlet in a lower portion of said essentially vertical reactor section provided for outputting said flow, and said essentially non- vertical reactor section is configured to receive oxidant and to efficiently oxidize organic material of said flow through supercritical water oxidation while said flow is flowed through said essentially non- vertical reactor section, wherein each of said essentially vertical reactor section and said essentially non-vertical reactor section is configured for oxidizing at least 5% of the organic material comprised in the flow. The Stenmark technology is believed incorporated into an Aqua Critox supercritical water reactor for treatment of sewage sludge; again there is no proposal to treat autoclaved organic material from MSW or sludge from anaerobic digestion of such a material.

Anaerobic digestion

The invention may further comprise supplying an organic-rich fraction of processed waste from the autoclave to an anaerobic digester, and recovering a methane- rich gas there from. The anaerobic digester advantageously operates under mesophilic or thermophilic conditions. Methane-rich gas may be supplied to at least one internal combustion engine (e.g. based on reciprocating pistons or a turbine) for generation of power and exhaust gas, and generating steam for said autoclave using the exhaust gas from said internal combustion engine. Recovered jacket water may be used for heating water be supplied to the autoclave and also water to be supplied to a steam generator of the autoclave or anaerobic digestion system. Recovered jacket water may also or independently be used to conduct anaerobic digestion at an elevated temperature e.g. to maintain mesophilic or thermophilic conditions

An anaerobic digestion plant may be operated under wet conditions, solids content being <15% e.g. 2-15%, as a further example about 10%. It may also be operated under semi-dry conditions with solids content 15-20%) or under dry conditions with solids content 30-40%>, but these possibilities are less preferred. Stirred anaerobic digestion tanks may each hold the autoclaved organic waste component for 15-30 days e.g. about 20 days, working at a content of about 10%> w/v solids content and each of liquid capacity about 2500m³, height 10m and diameter 21m. Gas may be collected overhead and passes via a common line a to gas scrubber and then to a compressor, compressed gas at least about 0.1 barg. e.g. about 0.25 barg. being output . Liquid heats the tanks via internal heating coils and returns. Digestate from the tanks is discharged and is further processed as described below. The tanks may be operated under mesophilic conditions e.g. at 35-40°C or under thermophilic conditions. The process may be configured to use acidogenic and methanogenic bacteria together in a single stage as in the disclosed embodiment, or in a further embodiment the process may be operated in two stages, a first acidogenic stage and a second methanogenic stage.

Methane-containing gas from the digestion tanks passes to gas storage tanks which can store typically some hour or hours output, 3750m³ at about 0.25 barg. Gas from the storage tanks flows to engines where it is combusted to generate power. The engines may have a rated output of e.g. 1.5MW each, discharging through their exhaust about 315GJ of heat per day with an exhaust temperature of about 500°C. Exhaust gas from the engines passes to a heater coil of an accumulator required on demand to deliver 3.25 tonnes of steam. It may be sized 13m in length and 2.5m diameter, giving a capacity of about 65m³.

Liquid from the digestion tanks is pumped by a pump as jacket water for the engines, and leaves them via line 132 at 110°C. A first branch line leads to heater coil of a hot well which stores water at 90°C. Water leaving the heater coil passes to a heating coil of a dryer and then returns as warm feed to the digestion tanks. A second branch line passes jacket water through a heater coil a of dilution tank for maintaining the contents thereof at about 90°C.

Solids-rich discharge from the digestion tanks passes to a discharge tank at the same volume flow as the liquid entering the digestion tanks. The discharge tank may receive about 48m³/hour of dilute slurry carrying about 60 tonnes per day of solids, the tank having typically a capacity of about 250m³. Dilute slurry is pumped from the tank and is combined with flocculent from a flocculent injection tank, the combined flow then being treated as described below.

Claims

1. Apparatus for the treatment of solid waste comprising:

autoclave treatment apparatus for steam treating the solid waste; and

a supercritical water oxidation reactor downstream of the autoclave treatment apparatus for converting a product stream to heat, water and carbon dioxide.

2. The apparatus of claim 1, which is configured for the treatment of any of

household waste or sorted fractions of household waste;

catering waste (including waste from restaurants or other catering facilities); biodegradable supermarket waste including food waste;

paper and biodegradable plastics waste,

partly or wholly biodegradable commercial waste;

slaughterhouse waste (optionally including blood from slaughterhouses);

animal by-product waste, shellfish waste, poultry product waste, or a mixture of any of the above.

3. The apparatus of claim 1 or 2, wherein the autoclave treatment apparatus comprises a pair of rotary autoclaves each having an interior for treating the solid waste.

4. The apparatus of any preceding claim, wherein the or each autoclave of the autoclave treatment apparatus is downwardly inclined towards its discharge end and has a door at the discharge end, means in said door being provided for injecting steam through said door via a plenum chamber in said door into the interior of said autoclave to treat the load, the plenum chamber communicating with the interior of the autoclave through at least one one-way device leading directly from the plenum chamber into the interior, the one-way device being configured to prevent solid waste entering the plenum chamber from the interior., said plenum chamber being defined between a region of the door and a plate secured to the door at a small spacing inwardly of said region, at least one outlet being defined in the plate, and said one-way device or devices being fitted to said outlet(s).

5. The apparatus of claim 4, wherein each autoclave has a plurality of the one-way devices providing parallel paths from the plenum chamber into the interior.

6. The apparatus of claim 5, wherein the cross-sectional area of the path or paths from the plenum chamber into the interior defined by said at least one one-way device is equal to or greater than the area of an inlet into the plenum chamber for injected steam.

7. The apparatus of any of claims 4-6, wherein the or each one-way device is of porous sintered metal.

8. The apparatus of any of claims 4-6, wherein the or each one-way device is a mushroom or poppet valve.

9. The apparatus of any of claims 4-8, wherein the door carries a rotary seal for connecting a steam pipe to a steam inlet in the door for injection of steam into the autoclave as the autoclave is rotated.

10. The apparatus of any of claims 4-9, wherein the door is hinged to a support frame of said autoclave for movement between one position in which a discharge opening of the autoclave is revealed and another position in which the discharge opening is covered.

11. The apparatus of claim 10, wherein the door is supported for hinged movement between open positions and a position spaced from and axially aligned with the discharge opening and is supported for translational movement between the spaced axially aligned position and the position in which the discharge opening is covered.

12. The apparatus of any of claims 4-11, wherein the or each autoclave has an inlet door for waste at its upper end, and an inlet in said door for water to be sprayed into the autoclave to condense steam therein.

13. The apparatus of any of claims 4-12, wherein the or each autoclave has generally helical internal flights and a drive configured to rotate the flights during steam injection in a direction such that the flights lift the waste from the discharge end into the body of the autoclave.

14. The apparatus of claim 13, wherein the or each autoclave further comprises load sensors at upper and lower ends of said autoclave for sensing load while the flights are lifting the waste from the lower end, equalization of the load at the upper and lower ends compared to the loads at the end of waste introduction indicating that lifting is taking place.

15. The apparatus of any of claims 4-13, further comprises pressure sensors at upper and lower ends of the autoclave for sensing pressure within the autoclave, substantial equality of pressure indicating that the steam has fully penetrated the load.

16. The apparatus of any of claims 4-15, wherein the or each autoclave has an axis of rotation which slopes forwardly and downwardly at an angle of 5-20°.

17. The apparatus of claim 16, wherein the or each autoclave has an axis of rotation which slopes forwardly and downwardly at an angle of 10-15°.

18. The apparatus of any of claims 4-17, wherein the or each autoclave is supported by a support frame in a fixed attitude.

19. The apparatus of any preceding claim, further comprising:

screening apparatus for separating a product stream from the autoclave treatment apparatus into a stream of organic-rich aqueous material and a reject stream of mechanically separable solids; and

the supercritical water oxidation reactor is arranged to receive the organic-rich aqueous stream and oxidise the organic material of said stream to water and carbon dioxide.

20. The apparatus of any preceding claim, further comprising a turbine for receiving a product stream from the oxidation reactor and generating electrical power.

21. The apparatus of any of claims 1-18, including:

at least one anaerobic digestion tank for digesting the organic-rich aqueous material;

wherein the supercritical water oxidation reactor is arranged to receive and oxidise sludge from the anaerobic digestion to water and carbon dioxide.

22. The apparatus of claim 21, further comprising a recovery system for recovering methane-containing gas from the or each digestion tank, at least one internal combustion engine for combusting the methane-containing gas and generating power, and a steam generator fed with combustion gas from the internal combustion engine for generating and accumulating steam for supply to the autoclave.

23. The apparatus of claim 36, wherein the steam generator comprises a steam accumulation tank.

24. The apparatus of claim 22 or 23, further comprising a recovery system for recovering jacket water from the internal combustion engine, a tank for water to be supplied to the autoclave, a second tank for water to be supplied to the steam generator and heaters in the first and second tanks for heating the water therein to near boiling using the heat of said jacket water.

25. A method of treating waste which comprises processing the waste in the apparatus of any preceding claim.

26. Apparatus for the treatment of solid waste comprising:

autoclave treatment apparatus for steam treating the solid waste; and a water oxidation reactor downstream of the autoclave treatment apparatus for converting a product stream to heat, water and carbon dioxide, said reactor being configured to operate closely below, at or above the critical temperature of water.

27. The apparatus of claim 26, further comprising a supply of oxygen or oxygen- enriched air for said oxidation reactor.

28. A method for the treatment of solid waste comprising steam treating the waste in an autoclave and oxidising a product stream downstream of said autoclave to heat, water and carbon dioxide using a water oxidation reactor, said reactor being configured to operate closely below, at or above the critical temperature of water.

29. The method of claim 28, further comprising supplying oxygen or oxygen- enriched air to said oxidation reactor.

30. The method of claim 28 or 29, wherein the product stream oxidised in the reactor is an organic-rich stream from the autoclave.

31. The method of claim 28 or 29, wherein an organic-rich stream from the autoclave is digested to produce hydrocarbon-containing gas and digestate, and the digestate is the product stream supplied to the reactor.