Pressurised recirculation of organic material

Abstract

An apparatus (10) for the pressurized recirculation of organic material comprising a reactor vessel (12) capable of being pressurized and in which both anaerobic digestion and aerobic composting of organic material may occur, the reactor vessel (12) having both an inlet (14) and an outlet (16) for organic material, together with a conveyor means (18, 20, 22, 28, 29, 30, 32, 34 and 36) to convey organic material to the inlet (14) and from the outlet (16), whereby organic material may be transferred between the outlet (16) and the inlet (14) to achieve recirculation and rearrangement thereof while maintaining a pressurized state. A method for the pressurized recirculation of organic material is also described.

Claims

1. A method for the pressurised recirculation of organic material, the method comprising the method steps of: pressurising a reactor vessel in which a volume of organic material has been positioned; treating the organic material sequentially through an anaerobic digestion stage and an aerobic composting stage, wherein the anaerobic digestion stage and the aerobic composting stage occur within the same reactor vessel; and recirculating the volume of organic material from an outlet of the reactor vessel to an inlet of the reactor vessel through at least one conveyor means comprising both a seal means and a valve means whilst maintaining the pressurised state, wherein the organic material is solids.

2. The method according to claim 1, wherein the organic material is rearranged.

3. The method according to claim 1, wherein the organic material is dewatered in the reactor vessel prior to recirculation and whilst under pressurised conditions.

4. The method according to claim 3, wherein additional dewatering of the organic material occurs as the organic material is recirculated.

5. The method according to claim 1, wherein the moisture content of the conveyed organic material is reduced to about 40 to 60% during recirculation to the reactor vessel.

6. The method according to claim 1, wherein arched or radial stress fields are formed in the recirculated organic material as a result of decreased consolidation pressure in a base of the reactor vessel, whereby approximately uniform flow of the organic material from the reactor vessel is facilitated.

7. The method according to claim 1, wherein the organic material is the organic fraction of municipal solid waste.

8. The method according to claim 1, wherein the anaerobic digestion stage includes introducing liquid to the organic material positioned within the reactor vessel to produce biogas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:

(2) FIG. 1 is a schematic representation of an apparatus for the pressurised recirculation of organic material in accordance with the present invention.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

(3) In FIG. 1 there is shown an apparatus 10 for the pressurised recirculation of organic material, for example the organic fraction of municipal solid waste (OFMSW). The apparatus 10 comprises a reactor vessel 12 having an organic material inlet 14 and an organic material outlet 16. Two arrays 18 of conveyors means, for example first outlet conveyors 20, are provided at the reactor vessel outlet 16. The first outlet conveyors 20 are arranged to convey material (not shown) from the reactor vessel 12 outwardly or laterally with respect to the outlet 16, and into second outlet conveyors 22 arranged to extend transversely with respect to the first outlet conveyors 20.

(4) The arrays 18 back onto one another and convey material in substantially opposed directions to respective opposed sides of the outlet 16. A single central conveyor 24 is provided beneath the point at which the arrays 18 meet. The central conveyor collects material from a central column or screen (not shown) in the reactor vessel 12 which is used for liquid drainage when submerged. During solids recirculation this column in effect grates the OFMSW as it travels downwardly. The solids that pass through the screen are collected by the central conveyor 24. The central conveyor 24 conveys material to an intermediate conveyor 26 that passes any material therein to the respective second outlet conveyor 22.

(5) The second outlet conveyors 22 are arranged to direct material to respective third, or dewatering press conveyors 28, from which it may then be passed through an intermediate conveyor 29, a fourth conveyor 30, a fifth conveyor 32, a sixth conveyor 34 and a seventh conveyor 36.

(6) The fifth conveyor 32 is fed by an eighth conveyor 38 with organic waste material from a materials recycling facility (not shown). A ninth conveyor 40 is provided, by which materials may be unloaded from the vessel 12 when the batch is complete.

(7) Each of the conveyors 22 and 29 are provided with dewatering means, for example 180 screens (not shown), by which the material being conveyed therein may be gravity dewatered. The dewatering press conveyors 28 remove the majority of the liquid in the organic material by physically pressing the material against a 360 screen (not shown). A series of fluid lines 42 are provided from these conveyors 22, 24, 28 and 29 feeding to a fluid outlet 44. Each of the lines 42 have provided therein a valve means 46 for control of fluid flow therethrough.

(8) A biogas outlet 58 is provided in the reactor vessel 12 and communicates with a biogas line 60. A series of fluid inlets 62 are provided in the reactor vessel 12 and are fed from a fluid inlet line 64 and a series of branches 66 therefrom. Each of the branches 66 have provided therein valve means 68 for control of fluid flow therethrough.

(9) The reactor vessel 12 and each of the conveyor means, for example conveyors 18, 20, 22, 24, 26, 28, 29, 30, 32, 34 and 36, such as are required in the potential recirculation of material from the reactor vessel 12, are capable of being pressurised and maintain that pressure during the recirculation of material from the organic material outlet 16 to the organic material inlet 14. For this purpose each of the conveyors 18, 20, 22, 24, 26, 28, 29, 30, 32, 34 and 36 are equipped with seal means, for example shaft seals and housing or casing seals at the connection points between consecutive housings and inspections ports. Further, there are valves 70 provided on each of the conveyors 38 and 40 that communicate with a feed end 72 of the fifth conveyor 32.

(10) In use, the organic fraction of municipal solid waste (OFMSW) from a materials recycling facility (MRF) are directed to a reactor vessel 12 in which a process such as that described in International Patent Application PCT/AU00/00865 (WO 01/05729) is to be conducted, where the OFMSW is to be exposed to sequential treatment through anaerobic digestion and aerobic composting stages.

(11) The anaerobic digestion stage involves the introduction of liquid to the OFMSW to create conditions optimal for the production of biogas. Biogas production may be increased by ensuring maximum flow of the liquid through the OFMSW. This is achieved by way of recirculation of the OFMSW whilst maintaining substantially anaerobic conditions. This requires the draining of free liquid from the reactor vessel 12 through the conveyors 22 and 24, and the fluid outlet 44. Then the OFMSW from within the reactor vessel 12 is recirculated from the material outlet 16, through the action of conveyors 18, 20, 22, 24, 28, 29, 30, 34 and 36, to the material inlet 14.

(12) This process results in the recirculation and rearrangement of the OFMSW and the improved penetration of liquid once that liquid is reintroduced to the reactor vessel. This in turn improves biogas production relative to an OFMSW that hasn't been recirculated in this manner. The ability to conduct this process under pressure, without having to vent the reactor vessel 12 (without opening the vessel to the atmosphere), allows the process to be conducted more quickly to achieve a given result when compared with the time taken to achieve that same result, in terms of biogas production, if using this process without recirculation or if using a static dry batch anaerobic digestion system of the prior art.

(13) It is also known that it is necessary to dewater the OFMSW before the reactor vessel 12 can be transitioned to aerobic conditions. The arrangement of the present invention is such that it allows for the moisture content of the OFMSW to be reduced to optimum content of about 40 to 60% as it transitions to aerobic conditions. In part this is achieved by way of gravity draining that occurs within the reactor vessel 12 and in part by way of additional dewatering. The additional dewatering is achieved by passing the OFMSW through conveyors 18, 20, 22, 28, 29, 30, 32, 34 and 36 such that any free liquid is drained via the 180 screens and also through mechanical dewatering achieved by way of press via conveyors 28 which squeeze the liquid from the material under pressure.

(14) It is envisaged that there may be provided more than a single inlet 14 and more than a single outlet 16 in the reactor vessel 12 without departing from the scope of the present invention.

(15) The recirculation of the OFMSW is understood to favourably contribute to the establishing of arched or radial stress fields in the OFMSW above the base of the reactor vessel 12 whereby the flow of that OFMSW is enhanced through the arrays 20 of first conveyors 18 and 20. This is caused by the reduction in consolidation pressure at the base of reactor vessel 12 through the formation of the arched stress fields.

(16) The ability to return recirculated, rearranged, and substantially dewatered OFMSW to the reactor vessel 12 under pressure is understood to be advantageous in that it allows the maintenance of a given moisture content during the aerobic composting stage. Further, as noted above, the reactor vessel 12 need not be opened to atmosphere and thereby continual processing is possible. Still further, the penetration of air into the OFMSW is improved, thereby improving aerobic activity and hence overall efficiency of the process.

(17) The recirculation of the OFMSW within the reactor vessel 12, whether under pressure or not (during the aerobic phase) is understood to enable the moisture content to be maintained, the generation of an arched or radial stress field above the base of the reactor vessel 12, and improved air penetration and a reduction in consolidation of the OFMSW.

(18) It can be seen from the above description that the apparatus and method of the present invention allow for improved results from processes for the treatment of organic wastes that utilise anaerobic digestion and/or aerobic composting in a vessel.

(19) The benefits of solids recirculation apparent to the Applicants in light of the above description include: (i) Improved biogas production through recirculation and rearrangement of the OFMSW and improved liquid penetration; (ii) Improved transition from the anaerobic digestion stage to the aerobic composting stage through faster dewatering of the OFMSW over conventional gravity drainage; (iii) The maintenance of moisture content at optimum levels during the aerobic composting stage; and (iv) The creation of radial stress fields that act to reduce consolidation pressure of the OFMSW, improving flow of the material through outlet 16 and greater porosity to allow better air flow through the OFMSW thus improving aerobic composting efficiency.

(20) Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention. For example, whilst the above example has been described in terms of the processing of OFMSW, the apparatus and method of the present invention are equally applicable to other sources of organic waste, including for example only, a combination of MSW, kitchen waste and green waste.