FIELD OF THE INVENTION

The present invention concerns the field of the processing and recycling of waste, especially mixed municipal solid waste (MSW). In particular, the present invention relates to the refining of MSW to produce methane using an anaerobic digestion process for the treatment of the biodegradable fraction of MSW.

BACKGROUND OF THE INVENTION

The term waste is intended to mean all products that are no longer of use and which are to be disposed of and any substance derived from human activities or natural cycles that is abandoned or destined to be abandoned. Municipal solid waste treatment and recycling systems have been studied for a long time, due to the always growing necessity of an effective, environment-friendly disposal and of a functional use of the waste as an energy source.

The main objective of waste treatment is to find methods which allow the maximum recovery of the waste products and with a minimal environmental impact. However, the practice of disposing of waste in landfill sites is considered to present a significant potential risk to the environment. In particular, the anaerobic decomposition of biodegradable waste in landfill generates methane, which is a strong greenhouse gas (21 times stronger than carbon dioxide). Consequently, the European Landfill Directive 99/31 /EC requires Member States to bring about a phased reduction in the amount of biodegradable MSW that is disposed of to landfill.

A number of alternatives are used for the treatment of biodegradable MSW including the following:

- Separation of the organic biodegradable fraction and composting to produce a soil conditioning product or a low grade compost like output that can be used as a landfill;

- Stabilisation of the biodegradable fraction to meet stability criteria permitting landfill disposal; - Mechanical segregation followed by anaerobic digestion to generate biogas as a supplementary fuel and stabilised residue suitable for land spreading.

Accordingly, land spreading in some form is currently the main alternative outlet for treated biodegradable MSW. However, there are some concerns, such as the potential environmental impact of the low grade compost like output, due to the presence of contaminants. Even if the low grade compost meets the biological stability criteria, it will continue to degrade slowly for very long periods of time in the landfill environment generating a potential long-term problem of methane release. Further, the costs of treating waste by this approach are high relative to the low grade end-product that is generated.

Another problem relating to the treatment of mixed MSW is that even if the separated organic biodegradable fraction can be treated as discussed above the remaining non-biodegradable fraction is disposed of in landfill sites.

US 2008006034 discloses a method and a system for recycling of municipal solid waste with the exploitation of the wasted solid recovery fuel (WSRF) for the production of electric energy and/or hydrogen. The results are achieved by means of the gasification of the WSRF in a reactor where the volatile and the inorganic components are combusted separately thus allowing contemporary the further treatment of the synthesised gas and the recovery of mineral and metallic molten granulates. In this method the "wet fraction", i.e. organic biodegradable fraction of MSW, is subjected to an aerobic treatment, which comprises accelerated decomposition of the organic substances, separation of the inert processing residues from the bio-mass and maturation and stabilization of the organic fraction to produce stabilized biowaste (grey compost). Thus, again a relatively low grade end-product is generated.

US 2009305390 A1 discloses a method for processing waste and producing methane, in which a chamber is filled with waste, in which it undergoes anaerobic degradation. According to the method, a large chamber and a small chamber are filled respectively with slightly organic waste and highly organic waste, and a liquid fraction generated by the degradation of the waste in the large chamber is introduced into the small chamber. The highly organic waste may comprise biodegradable fraction of MSW. However, US 2009305390 A1 does not teach or suggest what to do with the remaining non-biodegradable fraction of MSW.

WO 2008065452 A2 discloses a method of treating municipal solid waste comprising the steps of: shredding the waste to a first particle size; storing the shredded waste in an aerated storage bay for a first period of time; shredding at least some of the waste to a second particle size; and agitating the shredded waste in a rotating drum for a second period of time and then outputting the treated waste; wherein biological decomposition under aerobic conditions takes place in the aerated storage bay and the rotating drum. The waste that is treated by the method is dried as a result of the biological decomposition, so that the moisture content of the waste is reduced. Thus, the energy contained in the organic fraction of the MSW is used in the treatment process.

The prior art waste treatment processes are focusing either on the environmentally friendly disposal or functional use of the waste as an energy source. However, processes where overall positive energy balances of the waste treatment plant are achieved together with the possibility to refine valuable end products from the waste are lacking.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method that can process mixed MSW so that a proportion of the waste can be refined to form a solid recovered fuel and another portion of the waste can be used for the production of methane. The object of the invention is achieved by a method which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.

A further object of the present invention is to provide a process for the use of the solid recovered fuel (SRF) with consequent energy recovery with a minimal environmental impact. The recovered energy can be used for power generation or alternatively as a supplementary fuel for industrial processes, and particularly in cement manufacture. Still a further object of the present invention is to provide a suitable system for the achievement of a cost- and energy-effective waste refining process for MSW. An advantage of the method of the invention is that the amount of MSW that is disposed of to landfills may be remarkably reduced. Thus, the amount of uncontrolled methane generation by the anaerobic decomposition of biodegradable waste in landfill may be reduced. Also the carbon dioxide (CO ₂ ) production may be controlled and at least part of the produced CO2 may be recovered. Still another advantage is that valuable end products, such as liquefied biogas or methane, liquefied CO ₂ , granulated fertilizers, biocarbon, purified water, heat energy and/or electric power, may be obtained from the waste refining process.

Accordingly the present invention provides as a first aspect a waste refining method, wherein

a. municipal solid waste (MSW) is fed to a pretreatment wherein a biodegradable fraction is separated therefrom and recovered, b. the biodegradable fraction from step a) is fed to an anaerobic digestion process, wherein biogas and a liquid reject is produced, and the biogas containing methane is recovered,

c. at least a fraction of the remaining part of the waste from step a) wherefrom a biodegradable fraction has been separated is pyrolyzed in a carbonizer, which is used to produce biocarbon and heat, and d. the heat obtained from the carbonizer in step c) is at least partly used in the treatment of the biodegradable fraction of MSW, and/or the biocarbon produced in step c) is burnt in a burner and the heat obtained from the burner is at least partly used in the treatment of the biodegradable fraction of MSW.

The basic ideology behind the prior art processes has been waste treatment, which has been understood as avoiding negative effects of the waste by neutralising them. The aim has not been in the production of valuable end products, and therefore it has been possible to use the produced biogas for the production energy needed in the treatment plant. On the contrary, in the present invention the overall energy balance has a key role, because the aim is to produce methane and possibly other valuable end products.

A further remarkable advantage of the present invention is that the raw material fed to the refining process may be MSW from a landfill site, i.e. the present invention may be utilized for landfill mining. Thus, the present invention not only reduces the amount of MSW to be disposed of to a landfill, but actually makes it possible to empty the landfill from the MSW that has already been dumped therein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawing, in which Figure 1 shows a schematic picture of an embodiment of the waste refining process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a waste refining method, wherein municipal solid waste (MSW) is used as a raw material. The MSW is first fed in step a) to a pretreatment, wherein a biodegradable fraction is separated therefrom and recovered. In this pretreatment process the MSW may be separated to at least three different fractions comprising a biodegradable fraction; a solid recovered fuel fraction, and a solid non-burning fraction containing metals, glass, stones and sand. In a preferred embodiment of the invention, the MSW is segregated to following fractions: 1 ) biodegradable fraction, 2) solid recovered fuel fraction, 3) ferromagnetic metals, 4) other metals (especially aluminium and copper), 5) fines from screen, and 6) a fraction containing the rest of the MSW (PVC, glass, sand, stones, etc.).

Solid recovered fuel, also known as specified recovered fuel, is a fuel produced from municipal solid waste. SRF is a standardized product, which can be used alongside traditional sources of fuel in coal power plants, pyrolysis plants or a variety of ways to produce electricity.

After the pretreatment, the biodegradable fraction from step a) is fed to an anaerobic digestion process (step b), wherein biogas and a liquid reject is produced, and the biogas containing methane is recovered.

Biogas refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel. Biogas comprises primarily methane (CH ₄ ) and carbon dioxide (CO ₂ ). Biogas typically has methane concentrations around 50-60 %, but with advanced waste treatment technologies it is possible to produce biogas with 55-75% CH ₄ or even higher if purification techniques are used.

A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops. In the present invention biogas is produced by anaerobic digestion or fermentation of biodegradable materials separated from MSW. In addition to the biodegradable fraction of MSW, the feed stream to the anaerobic digestion process may contain any of the types of waste chosen from organic waste processing residues, food waste, garden waste, biomass, manure, waste from the agri-food industry and agricultural residues.

The gaseous or liquefied methane can be combusted and the energy release allows biogas to be used as a fuel. Biogas can be used as a low-cost fuel for any heating purpose. It can also be used in waste management facilities to run any type of heat engine to generate either mechanical or electrical power.

An example of typical raw biogas produced from digestion comprises roughly 60% methane and 29% CO ₂ the rest being typically nitrogen (N ₂ ), hydrogen (H ₂ ), hydrogen sulfide (H ₂ S), and oxygen (O ₂ ). Biogas as such is typically not high quality enough for selling it or using it as fuel gas for machinery. Also the corrosive nature of H ₂ S may destroy the internals of the plant. Therefore, in a preferred embodiment of the present invention the produced biogas is fed to an upgrading or purification process whereby contaminants in the raw biogas stream are adsorbed or scrubbed. There are also other possible methods for biogas purifications including water washing, pressure swing adsorption, Selexol adsorption and chemical treatment.

Water Washing is a method of purifying biogas, wherein raw biogas from the digester is compressed and fed into the scrubber vessel where passing water streams adsorb the gas contaminants leaving near pure methane. The gas is then dried by dessicant in the drier columns and it exits the system as more than 98% pure methane per unit volume of gas.

Pressure Swing Adsorption (PSA) is a purification method that separates the CO2, nitrogen, oxygen and water from the raw biogas stream by adsorbing gases at high pressure and desorbing them at low pressure as waste. The PSA system usually consists of 4 different adsorption columns working in sequence; adsorption, depressurizing, desorption and repressurizing. In this purification method the raw biogas is compressed and fed into the bottom of the adsorption column where it is purified. During this time the remaining columns regenerate, so that there is always one absorber column actively cleaning gas. PSA does not scrub hydrogen sulphide so this must be removed before it enters the compressor.

Using polyglycol (tradename Selexol) to purify biogas is similar to the water washing method with regeneration. Selexol can adsorb hydrogen sulphide, carbon dioxide and water. However, the energy required for regenerating the solution after adsorbing H ₂ S is high, so hydrogen sulphide is removed before the process.

Raw biogas can also be upgraded by various chemical reactions that remove the CO ₂ and other contaminants from the gas stream. The chemicals such as alkanolamines react at atmospheric pressure in an adsorption column with the CO2 and are regenerated afterwards with steam. The hydrogen sulphide must first be removed to avoid toxifying the chemicals.

In an embodiment according to the present invention, the biogas recovered in step b), which contains methane and CO2, is purified as disclosed above and methane and CO2 are separately recovered. If the method according to the present invention is utilized near by a landfill site or it is for another reason considered reasonable, biogas obtained from the landfill may be fed to the same purification process as the biogas recovered in step b). This has several benefits. First of all, more methane can be produced without any need for several purification units. It is also an advantage that the biogas from the landfill can be used for the production of methane without the need of burning it somewhere in a waste treatment plant to produce energy to the process.

Biogas can be compressed, much like natural gas, and used to power motor vehicles. Biogas is a renewable fuel, so it qualifies for renewable energy subsidies. In an embodiment of the present invention the recovered methane and/or CO ₂ are liquefied. Purified liquid biogas (LBG) is basically liquid methane produced from the biogas, and is often called also as "biomethane". This requires that the methane within the biogas is concentrated to the same standards as fossil natural gas. One of the disadvantages of the prior art processes has been that only part of the MSW has been treated and the prior art is lacking waste refing processes which would produce valuable end products from the major part of the MSW. Therefore the method according to the present invention comprises a further step (step c), wherein at least a fraction of the remaining part of the waste from step a) wherefrom a biodegradable fraction has been separated is pyrolyzed in a pyrolysis unit.

Pyrolysis is defined as the irreversible chemical change brought about by heat in the absence of oxygen. During pyrolysis, biomass or specifically in this invention SRF undergoes a sequence of changes and normally yields a black carbonaceous solid, called charcoal, along with a mixture of gases and vapours. Generally low temperatures and slow heating rates result in high yield of charcoal; this type of pyrolysis, in which charcoal production is maximized, is called carbonization.

Reactors used for SRF pyrolysis according to the present invention may be very different depending on the way the solids move through the reactor. In batch reactors there is no solid movement through the reactor during pyrolysis. Then there are moving bed reactors (shaft furnaces); reactors, wherein the movement is caused by mechanical forces (e.g., rotary kiln, rotating screw etc.); and reactors, wherein the movement is caused by fluid flow (e.g., fluidized bed, spouted bed, entrained bed etc.). Reactors, wherein the movement is caused by mechanical forces are preferred in the present invention, especially in the cases where carbonization of the SRF into biocarbon is main object.

Pyrolytic reactors may also be different based on the way heat is supplied to SRF. It is possible to use reactors, wherein part of the raw material burns inside the reactor to provide heat needed to carbonize the remainder. It is also possible to use reactors, wherein direct heat transfer from hot gases produced by combustion of one or more of the pyrolysis products or any other fuel outside the reactor is utilized. Another option is to use reactors, wherein indirect heat transfer through the reactor walls is utilized (i.e. external heat source due to combustion of one or more pyrolysis products or any other fuel). Examples of possible reactors include Hereshoff carbonizer, Pillard rotary carbonizer, Keil- Pfaulder converter, Cornell retort, and indirectly heated screw-fed converter such as Thompson converter.

The use of a conventional Thompson Converter type apparatus for the above purpose is based on the feeding of the matter to be processed to one or more screw conveyors provided in the process space of the apparatus, by which conveyor/s the matter to be processed is transferred in the longitudinal direction of the process space while being heated indirectly at the same time. The matter carbonized inside the screw conveyors by heat transferred from the conveyors to the matter to be processed is discharged from one end of the conveyors to a collecting conveyor that transfers the carbonized matter out of the process space. In a solution such as this the pyrolysis gas created inside the screw conveyors is conventionally carried within the matter to be processed in the travel direction thereof from the discharge end of the screw conveyors to a collection chamber and further on a connecting conduit to a combustion furnace below the screw conveyor space, where it is burned. Fuel gas leaves the combustion furnace to enter a screw conveyor space, where the heat contained in the fuel gas is transferred by convective heat transfer into the screw conveyors before being discharged from the process space through a discharge assembly.

In a preferred embodiment of the present invention the pyrolysis is done using a method, wherein carbon is separated by thermal treatment, in which method matter to be processed is brought by a feed arrangement to a conveyor arrangement connected to a process space that is substantially of a Thompson Converter type. The matter to be processed is made to move in the process space in longitudinal direction thereof by means of a conveyor arrangement closed in relation to the space, whereby pyrolysis gas formed by heat transfer from the process space into the matter to be processed contained in the conveyor system is conveyed into a combustion space provided in the process space for combustion of the gas, flue gas thereby formed being discharged from the process space by means of a discharge arrangement, and thermally treated matter is discharged from the conveyor arrangement for further processing. Especially, the pyrolysis gas is, firstly, burned by a continuous gas burner arrangement, and secondly, heat transfer of the conveyor system in the process space is carried out substantially by direct radiation from the flame of the gas burner arrangement and from the walls of the combustion space. As a preferred embodiment of the method of the invention, pyrolysis gas is conveyed within the conveyor arrangement by counter current towards feed end of the conveyor arrangement for transferring heat contained in the pyrolysis gas into the matter to be processed that is moving to the opposite direction and for feeding cooled pyrolysis gas into the gas burner arrangement. Broadly, there are two technological routes for producing charcoal briquettes from waste biomass and residues, (i) the briquetting-carbonization (B-C) option, and (ii) the carbonization-briquetting (C-B) option.

In briquetting-carbonization (B-C) option, the raw material is first densified and the densified product is next carbonized to produce charcoal briquette. An advantage of the B-C option is that the intermediate product of the process, uncarbonized biomass briquettes, can be used for certain applications as such. Another advantage is that conventional charcoal kilns can be used for carbonizing the briquettes. One of the disadvantages of this option is the high pressures involved in briquetting biomass resulting in high energy input to the process. In the carbonization-briquetting (C-B) option, the raw material is first carbonized (and crushed, if necessary) to obtain powdered charcoal, which is then briquetted using a suitable binder. An advantage of the C-B option is that energy input to the process is less than that in the B-C option since the briquetting process handles a much lower amount of mass and lower pressure is needed for briquetting charcoal, compared with uncarbonized biomass. Also the production of char fines as waste is eliminated. In the present invention the C-B option is preferred, if briquetting is required at all. For example, in one embodiment of the present invention, the produced biocarbon is used for the manufacturing of a fertilizer. This process has a separate granulation process at the end and therefore it is not necessary to from briquettes from the biocarbon.

Although it is advantageous that the pyrolysis in step c) occurs in the same plant wherein the anaerobic digestion plant is, it is also possible that this happens in another plant.

In a preferred embodiment of the invention the energy obtained from the pyrolysis unit in step c) is also used to produce electrical power, hot air, hot water or steam. The pyrolysis may be boosted with an auxiliary fuel substance. The auxiliary fuel substance may be selected from biomass, vegetable oil, fish oil, and animal fat. Such an auxiliary fuel substance may be utilized when SRF is carbonized. In such a case the auxiliary fuel substance is fed to the burner, which provides the heat for the gasification (carbonation) reaction. In the present invention the pyrolysis unit is a carbonizer, which is used to produce biocarbon and heat. In an embodiment the gas produced during carbonation is burned in carbonizer and the heat is partly used for the carbonisation reaction and partly for other purposes. In another embodiment of the invention the pyrolysis is partly used to gasify the SRF into synthesis gas. Syngas or synthesis gas is wood gas which is created by gasification of wood or other biomass. This type of gas is comprised primarily of nitrogen, hydrogen, and carbon monoxide, with trace amounts of methane. The syngas can be further used for the production of electric energy and/or hydrogen as well as in the synthesis of biodiesel, methanol or other chemicals.

In the method according to the present invention the heat obtained from the pyrolysis unit in step c) is at least partly used in the treatment of the biodegradable fraction of MSW, and/or the biocarbon produced in step c) is burnt in a burner and the heat obtained from the burner is at least partly used in the treatment of the biodegradable fraction of MSW. This is one of the important advantages that the present invention may offer.

The anaerobic digestion process requires energy (heat) to work. A possible energy source could be the produced biogas, which could be burnt and the heat conducted to the digestion process. However, the aim of the present invention is to produce methane and not to use it in the production thereof. Therefore, in the present invention the required heat is obtained from the pyrolysis unit in step c), wherein the SRF is carbonized, or from the burner, wherein the produced biocarbon is burned. Especially in cases where the biogas plant is nearby a coal-fuelled power plant, it may be advantageous to burn the produced biocarbon at the coal power station and use the heat produced therein in the treatment of the biodegradable fraction of MSW.

Another and even more severe problem associated with typical biogas plants is the liquid reject obtained from the anaerobic digestion process. This liquid reject is a major problem in almost all biogas plants, because it is difficult and/or expensive to dispose of. Drying of this liquid reject requires a lot of energy and therefore it has not been considered to be meaningful. However, in an embodiment of the present invention the liquid reject is conducted to a dewatering unit, wherefrom a solid humus stream and a reject water stream are obtained. The reject water stream may be evaporated in an evaporation plant and thus produce distilled water, and the rest may be combined with the solid humus stream obtained earlier and these can be both then dried in a thermal dryer, which utilises heat from the pyrolysis unit in step c), wherein the SRF is carbonized or the heat from the burner, wherein the produced biocarbon is burned. This thermal dryer unit will produce water vapour, which may be further used in the heating of the anaerobic digestion process.

In an embodiment of the invention the liquid reject from step b) is fed to a dewatering unit wherefrom the obtained solid humus is further dried in a thermal dryer and the dried solid humus is granulated. In another embodiment the dried solid humus is granulated together with additional minerals to produce dry granulate to be used as a fertilizer. It is also possible to further enhance the fertilizer by a method, wherein the pyrolysis unit used in step c) is a carbonizer and the obtained biocarbon is fed to the thermal dryer together with the solid humus, and the obtained dry product is granulated with or without additional minerals. The additional minerals that may be added to the granulation process include nitrogen, phosphorus and potassium.

A preferred embodiment of the over all process is shown in Fig. 1 , wherein two possible sources of MSW are disclosed: fresh MSW and MSW from a landfill site. One or both of these MSWs are fed to the pretreatment process from which biodegradable fraction and SRF are separated. Also metals and glass may be separately recovered. The biodegradable fraction is fed to an anaerobic digestion process (biogas plant), wherein biogas and a liquid reject are produced. The biogas is then fed to purification process optionally together with biogas obtained from a nearby landfill site. The purified biogas is then compressed and the liquefied methane and carbondioxide are separately recovered.

The liquid reject is fed to a dewatering and drying process, wherein the heat required is obtained from the pyrolysis unit. The heat may be transferred or conducted for example in the form of hot air, hot water or steam. The surplus heat from the above-mentioned dewatering and drying processes may be further used in the biogas plant. It is also possible that the heat from the SRF pyrolysis unit is directly conducted to the biogas plant as shown with one of the dotted lines in Fig 1 .

The obtained dry solid humus is then granulated and a dry granulated fertilizer is produced. The SRF from the pretreatment is fed to a pyrolysis unit and, based on the specific need, heat, syngas and biocarbon is produced. The heat produced may be further used to produce hot air, hot water, steam or electric power also for other purposes than those mentioned above.