Patent Application
20110036280
The present invention relates to a waste processing system.
Modern methods for the thermal destruction of waste, to meet environmental standards, are usually conducted on a large scale to be economical and safe. Therefore, in smaller local applications, waste management solutions are more limited. Incineration is a common method of waste destruction/reduction in larger applications, however, such a process has become socially unacceptable due to a large footprint (both carbon footprint and physical space required), high profile, potential for air and water pollution, and reliance on vehicles with associated emissions and CO2.
An example of a situation where smaller scale thermal destruction is desirable is on board an ocean going ship, at a hospital, or in a mobile situation such as for military applications where a base may be relocated with some regularity. Furthermore, such an operation can be utilized in any business that wishes to treat its waste at source.
Accordingly, the present invention seeks to provide a waste processing system for use where there is a local need for effective waste disposal and a potential to use energy generated by the process. More particularly, it is an object of the present invention to provide a relatively small scale integrated waste management system for thermal destruction of waste.
The waste processing system according to the invention comprises a compactor for densifying waste and expressing air, a pyrolyzer to receive waste delivered from said compactor, a gasifier receiving waste from said pyrolyzer, and an oxidiser receiving gas from the gasifier to be combusted.
It is intended that the invention is a continuous feed waste processing system.
Preferably the pyrolyzer processes waste therethrough by use of a screw arranged through a main tube.
Preferably ash is removed from the gasifier stage via an ash vessel wherein ash can accumulate.
In another aspect of the present invention there is provided a method of processing waste comprising densifying waste and expressing air therefrom in a compactor, delivering the waste from a compactor to a pyrolyzer, heating the waste for thermal decomposition of organic material by action of heat alone in the pyrolyzer, receiving waste from the pyrolyzer in a gasifier, gasifying waste in the gasifier, receiving the gas from the gasifier in an oxidiser, and oxidising the gas to be combusted.
Preferably energy from the output gas of the oxidizer is used to provide heat to the pyrolyzer. It is also preferred that excess heat remaining in the gas once it has completed the pyrolysis process is reclaimed. Heat reclamation can be by use of an indirect heat exchanger such as a waste heat boiler. There is also potential to use reclaimed heat for electrical generation, absorption chilling for refrigeration and/or air conditioning, space heating, process heating with hot water or steam and/or distillation or desalination.
Observable benefits of technology according to the invention include:
a, 4b and 4c show further section views taken from
As a pre-feed treatment, the waste is processed through the shredder 11 to homogenize it and act as a separator for contaminants, mainly consisting of materials that may be injurious to mechanical handling.
The greater the homogeneity of the waste feed, the smoother the operation of the thermal plant, leading to increased processing capacity of the plant and increased life of the components. Preferably, before entering the shredder 11, large items and other damaging materials are removed. Separation may be performed by hand or automated depending on the application and available manual resources.
Size reduction of the waste has an added advantage of increasing surface area and therefore the effectiveness of the pyrolysis process.
Once shredded the waste is compacted through a reducing cone using a screw compactor 12. This method of feeding has several benefits: it densifies the waste, making it easier to transport; it expresses air from the waste; it also provides a seal between atmosphere and the thermal process (the seal being formed by the waste itself). This seal ensures that air is not drawn into the process. If it were, combustion would be promoted rather than pyrolysis as intended to occur at the next step of the system. The compactor 12 provides a continuous feed (as opposed to a batch feed) which is beneficial to the process and eliminates the need for a rotary valve or lock hopper system.
The first stage of the thermal process is pyrolysis, defined as the thermal decomposition of organic material by the action of heat alone. No other reagents are utilized, just heat.
For delivery from the compactor 12 to the pyrolyzer 13 waste is fed along a joining duct 19 toward a further screw 18 located within pyrolyzer tube 17. The tube 17 is externally heated using gases from later in the process. In this manner the waste is heated from ambient temperature, whilst adding no other reagents. The heat first dries the waste then decomposes organic molecules to form a gas and vapor mixture mostly made up of water, carbon monoxide and carbon dioxide, hydrogen, methane and ethane. The balance of the material is left as a carbon char and ash.
Inert materials (e.g. metals and glass) are merely heated and pass through the tube unaffected.
As a general overview, the pyrolyzer 13 indirectly heats combustible waste material to a high temperature, e.g. to over 500° C., in the absence of air or oxygen. This is achieved by passing the combustible waste material through the heated tube 17, a pyrolysis tube, by means of an auger (e.g. screw 18). The pyrolysis tube 17 is contained within a pyrolysis chamber through which hot exhaust gases from the outlet of the oxidiser 15 are passed. These hot gases pass over the outside of the pyrolysis tube and transfer heat from the exhaust gases to the tube by convection and radiation. The hot tube 17 then transfers heat into the combustible waste material by conduction and radiation from the inner surface of the tube wall. The heat energy heats up the combustible waste material to approximately 600° C. and thermally degrades the material to pyrogas and char. The absence of air or oxygen prevents the combustible waste material from combusting within the pyrolysis tube 17.
The main benefit of the pyrolyzer 13 is that it produces an excellent feedstock for the gasifier 14, being dry, hot, pre-pyrolyzed and homogenous. This makes the operation of the subsequent gasifier 14 simpler and more efficient.
Preferably the pyrolysis chamber 13 comprises an insulating outer surface to retain heat and to improve the overall energy efficiency of the systems and methods according to the invention.
At the oxidizer end of pyrolyzer tube 17 the resultant char and ash is discharged into a refractory-lined vessel (the gasifier 14). The apparatus 10 can be operated either in pyrolysis mode, or in gasification mode.
When in pyrolysis mode, the resultant char and ash is stored in the gasifier 14 before being discharged into ash containers via another screw system 20 visible in
Trials of prototypes of the invention indicated that the pyrolyzed waste material at this stage of the process is reduced by mass to 30% of its original weight and wherein 25% of that 30% was metal and glass. Therefore, the waste product was <5% of its volume compared to the raw waste. The residue from over sixty large refuse sacks fitting into six buckets. Of course, these reduction ratios are dependent on the nature of the waste.
The gasifier 14 receives char and pyrogas from the pyrolysis tube 17 where char falls out of the end of the pyrolyzer 13 and forms a char bed in the bottom of the gasifier 14. In the preferred form the gasifier 14 is of an updraft gasifier type. Steam and air is injected at the bottom of the char bed and percolates upwards through the char undergoing various chemical reactions and reducing the steam and char to carbon monoxide and hydrogen commonly known as syngas. This reaction occurs at about 850° C. and is self regulating by means of the endothermic and exothermic nature of competing reactions and their different reaction rates at different temperatures. The syngas combines with the pyrogas in the freeboard above the gasifier char bed and is passed to the oxidiser 15 by pipe 21. Ash containing a small amount of unreacted carbon is discharged from the bottom of the gasifier into an airtight ash container to prevent uncontrolled ingress of air into the bottom of the gasifier 14.
The advantage of an updraft gasifier as utilized in the present system is that it is a simple design and less sensitive to particle size, homogeneity and moisture content than other types of gasifiers (e.g. compared to a downdraft or fluidised bed gasifier). The simple design makes the gasifier easier to operate, more reliable and cheaper to build which are all clear advantages for a compact system according to the invention. However, other types of gasifiers may be employed such as downdraft gasifiers and cross flow gasifiers depending upon the selected energy recovery option.
Preferably the gasification chamber 14 comprises an insulating outer surface to retain heat and to improve the overall energy efficiency of the invention.
It is not mandatory to actively gasify in the gasifier 14 for all waste processing systems according to the invention, but it will be necessary to actively gasify for applications where maximising energy recovery (heat or power) is required. Although large waste processing plants utilise steam gasification, it will be possible to opt for exhaust gas gasification since exhaust gas is more readily available than steam in smaller systems.
When in exhaust gas gasification mode the exhaust gases can be re-circulated to the bottom of the gasifier using a gasification fan (26) and recirculation pipework (27) (seen in
Alternatively, if the exhaust gases are extracted downstream of the indirect heat exchanger (at typically 200-300° C.), then the recirculation pipework can be shorter and insulated.
The gases generated in the pyrolyzer 13 and gasifier 14 are fed forward into the oxidizer 15, a refractory-lined combustion vessel maintained at approximately 1100° C. The gases are combusted by mixture with a controlled amount of air in this environment. This is a highly exothermic reaction, which is what maintains the temperature. Some products from this reaction go forward to the pyrolyzer 13, to be used to heat the waste in the pyrolysis stage. Such an aspect of the invention improves energy efficiency and economy.
As mentioned, the oxidizer 15 receives mixed pyrogas and syngas via pipe 21. The oxidizer 15 mixes syngas with air where it is oxidised to release chemical energy in the form of heat. Oxidizer outlet temperature is controlled by adjusting the amount of excess combustion air that is introduced into the oxidizer. Good combustion is achieved by ensuring that the syngas and combustion air is mixed well in a turbulent environment with a long residence time at a high temperature. Typically the oxidizer temperature is maintained at 1100° C., but it can be operated as low as 850° C. Preferably the residence time of the gases within the oxidizer 15 is greater than two seconds. Sufficient mixing and turbulence is achieved by injecting the combustion air into the oxidizer at high velocity (greater than 20 m/s) and directing the jet of combustion air (or plurality of combustion air jets) into the centre of the syngas injection port.
Preferably the oxidizer 15 comprises an insulating outer surface to retain heat and to improve the overall energy efficiency according to the invention.
To enable the process to start, a start-up burner 30 is located within the oxidiser 15. This is used to warm up the apparatus to operating temperature prior to waste feed. A conventional burner using any conventional fuel (gas or oil) can be used.
Since there is an excess of heat remaining in the gas once it has completed the pyrolysis process this may be reclaimed using a heat reclaimer 31 in the form of an indirect heat exchanger such as a waste heat boiler. The specifications of heat reclamation depend on the environment of the waste processing system.
Whatever gas cooling is used, with heat reclamation or not, an exit temperature of 200° C. for the gases from flue 16 is recommended, in order to avoid acid gas condensation and also so that the gas is at a good temperature for acid remediation, which local legislation may require.
There is potential for electrical generation from the reclaimed heat, alternatively it may be desirable to utilize the heat itself in uses relevant to a particular situation, like absorption chilling. Uses for excess heat include space heating or process heating with hot water or steam, air conditioning, distillation and/or desalination.
The use of sodium bicarbonate and a bag filter is recommended for the remediation of HCl, SO2 and particulate matter. However, this remains a choice of the end user, dependent on the legislative framework in which the plant will be expected to operate and the nature of the waste being processed.
Most preferably the system of the invention is maintained under negative pressure by the use of ID (induced draft) fans at the back of the plant (not illustrated in
The fans discharge gas via a flue (not illustrated) to atmosphere. Said flue 16 need only be about one meter taller than the roof of the structure housing the plant, provided there are no other higher occupied buildings in the immediate vicinity, dependent upon the legislative framework in which the plant will be expected to operate.
In other respects the function and operation of the components is analogous to the plant described above so the same reference numerals have been used (assuming a given component is visible). It should be noted with reference to
Referring to
Shredded material is compacted by a screw compactor 12 then fed to the pyrolyzer stage 13. Char and gas is delivered to the gasifier 14 where ash can be removed via a screw 20 to an ash bin 23 located (when assembled) outside the ISO container. For transit ash bin 23 may be housed within one of the containers, depending on available space.
Syngas from the updraft gasifier 14 is fed through pipe 21 to the oxidizer stage 15 where heated gas (mixed with air/oxygen) can be utilized to provide energy for heating the pyrolyzer stage.
Operation of the waste processing system is controlled and monitored via a control panel 24, preferably with a view screen visible through a sidewall of the ISO container. Operating parameters can be predetermined and alerts raised if conditions exceed certain limits. Adjacent the control panel in
An integrated system with separate pyrolysis, gasification and oxidation has a number of advantages over conventional incineration. Most combustion processes undergo pyrolysis, gasification and oxidation at the same place at the same time. By contrast, separate pyrolysis, gasification and oxidation in a compact system according to the invention allows much better control of each of the individual processes. For example, it can be ensured that all waste material is heated to a uniform temperature and that none of it is heated above 900° C. In conventional incineration on a hearth there are hot spots and cold spots. Some pieces of waste are not heated sufficiently and come out in the ash unburnt. Conversely some pieces of waste are overheated and release all their noxious constituents in the gaseous phase.
In the process described it is possible to ensure that all material is heated and thermally decomposed, but none of the waste material is overheated and the amount of pollutants that are volatized is less. In addition to this, the process combusts a medium calorific value gas in a highly controlled oxidizing environment. Again, there are no hot spots or cold spots compared to the combustion zone in a conventional mass burn incinerator. This results in complete combustion of the gases with lower concentrations of carbon monoxide and volatile organic compounds. A uniform oxidation temperature results in the generation of less temperature generated nitrogen oxides (thermal Nox) while the pyrolysis and gasification process subdue the generation of fuel generated nitrogen oxides (fuel Nox) due to their reducing nature. This means that the nitrogen oxide levels in the exhaust gas are typically lower than that of a mass burn incinerator.
Furthermore, the low off-gas velocity of an updraft gasifier results in a very low particulate loading in the syngas to the oxidizer.
The result of all these features is that the un-cleaned exhaust gas at the oxidizer outlet is cleaner than that of an equivalent mass burn incinerator prior to flue gas remediation.
In one form the system of the invention preferably includes a heat reclamation heat exchanger (e.g. a boiler). Such a boiler is designed to quench the exhaust gases from 450° C. to 200° C. as quickly as possible (typically 0.25 seconds). This is in order to minimise the potential for de-Novo synthesis of dioxins and furans. De-Novo synthesis is also minimised because of the effectiveness of the pyrolysis, gasification and oxidation system to destroy the pre-curlers of de-Novo sythesis.
As previously mentioned, the system is maintained at a slight negative pressure (typically <−5 mbarg) by an ID fan to prevent fugitive gaseous emissions. Although air leakage into the system is beneficial for environmental and health and safety issues, it is detrimental to the performance of the process and must be minimised. This requires careful specification and design of all flanges, connections, expansion joints and valves and instrumentation using common engineering practice.
Thermal expansion of components, as the system is heated up from ambient temperature to operating temperature, needs to be accommodated in the design. Of particular difficulty is the thermal expansion of the pyrolysis tube 17 which may expand by up to three inches. This is a concern because it is important that air is not leaked into the pyrolysis tube causing pyrolysis to cease and combustion to occur. The solution to this problem is to anchor the pyrolysis tube 17 to the gasifier 14 with a static flange and allow the tube 17 to expand towards the feed end. This means that the greatest thermal expansion occurs at the cold end of the pyrolyzer 13. A simple fabric expansion gaiter 22 can be used to seal between the pyrolysis tube 17 and the pyrolysis chamber 13 at the cold end. Also, because the feed system is static, the thermal expansion of the pyrolysis tube 17 towards the feed system needs to be accommodated. This is achieved by inserting the feed tube into the pyrolysis tube with a generous clearance to prevent jamming. A gas tight seal is achieved by using the fabric expansion gaiter.
The waste processing system of the present invention is defined as an “advanced thermal conversion process” (by the UK Department for Environment, Food and Rural Affairs—DEFRA).
The invention accepts a variable and broad range of wastes with a continuous rather than batch feed with no pre-treatment required and can dispose of waste at source. Furthermore, it generates very low levels of emissions, especially when considering its size.
Flue gas remediation can be easily added to ensure that the gaseous emissions meet any legislative requirements.
Most preferably the system has a small footprint, is light weight, highly automated with low skill required, can be skid mounted, quickly and easily started up and shut down, and easy to integrate with minimal servicing required.
In its preferred form the waste processing system of the invention is containerized as illustrated to provide mobility, i.e. equipment components are constructed so as to fit into two twenty-foot ISO containers. The benefit of this is rapid deployment and relocation in the field without the need for special haulage or cranes.
This application is based on provisional application Ser. No. 61/233,239, filed Aug. 12, 2009, all of the details of which are incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| 61233239 | Aug 2009 | US |
