WASTE PROCESSING SYSTEM

Abstract

The invention relates to an apparatus for producing syngas, typically from municipal waste. In particular, a gasifier is used in combination with a plasma furnace. The system is configured so that non-airborne char generated in the gasifier is removed from the system prior to delivery to the plasma furnace. This enhances the energy efficiency of the system whilst still yielding excellent yields of syngas.

Claims

1. An apparatus for processing a feedstock into syngas, the apparatus comprising: a fluidised bed gasifier adapted to receive a feedstock; and a free radical generator; wherein the gasifier is in fluid communication with the free radical generator such that gases and airborne char generated by the gasifier are conveyed to the free radical generator; wherein the gasifier comprises one or more outlets to permit the removal of non-airborne char from the apparatus; and wherein the fluid communication between the gasifier and the free radical generator is provided by a conduit, said conduit containing at least one oxidising agent inlet.

2. (canceled)

3. The apparatus of claim 1, wherein the free radical generator is a plasma furnace.

4. The apparatus of claim 1, wherein the free radical generator comprises one or more outlets to permit the removal of ash.

5. The apparatus of claim 1, further comprising a feedstock hopper for storing the feedstock prior to delivery to the gasifier, wherein the hopper is gas purgeable.

6. The apparatus of claim 1, further comprising a conveyor adapted to deliver the feedstock from the feedstock hopper to the gasifier.

7. The apparatus of claim 1, wherein the gasifier comprises a fluid bed and an inlet for delivering an oxidising agent to the fluid bed.

8. The apparatus of claim 7, wherein the oxidising agent is oxygen.

9. (canceled)

10. The apparatus of claim 1, further comprising a heat exchanger adapted to cool gas material exiting the free radical generator.

11. The apparatus of claim 1, further comprising a fluid pumping means adapted to control the flow of gas through the apparatus.

12. The apparatus of claim 1, further comprising a controller configured to receive one or more inputs indicative of one or more variables of the syngas production process; and, based on said input, control one or more components of the apparatus so as to ensure a constant rate of production of syngas.

13. A process of making syngas from a feedstock, the process comprising: i) delivering the feedstock to a fluid bed gasifier; ii) gasifying the waste in the presence of a first oxidising agent to produce a non-airborne char and a gas stream, the gas stream comprising a syngas and an airborne char; and iii) transferring the gas stream to a free radical generator, wherein the gas stream is transferred to the free radical generator via a conduit in which a second oxidising agent is added to the gas stream; wherein non-airborne chars and bottom ashes generated in the gasifier are not transferred to the free radical generator.

14. The process of claim 13, wherein the temperature of the gasifier is in the range 600° C. to 700° C.

15. (canceled)

16. The process of claim 13, wherein the temperature in the conduit is in the range 1000° C. to 1200° C.

17. The process of claim 13, further comprising the step of: iv) rapidly cooling the gas stream to a temperature of less than 600° C.

18. The process of claim 13, wherein the first and second oxidising agents each independently comprise at least 90% oxygen.

19. The process of claim 13, wherein the free radical generator is a plasma furnace.

20. The process of claim 13, wherein the process is controlled to produce syngas at a constant rate.

21. (canceled)

22. The process of claim 13, wherein the process is conducted at or below atmospheric pressure.

Description

DESCRIPTION OF FIGURES

[0060] FIG. 1 shows a schematic diagram of a preferred apparatus of the invention.

[0061] FIG. 2 shows a schematic diagram of the conduit between the gasifier and the free radical generator.

[0062] FIG. 3 shows a schematic diagram of the heat exchanger.

DETAILED DESCRIPTION

[0063] FIG. 1 shows a schematic diagram of a typical apparatus 1 of the invention. A feedstock is delivered to waste hopper 10. The feedstock hopper is not particularly limited in size but is equipped with an inlet 12 through which carbon dioxide gas can be administered during operation. The carbon dioxide displaces air present within the hopper and so purges substantially all nitrogen and oxygen contained within the hopper. A screw conveyor (not shown) transports purged waste along a sealed channel at a rate governed by the controller (not shown) to an opening 21 in the fluidised bed gasifier 20 positioned above the fluidised bed 23 such that the feedstock falls onto the fluidised bed 23. The conveyor is in communication with the controller (not shown) and the controller can adjust the rate of delivery of feedstock to the gasifier based on downstream process parameters. In particular, the rate of feed is calibrated so as to ensure a substantially consistent thermal input and production of syngas.

[0064] The gasifier 20 is a vertically aligned cylinder or cuboid with a height of 16 to 20m. It is constructed of refractory lined carbon steel. The gasifier 20 is heated to a temperature of around 800° C. and a mixture of oxygen gas and steam is injected into the gasifier 20 at the base of the gasifier via an inlet 25 so that the oxygen gas mixes with the bed 23 to create a fluid-like bed to which the feedstock is exposed. This creates a fluidised bed and the flow of oxygen and steam is controlled so as to produce a low superficial velocity. The gasified compounds produced in the gasifier, including syngas (i.e. a mixture of carbon dioxide, carbon monoxide, hydrogen and water), tars, airborne char, and fly ash exit the gasifier through outlet 27 into conduit 30. Non-airborne chars and bottom ashes are deposited in the fluidised bed and are periodically removed from the gasifier 20 via outlet 29, together with a portion of fluidised bed material (usually sand). This material is screened to remove large material (predominately non-reactive inorganic components) and the remaining material (sand and char) are returned to the gasifier for further processing. Periodically, the process will be halted and this system will be subject to blowdown where all of the material is rejected and replaced to prevent accumulation of the material in the fluidised bed.

[0065] Conduit 30 includes an oxygen inlet 31. The conduit is operated so that the oxygen gas administered thereto creates a temperature in the conduit such that gases leaving via gas outlet 35 have a temperature of approximately 1150° C. As can be seen from FIG. 2, the conduit 30 is a steeply inclined shaft comprising two oxygen inlets 31a,31b, having nozzles positioned opposite one another on the side walls 32a,32b of the conduit. The conduit is provided with thermal insulation 34, though this is usually in the form of a refractory lining, so as to aid in the maintenance of a consistent temperature within the conduit (and prevent damage to the conduit). Oxygen gas is injected via the oxygen inlets 31a,31b which reacts with the gas stream to generate heat 36 which aids in the maintenance of a constant temperature within the conduit 30 upstream of the oxygen inlets 31a,31b. The gases travel down the conduit 30 in the direction 37 indicated, leaving the conduit via outlet 35. The oxygen inlets 31a,31b include nozzles made from a robust metal material such an austenitic nickel-chromium-based superalloy e.g. Inconel (RTM).

[0066] The gases exiting conduit 30 are delivered to the plasma furnace 40 via a gas inlet 41. The plasma furnace includes a first electrode 43 positioned in the roof 45 of the plasma furnace and a plurality of second electrodes 47 in the base of the shell 49 of the plasma furnace. During operation, an electric arc is generated between the electrodes 43,47. The electric arcs generate high energies that result in the formation of free radicals. The oxygen free radicals formed are particularly effective at breaking down tars. Some fly ash accumulates in the base of the plasma furnace. This material can be removed either continuously or periodically using outlet 48. Some fly ash is transported with the gases exiting the plasma furnace. The location of the inlet and outlet of the plasma furnace are chosen to provide a residence time for the gases, tars and airborne char of about 3 seconds. This provides sufficient time for tar reformation, gasification of the airborne char and capture of fly ash. This is achieved by injecting the gas tangentially in order to create a circular flow around the furnace. The tangential injection promotes the motion of larger particles, such as fly-ash, towards the walls of the system which improves the likelihood that they will be captured. The plasma furnace 40 is cylindrical with the inlet port 41 located tangentially at one side and the outlet port 42 located either at the top or tangentially at the opposite side. The plasma furnace is made of refractory lined carbon steel. The outlet duct from the furnace is angled steeply upward to meet the waste heat boiler. The duct is constructed from refractory lined steel. It should be kept as short as possible to avoid fouling by fly-ash.

[0067] Gas exiting from the plasma furnace 40 is delivered to the waste heat boiler 50. The boiler comprises a first heat exchanger 53 and a further heat exchanger 55. The first heat exchanger rapidly cools the gases existing the plasma furnace to below 600° C. FIG. 3 shows a schematic view of a preferred embodiment of the first heat exchanger 53. The heat exchanger 53 is a horizontal carbon steel tube having walls 51 surrounded by a water jacket 52. The syngas enters via the inlet 54a, moves along the lumen 56 of the tube and is radiatively cooled by the walls 51 of the tube. Fly ash drops out into an ash box 57. Ash can be removed from the ash box 57 using a rotary valve 53. Syngas comprising a reduced fly ash content then leaves via outlet 54b. The second (and optionally third stage) heat exchanger comprises a set of horizontally mounted carbon steel fire tubes passing through the same cooling water system as the first heat exchanger. The gas is convectively cooled in these exchangers. Around 25% of the water in the cooling system is fed back to the gasifier and 75% is available for export to use in drying the feedstock, use in water gas shift reactions downstream or production of power.

[0068] Downstream of the boiler, there is provided an induced draft fan 60 configured to draw gases from the boiler and maintain the rate of flow of through the apparatus. The fan can be operated at a variable speed and is controlled by the controller (not shown). The fan will typically draw the gas at a rate of 15 m/s and will maintain a pressure in the upstream equipment of −5 mbar below atmospheric. The fan is also made from a robust metallic material such an austenitic nickel-chromium-based superalloy e.g. Inconel (RTM). As will be appreciated, the fan must endure harsh conditions and so must be hard wearing.

[0069] The filtration system is a dry gas filter, usually a carbon steel inverted pyramid containing ceramic filter elements through which the syngas is drawn to remove any remaining fly ash. The system is periodically flushed with carbon dioxide to knock ash from the filters into a collection bin at the base of the unit.

[0070] Downstream of the fine particulate filtration system is a measuring unit 80 which monitors various properties of the syngas. These include: temperature, composition, energy content, rate of flow and pressure. The measuring unit communicates this information to a controller which in turn, adapts the behaviour of the various components of the system so as to ensure a regular flow of syngas out from the apparatus. It is not easy to make detailed measurements of the syngas before this point because tars and fly ash would damage the measurement equipment. Therefore, it is typical for the control system to infer the composition and quality of the syngas in earlier stages of the process based upon measurements taken by the measuring unit 80 at the end of the process.

[0071] The calorific value and flow rate of the syngas are combined to calculate the thermal output from the process. The thermal output is used to modulate the feedstock addition rate to the gasifier. The flow rate, temperature and pressure from the system are monitored and will reduce the thermal output set point to ensure the gas flows are within tolerable limits for the equipment. The gas composition is monitored to determine if the gasification of feedstocks is proceeding properly. Each of these methods typically involves a dedicated algorithm using upstream temperatures, pressures and flows to estimate gas stream properties and to respond with a suitable modification of the apparatus' operation in order to a achieve a desired outcome.