The present invention relates to an exhaust gas treatment system for and method of treating exhaust gas, in particular from a waste treatment system for processing waste product, such as municipal solid waste (MSW) and processed waste, for example, refuse-derived waste (RDF), including human waste, waste which contains organofluorines, such as perfluorocarbons (PFCs), including perfluoroalkyl and polyfluoroalkyl substances (PFAS), such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS) and perfluorohexane sulfonic acid (PFHxS), and also fluorinated gases (F-gases). The waste product can be a solid, liquid or gas, or combinations thereof, and can be supplied in many forms, for example, sludge, plastic and biomass, and includes tires and flooring material and tiles.

PFAS are substances having a chain of linked carbon and fluorine atoms, and, because the carbon-fluorine bond is one of the strongest, these substances are recalcitrant and not easily degraded.

Waste product typically contains a significant percentage of water, which gives rise to problems in processing, especially in systems using high- temperature combustion. The present inventors have, however, recognized that this contained water can advantageously be employed in processing of the waste product itself, in that, if the contained water is extracted to generate supercritical water, this supercritical water can be utilized for supercritical water oxidation (SCWO), which has been found to reduce the PFAS content by greater than 99% (Krause et al, Supercritical Water Oxidation as an Innovative Technology for PFAS Destruction, Journal of Environmental Engineering, 148, 10.1061/(ASCE)EE.1943-7870.0001957). The PFAS content of the waste product would otherwise have to be converted thermally at high temperature, typically above 1200 C, and moisture arising from water in the waste product would have to be carefully regulated. In one aspect the present invention provides an exhaust gas treatment system, comprising: a carbon dioxide scrubber which receives an exhaust gas, optionally from a waste processing system; an absorbent supply which supplies absorbent to the scrubber, wherein the absorbent acts to remove carbon dioxide and residual pollutants from the exhaust gas; and an absorbent reformer which receives the carbon dioxide-rich absorbent from the scrubber and reforms the absorbent for re-use.

In another aspect the present invention provides a method of treating an exhaust gas, comprising: delivering an exhaust gas, optionally from a waste processing system, to a carbon dioxide scrubber; supplying absorbent to the scrubber from an absorbent supply, wherein the absorbent acts to remove carbon dioxide and residual pollutants from the exhaust gas; and reforming the absorbent from the carbon dioxide-rich absorbent which is received from the scrubber using an absorbent reformer for re-use.

Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which:

Figure 1 illustrates a waste processing system in accordance with one embodiment of the present invention;

Figure 2 illustrates a horizontal sectional view (along section I-I in Figure 1) of the system of Figure 1;

Figure 3 illustrates an exhaust gas treatment system in accordance with one embodiment of the present invention; and

Figure 4 represents the reduction in carbon dioxide emission to the atmosphere using the exhaust gas treatment system of Figure 3. Figures 1 and 2 illustrate a waste processing system in accordance with one embodiment of the present invention.

The system comprises a primary waste product supply 3 for supplying waste product, a secondary waste product supply 5 for supplying waste product, a drying chamber 7 which is supplied with the waste product from the primary waste product supply 3 and heats the waste product to dry the waste product and generate steam, a gasification chamber 9 which receives the waste product from the drying chamber 7 and in which the waste product is heated to generate synthetic gas or syngas, a treatment unit 11 for treating steam vented from the drying chamber 7, and a thermai converter 12 for generating a stream of heated gas for energy conversion.

In one embodiment the waste product is municipal solid waste (MSW) or processed waste, for example, refuse-derived waste (RDF), including human waste.

In one embodiment the waste product contains organofiuorines, such as perfluorocarbons (PFCs), including perfluoroalkyl and polyfluoroalkyl substances (PFAS), for example, perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS) and perfluorohexane sulfonic acid (PFHxS).

In one embodiment the waste product contains fluorinated gases (F-gases).

In one embodiment the waste product comprises a solid, liquid or gas, or combinations thereof.

In one embodiment the waste product is supplied in many forms, for example, sludge, plastic and biomass, and includes tires and flooring material and tiles.

In this embodiment the primary waste product supply 3 comprises a container 15, here in the form of a hopper, which contains waste product, in one embodiment in the form of a solid or sludge or mixtures thereof, and a waste feed 17 which feeds waste product from the container 15 to the drying chamber 7.

In this embodiment the waste feed 17 comprises a ram 19 which is actuated reciprocally between a first, material-receiving position, in which waste product can fall from the container 15 in front of the ram 19, and a second, feed position, in which the waste product ahead of the ram 19 is fed, here pushed, into an upstream end of the drying chamber 7. With this action of the ram 19, the feeding of fresh waste product into the drying chamber 7 causes waste product at a downstream end of the drying chamber 7 to be fed into an upstream end of the gasification chamber 9.

In this embodiment the drying chamber 7 comprises an elongate cavity 21 along which waste product is displaced with operation of the waste feed 17.

In this embodiment the drying chamber 7 includes a heated gas source 23 which supplies a heated gas into the cavity 21 thereof, which heated gas can be a recirculated exhaust gas, and an outlet 24, here including a plurality of vents 26, through which steam, in this embodiment flash steam, is vented from the drying chamber 7. With this configuration, the heated gas supplied to the drying chamber 7 and heat from the gasification chamber 9 acts to heat the waste product within the cavity 21 of the drying chamber 7.

In this embodiment the temperature of the drying chamber 7 is controlled such that the upstream end of the cavity 21 is maintained at a temperature of less than 600 C, which is sufficient to yield steam from the moisture within the waste product.

In this embodiment the treatment unit 11 heats the steam which is received from the drying chamber 7 to a temperature above 374 C and a pressure above 22.1 MPa, which represents the critical point, in order to generate supercritical water. In this embodiment the treatment unit 11 includes an inlet 27 which is fluidly connected to the outlet 24 of the drying chamber 7 and an outlet 28 which is fluidly connected to the thermal converter 12.

In this embodiment the treatment unit 11 includes a heated gas source 29 which supplies a heated gas to the treatment unit 11 to heat the received steam, which heated gas can be a recirculated exhaust gas.

In this embodiment the treatment unit 11 includes a reactor 30, which generates supercritical water from the received steam.

In this embodiment the treatment unit 11 includes an oxidant supply 31 which supplies an oxidant, here air, to the generated supercritical water. With the addition of oxidant, the supercritical water provides for supercritical water oxidation of material, here in the thermal converter 12, which, as noted above, has been found to reduce the PFAS content by greater than 99%.

In this embodiment the gasification chamber 9 comprises a cavity 33 having an outlet 34, here in an upper section thereof, and a floor assembly 35, here at lower section thereof, over which waste product is transferred.

In this embodiment the floor assembly 35 comprises a transfer mechanism 39 which provides a grate at the floor of the gasification chamber 9 and transfers waste product which is received from the drying chamber 7 through a gasification zone GZ.

In this embodiment the transfer mechanism 39 comprises a stepped assembly 45 over which waste product is transferred.

In this embodiment the stepped assembly 45 comprises a plurality of steps 47, here of fixed position, which are arranged in staggered downward relation, and a plurality of movable members 49 which are movable reciprocally in relation to the steps 47 to transfer waste product along and over the steps 47.

In this embodiment the steps 47 are arranged substantially in spaced, parallel relation, and the movable members 49 are configured such that upper and lower surfaces of the movable members 49 are in close relation to adjacent surfaces of the steps 47, whereby the action of withdrawing the movable members 49 acts to scrape material therefrom.

In this embodiment ones or groups of ones of the movable members 49 are movable independently of one another, so as to enable control of an amount of waste product in the gasification zone GZ. This manner of control allows readily for use with different kinds of waste product, with the rate of transfer being controlled accordingly.

In this embodiment the gasification chamber 9 includes a heated gas source 51 which supplies a heated gas to the gasification chamber 9, which heated gas can be a recirculated exhaust gas, and heats the waste product to generate synthetic gas or syngas, with the temperature within the gasification zone GZ being controlled so as to prevent combustion, here to a temperature of less than 600 C.

In this embodiment the heated gas source 51 supplies the heated gas to the gasification chamber 9 through the transfer mechanism 39.

In this embodiment the gasification chamber 9 includes a combustion zone CZ at a downstream region thereof, at which a temperature is maintained to provide for starved combustion of the waste product thereat, which waste product has passed through the gasification zone GZ and from which syngas has been extracted.

In this embodiment the gasification chamber 9 includes a flow restrictor 55, here in the form of an apertured refractory member, at the outlet 34 thereof through which the flow of the generated syngas and heated gas is restricted, in order to provide for controlled generation and supply of syngas from the gasification chamber 9.

In this embodiment the thermal converter 12 is fluidly connected to the outlets 28, 34 of the treatment unit 11 and the gasification chamber 9.

In this embodiment the outlet 28 of the treatment unit 11 is fluidly connected to the thermal converter 12 downstream of the outlet 34 of the gasification chamber 9.

In this embodiment the thermal converter 12 includes an elongate cavity 71, with the outlet 34 of the gasification chamber 9 being located at an upstream region of the cavity 71.

In this embodiment the thermal converter 12 includes a flow accelerator 73 which is fluidly connected to the outlet 34 of the gasification chamber 9 and acts to provide a flow of syngas of increased velocity from the gasification chamber 9.

In this embodiment the thermal converter 12 includes a burner 75 which acts to ignite and provide for burning of the syngas as delivered from the gasification chamber 9, and a suction fan 77 which acts to draw the syngas into the cavity 71 of the thermal converter 12.

In this embodiment the suction fan 77 is regulated to control temperature and residence time within the thermal converter 12.

In this embodiment the thermal converter 12 is controlled such that a temperature of at least 1000 C, in one embodiment at least 1100 C, is maintained in the cavity 71 thereof. In this embodiment the temperature within the cavity 71 of the thermal regulator 12 is maintained at a temperature of less than 1400 C. In this embodiment the secondary waste product supply 5 comprises a container 81, here in the form of a chamber, which contains waste product, in one embodiment in the form of a particulate, liquid or gas or mixtures thereof, and a waste feed 83 which feeds waste product from the container 81 into the cavity 71 of the thermal converter 12.

In one embodiment the container 81 is adapted separately to contain particulate, liquid and gas or mixtures thereof.

In one embodiment the waste feed 83 comprises one or more injectors 85.

With this configuration, the thermal converter 12, in addition to generating a stream of heated gas for energy conversion, acts further to destroy substances which are delivered in the gas flow from the gasification chamber 9, including residual hydrocarbons, organofluorines and fluorinated gases, and also waste product which is delivered directly by the secondary waste product supply 5 into the cavity 71 of the thermal converter 12.

Figure 3 illustrates an exhaust gas treatment system in accordance with one embodiment of the present invention.

The exhaust gas treatment system comprises a carbon dioxide scrubber 101 which receives an exhaust gas, in this embodiment from the waste treatment system of the above-described embodiment, and an absorbent, which acts to remove carbon dioxide and residual pollutants from the exhaust gas, an absorbent supply 103 for supplying absorbent to the scrubber 101, and a carbon dioxide reformer 105 which receives the carbon dioxide-rich absorbent from the scrubber 101 and reforms the absorbent and yields carbon dioxide for capture.

In this embodiment the scrubber 101 is a cyclone scrubber. In one embodiment the absorbent is provided as a spray.

In this embodiment the absorbent is an aqueous absorbent.

In one embodiment the absorbent is an amine.

In this embodiment the absorbent is monoethanolamine (MEA).

In this embodiment the reformer 105 reforms the carbon-dioxide absorbent by steam reforming to boil the absorbent, here with steam supplied from the waste treatment system, and yields carbon dioxide for capture, with the lean, reformed absorbent being recirculated to the absorbent supply 103.

In this embodiment the system further comprises a carbon dioxide extraction unit 107, which extracts carbon dioxide from the reformer 105 under vacuum and compresses the extracted carbon dioxide for recycling.

In this embodiment the system further comprises a filtration unit 109, which receives the residual water from the reformer 105 and filters residual pollutants therefrom.

With this configuration, the present inventors have recognized that, by utilizing steam, as an available by-product of the waste processing system, much reduced consumption of the absorbent can be achieved and the captured carbon dioxide can be recycled to provide a usable source of carbon dioxide.

Use of the embodied exhaust gas treatment system in the above-described waste processing system avoids significant carbon dioxide emission to the atmosphere as compared to incineration, with the present embodiment typically yielding a reduction in the atmospheric carbon dioxide emission of greater than 80%, as represented in Figure 4. Final ly, it will be understood that the present invention will be described in its preferred embodiments and can be modified in many different ways without departing from the scope of the present invention as defined by the appended claims.

For example, in the above-described waste processing system, the secondary waste product supply 5 could be omitted.