Process for incinerating biosolids containing per- and polyfluoroalkyl substances (PFAS)

Abstract

Discussed herein in a process for remediating biosolids containing per- and polyfluoroalkyl (PFAS) compounds in a grate incinerator having a combustion chamber divided into a relatively high temperature zone and a relatively low temperature zone by an insulated panel. Biosolids are loaded onto a grate and moved through the relatively low temperature zone where the temperature is maintained at a temperature sufficient to decompose PFAS compounds but less than a temperature that gives rise to produced ash softening or melting.

Description

[0001] PROCESS FOR INCINERATING BIOSOLIDS CONTAINING PER- AND POLYFLUOROALKYL SUBSTANCES (PFAS)

[0002] RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application Serial No. 63 / 676,472 filed on July 29, 2024, which is incorporated herein by reference in its entirety.

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to processes for reducing or destroying PFAS in substances, and particularly to a process for incinerating biosolids containing PFAS.

[0006] BACKGROUND OF THE INVENTION

[0007] Destroying PFAS substances in an incinerator is challenging due to a number of factors. First, PFAS compounds can withstand extreme conditions, including high temperatures. Their carbon-fluorine bonds are among the strongest in organic chemistry, requiring very high temperatures to break down effectively. Furthermore, many incinerators operate at temperatures that are not sufficient to completely break down PFAS and this leads to incomplete combustion. Furthermore, this can result in the formation of hazardous - byproducts or the release of partially degraded PFAS compounds into the environment.

[0008] There is, however, one type of incinerator that can be operated at temperature ranges that degrade or destroy PFAS compounds. These incinerators are known as grate-type incinerators. However, the problem with grate-type incinerators is that the temperature in the combustion chamber tends to melt or soften the ash produced in the course of incinerating the material or substances. When this occurs, the incinerating process has to be shut down in order that the softened or melted ash can be removed through a time-consuming and cumbersome cleaning process.

[0009] SUMMARY OF THE INVENTION

[0010] The present invention is directed to an incineration process for remediating PFAS in a material, such as biosolids, that avoids melting or softening the ash produced in the course of the process. Two temperature zones are provided in the combustion chamber of a grate incinerator, a relatively high temperature zone in an upper portion of the combustion chamber, and a relatively low temperature zone in a lower portion of the combustion chamber in and around the area occupied by a grate that carries and conveys material containing the PFAS through the combustion chamber. Temperature in the lower portion of the combustion chamber in and around the grate is maintained sufficiently high to degrade or transform the PFAS contained in the material supported on the grate, but not high enough to soften or melt the ash produced in the course of the incineration process. Temperature in the upper portion of the combustion chamber is maintained sufficiently high to further treat the more difficult to destroy compounds and any products of incomplete combustion that have volatilized during the incineration process prior to being discharged to the atmosphere.

[0011] In one embodiment, an insulated steel panel is disposed across the combustion chamber of the incinerator. Effectively, the insulated steel panel separates the combustion chamber of the incinerator into two zones, an upper zone and a lower zone. During the incineration process, the temperature in the upper zone is relatively high and, in one example, tends to be approximately 1 , 050-1 , 100°C. In the lower zone underneath the insulated steel panel, the temperature during an incineration process is maintained at approximately 800- 900°C. In the course of remediating PFAS-containing biosolids, the biosolids are conveyed on the grate into the lower zone of the combustion chamber of the grate incinerator. The biosolids are subjected to a temperature of approximately 800-900°C. As the grate moves through the lower portion of the combustion chamber, the biosolids are burned and in the process produces ash, and at these high temperatures decomposes or transforms the PFAS molecules and compounds. By maintaining and controlling the temperature in the lower zone of the combustion chamber, the process maintains a sufficient high temperature to degrade or transform the PFAS molecules and compounds but maintains the temperature below a threshold temperature that will cause the produced ash to melt or soften.

[0012] Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of the invention.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a simplified drawing of a grate incinerator illustrating biomass, supported on a grate, being transferred through the lower temperature zone, underneath an insulated steel panel.

[0015] DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0016] The present invention relates to an incineration process for remediating PFAS contained in biosolids. The principle underlying the invention is to create two temperature zones in the incinerator, a lower temperature zone in and around the grate, and an upper temperature zone in the upper portion of the incinerator. The process maintains the temperature in the lower zone in and around the grate at a temperature that is sufficient to degrade or transform the PFAS in the biosolids being incinerated, but yet at a temperature that will not result in the softening or melting of the ash being transferred through the incinerator on the grate. Figure 1 is a schematic illustration of a grate incinerator, the grate incinerator being generally referred to by the numeral 10. Details of the grate incinerator 10 are not dealt with herein in detail because the design and operation of a grate incinerator is well known and appreciated by those skilled in the art, and furthermore, details of the grate incinerator are not material to the present invention. However, a general discussion of the grate incinerator 10 is helpful in understanding and appreciating the present invention which is directed to a method for incinerating PFAS without causing the produced ash to melt or become soft.

[0017] Accordingly, grate incinerator 10 includes a combustion chamber 12 where waste, which in this case is biosolids, is burned at high temperatures. Grate incinerator 10, it includes a moving grate 14 disposed in the bottom of the combustion chamber 12. Moving grate 14 is mechanically driven and travels along the bottom area of the combustion chamber 12. It supports the biosolids while allowing air to flow upwardly through the grate 14. Ash produced during the course of burning the biosolids can fall through the grate 14, and typically falls into an underlying ash pit. In the schematic illustration of Figure 1 , the ash is directed into a conduit 20 and conveyed, by a screw conveyor or other type of conveyor, from the incinerator 10 to an appropriate outlet. Communicating with the moving grate 14 is a biosolids feed arrangement. This could entail various approaches and designs. For example, the incinerator 10 could have a hopper for receiving the biosolids. In the example shown in Figure 1 , there is a conduit 16 that includes a screw conveyor for conveying the biosolids into the incinerator 10. Disposed below the grate 14 is a fan 22 that directs air upwardly through the grate 14 into the combustion chamber 12. This supplies oxygen to support combustion and promotes efficient combustion. Incinerator 10 may include one or more additional fans for supplying air to the combustion chamber 12. In the example shown in Figure 1 , there is an additional fan 24. Also, the incinerator 10 is provided with one or more heat sources. In the example shown in Figure 1 , there is provided a burner 26 that in this example is a natural gas burner. As a part of the overall control system for controlling the temperature in various parts of the combustion chamber 12, there is provided a series of temperature indication and transmission instruments 28.

[0018] An insulated steel panel 18 extends across the combustion chamber 12 and divides the combustion chamber into an upper zone 12A and a lower zone 12B. Note that the insulated steel panel 18 is spaced above the moving grate 14. Effectively, the insulated steel panel 18 divides the combustion chamber 12 into two temperature zones, a relatively high temperature zone in the upper zone 12A of the combustion chamber and a relatively low temperature zone in the lower zone 12B of the combustion chamber. This allows for controlling the temperature particularly in the lower zone 12B. That is, the combustion chamber 12 can be operated at two different temperatures or two different temperature ranges. This is significant because the process of the present invention aims to maintain the temperature in the lower zone 12B within a temperature range that will decompose the PFAS in the biosolids, but yet will not be at a temperature so high that the ash produced in the combustion process will soften or melt. Various factors can impact the temperature distribution throughout the combustion chamber 12. In some cases, one can expect the temperature in the upper portion of the combustion chamber to be relatively high. This is where the combustion gases and flames rise due to the heat generated in the combustion process and the mixing of hot gases. In any event, the insulated steel panel 18 functions to at least partially isolate the lower zone 12B from the upper zone 12A. This, in part at least, enables the temperature within the lower zone 12B to be controlled within a range (800-900°C) and to be maintained at a lower temperature than the temperature in the upper zone 12A.

[0019] The provision of the insulated steel panel 18 is one example of creating two temperature zones in the combustion chamber 12. There are other ways and means for creating these two temperatures zones. That is, other types of barriers or panels can be installed in the combustion chamber and strategically placed therein such that two temperature zones are effectively formed. As another alternative, the two temperature zones could be achieved through advance control of the flue gas recirculation air and / or the use of additional burners. Such an approach would alleviate the need for a physical separation between the two zones.

[0020] The inventors have discovered that in a grate incinerator, such as that shown and described herein, the temperature in the lower zone can be controlled within a range of 800- 900°C. When the temperature in the lower zone 12B is controlled within this range, a typical temperature range in the upper zone 12A would typically be in the range of 1 ,000-1 ,500°C. Further, the inventors found that, by controlling the temperature in the lower zone 12B in the range of 800-900°C, the PFAS in the biosolids is substantially degraded or transformed, if not totally destroyed. Furthermore, it was found that by controlling the temperature in the lower zone 12B within the range of 800-900°C, the ash produced in the combustion process would not melt or soften, but instead would efficiently fall through the grate into an underlying ash collector or as suggested in Figure 1 , conveyed from the combustion chamber into a discharge conveyor. In carrying out the process, biosolids is loaded into the feed conveyor 16 and fed onto the grate 14 at an area in the vicinity of an inlet to the combustion chamber 12. Once the grate 14 is loaded with the biosolids, the grate moves the biosolids through the lower zone 12B of the combustion chamber 12. Once in the lower zone 12B, initial ignition takes place and the burning process begins and soon thereafter burning becomes self-sustaining. The moving grate 14 continues to move underneath the insulated steel panel 18 and through the lower temperature zone, and the biosolids carried on the grate are exposed to a temperature range of 800-900°C. Combusted biosolids that do not burn entirely is referred to as ash. In some incinerator designs, the ash falls through gaps in the grate and is collected in an ash collector or pit that lies below the grate. In the example schematically shown in Figure 1 , the ash is actually conveyed on the grate to a point of discharge where the ash is discharged into a screw-type conveyor, for example. As described above, by controlling the temperature below the insulated grate panel and in the lower zone 12B to a temperature range of 800-900°C, this ash remains solid.

[0021] There are numerous advantages of the process of the present invention over conventional approaches for degrading PFAS in biosolids. In many cases, existing furnaces or incinerators do not operate at temperatures high enough to treat PFAS. However, some conventional furnaces that do operate at temperatures high enough to treat PFAS will likely have issues with ash fusion resulting in high operating and maintenance costs. Other technologies, such as gasification and pyrolysis, must use additional treatment for flue gases. The present process carried out in a grate furnace can operate at sufficiently high temperatures to treat PFAS and at the same time control or minimize issues caused by ash fusion. Finally, the process of the present invention does not require additional treatment steps, (other than meeting the required air pollution control limits such as Mact LLLL) which is independent from PFAS destruction such as those that are often required in other conventional approaches.

[0022] The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments disclosed herein are therefore to be construed in all respects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

CLAIMSWhat is claimed is:1 . Process for remediating biosolids containing per- and polyfluoroalkyl (PFAS) compounds in a grate incinerator having a combustion chamber divided into a relatively high temperature zone and a relatively low temperature zone by a panel that extends through the combustion chamber, the process comprising: placing the biosolids containing PFAS on a grate; moving the grate and the biosolids thereon underneath the panel and through the relatively low temperature zone formed by the panel; as the grate and biosolids move through the relatively low temperature zone, incinerating the biosolids and in the process producing ash; in the course of incinerating the biosolids in the relatively low temperature zone, thermally decomposing or transform the PFAS compounds; and maintaining the produced ash in a solid form and preventing the produced ash from melting as the grate and biosolids move through the relatively low temperature zone by maintaining the temperature in the relatively low temperature zone at approximately 800-900°C.

2. The process of claim 1 including maintaining a temperature differential of at least 200°C between the relatively low and high temperature zone.

3. The process of claim 1 wherein the panel separating the relatively low temperature zone from the relatively high temperature zone in the combustion chamber comprises an insulated steel panel.

4. The process of claim 3 wherein there is provided a flue gas opening between an edge of the insulated steel panel and a wall of the combustion chamber.

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