ABATEMENT

Abstract

An abatement method is disclosed. The method comprises: supplying a combustion chamber of an abatement apparatus with an effluent stream containing a perfluoro compound, together with combustion reagents and a diluent; heating a combustion zone of said combustion chamber by reacting said combustion reagents to perform abatement of said perfluoro compound to stable by-products, said diluent being selected to remain inert during said abatement. In this way, the perfluoro or other compound is abated in the combustion chamber during the combustion of the combustion reagents, but without creating undesirable compounds such as, for example, NOx or other compounds.

Claims

1. A method, comprising: supplying a combustion chamber of an abatement apparatus with an effluent stream containing a perfluoro compound, together with combustion reagents and a diluent; heating a combustion zone of said combustion chamber by reacting said combustion reagents to perform abatement of said perfluoro compound to stable by-products, said diluent being selected to remain inert during said abatement.

2. The method of claim 1, wherein said diluent is selected to be unchanged during said abatement in said combustion zone.

3. The method of claim 1, wherein said heating raises a temperature of said combustion zone to greater than 1000° C., preferably to greater than 1300° C.

4. The method of claim 1, wherein said abatement apparatus comprises a nozzle for injecting a gas stream into said combustion chamber and said supplying comprises supplying said nozzle with said effluent stream, preferably wherein said supplying comprises supplying said nozzle with said combustion reagents and said diluent.

5. The method of claim 4, wherein said abatement apparatus comprises a pump which supplies said nozzle and said supplying comprises supplying said diluent as a pump purge gas.

6. The method of claim 1, wherein said abatement apparatus comprises a foraminous sleeve at least partially defining said combustion chamber for conveying a gas into said combustion chamber and said supplying comprises supplying said foraminous sleeve with said combustion reagents and said diluent.

7. The method of claim 1, comprising adjusting a ratio of flow rates of said diluent to said combustion reagents to provide a selected minimum destructive rate efficiency of at least one compound in said effluent stream.

8. The method of claim 1, wherein said combustion reagents comprise a fuel and an oxidant.

9. The method of claim 8, wherein said fuel comprises at least one of a hydrocarbon, methane, propane, butane and the like.

10. The method of claim 8, wherein said oxidant comprises oxygen, ozone and the like.

11. The method of claim 1, wherein said diluent comprises at least one of a noble gas and carbon dioxide, or a mixture of a noble gas and carbon dioxide, preferably a mixture of argon and carbon dioxide.

12. The method of claim 1, wherein said diluent comprises a mixture of argon and carbon dioxide in a ratio of around 80% of argon to around 20% of carbon dioxide by volume.

13. The method of claim 8, wherein said oxidant is mixed with carbon dioxide in a ratio of around 35% oxidant to around 65% carbon dioxide by volume.

14. The method of claim 8, wherein said oxidant is mixed with carbon dioxide in a ratio of around 37.5% oxidant to around 63.5% carbon dioxide by volume.

15. The method of claim 8, wherein said oxidant is mixed with argon in a ratio of around 20% to around 80% argon by volume.

16. The method of claim 8, wherein said fuel is mixed with carbon dioxide in a ratio of around 8% fuel to around 92% carbon dioxide by volume.

17. The method of claim 8, wherein said fuel is mixed with combined oxidant and argon in a ratio around 4.5% fuel to around 95.5% combined oxidant and argon by volume.

18. The method of claim 1, comprising recovering and/or recirculating at least some of said diluent from an exhaust stream of said combustion chamber and/or comprising recirculating said diluent together with at least one of said oxidant and said fuel as a contaminant to said combustion chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0037] FIG. 1 illustrates an abatement apparatus according to one embodiment;

[0038] FIGS. 2 to 5 show changes in CF.sub.4 DRE and NOx production under different conditions; and

[0039] FIG. 6 show changes in peak (stoichiometric) burning velocity versus O.sub.2 under different conditions.

DETAILED DESCRIPTION

[0040] Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments provide an arrangement whereby nitrogen or another compound, which would normally be present in a combustion or reaction chamber performing abatement on an effluent stream, and which would produce one or more undesirable by-products within the combustion chamber, is instead replaced by an inert compound which acts as a diluent to preserve the chemical and thermal conditions within the combustion chamber to maintain the appropriate conditions to perform abatement on the effluent stream without generating the undesirable by-products. For example, the avoidance of nitrogen during the abatement prevents the production of NOx, since the abatement would typically occur at temperatures where nitrogen would readily react with oxygen to produce NOx. By supplying a diluent which is typically inert or unreactive under the operating conditions within the combustion chamber, no undesirable NOx is formed. By adjusting the composition of the diluent, the destructive rate efficiency (DRE) of compounds in the effluent stream is increased while NOx generation is decreased, minimised or eliminated. In particular, a mixture of an inert (typically noble) gas and carbon dioxide helps to reduce NOx production and maximise destructive rate efficiency of compounds in the effluent stream. Accordingly, nitrogen is removed from the flame front. An inert gas such as argon or carbon dioxide is used a pump purge instead of nitrogen and/or an argon/oxygen/CH.sub.4 and/or carbon dioxide/oxygen/CH.sub.4 premix is used on the burner instead of air (which contain nitrogen). The carbon dioxide/argon and oxygen can be separated and recovered from the exhaust to be reused.

Abatement Apparatus

[0041] FIG. 1 illustrates an abatement apparatus, generally 10, according to one embodiment. The abatement apparatus 10 comprises a radiant burner which treats an effluent gas stream (which contains a purge gas) pumped from a manufacturing process tool, such as a semiconductor or flat panel display process tool, typically by means of a vacuum pumping system (not shown). The effluent stream is received at inlets 20. The effluent stream is conveyed from the inlet 20 to a nozzle 30 which injects the effluent stream into a cylindrical combustion chamber 40. Each nozzle 30 is located within a respective bore formed in a ceramic top plate 50 which defines an upper or inlet surface of the combustion chamber 40. An oxidant, in this example oxygen, is mixed with the effluent stream as it is conveyed from the inlet 20 to the nozzle 30. A fuel gas is conveyed to a concentric conduit which surrounds the nozzle 30 for delivery as a surrounding curtain into the combustion chamber 40. A fuel gas is also conveyed via a concentric lance 90 located within the nozzle 30 for delivery as an inject into the combustion chamber 40.

[0042] The combustion chamber 40 has sidewalls defined by an exit surface of a foraminous burner element 60 such as that described in EP 0 694 735. The burner element 60 is cylindrical and is retained within a cylindrical outer shell 70. A plenum volume 80 is defined between an entry surface of the burner element 60 and the cylindrical outer shell 70. A mixture of fuel gas, such as natural gas or a hydrocarbon, and an oxidant and purge gas is introduced into the plenum volume 80 via one or more inlet nozzles (not shown). The mixture of fuel gas and oxidant and purge gas passes from the entry surface of the burner element to the exit surface of the burner element for combustion within the combustion chamber 40.

[0043] The ratio of the mixture of fuel gas and oxidant and purge gas is varied to vary the temperature within the combustion chamber 40 to that which is appropriate for the effluent stream to be treated. Operating temperatures within the combustion chamber 40 start at around 800° C. to around 900° C. for some abatement processes. However, the temperatures can also be set to around 1300° C. to around 1500° C. for other abatement processes. At these temperatures, nitrogen present in the combustion chamber 40 can react to produce NOx. Also, the rate at which the mixture of fuel gas and oxidant and purge gas is introduced into the plenum volume 80 is adjusted so that the mixture will burn without visible flame at the exit surface of the burner element 60. The exhaust from the combustion chamber is vented into a downstream cooling chamber (not shown).

[0044] Accordingly, the effluent stream received through the inlets 20 and provided by the nozzles 30 to the combustion chamber 40 is combusted within the combustion chamber which is heated by the mixture of fuel gas and oxidant which combusts near the exit surface of the burner element 60 and forms a flame extending from the nozzles 30. Such combustion causes heating of the combustion chamber and provides combustion products, such as oxygen, typically within a range of 7.5% to 10.5% depending on the air/fuel mixture [CH.sub.4, C.sub.3H.sub.8, C.sub.4H.sub.10], provided to the combustion chamber 40. This heat and the combustion products react with the effluent stream within the combustion chamber 40 to clean the effluent stream. For example, SiH.sub.4 and NH.sub.3 may be provided within the effluent stream, which reacts with O.sub.2 within the combustion chamber 40 to generate SiO.sub.2, N.sub.2, H.sub.2O, NO.sub.x. Similarly, N.sub.2, CH.sub.4, C.sub.2F.sub.6 may be provided within the effluent stream, which reacts with O.sub.2 within the combustion chamber 40 to generate CO.sub.2, HF, H.sub.2O. The combusted effluent stream exhausts from the abatement apparatus 10 and comprises the treated stream.

Existing Operation

[0045] During existing operation, the purge gas supplied the effluent stream is nitrogen. When operating the abatement apparatus 10 (having four inlets 20 having a 6″ diameter and 3″ length) with the effluent stream containing 50 l/min of N.sub.2 leads to 43 ppm of NOx and a CF.sub.4 DRE of 94.2%.

Argon Substitution

[0046] In one embodiment, argon (or other noble gas) is substituted to replace nitrogen as the purge gas. FIGS. 2 to 5 show the changes in CF.sub.4 DRE and NOx production with variations in premix O.sub.2 (FIG. 2), variations in lance CH.sub.4 (FIG. 3), Argon flow (FIG. 4) and curtain CH.sub.4 (FIG. 5). In particular, FIG. 2 shows the effect of premix O.sub.2 flow on CF.sub.4 destruction and NOx formation with Argon at 50 l/min, CF.sub.4 1 l/min, curtain CH.sub.4 3 l/min and lance CH.sub.4 10 l/min; FIG. 3 shows the effect of lance CH.sub.4 flow on CF.sub.4 destruction and NOx formation with Argon at 50 l/min, CF.sub.4 1 l/min, curtain CH.sub.4 3 l/min and premix O.sub.2 15 l/min; FIG. 4 shows the effect of Argon purge flow on CF.sub.4 destruction and NOx formation with CF.sub.4 1 l/min, curtain CH.sub.4 3 l/min, lance CH.sub.4 6 l/min and premix O.sub.2 15 l/min; FIG. 5 shows the effect of curtain CH.sub.4 on CF.sub.4 destruction and NOx formation with Argon at 55 l/min, CF.sub.4 1 l/min, CH.sub.4 6 l/min and premix O.sub.2 15 l/min As can be seen particularly in FIG. 4, optimising the operating conditions of the abatement apparatus 10 when using argon (or other noble gas) leads to up to a 75% reduction in NOx. if all flows (pump purge, CF.sub.4, inject CH.sub.4, curtain CH.sub.4, inject O.sub.2) remain constant, the NOx generation when using argon (or other noble gas) is slightly higher than with nitrogen as the purge gas but the CF.sub.4 destruction efficiency is substantially improved, indicating that the production of NOx at the flame-front does not discriminate between upstream nitrogen (from the process gas or effluent stream) and downstream nitrogen (in the radiant burner combustion by-products). When using argon (or other noble gas) instead of nitrogen, the change in inject CH.sub.4 flow rate required to maintain the DRE achieved when using nitrogen (by maintaining a similar flame temperature) varies generally in proportion to the ratio of Cp (heat capacity at constant pressure) of argon (or other noble gas) relative to nitrogen. Hence, if it is assumed that the DRE for CF.sub.4 provides an indication of the flame temperature, then substituting argon for nitrogen and adjusting the inject flow rates leads to around a 75% reduction in NOx emissions for a similar flame temperature. Accordingly, recognising that the ratio of the specific heat capacities of argon and nitrogen is 0.72, inject CH.sub.4 and 0.sub.2 flows were reduced in a stepwise fashion, targeting 95% CF4 DRE. All other flows were kept constant. The result was an approximately 5-fold reduction in NOx compared to existing operating conditions. Hence, it has been demonstrated that while not entirely eliminating NOx, the use of argon as an inert gas purge gives a substantial reduction in NOx emissions and a moderate reduction in the consumption of fuel and oxygen.

[0047] When operating the abatement apparatus 10 (having four inlets 20 having a 6″ diameter and 3″ length) with the effluent stream containing 50 l/min of Ar leads to 61 ppm of NOx and a CF.sub.4 DRE of 98.3%. Accordingly, it can be seen that the presence of argon improves the DRE, but leads to an increase in the amount of NOx. This is because the specific heat capacity of argon differs to that of nitrogen. Ar Cp (J.mol-1.K-1) is 0.71× that of N.sub.2—which improves abatement. Ar Cp/Cv=γ is 1.67 compared to 1.4 for N.sub.2—which is bad for vacuum pumps. However, by optimising the inject conditions for Ar leads to an 80% reduction in NOx emissions compared to the existing operation using N.sub.2 described above.

Carbon Dioxide Substitution

[0048] Another inert gas of interest is carbon dioxide. CO.sub.2 is readily available and cheaper than argon. For example, typical current bulk gas prices per m.sup.3 are: N.sub.2 $0.16; CO.sub.2 $0.72; Ar $1.09. Furthermore, carbon dioxide should be better behaved in the vacuum pump as the ratio Cp/Cv or γ is high for monoatomic gases such as argon leading to significant heat of compression within the pumping mechanism. Hence, use of argon can lead to over-heating of the pump but this would be less likely with carbon dioxide. In embodiments, carbon dioxide is substituted to replace nitrogen as the purge gas.

[0049] In one embodiment, under standard inject conditions (for 50 l/min nitrogen purge), when operating the abatement apparatus 10 (having four inlets 20 having a 6″ diameter and 3″ length) with the effluent stream containing 50 l/min of CO.sub.2 leads to 19 ppm of NOx and a CF.sub.4 DRE of 12%.

[0050] In one embodiment, using 25 l/min of CO.sub.2, leads to 38 ppm of NOx and a CF.sub.4 DRE of 90%. In another embodiment, the inject conditions are scaled to give an equivalent burning velocity with the effluent stream containing 40 l/min of CO.sub.2 together with 40 l/min of O.sub.2 and 22 l/min of CH.sub.4 which leads to 40 ppm of NOx and a CF.sub.4 DRE of 98%. In a further embodiment, O.sub.2 remains at 40 l/min, but CH.sub.4 is reduced to 20 l/min which leads to 25 ppm of NOx and a CF.sub.4 DRE of 95%. These can be further optimised with flow rates of around 2 times that of nitrogen and 3.6 times that of argon, in line with specific heat capacity and burning velocity considerations. Hence, it can be seen that CO.sub.2 substitution also results in reduced NOx formation, but at the expense of increased CH.sub.4 and O.sub.2 usage, broadly in proportion to the specific heat capacities of N.sub.2, Ar and CO.sub.2.

[0051] When the pump purge was replaced with CO.sub.2 similar results were obtained—high CF.sub.4 DRE and low NOx emission, but significantly higher flows of injected methane and oxygen were required—at least twice the flow rates required for nitrogen. There is a correlation between these higher flows and parameters including the specific heat capacity of CO.sub.2 and the peak burning velocity of CH.sub.4/CO.sub.2/O.sub.2 mixtures

[0052] According, it can be seen that argon gives the best abatement efficiency and lowest NOx but is not preferred as a pump purge. Also, carbon dioxide, whilst suitable for use as a pump purge, requires approximately twice the injected methane and oxygen as nitrogen.

Combined Argon & Carbon Dioxide Substitution

[0053] As mentioned above, a high γ can lead to overheating and, in particular, a threshold value of γ can be established above which pumps are likely to overheat and seize. Mixtures as high as 75% argon (balance nitrogen) can be pumped successfully. The value of γ can be calculated for this mixture as the ratio of the components. From this, the proportions of argon and carbon dioxide can be calculated which, once blended, will have the same γ and can be pumped successfully. Those calculations show that a mixture of 81% argon/19% carbon dioxide would behave in a similar manner.

[0054] Therefore, CF.sub.4 abatement and NOx production measurements were also performed with this mixed Ar/CO.sub.2 purge gas. In one embodiment, under standard inject conditions (for 50 l/min nitrogen purge), when operating the abatement apparatus 10 (having four inlets 20 having a 6″ diameter and 3″ length) with the effluent stream containing 50 l/min of Ar & CO.sub.2 mix (81%:19%) leads to 12 ppm of NOx and a CF.sub.4 DRE of 97.5%. Again, the result was high CF.sub.4 DRE and low NOx. Inject flows were comparable to those used with standard nitrogen purges.

Combustion Chamber Operation

[0055] When operating the combustion chamber 40 with nitrogen, the lower flammable limit is achieved when the following are achieved N.sub.2/O.sub.2(79%/21%)/CH.sub.4 (5.2%). The peak burning velocity (21% O.sub.2) is 36 cm.s-1.

[0056] When operating the combustion chamber 40 with carbon dioxide, the lower flammable limit is achieved when the following are achieved CO.sub.2/O.sub.2(79%/21%)/CH.sub.4 (7%). The peak burning velocity (21% O.sub.2) is 1.4 cm.s-1.

[0057] The predicted stable operating conditions are with 8% CH.sub.4 in the pre-mix (1.2× the lower flammable limit) and a ratio of O.sub.2/O.sub.2+CO.sub.2 of 35% O.sub.2 which is a similar ratio of peak burning velocity to total flow.

[0058] The predicted exhaust composition (dry) burner only is 18.7% O.sub.2 with the balance CO.sub.2.

[0059] The predicted exhaust composition (dry) burner with four inlets in optimised high fire with argon purge is 7% O.sub.2, 50% Ar with the balance CO.sub.2.

[0060] In one embodiment, the burner is operated on Ar/O.sub.2/CH.sub.4 rather than the N.sub.2/O.sub.2/CH.sub.4 (fuel-air premix).

[0061] In another embodiment, the burner is operated on CO.sub.2/O.sub.2/CH.sub.4. In one embodiment, under standard inject conditions (for 50 l/min nitrogen purge), when operating the abatement apparatus 10 (having four inlets 20 having a 6″ diameter and 3″ length) with the effluent stream containing 130 l/min of CO.sub.2 together with 70 l/min of O.sub.2 giving a ratio of O.sub.2/(O.sub.2+CO.sub.2) of 35% O.sub.2 and 20 l/min of CH.sub.4 giving a ratio of CH.sub.4/(CH.sub.4+O.sub.2+CO.sub.2) of 9% CH.sub.4 leads to 98 ppm of NOx and a CF.sub.4 DRE of 98.7%.

[0062] In one embodiment, 17 l/min of premixed O.sub.2 with 13 l/min CH.sub.4 (10 l/min provided on the lance and 3 l/min provided on the curtain), leads to 32 ppm of NOx and a CF.sub.4 DRE of 93.5% (compared to 43 ppm of NOx and a CF.sub.4 DRE of 94.2% for the existing operation mentioned above).

[0063] As illustrated in FIG. 6, (which shows the burning velocity of methane in nitrogen-oxygen (upper curve) and carbon dioxide-oxygen (lower curve) mixtures) calculations show that by plotting the peak (stoichiometric) burning velocity versus O.sub.2 concentration for the N.sub.2/O.sub.2/CH.sub.4 and CO.sub.2/O.sub.2/CH.sub.4 systems, 37.5% O.sub.2 in CO.sub.2 has the same peak burning velocity as 21% O.sub.2 in N.sub.2 (air). The burner is typically operated at around 6% CH.sub.4—1.15 times the lower flammable limit of CH.sub.4 in air (5.2%). So, with the CO.sub.2/O.sub.2 system, the ideal CH.sub.4 concentration is around 8%—1.15 times the lower flammable limit which is 7%. A further consideration is the total volumetric flow through the burner. Using the above figures as a guide, revised conditions seek to maintain the ratio of burning velocity to volumetric flow between the two systems, suggesting that a value of 35% O.sub.2 in CO.sub.2 would be more appropriate.

Gas Recovery

[0064] The downstream cooling chamber may feed a recovery device (not shown). The recovery device can be any of a different number of devices such as a cryogenic distillation device, a pressure (vacuum) sing adsorption device, a ceramic or polymer membrane separation device which separates the gases and produces a at least one of a pure stream from the pump purge and an oxygen rich stream for the burner.

[0065] The exhaust stream of the burner, after wet scrubbing, will contain primarily CO.sub.2 and O.sub.2 at 100% relative humidity (at the temperature of the packed tower) often with traces of CO and other contaminants. One embodiment recycles this back to the burner as a diluent, being “made up” with O.sub.2 and CH.sub.4 to the required proportions. Combustible contaminants might be fully oxidised over a combustion catalyst such as “Hopcalite” (Molecular Products, Thaxted, UK supply a room temperature combustion catalyst based on a CuO/MnO.sub.2 mix; the product is called Moleculite).

[0066] In one embodiment, the exhaust from the burner, is passed to a separation unit configured to produce a stream of pure CO.sub.2 for purging the vacuum pumps along with an impure stream comprising CO.sub.2 and O.sub.2 to be used in the radiant burner as above. Recognising that CO.sub.2 is a by-product of burning hydrocarbon fuels, the system is self-sufficient; once primed, no additional CO.sub.2 is required.

[0067] if the pump purges contain a high proportion of argon, the exhaust from the burner, may be passed to a separation unit configured to produce a first stream of pure CO.sub.2, a second stream of pure argon and a third impure stream comprising predominantly O.sub.2 with residual argon. Some of the CO.sub.2 would be blended with the argon to be used as pump purge while the O.sub.2 rich stream could be used in the radiant burner as above. By returning the O.sub.2 rich stream as described above, the residual argon is not lost from the system.

[0068] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0069] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0070] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.