Patent Description:
Abatement apparatus for performing abatement are known and are typically used for treating an effluent gas stream from a manufacturing processing tool used in, for example, the semiconductor or flat panel display manufacturing industry. During such manufacturing, residual perfluorinated compounds (PFCs) and other compounds exist in the effluent gas stream pumped from the process tool. PFCs are difficult to remove from the effluent gas and their release into the environment is undesirable because they are known to have relatively high greenhouse activity.

Known abatement apparatus use combustion to remove the PFCs and other compounds from the effluent gas stream. Typically, the effluent gas stream is a nitrogen stream containing PFCs and other compounds. A fuel gas is mixed with the effluent gas stream and that gas stream mixture is conveyed into a combustion chamber that is laterally surrounded by the exit surface of a foraminous gas burner. Fuel gas and air are simultaneously supplied to the foraminous burner to affect flameless combustion at the exit surface, with the amount of air passing through the foraminous burner seeking to be sufficient to consume not only the fuel gas supply to the burner, but also all the combustibles in the gas stream mixture injected into the combustion chamber.

Documents <CIT>, <CIT>, <CIT> and <CIT> disclose abatement methods according to the preamble of claim <NUM>.

Although techniques exist for processing the effluent gas stream, they each have their own shortcomings. Accordingly, it is desired to provide an improved technique for processing an effluent gas stream.

According to a first aspect, there is provided 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 the combustion chamber by reacting the combustion reagents to perform abatement of the perfluoro compound to stable by-products, the diluent being selected to remain inert during the abatement, wherein the diluent comprises a mixture of a noble gas and carbon dioxide, preferably a mixture of argon and carbon dioxide.

The first aspect recognises that a problem with abating some compounds is that nitrogen present during the abatement can react to generate NOx or other compounds, which it is increasingly desirable to reduce in the exhaust of an abatement apparatus.

In one embodiment, the diluent is selected to be unchanged or unreactive during the abatement in the combustion zone. Accordingly, the diluent may fail to react and remains unchanged during combustion.

In one embodiment, the heating raises a temperature of the combustion zone to greater than around <NUM>.

In one embodiment, the abatement apparatus comprises a nozzle for injecting a gas stream into the combustion chamber and the supplying comprises supplying the nozzle with the effluent stream. Accordingly, the effluent stream may be injected together with the gas stream from a nozzle into the combustion chamber.

In one embodiment, the supplying comprises supplying the nozzle with the combustion reagents and the diluent. Accordingly, the combustion reagents and/or the diluent may also be provided by the nozzle into the combustion chamber.

In one embodiment, the abatement apparatus comprises a pump which supplies the nozzle and the supplying comprises supplying the diluent as a pump purge gas. Accordingly, the diluent may also be provided as a purge gas for a pump which supplies the effluent stream to the nozzle.

In one embodiment, the abatement apparatus comprises a foraminous sleeve at least partially defining the combustion chamber for conveying a gas into the combustion chamber and the supplying comprises supplying the foraminous sleeve with the combustion reagents and the diluent. Accordingly, the abatement apparatus may utilise a foraminous burner through which the combustion reagents and the diluent may be provided into the combustion chamber.

In one embodiment, the method comprises comprising adjusting a ratio of flow rates of the diluent to the combustion reagents to provide a selected minimum destructive rate efficiency of at least one compound in the effluent stream. Accordingly, the ratio of the diluent to the combustion reagents may be adjusted, changed or selected to achieve a desired destructive rate efficiency of a compound in the effluent stream.

In one embodiment, the reagents comprise a fuel and an oxidant.

In one embodiment, the fuel comprises a hydrocarbon, methane, propane, butane and/or the like.

In one embodiment, the oxidant comprises oxygen, ozone and/or the like.

In one embodiment, the diluent comprises a mixture of argon and carbon dioxide in a ratio of around <NUM>% of argon to around <NUM>% of carbon dioxide by volume.

In one embodiment, the oxidant is mixed with carbon dioxide in a ratio of around <NUM>% oxidant to around <NUM>% carbon dioxide by volume.

In one embodiment, the oxidant is mixed with carbon dioxide in a ratio of around <NUM>% oxidant to around <NUM>% of carbon dioxide by volume.

In one embodiment, the oxidant is mixed with argon in a ratio of around <NUM>% oxidant to around <NUM>% argon by volume.

In one embodiment, the fuel is mixed with combined oxidant and carbon dioxide in a ratio of around <NUM>% fuel to <NUM>% combined oxidant and carbon dioxide by volume. In other words, fuel (such as methane) is added to a carbon dioxide and oxygen mixture such that the fuel forms around <NUM>% of the total (i.e. CH<NUM>/[CH<NUM> + O<NUM> + CO<NUM>] = <NUM>%. It will be appreciated that for higher-carbon fuels (such as propane), the ratio of fuel will be lower to achieve similar combustion conditions. In one embodiment, the fuel is mixed with combined oxidant and argon in a ratio of around <NUM>% fuel to around <NUM>% combined oxidant and argon by volume. In other words , fuel (such as methane) is added to an argon and oxygen mixture such that the fuel forms around <NUM>% of the total (i.e. CH<NUM>/[CH<NUM> + O<NUM> + Ar] = <NUM>%. It will be appreciated that for higher-carbon fuels (such as propane), the ratio of fuel will be lower to achieve similar combustion conditions.

In one embodiment, the method comprises recovering at least some of the diluent from an exhaust stream of the combustion chamber.

In one embodiment, the method comprises recirculating the diluent.

In one embodiment, the method comprises recirculating the diluent together with at least one of the oxidant and the fuel as a contaminant to the combustion chamber.

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. According to the invention, a mixture of a 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. A mixture of a noble gas, such as argon, and carbon dioxide is used a pump purge instead of nitrogen and/or an argon/oxygen/CH<NUM> and/or carbon dioxide/oxygen/CH<NUM> 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.

<FIG> illustrates an abatement apparatus, generally <NUM>, according to one embodiment. The abatement apparatus <NUM> 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 <NUM>. The effluent stream is conveyed from the inlet <NUM> to a nozzle <NUM> which injects the effluent stream into a cylindrical combustion chamber <NUM>. Each nozzle <NUM> is located within a respective bore formed in a ceramic top plate <NUM> which defines an upper or inlet surface of the combustion chamber <NUM>. An oxidant, in this example oxygen, is mixed with the effluent stream as it is conveyed from the inlet <NUM> to the nozzle <NUM>. A fuel gas is conveyed to a concentric conduit which surrounds the nozzle <NUM> for delivery as a surrounding curtain into the combustion chamber <NUM>. A fuel gas is also conveyed via a concentric lance <NUM> located within the nozzle <NUM> for delivery as an inject into the combustion chamber <NUM>.

The combustion chamber <NUM> has sidewalls defined by an exit surface of a foraminous burner element <NUM> such as that described in <CIT>. The burner element <NUM> is cylindrical and is retained within a cylindrical outer shell <NUM>. A plenum volume <NUM> is defined between an entry surface of the burner element <NUM> and the cylindrical outer shell <NUM>. A mixture of fuel gas, such as natural gas or a hydrocarbon, and an oxidant and purge gas is introduced into the plenum volume <NUM> 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 <NUM>.

The ratio of the mixture of fuel gas and oxidant and purge gas is varied to vary the temperature within the combustion chamber <NUM> to that which is appropriate for the effluent stream to be treated. Operating temperatures within the combustion chamber <NUM> start at around <NUM> to around <NUM> for some abatement processes. However, the temperatures can also be set to around <NUM> to around <NUM> for other abatement processes. At these temperatures, nitrogen present in the combustion chamber <NUM> 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 <NUM> is adjusted so that the mixture will burn without visible flame at the exit surface of the burner element <NUM>. The exhaust from the combustion chamber is vented into a downstream cooling chamber (not shown).

Accordingly, the effluent stream received through the inlets <NUM> and provided by the nozzles <NUM> to the combustion chamber <NUM> 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 <NUM> and forms a flame extending from the nozzles <NUM>. Such combustion causes heating of the combustion chamber and provides combustion products, such as oxygen, typically within a range of <NUM>% to <NUM>% depending on the air/fuel mixture [CH<NUM>, C<NUM>H<NUM>, C<NUM>H<NUM>], provided to the combustion chamber <NUM>. This heat and the combustion products react with the effluent stream within the combustion chamber <NUM> to clean the effluent stream. For example, SiH<NUM> and NH<NUM> may be provided within the effluent stream, which reacts with O<NUM> within the combustion chamber <NUM> to generate SiO<NUM>, N<NUM>, H<NUM>O, NOx. Similarly, N<NUM>, CH<NUM>, C<NUM>F<NUM> may be provided within the effluent stream, which reacts with O<NUM> within the combustion chamber <NUM> to generate CO<NUM>, HF, H<NUM>O. The combusted effluent stream exhausts from the abatement apparatus <NUM> and comprises the treated stream.

During existing operation, the purge gas supplied the effluent stream is nitrogen. When operating the abatement apparatus <NUM> (having four inlets <NUM> having a <NUM>" diameter and <NUM>" length) with the effluent stream containing <NUM>/min of N<NUM> leads to 43ppm of NOx and a CF<NUM> DRE of <NUM>%.

In one embodiment, not in accordance with the invention, argon (or other noble gas) is substituted to replace nitrogen as the purge gas. <FIG> show the changes in CF<NUM> DRE and NOx production with variations in premix O<NUM> (<FIG>), variations in lance CH<NUM> (<FIG>), Argon flow (<FIG>) and curtain CH<NUM> (<FIG>). In particular, <FIG> shows the effect of premix O<NUM> flow on CF<NUM> destruction and NOx formation with Argon at <NUM>/min, CF<NUM><NUM>/min, curtain CH<NUM> <NUM>/min and lance CH<NUM> <NUM>/min; <FIG> shows the effect of lance CH<NUM> flow on CF<NUM> destruction and NOx formation with Argon at <NUM>/min, CF<NUM><NUM>/min, curtain CH<NUM> <NUM>/min and premix O<NUM> <NUM>/min; <FIG> shows the effect of Argon purge flow on CF<NUM> destruction and NOx formation with CF<NUM> <NUM>/min, curtain CH<NUM> <NUM>/min, lance CH<NUM> <NUM>/min and premix O<NUM> <NUM>/min; <FIG> shows the effect of curtain CH<NUM> on CF<NUM> destruction and NOx formation with Argon at <NUM>/min, CF<NUM><NUM>/min, CH<NUM> <NUM>/min and premix O<NUM> <NUM>/min. As can be seen particularly in <FIG>, optimising the operating conditions of the abatement apparatus <NUM> when using argon (or other noble gas) leads to up to a <NUM>% reduction in NOx. If all flows (pump purge, CF<NUM>, inject CH<NUM>, curtain CH<NUM>, inject O<NUM>) 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<NUM> 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<NUM> 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<NUM> provides an indication of the flame temperature, then substituting argon for nitrogen and adjusting the inject flow rates leads to around a <NUM>% reduction in NOx emissions for a similar flame temperature. Accordingly, recognising that the ratio of the specific heat capacities of argon and nitrogen is <NUM>, inject CH<NUM> and O<NUM> flows were reduced in a stepwise fashion, targeting <NUM>% CF4 DRE. All other flows were kept constant. The result was an approximately <NUM>-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.

When operating the abatement apparatus <NUM> (having four inlets <NUM> having a <NUM>" diameter and <NUM>" length) with the effluent stream containing <NUM>/min of Ar leads to 61ppm of NOx and a CF<NUM> DRE of <NUM>%. 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. K-<NUM>) is <NUM>. 71x that of N<NUM> -which improves abatement. Ar Cp/Cv = γ is <NUM> compared to <NUM> for N<NUM> - which is bad for vacuum pumps. However, by optimising the inject conditions for Ar leads to an <NUM>% reduction in NOx emissions compared to the existing operation using N<NUM> described above.

Another inert gas of interest is carbon dioxide. CO<NUM> is readily available and cheaper than argon. For example, typical current bulk gas prices per m<NUM> are: N<NUM> $<NUM>; CO<NUM> $<NUM>; Ar $<NUM>. 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, not in accordance with the invention, is substituted to replace nitrogen as the purge gas.

In one embodiment, under standard inject conditions (for <NUM>/min nitrogen purge), when operating the abatement apparatus <NUM> (having four inlets <NUM> having a <NUM>" diameter and <NUM>" length) with the effluent stream containing <NUM>/min of CO<NUM> leads to 19ppm of NOx and a CF<NUM> DRE of <NUM>%.

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

When the pump purge was replaced with CO<NUM> similar results were obtained - high CF<NUM> 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<NUM> and the peak burning velocity of CH<NUM>/CO<NUM>/O<NUM> mixtures.

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.

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 <NUM>% 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 <NUM> % argon/<NUM>% carbon dioxide would behave in a similar manner.

Therefore, CF<NUM> abatement and NOx production measurements were also performed with this mixed Ar/CO<NUM> purge gas. In one embodiment, in accordance with the invention, under standard inject conditions (for <NUM>/min nitrogen purge), when operating the abatement apparatus <NUM> (having four inlets <NUM> having a <NUM>" diameter and <NUM>" length) with the effluent stream containing <NUM>/min of Ar & CO<NUM> mix (<NUM>%:<NUM>%) leads to 12ppm of NOx and a CF<NUM> DRE of <NUM>%. Again, the result was high CF<NUM> DRE and low NOx. Inject flows were comparable to those used with standard nitrogen purges.

When operating the combustion chamber <NUM> with nitrogen, not in accordance with the invention, the lower flammable limit is achieved when the following are achieved N<NUM>/O<NUM>(<NUM>%/<NUM>%)/CH<NUM> (<NUM>%). The peak burning velocity (<NUM>% O<NUM>) is <NUM>.

When operating the combustion chamber <NUM> with carbon dioxide, not in accordance with the invention, the lower flammable limit is achieved when the following are achieved CO<NUM>/O<NUM>(<NUM>%/<NUM> %)/CH<NUM> (<NUM>%). The peak burning velocity (<NUM> % O<NUM>) is <NUM>. The predicted stable operating conditions are with <NUM>% CH<NUM> in the pre-mix (<NUM>. 2x the lower flammable limit) and a ratio of O<NUM> / O<NUM> + CO<NUM> of <NUM>% O<NUM> which is a similar ratio of peak burning velocity to total flow.

The predicted exhaust composition (dry) burner only is <NUM>% O<NUM> with the balance CO<NUM>.

The predicted exhaust composition (dry) burner with four inlets in optimised high fire with argon purge is <NUM>% O<NUM>, <NUM>% Ar with the balance CO<NUM>.

In one embodiment, the burner is operated on Ar/O<NUM>/CH<NUM> rather than the N<NUM>/O<NUM>/CH<NUM> (fuel- air premix).

In another embodiment, the burner is operated on CO<NUM>/O<NUM>/CH<NUM>. In one embodiment, not in accordance with the invention, under standard inject conditions (for <NUM>/min nitrogen purge), when operating the abatement apparatus <NUM> (having four inlets <NUM> having a <NUM>" diameter and <NUM>" length) with the effluent stream containing <NUM>/min of CO<NUM> together with <NUM>/min of O<NUM> giving a ratio of O<NUM> / (O<NUM> + CO<NUM>) of <NUM>% O<NUM> and <NUM>/min of CH<NUM> giving a ratio of CH<NUM> / (CH<NUM> + O<NUM> + CO<NUM>) of <NUM>% CH<NUM> leads to 98ppm of NOx and a CF<NUM> DRE of <NUM>%.

In one embodiment, <NUM>/min of premixed O<NUM> with <NUM>/min CH<NUM> (<NUM>/min provided on the lance and <NUM>/min provided on the curtain), leads to 32ppm of NOx and a CF<NUM> DRE of <NUM>% (compared to 43ppm of NOx and a CF<NUM> DRE of <NUM>% for the existing operation mentioned above).

As illustrated in <FIG>, (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<NUM> concentration for the N<NUM>/O<NUM>/CH<NUM> and CO<NUM>/O<NUM>/CH<NUM> systems, <NUM>% O<NUM> in CO<NUM> has the same peak burning velocity as <NUM> % O<NUM> in N<NUM> (air). The burner is typically operated at around <NUM>% CH<NUM> - <NUM> times the lower flammable limit of CH<NUM> in air (<NUM>%). So, with the CO<NUM>/O<NUM> system, the ideal CH<NUM> concentration is around <NUM>% - <NUM> times the lower flammable limit which is <NUM>%. 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 <NUM>% O<NUM> in CO<NUM> would be more appropriate.

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) swing 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.

The exhaust stream of the burner, after wet scrubbing, will contain primarily CO<NUM> and O<NUM> at <NUM>% relative humidity (at the temperature of the packed tower) often with traces of CO and other contaminants. One embodiment, not in accordance with the invention, recycles this back to the burner as a diluent, being "made up" with O<NUM> and CH<NUM> 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<NUM> mix; the product is called Moleculite).

In one embodiment, not in accordance with the invention, the exhaust from the burner, is passed to a separation unit configured to produce a stream of pure CO<NUM> for purging the vacuum pumps along with an impure stream comprising CO<NUM> and O<NUM> to be used in the radiant burner as above. Recognising that CO<NUM> is a by-product of burning hydrocarbon fuels, the system is self-sufficient; once primed, no additional CO<NUM> is required.

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<NUM>, a second stream of pure argon and a third impure stream comprising predominantly O<NUM> with residual argon. In accordance with the invention, some of the CO<NUM> would be blended with the argon to be used as pump purge while the O<NUM> rich stream could be used in the radiant burner as above. By returning the O<NUM> rich stream as described above, the residual argon is not lost from the system.

Claim 1:
A method, comprising:
supplying a combustion chamber (<NUM>) of an abatement apparatus (<NUM>) 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 and characterised in that said diluent comprises a mixture of a noble gas and carbon dioxide, preferably a mixture of argon and carbon dioxide.