Patent Application
20180326354
The invention relates to an improved process for the removal of NOX from exhaust gases.
Oxides of nitrogen are common by-products and/or desirable intermediates in a number of industrial processes, including the manufacture of chemicals, such as nitric acid, or combustion processes in air. Nitrogen oxides of the formula NO and NO2 are typically referred to together as NOX. NOX is a large scale pollutant and significant efforts have been made for the reduction of NOX in exhaust gas streams from processes in which they are produced. Processes for removal of NOX from gas streams are generally referred to in the art as DeNOX processes and the catalysts used therein as DeNOX catalysts.
One process used for the removal of NOx from gas streams is the selective catalytic reduction (SCR) process. One version of this process is disclosed in U.S. Pat. No. 7,294,321. In this selective catalytic reduction process, a combustion gas that contains a concentration of NOx and ammonia (NH3), which is typically added to the combustion gas as a reactant, is contacted with a catalyst that promotes the reduction reaction in which the NOx reacts with ammonia and oxygen to yield nitrogen and water.
Nitrous oxide (N2O) is a greenhouse gas and is considered to be a greater contributor to climate change by weight than carbon dioxide. In many countries limits on nitrous oxide emissions have been set and efforts have been focussed on developing methods to remove nitrous oxide from exhaust gases. Many of these efforts have focussed on identifying catalysts suitable for use in the catalytic decomposition of nitrous oxides. Processes for removal of N2O from gas streams are generally referred to in the art as DeN2O processes and the catalysts used therein as DeN2O catalysts.
Zeolite-supported iron catalysts, optionally also containing a noble metal such as Pt or Ru, have been described, for example in U.S. Pat. No. 5,171,553, WO2005110582 and Journal of Catalysis 243 (2006), 340-349. Other known nitrous oxide decomposition catalysts include those based on base metal oxides such as Co3O4, as described in U.S. Pat. No. 5,705,136 and Catalysis Communications 4 (2003) 505-509. A bulk metal oxide catalyst for the removal of nitrous oxide from waste gas is described in WO2015014863.
It is considered advantageous to be able to treat a gas stream containing both NOX and N2O in order to reduce the amounts of both NOX and minimise N2O in the treated gas stream. This may be carried out by subjecting said gas stream to a DeN2O process in the presence of a DeN2O catalyst and then subjecting the resultant stream to a DeNOX process in the presence of a DeNOX catalyst.
However, competing reactions occur in these processes which may reduce their efficiency in producing a treated stream low in both NOX and N2O. For example, treatment of a NOX-containing stream which is NO2-rich (containing more NO2 than NO on a molar basis) over a DeNOX catalyst may result in the formation of N2O.
It would be desirable to provide a robust process for the reduction of NOX from NOX-containing streams, in which the level of N2O in the treated stream is also minimised.
Accordingly, the present invention provides a process for the treatment of a NOX-containing gas stream, said NOX-containing gas stream containing NO2 and NO in a molar ratio of NO2:NO of at least 1:1, to remove at least a portion of the NOX contained therein, said process comprising:
i) providing an additional gas stream comprising NO to the NOX-containing gas stream, such that the molar ratio of NO2:NO in the NOX-containing gas stream is reduced to be less than 1:1; and
ii) then passing the NOX-containing gas stream through a catalyst bed comprising a deNOX catalyst under suitable conditions to reduce the level of NOX in the gas stream and thus produce a deNOX treated gas stream, said deNOX treated gas stream containing a reduced amount of NOX.
The present invention also provides a process for the treatment of a N2O- and NOX-containing gas stream to remove at least a portion of each of the NOX and the N2O contained therein, said process comprising:
i) passing the N2O- and NOX-containing gas stream through a catalyst bed comprising a deN2O catalyst under suitable conditions to reduce the level of N2O in said N2O- and NOX-containing gas stream and thus produce a deN2O-treated gas stream, said deN2O-treated gas stream containing a reduced amount of N2O;
ii) taking at least a portion of said deN2O-treated gas stream to provide a NOX-containing gas stream; and
iii) passing at least a portion of said NOX-containing gas stream through a catalyst bed comprising a deNOX catalyst under suitable conditions to reduce the level of NOX in the deN2O-treated gas stream and thus produce a deNOX-treated gas stream, said deNOX-treated gas stream containing a reduced amount of NOX;
wherein an additional gas stream comprising NO is provided to either or both of the N2O- and NOX-containing gas stream and the NOX-containing gas stream, such that the ratio of NO2:NO in the NOX-containing gas stream is less than 1:1.
The present inventors have surprisingly found that by decreasing the ratio of NO2:NO in a NOX-containing gas stream before subjecting it to treatment with a deNOX catalyst, the overall level of pollutants in the resultant deNOX treated gas stream, in the form of oxides of nitrogen, can be decreased.
The NOX-containing gas stream in the process of the invention may be any gas stream containing NOX. Preferably, the NOX-containing gas stream is derived from an exhaust gas stream, typically from an industrial process. Exhaust gas streams particularly suitable for use as the NOX-containing gas stream in the process of the present invention include exhaust gas streams from a process for the production of nitric acid.
Typically the NO2 content of the NOX-containing gas stream is in the range of from 500 to 10000 ppmv.
Typically the NO content of the NOX-containing gas stream is in the range of from 500 to 10000 ppmv.
When using the process of the invention, the ratio of NO2:NO in the NOX-containing gas stream, before the introduction of the additional gas stream comprising NO is at least 1:1, preferably greater than 1:1.
The NOX-containing gas stream is contacted with a catalyst bed comprising a deNOX catalyst under suitable conditions to reduce the level of NOX in said NOX-containing gas stream and thus produce a deNOX treated gas stream.
Any deNOX catalysts may suitably be used in the process of the present invention, for example those described in U.S. Pat. No. 6,419,889. An exemplary catalyst from U.S. Pat. No. 6,419,889 comprises a titania carrier and one or more metal compounds which metals are selected from the group consisting of vanadium, molybdenum and tungsten. Said catalyst typically has a surface area measured by nitrogen adsorption of between about 70 m2/g and about 99 m2/g. Said catalyst suitably has a bimodal pore distribution with more than 90% of the pore volume present in pores having a diameter of at most about 100 nm, which pore volume is considered to be the pore volume present in pores having a diameter between about 1 nm and about 104 nm. Further, said catalyst is obtainable by impregnating or deposition of the carrier with the metal compound(s) after extruding, drying and calcining the carrier.
Suitable conditions to reduce the level of NOX in the gas stream include a pressure in the range of from 0 kPa (gauge) to 1200 kPa (gauge) and a temperature in the range of from 140° C. to 400° C.
The deNOX treated gas stream will contain a reduced level of NOX (considering both NO and NO2 on a molar basis) compared to the NOX-containing gas stream. Preferably, the deNOX treated gas stream contains no more than 10% of the amount of NOX in the NOX-containing gas stream. More preferably, the deNOX treated gas stream contains no more than 5% of the amount of NOX in the NOX-containing gas stream. Even more preferably, the deNOX treated gas stream contains no more than 2% of the amount of NOX in the NOX-containing gas stream. Most preferably, the deNOX treated gas stream contains no more than 1% of the amount of NOX in the NOX-containing gas stream.
In a preferred embodiment of the invention, the NOX-containing gas stream is derived from a N2O- and NOX-containing gas stream. In said embodiment, the N2O- and NOX-containing gas stream in the process of the invention may be any gas stream containing N2O and NOX. Preferably, the N2O- and NOX-containing gas stream is an exhaust gas stream, typically from an industrial process. Exhaust gas streams particularly suitable for use as the N2O- and NOX-containing gas stream in the process of the present invention include exhaust gas streams from a process for the production of nitric acid.
In this embodiment, depending on the exhaust stream, the amount of N2O present will vary. For the exhaust stream from a nitric acid plant, typically the N2O content of the N2O- and NOX-containing gas stream is in the range of from 500 to 10000 ppmv, preferably in the range of from 500 to 2000 ppmv.
Further, in this embodiment, prior to the NOX-containing gas stream being passed through a catalyst bed comprising a deNOX catalyst under suitable conditions to reduce the level of NOX in the gas stream, the N2O- and NOX-containing gas stream is passed through a catalyst bed comprising a deN2O catalyst under suitable conditions to reduce the level of N2O in said N2O- and NOX-containing gas stream and, thus, produce a deN2O-treated gas stream, said deN2O-treated stream containing a reduced amount of N2O. At least of portion of said deN2O-treated gas stream is then used as the NOX-containing gas stream.
When using the process of the invention, the molar ratio of NO2:NO in the N2O- and NOX-containing gas stream, before any introduction of the additional gas stream comprising NO is typically at least 1:1, preferably greater than 1:1. However, in some embodiments in which the deN2O catalyst converts some NO to NO2, the ratio of NO2:NO in the N2O- and NOX-containing gas stream, before any introduction of the additional gas stream may be lower than this.
Other gases present in the NOX-containing and/or N2O- and NOX-containing gas stream, wherein the said gas stream or streams are derived from the exhaust stream from a nitric acid plant include, but are not limited to, nitrogen, H2O, oxygen and argon.
In the process of the present invention, a N2O- and NOX-containing gas stream may initially be passed through a catalyst bed comprising a deN2O catalyst under suitable conditions to reduce the level of N2O in the gas stream and thus produce a deN2O-treated gas stream, said deN2O-treated gas stream containing a reduced amount of N2O.
Any deN2O catalysts may suitably be used in the process of the present invention, including base metal catalyst and zeolite-supported iron catalysts, optionally also containing a noble metal such as Pt or Ru. Such zeolite-supported iron catalysts include those described in U.S. Pat. No. 5,171,553, WO2005110582 and Journal of Catalysis 243 (2006), 340-349. Suitable base metal catalyst have been described in U.S. Pat. No. 5,705,136, Catalysis Communications 4 (2003) 505-509 and WO2015014863.
Suitable conditions to reduce the level of N2O in the gas stream include a pressure in the range of from 0 kPa (gauge) to 1200 kPa (gauge) and a temperature in the range of from 350° C. to 650° C.
The deN2O-treated gas stream contains a reduced amount of N2O. Preferably, the deN2O-treated gas stream contains no more than 10% of the amount of N2O in the N2O- and NOX-containing gas stream. More preferably the deN2O-treated gas stream contains no more than 5% of the amount of N2O in the N2O- and NOX-containing gas stream. Even more preferably, the deN2O-treated gas stream contains no more than 2% of the amount of N2O in the N2O- and NOX-containing gas stream. Most preferably, the deN2O-treated gas stream contains no more than 1% of the amount of N2O in the N2O- and NOX-containing gas stream.
In the process of the present invention, an additional gas stream comprising NO is provided to either or both of (i) the NOX-containing gas stream before it is contacted with the deNOX catalyst and (ii) the N2O- and NOX-containing gas stream before it is contacted with the deN2O catalyst in the embodiment wherein a N2O- and NOX-containing gas stream is treated with a deN2O catalyst in order to form a deN2O-treated gas stream, at least a portion of which is used as the NOX-containing gas stream. This additional gas stream contains NO in such an amount and concentration that the resultant ratio of NO2:NO in the NOX-containing gas stream is less than 1:1, preferably no more than 0.8:1.
Preferably the additional gas stream comprising NO is another process gas stream produced in the process which produces either the NOX-containing or the N2O- and NOX-containing gas streams. In one particularly preferred embodiment, the NOX-containing or the N2O- and NOX-containing gas stream is an exhaust gas stream from an industrial process and the additional gas stream is another gas stream within that process. Most preferably, the NOX-containing or the N2O- and NOX-containing gas stream is an exhaust gas stream from a nitric acid plant and the additional gas stream is formed from at least a portion of an outlet stream from the ammonia burner in such a process.
The present invention is further illustrated in the preferred, but non-limiting, embodiments of the invention illustrated in
In
The invention will now be illustrated by means of the following Examples, which are not intended to limit the invention.
The examples were carried out by passing a gas stream containing NOx, N2O, NH3, N2, O2 and H2O over a DeNOx catalyst at 250° C. and at different NO/NO2 ratios. The composition of the gas streams and the results of the tests are shown in Table 1. For the Examples of the invention (2, 4, 6 and 7), extra NO was added to the gas stream in order to correspond to an additional gas stream comprising NO being added to the NOX-containing gas stream in these examples.
The DeNOx catalyst used in the test runs was S-096 catalyst (a vanadium on titania catalyst commercially available from CRI Catalyst Company). A nominal catalyst diameter of 3.2 mm was used in runs 1 to 4 and a nominal catalyst diameter of 1.0 mm was used in runs 5 to 8. The tests showed that, in the examples of the invention (2, 4, 6 and 7), ratios of NO/NO2 above 1:1 (corresponding to an additional gas stream comprising NO being added to the NOX-containing gas stream) result in no increase of the concentration of N2O over the catalyst being detected. However, for the comparative examples (1, 3, 5 and 8) with lower ratios (corresponding to no additional gas stream comprising NO being added to the NOX-containing gas stream) N2O concentration was increased over the deNOx catalyst.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15195220.7 | Nov 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/077904 | 11/16/2016 | WO | 00 |
