Steam Reformation The present invention relates to a process and a plant for reducing nitrogen oxides during the steam reformation, wherein a gaseous carbon carrier and water are reacted in a reactor to obtain a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide, wherein hydrogen, carbon monoxide and/or carbon dioxide is separated from the synthesis gas and the stack gas obtained in the reactor is passed over at least one catalyst stage and subjected there to a selective catalytic reduction.

Nitrogen oxides (NO _x ) is a collective term for gaseous oxides of nitrogen. Due to the many oxidation stages of nitrogen, there is a number of nitrogen-oxygen compounds, the most important ones are nitric oxide (NO), nitrogen dioxide (NO2) and dinitrogen monoxide (N ₂ O, laughing gas). Nitrogen oxides are the source of a multitude of environmental impacts, which is why they already have attracted attention in terms of environmental policy at an early stage. They are one of the main causes of acid rain, are trigger of the so-called summer smog with the resulting environmental impacts and act as strong greenhouse gases. In particular the laughing gas significantly contributes to the destruction of the ozone layer.

The main source for nitrogen oxides are combustion processes, in particular in motor vehicle traffic and in large-scale stationary industrial applications. For lowering the nitrogen oxide em issions, the selective catalytic reduction (SCR) has been developed. In this process, a reducing agent, in general ammonia (NH3), is admixed to the waste gas, whereby a comproportionation of the nitrogen oxides occurs on a heterogeneous catalyst, which results in the formation of water and nitrogen. Ammonia has the advantage that it is an easily available substance to be procured at low cost. Since ammonia is liquid at the usual ambient temperatures, it can easily be stored and transported.

Beside ammonia, further reducing agents have also been examined when de- veloping the SCR technology, which however have a lower reduction potential. DE 1 1 2006 003 078 T5 for example describes the selective catalytic reduction of nitrogen oxide with a mixture of hydrogen and carbon monoxide on a catalyst which has a palladium content of 0.1 to 2.0 %, a vanadium pentoxide (V2O5) content of 0.1 to 7 %, and an oxide carrier material with a large surface area. However, only NO can effectively be reduced with this catalyst.

US 2010/0092360 A1 describes a novel catalyst for the selective catalytic reduction of NO and partly also NO2 with hydrogen, wherein the catalyst consists of a mixture of platinum and palladium which is in contact with a solid carrier material mixed from magnesium oxide and cerium oxide. The conversion of NO is effected under strictly maintained oxidizing conditions in a helium atmosphere.

US 2009/0285740 A1 describes a selective catalytic reduction on a palladium catalyst which is applied onto a zirconium oxide sulfate. As reducing agent, there is also used hydrogen, wherein water and hydrogen are injected into the waste gas stream and a molar ratio of hydrogen and NO _x must be obtained in a range from 10 to 100, which necessitates very high amounts of hydrogen.

All these processes have in common that they can only be carried out success- fully under certain exactly defined conditions. In addition, often only NO, in part also NO2, but not N ₂ O, is reduced successfully. Moreover, there are difficulties in the storage of hydrogen and in the hydrogen transport, which is why hydrogen so far hardly could find acceptance as reducing agent in the technical application. A further reducing agent examined in the academic sector is methane. WO 2008/026002 A1 for example describes the conversion of nitric oxide (NO) to nitrogen dioxide on a metal zeolite catalyst, wherein a hydrocarbon is used as reducing agent. A conversion of other nitrogen oxides is not described.

The publication of Yong-Ki Park et al. in "Mechanistic study of SCR-NO with methane over Pd-loaded BEA zeolite", appeared in the Journal of Molecular Catalysis A, Chemical 158, 2000, pp. 173-179, describes an SCR process with methane, in which a gas likewise containing nitric oxide is converted on a BEA zeolite loaded with palladium.

These two processes have in common that with them only nitric oxide (NO) can be reduced. A reduction of further nitrogen oxides, in particular of laughing gas, is not described for these reducing agents either.

The classical SCR catalyst with NH ₃ as reducing agent is a doped vanadium pentoxide catalyst which is applied on titanium dioxide. With this catalyst, at least nitric oxide and nitrogen dioxide can be converted into dinitrogen and dioxygen with sufficiently high conversion rates. In this conventional catalyst system it is, however, problematic that the catalyst only has a low high- temperature resistance and therefore an undesired phase conversion of V2O5 from the anatase into the rutile modification occurs, and also the sublimation of vanadium. On the other hand, V2O5 is toxic. What is furthermore disadvantageous is the relatively high price and the high price volatility of the vanadium raw material. The actual and potential risks of the vanadium emission due to sublimation and other undesired side reactions are covered only in part. Due to the described problems of the conventional catalytic systems, the same no longer are permitted for example in the Japanese and in part also in the US automotive industry. It should therefore be expected that also in stationary ap- plications the use of vanadium no longer will be admissible in the future. Meanwhile, zeolitic systems therefore are also used in part. EP 0 955 080 B1 describes a process for producing a catalyst material by incorporating a metallic component into a synthetic zeolite material. However, this catalyst is so active that it already works without incorporating an additional reagent, which is why a number of side reactions are to be feared in large-scale industrial processes.

WO 03/105998 describes a process and an apparatus for reducing the NO _x content based on all components, wherein the waste gas to be cleaned is guided over two catalyst beds which both are filled with old loaded zeolites. As reducing agent, ammonia is used.

The publication "Uhde EnviNOx ^® Technology for NO _x and N ₂ O abatement - A contribution to reducing emissions from nitric acid plants" by M. Groves, A. Sasonow, Fifth International Symposium on Non-CO2-Greenhouse Gases, 30.6- 3.7.2009, Wageningen, describes a process in which the SCR system likewise is divided in two for an effective reduction. In a first step, the nitrogen oxides are reduced with ammonia as reducing agent and then in a second step lowered even further by using hydrocarbons. Nitrogen oxides in principle also are generated in all processes in which in at least one step air is heated to temperatures of more than 1000 °C, whereby radical reactions of the N ₂ molecules contained in the air occur to an increased extent. One of these processes is the so-called steam reformation, in which a carbon carrier heterogeneously catalyzed with water is converted to a synthesis gas consisting of hydrogen (H ₂ ) and carbon monoxide (CO) and carbon dioxide (CO ₂ ).

WO 2009/080937 A2 proposes a process for the selective catalytic reduction of nitrogen oxides in combustion waste gases, for example also from the steam reformation, in which ammonia is utilized as reducing agent. In a first step, a reactor is heated by a hot gas and in a second step the waste gas obtained in the furnace is reduced selectively. For this purpose, a part of the gas provided for heating is branched off and mixed with the reducing agent, before it is introduced into the catalyst stage. This allows an exact temperature control. By means of this process, at least the content of NO and N ₂ O can be reduced significantly.

However, the use of ammonia as reducing agent has a number of disadvantages in the stack gas denitration of a steam reformation. On the one hand, keeping ammonia on stock and dosing it in always is necessary. On the other hand, ammonia is classified as a toxic substance harmful to the environment, whose use requires special safety devices (ventilations, emergency showers, etc.). A minor emission of non-reacted ammonia cannot be excluded, which limits the performance of these systems and represents a potential environmental burden. In addition, the storage of the reducing agent in an ammonia tank requires additional space in the plant, wherein the space requirement can further be increased by safety distances possibly to be maintained due to explosion protection zones. Regularly filling up the ammonia tank also represents quite a considerable risk at the existing operating temperatures.

Therefore, it is the object of the invention to provide a process for the selective reduction of nitrogen oxides, in particular also of laughing gas, in the purification of the flue gases from a steam reforming plant, which does not employ ammonia as reducing agent, but in which all nitrogen oxides contained in the waste gas can be equally be converted.

In accordance with the invention, this object is solved with the features of claim 1 , according to which a gaseous carbon carrier and preferably vaporous H ₂ O are reacted in a reactor to obtain a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide. The reaction is an allothermal reaction, which is why the reactor either must be heated indirectly or be fired directly. For heating up the reactor, a further gaseous fuel is used. Both the gaseous carbon carrier and the further fuel preferably is methane or natural gas. The synthesis gas obtained in the reactor subsequently is split up into its constituents hydrogen, carbon monoxide and carbon dioxide or a defined mixture of these components is adjusted. The also produced flue gas is passed over at least one catalyst stage, where it is subjected to a selective catalytic reduction. In this catalyst stage, a part of the gaseous carbon carrier, a part of a mixture of the gaseous carbon carrier with the water or the steam, a part of the gaseous fuel utilized for heating the reactor, a part of the (depleted) synthesis gas and/or a part of the separated hydrogen is utilized as reducing agent. The above-mentioned disadvantages of ammonia as reducing agent no longer exist here. In addition, such process is maintenance-friendly, since there is no need of filling an ammonia tank.

The flue gas preferably has a CO2 content of 15 to 20 mol-%, a H ₂ 0 content of 15 to 20 mol-%, an O2 content of 0.5 to 5 mol-%, an argon content of about 1 mol-% and a NO _x content of 50 to 250 mg/m ³ . In a favorable configuration of the catalyst stage, the same contains a metallic component of one or more metals and/or metal oxides of the group including Cr, Cu, Mn, Mo, Fe, Co, Pd, Ru, Pt, Rh, Ti, Zr, V, W, preferably palladium, ruthenium and platinum. By using palladium, ruthenium and/or platinum in pure or oxidic form, disadvantages of a vanadium-containing catalyst can be avoided. In addition, preparing a metallic catalyst is much easier than preparing a zeolitic one.

Furthermore, it is favorable when the catalyst stage contains a zeolite in whose structure atoms of one or more metal(s) and/or metal oxide(s) of the group including Cr, Cu, Mn, Mo, Fe, Co, Pd, Ru, Pt, Rh, Ti, Zr, V, W, preferably iron, copper or cobalt, are present. In accordance with the invention, zeolite is understood to be a crystalline substance from the group of aluminosilicates with a uniform spatial ring structure of Si0 ₄ /AI0 ₄ tetrahedrons linked via oxygen atoms. There can be used any zeolite, but zeolites in a decimal or duodecimal ring structure and zeolites of the types MFI, FER, MOR and BEA are preferred. The zeolite used additionally is characterized by the exchange of one or more metals) or metal oxide(s). Useful exchange metals include for example copper, iron or cobalt and a doping with one or more transition or noble metals. The metals are present in the zeolitic ring structure, typically at characteristic centers, the alpha, beta or gamma positions. The invention includes exchange degrees of metals on all three positions, preferably with an exchange content between 2.5 and 15 wt-%, independent of whether this exchange occurs in the liquid or solid phase. The exchanged metal can be present in metallic form or as metallic oxides or in any possible mixture and as monomer or dimer.

The significant advantage of the zeolitic catalyst as compared to the metallic catalyst system is its higher temperature stability, which provides for a broader temperature system for the design and leads to a higher insensibility of the system with respect to unwanted or unusual operating conditions. Furthermore, zeolitic catalysts have a higher DeNOx activity, which results in a broader temperature window for the operating condition and a potentially smaller catalyst volume and correspondingly smaller reactors. In addition, zeolites have a simplified, fully synthetic raw material basis, which is why here an environmentally compatible catalyst composition (free from pollutants, noble and transition met- als) as well as lower raw material prices can be found. Another advantage of zeolites as catalysts for SCR reactions is a lower catalytic activity for undesired side reactions. In particular, under the classical SCR reaction conditions, vanadium-based products generate the undesired laughing gas, sulfur trioxide and gaseous vanadium. In particular the formation of the nitrogen oxide N ₂ 0, which just has to be decomposed, must be regarded as highly disadvantageous. Since temperatures in the range from 850 - 1200 °C, preferably 900 to 1 050 °C, in particular about 970 °C exist in the reactor, the flue gas exits from the reactor with temperatures of 150 °C to 200 °C above the reactor temperature. To recover at least a part of the heat of the flue gas, the gaseous carbon carrier, which is used as one of the educts, and/or the water, which is introduced into the reactor preferably in evaporated form, is guided in heat exchange with the flue gas. Thus, the gaseous carbon carrier and/or the water or the steam can be heated up further. With reference to the water, the amount of energy contained in the flue gas can also be utilized to evaporate the water. Upon entry into the catalyst stage, the temperature of the flue gas then lies between 200 and 500 °C; a range preferred for the SCR reactions for entry of the gas into the catalyst bed is 200 to 350 °C when using NH ₃ , and 150 to 250 °C when using H ₂ as reducing agent. Since even further impurities can be contained in the flue gas, it is also favorable to subject the flue gas to at least one further cleaning step. Such cleaning step for example can be a desulfurization or the filtration of the particles contained in the waste gas. To not unnecessarily load the catalyst for the SCR process, it may be expedient to provide both the desulfurization and the filtration before the SCR catalyst stage. However, in particular with an integrated heat recovery of the flue gas, this has the disadvantage that the temperature of the flue gas can fall so much that it no longer is high enough to completely run the SCR process. The advantage of an arrangement in which the SCR process is conducted in the second or even in the first position of the waste gas aftertreat- ment, therefore consists in that relatively high temperatures can be employed. However, a catalyst damage is put up with, which in particular with metallic catalysts has a negative influence on the useful life.

To achieve a sufficient distribution of the gaseous reducing agent, it was found to be particularly favorable to branch off part of the flue gas before the catalyst stage, to dose the reducing agent into this partial stream, and to then feed this mixture into the catalyst stage. In this way, locally high concentrations of the reducing agent and undesired storage operations are prevented. The invention furthermore comprises a plant for reducing nitrogen oxides in a steam reformation, which is suitable for carrying out the process according to the invention and includes the features of claim 8. This plant comprises a reactor into which either one conduit for supplying a mixture of H ₂ 0 and a carbon carrier opens, or two separate conduits for separately introducing H ₂ 0 and a carbon carrier. The synthesis gas formed in the reactor is supplied to a separating device via a discharge conduit. In addition, a waste gas conduit leads into a catalyst stage, wherein before or in the catalyst stage a dosing means is provided for introducing a reducing agent. The waste gas conduit is guided directly out of the reactor. Via a conduit, the dosing means is connected with one of the supply conduits of the reactor, a further supply conduit for a gaseous fuel for heating the reactor, the discharge conduit for the (depleted) synthesis gas formed and/or the discharge conduit for the separated hydrogen. The connection of the dosing means with the discharge conduit can be provided both before and after the separating device for separating the reaction products of the steam reformation.

In accordance with the invention, the dosing means is designed such that it is suitable for introducing a gas. Hence it differs from the dosing means of classical SCR plants which are operated with ammonia as reducing agent, since the ammonia is introduced as liquid either in its pure form or, for example in the vehicle sector, as urea solution.

It was furthermore found to be particularly advantageous that the dosing means is provided in a partial stream conduit branching off from the waste gas conduit and opening into the catalyst stage. ln addition, it was found to be favorable to use the catalyst either as bulk material of extruded cylindrical full-body extrudates or as honeycomb-shaped catalyst modules, in dependence on the pressure loss allowed on the process side. Systematic advantages of the honeycomb-shaped catalyst modules are the improved mechanical properties, a significantly lowered pressure loss on the process side, and a simplified filling and draining of the reactor. When using honeycomb-shaped modules, the replacement of parts of the catalyst bed also is possible and a scale-up is easier, since here, like in the microreaction technology, only a numbering-up must be effected.

Further developments, advantages and possible applications of the invention can also be taken from the following description of an exemplary embodiment and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

The only Figure shows the schematic diagram of a plant for steam reformation according to the invention with a downstream SCR catalyst stage. Into a reactor 10 according to the invention, methane preferably compressed with the compressor 2 is introduced via conduits 1 and 1 '. In principle, however, other carbon-containing gaseous carbon carriers, such as natural gas, light gasoline or methanol as well as biogas, are also conceivable. Methane, however, has the advantage that only few impurities thus are introduced into the plant. Preceding cleaning steps, such as the removal of acid gases, e.g. AGR - Acid Gas Removal or the Rectisol process, thus can be omitted.

Via conduits 3 and 3', water is introduced into the reactor 10 preferably as steam. Therefore, it is favorable to guide the water over a heat exchanger 4. To achieve a complete intermixture of the two educts, methane and water, it may be advantageous to first join the two educts in one common conduit, additionally intermix the same if necessary, and then jointly feed them into the reactor 10. Optionally, the water or the steam can partly or wholly be admixed to the carbon carrier before or after the compressor 2 and the entire educt mixture can be guided into the reactor 10 via conduit 1 '. Usually, the educt mixture or the individual educts is/are heated in a heat exchanger 8 before entry into the reactor 10.

The reactor 10 preferably is a fired tubular furnace. This tubular furnace is oper- ated with a fuel gas, preferably natural gas or methane or depleted synthesis gas (tail gas / off gas), which is introduced into the reactor 10 via the supply conduits 5 and 5'. By means of the heat exchanger 8, the fuel gas also can be heated in advance. Other, directly fired reactors are however just as conceivable as indirectly heatable reactors, and the steam reformation in particular can also be carried out in microreactors.

Via a discharge conduit 7, the synthesis gas obtained in the reactor 10 is discharged. This synthesis gas then is processed in one or more cleaning steps. For separating the products H ₂ , CO and CO2 the last process stage used is a separating device 40 for carrying out a catalytic, adsorptive, absorptive or cryogenic separation process or combinations of such processes, preferably a process of pressure swing absorption. Via conduit 41 , the carbon monoxide formed is discharged, via conduit 42 the carbon dioxide and via conduit 43 the hydrogen. Alternatively, a defined mixture of all said components is withdrawn as product (synthesis gas mixture).

The flue gas of the fired furnace withdrawn via conduit 1 1 passes through one or more not completely represented steps of heat recovery. Preferably, the flue gas can be used for heating water, methane and/or the fuel gas. Via conduit 15, the waste gas still loaded with nitrogen oxides then flows into an SCR catalyst stage 20, wherein the waste gas possibly is heated to the ideal operating temperature of 150 to 350 °C. Into this catalyst stage 20 a reducing agent is fed via conduit 22', wherein the quantity of this reducing agent can be controlled or regulated via the dosing device 21 . As reducing agent, the hydrogen can be withdrawn from the processed product stream behind the separating device 40. In principle, however, it is also conceivable to branch off the depleted synthesis gas stream 22. The reducing agent either can directly be introduced into the catalyst stage 20 via conduit 22' or parts of the waste gas stream to be cleaned optionally are branched off via conduit 15', which is connected with conduit 22'. As a result a bypass is formed, in which the dosing device 21 is positioned. Not only hydrogen, but also methane or natural gas can be used as reducing agent. For this purpose, a conduit must be guided from the supply conduit of the educt 1 and/or the fuel 5 to the dosing device 21 . The use of the educt or the fuel gas has the advantage that the yield of hydrogen or synthesis gas is not diminished by the waste gas cleaning method.

It is also possible to mix a plurality of reducing agents. Within a system with vanadium catalyst, all said reducing agents have a lower reduction potential than NH ₃ , but they have the advantage that they are already present in the plant.

Via conduit 23, the cleaned waste gas then is guided into a chimney 30 and escapes as stream 31 . Example

A typical steam reforming plant with a production of 45,000 Nm ³ /h of hydrogen includes a flue gas quantity of 173,000 m ³ /h. With pure natural gas firing, a NO _x content of 200 mg/m ³ (0.00667 mol/m ³ ) in the waste gas must be expected. In fireboxes, the substance quantity typically is composed of about 95 mol-% NO and 5 mol-% NO ₂ . In addition, N ₂ O concentrations between 2 and 500 vol-ppm, in particular between 20 and 80 vol-ppm can be found. A usual emission limit value for the entire nitrogen oxide load is about 100 mg/m ³ ; accordingly about 548 mol/h NO, 29 mol/h NO ₂ and the N ₂ O must be converted as completely as possible. The stoichiometric hydrogen demand according to the reaction equations:

2 NO + 2 H ₂ ^ N ₂ + 2 H ₂ O

and

2 NO ₂ + 4 H ₂ -» N ₂ + 4 H ₂ O thus is 606 mol/h or 13.6 m ³ /h, which corresponds to 0.03% of the production quantity.

In the flue gas of a firebox of a steam reforming reactor about 1 .2 vol-% oxygen must be expected. As compared to these 2076 m ³ /h of oxygen, about 12.8 m ³ /h of hydrogen are present. The amount of hydrogen in oxygen of 0.6 % falls below the lower explosion limit, which is about 4 % in air and 19 % in pure oxygen, respectively. The use of hydrogen as reducing agent hence does not lead to an explosive atmosphere. List of Reference Numerals

1 , r conduit

2 compressor

3, 3', 3" conduit

4 heat exchanger

5, 5' supply conduit

6 heat exchanger

7 discharge conduit

8 heat exchanger

10 reactor

1 1 conduit

13 conduit

15, 15' conduit

20 SCR catalyst stage

21 dosing device

22, 22' conduit

23 conduit

30 chimney

31 conduit

40 separating device

41 conduit

42 conduit

43 conduit