WO2012104205A1

WO2012104205A1 - Ammonia generator converting liquid ammonia precursor solutions to gaseous ammonia for denox-applications using selective catalytic reduction of nitrogen oxides

Info

Publication number: WO2012104205A1
Application number: PCT/EP2012/051296
Authority: WO
Inventors: Oliver Kröcher; Daniel PEITZ
Original assignee: Paul Scherrer Institut
Priority date: 2011-02-04
Filing date: 2012-01-27
Publication date: 2012-08-09

Abstract

The presented invention is an ammonia generator, constructed to convert liquid ammonia precursor solutions to gaseous ammonia. The precursor compound is decomposed in the liquid reducing agent solution at elevated pressures and temperatures. After conversion of most of the ammonia precursor compound to ammonia and other gases, the evolved gases are released to ambient pressure. Providing heat to the precursor compound solution without allowing it to boil enables good hydrolysis reaction conditions for the ammonia precursor compound. The high pressure in the ammonia generator keeps water mostly in its liquid state and in direct contact with the reactant ammonia precursor compound and the catalyst bed. The heat transfer is accelerated if water is still mostly liquid, as the heat consumed by the decomposition reactions is quickly replenished. Applying the presented principle according to claims 1 to 820 enables the design and production of a highly compact converter unit for continuous hydrolysis of ammonia precursor compounds to hot ammonia gas. The yielded ammonia gas can be introduced into the hot exhaust gas without causing cooling, thus enabling SCR of NOx at unprecedented low exhaust gas temperatures. In the ammonia generator described the formation of deposits due to incomplete precursor compound decomposition can be eliminated. Ammonia mixes much faster with the exhaust gas than the spray of a liquid reducing agent giving the described generator a significant increase in reaction rate and yield for DeNOx compared to previous designs for the SCR process. It may even be possible to replace a spray nozzle in an existing SCR system by a compact converter.

Description

Ammonia generator converting liquid ammonia precursor solutions to gaseous ammonia for DeNOx-applications using selective catalytic reduction of nitrogen oxides

The presented invention relates to a method and an apparatus comprising a separate ammonia generator to convert liquid ammonia precursor solutions into gaseous ammonia for NOx reduction (DeNOx) applications utilizing selective catalytic reduction of nitrogen oxides .

Emission guidelines for on-road, off-road and industrial

applications yielding exhaust gases have become ever more

stringent. Some of the undesired components in exhaust gases are the nitrogen oxides which are formed from nitrogen and excess oxygen during the combustion process at high temperatures. As these compounds cannot be reduced down to the demanded

concentrations by optimizing engine operation or the combustion process alone, a DeNOx system needs to be introduced. The most promising process, which has been already in commercial use for several decades in power plants and a few years in mobile

applications, is selective catalytic reduction (SCR) .

In the SCR process, ammonia gas reacts with the nitrogen oxide (NOx) gases on the SCR-catalyst to yield harmless nitrogen and water. In stationary applications, such as power plants, ammonia gas is injected into and mixed with the exhaust gas, which is not feasible for mobile applications due to safety issues of

transporting ammonia gas. Therefore, an ammonia precursor compound solution, such as urea in water is injected into the hot exhaust gas. The urea is then either decomposed on a special hydrolysis catalyst or on the SCR-catalyst itself to yield gaseous ammonia for the SCR reaction. Thus, the decomposition of the ammonia precursor compound yields the necessary ammonia, but also creates no issues with legislation as only a harmless ammonia precursor compound solution is transported.

This invention describes the production of ammonia gas and its introduction into the exhaust gas of combustion engines. However, it may also be used for any other application requiring a dynamic or constant flow of ammonia and/or hydrogen containing gas.

Obviously, this includes stationary applications of SCR or SNCR (selective non-catalytic reduction of NOx) in e.g. power plants, incineration facilities or boilers, but also less apparent

applications like the supply of a fuel cell with a CO-free

hydrogen and ammonia containing gas flux for direct use or use after reforming also the contained ammonia to hydrogen. In SCR systems utilizing liquid ammonia precursors it is hard to mix the ammonia precursor spray with the exhaust gases and

simultaneously avoid deposit formation in the region between the spray nozzle and the catalyst. Incomplete mixing of the reducing agent with the exhaust gas will result in a poor DeNOx

performance, as some channels of the SCR catalyst will not be supplied with the required amount of reducing agent. On the other hand an excess of reducing agent in other channels will lead to an unwanted ammonia slip. The decomposition of the ammonia precursor compound or compound mixture and the evaporation of the solvent it is dissolved in will draw a significant amount of heat from the exhaust gas.

Today's major challenge is to have a good reaction of ammonia gas with NOx gases at low temperatures on the SCR catalyst. The increasing need for a low temperature SCR process is also a result of the trend for using the hot exhaust gas as energy source for turbo-chargers, thereby lowering exhaust gas temperature by the conversion of heat to mechanical energy. Often the exhaust gas would cool down during the evaporation of solvent in the reducing agent solution, preventing the precursor from decomposing

completely and reducing the yield and rate of the reaction between produced ammonia with NOx. Thus, at low exhaust gas temperatures the dosing of ammonia precursor into the exhaust is often

unsuitable in mobile applications.

If gaseous ammonia is dosed into the system rather than spraying an ammonia producing solution, the exhaust gas could have lower temperatures than are used in the conventional SCR process and there would be enough heat to satisfy the conditions of the reaction on the SCR catalyst. Our invention is an ammonia

generator which can be operated independently of the exhaust gas temperature to provide hot ammonia gas for direct injection into the exhaust gas.

Existing systems: Most commercial systems and disclosed inventions for the

decomposition of liquid NOx reducing agents inherited a basic setup like in patent DE 40 38 054 Al concerning the way the reducing agent is introduced. In the cited disclosure, the

reducing agent solution is sprayed onto a devoted hydrolysis catalyst or the SCR catalyst itself for decomposition.

Consequently, the reducing agent solution is either situated in the gas phase or present as an aerosol, both being diluted with either exhaust gas and/or pressurized air while passing the decomposition catalyst. As a result, the ratio of reactants dosed per catalyst volume is much lower than the ratio which results when pumping the liquid reducing agent solution through a heated catalyst bed.

In patent DE 40 38 054 Al two of the main claims are: the

vaporization of the reducing agent to form droplets of less than 10 μπι and the vaporization of the reducing agent within 1 s on an evaporator .

Later amendments like DE 42 03 807 Al focused, for instance, on replacing the laminar flow of the hydrolysis catalyst, on which the reducing agent is sprayed, by a more turbulent flow within the channels to increase the conversion efficiency of urea to ammonia gas and carbon dioxide.

Recent patents like WO2008077587A1 have been dealing with

alternative reducing agents in various solutions and separating the exhaust gas flow from the ammonia production from the

decomposition of the precursor compounds. WO2008077587A1 and many other recent patents still contain a spraying apparatus involving either a gas to assist spraying and/or a gas into which the ammonia storage compound is sprayed and a subsequent evaporation and decomposition system.

However, there are also some inventions concerning the conversion of the reducing agent solution without spraying:

FR 29 36 957 Al and FR 29 36 958 Al describe a device compromising a chamber for receiving aqueous urea. The chamber is filled with a porous material with a catalytic coating (100 cm² surface area per 1 cm³) on the pores to decompose aqueous urea to ammonia. The chamber contains a heating element inside which is heated to 40- 200°C (FR 29 36 958 Al ) or to 200-400°C ( FR 29 36 957 Al ) . During heating in a first heating zone to 50°C or during heating up of the solution after insertion, urea is claimed to precipitate in the chamber, but due to the filling with a porous material (pore size 1 - 100 μπι) only small urea crystals can form. These urea crystals are claimed to decompose to ammonia during further heating. The device is operated at atmospheric pressure, or slightly elevated pressure to inject the produced gases into the exhaust duct.

In contrast, the presented method will utilize catalytic coatings or catalyst with surface areas which are larger by at least two, preferably three orders of magnitude. The heating will always be intended to be from the outside of the reactor, and restricted to a single zone. No porous material will be necessary to restrict the formation of urea crystals, and pressures used will be

significantly larger than atmospheric pressure. In addition, the use will not be restricted to urea solution, and, therefore, also the production of hydrogen along with ammonia is possible.

US 58 27 490 A describes a method for the conversion of aqueous urea solution (concentration of less than 20% urea) to ammonia. The method is primarily intended for the treatment of NOx in the exhaust gas stream of a boiler. While heating to a temperature of 350-650°F (177-343°C), the solution is pressurized to a pressure required to keep urea reaction products in the liquid phase, at least 300 psi (20.7 bar) . The heated solution is contacted with a catalyst for the conversion of urea to ammonium carbamate which is claimed to be practically equivalent to a conversion to ammonia. The catalyst is preferably composed of chromium or molybdenum as metal, metal oxide or compound, but possibly also of aluminum, cobalt, niobium, titanium or vanadium as metal, metal oxide or compound. Conversion of >90% of urea to ammonia is reported for a 5% urea aqueous solution at 260°C with 4.5 min reaction time.

Conversion of a 2.4% urea aqueous solution at 300 psi (20.7 bar) and 350°F (177°C) with a reaction time of 3.5 min yielded 80% ammonia on a Cr catalyst, 72% on Mb and just 48% on Al, 40% on Ti or 0% without catalyst.

In contrast, the presented method will use urea solutions with concentrations from 30% (by mass) onward, and focus on combustion engine exhaust gas. The catalysts which were reported to be the most active ones will not be considered preferably. The operation without catalyst which was reported to be very bad (0% yield) will be a feasible option in the presented method for urea

decomposition . WO 98 42 623 Al describes a method for the conversion of a

solution of urea, but also biuret or ammonium carbamate to

ammonia. The solution is pumped into a reactor which is heated. The heating is used to influence temperature and pressure in the reactor. While under pressure, ammonia and CO₂ are removed as gases from the reaction mixture, while the remaining solution is

recycled. Ammonia and CO₂ are then mixed into the process gas stream of a furnace, incinerator or power plant. The decomposition is enhanced by preferably oxides, ammonium or alkali salt of vanadium or molybdenum, but also chromium, tin, bimuth or boron or by "active surface solids" like activated carbon, activated silica, activated alumina and ion-exchange resins in their acid and basic form. The temperature for operation is preferably 130- 170°C, in general 110-200°C, the coupled pressure 20-120 psi (1.4- 8.3 bar), in general 20-500 psi (1.4-34.5 bar) .

In contrast, the presented method will not control pressure by the heating rate, but by mechanical means. Also, the solution will not be recycled, but completely inserted into the exhaust gas duct. The conversion temperature for urea solution will be significantly higher than 170° and even 200°C. The mentioned catalysts will not be preferably utilized, with the exception of alumina as

hydrolysis catalyst.

US 63 61 754 Bl describes a system for the conversion of urea, urea hydrolysis products, urea dimers and polymers, urea adducts or urea condensation products to yield ammonia gas without the formation of urea decomposition products in solid or molten form. Conversion is performed at 50-600 psi (3.4-41.4 bar) and

temperatures above 140°C, in order to be above the hydrolysis temperature of urea. A liquid catalyst may be added to the urea solution (e.g. phosphoric acid) or a solid catalyst may be present in the heating unit (e.g. aluminum oxides) .

In contrast, various other ammonia storage compounds will be utilized with the advantage of producing hydrogen in addition to ammonia to further increase the efficiency of NOx removal. No liquid catalysts will be considered, but some solid catalysts could be inserted in the reactor to accelerate the decomposition of the ammonia precursor compounds.

The objective of the present invention is to disclose a method and a compact apparatus for the efficient generation of ammonia from liquid ammonia precursors for the selective catalytic reduction of NOx in engine exhaust gas.

This objective is achieved by the use of aqueous ammonia precursor solutions containing 30-80% by mass of urea, ammonium formate, guanidinium formate, methanamide or mixtures thereof, and their catalyzed hydrolysis by heating under pressure in presence of a hydrolysis catalyst in a reactor. The product gas mixture from the described ammonia generator is first produced as gas flow apart from the engine exhaust gas. Only upon release from the reactor the product gas is mixed into the main exhaust flux. The position of the mixing must be upstream of the SCR catalyst but could be as far upstream as the exhaust manifold on the engine before entering a turbo charger. In the extreme scenario of insertion at the exhaust manifold before turbocharger, the turbine would enable good mixing of the product gas with exhaust gas while the high pressure inserted gas may add to the performance of the turbine. Addition of the product gas at a further downstream position of the exhaust gas duct could be very advantageous in order to raise the temperature of the exhaust gas flow before entering the SCR catalyst, thereby increasing DeNOx activity.

The production of ammonia "apart from the engine exhaust gas" shall mean that the engine exhaust gas stream does not flow through the ammonia generator, but its walls or other parts of it may be in direct or indirect (using e.g. a heat pipe) contact with the engine exhaust gas flow in order to transfer heat from the exhaust gas to the ammonia reactor. The heat transfer from the exhaust gas to the ammonia reactor may satisfy the complete need for decomposition heat or a part thereof.

Applying the presented principle enables the design and production of a highly compact converter unit for continuous hydrolysis of ammonia precursor compounds to hot ammonia gas. The yielded ammonia gas can be introduced into the hot exhaust gas without causing cooling, thus enabling SCR of NOx at unprecedented low exhaust gas temperatures. In the described ammonia generator the formation of deposits due to incomplete precursor compound

decomposition can be avoided. Hot ammonia gas mixes much faster and more homogeneously with the exhaust gas than the spray of a liquid reducing agent giving the described generator a significant increase in reaction rate and yield for DeNOx compared to previous designs for the SCR process. It may even be possible to replace a spray nozzle in an existing SCR system by a compact converter. The addition of the hot ammonia gas to the exhaust gas flow is

performed by a valve to control the amount of dosed ammonia. The signal for the valve originates from a control unit which may determine the amount of necessary ammonia either solely from engine operation parameters like load and rpms, from NOx sensors before and/or after the SCR catalyst or a combination of these techniques . In order to achieve a substantially complete hydrolysis of the ammonia precursor compound in the ammonia generator a choice with respect to the pressure and the temperature for the catalytic decomposition of the reducing agent solution is made in a way that hydrolysis of the ammonia precursor compound takes place

preferably in the liquid phase inside the ammonia generator while contacting the catalyst. Preferably, the decomposition of the ammonia precursor is

performed at high pressure to yield ammonia which is released as ammonia gas along with carbon dioxide gas and water vapor upon pressure release. Typically, the suppression of the boiling of the reducing agent solution is achieved by applying high pressures of 5 to 100 bar, depending on the operation temperature.

In some applications it can be desirable that the complete

conversion of the ammonia precursor compound to ammonia is

replaced by an incomplete conversion of the precursor compound with the production of a precursor compound gas and therefore requiring less heat input, particularly in dependence of the reactant material which is used as ammonia precursor material.

In order to limit the wear of the catalyst used in the ammonia reactor, a catalytic bed is arranged inside the ammonia reactor with a sufficiently large void volume to provide for a constant flow of liquid through the void volume; said catalytic bed

contains catalyst particles with of size from 0.01 mm to 10 mm, preferably 0.1 mm to 10 mm, more preferably 0.1 mm to 5 mm, most preferably 0.1 mm to 1 mm. Typically, the catalytic bed comprises either solid catalyst particles, catalyst coated on non-catalytic particles, catalyst coated on ceramic or metallic substrates or extruded catalysts. Catalyst materials may be chosen to be T1O₂, AI₂O₃, ZrC>₂, zeolite, metal organic frameworks (MOF) or mixtures thereof, preferably T1O₂, AI₂O₃, r0₂, zeolite, most preferably

T1O₂, AI₂O₃ and/or ZrC>₂. The catalyst may possibly be stabilized by WO₃ or S1₂O₃. The catalyst could be doped with metals, metal oxides or combinations thereof to facilitate the decomposition of ammonia precursor compounds other than urea. Metals or metal oxides for doping could be Au, Pt, Pd, Re, Ru, Ag, Rh as well as rare earth metals and/or transition metals, such as Cr, Ni, Cd, Cu, Mn, V, Co, Fe, Zn, Nb, Ta, Os, Ir, Sn, or alloys/mixtures thereof, preferably Au, Pt, Pd, Ag, Cu, Nb, more preferably Au, Ag, Pt, Pd, most preferably Au. The catalyst has a specific surface area

(determined by BET N2-physisorption) of at least 50 m²/g. If coated, at least 50 g of catalyst are deposited per Liter of support material. This implies a specific surface area of 2.5 m² per cm³ of supported catalyst at the minimum.

A further preferred embodiment of the present invention may contain a porous material, most preferred a metal frit, of

tailored length and pore size, a pressurizer (a valve which will open above a set pressure but close below the set pressure) or a controllable valve that is used at the outlet of the ammonia generator to create the necessary pressure. Instead of and/or in addition to a porous material a valve may be used at the outlet of the ammonia generator to create the necessary pressure and/or improve the dynamics of the ammonia dosage into the exhaust gas system.

The respective apparatus claims refer to the claims 9 to 17 listed below.

Preferred examples of the present invention are hereinafter described with reference to the attached drawings which depict in: Figure 1 a schematic overview of a system for the hydrolysis of an ammonia precursor material;

Figure 2 a plot of the conversion rate and the contents of the involved compounds as a function of the reactor wall temperature; and Figure 3 a plot of the conversion rate with different catalyst as a function of the reactor temperature.

Figure 1 shows a first system for the hydrolysis of the ammonia precursor material. A stainless steel tube with 10 mm internal diameter was sealed at one end with a 0.1 μπι diameter pore size stainless steel frit (9) . A high pressure valve with small flow diameter could have been chosen instead. A volume of 7.9 ml

(corresponding to a tube length of 100 mm) was filled with 300 μπι D50 diameter Y-AI₂O₃ (7) . A flow of 1 ml/min 32.5% urea (by mass) in de-ionized water (4) was pumped by a high pressure pump (5) into the packed bed via a capillary of 128 μπι internal diameter (6) . The tube was electro-thermally heated from the outside (8) to a temperature of 300°C (measured on the outside wall) and the solution inside the packed bed heated up. The expected boiling of water at this temperature was suppressed by the high pressure created by the frit at the exit (9) and the catalytic bed, so the liquid was heated up to a temperature sufficient for quantitative hydrolysis in the liquid phase. Using the hydrolysis catalyst packed bed which was wetted by hot water, the urea was completely decomposed to ammonia. If deposits of cyanuric acid, biuret, triuret, ammelide and/or other side products of the ammonia production via hydrolysis of the precursor were formed, they were hydrolytically decomposed under the previously described reaction conditions present in the ammonia generator.

In previous systems undesired side-products of the ammonia

production from the precursor inhibit the SCR process, however the ammonia generator decomposes these side products and prevents the inhibition of the process. The decomposition of the side-products from the ammonia production from the precursor in the ammonia generator would also prevent the plugging of the packed bed by deposits. Furthermore, the nitrogen contained in potentially formed side-products would be released as ammonia, improving the yield of ammonia gas. In the first series of experiments performed, the reaction products ammonia, carbon dioxide and water vapor were released as hot gases from the exit of the ammonia generator from the 1 ml/min input of 32.5% urea in water. The ammonia content was determined quantitatively by bubbling the gases through an acidic solution and titrating the resulting solution. Assuming 100% conversion of ammonia to ammonium in the acidic solution, the amount of gaseous ammonia produced by the generator was calculated backwards.

Additionally, product gas was quenched in acidic and basic

absorbing solutions and analyzed by HPLC for other compounds like urea, carbonate and side products. At T ≥ 250 °C, no urea, HNCO or byproducts could be detected. At T < 230 °C, the conversion was incomplete and significant amounts of urea and biuret were

detected (see Fig. 2) . As the reactor temperature was measured at the outside of the reactor wall, the effective temperature of the catalyst bed needed for complete conversion could have been lower than 250 °C. Also, the used Y-AI₂O₃ catalyst was chosen due the suited particle size and the good mechanical stability of the particles. T1O₂ and Zr0₂ have been determined to provide better decomposition activity in the gas phase than Y-AI₂O₃, which could lower the temperature needed for complete conversion.

The obtained results therefore may suggest a complete conversion of urea to gaseous products, provided that the reactor is

sufficiently heated. The gases water : ammonia : carbon dioxide were in a 6.83 : 2 : 1 ratio. The product gas mixture is then dosed into an exhaust gas stream (3) collected by the exhaust manifold (2) in a combustion engine (1) . The NOx produced by the combustion process will react with the dosed ammonia gas on the SCR-catalyst (10) .

A second system for the hydrolysis of the ammonia precursor material has been designed and constructed similar to the first one. The second system was also operated for several hours. At the exit end of a stainless steel tube with 10 mm internal diameter and 15 cm length (= 11.8 ml) a pressurizer (a valve which will open above a set pressure but close below the set pressure) with a set pressure of 50 bar was attached. The tube was heated from the outside, the temperature sensor for the feedback loop was

connected on the outside wall of the reactor 5 cm before the exit. Experiments with a thermocouple placed inside the reactor showed the temperature on the outside wall was identical to the inside temperature. A flow of 5 ml/min of an ammonia precursor fluid, either 32.5% urea (by mass) in water or 40% methanamide (by mass) in water was pumped through the inlet of the tubular reactor. The reactor was filled with different catalyst materials: T1O₂ anatase particles with diameters of 425-710 μπι or ZrO pellets with 3 mm diameter or AI₂O₃ particles of 0.8-1 mm or a T1O₂ anatase monolith (400 cpsi; stabilized with S1O₂) doped with 1.5% Au. Only

methanamide was tested with the Au-doped T1O₂ anatase monolith catalyst. An extra experiment was performed with urea solution without catalyst filling.

Some of the results are shown in Fig. 3. At a temperature below 240°C, less than 80% of the contained ammonia was released from urea solution. The catalyst filling actually only had a small effect on the release of ammonia from urea solution, it could be compensated by a temperature elevation of max. 10K.

Up to 180°C methanamide was decomposed on the Au-doped T1O₂ to the same degree as urea was at 40K higher temperatures. During the decomposition of methanamide, ammonia, CO₂, ¾0 and ¾ were

measured in the gas phase. Methanamide can be hydrolyzed to ammonia and formic acid. Formic acid can decompose to yield CO₂ and ¾ . The ammonia generator can therefore be used to produce a gas flow containing ammonia and hydrogen. Hydrogen has been shown to enhance the catalytic removal of NOx .

Ammonium formate and guanidinium formate are expected to show a similar decomposition product gas mixture, as they can be

decomposed to ammonia and formic acid, and formic acid could again be decomposed to yield H₂.

During the experiment, the pressure of 50 bar was not constantly sufficient to keep all components in the liquid phase. The vapor pressure of water is far below 50 bar at the presented

temperatures, but the produced CO₂ could not be kept completely in the liquid phase. However, the vast majority of CO₂ was still contained in the liquid phase, only a small fraction evaporated to form a gas phase. In case of methanamide, where hydrogen gas was formed, a gas phase evolved due to the limited solubility of hydrogen in water. As a consequence, the liquid and/or gas flow released at the pressurizer was fluctuating. The composition of the dosed gas flow changed only slightly, as the amount of products contained in the gas phase was only very small.

An application could contain a second pressure containment after the pressurizer. The operating pressure of the second containment would be below the pressure of the reactor, in order to enable a complete evaporation of the liquid phase. The gases would

therefore be uniformly mixed and could be dosed with constant gas flow .

Claims

1. A method for the selective catalytic reduction (SCR) of NOx in engine exhaust gas by the use of liquid ammonia precursors like aqueous solutions containing 30-80% by mass of urea, ammonium formate, guanidinium formate, methanamide or mixtures thereof ( 1 ) , characterized by the catalyzed hydrolysis of the ammonia precursor compound, being heated and pressed in a separate ammonia reactor apart from the engine exhaust gas stream and guiding the so- generated ammonia gas into the engine exhaust gas stream upstream of an SCR catalyst.

2. The method according to claim 1, characterized by the choice of pressure and temperature for the catalytic decomposition of the reducing agent solution in a way that hydrolysis of the ammonia precursor compound takes place preferably in the liquid phase.

3. The method according to claim 1, characterized by the

decomposition of the ammonia precursor at high pressure to yield ammonia, which is released as ammonia gas along with carbon dioxide gas, water vapor and hydrogen gas (if applicable) upon pressure release.

4. The method according to claims 1 to 3, characterized by the suppression of the boiling of the liquid ammonia precursor by applying high pressures of 5 to 100 bar, depending on the

operation temperature.

5. The method according to claim 3, wherein the complete

conversion of the ammonia precursor to ammonia is replaced by incomplete conversion of the precursor with the production of a precursor gas and therefore requiring less heat input in the separate ammonia reactor.

6. The method according to claims 1 to 4, characterized by a high hydrolytic decomposition efficiency of the ammonia precursor in the presence of solvents, in particular liquid water, at temperatures between 50 to 600°C, preferably between 150 to 350°C, by presence of a decomposition catalyst.

7. The method according to any of the preceding claims,

characterized by a catalytic bed ( 7 ) inside the ammonia generator with sufficiently large void volume to provide a constant flow of liquid through the void volume; said catalytic bed comprising catalyst particle of size from 0.01 mm to 10 mm, preferably 0.1 mm to 10 mm, more preferably 0.1 mm to 5 mm, most preferably 0.1 mm to 1 mm.

8. The method according to any of the preceding claims, wherein the catalytic bed comprises either solid catalyst particles, catalyst coated on non-catalytic particles, catalyst coated on ceramic or metallic substrates or extruded catalysts;

i) catalyst materials are T1O₂, AI₂O₃, Zr0₂, zeolite, metal organic frameworks (MOF) or mixtures thereof, preferably Ti0₂, AI₂O3, Zr0₂, zeolite, most preferably Ti0₂, AI2O₃ and/or Zr0₂;

ii) the catalyst is further preferably stabilized by WO₃ or S1₂O₃; iii) the catalyst is furthermore preferably doped with metals, metal oxides or combinations thereof to facilitate the

decomposition of ammonia precursor compounds other than urea;

iv) metals or metal oxides for doping are Au, Pt, Pd, Re, Ru, Ag, Rh as well as rare earth metals and/or transition metals, such as Cr, Ni, Cd, Cu, Mn, V, Co, Fe, Zn, Nb, Ta, Os, Ir, Sn, or

alloys/mixtures thereof, preferably Au, Pt, Pd, Ag, Cu, Nb, more preferably Au, Ag, Pt, Pd, most preferably Au;

v) the catalyst has a specific surface area (determined by BET N₂- physisorption) of at least 50 m²/g, and preferably in the coated version, at least 50 g of doped material are deposited per Liter of catalyst material which implies a specific surface area of 2.5 m² per cm³ of supported catalyst at the minimum.

9. The method according to any of the preceding claims, wherein a porous material, most preferred a metal frit, of tailored length and pore size or a valve that is used at the outlet of the ammonia generator to create the necessary pressure; instead of and/or in addition to a porous material preferably a valve is used at the outlet of the ammonia generator to create the necessary pressure and/or improve the dynamics of the ammonia dosage into the exhaust gas system.

10. An apparatus for the selective catalytic reduction (SCR) of NOx in engine exhaust gas by the use of liquid ammonia precursors like aqueous solutions containing 30-80% by mass of urea, ammonium formate, guanidinium formate, methanamide or mixtures thereof (1) , wherein the catalyzed hydrolysis of the ammonia precursor is performed in liquid solution, being heated and pressed in a separate ammonia reactor apart from the engine exhaust gas stream and guiding the so-generated ammonia gas into the engine exhaust gas stream upstream of an SCR catalyst.

11. The apparatus according to claim 10, characterized by the choice of pressure and temperature for the catalytic decomposition of a liquid ammonia precursor in a way that hydrolysis of the NH₃ precursor compound takes place preferably in the liquid phase.

12. The apparatus according to claim 10, characterized by the decomposition of the ammonia precursor at high pressure to yield ammonia, which is released as ammonia gas along with carbon dioxide gas, water vapor and hydrogen gas (if applicable) upon pressure release.

13. The apparatus according to claims 10 to 12, characterized by the suppression of the boiling of the liquid ammonia precursor by applying high pressures of 5 to 100 bar, depending on the

operation temperature.

14. The apparatus according to claim 12, wherein the complete conversion of the liquid ammonia precursor to ammonia is replaced by incomplete conversion of the precursor compound with the production of a precursor compound gas and therefore requiring less heat input in the separate ammonia reactor.

15. The apparatus according to claims 10 to 13, characterized by a high hydrolytic decomposition efficiency of the ammonia precursor in the presence of solvents, in particular liquid water, at temperatures between 50 to 600°C, preferably between 150 to 350°C, by presence of a decomposition catalyst.

16. The apparatus according to any of the preceding claims 10 to

15, characterized by a catalytic bed ( 7 ) inside the ammonia generator with sufficiently large void volume to provide for a constant flow of liquid through the void volume; said catalytic bed comprising catalyst particle of size from 0.01 mm to 10 mm, preferably 0.1 mm to 10 mm, more preferably 0.1 mm to 5 mm, most preferably 0.1 mm to 1 mm.

17. The apparatus according to any of the preceding claims 10 to

16, wherein the catalytic bed comprises either solid catalyst particles, catalyst coated on non-catalytic particles, catalyst coated on ceramic or metallic substrates or extruded catalysts; i) catalyst materials are Ti0₂, A1₂0₃, Zr0₂ zeolite, metal organic frameworks (MOF) or mixtures thereof, preferably Ti0₂, A1₂0₃, Zr0₂, zeolite, most preferably Ti0₂, AI₂O₃ and/or Zr0₂;

ii) the catalyst is further preferably stabilized by W0₃ or Si₂0₃; iii) the catalyst is furthermore preferably doped with metals, metal oxides or combinations thereof to facilitate the

decomposition of ammonia precursor compounds other than urea;

18. The apparatus according to any of the preceding claims 10 to 17, wherein a porous material, most preferred a metal frit, of tailored length and pore size or a valve that is used at the outlet of the ammonia generator to create the necessary pressure wherein instead of and/or in addition to a porous material a valve is used at the outlet of the ammonia generator to create the necessary pressure and/or improve the dynamics of the ammonia dosage into the exhaust gas system.

19. The apparatus according to any of the preceding claims 10 to 18 wherein the liquid ammonia precursor is pre-heated by cooling water or lubrication oil from the engine to reduce the heat requirement of the catalyst bed, preferably provided that the catalyst is sufficiently active, the catalyst bed is heated exclusively by cooling water and/or lubrication oil from the engine, rendering any other heat source like electrical heating or heat transfer from the exhaust gas unnecessary.

20. The apparatus according to any of the preceding claims 10 to 19 wherein the catalyst is coated on a metallic or semi-conducting substrate, which is electrically resistance heated.

21. The apparatus according to any of the preceding claims 10 to 20 wherein instead of one single tube containing the catalyst bed, two or several tubes containing catalyst beds are used in parallel to facilitate the heat transfer from a heat source to the catalyst bed (s) .

22. The apparatus according to any of the preceding claims 10 to 21, wherein the catalyst bed is heated by microwave irradiation or other electromagnetic radiation.

23. The apparatus according to any of the preceding claims 10 to 22 wherein a second pressure containment after the pressurizer is comprised wherein the operating pressure of the second containment is below the pressure of the reactor in order to enable a complete evaporation of the liquid phase allowing the gases to be uniformly

mixed and dosed with constant gas flow.