METHOD FOR EXHAUST GAS CLEANING OF PISTON ENGINES RUNNING ON MIXTURES OF HYDROGEN AND AMMONIA

Description

Field of the invention

[0001] The present invention relates to a method for purifying the exhaust gases of 4-stroke gasoline engines that are operated with mixtures of hydrogen and ammonia as fuel. Background of the invention

[0002] In the course of the energy transition and in the context of the discussion on the storage of excess electrical energy from solar and wind power plants (so-called power-to-gas), hydrogen and methane are primarily named as chemical energy carriers, which are intended to replace conventional natural gas and, in times of weak green electricity supplies, for example, to be converted back into electricity in gas-fired power plants or to be used for mobile applications.

[0003] Ammonia can also be used as a chemical energy storage medium, offering the advantage of inexpensive storage in liquid form combined with the resulting cost-effective transport options. The poor flammability of ammonia, which significantly complicates its use in heat engines, can be eliminated by adding hydrogen. Particularly in mobile applications, it is possible not only to carry the hydrogen in separate gas cylinders but also to produce the hydrogen itself by splitting ammonia into its elements. This eliminates the need to carry an additional tank or component. The nitrogen produced during the ammonia splitting process passes through the process as an inert gas.

[0004] Such mixtures of ammonia and hydrogen can be used as fuel in conventional gasoline engines, similar to methane, propane, biogas, or gasoline. The mixing ratio can be varied widely. To avoid unnecessary costs, the lowest possible hydrogen content is sought. However, too low a proportion leads to poor flammability of the gas mixture, coupled with reduced efficiency of the heat engine. Depending on operating conditions and engine design, the combustion of pure ammonia may still be possible. In today's conventional gasoline engines, gas mixtures of ammonia and hydrogen in a molar ratio of approximately 1:1 to 4:1 can be used as fuel without special measures. Even higher hydrogen contents are usable, but increase the cost of operating the system and entail the disadvantages of pure hydrogen operation, such as premature ignition or flashback into the intake section.

[0005] One of the key differences in the combustion of ammonia-hydrogen mixtures compared to the other fuels mentioned is that the combustion products generally contain a higher proportion of nitrogen oxides (NOx), since the nitrogen is already bound in the fuel. Therefore, appropriate exhaust gas purification is required, which is the subject of the present invention.

[0006] The operation of four-stroke spark-ignition internal combustion engines using ammonia-hydrogen mixtures as fuel is described in the application DE102009007021. Within the context of exhaust gas purification, a process is described here involving engine operation with a precisely stoichiometric fuel-to-air mixture ratio using a catalyst installed in the exhaust stream. The catalyst used is a type suitable for the reduction of nitrogen oxides with upstream injection of urea solution, a so-called SCR catalyst. These catalysts consist of oxides of titanium, tungsten, or vanadium, or even zeolites, as the active components.

[0007] US 2011 / 283960 describes a process in which an engine is powered by ammonia / hydrogen. The hydrogen is produced by ammonia cracking in a suitable ammonia cracking device, with the required heat (endothermic reaction) being extracted from the engine's exhaust stream. Since the temperature of the exhaust gas is generally insufficient for ammonia cracking, an ammonia oxidation catalyst is installed upstream of the cracking device, and additional ammonia is added to the exhaust gas upstream of this catalyst. The oxidation of the additional ammonia increases the temperature of the exhaust stream accordingly so that ammonia cracking can occur. For the oxidation catalyst to function at all, a sufficient amount of oxygen must be present to oxidize the additionally added ammonia. The engine is therefore operated in the lean range.

[0008] DE112011 / 101487T5 discloses a process in which an engine is operated with ammonia and an additional fuel, e.g., hydrogen. Exhaust gas purification is controlled by an ammonia and a NOx sensor in the exhaust stream. A lambda sensor is not provided for in this document.

[0009] US2014 / 311428A1 also addresses the purification of NH3 and NOx using an SCR catalyst, which requires a lean mixture for the operation of the piston engine. The engine is operated in such a way that specific ratios of NH3 and NOx in the exhaust gas are adjusted, allowing selective conversion even in the presence of oxygen. Control is implemented via NH3 and NOx sensors. Summary of the invention

[0010] The invention is a method according to claim 1.

[0011] The combustion of ammonia-hydrogen mixtures does not produce any carbon-containing pollutants in the exhaust gas such as soot, carbon monoxide, or hydrocarbons because the fuel does not contain any carbon. Relevant exhaust gas components, in addition to NOx, include ammonia and hydrogen itself, as well as the small amounts of N2O that may be formed, although this is of minor environmental significance. There is no limit for the release of hydrogen into the atmosphere; only safety considerations must be taken into account. There is currently no limit for ammonia in mobile applications, but corresponding regulations are expected in the future. NOx therefore remains the main pollutant, for which legal requirements must be met depending on the emission source.

[0012] When a gasoline engine operates with ammonia-hydrogen mixtures, the exhaust gas contains the components ammonia, hydrogen, NOx, and oxygen, in addition to the main components nitrogen and water vapor. Typically, gasoline engines operating with hydrocarbons are operated with a stoichiometric mixture of fuel and atmospheric oxygen (λ=1), where λ represents the ratio between the actual amount of oxygen present and the amount of oxygen required for complete combustion of the available fuel. When burning hydrocarbons, this ratio can fluctuate between approximately λ=0.9 (rich operation) and 1.5 (extremely lean operation).

[0013] When operating with ammonia-hydrogen mixtures, the components ammonia, hydrogen and NOx (as well as oxygen) appear in the exhaust gas on both sides of the stoichiometric point (λ=1). The systematic concentration curve is shown in Fig. 1as a function of the A-value. According to this, nitrogen oxides are still present in the exhaust gas even when the engine is running rich, and ammonia and hydrogen are still present even when the engine is running lean. This is caused by incomplete combustion of the gas mixture in the cylinder, e.g., due to the flame extinguishing before the cold cylinder wall, so that components present in the under-fueling are retained. NOx concentrations can reach values ​​in the range of 2000 - 4000 ppm.

[0014] The tests were conducted on a standard single-cylinder, 4-stroke engine with a displacement of 0.4 l and a compression ratio of 8:1. A λ-sensor (step sensor) and an exhaust gas catalyst were installed downstream of the engine. A standard transistor ignition system was used.

[0015] The following operating conditions were observed: Speed: 2000 - 2500 rpm Mechanical performance: 2.5 - 3.5 kW Temperature before and after catalyst: 300 - 600 °C Mixture setting: 0.9 < λ < 1.5

[0016] Ammonia-hydrogen mixtures with a hydrogen volume fraction of between 20 and 60%, corresponding to an ammonia content of between 80 and 40%, were used as fuel. Since the hydrogen was produced by ammonia cracking, a corresponding amount of nitrogen, equal to one-third of the hydrogen volume, was entrained through the engine as an inert gas.

[0017] To purify the exhaust gases according to the invention, a commercially available automotive exhaust catalyst suitable for gasoline engines is installed downstream of the internal combustion engine. This catalyst uses a precious metal-containing catalyst (doped with platinum, palladium, or rhodium), which is additionally doped with an oxygen storage element according to the invention. In this specific case, a catalyst with an oxygen storage function was used, suitable, for example, for a VW Golf gasoline engine with a 1.6-liter engine.

[0018] Fig. 2shows an example of the exhaust gas composition depending on the mixture composition (A-value) downstream of the catalytic converter. The example applies to a fuel composition with 40% hydrogen and 60% ammonia. The result shows a sharp increase in NOx concentration starting at λ = 1 with a fuel-lean mixture. In the range λ < 1, the NOx concentration drops below the detection limit. The NOx concentrations during lean-burn operation are of the order of magnitude of the values ​​upstream of the catalytic converter and may even exceed them because, during lean-burn operation, some of the ammonia still present can be oxidized to NO in the catalytic converter.

[0019] The basic NOx concentration curve is largely independent of fuel composition. Fuel composition primarily affects the ratio of ammonia to hydrogen in the exhaust gas when the engine is running on fuel-rich fuel.

[0020] Hydrogen and ammonia concentrations increase with a richer mixture starting at λ = 1. These components are more detectable at λ > 1, meaning that during lean-burn operation, these substances are almost completely converted in the catalyst. The results demonstrate that optimal exhaust gas purification can be achieved at λ = 1. The preferred range is 0.9 < λ ≤ 1, and the most preferred range is 0.98 < λ ≤ 1.

[0021] A slight deviation leads directly to an increase in the proportion of NOx or ammonia in the exhaust gas. If the λ value fluctuates around an average of 1 and a catalyst with an oxygen storage function is used according to the invention, the pollutants can still be largely eliminated.

[0022] Optimal exhaust gas purification is thus achieved with stoichiometric operation with λ=1. This result applies regardless of the hydrogen-to-ammonia mixture ratio selected for the engine fuel. This mixture ratio only affects the hydrogen-to-ammonia ratio in the engine exhaust and in the exhaust gas downstream of the catalyst, provided the latter is operated in fuel-rich mode. Since NOx reacts very quickly with both hydrogen and ammonia, the fuel composition has virtually no influence on the nitrogen oxide purification rate.

[0023] If the exhaust gases are to be free not only of nitrogen oxides but also of ammonia, high demands are placed on the quality of the lambda control. During normal engine operation with control via the λ sensor, the λ value cycles back and forth between values ​​greater than 1 and less than 1, resulting in an average value as close to 1 as possible.

[0024] With fluctuations around the mean value λ=1, a largely complete conversion of the pollutants can be achieved if, according to the invention, a component with an oxygen storage function (e.g., cerium oxide) is added to the exhaust gas catalyst. These processes are state-of-the-art in exhaust gas purification of gasoline engines.

[0025] The advantages of precious metal-based exhaust catalysts over the SCR catalysts mentioned in DE102009007021 are that NO reduction begins at lower temperatures and the reaction rate is significantly higher. Furthermore, the usual fluctuations during stoichiometric operation around the mean value λ = 1 have a less adverse effect on the pollutant content of the exhaust gases downstream of the catalyst than with SCR catalysts due to the oxygen storage function of precious metal-based catalysts.

[0026] The combustion of ammonia-hydrogen mixtures and the usual control to an average value of λ = 1 place significantly higher demands on the control technology than the operation of gasoline engines, because the nitrogen oxide content in the untreated exhaust gas is significantly higher than in gasoline engines. Consequently, the nitrogen oxide content downstream of the exhaust catalyst also increases much more steeply during the transition from λ = 1 to the lean range than in gasoline engines. Therefore, any slight deviation in the mixture ratio to values ​​of λ > 1 immediately results in high NOx emissions, unless these are compensated for by the oxygen storage function.

[0027] To avoid the aforementioned high demands on control technology, operation can also occur with a slightly rich mixture composition, e.g., in the range of 0.99 < λ < 1. A mean λ value as close as possible to, but below, 1 should be aimed for. With appropriate control settings, the ammonia concentration in the exhaust gas can be reduced to below 100 ppm without increasing nitrogen oxide levels to unacceptable levels. Such control is relatively easy to implement, since the λ sensors are very sensitive in this range.

[0028] Fig. 3 shows the conditions in the immediate vicinity of the stoichiometric point λ = 1, indicating the A-sensor voltage. Regulating the engine to a medium voltage of, for example, approximately 0.5 - 0.7 V at the sensor enables reliable removal of nitrogen oxides without allowing ammonia concentrations to rise to unacceptably high levels.

[0029] Another option for reducing ammonia concentrations during slightly rich operation is the additional use of an ammonia cracking catalyst, which is installed upstream or downstream of the precious metal-containing catalyst. The catalyst splits ammonia into its elements, thereby shifting the ammonia-to-hydrogen ratio in favor of hydrogen. Iron, nickel, or ruthenium deposited on a support material serves as the catalyst.

[0030] From a thermodynamic perspective, installing a cracking catalyst downstream would be more advantageous; however, considering the actual temperature conditions in the exhaust system, installing it upstream could also prove useful. The cracking catalyst could also be incorporated into the exhaust catalyst as an additional component.

[0031] Another way to avoid ammonia residues during slightly rich operation is to meter in a small amount of air after the previously described exhaust catalyst, so that the A value is then raised from just below 1 to above 1. Residual amounts of ammonia can then be oxidized in a downstream oxidation catalyst.

[0032] Precious metal-containing automotive exhaust catalysts can be used as oxidation catalysts. However, there is a risk that some of the ammonia will be reoxidized to nitrogen oxides. Therefore, catalysts containing copper, cerium, cobalt, nickel, iron, manganese, vanadium, molybdenum, or tungsten, as well as other transition metals or their oxides or metal-containing zeolites as active components, can also be used. These catalysts achieve a largely selective oxidation of ammonia to nitrogen and water, while suppressing nitrogen oxide formation (so-called SCO catalysis). Since state-of-the-art catalysts ensure a selectivity of over 90%, the oxidation process no longer produces unacceptable amounts of NOx.

[0033] Using a separate oxidation catalyst with air injection has the advantage that the quality of the λ-sensor control is minimal. A rich mixture is sufficient, even if relatively high ammonia concentrations remain after the first catalyst. Thus, according to the example given, regulation to approximately 0.8 V is sufficient. The oxidation catalyst can then also remove higher residual amounts of ammonia.

[0034] To completely eliminate the ammonia residues, it is only necessary to increase the exhaust gas to λ > 1 by adding air, whereby it is not important to maintain a specific value. Controlling the air flow is therefore straightforward. However, an excessive amount of air should be avoided to prevent excessive cooling of the exhaust flow. If necessary, a second λ sensor downstream of the oxidation catalyst can be used to detect whether the air admixture is sufficient. For example, the sensor voltage could be adjusted to 0.2 V or lower via the air flow, which ensures complete conversion of the ammonia residues. Another option would be to set a constant air flow that is sufficient at full load to increase the λ value set on the engine to above 1. The desired conditions would then also be met during partial load operation.The regulation for the oxidation catalyst could then be completely eliminated. Short description of the drawings

[0035] Fig. 1 shows the usual course of the exhaust gas composition of the gasoline engine without the use of an exhaust gas catalyst as a function of the mixture composition (λ value) when using ammonia-hydrogen mixtures as fuel. Fig. 2 shows the usual course of the exhaust gas composition of the gasoline engine using a conventional car exhaust catalyst as a function of the mixture composition (A-value) when using ammonia-hydrogen mixtures as fuel. Fig. 3shows the usual course of the exhaust gas composition of the gasoline engine using an exhaust gas catalyst as a function of the mixture composition (λ value) when using ammonia-hydrogen mixtures as fuel in the immediate vicinity of the stoichiometric point, indicating the voltage then present at the λ probe.

Claims

1. A method for the catalytic cleaning of the exhaust gases of a spark-ignition engine operated with mixtures of ammonia and hydrogen as fuel, characterised in that - the fuel of the piston engine consists of admixtures of hydrogen to ammonia, in such a way that a fuel with a proportion of ammonia between 20% und 100% is obtained and utilized, - the fuel can in addition contain nitrogen, if the hydrogen is obtained by cracking ammonia into its elements, - for the cleaning of the exhaust gases of the pollutants nitrogen oxides and ammonia, an automotive catalytic exhaust gas converter with platin, palladium, rhodium or other components that contain noble metals is used, wherein the catalytic converter also contains an oxygen-storing component, - wherein, on average over time, the operation of the combustion engine takes place from the slightly rich range to the stoichiometric range between 0.9 < λ ≤ 1, preferably between 0.98 < λ ≤ 1.

2. The method according to claim 1, wherein the operation of the combustion engine takes place under stoichiometric conditions at λ = 1, wherein, because of the pendulum operation about the stoichiometric point that is common for the λ regulation, the time-average value is taken here for λ.

3. The method according to claim 1, wherein in a first step the operation of the combustion engine takes place, on average over time, in the slightly rich range between 0.9 < λ < 1, preferably between 0.98 < λ < 1.

4. The method according to any one of claims 1-3, with an additional ammonia-cracking catalyst, wherein the catalyst contains iron, nickel, or ruthenium on a carrier as the active component.

5. The method according to claim 4, wherein the ammonia-cracking catalyst is provided downstream of the noble-metal containing catalyst.

6. The method according to claim 4, wherein the ammonia-cracking catalyst is provided upstream of the noble-metal containing catalyst.

7. The method according to claim 3, wherein the ammonia-cracking catalyst with the active components iron, nickel or ruthenium for cracking is admixed into the noble-metal containing catalyst in a way that there is no longer any spatial separation between the two catalysts.

8. The method according to claim 1 and claims 3-7, with additional admixture of a small amount of air in a second step after the automotive catalytic exhaust gas converter for transferring the exhaust gas to λ > 1, and downstream provision of a selective oxidation catalyst in order to remove residues of ammonia, wherein the oxidation catalyst contains copper, cerium, cobalt, nickel, iron, manganese, vanadium, molybdenum, or tungsten as well as other transition metals or their oxides, or zeolites containing metal, as active components.