Ammonia-based fuel for a compression engine, containing a combustion-enhancing additive

Abstract

Provided is a fuel for a compression ignition engine, comprising 95.0% to 99.9% by mass of ammonia and 0.01% to 5.0% by mass of an alkyl nitrate.

Description

[0001] Ammonia-based compression engine fuel containing a combustion-enhancing additive.

[0002] Field of invention

[0003] The field of the invention is that of fuels for compression ignition engines, more particularly engines for maritime applications. The fuel of the invention is one of the new fuels with reduced environmental impact, for example those commonly called "e-fuels" when they are produced from low-carbon electricity, low-carbon hydrogen and / or CO2. They are considered as a solution for the decarbonization of transport for heavy and long-distance mobility. The fuel of the invention, essentially composed of ammonia, is thus one of the ecological alternatives for replacing diesel and heavy fuel oil. The invention relates to an additive which, incorporated into the ammonia, ensures better ignition and faster combustion of the fuel in the engine.

[0004] State of the art

[0005] Ammonia (NH3) represents a low-impact alternative to the environment and a credible renewable fuel to soon replace diesel and heavy fuel oil for maritime transport. The combustion of NH3 generates only water and nitrogen and no emissions of carbon molecules (CO2, CO), nor soot particles. Ammonia is a compound whose production is proven and common, which makes its uses, transport and storage known. It is also a product whose cost remains reasonable enough to be used as a fuel. Ammonia as a fuel has a usable energy density in compression ignition engines but has a low cetane number. Its ignition in diesel engines remains problematic, especially at low engine speeds. Ammonia is stored under pressure (~9 bars) in the liquid state in a tank and injected in the liquid or gaseous state into the engine.

[0006] Various means for improving ammonia ignition in a compression ignition engine have been described.

[0007] The first method is the co-injection into the engine of a pilot fuel with ammonia. Prior art, such as patent application US 2022 / 0056856, describes methods for co-injecting a so-called "pilot" fuel, or oxygen and / or hydrogen with ammonia. This co-injection into a premixing chamber promotes the ignition of the ammonia. This type of method has the major drawback of requiring co-injection control according to the engine operating conditions, structural arrangements such as tanks and separate injection systems. Also, the use of a fuel as a pilot fuel generates CO2 emissions and unwanted soot.

[0008] The second method is a blend of fuels with ammonia. Fuels consisting essentially of a mixture of ammonia and dimethyl ether (DME) have been described in the prior art. DME is a synthetic fuel recommended as a replacement for diesel. These fuels are described in the article entitled "Ignition delay times of NH3 / DME blends at high pressure and low DME fraction: RCM experiments and simulations" (Combustion and Flame Volume 227, May 2021, Pages 120-134) and in the article entitled "Ignition delay time and laminar flame speed measurements of ammonia blended with dimethyl ether: A promising lowcarbon fuel blend" (Renewable Energy Volume 181, January 2022, Pages 1353-1370). These articles describe a fuel consisting essentially of a mixture of ammonia and dimethyl ether with a better ignition delay than ammonia alone. Ammonia / DME fuel contains a DME fraction greater than 2%, ideally close to 18%.This high level of hydrocarbon co-fuel in ammonia therefore deviates from the objectives of reducing CO2 emissions. On the other hand, DME is in a gaseous state at atmospheric pressure, which is a disadvantage for its storage and use.

[0009] The report of the R&D department of the American Oil Company (Continental Aviation and Engineering Corporation, Detroit, US, vol. II, no. 1054, 1967-05-01, pages 1-251,) concerns compounds that may be used in association with ammonia fuel. This technical report presents a series of tests to measure the ignition temperature by slow linear heating in a furnace of ammonia alone and in the presence of added compounds. A first series of tests identifies gaseous acetylene as the only effective additive to improve the combustion of ammonia in a compression engine. A second series of tests, to measure the ignition temperature by slow heating of ammonia alone or in the presence of additives, leads to the conclusion that amyl nitrate is the second worst additive (after potassium azide) for reducing the ignition temperature of ammonia.This report therefore does not predict good efficiency of fuels composed of ammonia and alkyl nitrate as a combustion-improving additive in compression-ignition engines.

[0010] The third method is the injection of an ignition-enhancing additive into the engine chamber prior to the injection of ammonia. Patent US8904994 describes, to improve the ignition of ammonia, the injection before the ammonia of a highly flammable compound which self-ignites at the time of the injection of ammonia. This compound can be a fuel of the GTL or DME diesel type (as previously described), a cetane-modifying sulfate, a nitro compound of acetone, ethylene, hydrazine, acetylene, with hydrazine being the preferred compound. These compounds all obviously have disadvantages because some of them are highly toxic and unstable and / or highly flammable or sulfurized.

[0011] On another level, in the field of hydrocarbon diesel or biodiesel fuels, 2-ethylhexyl nitrate (NEH) has long been known as a cetane improver for diesel fuel. A higher cetane value ensures lower fuel consumption, reduced particulate matter and NOx emissions, faster cold engine starting, reduced engine knocking and noise, and reduced engine wear. The reaction mechanism of NEH in the presence of a hydrocarbon diesel fuel has been studied, for example, in the scientific publication "The Autoignition Behavior of Surrogate Diesel Fuel Mixtures and the Chemical Effects of 2-Ethylhexyl Nitrate (2-EHN) Cetane Improver" (vol. 108, section 4: Journal of fuels and lubricants (1999), pp. 1029-1045). However, the reaction mechanism of its cetane-enhancing effect remains poorly understood and its use is based on empirical laws.For this reason, its effectiveness on fuels other than diesel fuels cannot be assumed. This additive is industrially produced and widely used in commercial diesel fuels. More than fifty thousand tons of NEH have been produced annually in Europe since the 1980s. Other alkyl nitrates can also be used as cetane improvers for diesel fuel. Figure 1 shows the similarity with NEH of the effects on the cetane index of different alkyl nitrates in addition to a standard diesel fuel (medium paraffinic (~40%) with a low natural cetane number but a standard response to the cetane improver). The cetane improving effect is obtained for an alkyl nitrate incorporation in the diesel fuel close to 0.03% by mass.

[0012] The present invention provides a solution for improving the ignition of ammonia in standard engines by adding an alkyl nitrate, such as NEH, to the ammonia fuel. It is unexpected that additives known to increase the cetane number of a hydrocarbon can be so effectively used to improve the ignition of a carbon-free fuel.

[0013] Summary of the invention

[0014] A compression autoignition engine fuel comprising about 95.0% to about 99.9% by mass of ammonia, and about 0.01% to about 5.0% by mass of a compound that significantly improves the ignition delay of ammonia.

[0015] Said compound is an alkyl nitrate or a mixture of alkyl nitrates.

[0016] The said compound has the advantage of being liquid at room temperature, not very flammable, and produced industrially.

[0017] The said compound, at these low levels in the presence of ammonia fuel, is therefore conventionally considered a fuel additive.

[0018] The addition of said liquid compound is carried out either by co-injection with ammonia in the liquid or gaseous state into a premixing chamber of the engine, or preferably by mixing with liquid ammonia prior to injection into the engine.

[0019] The said compound, of the alkyl nitrate type, previously reserved for improving the cetane index of diesel hydrocarbon fuels, is therefore used effectively and surprisingly as an additive to improve the ignition of ammonia.

[0020] Brief description of the figures

[0021] Figure 1 shows the effect of alkyl nitrates on the cetane number of a standard diesel fuel.

[0022] Figure 2 shows the ignition delay of ammonia as a function of temperature, for a richness (air / fuel) of 1 at a compression pressure Pc of 40 bars; left curve = fuel consisting of 100% ammonia; right curve = fuel consisting of 99.6% ammonia and 0.4% NEH.

[0023] Figure 3 shows the ignition delay of a fuel consisting of 99.8% ammonia and 0.2% NEH as a function of temperature, for a richness (air / fuel) of 1.5 at a compression pressure Pc of 30 bars.

[0024] Figure 4 shows the ignition delay of an ammonia-based fuel containing different amounts of NEH, for a richness (air / fuel) of 0.35, at a compression pressure Pc of 43.4 bar, over a temperature range from 1000K to 1100K.

[0025] Description of the invention

[0026] The present disclosure relates to a compression ignition engine fuel that comprises about 95.0% to about 99.9% by mass of ammonia and about 0.01% to about 5.0% by mass of a fuel ignition enhancing compound consisting of an alkyl nitrate. In one embodiment, the fuel comprises about 0.05% to about 2.0% by mass of said compound. In another embodiment, the fuel comprises about 0.1% to about 0.8% by mass of said compound. In one embodiment, the fuel of the invention consists of ammonia and said compound (and in this case, the amount of compound in the fuel is at least 0.1% by mass).

[0027] In another embodiment, when the sum of the amount of ammonia and the amount of compound is not equal to 100% by mass, the fuel may contain one or more other additives to make up the fuel to 100%, such as additives with preservative, anti-corrosion or detergency functions.

[0028] Said compound added to ammonia is chosen from one or more linear, branched or cyclic alkyl nitrates.

[0029] Said compound is more particularly chosen from linear alkyl nitrates comprising 4 to 36, advantageously 4 to 24 carbon atoms, branched alkyl nitrates comprising 4 to 36, advantageously 4 to 24 carbon atoms, cyclic alkyl nitrates (or cycloalkyl nitrates) comprising 5 to 18 carbon atoms, and mixtures thereof. In one embodiment, said compound is chosen from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-1-octadecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, a mixture of C9-C13 branched alkyl nitrates, and mixtures thereof. In one embodiment, the alkyl nitrate is 2-ethylhexyl nitrate alone or in a mixture with one or more other alkyl nitrates as defined above, advantageously the alkyl nitrate is 2-ethylhexyl nitrate alone.Mixtures of C9-C13 branched alkyl nitrates may be obtained from corresponding mixtures of C9-C13 branched alcohols, for example, alcohols available from Exxon under the trade name Exxal™. As a guide, a mixture of at least two branched alcohols selected from a C9 branched alcohol, a C branched alcohol may be prepared. i0 , a Cn-branched alcohol, a C-branched alcohol i2 and a C-branched alcohol i3 , then synthesize the corresponding alkyl nitrate mixture.

[0030] According to one embodiment, the compound consisting of an alkyl nitrate is mixed with liquefied ammonia (under pressure) in a tank which supplies an engine, to obtain the fuel according to the invention.

[0031] According to one embodiment, said compound and the liquefied ammonia are stored separately, and brought into contact with each other in an injector, thus forming the fuel according to the invention, before the latter is conveyed into the combustion chamber of the engine. According to one embodiment, said compound is stored separately from the ammonia and is co-injected with the liquefied or gaseous ammonia to form the fuel according to the invention in a premixing chamber of the engine.

[0032] In some embodiments, the fuel does not contain sugar such as, for example, fructose, glucose, or sucrose.

[0033] In some embodiments, the fuel does not contain farnesene (alpha or beta) and generally does not contain an isoprenoid.

[0034] The present disclosure also relates to the use of an alkyl nitrate (as defined above), in the proportions defined above, as an ignition enhancer for an ammonia-based (consisting of) fuel.

[0035] The present disclosure also relates to a method for improving the ignition of an ammonia-based (consisting of) fuel in an engine (in particular a combustion engine), the method comprising adding to the ammonia an alkyl nitrate (as defined above), in the proportions defined above. In one embodiment, the alkyl nitrate and liquefied ammonia are mixed in an injector. In one embodiment, the alkyl nitrate and ammonia, liquefied or gaseous, are mixed in a premixing chamber of the engine.

[0036] The present disclosure also relates to an engine (in particular a combustion engine) of a vehicle (such as a car, heavy goods vehicle, tractor, etc.) or a ship (such as an oil tanker, container ship, etc.) containing a fuel as defined above. The present disclosure also relates to a vehicle or ship comprising an engine as defined above.

[0037] The invention is illustrated by the following examples, given for information purposes only.

[0038] Examples

[0039] The improvement in the ignition delay of liquid ammonia was measured under test conditions equivalent to those described in the scientific article “Ignition delay times of NH3 / DME blends at high pressure and low DME fraction: RCM experiments and simulations” (Combustion and Flame, Volume 227, May 2021, Pages 120-134). The test laboratory engine is a rapid compression machine equivalent to the one described in this scientific article. It is a rapid compression machine for measuring the self-ignition delay of a mixture. This machine allows the mixture to be compressed in a very short time in order to obtain pre-established pressure and temperature conditions. The liquids are admitted into the tank through a different orifice than the gas inlet; the liquid quantities are measured using a syringe and a precision balance.The ignition delay dAI is defined according to the following formula in which Pc is the pressure applied to the injected fuel:.

[0040] Example 1

[0041] The ignition delay was determined as a function of the injection temperature (between 950K and 1100K) at a pressure Pc of 40 bars of a fuel consisting of 99.6% by mass of ammonia and 0.4% by mass of N EH and for a mixture richness of 1 with air. The given reference points of the ignition delay of ammonia alone (figure 2, left curve) are taken from the aforementioned article. In figure 2 (right curve) we observe a significant reduction of approximately a factor of 10 in the ignition delay of the fuel in comparison with ammonia alone. This reduction in the ignition delay with the fuel of the invention compared with ammonia alone is all the more significant as the temperature is low.

[0042] Example 2

[0043] The ignition delay of a fuel consisting of 99.8% by mass of ammonia and 0.2% by mass of NEH was determined as a function of the injection temperature (between 925K and 1000K) at a pressure Pc of 30 bars and for a mixture richness of 1.5 with air. The ignition delays of the fuel are less than 800 ms (figure 3) whereas under these test conditions ammonia alone does not ignite.

[0044] Example 3

[0045] The ignition delay of a fuel consisting of either ammonia alone, or 99.9% by mass of ammonia and 0.1% by mass of NEH, or 98.0% by mass of ammonia and 2.0% by mass of NEH, was determined at 3 temperatures (1000K, 1050K and 1100K), at a pressure Pc of 43.4 bars, for a richness of 0.35 of mixture with air. For the three temperatures, the ignition delays of the fuels are lower than those of ammonia alone (figure 4). The effectiveness of the addition of NEH on the ignition delay compared to ammonia alone (as already observed in example 1) is all the more important as the temperature is low. An optimal mass content of 0.25% of NEH is substantially reached by extrapolation of the curves in these three temperature conditions.

[0046] These examples show that the use of alkyl nitrate(s) in very low mass percentages can significantly improve the ignition delay of an ammonia-based fuel. Nothing could suggest that additives known to increase the cetane index of a diesel or biodiesel hydrocarbon could be used so effectively, in very small quantities, to improve the ignition of ammonia.

Claims

Demands 1. Fuel for compression-ignition engines comprising 95.0% to 99.9% by mass of ammonia and 0.01% to 5.0% by mass of an alkyl nitrate.

2. Fuel according to claim 1, comprising 0.05% to 2.0% by mass, in particular 0.1% to 0.8% by mass of alkyl nitrate.

3. Fuel according to any one of claims 1 and 2, which consists of ammonia and an alkyl nitrate.

4. Fuel according to any one of claims 1 to 3 wherein the alkyl nitrate is a linear alkyl nitrate comprising from 4 to 36 carbon atoms, a branched alkyl nitrate comprising from 4 to 36 carbon atoms, a cyclic alkyl nitrate comprising from 5 to 18 carbon atoms, or a mixture of these nitrates.

5. Fuel according to any one of claims 1 to 4, wherein the alkyl nitrate is selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-l-octadecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, a C9-C13 branched alkyl mixture, and mixtures thereof.

6. Fuel according to any one of claims 1 to 5 wherein the alkyl nitrate is 2-ethylhexyl nitrate.

7. A process for obtaining a fuel according to any one of claims 1 to 6, comprising mixing an alkyl nitrate and liquefied or gaseous ammonia.

8. A method according to claim 7, wherein liquefied ammonia and alkyl nitrate are mixed in an engine tank.

9. A method according to claim 7, wherein liquefied ammonia and alkyl nitrate are mixed in an injector.

10. A method according to claim 7, wherein liquefied or gaseous ammonia and alkyl nitrate are mixed in a premixing chamber of an engine.

11. Use of an alkyl nitrate, at a rate of 0.01% to 5.0% by mass, in an ammonia-based fuel, as an ignition enhancer for said fuel.

12. Use according to claim 11, wherein the alkyl nitrate is present in the fuel at a rate of 0.05% to 2.0% by mass, in particular 0.1% to 0.8% by mass.

13. Use according to claim 12, wherein the alkyl nitrate is as defined in any one of claims 4 to 6.

14. Method for improving ignition in an engine of a fuel comprising 95.0% to 99.9% by mass of ammonia, comprising the addition to liquefied or gaseous ammonia of an alkyl nitrate, at a rate of 0.01% to 5.0% by mass, preferably 0.05% to 2.0% by mass, preferably still 0.1% to 0.8% by mass, relative to the mass of the fuel.

15. Method according to claim 14, wherein the alkyl nitrate is as defined in any one of claims 4 to 6.

16. Method according to any one of claims 14 and 15, wherein alkyl nitrate and liquefied ammonia are mixed in an engine tank.

17. Method according to any one of claims 14 and 15, wherein alkyl nitrate and liquefied ammonia are mixed in an injector.

18. Method according to any one of claims 14 and 15, wherein alkyl nitrate and liquefied or gaseous ammonia are mixed in a premixing chamber of an engine.

19. Vehicle or ship engine containing a fuel as defined in any one of claims 1 to 6.

20. Vehicle or vessel comprising an engine as defined in claim 19.

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