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

Abstract

The invention relates to 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 or a mixture of alkyl nitrates.

Description

Title of the invention: Ammonia-based compression engine fuel containing a combustion-enhancing additive. Scope of the invention

[0001] The field of the invention is that of fuels for compression-ignition engines, more particularly engines for marine applications. The fuel of the invention is one of the new fuels with a 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 a solution for the decarbonization of heavy-duty and long-distance transport. The fuel of the invention, essentially composed of ammonia, is thus one of the environmentally friendly alternatives for replacing diesel and heavy fuel oil. The invention relates to an additive which, when incorporated into the ammonia, ensures better ignition and faster combustion of the fuel in the engine. State of the art

[0002] Ammonia (NH3) represents a low-impact alternative and a credible renewable fuel to replace diesel and heavy fuel oil in the near future for maritime transport. The combustion of NH3 generates only water and nitrogen and no emissions of carbon molecules (CO2, CO) or soot particles. Ammonia is a compound whose manufacture is well-established and common, making its uses, transport, and storage well-known. It is also a product whose cost remains reasonable enough for use as a fuel. Ammonia as a fuel has a usable energy density in compression-ignition engines but a low cetane number. Its ignition in diesel-type engines remains problematic, especially at low engine speeds. Ammonia is stored under pressure (~9 bar) in liquid form in a tank and injected into the engine in liquid or gaseous form.

[0003] Various means for improving the ignition of ammonia in a compression-ignition engine have been described.

[0004] The first method is the co-injection of a pilot fuel with ammonia into the engine. Prior art, such as US patent application 2022 / 0056856, describes methods for co-injecting a so-called "pilot" fuel, or oxygen and / or hydrogen, with ammonia. This co-injection into a premix chamber promotes the ignition of the ammonia. This type of method has the major drawback of requiring the co-injection to be controlled according to the engine's operating conditions. Structural modifications such as separate tanks and injection systems. Also, the use of a fuel as a pilot fuel generates undesirable CO2 emissions and soot.

[0005] The second means is a mixture of fuels with ammonia. Fuels consisting of a mixture essentially of ammonia and dimethyl ether (DME) have been described in the prior art. DME is a synthetic fuel advocated 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 primarily of a mixture of ammonia and dimethyl ether, offering a faster ignition time than ammonia alone. The ammonia / DME fuel contains a DME fraction greater than 2%, ideally close to 18%.This high level of a hydrocarbon co-fuel in ammonia therefore deviates from CO2 emission reduction targets. On the other hand, DME is in a gaseous state at atmospheric pressure, which represents a disadvantage for its storage and use.

[0006] The third method is the injection of an ignition-enhancing additive into the engine chamber prior to the injection of ammonia. US patent 8904994 describes, for improving the ignition of ammonia, the injection before the ammonia of a highly flammable compound that self-ignites upon injection of the ammonia. This compound may be a GTL or DME diesel fuel (as previously described), a cetane-modifying sulfate, a nitrated compound of acetone, ethylene, hydrazine, or acetylene, with hydrazine being the preferred compound. These compounds all obviously have drawbacks, as some are highly toxic and unstable and / or highly flammable or sulfurous.

[0007] On another level, in the field of hydrocarbon diesel or biodiesel fuels, 2-Ethylhexyl nitrate (2-EHN) has long been known as a cetane-enhancing additive 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 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-improving effect remains poorly understood, and its use is based on empirical observations. For this reason, its effectiveness in fuels other than diesel cannot be assumed. This additive is produced industrially 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-improving additives in diesel fuel. Fig. 1 shows the similarity to the NEH of the effects on cetane number of different alkyl nitrates in addition to a standard diesel fuel (medium paraffinic (-40%) having a low natural cetane number but a standard response to cetane improver).The cetane number improvement effect is obtained for an incorporation of alkyl nitrate in diesel fuel close to 0.03% by mass.

[0008] 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 used so effectively to improve the ignition of a carbon-free fuel. Summary of the invention

[0009] The invention relates to a compression auto-ignition 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 which significantly improves the ignition delay of ammonia.

[0010] Said compound is an alkyl nitrate or a mixture of alkyl nitrates.

[0011] Said compound has the advantage of being liquid at room temperature, and only slightly flammable, and industrially produced.

[0012] Said compound, at these low levels in the presence of ammonia fuel, is therefore conventionally considered to be a fuel additive.

[0013] The addition of said liquid compound is carried out either by co-injection with ammonia in liquid or gaseous state in a premixing chamber of the engine, or preferably by mixing with liquid ammonia prior to injection into the engine.

[0014] Said compound, of the alkyl nitrate type, previously reserved for improving the cetane number of hydrocarbon diesel fuels, is therefore used effectively and surprisingly as an additive to improve the ignition of ammonia. Brief description of the figures

[0015] [Fig-1] shows the effect of alkyl nitrates on the cetane number of a standard diesel fuel.

[0016] [Fig.2] shows the ignition delay of ammonia as a function of temperature, for an air / fuel ratio 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.

[0017] [Fig.3] shows the ignition delay of a fuel consisting of 99.8% ammonia and 0.2% NEH depending on temperature, for a (air / fuel) ratio of 1.5 at a compression pressure Pc of 30 bars.

[0018] [Fig.4] shows the ignition delay of an ammonia-based fuel containing dif Different quantities of NEH, for an air / fuel ratio of 0.35, at a compression pressure Pc of 43.4 bar, over a temperature range from 1000K to 1100K. Description of the invention

[0019] This disclosure relates to a compression-ignition engine fuel comprising approximately 95.0% to approximately 99.9% by mass of ammonia and approximately 0.01% to approximately 5.0% by mass of a fuel ignition-enhancing compound consisting of an alkyl nitrate or a mixture of alkyl nitrates. In one embodiment, the fuel comprises approximately 0.05% to approximately 2.0% by mass of said compound. In another embodiment, the fuel comprises approximately 0.1% to approximately 0.8% by mass of said compound.

[0020] 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).

[0021] In another embodiment, when the sum of the quantity of ammonia and the quantity of compound is not equal to 100% by mass, the fuel may contain one or more other additives to complete the fuel to 100%, such as additives with preservation, anti-corrosion or detergent functions.

[0022] Said compound added to ammonia is chosen from one or more linear, branched or cyclic alkyl nitrates.

[0023] Said compound is more particularly selected 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 selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-1-octadecyl nitrate, isononyl to isotridecyl nitrate, the Hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate, 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.

[0024] According to one embodiment, the compound consisting of an alkyl nitrate or a mixture of alkyl nitrates is mixed with liquefied ammonia (under pressure) in a tank which supplies an engine, to obtain the fuel according to the invention.

[0025] According to one embodiment, said compound and liquefied ammonia are stored separately, and brought into contact with each other in an injector, thus forming the fuel according to the invention, before it is conveyed into the combustion chamber of the engine.

[0026] According to one embodiment, said compound is stored separately from ammonia and is co-injected with liquefied or gaseous ammonia to form the fuel according to the invention in a premixing chamber of the engine.

[0027] This disclosure also relates to the use of an alkyl nitrate or a mixture of alkyl nitrates, in the proportions defined above, as an ignition enhancer for an ammonia-based (constituted) fuel.

[0028] The invention is illustrated by the following examples, given by way of example. Examples

[0029] 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 ex-periments and simulations" (Combustion and Flame, Volume 227, May 2021, Pages 120-134). The laboratory test engine is a rapid compression machine equivalent to that described in this scientific article. This rapid compression machine allows for the measurement of the auto-ignition delay of a mixture. This machine compresses the mixture in a very short time to obtain predetermined pressure and temperature conditions. The liquids are admitted into the tank through a different orifice than the gas inlet, and the quantities of liquids are measured using a syringe and a precision balance.

[0030] The ignition delay dAI is defined according to the following formula in which Pc is the pressure applied to the injected fuel: \ .. , éUJ = 1j — J Example 1

[0031] The ignition delay was determined as a function of the injection temperature (between 950K and 1100K) at a pressure Pc of 40 bar using a fuel consisting of 99.6% by mass ammonia and 0.4% by mass NEH, and for a mixture ratio of 1:1 with air. The reference points given for the ignition delay of ammonia alone ([Fig. 2], left curve) are taken from the aforementioned article. [Fig. 2] (right curve) shows a significant reduction of approximately a factor of 10 in the ignition delay of the fuel compared to ammonia alone. This reduction in ignition delay with the fuel of the invention compared to ammonia alone is even greater at lower temperatures. Example 2

[0032] The ignition delay of a fuel consisting of 99.8% by mass ammonia and 0.2% by mass NEH was determined as a function of the injection temperature (between 925 K and 1000 K) at a pressure Pc of 30 bar and for a mixture ratio of 1.5 with air. The fuel ignition delays are less than 800 ms ([Fig. 3]), whereas under these test conditions ammonia alone does not ignite. Example 3

[0033] The ignition delay of a fuel consisting of either ammonia alone, or 99.9% by mass ammonia and 0.1% by mass NEH, or 98.0% by mass ammonia and 2.0% by mass NEH, was determined at three temperatures (1000 K, 1050 K, and 1100 K), at a pressure Pc of 43.4 bar, for a mixture ratio of 0.35 with air. For all three temperatures, the ignition delays of the fuels are shorter than those of ammonia alone ([Fig. 4]). The effect of adding NEH on the ignition delay compared to ammonia alone (as already observed in Example 1) is greater at lower temperatures. An optimal mass percentage of 0.25% NEH is substantially achieved by extrapolating the curves under these three temperature conditions.

[0034] These examples show that the use of alkyl nitrate(s) in very low mass percentages significantly improves the ignition delay of an ammonia-based fuel. Nothing could have suggested that additives known to increase the cetane number 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 selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-l-octadecyl nitrate, isononyl to isotridecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate and mixtures thereof.

2. Fuel according to claim 1, wherein the alkyl nitrate is 2-ethylhexyl nitrate.

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

4. Fuel according to claim 1 or claim 2, which consists of 95.0% to 99.9% by mass of ammonia and 0.1% to 5.0% by mass of alkyl nitrate.

5. A method for obtaining a fuel according to any one of claims 1 to 4, comprising mixing alkyl nitrate and liquefied or gaseous ammonia.

6. A method according to claim 5, wherein liquefied ammonia and said alkyl nitrate are mixed in an engine tank.

7. A method according to claim 5, wherein the liquefied ammonia and said alkyl nitrate are mixed in an injector.

8. A method according to claim 5, wherein liquefied or gaseous ammonia and said alkyl nitrate are mixed in a premixing chamber of an engine.

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

10. Use according to claim 9, wherein 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%.

11. Use according to claim 9 or claim 10, 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 4 to 18 carbon atoms, or a mixture of these nitrates.

12. Use according to any one of claims 9 to 11, wherein the alkyl nitrate is selected from 2-ethylhexyl nitrate, cyclohexyl nitrate, dodecyl nitrate, n-nonyl nitrate, 2-tetradecyl-l-octadecyl nitrate, isononyl to isotridecyl nitrate, hexyl nitrate, 2-octyl nitrate, isononyl nitrate, 2-propylheptyl nitrate and mixtures thereof.

13. Use according to claim 12, wherein the alkyl nitrate is 2-ethylhexyl nitrate.

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