Fuel mixture for operating an internal combustion engine

Abstract

The present invention relates to a fuel mixture for operating an internal combustion engine, comprising CO2, methane, and at least 25 vol% hydrogen. The present invention also relates to a method for producing the fuel mixture, and to uses of the fuel mixture. Additionally, a method for operating an internal combustion engine is provided.

Description

[Technical Field]

[0001] This disclosure relates to a fuel mixture for operating an internal combustion engine, and in particular to a fuel mixture for operating a gas engine. This disclosure also relates to a method for producing the fuel mixture, as well as uses of the fuel mixture. This disclosure also relates to a method for operating an internal combustion engine. [Background technology]

[0002] Hydrogen is a clean fuel that can be produced from a variety of sources. These properties make it an attractive fuel for transportation and power generation applications.

[0003] The high reactivity of hydrogen, which leads to auto-ignition and rapid combustion, poses challenges to the use of hydrogen as a fuel in gas engines. One particular problem that limits the use of hydrogen as a fuel in gas engines is pre-ignition (also called auto-ignition or self-ignition). Pre-ignition is a phenomenon that causes a high local pressure rise of the knocking type, in which the hydrogen fuel begins to react or self-ignite locally already before the arrival of the flame front triggered by the ignition (spark).

[0004] Pre-ignition knocking should not be confused with detonation knocking, a rather common phenomenon, especially in gasoline engines. Detonation knocking typically occurs after the spark plug initiates the ignition event of the fuel mixture and results from the detonation of one or more pockets of fuel mixture outside the normal combustion or flame front envelope.

[0005] Detonation knock can be mitigated by adjusting the ignition timing, typically by retarding the ignition. This reduces combustion temperatures, an effect that carries over from one power cycle to the next, thus reducing the propensity for detonation knock. Because hydrogen fuel auto-ignites based on a different phenomenon and more randomly, advancing or retarding the ignition timing is not sufficient to mitigate pre-ignition knock. Therefore, an efficient means of mitigating pre-ignition knock in gas engines that utilize hydrogen fuel is needed. Summary of the Invention

[0006] A fuel mixture is provided that can avoid abnormal combustion events caused by rapid combustion and / or self-ignition of hydrogen. The fuel mixture includes inert CO2 molecules to prevent further flame propagation within the gas if some hydrogen molecules ignite too early. Utilizing biogas obtained from the early stages of biogas purification as a CO2 source not only results in cost and energy savings, but also widens the range of possible biogas uses.

[0007] According to one embodiment, there is provided a fuel mixture for operating an internal combustion engine, the fuel mixture comprising CO2, methane, and at least 25 vol% hydrogen.

[0008] According to another embodiment, there is provided a method of producing a fuel mixture for operating an internal combustion engine, the method comprising blending CO2, methane, and hydrogen to provide a fuel mixture comprising CO2, methane, and at least 25 vol% hydrogen.

[0009] Further provided is the use of the fuel mixture disclosed herein to operate an internal combustion engine.

[0010] According to yet another embodiment, there is provided a method of operating an internal combustion engine, the method comprising supplying into a combustion chamber of the internal combustion engine a fuel mixture comprising CO2, methane, and at least 25 vol% hydrogen. DETAILED DESCRIPTION OF THE INVENTION

[0011] The solution is described in more detail below with reference to some embodiments, which should not be seen as limiting.

[0012] Hydrogen is a zero-carbon fuel when burned with oxygen, provided that it is produced in a carbon-free process. Hydrogen fuel can be produced from methane or by the electrolysis of water. So-called green hydrogen is produced entirely from renewable resources. In a flame of pure hydrogen burning in air, hydrogen reacts with oxygen to produce water, releasing energy according to equation 1: 2H2(g) + O2(g) → 2H2O(g) + energy (1).

[0013] The released energy allows the hydrogen to be used as a fuel. Hydrogen can be used in much the same way as natural gas. It can be fed to fuel cells to generate electricity and heat, used in combined gas turbines to produce large amounts of centrally produced electricity, or burned to power combustion engines. Hydrogen is considered a major sustainable source of renewable energy. Hydrogen combustion produces no undesirable emissions other than trace amounts of nitrogen oxides, which are formed when residual nitrogen and oxygen in the air are heated in the combustion process.

[0014] Hydrogen has a wide flammability range and high burn rate for combustion in spark-ignition engines. The wide flammability range provides smooth engine operation with very lean mixtures. However, the extremely fast combustion of hydrogen at high loads caused by its high reactivity can lead to abnormal combustion, manifested as knocking, pre-ignition, and flashback. This abnormal combustion reduces engine reliability and limits engine power output.

[0015] Gaseous fuels have different combustion characteristics that correlate with the gas's knock tendency or knock resistance during combustion in an engine. The methane number is an index for classifying gaseous fuels according to their knock resistance. Methane gas has high knock resistance (low knock tendency) and has a methane number of 100. Hydrogen gas has low knock resistance (high knock tendency) and has a methane number of 0.

[0016] The hydrogen knock phenomenon has a dual nature. The first cause of hydrogen knock is the spontaneous combustion of hydrogen as a result of excessive temperature and pressure buildup at the end of the power stroke. The second mechanism of hydrogen knock, known as light knock, is thought to be caused by an unstable combustion process initiated by the spark plug. While the latter is less harmful to the engine than heavy knock, it is still an undesirable phenomenon, and so it is important to improve control of the combustion process in the region where it occurs.

[0017] Previously, exhaust gas recirculation (EGR) has been suggested to have a particular impact on charge and knock tendency in combustion chambers supplied with variable fuel quality, particularly those powered by natural gas or biogas. Due to the high proportion of inert gases, exhaust gas recirculation has been shown to have a knock-suppressing effect by inertizing the mixture in the combustion chamber, thereby slowing combustion. However, by using such an approach, all components contained in the exhaust gas, including undesirable components, end up in the combustion chamber. Furthermore, this approach complicates the engine system. EGR also typically reduces the overall efficiency of the engine.

[0018] The approach disclosed herein relies on providing a fuel mixture that can avoid abnormal combustion events caused by the rapid combustion of hydrogen. The fuel mixture contains inert molecules that can prevent further flame propagation within the gas if some hydrogen molecules ignite too early. In particular, the solution disclosed herein aims to mitigate pre-ignition knock caused by hydrogen auto-ignition.

[0019] Preferably, the inert shielding molecule has a sufficiently large size and a sufficiently rigid molecular structure to effectively shield the surrounding hydrogen molecules from the flame caused by autoignition in the cylinder. Carbon dioxide (CO2) and nitrogen (N2) are examples of such molecules. A CO2 molecule has a carbon atom covalently double-bonded to two oxygen atoms. The symmetry of the CO2 molecule is linear and centrosymmetric in its equilibrium geometry. Because of its centrosymmetric nature, the molecule lacks an electric dipole moment. CO2, with a larger size (approximately 0.33 nm) than N2 (0.3 nm), may be more efficient for shielding.

[0020] Also, increasing the lambda value (air-to-fuel ratio) can be helpful in trying to prevent hydrogen autoignition. The higher the air density, the higher the concentration of inert nitrogen as part of the supplied air. The inert N2 molecule is non-reactive and stable, which can have a positive effect on blocking the propagation of flames caused by hydrogen autoignition. A lean mixture also contributes to a slower flame propagation, thus reducing the risk of hydrogen knocking.

[0021] A fuel mixture for powering an internal combustion engine is provided, the fuel mixture comprising CO2, methane, and at least 25% by volume of hydrogen. Preferably, the fuel mixture consists essentially of CO2, methane, and hydrogen, with the amount of hydrogen being at least 25% by volume.

[0022] According to one embodiment, the CO2 and methane are derived from biogas. If the CO2 and methane are derived from biogas and the hydrogen is green hydrogen, the fuel mixture can be referred to as a renewable fuel mixture. If the hydrogen used is non-fossil based, a non-fossil fuel mixture is provided.

[0023] The presence of CO2 in the fuel mixture helps to avoid abnormal combustion events, such as auto-ignition, caused by the high reactivity of hydrogen, thereby improving controlled hydrogen combustion and engine reliability. According to one embodiment, the amount of CO2 in the fuel mixture is up to 15 vol%. Preferably, the fuel mixture contains 1 to 15 vol% CO2. It is beneficial to keep the amount of CO2 as low as possible because, as an inert molecule, CO2 does not burn further, and therefore its addition to the fuel does not provide energy but takes up volume. However, the amount of CO2 in the fuel mixture must be such that the desired effect is achieved.

[0024] According to one embodiment, the fuel mixture comprises 25 to 98 vol% hydrogen. For example, the fuel mixture may comprise 25 to 85 vol% hydrogen.

[0025] The fuel mixture may contain 1 to 50 vol% methane. Preferably, the fuel mixture contains at least 25 vol% hydrogen and at most 15 vol% CO2, with the remainder of the fuel mixture consisting essentially of methane. Thus, the fuel mixture may consist essentially of hydrogen, CO2, and methane. The term "consist essentially of" in this context means that at least 95 vol%, e.g., 98 vol%, of the total volume of the fuel mixture consists of hydrogen, CO2, and methane.

[0026] The amounts of CO2, methane, and hydrogen in the fuel mixture may be optimized for operating the internal combustion engine at 80% rated power. Rated power, or rated power, refers to the maximum power output of the internal combustion engine under normal operating conditions. According to one embodiment, the fuel mixture is optimized for operating the internal combustion engine with an engine speed ranging from 200 to 1500 rpm.

[0027] A method of producing a fuel mixture for powering an internal combustion engine is provided, the method comprising the steps of combining CO2, methane, and hydrogen to provide the fuel mixture.

[0028] As already mentioned, CO2 is inert and does not burn further. Therefore, its addition to a fuel mixture does not provide energy but takes up volume. Therefore, to increase the energy content of a fuel mixture, it is beneficial to blend CO2 with hydrogen gas as part of the mixture, which has a higher volumetric energy density when compared to hydrogen.

[0029] According to one embodiment, the CO2 is provided as part of the biogas, and the method therefore comprises blending hydrogen with a biogas comprising CO2 and methane to provide a fuel mixture comprising CO2, methane, and at least 25 vol% hydrogen.

[0030] Biogas refers to the gas produced from biodegradable organic matter via anaerobic digestion. With the help of a wide variety of bacteria, the organic matter is broken down, releasing a mixture of gases. Typically, biogas contains 50 to 85 vol% methane (CH4) and 15 to 50 vol% CO2. In addition to methane and CO2, biogas may also contain hydrogen sulfide (HS), ammonia (NH3), water vapor (HO), methylsiloxanes, nitrogen (N2), oxygen (O2), volatile organic compounds (VOCs), carbon monoxide (CO), and / or hydrocarbons. Biogas composition varies depending on the substrate composition as well as the conditions (temperature, pH, and substrate concentration) within the anaerobic reactor.

[0031] According to one embodiment, the method of producing a fuel mixture further comprises producing biogas from biodegradable organic matter via anaerobic digestion prior to combining the biogas with hydrogen.

[0032] Traditionally, contaminant components of biogas are removed prior to any final utilization. HS is harmful to the environment and is corrosive to metal parts of, for example, engines, pumps, compressors, gas storage tanks, and valves, thereby shortening their lifespan. Furthermore, when burned, HS produces sulfur dioxide and sulfuric acid, which are similarly corrosive and environmentally harmful compounds. Typically, the amounts of at least CO and hydrogen sulfide are minimized in the purification process. HS is removed because of its toxicity and corrosiveness, and CO is removed because it is a recalcitrant gas, which reduces the density and calorific value of the biogas.

[0033] Subsequently, or simultaneously with purification, the biogas is upgraded, i.e., the CO2 content of the gas is adjusted to form biomethane, a high-calorie gas with a methane content close to 100 vol%, such as 95 to 99 vol%. Conventional upgrading technologies, which rely on physical, chemical, and / or biological approaches, are rather energy- and chemically-intensive and therefore incur investment and maintenance costs as well as energy consumption.

[0034] For the methods disclosed herein, biogas is purified to at least reduce the H2S content to a level where there are no corrosive effects. In one example, the H2S content of the purified biogas is less than 500 ppm, or even less than 50 ppm. However, preferably, the biogas used is not upgraded, i.e., the CO2 content of the gas is not reduced, at least not significantly. This results in cost and energy savings in biogas production, since energy- and chemical-intensive upgrading technologies do not need to be fully utilized. In some cases, it may even be possible to completely omit the upgrading process. CO2 removal is the most expensive process in biogas purification and upgrading. The less CO2 removed from raw biogas, the lower the cost of biogas production.

[0035] The removal of H2S from raw biogas may be carried out by absorption, for example, using a scrubbing liquid. The separation principle of absorption is based on the different solubilities of various gas components in the scrubbing liquid. The scrubbing liquid may be an aqueous solution. The solubility of methane in water is 、 H2S has an even higher solubility than CO2, being 26 times less than CO2 at 25°C. Therefore, H2S can be removed prior to possible CO2 removal, which may even be desirable since dissolved H2S is highly corrosive and odor nuisances can cause operational problems. Instead of an aqueous scrubbing solution, an organic solvent may be utilized.

[0036] Thus, according to one embodiment, CO2 is provided as part of the biogas, the biogas having a CO2 content of 5 to 50 vol%, preferably 15 to 50 vol%, for example 15 to 40 vol%.

[0037] A method of operating an internal combustion engine is provided, the method comprising the step of supplying into a combustion chamber of the internal combustion engine a fuel mixture comprising CO2, methane, and at least 25 vol% hydrogen.

[0038] According to one embodiment, the fuel mixture is provided by mixing biogas containing CO2 and methane with hydrogen.

[0039] In one embodiment, the method further comprises measuring the CO content of the biogas prior to feeding the biogas into the combustion chamber. Based on the CO content, the proportions of biogas and hydrogen in the fuel mixture can be adjusted to efficiently utilize the CO contained in the biogas in ensuring engine performance. A higher CO content in the biogas means that a smaller proportion of biogas needs to be mixed with hydrogen, thereby enabling the internal combustion engine to operate with a fuel mixture having a higher proportion of hydrogen.

[0040] According to one embodiment, the fuel mixture comprises 25 to 98 vol% hydrogen, and / or 1 to 15 vol% CO2, and / or 1 to 50 vol% methane.

[0041] Biogas is considered carbon neutral because the carbon in it comes from organic matter that has captured carbon from atmospheric CO2 over a relatively short timescale. Therefore, when biogas is blended with green hydrogen, a 100% renewable fuel is obtained.

[0042] The biogas containing CO2 and methane can be mixed with hydrogen prior to supplying the fuel mixture to the combustion chamber of the internal combustion engine, or the biogas containing CO2 and methane and the hydrogen can be supplied separately to the combustion chamber of the internal combustion engine.

[0043] The fuel mixture may be supplied to the main combustion chamber along with the intake air through the intake valve. Alternatively, a portion of the fuel mixture may be supplied through a pre-combustion chamber and the remaining portion may be supplied through the intake valve. The fuel mixture supplied into the pre-combustion chamber is ignited therein, and the burning air / fuel mixture is then allowed to diffuse into the main combustion chamber. Typically, the pre-combustion chamber may have a volume that is 25 to 40% of the volume of the main combustion chamber. Alternatively, the fuel mixture may be supplied using direct injection into the main combustion chamber.

[0044] Because hydrogen and biogas are completely miscible and chemically unreactive with each other, a mixture of the two, once created, can be treated, stored, and fed to an internal combustion engine as a homogeneous gas, rather than necessarily requiring two separate systems. For example, an empty pressure vessel can first be filled with one gas (such as hydrogen) to a predetermined partial pressure, and then continued to be filled with the other gas (such as biogas) to a predetermined total pressure. As the second gas enters the pressure vessel, the two gases rapidly mix by diffusion and convection. Once the gases mix, they can remain mixed indefinitely.

[0045] The internal combustion engine to be operated by the method disclosed herein is preferably a reciprocating (piston) engine. Preferably, the internal combustion engine is designed as a gas engine. The internal combustion engine may be used to drive a vehicle. Alternatively, the internal combustion engine may be used as a stationary engine. The stationary engine can preferably be or is coupled to a generator to generate electrical power.

[0046] The fuel mixtures and operating methods disclosed herein are optimized for specific types of internal combustion engines, the characteristics of which are discussed below. The internal combustion engine may have an engine speed ranging from 200 to 1500 rpm. In terms of cylinder configuration, the engine may be an in-line engine or a V-engine having 4 to 16 cylinders. The cylinder bore may range from 135 to 460 mm. The cylinder output, i.e., the power generated by each individual cylinder, may vary from 180 to 1200 kW / cyl. The rated power, i.e., the power output, of an internal combustion engine typically ranges from 1100 to 20,000 kW, although it may be possible to operate the engine at a power rating of over 20 MW. The mean effective pressure of the engine may vary from 20 to 31 bar. The compression ratio of the engine may vary from 6:1 to 14:1. Preferably, the compression ratio is 8:1 to 12:1.

[0047] The proportions of the components of the fuel mixtures disclosed herein are optimized based on the particular characteristics of the internal combustion engine.

[0048] The solutions disclosed herein can be particularly advantageous for use in marine applications, as the portability of hydrogen fuel poses certain challenges. H2's very low boiling point (approximately -253°C) necessitates the use of high pressure and / or low temperatures for storage. High-density storage systems are problematic due to safety concerns. Also, high-pressure tanks weigh much more than the hydrogen they can hold. Because a portion of the hydrogen in the fuel is replaced with biogas, the methods disclosed herein can lower hydrogen consumption, thereby reducing the amount of hydrogen fuel required to be carried on board the vessel.

[0049] The presence of CO2 in the fuel mixture provides the advantage that better overall engine performance is achieved when using hydrogen as the primary fuel component, and problems of uncontrolled auto-ignition and / or knocking can be avoided.

[0050] Blending biogas with green hydrogen provides a renewable fuel mixture that is low in energy and material consumption as well as production cost associated with fuel production. Utilizing raw materials from the early stages of biogas purification results in cost and energy savings. For example, water consumption associated with the biogas purification process can be reduced because only H2S (if present) needs to be removed from raw biogas. Furthermore, energy consumption for gas compression, recirculation pumps, and water reclamation is reduced. Furthermore, blending raw biogas with hydrogen provides efficiency and simplicity to fuel mixture production because blending two gases (biogas and hydrogen) already provides the desired product, rather than requiring blending multiple gases.

Claims

1. A fuel mixture for operating an internal combustion engine, comprising: ２ , methane, and at least 25 vol% hydrogen.

2. The CO ２ and methane derived from biogas.

3. Up to 15 vol% CO ２ 3. The fuel mixture of claim 1 or 2, comprising:

4. 4. A fuel mixture according to any one of claims 1 to 3, comprising from 25 to 98 vol% hydrogen.

5. 1 to 15 vol% CO ２ 5. A fuel mixture according to any one of claims 1 to 4, comprising:

6. 6. A fuel mixture according to any preceding claim, comprising 1 to 50 vol% methane.

7. CO in the fuel mixture ２ 7. A fuel mixture according to any one of claims 1 to 6, wherein the amounts of methane and hydrogen are optimised for operating an internal combustion engine having an engine speed ranging from 200 to 1500 rpm.

8. 8. A method of producing a fuel mixture for operating an internal combustion engine, comprising: ２ , methane, and hydrogen.

9. CO ２ and mixing the hydrogen with biogas containing methane.

10. 10. The method of claim 9, further comprising producing the biogas from biodegradable organic matter via anaerobic digestion prior to combining the biogas with the hydrogen.

11. The biogas contains 5 to 50 vol% CO ２ 11. The method of claim 9 or 10, comprising:

12. 8. Use of a fuel mixture according to any one of claims 1 to 7 for operating an internal combustion engine.

13. 1. A method of operating an internal combustion engine, comprising: introducing into a combustion chamber of said internal combustion engine ２ %, methane, and at least 25 vol% hydrogen.

14. The fuel mixture is ２ and methane.

15. Prior to the step of supplying the biogas into the combustion chamber, ２ The method of claim 14 further comprising measuring the content.

16. The fuel mixture may contain up to 15 vol% CO ２ 16. The method of any one of claims 13 to 15, comprising:

17. The fuel mixture comprises: - 25 to 98 vol% hydrogen, and / or 1 to 15 vol% CO ２ and / or 1 to 50 vol% methane, 17. The method of any one of claims 13 to 16, comprising:

18. the hydrogen and / or CO in the fuel mixture prior to the step of supplying to the combustion chamber. ２ 18. The method according to any one of claims 13 to 17, further comprising the step of adjusting the proportion of methane.

19. 19. A method according to any one of claims 13 to 18, wherein the internal combustion engine is used to drive a vehicle or the internal combustion engine is used as a stationary engine.

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