WO2015056777A1

WO2015056777A1 - Device for promoting combustion of natural gas

Info

Publication number: WO2015056777A1
Application number: PCT/JP2014/077690
Authority: WO
Inventors: 高橋　栄一; 小島　宏一
Original assignee: 独立行政法人産業技術総合研究所
Priority date: 2013-10-17
Filing date: 2014-10-17
Publication date: 2015-04-23
Also published as: JP6281882B2; JPWO2015056777A1

Abstract

In the present invention, installed is a plasma reactor (2) for irradiating an air-fuel mixture of natural gas fuel and air with plasma and thereby partially oxidizing a portion of the methane molecules contained in the air-fuel mixture. A discharge voltage applied to the plasma reactor (2) is subject to feedback control by a control device (10) so that the total efficiency reaches a maximum or becomes optimal along with an exhaust gas property. According to the present invention, without the use of additional equipment for introducing hydrogen to a combustion chamber, irradiating a premixed gas of combustion air and natural gas containing methane with non-equilibrium plasma makes it possible to partially oxidize a small part of the methane gas contained in the natural gas and the like, thereby making it possible to improve the combustion properties during combustion and ignition through promotion of a low-temperature reaction of the methane molecules and the like during compression, to improve the overall thermal efficiency, and to inhibit discharge as an unburned gas component.

Description

Natural gas combustion accelerator

The present invention relates to a spark accelerating device, a homogeneous premixed compression self-igniting engine (HCCI engine), and a combustion accelerating device when natural gas is used as fuel in a DDF engine using both light oil and natural gas.

Natural gas has become increasingly important as an energy resource year after year due to the development of new supply sources such as shale gas and coal bed methane (CBM) in recent years, and a large proportion of the world's energy supply in the future. It is expected to bear The composition of these natural gases varies, but the main component is methane gas.
Methane gas is a hydrocarbon fuel in which the ratio of carbon atoms to hydrogen atoms is small, and has an advantage that the amount of carbon dioxide, which is a global warming gas, due to combustion is small compared to other hydrocarbon fuels.

It is also a gas that is difficult to self-ignite due to compression, and it is difficult to cause knocking, so it can be expected to improve thermal efficiency by using it in a high compression ratio engine. Is low.

For this reason, in order to improve the thermal efficiency of a natural gas engine, when lean combustion is attempted, there is a high possibility that methane gas will be released as an unburned gas component. However, since methane gas itself is said to have a global warming potential 20 times or more that of carbon dioxide, how to suppress the emission of methane gas as an unburned gas component is an urgent technical issue. It has become.

In order to efficiently burn such natural gas, a method of adding hydrogen has been known for a long time as reported in Non-Patent Document 1, for example. Combustion speed can be increased by mixing and burning hydrogen. The use of natural gas to which hydrogen has been added for an internal combustion engine has already been proposed as shown in Non-Patent Document 2, for example. In this case, equipment for supplying hydrogen separately becomes a practical problem.

On the other hand, reforming technology for converting hydrocarbon fuels such as methane into synthesis gas (mixed gas of hydrogen and carbon monoxide) has been known for a long time and is widely used in the petrochemical industry and the like now. The combustion speed of the synthesis gas containing hydrogen is faster than that of natural gas, and it is shown in Non-Patent Document 3, for example, that it is used for an internal combustion engine.

Many methods using plasma have been proposed to obtain synthesis gas. For example, Patent Document 1 proposes a reforming method for generating hydrogen molecules and carbon monoxide by causing a plasma reaction between non-equilibrium plasma and natural gas and water.
Patent Document 2 proposes a method for producing hydrogen and carbon monoxide by supplying hydrocarbon gas and carbon dioxide to thermal plasma generated by high frequency induction heating.
Furthermore, in Patent Document 3, for low-grade fuel gas containing methane gas, non-equilibrium plasma is used, hydrogen is generated by the plasma reaction of the methane and water, and the efficiency is improved by supplying it to the gas engine. A proposal was made. The abstract includes "a low-temperature plasma apparatus 105 that irradiates a part of methane contained in a low-grade fuel gas to hydrogen by a low-temperature plasma chemical reaction to generate a mixed gas containing methane and hydrogen, and the mixed gas. A combined system of a low-temperature plasma apparatus and a gas engine having a gas engine 103 using a gas as a fuel.

On the other hand, as an ignition method using non-equilibrium plasma itself, as disclosed in Patent Document 4, a method using microwaves or supplying chemically active species (radicals) using the nonequilibrium plasma disclosed in Patent Document 5 is used. A method for promoting ignition is proposed. The abstract of Patent Document 5 states that “non-equilibrium that generates chemically active species (radicals) that enhance the ignitability of the air-fuel mixture in the combustion chamber 13 or at least one of the intake passages 30 communicating with the combustion chamber 13. There is described that the plasma discharge means 50 and the photocatalyst 52 that absorbs at least part of the light emitted by the non-equilibrium plasma discharge means 50 during discharge and generates chemically active species (radicals) are described.

Japanese Patent Publication No. 2005-519729 JP 2008-247717 A JP-A-2005-240586 JP 2007-113570 A JP 2010-37950 A

As described above, improving the combustion rate by mixing hydrogen with methane gas, and reforming to a fuel containing hydrogen by reacting oxygen-containing molecules such as water vapor with hydrocarbon-based fuel Known as the prior art.
The technique described in Patent Document 3 is a technique in which this conventional technique is applied to a low-grade fuel such as biogas. By adding water vapor to a part of methane contained in the low-grade fuel gas, a low-temperature plasma chemical reaction is performed. By converting to hydrogen and adding this hydrogen to the combustion chamber, the combustion rate can be improved.

However, in order to increase the combustion rate, it is necessary to add several tens of percent of hydrogen to methane gas. For this reason, in the case of steam reforming as shown in Patent Document 3, a water tank and a vaporizer are required, which increases the weight and costs, and requires regular water replenishment. There's a problem. Even when other reforming methods are used, there is a problem that an additional reforming apparatus requiring high energy is required.

Further, in the method described in Patent Document 5 in which non-equilibrium plasma is generated in an intake pipe or an engine cylinder and ignition is promoted by chemically active species, particularly chemically active species (radicals) having a very short lifetime. The effect in the engine cylinder is not always clear when the fuel is supplied through the intake pipe, and there is no mention of methane.

Accordingly, an object of the present invention is to provide a non-equilibrium plasma in a premixed gas of combustion air and natural gas containing methane gas without using an additional facility for introducing hydrogen into the combustion chamber, such as a water vapor introducing device. Irradiation partially oxidizes some of the methane gas contained in natural gas, improves the final ignition and combustion characteristics through the promotion of low-temperature reactions during compression, and improves overall thermal efficiency It aims at improving and suppressing discharge | emission of methane gas etc. as an unburned gas component.

In order to achieve the above object, a combustion promoting device of the present invention is a combustion promoting device for an engine using natural gas fuel, and is contained by irradiating plasma to a mixture of natural gas fuel and air. Install a plasma reactor that partially oxidizes a part of methane molecules, etc., and set the discharge voltage applied to the reactor and the operation time of the related equipment to the maximum value for total efficiency or including exhaust gas characteristics. Feedback control was made so that

According to the present invention, since a plasma reactor for irradiating plasma to a gas mixture is provided and an intermediate product is generated by a partial oxidation reaction, ignition is achieved through a low-temperature reaction such as methane molecules with low power consumption. -It is possible to improve the isovolume of the engine by promoting combustion, thereby improving the overall thermal efficiency of natural gas and suppressing emission of methane gas as an unburned gas component.

FIG. 1 is a diagram showing an outline of an engine system including a control device according to the present invention. FIG. 2 is a schematic diagram of an optimum value search for the plasma reactor voltage (V) and ignition timing (θ) for obtaining the control map. FIG. 3 shows an outline of an experimental apparatus including a non-thermal equilibrium plasma discharge reactor apparatus and a rapid compression / expansion apparatus used for accelerating ignition / combustion by partial oxidation of methane by plasma irradiation of the premixed gas. FIG. FIG. 4 is a diagram showing an experimental result of a pressure waveform measurement value of a compression self-ignition experiment measured in the rapid compression / expansion device. FIG. 5 is a diagram showing an experimental result of a pressure waveform measurement value of a laser spark ignition experiment measured in the rapid compression / expansion apparatus.

The overall configuration for carrying out the present invention will be described. FIG. 1 shows the overall configuration. A non-thermal equilibrium plasma discharge reactor apparatus 2 is installed as a plasma reactor in an intake pipe 1 that supplies premixed gas to an engine. Although the engine is assumed to be electrically ignited in this embodiment, the engine can be similarly applied to ignition by pilot injection or ignition by laser spark.

The entire engine system basically includes a spark ignition (SI) device comprising a piston 3 and a cylinder 4, a spark plug 5 and a spark plug drive circuit 6 constituting a conventional gas engine, and a crank angle sensor 7 for detecting a crank angle. A non-thermal equilibrium plasma discharge reactor apparatus 2 that irradiates plasma to a premixed mixture of natural gas and air inside the intake pipe 1, and a discharge drive circuit 8 that is driven by applying a reactor voltage to the non-thermal equilibrium plasma discharge reactor apparatus 2; And a pressure gauge 9 for detecting engine combustion, or a shaft output meter, a spark plug drive circuit 6, a discharge drive circuit 8 for driving the non-thermal equilibrium plasma discharge reactor apparatus 2, and the like. The
The control device 10 obtains an operation map described below in order to improve the overall thermal efficiency of the entire engine or optimize the operation of the engine including the exhaust gas characteristics.

Normally, an SI ignition gas engine introduces a premixed gas into the engine under a certain rotational speed and load condition, compresses it with a piston 3, and ignites it with an ignition plug 5 at an appropriate time.
When a reactor voltage (V1) is applied to a non-equilibrium plasma discharge reactor apparatus by a discharge driving circuit 8 to be described later and a premixed gas partially oxidized is introduced into the engine, the low temperature reaction of methane in the compression process is promoted, Ignition advances. Therefore, the ignition timing by the spark plug 5 is retarded (θ1) to the ignition timing (MBT) that generates the maximum torque. Since the combustion is promoted, the isovolume is improved and the thermal efficiency is improved. The total thermal efficiency at this time is obtained by subtracting the power consumption of the non-thermal equilibrium plasma discharge reactor apparatus 2 from the net thermal efficiency of the engine. Similarly, θ 2 and θ 3 of MBT are determined for other voltages V 2 and V 3, etc., but considering the power consumption and exhaust gas characteristics of the non-thermal equilibrium plasma discharge reactor apparatus 2, the total as shown in FIG. By obtaining a contour map relating to thermal efficiency, the rotational speed, the optimum reactor voltage (Vopt) at the load, and the SI ignition timing (θopt) are determined.

Optimize (V, θ) in the same manner under other load and rotation speed conditions, and obtain an operation map used by the control device 10. Basically, the combustion is fast at a high load, so the reactor voltage V is low, the reactor voltage V is increased to promote the reaction at a high rotation speed, and the reactor voltage V is decreased at a low load and low rotation.

In addition, when applied to a compression self-ignition (HCCI) natural gas engine, unlike the above embodiment, since a device such as a spark plug is not used for ignition, the operation of the engine is controlled by a pressure gauge or a shaft output meter. The voltage is controlled so that ignition can be realized at an appropriate time.
Further, in the HCCI engine, only plasma irradiation may cause insufficient progress of the low-temperature reaction of methane accompanying compression and may not lead to self-ignition. In that case, it is necessary to raise the intake temperature of the premixed gas itself.

Next, the experimental results verifying that combustion characteristics can be improved by partially oxidizing part of methane gas will be described. In this experiment, a premixed mixture of methane gas and air is irradiated with plasma in a plasma reactor, and the gas is applied to a cylinder of a device that can simulate the single-cycle compression / expansion process of an engine called a rapid compression / expansion device (RCEM). After the introduction, ignition was performed in the compression / expansion stroke. With regard to the ignition method, experiments were carried out by both self-ignition by compression and ignition by laser spark.

Hereinafter, the equipment used in the experiment will be described with reference to FIG.
In this experimental facility, a packed bed (PB type) plasma reactor 11 was used as the plasma reactor in order to irradiate the premixed gas of methane gas and air with plasma.
The methane-containing gas and air are metered from the

mass flow controllers

12a and 12b that generate the premixed gas, and are introduced into the plasma reactor 11 as a mixed gas having a predetermined equivalence ratio.
In the plasma reactor 11, a discharge voltage having a predetermined voltage and frequency is applied, heated to a predetermined temperature via the electric heater 13, and then at a predetermined compression ratio in a cylinder inside the rapid compression / expansion device (RCEM) 14. Compression is performed.

Inside the plasma reactor 11, a glass tube having an inner diameter of 18 mm is filled with glass beads having a diameter of 2 mm. A metal gas pipe having a diameter of 6 mm was passed through the center of the plasma reactor 11.
A metal tape with a width of 3 cm stretched outside the glass tube was used as a ground electrode, and a high voltage was applied to the central metal gas pipe. The distance between the electrodes is 7 mm. An intermediate product such as formaldehyde was slightly formed in the premixed gas that passed through the plasma reactor 11.

The pressure inside the combustor was measured with a piezo-type pressure gauge. The upper part of the combustor has a quartz window, and laser light can be introduced. In this experiment, an Nd: YAG laser was condensed from this quartz window, and it was forcibly ignited by laser spark. .

The change of ignition characteristics of methane premixed gas by non-thermal equilibrium plasma irradiation was investigated. First, the experimental results of the influence of the voltage applied to the plasma reactor 11 and the equivalent ratio on the self-ignition characteristics by compression will be described, and then the combustion characteristics results after ignition by laser spark will be described.
FIG. 4 shows the time change of pressure due to self-ignition. When the initial temperature is 393 K and the equivalence ratio is fixed to 0.97, the discharge voltage applied to the plasma reactor 11 is set to 0, and from 15 kV The pressure rise characteristics obtained when the pressure is raised by 0.5 kV up to 18.5 kV are shown. It can be seen that compression self-ignition is achieved by irradiating the premixed gas with plasma by the plasma reactor 11, and the ignition timing can be controlled by the applied voltage.

The above has verified the self-ignition accompanying compression, but next, the state of combustion after spark ignition by laser irradiation is examined from the pressure waveform.
In this experiment, laser spark was ignited from the center of the combustor. However, when applied to an actual engine, a normal electric spark plug can be used.

In this experiment, the discharge voltage applied to the plasma reactor 11 was fixed at 16 kV.
FIG. 5 shows the pressure waveform of laser spark ignition. The ignition by the laser spark is performed at 40 ms in the figure. Comparing the case where the plasma irradiation is not performed, it can be seen that the combustion rate after ignition is increased when the irradiation is significantly performed.

As described above, in this example, a premixed gas mixture containing methane is irradiated with non-thermal equilibrium plasma to generate a partial oxidation reaction of a very small part, thereby generating an intermediate reactant, and controlling the low temperature reaction of methane. It proceeds without going through the starting reaction to promote ignition and combustion. The conversion electric field required in the plasma reactor 11 is about 100 Td, and the intermediate reactant generated by irradiation is generally very low, less than 0.1%.

The plasma reactor 11 can have various configurations, and a specific voltage and a flow rate of the premixed gas flowing through the plasma reactor 11 are determined under conditions for forming a necessary intermediate product. The amount of intermediate product required also depends on the ignition method of the engine.

As described above, according to the present invention, when natural gas is used as an engine fuel, the gas mixture is irradiated with plasma by a plasma reactor, and the discharge voltage at that time is maximized in total thermal efficiency or exhaust gas. Feedback control was performed to optimize the characteristics.
As a result, the overall thermal efficiency is improved at a low cost, and methane gas can be prevented from being discharged as an unburned gas component, so natural gas combustion that makes the best use of the low emission characteristics of carbon dioxide It can be expected to be widely adopted as a promotion device.

DESCRIPTION OF SYMBOLS 1 Intake pipe 2 Non-thermal equilibrium plasma discharge reactor apparatus 3 Piston 4 Cylinder 5 Spark plug 6 Spark plug drive circuit 7 Crank angle sensor 8 Discharge drive circuit 9 Pressure gauge 10 Controller 11 Packed bed type (PB type)

plasma reactor

12a, 12b Mass flow controller 13 Heater 14 Rapid compression and expansion device (RCEM)

Claims

An engine combustion accelerator using natural gas fuel,
By irradiating the natural gas fuel and air mixture with plasma, a plasma reactor that partially oxidizes a part of the contained methane molecules is installed,
A combustion promoting device, wherein the discharge voltage applied to the plasma reactor is feedback controlled so that the total thermal efficiency is maximized or optimized including exhaust gas characteristics.