Gas engine

A gas engine includes a generator coupled to an output shaft thereof, an intake passage to which a low-concentration methane gas (VAM gas) derived from mine venting is supplied, and a gas mixing unit mixing a high-concentration methane gas (CMM gas) to the low-concentration methane gas midway along the intake passage, so that a gas mixture of the low-concentration methane gas and the high-concentration methane gas is supplied to and burnt in a combustion chamber. A turbocharger is provided in the intake passage upstream of the gas mixing unit, and in the intake passage upstream of the turbocharger, is provided a mixture ratio adjusting unit adjusting a mixture ratio of the low-concentration methane gas and the air. The temperature or flow rate of intake gas flowing into the turbocharger is kept in a constant range by an intake controller.

TECHNICAL FIELD

The present invention relates to a gas engine that makes effective use of natural gas, biogas, or methane gas emitted from a coal mine or the like as intake gas or fuel.

BACKGROUND ART

The world's interest in restrictions on emissions of green house gases such as methane (CH4) and CO2has been increasing over the years. Methane, in particular, is 21 times more potent than CO2in driving the global warming, and therefore methane emissions to atmosphere must not be left unnoticed. In the meantime, a large amount of methane, as much as 10 to 40 Nm3(pure methane) per ton of coal, is being released during coal mining from a coal mine.

Coal mines emit two types of methane gas: CMM (Coal Mine Methane) gas (with a concentration of about 20 to 50 wt %) existing in and recovered from coal seams through degassing bore holes using vacuum pumps for safety reasons, and VAM (Ventilation Air Methane) gas (with a concentration of less than 1 wt %) released through venting from the mine tunnels and the coal face.

Therefore, effective use of the methane contained in the gases emitted from coal mines by capturing it before it is released to atmosphere will make significant economic and social contributions.

Patent Document 1 discloses a gas turbine capable of making use of gases with a methane concentration of below the explosion limit, such as landfill gas produced in the landfill, or the gases emitted from coal mines as noted above, as fuel.

Patent Document 2 discloses a gas engine for power generation using methane gas emitted from a coal mine as fuel. The gas engine power generating facility disclosed in Patent Document 2 will be described below with reference toFIG. 7.FIG. 7is a schematic illustration of a coal mine CM and a gas engine power generating facility200built near the coal mine CM. InFIG. 7, inside the coal mine CM are layers of coal seams C0and the coal seam being mined C1. A ventilation hole206is provided for communicating the inside of the coal mine with the outside.

In the coal face204of the coal mine, degassing bore holes208are drilled in the coal seam being mined C1, and CMM gas emitted from the bore holes208is sent to the gas engine power generating facility200through a pipe210disposed inside the ventilation hole206by means of a vacuum pump211. VAM gas b emitted from the coal mine through the ventilation hole206is sent to the gas engine power generating facility200through a pipe212. Electric power E and steam S generated through operation of the gas engine power generating facility200are sent to a utility facility202in the mine premises or further to other consumers.Patent Document 1: Japanese Patent Application Publication No. 2010-19247Patent Document 2: U.S. Patent Application Publication No. 2005/0205022

Gas engines that use methane gas as fuel are an internal combustion engine expected to be used widely in future because of their advantage that they cause very little environmental pollution. However, the amount of methane emissions from a coal mine varies largely depending on the time, and therefore how to maintain a stable amount of supply to the gas engine is an issue to be addressed.

The air-fuel mixture ratio, or an excess air ratio, needs to be maintained at an optimal level in order to reduce the concentration of NOXin the exhaust gas or for other reasons. Maintaining a predetermined excess air ratio, however, is not easy because of the large variations in the amount of methane emissions from a coal mine as mentioned above.

In a gas engine, the temperature of gas mixture supplied to the combustion chamber has to be kept in a constant range of from 40 to 45° C., as otherwise there is a possibility of abnormal combustion such as knock or the like. For this reason, the intake gas having higher pressure and temperature after passing through a turbocharger is kept in a constant temperature range by a charge air cooler (intercooler). With the use of the methane gas emitted from a coal mine, however, it is not easy to control the temperature of the gas mixture supplied to the combustion chamber because of the large variations in the amount of methane gas and performance limitations of the intercooler.

DISCLOSURE OF THE INVENTION

In view of such problems in the conventional techniques, an object of the present invention is to enable a power generating gas engine that uses methane gas as fuel to maintain an optimal excess air ratio to reduce NOXemissions, and to control to stably keep an optimal excess air ratio even though there are variations in the amount of methane gas.

Another object is to allow optimal control of the temperature of gas mixture supplied to the combustion chamber to prevent abnormal combustion such as knock or the like, and to allow stable temperature control of the gas mixture supplied to the combustion chamber even though there are variations in the amount of methane gas.

To achieve these objects, the gas engine of the present invention includes:

a generator coupled to an output shaft of the engine; an intake passage to which a low-concentration methane gas derived from mine venting is supplied; a gas mixing unit mixing a high-concentration methane gas to the low-concentration methane gas midway of the intake passage, so that a gas mixture of the low-concentration methane gas and the high-concentration methane gas is supplied to and burnt in a combustion chamber;

a turbocharger provided in the intake passage upstream of the gas mixing unit; an air mixing part provided in the intake passage upstream of the turbocharger and mixing air with the low-concentration methane gas; a mixture ratio adjusting unit adjusting a mixture ratio of the low-concentration methane gas and the air in the air mixing part; and an intake controller controlling the mixture ratio adjusting unit to keep the temperature or flow rate of intake gas flowing into the turbocharger in a predetermined range.

The apparatus of the present invention enables use of VAM gas emitted from a coal mine through venting as the low-concentration methane gas, and use of CMM gas emitted from the coal mine as the high-concentration methane gas, whereby emissions of methane, which is a greenhouse gas, from a coal mine to atmosphere can be reduced. The effective use of VAM gas produced through venting allows the consumption of fuel gas of the gas engine to be reduced. Namely, the consumption of the high-concentration methane gas can be reduced.

Applicable examples of the high-concentration methane gas supplied to the gas engine include the CMM gas, natural gas, biogas, by-product gas exhausted from plants and the like, and landfill gas.

The intake controller controls the mixture ratio adjusting unit to adjust the mixture ratio of the low-concentration methane gas and the air so that the temperature or flow rate of intake gas flowing into the turbocharger is kept in a constant range. Thus the excess air ratio can be controlled to be stable by the turbocharger, and the gas mixture temperature can be controlled to be stable by the intercooler provided downstream of the turbocharger. Accordingly, even when there are variations in the amount of methane gas, the excess air ratio and the gas mixture temperature can be controlled to optimal values speedily and precisely, so that combustion can be maintained stable.

The mixture ratio adjusting unit may be formed by flow rate control valves or the like provided in inlet passages of air and VAM gas, for example, so that the mixture ratio of air and VAM gas in the intake passage can be controlled by adjusting the degrees of opening of these valves.

The apparatus of the present invention may further include a bypass passage arranged in parallel with a turbine of the turbocharger that is disposed in an exhaust passage, the bypass passage allowing part of exhaust gas to bypass the turbine; an exhaust gas flow rate control valve controlling flow rate of the exhaust gas in the bypass passage; and a turbocharger controller controlling the exhaust gas flow rate control valve to control operation of the turbocharger, wherein the turbocharger controller controls flow rate of the intake gas passing through the turbocharger such that the gas mixture is supplied to the combustion chamber with a target excess air ratio.

The turbocharger controller controls the flow rate of the intake gas passing through the turbocharger, so as to achieve a target excess air ratio of the gas mixture supplied to the combustion chamber. The intake controller controls the temperature or flow rate, or both the temperature and flow rate, of the intake gas flowing into the turbocharger to be within a constant range in advance. Therefore, even when there are variations in the amount of methane gas, the excess air ratio can be controlled to an optimal value speedily and precisely, through the control of the excess air ratio by the turbocharger and through the temperature control by the intercooler provided downstream of the turbocharger, so that combustion can be maintained stable.

In the apparatus of the present invention, the intake controller may include an intake gas temperature control unit, and the intake gas temperature control unit may control the mixture ratio adjusting unit to adjust the mixture ratio of the low-concentration methane gas and the air to keep the intake gas in the intake passage upstream of the turbocharger in a constant temperature range that allows stable control by the turbocharger controller to achieve the target excess air ratio.

The temperature may be kept in a constant range of 20 to 25° C., for example, for the turbocharger controller to control the excess air ratio to a target value in a stable manner. Thereby, even when there are variations in the amount of methane gas, the temperature of the intake gas flowing into the turbocharger is made stable, so that the turbocharger controller can control the excess air ratio stably, and the intercooler downstream of the turbocharger can control the gas mixture temperature stably. Thus adequate emission performance (such as NOXemissions, etc) can be achieved, and the engine performance can be fully exploited without the possibility of abnormal combustion such as knock or the like.

In addition to the above configuration, the apparatus may further include a target excess air ratio correcting unit correcting the target excess air ratio, so that the target excess air ratio is changed by the target excess air ratio correcting unit when the intake gas flowing into the turbocharger cannot be controlled to stay in the constant temperature range despite the control of the mixture ratio of the low-concentration methane gas and the air by the intake gas temperature control unit.

Even though the mixture ratio of the low-concentration methane gas and the air is adjusted by the intake controller, the intake gas temperature may sometimes be uncontrollable depending on the temperature of VAM gas or air. When this happens, the target excess air ratio correcting unit corrects the target excess air ratio to a value appropriate for the operation at temperatures outside of the preset range, to allow the gas engine to operate with the corrected target excess air ratio. The target excess air ratio λ is corrected from λ=2.0 to λ=1.9, for example, with which the gas engine is controllable, so that the gas engine can run stably.

In the apparatus of the present invention, the intake controller may include an intake gas flow rate control unit, and the intake gas flow rate control unit may control the mixture ratio adjusting unit to keep the flow rate of the intake gas upstream of the turbocharger in a constant range that allows stable control by the turbocharger controller to achieve the target excess air ratio.

Thereby, even when there are variations in the amount of methane gas, the amount of the intake gas flowing into the turbocharger is made stable, so that the excess air ratio can be made closer to a target value by the turbocharger controller swiftly and precisely.

In the apparatus of the present invention, the mixture ratio adjusting unit may be controlled such that the low-concentration methane gas is supplied to the air mixing part always with a maximum permissible flow rate.

This allows maximum use of VAM gas as fuel of the gas engine, so that the methane emissions to atmosphere can be minimized. The effective use of VAM gas produced through venting allows the consumption of fuel gas of the engine to be reduced. Namely, the consumption of the high-concentration methane gas can be reduced.

The apparatus of the present invention may be configured to allow part of the high-concentration methane gas to be supplied to the intake passage upstream of the turbocharger and downstream of the air mixing part.

As the high-concentration methane gas is divided and supplied to the intake passage downstream and upstream of the turbocharger, the associated devices such as control valves that form the gas mixing unit midway of the intake passage for mixing the high-concentration methane gas can be divided and arranged at respective positions. The respective devices can be made small and lightweight as they are arranged at separate positions. As the associated devices such as control valves can be made small and lightweight, the problem of installation space can be resolved, and the component cost can be reduced.

The apparatus of the present invention is a gas engine including a generator coupled to an output shaft of the engine, an intake passage to which a low-concentration methane gas derived from mine venting is supplied, and a gas mixing unit mixing a high-concentration methane gas to the low-concentration methane gas midway of the intake passage, so that a gas mixture of the low-concentration methane gas and the high-concentration methane gas is supplied to and burnt in a combustion chamber. The gas engine further includes a turbocharger provided in the intake passage upstream of the gas mixing unit, an air mixing part provided in the intake passage upstream of the turbocharger and mixing air with the low-concentration methane gas, a mixture ratio adjusting unit adjusting a mixture ratio of the low-concentration methane gas and the air in the air mixing part, and an intake controller controlling the mixture ratio adjusting unit to keep the temperature or flow rate of intake gas flowing into the turbocharger in a predetermined range. The apparatus of the present invention enables use of VAM gas derived from mine venting as the low-concentration methane gas, and use of CMM gas emitted from the coal mine as the high-concentration methane gas, whereby emissions of methane, which is a greenhouse gas, from a coal mine to atmosphere can be reduced. The effective use of VAM gas produced through venting allows the consumption of fuel gas of the gas engine to be reduced. Namely, the consumption of the high-concentration methane gas can be reduced.

The intake controller may control the mixture ratio adjusting unit to control the temperature or flow rate of intake gas flowing into the turbocharger by adjusting the mixture ratio of air and VAM gas.

As the temperature or flow rate of intake gas flowing into the turbocharger is controlled in advance to be within a constant range, the excess air ratio can be controlled to be stable by the turbocharger, and the gas mixture temperature can be controlled to be stable by the intercooler provided downstream of the turbocharger. Accordingly, even when there are variations in the amount of methane gas, the excess air ratio and the gas mixture temperature can be controlled to optimal values speedily and precisely, so that combustion can be maintained stable.

BEST MODE FOR CARRYING OUT THE INVENTION

The illustrated embodiments of the present invention will be hereinafter described in detail. It should be noted that, unless otherwise particularly specified, the sizes, materials, shapes, and relative arrangement or the like of constituent components described in these embodiments are not intended to limit the scope of this invention.

A first embodiment of the apparatus of the present invention will be described with reference toFIG. 1toFIG. 3. The power generating gas engine of this embodiment is installed near a coal mine, and uses methane gas emitted from the coal mine as fuel gas and intake gas. InFIG. 1, the power generating gas engine10includes an engine body12having a plurality of (four inFIG. 1) combustion cylinders, inside which combustion chambers are formed, and a generator16coupled to an output shaft14of the engine body12.

An air mixing chamber (air mixing part)20is provided upstream of an intake pipe18connected to the engine body12. An air inlet pipe22and a VAM gas inlet pipe24are connected to the air mixing chamber20. Air a is introduced into the air inlet pipe22, while VAM gas b emitted through venting from the coal mine is introduced into the VAM gas inlet pipe24. VAM gas is a methane-containing gas emitted through venting from the mine tunnels and the coal face of the coal mine, and contains methane with a diluted concentration of less than 1 wt %. The air mixing chamber20contains gas/air mixture d, which is a mixture of air a and VAM gas b.

Flow rate control valves26and28are interposed in the air inlet pipe22and VAM gas inlet pipe24, respectively, their degrees of opening being controlled by an engine controller (intake controller)90A. The mixture ratio of the gas/air mixture d inside the air mixing chamber20is adjusted by controlling the degrees of opening of the flow rate control valves26and28. A compressor32of a turbocharger30is provided to the intake pipe18downstream of the air mixing chamber20. The compressor32is coupled to a turbine34provided to an exhaust pipe62to be described later via a rotating shaft36, for compressing the gas/air mixture d to be supplied to the combustion cylinders of the engine body12.

A charge air cooler (intercooler)38is provided downstream of the turbocharger30. Cooling water w is introduced to this intercooler38, so that intake gas that has passed through the turbocharger30is cooled down by heat exchange with this cooling water w, after which the gas is supplied to the respective combustion cylinders56ato56dvia a common intake pipe40and intake branch pipes42ato42d. Temperature control of the intake gas by the intercooler38is controlled by the engine controller90A.

Meanwhile, CMM gas c released from the coal mine is supplied to the engine body12via a fuel gas supply pipe44. CMM gas c is a methane-containing gas that exists in coal seams and is recovered from degassing bore holes208by a vacuum pump211as shown inFIG. 7for safety reasons, and contains a high concentration of about 20 to 50 wt % of methane. To the fuel gas supply pipe44are interposed a buffer tank46, a flow rate control valve48, and a gas compressor50, from the upstream side in this order. The degree of opening of the flow rate control valve48is controlled by the engine controller90A.

The fuel gas supply pipe44is divided into four fuel branch pipes52ato52d, which are respectively connected to the intake branch pipes42ato42d.

CMM gas c sent to the fuel gas supply pipe44is compressed by the gas compressor50, and supplied to the intake branch pipes42ato42dvia the fuel branch pipes52ato52d. The gas/air mixture d and CMM gas c are pre-mixed inside the intake branch pipes, and this gas mixture is supplied to the respective combustion cylinders56ato56das fuel gas g (seeFIG. 2). Flow rate control valves54ato54dare respectively provided to the fuel branch pipes52ato52d, their degrees of opening being controlled by the engine controller90A. The flow rate control valves54ato54dconstitute a gas mixing unit that forms connecting parts of the fuel branch pipes52ato52dto the intake branch pipes42ato42d.

Exhaust branch pipes58ato58dare respectively connected to the head parts of the combustion cylinders56ato56d. The exhaust branch pipes58ato58dare connected to a common exhaust pipe60, which is further connected to an exhaust pipe62. The turbine34of the turbocharger30is provided in the exhaust pipe62. Exhaust gas e coming out from the respective combustion cylinders56ato56dis exhausted through the exhaust branch pipes58ato58d, common exhaust pipe60, and exhaust pipe62. A bypass pipe64bypassing the turbine34is connected to the exhaust pipe62, and a flow rate control valve66is interposed in the bypass pipe64. The degree of opening of the flow rate control valve66is controlled by the engine controller90A.

Next, the structure of the head parts of the combustion cylinders56ato56dof the engine body12will be described with reference toFIG. 2. InFIG. 2, a piston70reciprocates inside each of the combustion cylinders56ato56d. A recess70ais cut in the upper face of the piston70, and a main combustion chamber m is formed above this recess70a. An injector case72is mounted in the center on the upper face of each of the combustion cylinders56ato56d. An injector76is mounted inside the injector case72, and a sub chamber s is formed below the injector76. A conduit78extending through the injector case72is connected to the injector76, so that pilot fuel p is supplied into the injector76through the conduit78.

In the upper face of the combustion cylinder on both sides of the injector case72are provided an intake port communicating with a corresponding one of the intake branch pipes42ato42dand an exhaust port communicating with a corresponding one of the exhaust branch pipes58ato58d. There are provided an intake valve80for opening and closing the intake port, and an exhaust valve82for opening and closing the exhaust port. An exhaust gas temperature sensor84is provided in each of the exhaust branch pipes58ato58dfor detecting the temperature of the exhaust gas. Measurements by the exhaust gas temperature sensors84are sent to the engine controller90A.

CMM gas c is added to the gas/air mixture d flowing in the intake branch pipes42ato42dfrom the fuel branch pipes52ato52dso that they are pre-mixed to form a fuel gas g, which is supplied from the intake ports into the combustion cylinders56ato56d. The fuel gas g inside the combustion cylinder is compressed by the piston70, and the high-pressure, high-temperature gas enters the sub chamber s through injection holes74drilled in the bottom of the injector case72. Meanwhile, pilot fuel p is injected from the injector76into the sub chamber s, and ignites the high-pressure, high-temperature fuel gas g. Flames thus generated inside the sub chamber s propagate through the holes74drilled in the bottom of the injector case72to the main combustion chamber m and the flames f spread in the main combustion chamber m.

The fuel gas g expands inside the main combustion chamber m and pushes down the piston70to rotate the output shaft14. Exhaust gas e produced by combustion is exhausted through the exhaust branch pipes58ato58d, common exhaust pipe60, and exhaust pipe62.

Referring back toFIG. 1, there are provided an rpm sensor85for detecting rotation speed of the output shaft14, and cylinder pressure sensors (not shown) for detecting pressure inside the main combustion chambers m of the combustion cylinders56ato56d, and measurements from all these sensors including the exhaust gas temperature sensors84are sent to the engine controller90A. An engine output control unit92controls the output of the gas engine10and combustion state in the main combustion chambers m based on the measurements.

There are also provided an intake gas temperature sensor87and an intake gas pressure sensor88in the intake pipe18between the turbocharger30and the air mixing chamber20for detecting the temperature and pressure of the gas/air mixture d flowing into the compressor32of the turbocharger30. Measurements by these sensors are sent to the engine controller90A.

In this configuration, a turbocharger controller94of the engine controller90A controls the degree of opening of the flow rate control valve66to adjust the flow rate of the exhaust gas flowing through the bypass pipe64. This controls the flow rate of the exhaust gas flowing through the exhaust pipe62, which controls the rpm of the turbine of the turbocharger30, and controls the flow rate of the intake gas flowing through the intake pipe18.

By thus controlling the flow rate of intake gas flowing into the turbocharger30, the excess air ratio λ of the gas mixture g fed into the combustion chambers is controlled to a target value.

The excess air ratio λ is determined as follows: First, the concentration of gas/air mixture d is calculated from the measurements of temperature and pressure of the gas/air mixture d by the intake gas temperature sensor87and the intake gas pressure sensor88. Next, the flow rate is calculated from the concentration. The flow rate of CMM gas c flowing through the fuel gas supply pipe44is calculated from the degree of opening of the flow rate control valve48. An approximate value λ′ of excess air ratio is obtained from these flow rate of gas/air mixture d and the flow rate of CMM gas c thus calculated. Since the gas/air mixture d includes the VAM gas b and thus contains methane, an accurate value of excess air ratio λ cannot be obtained by the above calculation method. However, since the VAM gas b has an extremely low methane concentration of, typically, less than 1 wt %, the gas/air mixture d inside the intake pipe18is regarded as air, and the calculated value λ′ is assumed to be λ (λ′≅λ).

InFIG. 1, TAirand QAirrespectively represent the temperature and flow rate of air a introduced from the air inlet pipe22, TVAMand QVAMrespectively represent the temperature and flow rate of VAM gas b introduced from the VAM gas inlet pipe24, and TV+Aand QV+Arespectively represent the temperature and flow rate of gas/air mixture d flowing into the turbocharger30. Note, QV+A=QAir+QVAM.

While the temperature of VAM gas b emitted from the coal mine is typically from 20 to 25° C. under atmospheric pressure, the temperature of gas/air mixture d flowing through the intake pipe18is raised by the compressor32. The temperature of the fuel gas g supplied into the main combustion chambers m affects the combustion state of the fuel gas g inside the main combustion chambers m. There is a possibility of abnormal combustion such as misfire or knock depending on the temperature of the fuel gas g. The temperature of gas/air mixture d also affects the excess air ratio λ, since the concentration of the gas/air mixture d varies depending on its temperature.

Therefore, the temperature of the fuel gas g supplied into the main combustion chambers m need to be controlled within a predetermined range, typically from 40 to 45° C. While the gas/air mixture d is cooled by the intercooler38downstream of the turbocharger30, the temperature of the gas/air mixture d flowing into the turbocharger30needs to be kept in a range of from 20 to 25° C. in consideration of performance limitations of the intercooler38and in order to achieve stable control of the excess air ratio. This intake gas temperature control procedure will be explained with reference toFIG. 3.

In this embodiment, the flow rate QV+Aof the gas/air mixture d supplied into the combustion chambers through the compressor32of the turbocharger30is controlled by the turbocharger controller94to a value with which a target excess air ratio λ0can be achieved. The target excess air ratio λ0is set to be 2.0, for example, to reduce the NOXconcentration of the exhaust gas e.FIG. 3is a flowchart showing the procedure of controlling the temperature TV+Aof the gas/air mixture d to a preset temperature TSUC(TSUC1<TSUC<TSUC2) when the turbocharger30is being controlled to achieve this target excess air ratio. TSUCis kept constant in a range of 20° C. to 25° C., for example.

InFIG. 3, the control starts at step S10, and when TV+A<TSUC1at step S12, the process goes to step S14. When TSUC1<TVAMat step S14, the intake gas temperature control unit96controls the degrees of opening of the flow rate control valves26and28to increase the flow rate of VAM gas QVAMand to reduce the flow rate of air QAIR. Thereby, the temperature TV+Aof gas/air mixture d is raised to fall within the preset temperature range. The flow rate QV+Aof gas/air mixture d, which is the sum of the flow rate of VAM gas QVAMand the flow rate of air QAir, is not changed.

If not TSUC1<TVAMat step S14, the process goes to step S16. If TSUC1<TAirat step S16, the flow rate of VAM gas QVAMis reduced and the flow rate of air QAiris increased. Thereby, the temperature TV+Aof gas/air mixture d is raised to fall within the preset temperature range.

If not TSUC1<TAirat step S16, it means that both the VAM gas temperature TVAMand the air temperature TAirare higher than TSUC1, so that the controller judges that the temperature of gas/air mixture d cannot be controlled to be within the preset range. An excess air ratio correcting unit98corrects the target excess air ratio to a value λ0′ appropriate for the operation when the temperature of gas/air mixture d is outside a predetermined range (e.g., λ0=2.0+0.1). The preset flow rate QSUCof gas/air mixture d is changed such as to achieve the corrected target excess air ratio λ0′ (QSUC→QSUC+i), and the turbocharger controller94controls the degree of opening of the flow rate control valve66to achieve the flow rate QSUC+iof gas/air mixture d.

If not TV+A<TSUC1at step S12, the process goes to step S18. If not TSUC2<TV+Aat step S18, it means that the gas/air mixture temperature TV+Ais within the preset range, so the process returns to step S12. If TSUC2<TV+Aat step S18, the process goes to step S20. If TVAM<TSUC2at step S20, the flow rate QVAMof VAM gas is increased and the flow rate of air QAiris reduced. Thereby, the temperature TV+Aof gas/air mixture d is lowered to fall within the preset temperature range.

If not TVAM<TSUC2at step S20, the process goes to step S22. If TAir<TSUC2at step S22, the flow rate QVAMof VAM gas is reduced and the flow rate of air QAiris increased. Thereby, the temperature TV+Aof gas/air mixture d is lowered to fall within the preset range.

If not TAir<TSUC2at step S22, it means that both the VAM gas temperature TVAMand the air temperature TAirare lower than TSUC2, so that the controller judges that the temperature of gas/air mixture d cannot be controlled to be within the preset range. The excess air ratio correcting unit98corrects the target excess air ratio to a value λ0″ appropriate for the operation when the temperature of gas mixture is outside a predetermined range (e.g., λ0″=2.0−0.1). The preset flow rate QSUCof gas/air mixture d is changed such as to achieve the corrected target excess air ratio λ0″ (QSUC→QSUC−i), and the turbocharger controller94controls the degree of opening of the flow rate control valve66to achieve the flow rate QSUC−1of gas/air mixture d.

In this embodiment, the intake gas temperature control unit96controls the temperature TV+Aof gas/air mixture d such that the flow rate QVAMof VAM gas b is always maximum within a permissible range. This is for making the maximum use of VAM gas.

According to this embodiment, VAM gas b emitted from the coal mine is utilized as intake gas of the gas engine10, while CMM gas c is utilized as fuel gas of the gas engine10, so that emissions of methane, which is an greenhouse gas, from the coal mine to the atmosphere can be reduced.

The excess air ratio λ of fuel gas g supplied to the combustion cylinders56ato56dcan be controlled to a target value by controlling the flow rate of exhaust gas e bypassing the turbine34of the turbocharger30by means of the turbocharger controller94. Therefore, production of NOXand the like in the exhaust gas e is reduced, as well as the engine performance can be fully exploited without the possibility of abnormal combustion such as knock or the like.

Since the gas/air mixture d of air a and VAM gas b is used as intake gas, the supply of intake gas to the combustion cylinders56ato56dcan be made stable by adjusting the amount of supply of air in accordance with the amount of supply of VAM gas b. Therefore, the amount of supply of intake gas to the combustion cylinders56ato56dcan be made stable even though the amount of VAM gas b varies largely.

Furthermore, the intake gas temperature is more easily controllable because it is controlled by introducing air a into the intake gas. Controlling the intake gas temperature before it is fed into the turbocharger30to be within the range of from 20 to 25° C. enables stable control of excess air ratio by the turbocharger controller94and of intake gas temperature by the intercooler38disposed downstream of the turbocharger30. Thus, even when there are variations in the amount of methane gas, optimal control of the excess air ratio, and of the temperature of the gas mixture supplied to the combustion chambers, is achieved speedily and precisely, to maintain stable combustion.

Even if the temperature of gas/air mixture d cannot be adjusted to be within a preset range despite the adjustment of mixture ratio of air a and VAM gas b in the intake pipe18, the gas engine can still operate without being hindered, since the excess air ratio correcting unit98changes the target excess air ratio to a value appropriate for the operation when the temperature of the gas/air mixture d is outside the predetermined range. While the target excess air ratio λ, if corrected from 2.0 to 1.9, for example, is more different from the theoretical value, and therefore may adversely affect the NOXemission control performance, it is set to a value that makes the gas engine controllable, so as to allow the gas engine to run stably.

The mixture ratio of air a and VAM gas b is adjusted such that the flow rate of VAM gas is maximum within a range in which the excess air ratio λ can be controlled to a target value and under conditions in which the gas/air mixture d can be kept in a preset temperature range. This allows the emission of VAM gas b to atmosphere to be maximally reduced, as well as maximum use of the energy of methane contained in VAM gas b, so that the consumption rate of CMM gas, which is a high-concentration methane gas, can be reduced.

A second embodiment of the apparatus of the present invention will be described with reference toFIG. 4andFIG. 5. This embodiment is an example of control of the excess air ratio λ to a target value when the temperature of gas/air mixture d is already within the preset range.FIG. 4illustrates an engine controller90B of this embodiment. The engine controller90B includes an intake gas flow rate control unit100, instead of the intake gas temperature control unit96and the excess air ratio λ correcting unit98of the engine controller90A, as compared to the engine controller90A used in the first embodiment. Other features of the structure of the engine controller90B are the same as the engine controller90A. The entire configuration other than the engine controller is also the same as the first embodiment.

The intake gas flow rate control unit100adjusts the degrees of opening of the flow rate control valves26and28based on the measurements of the intake gas temperature sensor87and the intake gas pressure sensor88, thereby adjusting the mixture ratio of air a and VAM gas b, so that the flow rate of gas/air mixture d flowing into the turbocharger30is adjustable. The procedure for adjusting the flow rate of gas/air mixture d of this embodiment will be described below with reference toFIG. 5.

FIG. 5is a flowchart for controlling the flow rate QV+Aof gas/air mixture d to fall within a constant range (QSUC1<QSUC<QSUC2). Namely, the flowchart illustrates an example of control for maintaining the flow rate QV+Aof gas/air mixture d within a control range with a lower limit of QSUC1and an upper limit of QSUC2, which is for achieving a preset target excess air ratio λ, as well as for making the flow rate QVAMof VAM gas b to be maximum available. QVAM2in the drawing represents a maximum supply limit of VAM gas b.

InFIG. 5, the control starts at step S30. If QV+A<QSUC1at step S32, the process goes to step S34. If QVAM<QVAM2at step S34, the flow rate of VAM gas b is increased to increase the flow rate of gas/air mixture d to be within the preset range. If not QVAM<QVAM2, then the flow rate of VAM gas is reduced to not more than QVAM2, while the flow rate of air a is increased to increase the flow rate of gas/air mixture d to fall within the preset range.

If not QV+A<QSUC1at step S32, then the process goes to step S36. If not QSUC2<QV+Aat step S36, it means that the flow rate of the gas/air mixture d is within the preset range, so the process returns to step S32and the same procedure is repeated.

If QSUC2<QV+Aat step S36, it means that QV+Ais out of the preset range, so the process goes to step S38. If 0<QAirat step S38, then the flow rate of air a is reduced, so that QV+Afalls within the preset range. If not 0<QAir, then the flow rate of VAM gas b is reduced, so that QV+Afalls within the preset range.

The flow rate of gas/air mixture d on the inlet side of the turbocharger30is maintained within a preset range in this manner. According to this embodiment, control of gas/air mixture amount on the inlet side of the turbocharger30by the intake gas flow rate control unit100allows the turbocharger controller94to control the excess air ratio speedily and precisely. The excess air ratio can be controlled to remain stable even when there are variations in the amount of methane gas.

Next, a third embodiment of the apparatus of the present invention will be described with reference toFIG. 6. InFIG. 6, a fuel gas branch pipe110is connected to the buffer tank46, and a gas mixer112is provided to the intake pipe18between the compressor32and the air mixing chamber20. The fuel gas branch pipe110is connected to the gas mixer112, so that part of CMM gas c is supplied from the gas mixer112to the intake pipe18. A filter114and a flow rate control valve116are interposed in the fuel gas branch pipe110. The degree of opening of the flow rate control valve116is controlled by the engine controller90A. Other features of the configuration, including the engine controller90A, are the same as the previously described first embodiment.

In the third embodiment, the flow rate control valves54ato54dand the gas mixer112constitute the gas mixing unit.

In this embodiment, control of the excess air ratio and temperature control of the gas/air mixture d are performed by similar operations as in the first embodiment. Part of CMM gas c is supplied to the intake pipe18through the fuel gas branch pipe110.

As part of CMM gas c is supplied to the intake pipe18in this embodiment, in addition to the advantages effects of the first embodiment, the flow rate of CMM gas c supplied from the fuel gas supply pipe44to the intake branch pipes42ato42dis reduced. Therefore, the intake branch pipes42ato42dand associated devices such as flow rate control valves54ato54dcan be made small so that they do not require much installation space, which will lead to an advantage of lower facility cost.

INDUSTRIAL APPLICABILITY

The power generating gas engine of the present invention can reduce greenhouse gas emissions through effective use of methane gas, and ensure stable combustion with an appropriate excess air ratio.