REGULATION METHOD OF A PREMIX GAS BURNER AND CONTROL AND REGULATION DEVICE FOR CARRYING OUT THE METHOD

Abstract

A regulation method of a premixed gas burner substantially and/or essentially fed by hydrogen H2 as well as a device to carry out such method is described.

Description

A regulation method of a premixed gas burner is described below.

A control and regulation device for a premixed gas burner is also described.

The hereinafter described method and device are suitable for being used for modulable power burners.

In particular, the hereinafter described method and device are suitable for carrying out the combustion of a premixed gas wherein the fuel gas is hydrogen.

The method and the device hereinafter described are used, in particular, in hydrogen-fed boilers for the production of hot water, for civilian uses.

In the production of hot water, for civilian uses, it is known to use gaseous fuels, typically light hydrocarbons, such as methane (CH₄).

To contain emissions of nitrogen oxides (NOx) it is known to resort to the premixing of the fuel gas with the combustion air.

To obtain a complete combustion of the fuel gas (and to minimise the emission of pollutants) it is also known to provide a quantity of air higher than the stoichiometric air, i.e. to work with excess air. In this regard, the air excess factor λ is defined as the pure number that defines the ratio between the actual air/fuel ratio of the mixture with respect to the stoichiometric air/fuel ratio.

However, an air excess λ leads to a reduction in the efficiency of the heat generator that uses the burner.

In the case of the combustion of light hydrocarbons, a good compromise to minimise the emission of pollutants without excessively penalising the loss of throughput, is obtained with an excess air factor λ having a value of about 1.25-1.35.

However, the use of light hydrocarbons (for example methane) as a fuel still entails an important pollution problem, represented by carbon dioxide emissions.

The use of hydrogen as a fuel gas, produced from renewable sources, seems a promising solution to reduce pollutant emissions from gas boilers.

However, hydrogen combustion is very different from that of light hydrocarbons.

In particular, the hydrogen molecule has a much higher combustion speed than the light hydrocarbon molecules (indicatively, the flame propagation speed of hydrogen is about seven times higher than the flame propagation speed of methane).

The high propagation speed of gas combustion causes a much greater risk of flashback than in the combustion of fuel gases without, or with low, hydrogen content.

In the case of hydrogen combustion, the flashback phenomenon may have consequences even worse than in the combustion of other fuel gases.

In extreme cases, the flashback of a burner that burns hydrogen may cause an explosion that may damage the burner itself and the entire appliance.

The inventors observe that in the combustion of hydrogen the risk of flashback is particularly significant at the time of air/gas mixture ignition.

The purpose of the inventors is to propose a solution that allows preventing, at least in part, the problems of the prior art.

In particular, an object of the inventors is to propose a solution for reducing the risk of flashbacks in the combustion of premixed hydrogen at the time of ignition of the burner.

A further object of the inventors is to propose a solution that allows managing a premixed hydrogen burner in a safe manner, also enabling to modulate the power with a wide range.

These and other objectives are achieved by means of a method for managing a premixed gas burner according to the provisions of the independent claim 1 and by means of a control and regulation device of a premixed gas burner according to the provisions of claim 6.

Further advantages may be obtained by means of the additional features of the dependent claims.

A possible example of a regulation method of a premix gas burner and of a device for controlling and regulating a premixed gas burner are hereinafter described with reference to the attached drawing tables wherein:

FIG. 1 is a diagram showing the regulation of the excess air in a burner ignition step;

FIG. 2 is a diagram showing the relation between the temperature of a burner, the thermal power and excess air;

FIG. 3 is a schematic view of a control and regulation device of a premixed burner wherein the mixing chamber is located downstream of the fan;

FIG. 4 is a schematic view of a control and regulation device of a premixed burner wherein the mixing chamber is placed upstream of the fan.

With reference to the attached drawing tables, FIG. 1 shows how the regulation of a premixed burner 4 occurs in the ignition step thereof, according to the invention.

Initially (section O-A of the diagram of FIG. 1) a fan 2 is set in motion at a predetermined rotation speed Vp to create an aeriform flow F_a that activates the burner 4.

Subsequently (section A-B) the aeriform flow F_a is added with a fuel gas flow F_g, mainly and/or essentially composed of hydrogen H₂, so as to obtain an air and fuel gas mixture M_ag with an excess air factor λ≥2.5.

In a possible embodiment, a sensor for the volumetric concentration of hydrogen in the air and fuel gas mixture M_ag (better described later) may be used to ensure that the excess air factor λ is the desired one.

It should be noted that the ignition of burner 4 may only take place once the excess air factor λ has taken the desired value.

The air and fuel gas mixture M_ag escaping from the holes of the burner 4 is ignited by means of an ignition device 5, for example a conventional ignition electrode 5. After the ignition, the air and fuel gas mixture M_ag continues maintaining a value of λ≥2.5 for a stabilisation time ts≥5 seconds (section B-C) of the flame.

The inventors have verified that carrying out the ignition of an air and fuel gas mixture M_ag with an excess air value λ≥2.5 and maintaining such excess air value λ for at least 5 seconds allows drastically reducing the risk of flashbacks at the time of ignition of the burner 4.

After the flame stabilisation time has elapsed, the excess air factor λ is progressively reduced, for example in a linear manner, until reaching a λ_target value of 1.3<λ<2.5.

Preferably the excess air factor λ drops to a value λ_target of 1.5<λ<2.0 (section C-D-E of the diagram in FIG. 1).

Once the flame stabilisation time has elapsed and after the reduction of the excess air value λ to the value λ_target, the air and fuel gas mixture M_ag may begin according to the required thermal power.

During the step of its normal operation, the temperature T_b of the burner 4 is cyclically monitored, for example at time intervals Δt, to check whether the temperature of the burner 4 remains within predetermined limits, corresponding to a maximum temperature T_sup (above which a flashback may occur) and at a minimum temperature T_inf (below which there is a risk of flame lift-off).

In defining the minimum T_inf and maximum temperature T_sup within which the burner 4 must remain, both the excess air value λ and the thermal power at which the burner 4 works is taken into account.

More precisely, with the same excess air λ, the minimum T_inf and maximum temperature T_sup values decrease monotonously as the working thermal power increases.

In the absence of operating anomalies, more precisely as long as the temperature T_b of the burner 4 remains within values comprised between the minimum T_inf and maximum temperature T_sup, the regulation of the thermal power takes place with an excess air factor λ_target.

In a possible embodiment, this excess air factor λ_target remains constant; in an alternative embodiment, the excess air factor λ_target varies depending on the thermal power, but always remaining in the range of the excess air values λ_target mentioned above, i.e. 1.3<λ<2.5, preferably 1.5<λ<2.0.

In case the working temperature T_b of burner 4 exceeds the predetermined maximum temperature value T_sup, the excess air factor λ_target is progressively increased by a predetermined value Δλ, for example intermittently, until the burner temperature T_b is brought back 4 below such value T_sup.

On the other hand, when the temperature T_b of the burner 4 drops below a predetermined minimum temperature value T_inf, the excess air factor λ_target is progressively reduced by a predetermined value Δλ, for example intermittently, until the temperature T_b of the burner 4 is brought back above such value T_inf.

The time interval Δt with which the temperature check T_b of the burner 4 and the possible correction of the excess air λ is carried out may vary according to the time constant of the system, more precisely according to the thermal inertia of the burner 4.

In a possible embodiment, such time interval Δt=1 second.

The inventors have verified that the periodic check of the temperature T_b of the burner 4 and any periodic regulation of the excess air λ of the burned mixture M_ag allow keeping the flame stable avoiding, in particular, the risks of flashback when the burner is operated for a long time at reduced powers, without excessively penalising the thermal throughput thereof.

The control and regulation method described above may be performed by means of a device 1, for controlling and regulating the operation of a premixed gas burner 4 suitable for burning a fuel gas substantially and/or essentially consisting of hydrogen H₂.

The device 1 comprises a first duct 11, or combustion air inflow duct F_a, a second duct 12, or fuel gas inflow duct F_g, a variable speed fan 2, having an intake 21, connected to the first duct 11, and a delivery 22.

The speed of fan 2 may vary according to the required thermal power.

The mixing of the air F_a coming from the first duct 11 with the fuel gas F_g coming from the second duct 12 takes place by means of a Venturi tube 3, which may be positioned downstream of the fan (FIG. 3) or upstream of the same (FIG. 4).

The second duct 12 is in communication with the narrow section of the Venturi tube 3, so that the fuel gas F_g is sucked. A conventional motorised (or more generally modulating) valve 7 is provided, for regulating the flow rate of the fuel gas F_g that passes through the second duct 12 and thus for regulating the excess air λ.

A third duct 13, or air mixture and fuel gas outflow duct M_ag is provided, located downstream of the Venturi tube 3 which feeds a burner 4.

The burner 4 may be a conventional burner of the perforated surface type, inserted inside a combustion chamber 41. An ignition device 5 (for example a known ignition electrode) is provided to ignite the air and fuel gas mixture M_ag escaping from the Venturi tube 3 and reaching the burner 4.

A conventional non-return valve 14 may be provided upstream of the burner 4.

A first sensor 61, located inside the first duct 11, allows measuring a physical characteristic of the air flow F_a.

In a possible embodiment, the first sensor 61 is an air mass flow sensor that allows detecting the mass flow rate of the air sucked by the fan 2.

A second sensor 62, located in the Venturi tube 3 or downstream of it, allows measuring the concentration of hydrogen H₂ present in the created air and fuel gas mixture M_ag.

The first and second sensors 61, 62 allow controlling the excess air factor λ of the mixture M_ag before it reaches the burner 4.

A third sensor 63, or temperature sensor, is able to detect the temperature T_b of the burner 4.

A fourth sensor 64 detects the flame presence on the surface of the burner 41.

Such fourth sensor 64 may be an optical sensor able to detect the presence of hydroxyl radicals OH.

Alternatively, said fourth sensor 64 may consist of a low thermal inertia thermocouple, suitable for measuring the temperature of the flame.

In both cases, the fourth sensor 64 serves only to detect the presence of the flame, but does not provide any detail on the quality of the combustion in place.

A regulator 8 is provided that receives the signals in input detected by the sensors 61, 62, 63, 64 and a control signal Sc indicative of the desired thermal power.

The regulator 8 is able to provide an ignition signal to the ignition device 5, a fan speed regulation signal 2 and a motorised valve opening regulation signal 7.

In the example shown, a fifth sensor 65 is provided, located inside the second duct 12, which allows detecting a physical quantity of the fuel gas flow F_g.

In the example shown, the fifth sensor 65 is a mass flow sensor of the fuel gas F_g.

In the illustrated device, the value of a is determined by the measurements of the mass sensor 61 of the air F_a and the hydrogen concentration sensor 62, while the temperature sensor 63 is only used to correct the value of λ if the temperature T_b of the burner 4 takes abnormal values.

The fifth mass sensor 65 of the fuel gas F_g, if present, allows checking whether the information provided by the first two sensors 61, 62 is correct: in other words, the signal provided by said fifth mass sensor 65 may be redundant with respect to the signals already provided by the first two sensors 61, 62, if the fuel gas F_g consists of pure hydrogen H₂; on the contrary, the signal provided to the regulator 8 by this mass sensor 65 is important for the purposes of the correct definition of the excess air value λ, in combination with the aforementioned sensors 61, 62, in the event that the fuel gas F_g consists of a mixture mainly of hydrogen H₂ but with quantities of other gaseous fuel inside, for example methane CH₄.

Claims

1. Regulation method of a premixed gas burner fed with hydrogen H2 that provides the steps of: a) starting a fan at a predetermined speed Vp to create an aeriform flow Fa;b) adding a fuel gas flow Fg, mainly and/or essentially composed of hydrogen H2, to said aeriform flow Fa so as to obtain an air and fuel gas mixture Mag with an excess air factor λ≥2.5;c) igniting said air and fuel gas mixture Mag,d) maintaining the combustion of said air and fuel gas mixture Mag for a stabilisation time ts≥5 seconds,e) progressively reducing the excess air factor λ up to a value λtarget of 1.3<λ<2.5.

2. Method, according to claim 1, comprising the further steps of: f) checking what the required thermal power is,g) regulating the flow rate of said air and fuel gas mixture Mag according to the required thermal power, maintaining the excess air factor λtarget.

3. Method, according to claim 2, comprising the further steps of: h) periodically checking, at each time interval Δt, the temperature Tb of the burner (4): if said burner temperature Tb exceeds a predetermined maximum temperature value Tsup, increasing the excess air factor λtarget by a predetermined value Δλ;while if said burner temperature Tb is lower than a predetermined minimum temperature value Tinf, decreasing the excess air factor λtarget by a predetermined value Δλ,i) periodically returning to point h).

4. Method, according to claim 1, wherein in said point e) the excess air factor λ is progressively reduced to a preferable value κtarget of 1.5<λ<2.0.

5. Method, according to claim 3, wherein in said point h) said time interval Δt is preferably equal to 1 second.

6. Device for carrying out the regulation method of a premixed gas burner, mainly and/or essentially fed by hydrogen H2, according to claim 1, comprising: a) a first duct or combustion air inflow duct Fa,b) a second duct or fuel gas inflow duct Fg,c) a variable speed fan having an intake connected to said first duct, and a delivery,d) a motorised valve to regulate the fuel gas flow rate Fg passing through said second duct,e) a Venturi tube for mixing the air Fa coming from said first duct with the fuel gas Fg coming from said second duct,f) a third duct or outflow duct of an air and fuel gas mixture Mag, located downstream of said Venturi tube,g) at least one burner, in particular a burner of the perforated surface type, connected to said third duct, said burner being inserted inside a combustion chamber,h) an ignition device to ignite said air and fuel gas mixture Mag that escapes from said burner,i) a first sensor or air mass flow rate sensor Fa, located inside said first duct,j) a second sensor or hydrogen concentration sensor H2, located in said Venturi tube or downstream of said Venturi tube,k) a fourth sensor to detect the presence of flame inside said combustion chamber,l) a regulator adapted to receive in input: a signal Sc indicative of the required thermal power,a signal indicative of the air mass flow rate Fa, provided by said first sensor,a signal indicative of the concentration of hydrogen H2 in said air and fuel gas mixture Mag, provided by said second sensor,a signal indicative of the presence of the flame, provided by said fourth sensor,and to provide in output: an ignition signal to the ignition device,a signal for the regulation of the speed of said fan, anda signal for the regulation of the opening of said motorised valve.

7. Device according to claim 6, further comprising a third sensor or temperature sensor, capable of detecting the temperature Tb of said burner, and wherein said regulator comprises an input adapted to further receive a signal indicative of the temperature Tb of the burner,and to provide in output: a signal for the speed regulation of said fan,a signal for the regulation of the opening of said motorised valve, anda signal for the regulation of the thermal power.

8. Device according to claim 6, further comprising a fifth sensor, or mass flow rate sensor of the fuel gas Fg, located inside said second duct, and wherein said regulator comprises an input adapted to further receive a signal indicative of the mass flow rate of the fuel gas Fg.

9. Device according to claim 6, wherein said fourth sensor is an optical sensor or a low thermal inertia thermocouple.

10. Method according to claim 2, wherein in said point g) said excess air factor λtarget remains constant.

11. Method according to claim 2, wherein in said point g) said excess air factor λtarget varies depending on the thermal power, always remaining in the range of said excess air values λtarget.