WO2023280923A1

WO2023280923A1 - Premix gas burner system and method

Info

Publication number: WO2023280923A1
Application number: PCT/EP2022/068751
Authority: WO
Inventors: Geert Folkers; Gerben VAN VLIET; Camillo Marino Josef Hogenbirk; Marc BUS
Original assignee: Bekaert Combustion Technology B.V.
Priority date: 2021-07-06
Filing date: 2022-07-06
Publication date: 2023-01-12
Also published as: NL2028637B1; EP4367442A1

Abstract

There is provided a premix gas burner system comprising a burner, a mixing chamber, and a control system. The mixing chamber is arranged to receive a flow of air and a flow of combustible gas to create a mixture. The combustible gas comprises at least 80% hydrogen. The burner has a surface forming a burner deck comprising burner deck portions and a separation surface arranged between the burner deck portions. The burner deck portions are adapted to combust the mixture in reaction zones extending over the burner deck portions. The burner deck portions are arranged relative to each other to prevent the reaction zones from extending over the separation surface. The control system is adapted to control the flow of air and the flow of combustible gas to modulate the burner to operate at a minimum load, an intermediate load, and a maximum load.

Description

Premix gas burner system and method

The invention relates to a premix gas burner system and a method for operating the premix gas burner system. In particular, the premix gas burner system is for combusting hydrogen.

Known premix gas burners systems receive a mixture of air and a fossil gas, such as methane (CH4), ethane (C2H6) or propane (C3H8). Premix burners have the advantage that the mass ratio of air and gas for the air-gas mixture can be set prior to igniting the mixture. As a result, a premix burner generate heat more efficiently than a non-premix burner.

The heat generated by the gas burner is then, for example, used to heat a building. To this end, the burner may be arranged in a heating appliance, which further comprises a heat exchanger. The heat exchanger comprises a channel that is filled with a fluid, typically water. The heat that is generated by the gas burner heats the water in the heat exchanger. This water is then circulated through the building in order to heat the building or parts thereof.

A known gas burner is described in European patent application no. EP2037175A2. The known gas burner has a tubular body provided with a plurality of small through openings. The through openings are made along the side surface of the tubular body. A flange is arranged at the base of the tubular body. An air-gas-mixture enters the gas burner via an inlet opening in the base of the tubular body and exits the gas burner via the through openings. After the gas exits the through openings, the gas is combusted.

A disadvantage of burning a fossil gas is that the carbon molecules in the gas combine with oxygen molecules to form carbon dioxide (CO2). Carbon dioxide is a greenhouse gas that contributes to climate change. A way to prevent the creation of carbon dioxide is to use hydrogen (H2) instead of a fossil gas. Hydrogen does not comprise carbon molecules, so no carbon dioxide is created when hydrogen is burned.

Simply providing hydrogen to the known gas burner would not be successful.

Hydrogen has a much larger combustion velocity than for example methane under most conditions. The combustion velocity is the rate of expansion of a flame front in a combustion reaction. The combustion velocity of hydrogen is between 20 and 350 cm/s, whereas the combustion velocity of methane is between 5 and 40 cm/s. When using hydrogen in the known gas burner, the high combustion velocity causes the flame of the hydrogen to move back into the gas burner through the openings. This effect is called flash-back. Flash-back may result in an explosion that causes overheating or overpressure, leading to damage to the gas burner or other components in the heating appliance. The known gas burner is typically used at different power levels. When there is a large demand for heat, the known gas burner operates at a high load. When there is only a little demand for heat, the known gas burner operates at a low load. For the high load, the flow rate of the combustible gas through the gas burner is higher than at the low load. Operating a gas burner at different loads is referred to as modulation.

However, when attempting to modulate the known gas burner with hydrogen, the high combustion velocity of the hydrogen causes the combustion reaction, i.e. , the flame, to move toward the burner deck when modulating at low loads. The result is a flash back and/or an unacceptable high temperature of the gas burner because the flame is on the burner deck or very close to the burner deck.

A similar problem occurs when stopping operation of the gas burner. When stopping the flow of the mixture, the flow rate of the mixture reduces. A typical mixture of air and methane has a ratio of 12:1, which means the volume of air in the mixture is 12 times the volume of methane. A typical mixture of air and hydrogen has a ratio of 3:1, which means the volume of air in the mixture is 3 times the volume of hydrogen. Stopping the hydrogen in the mixture reduces the flow rate with 25%, whereas stopping the methane would only reduce the flow rate with about 8%. When stopping the flow of hydrogen, the large decrease of the flow rate causes the flame, which is still present, to move towards the burner deck, which may cause flash back.

It is the objective of the invention to provide an improved gas burner for the combustion of hydrogen.

The objective is achieved by a premix gas burner system comprising a burner, a mixing chamber, and a control system. The mixing chamber is arranged to receive a flow of air and a flow of combustible gas to create a mixture. The combustible gas comprises at least 80% hydrogen. The burner is arranged to receive the mixture. The burner has a surface forming a burner deck comprising burner deck portions and a separation surface arranged between the burner deck portions. The burner deck portions are adapted to combust the mixture in reaction zones extending over the burner deck portions. The burner deck portions are provided with holes to provide the mixture to the reaction zones. The burner deck portions are arranged relative to each other to prevent the reaction zones from extending over the separation surface. The control system is adapted to control the flow of air and the flow of combustible gas to modulate the burner to operate at a minimum load, an intermediate load, and a maximum load. The maximum load is at least three times larger than the minimum load. The intermediate load is higher than the minimum load and lower than the maximum load. The control system is adapted to operate the burner with an exit velocity of the mixture exiting from the holes to the reaction zones. The control system is adapted to operate the burner with a velocity ratio between the exit velocity and a combustion velocity of the mixture that is larger at the minimum load than at the intermediate load.

According to the invention, the combustible gas is burned in reaction zones that are separated from each other by the separation surface. The holes through which the mixture flows out of the burner deck are only located in the burner deck portions, whereas there are no holes in the separation surface through which the mixture flows. This creates a higher exit velocity of the mixture compared to a gas burner that has no separation surface. The exit velocity is the velocity at which the mixture exits the holes. The high exit velocity of the mixture helps to maintain the reaction zone, i.e. , the flame, at a desired distance from the burner deck. When modulating from the intermediate load to a lower power level, the control system reduces the amount of hydrogen in the mixture, and increases or maintains the amount of air in the mixture to increase the velocity ratio between the exit velocity and the combustion velocity of the mixture. As a result of the increased velocity ratio the flame is kept at a desired distance from the burner deck. The reduced combustion velocity helps keeping the flame at the desired distance from the burner deck.

The combustion velocity of the mixture depends on the air-to-gas ratio l, the composition of the combustible gas, and the composition of the air, such as the amount of water vapor in the air, as well as the pressure and the temperature of the mixture. In an example, the combustion velocity is considered to depend only on the air-to-gas ratio l, because the air-to-gas ratio l has a relatively large contribution to the combustion velocity. The control system is adapted to operate the burner taking into account the combustion velocity of the mixture based on, for example, only the air-to-gas ratio l. The control system has, for example, a sensor to provide information about the air-to-gas ratio l. In another example, the control system is adapted to determine the air-to-gas ratio l based on a drive signal, for example to a valve for the flow of air, to a valve for the flow of gas, or to a fan that creates the flow of air. In an example, the control system comprises one or more sensors to obtain information about the flow of air, the flow of combustible gas, the composition of the combustible gas, and/or the composition of the air.

An excessive increase of the temperature of the burner deck is prevented, because the flame is at a distance from the burner deck, even at a low load. Further, because the flow of the relatively cool mixture is concentrated via the holes in the burner deck portions, and because there is no reaction zone above the separation surface, additional cooling is provided to the burner deck.

Because of the distance between the burner deck and the flame, and because of the additional cooling, the temperature of the burner deck is sufficiently low to prevent auto- ignition of the mixture inside the gas burner. Auto-ignition is very undesirable, because it may lead to damage of the gas burner. Hydrogen auto-ignites far more easily than for example methane. A temperature of 600°C of the gas burner may be sufficient to auto-ignite the hydrogen inside the gas burner. For methane, the temperature of a gas burner may be as high as 900°C. Also, the low temperature allows the use of materials that have less heat resistance to form the burner deck. Such materials are typically less expensive than materials which are resistant to high temperatures.

When stopping operation of the gas burner according to the invention, the load of the gas burner is reduced. While reducing the load, the amount of combustible gas is reduced, while the amount of air is increased to prevent flashback. When the control system completely stops with providing combustible gas to the gas burner, the control system provides sufficient air to the gas burner to keep the reaction zone at a desired distance from the burner deck until the reaction zone disappears due to the lack of combustible gas.

The gas burner, which will be further referred to as burner, is a body with holes through which the mixture flows. The side of the body at which the mixture is combusted is the burner deck. For example, the burner is a cylindrical burner. The mixture is provided into the burner via an inlet of the cylindrical burner. The mixture exits the cylindrical burner via the holes arranged on a lateral surface of the cylindrical burner. Optionally, the mixture exits the cylindrical burner via additional holes on an end surface of the cylindrical burner opposite to the inlet. In an example, the burner is a flat burner or a bowl-shaped burner having two opposite sides. The mixture flows from one side via the holes to the other side that forms the burner deck. The burner is made from a metal or a ceramic.

A premix gas burner is a type of burner that receives a mixture of air and a combustible gas, and combusts the mixture. The ratio of air and the combustible gas is set as desired to obtain a desired combustion. No additional air from outside the gas burner is added to the reaction zone.

The premix gas burner is a different type of burner than an atmospheric gas burner. In an atmospheric gas burner, the combustible gas exists holes in the burner deck. After the combustible gas exists the holes, the combustible gas is ignited. The holes in the burner deck are arranged to allow air from outside the gas burner to be drawn to the reaction zone. At the reaction zone, the combustible gas and the air from outside the gas burner are combusted. The atmospheric gas burner may have venturi tube to draw in some air into the gas burner, to mix the combustible gas with that air. However, even with a venturi tube, the holes still need to be arranged to allow air from outside the gas burner to be drawn to the reaction zone to provide sufficient air for a complete combustion. In case the atmospheric gas burner has a venturi tube, the air drawn in by the venturi tube is referred to as primary air, whereas the air from outside the gas burner drawn to the reaction zone is referred to as secondary air. The secondary air does not flow through the holes in the burner deck, but flows only on the side of the burner deck.

The premix gas burner and the atmospheric gas burner are very different types of gas burners. The atmospheric gas burner is typically less complex, because no components are needed to create the mixture with the desired ratio of combustible gas and air. The atmospheric gas burner has a relatively large burner deck, because space needs to be provided for the secondary air to reach the reaction zones. Also, the heating appliance needs to be adapted to allow the secondary air to reach the reaction zones inside the heating appliance. Compared with premix burners, atmospheric gas burners are typically less efficient and produce more NOx, like nitric oxide and nitric dioxide.

The premix gas burner, on the other hand, has a very high efficiency, because the mixture is created with the desired ratio of combustible gas and air to obtain the most efficient combustion. Further, the burner deck of the premix gas burner can be made very compact, because no secondary air needs to reach the reaction zones. As a result, a premix gas burner with a certain maximum power output is significantly smaller than an atmospheric gas burner with the same maximum power. The premix gas burner is typically used for high power appliances with a high efficiency. By creating the mixture with the desired ratio of combustible gas and air, a premix gas burner typically produces less NOx than an atmospheric gas burner.

The burner deck is a most outer surface of the burner. The burner deck is not enclosed by any part of the burner. After the mixture has passed the holes in the burner deck, the mixture does not flow through any other part of the burner. The reaction zones do not extend through any part of the burner, because the reaction zones extend over the burner deck portions, which are on the most outer surface of the burner. The mixture is combusted after the mixture has passed through the holes in the burner deck portions. Combusting the mixture before the mixtures has entered the holes, i.e. , when the mixture is still inside the burner, would cause an instable combustion, an unacceptable high temperature of the burner deck and/or a high risk of flash back.

Optionally, the burner is provided with a mesh, such as a nit. A mesh is formed from metal wires. A nit has the metal wires arranged in a knitted fashion, i.e., arranged in interconnected loops. The mesh is arranged as the most outer surface of the burner and forms the burner deck. The holes are formed in the mesh. Any structure with holes enclosed by the nit is not the burner deck, because such a structure is not the most outer surface of the burner. For example, such a structure is a distributor. A distributor is arranged inside the burner to distribute the mixture along the holes, so the flow rate of the mixture is the same or substantially the same through all holes in the burner deck portions. Without such a distributor, the mixture may flow at a high flow rate through holes in one part of the mesh, and with a low flow rate through holes in another part of the mesh. The distributor has a higher flow resistance than the holes in the burner deck portions to evenly distribute the pressure of the mixture inside the burner. Because of the evenly distributed pressure, the flow rate of the mixture is the same or substantially the same for all holes in the burner deck portions.

The mixing chamber receives the flow of air and the flow of combustible gas and mixes them to create the mixture. The mixing chamber is, for example, simply an empty chamber that provides a space for the air and the combustible gas to mix. The velocity with which the air and/or the combustible gas enters the mixing chamber may be sufficient to create the mixture. The mixing chamber has, for example, one or more bodies to improve mixing of the air and the combustible gas. For example, such bodies include stationary fins, or, pins, or funnels, or swirl creators. The mixing chamber accommodates, for example, a moveable component, such as a fan or a stirrer to create the mixture. The moveable component, for example, is driven by a motor. The mixing chamber is, for example, arranged outside the burner, partly inside the burner or completely inside the burner.

The burner deck portions are provided with holes to provide the mixture from the mixing chamber to the reaction zones. The holes are large enough to allow the mixture to pass through the holes at an acceptable pressure loss, but small enough to prevent flashback. The holes are for example round or slot-shaped or polygonal-shaped. The holes are for example round holes with a diameter in the range of 0.4 - 2.0 mm, for example 0.7 - 1.0mm, such as 0.8 mm. The holes are for example rectangular shaped holes with a length in the range of 0.5 - 2.0 mm, for example 0.7 - 1.0 mm, such as 0.8 mm, wherein the width is smaller than the length. For example, a burner deck portion comprises at least three holes, for example five or nine or sixteen holes. For example, each burner deck portion comprises at least three holes, for example five or nine or sixteen holes. By providing at least three holes, the stability of the reaction zone is improved.

The burner deck portions are larger than the surface defined by the position of the holes. The size of the burner deck portions is defined by the size of the reaction zone extending over the burner deck portion. The holes of a burner deck portion are arranged at one or more pitches. The pitch is a distance between two adjacent holes in a burner deck portion. The burner deck portion extends beyond half an average pitch from a hole near the edge of the burner deck portion. All burner deck portions have the same arrangement of the holes or at least one burner deck portion has a different arrangement of the holes than other burner deck portions.

The burner deck portions cover large portion of the burner deck. For example, the burner deck portions cover more than 50% or more than 60% or more than 80% of the burner deck. For example, the burner deck comprises a large number of burner deck portions, such as more than 20 burner deck portions, for example more than 50 or more than 100 burner deck portions. In case the burner deck is covered for a large portion with the burner deck portions and when there are a large number of burner deck portions, the burner is able to provide a large amount of thermal power while providing sufficient cooling to the burner deck.

The separation surface is the surface of the burner deck in between the burner deck portions. The separation surface separates the burner deck portions from each other, so as to prevent the reaction zones from different burner deck portions to come into contact with each other. No reaction zone extends over the separation surface, which means that the separation surface is not covered with a reaction zone. There is not a reaction zone directly in contact with the separation surface, nor does the separation surface face a reaction zone.

So, each burner deck portion comprises multiple holes which are adapted to provide the mixture to the reaction zone which extends over that particular burner deck portion. Due to the separation surface between the burner deck portions, the reaction zones that are associated with different burner deck portions are separated from each other. As a single reaction zone receives the mixture from a plurality of holes, inevitably the smallest heart-to- heart distance between holes in the same burner deck portion is smaller than the smallest heart-to-heart distance between a first hole in a first burner deck portion and a second hole in a second burner deck portion, which second burner deck portion is adjacent to the first burner deck portion.

In this arrangement, a flame base is present at each one of the holes that are associated with the same burner deck portion. From each flame base, a flame extends from outwards from the burner deck. At some distance from the burner deck, the flames that are associated with the holes of the same burner deck portion merge and/or interact with each other to form a single reaction zone. The holes that are associated with the same burner deck portion must be relatively close together, so the flames of these holes can interact with each other to form the single reaction zone.

An advantage of this arrangement with multiple holes feeding a single reaction zone is that this way, the reaction zone appears to be anchored to the burner deck in a stronger way, which makes that the reaction zone is more stable and for example less susceptible to blow- off. This allows a higher exit velocity of the mixture, and therewith increases the cooling of the burner deck by the mixture flowing through the holes, which may result in a lower burner deck temperature. This allows to operate the premix gas burner within a large range of operating conditions.

The control system is adapted to control the flow of air and the flow of combustible gas to operate the burner at the desired load. The control system is adapted to provide sufficient air in relation to the amount of combustible gas to create at least a stoichiometric combustion. For example, the control system comprises a valve arranged in a pipe providing the flow of gas. The control system is adapted to set the flow of combustible gas by setting the valve. The control system is able to stop the flow of combustible gas by closing the valve. The control system is able to operate the burner at the maximum load by completely opening the valve. For example, the control system comprises a valve arranged in a pipe providing the flow of air. The control system is adapted to set the flow of air gas by setting the valve. In an example, the control system is adapted to control the rotational speed of a fan. By increasing the rotational speed of the fan, the control system is able to increase the flow of air. By decreasing the rotational speed of the fan, the control system is able to decrease the flow of air. The control system is, for example, adapted to receive an input signal representative of a desired heat load. The control system adjusts the flow of air and the flow of combustible gas to achieve the desired heat load.

Even though the control system is adapted to control the velocity ratio, the velocity ratio may vary in operational use, for example, due to a change in temperature of the mixture. A higher temperature of the mixture may, for example, increase the exit velocity. In an example, the control system is adapted to take into account the temperature of the air, the temperature of the combustible gas and/or the temperature of the mixture when controlling the velocity ratio.

The flow of air contains, for example, outside air. For example, an air inlet collects outside air which flows via a duct to the premix gas burner system. The flow of air contains oxygen to react with the combustible gas. The flow of air may contain, besides oxygen, other gasses such as nitrogen, water vapor, carbon dioxide and inert gasses. The flow of air may contain part of the flue gas created by the reaction zones, for example to increase the temperature of the flow of air. Due to the composition of the flue gas, the amount of oxygen in the flow of air decreases. Due to the high temperature of the flue gas, the density of the flow of air decreases. For example, the flow of air comprises between 1-10% flue gas, for example, 1-5%. The flue gas in the flow of air is also referred to as recirculation gas, i.e., flue gas that is recirculated through the burner. The flow of air comprises, for example, air that is heated by the burner. In this example, the flow of air comprises air with an ambient temperature and air with a higher temperature. In this example, the density of the flow of air is high when the burner starts to operate, because no heated air is yet available. When the burner has been operating some time, the heated air causes the temperature of the flow of air to increase, which reduces the density of the flow of air.

The flow of combustible gas comprises hydrogen, for example at least 80% or at least 90% or at least 95% hydrogen. The percentage indicates the amount of volume of hydrogen in the combustible gas. In an example, the combustible gas is 100% hydrogen, with the exception of impurities and/or optionally an odorant and/or colorant and/or a small residue of a fossil gas like methane or ethane. Besides hydrogen, the combustible gas comprises, for example, a fossil gas such as methane or propane. Combusting a mix of hydrogen with a fossil gas creates significantly less carbon dioxide compared to combusting a fossil gas that is not mixed with hydrogen. It is possible that during operational use of the burner, the composition of the combustible gas changes. For example, the percentage of hydrogen in the combustible gas changes and/or the percentage of the fossil gas changes. For example, the premix gas burner system receives the combustible gas via a gas company. During a cold winter day, the gas company may not have sufficient hydrogen available because of the high demand of combustible gas. The gas company therefore decreases the percentage of hydrogen and increases the percentage of the fossil gas in the combustible gas. Depending on the composition of the combustible gas, the combustion velocity of the combustible gas may be different. The burner is adapted to safely combust the combustible gas in a range of compositions.

With “load” (expressed in kilowatt [k W]) is meant the quantity of energy supplied to the burner in a unit of time; the quantity of energy equals the mass flow rate multiplied with the calorific value of the combustible gas per unit of mass.

The maximum load is the highest load at which the premix gas burner system is designed to operate. For example, the maximum load is 24 kW for a premix gas burner system that is for use in a house. In an example, the maximum load is 100 kWfor a premix gas burner system that is for use in an apartment building or an office building. In another example, the maximum load is 5 kWfor a portable premix gas burners system.

The minimum load is the lowest load at which the premix gas burner system is designed to operate. The premix gas burner system is able to maintain the power output at the minimum load. The power output of the burner may be temporarily lower than the minimum load when the burner is being shut down. However, this lower power output is not considered the minimum load, because this power output cannot be maintained by the premix gas burner system. The minimum load is higher than zero kilowatt.

The maximum load is at least three times larger than the minimum load. This means, for example, that for a maximum load of 24 kW, the minimum load is 8 kW or less. For a maximum load of 100 kW, the minimum load is 33 kW or less. For a maximum load of 5 kW, the minimum load is 1.66 kW or less. When maximum load is three times larger than the minimum load, the modulation is 1:3. In an example, the premix gas burner is adapted to provide a maximum load that is five times or ten times larger than the minimum load. In this example, the modulation is 1:5 or 1:10.

The intermediate load is in between the minimum load and the maximum load. For example, the maximum load is 36 kW, the minimum load is 4 kW and the intermediate load is 8 kW. For example, the maximum load is 25 kW, the minimum load is 4 kW and the intermediate load is 6 kW. For example, the maximum load is 25 kW, the minimum load is 8 kW and the intermediate load is 10 kW. The flow of air and the flow of combustible gas are mixed in the mixing chamber to create a mixture. The control system is adapted to control the amount of air and/or the amount of combustible gas that flows into the mixing chamber to control the composition of the mixture. With “air-to-gas ratio” is meant the ratio of the amount of air in the mixture of air and combustible gas, relative to the theoretically stoichiometrically required amount of air for full combustion of the combustible gas. Preferably, the mixture has more air than required to obtain a stoichiometric combustion. The air-to-gas ratio for a stoichiometric mixture is l=1. Because the mixture preferably has more air than required to obtain a stoichiometric combustion, the air-to-gas ratio is preferably l>1. The air-to-gas ratio l may be alternatively referred to as excess-air-ratio. By creating a mixture with a l>1, the mixture is efficiently combusted, because all or substantially all the combustible gas is combusted. The flue gas contains no or hardly any combustible gas, whereas the flue gas contains excess air. Experiments has shown that for 1 <l<4, for example 1< l <3.5, the premix gas burner system is able to create stable reaction zones without flashback for the minimum load, the maximum load, and the loads in between. Increasing to an air-to-gas ratio l > 4 leads to a low temperature in the reaction zone at which in some cases not all hydrogen is combusted anymore. The control system sets the air-to-gas ratio l to a higher value at the minimum load than at the intermediate load. For example, the air-to-gas ratio l is 1.0 or 1.1 or 1.2 or 1.5 at the intermediate load, whereas the air-to-gas ratio l is 2.5 or 3.0 or 3.5 or 4.0 at the minimum load.

In an embodiment, a combined surface area of the holes is more than 1.0% and less than 7.0% of the surface area of the burner deck, preferably less than 6.0% or less than 5.0%.

According to this embodiment, the combined surface area of the holes is the sum of the surface areas that the holes occupy on the burner deck. Because more than 1.0% of the surface area of the burner deck is formed by the holes, there is enough surface through which combustible gas can flow to create stable reaction zones. By having less than 7.0% of the surface area of the burner deck formed by the combined surface area of the holes, the combustible gas exits the gas burner at a higher exit velocity than if more than 7.0% of the surface area of the burner deck were formed by the combined surface area of the holes. The exit velocity is sufficiently high to match the high combustion velocity of the combustible gas. Wth less than 7.0% of the surface area of the burner deck formed by a combined surface area of the holes, experiments have shown that a stable combustion of the combustible can be achieved, even when modulating the burner, i.e. when changing the power level of the burner. The burner deck includes the burner deck portions and the separation surface between the burner deck portions. The burner deck does not include a blank surface extending from an end of burner to the burner deck. The blank surface does not have holes and is not arranged between burner deck portions. The blank surface is provided to place the burner deck at a desired position on the burner.

By having the surface area of the burner deck that is formed by a combined surface area of the holes in the range of 1.0% - 7.0%, the reaction zone is stable and there is little risk that the combustion of the combustible gas will stop unexpectedly.

In an embodiment, the exit velocity at the minimum load is higher than at the intermediate load.

According to the embodiment, the control system is adapted to provide the mixture with a higher exit velocity at the minimum load than at the intermediate load. For example, the control system is adapted to provide a larger flow of air at the minimum load than at the intermediate load to create the higher exit velocity at the minimum load. By increasing the exit velocity, the flame is kept at a desired distance from the burner deck. This prevents the burner deck from becoming too hot.

In an embodiment, the burner deck portions comprise a first burner deck portion, a second burner deck portion and a third burner deck portion. The first burner deck portion is separated from the second burner deck portion by the separation surface in a first direction. The first burner deck portion is separated from the third burner deck portion by the separation surface in a second direction. The first burner deck portion is adjacent to the second burner deck portion and the third burner deck portion. The first direction and the second direction are different from each other. Optionally at least one of the burner deck portions comprises at least three holes.

In an embodiment, a hole of the first burner deck portion and a hole of the second burner deck portion that are closest to each other along the first direction are separated from each other by a first distance in the first direction. A hole of the first burner deck portion and a hole of the third burner deck portion that are closest to each other along the second direction are separated from each other by a second distance in the second direction. The sum of the first distance and the second distance is at least 15 mm.

According to this embodiment, the burner has sufficient cooling while still properly combusting the mixture when the sum of the distances of a hole in one burner deck portion relative to the closest holes in adjacent burner deck portions are at least 15 mm. There is a minimum distance between the holes of adjacent burner deck portions created by the separation surface. The inventors have discovered that a distance between two adjacent burner deck portions may be different in the first direction than a distance between two adjacent burner deck portions the second direction, for example, perpendicular to the first direction.

In an embodiment, each of the first distance and the second distance are at least 7.5 mm, for example, at least 10 mm or 15 mm or 20 mm. In an embodiment, the velocity ratio between the exit velocity and a combustion velocity of the mixture varies over the range of input power over which the premix gas burner operates. Thus, as seen over this input power range, at some point of this input power range, the velocity ratio has a minimum velocity ratio. The exit velocity is determined at the holes of the burner deck portions.

Optionally, in this embodiment, the minimum velocity ratio between the exit velocity and a combustion velocity of the mixture (as seen over the input range) is 12 or less, preferably 9 or less. Optionally, the minimum velocity ratio between the exit velocity and a combustion velocity of the mixture (as seen over the input range) is 7 or less, e.g. 4.8 or less. Tests have shown that the premix gas burner according to the invention works well under practical conditions with such velocity ratios.

In an embodiment, at least one burner deck portion comprises a two-dimensional pattern of holes. For example, the holes in this burner deck pattern are arranged in a rectangular pattern, a triangular pattern, a circular pattern or a star-shaped pattern.

Optionally, the majority of the burner deck portions comprises a two-dimensional pattern of holes, or even all of the burner deck portions comprise a two-dimensional pattern of holes. There are indications that the two-dimensional pattern provides more stable flame and/or less risk of blow-off.

In an embodiment, the premix gas burner system comprises a metal plate having the surface forming the burner deck. An equivalent diameter of at least one of the holes is larger than a thickness of the metal plate.

According to this embodiment, the burner deck is formed on a metal plate. The metal plate has a thickness that is smaller than an equivalent diameter of at least one of the holes. The equivalent diameter is the diameter of a round hole that has the same surface area as the hole. For example, a rectangular hole with a cross-section of 0.50 mm², has the equivalent diameter of a circle with the same cross-section of 0.50 mm², i.e. , an equivalent diameter of 0.8 mm. The equivalent diameter is along the surface forming the burner deck. For example, each hole has an equivalent diameter that is larger than the thickness of the metal plate. The thickness of the metal plate is for example less than 2 mm, for example less than 1 mm, for example less than 0.5 mm. Because of the arrangement of the burner deck portions and the separation deck, and because the control system operates the burner with an exit velocity that is larger at the minimum load than at the intermediate load, the temperature of the burner deck remains at an acceptable low level, and flash back is prevented. The low temperature allows the use of a thin metal plate to form the burner deck, which results in a small and cost-effective burner. If the temperature would be too high, the thin metal plate would cause the hydrogen inside the burner to auto-ignite. If the exit velocity would be too low, the reaction zone would be able to expand through the short depth of the holes. In comparison, in the prior art, thick ceramic burners are used. The thickness of such ceramic burners is typically more than 10 mm or more than 20 mm. The thickness of the ceramic burner provides thermal insulation to prevent auto-ignition, whereas the long depth of the holes through the thick ceramic burner provides flow resistance to prevent flash back. The metal plate may have any suitable form, such as a rectangular, disk-shaped, cylindrical, ogive or half-cylindrical.

In an embodiment, the control system is adapted to operate the burner with an air-to- gas-ratio that is inverse proportional with the load between the minimum load and the intermediate load.

According to this embodiment, the control system is adapted to decrease the air-to- gas-ratio when increasing the load from the minimum load to the intermediate load. Between the minimum load and the intermediate load, the relationship between the load and the air-to- gas-ratio is inverse proportional. Inverse proportional is, for example, inverse linear proportional or any other type of function that has only negative derivatives between the minimum load and the intermediate load. By having a larger air-to-gas ratio at the low load than at the intermediate load, the exit velocity at the low load is at a sufficiently high value to prevent flash back. In an example, the minimum load is 5 kW, the intermediate is 7 kW and the maximum load is 25 kW. The air-to-gas ratio at the minimum load is preferably at least 20% higher, even more preferably at least 30%, and even more preferably at least 40% higher than at the intermediate load.

In an embodiment, the premix gas burner system comprises a sensor for providing a signal representative of information about the reaction zones. The control system is adapted to evaluate a relationship between the information and the flow of air. The control system is adapted to adjust the flow of air and/or the flow of gas based on the evaluated relationship.

According to this embodiment, the sensor collects information about the reaction zones. The control system obtains the information and evaluates the information with information about the flow of air. Based on the evaluation, the control system adjusts the flow of air. The information of the sensor indicates, for example, that the combustion in the reaction zones is not optimal, for example, because the air-to-gas ratio is too high or too low. By adjusting the flow of air, the efficiency of the burner is improved.

In an embodiment, the premix gas burner system comprises a fan. The control system is adapted to control a speed of the fan to generate the flow of air. The control system is adapted to evaluate a relationship between the signal and the speed of the fan.

According to this embodiment, the control system determines a property of the reaction zone based on the signal from the sensor. For example, a property that indicates that the reaction zone requires more air or less air to efficiently combust the combustible gas. Based on the signal of the sensor, the control system drives the fan. By increasing the speed of the fan, the flow of air increases and vice versa. The fan is, for example, a radial fan or an axial fan or a centrifugal fan.

In an embodiment, the information about the reaction zones is at least one of a temperature, a concentration of oxygen, a concentration of combustible gas, and ionization signal.

According to this embodiment, the sensor provides information about one or more properties of the reaction zone. The temperature of the reaction zone is a measure, for example, for the efficiency at which the combustible gas is combusted. If the temperature is too high, too much nitric oxide is created. Nitric oxide is an environmental pollutant. If the temperature is too low, not all the combustible gas may be combusted, leading to a waste of the combustible gas and/or to a hazardous situation with unburned gas. The concentration of oxygen is a measure, for example, for the efficiency at which the combustible gas is combusted. If the concentration of oxygen is too high, this may indicate that not all the combustible gas is combusted. If the concentration of oxygen is too low, there may be a blockage in the flow of air or in the flow of the flue gas. By using information about the ionization signal, the control system is able to identify whether combustion takes place in the reaction zone.

In an embodiment, the control system is adapted to stop the flow of gas to the burner. The control system is adapted to increase at least one of the exit velocity and the velocity ratio of the mixture, while reducing the flow of combustible gas prior to stopping the flow of combustible gas.

According to this embodiment, the control system is adapted to stop operation of the burner, for example, in case there is no demand for heat. To stop operation of the burner, the control system stops the flow of gas to the burner, for example, by closing a valve in the conduit providing the combustible gas to the burner. The control system sends, for example, a drive signal to an actuator arranged in the valve. The actuator closes the valve. However, in case the flow of combustible gas would be stopped suddenly, the exit velocity of the mixture would reduce suddenly. The reaction zone would then be able to move towards the burner deck and cause flashback, because the combustion velocity exceeds the exit velocity. Because the control system is adapted to increase the exit velocity, or to increase the velocity ratio, or to increase both the exit velocity and the velocity ratio, while reducing the flow of gas prior to stopping the flow of gas, the exit velocity of the mixture is maintained at a sufficiently high value compared to the combustion velocity of the flame. This way, flashback is prevented or at least the chance that flashback occurs is reduced. For example, the control system increases the flow of air while reducing the flow of combustible gas to increase the exit velocity. In an embodiment, the control system is adapted to increase the exit velocity of the mixture, while reducing the flow of gas less than 5 seconds, preferably less than 2 seconds, more preferably less than 1 second prior to stopping the flow of combustible gas.

According to this embodiment, the control system increases the exit velocity shortly before stopping the flow of combustible gas. Preferably, the duration of increasing the exit velocity is as short as possible, to waste as little energy as possible. However, the duration needs to be long enough to prevent flashback. Experiments have shown that a duration of 1 second is sufficient to reduce the risk of flashback. Depending on operation parameters, such as the accuracy with which the flow of air and the flow of combustible gas are controlled, variations of the composition of the air and/or the combustible gas etc., the duration is preferably extended to a maximum of 2 seconds. The duration of 5 seconds is sufficient to reduce the risk of flashback or to prevent flashback, even in very unfavorable operation conditions. For example, the duration is in the range of 0.5-1 second, or 1-2 seconds or 2-5 seconds. Especially in case the burner is used to heat tap water, and the request for hot water is dictated by the tap, the duration is preferably as short as possible to minimize energy loss. Such short duration is in the range of 0.5-1 seconds or 1-2 seconds.

For example, the burner has a cylindrical shape. The burner deck portions are arranged along a circumference of the cylindrical shape and separated from each other by the separation surface.

According to this example, the cylindrical shape of the burner provides a large burner deck compared to the size of the burner. As a result, the premix gas burner system may be small, while it is able to provide a large maximum load.

In an embodiment, the control system comprises an input terminal to receive gas information about a property of the flow of combustible gas. The control system is adapted to adjust the flow of air and/or the flow of combustible gas based on the gas information.

According to this embodiment, the control system has an input terminal to receive the gas information. The input terminal is, for example, connectable to a data wire to receive the information in the form of a data signal. In another example, the input terminal is connectable to wireless data receiver to receive the data signal, for example via Wi-Fi or Bluetooth. The control system is for example connectable to the internet via the input terminal. The data signal is for example provided by a sensor arranged in a conduit that provides the combustible gas to the burner. The data signal originates, for example, from the gas company providing the combustible gas. The gas company indicates, for example, the composition of the combustible gas, such as the amount of hydrogen in the combustible gas. The input terminal is, for example, connectable with a gas meter through which the combustible gas flows. Based on the information from the data signal, the control system is able to adjust the flow of air and/or the flow of combustible gas to improve the operation of the premix gas burner system.

In an embodiment, the property of the flow of combustible gas is at least one of a gas composition, an amount of hydrogen, a gas pressure, and a temperature.

According to this embodiment, information about one or more properties of the combustible gas are received by the control system. The gas composition influences the combustion velocity of the mixture. For example, the combustion velocity is higher for a composition with a large amount of hydrogen and a small amount of methane, than a composition with a small amount of hydrogen and a large amount of methane. The gas pressure influences the exit velocity of the mixture. A large pressure results in a high exit velocity, whereas a low pressure results in a low exit velocity. The pressure may be low, for example, during cold winter days when there is a high demand for combustible gas. The gas pressure in a gas network may be lower than usual because of the high demand. The temperature influences the density of the combustible gas. By receiving information about the temperature of the combustible gas, the control system is able to estimate a relationship between the volume of the combustible gas and the mass of the combustible gas. In an example, the control system receives information about the temperature, the pressure and the composition to allow accurate control of the burner.

In an embodiment, the premix gas burner system comprises a heat exchanger and a heat exchanger sensor. The heat exchanger is adapted for transferring heat away from flue gas created by the reaction zones. The heat exchanger sensor is adapted to provide information about a property of the heat exchanger, for example temperature. The control system is adapted to adjust the flow of air and/or the flow of gas based on the information about a property of the heat exchanger.

According to this embodiment, the premix gas burner system uses information from the heat exchanger sensor to adjust the flow of air and/or the flow of gas. The heat exchanger is, for example, connectable to a conduit system, wherein the conduits are filled with water. Heat is transferred through the conduit system by circulation of the water. For example, the heat exchanger sensor provides a data signal to the control system representative of a temperature of water flowing into the heat exchanger, a temperature of water flowing out of the heat exchanger, and/or a pressure of water in the heat exchanger. The control system controls, for example, operation of the burner to achieve a desired temperature of the water in the heat exchanger.

In a second aspect of the invention, there is a method for operating a burner. The method comprises the step of - providing a mixture of air and a combustible gas to the burner, wherein the burner has a surface forming a burner deck comprising burner deck portions and a separation surface arranged between the burner deck portions, wherein the combustible gas comprises at least 80% hydrogen, wherein the burner deck portions are adapted to combust a mixture in reaction zones extending over the burner deck portions, wherein the burner deck portions are provided with holes to provide the mixture to the reaction zones, wherein the burner deck portions are arranged relative to each other to prevent the reaction zones from extending over the separation surface, wherein the method further comprises the steps of:

- operating the burner at a high load to generate an amount of thermal energy,

- operating the burner at a low load to generate a smaller amount of thermal energy than at the high load, wherein the high load is at least 3 times the low load,

- operating the burner at an intermediate load that is higher than the minimum load and lower than the maximum load,

- operating the burner with a velocity ratio between an exit velocity and a combustion velocity of the mixture that is larger at the low load than at the intermediate load, wherein the mixture exits from the holes to the reaction zones with the exit velocity.

Optionally, in the method according to the second aspect of the invention, a premix gas burner according to the invention is used.

According to the second aspect, by increasing the velocity ratio between the exit velocity and the combustion velocity at the low load, the exit velocity of the mixture from the burner remains sufficiently high compared to the combustion velocity to prevent flashback or to reduce the risk for flashback. It has a large benefit to operate the burner at the high load when there is a high demand for heat, and to operate the burner at the low load when there is only a small demand for heat. The high load is at least 3 times the low load, for example 4 times or 5 times or 10 times the low load. For example, the high load is 24 kW, whereas the low load is 8 kW or 4 kW or 2 kW. The high load is for example equal to the maximum load of the burner. The low load is for example equal to the minimum load of the burner.

Depending on the composition of the combustible gas, the combustion velocity of the mixture may depend on the air-to-gas ratio. The burner is operated at the low load with a first ratio between the exit velocity and the combustion velocity of the mixture with the air-to-gas ratio. The burner is operated at the intermediate load with a second ratio between the exit velocity and the combustion velocity of the mixture with the air-to-gas ratio. The combustion velocity may be different at the low load than at the intermediate load. The first ratio at the low load is higher than the second ratio at the high load. In an embodiment, the method comprises the step of:

- forming the mixture from a flow of air and a flow of combustible gas.

According to this embodiment, a flow of air and a flow of combustible gas is provided. The air and the combustible gas are mixed to create an air/gas mixture.

In an embodiment, the combustible gas comprises at least 80% hydrogen, for example at least 90%, at least 98% or 100%.

In an embodiment, the method comprises the step of:

- operating the burner at the low load with the exit velocity being higher than at the intermediate load.

In an embodiment, the method comprises the step of:

- operating the burner with the exit velocity that is inverse proportional with the load between the low load and the intermediate load.

In an embodiment, the method comprises the step of:

- determining information about the reaction zones,

- adjusting an amount of air and/or combustible gas in the mixture.

In an embodiment, the method comprises the steps of:

- determining information about the reaction zones,

- adjusting an amount of air and/or combustible gas in the mixture based on the information about the reaction zones.

The invention will be described in more detail below under reference to the figures, in which exemplary embodiments of the invention will be shown. The figures show in:

Fig. 1: a premix gas burner system according to an embodiment of the invention,

Fig. 2: a burner of the premix gas burner system according to the embodiment of Fig.

1.

Fig. 3: a cross-section of the burner of Fig. 2.

Fig. 4: a detail of the burner deck of the burner of Fig. 2 according to a second embodiment.

Fig. 5: a detail of the burner deck of the burner of Fig. 2 according to a third embodiment.

Fig. 6: a detail of the burner deck of the burner of Fig. 2 according to a fourth embodiment.

Fig. 7: a relationship of air-to-gas ratio versus the load according to the invention.

Fig. 8: a relationship of the ratio of the exit velocity and the combustion velocity versus the load according to the invention. Fig. 9: a relationship of the volume flow rate versus the load according to the invention.

Fig. 10: a method according to an embodiment of the invention.

Fig. 11: a method according to another embodiment of the invention.

Fig. 1 schematically depicts a premix gas burner system 100 according to an embodiment of the invention. The premix gas burner system 100 comprises a burner 102, a mixing chamber 104 and a control system 110. The mixing chamber 104 is arranged to receive a flow of air 121 and a flow of combustible gas 122 to create a mixture 123. The combustible gas comprises at least 80% hydrogen. The burner 102 is arranged to receive the mixture 123. The burner 102 has a surface 201 forming a burner deck 202 comprising burner deck portions 203 and a separation surface 204 arranged between the burner deck portions 203, which are shown in Fig. 2. The burner deck portions 203 are adapted to combust the mixture 123 in reaction zones 130 extending over the burner deck portions 203. The reaction zones 130 are schematically depicted with the dashed lines to indicate that the reaction zones 130 are separated from each other and that they are located at a distance from the surface of the burner deck 202. The burner deck portions 203 are provided with holes to provide the mixture 123 to the reaction zones 130. The burner deck portions 203 are arranged relative to each other to prevent the reaction zones 130 from extending over the separation surface 204.

The control system 110 is adapted to control the flow of air 121 and the flow of combustible gas 122 to modulate the burner 102 to operate at a minimum load, a maximum load and an intermediate load. The control system 110 is optionally adapted to modulate the burner 102 to operate at additional loads in between the minimum load and the maximum load. The maximum load is at least three times larger than the minimum load. The intermediate load is higher than the minimum load and lower than the maximum load. The control system 110 is adapted to operate the burner 102 with an exit velocity of the mixture 123 from the holes 300, as shown in Fig. 3, to the reaction zones 130, that is higher at the minimum load than at the intermediate load.

The flow of combustible gas 122 goes into the premix gas burner system 100 via a gas inlet 105. The gas inlet 105 is for example connected to a gas network. Via the gas inlet 105, the flow of combustible gas 122 passes a gas valve 112. The control system 110 is adapted to control the setting of the gas valve 112. The control system 110 is able to open and close the gas valve 112 to control the flow rate of the flow of combustible gas 122.

The flow of air 121 is created by a fan 111. The rotational speed of the fan 111 is controlled by the control system 110. The control system 110 controls the flow rate of the flow of air 121 by setting the fan 111 to a desired rotational speed. The fan 111 creates the flow of air 121 by sucking in air via an air inlet 106. The fan 111 directs the flow of air 121 to the mixing chamber 104. Alternatively, the mixing chamber 104 is arranged upstream of the fan 111.

In the mixing chamber 104, the flow of air 121 and the flow of combustible gas 122 are mixed to create the mixture 123. The mixture 123 flows from the mixing chamber 104 to the burner 102.

The premix gas burner system 100 comprises a heat exchanger 140. The burner 102 is arranged in the heat exchanger 140. The heat exchanger 140 is adapted for transferring heat away from flue gas created by the reaction zones 130. The heat exchanger 140 has channels filled with a fluid, such as water, to absorb the heat generated by the burner 102. The heated fluid is then used, for example, to heat a building. A heat exchanger sensor 141 is arranged in the heat exchanger 140. The heat exchanger sensor 141 is adapted to provide information about a property of the heat exchanger 140, for example temperature. The control system 110 is adapted to adjust the flow of air 121 and/or the flow of gas based on the information about a property of the heat exchanger 140.

The premix gas burner system 100 comprises a sensor 150 for providing a signal representative of information about the reaction zones 130. The control system 110 is adapted to evaluate a relationship between the information and the flow of air 121. The control system 110 is adapted to adjust the flow of air 121 and/or the flow of gas 122 based on the evaluated relationship. The information about the reaction zones 130 is at least one of a temperature, a concentration of oxygen, a concentration of combustible gas, and ionization signal. The control system 110 is adapted to control a speed of the fan 111 to generate the flow of air 121. The control system 110 is adapted to evaluate a relationship between the signal and the speed of the fan 111.

The control system 110 comprises an input terminal 151 to receive gas information about a property of the flow of combustible gas 122. The control system 110 is adapted to adjust the flow of air 121 and/or the flow of combustible gas 122 based on the gas information. The property of the flow of combustible gas 122 is at least one of a gas composition, an amount of hydrogen, a gas pressure, and a temperature.

Fig. 2 schematically depicts a burner 102 of the premix gas burner system 100 according to the embodiment of Fig. 1. The burner 102 has a cylindrical shape. The outer surface of the cylinder forms surface 201. A burner deck 202 is formed on the surface 201. The burner deck 202 comprises burner deck portions 203 and a separation surface 204. The burner deck portions 203 are arranged along a circumference of the cylindrical shape and separated from each other by the separation surface 204. The burner deck portions 203 form the surface area over which reaction zones 130 extend. In a reaction zone 130 the combustible gas containing hydrogen is combusted. When burning hydrogen, no flame is visible. In case the burner 102 combusts a combustible gas with hydrogen and a fossil gas, flames would be visible during operation of the burner 102 if the percentage of fossil gas is large enough. The burner deck 202 of the burner 102 in has multiple burner deck portions 203. The burner deck portions 203 are schematically indicated by black rectangles. The burner deck portions 203 are separated from each other by the separation surface 204 in two direction, i.e. , the x-direction and the z-direction. The two directions are perpendicular to each other. The embodiments described below show different possible arrangements of the burner deck portions 203.

Part of surface 201 is in between adjacent burner deck portions 203. This part of surface 201 is referred to as separation surface 204. The separation surface 204 is arranged to separate the burner deck portions 203 from each other. During operational use of the burner 102, the reaction zones 130 cover the burner deck portions 203, but do not cover the separation surface 204. The burner deck portions 203 are arranged relative to each other to prevent the reaction zones 130 from extending over the separation surface 204.

In this embodiment, the burner 102 is provided with a flange 205 for mounting the burner 102 in the heat exchanger 140. There is a distance along the surface 201 between the flange

205 and the burner deck 202. This part of the surface 201 is blind surface 206. The blind surface 206 is large enough to ensure that the burner deck 202 reaches far enough in the heat exchanger 140. The size of the blind surface 206 depends on the heat exchanger 140 that is used in combination with the gas burner 102. For example, the size of the blind surface

206 is in the range of 7 - 15 mm or up to 20 mm or up to 40 mm or more. The blind surface 206 also helps to prevent too much heat to be transferred to the flange 205. Too much heat on the flange 205 could cause thermal stress on the connection of the burner 102 to the heat exchanger 140 to overheat the thermal insulation of the heat exchanger 140. The blind surface 206 does not have holes through which combustible gas is supplied. The reaction zones 130 do not cover the blind surface 206. Also, the blind surface 206 is not between adjacent burner deck portions 203. The blind surface 206 has the burner deck portions 203 on one side and the flange 205 on the other side. Therefore, the blind surface 206 does not form part of the burner deck 202. A second blind surface 207 is provided near the top 208 of the burner 102. The surface 201 forms the second blind surface 207 near the top 208. The second blind surface 207 helps to prevent the top 208 from overheating by separating the top 208 from the burner deck 202. Similar to the blind surface 206, the second blind surface 207 is not covered by the reaction zones 130, nor is the second blind surface 207 between burner deck portions 203. The second blind surface 207 does not form part of the burner deck 202. Fig. 3 depicts a cross-section of the burner 102 of Fig. 2 along line A-A. Fig. 3 shows the reaction zones 130 is that are created on the burner deck portions 203 when the combustible gas containing hydrogen is combusted. The reaction zone 130 is the space in which most of the hydrogen, for example 95%, is converted into water. The separation surface 204 in between adjacent burner deck portions 203 is not covered by the reaction zones 130 during operational use of the gas burner 102.

The surface 201 is formed by a metal plate. In case of a cylindrically shaped gas burner 102, the sheet of metal is bent and/or rolled to create the cylindrical shape. On the opposite side of the surface 201, the flow of the mixture 123 is provided, schematically indicated with arrows. The mixture 123 enters the burner 102 via an opening through the flange 205 and flows into an inner space of the burner 102. The opening through the flange 205 has, for example, an anti-noise device. Via the inner space, the mixture 123 propagates through the holes 300 of the burner deck portions 203, which go through the surface 201. Where the combustible gas exits the holes 300, the combustible gas is ignited and several reaction zones 130 are created. The burner deck portions 203 are the part of surface 201 over which the reaction zones 130 extend.

The metal plate has a thickness 301. The holes 300 extend through the entire thickness 301 of the metal plate. An equivalent diameter of at least one of the holes 300 is larger than a thickness 301 of the metal plate.

Fig. 3 schematically indicates that the reaction zones 130 do not touch the burner deck portions 203 on the surface 201. However, depending on operating parameters, for example when operating at a low load, it is possible that the reaction zones 130 are touching the burner deck portions 203 on the surface 201 or are partly at an end part of the holes 300 near the surface 201.

A combined surface area of the holes 300 is more than 1.0% and less than 7.0% of the surface area of the burner deck 202, preferably less than 6.0% or less than 5.0%.

Fig. 2 shows a group of burner deck portions 203 as indicated by detail B. Detail B will be used to explain several embodiments of the invention by showing the arrangement of the burner deck portions 203. Any of these embodiments may be applied to the burner 102 as depicted in Fig. 2. Alternatively, any of these embodiments may be applied to a burner 102 with a different shape than a cylindrical shape. For example, the burner 102 is a flat gas burner or is a bowl-shaped gas burner or a line shaped gas burner or an inverted bowl shaped gas burner. In an embodiment, the surface 201 of the burner 102 has a shape as disclosed in W02004092647 or EP3064831 or W02009059933, hereby incorporated by reference, wherein on the surface 201 the burner deck 202 according to the invention is formed. Further, the top 208 of the cylindrical shape may be completely closed, so no mixture 123 is able to exit the burner 102 via the top 208. In an embodiment, the top 208 is provided with burner deck portions 203, so combustible gas can exit the top 208 to create one or more reaction zones 130 on the top 208. By having burner deck portions 203 provided on the top, gas exits the gas burner 102 not only at a circumference of the cylindrical shape, but also at the head of the cylinder shape. For example, the burner deck 202 according to the invention is applied to a burner 102 with a shape as shown in W02009/077505.

The burner deck portions 203 may be arranged rotational-symmetrically along a circumference of the cylindrical shape of the burner 102. The burner deck portions 203 may be aligned along the longitudinal axis of the cylindrical shape, i.e. , the z-axis. Alternatively, a burner deck portion 203 may be at an offset relative to adjacent burner deck portions 203 along the longitudinal z-axis. The burner deck portions 203 may be aligned along an axis that is at an angle with the longitudinal z-axis, for example at -45°, -30°, -20°, 20°, 30° or 45°.

Fig. 4 shows a second embodiment according to the invention. In this embodiment, detail B shows several burner deck portions 203. Each burner deck portion 203 has three holes 300. The three holes 300 are arranged in a triangle, i.e. one hole is at an offset from a line extending through the other two holes 300. The burner deck portions 203 are schematically indicated with triangles surrounding each group of three holes 300. Each of the burner deck portions 203 is separated from the other burner deck portions 203 by the separation surface 204.

The distance z1 is the distance between the of two holes 300 in the burner deck portion 203 along the longitudinal z-axis of the burner 102. Z1 may be in the range of 0.5-2.5 mm, for example 1 or 1.2 or 1.5 or 2.0 mm. The distance z2 is the distance along the longitudinal z- axis between the center of two adjacent holes 300 in two adjacent burner deck portions 203. Z2 may be in the range of 5.0 - 25 mm, for example, 6.2 or 7.5 or 10 mm. The distance x1 is the distance between the centers of two holes 300 in the burner deck portion 203 along the circumference of the cylindri cally shape burner 102. X1 may be in the range of 0.5 - 2.5 mm, for example 0.8 or 1 or 1.2 or 1.5 or 2.0 mm. The distance x2 is the distance between the center of two adjacent holes 300 in two adjacent burner deck portions 203 along the circumference of the cylindri cally shape gas burner 102. X2 may be in the range of 7 - 25 mm, for example, 7.7 or 10.3 or 11.3 mm. In an embodiment, z1 is 1.2 mm, z2 is 6.2 mm, x1 is 0.7 mm, x1 is 0.7 mm and x2 is 6.3 mm, wherein the diameter of the holes 300 is 0.8 mm.

The holes 300 in a burner deck portion 203 are arranged with respectively a first pitch p1, a second pitch p2 and a third pitch p3. In an example, all pitches p1-p3 have the same length, for example in the range of 2.0-2.4 mm. The edges of the burner deck portions 203 are indicated with the dashed lines. The edge of the burner deck portion 203 extends beyond the centers of the holes 300 with a pitch pa. The value of pitch pa is half the average value of pitches p1-p3.

Fig. 5 shows another further embodiment of the burner 102. Detail B of fig. 5 shows burner deck portions 203 that each have sixteen holes 300. The holes 300 are arranged in a 4x4 formation, i.e. in a rectangular arrangement. In the rectangular arrangement, four groups of four holes 300 are defined. The holes 300 of each group are aligned along the longitudinal z-axis. The four groups are arranged at the same position on the longitudinal z-axis at a different position on the circumference of the gas burner 102. The rectangular arrangement may be square, i.e., a distance between the holes 300 within a group is the same as a distance between the holes 300 in adjacent groups. The values for z1, z2, x1 and x2 may be same as described in the embodiment above. In an embodiment, z1 is 1.5 mm, z2 is 5.2 mm, x1 is 1.5 mm, x2 is 7.5 mm, wherein the diameter of the holes 300 is 0.4 mm.

The pitch p1 is the distance between two adjacent holes 300 along the circumference of the burner 102. The pitch p2 is the distance between two adjacent holes 300 along the z- direction. The pitch p3 is the diagonal distance between two adjacent holes 300.

The burner deck portion 203 extends beyond the holes 300 with a pitch pa. The pitch pa is the distance between the edge of the burner deck portion 203 and a center of a hole closest to the edge of the burner deck portion 203. The pitch pa is half the average pitch of p1-p3.

In an embodiment, the burner 102 comprises a combination of the burner deck portions 203 as disclosed in the embodiments of Fig. 4 and Fig. 5.

When using a punch to create the holes 300 in the metal plate during the manufacturing of the burner 102, it is beneficial to have the same pitch for p1 and p2. This way, a single punch is able to form all the holes 300. However, variations of the pitches p1 and p2 within a burner deck portion 203 or between burner deck portions 203 are possible.

For example, when applying laser cutting to provide the holes 300 in the metal plate, the laser cutting apparatus can be programmed to provide the holes 300 with a variation of the pitch p1 and/or pitch 2. A variation of the pitch p1 and/or pitch 2 may help to reduce noise produced by the gas burner 102 in operational use.

Fig. 6 shows yet another embodiment. Detail B of fig. 6 shows burner deck portions 203 that each have seven holes 300. The holes 300 are arranged with a larger hole in the center, surrounded by six smaller holes 300. The larger hole may have a diameter of for example 1.0 mm, whereas the surrounding six holes 300 have diameter of for example 0.8 mm. The values for z1 , z2, x1 , and x2 may be same as described in the embodiment above. The pitch p1 is the distance between two adjacent holes 300 in a burner deck portion 203. In an embodiment, z1 is 3 mm, z2 is 10 mm, x1 is 2.6 mm, and x2 is 8.7 mm. The burner deck portion 203 is substantially circular with a diameter of 3 times the pitch p1.

Based on the dimensions of the burner 102 and on the operating parameters, the size of the reaction zones 130 can be calculated or estimated. The reaction zone 130 extends beyond the holes 300 of a burner deck portion 203

In case, for example, all the holes 300 in a burner deck portion 203 are separated from each other with a single pitch. The burner deck portion 203 in Fig. 4 is such an example in case p1, p2 and p3 are equal to each other, or the burner deck portion 203 in Fig. 6 that only has one pitch p1 between all holes 300. In this example, the average pitch is equal to the single pitch. The burner deck portion 203 extends half the single pitch from the center of the holes 300 at the edge of the burner deck portion. In an example, the holes 300 in a burner deck portion 203 are arranged in a more complex layout. The average pitch depends on the layout of the holes 300. To determine the edge of the burner deck portion, the planar graph theory is for example used. Using the planar graph theory, lines are drawn from the center of each hole in a burner deck portion 203 to the centers of the other holes 300 in the burner deck portion 203, with the condition that none of the lines intersect each other, except at the centers of the holes 300. With this condition, each hole is connected via lines to adjacent holes 300. The average distance of all lines is then determined, by the minimum of all possible drawn planar graphs. The average distance represents an average pitch. The edge of the burner deck portion 203 is at a distance equal to half the average pitch from the center of the holes 300 nearest to the edge of the burner deck portion.

Fig. 7 depicts the relationship between the air-to-gas ratio l in dependency of the load of the burner 102. The x-axis shows the load of the burner 102 between the minimum load

700 and the maximum load 702 of the burner 102.

The control system 110 is adapted to provide the mixture 123 with a higher air-to-gas ratio at the minimum load 700 than at the intermedium load 701. The control system 110 is adapted to operate the burner 102 with the air-to-gas-ratio that is inverse proportional with the load between the minimum load 700 and the intermediate load 701. As is shown in Fig 7, the graph has a negative derivative between the minimum load 700 and the intermediate load

701

Fig. 8 depicts the relationship between the ratio of the exit velocity and the combustion velocity in dependency of the load of the burner 102. The x-axis shows the load of the burner 102 from the minimum load 700 to the maximum load 702. The y-axis shows the ratio of the exit velocity and the combustion velocity.

The control system 110 is adapted to control an exit velocity of the mixture 123 exiting from the holes 300 to the reaction zones 130. The control system 110 is adapted to estimate or determine a combustion velocity of the mixture 123, for example based on information about the air-to-gas ratio l of the flow of air 121 and the flow of combustible gas 122. The velocity ratio between the exit velocity and the combustion velocity at the minimum load 700 is higher than at the intermediate load 701. The velocity ratio at the minimum load 700 is higher than at the maximum load 702.

The control system 110 is adapted to stop the flow of combustible gas 122 to the burner 102. The control system 110 is adapted to increase the velocity ratio, while reducing the flow of combustible gas 122 prior to stopping the flow of combustible gas 122. For load values lower than the minimum load, the velocity ratio may not be well defined. For such low load values, there is, for example, not a stable combustion. Without a stable combustion, it is difficult or impossible to properly define the combustion velocity. That is why Fig. 8 does not show values for loads lower than the minimum load 700.

Fig. 9 depicts the relationship between the volume flow rate of the mixture 123 in dependency of the load of the burner 102. In this embodiment, the volume flow rate of the mixture 123 is larger at the minimum load 700 than at the intermediate load 701. The volume flow rate of the mixture 123 is larger at the maximum load 702 than at the intermediate load 701. Because the volume flow rate of the mixture 123 is larger at the minimum load 700 than at the intermediate load 701 , the exit velocity is larger at the minimum load 700 than at the intermediate load 701. The control system 110 is, for example, adapted to increase the exit velocity, while reducing the flow of gas less than 5 seconds, preferably less than 2 seconds, more preferably less than 1 second prior to stopping the flow of combustible gas 122.

Fig. 10 depicts a method for operating the burner 102 according to an embodiment of the invention. The method comprises the steps of:

Step 1: providing a mixture 123 of air 121 and a combustible gas 122 comprising at least 80% hydrogen to the burner 102.

Step 2: operating the burner 102 at a high load 702 to generate an amount of thermal energy.

Step 3: operating the burner 102 at a low load 700 to generate a smaller amount of thermal energy than at the high load 702, wherein the high load 702 is at least 3 times the low load 700, Step 4: operating the burner 102 at an intermediate load 701 that is higher than the minimum load and lower than the maximum load.

Step 5: operating the burner 102 at the low load 700 with an exit velocity of the mixture 123 that is higher than at the intermediate load 701. In the method, the combustible gas comprises at least 80% hydrogen, for example at least 90% or 100%.

Fig. 11 depicts a method for operating the burner 102 according to an embodiment of the invention. The method comprises the steps of: Step 1: providing a mixture 123 of air 121 and a combustible gas 122 comprising at least 80% hydrogen to the burner 102.

Step 3: operating the burner 102 at a low load 700 to generate a smaller amount of thermal energy than at the high load 702, wherein the high load 702 is at least 3 times the low load 700,

Step 4: operating the burner 102 at an intermediate load 701 that is higher than the minimum load and lower than the maximum load.

Step 5: operating the burner at the low load 700 with a velocity ratio of an exit velocity and a combustion velocity that is higher than at the intermediate load 701.

Claims

1. A premix gas burner system (100), comprising: a burner (102); a mixing chamber (104); and a control system (110), wherein the mixing chamber (104) is arranged to receive a flow of air (121) and a flow of combustible gas (122) to create a mixture (123), wherein the combustible gas comprises at least 80% hydrogen, wherein the burner (102) is arranged to receive the mixture (123), wherein the burner (102) has a surface (201) forming a burner deck (202) comprising burner deck portions (203) and a separation surface (204) arranged between the burner deck portions (203), wherein the burner deck portions (203) are adapted to combust the mixture (123) in reaction zones (130) extending over the burner deck portions (203), wherein the burner deck portions (203) are provided with holes (300) to provide the mixture (123) to the reaction zones (130), wherein the burner deck portions (203) are arranged relative to each other to prevent the reaction zones (130) from extending over the separation surface (204), wherein the control system (110) is adapted to control the flow of air (121) and the flow of combustible gas (122) to modulate the burner (102) to operate at a minimum load (700), an intermediate load (701), and a maximum load (702), wherein the maximum load (702) is at least three times larger than the minimum load

(700), wherein the intermediate load (701) is higher than the minimum load (700) and lower than the maximum load (702), wherein the control system (110) is adapted to operate the burner (102) with an exit velocity of the mixture exiting from the holes to the reaction zones, wherein the control system (110) is adapted to operate the burner (102) with a velocity ratio between the exit velocity and a combustion velocity of the mixture (123) that is larger at the minimum load (700) than at the intermediate load (701).

2. The premix gas burner system (100) according to claim 1, wherein a combined surface area of the holes (300) is more than 1.0% and less than 7.0% of the surface area of the burner deck (202), preferably less than 6.0% or less than 5.0%. 3. The premix gas burner system (100) according to claim 1 or 2, wherein the exit velocity at the minimum load (700) is higher than at the intermediate load (701).

4. The premix gas burner system (100) according to any one of the preceding claims, wherein the burner deck portions (203) comprise a first burner deck portion, a second burner deck portion and a third burner deck portion, wherein the first burner deck portion is separated from the second burner deck portion by the separation surface (204) in a first direction (z), wherein the first burner deck portion is separated from the third burner deck portion by the separation surface (204) in a second direction (x), wherein the first burner deck portion is adjacent to the second burner deck portion and the third burner deck portion, wherein the first direction (z) and the second direction (x) are different from each other.

5. The premix gas burner system (100) according to claim 4, wherein a hole (300) of the first burner deck portion and a hole (300) of the second burner deck portion that are closest to each other along the first direction are separated from each other by a first distance (z2) in the first direction (z), wherein a hole (300) of the first burner deck portion and a hole (300) of the third burner deck portion that are closest to each other along the second direction are separated from each other by a second distance (x2) in the second direction (x), wherein the sum of the first distance and the second distance is at least 15 mm.

6. The premix gas burner system (100) according to claim 5, wherein each of the first distance (z2) and the second distance (x2) are at least 7.5 mm, for example, at least 10 mm or 15 mm or 20 mm.

7. The premix gas burner system (100) according to any one of the preceding claims, comprising a metal plate having the surface (201) forming the burner deck (202), wherein an equivalent diameter of at least one of the holes (300) is larger than a thickness (301) of the metal plate.

8. The premix gas burner system (100) according to any one of the preceding claims, wherein the control system (110) is adapted to operate the burner (102) with an air-to- gas-ratio that is inverse proportional with the load between the minimum load (700) and the intermediate load (701).

9. The premix gas burner system (100) according to any one of the preceding claims, comprising a sensor (150) for providing a signal representative of information about the reaction zones (130), wherein the control system (110) is adapted to evaluate a relationship between the information and the flow of air (121), wherein the control system (110) is adapted to adjust the flow of air (121) and/or the flow of gas (122) based on the evaluated relationship.

10. The premix gas burner system (100) according to claim 9, comprising a fan (111), wherein the control system (110) is adapted to control a speed of the fan (111) to generate the flow of air (121), wherein the control system (110) is adapted to evaluate a relationship between the signal and the speed of the fan (111).

11. The premix gas burner system (100) according to claim 9 or 10, wherein the information about the reaction zones (130) is at least one of a temperature, a concentration of oxygen, a concentration of combustible gas, and ionization signal.

12. The premix gas burner system (100) according to any one of the preceding claims, wherein the control system (110) is adapted to stop the flow of combustible gas (122) to the burner (102), wherein the control system (110) is adapted to increase at least one of the exit velocity and the velocity ratio of the mixture, while reducing the flow of combustible gas (122) prior to stopping the flow of combustible gas (122).

13. The premix gas burner system (100) according to claim 12, wherein the control system (110) is adapted to increase the exit velocity of the mixture, while reducing the flow of gas less than 5 seconds, preferably less than 2 seconds, more preferably less than 1 second prior to stopping the flow of combustible gas (122).

14. The premix gas burner system (100) according to any one of the preceding claims, wherein the control system (110) comprises an input terminal (151) to receive gas information about a property of the flow of combustible gas (122), wherein the control system (110) is adapted to adjust the flow of air (121) and/or the flow of combustible gas (122) based on the gas information.

15. The premix gas burner system (100) according to claim 14, wherein the property of the flow of combustible gas (122) is at least one of a gas composition, an amount of hydrogen, a gas pressure, and a temperature.

16. The premix gas burner system (100) according to any one of the preceding claims, comprising a heat exchanger (140) and a heat exchanger sensor (141), wherein the heat exchanger (140) is adapted for transferring heat away from flue gas created by the reaction zones (130), wherein the heat exchanger sensor (141) is adapted to provide information about a property of the heat exchanger (140), for example temperature, wherein the control system (110) is adapted to adjust the flow of air (121) and/or the flow of gas based on the information about a property of the heat exchanger (140).

17. Method for operating a burner (102), wherein the method comprises the step of

- providing a mixture (123) of air and a combustible gas comprising at least 80% hydrogen to the burner (102), wherein the burner (102) has a surface forming a burner deck (202) comprising burner deck portions (203) and a separation surface (204) arranged between the burner deck portions (203), wherein the burner deck portions (203) are adapted to combust a mixture (123) in reaction zones (130) extending over the burner deck portions (203), wherein the burner deck portions (203) are provided with holes (300) to provide the mixture (123) to the reaction zones (130), wherein the burner deck portions (203) are arranged relative to each other to prevent the reaction zones (130) from extending over the separation surface (204), wherein the method further comprises the steps of:

- operating the burner (102) at a high load (702) to generate an amount of thermal energy,

- operating the burner (102) at a low load (700) to generate a smaller amount of thermal energy that at the high load (702), wherein the high load (702) is at least 3 times the low load (700),

- operating the burner (102) at an intermediate load (701) that is higher than the minimum load (700) and lower than the maximum load (702),

- operating the burner (102) with a velocity ratio between an exit velocity and a combustion velocity of the mixture (123) that is larger at the low load (700) than at the intermediate load (701), wherein the mixture exits from the holes to the reaction zones with the exit velocity.

18. Method for operating a burner (102) according to claim 17, comprising the step of:

- forming the mixture (123) from a flow of air (121) and a flow of the combustible gas.

19. Method for operating a burner (102) according to claim 17 or 18, wherein the combustible gas comprises at least 80% hydrogen, for example at least

90%, at least 98%, or 100%.

20. Method for operating a burner (102) according to any one of claims 17-19, comprising the step of: - operating the burner (102) at the low load (700) with the exit velocity being higher than at the intermediate load (701).

21. Method for operating a burner (102) according to any one of claims 17-20, comprising the step of: - operating the burner (102) with an air-to-gas-ratio that is inverse proportional with the load between the low load (700) and the intermediate load (701).

22. Method for operating a burner (102) according to any one of claims 17-21, comprising the steps of: - determining information about the reaction zones (130),

- adjusting an amount of air and/or combustible gas in the mixture (123) based on the information about the reaction zones (130).