NL2019559B1

NL2019559B1 - Micro machined fuel gas combustion unit

Info

Publication number: NL2019559B1
Application number: NL2019559A
Authority: NL
Inventors: Zhao Yiyuan; Veltkamp Henk-Willem; Zeng Yaxiang; Conrad Lötters Joost; John Wiegerink Remco
Original assignee: Berkin Bv
Priority date: 2017-09-15
Filing date: 2017-09-15
Publication date: 2019-03-28
Also published as: WO2019054872A1

Abstract

The invention relates to a micro machined fuel gas combustion unit, comprising a substrate with at least one inlet for supplying fuel and oxygen-containing gas; a micro machined combustion tube connected to said inlet and arranged for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; and at least one an outlet connected to said micro machined combustion tube for discharging waste gases produced in the combustion reaction. According to the invention said micro machined combustion tube is substantially thermally isolated to prevent heat loss to the surrounding, and the micro machined combustion tube has four walls defining a substantially rectangular cross section, wherein at least two opposing walls of the four walls of said combustion tube are provided with a reinforcing supporting structure.

Description

Title: Micro machined fuel gas combustion unit

Description

The invention relates to a micro machined fuel gas combustion unit, comprising at least one inlet for supplying fuel and oxygen-containing gas; a micro machined combustion tube connected to said inlet and arranged for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; and at least an outlet connected to said micro machined combustion tube for discharging waste gases produced in the combustion reaction.

Such a unit is known, for example, from US2016195482 where it is incorporated in a device for determining the combustion value of a fuel. With the device of the prior it is possible to supply the fuel to be measured to the device in a continuous manner, mix said fuel with the oxygen-containing gas being supplied and subsequently combust the fuel (whether or not continuously), and thus determine the combustion value of the fuel.

The Wobbe Index (Wl) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities. An integrated Wobbe Index (Wl) meter is desired in industries such as the central heating systems and fuel gas supplies in many countries. Miniaturized on-chip fuel gas combustion and local temperature sensing facilitate the measurement of the Wl and determine the exchangeability of different fuel gases.

One of the challenges in micro machined combustion units is to establish and maintain the microscale combustion process in the microchannel. Microflames may suffer from flame extinction, for example.

It is therefore an object of the invention to provide an improved micro machined fuel gas combustion unit, in particular with improvements with respect to flame extinction.

To this end, the invention provides a micro machined fuel gas combustion unit of the aforementioned kind, comprising a substrate with at least one inlet, a micro machined combustion tube, and at least one outlet. The micro machined combustion tube is substantially thermally isolated to prevent heat loss to the surrounding. This may be obtained by providing a thermally insulating layer about the combustion tube. In one embodiment, the thermally insulating layer is a fluid layer, such as a gas (air). In a preferred embodiment, the combustion tube is a free hanging mechanical structure, with which the thermally insulating layer is already established. The micro machined combustion unit according to the invention has four walls defining a substantially rectangular cross section, wherein at least two opposing walls of the four walls of said combustion tube are provided with a reinforcing supporting structure.

By providing a reinforcing supporting structure in two opposing side walls, it becomes possible to provide a larger span between these side walls, and with this the cross sectional area of the tube may be increased without adversely affecting the mechanical strength of the tube. With the increased cross sectional area, large internal volumes of the combustion tube compared to the outer surface area of the tube are obtained, which leads to a relatively low surface-to-volume ratio. This is beneficial, as will be explained below.

It was found that in microchannels in the prior art, where the characteristic diameter is smaller than the critical quenching diameter, the huge surface-to-volume ratio induces quenching of the flame. Due to the relatively large surface-to-volume ratios in micro/meso-channels, gas burning in a small channel suffers from flames extinction that is induced by thermal quenching and radical quenching.

It was found that to overcome the radical quenching, channel inner wall materials should be chemically inert to avoid radical adsorption and further recombination to cause radical extinction.

It was furthermore found that to overcome thermal quenching, thermal loss to the environment should be minimized to ensure to be smaller than the heat generated from combustion. With the reinforcing supporting structure according to the invention, it is possible to provide a large channel with a relatively big cross-sectional area that is necessary to maintain a continuous flame propagation to obtain the adiabatic flame temperature. This way, the micro flames stability is improved and the object of the invention is achieved.

The reinforcing supporting structure in the two opposing side walls may comprise trenches that are formed in said substrate, and that are filled with a material that is different from the material of said substrate. This way the filled trenches act like pillars increasing the mechanical strength of the combustion tube.

Further embodiments of the invention will be explained below.

The substrate may be an Silicon on Insulator (SOI) substrate or similar substrate. The substrate comprises a device layer, a buried layer, such as a BOX layer, and a handle layer. In an embodiment, the Silicon On Insulator substrate comprises a device layer having a thickness of approximately 50 pm, a BOX layer having a thickness of approximately 200 nm and a handle layer having a thickness of approximately 400 pm. The micro machined combustion tube is in this embodiment at least partly provided in the device layer. A wall of the combustion tube may be bound or formed by at least part of the buried layer, in particular the BOX layer. The reinforcing supporting structure according to the invention is at least partially made from a material that is different from the device layer material to create a mechanical stable channel.

The reinforcing supporting structure may comprise a pillar element. The pillar element may extend over the entire height of the side wall. The reinforcing supporting structure may comprise or consist of a polysilicon material.

The micro machined combustion tube may be a free standing tube for providing thermal isolation. The tube may be provided in an opening of a substrate, and may be connected with two ends to said substrate. This way, the tube forms a beam with two fixed supports with respect to said substrate. Of course, more connections to the substrate may be provided, for providing improved mechanical strength, whilst maintaining thermal isolation.

The connection of the combustion tube to the substrate may be made by means of flexure structure elements that are designed to both suspend the floating channels for mechanical strength, and deform flexibly to relax the induced thermal stress during combustion.

In an embodiment, at least one wall connecting said opposing walls comprises a membrane structure. This membrane structure connecting said opposing walls, which have said reinforcing supporting structure, functions a span between said opposing side walls. These membranes may be several millimeters long and wide and can be connected to the reinforcing supporting structure for better mechanical strength. The membrane structure may comprise a silicon rich silicon nitride (SiRN) membrane, in particular a low-stress SiRN membrane, although other materials are conceivable as well. In an embodiment, the thickness of the membrane may be in the range of 0.1 pm to 10.0 pm. The thickness may be different for different membranes.

In a particular embodiment, one of the membranes has a thickness of approximately 1.6 pm and the other of approximately 3.2 pm.

In an embodiment, at least one of the four walls of the micro machined combustion tube comprises a heater element. The heater element is arranged for heating up at least a wall of the combustion tube to provide excess enthalpy.

In an embodiment, each of said two opposing walls comprise at least one heater element. This provides for an easy way to provide excess enthalpy to the fuel gas in the combustion tube.

In an embodiment, at least one wall connecting said two opposing walls comprises said heater element.

Said heater element may be a silicon and/or platinum heater. The heater (or heaters) may be embedded in the side walls of silicon-rich silicon nitride (SiRN). Additional platinum heaters and temperature sensors may be deposited on top of the structures for accurate heat management.

According to an aspect, a device for determining the combustion value of a fuel is provided, the device comprising: - a micro machined fuel gas combustion unit according to the invention and as described above; - a measurement unit for measuring at least a measure of the amount of energy released by the combustion reaction; and - a control unit connected to said measurement unit and arranged for determining the combustion value of the fuel based on the measured amount of energy released by the combustion reaction.

With the combustion unit according to the invention, such a device becomes more reliable as thermal and radical quenching is reduced.

The combustion unit may be provided a chip, in particular a system chip with a silicon substrate provided on a carrier. In this embodiment the device comprises a system chip that is provided with said combustion unit connected to the fuel inlet and the gas inlet, which combustion unit is provided with a combustion chamber for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; a gas outlet connected to the combustion chamber for discharging waste gases produced in the combustion reaction; as well as means for measuring at least a measure of the amount of energy released by the combustion.

To determine a measure of the combustion value of the fuel to be measured, it is preferred to measure a temperature increase resulting from the combustion. In one embodiment this can be realised in that the device is provided with at least one temperature measuring element which may be disposed on or near the combustion unit. The means for measuring at least a measure of the amount of energy released upon combustion thus comprise a temperature measuring element. In one embodiment, the temperature measuring element is disposed at a distance from the combustion chamber, such that the temperature measuring element will be exposed to smaller temperature increases. The temperature measuring element may be an integrated platinum resistance sensor.

In an embodiment, the device comprises at least one flow measurement unit provided upstream of the micro machined combustion unit. The device according to the invention preferably comprises a flow measurement unit for determining the density and/or the flow rate of the fuel. A suitable flow measurement unit is a flow measurement unit of the Coriolis type, for example, whose construction and operation are known per se to those skilled in the art, as follows from EP 1 719 983, for example. Using such a flow measurement unit, the flow rate (mass flow and/or volume flow) of the fuel to be measured can be determined in a relatively inexpensive and reliable manner. Said determination can take place just before the fuel reaches the combustion chamber, which will further increase the precision of the device. Additionally, the flow measurement unit of the Coriolis type is suitable for determining the density of the fuel to be measured while the fuel to be measured is being supplied. Thus it becomes possible to measure the density also momentarily, so that variations in said density can be taken into account in the determination of the combustion value, if desired, which further increases the precision. The use of the aforesaid flow measurement unit, for example of the Coriolis type, makes it possible to realise a relatively compact flow measurement unit, so that the entire device can be relatively small. Another advantageous aspect is the fact that such a sensor is relatively inexpensive, since comparable technologies may be used to incorporate these units into a single substrate / chip. The flow measurement unit of the Coriolis type thus makes it possible to realise a compact and manageable construction, and in addition to that it is relatively inexpensive.

An additional advantage of a flow measurement unit of the Coriolis type is that it is very suitable for measuring the flow rate and/or the density both of a gaseous fuel and of a liquid fuel, or even of combinations thereof. Thus it is possible to determine the combustion value both of gaseous and of liquid fuels.

As indicated before, it is advantageous when the control unit is arranged for determining the Wobbe index of the fuel.

In an embodiment, the measurement unit comprises an oxygen sensor. The oxygen sensor may in one embodiment be provided downstream of the combustion chamber for determining a measure of the amount of residual oxygen in the combustion gas. Alternatively or additionally, catalytic detection of non-combusted components may take place. Such an embodiment can be of relatively simple and compact construction.

The invention will next be explained by means of the accompanying drawings and description of the figures. In figures 1-9 different stages of fabricating a micro machined channel from a substrate are shown, and figures 10-15 show how a micro machined fuel gas combustion unit according to the invention may be made. In particular it is shown:

Fig. 1 - a schematic cross sectional view of a substrate, such as a Silicon on Insulator wafer, wherein a hard mask is formed using wet thermal oxidation;

Fig. 2 - Patterning of the hard mask via RIE, and trench formation via BOSCH DRIE etching;

Fig. 3 - BOX protection with parylene-C via CVD;

Fig. 4 - Parylene-C etching with O2 plasma and hard mask stripping in BHF;

Fig. 5 - Parylene-C stripping in piranha, dry thermal oxidation of SI to create isolation layer, and trench filling with polycrystalline Si via LPCVD;

Fig. 6 - Patterning of isotropic etch mask with RIE;

Fig. 7 - Isotropic etching of Si to create channels;

Fig. 8 - Deposition of low-stress SiRN via LPCVD;

Fig. 9 - Release Etching of channels with isotropic etching;

Fig. 10 - A schematic cross sectional view of a substrate, such as a Silicon on Insulator wafer, with a hard mask, that is used for forming a combustion unit according to the invention;

Fig. 11 - Formation of a plurality of desired trenches;

Fig. 12 - Filling of the plurality of trenches;

Fig. 13 - Etching of the inlet hole and channels;

Fig. 14 - Sealing the channel and sputter metal on top, etching the outlet from the combustor chamber ceiling membrane;

Fig. 15 - Etching side and bottom cavities to fully release the channels from the substrate.

Figures 1-9 schematically show an embodiment of a method of fabricating a micro machined channel, using Trench-Assisted Surface Channel Technology (TASCT), which may be used in fabricating a micro machined combustion unit according to the invention.

In general, the method comprises the steps of: - Providing a substrate 11 of a first material (Fig. 1); - Forming at least two trenches 21, 22 in said substrate 11 by removing at least part of said substrate 11 (Fig. 2); - Forming at least two filled trenches 31, 32 by providing a second material different from said first material and filling said at least two trenches 21,22 with at least said second material (Fig. 5); - Forming an elongated cavity 51 in between said filled trenches 31, 32 by removing part of said substrate 11 extending between said filled trenches 31, 32 (Fig. 7); and - Forming an enclosed channel 5 by providing a layer of material 61 in said cavity 51 and enclosing said cavity 51 (Fig. 8).

As shown in Fig. 9, said enclosed channel 5 may be partially released from said substrate by defining an exterior of said channel 5 by removing at least part of said substrate 11. By removing part of said substrate for defining an exterior of said channel, the fabrication of free-hanging, mechanical stable and thermally isolated channels may be realized.

The process will now be described in more detail.

Fig. 1 shows that an SOI wafer 11 (having for example a 50 pm device layer 13, a 200 nm BOX layer 14, and a 400 pm handle layer 15) is oxidized via wet thermal oxidation for creating a hard mask 12. This Si02 layer 12 is patterned with 3 pm wide trenches to create the actual mask 12.

Fig. 2 shows that the high aspect ratio trenches 21,22, which may for example be 3 pm wide, are etched completely down to the BOX layer 14 with a Bosch process using a DRIE plasma system.

Then, referring to Fig. 3, the trenches 21, 22 are filled with a polymer 71, in particular parylene-C, which in the embodiment shown is deposited as a conformal 2 pm thick layer via CVD. The chosen thickness is, in this case, more than half the trench 21, 22 width, ensuring full filling of the trenches 21, 22.

Fig. 4 shows that the surface parylene-C is etched back using an 02 plasma in a barrel etcher. Advantageously, etching on the surface has a higher rate than etching inside the trenches, which means that the Si02 hard mask 12 is stripped in BHF (7:1 NH4F:HF) whilst protecting the BOX layer 14. The remaining polymer 71 is stripped away in piranha solution (3:1 H2S04:H202).

In the next step, shown in Fig. 5, the trenches are refilled. In an embodiment the trenches are filled with a material comprising at least one non-Silicon layer. In the embodiment shown the trenches are refilled with a multilayer system consisting of a thin Si02 etch-stop layer 35 grown via dry thermal oxidation and subsequent filling via LPCVD of polycrystalline Si 36. This way, filled trenches 31, 32 are obtained. Of course, other materials such as Silicon Nitride are conceivable as well. In that case, the Silicon Nitride may already function as an etch-stop 5 layer for some etching techniques, meaning that the additional Si02 35 layer is not necessary anymore. Thus in effect, the trench may be filled, in an embodiment, with a filling material, which may be a layered material or a single material. The filling material may function as an etch stop layer for specific etching techniques.

As shown in Fig. 6 a slit pattern 41, which will be used as isotropic etch mask, is etched between two adjacent trenches 31, 32 with RIE.

Now referring to Fig. 7, it is shown that the Si of the device layer 13 is etched away through the slits 41 with an isotropic gas phase process, stopping on the Si02 etch-stop 35. This way the cavity 51 is formed.

After etching away the Si 13 for forming the cavity 51, the inner channel wall of the channel 5 is formed via LPCVD of low-stress silicon rich silicon nitride 61, which is conformally grown to a thickness slightly more than half the slit 41 width, ensuring full closure of all slits 41 (see Fig. 8).

As an optional last step, the channels are completely etched free from the top and the bottom, with a suitable etching process, for example an isotropic gas phase etch or semi-isotropic RIE etch.

The TASCT process as described herein may start with a single SOI wafer and may use XeF2 to etch channel 5 through slits 41 arrays and release channel 5 from the bulk substrate 11, due to XeF2 has fast etch rate and high selectivity for silicon over silicon dioxide. Other etching techniques are possible as well. In the depth direction, the Box layer 14 can act as the silicon etch stop through the slits 41, therefore channels 5 are confined in the device layer 13. In the planar direction, high aspect-ratio trenches 21, 22 are etched in the device layer 13, all the trench walls 31, 32 are coated with thin layer of thermal oxide 35 as XeF2 etch stop. Therefore, within the trenches 31, 32 confined device layer 13, through the slits 41 arrays channels 5 can be etched and result with the designed shape and sizes. Outside the trench confined channels, the handle layer 15 and device layer 13 silicon can be etched by XeF2 until reaching the etch stop, which may be the thermal oxide trench walls 35.

As will be seen, the side walls of the channel 5 formed may be given a heater function. To this end, highly doped device layer silicon 13 is encapsulated/sandwiched within two refilled trenches 31, and these trenches all have thermal oxide coatings 35 and can provide electrical isolation from the bulk silicon substrate 13. In this way, by heating up the channel from the sidewall directions, thermal loss to the environment can be minimized.

The channel 5 top and bottom surfaces may be made from low-stress SiRN membranes, they can be connected by the pillars and sidewalls made from trenches 31, 32 to achieve good mechanical strength. Therefore these thin membranes can be several millimeters long or wide.

The top and bottom membrane thickness may be determined by the width of the rectangular slits 41. Heaters and sensors can be placed on top of the channel ceiling surface to efficiently heat up the channel 5 from the top and sense temperature profile. The bottom membrane may be made very thin and transparent which gives good access to microscopic views of flame location in the channel.

The method described above also allows springs and suspensions structures to be made. In a first embodiment, channel structure or solid silicon in the device layer may be used, as they are defined by trenches, therefore any desired shapes such as serpentine springs can be made. Second choice is using a thin membrane of 500nm thick TEOS, which functions as the slits hard mask, as the spring or suspension by etching all the silicon beneath it by XeF2. In conclusion, with the method according to the invention a lot of freedom in designing the desired shape and size for the flexure and suspension is possible, due to the high selectivity of XeF2 over silicon than silicon dioxide.

The channels obtainable with the method described above may be made with a relatively large cross-sectional area, which is advantageously in terms of fluid dynamics (boundary layer, laminar/turbulent flow, flow development).

According to the invention, the method allows the fabrication of a small sized combustion chamber to burn gas blends on chip and measure adiabatic flame temperature. With additional integration of a micro-Coriolis sensor, it is possible to determine the Wobbe Index of any gas blend. In particular when a free-hanging channel is used, thermal heat loss to the environment may be minimized to ensure to be smaller than the heat generated from combustion. In general, a large channel with bigger cross-sectional area may be formed, as this is advantageous to maintain a continuous flame propagation to obtain the adiabatic flame temperature. To overcome the radical quenching, channel inner wall materials may be made chemically inert to avoid radical adsorption and further recombination to cause radical extinction.

Fabrication of a micro machined combustion unit according to the invention uses process steps as described with reference to Figs. 1-9, and will next be explained with reference to Figs. 10-15.

Fig. 10 shows the SOI wafer 111 with device layer 113, BOX layer 114 and handle layer 115.

Fig. 11 shows how a number of trenches 121-127 are formed in the device layer 113. To this end, a hard mask (not shown here, compare reference 12 in Figs. 1 and 2) may be provided on top; the trenches 121-127 may be formed by means of BOSCH DRIE etching; parylene-C may be provided in the trenches via CVD; O2 plasma may be used for parylene-C etching and piranha may be used for parylene-C stripping, as described with respect to Figs. 3-4.

As shown in Fig. 12, the trenches 121-127 may be filled for creating filled trenches 131-137. As described before, this process may use a multilayer system consisting of a thin Si02 etch-stop layer grown via dry thermal oxidation and subsequent filling via LPCVD of polycrystalline Si. This way, filled trenches 131-137 are obtained. Due to the filling of the trenches, a top multilayer 236 is provided on top of the substrate. This top layer is similar to the layer 35, 36 as shown in Fig. 5. For reasons of clarity it is shown here as a single layer, and is shown separated from the filled trenches, although in fact the trenches 131-137 and the top multilayer 236 are integrally connected.

Fig. 13 shows how a mask is provided in the top multilayer 236, with which the combustion chamber 151 and an outlet 154 may be produced. To this end, the process as described with reference to Fig. 5, 6 and 7 may be used. Inlet 153 may be made in a similar way.

Fig. 14 shows the creation of (partially) enclosed channels 151, 154 by filling with a layer of material (61, see Fig. 8). For reasons of conciseness, this layer of material is indicated with reference sign 336 in Fig. 14; however this layer 336 may be a multilayer, similar to the layers 35, 36, 61 as shown in Fig. 8. Fig. 14 also shows the formation of heaters 201, 202 and 203 in side walls and top wall of the combustion chamber 151. For the side walls this may comprise that trenches 133, 135 actually consist of two closely spaced trenches that are refilled. Encapsulated /sandwiched within two refilled trenches 31 is provided a highly doped device layer silicon 13. These trenches all have thermal oxide coatings 35 and can provide electrical isolation from the bulk silicon substrate 13. In this way, by heating up the channel from the sidewall directions, thermal loss to the environment can be minimized.

Fig. 15 shows how the combustion chamber 151 and outlet 154 are released from the surrounding substrate. This way an isolated combustion chamber 151 is obtained.

The process described above with respect to Figs. 10-15 may use a single SOI wafer where channels are realized in the device layer. The fabricated channels 153, 151, 154 comprise a rectangular cross-section. High aspect ratio trenches of 3pm wide and 50 pm deep, for example, are used to fabricate these large channels. It will be understood that other ratios and dimensions are conceivable as well. These deep trenches 121-127 can be refilled with polysilicon to function as pillars to support large membranes. In an embodiment the trenches are filled with a material comprising at least one non-Silicon layer. These refilled trenches 131-137 may also be the channel 151 sidewalls and define the desired channel shape and width, in particular when the channel sidewall functions as a etching stop layer. The height of the channel side walls may be 50 pm, as defined by the device layer 113 thickness of the SOI wafer 111. The top and bottom of the channels are formed by low-stress SiRN membranes, which may have a thickness of 3.2 pm and 1.6 pm, respectively. These thin membranes can be several millimeters long and wide and can be connected by the pillars for better mechanical strength. The structures may be released from the substrate 111 by selective isotropic etching of the handle layer 115 and device layer 113 (see Fig. 15). The result is a thermally stable, chemically inert and strong structure with good thermal insulation. Highly doped silicon heaters (201, 203) are defined by the refilled trenches 133, 134 to heat up the combustor chamber 151 side walls.

Claims

Micro-machined fuel gas combustion unit, comprising a substrate with: - at least one inlet for supplying fuel and oxygen-containing gas; - a micro-machined combustion tube connected to the inlet and adapted to chemically react the fuel to be measured therein with the oxygen-containing gas in a combustion reaction, the micro-machined combustion tube being substantially thermally insulated by means of a thermally insulating layer for preventing heat loss to the environment; and - at least one outlet connected to the micro-machined combustion tube for discharging waste gases from the combustion reaction; wherein the micro-machined combustion tube has four walls that substantially define a rectangular cross-section to form a channel, wherein at least two opposite walls of the four walls of the combustion tube are provided with a reinforcing support structure and wherein the channel inner wall materials are chemically inert.

The micro-machined fuel gas combustion unit of claim 1, wherein the reinforcing support structure comprises polycrystalline silicon.

Micro-machined fuel gas combustion unit according to claim 1 or 2, wherein at least one wall connecting the opposite walls to each other comprises a membrane structure.

The micro-machined fuel gas combustion unit of claim 3, wherein the membrane structure comprises silicon-rich silicon nitride.

Micro-machined fuel gas combustion unit according to claims 1-4, wherein at least one of the four walls of the micro-machined combustion tube is provided with a heating element.

The micro-machined fuel gas combustion unit according to claim 5, wherein each of the two opposite walls comprises a heating element.

Micro-machined fuel gas combustion unit according to claim 5 or 6, wherein at least one wall connecting the two opposite walls with each other comprises a heating element.

A micro-machined fuel gas combustion unit according to claim 5, 6 or 7, wherein the heating element is a silicon heater or platinum heater.

The micro-machined fuel gas combustion unit according to any of the preceding claims, wherein the reinforcing support structure is configured to act as a heating element.

Device for determining the combustion value of a fuel, the device comprising: - a micro-machined fuel gas combustion unit according to one of the preceding claims; - a measuring unit for measuring at least one measure of the amount of energy released during the combustion reaction; and - a control unit which is connected to the measuring unit and which is adapted to determine the combustion value of the fuel based on the measured amount of energy released during the combustion reaction.

Device as claimed in claim 10, wherein the measuring unit comprises at least one temperature sensor which is placed on or near the micro machined combustion tube.

12. Device as claimed in claim 10 or 11, wherein the device comprises at least one flow measuring unit which is placed upstream of the micro machined combustion tube.

The device of claim 12, wherein the flow measurement unit is of the Coriolis type.

Device as claimed in any of the foregoing claims 10-13, wherein the control unit is adapted to determine the Wobbe index of the fuel.

Device as claimed in any of the foregoing claims 10-14, wherein the measuring unit comprises an oxygen sensor.