NL2013536B1 - Method to provide a heated gas. - Google Patents

Abstract

The invention is directed to a method to provide a heated gas of a certain desired mass flow by compressing air to obtain compressed air in a compressor connected to a rotatable shaft, combustion of a fuel with the compressed air in a burner to obtain compressed combustion gas and expanding the compressed combustion gas via a turbine rotor connected to the rotatable shaft to obtain the heated gas. At a low mass flow of heated gas the shaft is driven by an electrical motor connected to the shaft. At a high mass flow of heated gas electrical energy is generated in a generator connected to the shaft.

Description

METHOD TO PROVIDE A HEATED GAS

The invention is directed to a method to provide a heated gas of a certain desired mass flow by combustion of a fuel.

Such methods are generally known. Processes which make use of a heated gas are for example boilers. In such a boiler water is heated and evaporated by indirect heat exchange with the heated gas. The heated gas is typically provided by combustion of a fuel in a burner to obtain a heated gas. The burners in such processes are specially designed for a certain mass flow of fuel and air or any other oxygen containing gas at which the emission of NOx gasses is minimised. The air as provided to the burner is typically slightly compressed by means of a ventilator up to about a pressure ratio of 1.2, i.e. 1.2 times the absolute ambient pressure. At lower or higher mass flow the more NOx gasses are formed.

For some applications it is desirable that the process for making the heated gas can be turned down with respect to the heating power represented by the temperature and flow of the heated gas, i.e. operated at a significant lower fuel mass flow. In the above example this would be desirable in a situation wherein less steam is required. This however raises all kind of issues like higher NOx emissions and a lower flame stability. The lower flame stability is less desirable in view of safety issues and burner availability.

The present invention aims at providing a method which can prepare a heated gas in a wide range of mass flows without or at least with less of the above described NOx emissions and/or flame stability issues.

This is achieved with the following method. Method to provide a heated gas of a certain desired mass flow by compressing air to obtain compressed air in a compressor connected to a rotatable shaft, combustion of a fuel with the compressed air in a burner to obtain compressed combustion gas, expanding the compressed combustion gas via a turbine rotor connected to the rotatable shaft to obtain the heated gas, wherein at a low mass flow of heated gas the turbine rotor is driven by an electrical motor connected to the shaft and wherein at a high mass flow of heated gas electrical energy is generated in a generator connected to the shaft.

Applicant found that the method can provide a wide range of mass flows for heated gas while maintaining a substantially constant flame characteristics in the burner. This in turn results in that the NOx emissions can be kept to a minimum and that the flame stability is not negatively affected. It is found that in the method the velocity in the burner can remain substantially constant while the mass flow of the heated gas is increased by a factor 3. The constant gas velocity in the burner results in that the flame characteristics in the burner are not substantially changed. A further advantage is that at high heated gas mass flow electricity is generated. Further advantages will be described in relation with the preferred embodiments described below.

In the method at a high mass flow of the heated gas the turbine rotor delivers more energy than the energy required to compress the gas via the common shaft. This surplus of energy is converted into electricity using a generator connected to said common shaft. At lower mass flow the energy delivered by the turbine rotor will not be sufficient to compress the gas to the desired pressure. In that mode of operation the required surplus of energy is delivered by an electrical motor connected to the shaft.

The absolute pressure of the compressed air at a high mass flow is suitably between 200 and 500 kPa and wherein the pressure of the heated gas is suitably lower than 10 kPa above the ambient pressure. Suitably the herein the pressure ratio of the compressed air is between 2 and 5. The temperature of the heated gas may be above 200 °C, suitably above 300 °C and preferably above 500 °C.

The compressor may be an axial compressor having one or more stages. The compressor is preferably a centrifugal compressor and even more preferably a single centrifugal compressor. Centrifugal compressors are preferred because of their simplicity because they require less stages to obtain the same pressure rise as compared to an axial compressor.

The burner is suitably of the so-called can type. The burner is suitably part of a combustor comprising of a combustor space extending axially from the axis of the shaft. The combustor space is provided with the burner at one end and a circular slit as discharge opening at its opposite downstream end through which discharge opening the compressed combustion gas is discharged to the turbine rotor. Optionally more than one burner may be provided at the end of the combustor space. The compressed combustion gas as generated in such a burner or burners will flow in a net axial direction towards the turbine rotor in the combustor space.

The axially extending space of the combustor space may have any design. Its cross-sectional area may for example be circular, elliptical or rectangular. The cross-sectional area may increase or decrease along the length of the combustion space axis. Suitably the combustor space is a tubular space.

The burner may be centrally positioned when a single burner is present. When multiple burners are present it is preferred to arrange the burners in a somewhat symmetrical manner to assist in the creation of an uniform combustion flow in the combustion space from the burners to the discharge opening. The direction of the burner, i.e. the direction of the combustion gasses as exiting from the burner, may be along the combustor space axis in case a single burner is used. In case multiple burners are present the direction of the burner may be parallel to the combustor axis or under an angle with this axis.

The position of the burner or burners at the end of the combustor space allows fast dismounting for service purposes. Further it is found that when a single burner is axially aligned with the turbine rotor and shaft optimal inflow conditions are created for the turbine rotor.

Suitably at the downstream end of the combustor space a gas diverting body is present, which body is positioned along the combustor axis. The body and wall of the combustor space define a combustor gas pathway for compressed combustion gas flowing towards the turbine rotor. Such a gas diverting body, sometimes referred to as a dome, may have a cross-sectional design being or resembling the half of an ellipse, wherein the upper end of the ellipse is positioned closest to the burner on the combustor axis.

The axially extending combustor space has a closed end provided with one or more burners, an opposite end provided with a circular slit as discharge opening and optionally a dome and a combustor space wall running in the direction of the combustor axis and connecting both said ends. In the method according to the invention it is preferred that the compressed gas is first used to cool the exterior wall of the combustor space wall before being fed to the burner. This will result in a increased temperature of the compressed gas which is advantageous for an efficient combustion in the burner. To achieve such cooling it is preferred that around the combustor space wall a second shell is placed defining a cooling zone between said wall and second shell.

The cooling zone is provided with an inlet for compressed air and one or more outlets for compressed air, wherein at least one of these outlets is to provide compressed air to the burner. In the method it is preferred that part of the compressed gas is directly fed to the combustor space resulting in a mixture of compressed air and compressed combustion gas. This is suitably achieved by providing openings in the wall of the axially extending space as outlets for compressed air. In case of a tubular space these openings may be arranged in a circle along the circumference of the tubular wall. The inlet for compressed air is suitably positioned such that the compressed air flows along as much of the combustion space wall in order to cool the wall. The cooling space may be provided with guiding vanes to create a pathway in the cooling zone which enhances the cooling and converts any radial movement of the compressed air as it is supplied to the cooling space to a direction parallel to the axis of the combustion space and towards the end which is provided with the one or more burners.

The apparatus is provided with a turbine rotor. This may be a single rotor or a multiple rotor. Preferably a single rotor is used to obtain a more simple design. The rotor itself may be an axial turbine rotor. Optionally the rotor may have a radial component. Such a radial component may direct the combustion gas towards the direction of a radially and axially extending flow path of a diffusor as will be described below.

The gas after passing the turbine rotor is preferably collected in a collecting space having a tangentially positioned outlet for the heated gas. Because the gas after passing the turbine rotor will flow substantially in an axial direction a diffusor and volute type collecting space is preferably used to collect the gas and divert the direction of the gas flow from a substantial axial flow to a radial flow. Suitably part of the kinetic energy of the gas as it passes the diffusor will be converted into pressure. This is achieved by passing the gas via a flow path which extends axially away from the combustor and extends radially outward towards a circular outlet, which circular outlet is fluidly connected along its circumference to a receiving space. In the receiving space the gas is collected and redirected to a circular flow and tangentially discharged from said collecting space.

The design of the axial and radial diverting diffusor allows that the generator and/or electrical motor can be tightly arranged between turbine rotor and compressor. This results in a short shaft length which is advantageous for obvious reasons.

The diffusor is suitably comprised of a circular inlet opening fluidly connected to the discharge outlet of the turbine rotor. The diffusor is comprised of a flow path for combustion gas which extends axially away from the combustor and extends radially outward towards a circular outlet. At its upstream part the flow path may be provided with a bend to direct the expanded combustion gas from a substantially axial direction as it is discharged from the turbine rotor to the above described path way which extends both radially and axially. The radius of such a bend should not be too small in order to avoid unnecessary additional pressure drops and not too large in order to avoid a very large apparatus. The inlet area of the bend and thus also of the circular inlet opening of the diffusor is suitably equal or greater than the and outlet area of the bend.

Part of the flow path for the combustion gas of the diffusor is suitably defined by two frusto conical walls. This part will thus extend both in a radial and axial direction when viewed in the direction of the gas as its passes the diffusor. These walls may run parallel wherein the distance between the two walls remains constant along the length of the flow path. The distance may also vary, for example decrease or increase, along the length of the flow path.

The angle between the general direction of the flow path between the two frusto conical walls and the combustor axis is between 20 and 60°. The angle of the general direction is the numerical average of the angle of the two frusto conical walls with the combustor axis. In case additional elements, such as a generator and compressor are to be fixed to the shaft it is preferred that this angle is greater than 30°, more preferably greater than 40°. The angle is preferably smaller than 50° to achieve higher efficiencies.

Preferably the area ratio between the end and the start of the flow path between the two frusto conical walls is above 1.5, more preferably above 1.8 and preferably below 6 and more preferably below 3. The area is here defined as the cross sectional area of the flow path as present the two frusto conical wall parts and not including the optional bends. Preferably the increase of this area in the direction of the flow path is low enough to avoid that the boundary layer of the gas separates from the wall to form eddies, also referred to as diffusor stall. These eddies will block the flow in the diffusor and are preferably avoided. This increase in area will be dependent of the above described general direction of the flow path, the length of the flow path and the positioning of the two walls relative with respect to each other. The two frusto conical walls may run parallel, divert or convert. The distance between the frusto conical walls may increase in the flow direction, wherein a diffusion angle y is suitably below 20°, preferably below 10° and even more preferably below 5°. Applicants found good results with an angle close to zero. The angle may also be slightly negative, wherein the two walls converge slightly in the flow direction. The diffusion angle is herein twice the angle of one frusto conical wall with the average direction of the flow path. The length of the part of the flow path between the two frusto conical walls may be limited by the available space. Further the length is determined by the above mentioned area ratio, general direction of the flow path, and the positioning of the two walls relative with respect to each other.

The length of the part of the flow path between the two frusto conical walls is limited by the available space. Further the length is determined by the above mentioned area ratio and the chosen diffusion angle.

The diffusor is also provided with an circular outlet fluidly connected along its circumference to a receiving space. The receiving space is designed to direct the axially and radially flowing gas in a single circular moving gas flow. Preferably the receiving space has the design of a so-called volute. In such a volute combustion gas as discharged from the outlet opening of the diffusor will be collected. The volute will have an increasing cross-sectional area in a radial direction such that the velocity of the combustion gas in the volute remains substantially the same while combustion gas is collected along the circular outlet opening. The receiving space may have one or more outlet, suitably tangentially directed, openings for discharge of the combustion gas to other process apparatuses, such as a boiler or a further burner. The diffusor and collecting space as described above thus results in that the axial outflow or axial outflow component of combustion gas as it is discharged from the turbine rotor is deflected in a radial direction.

Applicants found that the above design of combustor and diffusor and collecting space result in a better mixing of fuel and compressed air, a lower NOx emissions, a better stability of the flame and a lower heat load on the wall of the axially extending space. Furthermore because the turbine rotor, the axially extending space and burner are aligned very low loss of energy is obtained of the pressurised combustion gas.

The electrical motor and generator may be separate devices. Preferably the electric motor and the generator is the same. In such a preferred embodiment the generator is equipped to either generate electric power or deliver a shaft torque to the shaft such as for example described in US2002/0070716. Suitably use is made of a converter in combination with a combined generator-electric motor. Such a converter may comprise of a grid-side converter and a gen erator/motor-side converter. The grid side converter has two operating modes. In one mode it can transform DC current as generated by the generator/motor-side converter into grid alternating current at a high mass flow of heated gas. In another mode it can convert grid alternating current into DC current which is converted into electrical motor alternating current used when the combination is operated as an electrical motor at the low mass flow of heated gas. The grid side converter can perform both conversions. The generator/motor side converter can suitably additionally control the rotational speed of the generator by changing the frequency of the current at the generator/motor side. The generator/motor side converter and grid side converter may also be combined into one apparatus. The generator-motor combination is suitably connected to the electrical grid or any other external source of electricity via the above converter. The generator may comprise of a rotor as fixed to the shaft and equipped with permanent magnetic materials. Around the rotating rotor a stator is positioned with electric spools in which electricity is generated or supplied.

Applicants found that the method may suitably be performed in an apparatus as described above and wherein the sequence along the shaft is turbine rotor, generator-motor and compressor. This sequence is advantageous because it results in a very compact apparatus. Furthermore because no gear box is present between shaft and generator a more efficient apparatus is obtained. The compressor is provided with an outlet for compressed air which is fluidly connected to the combustor. The outlet for compressed air may be fluidly connected to the combustor via a compressed air flow path and wherein no heat exchanger is present in the compressed airflow path. This is advantageous when the expanded combustion gas as obtained in the collecting space is further used to for example heating purposes.

The heated gas may comprise non-converted oxygen. Such a heated gas may be combusted with a fuel in a further combustion step to obtain a further heated gas. This allows an even higher range of operation. Preferably the further combustion step is only used when a higher energy demand is required. At the lower end of the energy demand the, i.e. at a low mass flow of heated gas, it may be preferred to omit the combustion in the burner and combust the expanded gas in another separate burner suited for the very low mass flows.

These measures thus further expand the range of operation with regard to the energy demand.

The heated gas or the further heated gas is suitably used directly or indirectly to heat a liquid or gaseous medium or object by heat exchange . Examples of such applications are boilers wherein water is indirectly heated to produce steam, dryers, wherein objects are directly contacted with the heated gas and evaporators such as for example distillation column reboilers and distillation column feed furnaces. The boilers may be fire-tube boilers, such as for example a two-pass or three-pass boiler, or water-tube boilers. The invention is also directed to a process to prepare steam in a boiler wherein a heated gas is used as prepared by the method according to the invention.

The invention shall be illustrated making use of Figures 1-2. Figure 1 shows an apparatus in which the method of the invention is preferably performed. In Figure 1 shows a can-type burner 1 as part of a combustor 2. The combustor 2 is comprised of a tubular combustor space 3 extending axially from the axis 4 of the shaft 5 and provided with the burner 1 at one end 6 and an circular slit 7 as discharge opening at its opposite downstream end 8. Through slit 7 the compressed combustion gas flows to the turbine rotor 9. At slit 7 stator vanes may be present. The apparatus is further provided with a single stage centrifugal compressor 10 connected to rotatable shaft 5. In compressor 10 air is compressed to obtain compressed air which compressed air is routed via a compressed air flow path to the burner 1 via conduit 11 to an annular space 12 where the compressed air is used to cool the exterior wall 13 of the tubular combustor space 3 before being fed to the burner 1 via conduit 14. Part of the compressed air is supplied directly to the combustor space 3 via openings 17. Burner 1 is further provided with a fuel supply conduit 16. Combustor space 3 is provided with a dome 18 at end 8.

The compressed combustion gas as present in combustor space 3 is expanded via turbine rotor 9 connected to the rotatable shaft 5 to obtain the heated gas. The heated gas after passing the turbine rotor 9 will flow substantially in an axial direction via a diffusor 19 and volute type collecting space 20. The diffusor 19 will divert the direction of the heated gas flow from a substantial axial flow to a more radial flow. The diffusor 19 has a flow path positioned between an inner 22 and outer 23 frusto conical wall. At the end of the flow path a circular outlet 24 is present which fluidly connects along its circumference the flow path to the receiving space 20. In the receiving space 20 the heated gas is collected and redirected to a circular flow and tangentially discharged from said collecting space. As shown the receiving space 20 has an increasing cross-sectional area along its circumference wherein the tangentially arranged outlet is positioned at the larger cross-sectional part of space 20.

The radially and axially extending diffusor 19 allows the positioning of the combined electrical motor and generator 25. The combined electrical motor and generator 25 has a permanent magnet 26 connected to the shaft 5 and a windings in stator 27 in which either electricity is generated or provided to.

Example 1

In an apparatus according to Figure 1 electricity was provided to achieve a rotation of the shaft of 11000 rpm such that air was increased in pressure from ambient to 125 kPa above ambient. This compressed air was used to combust natural gas in the burner. The mass flow of heated gas was 0.976 kg/s and the volume flow of gas in the combustion space was 0.73 l/s.

The electricity provided to electrical motor 25 was increased resulting in an increased mass flow of heated gas. At 19000 rpm the apparatus did not require any external electricity and at even higher rpm the apparatus started to generate electricity and electrical motor 25 acted as generator 25. The pressure ratio (pressure of compressed gas after compressor to ambient pressure), mass flow of heated gas as obtained in the apparatus and volume flow of gas in the combustion space is presented in Figure 2. From thus Figure it can be seen that the volume flow in combustion space 3 only slightly increases while the mass flow of heated gas significantly increases. It is found that the flame stability of the burner over this large range of mass flows remains good. This may be explained by the relatively constant volume flow in the combustion space 3.

Claims (9)

A method for providing a heated gas with a specific desired mass flow rate, by compressing air to obtain compressed air in a compressor connected to a rotating shaft, by burning a fuel with the compressed air in a burner , in order to obtain compressed combustion gas, causing the compressed combustion gas to expand by using a turbine rotor connected to the rotating shaft to obtain the heated gas, wherein, at a small mass flow rate of the heated gas, the shaft becomes driven by an electric motor connected to the shaft, and where, at a high mass flow rate of the heated gas, electrical energy is generated in a generator connected to the shaft, the "can-type" burner being a component is of a burner which comprises a combustion space which extends axially with respect to the rotating axis, provided with the burner a at one end thereof and from a circular trunk as a discharge opening at the opposite, downstream end thereof, wherein through this discharge opening the compressed combustion gases flow to the turbine rotor, the combustion gases being collected in a receiving space of the volute type after passing through the turbine rotor which has a tangentially positioned outlet for the heated gas and wherein the turbine rotor, generator and compressor are connected to the rotating shaft in this order. Method according to claim 1, wherein the pressure ratio of the compressed air is between 1 and 5. The method of claim 1 or claim 2, wherein the compressor is a single-stage centrifugal compressor. A method according to any one of claims 1-3, wherein the compressed gas is first used to cool the outside of the combustion space, and this before being led to the burner. A method according to any one of claims 1-4, wherein a part of the compressed gas is led directly to the combustion space, resulting in a mixture of compressed air and compressed combustion gases. The method according to any of claims 1 to 5, wherein the generator is used as the electric motor in the case of a low mass flow rate of the heated gas. The method of any one of claims 1 to 6, wherein the heated gas comprises unreacted oxygen, wherein the heated gas and a fuel are burned in an additional combustion step to obtain an additional heated gas. A method according to any of claims 1 to 7, wherein the heated gas or the additional heated gas is used directly or indirectly to heat a liquid or gaseous medium or object by means of a heat exchange. A method of producing steam in a boiler, wherein a heated gas is used which is obtained by a method according to any one of claims 1 to 6.