EP1834131A1

EP1834131A1 - Method for burning a liquid fuel by variable speed spraying

Info

Publication number: EP1834131A1
Application number: EP05825841A
Authority: EP
Inventors: Rémi Tsiava; Patrick Recourt; Bertrand Leroux
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2004-12-31
Filing date: 2005-12-19
Publication date: 2007-09-19
Also published as: FR2880409A1; FR2880409B1; WO2006072723A1; JP2008527291A; CN100582574C; CN101091089A

Abstract

The invention concerns a method for burning a liquid fuel including steps for producing a spray (4) of the liquid fuel by injecting a spray gas in contact with the liquid fuel, then in contacting the resulting spray with an oxidant (7) so as to burn said spray. Said method is characterized in that the spray gas is injected with variable speed.

Description

Method of burning liquid fuel by atomization at variable speed

The present invention relates to a method of burning a liquid fuel.

When setting up a combustion system for an industrial furnace, for example, one of the major concerns is to minimize the discharge of atmospheric pollutants so as to meet the environmental standards in force, while respecting the needs of the heated process in terms of heat transfer. NOx nitrogen oxides are one of the most environmentally polluted pollutants, and minimizing their emission is a major technical problem. Several solutions can be considered to limit NOx emissions, namely primary type measures to reduce NOx formation during the combustion itself, and secondary type measures to remove NOx from post-combustion effluents. . It should be noted that the secondary type of measurements generally require the installation of extremely expensive means, and all the more so that the rate of NOx released to reach is low. The primary measures are aimed mainly at reducing the formation of nitric oxide NO, the major constituent of NOx, whose main mechanism of formation is a radical thermal mechanism mainly dependent on the partial pressures of oxygen and nitrogen, the temperature of the medium. and the residence time of the reactants in the combustion zone. The case of oxycombustion, that is to say a combustion in which the oxidizer is oxygen, is therefore particularly problematic, this type of combustion being widespread in the industry. According to the thermal mechanism proposed by Zel'dovich in 1947, the speed of formation of NO is written:

with k an activation constant, R the perfect gas constant, E ₃ the activation energy of the reaction, T the local temperature, [OJ and [N] being the local oxygen and nitrogen concentrations. In order to minimize the rate of NO formation and, consequently, the amount of NO formed, it is known to reduce the local nitrogen concentrations by using as oxygen enriched air oxygen, or even pure oxygen. However, the presence of nitrogen can not be totally excluded from the combustion process, it can also be introduced into the medium by infiltrations of air or by the fuel itself. In addition, the temperatures reached by certain processes can lead to a significant formation of NO, despite a low concentration of nitrogen in the environment.

Additional solutions have therefore been developed and in particular solutions consisting of diluting the reagents, mainly the oxidant, with a low-reactivity gas such as CO ₂ or by allowing the recirculation of the exhaust gases in the combustion zone, for example . The recirculation of the exhaust gas is more precisely implemented by injecting the majority of the oxidant at a relatively large distance from the fuel, a small amount of oxidant being however injected near the latter to ensure in particular the stabilization of the flame. This concept of strong separation of the fuel and oxidant jets is, however, limited to air / fuel gas combustion.

Patents EP 0 524 880 and US Pat. No. 5,522,721 describe another process for reducing NOx called oscillating combustion. Such a process consists in oscillating the speed of the oxidant or fuel so that the stoichiometry of the reagents deviates from 1, thus leading to a decrease in the flame temperature and therefore to a reduction in the formation of NOx. In addition, the frequency, amplitude, and oscillation phases can be adjusted to also limit the formation of carbon monoxide. It should be noted, however, that the oscillating combustion processes involve the combustion of a gaseous fuel and are not directly applicable to the combustion of a liquid fuel.

Indeed, in two-phase combustion, it is first necessary to atomize the liquid fuel by means of an atomization gas injected so as to destabilize the flow of liquid fuel and form a fuel spray having a increased contact surface with the oxidant, thus promoting combustion. It is then necessary that the drops of liquid fuel thus formed evaporate before it burns with the oxidizer. The mixing of the reactants therefore depends on the characteristics of the fuel spray, in particular on the size of the fuel drops formed. It has been found that the application of the oscillating combustion principles previously described for gaseous fuels does not lead to satisfactory results in the case of a liquid fuel: indeed, the oscillation of the oxidant flow requires the setting up of extremely expensive and relatively inefficient devices and setting up an oscillation of the liquid fuel flow does not allow a sufficiently precise control of the spray formed.

The object of the present invention is to overcome the aforementioned drawbacks, and more particularly to allow the application of the oscillating combustion principle to a liquid fuel, and for that purpose consists in a method of combustion of a liquid fuel comprising the steps of produce a fuel spray liquid by injecting an atomizing gas with the nontact liquid fuel, then put the spray thus produced in contact with an oxidizer so as to proceed to the combustion of the spray, the atomization gas being injected:

with a variable speed varying around a nominal value of speed for which the combustion of the fuel is complete, and

at a maximum speed less than 1.2 times the nominal value of the atomization gas speed. The term variable speed should be understood to mean that the rate at which the atomizing gas is injected oscillates between two extreme values in a regular manner over time. The curve of the injection speed as a function of time may have a sinusoidal or crenellated profile, for example. Thus, by varying the speed of the atomizing gas, the resulting spray has variable physical characteristics, especially in terms of the size of the formed drops, and therefore also modified combustion properties. More specifically, the increase in the speed of the atomization gas ensures the formation of a fuel spray having a smaller drop size, and therefore a better combustion, while the decrease in the speed of the atomizing gas will cause the formation of a spray having a larger drop size, and therefore a less efficient combustion. In this way, it is possible to control the physical properties of the fuel spray so as to vary the local combustion conditions, especially the flame length, and thus influence the NOx formation conditions. Indeed, the variation of the flame length makes it possible to increase the transfer area to the heated load by avoiding creating zones of high temperature, generating NOx. The atomizing gas is injected at a speed varying around a nominal value of speed for which the combustion of the fuel is complete. More precisely, the nominal speed of the atomizing gas is determined for a given liquid fuel flow. Thus, varying the speed of the atomizing gas around a nominal value ensuring complete combustion optimizes fuel consumption and to vary the combustion properties around this optimum value. For a given burner geometry, the fuel velocity is deduced from the required power and the lower heating value (LHV) of the fuel. The nominal speed of the atomizing gas is generally chosen so that the mass flow rate of the atomizing gas is equal to 0.3 times the mass flow rate of the fuel. According to the invention, the atomizing gas is injected at a maximum speed of less than 1.2 times the nominal value of the atomization gas speed. Thus, the risks of striking the flame are reduced. Preferably, the atomizing gas is injected at a minimum speed greater than 0.7 times the nominal speed of the atomization gas. Thus, the risks of poor combustion are reduced. So advantageously, the atomization gas is injected with a speed whose variation frequency is between 0.1 and 10 Hz. Preferably, the variation of the injection speed of the atomization gas is obtained by piloting a flow control valve.

According to an alternative embodiment, the liquid fuel is pulsed with a variable speed. Indeed, the variation of the liquid fuel flow rate associated with a variation of the atomization gas makes it possible to better control the physical characteristics of the liquid fuel spray. Advantageously, the liquid fuel is pulsed with a speed varying in phase with the injection speed of the atomizing gas. The purpose of this double variation is to maintain a droplet size of the fuel spray substantially constant. The quantity of liquid fuel delivered is nevertheless modified, which affects the stoichiometry of the combustion in addition to the flame length. In the case of a variation around a nominal value for which the combustion is complete and meets the need for the heated process, these conditions will be maintained during variations. Preferably, the speed variation of the liquid fuel is obtained by controlling a flow control valve.

The implementation of the invention will be better understood with the aid of the detailed description which is set out below with reference to the appended drawing in which:

- Figure 1 is a schematic representation of a method according to a first embodiment of the invention wherein the speed of the atomizing gas varies during the combustion.

FIG. 2 is a curve showing the variation of the flame length as a function of the flow rate of the atomizing gas in the process of FIG. 1.

FIG. 3 is a schematic representation of a method according to a second embodiment in which the speed of the liquid fuel varies in phase with the speed of the atomization gas during the combustion.

FIG. 4 is a curve showing the variation in flame length as a function of the liquid fuel flow in the process of FIG. 3.

A combustion process, as shown in FIG. 1, uses a spray atomizer 1 with external mixing (also called assisted atomizer). This atomizer 1 comprises a liquid fuel supply 2 and a supply of atomizing gas 3 and delivers a spray 4 of liquid fuel. More specifically, the supply of atomizing gas 3 is equipped with a control valve 5 controlled by a controller 6 and able to modify the flow rate, and therefore the speed, of the atomizing gas. This spray 4 is then brought into contact with oxygen 7 supplied by an oxidizer feed 8 to proceed with the combustion of the fuel. Initially, the flow rate of the atomizing gas is adjusted so as to obtain a stable flame and a complete combustion for a given liquid fuel flow. This nominal flow rate m _{g name} of the atomizing gas corresponds to a nominal speed U _{9 nom} . For these values, the flame resulting from the combustion has a nominal flame length L _{f nom} . During the implementation of the method according to the invention, the controller 6 controls the control valve 5, so that the variation of the flow rate of the atomizing gas as a function of time has a sinusoidal appearance of period T represented on curve 9, the flow rate oscillating between a maximum flow rate value m _{g mgχ} and a minimum flow rate value m _{g min} , respectively corresponding to speeds U _{g max} and U _{g min} of the atomizing gas. The flow m _f , and therefore the velocity U ₁ , of the liquid fuel are assumed to be constant. During the oscillation of the atomizing gas flow, the flame length varies accordingly between a value L _{1 max} and a value L _{1 min} . The variation of flame length increases the surface of the thermal transfers and avoids the formation of high temperature zones. However, it is advisable to ensure the stability of the flame and the quality of the combustion during the variation of the flow of the atomizing gas. In order to avoid the separation of the flame and its extinction and to ensure sufficient vaporization of the drops of fuel, the speed of the atomization gas is oscillated between a lower limit U _{9 min} set at 0.7 times the nominal speed and an upper terminal U _{9 max} fixed at 1, 2 times the nominal speed. The valve is actuated by the controller 6 so as to change the flow between the corresponding values m _{g min} and m _{g max} . The variation of the flow rate of the atomizing gas as a function of time is represented by the curve 9 having a sinusoidal profile of period T.

FIG. 2 represents the flame length variation L _f as a function of the flow rate of the atomization gas m _g for the case where the atomization gas used is air and the liquid fuel has a speed of 2.3 m. s ^"1. The nominal value of velocity of the atomizing gas to obtain a stable flame and full combustion was determined as being equal to 99 ^{ms" 1.} Given the above equations, the speed of the atomizing gas thus varies between 69 ms ^-1 and 118 ms ^-1 . Under these conditions the flame length varies between 3.2 m and 4.8 m, for a nominal flame length of 4 m. This range of flame lengths allows the coverage of a large area, thus ensuring increased heat transfer, and therefore greater energy efficiency while avoiding high temperature zones generating NOx.

According to a variant of the invention, as represented in FIG. 3, the atomizer 1 differs from the atomizer of FIG. 1 only in that the liquid fuel supply is equipped with a controlled regulation valve 50 by the controller 6. The controller 6 then controls the regulating valve 50, so that the variation of the flow rate of the liquid fuel as a function of time has a bearing sinusoidal period T represented on the curve 11, oscillating between a maximum flow rate m _fmgχ and a minimum flow rate m _fmin , respectively corresponding to U _{f max} and U _{f min} fuel. In addition, the controller also synchronizes the control valves 5, 50 so that the curves 9, 11 are in phase. This double phase oscillation makes it possible to keep the size of the drops, and therefore most of the physical characteristics, including the vaporisation time, of the fuel spray 4 constant. Thus the combustion as obtained for the nominal conditions, that is to say a complete combustion, is maintained during flow variations.

As minimum and maximum values of flow and velocity for atomizing gas have been defined, minimum and maximum values of flow and velocity for the fuel are defined. In order to ensure a constant drop size and the uniformity of the atomization mode, the speed of the liquid fuel is oscillated between a minimum speed equal to 0.3 times the nominal speed and a maximum speed equal to 1. 7 times the rated speed.

The flame length variation L _f as a function of the liquid fuel flow is shown in FIG. 4. As for the example represented in FIG. 2, the atomization gas used is air and the liquid fuel has a nominal speed 2.3 ms ^"1. the liquid velocity of the fuel thus varies between 0.7 ^{ms" 1} and 3.9 ms ^"1. the nominal speed value of the atomizing gas to obtain a stable flame and complete combustion was determined to be 99 ms ^-1 . In view of the above equations, the speed of the atomizing gas thus varies between 67 ms ^-1 and 118 m / s, in which case the flame length varies between 1.7 m and 6 m, for a nominal flame length of 4 m.

Although the invention has been described in connection with particular embodiments, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims

1. A method of burning a liquid fuel comprising the steps of producing a spray (4) of the liquid fuel by injecting an atomizing gas in contact with the liquid fuel, and then putting the spray thus produced in contact with an oxidizer (7) so as to carry out the combustion of the spray, the atomizing gas being injected with a variable speed varying around a nominal speed value for which the combustion of the fuel is complete, said method being characterized in that the atomizing gas is injected at a maximum speed of less than 1.2 times the nominal value of the atomization gas velocity.

2. Combustion method according to claim 1, characterized in that the atomizing gas is injected at a minimum speed greater than 0.7 times the nominal speed of the atomizing gas.

3. Combustion method according to claim 1 or 2, characterized in that the atomizing gas is injected with a speed whose variation frequency is between 0.1 and 10 Hz.

4. Combustion method according to any one of claims 1 to 3, characterized in that the variation of the injection speed of the atomizing gas is obtained by controlling a flow control valve.

5. Combustion method according to any one of claims 1 to 4, characterized in that the liquid fuel is pulsed with a variable speed.

6. Combustion method according to claim 5, characterized in that the liquid fuel is pulsed with a speed varying in phase with the injection speed of the atomizing gas.

7. Combustion method according to any one of claims 5 or 6, characterized in that the speed variation of the liquid fuel is obtained by controlling a flow control valve.