Introduction

The invention relates to a combustion apparatus, to a fuel burner for a combustion apparatus, and to a method of operating a fuel burner/ combustion apparatus. The invention in particular relates to an apparatus and method for improving oxyfuel burner performance by controlling the mixing of oxygen and fuel using adjustable direct injection lances.

Background

In conventional fossil fuel fired combustion equipment as applied to industrial and utility boilers for steam generation the oxygen required to burn the fuel is supplied via atmospheric air. The combustion of the fuel in air releases the chemical energy stored in the fuel as heat, which is then transferred to the water in the boiler to generate steam. Typically the air is supplied to the boiler via burners, which are typically of low NOx design, and via overfire air ports. The boiler may have one or, more typically, a number of burners and overfire air ports. The burners will typically comprise a fuel stream surrounded by one or more air streams; the air streams may be swirled, the swirling air providing the stabilisation of the flame on the burner. The air may be supplied individually to each burner or overfire air port via ducts, or to a group of burners or overfire air ports via a common plenum typically known as a windbox.

In the burner (or burners), the efficiency of combustion (as indicated by the level of carbon monoxide and unburned carbon that remains after combustion is complete) and propensity for the formation of nitrogen oxides (NOx) are related to the quantity of oxygen provided and the rate of mixing of the oxidising media with the combusting fuel. Since the concentration of oxygen in air is fixed, it is necessary to adjust the overall amount of air and/or the proportion of air to the individual registers to achieve the optimum burner performance.

In one implementation of the oxyfuel technology for fossil fuel fired combustion equipment, the combustion process will utilise a recycled flue gas stream to which is added upstream of the burners a pure, or nearly pure, oxygen injection stream to produce a single comburant gas which gives combustion process performance equivalent to that of conventional air firing. The oxygen concentration of the comburant gas is variable, and increases with reducing flue gas recycle rate. This approach is sometimes referred to as "simulated air" firing. A proportion of oxyfuel plant is expected to retain an air firing capability.

Summary of Invention

According to a first aspect of the present invention, there is provided a burner for a combustion apparatus comprising:

a primary, fuel input conduit defining an inlet for a fuel supply and an outlet for supplying fuel to a combustion site;

a secondary, combustion gas input conduit defining an inlet for a first gas supply and an outlet for supplying combustion gas to a

combustion site;

a first combustion gas supply means fluidly connected to an inlet of the combustion gas input conduit; and

a second combustion gas supply means adapted to supply an oxygen rich gas directly to the combustion site generally at the outlet of the combustion gas input conduit; wherein the second combustion gas supply means comprises at least one lance having an elongate portion extending along the burner in a lance elongate direction parallel to a burner centreline and an outlet portion adapted to outlet at least a part of the oxygen rich gas in a direction at an angle to the lance elongate direction.

The term "oxygen rich gas" is intended to cover a gas supply that includes an oxygen content by volume substantially higher than air. In particular preferably the second gas supply comprises substantially pure oxygen. The first gas supply means may be adapted to supply a first gas other than air, for example comprising a gas composition having a reduced oxygen concentration relative to air and/ or a reduced nitrogen

concentration relative to air. For example the first gas supply comprises a mixture including recycled flue gas and the first gas supply means is adapted to supply such a mixture to the combustion gas input conduit. Preferably, the first gas supply is not enriched with oxygen.

Thus, in the preferred case, the first gas supply means comprises a supply of reduced oxygen levels. A suitable combustion mixture is created by supplying an oxygen enriched second gas supply. The resultant combustion mixture thus preferably comprises oxygen enriched recycled flue gas, with optional further component gases. In other words, the firing mode comprises an "oxyfuel" firing mode. However, by contrast with other burner arrangements, the addition of the pure, or nearly pure, oxygen occurs neither upstream of nor within the burner, but at the burner exit.

This offers enhanced control of oxygen injection to each individual burner.

A second gas supply, preferably comprising pure, or nearly pure, oxygen may be directly injected into an individual burner via the second gas supply means. The second gas supply means comprises at least one lance acting as a conduit for passage of the second gas supply of oxygen rich gas from a suitable source at one end of the lance to the combustion site at the other end of the lance. To act as such a conduit the lance defines a gas flow conduit and with an elongate portion extending along the burner in a lance elongate direction parallel to a burner centreline axis. It is distinctly characterised by provision of an outlet portion towards an end of the lance distal of the oxygen rich gas supply and in the vicinity of the combustion site that is adapted to supply at least a part of the oxygen rich gas in an injection direction at an angle to the lance elongate direction. Thus, the lance directs at least some of the oxygen rich gas off axis at the point of outlet (that is, in a direction at an angle to the parallel with the burner centreline axis). The output of such a lance is thus asymmetric and off-axial to the axial centerline longitudinal direction in the burner. This makes the injected supply directional which is a key aspect of the control of the present invention.

The lance is further adapted such that the off-axis injection direction is variable dynamically during use, most conveniently at least in that the lance is rotatable about an elongate axis. This changes the injection direction of the lance in simple manner. The lance may also be

translatable longitudinally. The flow rate into a lance may be variable.

By this means the point at which the oxygen rich gas and the fuel carrying streams intersect, and the level of swirl imparted by the angled oxygen streams, can both be adjusted. The rotation of a lance, and especially of an array of lances as a group or individually, therefore provides the ability to control the rate at which the oxygen rich gas mixes with the fuel and therefore the rate of combustion and heat release along the path of the flame. It is the asymmetrical nature of the lance outlet and the consequent off-axis injection direction in combination with the ability at least to rotate the lance to change the directionality of the off-axis outlet flow that enables the burner to achieve this controlled variability in the oxyfuel process.

The invention is intended to encompass a burner having a lance with an outlet portion that is adapted to outlet at least some of the oxygen rich gas in a gas injection direction at an angle to the lance elongate direction. So long as a lance does this, the required directionality of injection is obtained. In particular the invention does not preclude the use of lance(s) that also outlet at least some of the oxygen rich gas in a gas injection direction parallel to the lance elongate direction and/ or the use of directional and non-directional lances in the same burner.

The lance in particular may be so adapted by provision of asymmetrically arrayed outlet aperture(s), asymmetrically configured outlet portion structure(s) or combinations thereof.

For example, a lance outlet portion may define a conduit portion in deviating from the lance elongate direction. In this way, the part of the lance proximal the injection site is itself diverted off-axis into a gas injection direction at an angle to the lance elongate direction.

Additionally or alternatively, one or more outlet apertures may be provided on an outlet portion end face, which end face is at an angle to the perpendicular to the lance elongate direction. Thus, even if the lance conduit itself is entirely axially disposed, the outlet aperture(s) tend to outlet gas in a manner which is asymmetrically off-axis in an injection direction at an angle to the lance elongate direction.

Additionally or alternatively, one or more outlet apertures may be provided on a wall of the outlet portion upstream of an end face. Such aperture(s) may be asymmetrically arrayed to provide injection of at least a part of the gas flow in a manner which is asymmetrically off-axis.

In a particularly preferred case, the outlet portion will feature one or more outlet apertures which are asymmetrically structured and/ or located and/ or directed in such manner as to create such asymmetric off-axial outlet flow. For example the end portion of the lance will feature one or more outlet holes on a sloped end face (a face deviating from a direction perpendicular to the lance elongate direction) plus, optionally, further holes along the lance length. The size of the holes may be the same or different.

Such a configuration means the lance itself can have a simple elongate structure disposed to lie parallel to the burner centreline axis but still generate the required asymmetric injection at the oxygen rich gas injection location.

Preferably the second gas supply means comprises a plurality of lances arranged around the fuel stream defined by the primary conduit. Preferably, the second gas supply means comprises a plurality of lances arranged around the fuel stream defined by the primary conduit in generally evenly spaced manner.

Preferably, the second gas supply means comprises a plurality of lances arranged around the fuel stream defined by the primary conduit

circumferentially (that is to say, the plurality of lances are arrayed around the circumference of a notional annulus centred on the centreline of the primary conduit. Preferably, each lance has an independently adjustable gas supply and/ or is independently directionally adjustable (for example, being independently rotatable and or translatable) to control the fuel / oxygen mixing location and intensity.

Preferably, the first gas supply has a similarly independently controllable adjustable gas supply with respect to an individual burner or burner conduit register to provide the balancing inert mass of the first gas to moderate the flame temperature.

The invention provides a means of optimising the combustion process with regard to combustion efficiency, emissions, and flue gas recycle rate within oxyfuel technology. The resultant mixture of combustion gas supplied to the combustion site by a burner in accordance with the invention (including, where applicable, the content of any primary gas and/ or higher order gas streams) may have any proportion of oxygen capable of supporting combustion at the first site. In a preferred case the resultant mixture has a proportion of oxygen which generally gives combustion process performance equivalent to that achieved with air, for example being around 20 to 50%. That is, the firing mode comprises a "simulated air" oxyfuel firing mode

The implementation of the invention leads to the creation of an "advanced oxyfuel burner" in particular based on the use of "simulated air".

Preferably a further gas supply means is adapted to supply gas

additionally to the fuel input conduit such that the gas at least assists in transporting the fuel to the combustion chamber. Preferably, both of the first combustion gas supply means and the further gas supply means are adapted to supply a mixture including recycled flue gas, for example in that both are fluidly linked to a recycled flue gas supply means. Thus, the further gas supply means comprises a supply of a primary flue gas recycle (PFGR) stream, and the first combustion gas supply means comprises a supply of a secondary flue gas recycle (SFGR) stream in familiar manner. In a preferred embodiment, the PFGR stream is enriched by oxygen upstream of the burner. By contrast in a preferred embodiment, the SFGR stream is not enriched by oxygen upstream of the burner, but by the oxygen supply at the burner exit.

The burner may comprise further fluidly independent combustion gas input conduits, for example comprising additional tertiary or higher order input conduits, fluidly connected to additional gas supply means for supplying combustion gas to a combustion site defined by the burner, for example directly to the burner or otherwise to a combustion apparatus in which the burner is located. Preferably, the additional gas supply means for supplying combustion gas to such additional tertiary or higher order input conduits are adapted to supply a mixture including recycled flue gas, for example in that they are fluidly linked to a recycled flue gas supply means. Thus, the additional gas supply means comprises a supply of a tertiary (or higher order) flue gas recycle stream. In a preferred embodiment, the stream is enriched by oxygen upstream of the burner, for example to approximate simulated air concentrations. In a typical arrangement, a primary fuel input conduit may extend along a burner, a secondary combustion gas input conduit may be disposed outwardly of and for example annularly arrayed about the primary fuel input conduit, and higher order combustion gas input conduits, where present, may be disposed outwardly of and for example annularly arrayed about the secondary input conduit in familiar manner. The primary input conduit may be a central conduit extending generally axially along the burner, for example on a burner centreline. Alternatively, the primary input conduit may itself be disposed about a central conduit, for example annularly, with the central conduit serving another purpose. The primary input conduit is still preferably nearer to the centre line than the secondary and higher order conduits, but the primary stream does not necessarily flow along the centreline itself. A conduit may include suitable swirl generation structures to impart an axial swirl to a gas supply therein. In a particularly preferred embodiment, the burner comprises:

an axially extending primary fuel input conduit defining an inlet for a fuel supply and an outlet for supplying fuel to a combustion site, and having a primary recycled flue gas supply means to supply recycled flue gas to the inlet;

a secondary combustion gas input conduit disposed outwardly of and for example annularly surrounding the primary fuel input conduit, defining an inlet for a gas supply and an outlet for supplying gas to a combustion site, and having a secondary recycled flue gas supply means fluidly connected to the inlet, and a plurality of oxygen rich gas supply means adapted to supply an oxygen rich gas directly to the combustion site generally at the outlet arrayed around the fuel stream, wherein the second combustion gas supply means comprises at least one lance having an elongate portion extending along the burner in a lance elongate direction parallel to a burner centreline and an outlet portion adapted to outlet the oxygen rich gas in a direction at an angle to the lance elongate direction; and further preferably

a tertiary combustion gas input conduit disposed outwardly of and for example annularly surrounding the primary and secondary conduits, defining an inlet for a gas supply and an outlet for supplying combustion gas to a combustion site, and having a tertiary recycled flue gas supply means fluidly connected to the inlet.

The axially extending primary fuel input conduit preferably extend along a burner axial direction about the burner centreline. The primary conduit may comprise a central cylinder. Alternatively the primary conduit may comprise an annular conduit disposed around a central cylinder symmetrically about the burner centreline. Preferably the gas supply means includes varying means, such as a baffle or valve means, for varying the proportion of gas supplied to one or both or all where applicable of the fuel input conduit and the secondary and higher order combustion gas input conduits. Optionally, a further oxygen rich gas supply means is provided to supply oxygen rich gas into the primary and/ or tertiary recycled flue gas supply upstream of the burner.

In a more complete aspect of the present invention, there is provided a combustion apparatus comprising:

a combustion chamber; and

at least one and preferably a plurality of burners as hereinbefore described located so as to define combustion sites within the combustion chamber.

Preferably the combustion apparatus comprises a boiler for generating steam.

Preferably the fuel used is coal, most preferably pulverised coal. Preferably the combustion apparatus includes a flue gas recirculation conduit. Preferably the flue gas recirculation conduit is fluidly connected to a flue gas supply means and/ or to at least one of the primary fuel input conduit and the secondary and/ or higher order combustion gas input conduits such that a mixture including flue gas may be supplied to the combustion chamber. Preferably the flue gas recirculation conduit is fluidly connected to both of the fuel input conduit and the/ each

combustion gas input conduit. According to a further aspect of the present invention, there is provided a method of firing a burner in a combustion apparatus comprising the steps of:

supplying fuel to primary, fuel input conduit of the burner;

supplying a first gas to an inlet of a secondary, combustion gas input conduit of the burner;

supplying a second, oxygen rich gas directly to the combustion site generally at the outlet of the combustion gas input conduit via at least one lance having an elongate portion extending along the burner in a lance elongate direction parallel to a burner centreline and an outlet portion adapted to outlet at least part of the oxygen rich gas in a gas injection direction at an angle to the lance elongate direction;

causing combustion of fuel to occur at the combustion site while controlling the injection direction to control the point at which first and second gas streams intersect.

Preferably, the injection direction is controlled at least in that the lance is rotatable about an elongate axis. The lance may also be translatable longitudinally. The flow rate into a lance may be variable. In typical operation, the fuel is entrained in the first combustion gas, and the method enable the point at which the oxygen rich gas and the fuel carrying streams intersect, and the level of swirl imparted by the angled oxygen streams, to be adjusted in simple and convenient manner.

In this way fuel and a combustion gas mixture are delivered to a

combustion site generally at an outlet of the combustion gas input conduit; the combustion gas mixture comprising a mixture of the first gas and the second gas being formed at the outlet of the combustion gas input conduit. Preferably the first gas comprises a composition having a reduced oxygen concentration relative to air and/ or a reduced nitrogen concentration relative to air. For example the first gas supply comprises a mixture including recycled flue gas. Preferably, the first gas is not enriched with oxygen. A suitable mixture is created rather by supplying an oxygen enriched and for example substantially pure oxygen second gas directly at the burner exit. Preferably combustion gases are supplied to the

combustion site, via primary, secondary and where applicable additional supply streams, in such manner that the resultant mixture has a proportion of oxygen that produces combustion process performance equivalent to that achieved with air, for example being around 20 to 50%.

Other preferred aspects of the method, and in particular modes of operation and gas compositions, will be appreciated by analogy. Summary of Figures

The invention is described by way of example only with reference to figures 1 and 2 of the accompanying drawings in which:

Figure 1 is a schematic of a conventional air firing burner;

Figure 2 is a schematic of the oxyfuel process; Figure 3 is a schematic of a modified burner embodying the principles of the invention;

Figure 4 is a close up view of alternative arrangements of lance outlet for use in the modified burner of figure 3.

Specific Description

Figure 1 is a schematic of an air firing burner of conventional design.

Figure 2 is a schematic of the oxyfuel process into which either a burner of conventional design such as illustrated in figure 1 or a burner in

accordance with the invention may be incorporated.

In a conventional air fired burner the combustion air, containing the required oxygen to burn the fuel, is supplied by means of a forced draught fan (FD fan) to individual burners via air ducts, or to groups of burners through a windbox. In the example burner (1 ) shown in figure 1 , there are three separate air streams in the burner; the primary air (PA) which conveys the coal, the secondary air (SA), and the tertiary air (TA); specific burner designs may have fewer or more air streams. In the example shown, the primary air (PA) stream follows the burner axis (11 ) and the secondary air (SA), and the tertiary air (TA) streams are axially directed in ducts concentrically therearound. The burner (1 ) is fired through an outlet in a furnace wall (FW). In the example shown in figure 1 and in the embodiment of figure 3 the primary air stream is in a central conduit. The invention is not limited to such arrangements. One alternative design option involves moving the primary air (or PFGR/Oxygen/Fuel mix) to a conduit annularly parallel to the central line, but with a central cylinder along the axis used for other purposes (core air and/or for oil/gas igniters). In this case, the primary stream would still be nearer the centre line than the secondary and tertiary streams, but wouldn't flow along the actual centerline.

Dampers (3) control the division of the air between the secondary and tertiary streams (SA, TA). The secondary and tertiary air streams may be swirled, and the extent of swirl may be adjustable. Swirl devices (5) are provided downstream of the dampers for this purpose. Optimisation of the burner with respect to combustion efficiency, emissions, and flame stability is achieved by variation of the total quantity of the air supplied to the burner, the division of the air between the various streams, and the level of swirl applied.

In the "simulated air" exemplification of oxyfuel firing shown in figure 2, flue gas is recycled to the coal pulverising mill (21 ) (primary flue gas recycle, or PFGR) and to the windbox (23) containing the burners (not specifically shown in figure 2) (secondary flue gas recycle, or SFGR) by means of dedicated fans (respectively the primary flue gas recycle fan 25 and the secondary flue gas recycle fan 27). There are numerous detailed variants of this process, but they all follow the same generic method.

The composition of the recycled flue gas is related to the combustion process, but the stream extracted from the boiler exit will contain low levels of oxygen, typically less than 5% by volume, and insufficient to support combustion. Pure, or nearly pure, oxygen is introduced into the PFGR and SFGR streams to provide the oxidant required to combust the fuel. The composition of the PFGR and SFGR streams will depend upon the detailed implementation of the oxyfuel technology, but typically the PFGR will contain around 20 to 25% by volume of oxygen or higher, whereas the SFGR will contain a significantly higher oxygen

concentration, for example 25 to 50%. The exact concentration levels will be dependant upon a number of factors including the overall furnace stoichiometry, the quantity of flue gas that is recycled to the boiler, the amount of combustion generated moisture that is removed from the recycle stream, the amount of air that leaks into the process, etc.

The streams supply burners (not specifically shown) via the windbox (23) to fire the furnace/ boiler (31 ) in generally known manner, with flue gases being drawn off via a particulate removal system (33) to remove solids (ash, etc, 34) and drawn by means of an ID fan (35) to a stack or capture stage (37) as will be familiar.

Figure 3 is a schematic of an example embodiment of oxyfuel burner (41 ) in accordance with the invention. This is suitable for incorporation into a general oxyfuel process such as, but not limited to, the process

exemplified in figure 2.

In this embodiment it is proposed that the addition of the pure, or nearly pure, oxygen occurs neither upstream nor within the burner, but at the burner exit. In the advanced oxyfuel burner, coal is transported with primary flue gas recycle (PFGR) along the burner centreline axis (51 ). The oxygen concentration in this stream may typically be enriched to an oxygen level equal to or less than 21 % by volume, or some other value.

Within the concentric arrangement proposed, a secondary annular register (44) carries a proportion of secondary flue gas recycle (SFGR) to which no additional oxygen has been supplied. Furthermore the secondary annulus accommodates a series of oxygen injection lances (45) spaced around the circumference of this annulus for the direct introduction of oxygen at the confluence of the primary and secondary streams. The main part of the length of each lance (45) extends axially parallel to the burner centerline axis and along the secondary annulus.

Each lance has an outlet formation at the end portion proximal the confluence of the primary and secondary streams which is so configured as to direct at least some of the outlet flow away from the axial direction (that is, the lance elongate direction parallel to the burner centerline axis). In the example of figure 3 the end portion of the lance will feature one or more outlet holes which are asymmetrically structured and/ or located and/ or directed in such manner as to create such asymmetric off-axial outlet flow. For example as shown in figure 3 the end portion of the lance will feature one or more outlet holes on a sloped face (a face deviating from a direction perpendicular to the lance elongate direction) plus, optionally, further holes along the lance length. The size of the holes may be the same or different.

Each lance will have the capability for it to be rotated; by this means the point at which the oxygen and fuel streams intersect, and the level of swirl imparted by the angled oxygen streams, can be adjusted. The rotation of each lance, as a group or individually, therefore provides the ability to control the rate at which the oxygen mixes with the coal and therefore the rate of combustion and heat release along the path of the flame. It is the asymmetrically acting outlets acting such as to create asymmetric off-axial outlet flow in combination with the ability to rotate the lance to change the directionality of the asymmetric off-axial outlet flow that enables the burner to achieve this controlled variability in the oxyfuel process.

On the outside of the secondary annulus is a tertiary annulus (46). The tertiary annulus contains a means of imparting a swirl on the flow in the form of a swirl device (49), introduces the remaining SFGR to the flame and provides ballast and a further means of flame temperature control. Typically the swirl device (49) in the tertiary annulus (46) will have the capability to vary the level of swirl imparted. Although in the embodiment specific swirl devices are provided, it is conceivable that a burner in accordance with the invention could use the directional lances to complement (or even to replace) some of the swirler function inside the burner Damper devices (43) are located on the secondary annulus barrel; the adjustment of these dampers allows the proportion of SFGR supplied to the secondary and tertiary annuli to be adjusted.

Alternative possible means for providing asymmetrically acting outlets acting such as to create asymmetric off-axial outlet flow from the end portion of the lance are considered in the figure 4 schematic inserts.

Figure 4a is merely an enlargement of the end of the lance in figure 3. The end face of the lance is a single bevel (a face deviating from a direction perpendicular to the lance elongate direction). A simple aperture in the face thus generates directional flow.

Figure 4b illustrates a more complex end structure with more than one face. In such case at least one face slopes from a direction perpendicular to the lance elongate direction to create directional flow. Of course, this simple schematic is merely illustrative of alternatives which could have multiple faces at multiple angles. In Figure 4c the end portion of the lance itself deviates from an axial direction. In such a case even a simple aperture in a perpendicular face generates an off-axis outlet flow. Combinations of such arrangements, optionally with further aperture outlets in the elongate lance wall proximal the end face, and of any other arrangements producing directional off-axis outlet flow, can be considered without departing from the principles of lance design exploited by the invention. In particular, although the figures show planar faces,

alternatives wherein faces are concave or convex may be considered. Similarly, although faces are shown for simplicity with a single aperture, there are likely in practice to be plural arrays of apertures in angled faces and/ or cylinder walls in the manner of a pepperpot. Arrangements of aperture and end structure may be such that some flow is off-axis and some is axial. Additionally, axial injection lances may also be provided. The condition for the invention is satisfied so long as there is at least some off-axis flow the direction of which can be affected by rotation of the asymmetric lances.

In one variation of the above description of the advanced oxyfuel burner a fraction of the pure, or nearly pure, oxygen is additionally supplied upstream of the burner to increase the oxidant concentration to a level higher than that exiting the boiler, but not sufficient to supply the required oxidant to completely combust the fuel.

In a second variation of this invention, there may be supplemental addition of pure or nearly pure oxygen to the tertiary annulus upstream of the swirl generation device. In a third variation of this invention, the amount of pure or nearly pure oxygen supplied to an individual burner will be controlled, so as to allow the biasing of the stoichiometry between burners to optimize the overall system performance with regard to combustion efficiency, emissions, flame stability, and gas composition at the furnace walls.

The invention provides a number of potential advantages including some or all of the following. The mixing point of the injected oxygen stream with the fuel stream can be controlled by the use of the directional (for example, in the embodiment, the chisel-faced) injection lances and variable swirl recycled flue gas.

The mixing point of the injected oxygen and fuel can further be controlled by the rotation of the injection lances.

The oxygen flow to individual burners can be controlled by adjusting the supply to each burner's injection lances, thereby allowing the biasing of stoichiometry between burners.

The use of the three methods above described can optimise combustion and alleviate corrosion on the walls of a fossil fuel fired furnace.

The use of the three methods above described can reduce the overall rate of flue gas recycle, thereby reducing the size of certain items of equipment such as particulate removal plant, fans, etc., reducing the power consumption of auxiliary equipment such as recycle fans, and reducing efficiency losses from the combustion process.