Oil burners

Abstract

An oil burner 14 has an oil supply interface 102 at a first end and a firing tip 104 at a second end, with the second end including a control pintle 116 arranged to control a flow of oil out of the fi

Description

TECHNICAL FIELD The present invention relates to oil burners for use in biomass power stations. BACKGROUND Biomass power stations burn biomass (e.g. wood) in a furnace to produce heat, which is used to heat steam that powers a turbine to produce electricity. The biomass fuel is typically burned in a pulverised form (referred to as pulverised fuel or PF). A typical biomass power station unit furnace features multiple PF burners (e.g. up to 48 or more) that burn PF to produce heat. The number of PF burners that are active at any given time can be controlled to vary the power output of the power station unit. To ignite reliably a pulverised fuel burner on start-up biomass boilers typically include auxiliary burners which burn a different fuel to provide an initial flame (e.g. one for each PF burner). These burners can also help with flame stability when lower volumes of biomass fuel are being burned by the PF burners. A common choice for these auxiliary burners is an oil burner. An oil burner for use in a biomass furnace boiler is typically elongate, extending from an oil supply end located outside of the furnace, to a tip located inside of the furnace adjacent to its corresponding PF burner. Oil is supplied to the tip through internal oil flow channels. An oil burner in a biomass furnace does not fire continuously. There may be reasonably long periods of time in which the oil burner is not firing, for instance in biomass power station units used to meet a base load of electricity demand in which the number of active PF burners does not change very often. Conversely, in biomass power station units whose output varies more frequently (e.g. to adapt to changing peak electricity loads), the oil burners will fire multiple times each day as PF burners are stopped and started. It is important for the oil burners to be able to fire reliably whenever they are needed, whether this is after a long period of nonfiring or shortly after a previous firing. When an oil burner is not firing, stagnant oil in the internal fuel channels is at risk of stagnating and solidifying, e.g. due to cool air flowing into the furnace carrying heat away from the elongate oil burner. Moreover, oil near to the tip of the oil burner could become very hot due to the heat of combustion in the furnace. These effects could cause the re-lighting of the oil burner to be unreliable. It has been proposed to address this by continuously circulating oil through the internal channels of an oil burner, even when it is not firing, to dissipate heat from the tip and maintain oil throughout the burner at an operating temperature. However, it has proved challenging to accomplish this whilst maintaining reliable control over oil flows in the firing mode, e.g. due to limitations on control hardware that can practically be positioned near to the extreme conditions experienced by the tip. The present invention seeks to address this problem. SUMMARY When viewed from a first aspect the present invention provides an oil burner for supporting biomass combustion, the oil burner comprising: an oil supply interface at a first end; a firing tip at a second end; a control pintle at the second end arranged to control a flow of oil out of the firing tip; a first oil channel extending from the oil supply interface to the firing tip; a second oil channel extending from the oil supply interface to the control pintle, wherein the control pintle comprises a circulation oil channel connecting the second oil channel to the first oil channel; and a non-return valve arranged to prevent oil flow from the first oil channel to the second oil channel via the circulation oil channel but to permit oil flow from the second oil channel to the first oil channel via the circulation oil channel; wherein the control pintle is arranged to: permit oil to flow from the first oil channel out of the firing tip when a pressure differential between the first oil channel and the second oil channel is positive and greater than a firing threshold; and direct oil from the second oil channel to the first oil channel via the circulation oil channel and prevent oil from flowing out of the firing tip when a pressure differential between the first oil channel and the second oil channel is negative. It will be recognised by those skilled in the art that the use of a non-return valve to allow oil to flow in only one direction through the circulation oil channel to the first oil channel can improve the reliability of the oil burner. The Applicant has recognised that preventing oil flow from the first oil channel through the circulation oil channel can mitigate pressure loss and thus cause the control pintle to open more readily. This may allow the oil burner to fire more reliably at lower oil pressure differentials between the first oil channel and the second oil channel. The arrangement of the oil channels, the control pintle and the non-return valve allows the oil burner to be operated in a desired mode by adjusting oil flow into the first and second oil channels at the oil supply interface. Creating a sufficiently positive pressure differential between the first and second oil channels (i.e. a pressure in the first oil channel that is sufficiently higher than the pressure in the second oil channel) causes the pintle to open, allowing the oil burner to fire. This is the oil burner operating in a firing mode. Conversely, creating a negative pressure differential between the first and second oil channels (i.e. a higher oil pressure in the second oil channel) causes the pintle to close and for oil to circulate through the first and second oil channels instead. This is the oil burner operating in a circulating mode. The use of a non-return valve (sometimes also referred to as a “check valve”) to mitigate pressure loss in the firing mode means that the ability to fire the oil burner at lower pressures can be enhanced without adversely impacting oil flows in the circulating mode. In contrast, for instance, boosting a pressure difference in the firing mode by constraining bidirectional flow with a metering screw presents problems, because using a small metering orifice to maintain a sufficient pressure difference to keep the pintle in the open position in a firing mode (particularly in a low firing mode) may prevent a required level of circulating flow in the circulating mode. The negative pressure differential between the first oil channel and the second oil channel may provide sufficient to move the control pintle into a closed position where oil is prevented from flowing out of the firing tip. However, in a set of embodiments, the oil burner comprises a biasing member arranged to bias the control pintle towards a closed position in which oil is prevented from flowing out of the firing tip. In a set of embodiments, the biasing member comprises a spring. The strength of the biasing member (i.e. a magnitude of biasing force provided by the biasing member) may at least partially define the firing threshold pressure differential. The pintle may comprise a tapered portion (e.g. at or near a front of the pintle). The tapered portion may be arranged to seat in the firing tip to allow oil discharge from the tip in a firing mode and prevent oil discharge in a circulating mode (i.e. to open or close the firing tip). The pintle may comprise one or more surfaces (e.g. a front face) on which oil pressure in the first oil channel acts to generate a force urging the control pintle into an open position (i.e. a firing position in which oil can flow out of the firing tip). It will be recognised that said surface(s) thus cause the control pintle to open when there is sufficient oil pressure in the first oil channel to overcome any opposing forces (i.e. force from oil pressure in the second oil channel and / or a biasing force from a biasing member). The pintle may be elongate. The circulation channel may extend parallel to a longitudinal axis of the pintle. The pintle may comprise one or more outlet orifice(s) which open onto the first oil channel and define a first end of the circulation channel. The outlet orifice(s) may extend radially (i.e. perpendicular to a longitudinal axis of the pintle). The pintle may comprise one or more inlet orifice(s) which define a second end of the circulation channel. The inlet orifice(s) may open onto the second oil channel, potentially via the NRV as explained below. In a set of embodiments, the control pintle comprises at least part of the NRV. The NRV may be positioned directly adjacent the circulation channel (e.g. adjacent one or more inlet orifices at a second end of the circulation channel). However, it will be appreciated that the non-return function of the NRV could be effectively obtained with a different positioning (e.g. the NRV could be located at another point within the second oil channel). In a set of embodiments, the NRV comprises a ball-type non-return valve. In other words, the NRV may comprise: an enclosure comprising a first opening in fluid communication with the first oil channel (e.g. via the circulation channel) and a second opening in fluid communication with the oil supply interface (e.g. via the second oil channel); and a captive ball (e.g. a ball bearing) within the enclosure; wherein the captive ball bearing is arranged to seal the second opening in response to oil flowing from the first opening toward the second opening, but to allow oil to flow from the second opening to the first opening. The second opening may comprise a tapered internal surface against which the ball is pressed by oil flowing from the first opening toward the second opening. The NRV may comprise an oil passage slot that allows oil to flow past the captive ball when flowing from the second opening to the first opening. However, it will be recognised that a ball-type NRV is not essential and alternatively the NRV may comprise any appropriate NRV arrangement known in the art perse. The oil burner may burn oil using oxygen from a separate source (e.g. primary air from an associated PF burner). However, in a set of embodiments, the oil burner comprises an air supply arrangement arranged to deliver combustion air to the firing tip (sometimes referred to as “core” air). The air supply arrangement may comprise a core air tube. The core air tube may extend alongside the first and / or second oil channels. In some embodiments, one or more parts of the oil burner may be located inside the core air tube. For instance, the oil burner may comprise a burner assembly located inside the core air tube that comprises the firing tip, the control pintle and at least part of the first and second oil channels. The oil burner may be elongate. In a set of embodiments, first and second ends of the oil burner are separated by at least 1 m, at least 2 m or at least 3 m (e.g. by approximately 4 m). In a set of embodiments, the oil burner comprises a single first oil channel. However, in some embodiments, the oil burner comprises a plurality of first oil channels that each extend from the oil supply interface to the firing tip. Correspondingly, the oil burner may comprise a single second oil channel, or a plurality of second oil channels that extend from the from the oil supply interface to the control pintle. Because oil only flows in the second oil channel(s) in the circulating mode (e.g. when flow rates may be lower), the second oil channel(s) may have a lower flow capacity than the first oil channel(s). In a set of embodiments, the first oil channel (or at least one first oil channel if there are several) is located radially outside the second oil channel (or at least one second oil channel). In other words, the second oil channel may be located further inside the oil burner than the first oil channel. This may increase the degree to which the second oil channel is isolated from surrounding environmental conditions (e.g. cool air in a surrounding core air tube). This may be useful for mitigating excessive cooling of oil in the second oil channel (where oil flow rates may be generally lower than in the first oil channel). In a set of embodiments, the oil burner comprises a pressure atomised oil burner, i.e. where the atomisation of oil for burning is produced by the pressure of the oil itself, rather than any separate atomisation mechanism (e.g. air or steam). In a set of embodiments, the oil burner comprises a control block arranged to control the supply of oil into to the oil burner at the oil supply interface. In a set of embodiments, the oil burner comprises one or more pumps arranged to generate oil pressure for oil supplied to the oil supply interface. In a set of embodiments, the oil burner comprises an electrical spark igniter. In some embodiments, the oil burner comprises a two-stage ignition system comprising an electrical spark igniter arranged to ignite a gas flame that is arranged to ignite the oil flame. The oil burner may be suitable for use with a variety of different oils including but not limited to: heavy fuel oil, hydrogenated vegetable oil, or any other biofuel oil. The present invention extends to a method of operating the oil burner disclosed herein to support biomass combustion, the method comprising: operating the oil burner in a firing mode by supplying oil at the oil supply interface to produce a pressure differential between the first oil channel and the second oil channel that is positive and greater than a firing threshold; and operating the oil burner in a circulating mode by supplying oil at the oil supply interface to produce a pressure differential between the first oil channel and the second oil channel that is negative. In other words, the oil burner may be operated in a firing mode by supplying oil to the first oil channel at a sufficiently higher pressure than the second oil channel, and in a circulating mode by supplying oil to the second oil channel at a higher pressure than the first oil channel. Whilst a single firing mode may be sufficient for some implementations, a greater degree of control may be desired, e.g. to provide different strengths of flame for ignition and stabilisation functions. In a set of embodiments, the method comprises: operating the oil burner in a low firing mode by supplying oil at the oil supply interface to produce a first pressure differential between the first oil channel and the second oil channel that is positive and greater than a firing threshold; and operating the oil burner in a high firing mode by supplying oil at the oil supply interface to produce a second pressure differential between the first oil channel and the second oil channel that is positive and greater than the first pressure differential. The oil burner may be operated in a firing mode (e.g. the low or high firing mode) to ignite an associated biomass pulverised fuel burner. The oil burner may be operated in a firing mode (e.g. the low or high firing mode) to stabilise an associated biomass pulverised fuel burner. The oil burner may be operated in the circulating mode when an associated biomass pulverised fuel burner is not activated and / or when its associated PF burner is stably ignited. The present invention extends to a biomass power station unit comprising: a furnace comprising a plurality of biomass pulverised fuel burners arranged to burn biomass pulverised fuel to produce heat, and an oil burner as disclosed herein associated with each of the biomass pulverised fuel burners; a boiler configured to use the heat from the plurality of biomass pulverised fuel burners to heat steam; a turbine configured to be powered by the steam from the boiler; and a generator configured to be driven by the turbine to generate electricity. The present invention extends to a method of operating the biomass power station unit comprising: burning biomass pulverised fuel in the plurality of biomass pulverised fuel burners to produce heat; using the heat from the plurality of burners to heat steam in a boiler; using the steam from the boiler to power a turbine; driving a generator with the turbine to generate electricity; and operating the plurality of oil burners to support biomass combustion by the biomass pulverised fuel burners. Each oil burner may be operable to ignite and / or stabilise a flame of its associated pulverised fuel burner. The method may comprise operating an oil burner in a firing mode to ignite and / or stabilise an associated PF burner. The method may comprise an oil burner operating in a circulating mode when its associated PF burner is not activated and / or when its associated PF burner is stably ignited. In a set of embodiments, each oil burner is positioned coaxially inside its associated PF burner. One or more of the biomass pulverised burners may be operable to produce at least 2 MW of heat power, at least 5 MW of heat power or at least 10 MW of heat power. The biomass pulverised fuel may comprise wood. Additionally or alternatively, the biomass pulverised fuel may comprise agricultural residues. Additionally or alternatively, the biomass pulverised fuel may comprise purpose-grown agricultural fuel crops. In a set of embodiments, the biomass power station unit is operable to generate at least 100 MW of electricity, at least 200 MW of electricity, at least 400 MW of electricity or at least 600 MW of electricity. The present invention extends to a biomass power station comprising one or more biomass power station units as disclosed herein. The present invention also extends to a method of operating a biomass power station to generate electricity comprising operating one or more biomass power station units to generate electricity according to a method disclosed herein. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments, it should be understood that these are not necessarily distinct but may overlap. BRIEF DESCRIPTION OF DRAWINGS One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which: Figure 1 is a schematic diagram of a biomass power station unit according to an embodiment of the present invention; Figure 2 is a schematic diagram of the furnace of the power station unit; Figure 3 is a schematic diagram of an oil burner according to an embodiment of the present invention, operating in a firing mode; Figure 4 is a schematic diagram of the oil burner operating in a circulating mode; and Figures 5-7 are schematic diagrams of the control pintle of the oil burner. DETAILED DESCRIPTION Figure 1 is a simplified schematic view of a biomass power station unit 2 operable to produce electrical power by burning biomass. The power station unit 2 comprises a furnace 6, a boiler 8 and a turbine 10 and a generator 11. The furnace 6 receives biomass pulverised fuel (PF, e.g. from one or more pulverising mills), and burns it to produce heat. The heat is used by the boiler 8 to heat steam. The heated stem powers the turbine 10, which drives the generator 11 to produce electricity. Figure 2 shows the furnace 6 in more detail. The furnace 6 comprises a plurality of pulverised fuel burners 12 that receive biomass PF and air, and burn the PF to produce heat. The furnace 6 shown in Figure 2 has eight PF burners 12, although it will be appreciated that this is only exemplary and in practice more or fewer PF burners 12 may be used. In one example, a biomass furnace includes 48 PF burners arranged in eight rows of six. The amount of electricity produced by the turbine 10 and generator 11 can be controlled by activating different numbers of the PF burners 12 in the furnace 6 (and, to some extent, by varying the amount of PF fuel that is sent to the PF burners 12). To ensure that each PF burner 12 is reliably ignited each time it is activated, the furnace 6 further comprises a plurality of oil burners 14. One oil burner 14 is positioned coaxially inside each PF burner 12. The oil burners 14 burn oil (e.g. heavy fuel oil or hydrogenated vegetable oil) to produce an initial flame. This initial flame ignites the corresponding PF burner 12 when that PF burner 12 is activated. The flame provided by the oil burners 14 can also help to maintain a stable PF flame when lower volumes of biomass fuel are being burned. The oil burners 14 themselves are ignited with an electrical spark igniter (not shown). The oil burners 14 do not fire continuously. Instead, they only fire when a corresponding PF burner 12 needs to be ignited or stabilised. In a typical biomass power station unit used for non-baseload generation, each oil burner 14 may be fired on multiple separate occasions each day. The structure and operation of an oil burner 14 will now be described with further reference to Figures 3 and 4. Figure 3 shows the oil burner 14 in a firing mode. Figure 4 shows the oil burner 14 in a circulating mode. The oil burner 14 is elongate, measuring around 4 m from an oil supply interface 102 at a first end to a firing tip 104 at the other end. The oil burner 14 comprises a burner assembly 106 located within a core air tube 108. The burner assembly 106 delivers oil for combustion to the tip 104. The core air tube 108 delivers air for combustion to the tip 104. Air is driven into the core air tube 108 by a core air system (not shown) located outside of the furnace 6. The burner assembly 106 comprises the oil supply interface 102, which connects to a control block 110. The control block 110 controls the supply of oil into and the return of oil from the oil burner 14. Oil pressure may be produced by one or more pumps not shown. The burner assembly 106 also comprises a first oil channel 112, a second oil channel 114, a control pintle 116, and a biasing spring 119. The control pintle 116 comprises a non-return valve 118. The first oil channel 112 is located towards the radial outside of the burner assembly 106, i.e. adjacent the core air tube 108. The first oil channel 112 extends from the oil supply interface 102 to the firing tip 104. When the control pintle 116 is in an open position, retracted from the tip 104, as it is shown in Figure 3, oil can flow from the first oil channel 112 and spray out of the firing tip 104 (where it burns to produce a flame), as indicated by the dashed arrows. When the control pintle 116 is in a closed state, engaged with the tip 104, as shown in Figure 4, oil is prevented from spraying out of the firing tip 104 and no flame is produced. The biasing spring 119 pushes the control pintle 116 towards the closed state. The second oil channel 114 is located at the centre of the oil burner 14 (i.e. radially inside the first oil channel 112). The second oil channel 114 extends from the oil supply interface 112 to the control pintle 116. The control pintle 116 comprises a circulation oil channel 120 which extends from radial orifices 122 open to the first oil channel 112 to the second oil channel 114 via the non-return valve (NRV) 118, which is located at the rear of the pintle 116. The NRV 118 permits the flow of oil in only one direction, from the second oil channel 114 into the circulation oil channel 120 (i.e. from left to right in Figures 2 and 3). The non-return valve 118 prevents oil flowing from the circulation oil channel 120 back into the second oil channel 114. The control pintle 116 also comprises a tapered front portion 121. The tapered front portion 121 seats in the firing tip 104 to allow or prevent oil discharge from the tip. The control pintle 116 comprises a front face 123. Oil pressure in the first oil channel 112 acts on this surface to generate a force urging the control pintle 116 towards the open position. The oil burner 14 is operable in three modes: a low firing mode, a high firing mode and a circulating mode. The operation of the oil burner 14 in these modes is controlled by changing the flow of oil into the first and second oil channels 112, 114, i.e. using the control block 110. To operate the oil burner 14 in the circulating mode of operation as pictured in Figure 4, oil is controlled to flow into the second oil channel 114 at the oil interface 102 and toward the tip 104. Oil then flows through the NRV 118, the circulation oil channel 120 and the radial orifices 12 and into the outside first oil channel 112 before returning to the oil interface 102. In this configuration, the oil in the second oil channel 114 is at a higher pressure than the oil in the first oil channel 112, as there is a pressure drop when the oil is forced through the circulation oil channel 120. There is thus a negative pressure differential between the first and second oil channels 112, 114, which produces a force on the pintle 116 in the same direction as the biasing force from the spring 119. These forces push the pintle 116 into the closed position shown in Figure 4, preventing any oil from flowing out of the tip 104. In the circulating mode, the oil burner 14 thus does not ignite. Instead, oil from the supply interface 102 simply circulates through the second and first oil channels 114, 112. This circulation helps to dissipate heat from the tip 104 and avoid any cool areas away from the furnace 6 in which oil might otherwise solidify. These effects may be achieved with a relatively low oil flow rate in the circulating mode. To operate the oil burner 16 in the low firing mode, oil is controlled to flow into the outside first oil channel 112 at the oil interface 102 and toward the tip 104 (i.e. in a reverse direction compared to the circulating mode). The NRV 118 prevents oil from flowing in this direction through the circulation oil channel 120 into the second oil channel 114. The oil pressure in the first oil channel 112 is thus now higher than that in the second oil channel 114, i.e. there is a positive pressure differential between the first and second oil channels 112, 114. This produces a force that opposes the spring force from the biasing spring 119. In the low firing mode, oil is controlled to flow toward the tip 104 along the outside first oil channel 112 to produce a pressure differential that is above a firing threshold (e.g. above a pressure required to overcome the biasing spring force). This causes the pintle 116 to move into an open position (shown in Figure 3). Oil from the first oil channel 112 sprays out of the tip 104 where it ignites to produce a flame. To operate the oil burner 6 in the high firing mode, oil is controlled to flow toward the tip 104 along the outside first oil channel 112 at a higher pressure than in the low firing mode, i.e. to produce a pressure differential that is even further above the firing threshold. This causes the pintle 116 to open even further and thus allow more oil to spray out of the tip 104, producing a larger flame. In the low and high firing modes, the NRV 118 prevents oil from flowing in the second oil channel 114. However, the rate of oil flow is higher in both firing modes than in the circulating mode. As such, the oil burner 14 does not suffer from issues with cold oil in the second oil channel 114 in the firing modes because the relatively high flow of oil in the first oil channel 14 can sufficiently distribute heat along the oil burner 14 to avoid cold spots in the second oil channel 112. Because oil flow from the first oil channel 112 to the second oil channel 114 is prevented in the firing modes, the degree to which the control pintle 116 opens in the firing modes is determined only by the relative forces of the biasing spring and the oil pressure in the first oil channel 112. In contrast, if no NRV 118 was present and a small flow of oil was permitted to flow through the control pintle 116 in both directions, the oil pressure lost through the pintle 116 in the firing mode would complicate control of pintle position and potentially require constraining the amount of flow allowed in the circulating mode. Figures 5-7 show the control pintle 116 in more detail. The dot-dashed line labelled F7 in Figure 5 illustrate the plane of the cross section shown in Figure 7. Similarly, the dot-dashed lines labelled F5 and F6 in Figure 7 show the planes of the cross sections shown in Figure 5 and Figure 6 respectively. As explained above, the pintle 116 comprises the circulation oil channel 120, the tapered front portion 121, the front face 123, the radial orifices 122 and the NRV 118. The NRV 118 comprises a tapered oil inlet 124, a captive ball bearing 126 and an oil passage slot 128. The oil passage slot 128 is narrower than the diameter of the circulation oil channel 120 as shown in Figure 7. Figure 5 shows the control pintle 116 when an oil pressure in the second oil channel 114 is greater than an oil pressure in the first oil channel 112 (i.e. corresponding to the circulating mode shown in Figure 4). Oil flowing from the second oil channel 114 pushes the ball bearing 126 against the opening of the circulation oil channel 120. However, the oil passage slot 128 allows oil to nevertheless pass around the ball bearing and into circulation oil channel 120 (and then on to the first oil channel 112). Figure 6 shows the control pintle 116 when an oil pressure in the second oil channel 114 is lower than an oil pressure in the first oil channel 112 (i.e. corresponding to the firing mode shown in Figure 3). In this scenario, oil pressure from the first oil channel 112 pushes the ball bearing 126 against the tapered oil inlet 124, sealing it and preventing any oil flowing into the second oil channel 114. While the invention has been described in detail in connection with only a limited 5 number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the scope of the invention. Additionally, while various embodiments of the invention 10 have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 15

Claims

1. An oil burner for supporting biomass combustion, the oil burner comprising: an oil supply interface at a first end;a firing tip at a second end;a control pintle at the second end arranged to control a flow of oil out of the firing tip;a first oil channel extending from the oil supply interface to the firing tip;a second oil channel extending from the oil supply interface to the control pintle, wherein the control pintle comprises a circulation oil channel connecting the second oil channel to the first oil channel; anda non-return valve arranged to prevent oil flow from the first oil channel to the second oil channel via the circulation oil channel but to permit oil flow from the second oil channel to the first oil channel via the circulation oil channel;wherein the control pintle is arranged to:permit oil to flow from the first oil channel out of the firing tip when a pressure differential between the first oil channel and the second oil channel is positive and greater than a firing threshold; anddirect oil from the second oil channel to the first oil channel via the circulation oil channel and prevent oil from flowing out of the firing tip when a pressure differential between the first oil channel and the second oil channel is negative.

2. The oil burner of claim 1, comprising a biasing member arranged to bias the control pintle towards a closed position in which oil is prevented from flowing out of the firing tip.

3. The oil burner of claim 1 or 2, wherein the control pintle comprises at least part of the non-return valve.

4. The oil burner of any preceding claim, wherein the non-return valve is positioned directly adjacent the circulation channel.

5. The oil burner of any preceding claim, wherein the non-return valve comprises a ball-type non-return valve.

6. The oil burner of any preceding claim, comprising an air supply arrangement arranged to deliver combustion air to the firing tip, the air supply arrangement comprising a core air tube.

7. The oil burner of any preceding claim, comprising a burner assembly located inside the core air tube that comprises the firing tip, the control pintle and at least part of the first and second oil channels.

8. The oil burner of any preceding claim, wherein the oil burner is elongate and the first and second ends of the oil burner are separated by at least 2 m.

9. The oil burner of any preceding claim, wherein the first oil channel is located radially outside the second oil channel.

10. A method of operating the oil burner of any preceding claim to support biomass combustion, the method comprising:operating the oil burner in a firing mode by supplying oil at the oil supply interface to produce a pressure differential between the first oil channel and the second oil channel that is positive and greater than a firing threshold; andoperating the oil burner in a circulating mode by supplying oil at the oil supply interface to produce a pressure differential between the first oil channel and the second oil channel that is negative.

11. The method of claim 10, comprising:operating the oil burner in a low firing mode by supplying oil at the oil supply interface to produce a first pressure differential between the first oil channel and the second oil channel that is positive and greater than a firing threshold; andoperating the oil burner in a high firing mode by supplying oil at the oil supply interface to produce a second pressure differential between the first oil channel and the second oil channel that is positive and greater than the first pressure differential.

12. A biomass power station unit comprising:a furnace comprising a plurality of biomass pulverised fuel burners arranged to burn biomass pulverised fuel to produce heat, and an oil burner of any of claims 1-9 associated with each of the biomass pulverised fuel burners;a boiler configured to use the heat from the plurality of biomass pulverised fuel burners to heat steam;a turbine configured to be powered by the steam from the boiler; anda generator configured to be driven by the turbine to generate electricity.

13. The biomass power station unit of claim 12, wherein each oil burner is positioned coaxially inside its associated biomass pulverised fuel burner.

14. The biomass power station unit of claim 12 or 13, wherein the oil burners are operable to ignite and / or stabilise a flame of its associated pulverised fuel burner.

15. The biomass power station unit of any of claims 12-14, operable to generate at least 100 MW of electricity.

16. A biomass power station comprising one or more biomass power station units as claimed in any of claims 12-15.

17. A method of operating a biomass power station unit as claimed in any of claims 12-15, the method comprising:burning biomass pulverised fuel in the plurality of biomass pulverised fuel burners to produce heat;using the heat from the plurality of burners to heat steam in a boiler;using the steam from the boiler to power a turbine;driving a generator with the turbine to generate electricity; andoperating the plurality of oil burners to support biomass combustion by the biomass pulverised fuel burners.

18. The method of claim 17, comprising operating an oil burner in a firing mode to ignite and / or stabilise an associated biomass pulverised fuel burner.

19. The method of claim 17 or 18, comprising operating an oil burner in a circulating mode when its associated PF burner is not activated and / or when its associated PF burner is stably ignited.s

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