EP3583356B1

EP3583356B1 - Method for firing a biofuel

Info

Publication number: EP3583356B1
Application number: EP18707851.4A
Authority: EP
Inventors: Scott Carpenter MILLIGAN; Todd David Hellewell; Robert Kunkel
Original assignee: General Electric Technology GmbH
Current assignee: General Electric Technology GmbH
Priority date: 2017-02-17
Filing date: 2018-02-01
Publication date: 2023-01-11
Anticipated expiration: 2038-02-01
Also published as: JP2020510803A; EP3583356A1; WO2018149651A1; JP7027435B2; PL3583356T3; CN110476015A; US20180238541A1

Description

TECHNICAL FIELD

Embodiments of the invention relate generally to energy production, and more specifically, to a method for firing a biofuel.

DISCUSSION OF ART

As demand for renewable energy sources continues to grow, biofuels are increasingly used in the production of energy. In particular, many electrical power plants, also referred to hereinafter simply as "power plants", burn biofuels to produce steam, which in turn powers a steam turbine generator. In many such power plants, the biofuel is burned on a stoker grate within a combustion chamber. Burning biofuel on a stoker grate, however, can potentially create a relatively unpredictable and/or uncontrolled combustion reaction for a given amount of biofuel. The terms "predictable" and "unpredictable", as used herein with respect to a combustion reaction, refer to the likelihood that the combustion reaction will follow a predicted/calculated rate and/or stoichiometry. Typically, the more unpredictable/predictable a combustion reaction, the harder/easier it is to control the stoichiometry of the combustion reaction, and the greater/lesser the amount of mono-nitrogen oxides ("NOx") produced from the combustion reaction. Accordingly, stoker grate-based power plants, which as stated above tend to create unpredictable combustion reactions, generally create high amounts of NOx.
As NOx has been determined to contribute to the formation of acid rain, many governments have defined NOx emission limits for biofuel burning power plants. In order to meet such NOx emission limits, many biofuel burning power plants employ both selective non-catalytic reducers ("SNCRs") and selective catalytic reducers ("SCRs"). SNCRs and SCRs, however, are resource intensive and expensive to operate. Moreover, many SNCRs rely on ammonia ("NH3") injection into an emitted flue gas for NOx reduction. Using NH3 to reduce NOx under the wrong temperature conditions, however, risks NOx formation and/or NH3 slip in the emitted flue gas.
Document US 6 269 755 B1 discloses an industrial burner for combustion of biomass. Said industrial burner is designed to achieve high turndown ratios and high heat release ratios. A combustion chamber disclosed in this document comprises three stages with the same amount of air provided to each of them. Document US 4 398 477 A discloses a combustion furnace that avoids discharging large amount of dust into the environment and uses combustion gases as a direct heat source, i.e., not requiring a heat exchanger. This combustion furnace comprises two chambers wherein the second chamber receives more air than the first one. Document US 3 831 535 A discloses a system with a combustion chamber and a blending chamber. The combustion chamber burns a biofuel and the blending chamber burns gases produced from drying veneer. Document US 2006/225424 A1 discloses a cyclonic combustor for a biofuel. The cyclonic combustor includes an ignition zone, a combustion zone and a dilution zone. Air is provided in the ignition zone and in the combustion zone in stoichiometric amounts to lower emission of nitrogen oxides. The dilution zone is provided with air to dilute and cool down combustion gases so that said combustion gases can be used in a gas turbine.
What is needed, therefore, is an improved system and method for firing a biofuel.

BRIEF DESCRIPTION

According to the invention, a method of firing a biofuel is provided as claimed in claim 1.
Within the scope of this disclosure, a system for firing a biofuel is provided. The system includes a combustion chamber having a first stage and a second stage. The combustion chamber is operative to provide for combustion of the biofuel in a suspended state while flowing from the first stage to the second stage. The combustion chamber further has a first injector and a second injector operative to introduce a first air stream and a second air stream into the combustion chamber at the first stage and at the second stage, respectively, to facilitate combustion of the biofuel.
According to the invention, a non-transitory computer readable medium storing instructions is provided as claimed in claim 5.

DRAWINGS

The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:

FIG. 1 is a block diagram of a system for firing a biofuel, in accordance with an embodiment of the invention;
FIG. 2 is a diagram of a combustion chamber of the system of FIG. 1, in accordance with an embodiment of the invention; and
FIG. 3 is a cross-sectional view of a firing layer of the combustion chamber of FIG. 2, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference characters used throughout the drawings refer to the same or like parts, without duplicative description.
As used herein, the terms "substantially", "generally" and "about" indicate conditions within reasonably achievable manufacturing and assembly tolerances, relative to ideal desired conditions suitable for achieving the functional purpose of a component or assembly. The term "real-time", as used herein, means a level of processing responsiveness that a user senses as sufficiently immediate or that enables the processor to keep up with an external process. As used herein, "electrically coupled", "electrically connected", and "electrical communication" mean that the referenced elements are directly or indirectly connected such that an electrical current, or other communication medium, may flow from one to the other. The connection may include a direct conductive connection, i.e., without an intervening capacitive, inductive or active element, an inductive connection, a capacitive connection, and/or any other suitable electrical connection. Intervening components may be present. As also used herein, the term "fluidly connected" means that the referenced elements are connected such that a fluid (to include a liquid, gas and/or plasma) may flow from one to the other. Accordingly, the terms "upstream" and "downstream", as used herein, describe the position of the referenced elements with respect to a flow path of a fluid and/or gas flowing between and/or near the referenced elements. Further, the term "stream", as used herein with respect to particles, means a continuous or near continuous flow of particles. As also used herein, the term "heating contact" means that the referenced objects are in proximity of one another such that heat/thermal energy can transfer between them. As further used herein, the terms "suspended state combustion" refers to the process of combusting a fuel suspended in air.
Additionally, while the embodiments disclosed herein are primarily described with respect to power plants, it is to be understood that embodiments of the invention may be applicable to any apparatus and/or method that needs to limit and/or eliminate NOx emissions resulting from the combustion of a biofuel, e.g., an incinerator.
Referring now to FIG. 1, a system 10 for firing a biofuel (12, FIG. 2), e.g., bagasse, switchblade and/or other grasses, wood, peat, straw, and/or other suitable biofuels, in accordance with embodiments of the invention is shown. The system 10 includes a combustion chamber 14, and may further include a controller 16 having at least one processor 18 and a memory device 20, a mill 22, an SCR 24, and/or an exhaust stack 26. As will be appreciated, the system 10 may form part of a power plant 28 where the combustion chamber 14 is incorporated into a boiler 30 which produces steam for the generation of electricity via a steam turbine generator 32.
As will be understood, the mill 22 is operative to receive and process the biofuel 12 for combustion within the combustion chamber 14, i.e., the mill 22 shreds, pulverizes, and/or otherwise conditions the biofuel 12 for firing within the combustion chamber 14. For example, in embodiments, the mill 22 may process the biofuel 12 to particles sizes less than or equal to about 2 mm. In other embodiments, the mill 22 may process the biofuel 12 to particles sizes less than or equal to about 1 mm. The mill 22 may be a non-screened styled hammer mill integrated with a flash drying column disposed at the inlet of a beater wheel exhaust fan. The processed biofuel 12 is then transported/fed from the mill 22 to the combustion chamber 14 via conduit 34.
The combustion chamber 14 is operative to receive and to facilitate combustion of the biofuel 12, which results in the generation of heat and a flue gas. The flue gas may be sent from the combustion chamber 14 to the SCR 24 via conduit 36. In embodiments where the combustion chamber 14 is integrated into the boiler 30, the heat from combusting the biofuel 12 may be captured and used to generate steam, e.g., via water walls in heating contact with the flue gas, which is then sent to the steam turbine generator 32 via conduit 38.
The SCR 24 is operative to reduce NOx within the flue gas prior to emission of the flue gas into the atmosphere via conduit 40 and exhaust stack 26.
Turning now to FIG. 2, the combustion chamber 14 has two or more stages 42 and 44, and one or more wind boxes 46 and 48 each having a plurality of nozzles/ injectors 50, 52, and 54. While FIG. 2 depicts the stages 42 and 44 as discrete and spaced apart, it will be understood that, in embodiments, the stages 42 and 44 may be continuous and flush with one another, i.e., the stages 42 and 44 may smoothly transition from one 42 to the next 44. As shown in FIG. 2, a first set of wind boxes 46 have nozzles 50 and 52 that are operative to introduce the biofuel 12 and a first air stream 56 into the first stage 42. As will be appreciated, the first air steam 56 may be generated from both primary and secondary air supplies. For example, in embodiments, nozzles 50 may introduce the biofuel 12 into the first stage 42 via primary air, while secondary air is introduced into the first stage 42 via nozzles 52. Particles of the biofuel 12 may be introduced into the combustion chamber 14 via nozzles 50 at a slip speed, with respect to primary air, of between about 0-30,48 meters/second (0-100 feet/second). As used herein, the term slip speed refers to the difference in the velocity of the particles of the biofuel 12 and the velocity of the primary air transporting the biofuel 12 via nozzles 50. In embodiments, 100% of the biofuel may be injected by the nozzles 50 within the first stage 42.
As also shown in FIG. 2, the nozzles 50 and 52 may be arranged into one or more firing layers 60, 62 and 64, i.e., groups of nozzles 50 and 52 disposed at and/or near the same position along a vertical/longitudinal axis 58 of the combustion chamber 14. For example, a first firing layer 60 may include nozzles 50 that introduce the biofuel 12 and primary air, a second firing layer 62 may include nozzles 52 that introduce secondary air, and a third firing layer 64 may include nozzles 50 that introduce the biofuel 12 and primary air. While the firing layers 60, 62, and 64 are depicted herein as being uniform, i.e., each firing layer 60, 62, and 64 includes either nozzles 50, that introduce only primary air and the biofuel, or nozzles 52, that introduce only secondary air, it will be understood that, in embodiments, an individual firing layer 60, 62, and 64 may include both nozzles 50 and nozzles 52. Further, while FIG. 2 shows three firing layers 60, 62, and 64 in the first stage 42, it will be understood that embodiments of the invention may include any number of firing layers in the first stage 42.
Upon introduction into the first stage 42, the biofuel 12 and first air stream 56 are ignited such that the biofuel 12 combusts while in a suspended state. As further shown in FIG. 2, the second stage 44 is downstream of the first stage 42 with respect to the combusting biofuel 12. Thus, due to convective forces generated by combusting the biofuel 12, particles of the biofuel 12 rise, or otherwise move, within the combustion chamber 14 as they undergo combustion such that they flow from the first stage 42 to the second stage 44. In other words, the combusting biofuel 12 forms a fireball 66 that spans the vertical axis 58 from the first 42 to the second 44 stage.
As further shown in FIG. 2, a second set of wind boxes 48 have nozzles 54 that are operative to introduce a second air stream 68, e.g., closed coupled over-fired air and/or separated over-fired air, into the second stage 44. Accordingly, as used herein, the terms "staged air", "air staging", "staged combustion", and "staging the combustion" refer to above described splitting of the air consumed by the combustion of the biofuel 12 between the first 42 and second 44 stages via the first 56 and the second 68 air streams. As will also be appreciated, nozzles 54 may be arranged into one or more firing layers 70 and 72 similar to nozzles 50 and 52 and firing layers 60, 62, and 64.
Moving now to FIG. 3, a cross-sectional view of firing layer 60 is shown. As will be appreciated, in embodiments, the biofuel 12 may be tangentially fired, i.e., the biofuel 12 is introduced into the first stage (42 in FIG. 2) via nozzles 50 at an angle Ø formed between the trajectory of the primary air component of the first air stream 56, and a radial line 74 extending from the vertical axis 58 to the nozzles 50. In other words, the nozzles 50 inject the biofuel 12 via the primary air component of the first air steam 56 tangentially to an imaginary circle 66, representative of the fireball, that is centered on the vertical axis 58. In certain aspects, the angle Ø may range from 2-10 degrees. While FIG. 3 depicts the nozzles 50 within the first firing layer 60 as disposed within the corners of the combustion chamber 14, in other embodiments, the nozzles 50 may be disposed at any point within the firing layer 60 outside of the fireball 66. As will be understood, the nozzles (50, 52 and 54 in FIG. 2) of the other firing layers (62, 64, 70, and 72 in FIG. 2) may be oriented in the same manner as the nozzles 50 of first firing layer 60 shown in FIG. 3. Accordingly, upon leaving the nozzles 50, the particles of the biofuel 12 follow a helix shaped flight path 76, e.g., a corkscrew, within the fireball 66 as they flow from the first stage 42 to the second stage 44. In other words, tangentially firing the biofuel 12 causes the fireball 66 to rotate about the vertical axis 58.
Returning back to FIG. 2, as will be appreciated, the helix shaped flight path 76 provides for a more controlled combustion reaction for particles of the biofuel 12 over traditional stoker grate firing methods. In particular, the helix shaped flight path 76 causes the flue gas to circulate about the "eye"/center of the fireball 66, i.e., the vertical axis 58, which dilutes the oxygen concentration within the fireball 66, thereby retarding the combustion reaction and/or temperature. Further, staging of the combustion reaction additionally retards the combustion reaction and/or combustion temperature, which also increases the de-NOx performance of the combustion reaction. As will be appreciated, the aforementioned provides for better control over the stoichiometry of the combustion reaction. In particular, the first 56 and the second 68 airstreams can be adjusted to regulate the stoichiometry of the combustion reaction in a predictable manner so as to limit the generation of NOx.
Accordingly, the first air stream 56 provides a greater than or equal amount of air consumed by the combustion reaction than does the second air stream 68. For example, in embodiments, the first air stream 56 provides about 50-70% of the air consumed by the combustion of the biofuel 12, which in turn regulates the stoichiometry of the combustion reaction of the biofuel 12 in the first stage 42 to between about 0.6-0.8. As will be understood, the second air stream 68 provides the remaining air consumed by combustion reaction, which in turn regulates the stoichiometry of the combustion reaction in the second stage 44 to less than or equal to about 1.2. As will be understood, regulating the stoichiometry of the first 42 and the second 44 stages via staging of the combustion reaction, i.e., staging of the introduction of the first 56 and second 68 air streams, drives nitrogen ("N") out of the biofuel 12 to become molecular nitrogen ("N2") within the first stage 42, as opposed to forming NOx as typically occurs in the unpredictable stoichiometric conditions associated with traditional stoker grate methods. In other words, in embodiments, the nitrogenous species are released from the volatile matter of the biofuel 12 and subsequently reduced to N2 by hydrocarbon intermediates within the first stage 42.
Thus, in certain aspects of the invention, combusting the biofuel 12 may result in about 12381, 88Kg/Kwh (0.08 lb/MBtu) of NOx prior to treatment of the flue gas by the SCR (24 in FIG. 1) and/or without the use of support fuel. As such, the SCR 24 may further reduce the NOx within the emitted flue gas to less than or equal to about 1547, 72 Kg/Kwh (0.01 lb/ MBtu) without the use of an SNCR.
Finally, it is also to be understood that the system 10 may include the necessary electronics, software, memory, storage, databases, firmware, logic/state machines, microprocessors, communication links, displays or other visual or audio user interfaces, printing devices, and any other input/output interfaces to perform the functions described herein and/or to achieve the results described herein, which may be executed in real-time. For example, as stated above, the system 10 may include at least one processor 18 and system memory / data storage structures 20 in the form of a controller 16 that electrically communicates with one or more of the components of the system 10. The memory may include random access memory ("RAM") and read-only memory ("ROM"). The at least one processor may include one or more conventional microprocessors and one or more supplementary co-processors such as math co-processors or the like. The data storage structures discussed herein may include an appropriate combination of magnetic, optical and/or semiconductor memory, and may include, for example, RAM, ROM, flash drive, an optical disc such as a compact disc and/or a hard disk or drive.
Additionally, a software application that provides for control over one or more of the various components of the system 10 may be read into a main memory of the at least one processor from a computer-readable medium. The term "computer-readable medium", as used herein, refers to any medium that provides or participates in providing instructions to the at least one processor 18 (or any other processor of a device described herein) for execution. Such a medium may take many forms, including but not limited to, non-volatile media and volatile media. Non-volatile media include, for example, optical, magnetic, or opto-magnetic disks, such as memory. Volatile media include dynamic random access memory ("DRAM"), which typically constitutes the main memory. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, a RAM, a PROM, an EPROM or EEPROM (electronically erasable programmable read-only memory), a FLASH-EEPROM, any other memory chip or cartridge, or any other medium from which a computer can read.
While in embodiments, the execution of sequences of instructions in the software application causes the at least one processor to perform the methods/processes described herein, hard-wired circuitry may be used in place of, or in combination with, software instructions for implementation of the methods/processes of the present invention. Therefore, embodiments of the present invention are not limited to any specific combination of hardware and/or software.
It is further to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. Additionally, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope.
According to the invention, a method of firing a biofuel is provided. The method includes: introducing the biofuel into a combustion chamber having a first stage and a second stage; combusting the biofuel in a suspended state while flowing from the first stage to the second stage; and introducing a first air stream and a second air stream into the combustion chamber at the first stage and at the second stage, respectively, to facilitate combustion of the biofuel. In certain embodiments, introducing the biofuel into a combustion chamber further includes tangentially firing at least some of the biofuel at the first stage. In certain embodiments, the biofuel has a maximum particle size less than or equal to about 2 mm. The first air stream provides a greater than or equal amount of the air consumed by combustion of the biofuel than the second air stream and the first air stream provides about 50-70% of the air consumed by combustion of the biofuel. In certain embodiments, combusting the biofuel produces less than or equal to about 12381, 77 Kg/Kwh (0.08 lb/MBtu) of NOx. In certain embodiments, the biofuel is at least one of bagasse, wood, peat, straw, and grass.
Other embodiments not covered by the present invention provide for a system for firing a biofuel. The system includes a combustion chamber having a first stage and a second stage. The combustion chamber is operative to provide for combustion of the biofuel in a suspended state while flowing from the first stage to the second stage. The combustion chamber further has a first injector and a second injector operative to introduce a first air stream and a second air stream into the combustion chamber at the first stage and at the second stage, respectively, to facilitate combustion of the biofuel. In certain embodiments, first injector is operative to introduce at least some of the biofuel into the combustion chamber at the first stage via tangential firing. In certain embodiments, the system further includes a mill operative to provide the biofuel to the combustion chamber at a maximum particle size less than or equal to about 2 mm. In certain embodiments, the first air stream provides a greater than or equal amount of the air consumed by combustion of the biofuel than the second air stream. In certain embodiments, the first air stream provides about 50-70% of the air consumed by combustion of the biofuel. In certain embodiments, the system further includes a selective catalytic reducer that is operative to limit NOx emissions resulting from combustion of the biofuel to less than or equal to about 1547, 72 Kg/Kwh (0.01 lb/MBtu) . In certain embodiments, the biofuel is at least one of bagasse, wood, peat, straw, and grass.
The invention also provides for a non-transitory computer readable medium storing instructions. The stored instructions are configured to adapt a controller to: introduce a biofuel into a combustion chamber having a first stage and a second stage; combust the biofuel in a suspended state while flowing from the first stage to the second stage; and introduce a first air stream and a second air stream into the combustion chamber at the first stage and at the second stage, respectively, to facilitate combustion of the biofuel. In certain embodiments, at least some of the biofuel is introduced into the combustion chamber at the first stage via tangential firing. In certain embodiments, the biofuel has a maximum particle size less than or equal to about 2 mm. The first air stream provides a greater than or equal amount of the air consumed by combustion of the biofuel than the second air stream and the first air stream provides about 50-70% of the air consumed by combustion of the biofuel. In certain embodiments, combustion of the biofuel produces less than or equal to about 12381, 77 Kg/Kwh (0.08 lb/MBtu) of NOx.
Accordingly, by combusting the biofuel in a suspended state and staging the introduction of air consumed by the combustion reaction, some embodiments of the invention generate significantly lower amounts of NOx than traditional methods of firing biofuels, e.g., stoker grates. In particular, some embodiments of the invention are able to achieve NOx emissions as low as about 0.08 lb/MBtu without the use of an SCR, and as low as about 1547, 72 Kg/Kwh (0.01 lb/MBtu) with an SCR unaccompanied by an SCNR. By eliminating the need for an SCNR to reach about 1547, 72 Kg/Kwh (0.01 lb/MBtu) emitted NOx, some embodiments of the invention greatly reduce the operational costs of an encompassing power plant. Additionally, by achieving NOx emissions as low as 12381, 77 Kg/Kwh (0.08 lb/MBtu), the SCR of some embodiments of the invention may be smaller than those typically used in traditional biofuel power plants.
Further, the tangential firing of the biofuel 12 in some embodiments causes the combustion reaction to occur "globally", i.e., uniformly, within the first stage 42. Thus, some embodiments provide for the ignition and/or mixing of the biofuel 12 and first air stream 56, as well as improved flame stability, without the need for localized, high turbulence injections of fuel and air.
While the dimensions and types of materials described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Moreover, in the following claims, terms such as "first", "second", "third", "upper", "lower", "bottom", "top", etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted as such, unless and until such claim limitations expressly use the phrase "means for" followed by a statement of function void of further structure.
This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable one of ordinary skill in the art to practice the embodiments of invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims.
As used herein, an element or step recited in the singular and proceeded with the word "a" or "an" should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to "one embodiment" of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments "comprising", "including", or "having" an element or a plurality of elements having a particular property may include additional such elements not having that property.
Since certain changes may be made in the above-described invention, without departing from the scope of the invention herein involved, it is intended that all of the subject matter of the above description shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.

Claims

A method of firing a biofuel (12) comprising:
• introducing the biofuel (12) into a combustion chamber (14) having a first stage (42) and a second stage (44),

• combusting the biofuel (12) in a suspended state while flowing from the first stage (42) to the second stage (44), and

• introducing a first air stream (56) and a second air stream (68) into the combustion chamber (14) at the first stage (42) and at the second stage (44), respectively, to facilitate combustion of the biofuel (12), and

characterized in that

the first air stream (56) provides a greater than or equal amount of the air consumed by combustion of the biofuel (12) than the second air stream (68), and

wherein the first air stream (56) provides 50-70% of the air consumed by combustion of the biofuel (12).
The method according to claim 1, characterized in that introducing the biofuel (12) into a combustion chamber (14) further comprises tangentially firing at least some of the biofuel (12) at the first stage (42).
The method according to claim 1 or 2, characterized in that the biofuel (12) has a maximum particle size less than or equal to 2 mm.
The method according to any of claims 1 to 3, characterized in that the biofuel (12) is at least one of bagasse, wood, peat, straw, and grass.
A non-transitoiy computer readable medium storing instructions configured to adapt a controller (16) to:
• introduce a biofuel (12) into a combustion chamber (14) having a first stage (42) and a second stage (44),

• combust the biofuel (12) in a suspended state while flowing from the first stage (42) to the second stage (44), and

• introduce a first air stream (56) and a second air stream (68) into the combustion chamber (14) at the first stage (42) and at the second stage (44), respectively, to facilitate combustion of the biofuel (12), and
characterized in that the first air stream (56) provides a greater than or equal amount of the air consumed by combustion of the biofuel (12) than the second air stream (68), wherein the first air stream (56) provides 50-70% of the air consumed by combustion of the biofuel (12).
The non-transitoiy computer readable medium according to claim 5, characterized in that at least some of the biofuel (12) is introduced into the combustion chamber (14) at the first stage (42) via tangential firing.
The non-transitory computer readable medium according to claim 5 or 6, characterized in that the biofuel (12) has a maximum particle size less than or equal to 2 mm.