Patent Application
20180238541
Embodiments of the invention relate generally to energy production, and more specifically, to a system and method for firing a biofuel.
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.
What is needed, therefore, is an improved system and method for firing a biofuel.
In an embodiment, 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 another embodiment, 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.
In yet another embodiment, a non-transitory computer readable medium storing instructions is provided. 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.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:
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
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
As also shown in
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
As further shown in
Moving now to
Returning back to
Accordingly, the first air stream 56 may provide 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 may provide 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 0.08 lb/MBtu of NOx prior to treatment of the flue gas by the SCR (24 in
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.
For example, in an embodiment, 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. 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, combusting the biofuel produces less than or equal to about 0.08 lb/MBtu of NOx. In certain embodiments, the biofuel is at least one of bagasse, wood, peat, straw, and grass.
Other embodiments 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 0.01 lb/MBtu. In certain embodiments, the biofuel is at least one of bagasse, wood, peat, straw, and grass.
Yet still other embodiments provide 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. 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, combustion of the biofuel produces less than or equal to about 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 0.01 lb/MBtu with an SCR unaccompanied by an SCNR. By eliminating the need for an SCNR to reach about 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 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, or if they include equivalent structural elements with insubstantial differences from the literal languages 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 spirit and 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.
