Patent Application
20170138634
1. Field of the Invention
This invention relates generally to a boiler and burner system for the production of hot water or steam. More particularly, this invention relates to the production of hot water or steam using a firetube boiler and burner system that are designed to produce ultra low NOx emissions and higher efficiency at the same time. The firetube boiler and the burner of this invention are specifically designed to work with each other.
2. Description of the Related Art
Boilers are widely used for the generation of hot water and steam. A conventional boiler (excluding Heat Recovery Steam Generator or HRSG) comprises a furnace in which fuel is burned, and surfaces typically in the form of steel tubes to transfer heat from the flue gas to the water. A conventional boiler has a furnace that burns a fossil fuel or, in some installations, waste fuels or biomass derived fuels. According to the website of Britannica, the first boiler with a safety valve was designed by Denis Papin of France in 1679; boilers were made and used in England by the turn of the 18th century. Most conventional boilers are classified as either firetube or watertube types. In a firetube boiler, the water surrounds the steel tubes through which hot flue gases from the furnace flow. In a watertube boiler, the water is inside the tubes with the hot flue gases circulating outside the tubes. One example of firetube boilers is Scotch. Marine firetube boilers. There have been relatively few innovations in the designs of firetube boilers in the last few decades. The recent innovations have been focused on low NOx burner technologies. However, innovations in the burners have been hampered by the lack of innovations in the firetube boilers.
NOx is a recognized air pollutant. Regulations on NOx tend to get more stringent in densely populated areas of the world. In some areas, local regulations require low NOx or even ultra low NOx emissions in the exhaust from the combustion processes of the boilers. Various low NOx and ultra low NOx burners are available in the market to meet these requirements. A review of typical NOx reduction methods can be found in the article “NOx emissions: Reduction Strategy” in “Today's Boiler” magazine Spring 2015 by Jianhui Hong. FGR (Flue gas recirculation) is a commonly used technique for NOx reduction. In one common implementation called “Induced FGR”, flue gas is drawn through a pipe or duct to the inlet of a blower and mixed with the combustion air by using the blower wheel as a mixing device. The flue gas is typically at a higher temperature than the ambient air. The introduction of flue gas into the blower can sometimes lead to condensation, corrosion, and heat damage to the burner equipment. For example, condensation on the ignition system could render it inoperable. Corrosion to the internal parts of the blower and the burner head can occur. Heat and condensation from the flue gas can damage the flame scanners, which is part of the burner management system. The heat can also transmit through the shaft of the motor and damage the motor if the shaft is not properly cooled.
According to the Perry's Chemical Engineers' Handbook (7th Edition) Section 10-46, the horsepower requirement for a blower is determined by the multiplication of two factors, the volumetric flow rate through the blower in cubic feet per minute, and the blower operating pressure in inches water column. Induced FGR increases both the volumetric flow rate through the blower and the pressure drop through the burner and the boiler (hence increasing the blower operating pressure), and therefore greatly increases the horsepower requirement for the blower motor. If the flue gas does not have to go through the blower wheel, the horsepower requirement of the motor does not have to be penalized for the extra volumetric flow rate of the flue gas.
U.S. Pat. No. 5,407,347A teaches an apparatus and method for reducing NOx, CO and hydrocarbon emissions when burning gaseous fuels. The advantage of this invention is that ultra low NOx emission can be achieved at relatively low oxygen level (such as 3% dry volume basis) in the flue gas. The shortcoming of this technology; is that a large amount of FGR (up to 40% of combustion air by mass) is required to achieve <9 ppm NOx emissions. In addition, the rapid mixing design requires large pressure drops across the swirl vanes in the combustion air pathway near the burner head. Since mixing rate slows down with flow velocity, this design also has a limited turndown for ultra low NOx performance. Due to the large amount of FGR and the high pressure drop the air/FGR mixture has to overcome, a larger motor and a larger combustion air blower are required compared to some other alternative ultra low NOx burner technologies. The larger motor means higher initial capital costs, higher electricity consumption and higher noise during the burner's operation. In the state of California in particular, operators of boilers often dislike use of FGR, perhaps due to the concerns of earthquake and the additional mandatory structural inspection related to the field installation of the FGR pipe. U.S. Pat. No. 6,776,609 also discussed the motor size penalty problem in details related to the use of Induced FGR.
Another commonly used technique for ultra low NOx is called “lean premixed combustion”. U.S. Pat. No. 6,776,609 was intended to teach a method for operating a burner with FGR, but it also discussed the disadvantages of the lean premixed combustion method based on fiber matrix. It disclosed that “Alzeta Corp. of Santa Clara, Calif. sells a burner for use in food processing and other industries that utilizes only excess combustion air (no FGR) to achieve the flame dilution necessary for 9-ppm NOx emissions. A dilution level of 60% on a mass basis is required”. Using the same dilution principle, Power Flame Corporation offers a product called Nova Plus. It uses metal fiber matrix elements for the “fully premixed surface stabilized combustion”. As recent regulations in some counties of California requires 6 or 7 ppm NOx, the dilution air level may be higher than 60%, which is the level of dilution required for 9 ppm NOx.
Another technique for ultra low NOx relies on lean premixed combustion, but it does not use fiber matrix elements. Sellers Manufacturing, a subsidiary of Green Boiler Technologies, sells an S-Series boiler that generates 9 ppm NOx level using lean premixed combustion based on injection nozzles instead of fiber matrix elements. A fully premixed air/fuel mixture goes through multiple injection nozzles at relatively high velocity to resist flashback to areas upstream of these injection nozzles.
The shortcomings of the “lean premixed combustion” technique are well recognized in the combustion community: low thermal efficiency due to the very high excess air level and the resultant very high oxygen level in the flue gas (9% oxygen is typical), and the extra electricity consumption due to the extra excess air for the dilution effects. The large amount of excess air was intended to reduce the peak flame temperature by dilution effects. The extra dilution air carries additional heat into the atmosphere (wasted heat) when the exhaust is vented, and causes a reduction of thermal efficiency.
In view of the foregoing, there exists a need for an improved method and apparatus for production of hot water and steam that can produce low NOx (including ultra low NOx) emissions, low electricity consumption for the motor and high thermal efficiency at the same time.
It is a general object of the present invention to provide a method and apparatus for the production of hot water or steam in a firetube boiler and burner system that produces low NOx emissions, low electricity consumption and high thermal efficiency.
A more specific object of the present invention is to provide a method and apparatus for the production of hot water or steam in a firetube boiler and burner system that produces ultra low NOx emissions in the flue gas, low oxygen level in the flue gas which leads to higher thermal efficiency, low horsepower requirement for the blower motor for the burner.
These objects are achieved by a method of producing hot water or steam, comprising the steps of, producing a first flue gas in a first stage of a burner associated with a firetube boiler; passing at least a portion of said first flue gas through a first pass and a second pass of the boiler, wherein each of said first and second passes comprises at least one firetube; routing said portion of said first flue gas to a second stage of said burner to reduce the flame temperature and NOx emissions of said second stage of said burner; producing a second flue gas in said second stage of said burner; passing said second flue gas through a third pass of the boiler, wherein said third pass comprises at least one firetube.
These objects are achieved by an apparatus for producing hot water or steam, comprising a firetube boiler and a burner, said firetube boiler comprising a shell substantially cylindrical in shape, the shell having a front end, a back end, and a water entry port; a plurality of firetubes disposed in the shell and substantially extending the length of the shell between the front end and the back end, said firetubes form a first pass, a second pass and a third pass in the boiler, wherein said first pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; said second pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the back end to the front end; said third pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; and said burner located in the vicinity of the front end of said firetube boiler comprising of a first stage that produces a first flue gas; a second stage that produces a second flue gas; wherein the first flue gas goes through said first and second passes of said boiler, and is routed to the second stage of the burner to reduce the flame temperature and NOx emissions of said second stage; said second stage produces a second flue gas that goes through said third pass of said boiler.
Additional objects and features of the invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.
Identical reference numerals throughout the figures identify common elements.
For the purpose of this invention, a firetube shall mean a tube that allows a flue gas to flow inside, and shall also include a tube that allows combustion to occur inside, even if combustion may be in progress in the majority of the length of the tube. For example, a Morrison tube, or a furnace tube, whether corrugated or not, is considered a firetube for the purpose of this invention. No distinction is made between a furnace and a firetube.
A burner shall mean a device to produce one or more flames in the firetube boiler of the current invention in a controlled manner, taking inputs from at least one fuel source and an oxidizer source such as air. The two stages of the burner disclosed in this invention could arguably be referred to as two separate burners by anyone skilled in the art, at least for some of the embodiments. But such change of nomenclature does not create a new invention outside the scope of the current invention.
A burner 10 has a first stage 11 and a second stage 12. Each of these two stages of the burner 10 is supplied with fuel and combustion air in a proper air/fuel ratio so that combustion can be sustained, a means of ignition to start combustion, and a means for flame monitoring to ensure safety. For clarity and simplicity of illustration, the details of these two stages of the burner are omitted in
The first stage 11 of the burner 10 produces a first flue gas in firetube 33, which is the first pass of the boiler. The first flue gas flows in the first pass in the direction from the front end 6 to the back end 7. The first flue gas exits from the first pass into a chamber 50 affixed to the back end 7, then goes through firetube 35, which is the second pass of the boiler in the direction from the back end to the front end, and discharges into a chamber 20 affixed to the front end 6. The second stage 12 is disposed in chamber 20 to produce a flame. The first flue gas in chamber 20 is mixed with the combustible from the second stage 12 and functions to reduce the flame temperature of the second stage 12, and thus reduce NOx emissions from the second stage 12 of the burner 10. Using flue gas to reduce NOx emissions is well understood in the combustion community. The advantages of the current invention include three aspects: firstly the flue gas does not go through the wheel of any blower, thus reducing the motor horsepower requirement and electricity consumption of the motor; secondly the use of flue gas helps reduce the oxygen level in the flue gas while maintaining the desired low NOx levels, thus improves the thermal efficiency of the boiler; thirdly there is no need for an external FGR pipe. Elimination of the external FGR pipe is advantageous as is discussed in the section of “Description of Related Art”.
The second stage produces a second flue gas in firetube 38, which is the third pass of the boiler. The second flue gas exits the third pass and discharges into a chamber 60 affixed to the back end 7, and goes through a group of firetubes 39, which are the fourth pass of the boiler. The second flue gas exits the fourth pass, and discharges into a flue gas collection chamber 70 affixed to the front end 6, and is vented out of the boiler through flue gas outlet 80.
The stages 11 and 12 of the burner 10 are located in the vicinity of the front end 6. The observation ports 52 and 62 are located in the vicinity of the back end 7. Ports 52 and 62 allow manual observation of the flames in the first pass and the third pass, respectively. For simplicity, insulation and refractory materials as commonly used for boilers are not shown in any figures in this invention.
It is well known that burners can be classified as premix type or diffusion type (also known as non-premix type), depending on whether the fuel and air is mixed well before combustion is initiated. Each of the stages 11 and 12 in
Burner 10 can be operated in several modes. In a first mode, the first stage 11 and second stage 12 are both in operation, converting fuel and air into flue gas and generating heat. This is the normal mode of operation where low and ultra low NOx emissions are desired. In a second mode, the first stage 11 is in operation, and the second stage 12 is turned off. In this mode of operation, the flue gas from the first stage 11 still goes through the first, second, third and fourth passes of the boiler, but the fuel supply to the second stage is turned off. The combustion air supply to the second stage of the burner is kept on but modulated to a minimal flow rate, just to prevent the flue gas from the second pass of the boiler to back flow in the combustion air duct leading to the second stage and cause damages. This is the mode of operation when extremely high turndown is desired for the boiler. Caution should be used to limit the maximum turndown to avoid condensation in the firetubes, if the boiler is not designed as a condensing boiler. In a third mode of operation, the first stage 11 is turned off, and the second stage 12 is in operation. To prevent back flow in the first and second passes, the fuel supply to the first stage is shut off, and certain amount of combustion air is supplied to the first stage. This may be an acceptable mode of operation when the requirement for NOx emissions is not stringent, and the full capacity of the boiler is not required. This mode of operation does not take full advantage of the current invention, and is generally not recommended.
The third and fourth passes (firetube 38 and firetubes 39) of the boiler in
It is common in the firetube boiler industries to have one-pass, two-pass, three-pass and four-pass conventional firetube boilers. Additional passes can be added to the boiler in
It is common in the firetube boiler industries to have dry back and wet back designs.
Even though the first pass, the second pass and the third pass in
In
A small fraction of the combustion air supplied by blower 1 goes through air duct 3D, and goes into the first stage 11 of the burner 10, to increase the air/fuel ratio of the premixed mixture of air and fuel so that the oxygen level in the first flue gas is at a higher level such as 7-10%. The first stage of the burner utilizes the lean premix technique commonly used in the conventional ultra low NOx burners. The high excess air was necessary for lowering the flame temperature, which in turn suppress NOx formation. The higher oxygen level in the first flue gas allows the NOx emissions in the first flue gas to reach ultra low NOx levels. However, this higher level of oxygen in the first flue gas does not result in lower efficiency of the boiler, because at least some of the oxygen in the first flue gas will be consumed in the second stage of the burner. The oxygen level in the second flue gas is maintained at lower levels such as 1-6% dry volume based, preferably at 1-3% dry volume based.
The first stage 11 produces the first flue gas in firetube 33, which is the first pass of the boiler. The first flue gas then goes through an U-shaped tube 50A and passes through firetube 35, which is the second pass of the boiler. The first flue gas is then discharged into chamber 20. The nozzles 22 of the second stage 12 are disposed in chamber 20, so that the first flue gas from the second pass can be distributed substantially evenly to the five nozzles 22 to help suppress the formation of thermal NOx of the second stage. Tube 50A is connected to a bypass tube 39. Both tube 50A and bypass tube 39 could be further equipped with valves so that a portion of the first flue gas could be directed through the bypass tube 39 and discharged into chamber 70, instead of going to second pass of the boiler. When bypass tube 39 is shut off completely, all the flue gas exiting the first pass is directed though tube 50A to the second pass.
The first stage 11 is ignited by an ignitor through port 26 to produce a flame in chamber 24. This flame can start outside the firetube 33, and ends typically somewhere within firetube 33. Chamber 24 is disposed in chamber 20. Chamber 24 has small ports communicating with chamber 20 so that a flame associated with nozzle 21 in chamber 24 can ignite the adjacent nozzles 22. Nozzles 22 are disposed close enough to each other so that cross-ignition can occur. This cross-ignition avoid the need for an ignitor for each nozzle within the second stage 12.
The flames associated with the second stage 12 start within chamber 20 but outside of firetubes 38, and typically ends somewhere within firetubes 38. Firetubes 38 are the third pass of the boiler. The flue gas from the second stage exits from the third pass into a flue gas collection chamber 70, and is routed through a flue gas outlet 80 to other devices such as a chimney or an economizer.
Chamber 24 directs the flame from the first stage into firetube 33, and creates a pressure drop between the front end of firetube 33 and the front end of firetube 35. This pressure drop is necessary to drive the flue gas to travel from right to left in firetube 35 and into fire chamber 20. Chamber 24 may have small openings to allow detection of the flame within chamber 24 using a scanner mounted on port 28.
We now turn our attention to the production of steam in
As is well understood in the boiler industry, if hot water production is desired instead of steam, steam outlet 48 in
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, the thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
