Patent Application
20100242545
The present invention relates to staging of combustion in glass melters to reduce the formation of NOx.
Legislation and community concerns require the reduction of NOx levels. The use of oxygen to accomplish same is one option in reducing said levels.
For a more complete understanding of embodiments of the present invention, reference may be had to the following drawing figures taken in conjunction with the description of the embodiments, of which:
Furnace and system embodiments of the present invention reduce the amount of NOx formed in glass furnaces, such as for example end-port regenerative furnaces.
The furnace and system embodiments provide an efficient means of partially firing a furnace with oxygen and reducing NOx levels.
Referring to
Chargers 16, 18 are connected to the furnace 10 for communication with the combustion chamber 14 so as to provide a feed of the raw glass forming materials or other charging material (not shown) to the furnace 10, and in particular the combustion chamber 14.
A downstream end of the furnace 10 includes a discharge port 11 or end of the furnace where the melted glass is withdrawn, often referred to as a throat. A pair of burners 20, 22 are disposed for operation at the downstream end of the furnace 10. The burner 20 may be an oxy-fuel burner for use with oil or gas fuel. The burner 20 can be operated as a fuel rich burner, i.e. a burner with an excess amount of fuel; or as an oxidizing burner, i.e. a burner using excess amounts of oxygen (O2). The burner 22 is also mounted at the downstream end of the furnace 10, for example at an opposed side of the furnace wall 12 as shown in the FIGS., so that the discharge from the burner 22 is in registration with burner 20. The burner 22 may also be operated on oil or gas fuel and can be run fuel rich or fuel lean/oxidizing (as defined above with respect to the burner 20). Both of the burners 20, 22 are constructed and arranged for cyclical operation. That is, the burners 20, 22 can operate alternatively as fuel rich burners or oxidizing burners.
At an end of the furnace 10 opposed to the discharge port 11 there is disposed a pair of regenerators shown generally at 24, 26. Each of the regenerators 24, 26 is connected to a corresponding port, each of which is in communication with the combustion chamber 14. That is, the regenerator 24 is connected to port 24A. The regenerator 26 is connected to port 26A. The regenerators 24, 26 have fuel injectors (not shown) at the ports 24A, 26A which operate on oil or gas fuel, as the melting application requires. Arrows at the ports 24A, 26A indicate flow with respect to their associated regenerators and ports, and the operation of the furnace 10.
While in operation (i.e. not including the short length of time each regenerator 24, 26 is switching from exhausting to firing, and vice versa) one of the regenerators 24, 26 is firing (at a firing port), while the other of the regenerators 24, 26 is exhausting (at an exhaust port). Each of the ports 24A, 26A is equipped with fuel injectors (not shown) which operate only when the corresponding one of the ports is in firing mode. When in firing mode, combustion air flows through the regenerator and is preheated so that a high combustion temperature can be achieved for efficient operation of the furnace 10. The preheated air flows through the firing port and into the combustion chamber 14 where it reacts with the fuel from the firing port fuel injectors creating a flame. The flame heats the furnace structure and glass (not shown) to be melted. The exhaust port passes the hot furnace exhaust gases into a second regenerator which is heated up by the passage of these gases. After a period of 10-30 minutes (more typically 15-25 minutes) the flows of gases through the ports are reversed, so that combustion air now flows through the preheated regenerator (ie, the one that was previously exhausting) and the hot furnace exhaust now flows out through the heat depleted regenerator (i.e., the one that was previously firing) so as to recover waste energy.
More particularly and referring to
This process will operate for approximately 15-25 minutes before the process is reversed as discussed below with respect to
Referring to
The burners 20, 22 may be mounted in the breast walls, that is, along sides of the furnace 10 parallel to the initial flame direction from the regenerators 24, 26, and/or in a crown of the furnace. Alternatively, the burners 20, 22 can be mounted in an end wall 15 as shown at 20A,22A, i.e., opposed to the ports 24A, 26A and proximate the discharge port 11.
Burners 28, 30 show an approximate position for such burners in the crown of the furnace 10. One or a plurality of the burners 28, 30 arranged in pairs along the furnace crown may be used. The burners 28, 30 will fire approximately perpendicular to an initial flame direction from the regenerators 24, 26 so that the oxy-fuel flame of burners 28, 30 fires proximate or into the respective combustion footprint 25, 29, i.e. the burner 28 firing into the footprint 25, while the burner 30 fires into the footprint 29.
It is possible to have a combination of breast wall 20, 22 and roof mounted burners 28,30 in a given furnace. This has the advantage of providing better mixing and consequently efficient reaction between the fuel rich and oxidizing streams. For example, in
In this invention any oxy-fuel burners on the firing side of the melter would be operated fuel rich, and oxy-fuel burners on the exhaust side will be operated fuel lean.
In summary, a firing side of the furnace 10 will have the oxy-fuel burners firing in a fuel rich manner, i.e. with insufficient oxygen for complete combustion. At an opposed side of the furnace 10 that is being used to exhaust the combustion products, i.e. the exhaust side of the furnace, burners at the opposed side will be fired with an excess of oxygen so as to combust as completely as possible any fuel remaining from the incomplete combustion from the firing side of the furnace. The cycling between the regenerators 24, 26 and the burners 20, 22 can be done at intervals of 15-25 minutes, for example.
At least one of the oxy-fuel burners 20,22 will be in operation on each of the firing and exhaust sides of the furnace 10. Such burners 20,22 should be sufficiently spaced from the exhaust ports 24A,26A so that there is sufficient time and space available for reaction to take place between the excess fuel from the firing side of the furnace 10 and the excess oxygen from the oxy-fuel burner on the exhaust side of the furnace 10. Furthermore, the fuel rich oxy-fuel burner may be disposed on the firing side of the furnace 10 located to create a fuel rich mixture in the applicable one of the combustion footprints 25,29 at the peak temperature regions in the furnace 10, i.e. at a furnace hot spot. Accordingly, suitable locations for these burners 20,22 are approximately between 20%-80% of the length of the glass melter from ports 24A,26A.
The oxy-fuel burners 20,22 may be disposed in end-port furnaces at a location 70-80% of the furnace length from the ports 24A,26A. At this position furnace crown temperatures are at or near their maximum in the furnace 10, which is commonly referred to as the furnace hot spot. At the hot spot there is typically an upwelling of low density heated glass from a bottom (not shown) of the furnace 10. On a surface of the glass bath (not shown) at the hot spot, the glass is further heated and the upwelled glass is forced partially towards the firing 24A and exhaust ports 26A, and partially toward the glass discharge port 11 or throat. Surface movement of the glass melt towards the firing port 24A and exhaust port 26A helps to restrict movement of any batch material towards the glass discharge port 11 before said batch material has been sufficiently melted. This upwelling of glass and resulting convection currents in the glass bath necessary for high performance operation of the furnace are promoted by maintenance of the hot spot. By using oxy-fuel burners 20, 22 at or near the hot spot additional energy is imparted directly to the desired hot spot to thus maintain the location of the hot spot through the firing cycle of furnace operational changes. Furnace stability is improved by maintaining and controlling the hot spot. The hot spot is an effective location for the addition of oxy-fuel energy because it additionally improves or reinforces the natural melting processes in the combustion chamber 14 of the furnace.
There is however a maximum amount of oxy-fuel energy that can be introduced to the hot spot before the crown superstructure temperatures are raised excessively. In addition, the flow paths of combustion products 27, 31 indicate that were a fuel rich region to be introduced on the firing side near the hot spot, ie, from the burner 20 on
With regard to NOx in such a system, NOx formation is inhibited in fuel rich regions due partially to the absence of oxygen and as such, if a size of the fuel rich region is increased then the final amount of NOx produced would be reduced. Therefore, to reduce NOx formation the size of the fuel rich region is increased which is achieved by the use of the oxy-fuel burners 20,22 closer to the firing and exhaust ports 24A, 26A.
As a result of the need for fuel efficient operation, the combustion reactions are essentially complete prior to exit of the combustion products 27,31 into the respective exhaust port 24A,26A. Consequently, additional oxy-fuel energy is not introduced proximate the exhaust port as space and time is needed for combustion to occur and be completed. Furthermore, the fuel rich combustion products following the paths 27,31, need to mix, interact and react as completely as possible with the oxygen rich flames at the respective exhaust side of the furnace so that the excess fuel in the fuel rich combustion products is consumed as much as possible within the furnace. Such mixing, etc. at a distance of 30-40% of the melter length from the ports 24A, 26A is a good location, as this is sufficiently distant from the ports 24A, 26A to prevent unreacted oxidizing gases or fuel rich combustion products from exiting the furnace 10 and for heat to be liberated within the furnace.
The furnace and system of the present invention will reduce the NOx emissions from, for example, end-port furnaces by the use of oxy-fuel burners 20,22 in a staged manner. The system obviates the need for costly secondary NOx abatement equipment to be mounted to the furnace 10.
Use of oxygen in furnaces furthermore enables increased production rates for the furnace 10 and allows the furnace to continue to operate where the primary air-fuel combustion is deteriorating. Use of the system embodiments reduces the impact of NOx emissions, allows greater utilization of furnace equipment and profit to the customer, while avoiding capital expenditure associated with furnace modification or repair.
Switching the burners 20, 22 from fuel rich to oxidizing (fuel lean) avoids the need for turning the burners 20, 22 on and off and thus, reduces thermal cycling of components which can lead to failure and the need for secondary cooling media for the burners 20, 22.
There is accordingly provided the embodiment of an end-port regenerative furnace, comprising a housing; a combustion chamber within the housing; first and second regenerators each disposed in communication with the combustion chamber, the first and second regenerators each constructed and arranged to alternate between a firing mode during which fuel is injected proximate preheated combustion air passing to said combustion chamber, and an exhaust mode during which hot combustion products circulating in the combustion chamber are removed from the combustion chamber; a first burner assembly in communication with the combustion chamber for coaction with the first regenerator during a corresponding one of the firing mode and the exhaust mode; and a second burner assembly in communication with the combustion chamber for coaction with the second regenerator during a corresponding one of the firing mode and the exhaust mode, wherein the first and second regenerators alternate between the firing and exhaust modes for successive time intervals to alternate the flow of the hot combustion products and the exhaust.
There is also provided the embodiment of a method, i.e. in an end-port regenerative furnace having first and second regenerators each of which is operable in a firing mode and an exhaust mode, a method comprising: providing a first burner adapted to operate in a fuel-rich mode and a fuel-lean mode concurrent with the operable mode of the first regenerator; providing a second burner adapted to operate in a fuel-rich mode and a fuel-lean mode concurrent with the operable mode of the second regenerator; operating the first regenerator in the firing mode and the first burner in the fuel-rich mode; operating the second regenerator in the exhaust mode and the second burner in the fuel-lean mode; alternating the operable modes of the first and second regenerators and the first and second burners, wherein the first regenerator is operable in the exhaust mode and the first burner is operable in the fuel-lean mode, and the second regenerator is operable in the firing mode and the second burner is operable in the fuel-rich mode; and cycling the operable modes of the first and second regenerators and the first and second burners for successive time intervals to provide cyclical flows between the first and second regenerators.
A further embodiment includes operating the first regenerator and the second regenerator simultaneously.
It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the present embodiments as described and claimed herein. It should be understood that the embodiments described above are not only in the alternative, but may be combined.
