Patent Application
20160003543
The present invention relates to a glass melting regenerative furnace to reduce the formation of NOx and increase glass melting capacity.
Legislation and community concerns require the reduction of NOx levels. The use of oxygen to accomplish same is one option in reducing said levels.
One such furnace is disclosed in WO 2010/114714. This discloses a fuel burner, which is preferably arranged on the hotspot of the furnace or in its proximity and is preferably operated substoichiometrically i.e., fuel-rich, located downstream from a burner port operated to form a combustion flame. A further fuel burner is arranged further downstream, which is operated superstoichiometrically, i.e., oxygen-rich. In practice, the two fuel burners are preferably located on the opposing side walls of the furnace in front of the outlet port for liquid glass. Oil or gas can be used as the fuels. The use of the fuel-rich, substoichiometrically operated burner results in an expansion of the fuel-rich areas in the furnace, so that the formation of NOx is decreased because of a lack of oxygen and because of the produced CO. Further downstream and therefore in the direction of the exhaust gas side of the furnace, the second oxygen-rich and therefore superstoichiometrically operated fuel burner ensures thorough mixing of the exhaust gases with an oxygen-rich flame and therefore the most complete possible post-combustion of incompletely combusted fuel components. The two fuel burners, which are also referred to as “hotspot burners”, are preferably arranged opposite one another in the third of the longitudinal walls of the furnace located further downstream, and are therefore located in the area of the U-shaped reversal of the combustion gas stream in the furnace. Alternating furnace operation Is readily possible, in that the fuel burners are switched from substoichiometric to superstoichiometric operation (or vice versa).
This furnace has been found to provide significantly reduced NOx exhaust gas values.
A development of this idea is disclosed in EP 2 508 827. In this case, additional gas flow streams which may be pure oxygen are injected into the charging end of the furnace. This gas is introduced at high speed to provide good mixing with the combustion gas which reduces the flame temperature resulting in a further reduction of the NOx concentration.
According to a first aspect of the invention, there is provided a furnace as defined in claim 1.
The present invention effectively takes the opposite approach to that described in EP 2 508 827 in that the pure streams are injected at the opposite end of the combustion chamber. It transpires that the arrangement of EP 2 508 827 changes the hot spot location adversely necessitating the use of hot spot burners to give an acceptable temperature profile. This requires the use of more oxygen. Installing lances which inject pure fuel gas and pure oxygen into the hot spot area, results in a significant reduction in energy or corresponding increase in throughput. By injecting a pure gas stream at the hot spot there is improved control of heat to the bridgewall and its temperature. It is known that oxygen or at least a high partial pressure/concentration at the glass surface will generate a foam layer which insulates the glass and the heat transfer. Conversely a reducing zone will suppress the foam layer. In following the U flame the gas on the firing side will reduce the foam layer and then will combust through non visible flame with the pure oxygen over the raw batch materials where foaming is not an issue.
For optimum positioning, the first lance assembly and second lance assembly are each disposed on an opposing side wall of the housing. Alternatively, they may be disposed in a crown of the housing.
Preferably, the first lance assembly and the second lance assembly are each disposed in less than forty percent, preferably less than a third, and more preferably less than a quarter of the way along the combustion chamber from the discharging end to the charging end. This places the lances closer to the hot spot.
Each first and second lance assembly may comprise a single lance. However, preferably, at least one of the first and/or the second lance assemblies comprise a plurality of lances.
The lances may be configured such that they can only operate in a pure fuel gas or pure oxygen operating mode. However, preferably, at least one of the first and second lance assemblies is arranged to operate in a fuel gas rich and/or and oxygen-rich mode. This allows greater flexibility of operation.
At least one of the first and second lance assemblies may have separate ducts for the injection of fuel gas and oxygen. In this case, the supply of two gases can be maintained entirely separately. Alternatively, at least one of the first and second lance assemblies has a common duct for the injection of fuel gas and oxygen. Such an arrangement requires the gases to be fed separately to the common duct.
According to a second aspect of the present invention, there is provided a method as defined in claim 9.
The time interval is preferably from 10 to 30 minutes, Preferably, also, no pure oxygen or pure fuel is injected into the third of the combustion chamber closest to the charging end in order to avoid adversely affecting the hot spot.
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 increase the throughput (pull) of glass and 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/injecting a furnace with fuel gas on firing side with oxygen on the exhaust side to increase heat transfer to hot spot to enable increase in throughput (pull) of glass and reducing NOx levels.
Referring to
Chargers 18, 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.
It is well known to those skilled in the art that oxy-fuei burners can be positioned in a furnace at positions similar to those shown at locations 20 and 22 to increase pull rate. The implication of firing oxy-fuel burners in this zone is the generation of foam due to the localized high partial pressure/concentration of oxygen. During the furnace reversals it is common for the oxy-fuel burners to continue operating with injection of oxygen and fuel.
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 lances 20, 22 are disposed for operation at the downstream end of the furnace 10 (i.e., within the downstream end of the combustion chamber) The lance 20 may be used for injecting 100% fuel gas on one reversal (firing side) and used for injecting oxygen on other reversal (exhaust side). The lance 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 lance 22 is in registration with lance 20. The lance 22 may be used for injecting 100% fuel gas on one reversal (firing side) and used for injecting oxygen on other reversal (exhaust side). Both of the lances 20, 22 are constructed and arranged for cyclical operation. That is, the lances 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, 28, Each of the regenerators 24, 28 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 28 is connected to port 26A. The regenerators 24, 28 have fuel injectors (not shown) at the ports 24A, 28A which operate on oil or gas fuel, as the melting application requires. Arrows at the ports 24A, 28A 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, 28 is switching from exhausting to firing, and vice versa) one of the regenerators 24, 28 is firing (at a firing port), while the other of the regenerators 24, 28 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 lances 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, 28, and/or in a crown of the furnace. Alternatively, the lances 20, 22 can be mounted in an end wall 12 as shown at 20A,22A, i.e., opposed to the ports 24A, 26A and proximate the discharge port 11. They may not, however, be mounted anywhere other than the half (measured along the length of the furnace) of the furnace closest to the wall 12.
Lances 28, 30 show an approximate position for such lances in the crown of the furnace 10. One or a plurality of the lances 28, 30 arranged in pairs along the furnace crown may be used.
if is possible to have a combination of breast wall 20, 22 and roof mounted lances 28,30 in a given furnace. This has the advantage of providing better mixing and consequently efficient reaction between the fuel and oxygen streams. For example, in
In this invention any lances on the firing side of the melter would be operated fuel gas, and lances on the exhaust side will be operated oxygen.
In summary, a firing side of the furnace 10 will have the lances firing in a fuel gas 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, lances at the opposed side will be 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, 28 and the lances 20, 22 can be done at intervals of 15-25 minutes, for example.
At least one of the lances 20,22 will be in operation on each of the firing and exhaust sides of the furnace 10, Such lances 20,22 should be sufficiently spaced from the exhaust ports 24A,28A 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 oxygen lance on the exhaust side of the furnace 10. Furthermore, the fuel gas lance 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.
The fuel gas and oxygen lances 20,22 are disposed in the half of the furnace closest to the wall 12. 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 fuel gas and oxygen lances 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 from fuel gas and oxygen lances 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 lance 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 fuel gas and oxygen lances 20,22 closer to the firing and exhaust ports 24A, 28A.
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 energy from the fuel gas and oxygen lances 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 streams 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.
The furnace and system of the present invention will reduce the NOx emissions from, for example, end-port furnaces by the use of fuel gas and oxygen lances 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 lances 20, 22 from fuel gas to oxygen avoids the need for turning the lances 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 lances 20, 22.
It is well known to those skilled in the art that oxy-fuel burners can be positioned in a furnace at positions similar to those shown at locations 20 and 22 to increase pull rate. The implication of firing oxy-fuel burners in this zone is the generation of foam due to the localized high partial pressure/concentration of oxygen. The impact of increasing the foam layer is to insulate the bulk of the glass and reduce the heat transfer. During the furnace reversals it is common for the oxy-fuel burners to continue operating with injection of oxygen and fuel. The injection of 100% fuel gas in this zone reduces the localized concentration and partial pressure of oxygen and creates a localized fuel rich zone at the hot spot location. This small localized zone of fuel has the impact to suppress the foam layer thus reducing the insulating effect and increasing the heat transfer. The bulk of the gas injected dissociates into Carbon and OH radicals which burns adjacent to the bridge wall 15. This fuel gas provides additional heat to the bridge wall 15 and is an additional means of controlling this temperature. This temperature is impacted by the level of foam thickness and clearly the thinner the layer of foam the more heat that will be radiated from the wall 15 into the glass.
It is well known to those skilled in the art that oxy-fuel burners can be positioned in a furnace at positions similar to those shown at locations 20 and 22 to increase pull rate. These burners typically fire continuously during a furnace reversal when there is no fuel from the port burners. The net effect of having an oxy-fuel burner in an air environment is that the NOx increases during the reversal. In the present case, during reversal both lances 20 and 22 can operate in fuel gas mode (i.e., fuel gas only) and as a result there is a significant reduction in NOx at this point.
The lances 20 and 22 have the potential to have a single conduit or alternatively to have two distinct conduits one for fuel gas and another for oxygen.
If will be known by one skilled in the art that the furnace efficiency is directly proportional to the excess oxygen. In the present case, the excess oxygen in the exhaust regenerator can be used to modulate the combustion air and/or the oxygen injected. The higher the volume of oxygen the lower the volume of combustion air. The lower the volume of combustion air the more the staging of the primary fuel burners.
The more staging of the burners, the lower the NOx and typically the higher the heat transfer. The addition of oxygen lowers the combustion air flow which increases the air preheat. Lower combustion air volume will delay mixing of the primary natural gas. The natural gas injected by the lancing 20 is secondary fuel staging with completion of combustion by the secondary oxygen stream 22. The system utilizes both fuel and oxygen staging.
In a recent case study an end-fired furnace of capacity greater than 200MTPD was operated with this arrangement. A record pull increase was achieved by injecting approximately 5% of the natural gas through the lance 20 and an equivalent volume of oxygen on the exhaust lance 22. This achieved a record maximum pull that was approximately 3% higher than previous maximum. This max pull was achieved with NOx comparable to that at 20% lower capacity.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1303283.4 | Feb 2013 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/053660 | 2/25/2014 | WO | 00 |
