Stable and efficient combustion within the forehearth and distributor is essential to ensure that consistent operation and efficient glass production is maintained. Here we describe the importance of burner nozzles in this process and how converting from stainless steel to ceramic burner nozzles can achieve greatly improved results.
BURNER NOZZLES
The burner nozzle is the point of interaction between the forehearth or distributor and the combustion system. The air/gas mixture supplied by the combustion system flows through the burner nozzle into the forehearth or distributor zone where it burns in the combustion chamber above the glass (See Fig 1).
The burner nozzle diameters are normally specified based on the forehearth or distributor width. The burner nozzle diameters would typically range between 4.0mm and 9.0mm in increments of 0.5mm. The burner nozzle diameter is sized to provide sufficient heat input in order to maintain the required glass temperature at the specified forehearth or distributor section minimum load condition as well as adequate temperatures under no load conditions when no glass is following.
The burner pipe and nozzle assembly is designed to seal against both the inside face and the outside face of the refractory burner block to ensure that all combustion takes place within the forehearth and to prevent any excess air in-leakage around the burner nozzle which would weaken the air/gas mixture, reduce the flame temperature, increase the flue losses and thereby reduce the combustion efficiency. (See Fig 1).
Burner nozzles are traditionally manufactured from a stainless steel material. This allows the nozzle to withstand the temperatures within the burner block. However, the burner nozzle is cooled by the flow of the air/gas mixture through the inside of the nozzle and the high thermal conductivity of the stainless steel means that the heat at the tip of the nozzle from the forehearth atmosphere is quickly transferred along the steel nozzle away from the nozzle tip. This results in a relatively low surface temperature at the tip of the burner nozzle providing a cold surface on which volatile materials from the glass, such as soda vapour, can condense. These volatile materials are carried to the burner nozzle from the forehearth combustion chamber atmosphere by the action of re-circulating gases around the burner flame (See Fig1). Over time these condensates gradually build up on the surface of the burner nozzle causing it to reduce in diameter and, in the absence of proper preventative maintenance, it can eventually become completely blocked.
Figure 2 shows the condition of burners removed from a forehearth by PSR engineers in which problems were being experienced in achieving adequate operating temperatures for correct glass production. The burners were reported to be clean. This forehearth operates at low tonnages and high temperatures and it was stated that the burners were cleaned out in-situ every twelve months. In this case, following removal, it was determined that the last scheduled burner cleaning had not been carried out and the burners had not been cleaned for more than 18 months resulting in the operating problems. Following replacement of the burners with spare burners, originally supplied as part of a recommended spare parts package with the forehearths to assist with regular maintenance, the combustion system performance was returned to its original, as commissioned, state.
This progressive reduction in diameter of burner nozzles will, with any pre-mixed combustion system, eventually result in a change in the air/gas ratio unless corrected. The air/gas ratio will become gas lean with the excess air reducing the flame temperature, increasing the flue losses and thereby reducing the combustion efficiency. This will initially result in an increase in fuel consumption to achieve the same operating temperatures. However, if this deterioration is allowed to continue further without the proper burner maintenance then the reduction in burner nozzle diameter will reduce the firing capacity and eventually result in an inability to adequately control or achieve the required forehearth operating temperatures. As it is also only the flow of the air/gas mixture through the burner nozzles that keeps them cool, complete blockage of the burner nozzles can result in overheating of the burner nozzles and backfiring of the air/gas mixture in the burner manifolds and pipe work resulting in damage to the pipe work and combustion equipment if left uncorrected.
The degree of condensate build up and the frequency of burner nozzle maintenance required will depend on a number of interrelated factors including glass composition, glass colour, burner nozzle size and operating temperatures.
Coloured glasses such as amber and green will generally produce more condensates than white flint glass and the burner nozzles will require more regular maintenance. Larger diameter burner nozzles used on wider forehearths and distributors are less likely to block up than smaller diameter nozzles and therefore require less maintenance.
Forehearths operating at high temperatures and low tonnages will result in greater evolution of the volatile materials from the glass which will then be present in greater concentration in the forehearth atmosphere and more readily available to condense on any cooler surfaces.
Forehearths operating at lower glass temperatures and higher tonnages will have a lower concentration of volatiles in the forehearth atmosphere but will have cooler surfaces on which the volatile materials can condense.
Colourant forehearths in which the colourant material is added to white flint glass as a relatively low melting point frit are particularly volatile and the condensates from these materials readily build up on the relatively cold areas of a colourant forehearth such as the exhaust dampers, peepholes and burner nozzles and are very corrosive.
BURNER MAINTENANCE
In order to maintain efficient combustion conditions inside the forehearth or distributor, the burners must be regularly cleaned to maintain the correct burner nozzle diameter and thereby the correct air/gas ratio. This could be done in-situ by removing the plug from the rear of the burner and using a rod with a drill of the correct burner nozzle diameter to ream out the burner nozzle without removing the burner from the burner manifold. However, this method is not recommended by PSR as it results in the condensate materials formed on the burner nozzle being pushed into the glass, and does not guarantee that the face of the burner nozzle and the opening in the refractory burner block is clean.
PSR recommend that the complete burners be replaced during job changes or other machine stoppages and the burners removed be thoroughly cleaned by grit blasting and drilling out in the workshop so that they are completely clean and ready for the next scheduled burner replacement. Spare burners of each forehearth or distributor section type are normally included in our recommended spares package to allow this method of burner maintenance.
However carried out, burner maintenance is a time and labour consuming process which must be correctly documented and controlled to prevent operating problems and between the scheduled burner cleaning the combustion system efficiency is continuously, gradually reducing from the optimum value. In our experience this essential maintenance is, in many cases, not carried out as often as required resulting in diminished forehearth operation.
CERAMIC BURNER NOZZLES
The solution to this problem, to eliminate the build-up of condensates and subsequent blockage of the burner nozzles without the need to regularly change and clean the burners, is to use a ceramic material for the burner nozzle instead of stainless steel. Due to the lower thermal conductivity of the ceramic material compared with stainless steel, the heat at the ceramic burner nozzle tip is not conducted along the nozzle away from the nozzle tip as quickly as with a stainless steel nozzle. The ceramic nozzle tip therefore operates at a higher temperature greatly reducing or eliminating the tendency for any volatile materials from the glass to condense and build up on the ceramic nozzle surface. This prevents the reduction and eventual blockage of the burner nozzle, thereby continuously maintaining the air/gas ratio and combustion efficiency whilst reducing or eliminating the need for burner maintenance. Other normal combustion system maintenance, such as regularly replacing combustion air fan filter media, still needs to be carried out to prevent foreign materials such as dust entering the combustion system and potentially blocking the burner nozzles.
PSR CERAMIC BURNER NOZZLES
The PSR ceramic burner nozzles are manufactured from a 95% dense alumina. It is an exceptionally strong material, to the extent that it is not practical to drill out the ceramic nozzles to the required diameters. They are typically manufactured in diameters at 0.5mm increments from 4.0mm up to 9.0mm and each different diameter must be cast using it’s own specific mould.
This inherent strength of the material is important to ensure that the nozzles can withstand the rigours of day to day operation in a glass factory. Under both cold and hot conditions these nozzles must be extremely durable to withstand heavy loads such as from workers inadvertently standing on the burners and burner manifolds to access components on the forehearth superstructure.
The nozzles were designed to function in exactly the same way as the stainless steel burner nozzles which preceded them. Having exactly the same exterior nozzle diameter and nozzle length from the burner nozzle holder to the tip of the nozzle ensures that the nozzle seals against the inside face of the burner block and the burner nozzle holder seals against the outside face of the burner block in the same way that a stainless steel nozzle does. The use of the ceramic burner nozzles will ensure that the combustion system efficiency set during commissioning can be continuously maintained.
The PSR ceramic burners have now been installed in glass factories producing a variety of glass colours under various different operating conditions. They have been installed on a colourant forehearth frit addition section where the presence of volatile materials from the addition of the frit material would ordinarily result in the need to regularly maintain the burners to ensure that operating conditions are maintained.
Figure 3 shows a row of these burners which were removed from a colourant forehearth frit addition section after six months’ operation. The burners were removed following cool down of the forehearth for a furnace rebuild.
One particular project on which the benefits of ceramic nozzles over stainless steel nozzles was of particular importance, consisted of a forehearth and distributor for production of electrical insulators for overhead power lines. The extreme thermal toughening process which the glass insulators are subjected to means that the quality requirements for the glass are extremely high, particularly with respect to foreign particles in the glass. Even microscopic inclusions in the glass can cause the insulators to shatter when undergoing this toughening process. Therefore, any places where materials could build up and then enter the glass, such as the condensates on a burner nozzle, are of serious concern. The low forehearth operating temperatures of 1050°C to 1080°C at the spout entrance position also provided conditions under which the build-up of condensates would be of additional concern. This was also the first time that the ceramic burner nozzles had been installed on a forehearth and distributor cold during a furnace rebuild and warmed-up to operating temperatures. When the burners are installed cold and heated up with the forehearth and distributor, the nozzles are subjected to additional stresses due to the expansion and movement of the burner blocks and the re-setting of the burner manifolds to re-align the burners with the burner blocks. No breakage of the ceramic burner nozzles occurred and no other problems were experienced and they have currently been in operation for 14 months without any problems.
The ceramic burner nozzles have now been adopted as our standard supply with all new forehearth and distributor installations.
CONVERSION FROM STAINLESS STEEL TO CERAMIC
The PSR ceramic burner nozzles are not only an addition to our new forehearth and distributor packages as standard, they can also be installed on existing PSR MR-5000 combustion systems as well as other forehearth systems. Upgrading to PSR ceramic burner nozzles from steel burner nozzles previously supplied by PSR requires replacement of the burner nozzle holder including additional internal sealing gaskets and washers for the ceramic nozzle and the steel burner nozzle with the equivalent ceramic burner nozzle. The overall length of the burner pipe remains the same and the burner casting and burner pipe can be reused. The conversion would normally be carried out by converting spare burners supplied by PSR in a recommended spares package for regular maintenance from steel to ceramic burner nozzles, installing them during a scheduled burner replacement and then continuing the process with the burners removed until all the forehearth burners have been converted.
To upgrade to PSR ceramic burner nozzles from steel burner nozzles on a different forehearth and distributor combustion system would require the information detailed in the drawing in Fig.5.
Using this information, PSR can determine whether our standard ceramic burner nozzles are compatible with the customer’s existing burner configuration.
Additionally, PSR can determine whether the ceramic nozzles and nozzle adapters are compatible with the existing burner pipe in which case they can be supplied as an upgrade kit to be installed using the existing burner pipe. If they are not compatible with the existing burner pipe then they can be supplied as a complete burner assembly. To reduce time during conversion from steel to ceramic burner nozzles we would recommend that spare burners be converted from steel to ceramic nozzles if available or spare burners be supplied with ceramic nozzles for complete combustion zones. Once these spare burners are installed and commissioned on these complete combustion zones, the burners removed can be converted from steel to ceramic nozzles and are then available for installation on subsequent combustion zones to be converted.
