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Metal Casting Technologies : September 2006
74 TECHNICAL FEATURE INTRODUCTION ajor industrial processes require combustion in different forms. The burner is the device that is used to combust fuels with an oxidizer and to control the heat transfer into a furnace in which these processes take place. An essential part of a burner is the refractory burner tile either made of ceramic fiber or refractory pre-fabricated shapes. A burner tile is a complex functional refractory product and inaccurate operation can cause downtime costs which exceeds the costs for the burner equipment by far. Environmental aspects and more stringent regulations have changed the philosophy of burner design in the last 20 years. Burner design is now a compromise between efficient mixing of fuel and oxidizer and the necessity to reduce the total fuel consumption and pollutant emissions, which requires moderate low flame temperatures in order to reduce NOx. These designs have led to the development of tailor made solutions for certain applications and to highly sophisticated burner designs with tiles of complex shapes, figure 1. This paper discusses the technical requirements of refractory materials for burner tile applications and evaluation methods in order to generate thermo- mechanical data as an engineering tool for the design of burner tiles. Furthermore the influence of two different binder systems will be described in this context. BURNER TILE DESIGN The design of a burner tile plays an important role in the performance of a burner. The tile ■ forms the desired flame pattern ■ avoids flame radiation towards the burner casing ■ forms the orifice that controls the flow of air for the combustion reaction ■ stabilizes the flame due to the thermal insulating properties of the refractory material Regarding the burner design, the combustion engineer has to consider the characteristic of the ceramic burner tile material in order to obtain stable combustion process conditions. The main design problems are: ■ The tile is restrained against complete, free, thermal expansion by the external structural steel casing or anchoring. The designer has to consider the physical characteristics of the refractory material by definition of the thermal restraint stresses. ■ Sharp inside 90°edges lead to material stress peaks in service. ■ The wall thickness of the burner tile is not uniform under isotherm conditions and stresses may concentrate in the thinner areas. Hence critical internal strain controlled loads can occur. ■ The design of the tile must be in accordance with the occurring temperature range and the plastic / elastic response of the refractory material within this range. ■ Part of the tile is cooled by the fuel and oxidizer, another part is heated by the flame and the furnace. As a consequence the tile has to cope with large temperature differences. In many burners this may be over 10000C. ■ In case the burner is stopped and the furnace is hot, then the tile will heat-up quickly and damage caused by thermo-shock is often observed. Figure 1 shows a low emission flat flame burner of complicated tile shape with sharp edges and additional fuel jets in order to reduce NOx. The purpose of the geometry is to control a precise oxidizing process of the fuel (1) The wall thickness is not uniform and the jets probably keep parts of the tile cold. This is an example for a burner with a critical tile design. Technical requirements of refractory materials for burner tile applications - the advantage of a phosphate binder system M Jens Decker, Tom Atkins, David Mintz -- Stellar Materials, Boca Raton, USA Karel Hoeksma, Roy Ovink -- Corus Research, IJmuiden, The Netherlands Figure 1. Flat Flame Burner (courtesy of John Zink Company) www.metals.rala.com.au