Material properties, peculiar for household burners, studied in the FlexHEAT project
Contribution: Politecnico di Torino (POLITO)
Specification definition of a high temperature materials should be drawn according
to the industrial operating requirements, dealing both with high temperature
performance and with workability needs. A literature survey was conducted in order
to identify adequate materials suitable for household burner materials, and
failure mechanisms, important for this application. An overview of the possible
materials was outlined and preliminary candidate and reference materials can now
be selected according to different applications foreseen for the project.
1. Overview of the materials-related problems in burner application
Traditional pre-mixed gas burners for domestic applications, such as central heating systems or hot water boilers consist of a thin sheet of metal, perforated with slits and/or holes, through which the fuel/air mixture passes. Combustion takes place just outside the surface of the burner plate. The flue gas consists mainly of CO2, H2O, and to a lesser extent CO, unburned hydrocarbons, NOx, dust and degradable compounds, originating from the burner surface.
Two typical burning modes are: the radiant mode and the blue flame mode. In radiant mode combustion occurs near or in the metal surface, resulting in high surface temperatures. In the blue mode flame is situated above the burner surface,which is then less exposed to high temperatures. The burner material design has to account for typical operating conditions, e.g. temperature cycles depending on the use,modulating or on/off operation, heating/hot water for sanitary uses, etc.
For the conventional burners (blue mode), the surface temperature during combustion is approximately 200℃, causing pre-heating of the gas mixture. The temperatures in flames vary with the height, from about 200℃ up to temperatures of 1700 to 2000℃, in a very short distance. In the blue mode, due to limited temperatures and oxidising conditions the surface is usually protected by a passive oxide scale. The experience is that blue mode domestic heating appliances last about 10-15 year or 1500 burning hr/yr at 5000-10000 cycles/yr .
In the radiant mode the temperature of burner surface is normally between 800-900℃, depending on the design and operation, but in case of poor air/fuel mixing, non-uniform flow and back radiation from the heat exchanger, temperatures as high as 1100℃ can be reached. Since the radiant mode temperatures place high demands on the burner material, as compared to the blue mode ones, overall degradation or break-down of burners can result in an increase in emissions and decrease in efficiency . The radiant conditions can result in a dramatic life time reduction due to several degradation mechanisms such as high temperature corrosion, spalling of protective oxide scales, creep deformation, and thermal and thermo mechanical fatigue.
In domestic boilers, the most promising marketing sector, 1:5 power modulation should be achieved in order to cope with two different needs: apartment heating (requiring low power generation, i.e. below 5 kW), and hot sanitary water production (exceeding 25 kW). Strong marketing requirements are forcing towards developing burners enabling a 10:1 turndown ratio, as a consequence of the higher and higher insulation of new apartments, which imply power needs approaching just 2-3 kW. This is exactly one of the major objectives of the present project, to be obtained by forcing the burner operation more deeply in the low power radiant operating regime.
The lifetime of metallic burners depends among other factors on the formation of protective oxide scales. The rate of oxidation, the morphology and the adhesion characteristics of the grown oxide layer mainly depends on a complex interplay among composition, microstructure and surface condition of the alloy with the thermo chemistry of the environment and the temperature of reaction.
Protection of the underlying metal is most often accomplished by the formation of a continuous scale over the surface, such that it serves as a barrier preventing the corrosive media from further penetrating towards the remaining unoxidised metal. The chemical composition of alloy designed for high temperature applications have been generally established so that such protective oxide are of either Cr2O3 or Al2O3, since both these oxides can offer excellent protective capabilities due to their slow growth rates and thermodynamic stability. Furthermore, these scales are subject to healing if cracks are formed in the scales. The Cr2O3 scale offers some protection against further oxidation, but is less cohesive with the substrate and tends to spall under thermal cycling exposing new fresh metal to attack. For applications at very high temperatures (a practical limitation is for temperature exceeding around 1100℃) the loss of chromium from the scale as the volatile CrO3 becomes increasingly significant and alumina-forming alloys are therefore preferred [4-7].
Cooling from high temperature to room temperature can cause thermal fatigue between scale and substrate, and spalling of oxide scale due to thermal shock. Spalling and enhanced oxidation will result in accelerated metal losses, and thinning of the metal sheet. Ultimately failure can occur either by insufficient mechanical strength or even excessive perforation of the plate . In addition to enhanced oxidation other degradation mechanisms of the metal can be due to creep and hot spots. Hot spots create local stresses. This leads to buckling, cracking and also enhanced spallation of oxide layers.
2. General considerations on the most commonly used materials in this field of application
Most burners are made of 316, 430 or 441 steel, FeCrAlloys are used for temperatures up to 1100℃ and are mostly used as weaved inserts. High alloyed CrNi steels and ceramic materials (e.g. Al2O3, SiC and SiSiC) are also used at temperatures exceeding 1000℃. Most austenitic high temperature stainless steel grades (309, 310, 321Ti, 330, 153MA, 253MA and 353MA) can suffer from a common disadvantage, an embrittlement when used in the temperature range 550 to 850℃.
The ferritic stainless steels of the AISI 400 series (430, 441and 444) are more susceptible to various embrittlement mechanisms after prolonged exposure. At temperatures of about 370 to 510℃, the alloys tend to suffer from the so-called 475℃ embrittlement. Due to enhanced precipitation of carbides at temperatures between 480 to 650℃ the alloys show loss of ductility. In view of their lower alloy content and lower cost, ferritic steels are affected by many factors including temperature, time, type of service (cyclic or continuous) and atmosphere .
The FeCrAlloy is a modified stainless steel based on a ferritic steel with 5-8% aluminium, 17- 22% chromium. The high aluminium content is the first reason for the extremely good high temperature mechanical and oxidation behaviour of this metal alloy grades. FeCrAlloys are widely used for long service life industrial heaters, and other major high temperature applications.
NiCr-alloys (Hastelloy X, Inconel 602 CA, Haynes 230, etc.) offer a good hot strength combined with ease of fabrication. The limitations of NiCr alloys operating in air are primarily due to the formation of Cr2O3 on the surface. Although this scale offers some protection against oxidation, it is less cohesive with the substrate and tends to suffer from extensive exfoliation.
3. Materials data survey
Different types of materials have been considered in this literature survey. The first selection criteria was based on the fact that both ferritic and austenitic materials had to be present in the analysis for the following reasons:
- Ferritic and austenitic materials are currently used in the production of blue-flame burners.
- They do have some common characteristics, such as the thermal expansion, the recrystallization behaviour and the hardening effects.
Moreover, since all high temperature alloys used in corrosive/oxidizing atmospheres have the peculiarity of forming a spontaneous self protecting oxide layer, both chromia and alumina forming alloys were investigated in the present analysis. The adhesion and the resistance to foiling off of the oxide can determine the success in the application of the alloy in oxidising/corrosive medias at high temperatures.
Other two important parameters for the definition of the materials to be characterised is represented by their cost and workability. These aspects necessarily influences the burners' design and therefore has to be taken into account. Creep resistance has been considered, in order to be able to compare the different behaviours of burners applications: higher ductility rather than failure resistance.
As far as the technical aspects are concerned, the most appropriate material should have good high temperature mechanical properties in junction with high oxidation resistance. Materials, which become very ductile and easily flow at high temperatures, are inappropriate when burner shape matters: if any constraint to such material free flow is imposed by the design, intense buckling may occur resulting in a fast loss of the burner geometry. This means that such a material can be safely used in burners where no constraint are imposed on the material at high temperature, e.g. cylindrical geometries, but has to be definitively avoided for flat geometries, where typically severe constraints are imposed by the item design.
According to the so far discussed principles and the requests imposed by the two burner design (atmospheric premix and fully-premixed fan assisted), as a very preliminary analysis it can be said that: it is probably better to think to the cheap and easy to work materials for the first type of burners, where no critical degradation mechanisms occur; for the second burner type, the competition may be harder as the technical performance has to be high, but the costs has to be kept as much low as possible; therefore, a critical analysis of the performance of at least four materials pertaining to the medium to high class has to be performed.
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