Concept and project objectives
Industrial production is a major consumer of energy. The energy balance of the EU-27 in 2005 as reported by Eurostat shows a total end-energy consumption in the industry sector of more than 13.000 PJ representing 28 % of the overall end-energy consumption in all sectors. Based on data of the same year for Germany (Source: German Federal Ministry of Economics and Technology) one can asses that ca. 60% of the industrial end-energy use is spent for industrial process heating (total energy consumption in the German industrial sector of 2.550 PJ and 1.562 PJ for process heat), while more than 80 % of process heat is covered by fossil fuels. Assuming that the German distribution is representative and can be extrapolated to the European dimension, a total of more than 6.000 PJ are spent for process heat generated by burning of fossil fuels on the European level, representing more than 12% of the total end-energy consumption of EU-27 and at the same time approx. 9.7 % of the total fossil fuel consumption.
Consequently, an efficiency increase in process heating of only a few percent would result in a significant reduction of energy consumption and CO2-emissions at the European level.
Industrial branches with high energy consumption are for example iron and steel making, metal processing and ceramics production. Most of the processes are operated at a high temperature level. Heat recovery at this high temperature level is essential in terms of efficiency. The most common way for heat recovery is preheating of air or other gas flows by using the energy of the hot waste gas flows. Recuperative or regenerative heat exchanger systems, which may be integrated in the burner assemblies, are commonly used for this purpose.
Among the different heat recovery systems centralized systems like central recuperators show the lowest efficiency increase in practice. This is due to the long collection and distribution pipes needed and the associated heat losses at the high temperature level.
The use of burners equipped with recuperators results in a higher efficiency increase. However, the efficiency of recuperators is limited by the overall volume and burner length restrictions and the required heat exchanger surface for an almost complete heat recovery.
The highest efficiency can be reached using regenerative burners. However, the complexity of regenerative burner systems is higher, due to the discontinuous alternately mode of operation utilizing several regenerator matrices.
In all cases it can be presumed that a 100 K decrease of the flue gas temperature after heat recovery results in an estimated reduction of fuel consumption of about 5 %.
The heat exchangers used nowadays are mostly built out of high temperature steel with corresponding temperature limitations. The use of ceramic materials allows operation and heat recovery at higher temperatures and subsequently higher process efficiency.
Ceramics also show a very good corrosion resistance, although application restrictions due to dust transported with the gas flows may occur in practice. Heat recovery devices should be directly integrated into the thermoprocessing facility, in order to minimize thermal losses and utilize the energy in an immediate manner.
Despite the obvious benefits of ceramic heat exchanger components for heat recovery at high temperature applications, the penetration of such technologies in industrial furnaces is relatively low, due to the comparatively high prices and large size of such components. Only a few very simple ceramic heat exchanger geometries with large dimensions are used up to now, due to several manufacturing and operational obstructions.
Ceramic recuperators used nowadays in industrial burners are tubular. Common recuperator lengths are in the order of 0.5 m and hence longer than the thickness of the furnace wall. Heat enhancement with structured surfaces is limited due to limitations of the associated ceramic production technologies. Given the technological obstructions and the dimensional limitations the level of heat recovery is limited. State of the art recuperative burners show an air preheating and flue gas temperature level after the recuperator in the range of 500 to 700 °C resulting in heat losses through the flue gas of ca. 25 – 35 % [Gas-Wärme-International 56(2007), 6, pp. 425-428].
Regenerative burners (typical nominal power range ca. 200 kW) operate with higher preheating temperatures of the combustion air. Due to the larger surface the regeneration temperature reaches 85 – 95 % of the flue gas temperature.
The aim is to develop a new generation of ceramic heat exchangers for high temperature heat recovery with the target of significantly reducing the size and weight as well as also the price of such components by simplifying the manufacturing process and allowing a higher flexibility in the heat exchanger geometry.
The use of precursors/template materials taken from the textile industries and a subsequent ceramic conversion is proposed as the main technological path for reaching the above objectives. The proposed route will lead to an increase in freedom of the geometric design at low costs for shaping. The development/refinement of the conversion process for such materials into a thermal-shock resistant gas-tight ceramic (e.g. silicon infiltrated silicon carbide) and the multi-objective optimization in terms of size, geometry, material and production costs is the major challenge of the proposed project.
A complete ceramic heat exchanger component shaped by textile technologies is targeted. However, the combination/junction of existing robust ceramic components already applied in industrial furnaces, like silicon infiltrated SiC tubes, with compatible ceramic heat enhancement elements, built through the textile technology based manufacturing process, allows a robust construction in terms of application safety as an intermediate technology development step. At the same time a significant size reduction or increase of the heat recovery level can be achieved due to the higher heat transfer by the fine shaped and geometrically flexible heat enhancement elements.
The S&T objectives of the project may be summarized in the following:
• Utilization of “cheap” textile technologies for green material shaping (precursor, template) of heat exchanger elements and conversion to robust long term stable (>40.000 h) ceramic heat exchanger units for industrial furnaces
• Increase of air preheating temperatures through direct heat recovery up to levels of > 1000 °C, or
• At least 50% size reduction of ceramic heat exchangers for industrial furnaces at the same performance
• Development and proof of concept of innovative application schemes leading to energy savings possible with the new enhanced heat transfer ceramic heat exchanger components (e.g. high temperature inert or process gas convective heating as described later).
• Optimizing the ceramic conversion technology and heat exchanger design, in order to achieve same costs as with nowadays ceramic heat exchanger technologies already at the early market introduction, however at significantly more compact size and improved performance
The expected impact of the development in terms of energy savings and greenhouse gas emission reduction is significant, keeping in mind the share in energy consumption of industrial process heating.