Thermal oxidation contributes to a considerable reduction of potential condensation, which results in significantly minimized drying system maintenance expenses.
The compositions of pastes used to metalize solar cells may vary greatly. This applies not only to the actual metal content (silver or aluminum powder), but to other additives as well, which are decisive with regard to the printing properties of the paste, as well as its baking and sintering characteristics. These materials may amount to as much as 25% by weight, whereas the greatest portion consists of the organic medium into which the solid materials are dispersed (powdered metal, metal oxides and inorganic binders, e.g., glass frit).
After being printed onto the solar cell at temperatures ranging from 200-350°C, the pastes are usually dried and baked/sintered into the solar cell during a second step at a temperature of greater than or equal to 800°C. Vapor/smoke forms while the pastes are being dried, which must be reliably exhausted from the drying system’s process chamber in order to avoid contamination of the system. The vapor/smoke consists of volatile constituents of the organic medium, which may be made up of various organic liquids that can additionally contain thickeners and stabilizers.
Examples of organic liquids include alcohol (texanol) and alcohol-ester (acetic acid and propionate), terpenes (pine oil, terpineol), resin solutions (methacrylate polymer), ethyl cellulose solutions in solvent (e.g., terpineol) and monobutyl ether from ethylene glycol monoacetate. Ethyl cellulose in terpineol, in combination with a thickener mixed with butyl carbitol acetate, is a preferred organic medium.
Separator systems
As a rule, operators of drying systems are not familiar with the exact composition of the utilized metallizing paste and its volatile constituents. For this reason, it’s not possible to customize a filter/separator for the vapor/smoke, which is thus required to be suitable for the separation of a broad range of various contents.
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| Figure 1. Contamination in a condensate separator. |
Condensate separators have proven their worth in actual manufacturing practice. Gases/vapors are condensed onto the cold surfaces of heat exchangers in these systems, and the condensate is caught in suitable containers. The disadvantage of these systems involves significant maintenance work in the form of periodic (e.g., weekly) manual cleaning. Figure 1 shows contamination in a condensate separator after one week of 3-shift operation. In some cases, several liters of condensate accumulate.
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| Figure 2. Thermal reactor. |
To minimize maintenance work, we have implemented the well known method of thermal oxidation for solar drying systems. Thermal oxidation is a process during which the volatile organic constituents and the hydrocarbons in the metallization pastes combine with oxygen, and are decomposed for the most part into water vapor and carbon dioxide. The goal is to burn the long-chain molecules in the vapor/smoke, and to transform them into readily volatile, condensable substances. These can then be discharged from the system in a trouble-free fashion. As a result, condensate forming potential in the drying system is drastically reduced.
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| Figure 3. Raw gas upstream from the thermal reactor. |
Thermal oxidation is started by heating the exhaust gas to a temperature of greater than 750°C. The molecules disintegrate at these high temperatures and combine with atmospheric oxygen that is present within the system. To be able to oxidize hydrocarbons in an energy-efficient manner at low reaction temperatures of ≤500°C, catalyzers are installed downstream from the heating chamber. This results in a thermal reactor that is dimensioned for solar dryers such that a gas exchanger of greater than 40-fold assures reliable removal of vapor/smoke which occurs in the process chamber.
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| Figure 4. Contaminant reduction results. |
Thermal oxidation
Thermal oxidation contributes to a considerable reduction of potential condensation, which results in significantly minimized drying system maintenance expenses. Figure 2 is a schematic diagram of the thermal reactor (oxidation unit). Contaminated, hot raw gas is exhausted from the solar dryer’s process chamber and is fed directly through the reactor’s electrical heater, in order to heat it up to the necessary temperature of 500°C. The gas is subsequently fed through a special granulate pack, which serves two purposes. On the one hand, the granulate catalyzes the combustion process, i.e., it reduces the combustion temperature; and on the other hand, a portion of the combustion products are absorbed and combine with the granulate. The arrangement of the heater and the granulate pack results in a compact, thermal reactor, which requires very little additional energy. All of the necessary temperatures, as well as gas flow, are monitored.
The concept of catalytic combustion was discovered during the last century by Johann Wolfgang Doebereiner. The chemistry professor from Jena, Germany, had already developed a catalytic lighter in 1823. He understood, for example, the catalytic effect of platinum metals. A host of catalytic materials is well known today. A catalyzer accelerates combustion without being consumed itself, and reduces the reaction temperature. Reducing the reaction temperature results in considerable energy savings, and is a major economic advantage of catalytic oxidation The Table provides an overview of the catalytic combustion temperatures of various organic compounds.
Because a scarcely determinable variety of substances can escape from metallizing pastes, the reaction temperature must be kept in the upper range (500°C). This assures that all of the substances contained within the raw gas can be thermally decomposed/oxidized. The contaminant distribution depicted in Fig. 3 was revealed during comparative measurements performed on soldering pastes.
Thermal oxidation systems can be used for nearly all organic contaminants, and thus represent a good, broad-ranging alternative to condensate separators. The primary advantage in comparison with condensation is significantly reduced maintenance work.
The technical layout of the oxidation unit is based on the basic principle of lowest possible energy consumption. In implementing this concept, we have been able take advantage of favorable experience with pyrolysis units used in reflow soldering systems. Comparative measurements reveal a very high degree of separation (Fig. 4).
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Measurements have demonstrated that the limit values specified by the technical instructions on air quality control for contaminants in process air can be adhered to reliably under actual manufacturing conditions by means of thermal oxidation. The main advantages of thermal oxidation are:
- Reduced maintenance expenses
- Can be used universally for various metallizing pastes
- Time-tested, broad-ranging process
- Rugged system technology
- Minimal energy consumption thanks to granulate technology
- Low operating costs
- Reliable adherence to legal emissions regulations
- Thermal oxidizer with granulate technology leads to less energy consumption and a cleaner process
Conclusion
During the drying of metallization pastes, a great quantity of residue is generated. Thermal oxidation is able to reduce the growth of condensate. In connection with an adsorbent granulate, the reaction temperature can be decreased, which reduces the energy consumption for the separation process.
Hans Bell studied physics-crystallography at Humboldt U. and received his doctorate at the Technical U. of Munich and is head of the Development and Technology department at Rehm Thermal Systems, www.rehm-group.com; ph.: +49 7344 9606 29; email: [email protected]
