Transmission and distribution utilities have become a significant source of waste materials including scrap components such as porcelain insulators and bushings, instrument transformers, protective equipment, etc. However, there are currently few if any standardized policies for dealing with these in an environmentally-sensitive and economically-justifiable manner. Landfill has therefore typically been the dominant manner used to dispose of them. This INMR article from 2007, contributed by Dr. Robert Sekula of ABB Corporate Research in Poland, offered recycling options for a variety of insulating components and equipment. It also summarized relevant laboratory testing in this field.
Ceramic as well as thermoset plastic materials are used widely in the electrical industry due to their dielectric properties and mechanical strength. They therefore find application in products such as insulators, bushings, instrument transformers, etc. Appropriate waste management and recycling policies should be established for these types of products at the end of their service lives.
For example, legislation in Poland is based on European directives which propose that policies should ideally be concentrated on those wastes which fulfill basic requirements including serving as raw materials for or somehow improving a particular production process.
Three streams of wastes have been investigated in this research: plastics (thermosets), ceramic insulators, and silicone-housed surge arresters. Large volumes of these are continually being generated by utilities from their overhead lines and also from substations.
It should also be pointed out that in many products from the electrotechnical industry, different metals are embedded either in cured thermosets or in silicone materials. Their recovery could prove a very useful recycling opportunity.
2. Current Status of Utilization
The following sections provide general information regarding present policies for the streams of waste products investigated.
2.1. Scrap Thermosets
The most widely used scrap method for thermosets is landfill. Currently, it is estimated that more than 90 percent of these wastes are being buried. Recognizing the growing volume of these types of waste materials and the limited space for disposing of them, more sophisticated, recycling-oriented technologies should be promoted.
For example, instead of landfill, the following options are currently being considered for utilizing thermoset waste:
• application as a construction material
• energy recovery
The addition of cured epoxy-based scrap to concrete or asphalt construction materials has often been considered as a simple means for their disposal. However, keeping in mind that the relative amounts of such materials are small and the waste streams distributed geographically, there is a very limited economic benefit in such a solution.
Epoxy scrap often originates from previous use in different types of electrical equipment. As a result, internal metallic parts (e.g. cores, windings) are typically embedded in the bulk epoxy and require some form of pre-treatment of the scrap in order to remove them.
Considering the energy content of thermosets (i.e. LHV from 10 to 20 MJ/kg, depending on filler content), energy recovery through combustion can prove a worthwhile option. Such treatment, however, generates large amounts of inorganic material in the form of filler and which must therefore be handled in some economic manner. From the pollution point of view, combustion of thermosets is relatively safe, if properly defined hazardous emissions can be limited.
The last option, namely degradation, refers to the reduction of plastics to lower molecular weight materials through application of such processes as photo-degradation, chemical degradation or biodegradation. Pyrolysis is one specific type of chemical degradation which is considered in this article.
2.2. Scrap Ceramic Insulators
Up to now, the issue of somehow utilizing scrap porcelain insulators has not been considered too broadly. When such scrap has been generated, it has often been ground up for use in road aggregates, ice melt or outdoor tiling. Some American power utilities have even donated scrap insulators to local artists who use them to create sculptures or other household objects.
In some cases, used insulators can also be re-utilized as raw material for new insulator manufacturing. But, most typically, they are either stored on-site at substations, very often without any planned disposal solution, or just buried.
An additional factor when considering utilization of scrap ceramic insulators is that polymeric types (i.e. made with silicone rubber) are now broadly being used to replace traditional glass and porcelain units. The main reasons for this include lighter weight, easier installation, less vulnerability to vandalism and superior performance in polluted areas.
Taking into account the clearly less-than- optimal current situation for disposal of used ceramic insulators, one conclusion seems obvious: there will be a growing need to develop a more versatile and efficient waste management strategy to reflect growing volumes of such of scrap. In fact, this was one of the main reasons for seeking a new approach to solve this problem.
3. Proposed Re-cycling Solution for Thermosets
Any recycling option for thermoset (i.e. epoxy and silicone) based components must take into account that valuable metals such as copper and aluminum are very often embedded in the material. Therefore, any proposed recycling methodology should ideally result in a high quality of the recycled parts.
Based on both ecological and economic considerations, pyrolysis is viewed as the ideal process. Basically, it offers thermal degradation in an oxygen-free environment, resulting in:
• pyrolytic gas
• liquid by-products
• solids (char, mineral fillers, metals)
3.1. Epoxy Scrap Products
Such a pyrolysis was carried out using the experimental arrangement shown in Figure 2. A pyrolytic reactor equipped with electric heaters and thermocouples was the main element of the system. The design of the reactor allowed regulating the temperature inside over a wide range. This was achieved through the use of a type K thermocouple connected to the temperature regulating system.
A condenser, de-mister and cyclone were the main components of the purification system for the pyrolysis gas which was burned off in a special combustion chamber. Liquid products were separated by condensation and collected in the demister and cyclone.
The minimum time required for full pyrolysis was about 3 hours for one feed of the processed material and the maximum time was approximately 5 hours. Low temperature pyrolysis was carried out at 450° C, while the high temperature process was performed at 750° C and 850° C.
In total, three different sets of experiments were conducted to establish the optimum process parameters so as to assure a high decomposition of the organic material (i.e. a low carbon content in the solid residues) as well as high quality of any recycled metallic parts.
Sample results obtained from well-defined pyrolytic recycling are presented in Table 1 (gaseous products) and Table 2 (solid products).
Based on these experiments, it was demonstrated that pyrolysis can be used effectively for thermal utilization of resin wastes with simultaneous recycling of their metallic components. Pyrolysis by-products such as gas and oils can then be utilized as fuels in the next step of this technology for waste utilization.
3.2. Scrap Surge Arresters
Over the past two decades, silicone-housed surge arresters have found broad application in a wide range of medium and high voltage networks. Since this type of silicone rubber material is relatively new, there is at present no commercial method allowing efficient utilization/ recycling of used surge arresters.
Recycling investigations have already been performed for insulation made from liquid silicone rubber (LSR). Due to the nature of this material, the number of efficient decomposition methods is limited and combustion (incineration) is typically used. This process, however, is characterized, by very high temperatures (i.e. over 900° C) and its application in recycling surge arresters is not acceptable since it would cause degradation of the zinc oxide and emission of heavy metals.
At the same time, it should be mentioned that, from a purely energy point of view, combustion of silicone results in a very high release of heat, e.g. 17,000 kJ/kg. By comparison, the heat of combustion of hard coal equals 25,000-30,000 kJ/kg.
With the above in mind, the only rational option for thermal degradation of silicone is based on pyrolysis. The same experimental set-up as in the case of epoxy scrap has been employed. Again, different temperature scenarios have been evaluated so as to optimize process parameters and to recycle high quality internal parts (mostly the zinc oxide varistors). These experiments have now resulted in the development of an efficient procedure (see Figure 3).
4. Proposed Solution for Ceramic Insulators
As mentioned above, studies aimed at developing interesting utilization techniques for scrap ceramic insulators have so far been quite limited. Generally-speaking, porcelain is not a hazardous material for the environment and landfill is therefore used broadly.
However, given the increasingly limited space for such an option and the growing fees associated with it, novel utilization methods were explored. These investigations started with a detailed analysis of the composition of the porcelain typically used for electrical insulation.
Analyzing the data, one can see that all these various metal oxides are also typically present in Portland cement compositions (see Table 4). This was then the driving factor in development of one potentially interesting utilization option for scrap ceramic insulators, namely application in a cement rotary kiln.
This application is based on grinding the porcelain and injecting the resulting powder (preferably in a mixture with pulverized coal) into the flame inside the cement rotary kiln (i.e. at the hot end). There has even been the suggestion that doing so will promote catalytic reactions on the surfaces of particles leading to reductions in nitrous oxide (NO) emissions.
As mentioned, the composition of insulators is close to that of the cement raw material. Therefore, addition of milled insulators to the kiln should be acceptable due to an expected increase in yield.
In order to verify this, laboratory tests were performed whose goal was to evaluate the influence of introducing into the flame powder obtained from grinding scrap ceramic insulators. Three types of ceramic bulk material (i.e. 110, 120, 130) were tested. To assure a high surface area of the resulting powder, the insulators were milled to grain sizes below 200 mm.
A laboratory furnace was the main element of the experimental test setup (see Figure 4). It had a 1.3 m long combustion chamber with an inside diameter of 0.25 m and was equipped with an oil burner having a thermal capacity of 7 kW. The furnace also had 10 sampling ports along the combustion chamber.
Diesel oil was mixed with ceramic powder (3 g. of powder/litre) in a mixing tank and this mixture was supplied to the burner. A TESTO 350 analyzer and water-cooled probe were used to measure flue gas concentration. The temperatures of flame and flue gases were measured with type S thermocouple and all measurements were made along the furnace axis.
Due to very low NO2 concentrations (in the range of 1 to 10 mg/m3) the studies were focused mainly on NO. To increase the amount of nitrogen oxides generated in the process, up to 10 percent by weight of pyridine (C5H5N) was added to the diesel oil. This allowed NO concentrations in the flue gases to exceed levels of 2000 mg/m3.
The experiment on oil combustion allowed the influence of ceramic powder injection on the concentration of nitrogen oxides to be observed, but mainly those coming from the fuel source. To assess the effect of this process on thermally-generated nitrogen oxides, additional tests were carried out with the same equipment but this time using natural gas as fuel. In this case, the oil burner was removed and a natural gas burner (having a 6 kW thermal capacity) was installed alongside the powder injection system.
Sample data from these experiments are presented in Figures 5 and 6. All NO concentrations have been recalculated for a 3 percent oxygen level to allow better comparison of the results for combustion tests with and without powder injection.
It can readily be seen that introduction of ceramic powder to the flame affects NO concentration and a significant reduction has been achieved, in many cases by more than 50 percent. A beneficial effect in regard to NO emissions was observed for both fuels – oil and natural gas. This means that injection of the ceramic powder to the flame zone influences both fuel and thermal nitrogen oxides.
The most probable explanation for this phenomenon is that components in the ceramic powder have some catalytic effect on NO. Probably it is the same effect which was observed by other researchers, where NO can be destroyed in the presence of a reducing agent (e.g. CO). However, the influence of metal oxide concentrations from the ceramic powder on the reaction kinetics of NO and CO on the powder’s surface is not yet well defined.
Laboratory-based investigation confirms that technologies are presently available for utilization (recycling) of different types of scrap insulating components. Indeed, all the specific techniques discussed here have already been patented by ABB.
Now, in order to ensure successful commercialization of these proposed solutions, appropriate policies must be put in place relating to efficient collection of such scrap components. This should be done by waste disposal organizations, ideally with the strong support of local governments.