Pyrolysis Of Automotive Shredder Residue
Automotive shredder residue (ASR) is a problematic mixture of materials that in an ideal world would be recycled. This has not been possible due to the difficulty of finding suitable uses for a variable material which may contain some toxic contaminants. Clearly, ASR needs a new processing solution.
Pyrolysis has been identified by the waste management industry is a process which has many advantages to offer in rendering ASR into materials which can be re-used or disposed of safely. However, despite many proposals and studies over the last five or more years, it is still considered unproven for this purpose in most countries.
Factors that have proven critical to its development have been ASR composition, heavy metal contamination, high chlorine levels, and competition with alternative emerging technologies.
In the waste management industry, pyrolysis and gasification are generally still considered emerging technologies and unproven until a particular technology has been shown to have been running successfully at full scale, or close to full scale, for many years.
Although pyrolysis and gasification are well known in activities such as the conversion of coal into town gas, the technology has only since the turn of the century been considered for its use in waste management. To the casual onlooker the apparently inconsequential shift from taking in homogenous, well-characterised traditional feedstocks to using heterogeneous, variable waste stream feedstocks has proven to quite the opposite.
Demonstrator plant operators have found the move to successful commercial status, very hard to achieve. Shredder residue is proving to be a complex waste stream which makes it difficult to process.
On rare occasions constant supplies are available to provide a secure supply. In these instances the shredder residue has been well characterised and a number of papers report proportions of sulphur, chlorine, heavy metals and contaminant oils arising from elastomers, PVC, metals and car fluids respectively. Unfortunately, these can vary significantly in the feedstock from hour to hour. Not surprisingly this requires demanding design features to actively and continually adjust the pyrolysis process.
This is understandable when one considers the types and ages of vehicles from which the ASR is being produced. Plus, many processors also accept additional scrap feed from white goods and light iron.
The moisture content also varies and so does the all-important energy content. Only when all these factors are put together does the full degree of difficulty become apparent within the design of appropriate thermal processes.
Yet, the pressure to achieve commercial processes to deal with such complex waste feedstock is growing year by year. This is especially true for shredder residues (SR) because they are more and more frequently considered unsuitable for landfill disposal, and even where accepted the price is rising rapidly. When landfill operators will not accept SR or char, it is the potential leaching of the contaminant metals which causes the problem.
This problem can be avoided when designing an ASR pyrolysis process by using the large amounts of waste heat available to vitrify the char. Several processes incorporate this idea.
Another possibility is co-combustion in cement kilns, however, again the level of metals remaining in ASR char is generally excessive for the cement industry. Chlorine levels are also a problem. These come mainly from PVC and other plastics.