From an historical point of view, the golden age of coal tar distillation ended in the second half of the XXth’s century. At the time, the downstream chemical industry shifted from coal tar recovered molecules to petroleum based alternatives now available in cheaper and larger volumes. Despite this shift the distillation of the coal tar recovered as a downstream product of the coke oven batteries kept going on. The valorization of coal tar by means of distillation related processes allow the recovery of various types of chemical oils and of coal tar pitch. The usage of coal tar pitch as a non substitutable binder for the production of anodes for aluminum smelters is one of the key factors contributing to maintain and even develop that kind of process as a key industrial step in a large picture.
By nature, the coal tar feedstock is composed to hundred percent by polycyclic aromatic hydrocarbons (PAH). This intrinsic nature of the PAH compounds present in the coal tar, in its derived oils or in the CTP is however a source of concern for the industry. This point of view is widely shared whatever country is considered for the operation of a distillation train or for the use of the products recovered. The operation of coal tar distillation plants is indeed considered has a source of impact as well globally for broad environmental performance indicators than locally for the consequences of direct and occupational exposures. In this context, proposing a sound evaluation of the environmental impact of a coal tar distillation project systematically becomes part of the decision process.
Despite, or in fact perhaps because of, the exceptionally long occupation of usual coal tar distillation sites, often operated over decades, the few publically available data related to the analysis of environmental impact of coal tar distillation sites do not appear as being usable as such for a fair evaluation of any impact analysis. Indeed the fairly long occupation of coal tar distillation sites (quite often over several generations of workers lifetimes), the imbrications of these units within production complexes (including coke oven batteries, steel mills or carbon black plant) and the evolution of operational practice and engineering standards tend to distort any return of experience. The picture draw from these past experiences will quite often be far darker than it could be expected for a green field unit.
In order to propose an evaluation of the environmental impact of a green field coal tar distillation project, a life cycle analysis methodology was applied for rating a typical project for a 300 000 MTPY coal tar distillation plant. The scenario was built around a unit designed on a multi flash technology with steam stripping for coal tar distillation. The operation was considered over 30 years including effect of maintenance and replacement of pieces of equipment concerned by corrosion ageing. The contribution of end life has been defined through a scenario consisting into a full remediation of the site, soils and underground waters included. The selection of the technologies and remediation strategies was made on the basis of the last reports and advances on the topic and while considering no underground sequestration of contaminated soils thus leading to a most impacting scenario. The data used for the rating of the various contributors to the environmental impact and life cycle assessment were taken the Ecoivent database (2.0) used through the Quantis software for life cycle assessment (Quantis Suite 2).
Based on the obtained results, a fair picture of the environmental impact of a coal tar distillation plant was obtained. It showed that as for most industrial processes the operation part represented the majority of the impacts with 87 to 97% of the impact for the five performance criteria used in Quantis Suite (Climate change, Human health, Ecosystem, resource usage, water quality). The impact of end life was especially rated as being far inferior to what was expected if based on over anticipation guided by the often quite spectacular consequences of tar spillages or CTP soil pollution. On the basis of these results, the effect of coal tar distillation technology selection was then evaluated through the scope of all technology dependant operation related factors: energy use (direct fossil energy, steam and electric power), waste water production and overall footprint. This evaluation allowed rating, for a fixed tar feedstock and a given productivity target of the most representative tar distillation processes operate nowadays: bottom circulation distillation, multi-flash distillation and hybrid processes such as Fives’s CTD processes. In order not to denature the comparison all evaluations were done on process design and equipment selection made on the basis of the best available technologies and EC BREFs’ and regulations. The results obtained teaches about the effect of each process philosophy, for a given production background, on the overall performance of the coal tar distillation both in terms of environmental impact and operational expenditures. The learning can be used within the frame of other projects including brown field and revamp projects.