ENVIRONMENTAL IMPACT OF ENERGY RECOVERY FROM WASTE TIRES

Compared to coal, tires have a high heat content and low levels of moisture and nitrogen, and thus are ideally suited for energy recovery through combustion. This process, however, generates gaseous pollutants and solid waste materials, which must be disposed of or reused as secondary raw materials. Studies have shown that substantial reductions of some environmental pollutants (e.g., NOx emissions) can be achieved by partially replacing conventional fuels with waste tires. On the other hand, using tires as fuel may lead to considerable increases in the levels of other pollutants. The geochemical effects on solid and gaseous combustion products are more or less pronounced depending on fuel composition, conditions of combustion, type of facility, and effectiveness of air pollution control devices. Thus, the use of tire fuel has environmental impacts that must be weighed against the benefits of reducing the large volume of waste tires in the global waste stream.

This study reports emission data obtained from an experiment carried out at the Purdue University power plant, where two different sets of fuel were combusted at the same conditions. Compared to combustion of pure coal, the use of a mixture of 95 wt% coal + 5 wt% TDF (tire-derived fuel) leads to higher emissions of various metals, most notably Zn (increase from 55 g/h to ~2.4 kg/h). To better characterize the emissions, particulate matter was collected on filter paper that was inserted into the top part of the smokestack. The collected particles thus represent the fly ash fraction that would have escaped into the atmosphere. SEM and TEM investigations revealed that these fugitive particles consist of amorphous (mainly Al-Si-glass) and crystalline material. The crystalline material comprises lime and mullite, and a variety of mostly euhedral S-O phases (inferred to be sulfates), including sulfates of Ca, Pb, Fe-Zn (pure coal), and Zn (TDF+coal). The Zn and Fe-Zn sulfates range in size from a few nm to >100 &mum. The large crystals must have crystallized only after the flue gas has passed through the air pollution control devices within the power plant. Detailed characterization of these particles is essential to assess the interaction between crystals and atmospheric moisture or rain, and thus the environmental impact of tire combustion.