Optimizing H_2 Production from Waste Tires via Combined Steam Gasification and Catalytic Reforming

摘要

The disposal of waste tires represents a relevant problem within the waste management strategy of the European community: more than 300 000 000 tires are estimated to reach their end of life each year in the 27 member states of the European Union and comparable amounts are found in North America, Latin America, Asia, and the Middle East. The global total is ~1000000000 and rising each year. (Source: European Tyre Recycling Association (ETRA), 2006.) It is well-known that scrap tires possess high volatiles and low ash contents, together with a heating value that is higher than coal and biomass. These properties make them an ideal material for alternative thermal processes, such as pyrolysis and gasification, which can be finalized both to energy and material recovery. Within this frame, the present work is related to experimental tests and has obtained results of a combined process of scrap tire steam gasification and syngas catalytic reforming, with the aim of exploring the possible utilization of syngas for fuel cell applications. Four catalysts have been used for the experimental tests: two natural mineral products (olivine and dolomite) and two commercial nickel-based catalysts. Experimental data show that whether olivine or dolomite is used directly into the reactor to carry out the steam gasification, the char and gas yields increase with respect to the sole tire gasification (the char production varies from 41.2% w/w without catalysts to 59% w/w and 47.9% w/w, using olivine and dolomite, respectively; the gas production varies from 60.8% w/w for the sole tire gasification to 63.5% w/w with olivine and 84% w/w with dolomite). Then, while the olivine shows a stronger effect on the char production, the dolomite seems to be more effective on die gas yield. Moreover, both the catalysts promote a higher hydrogen production, which varies from 51.6 vol % for the sole tire gasification to 65.6 vol % and 57 vol % using, respectively, dolomite or olivine, basically because of the enhanced cracking of methane and the other hydrocarbons (the methane content decreases from 27.6 vol % for the sole tire gasification to 11.3 vol % and 20.8 vol %, using dolomite or olivine, respectively). Regarding both dry and steam reforming, the experimental tests show that the catalytic step, tested by varying the temperature, the catalytic substrates, and the reactive atmosphere, promote the production of a high hydrogen-rich gas, already at the lower tested temperature. It has been seen that the stronger effect for the increase of hydrogen content is for steam reforming condition and using a commercial nickel catalyst instead of Ni/olivine: under such conditions, the hydrogen content increases, from 51.6 vol % before the reforming up to 78 vol % at 650 ℃. With regard to gas production, a strong increase of the flow, mostly due to the effect of the cracking reactions, is registered as well and, more in detail, the gas production increases from 0.8 m_3 kg~(-1) fed, before the reforming, up to 1.0 m_3 kg~(-1) fed and 1.S m_3 kg~(-1) fed, respectively, for dry and steam reforming at 7S0 ℃ and using Ni olivine catalyst, and up to 1.12 m_3 kg~(-1) fed and 1.91 m_3 kg~(-1) fed, for dry and steam reforming at 750 ℃, respectively, and using a commercial Ni catalyst The adopted operating conditions allow one to obtain an appreciable amount of char, whose high carbon content suggest its further exploitation both as activated carbon (after activation process) and as a carbon source for synthesis reactions.