Volatile and semivolatile emissions from the pyrolysis of almond shell loaded with heavy metals

Highlights: • Metals did not change the main degradation pathways of thermogravimetric curves. • Nickel-loaded sample presented the highest H₂ and CO yields in the pyrolysis gases. • Nickel is the most effective in the higher aromatic tar reduction. • Higher carcinogenic potency values were found in native and chromium-loaded samples. Heavy metal-loaded almond shell was subjected to pyrolysis to understand the effect of the presence of different heavy metals on its thermal degradation. Pyrolysis behavior of native and metal-loaded samples was studied by thermogravimetric analysis. Similar shapes of thermogravimetric curves indicate that the presence of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and lead (Pb) did not change the main degradation pathways of almond shell. However, the temperature at which the decomposition in each stage takes place at a higher rate and char yield was considerably modified by the presence of Cr and Ni. Then, pyrolysis tests of the almond shell samples were performed in a moving tubular reactor at 700 °C. Gases and volatile organic compounds were collected using Tedlar bags and semivolatile organic compounds were collected using a resin as adsorbent. Significant changes were obtained in the composition of the gaseous fraction as a result of the metal impregnation. The main changes in the composition of the gas were observed for Ni-loaded sample, which presented the highest H₂ and CO yields. Also, the yields of most of the light hydrocarbons decrease in the presence of metal, while the rest remain quite similar. The total PAH yields reached 103 μg/g for nickel-loaded sample (NiAS), 164 μg/g for copper-loaded sample (CuAS), 172 μg/g for lead-loaded sample (PbAS), 245 μg/g for native sample (AS), 248 μg/g for cadmium-loaded sample (CdAS) and 283 μg/g for chromium-loaded sample (CrAS). Nickel is the most effective in the higher aromatic tar reduction, followed by Cu and Pb, whereas the presence of Cd does not affect the total emissions of PAHs. Finally, the carcinogenic potency of the samples was calculated. Native sample and the sample loaded with Cr presented slightly higher values associated to the presence of small amounts of benzo(a)pyrene.