Kinetic modelling of VOC catalytic steam pyrolysis for tar abatement phenomena in gasification/pyrolysis technologies

摘要

Tar elimination and hot-gas conditioning in thermochemical conversion processes, i.e. thermal gasification, pyrolysis of heterogeneous materials involved two main classes of catalysts and/or additives: metallic and mineral oxides. This communication focused on the experimental kinetic data on catalytic steam cracking of vaporized toluene (P_(C_7H_8) = 0.93-1.15 kPa, H_2O/C_7H_8= 2.67 mol/mol, space-time tau= 0.067-0.127 kg h (Nm~3)~(-1)) as a tar-derived species and/or Volatile Organic Compound (VOC). Toluene (C_7H_8) has been chosen as a model formula for reactive tar-derived one-ring species determined from tar constituents. Gaseous product distribution data were obtained at atmospheric pressure (101.3 kPa), steam pyrolysis temperature range of 923-1223 K and GHSV 1200-2300 Nm~3 (m~3h)~(-1). The overall catalytic pyrolysis of toluene over a commercial available metal based catalyst NiMo/gamma-Al_2O_3 was compared to the pyrolysis in presence of basic non-metallic mineral additives, i.e. Norwegian (Norsk Hydro) dolomitic magnesium oxide [MgO], Swedish low surface quicklime [CaO], and calcined dolomite [CaMg(O)_2]. The operational conditions were applied without internal or external mass-transfer limitations. Kinetics for the pyrolysis could be described by first-order reactions for all the studied additives. The influence of hydrogen gas (30 vol percent, tau= 0.150 kg h (Nm~3)~(-1)) and water vapor (P_(H_2O) = 4.51-21.41 kPa, tau= 0.092-0.270 kg h(Nm~3)~(-1)) in vaporized toluene cracking runs over low surface quicklime [CaO] was determined. A mechanistic model of the Langmuir-Hinshelwood type describing toluene decomposition was also developed.