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

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 (PC7H8=0.93–1.15 kPa, H2O/C7H8=2.67 mol/mol, space-time τ=0.067–0.127 kg h (Nm3)−1) as a tar-derived species and/or Volatile Organic Compound (VOC). Toluene (C7H8) 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 Nm3 (m3 h)−1. The overall catalytic pyrolysis of toluene over a commercial available metal based catalyst NiMo/γ-Al2O3 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%, τ=0.150 kg h (Nm3)−1) and water vapor (PH2O=4.51–21.41 kPa, τ=0.092–0.270 kg h (Nm3)−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.