Waste Tire Rubber Gasification Using Air Steam for Partial Oxidation and N₂ as Carrier Gas Available to Purchase

The high calorific value of rubber in waste tires (RWT) and its composition, similar to that of coal, make it a promising resource for thermal treatment. This paper assesses the potential of RWT for the production of syngas through thermal gasification using air and steam as oxidizing agents. Subsequent purification of the obtained syngas and its conversion to ammonia were also evaluated. The analysis started by grinding RWT samples and submitting them to thermogravimetric analysis (TGA), ultimate analysis (UA) and proximate analyses (PA). Through TGA data, a pyrolysis Activation Energy of 110.5 KJ mol⁻¹ was determined using isoconversional methods. The results from UA and PA were used to estimate empirical formulae, high heating value (HHV), and formation enthalpy. These data were entered in the simulation package ASPEN Plus® where a sensitivity analysis was carried out using the following models: RGibbs (for equilibrium reactions) and Non-Random Two Liquids (NRTL) coupled with UNIFAC for phase equilibrium calculations. As a result, the effects of equivalence ratio (ER) and steam to fuel ratio (W:F) on adiabatic temperature, gas composition, and energy recovery were determined. It was shown that W:F lower than 0.6 should be used, since the additional water shifted equilibrium towards oxidation of syngas components. Additionally, an ER range from 3.0 to 5.5 should be selected due to the presence of maximum values for both concentration and syngas output molar flow. Optimal operation conditions were estimated around an ER of 4.0 and a W:F of 0.5. Further simulation considered purification of the gas stream from gasification and its conversion to pure sulfur and ammonia. The purification plant, a selexol-based SOx and COx absorption, produced two streams: gas and liquid. The gas stream (containing mainly H₂, CO, traces of produced hydrocarbons, some CO₂ and N₂ from air) entered a Water Gas Shift (WGS) unit. The liquid stream (containing the solvent that carries H₂S, CO₂ and SO₂) was distilled to recover selexol. The gas resulting from this distillation, was entered to a Catalytic Claus reactor to recover elemental sulfur. As a result, 16.6 kg h⁻¹ of pure sulfur (which constitutes approximately 88% of the sulfur pollutants in RWT) was recovered. The N₂-H₂ mixture exiting the WGS was fed to a Haber Botch where 1552 kg h⁻¹ of ammonia were obtained from an initial 1250 kg h⁻¹ RWT feed. This study was concluded with a cost analysis which led to calculate a cost of 0.19 USD$/MT of ammonia produced through this innovative route that uses RWT rubber as feedstock.