Thermodynamics of hydrogen production from urea by steam reforming with and without in-situ carbon-dioxide sorption.

The thermodynamic effects of molar steam to carbon ratio (S:C), of pressure, and of having CaO present on the H2 yield and enthalpy balance of urea steam reforming were investigated. At a S:C of 3 the presence of CaO increased the H2 yield from 2.6 mol H2/mol urea feed at 940 K to 2.9 at 890 K, and decreased the enthalpy of bringing the system to equilibrium. A minimum enthalpy of 180.4 kJ was required to produce 1 mol of H2 at 880 K. This decreased to 94.0 kJ at 660 K with CaO-based CO2 sorption and, when including a regeneration step of the CaCO3 at 1170 K, to 173 kJ at 720 K. The presence of CaO allowed widening the range of viable operation at lower temperature and significantly inhibited carbon formation. The feasibility of producing H2 from renewable urea in a low carbon future is discussed.