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

Abstract

The thermodynamic effects of molar steam to carbon ratio (S:C), of pressure, and of having CaO present on the H 2 yield and enthalpy balance of urea steam reforming were investigated.At a S:C of 3 the presence of CaO increased the H 2 yield from 2.6 mol H 2 /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 mole of H 2 at 880 K. This decreased to 94.0 kJ at 660 K with CaO-based CO 2 sorption and, when including a regeneration step of the CaCO 3 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 H 2 from renewable urea in a low carbon future is discussed.

SOFC fuelled with reformed urea

“…In Fig. 6 the obtained results in dots are compared with reference [39], showing a good agreement. The figure reports overall enthalpy, included reaction heating, in terms of kJ per each mole of urea.…”

“…Performance reduction caused by urea dilution can be quantified easily and it results independent from temperature, confirming that water content is the main cause of decay. The experimental activities suggests that the best performing composition is that with the S:C = 1 and not the one with S:C = 3 as found in reference [39].…”

“…System scheme. Dupont et al [39] performed a theoretical study of urea decomposition at SOFC operational temperature, considering water addition to enhance steam reforming as primary decomposition pathway and to manage and optimize carbon content in the compound. Theoretical steam reforming of urea is:…”

“…S:C equal to zero was not considered due to the high risk of carbon deposition. The three selected values of S:C were: S:C = 1 (stoichiometric), S:C = 3 (the best performing in [39]) and S:C = 6.92 (as in AdBlue). For the nine test conditions above, the molar compositions were calculated.…”

Steam reforming of shale gas with nickel and calcium looping

“…Overall, the effect of temperature on the surface area, carbon concentration on the surface of the catalyst and the condensate was not obvious. The absence of any major difference in the solid carbon concentration on the surface of the catalyst during SE-SR process was not surprising because all the temperatures were investigated at S:C 3, which thermodynamically inhibits solid carbon deposition [9,65].…”