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The present industrial production of hydrogen, based on fossil fuels, contributes in green-house gas ( CO2) emissions and in wasteful energy consumption. Research has been recently focused to an alternative concept, which combines both reforming reaction and in situ CO2 separation in order to increase the efficiency of the current process. Such process is known as Sorption Enhanced Reforming (SER). The presence of a CO2 sorbent material in the reformer reactor boosts the feedstock (natural gas) conversion and leads to higher product quality. Development of a sorbent material with constant capture and regeneration ability is the key point for the economical and waste management efficiency of the process.

The scientific result focuses on the development of a CaO-based CO2 sorbent with high sorption capacity and long life-time for high temperature applications, such as SER. The optimum CO2 sorbent was found to be CaO-Ca12Al14O33 (85-15 wt%). The active component of the material is CaO while Ca12Al14O33 provides a stable framework inhibiting deactivation of CaO.

A Scanning Electron Microscopy, SEM, study of CaO-Ca12Al14O33 (85:15) revealed that this sorbent material can be visualized as consisting of a number of small grains (Fig 1). The macropores surrounding these grains facilitate the gas diffusion to the various grains.

Fig 1. SEM micrograph of the CO2 sorbent material CaO-Ca12Al14O33 (85:15)

 

X-ray Diffraction, XRD, was also employed to study the crystal phases which are present in this new sorbent material. The XRD pattern is shown in Figure 2. All characteristic peaks of CaO (2θ = 32.2, 37.35, 53.85, 64.15, 67.3) and Ca12Al14O33 (2θ = 33.41, 41.21, 55.22, 57.52) were clearly detected. The absenceof any other Ca-Al mixed phases or hydrated mixed structures proved the formation of the desired CaO-Ca12Al14O33.

Fig 2. XRD pattern of the CO2 sorbent material CaO-Ca12Al14O33 (85:15)

The concept of Sorption Enhanced Reforming,SER, is based on Le Chatelier’s principle, according to which the conversion of reactants to products and the rate of the forward reaction in an equilibrium controlled reaction can be increased by selectively removing some of the reaction products from the reaction zone.

In SER this principle is applied by using a CO2 sorbent (e.g. CaO) (reaction 3) in order to shift reaction (2) and consequently reaction (1) to hydrogen production side. As the sorbent is effectively consumed in reaction 3, the process is inherently dynamic in operation, requiring a regeneration step. In addition, the sorbent must maintain its activity through many cycles for the process to be economically viable.

 

CH4 + H2O -> CO + 3H2     ΔH298K  = 206,2kJ / mol (1)

CO + H2O – >CO2 + H2    ΔH298K = – 41,2 kJ / mol (2)

CaO + CO2 -> CaCO3   ΔH298K =- 178 kJ / mol  (3)

Posted at 7, April, 2011

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