Hydrocarbons for Hydrogen Generation - Hydrogen Generation, Storage, and Utilization - page 17

2.5.2 Mechanistic Aspects of Glycerol Reforming Reactions

The overall reforming process involves a number of complex reactions, some
of which lead to the formation of byproducts. The primary reactions are given
by [13]:

C H O → 3 CO + 4 H ∆ H = 251 kJ mol ⋅

- 1

(2.13)

3 8 3 2

CO H O CO + → + H ∆ H =− 41 kJ mol ⋅

- . 1

(2.14)

2 2 2

Equation 2.13 represents glycerol decomposition in the presence of water,
whereas as Equation 2.14 denotes the water gas shift reaction. The sum of
the two reactions is the overall reaction represented in Equation 2.10. No
oxygen is required for this reaction. High temperatures, low pressures, and
high water/glycerol ratios favor H 2 production.
Some of the side reactions, including methane formation, are represented
below:

CO + 3 H → CH + H O ∆ H =− 206 kJ mol ⋅

- 1

(2.15)

2 4 2

CO + 4 H → CH + 2 H O ∆ H =− 165 kJ mol ⋅

- . 1

(2.16)

2 2 4 2

The formation of CH 4 is favored at high pressures, low temperatures, and
low water/glycerol ratios. The methane formed can react with CO 2 to produce
CO and H 2 . Other side-reactions can lead to coke formation on the surface
of catalysts, and some of which are represented by the following reactions:

2 CO CO → + C ∆ H =− 171 5 . kJ mol ⋅

− 1

(2.17)

2

CO H + → H O C + ∆ H =− 131 kJ mol ⋅

- . 1

(2.18)

2 2

In addition, CH 4 decomposition and reaction between CO 2 and H 2 can also
result in coke formation. To minimize methane and coke formation and
enhance H 2 production, the reactions are usually carried out at high tempera-
ture (>627°C), a pressure of 0.1 MPa, and a water/glycerol molar ratio of
9 [11].
Similar mechanistic and thermodynamic studies have been conducted for
other reforming processes, for example, those involving aqueous water and
H 2 O 2 . Oxidation plays an important role in these cases.

2.5.3 Catalytic Reforming of Glycerol

Similar to reforming of methane and methanol or ethanol, catalysts can
accelerate the glycerol reforming process. Catalysts such as Ni, Co, Pt, Ru,