168
Water for Energy and Fuel Production
APR of cellulose can form hydrogen by the following reaction [1–6,16]:
C O H + 6 H O → 6CO + 12H
(6.8)
6 6 12
2
2
2
Also, dehydration/hydrogenation results in the formation of alkanes as
C O H + 7H 2 → C H 14 + 6 H O
6 6 12
6
2
(6.9)
which gives the combined reaction as
(6.10)
Alkanes contained 95% of the heating value and only 30% of the mass of the
biomass-derived reactant.
Davda et al. [16] proposed that a way to increase hydrogen selectivity from glucose is to operate in two stages: (1) to carry out the low-temperature hydrogenation
step followed by the high-temperature reforming process and (2) to co-feed hydrogen with liquid reactant stream to the reforming reactor. This co-feeding argument
leads them to propose a reactor scheme shown in Figure 6.4 to obtain the product of
desired specification using APR [16].
For biomass application, APR of glucose is very important because it is the
basic sugar component of all starch and carbohydrates [1–6,16,48]. The hydrogenation of glucose leads to the formation of sorbitol, and both glucose and sorbitol
can be reformed to form carbon dioxide and hydrogen. As the glucose concentration in the feed increases, the hydrogen selectivity decreases. Also, these reactions are favored at low temperatures. The reforming of both glucose and sorbitol
can occur on Pt and Ni–Sn alloy by cleavages of C–C bonds followed by the
water–gas shift reaction [1–6,16]. The alkanes are produced on the acidic sites of
metals from both glucose and sorbitol. Glucose also produces acids, aldehydes,
and so on through homogeneous side reactions. Since undesirable side reactions
are first order with respect to glucose and the desirable reactions have a fractionorder dependence on glucose, an increase in glucose concentration reduces hydrogen selectivity [1–6,16]. The hydrogenation of glucose to sorbitol also occurs at
a higher rate at low temperature (400 K) and high hydrogen pressure. Recently,
Wen et al. [39] examined the catalytic properties of Ni on alumina for the APR
of glucose.
6.5.4.3 aPr of Biomass and Cellulose (secondary Feedstock)
APR has also been tested on biomass and cellulose waste paper [37,45]. Valenzuela
et al. [37] studied APR of woody biomass in a batch reactor. In this study, APR was
used to produce hydrogen from actual biomass. The experiments were carried out
in a 100 mL Parr micro reactor heated to 225°C. Both acid hydrolysis of woody
biomass and subsequent APR of soluble molecules by a Pt/Al 2 O 3 catalyst were
carried out in a single reactor. The experiments showed that increasing the acid
concentration from 1% to 5% resulted in more than a twelve-fold increase in H 2
concentration in the product gas. However, hydrogen accounted for only 18% of
1 6
3 5
2 5
6 6 12
6 14
2
.
.
.
C O H
C H
CO
HO 2
→
+
+