Environmental Engineering Reference
In-Depth Information
straw. A bioethanol plant in Ulmea (Sweden) is running using waste stream of
cellulose-based materials and another pilot plant production for the preparation of
bioethanol from lignocellulosic materials (e.g. Norway Spruce) has recently started
production [95].
Linear bioalcohol mixtures
. Mixed linear alcohols (i.e., mixtures of mostly
ethanol, propanol and butanol, with some pentanol, hexanol, heptanol and octanol)
can also be produced from syngas in a similar way to that described for methanol
and ethanol [96]. One of such linear alcohol mixtures denoted as Ecalene
TM
is cur-
rently registered with the US Environmental Protection Agency per 40 CFR 79.23
as a fuel blending additive [96].
Synthetic Biofuels
Synthetic biofuels can be defined as fuels prepared from syngas via different pro-
cesses. Bridgwater and Demirbas have recently reported comprehensive overviews
of the development of these technologies for the preparation of biofuels [87, 89].
Under this headline we can include a selection of some interesting options
such as biofuels obtained by steam reforming, HydroThermalUpgrading (HTU) and
Fischer-Tropsch Synthesis (FTS).
Biofuels obtained by steam reforming
. Steam reforming can be applied to vari-
ous solid waste materials including organic waste, sewage sludge, waste oils, black
liquor and agricultural waste to produce biofuels [89]. Steam reforming of natural
gas (often referred as steam CH
4
reforming) is the most common method to produce
commercial H
2
[89].
Biohydrogen can therefore be produced from a biomass feedstock via conven-
tional gasification at high temperatures to syngas to obtain methane (reaction 4,
Fig. 8.9) and subsequent steam CH
4
reforming at high temperatures (700-1100
◦
C)
using Ni supported catalysts (e.g. Ni/Al
2
O
3
, Ni/MgO) at 3-25 bar pressure
(Fig. 8.10, reaction 1) [89, 97]. For the production of high purity H
2
, the reform-
ing of the biofuel that includes multiple catalytic steps is followed by two water
gas-shift (WGS) reaction steps (Fig. 8.10, reaction 2), a final CO purification and
removal of the remaining CO
2
by pressure swing adsorption or ceramic membrane
separation [89, 97].
Alternatively, the gasification step of biohydrogen can also be performed in
supercritical water (in a similar way to that of the bio-SNG) with the advantages
of the direct use of wet biomass without drying and a high gasification efficiency
at lower temperature [89, 98]. However, the cost of H
2
production using this tech-
nology is several times higher than the current price of H
2
obtained from steam
reforming [89].
(1)
+
+
3H
2
CH
4
H
2
O
CO
Fig. 8.10
Steam CH
4
reforming (1) and WGS (2)
reactions for the preparation
of biohydrogen
(2)
CO
+
H
2
O
CO
2
+
H
2
