Environmental Engineering Reference
In-Depth Information
upgraded to biomethane by removing CO
2
and other undesired elements, like H
2
S
(Mozaffarian et al., 2004a; Mozaffarian et al., 2004b). Biomethane can be used as a
transportation biofuel, as a stationary biofuel for electricity and heat generation or can be
transported through the existing gas infrastructure and substituting natural gas in all its
existing applications.
3.3.2. Syngas
Syngas is the product of the gasification process. It is mainly made of CO, H
2
, CH
4
and
CO
2,
but it may also contain different contaminants such as nitrogen, particulates, con-
densable tars, alkali compounds, H
2
S, HCl, NH
3
, HCN and COS. Since these contaminants
can lower activity in the FT or other chemical synthesis due to catalyst poisoning, the syngas
undergo a cleaning process in which the contaminats are removed and its main components
can be tailored to the needs of the following conversion processes, by means of methane
reforming (which converts CH
4
with steam to CO and H
2
), water gas shift reaction (adjusts
the H
2
/CO ratio by converting CO with steam to H
2
and CO
2
) and CO
2
removal (with an
amine). The main products that can be obtained from syngas are: ethanol, methanol,
hydrogen, ammonia, FT-fuels, acetic acid, formaldehyde and others (Dybkjaer and
Christensen, 2001; Hamelinck et al., 2004; Spath and Dayton, 2003).
3.3.3. Hydrogen
Hydrogen can be produced by syngas after a water shift reaction process (CO + H
2
O →
H
2
+ CO
2
), by methane after a steam reforming process (CH
4
+ H
2
O → 3 H
2
+ CO), by
alkaline water electrolysis (H
2
O → H
2
+ ½ O
2
) or by fermentation of suitable substrates and
microorganisms (Spath and Mann, 2001; Spath and Mann, 2004; Stojic et al., 2003).
Hydrogen can be used either as a fuel or as a chemical reducing agent.
3.3.4. C6 sugars
C6 sugars are the most abundant renewable resource available. Glucose is the most
important C6 sugar and, in Nature, is present in cellulose, starch or as a free monomer. Other
C6 sugars, present in hemicellulose, are arabinose and galactose. Glucose may be used as
substrate for producing energy and material product through mainly 3 pathways: a)
hydrogenation, leading to sorbitol and 1,2-propylene glycol, b) acid treatment, leading to 5-
Hydroxymethylfurfural, levulinic acid and their derivates, c) fermentation, leading to ethanol,
other alcohols and organic acids (such as lactic acid, acetic acid, citric acid and others)
(Kamm et al., 2006b).
3.3.5. C5 sugars
The most common C5 sugars in biomass feedstocks are xylose and mannose (with xylose
having a dominant position). They can be found in the hemicellulose fraction of
lignocellulosic raw materials or as free monomers. As in the previous case, there are three
main chemical conversion pathways for producing fuels or chemicals from xylose:
(a) hydrogenation, producing xylitol, (b) acid treatment producing furfural and its derivates,
(c) fermentation, producing bioethanol and other chemicals (Kamm et al., 2006b).
