Environmental Engineering Reference
In-Depth Information
Typically, a biomass-derived material contains substances constituted by car-
bon, hydrogen and oxygen atoms. As an example, the simplified not balanced
chemical equation representative of the overall gasification process for a reference
substance such as glucose is:
C
6
H
12
O
6
þ
O
2
þ
H
2
OCO
þ
CO
2
þ
H
2
þ
other species
ð
2
:
19
Þ
The exhaust gases contain CH
4
,N
2
,H
2
O, tar, acidic and basic compounds
(NH
3
, HCN, H
2
S) considered as impurities. Tar conversion has to be controlled to
maximize the reliability of mechanical equipments and to assure the operation of
the successive clean-up catalytic steps for final hydrogen separation and purifi-
cation [
64
]. This step involves the utilization of additional steam and selective
catalysts, affecting the overall efficiency of the process [
65
]. The operation with
oxygen instead of air may improve the efficiency of the process but it suffers the
costs associated with air liquefaction process, necessary for O
2
/N
2
separation.
The current industrial concept for biomass gasification is conditioned by several
problems, i.e. heterogeneity of material availability, relatively high costs of col-
lection and transporting the feedstock, and a relatively low thermal efficiency due
to the vaporization cost of the moisture contained in the biomass. In order to lower
capital costs many efforts are addressed towards the development of advanced
membrane technologies able to separate oxygen from air (when the gasifier utilizes
oxygen), replacing the cryogenic process of air liquefaction, and separate and
purify hydrogen from the produced gas stream [
66
].
Similar to coal, biomass gasification technology seems to be more appropriate
for large-scale, centralized hydrogen production, due to the nature of handling
large amounts of biomass and the required economy of scale for this type of
process, and it may be relevant in specific geographic zones where this feedstock is
readily available. However, it will be also useful to explore the future possibilities
to use biomass for improving economics of distributed and/or semicentral
reforming processes. In this respect, heterogeneous waste and in particular
municipal rubbish could represent an important feedstock, if thermally pretreated,
in medium-sized power plants.
2.1.1.6 Thermochemical Methods
The possibility to transform directly a high-temperature thermal source to chem-
ical energy makes quite attractive the water thermolysis process. This approach
represents a direct route for conversion of heat associated with a primary source
into hydrogen without intermediate steps; the constraint is that theoretically
attractive efficiency can be obtained only if primary sources producing high-
temperature energy are used.
Severe engineering barriers are correlated to the very high temperatures nec-
essary to split water exclusively by heat, together with the problems connected to
heat extraction and thermal management. These problems require the practical
