Environmental Engineering Reference
In-Depth Information
cross this boundary and link the process with the supply chain. Also, some main
characteristics of the process itself must be specified, such as preferred proces-
sing routes and key technologies to implement such routes. Available key tech-
nologies serve as building blocks at this level.
b.
Knowledge
Knowledge (in the form of patents, empirical data, diagrams, heuristic design
rules, and models) has to be gathered about the properties of the feeds, avail-
ability of feeds, product properties and market demand, available key conver-
sion technologies, and their performance.
knowledge, such as awareness
of public perception of certain feeds and products is also vital information for
creating a process that meets public acceptance. Last but not least, analysis of
the performance of competitors
“
Soft
”
plants and their plans for the future is advised.
When making a design, it is necessary to know (or estimate) the chemical
composition of the biomass and its variability over time (seasons). In addition
to organic matter, biomass contains water, sand, and minerals. The moisture
content of the biomass feed is quite important for the energy economy in a proc-
ess. Furthermore, the molecular composition of the organic material is not uni-
form. The average elemental composition of an organic molecule is given by
CH
x
O
y
(
x
hydrogen and
y
oxygen atoms per carbon atom). These (fractional)
atom numbers
x
and
y
vary over a certain range. The feed composition can
be characterized by a distribution function for the fraction of organic molecules
in the feed having
x
hydrogen and
y
oxygen atoms: 0
'
≤
f
(
x
,
y
)
≤
1,
x
min
≤
x
≤
x
max
,
y
min
≤
y
≤
y
max
.
c.
Synthesis
Decision items in the synthesis step are shown in a so-called input
output dia-
gram (see Figure 7.7). The decisions on suitable feeds and products can only be
made in connection with the choice for a key processing technology. Reversely,
given feeds and products limit the choice of suitable technologies. The mode of
production depends on the production capacity and the way the products are
formed and used. From an economic perspective, a continuous mode of operation
is already preferred when the mass flow capacity of a process exceeds 5 kt
-
year
−1
(Douglas, 1988). A flexible continuous mode is also required for uninterrupted
power supply to society, though some of the contributing processes to the power
production may be of the on/off type (wind energy) or are daily varying (e.g.,
solar energy), necessitating internal energy storage. The uptime of a process
(i.e., the time that a process functions properly and can make products) depends
on the physical state of its feed. Practical experiences indicate that for well-
maintained gas
year
−1
is possible
(1 year = 8760 h), while for solid processing units and batch processes, an uptime
of 6000
-
liquid plants, an uptime of well over 8150 h
year
−1
is more realistic (Douglas, 1988).
In gasification and combustion process design, the way of oxygen supply
for partial combustion of the biomass and cofeeds is an important factor. Air
is free of cost, but it brings along 80% of inert nitrogen, which has to be pumped
through the process. This enlarges the required volumes of piping and equipment
at a very significant additional investment cost. As an alternative, one can enrich
-
7000 h
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