Environmental Engineering Reference
In-Depth Information
Example 17.3 Production capacity of an SNG plant
The SNG plant in Gothenburg has a production capacity of 20 MW biogas. The
lower heating value of the biogas is 35 MJ
m −3 , and that of gasoline is 33 MJ
L −1 .
a. What is the yearly production of biogas in m 3 ?
b. The biogas produced will be used as a transportation fuel. How many cars
can approximately drive on this gas?
Solution
a. 20 MW of biogas is equivalent to 20 MJ
s −1 . Dividing by the lower heating
s −1 . There are 8760 h in a year, but it is
reasonable to assume an uptime for the plant of 8000 h (see also Chapter 7).
This means a production of 16 Mm 3 of biogas per year.
b. Assuming that a car drives on average 18,000 km
value gives a production of 0.571 m 3
year −1 and uses 1 L of gas-
oline per 14 km, this gives a yearly gasoline use of roughly 1300 L, which is
equivalent to an energy use of 43 GJ. This corresponds to ca. 1200 m 3 of bio-
gas per year. Thus, the plant can provide approximately 13,000 cars with fuel.
17.4 METHANOL SYNTHESIS
Instead of producing alkanes via FTS, syngas can also be converted into methanol.
Methanol is an important chemical. It mostly serves as an intermediate for other che-
micals, especially formaldehyde, but it can also serve as a liquid transportation fuel.
The reaction equation for the formation of methanol from syngas is
mol −1
CO+ 2H 2
CH 3 OH
Δ r H =
91 kJ
ð
RX
:
17
:
4
Þ
The formation of methanol is an exothermal equilibrium reaction. The equilibrium
shifts to the right with decreasing temperature and shifts to the left with increasing tem-
perature. In addition, the equilibrium shifts to the right with increasing pressure. There-
fore, it is desirable to carry out the reaction at high pressure and low temperature.
However, too low a temperature would lead to a very low reaction rate. Typical con-
ditions used in practice are 490
100 bar. In the past, operation at higher
pressure has been applied as well, but this is less favorable because of high investment
and operating costs. In the industrially used temperature and pressure ranges, the con-
version in the reactor is rather low (around 50%) (Fiedler et al. 2011), which means that
a large portion of the syngas needs to be recycled. The reaction rate can be increased by
feeding a mixture of syngas and CO 2 to the reactor. Simultaneous to the methanol
formation (RX. 17.4), the water
560 K and 50
gas shift reaction (RX. 17.2) will take place. Accord-
ing to Lee (1990), CO 2 concentrations of up to 7 vol.% can improve the conversion to
methanol. Higher levels of CO 2 decrease the conversion (Phillips et al., 2011).
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