At the same time, the hydrogen requirement drops below 0.2 t/t in most cases if the product contains oxygen. Producing oxygen-free products from CO₂ requires more hydrogen to remove both oxygen atoms. In addition, removing oxygen from the CO₂ results in products with lower molecular weight, which further increases the H₂ requirement, at least if expressed per tonne of product. Figure 8 shows the HUI for each of the nine technologies considered. The carbonation, urea, polyol, and PPC technologies require no hydrogen because the CO₂ molecule is bound to another molecule without prior oxygen removal. Note that the CUI of the FT process tested is close to 0.52 t H2 /t Product , which is considerably higher than the theoretical intensity of around 0.44 t/t (see Figure 7 ). This is because a significant purge of a syngas stream is applied in the specific process set-up considered in our study (Zang et al ., 2021). In Figure 7 , the hydrogen in that syngas stream has been deducted from gross hydrogen intake, which reaches 0.57 t/t. Even so, the net HUI remains higher than the theoretical value due to the presence of CO in the purge stream. The conversion of CO₂ to CO, which is then purged, requires hydrogen and increases the ratio of hydrogen consumed to FT product produced. The Oxo synthesis process generates butanal (also known as butyraldehyde, C₄H₈O) from CO₂, hydrogen, and propylene. The use of propylene, which adds hydrogen and carbon to the product, results in the HUI of this process being only a fraction of the theoretical 0.31 t H2 /t butanal HUI based on the production of butanal from only H₂ and CO₂. The same applies to the polyol and polypropylene carbonate (PPC) products, which
2030 Scenario 2050 Scenario
Electricity Fuel/steam
0.26 0.14
0.00 0.00
in the technologies investigated use propylene oxide, in addition to hydrogen and CO₂. Table 3 Electricity and fuel/steam emission factors, tCO₂/MWh
Carbon utilisation: impact on operating revenue
The utilisation of CO₂ is assumed to generate a revenue stream. Figure 9 shows the CUI for each technology. In general, higher CUIs are preferred. The CUI data in Figure 9 includes a correction for the emissions related to electricity and fuel/ steam that the process requires or exports. Different electricity and fuel/steam emission factors are assumed for the two cases (see Table 3 ). Therefore, Figure 9 shows two series of bars. In the 2050 scenario, zero-carbon power and fuel are assumed to be available. The hydrogen import is assumed to have a zero carbon intensity (CI). If the CI of imported hydrogen is significant, then the CUI of the processes will drop significantly. The utility balance and the impact on the CUI will be discussed in more detail in Part 2. There is an inverse correlation between CUI and HUI. As mentioned previously, the inclusion of feeds other than CO₂ and hydrogen (oxygen, PO, and propylene) dramatically reduces the relative hydrogen demand. However, it also reduces the CUI. The carbonation technology, in particular, has
4
FT
3
CH
0.6 0.5 0.4 0.3 0.2 0.1
FT
CH
2
Carbonation
1
Urea
PPC
Carbonation
0
MeOH
Xylenes
Oxo (butanal)
Polyols
Urea
PPC
0.0
Oxo (butanal)
Xylenes
MeOH
Polyols
‘2030’
‘2050’ (zero carbon imports)
Figure 8 H₂ utilisation intensity for each technology
Figure 9 Carbon utilisation intensity of the investigated technologies
www.decarbonisationtechnology.com
36
Powered by FlippingBook