Marco Doctrinario

The hydrogen production pathways are compared to de-centralised electrolysis using grid electricity, which is set as the baseline 100% reference. In the GWP impact category, pre- sented in Figure 5.10, the baseline category is the largest emitter of greenhouse gases, fol- lowed closely by the centralised electrolysis version of the pathway which operates at a greater efficiency and reduces the GWP impact by 4%.

Interestingly, electrolysis powered by electricity generation from natural gas has the same GWP result of 68% for both centralised and decentralised configurations which reflects the effect that a slight efficiency difference can have for a more carbon-intensive electricity sup- ply. As expected the GWP of wind powered electrolysis is near zero, with only a small amount of GWP emissions occurring during the manufacturing processes.

The SMR pathways show a reduction of over 70% from the grid electrolysis baseline. Biswas et al. [104] found a similar ranking of the results, with different proportionality between the technologies. However, the methodology of the study conducted by Biswas et al. used a different functional unit and the results were expressed in different impact category units. The relative performance of hydrogen production methods can be compared within each study but absolute results comparisons between the studies is not practical.

The Ethene by-product hydrogen pathway which is derived from the oil refinery by-product gas is included in the results for purposes of comparison and completeness, but is of little relevance to a future HFCB because that by-product hydrogen source is no longer available. When observing the Ethene by-product results it must be considered that allocation tech- niques were used, which turned out to result in a relatively small fraction of the refinery’s environmental and energetic burden being allocated to the by-product hydrogen.

The AP result, presented in Figure 5.11, shows that a change from the grid mix to natural gas electricity generation produces a 75% reduction in acidification potential, which can be attributed to the emissions of the coal-dominated electricity grid. The SMR processes achieve a reduction of nearly 90% from the baseline which reinforces the relatively lower environmental impact of natural gas as a primary energy source.

The POCP results are presented in Figure 5.12 which show a similar pattern to the other impact categories that have been discussed. For purposes of context, it is worth noting that the POCP impact category is a contributor to smog, and the POCP emissions for a hydrogen bus occur upstream in the fuel production processes.

For a project like the Perth CAT where the buses operate in the high-density inner city it may be more desirable for emissions to flow from the industrial regions where hydrogen can

5.3. Hydrogen production pathway scenario analysis 163

Figure 5.10:Comparison of hydrogen pathways: Global Warming Potential.

164 Chapter 5. Results and Discussion

Figure 5.12:Comparison of hydrogen pathways: Photochemical Ozone Creation Potential.

be produced at the large scale, as opposed to the tailpipe emissions which cause inner-city smog.

The EP result, presented in Figure 5.13, also primarily effects emissions to water and land. These results indicate that once again, the grid mix sets a very high emissions baseline. The ODP result, presented in Figure 5.14, illustrates that the comparison of technologies on an ozone depletion basis again yields a very different result, with grid electrolysis exceeded by the renewable sources which include greater trace amounts of ozone depleting emissions in the manufacturing phase.

An interesting result in the ODP data is that the emissions from the decentralised SMR pro- cess is more than double the emissions of the centralised SMR process. An in-depth analysis of the ODP results data reveals that a large component of the emissions for the central SMR process is due to truck transportation of the hydrogen from the central SMR plant to the bus depot which is undertaken by specialised hydrogen transport trailer, however this is far exceeded by the replacement parts required for the decentralised SMR process.

A decentralised SMR plant requires much more maintenance per unit of gas produced than a centralised plant, and for this reason the ODP emissions which mainly arise from manu- facturing processes are much higher.

In terms of primary energy demand from non-renewable resources, presented in Figure 5.15, the energy required by the electrolysis processes from both the electricity grid and natural gas generation are the least efficient means of hydrogen production. Electrolysis from wind nearly eliminates the non-renewable primary energy demand, as expected. If natural gas is

5.3. Hydrogen production pathway scenario analysis 165

Figure 5.13:Comparison of hydrogen pathways: Eutrophication Potential.

166 Chapter 5. Results and Discussion

to be used as a feedstock for hydrogen production then the results show that SMR is a much more efficient conversion process than natural gas electricity generation and electrolysis.

Figure 5.15:Comparison of hydrogen pathways: Primary Energy Demand (non-renewable).