Based on economic and design considerations, the two-stage dewatering process of chitosan 2 and disc-stack centrifugation is recommended. The recommendation is based on an overall study of the process steps: cultivation, dewatering, extraction and biodiesel production. The net emission results for the complete process are

shown in Table 7. The results from the Scope 2 emission audit indicate that the over­all emissions from the HTR and ELR options are by a factor of 10 greater than that for the RP option. As seen in Fig. 15, the majority of the emissions for the HTR and ELR are due to emissions from the cultivation stage (98 and 97% of total emissions, respectively), whilst the emissions from the cultivation stage for the RP only amounts to 33% of total emissions. If the cultivation emissions were ignored, as seen in Fig. 16, the data indicate that to dewater, extract and produce biodiesel more emis­sions are produced for the RP, than for the ELR or the HTR. This is due to the emis­sion rating to dewater large volumes of less concentrated algae culture from the RP.

The fundamental importance of this project is CO2 biosequestration. By captur­ing CO2 , the process reduces the overall emissions which would otherwise be released into the atmosphere. This would reduce the number of permits the facility or an industry is required to obtain. The financial savings analysis shows that the HTR and ELR options save $2.18 million (87,000 permits) whilst the RP saves $2.04 million (82,000) per year. The higher permit saving for the HTR and ELR

150000

100000

50000

0

-50000

-100000

■ Net NGER □ Net CPRS

Fig. 17 The net emissions according to the CPRS and NGER requirements options is due to the lower Scope 1 emissions resulting from their higher CO2 cap­turing efficiencies. In terms of an overall outcome, the best option is RP cultivation followed by the two-stage dewatering process. As seen in Fig. 17, the HTR and ELR actually produce more emissions than the RP due to the high cultivation emission rating, whilst the RP has an overall negative emissions rating. As the use of HTR or ELR produces significantly more emissions than the RP, it has a greater negative environmental impact.

2 Conclusion

In terms of the design outcomes, the HTR and ELR appeared to be attractive options due to their ability to achieve high biomass concentrations during cultivation. However, the fixed capital cost involved with the HTR and ELR are up to 493% greater than that for the RP. The tubular reactors are difficult to scale-up due to issues of dark zones and dissolved oxygen build-up. Therefore, the number of units required proved to be significant. The results of the carbon audit indicated that the overall emissions from the HTR and the ELR were greater than that for the RP by a factor of 10. This was largely due to energy consumptions associated with the use of airlift pumps.

In dewatering, a two-stage process involving flocculation preceding centrifuga­tion heavily reduced energy consumption with high reduction in emissions, com­pared with a single-stage process. The carbon study also indicated that dewatering using a two-stage process was more attractive as culture volume increased, even with a low-efficiency flocculant. In the extraction and transesterification stages only one design alternative was investigated, and on the basis of 50,000 tonnes biomass processing, the results were identical for the different cultivation system.

The overall findings from this study indicate that a RP cultivation stage, followed by a two-stage dewatering process is the optimum alternative. However, the eco­nomic study showed that this option is not feasible presently due to an excessively high cost of production of $74/L of biodiesel, which leads to an annual operating loss of $190 million. The carbon audit, however, indicates that the process is carbon neutral, capturing ~49,000 net tonnes of CO2-e. In terms of environmental impact, the project is attractive. However, it would not be financially viable, as the value of carbon permits is only $2.0 million.