CONCLUSIONS

By the redesigning and reconfiguration of the sonochemical reactor to conduct the sonochemical treatment of biomass, it was concluded that the process was successfully optimized. The hypothesis that the partial oxidation, sonolysis and lime degradation process using ultrasound can be optimized was therefore confirmed. The hydrogen production shows a 58% improvement, the carbon monoxide showed a 92 % improvement and the carbon dioxide was reduced by 49% from the work conducted by Beyers (2011). As a result, it was also concluded that the addition of argon positively influenced the sonochemical reactions. It was concluded that the reduction in carbon dioxide is due to not feeding any oxygen to the system under the assumption that enough oxygen is present in the air that is initially dissolved in the liquid. This assumption was confirmed by the presence of an increased amount of nitrogen present after the reaction took place, than in the beginning – showing that dissolve air was present and degassing of the sample occurred. The ratio of carbon monoxide to hydrogen was not optimized and a maximum of only 0.438 was achieved and the hypothesis that a ratio of 2 can be achieved was, therefore, not confirmed. All the product gas analysis was conducted by means of gas chromatography and as a result, the GC was successfully commissioned and calibrated.

As it was found that more hydrogen is produced at higher pressures (whilst other parameters are kept constant), it can be concluded that the static pressure increase resulted in smaller cavitational bubble sizes which lead to an increased in the extent of the hydrogen production reaction.

Representative sludge samples were analysed to characterize the composition of the sludge prior to conducting the experiments. Due to limited time and resources, it was not possible to fully characterise the sludge after experiments were conducted and only prioritized analysis occurred in terms of HHV.

The reactor design optimization was also concluded as successful due to the significant improvement in the product yields as well as repeatability of the experiments. It was found that the application of cooling to the reactor ensured that no water vapour formed during the experiments and the experiments could be conducted for a longer period of time as boiling of the contents was

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successfully avoided. All experiments were conducted at least twice and based on the adequate model fit for hydrogen production; it was found that the process conveyed good reproducibility. A central composite design was conducted and from the results, it was concluded that both hydrogen and carbon monoxide yields increased with an increase in time and amplitude. The hydrogen to carbon monoxide ratio varied considerably and did not reach the desired ratio of 2. The energy potential (HHV) in the feed changed with only 3% at the optimum reactions conditions and it was concluded that the presence of reactive carbon in the feed is not the limiting factor for the partial oxidation of biomass using ultrasound. From the central composite design, the hydrogen production model adequately described the system and identified that time, amplitude and pressure influences the hydrogen yield. The rate equation for hydrogen and carbon monoxide production was found to be zero order and therefore, independent of feed concentration. This supports the small change in the HHV of the feed for the experiment that should have induced the most significant effect on the feed. The hydrogen production model, a manual statistical solution as well as physical testing yielded that the maximum possible hydrogen production under the current conditions, was a value of 0.16 mole % hydrogen at a pressure of 1.42 barg, 17.97 minutes and 100 % amplitude.

It was also concluded that the reactor only operated at a 36% efficiency due to the actual energy intensity reaching an average value of 0.52 W/m2 and the actual design intensity was 1.44 W/m2. The energy intensity was also found to be directly proportional to the amplitude of operation.

From the control experiments, it was concluded that hydrogen in produced during the sonolysis of distilled water as a result, this should also occur during the partial oxidation of the biomass sludge as the sludge consists mostly of water. This was found to be the largest contributor to hydrogen production during this study. It was also concluded from the control experiments that carbon monoxide was formed by the thermal degradation of lime into calcium oxide and carbon dioxide, followed by the Boudouard reaction to produce carbon monoxide.

Based on the production of no methane during the course of this study, the sonochemical process can be tied into the GTL process after the steam reforming unit. Due to the relatively high carbon dioxide content, the process will need to join the main feed gas stream that is past into the carbon dioxide removal unit before it enters the GTL process to correct the desired feed gas ratio.

Based on the very low syngas yields, the low hydrogen to carbon monoxide ratio in comparison to the required ratio of 2 as well as the high energy intensity required for this process, it was concluded

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that the partial oxidation of biomass sludge in a sonochemical reactor is not feasible as an alternative technology to conventional steam gasification. The process yield of 0.002 Nm3/kg feed resulted in the cost of syngas production at this yield to reach R 19.98/Nm3. As a result, this production process was not comparable to conventional steam gasification that delivered a yield of 0.67 Nm3/kg which resulted in a syngas price of R1.48/ Nm3. This process was therefore not economically feasible as an alternative to steam gasification. The operating costs of the sonochemical unit would be nearly ten times that of steam gasification and is therefore concluded to not be a competitive technology to conventional steam gasification.

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