Espacios naturales protegidos (incluida Red Natura)

At the optimum conditions determined by the central composite design, it was decided to conduct some experiments to determine if the product gas composition is influenced by any external factors. It was decided to test distilled water as a control experiment and to test the product composition when sludge that had a zero HHV value is used as feed. This is termed “Old Sludge” in Figure 38. The results from these control tests were then compared with the product composition of the optimized factors of the current study. The reactor system was repeatedly flushed with distilled water; all residues was scrubbed off and pipes where flushed with argon. The reactor was then washed with acetone and dried using compressed air to ensure all impurities were removed from the reactor surface. This process was repeated between each experiment run in the set of control experiments. The student ensured, to the best of her ability, that the system was free from any residual sludge from the experiment conducted with sludge from sample batch-B. “New sludge optimum” refers to the maximum sludge production using sludge from sample batch-B.

y = -0.2368x + 9.3843 R² = 0.6249 -2 0 2 4 6 8 10 12 14 0.0000 5.0000 10.0000 15.0000 20.0000 25.0000 30.0000 35.0000 40.0000 45.0000 1/[CO] Time (min)

2nd Order Kinetics

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Figure 38: Results from Control Experiments using distilled water, previously sampled sludge as well as new sludge at optimum conditions

The “inactive” sludge was tested in two separate experiments to test repeatability. The product gas was found to contain 0.15 mole % hydrogen in the first experimental run and 0.11 mole % hydrogen in the second experiment. It can be seen in Figure 38 that the carbon monoxide production from the “inactive” sludge experiments was found to be 2.0 and 2.11 mole %. This value is more than double the result obtained during treatment of sludge from sample batch-B, as seen on the right hand side of Figure 38.

The distilled water run was repeated to test repeatability. The hydrogen mole % in the product gas was found to be 0.13 and 0.12 mole % and that of carbon monoxide was found to be 0.45 and 0.47 mole %. The hydrogen production for the “inactive” sludge, distilled water and for the optimal

run using the “active” sludge varied between 0.11 mole % to 0.15 mole %. The mean was 0.13 mole % with a standard deviation of only 0.02 mole %. This indicates that very similar amounts

of hydrogen were formed during all experimental runs during these control tests. As discussed in the literature review section of this report, water exposed to high intensity ultrasound undergoes sonolysis. This would result in the production of hydrogen and hydrogen peroxide. As the sludge consisted of 98% water, it is expected that the sonolysis of water would take place. Due to no

0.153 _{0.111 0.131 0.124 0.116} 2.003 2.112 0.453 0.469 0.657 0.000 0.500 1.000 1.500 2.000 2.500

Old Sludge 1 Old Sludge 2 Water 1 Water 2 New Sludge Optimum

Mole

%

Optimum Conditions using different feeds

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significant difference in the hydrogen concentration of the product gas utilizing different feed material, it can be concluded that all hydrogen production is as a result of the sonolysis of water and not due to the partial oxidation of biomass.

The carbon monoxide produced during the control experiments ranged from 0.45 mole % and 2.11 mole %. The average carbon monoxide was found to be 1.14 mole %, but with standard deviation of 0.84 mole %. With such a large standard deviation, the effects of carbon monoxide production cannot be ascribed to a single source. When considering the “inactive” sludge experiments, it was found that the sludge mostly consisted of lime that is used in the unit for pH control. Lime can be in the form of CaO or CaCO3, depending on the conditions the lime has been exposed to. Calcium carbonate undergoes thermal decomposition and forms calcium oxide and carbon dioxide (Thermal Decomposition of Calcium Carbonate, 2013). The carbon dioxide produced during the thermal decomposition of the lime then undergoes the endothermic Boudouard reaction where carbon dioxide reacts with carbon to form carbon monoxide. See equation reaction 1 in Table 3.

The presence of carbon monoxide during the experiments using the “inactive” sludge is therefore not due to the partial oxidation of biomass, but the due to the thermal degradation of calcium carbonate and consequent Boudouard reaction at the extreme localized reaction conditions.

The amount of carbon monoxide produced using the “active” sludge is 0.2 mole % more than when only using water. Due to the relatively high HHV of the “active” sludge, it confirmed that the carbon monoxide formed was due to the partial oxidation of biomass. The presence of carbon monoxide at the end of the experiment using only distilled water can possibly be ascribed to an impurity analytical error due to exposing the methanizer to high carbon monoxide concentration. Some residual carbon monoxide from previous experiments could potentially remain in the system. Air also has a carbon monoxide level of 0.5-3 ppm and as a result, when the water was exposed to air, carbon monoxide could have dissolved in the water. The solubility of carbon monoxide in water is 0.044 g/kg water at 0 °C and 0.03 g/kg water at 18 °C. The temperature change in the reactor due to the cavitational events could have potentially allowed for the dissolved carbon monoxide to be released from the liquid phase as a result of the different solubilities at different temperatures. Another possible cause for the larger than expected result for carbon monoxide in the distilled water run, is the presence of organic carbon in the water. The total organic carbon (TOC) was not measured and as a result, could potentially be the cause for the presence of carbon monoxide in the

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product gas after exposure to ultrasound. Tap water is expected to have a TOC up to 10mg/L. Distilled water was used in the experiments, but as no TOC measurement was taken, it cannot be ruled out as a possible cause. When the TOC is higher than 10 mg/L, it would result in a brownish tint to water, but it is not uncommon that levels up to 10mg/L appear clear in normal tap water.