Msc: Sustainable Engineering : Chemical and Process Engineering
Results
As can be seen from the stream results shown in Table 1, the thermal hydrolysis process has been successful in amplifying the AD process by completely converting the cellulose and hemicellulose in the system to create biogas and other products. The process does however generate a portion of furfural and xylose which act as rate inhibiting agents that can slow down the AD process, however only a small proportion of these two products are generated which only minorly affects the biogas production.
Table 1: Results of thermal hydrolysis pre-treatment.
Figure 1 shows the effects on the methane production via an increase in temperature of the plug flow reactor, starting in the lower mesophilic range of 30ᵒC and going up to the higher thermophilic range of 70ᵒC. There is a clear increase in methane production with temperature increase, going up to 30% at 70ᵒC when compared to 30ᵒC. This trend agrees with the literature which shows a similar trend of increasing methane production in the thermophilic range when compared to the mesophilic range. (Gebreeyessus & Jenı́Ček, 2016)
Figure 1: effect of temperature on the flowrate of methane produced from the plug flow reactor
Figure 2: Effect of temperature on the composition of biogas in the plug flow reactor. (note: the remaining percentage comprises of CO2).
However, we must also consider the composition of the biogas as it leaves the plug reactor. An increase in temperature can also lead to an increase in carbon dioxide, which can affect the purity of the biogas when it is upgraded into biomethane. Figure 2 demonstrates the effect of increasing the temperature on the composition of the biogas. The graph shows when there is an increase in the temperature, the methane gas produced has a lesser share in the makeup of the biogas. In order to create a biomethane gas suitable for injection into the grid, the gas needs to contain a purity of 90%. As will be seen in the purification section, we need a methane percentage of 55%. Therefore, a temperature of 55ᵒC is the most suitable as it secures this ratio.