Power consumption

The two main objectives of AD are to produce biogas which can be used as a sustainable energy source and treat waste streams by reducing its organic matter content to acceptable levels. Digestate that meets the standards set out in the Publicly Available Specification (PAS) 110 is regarded as fully recovered and no longer considered as a waste material, which can be sold and/or used as a ‘bio- fertiliser’ (WRAP, 2014). Commercial AD systems can therefore generate profit from the sale of biogas and bio-fertiliser as well as the provision of a service to treat waste material. However, these products/services are high volume, low value which requires a strong focus on capital and operating costs to enhance the commercial potential of the endeavour. A major operating cost associated with AD is power consumption of the facility, particularly for agitation and temperature control.

Theoretical power densities (P/Vs) were calculated for the STR and OBR using the relationships defined in equations 7 and 8, respectively, for agitation intensities. The estimated P/Vs and agitation parameters required for each of the conditions in both digesters during the study are summarised in Table 6.3.

Table 6.3: Theoretical power density (P/V) requirements (W/m3) to achieve the agitation intensities in this study for both digester designs. Intermittent (int.) agitation.

Condition OBR STR P/V (W/m 3 ) Xo (mm) f (Hz) Reo rpm OBR STR 0 10.3 0.5 1610 40 (int.) 3.0 0.2 1 10.3 0.5 1610 40 (int.) 3.0 0.2 2 10.3 0.5 1610 40 3.0 2.0 3 10.3 0.5 1610 40 3.0 2.0 4 20.5 0.5 3220 80 23 18 5 20.5 1.0 6440 160 190 150 6 20.5 0.5 3220 80 23 18 7 20.5 0.5 3220 80 23 18 8 20.5 0.5 3220 80 23 18 9 20.5 0.5 3220 80 (int.) 23 1.2

The P/Vs described in Table 3 indicate that the digesters were subjected to similar levels of power input for the period of the study focused on the effects of agitation intensity and feed rate (conditions 3-8). Figure 6.14 shows the methane yields obtained from both digesters at the three P/Vs tested.

128 Figure 6.14: Methane yields in terms of feed volume (solid) and VS added (dotted) achieved at

increasing power densities (W/m3) for the OBR (black) and STR (red). Error bars represent +/- 1 standard deviation from the measurement with the highest error (gas composition).

Both digesters achieved similar methane yields at lower P/Vs which were maximised for ~20 W/m3. The OBR maintained this yield at the highest agitation intensity tested which required ~190 W/m3, however; the STR demonstrated a significant reduction in the yield at a significantly lower P/V of ~150 W/m3. This reduction in the yield suggests initiation of process destabilisation and is consistent with previous work which has shown that OBR technology is able to generate lower average shear environments compared to STR technology (Ni et al., 2000). The lower shear environment is conducive to floc formation, which enhances syntrophic relationships and kinetic effectiveness (Schink and Stams, 2006). High shear environments generated in the STR near the impeller disrupt floc formation which results in process destabilisation and a reduction in methane yields. OBRs are therefore able to provide a significantly larger agitation intensity range for AD processes without inhibiting methane production.

The previous analysis was done with theoretical P/V calculations. However, the actual power consumption required for agitation was measured with an energy monitor (efergy, engage hub 1.1). For conditions 3, agitation in the OBR and STR required 1822 and 17 W/m3, respectively. In comparison, the theoretical P/V requirements were significantly less than those actually used, especially for the OBR.

0.00 0.10 0.20 0.30 0.40 0.50 0 1 2 3 4 5 6 7 8 9 0 50 100 150 200 m 3 m e th a n e /k g V Sa d d e d L m e th a n e /L F e e d Power Density (W/m3₎

OBR (feed) STR (feed)

129 Equipment is never 100% energy efficient as losses are produced through heat and sound, for example. Therefore, the theoretical power consumption calculated is the absolute minimum which is never achieved in practice. Power consumption in the OBR was orders of magnitude higher than those calculated and used in the STR, which was unexpected because OBR technology is cited as being energy efficient (Abbott et al., 2014b, Jambi et al., 2013). However, the oscillating pump used was oversized for the equipment, so was operated at only 3% of its maximum output. This exacerbates energy losses as a significant amount of energy is required to merely keep the pump on stand-by. This highlights the need to ensure equipment meets specification requirements, thereby minimising energy consumption.

Another major source of power consumption associated with AD facilties is temperature control. Many facilities operate in mesophilic (20-45oC) or thermophilic (49-57oC) temperature ranges to maximise biogas production, which requires (especially in the UK) heating apparatus. An energy monitor (efergy, engage hub 1.1) was used to directly measure the power consumption of the temperature control units used for both digesters. The results showed that the OBR and STR required 44 and 5 kWh/day, respectively, to maintain the set point temperature (~36oC). This translates as 89% less power consumption for the STR compared to the OBR.

The tubular design and in this instance, the material used (stainless steel), of the OBR resulted in a much greater heat loss compared to the STR. At 20 m in length, the OBR has a surface area of ~3.1 m2 compared to the STR at ~0.57 m2. Both digesters have a volume of 40 L equating to surface area to volume (SA:V) ratios of 77.5 and 14.3 m-1 for the OBR and STR, respectively; a reduction of 82% in the STR compared to the OBR. The OBR also requires a larger pump to circulate water around the jacketed columns. These aspects of the OBR result in significantly increased power consumptions required for temperature control, which is undesirable for commercial facilties. This could be mitigated through digester lagging to reduce temperature loss; increasing the OBR diameter to reduce SA:V ratios; and/or housing the entire OBR unit in a closed vessel which is maintained at the desired temperature. However, it appears difficult to achieve similar power consumption requirements for temperature control to those digesters based on STR technology without significant design changes to the present OBR.

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