6.2 Feedstock costs and other operating costs for biogas plants
The feedstock costs for AD biogas systems depend on the source. For waste streams such as manure and sewage, there are typically very low or zero costs for the raw feedstock onsite, as collection and storage systems are required in any event. In these cases, the only costs incurred are operational and the amortised capital costs. In many developed countries, feedstock costs may be negative, as the biogas plant is being paid to dispose of wastes and these revenues sometimes exceed the revenues from biomethane sales. However, for a large centralised biogas plant that is collecting feedstock from surrounding farms, transport costs are often an important consideration.
Energy crops are sometimes purchased to increase the scale of the biogas plant or for the properties they bring to the AD process (e.g. increased yield or more stable digestion). In these cases, the costs of feedstock may quickly become an important component of overall costs. For instance, the plants analysed in Figure 6.4 show the estimated impact of feedstock costs on bi-ogas production costs. These are around USD 0.19/Nm3 of biogas produced when maize silage is purchased at USD 45/tonne and used for 90% of the feedstock and 10% comes from wastes. This is equivalent to around USD 11.4/GJ of biogas and around USD 20.8/GJ of bi-omethane assuming the raw biogas has a 55% methane content.
The main non-feedstock costs for AD systems producing biogas are thermal energy for the process, with important contributions from electricity, maintenance and personnel costs (Figure 6.4). Process energy requirements can vary by AD system and feedstock for digestion, but are typically 7-15% of the biogas produced including electricity and thermal energy (Murphy, 2011 and Salter, 2008). The thermal energy required is to raise the feedstock substrate to the temperature required in the digester. Like many biochemical processes, higher temperatures result in faster reactions and throughput. However, the temperature selected for AD is typically a compromise between the optimal biochemical temperature and the economics of heating the digesters. The electricity is required for pumping, feedstock, handling, controls etc.
The operating costs for the upgrader are dominated by the costs for the electricity or heat used. Significant costs also arise from maintenance and other requirements (Figure 6.5). Table 6.3 presents the physical requirements for electricity, heat, water and chemicals for the main upgrading options to remove CO2, while Figure 6.5 presents the operating costs for the upgrading as a function of the raw biogas processing capacity of the upgrader. These operating costs exclude compression, as this will depend on the distribution route for the plant. However, given that vehicles will require biomethane at 200 bar, compression energy of around 0.2kWh/ Nm3 will be required (Bauer, 2013)37 somewhere between upgrading and fuelling.
Figure 6.4: Operating costs for AD biogas by feedstock and size
Source: Urban, 2009.
Table 6.3: Selected biogas upgrader systems, their inputs and characteristics
Source: Bauer, 2013.
Figure 6.5: Operating costs for biogas up graders by t ype and size
Source: Urban, 2009.
37 The energy required to compress biomethane from 1 bar to 250 bar is around 0.23 kWh/Nm3. However, the exact figure for biom-ethane will vary given that different upgraders operate at different pressures and temperatures.