Most studies regarding flexible biogas production focus on the final biogas and methane yield and the influence of feedstock management on these parameters.
Nevertheless, ample amounts of feedstock, especially fast degradable carbohydrates, can be a stress factor for anaerobic digestion (AD) causing process failure in the worst case. It was also shown that significantly higher biogas and methane yields can be achieved with intermittent supply versus continuous (hourly) supply.
Different approaches to flexible biogas production have been tested concerning the frequency of feeding events, the organic loading rate, and different feedstock mixtures. Various studies have already tested the applicability of intermittent feedstock supply to produce readily available methane within a short amount of time. Mixing fast degradable carbohydrates with fibrous feedstocks can furthermore improve their decomposition promoting microbial activity of all degradation stages, eventually also changing the share of intermediates. In contrast to fiber-rich carbohydrates, hydrolysis proceeds immediately if it has not already started during the ensiling process. They are easily degraded to volatile fatty acids (VFA) to form the final product methane later. This operation mode is generally performed with feedstocks with a high share of fast degradable carbohydrates like sugar beet silage. Supported with prediction models validating weather development as well as electricity production from other renewable sources against energy consumption, biogas can be used for electricity generation. According to the demand of the energy market, biogas production is increased and decreased with intermittent feedstock supply. In addition to technical flexibilisation attempts (storing biogas and controlling its combustion), also biological flexibilisation can be implemented through feedstock management. On-demand (flexible) biogas plants can compensate for the lack of predictability of wind and solar power and thus balance the power grid. In addition to wind power and photovoltaics, biogas is an important renewable energy source, especially as electricity and heat can be produced independently of the weather, daytime, and season. High-resolution monitoring showed the instant dynamics of VFA production after intermittent sugar beet silage supply and the cumulative impact during increasing process disturbance. The data revealed a strong negative exponential relationship between VFA concentrations and biogas and methane yields, respectively. At the highest organic loading rate, successive VFA accumulation escalated after 25 days (50 feeding periods) at 3.5 kg VS m −3 day −1, causing process failure with propionic acid concentrations exceeding 3500 mg L −1.
The VFA concentrations increased immediately after each input of sugar beet silage but levelled down until the next feeding period. The reactors differed in the amount of sugar beet silage and thus in total organic loading rate from 2.0 to 3.5 kg VS m −3 day −1. The correlation of VFA loading and the biogas process was tested in four anaerobic continuously stirred tank reactors supplied with maize silage hourly and with sugar beet silage twice a day at a 12-h interval. Their fast degradability can impair process stability. The dynamics of volatile fatty acid (VFA) formation and further degradation to methane production are of special interest when providing high portions of fast degradable carbohydrates. Such biogas production through feedstock management is a promising possibility but requires close monitoring. The future of biogas production will be characterized by on-demand provision to compensate the unpredictability of solar and wind power.
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