Abstract
The future of biogas production will be characterized by on-demand provision to compensate the unpredictability of solar and wind power. Such biogas production through feedstock management is a promising possibility but requires close monitoring. 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. Their fast degradability can impair process stability. 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. The reactors differed in the amount of sugar beet silage and thus in total organic loading rate from 2.0 to 3.5 kgVS m−3 day−1. The VFA concentrations increased immediately after each input of sugar beet silage but levelled down until the next feeding period. At the highest organic loading rate, successive VFA accumulation escalated after 25 days (50 feeding periods) at 3.5 kgVS m−3 day−1, causing process failure with propionic acid concentrations exceeding 3500 mg L−1. The data revealed a strong negative exponential relationship between VFA concentrations and biogas and methane yields, respectively. High-resolution monitoring showed the instant dynamics of VFA production after intermittent sugar beet silage supply and the cumulative impact during increasing process disturbance.
Highlights
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
S often already contains a high amount of organic acids and alcohols, which arise through hydrolysis that already starts during storage [28]
This study showed with high-resolution monitoring that volatile fatty acid (VFA) are produced within minutes after intermittent supply of fast degradable feedstocks like sugar beet silage
Summary
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. On-demand (flexible) biogas plants can compensate for the lack of predictability of wind and solar power and balance the power grid. Various studies have already tested the applicability of intermittent feedstock supply to produce readily available methane within a short amount of time. Biomass Conversion and Biorefinery to flexible biogas production have been tested concerning the frequency of feeding events, the organic loading rate, and different feedstock mixtures [4, 10,11,12]. It was shown that significantly higher biogas and methane yields can be achieved with intermittent supply versus continuous (hourly) supply [12,13,14]. Ample amounts of feedstock, especially fast degradable carbohydrates, can be a stress factor for anaerobic digestion (AD) causing process failure in the worst case [3, 5]
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