Degradation efficiency of agricultural biogas plants--a full-scale study.

The degradation efficiency of 21 full-scale agricultural CSTR biogas plants was investigated. The residual methane potential of the digestion stages was determined in batch digestion tests (20.0 and 37.0 °C). The results of this study showed that the residual methane yield is significantly correlated to the HRT (r=-0.73). An almost complete degradation of the input substrates was achieved due to a HRT of more than 100 days (0.097±0.017 Nm(3)/kg VS). The feedstock characteristics have the largest impact to the degradation time. It was found that standard values of the methane yield are a helpful tool for evaluating the degradation efficiency. Adapting the HRT to the input materials is the key factor for an efficient degradation in biogas plants. No influence of digester series configuration to the VS degradation was found. The mean VS degradation rate in the total reactor systems was 78±7%.

Development of an in-line process viscometer for the full-scale biogas process.

An in-line viscometer was developed to determine the rheological properties of biogas slurries at a full-scale biogas plant. This type of viscometer allows the investigation of flow characteristics without additional pretreatment and has many advantageous aspects in contrast to the rotational viscometer. Various effects were studied: alterations in the feedstock structure, increasing total solid (TS) of the slurry and the disintegration of the feedstock on the rheological properties. The results indicate that the Power-Law model is sufficient for the description of the flow curve of biogas slurries. Furthermore, the use of more fibrous materials increases in viscosity. The increase in TS of 10.1-15.1% resulted in a sharp increase of the viscosity. The mechanical disintegration of the feedstock positively influenced the rheological properties, but the effects were more apparent at higher TS.

Effect of substrate pretreatment on particle size distribution in a full-scale research biogas plant.

The objective of this study was to investigate the pretreatment effects of high-fibre substrate on particle size distribution in a full-scale agricultural biogas plant (BGP). Two digesters, one fed with pretreated material and one with untreated material, were investigated for a period of 90days. Samples from different positions and heights were taken with a special probe sampling system and put through a wet sieve. The results show that on average 58.0±8.6% of the particles in both digesters are fine fraction (<0.063mm). A higher amount of particles (13.1%) with a length >4mm was measured in the untreated digester. However, the volume distribution over all positions and heights did not show a clear and uniform distribution of particles. These results reveal that substrate pretreatment has an effect on particle size in the fermenting substrate, but due to the uneven distribution mixing, is not homogeneous within the digester.