PhD thesis
Examining European semi-natural grassland silages and urban green cut as input sources for the integrated generation of solid fuel and biogas from biomass

Publication Details
Hensgen, F.
Kassel Univ. Press
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Interdisciplinary Research on Climate Change Mitigation and Adaptation

This thesis describes the use of two different input materials along the process chain of the IFBB process. The first part of this thesis (Chapter 3) investigated the use of green cut material that is nowadays most often used for compost production and is built from a herbaceous fraction consisting to a large part of lawn cut, leaves, straw, hay, fine hedge cut, and a shrub fraction consisting of woody parts, branches, leaves, roots and the coarse part of hedge cut. The use of these fractions and their mixtures were investigated in the IFBB process and compared to the use in direct combustion. The samples were taken in August and October, subjected to a hydrothermal conditioning treatment at two temperatures and dewatered by a screw-press. Mass flows of elements unfavourable for combustion were investigated as well as the concentrations of these elements in the press cake. Ash softening temperature and heating value of the press cake were calculated, for the press fluid anaerobic digestion tests were done to assess their potential as a biogas substrate. An energy balance was carried out for the IFFB system in comparison to direct combustion. Considering the results, the methane yields of press fluids were between 254 (shrub material, 40°C) and 359 (grass silage, 40°C) LN CH4 kg-1 VS and degrees of degradation ranged between 59% (shrub material, 40°C) and 81% (grass silage, 40°C). There was no statistically significant effect of temperature during hydrothermal conditioning or type of input material to be found. Through high mass flows of minerals detrimental to combustion into the PF, the fuel quality of the urban green cut was considerably improved by the IFBB system; the PC showed significantly reduced contents of K, Cl, Mg, S and P, no significant reductions occurred for Ca and N. The calculated ash softening temperature was increased. The IFBB system at 40° C temperature of hydrothermal conditioning showed a better energy balance than the direct combustion. The second part of the study investigated the use of grassland silage from 18 European semi-natural grassland sites, 6 placed each in Germany, Wales and Estonia for the IFBB energy conversion system in comparison to whole crop digestion and hay combustion. Chapter 4 showed the results for the mass flows of mineral elements within the IFBB system and also for fuel quality and the results of multiple linear regression models for the influence of botanical and chemical parameters on solid fuel quality. Chapter 5 showed the mass flows of organic compounds and their concentration in the IFBB press cake, as well as the results of anaerobic digestion tests with the press fluids and finally a calculation of gross energy yields achievable with the IFBB system in comparison to whole crop digestion and hay combustion. The results showed that semi-natural grassland silage is a suitable input material to be used in an IFBB plant. The concentrations of investigated elements were significantly reduced. Correlations between botanical and chemical parameters (NDF) of the silage and mass flows of plant compounds (R²: 0.21-0.63) as well as the fuel quality of the press cake (R2: 0.34-0.79) could be shown. Higher NDF concentrations and higher grass covers led to better fuel properties, while higher legume covers decreased fuel quality. The IFBB treatment effectively increased organic dry matter and fibre content and lowered CP and NFC concentration in the press cake and produced a highly digestible press fluid with methane yields between 312 and 405 LN CH4 kg-1 VS that were not significantly influenced by different habitat types. Area related methane yields ranged from 55 to 330 mN³ CH4 ha-1 and were significantly influenced by habitat type. Regression analysis did not show any relationship between substrate related methane yield and any organic compound concentration of the silage or botanical parameter of the vegetation, i.e. cover estimates of functional groups (legumes, grasses and forbs). Considering the energy yield of European semi-natural grassland silages, gross energy yields for the IFBB were in the range of hay combustion and outperformed whole crop digestion. The energy yields were affected by botanical and technical parameters i.e. habitat type and conversion system. In conclusion it can be stated that the IFBB system offers the opportunity to use heterogeneous, fibrous and mineral rich biomasses as a source for renewable energy that are nowadays not used for energy production.


Last updated on 2019-25-07 at 17:04