Emma Kreuger

Ensiling of crops for biogas production: effects on methane yield and total solids determination

BackgroundEnsiling is a common method of preserving energy crops for anaerobic digestion, and many scientific studies report that ensiling increases the methane yield. In this study, the ensiling process and the methane yields before and after ensiling were studied for four crop materials.ResultsThe changes in wet weight and total solids (TS) during ensiling were small and the loss of energy negligible. The methane yields related to wet weight and to volatile solids (VS) were not significantly different before and after ensiling when the VS were corrected for loss of volatile compounds during TS and VS determination. However, when the TS were measured according to standard methods and not corrected for losses of volatile compounds, the TS loss during ensiling was overestimated for maize and sugar beet. The same methodological error leads to overestimation of methane yields; when TS and VS were not corrected the methane yield appeared to be 51% higher for ensiled than fresh sugar beet.ConclusionsEnsiling did not increase the methane yield of the studied crops. Published methane yields, as well as other information on silage related to uncorrected amounts of TS and VS, should be regarded with caution.

Bioconversion of industrial hemp to ethanol and methane: The benefits of steam pretreatment and co-production

Several scenarios for ethanol production, methane production (by anaerobic digestion) and co-production of these, using autumn harvested hemp as substrate, were investigated and compared in terms of gross energy output. Steam pretreatment improved the methane production rate compared with mechanical grinding. The methane yield of steam pretreated stems was similar both with and without pre-hydrolysis with cellulolytic enzymes. Co-production of ethanol and methane from steam pretreated stems gave a high yield of transportation fuel, 11.1-11.7 MJ/kg processed stem dry matter (DM); more than twice that of ethanol production alone from hexoses, 4.4-5.1 MJ/kg processed stem DM. Co-production from the whole hemp plant would give 2600-3000 L ethanol and 2800-2900 m(3) methane, in total 171-180 GJ per 10,000 m(2) of agricultural land, based on a biomass yield of 16 Mg DM. Of this, the yeast and enzymes from ethanol production were estimated to contribute 700 m(3) (27 GJ) of methane.

Temperature-controlled structural alterations of an RNA thermometer.

Thermoresponsive structures in the 5′-untranslated region of mRNA are known to control translation of heat shock and virulence genes. Expression of many rhizobial heat shock genes is regulated by a conserved sequence element called ROSE for repression of heat shock gene expression. This cis-acting, untranslated mRNA is thought to prevent ribosome access at low temperature through an extended secondary structure, which partially melts when the temperature rises. We show here by a series of in vivo and in vitro approaches that ROSE is a sensitive thermometer responding in the physiologically relevant temperature range between 30 and 40 °C. Point mutations predicted to disrupt base pairing enhanced expression at 30 °C. Compensatory mutations restored repression, emphasizing the importance of secondary structures in the sensory RNA. Only moderate inducibility of a 5′-truncated ROSE variant suggests that interactions between individual stem loops coordinate temperature sensing. In the presence of a complementary oligonucleotide, the functionally important stem loop of ROSE was rendered susceptible to RNase H treatment at heat shock temperatures. Since major structural rearrangements were not observed during UV and CD spectroscopy, subtle structural changes involving the Shine-Dalgarno sequence are proposed to mediate translational control. Temperature perception by the sensory RNA is an ordered process that most likely occurs without the aid of accessory factors.

Comparison of Reactor Configurations for Biogas Production from Energy Crops

Biogas production from a mix of easily fermentable and fibrous energy crops was evaluated and compared for two different process types. The processes were a batch two-stage leach bed combined with an upflow anaerobic sludge blanket (UASB) process and a semi-continuous one-stage continuously stirred tank reactor (CSTR) process The specific methane yields after a processing time of 29-30 days were 248 and 370 ml methane g -1 added volatile solids (VSadded) in the leach bed- UASB and the CSTR processes respectively. The average VS- removal rate of the leach bed-UASB system was initially faster, but when that process became limited by solids hydrolysis, the VS-removal rate dropped to less than half the initial rate, and the overall degradation was slower than for the CSTR. In conclusion, the biogas production was influenced significantly by the reactor configuration, but the effect of crop particle size and the influence of inoculum and microbial adaptation remain to be investigated.