• Energy Research

This study examined the potential for partly substituting dairy cow manure (DCM) with solids from solid to liquid separation of acidified dairy cow manure (SFDCM) during thermophilic anaerobic digestion. Three different substituting levels with a maximum of 30% substitution were tested. All digesters substituting DCM with SFDCM showed a stable biogas production with low volatile fatty acid concentrations after a short transition period. An increased methane yield in terms of digester volume compared to DCM alone was obtained with increasing amount of SFDCM and about 50% more methane was achieved when 30% of DCM was substituted with SFDCM. The digestates were subsequently digested in a post digestion, during which the methane yield increased proportionally with increasing amounts of SFDCM. It can be concluded that SFDCM is a suitable biomass for co-digestion and can be used to increase methane yield in terms of digester volume at ratios up to at least 30%.

Abstract Searching for available biomasses is a key factor to promote renewable energy, especially biogas. This paper examines the biogas potential of biomass from Danish buffer strips with different types of natural vegetation and the possibility of using the functional trait composition of the vegetation and the nutrient composition of the biomass as indicators for the quantity of the biomass and its biogas potential. Biomass was harvested from 73 plots in 2014 and 2015 and biomass yield, nutrient content (CNP) and methane yields were determined. Prior to harvesting, the plant communities were characterised in terms of species composition, and a cluster analysis was performed to identify species assemblages in terms of vegetation clusters. Four ecologically meaningful clusters were obtained: a tall herb fringe community, a tall grass community, a low herb and grass community and a rich fen/wet meadow community. The biomass yield was numerically higher in the tall herb fringe and the tall grass communities, while the specific methane yield was higher in the tall grass and low herb and grass communities. When all samples were analysed together, the biomass yield was positively correlated with some of the vegetation traits (Ellenberg value soil reaction and leaf dry matter content) and negatively with nitrogen and phosphorus contents in the biomass. In addition, a negative correlation was observed between the biomass yield and the specific methane yield. In contrast, the biomass yield was positively correlated with the methane yield per ha, indicating that even small differences in biomass yield can be more relevant for the methane yield per ha than differences in specific methane yields in natural vegetation. Biomass from tall herb fringe, tall grass and low herb and grass communities is suitable to be incorporated into the anaerobic digestion supply chain; and in these communities it could be possible to predict methane yield per ha, especially in tall grass community, using vegetation traits (Ellenberg values and Grimes life strategy) and phosphorus content in the biomass.

Lignocellulosic biomasses such as wheat straw are widely used as a feedstock for biogas production. However, these biomasses are mainly composed of a compact fibre structure and therefore, it is recommended to treat them prior to its usage for biogas production in order to improve their bioavailability. The aim of this work is to evaluate, in terms of performance stability, methane yield and economic feasibility, two different scenarios: a mesophilic codigestion of wheat straw and animal manure with or without a low-energy demand alkaline pre-treatment (0.08gKOHgTS-1of wheat straw, for 24h and at 25°C). Besides this, said pre-treatment was also analysed based on the improvement of the bioavailable carbohydrate content in the untreated versus the pre-treated wheat straw. The results pointed out that pre-treated wheat straw prompted a more stable performance (in terms of pH and alkalinity) and an improved methane yield (128% increment) of the mesophilic codigestion process, in comparison to the "untreated" scenario. The pre-treatment increased the content of cellulose, hemicellulose and other compounds (waxes, pectin, oil, etc.) in the liquid fraction, from 5% to 60%, from 11.5% to 39.1% TS and from 57% to 79% of the TS in the liquid fraction for the untreated and pre-treated wheat straws, respectively. Finally, the pre-treated scenario gained an energy surplus of a factor 13.5 and achieved a positive net benefit of 90.4€tVS-WS-1d-1, being a favourable case for an eventual scale-up of the combined process.

Abstract The effect of co-digestion of grass with cattle manure (CM) on digestate composition, methane (CH 4 ) yield and energetic and economic balances was investigated by using three different harvesting technologies in two continuous experiments. Both experiments performed at thermophilic temperature and a 25-day hydraulic retention time. Results showed that the addition of 5% grass in fresh matter increased the volumetric CH 4 production by around 20% and the organic matter content of the digestate, decreased the protein content and did not affect the N:P:K ratio. Residual CH 4 production from the digestate increased and a 6% decrease in the CH 4 concentration in the biogas was observed when grass was added to reactors. The best CH 4 yield was achieved when excoriated grass was added, with increments of 20% and 35% compared to mono-digestion of CM in 15-L and 30-m 3 reactors, respectively. Our results showed that the extra energy required for mixing and harvesting are the two operations with the highest energy inputs when grass is used for biogas, however in spite of the extra energy used, co-digestion of CM with grass is very favorable both, from an energetic and economic point of view, with excoriation being most favorable compared to other harvesting technologies.

Anaerobic co-digestion of cattle manure (CM) with shredded or briquetted wheat straw (SS and BS, respectively) was evaluated in thermophilic continuously stirred tank reactors (CSTR) in two experiments (lab and full-scale). Three lab-scale CSTR (15 l) were used with 20 days hydraulic retention time (HRT); one was fed with CM and the other two with mixtures of CM (95% of fresh matter, FM) and SS or BS (5% FM). In the second experiment, two full-scale CSTR (30 m3) were operated with 25 days HRT; one reactor was fed with CM and the other with CM + BS (9% FM). Ultimate CH4 yield was analysed from each substrate. Biochemical CH4 potential at 21 days for CM, SS and BS were 128; 187 and 200 lSTP [CH4] kg−1 [VS]. Anaerobic digestion of CM, CM + SS and CM + BS in lab-scale reactors yielded 165; 214 and 217 lSTP [CH4] kg−1 [VS]. In full scale-reactors, CM and CM + BS yielded 264 and 351 lSTP [CH4] kg−1 [VS]. Increments of 31 and 33% on CH4 yield were achieved in CM + BS compared to CM in lab and full-scale reactors, respectively. Regarding the energy balance, the energy yields were the same for both reactors using straw as co-substrate (CM + SS and CM + BS) after subtracting the energy consumption of the pretreatment, corresponding to 1100 kWh of net energy output. However, briquetting technology could be advantageous for biogas plants where the straw might be transported over longer distances, due to reduction of the transportation costs.

Alkaline pretreatment of lignocellulosic biomass has been intensively investigated but heavy water usage and environmental pollution from wastewater limits its industrial application. This study presents a pretreatment technique by in-situ injection of potassium hydroxide concentrations ranging from 0.8% to 10% (w/w) into the briquetting process of wheat straw and meadow grass. Results show that the biomethane yield and hydrolysis rate was improved significantly with a higher impact on wheat straw compared to meadow grass. The highest biomethane yield from wheat straw briquettes of 353mL.g-1 VS was obtained with 6.27% (w/w) potassium hydroxide injection, which was 14% higher than from untreated wheat straw. The hydrolysis rates of wheat straw and meadow grass increased from 4.27×10-2 to 5.32×10-2d-1 and 4.19×10-2 to 6.00×10-2d-1, respectively. The low water usage and no wastewater production make this a promising technology.

This study evaluated the effects of harvesting managements with two-cuts (2C) and three-cuts (3C) per year for subsequent specific methane yield (SMY) and methane yield per hectare (MYPH) of festulolium and tall fescue cultivated on a riparian fen peatland in a block-designed field experiment (n = 3). For the 2C managements, three timings of the first cut were implemented corresponding to growth stages of pre-heading (2C-early), inflorescence emergence (2C-mid), and flowering (2C-late). Anaerobic digestion batch assays with biomass samples were run for 68 days, showing that 90% of total methane (CH4) was produced within 38 days. Specific methane yield ranged from 315 to 464 NL CH4 kg−1 volatile solids (mean, 393 NL). On average, SMY of the final cut biomass was 13% lower than the first cut biomass. Methane yield per hectare ranged from 5277 to 6963 Nm3 CH4 ha−1 (mean, 6265 Nm3) and was predominantly influenced by biomass yield since SMY only deviated modestly in relation to harvest management (crop maturity). Methane yield per hectare of festulolium under 3C and 2C-late management were significantly higher than 2C-early and 2C-mid managements, whereas the harvesting managements did not influence MYPH of tall fescue. The levels of SMY and MYPH in the present study represented high-end of reported values due to a combination of high activity of the biogas inoculum and a high productivity of festulolium and tall fescue at the riparian fen peatland.

Anaerobic co-digestion of agricultural by-products or wastes with complementarity characteristics is commonly used to enhance methane yield. This study firstly explores the possibility of co-digesting grass and forb species (white clover, chicory and plantain) differing in nutrient composition in enhancing methane yield. This was examined with two inocula (a cattle manure-based inoculum and a grass-based inoculum) in a batch assay. Results showed that co-digesting grass and forbs synergistically enhanced methane yield potential on average by 31 L kg−1 volatile solids (+11%) and reduced lag phase time by 0.8 day in the grass-based inoculum, but not in the cattle manure-based inoculum. Mixtures containing plantain showed more consistent synergistic effect than chicory. Synergistic effects were attributed to more balanced nutrient composition (especially C/N ratio) in grass-forb mixtures. We demonstrate that anaerobic co-digestion of grass and forbs is feasible for enhancing methane yield, which promotes the utilization of multi-species grasslands for bioenergy production.