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The total theoretical biomethane resource of cattle slurry and grass silage in Ireland was estimated using the most up to date spatially explicit data available. The cattle slurry resource (9.6 PJ) was predominantly found in southern and north-eastern regions while the grass silage resource (128.4 PJ) was more concentrated in western regions. The total biomethane resource of cattle slurry and grass silage was equivalent to 6% and 76% of total natural gas consumption in Ireland in 2014/15, respectively. A sequential optimisation model was run to determine where to source cattle slurry and grass silage from, for 42 potential biomethane plant locations in Ireland. The concept was to maximise plant net present value (NPV) and develop locations in order of plant profitability. The impact of plant size, grass silage price, volatile solids ratio (VSR) of grass silage to cattle slurry, and incentive per unit energy of biomethane was assessed in 81 separate scenarios. The results indicated that total biomethane production from plants with a positive NPV ranged from 3.51 PJ/a to 12.19 PJ/a, considerably less than the total resource. The levelised cost of energy (LCOE) of plants was also calculated and ranged from ca. 50.2 €/MW h to ca. 109 €/MW h depending on the various plant parameters. LCOE decreased with increased plant size and ratio of grass silage to cattle slurry. The relationship between grass silage price and LCOE was assessed. In the median scenario (33 €/t wwt grass silage, VSR of 4, 75,000 t wwt /a plant size, 60 €/MW h incentive) cattle slurry was sourced within 6.4 km of the facility while grass silage was sourced within 10.5 km of the facility. A high level assessment of the carbon dioxide intensity of biomethane from the median scenario was conducted and showed a potential greenhouse gas reduction of 74–79% when compared to natural gas. [ABSTRACT FROM AUTHOR]

The technical feasibility of long term anaerobic mono-digestion of two brown seaweeds, and co-digestion of both seaweeds with dairy slurry was investigated whilst increasing the organic loading rate (OLR). One seaweed was natural ( L. digitata ); the second seaweed ( S. Latissima ) was cultivated. Higher proportions of L. digitata in co-digestion (66.6%) allowed the digester to operate more efficiently (OLR of 5 kg VS m −3 d −1 achieving a specific methane yield (SMY) of 232 L CH 4 kg −1 VS) as compared to lower proportions (33.3%). Co-digestion of 66.6% cultivated S. latissima , with dairy slurry allowed a higher SMY of 252 L CH 4 kg −1 VS but at a lower OLR of 4 kg VS m −3 d −1 . Optimum conditions for mono-digestion of both seaweeds were effected at 4 kg VS m −3 d −1 . Chloride concentrations increased to high levels in the digestion of both seaweeds but were not detrimental to operation. [ABSTRACT FROM AUTHOR]

This study investigated the feasibility of mono-digesting grass silage, dairy slurry and the co-digestion of the two substrates at a range of concentrations with a specific focus on digester performance while increasing organic loading rate (OLR). The results show that the higher the proportion of grass silage in the substrate mix the higher the specific methane yield (SMY) achieved. Optimum conditions were assessed for 100% grass silage at an OLR of 3.5 kg VS m −3 d −1 generating a SMY of 398 L CH 4 kg −1 VS equating to a biomethane efficiency of 1.0. For co-digestion of grass silage with 20% dairy slurry the optimum condition was noted at an OLR of 4.0 kg VS m −3 d −1 generating a SMY of 349 L CH 4 kg −1 VS and a biomethane efficiency of 1.01. Hydraulic retention times of less than 20 days proved to be a limiting factor in the operation of farm digesters. [ABSTRACT FROM AUTHOR]

This study investigated the effect of trace element addition to mono-digestion of grass silage at high organic loading rates. Two continuous reactors were compared. The first mono-digested grass silage whilst the second operated in co-digestion, 80% grass silage with 20% dairy slurry (VS basis). The reactors were run for 65 weeks with a further 5 weeks taken for trace element supplementation for the mono-digestion of grass silage. The co-digestion reactor reported a higher biomethane efficiency (1.01) than mono-digestion (0.90) at an OLR of 4.0 kg VS m −3 d −1 prior to addition of trace elements. Addition of cobalt, iron and nickel, led to an increase in the SMY in mono-digestion of grass silage by 12% to 404 L CH 4 kg −1 VS and attained a biomethane efficiency of 1.01. [ABSTRACT FROM AUTHOR]

Highlights: [•] A biomethane potential (BMP) assessment of grass silage yielded 107m3 CH4 t−1. [•] Mono-digestion of grass silage is problematic due to mineral deficiency. [•] Co-digestion of slurry and grass led to reduced methane yields of between 4% and 11%. [•] 1.1% of grasslands in Ireland can generate 10% renewable energy in transport. [•] 170 digesters treating 10,000ta−1 of grass and 40,000ta−1 of slurry are required. [ABSTRACT FROM AUTHOR]

On-farm anaerobic digestion plants served by virtual biomethane pipelines may be used in areas with no gas grid infrastructure access to improve manure (slurry) management and to produce and deliver renewable gas. A modelling analysis of a simulated virtual biomethane pipeline is presented whereby gas is collected from 100 on-farm biogas plants fed with slurry and grass silage feedstocks and delivered to a market entry point (depot). In total, 7 scenarios are presented in the analysis. In scenarios 1–4, the biogas is upgraded on-site by the digester operator, the biomethane is collected by the haulage vehicle(s) and transported to a single depot. The impact of an additional depot on vehicle routing is investigated in scenarios 5 and 6. Scenario 7 investigates the use of a mobile-upgrading and compression unit, which would allow biogas plant operators to forego the installation and operation of biogas upgrading facilities. In scenario 3 (which considers the largest capacity biomethane haulage vehicles and a single delivery depot) all 100 anaerobic digestion plants are visited over the course of one week using 4 routes, with a total aggregated distance travelled of 205 km. Scenario 7 requires a total routing distance in excess of 613 km to visit all 100 biogas plants and requires long routing times and periods of downtime at each site due to the slow upgrading speed of the mobile unit. The weekly greenhouse gas emissions due to vehicle routing from each scenario are calculated. Assuming diesel-fuelled haulage vehicles, scenario 7 routing results in 431 kg CO 2-eq week−1, a 167% increase in vehicle route emissions compared to scenario 3 (161 kg CO 2-eq week−1). Additionally, whilst the mobile-upgrading unit may alleviate financial challenges, they carry significant logistical challenges, and their resultant higher routing emissions will impact the sustainability of the collected biomethane. Biomethane fuelled haulage vehicles may address this concern if the biomethane is sustainably sourced. • Use of large biomethane haulage vehicles allows for shorter overall distance. • Mobile-upgrading units spend an average of 6 h upgrading biogas from on-farm AD. • Mobile-upgrading units spend 17 h week−1 upgrading biogas from 60 kW e digester. • Total routing distance may significantly influence biomethane sustainability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electric field and current density were enhanced by modified cathodic surfaces. • Application of modified cathodes boosted biomethane yield by 97%. • Increased biomethane yield equated to a 6.5-fold return on electrical energy input. • Biochar increased CH 4 yield in anaerobic digestion with/without electrode assist. • The microbe-electrode interface facilitated interspecies electron transfer. Integrating microbial electrolysis cells and anaerobic digestion (MEC-AD) improves upon conventional anaerobic digestion for biomethane production due to the in-situ provision of electrochemically produced reducing power (such as hydrogen). However, the electron transfer behaviour at the microbe-electrode interface remains unclear. This study assessed the micro-scale interface modification of a carbonaceous bioelectrode, leading to an enhanced local electric field, which was postulated to stimulate electro-methanogenesis. The effectiveness of incorporating electro-conductive biochar to enhance interspecies electron transfer was also evaluated for its potential to boost biomethane production. The findings revealed a notable increase in biomethane yield and methane content within the biogas of the MEC-AD system, with improvements of 96.8 % and 32.5 %, respectively, compared to conventional anaerobic digestion when co-digesting grass silage and cattle slurry. The enhancement was ascribed to the accumulation of charges and an intensified local electric field on the surface of the etched biocathode, thereby facilitating interfacial electron transfer. Incorporating 10 g/L of biochar to conventional anaerobic digestion resulted in a 7.9 % increase in biomethane yield compared to conventional anaerobic digestion. Overall, the heightened energy yield of biomethane by the MEC-AD system (featuring the modified graphite cathode) resulted in a 6.5-fold increase compared to the additional electrical energy input. This underscores the catalytic significance of the electricity in AD system. [ABSTRACT FROM AUTHOR]

Grass silage and cattle slurry are the most abundant feedstocks for biogas in an Irish context. This work describes how to optimize their biomethane yield. Hydrolysis and biomethane yields from co-digestion of grass silage and slurry were enhanced in a biochar-supplemented two-stage digestion process. Biochar addition at the optimal dosage of 10 g/L in batch two-stage digesters led to the highest methane yield of 253 L per kilogram (kg) volatile solid (VS), which was 24% higher than that from two-stage digesters without biochar supplementation. For a batch single-stage digester, the biomethane yields were lower than the two-stage but 10 g/L biochar addition led to an increase to 218 L/kg VS as compared to 198 L/kg VS for the group without biochar supplementation. Volatile fatty acid accumulation increased with the enhancement of biomethane production in the biochar-supplemented two-stage reactors. The addition of biochar in single-stage and two-stage anaerobic digesters presented positive effects on the electrochemical properties of the digester contents, reflected by the increased charge storage capacity, reduced solution resistance and improved digester content conductivity. The methane yields in two-stage anaerobic digestion were shown to be strongly correlated (R2 = 0.92) with the concentration of extracellular polymeric substances. This evidenced that the interlaced conductive network established by extracellular polymeric substances and biochar was essential for direct interspecies electron transfer and biomethane production. This study demonstrated that biochar-supplemented two-stage anaerobic digestion is an optimal route for energy recovery from grass silage and cattle slurry. [Display omitted] • Co-digestion of on-farm feedstock was enhanced in a two-stage process with biochar. • Biochar at 10 g/L in two-stage digestion maximized CH 4 yield at 253 L/kg VS. • Biochar at 10 g/L in single-stage digestion achieved a CH 4 yield of 218 L/kg VS. • Biochar improved sludge conductivity to facilitate interspecies electron transfer. • CH 4 yield highly correlated with extracellular polymeric substances concentration. [ABSTRACT FROM AUTHOR]

On-farm feedstocks such as grass silage and cattle slurry present recalcitrant characteristics that can limit microbial conversion in biofuel production. Introducing a biochar supplement in two-stage anaerobic digestion may facilitate feedstock hydrolysis and improve energy yields in biohydrogen and biomethane production. The biomethane potentials were first investigated in batch trials without biochar supplement; results indicated a biomethane yield of 230 L per kilogram (kg) volatile solid (VS) in single-stage digestion and 275 L/kg VS in two-stage digestion. In continuous trials, operated at an organic loading rate of 4.0 g VS/L/d, the second-stage digester in two-stage digestion showed a methane yield of 237 L/kg VS with 10 g/L biochar addition; this was 7% higher than the second-stage digester without biochar addition. At the same loading rate of 4.0 g VS/L/d, the biomethane yield in continuous single-stage digestion with 10 g/L biochar addition was 212 L/kg VS; this was 3% higher than the single-stage digester without biochar addition. Biochar was found to enhance the hydrolysis of recalcitrant solid components in the hydrogen-producing phase, promote biomethane production in methanogenesis, and stabilize the digestion process. The highest energy yield of 8.5 MJ hydrogen and methane per kg VS was achieved in the two-stage digestion with 10 g/L biochar addition at a loading rate of 4.0 g VS/L/d. The results demonstrated that the application of a biochar supplement could effectively enhance gaseous biofuel production in two-stage anaerobic digestion. [Display omitted] • Co-digestion of silage and cattle slurry was compared in single and two stage. • Biochar enhanced hydrolysis and acidification in continuous two-stage process. • 10 g/L biochar enhanced CH 4 production and stabilized continuous digestion process. • Two-stage digestion increased H 2 and CH 4 production as compared to single stage. • The highest energy yield was 8.5 MJ/kg VS at 4.0 g VS/L/d in second-stage digestion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Decentralised biomass is defined as biomass in excess of 15 km from the gas grid. • Over 17% of national on-farm biomass is decentralised but within 15 km of industry. • On-farm biomethane resource is equivalent to 200% of industry gas use in Ireland. • Industry heat demand in two studies can be met by biomethane produced within 2.5 km. A methodology was developed to determine the location and biomethane potential of on-farm biomass feedstocks and furthermore, the quantity of this biomass situated away from an existing gas grid network. The development of decentralised systems which can be integrated with large industry in such locations offers a potential alternative deployment opportunity to centralised gas grid injection. The total biomethane resource from all on-farm feedstocks in a chosen case country (Ireland) was estimated at 67 PJ. Thematic maps were developed to show the quantity of each on-farm feedstock in different regions of Ireland and their distance from the existing gas network. Approximately 17% of the on-farm biomethane resource in Ireland, though in excess of 15 km from the grid, was within a 15 km radius of a large energy user. Two large energy users were presented as case studies, of which their entire heating demand could be met by the on-farm biomethane resource within a 2.5 km radius of each site. Coupling decentralised biomass with large industry users can potentially offer an alternative energy management strategy. [ABSTRACT FROM AUTHOR]