• Energy Research

Abstract 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 €/twwt grass silage, VSR of 4, 75,000 twwt/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 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 €/twwt grass silage, VSR of 4, 75,000 twwt/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 This study investigated in-situ and ex-situ biological methanation strategies for biogas upgrading potential. The addition and circulation of hydrogen with a ceramic gas diffuser unit revealed positive effects on the methanogenic process. A short-term maximum methane productivity of 2.5 L CH4 per L reactor volume per day (LVR−1 d−1) was obtained in-situ. Adverse effects of elevated dissolved hydrogen concentrations on acetogenesis became evident. Ex-situ methanation in a reactor subjected to gas recirculation for recurrent 24 h periods achieved methane formation rates of 3.7 L CH4 LVR−1 d−1. A biomethane with methane concentrations in excess of 96% successfully demonstrated the potential for gas grid injection. A theoretic model supplying gases continuously into a sequential ex-situ reactor system and steadily displacing the upgraded biogas confirmed similar methane formation performance and was advanced to a full-scale concept. Gas conversion efficiency of 95% producing biomethane at 85% methane content was attained. A hybrid model, where an in-situ grass digester is followed by an ex-situ reactor, is proposed as a novel upgrading strategy.

Abstract This study investigated in-situ and ex-situ biological methanation strategies for biogas upgrading potential. The addition and circulation of hydrogen with a ceramic gas diffuser unit revealed positive effects on the methanogenic process. A short-term maximum methane productivity of 2.5 L CH4 per L reactor volume per day (LVR−1 d−1) was obtained in-situ. Adverse effects of elevated dissolved hydrogen concentrations on acetogenesis became evident. Ex-situ methanation in a reactor subjected to gas recirculation for recurrent 24 h periods achieved methane formation rates of 3.7 L CH4 LVR−1 d−1. A biomethane with methane concentrations in excess of 96% successfully demonstrated the potential for gas grid injection. A theoretic model supplying gases continuously into a sequential ex-situ reactor system and steadily displacing the upgraded biogas confirmed similar methane formation performance and was advanced to a full-scale concept. Gas conversion efficiency of 95% producing biomethane at 85% methane content was attained. A hybrid model, where an in-situ grass digester is followed by an ex-situ reactor, is proposed as a novel upgrading strategy.

Abstract Algae have emerged as a sustainable feedstock for gaseous biofuel production (such as hydrogen and methane). Fermentative sugars and amino acids can be obtained after suitable pretreatment and hydrolysis of algae. However, binary interactions between the carbonyl group ( C O) in sugars and the amino group ( NH2) in amino acids possibly occur during thermal pretreatment, resulting in deficient hydrolysis and fermentation performance. In this study, algae-derived glucose and glycine as model substrates were subjected to thermochemical treatment (135 °C for 15 min) under neutral, acid and alkaline conditions to assess their decomposition routes and the associated implications on sequential biohydrogen and biomethane fermentation. Acid treatment mainly resulted in direct decomposition of glucose into 5-methylfurfural (C6H6O2, 34.4% of peak area). While thermal treatment with deionized water and alkaline led to the formation of nitrogen-containing Maillard compounds, namely 1-azido-4-dimethylaminobenzene (C8H10N4, 33.1%) and 2,3,5-trimethylpyrazine (C7H10N2, 49.0%), respectively. Untreated glucose/glycine yielded a biohydrogen production of 171.9 mL/g, while alkaline treatment exhibited a biohydrogen yield of only 5.9 mL/g due to the great loss of fermentable substrate. The total energy conversion efficiency (ECE) of 71.1% was achieved through the second-stage biomethane fermentation of untreated glucose/glycine. Comparatively, alkaline treatment significantly inhibited the total energy recovery with an ECE of 31.9%. The findings of this study suggested that optimised pretreatment strategy for algae needs to be developed to avoid fermentable compounds loss and achieve a higher ECE.

Abstract Algae have emerged as a sustainable feedstock for gaseous biofuel production (such as hydrogen and methane). Fermentative sugars and amino acids can be obtained after suitable pretreatment and hydrolysis of algae. However, binary interactions between the carbonyl group ( C O) in sugars and the amino group ( NH2) in amino acids possibly occur during thermal pretreatment, resulting in deficient hydrolysis and fermentation performance. In this study, algae-derived glucose and glycine as model substrates were subjected to thermochemical treatment (135 °C for 15 min) under neutral, acid and alkaline conditions to assess their decomposition routes and the associated implications on sequential biohydrogen and biomethane fermentation. Acid treatment mainly resulted in direct decomposition of glucose into 5-methylfurfural (C6H6O2, 34.4% of peak area). While thermal treatment with deionized water and alkaline led to the formation of nitrogen-containing Maillard compounds, namely 1-azido-4-dimethylaminobenzene (C8H10N4, 33.1%) and 2,3,5-trimethylpyrazine (C7H10N2, 49.0%), respectively. Untreated glucose/glycine yielded a biohydrogen production of 171.9 mL/g, while alkaline treatment exhibited a biohydrogen yield of only 5.9 mL/g due to the great loss of fermentable substrate. The total energy conversion efficiency (ECE) of 71.1% was achieved through the second-stage biomethane fermentation of untreated glucose/glycine. Comparatively, alkaline treatment significantly inhibited the total energy recovery with an ECE of 31.9%. The findings of this study suggested that optimised pretreatment strategy for algae needs to be developed to avoid fermentable compounds loss and achieve a higher ECE.

Biofuels have had bad press in recent years. There are primarily two distinct issues. The biofuel crops with the best yields (such as sugarcane or oil palm) grow in tropical countries where habitat destruction has occurred in association with the biofuel system. First generation indigenous energy crops commonly used for transport fuel in Europe (such as rapeseed and wheat) have low yields and/or the energy balance of the associated biofuel system is poor. This paper shows that grass is a crop with significant yields and grass biomethane (a gaseous renewable transport biofuel) has a very good energy balance and does not involve habitat destruction, land use change, new farming practices or annual tilling. The gross and net energy production per hectare are almost identical to palm oil biodiesel; the net energy of the grass system is at least 50% better than the next best indigenous European biofuel system investigated. Ten percent of Irish grasslands could fuel over 55% of the Irish private car fleet.

Biofuels have had bad press in recent years. There are primarily two distinct issues. The biofuel crops with the best yields (such as sugarcane or oil palm) grow in tropical countries where habitat destruction has occurred in association with the biofuel system. First generation indigenous energy crops commonly used for transport fuel in Europe (such as rapeseed and wheat) have low yields and/or the energy balance of the associated biofuel system is poor. This paper shows that grass is a crop with significant yields and grass biomethane (a gaseous renewable transport biofuel) has a very good energy balance and does not involve habitat destruction, land use change, new farming practices or annual tilling. The gross and net energy production per hectare are almost identical to palm oil biodiesel; the net energy of the grass system is at least 50% better than the next best indigenous European biofuel system investigated. Ten percent of Irish grasslands could fuel over 55% of the Irish private car fleet.

Abstract Six EU gas grids have a target of 100% substitution of natural gas with renewable gas by 2050. This industry will start with biogas upgraded to biomethane. The biomethane resource and location of waste substrates (such as agricultural slurries, slaughterhouse waste, milk processing waste, and source separated household organic waste) were determined using the most recent spatially explicit data for Ireland. The total biomethane resource was estimated equivalent to: 7.6% of natural gas usage, 7% of energy in transport; 52% of the fuel usage in heavy goods vehicles in 2013. In terms of natural gas usage it corresponded to 26.5% of industrial gas use, and 52% of residential natural gas use. The resource as a source of thermal energy is equivalent to wood chips from 16.5% of arable land under short rotation coppice willow. Thematic maps illustrating the location of each resource were developed to highlight regions of significant biomethane production potential. The regions with the greatest resource of cattle slurry are located in the south and east of the country; sheep manure resources are concentrated on the western seaboard, while the largest biomethane resource from household organic waste is found in urban and city areas (63% of household organic waste biomethane resource).