No abstract

No abstract

IntroductionThe target set by the EU Renewable Energy Directive (2009/28/EC) [1] requires a 20% energy share from renewable sources by 2020. Thus, exploring alternative, environmentally benign and energy efficient systems has become the focus of governmental policies and industrial as well as academic research.Biogas is a renewable energy source that can be produced from the anaerobic digestion (AD) of biomass, such as sewage sludge, municipal solid waste, agricultural wastes, and energy crops. Biogas consists of around 50-60% methane (CH4), 40-50% carbon dioxide (CO2) and some minor constituents, such as hydrogen sulphide (H2S) and water. The use of biogas for energy production could displace fossil fuels, reduce greenhouse gas emissions and decrease dependence on imported energy [2].Upgraded biogas, termed bio-methane, is typically composed of ~97% CH 4 and ~3% CO 2 , and is converted to the same standard as natural gas through removal of CO 2 (upgrading) and other impurities (cleaning).Another route for upgrading biogas to bio-methane involves the chemical transformation of CO 2 to CH 4 by 2 the Sabatier reaction (equation 1); the hydrogen (H 2 ) in the reaction is usually obtained from water (H 2 O) electrolysis (equation 2). This combined pathway could have an important impact on the global carbon cycle [3]. There are various utilization pathways for both raw and upgraded forms of biogas [4]; commercial methods include electricity and heat generation via combined heat and power (CHP) units, electricity generation via fuel cells, and conversion to mechanical energy for transport via internal combustion engines (ICEs). Bio-methane can be injected into the gas grid, and/or converted to compressed renewable natural gas or liquefied renewable natural gas (referred to in this paper as CNG and LNG respectively) to serve as a transport fuel. Biogas can also be reformed to syngas (CO and H2) for liquid fuel production via Fischer Tropsch (FT) synthesis (see [5] for further details).As with any new energy system, countries are faced with ongoing challenges when designing the most optimum pathway to ensure sustainable development and sufficient energy supply [6]. In practice, many European countries have successfully integrated biogas into their energy sectors via different utilization routes. The annual energy production from biogas is around 42 TWh in Germany (the highest production in the EU), 9 TWh in UK, and 2.8 TWh in France; in each of these countries the biogas is mainly used for electricity generation [7]. Sweden produces around 1.7 TWh from biogas and 44% of biogas production is upgraded to bio-methane and used as vehicle fuel [8]. In Italy, biogas is mainly used for power generation while other pathways such as grid injection and CHP require further exploration [9]. However, although there are various options for biogas utilization, there is limited guidance in the literature on the selection of the optimum route. A number of papers focus specifically on biogas utilization as a vehicle fuel [9, 10] while oth...

No abstract

Farm incomes in Ireland are in decline and many farmers would operate at a loss in the absence of subsidies. Agriculture is responsible for 27% of Ireland's greenhouse gas emissions and is the largest contributing sector.Penetration of renewable energy in the heat and transport sectors is falling short of targets, and there is no clear plan for achieving them. The anaerobic digestion of grass to produce biogas or biomethane is put forward as a multifaceted solution, which could help meet energy and emissions targets, reduce dependence on imported energy, and provide additional farm income. This paper addresses the economic viability of such a system. Grass biogas/ biomethane fares poorly under the current combined heat and power tariff structure, which is geared toward feedstock that attracts a gate fee. Tariff structures similar to those used in other countries are necessary for the industry to develop. Equally, regulation should be implemented to allow injection of biomethane into the gas grid in Ireland.Blends of natural gas and biomethane can be sold, offering a cost-competitive green fuel. Sale as a renewable transport fuel could allow profi tability for the farmer and savings for the consumer, but suffers due to the lack of a market. Under current conditions, the most economically viable outlet for grass biomethane is sale as a renewable heating fuel. The key to competitiveness is the existing natural gas infrastructure that enables distribution of grass biomethane, and the renewable energy targets that allow renewable fuels to compete against each other. many farmers are seeking opportunities for farm diversification and alternative sources of income, and the National Development Plan 2 recognizes as a key task the promotion of Modeling and Analysis 520 Modeling and Analysis: Grass biomethane -an economically viable biofuel? the diversifi cation of the rural economy. A move away from conventional farming can be further supported by the fact that all grass-based farming systems (beef, dairy, and sheep) produce large quantities for the export market. Th is is especially true in the beef sector, which had self-suffi ciency values of over 600% for each year in the period 2000 to 2008. 3,4 Greenhouse gas emissions in agriculture Agriculture was responsible for 27.3% (18.4 Mt CO 2eq ) of Ireland's greenhouse gas (GHG) emissions in 2008 and was the single largest contributing sector, followed by energy industries (21.8%), transport (21.1%), industry and commercial (16.9%), residential (11.2%), and waste (1.6%). 5 Enteric fermentation in non-dairy (i.e. beef) cattle is the largest contributor to agricultural emissions and is one of the biggest GHG emission sources in the country. 6 Ireland has the highest ratio of cattle to human population in the European Union (from Eurostat data 7,8 ) and the importance of the agricultural sector is largely responsible for the country's relatively high per capita GHG emissions; in 2007 per capita emissions were 15.9 t CO 2eq compared with 10.2 t CO 2eq for the EU27. 9 Th e Nati...