1 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU2 Faculty of Engineering, SDU3 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU
- for transport and for heat and power production under displacement of natural gas based heat works and marginal electricity in northern Germany
This paper presents an environmental Life Cycle Assessment (LCA) of biogas produced from both maize silage (1) and animal manure (2) based on the technologies developed at Xergi A/S in Aalborg, Denmark. The LCA comprises both environmental impacts (with focus on global warming impacts) and impacts on resource consumption and covers utilisation of the produced biogas for either heat and power generation (A) or for transport (B) in an upgraded (cleaned) and compressed form. In biogas heat & power scenarios, the generated heat is assumed to replace natural gas based heat works, whereas the generated power will replace marginal power on the grid. The study is comparative and shows the environmental consequence of making biogas instead of the alternative use of the substrate. Biogas from manure is, thus, compared to the conventional storage and use of the manure as agricultural fertilizer, and biogas from maize silage is compared to using the same agricultural land for other bioenergy purposes, i.e. growth of maize for bioethanol production, growth of rapeseed for biodiesel production and growth of willow for heat and power production allowing to compare Xergi's biogas to other biofuels. The assessment, thus, comprises: 1. Biogas made from whole-crop maize (silage): 1A Biogas used for heat & power, and 1B Biogas cleaned, compressed and used for transport 2. Biogas made from animal manure: 2A Biogas used for heat & power, and 2B Biogas cleaned, compressed and used for transport 3. 1st generation biodiesel made from rapeseed 4. 1st generation bioethanol made from maize kernels 5. 2nd generation bioethanol made from whole-crop maize 6. Willow production for power and heat production In this context, 1st generation biofuels are defined as biofuels based on raw materials that alternatively could be used as food, whereas 2nd generation biofuels are based on energy crops, residues and waste streams. The environmental assessment is based on the EDIP method (Wenzel et al., 1997) and further up-dates of this method (Weidema et al. (2004), Weidema (2004), Stranddorf et al. (2005)) which are in agreement with the standards of the International Organisation for Standardisation, ISO. Moreover, the study is conducted according to the principles of consequential LCA, which is today's best scientific practice. It implies that the LCA is comparative and dedicated to identify the environmental consequence of choosing one alternative over the other. The consequential and comparative approach ensures that all compared alternatives are equivalent and provide the same services to society, not just regarding the primary service, which in this case is a specified transportation service together with a heat and power production, but also on all secondary services. Secondary services are defined as products/services arising e.g. as co-products from processes in the studied systems, and in the case of biofuels, such secondary services can typically be energy-services (electricity and/or heat) and animal feed. The consequential LCA ensures equivalence on all such services by identifying and including the displacements of alternative products that will occur when choosing one alternative over the other. Biomass has become a priority resource to substitute fossil fuels in the energy sector (heat & power) and is increasingly seen to be so in the transport sector as well. In e.g. Denmark, wood chips, wood pellets, and straw are increasingly used to substitute fossil fuels for heat & power production. Moreover, it has been shown that the amount of biomass, that is or can be made available for energy purposes, is limited compared to the potential use of it for fossil fuel substitution in the energy (heat & power) and transport sector as a whole (Jensen and Thyø, 2007), and so is the fraction of agricultural land that can be made available for energy crops. Any area of land that is made available for energy purposes has, thus, a potential customer in both the heat & power sector and the transport sector. As such, any use of such biomass for transport fuels will happen at the expense of using it for heat & power and, thus, with the consequence of using an equivalent amount of fossil fuels there. Moreover, any use of biomass for biofuels will require subsidies for a long period ahead (and covering the time perspective of this study), and money to support a given biofuel or technological pathway is limited as well. Therefore, any use of biomass for energy purposes or of money to support biomass for energy purposes will happen at the expense of an alternative use of the same biomass, land, and/or the same money. The situation to be modelled in a consequential LCA approach is, thus, clear: the use of the limited amount of agricultural land will happen at the expense of utilisation of agricultural land for alternative uses. The unit for greenhouse gas emissions is ton CO2-equivalents, and the unit for fossil fuel consumption is PR, standing for person reserves, which is a common unit for assessing resource consumption based on their scarcity and supply horizon. The scenarios 2A and 2B of manure based biogas are included in the comparison, however, it should be emphasised that they are "stand alone", while the rest of the scenarios are each others alternatives e.g. the prioritising of utilizing land for one option shall be seen to happen at the expense of the other options. Biogas based on manure is not an alternative strongly correlated to the other scenarios, because it does not include any utilization of agricultural land. However, since it provides the same services to society as the other scenarios, it still compares to them and enters into the overall prioritisation of which type of bioenergy technology society should promote with subsidies and other incentives. The conclusion of this comparison is unambiguous: biogas from manure implies by far the highest reduction of greenhouse gas emissions per unit of services provided to society. This being due to the fact that it implies CO2 reductions not only from the fossil fuel replacement by the generated biogas, but equally significantly from the reduced methane emissions from manure storage, reduced nitrous oxide emissions from soil application of the manure and improved plant availability of the nitrogen in the manure. The brief and overall conclusions on manure based biogas can, thus, be expressed as: Biogas from manure stands out as having very high reduction in greenhouse gas emissions and very high fossil fuel savings compared to the conventional storage and soil application of the manure. Environmentally and in terms of resource savings, manure should be utilised for biogas production prior to the soil application. Biogas from manure stands out as having much higher reduction in greenhouse gas emissions as the other bioenergy types and equal savings in fossil fuels. As cost aspects point to the same direction, manure based biogas should have the highest priority of all the compared bioenergy types. The other scenarios are strongly correlated by their competition for the same agricultural land. Based on the comparative approach, the LCA shows that environmentally and in terms of fossil fuel savings, energy crops should be prioritised for heat and power purposes either 1) through a preceding biogas generation or 2) by direct incineration or gasification, the two leading to almost equal CO2 reductions and fossil fuel savings. Energy crops converted directly into a transport fuel implies significantly lower CO2 reductions due to the energy losses in the conversion processes. The brief and overall conclusions on maize based biogas can, thus, be expressed as: Among the compared types of bioenergy requiring agricultural land and energy crops, biogas from maize silage and heat and power from willow imply the highest reductions in greenhouse gas emissions and the highest fossil fuel savings. Environmentally and in terms of fossil fuel savings, land for energy crops should, thus, be prioritised for crops for heat & power or for biogas. The explanation of this outcome of the LCA can be found within 3 main reasons: 1. The yield of the energy crop per hectare of land 2. The fossil fuel substitution efficiency, including the energy efficiency of the conversion of the calorific value of the crop's dry matter content 3. The energy infrastructure aspects of the bioenergy technology The explanation within these 3 categories of why the rape seed biodiesel and the 1st and 2nd generation bioethanol comes out with lower CO2 reductions and fossil fuel savings are given below. Rape seed biodiesel: Rape has a very low energy yield per hectare, and this is the one reason for rape seed biodiesel to come out as the environmentally least preferable of the biofuels. Prioritising land for rape through choosing (and subsidising) rapeseed biodiesel for transport means depriving society the higher yield of other energy crops on the same land. There is no sign that this will change. The conversion efficiency of the rape seed oil to the biodiesel is comparably high, i.e. only 10% conversion loss or less. There are no infrastructure disadvantages. Bioethanol: The yield of maize per hectare is the highest among the compared energy crops, and in this study, the bioenergy technologies using maize have for this reason an inherent advantage. For the first generation bioethanol, however, the advantage is of course lost when the stover is not used for energy purposes. On the energy conversion, however, the bioethanol technologies have large losses and an inherent disadvantage: Firstly (for the 2nd generation technology), a thermal pre-treatment of the maize stover is required, and this implies an energy consumption. Secondly, the metabolism of the ethanol fermentation is not as efficient as the methane fermentation, and much remains unconverted to ethanol in terms of metabolic side-products and un-degraded residues. It implies among other things that energy must be spent on drying/dewatering in order to render the residues suitable for subsequent incineration or gasification based energy conversions. Thirdly, energy is needed to separate the ethanol from the fermentation liquor, requiring a distillation process. The biogas has the inherent advantage of leaving the fermentation liquor voluntarily. On the infrastructure side, finally, the bioethanol technologies have an inherent requirement of being very large scale, mainly due to the necessity of the distillation to be large scale; in small scale the cost of bioethanol becomes much worse and detrimental to any real life implementation. It implies that bioethanol cannot enter into a decentralised heat & power production infrastructure and, thus, cannot, like biogas, realise the multiplication effect of full heat utilisation at the same time as delivering the electricity to the grid under marginal electricity replacement. The assessment is robust to changes in boundary conditions including the key issues for the sensitivity of the results. The most crucial boundary condition behind the assessment in this LCA is the acknowledgement of the fact that energy crops/land for energy crops will be a constrained resource and require subsidies in order to reach any utilisation for energy purposes, with the implication that any use of land for energy crops should be assessed against the lost opportunity of using it for other purposes in the fulfilment of the same aims.
11th Annual Meeting. Nato Science for Peace and Security Pilot Project on Clean Products Nad Processes, 2008, p. 51-55
Life Cycle Assessment, biogas, biofuels, manure, carbon footprint
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11<sup>th</sup> Annual Meeting, NATO Science for Peace and Security Pilot Project on Clean Products and Processes, 2008
Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit