System expansion for handling co-products in LCA of sugar cane bio-energy systems: GHG consequences of using molasses for ethanol production
Highlights
► A challenging issue in LCA is how to account for co-products’ environmental burdens. ► The two most commonly used procedures are system expansion and allocation. ► System expansion appears to be more appropriate than allocation. ► Indirect land use change is a consequence of diverting molasses from feed to fuel. ► The inclusion of land use change worsens the GHG balance of molasses ethanol.
Introduction
One of the main policy rationales for promoting bioenergy as a viable alternative source of energy is its potential to reduce greenhouse gas (GHG) emissions. Bioenergy can be produced from different biomass producing sources, e.g. plants, animal, organic waste etc., and can take the form of gaseous/liquid/solid fuels, heat, or electricity. The liquid fuels in the ‘fuel’ group, commonly referred to as biofuels, include ethanol and biodiesel whose production is growing driven by worldwide efforts to reduce oil dependence in transport. Due to their carbon–neutral nature, these two biofuels are in general thought to be more climate friendly than gasoline and diesel, though this has been questioned recently when GHG implications of land use change are taken into account [1], [2]. As the demand for biofuels increases, so does the percentage of cropland to be devoted to the production of fuel rather than food. This would result in land use change and the related carbon emissions will offset the carbon ‘savings’ from substituting biofuels for conventional fuels. Only in the past few years has land use for feedstock production been increasingly considered a key factor in determining biofuel sustainability.
The biofuel receiving the most attention today is ethanol, which can be produced from a wide variety of feedstocks. Until recently, several published studies showed that ethanol from food crops (e.g. corn, wheat, sugar cane) can offer GHG savings compared to gasoline [3], [4], [5], [6], [7]. However, due mainly to methodological problems in environmental impact assessment of land use and probably to assumptions behind the analysis, virtually all of these studies failed to include such important variable in their final results.
Worldwide, life cycle assessment (LCA) is recognized as a standardized and structured method for evaluating the environmental impacts arising throughout the entire life cycle of a product, process or activity. A challenging issue in LCA nevertheless is the selection of methods to allocate the environmental burden of a specific production system between products and co-products. It can be regarded as crucial since using different methods would produce different results and consequently different interpretations. In relation to allocation, the first priority as recommended by ISO 14044 [8] is to avoid allocation whenever possible by dividing the unit process to be allocated into sub-processes, or expanding the product system to include the additional functions of the co-products. Otherwise, allocation for the system can be done in such a way that it reflects the physical properties or the relative economic values of co-products. There is a relationship between the choice of method, allocation or system expansion, and the choice of LCA approach, attributional or consequential [9]. The attributional approach deals with co-product allocation by partitioning the environmental impact related to the product using allocation factors based on mass, energy or economic value. The consequential approach, seeking to capture change in environmental impact as a consequence of actions, avoids co-product allocation by system expansion.
In the context of a growing interest in ethanol from a wide range of feedstocks, recent analyses have been undertaken to assess the environmental costs and benefits of ethanol production from molasses, a co-product of sugar production. In most of the few published works dealing with co-product issues in sugar cane systems, the environmental costs of molasses were accounted for using either economic allocation alone [10], [11], [12], [13] or both economic allocation and system expansion [14], yet only in one study [15], was neither allocation nor system expansion applied. Since the ISO standard [8] recommends to avoid allocation by expanding system boundaries, it can be inferred that system expansion is preferred to allocation but the task is to identify close substitutes for the co-product considered and their product systems. This study aims to explore the applicability of system expansion to avoid allocation between sugar and molasses as two important products in a typical sugar cane system. The procedure is essential for environmental assessment of ethanol from molasses where the consequential approach appears to be best suited, especially when the debate over whether crops should be used for food/feed or fuel has been central to policy thinking on the adoption of biofuels as a sustainable energy source. An interesting case study would be Thailand which is one of the world’s main cane molasses producers [16] and where an ethanol demand for domestic consumption of E10 and E20, i.e. the 10:90 and 20:80 (v/v) ethanol–gasoline blends, respectively, has taken up 21% of the total national molasses produced in 2008 [17].
Section snippets
Goal and scope definition
The objective of this study is to verify the procedures of system expansion and allocation against the estimate of GHG emissions associated with ethanol production from molasses in a baseline and an alternative abatement scenario. The study also includes a sensitivity analysis to investigate the issue of ‘indirect land use change’ arising from an increased demand for land to grow crops to fill the gap in feed supplies when molasses is diverted to fuel production. The three GHGs considered are CO
GHG emissions estimate for sugar and molasses: allocation versus system expansion
From the emission rate of 39.9 kg CO2e/t cane (see Table 1), three ways of handling co-products are used to estimate GHG emissions embodied in sugar and molasses. The corresponding results are shown in Fig. 2.
Clearly from the figure, economic allocation assigns a major portion of GHG emissions to sugar, largely leaving aside its co-product molasses: 90.1% versus 9.9% and 94.7% versus 5.3% under the condition with and without the effect of ethanol demand (i.e. after and before the promotion of
Conclusions
From the present analysis results, main conclusions are as follows.
- 1.
Of different LCA procedures to handle co-products, system expansion yields the highest estimate of GHG emission load embodied in molasses, followed by allocation based on mass/energy content and allocation based on market value. This explains why the calculation resulting in the worst GHG performance of ethanol corresponds to the use of system expansion. However, no matter whether system expansion or allocation is used, a
Acknowledgments
This study was financially supported by (1) the Department of Agroecology, Aarhus University, Denmark; and (2) the Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand.
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