Over the past two decades, much work has focused on methodology to assess the environmental impact of processes and products. A number of these approaches are summarized in Table 9.2, indicating the methodology and nature of the assessment. It must be noted that while initially bioprocesses and energy processes from renewable resources were assumed to be preferential with respect to lower

TABLE 9.2

Approaches to the Quantification of Environmental Sustainability of Process Options

environmental impact, it has been demonstrated clearly that this does not necessarily hold; hence, objective assessment of the environmental burden of each process is essential in product and process selection, in a similar manner to that used to ensure economic feasibility.

Life cycle assessment (LCA) systematically identifies environmental impact and opportunities to minimize it, and evaluates these (Curran 2000). It is supported by a strong literature database and a well-defined methodology. A track record exists for its use in the environmental assessment of biofuels (Kaltschmitt et al., 1997; Kim and Dale, 2005; von Blottnitz and Curran, 2007; Harding et al., 2008; Evans et al., 2009). In conducting the LCA, setting the goal and scope of the study allows for selecting a functional unit for comparison and setting the system boundaries. A full inventory of the process flowsheet is required, including all raw materials and energy, and all emissions and products generated. Data are preferably obtained from operating plants; where this is not feasible in new process development, data are obtained experimentally, from the literature or through modeling, and validated through material and energy balancing. Typically, a cradle-to-gate approach is used where the products formed are the same. Where the products formed differ from the existing product and result in different emissions and by-products on use, a cradle — to-grave approach is needed to consider product use and disposal. In both cases, the raw material and energy requirements are expanded to include their pre-processing, taking into account extraction from abiotic reserves, cultivation, agricultural pro­cesses, etc. Typically, the impact of construction of the process plant and equipment is negligible with respect to the impact of the operating plant. In new technology environments, this should be verified. This has been demonstrated for algal bio­diesel in all categories except land use (Lardon et al., 2009). Where reactors having a short life span are used (e. g., polyethylene bags or PVC linings), these need to be included in the analysis. For multiple products or by-products, as in the biorefinery, environmental burden allocation or substitution is required to allocate the overall burden representatively across the products formed. Burden allocation may be done based on the mass or volume ratio of useful products or, in some cases, based on cost. According to ISO (International Organization for Standardization) guidelines, substitution is preferred where possible; that is, the additional product or by-product is accounted for through the inventory typical of its conventional process route. This handling of multiple products is important as typically the production of multiple biofuels has been shown to increase the material and energy efficiency and process economics of biomass utilization (Kaparaju et al., 2009).

Life cycle inventory (LCI) data are used in life cycle impact assessments (LCIAs), typically using appropriate software to group the impacts into a manageable set of impact categories (mid-point categories), such as abiotic depletion, global warming, eutrophication, acidification, toxicity, etc. These may be further grouped into end­point categories, such as human health, climate change, and ecosystem quality, where appropriate.

The importance of the holistic study, considering all aspects of resource utilization and emission generation, is demonstrated through early-stage biofuel analyses where the carbon benefits of land use were counted for first-generation biofuels; however, the emissions caused by clearing of the land to grow new feedstock (land-use change) were not estimated (Searchinger, 2008). Fargione (2008) determined that the greenhouse gases (GHGs) released from changing natural habitats to biofuel cropland were several-fold greater than the offset from displacing fossil fuels, and hence a “carbon payback time” was defined to determine the time required before a true reduction in GHG resulted. This example drives home the need for an integrated assessment of environmental impacts.