23-Chem-B10 Life Cycle Assessment (LCA) · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B10, Life Cycle Assessment (LCA) — December 2013. 3 hours, Closed-Book Exam (Casio/Sharp approved calculator and one double-sided aid sheet permitted). Question 1 is mandatory; any three (3) of the remaining four (Questions 2–5) constitute a complete 100-mark paper, and only the first four questions as they appear in the answer book are marked. All five questions (and, in Question 5, all five sub-parts) are solved below for completeness.
Reference texts: Baumann & Tillman, The Hitch Hiker's Guide to LCA; Graedel & Allenby, Industrial Ecology and Sustainable Engineering; Kemp, Pinch Analysis and Process Integration, 2nd ed.; Mackay, Multimedia Environmental Models: The Fugacity Approach, 2nd ed.; Davis & Cornwell, Introduction to Environmental Engineering.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A Life Cycle Assessment is only as meaningful as the boundary drawn around the system being studied and the basis chosen for comparing alternatives. The system boundary defines which life-cycle stages, unit processes, and even geographic/temporal scope are included in the inventory — common choices are cradle-to-gate (raw material extraction through the factory gate), gate-to-gate (a single process step), and cradle-to-grave (extraction through end-of-life disposal or recycling). The functional unit is the quantified performance basis on which competing systems are compared (e.g., "delivering 1000 L of a beverage to the consumer" rather than "one bottle"), and it must be chosen so that it fairly represents the service each alternative actually provides, not merely a convenient physical unit of one of them.
Worked example — reusable glass vs. single-use PET beverage bottles. If the system boundary is drawn narrowly as gate-to-gate manufacturing only, and the functional unit is chosen naively as "one bottle produced," glass loses badly: a glass bottle requires roughly 5–10 times the raw material mass and furnace energy of an equivalent PET bottle, and is heavier to transport, so per unit produced its embodied energy and transport-fuel burden are far larger. A comparative LCA stopped there would recommend PET. But if the boundary is widened to cradle-to-grave and the functional unit is corrected to the true service being delivered — "1000 L of beverage delivered to the consumer over the product's useful life" — the picture reverses: a returnable glass bottle in a deposit-return system is typically reused 20–40 times before being recycled, so its large per-bottle production burden is amortized over many trips, while every PET bottle is manufactured once, used once, and then enters an energy-intensive recycling stream (or, worse, landfill/incineration) after a single trip. Depending on the assumed number of reuse trips and the local energy mix used for washing/refilling versus PET recycling, the cradle-to-grave, correctly-scaled comparison can show the glass system with a lower total energy and GHG burden per 1000 L delivered — the exact opposite conclusion from the narrow-boundary study. This illustrates why boundary and functional-unit selection are not bookkeeping details: they can determine which product "wins" a comparative LCA, and a report that omits or buries these choices is not defensible engineering judgement.
The general phases associated with a product's development and distribution are: (1) raw material acquisition (extraction/harvesting of virgin or recycled feedstocks); (2) materials processing (refining/converting feedstocks into engineering materials, e.g. resin pellets, glass cullet, sheet metal); (3) manufacturing/assembly of the product itself; (4) distribution/transportation to the point of sale or use; (5) use phase (operation, maintenance, consumption by the end user); and (6) end-of-life management (reuse, recycling, energy recovery, or landfill disposal).
Continuing the beverage-bottle example, consider the end-of-life phase in a comprehensive LCA. Inputs to be investigated include the energy and water needed to collect, sort, and either wash-and-refill (glass) or shred/reprocess (PET) the used containers, along with any make-up virgin material needed to replace losses in the recycling loop. Outputs to be investigated include air emissions from the collection-truck fleet and from reprocessing furnaces/extruders, wastewater from bottle-washing (glass) or the PET wash/decontamination line, solid residues that cannot be recycled (broken glass fines, contaminated PET flake) that are diverted to landfill, and the avoided burden credited for material displaced from virgin production (a common and consequential LCA modelling choice, since the size of this credit strongly affects which system appears superior).
Working from the end-of-life inventory built in (b), the Life-Cycle Impact Assessment proceeds in three steps. Classification sorts each inventory flow into the impact category (or categories) it contributes to: CO2 and CH4 from collection-truck fuel and reprocessing-furnace combustion are classified under global warming; NOx and SOx from the same combustion sources are classified under acidification and respiratory/particulate impact; wash-water effluent (BOD, suspended solids) is classified under eutrophication/aquatic toxicity; and landfilled glass fines/PET residue are classified under resource depletion/land use. Characterization converts each classified flow into a common unit within its category using established equivalency factors — e.g. CH4 is multiplied by its 100-year Global Warming Potential (≈25× CO2) to produce a single "kg CO2-equivalent" score per category, and NOx/SOx are converted to "kg SO2-equivalent" acidification potential. Evaluation (normalization and weighting) then combines the several characterized category scores into an overall comparative judgement, e.g. normalizing each category against a regional per-capita reference load and applying stakeholder-agreed weights to global warming vs. acidification vs. resource depletion, so that a single relative ranking of "glass system" vs. "PET system" per 1000 L delivered can be reported.
Assumptions to state explicitly: a 100-year GWP time horizon (rather than 20-year, which weights CH4 far more heavily); an average (not marginal) electricity-grid emission factor for the washing/reprocessing energy; a specific assumed number of reuse trips for the glass system (the single most sensitive parameter in this comparison); and equal severity weighting between categories unless a specific regional priority (e.g. local air-quality non-attainment) justifies otherwise.
| Compound | Primary environmental concern |
|---|---|
| CO2 | Principal anthropogenic greenhouse gas; long atmospheric residence time (centuries) drives global climate change via radiative forcing. Used as the reference (GWP = 1) for all other greenhouse gases. |
| NOx (NO, NO2) | Precursor to ground-level (tropospheric) ozone and photochemical smog via reaction with VOCs in sunlight; contributes to acid rain (nitric acid formation) and is itself a respiratory irritant at ambient concentrations. |
| SOx (mainly SO2) | Oxidizes in the atmosphere to sulfuric acid, the dominant historical cause of acid rain/acid deposition (soil and surface-water acidification, forest and aquatic ecosystem damage); also a direct respiratory irritant and fine-particulate (sulfate aerosol) precursor. |
| VOC's | Precursor (with NOx) to photochemical smog and ground-level ozone; many individual VOC species (e.g. benzene, formaldehyde) are additionally toxic or carcinogenic in their own right, so VOC control addresses both a regional smog concern and, for specific compounds, a direct human-health concern. |
