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18-Env-A6 Solid Waste Engineering and Management · May 2017

Question 14 of 16: Energy Content of Refuse

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Examination, May 2017 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, non-communicating calculator permitted. All 16 questions constitute a complete paper (100 marks total).

Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management; ISO 14040/14044, Environmental Management — Life Cycle Assessment.

Q6 below is solved from the six printed per-component rows, which are unambiguous exam-given data — see the callout at Q6 for the arithmetic. Table 2's "5.800 kJ/kg" organics value (period instead of comma) is read as 5,800 kJ/kg.

Question 14: Energy Content of Refuse (7 marks)

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.

Given. A refuse mixture: 50% paper (16,300 kJ/kg), 20% metal + glass + ash combined, and the balance (30%) food and other organic wastes (5,800 kJ/kg).

Find. The overall (weighted-average) energy content of the mixture, kJ/kg.

Approach. Mass-weighted average of each fraction's typical energy content, treating the metal/glass/ash fraction as essentially non-combustible.

Check: the table prints "5.800" (period, not comma) for organics; it is read as 5,800 kJ/kg, matching the comma-separated convention used for every other value in the table. Metal/glass/ash are taken as ≈0 kJ/kg (standard simplification — these materials are essentially inert relative to paper or food waste).

  1. Weight each fraction by its typical energy content. $$E = 0.50(16{,}300) + 0.20(0) + 0.30(5{,}800)\ \text{kJ/kg}$$ $$E = 8{,}150 + 0 + 1{,}740 = \boxed{9{,}890\ \text{kJ/kg}}$$

This lands close to the table's own "refuse as collected" figure of 10,500 kJ/kg, a useful sanity check on the composition assumed. The result is also a useful design check for a waste-to-energy facility evaluating this same feedstock: if the boiler's design fuel value assumes a higher-paper composition than 50%, actual delivered refuse at this composition would under-perform the design heat rate, so a facility developer should confirm the assumed feedstock composition against actual local waste-characterization data before committing to a specific boiler thermal rating.

QuantityValue
Energy content of refuse9,890 kJ/kg