Energy Content of a Refuse Mix: 18-Env-A6 Solid Waste Engineering and Management, December 2016, Q16

Question 16: Energy Content of a Refuse Mix (5 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.

Component	Mass fraction	Energy content (Table 1)
Paper	50%	16,300 kJ/kg
Metal, glass & ash (non-combustible)	20%	0 kJ/kg
Food & other organic wastes (balance)	30%	5,800 kJ/kg

Find. The weighted-average energy content of the refuse mix, in kJ/kg.

Approach. Take a mass-weighted average of each component's own Table 1 energy content, treating the metal/glass/ash fraction as non-combustible (zero energy contribution) and the balance of the mix (100% − 50% − 20% = 30%) as food/organic waste at Table 1's per-unit-weight-of-organics value.

Establish the balance fraction. Food & other organics $= 100\% - 50\% - 20\% = 30\%$.
Weight each component's energy content by its mass fraction. Paper: $0.50 \times 16{,}300 = 8{,}150\ \text{kJ/kg}$. Metal/glass/ash: $0.20 \times 0 = 0\ \text{kJ/kg}$. Organics: $0.30 \times 5{,}800 = 1{,}740\ \text{kJ/kg}$.
Sum the weighted contributions. $E_{mix} = 8{,}150 + 0 + 1{,}740 = \boxed{9{,}890\ \text{kJ/kg}}$.

A literal reading of $5.800\ \text{kJ/kg}$ would be physically absurd for a combustible organic material (below the energy content of the metal/glass/ash fraction it is meant to exceed), so the thousands reading is used throughout.

Quantity	Result
Energy content of the refuse mix	9,890 kJ/kg

Question 16 of 20: Energy Content of a Refuse Mix

Question 16: Energy Content of a Refuse Mix (5 marks)