18-Env-A6 Solid Waste Engineering and Management · May 2013

Question 13 of 17: Landfill Gas for Home Heating

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

Notes on this paper

National Examination, May 2013 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, one letter-sized aid sheet permitted. All 17 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.

Check: several questions on this paper (Q1, Q13, Q14, Q17) supply an incomplete data set and explicitly invite the candidate to "make and state" assumptions. Every assumed value below is called out where it is introduced and is chosen from standard solid-waste-engineering practice; the governing METHOD, not the specific assumed number, is what the exam is testing.

Question 13: Landfill Gas for Home Heating (8 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. Population = 100,000; landfill has received waste for 3 years and gas recovery is steady/ongoing; landfill gas is 55% CH₄; 50 homes, each averaging 100×10⁶ kJ/yr, with a coldest-month peak of 2.5× the annual average.

Find. Whether the landfill gas supply can meet the homes' peak (coldest-month) heat demand.

Steady-state mass/energy balance: accumulated MSW generates landfill gas, whose methane fraction supplies recoverable heat.

Approach. Estimate the total waste in place, apply an assumed specific landfill-gas yield to get the average gas (and CH₄) production rate, convert to available heat via the methane heating value, and compare against the homes' peak heat demand.

Check: no waste-generation rate or landfill-gas yield is given in this question. The waste-generation rate is taken from this paper's own Table 2 (Q16) — 2.9 kg/capita/day, "Total MUNICIPAL" — since it is the only Canadian-typical figure the exam itself supplies. The specific LFG yield (10 m³ gas/tonne of waste in place per year) and the CH₄ heating value (37,300 kJ/m³, reusing the paper's own natural-gas figure from Q17, since natural gas is predominantly methane) are standard assumptions, checked for sensitivity below.

Total waste placed in 3 years. $$m = 100{,}000 \times 2.9\ \text{kg/cap/d} \times 365\ \text{d/yr} \times 3\ \text{yr} = 317{,}550\ \text{t}$$
Average landfill gas and methane production rate. Assuming a steady specific yield of 10 m³ LFG/tonne/yr: $$Q_{gas} = \dfrac{317{,}550 \times 10}{365} = 8{,}700\ \text{m}^3/\text{d}, \qquad Q_{CH_4} = 0.55 \times 8{,}700 = 4{,}785\ \text{m}^3/\text{d}$$
Available heat energy. $$\dot{E}_{avail} = 4{,}785\ \text{m}^3/\text{d} \times 37{,}300\ \text{kJ/m}^3 = \boxed{1.785\times10^{8}\ \text{kJ/d}}$$
Peak (coldest-month) heat demand of the 50 homes. $$\dot{E}_{peak} = \dfrac{50 \times 100\times10^{6}\ \text{kJ/yr}}{365\ \text{d/yr}} \times 2.5 = \boxed{3.42\times10^{7}\ \text{kJ/d}}$$

The average available landfill gas heat energy (1.785×10⁸ kJ/d) exceeds the peak home-heating demand (3.42×10⁷ kJ/d) by a factor of about 5.2 — yes, there is enough landfill gas available, with substantial margin. The breakeven LFG yield (the minimum specific yield that would just meet peak demand) is only about 1.9 m³/tonne/yr — well below the 10 m³/tonne/yr assumed and below typical published ranges for actively decomposing MSW — so the conclusion is robust even if the assumed yield is significantly too high.

Quantity	Value
Total waste in place (3 yr)	317,550 t
Average available heat from LFG	1.785×10⁸ kJ/d
Peak (coldest-month) home heat demand	3.42×10⁷ kJ/d
Margin at peak demand	≈5.2×
Conclusion	Sufficient landfill gas is available

Topic: Landfill gas utilization for beneficial energy use.

Key relations: $\dot{E} = Q_{gas}\,f_{CH_4}\,\text{HHV}_{CH_4}$; compare against peak, not average, demand for a firm-supply design check.

Why this works: Landfill gas generation is relatively steady year-round (driven by microbial decomposition, not weather), while heat demand is strongly seasonal — the binding constraint for "will it be enough" is always the coldest-month peak, not the annual average.

Common pitfall: comparing average gas supply to average heat demand and concluding "sufficient" while missing that the coldest month alone could exceed supply if the margin were smaller than found here.

Source: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management, Ch. 13 (LFG generation and recovery); CCME, Guidance Document on Landfill Gas Management.

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