SME
Environmental Engineering · Problem type
Heating value of mixed waste
Use this for “Waste is 40% paper at 16 MJ/kg, 15% plastic at 33 MJ/kg, 25% food waste at 5 MJ/kg and 20% inert. Find its heating value.”
Chart
Paper: 6.4 MJ/kg
Plastic: 4.95 MJ/kg
Food: 1.25 MJ/kg
This is an example. Type your own values to start a new problem.
HV (MJ/kg)
=
wₚ (%)
×
HVₚ (MJ/kg)
+
wₚₗ (%)
×
HVₚₗ (MJ/kg)
+
wf (%)
×
HVf (MJ/kg)
Each w is that part’s share of the waste by mass and each HV its heating value: p for paper, pl for plastic, f for food.
Eₚ
Paper energy
calculated · MJ/kg
Eₚₗ
Plastic energy
calculated · MJ/kg
Ef
Food energy
calculated · MJ/kg
Formulas
Eₚ
=
wₚ
×
HVₚ
100
6.4
=
40
×
16
100
Eₚₗ
=
wₚₗ
×
HVₚₗ
100
4.95
=
15
×
33
100
Ef
=
wf
×
HVf
100
1.25
=
25
×
5
100
HV = Eₚ + Eₚₗ + Ef
12.6 = 6.4 + 4.95 + 1.25
Assumptions
•
Shares are by mass of the waste as fired; the rest of the mix is inert and gives no energy.
•
Heating values are as-fired, moisture included, and the same basis for every part.
•
Each part burns as it would alone, so the energies of the parts add.
Step-by-step
WE KNOW
wₚ = 40%, wₚₗ = 15%, wf = 25%, HVₚ = 16 MJ/kg, HVₚₗ = 33 MJ/kg, HVf = 5 MJ/kg
FIND
paper energy (Eₚ), plastic energy (Eₚₗ), food energy (Ef), heating value of the mix (HV)
Step 1 · Find paper energy (Eₚ)
Use Eₚ = wₚ × HVₚ ÷ 100
The paper in each kg of the mix times the energy in a kg of paper.
Eₚ
=
40
×
16
100
Eₚ
=
640
100
Eₚ = 6.4 MJ/kg
Step 2 · Find plastic energy (Eₚₗ)
Use Eₚₗ = wₚₗ × HVₚₗ ÷ 100
The plastic in each kg of the mix times the energy in a kg of plastic.
Eₚₗ
=
15
×
33
100
Eₚₗ
=
495
100
Eₚₗ = 4.95 MJ/kg
Step 3 · Find food energy (Ef)
Use Ef = wf × HVf ÷ 100
The food waste in each kg of the mix times the energy in a kg of food waste.
Ef
=
25
×
5
100
Ef
=
125
100
Ef = 1.25 MJ/kg
Step 4 · Find heating value of the mix (HV)
Use HV = Eₚ + Eₚₗ + Ef
The energies of the parts add to the energy in a kg of the mix.
HV = 6.4 + 4.95 + 1.25
HV = 12.6 MJ/kg
Check
Put the numbers back in. Both sides should match.
6.4
=
40
×
16
100
✓
4.95
=
15
×
33
100
✓
1.25
=
25
×
5
100
✓
12.6 = 6.4 + 4.95 + 1.25 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.