Plasma gasification
as a
viable waste-to-energy treatment of MSW
Larry Gray
MANE 6960 – Solid and Hazardous Waste Prevention and Control Engineering
Rensselaer Hartford
Hartford, CT, USA
April 24, 2014
Waste-to-Energy Processes
 Incineration
 Oxidizing reaction
 Temperatures 850°C - 1200°C
 Excess air for complete combustion
 CO2, H2O and heat
 Gasification [Pyrolysis]
 Reducing reaction
 Temperatures 400°C - 900°C
 Air < stoichiometric air
[Pyrolysis - thermal decomposition in absence of air]
 CO, CO2, H2 H2O CH4 and some heat
 Partial combustion provides heat to sustain process
 Plasma gasification
 Reducing reaction
 Temperatures 1500°C - 5000°C
 Air < stoichiometric air
 CO, CO2, H2 H2O CH4 and heat
 Requires electricity input ( 1200 – 1500 MJ / tonne of waste), 15% - 20% of gross output energy
2
Plasma Gasification Furnace
(source: Zhang et al., 2012)
3
(source: Zhang et al., 2013)
Plasma Gasification Process
 Plasma
 Heating a gas to very high
temperatures where molecules and
atoms ionize
 Thermally and electrically
conductive
 Plasma torches
 Electric arc
 Concentric flow of air from torches
to form plasma
 Secondary air fed into melting
chamber to control gasification
 Steam can be fed into furnace to
enhance syngas yield
4
Gasification Process
Gasification of MSW
𝐶𝐻𝑥 𝑂𝑦 + 𝑤H2O + 𝑚 𝑂2 + 3.76𝑁2
→ 𝑛1 𝐻 + 𝑛2 CO + 𝑛3 𝐶𝑂 + 𝑛4 𝐻2 O + 𝑛5 𝐶𝐻 + 𝑛6 𝑁2 + 𝑛7 𝐶
2
4
The Boudouard reaction:
𝐶 + 𝐶𝑂2 ↔ 2𝐶𝑂
The water – gas reaction:
𝐶 + 𝐻2 𝑂 ↔ 𝐶𝑂 + 𝐻2
The methanation reaction:
𝐶 + 2𝐻2
Water-Gas Shift:
Gasification enhanced with steam:
Solving for 7 unknowns:
(3) mass balance equations (C, H, O)
(3) equilibrium constant equations
(1) energy balance equation
5
2
↔ 𝐶𝐻4
𝐶𝑂 + 𝐻2 𝑂 ↔ 𝐶𝑂2 + 𝐻2
𝐶𝐻4 + 𝐻2 𝑂 ↔ 𝐶𝑂 + 3𝐻2
Plastics and Rubber are Best
Feedstock for Plasma Gasification
Ulitimate Composition - Dry Basis
Waste Category
heterogeneous MSW *
Paper
Wood
Food waste
Textiles
Plastic
Rubber
%C
%H
57.1
7.6
43
6.0
49.5
6.0
45.4
6.9
55
6.6
76.3 11.5
78
10
%O
33.3
43.8
42.7
32.2
31.2
4.4
0
%N
2
0.36
0.2
3.3
4.6
0.26
2
%S
0
0.17
0.1
0.32
0.15
0.2
0
HHV,
Ash
Moisture (% as received
(% weight)
weight)
(KJ/kg)
25
35%
24,198
6.3
24%
13,414
1.5
2%
16,715
11
65%
7,229
2.5
27%
16,049
5.3
13%
33,264
10
2%
31,285
* Original values in the GasifEq model representing heterogeneous MSW
6
Plastics / Rubber – highest efficiency
Cold Gas Efficiency = η = ṁ syngas* LHVsnygas / (ṁ waste * LHVwaste + PPlasma)
where ṁ
= mass flow rate of syngas and solid waste
Pplasma = electrical power for plasma torch
Cold Gas
Efficiency
7
Sensitivities
Change in Air Flow
Change in Temperature
Change in Moisture
8
Summary
 Lower emissions – less amount of air to clean
 Reduced volume of waste - 6% to 15% of original volume
 Very good means to disposing of hazardous and medical waste
 Vitrified slag is inert and could be used as filler material
 For best production of syngas and best efficiencies use MSW
feedstock of plastics, rubber
 Syngas produced can be used to serve a variety of energy needs
 Use heat from syngas for district heating
 Electrical power generation
 Fuel cells
 Make liquid fuels
9