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Overview of thermal technologies for waste to energy applications
Sonja Boshoff
Bioprocess Engineering Research Group, University of Stellenbosch
7 April 2016
Introduction
Background to thermal technologies
Waste streams
Thermal technologies
Combustion
Gasification
Pyrolysis
Conclusions
2
Content
Thermal technologies
4
Conversion process occurring at relatively high temperatures causing modifications in the chemical
structure of the processed material.
Combustion Pyrolysis
Gasification
5
Waste
Pre-treatment, transport and storage
Conversion to secondary energy carrier
Combustion
Thermochemical processing
Pyrolysis Gasification syngas
Gaseous fuel
Thermal energy
Liquid fuel
pyrolysis oil
Figure 1: Main thermal waste-to-energy conversion technologies (simplified from Kaltschmitt and Reinhart, 1997).
Why thermal technologies?
6
Robust, efficiently convert range of feedstocks Addresses seasonal/regional variability issues. Utilizes entire waste feedstock. Reduce mass 70-80% and volume 80-90%.
Thermal conversion provides for a range of fuel opportunities Ethanol, mixed alcohols, oxygenates Hydrocarbons including gasoline, diesel Syngas
Waste management hierarchy
7
Figure 1: Waste management hierarchy as part of The National Waste Management Strategy (NWMS) (National Environmental Management: Waste Act, 2008).
MSW Municipal solid waste/commercial waste IW Industrial Waste IHW Industrial Hazardous Waste
Waste streams
8
RDF
Comparison of technologies
10
Combustion Gasification Pyrolysis
Temperature (°C) 800 – 1450 500 – 1800 250 - 900
Pressure (bar) 1 1 - 45 1
Atmosphere Air O2, H20 Inert, N2
Stoichiometric ratio >1 <1 0
Products:
Gas phase CO2, H2O, O2, N2 H2, CO, CO2, CH4, H2O, N2
H2, CO, H20, N2, HC
Solid phase Ash, slag Ash, slag Ash, coke
Liquid phase Pyrolysis oil, water
Conventional reactors
11
Combustion Gasification Pyrolysis
Grate Fixed bed Fixed bed
Mobile Updraft Fluidised bed
Fixed Downdraft Bubbling/Stationary
Rotary kilns Crossdraft Re-circulating
Counter current Fluidised bed Moving bed
Co-current Stationary/Bubbling Entrained flow
Fluidised beds Circulating Rotary kiln
Stationary/Bubbling Cyclone Ablative
Circulating Entrained flow
Rotating
Combustion
13
Combustion
Grate
Mobile
Fixed
Rotary kilns
Counter current
Co-current
Fluidised beds
Stationary/Bubbling
Circulating
Rotating
Oxidation of combustible materials in the waste
Combustion: considerations
14
Combustion
Grate
Mobile
Fixed
Rotary kilns
Counter current
Co-current
Fluidised beds
Stationary/Bubbling
Circulating
Rotating
Air Stoichiometric excess (1.2 - 2.5). High production of flue gas (4-10 Nm3/kg).
Waste Bottom ash = 10-50 wt.% of waste input. Fly ash = 1-5 wt.%
Emissions Severe environmental pollution can result. Flue gas cleaning contribute 15-35% of TCI. Stages to comply with legislation.
Grate combustion reactors
15
Combustion
Grate
Mobile
Fixed
Rotary kilns
Counter current
Co-current
Fluidised beds
Stationary/Bubbling
Circulating
Rotating
Simplistic design, robust, low maintenance Reactor most often used. Different size wastes. Treatment capacity = 120 MW. High capacity reduces specific cost/ton WASTES: MSW, non hazardous wastes, sewage sludge, medical wastes. DISADVANTAGES: Not suited for powders, liquid wastes. Moving: higher complexity, maintenance. Fixed: some require support fuel.
Rotary kiln combustion reactors
16
Combustion
Grate
Mobile
Fixed
Rotary kilns
Counter current
Co-current
Fluidised beds
Stationary/Bubbling
Circulating
Rotating
Cylindrical vessel slightly inclined on its horizontal axis, robust design. Internally lined with refractory material -withstand higher incineration temperatures. Treatment capacity = 30 MW. WASTES: Industrial hazardous waste, medical waste, liquids, gaseous, sludge.
DISADVANTAGES: Significantly lower throughput. Accepted solid waste streams more limited.
Fluidised bed combustion reactors
17
Combustion
Grate
Mobile
Fixed
Rotary kilns
Counter current
Co-current
Fluidised beds
Stationary/Bubbling
Circulating
Rotating
Vertical cylinder, lined combustion chamber with fluidized bed of inert material. Fine, homogenous waste. Fuel preparation is required. Stable operation. Treatment capacity = 90 MW. WASTES: Sludge, RDF, lignocellulosic waste biomass.
DISADVANTAGES: Higher flying ash quantities. Stationary: careful operation required. Circulating: cyclone required to conserve bed material.
Waste combustion scenarios
18
Heat and/ or Power
Steam Examples
Heat generation Saturated -Co-combustion with coal. -Combustion with highly specialised technologies.
Power generation Superheated -Combustion in dedicated boiler with condensing turbine.
Combined Heat and Power
Superheated -Combustion in dedicated boiler with back pressure/extraction condensing turbine.
Indaver Doel: details
20
3 Grate incinerator lines:
Non recyclable MSW and commercial waste @400 000 tpa.
Fluidized bed incinerator
Non recyclable MSW and industrial sludge @600 000 tpa.
Electricity to 170 000 households and 30tph steam to a neighbouring company Ineos that produces chemicals.
Gasification
22
Partial oxidation of waste into a combustible gas mixture at high
temperature.
Gasification
Fixed bed
Updraft
Downdraft
Crossdraft
Fluidised bed
Stationary/Bubbling
Circulating
Cyclone
Entrained flow
Gasification: considerations
23
Gasification
Fixed bed
Updraft
Downdraft
Crossdraft
Fluidised bed
Stationary/Bubbling
Circulating
Cyclone
Entrained flow
Atmosphere Oxidant lower than stoichiometry (<1). Air, Oxygen, Steam.
Syngas Cleaner fuel. Contains char- requires cleaning. Flexible fuel. Preparation of waste often required.
Fixed bed gasifiers
24
Gasification
Fixed bed
Updraft
Downdraft
Crossdraft
Fluidised bed
Stationary/Bubbling
Circulating
Cyclone
Entrained flow
Different reactor zones. Simpler, less expensive. Syngas with lower heating value. WASTES: paper industry wastes, packaging wastes, MSW.
Fluidised bed gasifiers
25
Gasification
Fixed bed
Updraft
Downdraft
Crossdraft
Fluidised bed
Stationary/Bubbling
Circulating
Cyclone
Entrained flow
No reactor zones due to moving bed. More complicate, more expensive. Syngas with higher LHV. WASTES: RDF/SDF, sewage sludge, packaging fuel, ASR, hazardous wastes.
Gasifiers
26
Gasification
Fixed bed
Updraft
Downdraft
Crossdraft
Fluidised bed
Stationary/Bubbling
Circulating
Cyclone
Entrained flow
DISADVANTAGES: High operational and maintenance costs (fluidised bed-lower than other gasifiers) High skill level required. Less widely proven. Limited waste feed accepted. Pre-treatment of waste is costly.
Pyrolysis
30
Pyrolysis
Fixed bed
Fluidised bed
Bubbling/Stationary
Re-circulating
Moving bed
Entrained flow
Rotary kiln
Ablative
Breakdown of organics at lower temperature, in the absence of oxygen.
Pyrolysis: considerations
31
Process Absence of oxidising agent. Slow, vacuum and fast. Complex. Products Liquid Gas Char
Preparation of waste often required Drying Size reduction
Pyrolysis
Fixed bed
Fluidised bed
Bubbling/Stationary
Re-circulating
Moving bed
Entrained flow
Rotary kiln
Ablative
Fluidized bed pyrolysis reactors
32
Simple reactor configuration. High heat transfer rates. Very good solids mixing. Good gas to solids contact. WASTES: RDF/SDF, high metal inert streams, shredder residues, plastics.
Pyrolysis
Fixed bed
Fluidised bed
Bubbling/Stationary
Re-circulating
Moving bed
Entrained flow
Rotary kiln
Ablative
Rotary kiln pyrolysis reactors
33
Low heat transfer rates Particle size limit < 2 mm Limited gas/solid mixing WASTES: RDF/SRF, plastics, paper residues.
Pyrolysis
Fixed bed
Fluidised bed
Bubbling/Stationary
Re-circulating
Moving bed
Entrained flow
Rotary kiln
Ablative
Pyrolysis reactors
34
Pyrolysis
Fixed bed
Fluidised bed
Bubbling/Stationary
Re-circulating
Moving bed
Entrained flow
Rotary kiln
Ablative
DISADVANTAGES: Lower throughputs as combustion Process control and engineering is critical High skill level required Not widely proven Limited waste feed accepted High pre-treatment, operation and capital cost. End-use/market required for products
36
RWE Contherm: details
Two rotary kiln lines @ 50 000 tpa each Produces coke, gas and metallic stream Wastes
waste paper, paper production residues packaging wastes mechanical/biological domestic refuse high caloric industrial wastes
Pyrolysed substitute replaces up to 10% of coal used at 800 MW power plant Fuel reduction 0.5-1.0 ton coal per ton RDF
Conclusions
38
Thermal processing of waste
part of a sustainable waste management system.
Combustion main thermal technology for waste to energy.
Gasification and pyrolysis
advantageous, but often more costly, complicated
processes
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