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

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Content

Introduction

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Thermal technologies

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Conversion process occurring at relatively high temperatures causing modifications in the chemical

structure of the processed material.

Combustion Pyrolysis

Gasification

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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?

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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

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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

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RDF

Thermal technologies

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Comparison of technologies

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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

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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

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Combustion

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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

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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

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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

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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

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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

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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.

Case study: Indaver Doel

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Indaver Doel: details

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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

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Gasification

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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

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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

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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

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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

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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.

Case study: SVZ Schwarze Pumpe

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SVZ Schwarze Pumpe: details

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Pyrolysis

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Pyrolysis

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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

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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

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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

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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

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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

Case study: RWE Contherm

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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

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Conclusions

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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

Thank you.

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