Thermal-treatment-of-solid-waste.pdf

Thermal Treatment of Solid Waste Engr. Mary Jane C. Calagui, PhD

Terminology Thermal treatment (or incineration)- a range of processes where temperature is used to reduce the volume of waste and to render it harmless. • Waste to Energy (WTE): as above, with the recovery of heat energy to produce steam and/or generate electricity. • Conventional WTE: mass burn, fluidized bed, modular, rotary kiln, (refuse derived fuel) • Advanced WTE: gasification, pyrolysis, plasma

The role of thermal treatment • Waste volume reduction, preservation of landfill space – Does NOT replace the need for a landfill • Energy recovery from the solid waste stream • Destruction of contaminants • Reducing waste transportation requirements • Dealing with waste here and now

Large and Small-scale Incineration Combustion Requirement: 3T’s of combustion Chemical properties of SW: -Ultimate Analysis Proximate Analysis Energy content- bomb calorimeter Fusing point of ash

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Combustion Theory BASIC PRINCIPLE FOR MASS BALANCE

Waste (C, H,O, S, N, Inert) Air (O2, N2)

Incinerator C + O2 → CO2 H2 + 1/2 O2 → H2O S + O2 → SO2 2 O2 (fuel) combines with O2 inNair N2 (fuel) combines with N2 in air

Flue gas CO2 H 2O SO2 Excess O2 N2

Inert

BASIC PRINCIPLE FOR ENERGY BALANCE Incinerator Waste CV Heated Air, Tin

Enthalpy of reactions Heat loss by radiation to the walls of the chamber Latent heats of evaporation of H2O Heat loss in unburned carbon Sensible heat in residue

Flue gas, Tflue

Inert, Tinert

Is Incineration Sustainable? Large-scale incineration

Small-scale incineration

Source: WASTE INCINERATION (2010)11 and AFRICAN DEVELOPMENT BANK (2002)

Is Incineration Sustainable? • In the hierarchy of integrated solid waste management, incineration is not considered as a very sustainable method. • With a highly developed system, energy recovery is possible. • The last desirable is landfilling (leaching, smouldering). • Open burning of waste must be avoided. Source: WASTE INCINERATION (2010)12 and AFRICAN DEVELOPMENT BANK (2002)

2. How it can optimise SSWM

Negative Effects of Open Burning (and Landfills)

Source: GREENPEACE (2008)

Heavy air pollution due to open fires or smouldering.

•Leachate leads to 13 groundwater contamination.

2. How it can optimise SSWM

Heavy air pollution due to open fires or smouldering.

Direct inhalation of toxic substances by the local community, dumpor landfill workers.

Source: GREENPEACE (2008)

Accumulation of toxic substances along the food chain.

Source: KLOHN and FROEHLICH (2011)

•Leachate leads to 14 groundwater contamination.

Source: GREENPEACE (n.y.)

Negative Effects of Open Burning (and Landfills)

2. How it can optimise SSWM

Controlled Incineration Large-scale incinerators

Source: GREENPEACE (2008)

Small-scale incinerators

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2. How it can optimise SSWM

Controlled Incineration Large-scale incinerators

Source: GREENPEACE (2008)

Benefits of incineration • No landfills necessary • No leachate • No open fires • Less air pollution • Disinfection (i.e. of medical waste) • Break down of some hazardous chemicals Small-scale incinerators

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2. How it can optimise SSWM

Controlled Incineration Large-scale incinerators BUT • •

Toxic flue gases Residual ash still has to be disposed of safely

Source: GREENPEACE (2008)

Small-scale incinerators

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3. Design Principles

Which Incineration Technique is Affordable? Large-scale incinerators

• • • • •

Big loads (50 - 1000 tons/day) Size of waste does not matter Waste-to-Energy Experts for O&M Expensive

Small-scale incinerators

•

Small loads (12 – 100 kg/hour)

•

Requires small-sized waste or it has to be shredded before

•

Can be built with local available material

•

Pre-fabricated products

• Trained labours for O&M 18

3. Design Principles (Large-scale)

Mass-burn Incinerator Holding area

1. 2. Grabbed and dropped into a hopper 3. Incinerator (approx. 800 ºC), 50 to 1000 tons per day 4. Waste-to-Energy system (boilersteamturbine) 5. Collection point for heavy ash, extraction of metals 6. Scrubber reactor for the extracton hazardous pollutants (e.g. SO2 and dioxins) 7. Fine particulate removal system 8. Chimney

to Nr. 6

from Nr. 5

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Source: BBC (2009)

3. Design Principle (Large-scale)

Fluidised-bed Incinerator • Bed of limestone or sand

• Circulating or bubbling technology • Capacity of 50 to 150 tons/day • Energy recovery system applicable

The scheme of a fluidised-bed incinerator 20 (bubbling bed). Source: EISENMANN (n.y.)

3. Design Principles (Large-scale)

Modular Incinerator • Prefabricated modules, in general 1 – 4 units • Capacity of 5 to 120 tons/day and unit • Energy recovery system applicable • Used in smaller communities • 1st chamber: low interior gas velocities under controlled temperature conditions • 2nd chamber: completes the oxidation reactions of the combustible products

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Source: UNEP (2005)

3. Design Principles (Large-scale)

Sludge Incineration • Reduces the volume of dried sludge • Destroys pathogens and toxic organic chemicals • Solution if there is no land available for sludge disposal

A possible design how sewage sludge incineration can be integrated in a MSW incineration plant. Source: PUTZMEISTER (2000) and WASTEWATER SYSTEMS (n.y.) 22

4. Treatment Efficiency (Large-scale)

Pollution Removal •

Weight reduction up to 75 %.

•

Volume reduction up to 90 %.

• Breaks down some hazardous, non-metallic organic wastes. •

Destroys bacteria and viruses.

• Efficient flue gas cleaning systems. Health Aspects and Pollution •

Complex air pollution control system.

•

Risk of environmental pollution or health risk from modern facilities are low.

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5. Operation and Maintenance (Large-scale) •

Maintenance and servicing by trained technicians.

•

When incineration is done in a manner that has low adverse health and environmental impacts it is expensive.

•

When it is done poorly (with low financial costs) it can be expensive in terms of human health and environmental impacts.

Control room of a waste incineration plant.

Source: MAUELL (n.y.)

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6. Applicability (Large-scale)

• Where land for landfilling is scarce • Where technical know-how is available • Where capital costs as well costs for O&M can be covered

Waste incineration plant in Switzerland.

Source: GEVAG (n.y.)

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7. Pros and Cons (Large-scale)

•Advantages:

•Disdvantages:

o No landfills required

o High investment, operation and maintenance costs

o Substantial reduction of the weight (up to 75%) and volume (up to 90%) of solid waste o Waste-to-energy (production of electricity and heat) o Disinfection o Some hazardous chemicals are destroyed o Some precious elements (e.g. metals, phosphorus may be recovered from the ashes)

o Risk of emissions which endanger human health and environment (gases, leaching into groundwater)

o Loss of organic substances such as kitchen waste or green waste from gardening (compared to composting)

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8. Design Principles (Small-scale)

Low-cost Medical Waste Incinerator • Simple two-chamber incinerator •

Temperatures of 800 ºC or higher

•

Capacity of 15 kg/hour

•

Locally constructed with bricks and steel components

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Source: PRACTICAL ACTION (2000)

8. Design Principles (Small-scale) •

Pre-fabricated Products Several types • In every location possible (e.g.

•

Capacity of 12 - 100 kg/hour

•

All kind of wastes (e.g. medical, slaughter, household waste)

small community, hospital, farm, truck stop) • Capacity of 12 - 100 kg/hour

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Source: INCINER8 (2004) and MAVI DENZ (n.y.)

9. Treatment Efficiency (Small-scale)

Pollution Removal

•

Significant weight reduction and volume reduction.

•

Breaks down some hazardous, non-metallic organic wastes.

•

Destroys bacteria and viruses.

•

Modern designs avoid toxic emissions.

Health Aspects and Pollution • Risk of environmental pollution or health risk from are much lower than open burning of waste. •

No more waste piles and backyard burning.

•

Operator should wear protection equipment during operation and maintenance. 29

10. Operation and Maintenance (Small-scale) •

Operator must be trained for the incinerator in use. This avoids accidents, injuries and damages.

•

Protection equipment during O&M is required: heat resistant gloves and boots, a respirator mask, safety goggles, clothes that cover the body and a helmet.

•

It is important that the incinerator reaches the optimal temperature for an optimal performance.

•

Regular inspection and to enlarge the life cycle and avoid damages are necessary.

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Source: PATH (2010)

11. Applicability (Small-scale)

• It is applicable in every location for almost all kinds of waste. • It especially avoids open burning, littering in the streets and helps to make harmful (medical) waste non-toxic.

Source: HEALING TALKS (2008)

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12. Pros and Cons (Small-scale)

Example of a comparison…

•Advantages:

•Disadvantages:

o No landfills required

o Risk of emissions which endanger human health and environment (flue gas)

o Substantial reduction of weight and volume of solid waste o Breaks down chemical toxics and destroys pathogens (e.g. medical waste) o There are many different products in all price ranges

o Loss of organic substances such as kitchen waste or green waste from gardening

o Risk of malfunction if operators are not instructed

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WTE Facility • • • • •

Starved air/pyrolysis Fluidised bed Multiple hearth Moving grate furnace Rotary kiln

1. Pyrolysis • No stoichiometric air for combustion is supplied • Heat dependant •Size reduction of waste required prior to incineration •Partial combustion generating gases that can be utilised as syn-gas (fuel) •Not preferred for MSW incineration (heterogeneous)

2. Starved air - 70-80% stoichiometric air - requires size reduction - off gas burnt in secondary chamber

3.

Fluidised Bed

• Uses high pressure and high velocity air through sand-bed • Waste requires segregation and size reduction • Small feed rate of waste to ensure combustion efficiency 4 • • • • •

Rotary Kiln Incinerator Rotary Drum lined with refractory Can incinerate a wide variety of waste Residence time can be controlled very easily Uniform contact of waste with air ensuring complete combustion Disadvantage – high particulate emission, air requirement and heat loss.

5. Multiple hearth • Use to burn sludges • Hearth at different level equipped with rotating shafts for combustion efficiency • High excess air required 75 – 125% 6. Moving grate • Waste move on sloping rocking grate in primary chamber • Direct fire beneath the grate ensuring high combustion efficiency • Flue gas is burnt in secondary chamber requiring high additional fuel

• High cost in design, installation, power requirement and maintenance