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Technological Options for Energy Recovery from Waste

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Basic Techniques of Energy Recovery from Waste

Energy can be recovered from the organic fraction of waste (biodegradable as well as non-biodegradable) through thermal, thermo-chemical and biochemical methods.

A brief description of the commonly applied technologies for energy generation from waste is as follows

Anaerobic Digestion/Biomethanation

In this process, the organic fraction of the waste is segregated and fed into a closed container (biogas digester). In the digester, the segregated waste undergoes biodegradation in presence of methanogenic bacteria and under anaerobic conditions, producing methane-rich biogas and effluent. The biogas can be used either for cooking/heating applications, or for generating motive power or electricity through dual-fuel or gas engines, low-pressure gas turbines, or steam turbines. The sludge from anaerobic digestion, after stabilization, can be used as a soil conditioner. It can even be sold as manure depending upon its composition, which is determined mainly by the composition of the input waste.

Combustion/Incineration

In this process, wastes are directly burned in presence of excess air (oxygen) at high temperatures (about 800°C), liberating heat energy, inert gases, and ash. Combustion results in transfer of 65%–80% of heat content of the organic matter to hot air, steam, and hot water. The steam generated, in turn, can be used in steam turbines to generate power.

Pyrolysis/Gasification

Pyrolysis is a process of chemical decomposition of organic matter brought about by heat. In this process, the organic material is heated in absence of air until the molecules thermally break down to become a gas comprising smaller molecules (known collectively as syngas).

Gasification can also take place as a result of partial combustion of organic matter in presence of a restricted quantity of oxygen or air. The gas so produced is known as producer gas. The gases produced by pyrolysis mainly comprise carbon monoxide (25%), hydrogen and hydrocarbons (15%), and carbon dioxide and nitrogen (60%). The next step is to ‘clean’ the syngas or producer gas. Thereafter, the gas is burned in internal combustion (IC) engine generator sets or turbines to produce electricity.

Landfill Gas recovery

The waste dumped in a landfill becomes subjected, over a period of time, to anaerobic conditions. As a result, its organic fraction slowly volatilizes and decomposes, leading to production of ‘landfill gas’, which contains a high percentage of methane (about 50%). It can be used as a source of energy either for direct heating/cooking applications or to generate power through IC engines or turbines.

 

Analysis of Key Aspects of Waste to Energy Technologies

 

Criteria

Incineration

Anaerobic Digestion

Gasification/Pyrolysis

A

Feedstock

 

Industrial

 

Liquid

Not suitable

Suitable

Not suitable

 

Solid

Suitable

Not suitable

Suitable

          Urban

 

Liquid

Not suitable

Suitable

Not suitable

 

Solid

Suitable

Suitable

Suitable

         Farm

 

Poultry

Suitable

Suitable

Suitable

 

Cattle

Suitable

Suitable

Suitable

B

Technology features

 

Technology status

 

 

 

 

Industrial

Proven

Proven

Emerging

 

Urban

Proven

Proven

Emerging

 

Farm

Proven

Proven

Proven

 

Energy efficiency

85-90% (Based on calorific value)

50-60% (Based on volatiles)

90-95% (Based on calorific value)

C

Operating conditions

 

System configuration

Complex

Simple

Complex

 

Process Flexibility

Low

Good

Low

 

Modular

Yes

Yes

Yes

D

Capital, O & M costs

 

Relative capital cost

Very high

Medium-high

Very High

 

O & M

High

Low

Limited

 

Commercial viability

Less viable than others owing to costly downstream air pollution control

Readily viable

Varies considerably

 

Captive power requirements

Significant (25-30%)

Low (5%)

Variable (5-20%)

 

Area requirements

Elaborate

Compact

Compact

E

Environmental impacts

Can be minimized, but requires expensive technology investments

Minimum

Can be controlled to a significant extent

F

Socio-economic impacts

 

Public acceptability

Not fully satisfactory

Satisfactory

Satisfactory

 

Waste disposal

Complete, except for ash to landfill.

Complete except for

sludge stabilization

Complete, except for ash

Source: MNRE (http://mnre.gov.in/nmp/technology-we.pdf), with additional research from EAI

 

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India Waste to Energy