Technology Part

1.  Is municipal solid waste (MSW) considered biomass?

A major constituent of MSW is biomass (domestic and market). It could also include waste such as plastic, footwear etc. Of the total organic wastes, these constitute a few percent of in towns (and villages) and less than 10 % in cities.

2.  What are the different WTE Technologies?
Biomethanation, Incineration and Gasification.

3.  What is the amount of MSW generated by the average Indian. citizen and how much of it is suitable for combustion?
According to the estimates (Technical Master Plan for Development of Waste-to-Energy in India. Technical Memorandum on Shelf of Viable Projects, August 2003, Montgomery Watson), the per capital generation is 0.3 - 0.4 kg (towns), 0.5 kg (cities) of which nearly 60 – 70% would be organic combustibles. 

Typical Indian MSW Composition

Description Percent by weight
Vegetable, leaves 40.15
Grass 3.80
Paper 0.81
Plastic 0.62
Glass / ceramics 0.44
Metal 0.64
Stones / ashes 41.81
Miscellaneous 11.73

4.  How much energy is contained in waste?
The energy content depends on the constituent of the waste. The energy density of waste generated in Indian towns and cities is about 15 - 16 MJ/kg of separated dry organic waste.

5.What are the aims and advantages of biomass gasification over other waste-to-energy technologies?

Merits compared to Bio-methanation and Land -Fill routes:

    Bio-methanation route is suitable only for bio-degradable such as vegetative matter with high moisture content and can’t accept non-bio-degradable such as plastic, foot wear etc.
    In biomethanation the organic matter is not fully converted, hence the power production per ton of waste is lower.
    Relatively large Land area for the land-fill would be required for dumping significant fraction of the unconsumed plastic and other material that are not fit for recycling. Whereas in gasification route the residue is ash with some carbon, that could be used as manure or if used as a land-fill calls for far less area of the land needed.
    Bio-methanation process being sensitive to the operating temperature the process affects quantity of gas production and to compensate for this, the project engineering cost would become further high. Whereas, gasification process being a thermo-chemical process does not face such limitation.

Merits compared to Combustion/incineration route:

    Gasification route is the lower cost option at power level up to and below 3.0 MWe capacity.
    Combustion route can accept chlorinated based polymers but requires expensive pollution control devices to keep Dioxins and Furans within the existing pollution control board norms. In well designed plants, the cost of clean up of the gases downstream of the boiler is comparable to the plant cost itself. Whereas in gasification, much of these emissions are separated within the gasification island, are concentrated to enable separate handling and therefore the technology is more benign and attractive compared to others. The cost of clean-up in the gasification system is about a third of the costs in the case of incineration even if gaseous stream is handled, since the volume throughput of the gases to be treated is about a third of the volume throughput in the case of incineration.
    Auxiliary fuel might be required to sustain the process in the boiler in combustion route, whereas gasification system can work on 100% RDF briquettes.
    The combustion route requires large quantity of water which is not the case so in the gasification route.
    In the combustion route there could be issue of fly ash and thereof associated treatment/handling system would be required in the event of RDF briquette contains higher ash content. Fly ash problem doesn’t exist in the gasification technology. All the ash is collected in bins and handled separately.
    Gasification route is the best route in the event RDF briquette contains inorganics such as chlorine and potassium since much of this is removed within the gasification island. Whereas in the combustion route there is a danger of inorganic vapour fouling and corroding the boiler tubes.

 6.  How much landfill space do we save and how much electricity do we generate?
Nearly 80 % of the land space can be saved. A 1.0 MWe requires roughly a waste of 120 – 130 ton /day. This plant can generate about 7000 million units of electricity per year.

7.  What is the amount of ash generated through gasification route?
This depends on the ash/inert content in the MSW. The ash content should be brought to less than 10 %.

8.  What are the constituents of ash?
This depends on the constituents in the MSW. If it is domestic waste, then the ash should contain a small fraction of carbon and inorganics such as silica, and other oxide of salts depending upon the raw material.

 9.  Can WTE ash be used for anything useful?
Rich in inorganics it can be used as a soil conditioner provided it does not contain any hazardous materials like heavy metals, etc.

10.  What happens to ash that is not used?
Can go for landfill.