Energy from waste – what an attractive proposition!

I can remember when the refuse collectors went on strike in the city of Liverpool UK. The streets stank! Overflowing bins, boxes open to the air, plastic bags splitting and spewing. It was disgusting and potentially dangerous.

And waste disposal is a worldwide problem. Mountains build up, landfill sites fill up – how enticing the thought of converting it all into energy.

And now even some of the politicians are beginning to realize that yes – we do have a problem with waste disposal and with energy supplies with an increasing world population.

Did you know that…

4 tons of waste equals 1 ton of oil.

2 tons of waste equals 1 ton of coal.

And as society in general is becoming more aware of green issues to the industry of waste disposal is also evolving to meet demands.

What is Waste?

Waste is what we throw away! And there are lots of kinds of waste including household, demolition, biomedical and nuclear. We will leave the disposal of hazardous waste (nuclear, toxic) for another time

How can we manage waste?

  • Reduce
  • Reuse
  • Recycle

Reduce

The best option is to reduce the amount of waste in the first place – and although we can be described as a throw-away society we are becoming more open to less. One example is in the way of less use of plastic bags at the supermarkets, but the packaging companies have a great deal to answer for.

Reuse

Reuse is the next best option – shopping bags being a good example.

Recycle

Recycling waste is the production of usable heat and the generation of electricity from waste.

Waste management includes the collection, the transport and disposal of waste.

Energy-from-waste-plant

Here the waste is converted into heat and electricity. There are various methods of doing this, the commonest being burning the waste at high temperatures to produce steam which drives a turbine to produce electricity. Or we can produce methane, methanol, ethanol or synthetic fuels

The waste can be divided into biomass – materials such as food waste, paper and cardboard, leather, leaves, wood, grass cuttings and so on. And about 2/3 household waste in America is biomass. And then there are the plastics which can burn and those materials which cannot burn like glass and metals.

In America about half of household waste is landfilled, a quarter recycled, and the rest burned with energy recovery or composted. In 2016 in America 30 million tons of waste was burnt to produce 14 billion kilowatt-hours of electricity.

And many of the big landfill sites produce electricity from the methane gas which comes from the decomposition of biomass there. As more waste is combusted the volume of landfill rubbish is reduced by around 87%.

Recycling Methods?

  • Combustion
  • Gasification
  • Pyrolysis
  • Anaerobic digestion

Combustion

Burn the waste. This produces heat for our buildings and steam to drive a turbine to generate electricity. Suitable materials include wood, paper, textiles and plastics. This is perhaps the most efficient of the methods used.

The problems:

Pollutants are emitted. Modern recycling plants work to strict emission controls.

The “Destructor” or first incinerator was built in Nottingham UK in 1874.

Then in 1885, the first US incinerator was built Governors Island in New York.

Gasification

As the name suggests the purpose here is to produce useful gases from waste. We burn rubbish like packaging, bottles, textiles at very high temperatures, they combine with oxygen or steam. This produces a gas to be used as motor fuel, fertilizers or it can be converted into electricity.

 Worldwide biofuels are providing us with energy and more plants are opening. In America, the Biofuel Energy Corporation of Denver, in 2008 had plants in, Nebraska, Minnesota.

Problems with getting energy from waste

Gasification is not very efficient since the process itself requires energy. In addition, the reactors need regular cleaning – which means they are closed down for a spell. Similar pollutants to the combustion method are also emitted as burning is part of the process.

Pyrolysis:

Pyrolysis also needs high temperatures, but less high – but in an atmosphere devoid of oxygen. As the temperature is lower the emission of pollutant is also lower

Both gasification and pyrolysis release CO2, whether the waste used was plastic or biological. However, efficiency can now reach 75%

Anaerobic digestion

Landfill sites – we can extract the biogas from landfill sites. However, as less and less organic waste goes into landfill this method is declining, although it still has a part to play – in the UK it produced 3.04TWh in one year.

Anaerobic plants – the waste is put in into a tank and denied access to oxygen. The waste breaks down into a biogas and fertilizer.

This method has huge potential. 5.5 million tonnes of food waste could produce energy sufficient for 164,000 households – and save up to 0.35 tonnes of CO2 compared with simple composting. Not an inconsiderable amount!

Fermentation

Another waste to energy technology is fermentation. Here instead of grapes and wine, biomass produces ethanol as the sugar in the waste is converted into carbon dioxide and alcohol.

The Plastic Problem

plastic bottles
plastic bottles plus

And what of the plastics filling the oceans and found inside each of us?

Both pyrolysis and gasification can be used to produce energy, and they can use plastics.  But establishing pyrolysis plants requires considerable investment and education.

The Problem with Pollutants

Burning waste produces pollutants. These include very fine particles which we can breathe in and are deleterious to our health. They include heavy metals, acid gases, and graces of dioxin. However, these emissions, properly treated, are released in very small amounts but they may escape through the chimneys. They may be acidic and cause rain to turn acid – this was a problem in the 1980s. They installed lime scrubbers to neutralize the acids in the smoke and now the incinerators have primary and secondary combustion chambers which are so efficient at removing pollutant that the old lime scrubbers on the smokestacks may no longer be required.

There are stringent emission controls in place. These include limits on sulfur dioxide, nitrogen oxides, dioxins and heavy metals – and the newer plants have these in place. They can reduce the volume of waste by 95%, and even recover some of the heavy metals for reuse.

It is claimed that emissions from waste plants may be healthier to breathe than the air in many cities – not sure how impressive that is when you look at pollution levels in some places though.

There is also the toxic ash, which is left as after the waste has burnt, which needs to be disposed of in hazardous waste plants and finally the bottom ash which can be reused e.g. in road building and repair.

You may like to take a 4-minute journey round a modern waste-to-energy plant

Conclusion

It’s a nice thought – turning all our waste into energy – and it makes good sense. The problem of waste disposal is increasing as mountains of rubbish build up and landfill sites become full. But by reducing the waste in the first place, reusing what we can and then recycling the waste to produce heat for our buildings and electricity we have an almost green energy as well as solving the waste disposal crisis.

Why not test yourself in our renewable energy quiz?

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