A powerful counterpoint to our mounting garbage is the clean energy that can be harnessed from it – once communities understand the wonders of modern technology.
If something has a bit of value left in it, it shouldn’t be chucked away. But that’s not the world we live in, where households wheel out huge bins of stuff each week so their contents may never be seen again.
All that waste going to waste is a bit of a waste, when you think about it.
If household garbage is fed into the right technology its calorific value can be released by intense heat and converted to kilojoules per second that can then be converted to kilowatt-hours of electricity.
The immediate concern among city-dwellers, however, is that they don’t like incinerators. On his ride to work your EcoGeneration correspondent pedals past two former incinerator sites that were transformed into sweet-smelling parkland many decades ago*.
But things have changed since those dirty old times, says Ali Abbas, associate professor at the University of Sydney’s School of Chemical and Biomolecular Engineering. “We will see waste-to-energy plants being installed and they will generate electricity – that’s the value they provide.”
It’s very early days for the waste-to-energy sector in Australia. A plant is being built near Perth, Western Australia, and plans are in progress for plants in Victoria, Queensland and possibly NSW. “We will see this industry growing,” Dr Abbas says.
Incineration versus gasification
The recovery of energy from all the trash society gleefully casts aside makes waste-to-energy sound like modern alchemy, but energy can never be created from nothing. The plant under construction in Perth is a proven moving grate technology, where material is incinerated. Heat is used to drive a turbine, ash and metal are sifted away to be put to use or recycled and emissions are captured and removed.
An alternative to incineration is gasification, where feedstock is heated to a point where energy can be recovered from the resultant gasses. A variety of gasification known as “slag-in” runs at up to 1,600˚C, where the “slagged” ash separates into bottom ash and fly ash. Bottom ash is vitrified, or transformed by heat into “a kind of molten rock”, Dr Abbas says, and fly ash is exhausted upwards and collected.
Gasification is not incineration. “You control the oxygen feed and the temperature in such a way that you get different product coming off,” Dr Abbas says. The process is started using fossil energy and internal energy is managed to reach a steady state.
The feedstock for waste-to-energy plants comes from the ubiquitous kerbside red bins, which alongside the usual wide variety of rubbish can also include some organics. Around 40-60% is moisture content, Dr Abbas says. The goal is to separate out all recyclables beforehand.
End product electricity
The primary product of a waste-to-energy plant is electricity, which is useful to everyone, but an efficient project will extract value from ash, liquid leachate and gas coproducts. Before feedstock is fed into the heating process, whether it be gasification or incineration, liquid leachate is extracted so that water can be recovered from it, along with nutrients and possibly some minerals.
The exhaust graduates through various steps where pollutants are extracted before exiting a stack as “relatively clean exhaust”.
In his lab at Sydney University Dr Abbas is exploring the possibilities of using bottom ash, or “slag”, as a construction material, incorporating the coproduct in concrete products. “We make different recipes of concrete, where we target utilisation of the waste and also construction properties, like compressive strength of the concrete product,” he says.
Cubes of pale grey concrete are stacked alongside a 3D printer the researchers are planning to use to craft objects using ash-cement compound.
Along with cement, the ash can also be mixed with waste glass and carbon dioxide so that waste is locked into a concrete product. “You can sink some carbon, not a lot,” says Dr Abbas, who expects about 10-15% of cement can be replaced by ash in concrete used in construction. Using landfill waste to offset emissions produced by cement achieves a double offset effect, he says.
An end to trash
We live in a throw-away society, and regardless how we got here over the last 100 years of ever steepening consumerism we can be sure the habit of generating rubbish will be hard to break. Everything comes wrapped in something, for a start, and the goods we buy are never made to last.
Ideally, the best way to avoid waste is to avoid buying something that will one day become waste. The second best solution is to find another use for whatever it is that’s about to hit the trash bin. “Having two lives will automatically increase an item’s lifecycle efficiency,” Dr Abbas says. “If you find a third life for it, even better.” Recycling comes next, and further down the hierarchy is disposal. That is where landfill and waste-to-energy compete for raw materials. “As a dealer in the waste business you can choose to install landfill or a waste-to-energy plant. It’s the same level in the hierarchy.”
Weighing the two options, a waste-to-energy plant gets ticks for generating electricity, eliminating volumes of waste, as a source of useful ash and leachate coproducts and as a way of decontaminating waste – ridding it of bacteria and possible viruses.
Waste-to-energy technology delivers about 30% efficiency, Dr Abbas says, compared with 30-40% for a coal-fired power plant. Demonstration waste-to-energy plants capable of 10 tonnes a day can be built in a few months, and larger plants in one to two years. The Kwinana plant in Perth will be able to manage 400,000 tonnes a year; plants overseas can manage up to 2 million tonnes a year.
The Sydney Uni Waste Research Hub is building a “waste atlas” map of where waste is generated across Australia to help inform decisions on installations of waste-to-energy plants. Locally, a rule of thumb says a plant would need to process at least 300,000 tonnes a year to be feasible. Other than the income earned form electricity sold, gate fees in the hundreds of dollars a tonne contribute to revenue.
The circular economy
It sounds like the answer, but what if waste-to-energy plants became commonplace just as society found a way to get by without producing mountains of the stuff? The river of feedstock could turn into a trickle.
Like many of his peers, Dr Abbas eyes an emerging “circular economy”, perhaps another 30 years away, where consumption is far more efficient and the appetite for landfill and waste-to-energy wanes.
For the past 20 years the volume of rubbish sent to landfill has grown about 8% a year. “That’s about six times population growth,” Dr Abbas says. “We still have a waste generation problem, and we can’t export it anymore.”
Plants are needed until at least 2040 to 2050, he says, when a predicted circular economy will slowly kick in, with its stringent approach to repurposing and far lower tolerance for trash. “Design for reuse and repurposing will become one of the cornerstones of future engineering,” he says, going hand in hand with an economy that is tuned more to leasing and sharing than a mantra of continual growth.
“We will not transition if we don’t think of the economics, the policy, sustainability, the regulations, the law, behavioural science in one silo together – that’s a very important message.”
* He also rides past Sydney’s White Bay coal-fired power station, which has been left to rot since it was switched off in 1983.