Bristling from the ‘cap’ of the now-closed Narre Warren landfill on the outskirts of Melbourne are clusters of black pipes, each leading down to a gas-wells network, drawing gas from the depths of 15 years worth of urban waste. In the past the gas, a mixture of methane and carbon dioxide, would have escaped into the atmosphere, adding to the greenhouse gas burden or, just as destructively, leached into the surrounding soil, killing vegetation and creating potentially explosive gas reservoirs.
Today, in line with EPA Victoria requirements, almost all landfill gas is captured in some way. Much of it is flared – burned off to convert the methane into carbon dioxide – because methane is 21 times more toxic than carbon dioxide as a greenhouse gas. But increasingly, landfill gas is being harnessed as a valuable fuel. Around Australia there are now 54 landfill-gas-driven power stations, operated by a number of companies including Energy Developments, Veolia and Sita. At Narre Warren the gas is harvested by Energy Developments for electricity, some of which is used to heat a commercial rose nursery and the remainder is fed into the grid, helping to power the equivalent of 10,000 homes.
However, as appealing as it sounds, running a power station from landfill gas is not as straightforward as from a coal-fired or hydro-energy plant. Dr Julia Lamborn, a senior lecturer in civil engineering at Swinburne University of Technology’s Centre for Sustainable Infrastructure, explains that the emission of landfill gas not only varies from landfill to landfill, but also varies over the life of the landfill. Being able to accurately predict how much gas a landfill site is likely to deliver over its lifespan is essential if an associated power station is to be profitable. She says an operator needs to be able to match landfill gas generation with the number of electricity units needed. Too many units means the operation is not cost efficient, too few means not enough gas is used.
To date, this has been an inexact science – at least when it comes to actual large-scale landfills. Dr Lamborn says that current planning models have ranged from 90 per cent to as much as 4000 per cent off the mark, making them next to useless as predictors.
A range of factors determines the amount of gas available and how quickly it is emitted. These include the actual composition of the waste, its moisture content, plus a host of other factors. There are several sophisticated predictive models that work within highly controlled laboratory conditions, but experience has shown they do not adapt readily to an actual landfill. “You can have very high accuracy in a laboratory-based model of one cubic metre or so, but that’s useless unless you can scale that model up to landfill size models of, say, three million cubic metres,” Dr Lamborn says.
This was the challenge that drove her project, ‘Modelling Landfill Degradation Behaviour’, as part of her recently awarded PhD. The objective was to find a model that can strike a balance between the lab-based accuracy and the simplicity required by a landfill operator.
The work is important because among the energy options currently available, landfill gas can be quite cost-competitive. A report by the Australian Bureau of Agricultural and Resource Economics (ABARE) estimates the long-run average cost of landfill gas at $35.59 per megawatt hour. By comparison brown coal costs about $38/MWh, wind power varies from $55 to $80, natural gas is $35 to $45 and hydro averages about $62/MWh. The average urban landfill site, such as at Narre Warren (a filled-in bluestone quarry, 45 metres deep in the centre) is expected to be able to supply commercial volumes of gas for 40 to 50 years. The Narre Warren site has been generating electricity since 1993.
Working as part of the International Waste Working Group, a group of about 50 scientists and engineers looking at issues dealing with landfill modelling, Dr Lamborn has had access to a number of working models and landfill sites – including the Narre Warren landfill – which, being one of the first landfill power stations in Australia, has been closely monitored over its lifespan, as well as others in the US and the UK. By comparing these real-life models with complex laboratory models, Dr Lamborn’s research aims to determine the most useful input parameters to predict how much gas is likely to be generated from landfill sites. “What I’ve tried to do is to find a balance between the accurate but complex modelling of the lab-based models, and the simple but inaccurate real-life models.”
Finding the bridge between these models has been an important step in making landfill gas extraction more viable, says Associate Professor Ian Harding, was was Dr Lamborn’s PhD supervisor in Swinburne’s Life and Social Sciences faculty. “There’s all too often a huge gap between what theoretical scientists are doing and what happens on the ground. Julia’s research helps to translate the complex mathematical models into something that is going to be useful to the person running the tip – who is generally not a scientist and doesn’t have the time or inclination to look at complex mathematical models.”
Energy Developments’ operations manager for Victoria and South Australia Maurie Morrone says the company welcomes any research that helps improve power station efficiency. “There’s always a balance between simplicity and accuracy,” he says. “The more reliable and accurate the model, the better you can tailor your infrastructure (to the gas emissions predicted).”
Dr Lamborn admits it is just a small piece in a big puzzle, but it is a significant one. If landfill-gas power stations can be made profitable it will be a huge step forward in the twin objectives of reducing greenhouse gases and harnessing a valuable resource for producing renewable energy. “It’s great to be involved in something so valuable,” she says. “Any way we can reduce our dependence on brown coal – one of our biggest emitters of greenhouse gases – has to be good.”
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