New source of power at landfill
Stories by TAN CHENG LIPictures by KAMAL SELLEHUDDIN
A WASTE landfill is not only to dispose of our daily discards. It can also provide a valuable commodity – energy. Gas emanating from dumps can be collected and combusted to produce electricity, as is happening at the Ayer Hitam Landfill in Puchong, Selangor.
At the landfill, one of the largest in the country as the bulk of Klang Valley trash ends up there, the mound of waste reaches 40m in height. The five million tonnes of rubbish contained within have been piling up since 1995, and all the decomposition going on inside the mound results in gases that not only smell bad, but are also hazardous to man and the environment.
For the past year, landfill operator Worldwide Landfills Sdn Bhd has been recovering the gas to produce electricity which is then sold to Tenaga Nasional Bhd (TNB), a scheme conducted jointly with TNB Energy Services Sdn Bhd (a TNB subsidiary).
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ZAMRI ABDUL RAHMAN: ‘Recovering the gas may not be commercially attractive but we should do for environmental reasons. It should be a part of landfill management.’ |
All these are possible only because it was pre-planned. While almost all of the 168 known waste disposal sites in the country are essentially open dumps, the Ayer Hitam Landfill is an engineered one – the country’s first when it was built in 1995. Thus the huge pit where all the waste is buried has an impermeable liner to prevent polluting seepage into the ground. Pipes laid out at the base collect leachate (wastewater from decaying waste), while wells enable gases to disperse.
Worldwide Landfills general manager Zamri Abdul Rahman says some 1,000 tonnes of rubbish were disposed daily at Ayer Hitam between 1995 and 2002; the amount has since tripled. Waste collected from four Selangor municipalities (Ampang Jaya, Subang, Petaling Jaya and Shah Alam), City Hall and Putrajaya ends up there.
Zamri says a high percentage of organics in the waste (95%) and the local wet climate work together to trigger quick production of landfill gas, which typically consists of 50% to 55% methane, 45% carbon dioxide and traces of hydrogen sulphide and volatile organic compounds.
Landfill gas has to be managed, asserts Zamri, and not only because it smells. Methane ignites easily and kills vegetation as it displaces oxygen from the root zone. Landfill gas can migrate in the ground and if it accumulates in buildings, poses risks of explosions.
It also contains toxic and carcinogenic substances, as well as volatile organic compounds that contribute to smog. Hence, capturing landfill gas for power generation is a wise move; it not only recycles the gas into energy but also avoids a myriad of health and environmental risks.
Furthermore, recovering and converting methane and carbon dioxide – both of which are greenhouse gases – into energy prevent their escape into the atmosphere. Methane is a particularly potent greenhouse gas; its global-warming potential is 21 times greater than that of carbon dioxide.
In 2001, Canada’s National Office of Pollution Prevention found that the 41 landfills that captured landfill gas saved seven tonnes of carbon dioxide annually – that is similar to removing 1.5 million cars from the road.
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Decaying waste in landfills generate highly combustible and heat-trapping gases such as carbon dioxide and methane which are now tapped through concrete wells such as this one and used to generate electricity. |
At Ayer Hitam, Zamri says, gaseous emissions were initially low but to prevent their build-up in the ground, the gas was vented through stacks driven into the landfill. When the waste volume grew, the gas was burned by connecting a mobile flare to gas wells. At the same time, a feasibility study on power generation commenced. After two years of monitoring confirmed the quantity and quality of the gas, commercial power generation started.
Gas wells are spread out some 30m to 40m apart to cover the site. Concrete 3m-long pipes with diameters of 90cm each are stacked on top of one another as waste piles up, to form the wells. Once the gas flow reaches sizeable amounts, a gas well head is installed to recover the gas and channel it through a piping system to the power plant.
In the plant, the gas first passes through an extraction system in which it is filtered and dried. It is then burned in two internal combustion engines with an installed capacity of 2megawatt (MW) to produce electricity at 450 volts. The power is then diverted to a transformer and stepped up to 11 kilowatt before transmission to a TNB sub-station. Fortunately, one such station sits just outside the landfill; this avoids the need for long transmission cables that would otherwise incur cost and power losses.
“It is a straightforward process,” says Zamri. “The gas engine is similar to that of a diesel engine and it is unnecessary to process the gas to remove trace gases.” Combustion of landfill gas does emit oxides of nitrogen and sulphur but Zamri attests that improved gas engine design has kept these to within standards.
To produce 1MW of power, the landfill must constantly produce 600cu.m of gas containing 55% to 65% of methane. To date, the plant has produced 1.3 million kilowatt/hour of electricity but production is not continuous as landfill operations take precedent. For instance, gas recovery is stopped when refuse is being tipped onto the site. The erratic electricity production prevents its usage within the landfill as the facility requires reliable energy supply, particularly to run the leachate treatment plant.
Zamri says when the landfill ceases to operate, power production will be non-stop and expanded to 5MW. It is expected to expel gases for years to come, so he estimates that power generation should last some 15 years. “Commercial production of electricity may be uncertain after that as the gas volume may be inconsistent and its composition may change.”
Eco-benefits
The waste-to-power scheme has stood Worldwide Landfills in good stead – it will be setting up gas power plants in two major dumps in Manila, the Philippines. Locally, some landfills have started proper venting and flaring of landfill gas but Zamri believes capturing emissions should be standard operating procedure for landfills. Admittedly, gas recovery will be difficult in the case of open dumps. “We will need more and longer feasibility studies as we are unsure about their contents and condition.”
Landfill gas is one of the viable alternative fuels identified in the government’s Small Renewable Energy Power Programme which encourages use of renewable energy in power generation. Producing power from rubbish appears to have enormous potential, particularly since Malaysians’ throwaway habit continues unabated.
The Ministry of Energy, Water and Communications estimates that landfill gas can generate RM4mil worth of energy annually. A study commissioned by the Malaysian Energy Centre in 2000 at the Taman Beringin Landfill in Jinjang, Selangor, found it to be a potential site for gas power generation. The dump has accumulated some 3.5 million tonnes of waste.
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At the Ayer Hitam Landfill in Puchong, Selangor, leachate (polluting wastewater from decaying waste) is treated before it is discharged into a stream. The hillock on the left is where waste has been dumped, compacted and layered with soil since the landfill opened in 1995. |
The TNB offer price of 16.5 sen per kilowatt-hour under the renewable energy power provider agreement is hardly a good rate considering the RM10mil invested in the privatised project but for Zamri, the benefits of the scheme outweighs its cost as landfill gas is now managed and not wantonly discharged.
“Recovering the gas may not be commercially attractive but we should do it not for commercial but for environmental reasons. It should be a part of landfill management.”
Hazardous pollutantsLANDFILL gas is about 50% methane and 50% carbon dioxide, with varying amounts of nitrogen, oxygen, hydrogen sulphide and an assortment of non-methane organic compounds (NMOCs).NMOCs usually make up less than 1% of landfill gas but consist of many hazardous air pollutants (HAP), volatile organic compounds (VOC), which react with sunlight to form ground-level ozone or smog, and halogenated compounds (typically chlorine, fluorine or bromine which when combusted in the presence of hydrocarbons, can recombine into highly toxic compounds such as dioxins and furans). Nearly 30 organic HAPs have been identified in landfill gas, including benzene, toluene, ethyl benzene, and vinyl chloride. Exposure to HAPs can lead to adverse health effects. How are NMOCs generated in landfill gas? They are contained in discarded items such as household cleaning products, materials coated with or containing paints and adhesives, and other items. During decomposition, NMOCs can be stripped from the waste by methane, carbon dioxide and other gases and carried in landfill gas.
Source: US Environmental Protection Agency
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