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  • Aug 23, 2014
  • Updated: 3:13am

Two ways to burn our trash, both with flaws

PUBLISHED : Sunday, 22 April, 2012, 12:00am
UPDATED : Sunday, 22 April, 2012, 12:00am

In its quest to build an incinerator to help dispose of Hong Kong's rubbish, the government has opted for tried-and-tested over state-of-the-art.

The rubbish-burning facility that the government hopes will finally win over lawmakers would use a 'moving grate' system, in which a grate carries a steady flow of waste through a high-temperature furnace. Present in more than 900 plants globally, it is one the world's most common waste-to-energy technologies.

But the system has drawn criticism from some environmentalists who prefer more advanced technologies, such as plasma gasification, an option that officials rejected.

Hong Kong closed the last of four incinerators in 1997, after the colonial government weighed the effects the facilities had on the environment and public health. But they are being considered once again, and a proposal to build an incinerator at Shek Kwu Chau south of Lantau, at a cost of HK$15 billion, has been heavily pushed by the environment chief, Edward Yau Tang-wah. Yau argues the city has no other viable alternative to incineration.

The project will fall to the next administration after the Environment Bureau withdrew its funding request from the Legislative Council on Friday amid a lack of support from lawmakers. Incoming chief executive Leung Chun-ying has yet to officially state his view on the plan, although Yau says the new leader will have no choice but to support it.

Professor Poon Chi-sun, director of the Polytechnic University's research centre for environmental technology and management and a technical adviser to the government, backs the grate system.

Such a facility could reduce the city's overall waste volume by about 90 per cent and provide much needed efficiency, since existing landfills were expected to be full by 2018 if they were not expanded, Poon says.

'I recommend Hong Kong's first plant be one that uses the moving grate technology,' he says. This is based on comparing the reliability, robustness, economics and environmental impact of the different technologies.'

Poon says other emerging incineration methods, such as plasma gasification, could be considered if a second plant was needed in the future, presumably after such technologies became more mature. He nonetheless concedes that the ash produced by a moving-grate incinerator was a concern, though one that could be addressed.

The government also weighed costs in its decision. A government-commissioned study found a moving-grate incinerator would be cheaper to build and run.

The government had said it wanted a 'state-of-the-art' facility to burn 3,000 tonnes of municipal solid waste daily, which would make the incinerator the biggest of its kind in the world. An advisory group at first recommended the project should adopt a multi-technology approach because of the complex nature of the city's waste, but a 2009 review concluded moving-grate technology should play a central role.

The technology is used in incinerators throughout the world, including in 420 of Europe's 450 facilities. Environmentalists oppose the method because it requires a constant flow of trash to keep the fires burning, a process they say would discourage waste-reduction efforts and possibly motivate the city to generate more waste.

In such a system, waste moves through a combustion chamber heated to 850 degrees Celsius to achieve the most efficient and complete burning possible. Heat from the burning waste runs a boiler, which produces steam to create electricity.

The heaviest ash produced by burning the rubbish - called bottom ash - falls into a collection point and is passed over with an electromagnet to extract metal for recycling. Flue gases containing fine ash are then passed through a scrubber reactor to remove dioxins and other compounds that contribute to acid rain.

The flue gas - heated to 200 degrees - is passed into the flue-gas-cleaning system, which removes fine particulate before releasing it through the chimney stack

The government says the plant would boast 'the most advanced flue-gas-treatment system' in the world. Nonetheless, environmentalists say they are concerned smoke and ash would pose a health risk, especially to nearby residents.

The government also studied plasma gasification, a more advanced, chemical approach. It relies on a high-energy plasma arc that creates temperatures ranging from 3,000 degrees to 7,000 degrees to break down organic matter primarily into syngas - a mixture of carbon monoxide and hydrogen - and solid waste.

An electrode is lowered into the plasma converter and gas and solid material is fed into the chamber.

Clean exhaust gases are produced which are then used to power generators. The rest of the material solidifies into what is called plasma rock.

The rock is broken down and used in construction and road building.

While the technology shows promise, not enough is known about its emissions, Poon says. Plasma gasification may seem better for the environment because it produces less ash and emission than other incineration methods, but its long-term reliability has yet to be proven.

A moving grate system can handle almost all kinds of municipal solid waste, while plasma gasification requires the waste to be sorted and ground beforehand to ensure a good thermal process.

'The current discussion by lawmakers has become politicised. It's not rational. It doesn't get us any closer to solving the waste problem,' Poon says. 'There's nothing we can do about it except to wait, but we don't have much time. People may say the governments in the past did not do enough, but that doesn't change the fact that it's a problem that has to be dealt with now. Whatever the history, we have to do both: incineration and waste reduction. Not either.'

18,000

The tonnage of municipal solid waste generated daily in Hong Kong. About half is recycled

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