A waste of energy

Last month, Dubai Municipality revealed it was studying a AED20billion household waste-to-energy programme in a bid to tackle its growing problem.

The amount of rubbish sent for disposal in the emirate more than tripled between 2000 and 2007, rising from 1million tonnes to 3.5million tonnes. In the same period, industrial and construction waste jumped from 3 million tonnes to 10.5million.

Lethal landfill

The vast majority of this waste is sent to five landfill sites around the emirate. But these malodorous dumping grounds are now in areas where housing and investment have crept in, meaning the amount of waste sent there needs to be reduced.

Landfill sites produce vast amounts of methane gas as materials degrade. Methane is a potent greenhouse gas, some 23 times more harmful than carbon dioxide (CO2). Across Europe, landfill sites are being severely restricted in accordance with new regulations governing greenhouse gas emissions.

In response, local authorities are increasingly turning to incineration as an effective solution for disposing of household waste, which cannot be recycled or is uneconomic to sort.

Incinerators or thermal recycling plants release energy from household and industrial waste that can then be used for power generation and other applications.

“Landfill just stores the problem – it does not solve it,” says Michael Sandeman, a consultant for German waste incineration firm, Jakob Stiefel. “But waste can be part of our energy resources.”

“Thermal recycling is the complete combustion of household waste, which has a usable by-product. That by-product is energy. It is not enough to solve the vast amounts of electricity needed in this region, but it can make a very useful contribution,” he adds.

Clean energy

Incinerators are increasingly seen as much more environmentally friendly than landfill sites. Burning waste produces 50% less CO2 per kilogramme than dumping it.

The plants provide their own energy supply and, even though there may be up to 60 000 tonnes of waste in the delivery bunker awaiting combustion, they do not give off unpleasant odours.

Intakes suck air from the bunker to supply the incinerator, which creates lower pressure in the bunker so no gases escape. The gases are burnt together with the waste.

As the waste burns, the smoke and gases transfer their heat energy to the boiler, which produces steam. The gases are constantly monitored by computers which in turn control the supply of air. The air temperature supply is also controlled to ensure a complete burn of the gases.

In a modern thermal recycling plant up to 5 000 points are linked and monitored by computers, according to Sandeman.

The waste gases pass through electrostatic and dust filters before being cleaned and rewashed to ensure all pollutants are removed prior to being discharged. Some of the ash deposits can be reused for instance in road building.

The steam is typically used to generate electricity using conventional steam turbines. But the steam can also be used in desalination or district cooling processes.

Some of the steam is reserved to generate the power for the waste-to-energy plant itself. In Singapore, there is even a thermal recycling plant that uses the steam to drive a reverse osmosis water treatment plant to supply water for the boilers.

Similar plants can also be used to convert industrial waste, sewage sludge and biomass into energy.

“It is a high-tech solution to a low-tech problem,” says Sandeman, who estimates that as much as 5% of Dubai’s electricity requirements could be met through burning waste.

Sustainable fuel

Burning five tonnes of household waste is equivalent in energy terms to about one tonne of oil. Large waste-to-energy plants can generate upwards of 100MW of power.
 

The plant size is only limited by the amount of waste available and how much waste is incinerated every hour. A plant that processes one million tonnes of waste each year could fuel a combined power plant, producing close to 120MW.

The price tag of a plant capable of handling 250 000 tonnes of waste each year is around 140million (US$222million), according to Jakob Stiefel. While the cost of burning one tonne of waste is said to be between US$90-300, depending on the size of the plant.

“The primary idea behind waste recycling is to treat a product which we produce,” says Sandeman.

“Energy is a by-product. One would never build a household waste incinerator to make money and compete with oil, gas or solar powered plants. We have a waste problem and by using the by-product we use that waste problem to our advantage.”

Waste-to-energy case study

An energy recovery plant on the Shetland Islands, 100 miles north of the UK, generates hot water for a district heating scheme by incinerating waste.

The plant burns 22 000 tonnes of domestic, commercial, clinical and industrial waste per year, generating 7MW of energy.

Non-hazardous waste is tipped into the plant’s delivery bunker before it is mixed to become a homogenous fuel for the furnace. It is then deposited into the furnace feed-chute by an overhead crane grabber at a rate of around 3 tonnes per hour.

The waste is pushed into the furnace, where it starts burning with the radiant heat from the waste already in the furnace. The waste burns at 1 100-1 200 degrees Celsius on a three-sectioned movable grate system, which ensures the complete combustion of the waste.

The bottom ash from the furnace and any metal objects from the waste are quenched and conveyed into skips. The bottom ash is sold on for landfill cover and the ferrous metals are separated off with a magnetic separator and then sent for recycling.

Fly ash and gases from the furnace pass into the post combustion chamber where they are held for at least two seconds at more than 850 degrees Celsius to ensure the removal of dioxins. Gas oil burners are on standby to ensure that the temperature does not drop below this level.

A 50-tonne water boiler is situated above the post combustion chamber. An induced cyclone in this chamber removes around 90% of the larger particulate generated by the furnace.

The hot gases enter the three-pass boiler to heat water to 115 degrees Celsius, which is then distributed via the district cooling system. When the heat from the water has been used it returns from the town at 55 degrees Celsius to be reheated.

The furnace gases are then reduced to 180 degrees Celsius before undergoing vigorous gas cleaning processes.

In the first stage, an electrostatic precipitator operating at a high voltage ionises and removes small particulate. A wet scrubber then reduces the acid gases in series of water and alkali sprays.

This produces a high concentrate acid effluent, which is then treated in a water treatment plant before being discharged as salty water. The neutralisation process uses lime milk to counterbalance the high acidic effluent and this generates a filter cake for disposal. Additional compounds are added in the water treatment process to remove metals.

The final stage of gas cleaning is performed in the bag house filter. This is a final removal of acid gases and dioxins. Activated carbon and lime is blown into a reactor just in front of the bag filter – the carbon molecules are porous and so absorb any remaining dioxins. The carbon and lime powder coats the filter bags meaning that the gases have to pass through them.

Deposits from the post combustion chamber, electrostatic precipitator, bag filter and water treatment are classified as special waste and have to be disposed of in a special waste landfill site.

The cleaned flue gases are then released into the atmosphere at around 100 degrees Celsius through a 46m high stack. All gases are continuously monitored to ensure they do not exceed permitted levels.

The £10 million plant started up in 1998 and supplies district heating to some 900 homes and businesses. The plant is designed to operate for 7 800 hours per year on a 24 hour per day, 7 day per week basis.