Waste recovery
Incineration- The Heating Power of Refuse
08/06/2010
Incineration is not only a way to get rid of waste- it also generates energy.
Many different types of waste, from households, industry or agriculture, can be converted into heat or electricity.
© AFP / Patrick Valasseris
On the Way to the Plant
In Europe, each person produces 1 kg of household waste per day on average. This is two times less than an American, but much more than an inhabitant of a developing country.
Apart from recycling operations, about 70% of our waste is combustible (paper, cardboard, putrescible waste, textiles, plastic, etc.). Incineration is first and foremost a way of eliminating a sizeable amount of this waste- 90% of the initial volume goes up in smoke. This represents an advance in health terms compared to the huge landfills of the past. In France, recoverable waste can no longer be sent to landfill since July 2002.
However, incineration is also a means of generating energy from waste.
An incineration plant includes a furnace and an afterburner. The waste is burned in the furnace in a process called pyrolysis, which produces combustible gases. These are burned in the afterburner at temperatures of 800-900°C. A heat exchanger -in which very hot water or steam circulates- is used to recover energy from the smoke emitted.
5-7 tons of waste are needed to produce the equivalent of a ton of fuel. For example, the energy produced in the three incineration plants in the Paris region (Syctom) is enough to heat 300,000 homes per year.
By using its potential for energy recovery from waste, Europe could supply electricity to 17 million households and heat to 24 million. Denmark is the European leader in electricity and heating production per capita in this category. This country’s 30 plants consume 3.5 million tons of waste per year, meeting 5% of domestic electricity and 20% of domestic heating requirements.
Africa is also starting to produce electricity from household waste. In 2010, construction on the first African plant to produce electricity from household waste started in Ifrane (Morocco). Construction is set to last 24 months.
• It increases if glass and metals are removed (because these materials do not burn). If wet, fermentable waste is removed (kitchen waste, etc.).
• It decreases if paper and cardboard are removed.
What is the Energy Efficiency after Incineration?
Two types of energy are produced in an incineration plant- heat and electricity.
Each type has its own production process and they have different energy efficiency ratings.
• Heat production: This involves simply heating water through waste combustion. This process has good energy efficiency- 70-80% of the combustion heat is recovered after incineration, i.e. about 1,500 kWh of thermal heat per ton of refuse.
However, after producing this heat, a problem remains. The heat needs to be used near the incineration plant. In winter, heating requirements solve this problem. But in summer, it is harder to find ways to use it, even taking into account demand from some industries. All told, energy efficiency throughout the year is lower than the average theoretical efficiency, estimated at 75%.
• Electricity production: The exchanger has to contain steam at the highest possible pressure. This steam is directed to a turbine that drives an electric generator. The electricity produced can be supplied to the grid throughout the year. Energy efficiency is about 20-25% (300-400 kWh).
It is estimated that by recovering 50% of all household waste in France, about 1% of the country’s energy consumption requirements could be met. This solution is therefore very far from meeting requirements.
Moreover, the smoke from incinerating waste is highly toxic. The resulting gas must therefore be filtered and neutralized (because of its acidity) before being discharged into the atmosphere. Atmospheric pollution levels from incineration plants are particularly high.
Non-Household Waste Can also Be Recovered
Aside from households, many industries produce waste whose value can be tapped with incineration.
• Special industrial waste: Industrially-produced hydrocarbon residue, tar, used solvents, and paint sludge. This waste can be converted into heating or electricity just like household waste in special incineration plants. It can also be burned in cement plants, which have high energy requirements.
• Agricultural waste: As with the other types of waste mentioned above, this type of waste produces heating or electricity through incineration.
Agricultural waste is mainly straw from wheat, corn, and rice, the three most commonly grown cereals worldwide. One hectare can yield 2-6 tons of straw. The energy potential of straw is 16 MWh/ha. 3 kg of straw is the energy equivalent of 1 liter of fuel.
However, straw has one major disadvantage: it takes up a lot of space- 4-8 times more than wood for the same energy value. Therefore, this fuel has high transport and storage costs.
Moreover, some straw has to be left in the fields to avoid soil depletion. If replaced by fertilizer, all the energy benefit of the operation would be lost.
• Agro-industrial waste: This type of waste comes mostly from sugar-beet mills and oil mills. In Senegal, almost 10% of electricity is produced from cane sugar or peanut residue. In South-East Asia, coconut and palm nut shells are burned after extracting the oil.
In pulping plants, after extracting cellulose, an organic substance that makes up the cell walls of wood, the "black liquor" – i.e. the residue from the pulping process- is burned. This provides more than enough to cover the plants’ steam and electricity requirements.
