Household rubbish
In Europe, each person produces on average one kilogram of household waste per day (half that of an American, but a lot more than an inhabitant of a developing country!). If recycling is not adopted as a policy, about 70% of this waste can be burnt (paper and cardboard, decomposable organic matter, textiles, plastics …). Incineration of waste is first of all a means of eliminating a large part of the volume: 90% of the initial volume goes up in smoke, which is at least a lot healthier than the enormous tips of the past. In France, direct disposal into tips of waste that can be treated and given value has been forbidden since July 2002. But incineration is also a means of producing energy from waste!
An incineration plant consists of an oven and a post-combustion chamber. In the oven, the waste undergoes decomposition by heat (pyrolisis), producing combustible gases. These gases are burnt at 800 to 900°C in the post-combustion chamber. 5 to 7 tons of waste are required to produce the equivalent of 1 ton of fuel oil. If recycling is carried out before incineration, the calorific content of the waste changes:
- It is increased if glass and metals (which do not burn) are recycled and the same is true of damp fermentable waste (for example kitchen waste),
- It is reduced if paper and cardboard are recycled.
The energy is recovered from the fumes at the outlet of the oven, using a heat exchanger in which water or superheated steam circulates.
To produce heat, water is sufficient. The heat exchanger yield is very good: 70 to 80% of the heat of combustion is recovered, about 1500 thermal kWh per ton of waste. The key is to find a user for the heat close to the incineration plant. In winter, there is little problem thanks to normal heating requirements. But in summer, it is difficult to find someone who will take the heat produced, even if certain industrialists have requirements! The result is that the energy yield over the year is not as high as the average theoretical yield of 75%.
To produce electricity, the heat exchanger must contain steam at as high a pressure as possible. This steam is fed into a turbine that drives an electrical generator. The electricity produced can be fed into the distribution network all the year round. But the energy yield is a lot lower: only 20 to 25% (300 to 400 kWh per ton of rubbish).
To solve this problem, a system of co-generation (electricity + heat) can be installed. In such a system, the residual heat of the steam coming out of the turbine is used, enabling the total yield of a co-generation unit to attain 50 to 60%.
It is calculated that if half the household rubbish in France were to be treated
in this way, about 1% of the total energy requirement of the country would be obtained.
The fumes from waste incineration are very toxic.
They must be filtered and neutralised (they are very acidic) before being discharged
into the atmosphere. In order to provide us with adequate protection, the atmospheric pollution norms applicable to incineration plants are particularly strict.
Special industrial waste
This consists of hydrocarbon residues, tars, used solvents, paint residues … produced by industry. They can be transformed in special incineration plants, into heat or electricity, in the same way as household rubbish. They can also be burnt in cement factories, which are very greedy in energy terms.
Agricultural and industrial food processing waste
This waste can also produce heat and electricity when it is burnt.
Agricultural waste consists mainly of the three cereals most commonly cultivated throughout the world: wheat, maize and rice. Between two and six tons of straw are obtained per hectare. The energy potential of straw is 16 MWh/ha. 3kg of straw are therefore the equivalent in energy terms of 1 litre of fuel oil. But straw has a major disadvantage compared to wood: it takes up a lot of volume, 4 to 8 times as much as wood for the same energy content. It is therefore a
fuel
that is expensive to transport and store. Moreover, some of the straw should be left in the fields if soils are not to become too impoverished: if more fertiliser has to be used all the energy benefit of the operation will be lost.
Food processing waste comes primarily from sugar and oil extraction plants. In the Senegal: almost 10% of the country’s electricity production results from
the use of
cane sugar and groundnut waste. In Southeast Asia, the shells of coconuts and palm nuts, from which oil has been extracted, are burnt.
In paper-pulping plants, the “black liquor” is burnt; this is the residue from the treatment of wood after cellulose extraction. In this way, the
steam and electricity requirements of these planets are satisfied. There may even be a small surplus. |