If you burn 100 tons, for example 20 tons of residual ash, where does the rest go? Different sources contradict each other; one says that mainly water comes out of the chimney, others claim that the 80 tons of burnt waste comes out with it. What exactly comes out and where did the 80 tons of waste in this example go?
Answer
This question uses Lavoisier’s law as its underlying idea: conservation of mass during a chemical reaction. As long as we don’t have nuclear reactions, this is indeed a very good starting point.
The answer to your question depends on the composition of the waste.
Water is a strong argument. Water is relatively heavy. If you burn “wet” waste, a large part of the weight loss will indeed be water. However, it is also very inefficient to incinerate wet waste. Evaporating the water requires a lot of energy, and it simply goes out of the fireplace with it. It is much more efficient to dry the waste first and then burn it.
Even with dry waste there will still be a large weight loss compared to the residual ash. For example, look at the weight of dry firewood compared to the residual ash. Where did that weight go? The main reaction products are: H2O (water), CO2 (carbon dioxide), CO (carbon monoxide), and C (carbon, “soot”).
I think you can generally say that a large part of the mass passes through the chimney via the carbon, and especially in the form of CO2.
Have you ever considered where a tree gets its mass? A tree gets its mass from CO2 in the atmosphere. A tree literally grows from air. Incineration is actually also a form of recycling, albeit a very slow one in relation to the speed and scale with which we consume humans.
Answered by
dr. ir. Nico Smets
Engineering Sciences
Avenue des Pélain 2 1050 Ixelles
http://www.vub.ac.be/
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