Yes: using LPG will help clean up carbon dioxide

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 July 2020

How many kilos of firewood can produce the same amount of energy as a kilo of liquified petroleum gas (LPG)? Last week we saw that the energy content of LPG is 50 megajoules per kilogram (MJ/kg) while that of firewood is 20MJ/kg. However, while a gas burner is about 60 per cent efficient, fireplaces are only 15 percent at best.

Thus, a kilogram of LPG produces about 30MJ of useful energy while a kg of firewood gives only 3MJ. In other words, LPG you need 10kg of wood to get the same amount of energy as you would get from 1kg of LPG.

Because of the high inefficiency of open fires, most people prefer to convert wood into charcoal and then use an efficient jiko (stove). The famous Kenya Ceramic Jiko operates at about 60 per cent efficiency – the same as an LPG stove.

Now, charcoal yields about 30MJ/kg, so it would take about 1.7kg of this fuel to produce the same amount of energy as a kilogram of LPG. However, when we consider that it takes about 10kg of wood to produce 1kg of charcoal, it turns out that, taking this route, one needs 17kg of wood to get the same heat as 1kg of LPG.

Clearly, LPG generates a lot more energy than wood-based fuel. So, we can now move on to the question that started all these calculations: can the carbon dioxide emitted by LPG can be absorbed by the trees saved by using it?

A mature tropical tree measuring about 50m in height will produce about 5,000kg (5 tonnes) of firewood. Burning it would generate 100,000MJ of energy; but, since the fireplace is 15 per cent efficient, only 15,000MJ will be available for use.

So, now we ask: how much LPG will produce 15,000MJ of usable energy? At 50MJ/kg, we need 300kg of the gas. But the burner is 60 per cent efficient, so, the total quantity required is 500kg of LPG.

From last weeks data, a kilogram of LPG emits 3kg of carbon dioxide when burnt. Therefore, 500kg of this fuel will produce 1,500kg of carbon dioxide. Can the tree absorb this much carbon dioxide in its lifetime?

Wood fuel is said to be carbon-neutral because, when burned, it emits the carbon dioxide that the tree had absorbed while growing – nothing more. Now, burning a kilogram of firewood produces 2kg of carbon dioxide.

So, a 5,000kg tree would emit 10,000kg of carbon dioxide. This is the quantity that the tree had absorbed while growing. Bear in mind, also, that the root roots are normally not used as firewood but they are very much a part of the tree. So, the 10,000kg is a minimum.

Comparing 10,000kg absorbed by the tree to the 1,500kg emitted by LPG, it is clear that LPG is far better than firewood or charcoal. But this opens another line of enquiry: can a firewood powered power station be good for the environment?