Can LPG use save enough trees to clean the environment?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 June 2020

On the Island of Hispaniola in the Caribbean Sea, the boundary between Haiti and the Dominican Republic stands out dramatically. The Dominican side is thick, lush, green forest while the Haitian side is dry, brown soil. The change is not gradual but abrupt on the boarder line: it is clearly visible in the satellite view of Google Maps – check it out.

One of the explanations for this dramatic difference is that the two countries adopted different policies on liquified petroleum gas (LPG – domestic cooking gas). Haiti saw LPG as a good source of government revenue and introduced high taxes. The Dominican Republic saw it as a social right and removed the taxes.

As a result, the price of LPG in Haiti is several times higher than in the Dominican Republic. Consequently, nearly every home in Dominican cooks with LPG while only the wealthiest people in Haiti can afford it – the majority poor use firewood and charcoal.

Several decades down the road, Haitians have depleted their forest cover while the Dominicans have preserved and even increased theirs. This observation leads to an interesting question: is LPG good for the environment? Can the saved trees soak out all the carbon dioxide emitted by the burning gas?

To find out, we need to work out the mass of carbon dioxide emitted by LPG and compare it to the quantity absorbed by the trees that would generate an equivalent amount of energy.

The following basic data is required. First, what are the energy densities of LPG and firewood? That is how many joules do we get when we burn one kilogram of each fuel. LPG generates about 50 million joules (mega joules, or MJ) while firewood produces 20MJ.

Secondly, what are the efficiencies of the stoves used for burning these fuels. This is important because some of the energy generated goes to waste depending on the design of the burner.

Domestic LPG burners are about 45 to 70 per cent efficient; we can work with the middle value of about 60 per cent. Modern firewood burners are much better. The type used in schools and other institutions are up to 80 per cent efficient.

However, most villagers use open, three-stone cooking fires. These are very wasteful; at best, they can reach about 15 per cent efficiency. That is, they waste 85 per cent of the Energy.

The alternative is to convert the wood into charcoal and then use this in an efficient stove. Modern, insulated jikos can get to about 60 per cent efficiency – same as the LPG stove. However, although charcoal generates more energy (30MJ/kg) than firewood (20MJ/kg), it takes about 10kg of wood to make 1kg of charcoal!

We these numbers at hand, we are now ready to work out the answer to the original question: Can the saved trees soak out all the carbon dioxide emitted by the burning gas? We shall do that next week.