How wood fired power station can increase forest coverage

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

12 July 2020

Suppose we wanted to build a 1 mega-watt (1MW) wood fired electricity generating power station: how much firewood would it consume? In the last two previous articles, we have seen that firewood can generate 20 mega-joules (MJ) of energy per kilogram. We also established that, whereas domestic open fireplaces are very inefficient (15 per cent), commercial, heavy-duty firewood stoves are available with efficiencies ranging at about 80 per cent.

At this higher level of efficiency, 1kg of firewood yields about 16MJ of useful energy. So, the question comes down to: how much will be consumed to generate 1MW? Now 1MW simply means 1MJ per second. Therefore, since 1kg of fuel will generate 16MJ, it will keep the power station running for 12 seconds.

That is, every second, the station will consume just 62.5 grammes of firewood. This looks small, but it accumulates to 225kg per hour, or 5,400kg per day. This comes to approximately one mature tropical tree (about 50m in height) every day!

This does not seem sustainable. If we cut one tree each day in order to get 1MW of electricity, we shall soon deplete the forests! We would need to plant trees to supply fire wood to the power station. But trees take about 10 years to mature. So, how many should we plant?

The answer is simple: one tree is consumed each day, so one tree must be planted to replace it. However, the new tree will take ten years before it is ready for cutting. Therefore, we would need 3,650 trees at various stages of growth to keep the power station running consistently.

But that might not be enough. Experience shows that, in a well-managed tree plantation, about one-third of the seedlings die within the first year. In many instances, the survival rate to maturity is about 50 per cent. This is the reason why forestry experts insist that we plant two trees to replace the one we cut.

Taking that into consideration, we find that we should actually plant two tree each day, thus we need a plantation with at least 7,300 trees. How much land would that need?

Most trees do well when planted about 3m apart. This gives about 1,000 trees per hectare, or about 400 per acre. So, to plant 7,300 trees, we would require about 18 acres of land.

For best results, trees should be planted at the onset of the long rains in March or April. So, instead of planting two trees each day, it would be better to plant 730 seedlings at a go each year.

So, if you have 20 acres of idle land, you may consider planting trees and building a firewood driven power station. But is it worth it? Well, the 1MW output will easily supply 1,000 households with continuous electricity-day and night. Not to mention that it will create 20 acres of woodland that didn’t exist before. So, it worthy of consideration.