The use of biomass for fuel has been advocated as a means of reducing carbon dioxide emissions, but some have argued it does nothing of the sort. There was a recent article in Physics World that discusses this issue, and here is a summary. First, the logic behind the case is simple. The carbon in trees all comes from the air. When the plant dies, it rots, releasing energy to the rotting agents, and much of the carbon is released back into the air. Burning it merely intercepts that cycle and gives the use of the energy to us as opposed to the microbes. A thermal power station in North Yorkshire is now burning enough biomass to generate 12% of the UK’s renewable energy. The power station claims it has changed from being one of the largest CO2 emitters in Europe to supporting the largest decarbonization project in Europe. So what could be wrong?
My first response is that other than a short-term fix, burning it in a thermal power station is wrong because the biomass is more valuable for generating liquid fuels, for which there is no alternative. There are many alternative ways of generating electricity, and the electricity demand is so high that alternatives are going to be needed. There is no obvious replacement for liquid fuels in air transport, although the technology to make such fuels is yet to be developed properly.
So, what can the critics carp on about? There were two criticisms about the calculated savings being based on the assumptions: (a) the CO2 released is immediately captured by growing plants, and (b) the biomass would have rotted and put its carbon back into the atmosphere anyway. The first is patently wrong, but so what? The critics claim it takes time for the CO2 to be reabsorbed, and that depends on fresh forest, or regrowth of the current forest. So replanting is obviously important, but equally there is quite some time used up in carbon reabsorption. According to the critics, this takes between 40 and a hundred years, then it is found that because biomass is a less energy-dense material during combustion, compared with coal you actually increase the CO2 emissions in the short-term. The reabsorption requires new forest to replace the old.
The next counter-argument was that the block should not be counted, but rather the landscape – if you only harvest 1% of the forest, the remaining 99% is busily absorbing carbon dioxide. The counter to that is that it would have been doing that anyway. The next objection is that older forests absorb carbon over a much longer period, and sequester more carbon than younger stands. Further, the wood that rots in the soil feeds microbes that otherwise will be eating their way through stored carbon in the soil. The problem is not so much that regrowth does not absorb carbon dioxide, but rather it does not reabsorb it fast enough to be meaningful for climate change.
Let us consider the options where we either do it or we do not. If we do, assume we replant the same area, and fresh vegetation is sufficient to maintain the soil carbon. In year 1 we release x t CO2. After year 40, say, it has been all absorbed, but we burn again and release x t CO2. By year 80, it is all reabsorbed, so we burn again. There is a net x t CO2 in the air. Had we not done this, in each of years 1, 40 and 80 we burn kx t CO2, giving us now 3kx t CO2, where k is some number <1 to counter the greater efficiency of burning coal. Within this scenario eventually the biofuel must save CO2. That we could burn coal and plant fresh forests is irrelevant because in the above scenario we only replace what was there. We can always plant fresh forest.
Planting more works in both options. This is a bit oversimplified, but it is aimed to show that you have to integrate what happens over sufficient time to eliminate the effect of non-smoothness in the functions, and count everything. In my example above it could be argued I do not know whether there will be a reduction in soil carbon, but if that is troublesome, at least we have focused attention on what we need to know. It is putting numbers on a closed system, even if idealized, that shows the key facts in their proper light.