Why renewable ‘biomass’ is not as good for the climate as we’d hoped

Why renewable ‘biomass’ is not as good for the climate as we’d hoped

Burning biomass to generate electricity is officially listed as being ‘carbon neutral’. But claims that it helps prevent climate change are not as straight-forward as they seem.

WHILST I HAVE BEEN living in New England for the past seven years, a new industry has popped up in Maine and the southeastern US — the wood pellet industry.

It has revitalised the sluggish demand for forest products in these regions. Biomass-powered utility plants in Europe are asking for the delivery of more than 5 million tonnes of wood pellets annually, which are burned to generate electricity. In the urge to fulfill its commitment to reducing carbon dioxide (CO2) emissions to the atmosphere, the European Union counts all biomass-energy as carbon-neutral. Australia would like to do the same.

If wood pellets are derived from construction debris, discarded shipping pallets and the debris left from forestry operations, they are probably helpful to reducing CO2emissions. All these materials are likely to decompose or get burned within a short period of time. We might as well burn them instead of coal.

Burning some forms of live biomass also helps to reduce CO2 emissions, if it is derived from short-lived plants, such as weedy grasses and short-rotation forest trees.

Unfortunately, the heat content of woody biomass, about 3,500 kWh/tonne, is less than in coal (7,500 kWh/tonne), so about twice as much wood must be burned to generate the same amount of electricity, resulting in higher CO2 emissions. Nevertheless, short-lived plants are often fast-growing and reabsorb CO2 from the atmosphere within a year or so.

When pellets are derived from large and older trees, it’s a different story, which is difficult to unfold. And once they are in the form of pellets, no one knows whether the pellets were derived from waste materials or whole trees. When whole trees are pelletised, the carbon in their biomass is released to the atmosphere when they are burned. For decades, the carbon contained in the small trees that replace them is much less — so there is an overall release of CO2 to the atmosphere — a carbon debt that is repaid over decades.

Anticipating a climate ‘tipping-point’, all nations are trying to reduce carbon dioxide emissions during the next several decades.

So, how much forest we have and the time it takes for forests to recover is the essence of any argument about whether pellets are good or bad for the environment. The carbon content of Australian forests must show little change before burning woody biomass could be considered carbon-neutral and not reported as yielding net CO2 emissions in national accounting to the international conventions that monitor greenhouse-gas emissions of all nations.

If all woody biomass is considered carbon-neutral, we are likely to see increased logging of the remaining old-growth forests — important wildlife and biodiversity habitat worldwide. Pellet production is increasing from the northern, boreal forests in Canada and the US. And, international demand for such biomass will undoubtedly lead to deforestation elsewhere. A worldwide reduction in standing forest biomass translates to greater CO2 in our atmosphere for decades.

All this casts a shadow on another good intention to wean society from its diet of fossil fuels and reduce carbon dioxide emissions to the atmosphere. Coal is bad for CO2 emissions and a variety of other air pollutants that affect human health. Wind, tidal, and solar power emit almost nothing and should dominate our renewable portfolio. Compared to coal, wood has less energy per unit of its weight, so more of it must be burned to displace coal from the utility industry. But wood derived from whole trees, it may actually increase CO2 emissions to the atmosphere.

Pellets provide a rich profit for the industry, but it is too simple to think that all pellets hit the target we are aiming for — a carbon-neutral economy.

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