Agenda: Why biomass is unlikely to make a significant contribution to the world’s energy needs

Posted on: Aug 04, 2016 - 09:59 AM | Market Trends

CLOSE examination of the power from biomass argument is sobering. The majority of the world’s biomass fuels for national scale power generation are derived from trees. Since trees are replaceable these fuels are considered “renewable” hence UK power stations running on wood pellets can qualify for subsidies.

It is argued that the carbon released by burning biomass was previously absorbed from the atmosphere and is part of the planet’s carbon cycle so if taken from sustainable forests the CO2 released when burning will be re-absorbed by new plantings rendering biomass energy carbon neutral. This ignores the huge amounts of energy used in pellet production and transportation which generates its own CO2 and needs to be considered part of the Life Cycle Assessment. (LCA).

For the carbon neutral LCA argument to be plausible the lifecycle has to be “sustained” and replanting needs to exceed harvesting levels. With CO2 generated in the UK, any sustainable carbon neutralising remains dependent on the country of source – primarily the US – so sustainability and renewability is not within our control. As Government subsidies continue can we be confident that they or the Green Investment Bank are being watchful?

Consider too that biomass residues and dead trees left in stripped/abandoned forests will rot, producing their own CO2 with no trees left to absorb it and also methane which is a greenhouse gas around 20 times more potent than CO2.

Ignoring replanting, it is an inconvenient truth that CO2 emissions from wood burning for the energy produced are far higher than from oil and gas, and about the same as from coal.

The Department of Energy & Climate Change (DECC) in a 2014 study noted that the relevant renewable energy directive setting the emissions standard for qualification for subsidies does not take full cognisance of carbon debt. This occurs when a stand of trees is harvested and its carbon storage capability reduces/disappears for many years. This has a significant impact on the total greenhouse gas releases over those years until forests have re-grown. DECC assessments are now based on theoretical 40 and 100 year impacts.

Given these timescales their assessment and modelling of risks naturally becomes progressively uncertain however some of the underlying hard energy figures of biomass remain damming.

DECC assess that the energy input requirement to make and deliver wood pellets ranges from 13% to 96% of the final delivered energy of the pellets depending upon the source of the wood, the levels of fossil fuels used in pellet processing, including the drying and the shipping distances to the generator. As 80% of UK pellets are from the US and Canada we are at the higher end of these figures. Notably, too, diesel and heavy fuel oil powered ships produce much more CO2 than burning natural gas.

Freshly cut trees have a moisture content between 30-65% and power stations recommend around 10% for firing. Drying requires 750kWh of energy for each tonne of moisture removed – around 1/6th of the annual electrical energy consumption of the average home.

DECC confirmed that biomass energy input is much greater than most other electricity generating technologies such as gas, coal and nuclear and they project that we could be soon needing to import more pellets as UK biomass generation grows.

Even compared to coal there are notable downsides to burning wood pellets as they produce serious amounts of highly toxic dioxins and nitrous oxides; they have only half the energy by volume so we need to burn twice as much to generate the same energy.

With regard to sustainability and renewability, the total existing US hardwood resource in energy terms is equivalent only to around 3 months of present total US energy usage so biomass is unlikely to ever make a significant contribution to the world’s energy needs.

We already have several large plants partly converted to utilise wood pellets, principally Drax and Ironbridge, which previously were 100% coal burning. Drax uses around 1m tonnes/year of US pellets and its reported estimated “green” subsidy is £10m per week and totals more than £1000m to date.

These hard facts held against the biomass industry argument that the CO2 produced (at levels similar to coal and much more than oil or gas, per unit of electricity produced) is captured by planting more trees suggest that it is more logical to continue planting trees and stop burning subsidised biomass.

David Watson is a retired chartered electrical engineer with 35 years' experience in international energy engineering projects.


Credit: David Watson The Herald Scotland


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