Merton Rules over biomass confusion

Biomass is on everyone’s list of environment friendly and sustainable energy sources, even though it involves burning fuel. Many developers and local authorities believe installing a biomass boiler is the cheapest way to comply with the Merton Rule requirement for a percentage of a new building’s energy to be generated on-site.

The theory goes that when you burn biomass to create heat, all you are doing is accelerating the release of carbon that would happen anyway if the biomass were left to decay. But burning biomass immediately releases carbon dioxide in huge quantities, whereas leaving it to rot releases CO2 much more slowly. Some rotting substances, particularly in unmanaged woodlands and wetlands, decay into peat, locking up much of their CO2 until this is released by burning. If the peat is not burned, the CO2 does not get released.

Also, I am not convinced that the replanting element of biomass production is on a scale sufficient to soak up all the CO2 released by the system. Burning biomass releases the carbon that the plants absorbed in growing; if you cut down a mature forest and burn it, you have to replant on a scale sufficient to ensure that you soak up the total carbon released, including the carbon used in transporting the biomass.

The carbon cycle is not completely closed when burning biomass. The industry estimates the carbon not recycled to be only about 5%, but I suspect that is an optimistic figure.

Most biomass boilers in this country use wood pellets. The fear of supply problems often results in developments having a back-up gas boiler. I imagine that in many cases the gas boiler ends up being more than just a back-up.

The carbon cycle is not completely closed when burning biomass

Burning wood pellets is not benign. Wood smoke contains inorganic gases such as carbon monoxide but also organic gases, particulates and organic compounds (such as benzo-pyrenes) which are carcinogens. True, most of the gases, particulates and compounds can be filtered out of the smoke before it reaches the flue. But efficient filtering depends on a rigorous and effective maintenance programme; we know from our experience with gas boilers that these are not always in place. And however good the filtration, there will always be some leakage to the atmosphere.

Maintenance is important with all burning devices. For biomass boilers it must include not only checking the flue and filtering system but also the regular clearing of wood ash. The ash can be recycled into fertiliser but in Finland and Sweden, where wood combustion is high, less than 10% of the ash is recycled because it has to be compacted and granulated before it is used to reduce health risks. The rest goes into landfill. Also, some ash contains cadmium and other heavy metals, so it will poison rather than fertilise.

Let us assume, then, that the biomass wood-burning boilers all have good working filters. Will that be enough? The filter will let through wood smoke particles unless it has an expensive system of smoke washing. The particles are too small to be filtered by the nose and the upper respiratory system so, like tobacco smoke, they will end up deep in the lungs. Many common conditions, such as asthma, bronchitis and emphysema, may be aggravated or, in the long term, created by the particles. The US Environmental Protection Agency suggests that the lifetime cancer risk from wood stove emissions may be 12 times greater than the equal amount of cigarette smoke.

There are technologies that we should use before biomass: thermal solar, geothermal and wind energy are benign. Each has different carbon paybacks as opposed to the attempted closed carbon cycle of a biomass system. But, for example, the carbon used in making, transporting and installing most thermal solar systems is recovered in less than two years.