Biochar may be tonic for washed-out earth

The post-cyclone problem: clogging forestry slash and silt-sti ed soil. The possible solution: burning the slash and using the resulting material to enhance the soil structure. Richard Rennie reports.

SEDIMENT and slash from Cyclone Gabrielle have created a giant headache for local authorities and landowners to deal with – but biochar creation may provide some solutions to dealing with both.

Biochar is the charcoal produced through the process of pyrolysis, or the burning of the product in an oxygen-free or air-limited environment. Found naturally in soils as a result of historical fire cycles, this pyrogenic carbon does not break down easily. It offers a means of enhancing soil structure, and most importantly has been proven to sequester carbon.

Overseas markets valuing biochar through carbon credits have developed rapidly, particularly in Australia, the United States and the European Union.

Traditional processes have involved smouldering the biomass in earth mounds, often leaving it to decompose naturally, but modern incinerator technology has made wholesale biochar creation a reality.

Phil Stevens, Biochar Network NZ’s deputy chair, said the group has high hopes for the role it could play in helping deal with Cyclone Gabrielle’s forest waste and slash legacy.

Estimates are in the Gisborne region alone 400,000 cubic metres of slash and waste washed down after the event.

Hopes for biochar playing a role were recently supported in a ministerial paper on the slash and waste residue that cited biochar as a means to deal with the problem and also help remediate the poor quality soils the waste has come off.

“Ultimately it is easier to deal with the waste stream at the source, on logging sites, where turning it into biochar becomes a standard part of the forestry practice,” Stevens said.

He said the burning treatment to turn wood mass into biochar reduces the volume on a four to one basis, helping deal with the problematic issue of having to truck high volumes of lower value waste material.

The Biochar Network has just completed a demonstration in Canterbury with Pukaki Forestry for using the technique to deal with massive windrows of wilding pines removed through the MacKenzie Country.

“Really you can go anywhere in New Zealand and there are councils and landowners having to contend with surplus biomass. That includes wilding pines, forest waste or willows being removed from river systems.”

Stevens said incinerator technology is advancing quickly.

In the US a semi-trailer air curtain burner has been developed.

“These things are beasts. You take them to the site and feed them organic matter and it produces biochar continuously.

In an eight-hour day a tonne of biochar is produced from four to five tonnes of biomass.”

He said such tech is ideal for dealing with the East Coast forest slash and waste.

“So much of it is larger logs and it is wet, and this leads to problems with handling, so what you can do with it is pretty limited anyway.”

In a closed-loop approach, the biochar produced from the waste can then be repatriated onto the land it has come from, often country that is of poor soil quality and in need of remediation. Biochar’s porosity and surface area can provide better structure and aeration to the benefit of the soil to which it is applied.

Stevens also pointed to the international markets rapidly developing for biochar carbon credits, although the network cautions that any scheme needs to ensure the carbon stored in biochar is taking account of the complete lifecycle of production and application. Inefficient production without proper combustion can produce a poor-quality product and deliver negative environmental effects.

Stevens said the network has been having several conversations with entities in the Tairāwhiti district, particularly for applying it to silt-affected soils.

Biochar also offers an additional output stream for biofuel, with equipment capable of heating a boiler also producing biochar at the end of the heating process.