Flame cap burning to protect airsheds and store carbon

Look closely at the smoke in the animation above. Watch how it is pulled into the active zone of flames and turbulence, where it is fully combusted and disappears. What you are seeing is the flame cap in action: a zone of high temperature and mixing with air that is the key to both effective pyrolysis and minimisation of emissions. This fire is treating wilding pine slash in a pit (3x5 m, 1.7 m depth) in glacial till next to Lake Pukaki in June 2023. A sizable pile of branches and logs was pyrolysed on the day, producing a yield of biochar and virtually no smoke. Read on to see why and how it was done.

The backstory

Wilding conifers are a massive ecological threat across many of the iconic landscapes of the interior of the South Island. In Central Otago and the Mackenzie Country, escaped Pinus contorta from plantations and shelterbelts established several decades ago have spread and colonised large swaths of pasture and tussock rangelands. The pines form dense stands and suppress native vegetation, deplete soils of their fertility, and pose an outsized wildfire hazard in an environment that has not evolved for this. To compound matters, when these trees do burn, any of their seeds that are deposited by wind will sprout readily on the ashen surface, as this is the natural adaptation of the the species in its original habitat. This sets up a recurring cycle of emergence that resists control -- mechanical, chemical or grazing pressure can suppress the seedling regrowth but as long as there are mature trees in the vicinity it's a losing battle. The trees grow quickly in what amounts to an ideal environment, and can produce cones in as little as three to five years.

When a solution leads to another problem

DOC, affected landowners, and regional councils have all brought extensive resources to bear on the issue, but in the process of clearing the pines a secondary problem has come to the fore: Slash piles and windrows now extend across hundreds of hectares, an obstacle to pastoral farming and a fire hazard if left in place, but a nuisance and detriment to local air quality when they are set alight in traditional pile burns. Because of the topography of many of the glacial basins in the high country, a day calm enough for safe burning will often have an inversion layer that traps smoke and particulates near the ground. This can affect air quality to the point where residents are forced to stay indoors, and for a site with extensive piles of slash the social licence to dispose of the material via open burning can become a contentious topic.

Leaving the windrows in place, as seen in the satellite image, is a frustrating option for landowners, as they can take many years to slowly decay and limit the ability to do anything, whether the goal is to return to pastoral farming or revegetate with natives. Meanwhile, all the carbon the trees stored as they were growing is returning to the atmosphere, even though there's a better place for it.

Charmakers to the rescue

in 2023, BNNZ gathered a team of experts to run a demonstration burn at Pukaki Downs, where wilding control is the single greatest expenditure on the property. The site was assessed for suitability, a pile was nominated for sacrifice, and invitations went out to neighbouring stations and councils.

On 23 June, with a freezing fog drifting from the lake, BNNZ treasurer Ben Elms confirmed pit dimensions as Rupert the digger driver planned the next series of scoops.

Sam from Black Moa Biochar loaded the pit with the initial charge of kindling.

Kindling stack awaiting ignition. We lit the fire from the top to minimise smoke and encourage fast involvement of the fuel. As soon as the first embers formed, we began adding more wood from the slash pile-- fast enough to encourage flames but not so much as to cool the combustion zone, as this would cause smoke formation. Once there was a layer of coals across the entire pit, the digger was used to add larger quantities of fuel, still keeping pace with the flame cap. Pieces of timber up to 30 cm in diameter were being added by the middle period of the burn.

Note how the flames cover the entire top surface of the burning material. Virtually no smoke is being emitted, because a zone of rolling turbulence above the flame cap carries it back into the active part of the fire. Wood smoke is merely unburned products of combustion escaping the process, so by reincorporating these along with plentiful oxygen back into the hottest part of the fire, we eliminate the problem simply by working with the fire instead of against it.

The flame cap serves another critical function: By using the available oxygen in the air column above the fire, it prevents the carbon in the char bed from being fully burned up. This is why biochar produced using flame cap methods can be comparable in quantity and yield to that made in closed retorts and kilns -- it's mostly a function of feedstock and fire management.

As the pit reached capacity, we snuffed the burn by pushing the excavated dirt over the top with the digger. Residual heat in the core continued the pyrolysis process for several more hours. Smaller burns can be quenched with water, but this requires a sizable quantity and a means of delivery.

A few months later, Rupert excavated the pit and we got to see the outcome. Only a few of the largest pieces put into the fire late in the day were not fully carbonised. Half of the carbon embodied in the slash is now effectively locked up for millenia and can be spread around the site to improve the soil and speed the return to productive pasture.

Next steps

The team from BNNZ is working collaboratively to do a larger series of trials, scaling up the pit method to a trench that can be used on a windrow site. We are available to offer advice and consultation to landowners, councils, and wilding control groups to help with management of residues, and would welcome opportunities for education and greater public awareness of the benefits of flame cap burning for field biomass disposal in general -- generations of farmers and land managers have been doing it wrong, to put it bluntly, and downwind neighbours don't have to suffer the airborne effects of open fires managed poorly. We can have the multiple advantages of reduced emissions, better airshed quality, soil and water quality improvement, and effective carbon sequestration simply by rethinking how we work with fire.