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New Zealand is uniquely placed due to its location and land use to make use of the biological short carbon cycle to offset and reduce its carbon emissions. With its high solar radiation and rainfall, it is one of the few places in the world where trees can be grown as a commercial crop with a 25 to 30-year rotation for radiata pine. New Zealand already has a substantial forestry industry based almost exclusively around plantation forests as opposed to old forest logging required in much of the rest of the world to supply sufficiently large trees.
Trees are natural carbon dioxide absorbers. Through photosynthesis they take in carbon dioxide and water to produce long chain sugars and oxygen. This carbon dioxide absorbing effect is recognized in international carbon reduction treaties with new plantings seen as an offset for emissions from other sources.
The substantial plantation forestry industry in New Zealand presents two very important opportunities for carbon emission reduction and sequestration. Wood is in effect solid solar energy that can be stored until that energy needs to be released. Wood has long fuelled human existence, with fires providing heat enabling cooking of food (aiding digestion and allowing a greater range of food to be eaten) and allowing human expansion into colder geographical areas. With the discovery of fossil fuels wood dropped out of favour in the developed world due to its lower energy content compared to coal and the ease of use of natural gas and liquid fuels. Wood is still the most widespread energy source in the third world.
Industrial heat from wood for steam and electricity is a well-developed technology with wood being the second largest renewable energy source in NZ for industry after hydroelectricity mainly due to its widespread use in the wood processing industry. The wood processing industry makes use of waste wood from processing operations to provide this energy, solving a waste disposal problem at the same time. There is, however, a substantial amount of waste wood that is not utilised or recovered, particularly from the harvesting operations. This unrecovered wood or “slash” remaining at the site of felling operations can present significant negative environmental effects if this waste wood is disturbed in high rainfall events. This was most widely reported following heavy rain during Queen’s Birthday 2018 with the slash depositing on the beaches of Tolaga Bay, damaging properties and infrastructure on the way to the coast. Similar events have also been recording in Northland, Coromandel, Bay of Plenty, Tasman and the Nelson Marlborough districts. Another event occurred in the Gisborne region affecting Tolaga Bay again in June 2020.
Generally, the carbon absorption effect of trees is deemed to have occurred when the trees are planted, and the emission of carbon is deemed to occur when the trees are harvested. This does not follow actual carbon absorption with the greatest rate of carbon absorption occurring in middle aged rapidly growing trees with very little absorption occurring from newly planted seedlings and absorption slowing again when trees reach maturity. Emission of the carbon likewise is not instantaneous when the trees are chopped down with 100-year-old Kauri villas testament to the fact that some carbon in wood is still contained many years after felling.
The main reason trees are deemed to emit all their stored carbon when they are felled is that wood is not stable in the form of wood. It is attacked by insects, bacteria and fungi along with being readily flammable, all of which return the carbon to the atmosphere in what is known as the short carbon cycle. If wood could be turned into a form that was stable for a longer time period and resistant to these factors then it could form a potential source of carbon sequestration, the movement of carbon from the short carbon cycle to the long carbon cycle which is the one that fossil fuels are in.
Developing technologies for carbon sequestration focus on the extraction of gaseous carbon dioxide from combustion exhaust gasses (generally of fossil fuels) followed by purification (and potentially liquification) then injection into underground reservoirs that will hopefully be sealed well enough to prevent leakage back into the atmosphere. This could result in carbon neutral fossil fuels albeit at significantly greater cost and complexity than current fossil fuel combustion facilities. There is, however, an old technology that could prove even more effective: charcoal or in this case biochar, which produces a solid high carbon product that is inherently stable.
When a tree is harvested it is predominantly for the high value wood for furniture and building products, with pulp and paper returning less, and waste stream wood from these process operations often being utilised for energy. There is also the remaining waste slash uneconomic to recover from the forest harvesting areas. This slash could be used as fuel or converted into a stable form of carbon; moving that carbon from the short carbon cycle to the long carbon cycle, pushing carbon from the atmosphere back into the ground, the reverse of extracting and burning fossil fuels.
Biochar is produced by the process of pyrolysis. Biomass such as waste wood residues are heated in the absence of oxygen, driving off water and the volatile components of the wood, leaving behind a solid residue with high carbon content. The gas driven off is combustible, providing surplus energy on top of that required heat the wood residues, so it is self-sustaining and an energy source. Biochar can be considered ‘sequestered carbon’ upon production. Only combustion will quickly return these carbon molecules to gaseous atmospheric carbon dioxide. By most definitions biochar is never a fuel (that would be charcoal). Biochar could be buried or dropped in the ocean, leaving the carbon locked up. However, biochar has a wide range of useful properties for agriculture and ecology that can provide a cascade of benefits along with carbon sequestration.
Biochar has an open structure with very high surface area acting as a sieve. Carbon has an affinity for organic molecules, (why activated carbon is used to remove off flavours) allowing biochar to act as a physical and molecular sieve for water passing through it. The high surface area and porosity creates additional biological environments for beneficial soil bacteria along with the ability to hold water giving soil that it is added to better water holding capacity and tolerance to drought conditions. As a soil improver biochar could be added back to plantation forest areas to return carbon and nutrients; however, it is potentially even more beneficial to soils in pastoral and horticultural areas.
Energy and biochar production from wood waste can provide ‘carbon negative’ electricity and industrial heat, along with the carbon sequestering biochar, using technologies that are well developed and currently available in New Zealand. A potential hazard in the form of forestry waste can be removed from the landscape and additional jobs will be created in rural regions. A reduction in fossil fuel use improves New Zealand’s balance of trade position and wood energy places a floor under the price of unprocessed low-grade logs ensuring continued expanded employment in the forestry industry even in times of low international demand.
While this introduction has focussed on forestry industry waste for energy and biochar, it is possible to generate biochar from most biomass. Instead of converting marginal farmland to forestry, fast growing, high dry matter crops could be grown and processed into biochar. This results in ongoing carbon sequestration rather than plant and forget forestry farming for carbon credits. It also avoids the negative long term economic and environmental effects of forestry farming for carbon credits.
In terms of future development opportunities there are numerous promising areas
- Wood gas is a potential source of green hydrogen (wood gas is up to 20% hydrogen depending on processing conditions)
- Biochar studies have shown potential benefits in horticulture and animal husbandry above and beyond the effects mentioned above.
- Biochar mixed with animal feed, leaves the animal with the manure, inoculated with bacteria. Taken below ground by insects (dung beetles) and invertebrates (worms) this adds carbon to the soil without mechanical effort and the inoculated biochar aids with the return of manure nutrients to the soil.
- Biochar has shown potential ability to reduce fertiliser and nitrogen leaching into waterways, so if combined with fencing and riparian plantings could assist with rehabilitating rivers and streams to achieve swimmable rivers and recovery of endangered native fish.
- Biochar has shown potential to reduce NOx release to the atmosphere from applied nitrogen to soil.
- Biochar can be made from almost any biological waste stream including contaminated ones and sewage sludge. Biochar added to concrete or asphalt locks up carbon and the contaminants, while having the potential to improve some physical properties. It also replaces sand or aggregates in these products reducing mining demand.
- Biochar made with cogeneration of electricity in rural regions could be a form of regional development and distributed energy production.
- 55 uses of biochar from Biochar Journal Website https://www.biochar-journal.org/en/ct/2-The-55-uses-of-biochar
- Biochar through a combination of these methods could be extremely useful in allowing for the continuation of pastoral and horticultural farming in New Zealand in a low carbon world, while also improving New Zealand’s natural environment. We can have our milk and honey and eat it too.
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