Wilding trees recognised as an energy source

Forestry consultant Jim Childerstone is pleased the potential of wilding trees for energy is being recognised.

It's taken some time but the Queenstown Lakes District Council has finally recognised the potential of wilding trees as an energy source (ODT, 3.12.12). Initially, this involves the removal of trees, processing into chip fuel and transport to users with newly installed or converted boilers suitable for producing heat energy. That would seem the easy bit. But it takes considerable investigation into the most cost-effective methods of achieving this.

The council has received some Eeca (Energy Efficiency and Conservation Authority) funding to start the ball rolling, initially to make contact with potential heat energy users within the Lakes district and Central Otago. It's simple enough to point out the advantages of woody biomass; clean, carbon neutral and a renewable resource. It's proven by users within the Dunedin Energy Hub that fuel costs can be halved through conversion from other forms of energy production (lpg, electricity or diesel).

There's no problem there once potential users can be enlightened on the benefits. Regional councils are beginning to accept that properly installed boilers using wood chip remain well under regulatory health emission levels. This virtually eliminates coal as the other cheap source.

Having worked on investigations into the logistics of wood-fuel conversion to heat energy, in both commercial plantation harvest residues and wilding conifer infestation (Lakes and Mackenzie districts), for the past two years, I have no doubt that there is considerable potential from both sources.

The logistics faced by the QLDC is the available biomass in hectares, how to get at it - taking into account contour, clearance/logging costs, distance from users - the best methods for processing, type of plant, access on to processing sites (skidsites, landings), age of wilding establishment, size variations, possible recovery of commercial grade saw log, roundwood and potential volumes from these sites.

Having lately carried out voluntary preliminary investigations on selected sites in the area, I came to the conclusion more research was needed on assessing volumes, clearance and harvest methods, chipping or hogging and handling costs, operating on site or central processing areas handy to town. Much of this will relate to the volumes of material in demand.

The prospect of supplying chipped fuel for a boiler to feed heat energy to the Lakes District Hospital, Queenstown Airport, swimming baths and events centre was initially turned down due to the cost of renewing or converting existing boilers and infrastructure to supply the complex with heat. Also, at the time it was thought the fuel source would not be clean enough. However, since then, new machines are able to separate fines [wood residue] and other defective materials.

This project may have required a 6MW to 7MW boiler needing something in the region of 5000 to 6000 tonnes a year of chip fuel for continuous heating. The University of Otago has commissioned a 1.1MW boiler to heat the College of Education using chipped fuel from local forests. Then it gets to the finer points of which part of the tree is most suitable. The butt or solid sections (stemwood) produces the best chip. But it can equally be sold as firewood.

On the other hand, the rest of the tree - tops, offcuts, branches and slash - are of nil value and a nuisance as well as a fire danger when it dries out. Modern chipper/hoggers can handle that. So can more recent boilers now available from Europe and the United States. Then there is the question of the most suitable moisture content of the chipped fuel. Ideally, anything below 30% gives a better calorific value than recently felled trees at around 55%. In dry conditions, wood residue could drop below these levels within a few months before processing. I had moisture-tested branchwood left nearly a year on skidsites after harvest, with readings from 15% to 25%. This gives similar calorific value in delivered heat from a furnace as lignite coal.

Most available areas of wildings are at lower levels and, with older trees, costs of removal can be mitigated through recovery of commercial grades. One scenario being considered for Mackenzie Country wildings is a possible $5 per tonne stumpage to landowners.

In the end, this could prove a win-win situation for both landowner and chip producers if finally a project gets the go-ahead. It would also be a win-win situation for the environment. On a continuous cycle, cutover areas could be replanted with non-invasive species under forest management regimes.

A logging operation is under way above Queenstown on terraces below Bobs Peak by Dave Collett Logging, which specialises in wilding tree logging, felling Douglas fir and pine. The area is to be replanted with Sequoia dendrum.

Wilding trees

There's no such tree as Sequoia dendrum. Could be Sequoia sempervirens (coast redwood) or Sequoiadendron giganteum (giant redwood).