Energy shavings

Croydon council has found partnership working invaluable in developing the UK's first TreeStations - where tree surgery waste is processed into fuel, providing heating for the community. Tom Idle reports

Since 1995, environmental staff at the London Borough of Croydon Council have been working with BioRegional, thinking up ways to improve the management of the area's woodland and trees, and to make best use of the timber. And the work paid off. In 1999, it became the first local authority in the world to have its street and park trees certified as a well managed urban forest under the Forest Stewardship Council system.

Sawlogs and smaller-diameter timber are produced from the woodlands with logs and chip coming from tree surgery work - known as arboricultural arisings. Meanwhile, the development of the BedZED eco village (a unique housing development which we have covered in these pages - visit www.bioregional.com for more details) has given the opportunity to set up wood-chip production in Croydon.

BedZED (Beddington Zero Energy Development) is the largest development of its kind in England so far and included a wood-chip-fuelled combined heat and power (CHP) plant as part of the energy strategy. A fuel requirement of 1,100 tonnes a year of wood chip at 30% moisture content was predicted. This matched estimates of the quantity of logs dropped off by tree surgeons at Croydon's green waste yard. More widely a study carried out in 2001 of arboricultural arisings estimated that 100,000 tonnes of arboricultural arisings were available annually from contractors working for local authorities across London. And in 2001 trials for the production of wood chip fuel from arboricultural arisings started at Croydon.

As work on the project proceeded, it became clear that other opportunities for using wood chip in boilers for heating were emerging. The scope of the project was broadened and the planned capacity of the site was increased to allow for this.

The Croydon TreeStation is one of the first sites in the UK where arboricultural arisings are being processed into fuel suitable for use in smaller wood-chip boilers. The quality requirement for small boilers is more exacting than for larger ones, with tight limits on chip size distribution and moisture content. The key features at the Croydon TreeStation which may be relevant for others contemplating wood-chip production are:

  • The wide range of sizes encountered in tree-surgery waste, requiring flexible equipment
  • The relatively small scale of operation envisaged, drawing material from the local area
  • The emphasis on high-quality chip suitable for use in all boilers
The initial aim of the project was to supply 1,100 tonnes a year of wood chip suitable for use in the BedZED CHP; the ideal chip size being 25 x 25mm.

The CHP was scheduled to start operating in early 2002, and it was clear that wood-chip production at Croydon would not be up and running by this date. To meet the anticipated demand a search for local sources of wood chip was made. Very few sources of wood chip suitable for fuel were found that could consistently meet the specification. The main problems were the presence of over-long slivers or a high proportion of fines, both of which could lead to blockages and bridging in the gasifier. The lack of high-quality wood chip in the region reinforced the need to develop its own supply at Croydon.

The material delivered into the yard at Croydon is typical of tree surgery arisings countrywide, a mix of logs and smaller leafy tops and branches chipped by tree surgeons on site, principally to reduce volume. Both can be used for fuel provided they are appropriately processed.

Chips produced by tree surgeons on site are very variable. Much depends on the model and condition of the chipper. A well maintained and adjusted machine with sharp blades will produce high-quality chip but many chippers are used only for volume reduction and chip quality is of no concern to the operator. A chipper in poor condition produces a higher proportion of fines and slivers. Very poor quality chip is not suitable for fuel.

The 2001, chip production trails were labour intensive. Arboricultural waste timber was received into the yard and cross cut with a chainsaw to a maximum of 1.5m long. They were then split using a manually operated vertical splitter into pieces less than 152mm (6in) diameter.

The timber was stacked for a minimum of six months to reduce moisture content to below 35% then chipped using a manually fed Laimet HP21 chipper. Although the chipper was rated to chip 152mm diameter logs, the irregular shape of the logs that were being chipping meant that the practical limit was 127mm or less. The early trials demonstrated that manually intensive production of wood chip using a relatively small chipper was not viable for a number of reasons:

  • Production rates were too low
  • The cost was too high, over £45 a tonne before delivery
  • Much heavy labour was involved, making it unlikely that anyone would want to work full-time on wood chip fuel production

Trials of chippers were directed to larger-scale machines equipped with loading cranes with the aim of reducing the amount of splitting and labour required. There are three types of chipper in common use in the UK with the cutting blades mounted on a disc, a drum or a cone.

Changing markets

As the project approached construction on site, it became clear that the CHP at BedZED could not be relied upon to provide a market for the chip as intractable technical problems continued. New markets for the chip had to be found.

Slough Heat and Power provided an immediate outlet but at a price that was insufficient to provide for further development of the site without a significant gate fee. Longer-term prospects for use as a heating fuel locally were good, with Croydon Council promoting renewable energy through its planning guidance which made specific reference to biomass.
Serving local markets has several advantages:

  • Lower delivery costs, lower CO2 emissions
  • Lower gate fees
  • Reduced reliance on a single customer
  • Higher profile, meeting more community objectives

Having a reliable local fuel source gives developers the confidence needed to opt for biomass heating. The site was planned as a demonstration unit to boost the profile of wood chip as fuel with visits by architects, planners, local authority personnel, private housing developers, housing associations and other people interested in setting up wood fuel production or installing wood-chip heating.
They were already planning to produce high-quality wood chip so few changes were required to serve the small boiler market. The main change is the requirement for chip at 30% moisture content or below, in contrast to the BedZED CHP which could accept chip at 45% moisture content. Trials are planned for low-cost methods of drying material chipped green.

Conclusion

It is still early to reach a firm conclusion about the commercial viability of the site.
The TreeStation was set up just ahead of a rise in interest in wood-chip heating in London, and is proving a valuable resource as a demonstration unit.

More than 60 people have visited the site either as part of seminars and study tours or as individuals interested in sourcing wood chip and specifying wood-chip heating systems. BioRegional has also used the TreeStation as a starting point for discussions with other local authorities about starting wood-chip fuel production.

The site has certainly stimulated interest in wood-chip heating in and around Croydon. It has also supported a package of other policy and planning measures such as the London-wide requirement for 10% on-site renewable energy generation on larger developments.

www.bioregional.com
www.croydon.gov.uk

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