Coppice Willow

 

The activity of growing coppice willow as a renewable energy fuel crop has been widely promoted as the answer for gasification projects. How this assumption was made without wide consultation with gasifier manufacturers will remain a mystery, but it is a decision and if successfully implemented can contribute substantial amounts to the U.K. grid network.

Based on the set aside land programme implemented by an E.U. edict, a census shows that potentially 100,000 30kWe and 25,000 100kWe gasifier systems could be installed generating some 25TWh/year. This is equivalent to Drax, the largest thermal station in the U.K.

While the logistics associated with such a system are considerable, dispersed generation using the grid connection at the farm gate and having that power on stream in less than 30 minutes should be a powerful motivator to make it happen. The costs I have been told are only a small percentage of the annual U.K. power budget.

As a fuel crop, willow is very wet when cut and drying must commence straight out of the field. Failure to do so results in rapid decomposition and the fuel stack ends up a compost heap. Air flow through the stack is critical and the end result is a lightweight bulky fuel with high sodium and potassium content. This forms clinker if gasified incorrectly.

Harvesting this fuel and its final piece size is controlled by the cutter speed and rate of feed, and the blades must be very sharp. The principle is to cut a piece without shattering the stem, or all you get is a mulch or pin chip, useless for engine gasifier fuel.

Fluidyne were asked to investigate coppice willow for gasification by a U.K. landfill company in 1993, and after trials in New Zealand, a project was implemented at Long Ashton in the U.K. This project was to establish the actual gasifying parameters of the coppice willow and sort out the unknown aspects of harvesting and drying. Since this project was initiated further work on fine chip fuels has been conducted in New Zealand and Germany by Fluidyne in preparation for larger gasified engine CHP systems.
At Long Ashton, a Champion Rotary Crop Header was used on a Claas tractor, and the chopped fuel was then blown into a trailer. The blown distance can be used to drop out all the fines which have to be separated before gasification.

The Irish single row harvester also blows the chopped fuel into the trailer, and ongoing development work is to perfect the piece size without shattering.

Other harvesting techniques are used to cut and bundle mainly for planting stock, but drying in bundles, then chipping may cause sizing problems.

The success of using coppice willow for a gasifier fuel will depend on its local application and not as a source for large central power generation. Transporting large volumes of low density fuel has its own problems and in very large cropping situations drying and storage becomes a major activity. Clearly supplementary chip wood will be needed for all year generation and that then requires the gasifier to handle multi fuel, for the two fuels have very different characteristics, and then again across hard and soft woods.

All comments regarding coppice willow relate to gasification for engine CHP systems and does not apply to combustion applications.



Champion Rotary Crop Header
Irish Single Row Header
Original Coppice Willow N.Z. test sample 1993
Long Ashton original sample 1994
Irish sample first trial cutting 2000
Irish wood chip comparison