Energy Sources

Woody biomass for use as an energy feedstock comes from a number of sources:

• Forestry residue - slash, tops and unmerchantable stemwood from trees harvested for saw or pulp logs.  Forest residue may include the cutover (those left at the stump) depending on location of harvest or material brought to the landing during the harvest operations.
• Wood processing residues - bark, sawdust, shavings, offcuts, etc. from processed wood for pulp, panel board, construction timber, furniture, etc. and black liquor from pulp plant residues.
• Woody crop plantations - short rotation crops grown specifically for energy purposes, possibly in association with land disposal of sewage and industrial effluent.
• Firewood - from dead trees, prunings, tree removal and a range of other sources used as firewood.

Cutover remains on the forest floor when trees are cut or trimmed, and has to be collected.  Forest residues produced at the landing with whole tree harvesting require preparation and loading for transport.  A range of preparation and transport arrangements are possible with resulting cost differences.

For information on forest residues click here

Various management regimes are possible for short rotation tree crops.  Short rotation woody crops grown intensively under a coppice regime are a means of sustaining biomass supply.  Tree crops can also be grown in association with land-based wastewater treatment, the trees taking up nutrients in the course of treating the effluent.  There has been some research into species selection and breeding programmes, and into hydraulic loading rates of effluent onto various energy crops and soil types.

Forest harvesting systems, have been developed overseas and some plant is in operation within New Zealand.  The plant is generally used to scavenge harvest landings for material to produce woodchips but can also be used for extracting cutover, or possible for short rotation plantations.

Techniques for drying, handling and storage of the material have been developed for a number of local applications.  Wider development will occur once the relative economics improve.

Wood residues and woodchips (or wood comminuted into smaller chunks) can be mechanically fed into suitable heating plant.  The resultant heat can be used directly or to raise steam for process needs or for electricity production via a steam turbine.  The mechanical handling and burning of wood is a proven technology.

A diverse range of technologies exists to convert woody biomass to useful energy, including combustion, gasification, pyrolysis and hydrolysis/fermentation systems. 

• Combustion processes for heat applications consume most of the biomass for energy in New Zealand.  Combustion can be in sloping grate or fluidised bed boilers. Although combustion is a mature technology, refinements relating to emissions control and efficiency continue.  Most biomass combustion systems installed in New Zealand use grate technology and are available for a wide range of applications, however plant with improved efficiencies and able to handle fuel with higher moisture contents such as fluidised bed combustion plants have recently been introduced.

• Gasification technologies have reached the commercial evaluation phase with several plants overseas undergoing detailed evaluation and monitoring.  Gasification, as a technology, has been proven for coal applications (though is still not widespread) and is currently being adapted for biomass.  The gas produced (“syngas”) is a mixture of carbon monoxide and hydrogen, with a low to medium heating value.  Gas cleaning issues (particularly related to silica content and tars) are now being addressed in MW-scale demonstration plant.  The technology is progressing to full large-scale commercial uptake, and is expected to take a more dominant position as a future means of large-scale energy conversion once clean-up problems are overcome.

Cogeneration of heat and power is particularly efficient where there is a demand for the heat.

For further information click here

Co-firing of biomass with coal presents an effective means of displacing a small portion of fossil fuels (3 -8 %) at minimal cost for heat generation.