Rail loading wood pellets



Ian Atkinson, Engineering Director, Spencer Group, Hull, UK


Large scale imports into the UK of wood pellets are primarily driven by those generators who have converted power station units to burn wood. UK legislation requires the fuel to come from a sustainable source; sustainable generally means from ‘farmed or managed forests’ with an audited regime of re-planting and a harvesting cycle of 60 to 80 years.


To date, RWE, E.ON, and more notably Drax, have been, or are in the process of, converting generating units to operate on biomass – predominantly wood pellets, with Lynemouth Power looking like being the next to do so. One of the quirks of the mathematics is that in round numbers 1m3/h of wood pellets will produce the heat to raise enough steam to generate 1 MWh of electricity. A 500MW unit will consume approximately 500m3/h or 300 tph of pellets, and while operating for 8000 hours per year, a unit will consume approximately 2.4 million tonnes of wood pellets. If all conversion plans come to fruition, a market for some 19.5 million tonnes per year of wood pellets will have been created. However, in reality, this is more likely to be 12 to 13 million tonnes per year. Currently tonnages imported into the UK are rising. Imported wood fuel in 2012 totalled 3.5 million tonnes, and even by conservative estimations, by 2016 the demand looks to grow to four times that number.

Logistical issues

To illustrate the logistical challenges using the lower figure of 12 MTPA, this equates to the bulk carrying capacity of some 207 Panamax vessels a year delivering the fuel to UK ports for offloading, storage and forward shipment to the power stations. Existing port infrastructure is geared up for these tonnages of coal, but biomass and particularly wood pellets pose their own set of unique challenges. To comply with legislation, the pellet producers are prohibited from using any form of artificial binder. Pellets are formed at such a pressure that friction heating melts the lignin in the cell walls to form a natural binder. This bond is not particularly strong, so from the moment the pellet is formed, it starts to degrade. By the time pellets reach the UK, up to 10% of the cargo may have reverted to sawdust. Another major problem is that pellets swell and revert back to sawdust if they get wet.

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