The supply of biomass is a key area, which is not the subject of this thesis. However, it should be realised that the quantities of biomass involved are enormous. A 660 MWe boiler at Drax with a thermal efficiency of 40% requires 1650 MWth of biomass and with a typical GCV of 18 MJ/kg this is 91.7 kg/s or for a typical 10 hour use per day is 3.3 kTonnes per day. For a typical bulk density of 300 kg/m3 this is 11,000 m3 per day. A one week supply; which is the minimum fuel reserve; is thus 77,000 m3. At Drax power station pellet storage silos of 100,000 m3 have been constructed. These are the world’s largest silo stores and they need to be filled every week. Drax intends to have three of their 660MW boilers operating on biomass by 2017 and this will be the UKs largest source of biomass electricity. Three boilers will remain on coal.
The sourcing of these large quantities of biomass is on a very large scale and includes ships arriving fully loaded with pellets, some from the USA – but they are sourced from all over the world, a ship arrives every day and is unloaded onto trains that take the biomass to Drax. This uses the same system as for delivering imported coal to Drax.
For coal the bulk density is higher so the storage volume required is less. However, coal can be stored in the open as it does not absorb rain water excessively. Biomass cannot be stored in the open as pellets, as they absorb moisture and rot and hence there is the necessity to build large fuel storage silos. This storage creates auto-ignition hazards and dust explosion hazards during the filling of the store and during the extraction of the biomass for feeding to the mills, as this also creates a biomass power explosion hazard. The transport of the biomass to the mills and from the mills to the burners creates further explosion risks. Unfortunately, these risks are shown to be high as there have been several biomass power plant explosions.
This is illustrated by the compilation of recent accidents, most of which occurred once changes to original system had been made or when working with biomass for the first time.
2005, Chetwynd mill, British Columbia, Canada. Work on a shutdown burner created a cloud of dust that was ignited by cutting torches. At least one worker was injured and taken to hospital. (Hoekstra, 2012 )
2008, Pacific Bioenergy’s pellet plant, Prince George, Canada. An explosion in the pellet plant in March. (Hoekstra, 2012 )
February 2008 , Imperial Sugar Company, Georgia. An overheated bearing on a conveyor initiated an explosion and fire lead to 14 fatalities. (CSHIB, 2008)
June 2009, University of South Carolina’s wood-burning boiler, an explosion followed two previous smaller explosions and a series of mechanical breakdowns. (Wayne, 2011)
August 2009, Pinnacle Pellet in Armstrong, Canada. The company experienced an explosion at its Williams Lake plant. That explosion was caused by a combination of air, dust and a spark, said the company. (Hoekstra, 2012 )
February, 2010, Brilon, Germany. A biomass plant exploded killing three workers and causing a subsequent fire. (Forum)
December 2010, Pacific Bioenergy’s pellet plant in Prince George in
Canada. An explosion caused extensive damage where dust was cited as a factor ignited by a spark (Hoekstra, 2012 ).
January 2011, Tolko’s Soda Creek sawmill, Williams Lake, Canada. An explosion was caused by dust in one of the mill’s motor control centres.
(Hoekstra, 2012 )
February 2011, Babine Forest Products, Canada. A small explosion took place that was fed by unusually dry sawdust, according to a B.C. Safety Authority report. (Hoekstra, 2012 )
20 June, 2011, Georgia Biomass plant. A dust explosion was caused by an overheated roller/bearing assembly in a pelletizer that sparked causing the explosion at the factory that had been online for just over a month. (Stepzinski, 2011)
April 2011 Pinnacle Pellet in Armstrong, Canada. Explosion was caused a fire that quickly spread into the basement and into the attic. (Hoekstra, 2012 )
30 October, 2011 Tyneside port biomass storage facility in South Shields stored biomass, which is used at Drax power station Yorkshire. 25 tonnes of which caught fire in storage. (BBC, 2011)
February, 2012 fire at Tilbury biomass power station burnt for days and needed 100+ firemen to control the fire, caused by run-away heating in a hopper. (Mail, 2012)
A very recent tragic incident of wood floor mill explosion in UK (17 July 2015) was the Bosley Mill sawdust explosion in Macclesfield. There were 4 deaths and the plant was almost completely destroyed. (BBC, 2014)
When biomass power stations were first developed in the UK the operators intended to use biomass delivered as logs or bales of hay. However, it was found that the milling of the biomass on site was a key problem area and that different mills were required for wood and agricultural biomass. A problem with the use of whole logs was that of transport. Even dried logs would have around 5% moisture
and this would mean that for a ship the transport costs were paying for water to be moved to the power station. Thus it was realised that the wood should be dried. Also logs do not fill a closed volume easily and this led to the transport costs being too high as the mass of biomass moved per ship or lorry load was too low.
The solution was to move the pulverisation and biomass drying operations to the source of the biomass – the forest or near a group of farms. This has been done for the large power stations such as Drax where large biomass pulverisation plants have been built on the forest sites in the USA used for sourcing the biomass. The pulverised biomass is then dried in a fired kiln and then compressed into pellets. The shipping of dried pellets reduces the transport of water and also the packing density of pellets is greater than that of logs, so a ship of the same volume carries greater biomass energy in pellet form. These pellets at the power station are fed directly to the coal mills where the pellets are broken up to yield the pulverised particles that the pellets were manufactured from. This process has been reproduced in the present work with biomass supplied as pellets broken up in a small mill at Leeds so that the particles investigated were typical of those being burnt in power stations. It will be shown that these particles are relatively large and this led to a theme of this research on the influence of particle size on biomass dust cloud flame propagation.