- Pulverised fuel ash
Pulverised fuel ash (shortened to PFA), is a by product of pulverised fuel (typically
coal ) firedpower station s. The fuel is pulverised into a fine powder, mixed with heated air and burned. Approximately 18% of the fuel forms fine glass spheres, the lighter of which (c. 75 %) are borne aloft by the combustion process. They are extracted from the flue gasses by cyclones andelectrostatic precipitation .The resultant material is used as engineering fill and as a component for concrete. It has been widely used, particularly in the UK, for concrete block production. The blocks are lightweight and have excellent thermal insulation properties. PFA can undergo a
pozzolanic reaction and become brittle over time. PFA has a fine dust texture and is grey in colour.When newly produced the dust is strongly
alkaline ; apH as high as 11 is known, and >9 is normal. It leaches a solution dominated bysodium andsulfate , with enoughboron (>15mg l-1) to kill most plants, though coastal species often tolerate the salinity and boron to grow on young ash lagoons. In dry conditions these solutes rise to the surface to form a hard salt crust, impeding all plant growth, though hardy grass species such as "Vulpia myuros " can later colonise it.As the ash weathers, its salinity, boron level and pH all fall; the former two are largely removed from surface layers after 5 years outdoors, while pH declines towards 7 in a generally linear fashion at a rate of about 1 pH unit per 20 years. The floral succession approximates to that of a coastal dune system, without the wind-blown deposition, so salt-tolerant plants are replaced by an attractive sward of
legume s and perennials before turning tobirch /willow scrub woodland. A notable feature are the "Dactylorhiza " orchids which often form spectacular colonies 10-20 years post dumping, only to fade away again as the woodland thickens.Hydroseeding is often used to establish vegetation onto PFA due to the inhospitable conditions of most sites.ee also
*
Fly ash - A general term for Ash from Power Stations (not necessarily PFA)External links
* [http://www.ukqaa.org.uk UK Quality Ash Association] A web site providing further information on the applications for PFA.
* [http://www.chezshaw.demon.co.uk/work/research/pfa/pfa.html PFA Biodiversity] A web site about the biodiversity value of PFA.
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