We’re all familiar with the stylish TV ads that Bord na Mona have to promote their briquettes, and we’re possibly envious also: blazing fire, attractive company… But spare a thought for the poor guy who must clean out the grate the morning after. All that ash!!!

That ash is the mineral matter which remains after the organic component of peat has been burned. It is one of two components of peat, along with moisture content, that those in Bord na Mona take seriously to ensure the production of high quality briquettes.

Ash levels must be closely monitored "on-line" (during production) and kept to a minimum. Peat contains anywhere from 2-12% ash; 4% is the ideal quantity because:

• Excess ash results in poor quality, bad burning fuel.

• Ash wears out factory components quickly, leading to excess factory downtime and raised costs.

With the assistance of the physics department in the University of Limerick, Bord na Mona now employs a novel system for the instantaneous analysis of ash on moving conveyors. This non-invasive system, which provides reliable, instantaneous data is based on the physical principle of gamma ray attenuation through matter. Using gamma radiation from two different sources - Caesium 137(Cs137) as a hard source (662 keV) and Americium 241 (Am241) as a soft source (~60 keV), and taking into account the fact that the mass attenuation coefficient m is a measure of the amount of mineral matter in the peat, one can determine a percentage ash value.

The radiation sources are housed in a stainless steel and lead container, and the beams are collimated as they exit the casing underneath the conveyor belt. The radiation passes through the belt and the peat it carries, and enters a scintillation detector located directly above the belt and source. The beam of gamma photons attenuated by the peat is then detected in the scintillation crystal where their energies are converted into photoelectrons. These photoelectrons are amplified by a photomultiplier/amplifier assembly, and the Am241 and Cs137 pulses are separated by an analyser card.

These two streams of pulses carry different information regarding the peat. Radiation from Cs137, being the more energetic, is only attenuated by the height of the peat, whereas that from Am241 is attenuated both by the height and the mineral content. This fact allows us to extrapolate a value for ash attenuation and thus a percentage.

Data analysis is performed in a programmable controller by a complex STEP5 program, and linked to two slave PCs which give an instantaneous ash value for each wagon of peat being tipped into the factory.

Contact: Leah Wallace, Department of Physics, University of Limerick;
E-mail: lmw@gemini.physics.ul.ie