Abstract:
Economic utilisation of biogas arising from sewage sludge is hampered by the
need to remove siloxanes, which damage gas engines upon combustion. This
thesis applies on-line Fourier transform infrared spectroscopy to measure
siloxanes in biogas upstream and downstream of the activated carbon vessels
designed to adsorb siloxanes. On-line analysis provides accurate
measurement of siloxane concentrations with a detection limit below the
siloxane limits set by engine manufacturers, high data intensity and timely
identification of breakthrough. Cost savings of up to £0.007 kWh-
1
may be
realised compared to existing grab sampling. Using on-line analysis, the
performance of full-scale and bench-scale carbon vessels were measured.
Full-scale carbon contactors are typically operated at Reynold’s numbers close
to the boundary between the laminar and transitional regimes (Re = 40 - 55).
This thesis demonstrates, at full- and bench-scale, that increasing the Reynold’s
number to site the adsorption process in the transitional regime increases
media capacity, by 36% in dry gas and by 400% at 80% humidity. It is
postulated that the change in gas velocity profile which occurs as Reynold’s
number increases reduces the resistance to siloxane transport caused by gas
and water films around the carbon particles, and therefore increases the rate of
the overall adsorption process. In the laminar regime (Re = 31) increasing
humidity from zero to 80% led to the classical stepwise reduction in adsorption
capacity observed by other researchers, caused by the increasing thickness of
the water film, but in the transitional regime (Re = 73) increasing humidity had
no effect as no significant water film develops. It is therefore recommended that
siloxane adsorption vessels should be designed to operate at Reynold’s
numbers above 55. By choosing a high aspect ratio (tall and thin) both
Reynold’s number and contact time can be optimised.