Characterization of full-scale carbon contactors for siloxane removal from biogas using online Fourier transform infrared spectroscopy

Abstract

In this study, online Fourier transform infrared (FTIR) spectroscopy has been used to generate the first comprehensive characterization of full-scale carbon contactors for siloxane removal from biogas. Using FTIR, two clear operational regions within the exhaustion cycle were evidenced: an initial period of pseudo-steady state where the outlet siloxane concentration was consistently below the proposed siloxane limits; and a second period characterized by a progressive rise in outlet siloxane concentration during and after breakthrough. Due to the sharp breakthrough front identified, existing detection methods (which comprise field sampling coupled with laboratory-based chromatographic determination) are insufficiently responsive to define breakthrough, thus carbon contactors currently remain in service while providing limited protection to the combined heat and power engine. Integration of the exhaustion cycle to breakthrough identified average specific media capacities of 8.5-21.5 gsiloxane kg(-1)GAC, which are lower than that has been reported for vapour phase granular activated carbon (GAC). Further speciation of the biogas phase identified co-separation of organic compounds (alkanes and aromatics), which will inevitably reduce siloxane capacity. However, comparison of the five full-scale contactors identified that greater media capacity was accessible through operating contactors at velocities sufficient to diminish axial dispersion effects. In addition to enabling significant insight into gas phase GAC contactors, the use of FTIR for online control of GAC for siloxane removal is also presented.

Understanding the effects of the origin, occurrence, monitoring, control, fate and removal of siloxanes on the energetic valorization of sewage biogas—A review

“…Engine manufacturers claim higher siloxane limits in biogas than other ECS, but there is a wide range of limits depending on the manufacturer, ranging from 12 mg/m 3 (Jenbacher) to 28 mg/ m 3 (Caterpillar) or 30 mg/m 3 (Waukesha) [6,69,129].…”

“…For example, a relative humidity of 50-70% can reduce the siloxanes adsorption capacity a factor of 10 or greater [4,106], probably as a result of water adsorption and formation of hydrogen bonds with oxygen functional groups on the adsorbent material surface, which block the adsorption sites. Moreover, Hepburn et al [69] showed that bed dimensioning (height-diameter ratio) also plays an important role in the siloxane adsorption capacity, concluding that operation at high Reynolds number favoured siloxane adsorption.…”

“…Monitoring results from six field cases in Finland (three landfill sites and three WWTP) were reported, but no economic comparison between on-line monitoring and high frequency off-line analysis was included. More recently, an on-line FT-IR was directly used to measure the siloxanes concentration at the inlets and outlets of biogas adsorption beds [69], thus giving real-time protection to the downstream energy conversion equipment (an internal combustion engine in that case). Cyclic and linear siloxanes were analysed at 798-817 and 837-867 cm À 1 , respectively.…”

“…The most common adsorbents used to remove siloxanes are activated carbons (AC), which have been widely implemented to treat synthetic gas and real landfill/sewage gas due to their excellent adsorption capacities [2,4,6,8,11,69,[94][95][96][99][100][101][102][103][104][105]. Inorganic adsorbents, such as silica gel, zeolites and alumina, and other polymeric adsorbents, such as polyurethane foams, have also been widely employed [4,8,70,[94][95][96]106].…”

On‐line management of siloxane breakthrough in carbon contactors using changepoint detection

“…) or of carbon bed breakthrough (Hepburn et al . ). In addition, there is a lag time between sampling and receiving the results (Hepburn et al .…”