Alternative low-cost solid media for scrubbing of hydrogen sulphide from piggery biogas

Australian pig producers are increasingly using biogas from on-farm manure management for shed heating and electricity generation. However, hydrogen sulphide (H₂S), a toxic and corrosive gas ingredient in raw piggery biogas, is currently impeding further adoption of biogas technology. To remove H₂S, biogas is typically passed through a packed column containing a commercial solid medium with an active ingredient (such as iron) which reacts with and sequesters H₂S while allowing the treated biogas to pass to the point of use (Skerman et al. 2012). However, periodic replacement of media (when spent) represents a significant operating cost for pig producers using biogas, consuming as much as 20% of the financial benefit of using the biogas. The aim of this laboratory-scale study, batch H₂S sorption/reaction was to evaluate and compare the performance of a commercial scrubbing medium with that of several low-cost, agricultural and industrial by-products.

Experiments involved passing a pre-humidified standard gas (Encore Automation Pty Ltd; WA) with 2,000 ppm H₂S in high purity nitrogen, through the various media, suspended on stainless steel mesh, in a PVC pipe canister (internal diameter 29.8 mm). In-line sensors (Alphasense H2S-BE, Great Notley, CM77 7AA; UK), which had been cross-calibrated with standard gases, were used to measure H₂S concentration in the treated gas discharged from the canister over time. The media tested in the experiments included cg₅^® commercial iron-oxide pellets (Clean-Gas, ACP Technologies Inc, USA), and the alternative media: granular steel furnace slag (<5 mm), red soil (krasnozem/red ferrosol, Toowoomba, QLD), commercial compost (Naturegrow – Amgrow Pty Ltd, QLD), composted feedlot manure (Kerwee feedlot, Jondaryan; QLD) and biochar (Green waste 550, Pacific Pyrolysis Pty Ltd, NSW). The alternative media were passed through a 2 mm sieve prior to testing, to remove any coarse fragments.

A measured H₂S of 0 ppm indicates that the media had effectively removed all of the H₂S in the canister inflow, while H₂S >0 ppm (following breakthrough) indicates that a portion of the H₂S in the canister inflow had not been absorbed/removed by the media and had been emitted through the canister exit (Fig. 1). The results showed that the cg₅^® commercial iron-oxide pellets vastly outperformed the alternative media, with a substantially higher H₂S loading before breakthrough of H₂S occurred. However, the red soil showed noteworthy performance. These results suggested that, because of the lower loading capacity, the alternative media would probably require a significantly larger scrubbing column and more frequent medium replacement, compared to a commercial medium. The red soil medium appeared to warrant further investigation, especially for use as a secondary polishing step to treat lower residual levels of H₂S following primary biological scrubbing.

Fig. 1.  Experimental data for: (a) various alternative media, red soil (■), composted feedlot manure (☐), biochar (×), commercial compost (▵), granular slag (); and (b) for the commercial media cg₅^®, showing the measured H₂S concentrations in the treated gas exiting the scrubber canister vs cumulative H₂S fed into the canister. Note the different extents of scale on the horizontal axes.