Behaviour and adsorptive removal of siloxanes in sewage sludge biogas

Abstract

We investigated the behaviour of siloxanes, which adversely affect biogas engines, as well as their concentration levels in sewage sludge biogas in Japan. We also performed experiments on the absorptive removal of siloxanes using various adsorbents and determined the main adsorbent characteristics required for the removal of siloxanes. The results of our study on the concentration and composition of siloxanes in biogas were similar to previous reports. Moreover, we found that the concentration of siloxanes changes in relation to the outside air temperature based on real-time measurements of siloxanes using a continuous analyser. We further speculated that the continuous analyser would accurately indicate the siloxane concentration in model biogas but overestimate the siloxane concentration in actual biogas because of positive interference by VOCs and other biogas components. In the siloxane adsorption experiment, the equilibrium uptake of both cyclic siloxanes, D4 and D5, was positively related to the BET-specific surface area of the adsorbents and the fraction of the external surface area taken up by relatively large diameter pores. We attributed the adsorption results to the fact that the siloxane molecules are generally larger than micropores; therefore, they are less susceptible to adsorption to micropores. Based on these results, we concluded that adsorbents with large BET-specific surface areas, especially those with a high external specific surface area and pores of relatively large diameters, are desired for the removal of siloxanes.

Adsorption of octamethylcyclotetrasiloxane on silica gel for biogas purification

“…The adsorption capacities determined experimentally are in agreement or even better than literature data concerning siloxane removal by silica gel [11,15,18,19].…”

“…They postulated that the siloxane concentrations may vary on a daily basis due to adsorption onto the gas collection pipework during the cooler night, followed by desorption when the sun comes up during the morning. Oshita et al [18] also found that the concentration of siloxanes changes in relation to the outside air temperature in a biogas from a waste water treatment plant. They concluded to an adsorption on the inner surface of the gas pipe or gasholder when the external temperature decreases.…”

“…Experimental set-up for dynamic adsorption tests D4 (C 8 H 24 O 4 Si 4 ) was chosen as a representative of VOSiCs because it is one of the major silicon compounds present in both landfill gas and sewage gas [3,12,18]. Tests were conducted with a D4 content of 30 ppmv (%400 mg/Nm 3 ).…”

“…One major way to remove VOSiCs is adsorption [12][13][14][15][16][17][18][19][20][21][22]. In this paper, dynamic laboratory-scale experiments were conducted on a synthetic polluted gas stream to compare three adsorbents for the removal of octamethylcyclotetrasiloxane (D4): an activated carbon (AC), a zeolite (Z) and a silica gel (SG).…”

Techniques for Siloxanes Removal from Biogas Produced by Sewage Sludge Digestion

“…The performance of adsorption is highly dependent on the molecular size of each siloxanes and the pore distribution of adsorbents, the carbon-based adsorbents with micro-pores are more efficient at removing L2 than D4 or D5, while non-carbon adsorbents that have meso or macro-pores are proper for D4 and D5 adsorption [9]. It was concluded that the adsorbents with large BET-specific surface areas and pH value are desired for the removal of siloxanes [10,11]. The siloxanes removal efficiency could reach 90-99% by solid adsorption technology, while the adsorbents are usually non-selective and the humidity will decrease the treatment efficiency of activated carbon.…”