(270d) Siloxane Removal from Biogas Using a UV Photodecomposition Technique

Authors: 
Tsotsis, T., University of Southern California
Divsalar, A., University of Southern California
Divsalar, H., University of Southern California
Dods, M. N., University of Southern California
Siloxane Removal from Biogas Using a UV Photodecomposition Technique

Containing typically >50% methane, biogas is considered today as one of the sustainable (“green”) sources for electricity and power generation. Unfortunately, the presence of NMOC compounds in biogas has prevented its widespread use for power generation. Si-containing compounds known as siloxanes are a particularly troublesome class of trace impurities found in biogas; they turn into silica (SiO2) particulates during their combustion in engines during power generation that deposit as a thin layer inside the engine, which then requires frequent maintenance. The conventional systems to remove the siloxanes include adsorption, absorption, but these techniques are not all that effective to remove the siloxanes from biogas prior to its combustion. Our research group has proposed, instead, a simple, efficient and cost-effective technique for siloxane compound removal that employs a UV photodecomposition reactor (PhoR), which in addition also removes a wide variety of other non-methane organic compounds (NMOC) from biogas. The technique involves irradiating the siloxanes with UV light at wavelengths <200 nm, which cleaves the Si-C bonds of the siloxane molecule to yield Si and CH3 radicals. When oxygen is present, the UV light also helps to cleave the O=O bond yielding two oxygen radicals, which are then available to react with the Si radicals from siloxane decomposition to form SiO2 solid materials. Landfill gas (LFG) is an ideal medium to apply the approach to decompose and to remove the siloxanes, because of small concentration of O2 (1-3%) that it contains; in addition, the absorption spectra of methane (the main component of LFG) are located in the IR region, and thus it is not affected by the UV radiation. We have carried-out lab-scale experimental studies of the use of PhoR with simulated LFG containing linear (L2 and L3) and cyclic (D4) siloxanes as well as air contaminated with the same trace compounds; these studies have helped to show the effectiveness of this method to decompose such siloxanes in both carrier media.

We have also developed a 3-D PhoR model that can accurately account for all transport and reaction phenomena that occur in the reactor, and we have utilized ANSYS® Fluent in order to solve the relevant equations. The model results have been compared with the lab-scale experimental data, and this has validated the model’s ability to describe the PhoR behavior. Using this model, we designed a scaled-up PhoR system for field-testing of the technology, with the key goal to validate its ability to treat real LFG in a practical setting. The field-scale PhoR proved again to be quite efficient in removing the siloxanes in the real landfill environment, with high siloxane removal rates attained in the presence of numerous other NMOC compounds, including sulfided and halogenated species one finds in real LFG.. These positive findings have led us to propose a commercial-scale PhoR system, competitive to conventional adsorption systems that can be practically applied in existing landfill plants to obtain high siloxanes removal rates without associated secondary emissions. We have performed a technical and economic analysis (TEA) of this system by investigating both its capital and operating costs, and have compared those costs with the corresponding costs of Adsorption Systems. The TEA results are based on simulation results using the data-validated PhoR model, quotes from various vendors and data provided by our industrial partners. Considering the total cost for these two systems, one can state that the PhoR system costs are quite competitive to the adsorption method, which is the current state of the art for landfill plants.