Conquering Siloxane in Digester Gas Fueled, Engine Driven Cogeneration Systems

Abstract

Organic silicon polymers (siloxanes) are increasing in use and are increasingly being detected at municipal sewage treatment facilities and landfills. Several forms of siloxanes are volatile and have been detected in gas evolved from the anaerobic digestion process at many publicly owned wastewater treatment plants (WWTPs). The presence of siloxane compounds in digester gas used to fuel internal combustion engines that generate electricity and heat power (cogeneration) causes severe engine maintenance problems that are related to silicon deposits in piston cylinder and valve head assemblies. At the Annacis Island WWTP in Vancouver, British Columbia and at the Alvarado WWTP in Union City, CA, siloxane related engine maintenance problems lead to decisions to terminate digester gas fueled engine operations.Both plants were motivated to quickly identify and implement systems to mitigate siloxane related engine maintenance problems. After quick review of available options, both plants implemented digester gas treatment systems consisting of gas dehumidification followed by activated carbon adsorption. Activated carbon systems were simple and relied on replacement of exhausted carbon as opposed to on-site regeneration. Activated carbon was extremely effective at eliminating siloxanes from digester gas and carbon life was as expected or better. Removal of siloxane resulted in reduction of build up of silicon in engine oil and dramatic improvements in engine operation as well as reduction in maintenance costs. Maintenance cost reductions greatly exceeded the cost of operating the siloxane removal system.

Energy and Resource Recovery from Sludge

“…The activated carbon can be regenerated to a high capacity using microwave technology . Annacis Island WWTP in Delta (British Columbia, Canada) opted for off-site regeneration because their monthly carbon consumption would not justify the additional complexity, maintenance costs and safety concerns of adding a regeneration system on-site (Slezak et al, 2002).…”

“…Schweigkofler and Niessner (2001) observed siloxane removal using silica gel on bench and pilot scales. (Slezak et al, 2002) 17000 m3/day (600,300 scf/d) One unit, 820kg media. Use gas compression, condenser/moisture removal, reheating, and particle filter Protect Gas Engines Annacis Island, Vancouver, BC (Slezak et al, 2002) 45000m3/day (1,589,000 scf/d) One unit with 500kg media.…”