A storage tank is only as good as its cover.
Less than three years ago, a storage tank cover manufacturer in Fredericton, NB, Canada, introduced its structurally supported “geomembrane” system for a project near Columbus, Ohio.
While the firm generally designs its covers for odor control, algae control, gas collection and heat loss control, this particular design also had some impressive breadth to it, covering two 65´-diameter storage tanks. What really caught the eye were its retractable roofs, which allowed easy access into the tank’s interior for maintenance and inspection.
About 8,800 miles away in Thailand, Geomembrane Technologies’ cover systems use special features to provide adequate biogas withdrawal. For example, floating beams installed directly into the tanks at strategic points enabled the tank covers to capture biogas from the anaerobic digestion of waste streams at various plants belonging to Thai Beverage Energy.
Using biogas for process heat
Used as a substitute for natural gas and fuel oil, the biogas finds its way into factory boilers as a source for process heat. Cost-savings on energy is considerable. Each of the 5 plants is designed to treat up to 600 cubic meters, or 158,000 gallons of wastewater from ethanol distilleries.
Approximately 50 percent of the energy produced from biogas is said to be enough to power a distillery’s boilers, eliminating the Thai’s previous reliance on petroleum fuel oil.
The cover system is part of proprietary equipment and engineering support for anaerobic reactors with biogas recovery and use. The new anaerobic facilities treat wastewater and convert remaining degradable organic components to biogas as an energy source, displacing fossil fuels.
In August 2009, Geomembrane Technologies announced its intent to participate in a project that qualified for the carbon credit market under the United Nations Framework Convention on Climate Change. Environmental benefits include a reduction in greenhouse gas emissions, conservation of groundwater resources, contamination prevention of groundwater sources, and a reduction in fuel oil use and other non-renewable energy forms.
Covering the bases
Companies like CST Industries make storage tank covers to suit many options, starting with the most common roof design, the externally supported roof. ESRs can be made from glass, stainless steel or epoxy. Having a smooth internal roof surface and no rafters, this type is used when moderate to high pressure or vacuum design limits are expected. Heavy load conditions expected from mixers and other ancillary equipment typically calls for a design of this kind.
On the other hand, an all-aluminum, pressure dome design can accommodate light-load conditions. This free-span type resists the kind of corrosive conditions that typify digester applications, where the digester’s gas zone is considered the tank’s most corrosive area.
Smaller diameter tanks can be outfitted with a stainless “knuckle” roof for lighter pressures.
Fabricated from stainless steel for gas zone longevity, this type of roof is used when the tank has no load-bearing requirements. Dual membrane covers are designed to work in situations where gas pressures run low to moderate, and where the design does not require that the cover be made from fixed steel or aluminum.
Keeping biomass dry
In today’s bioenergy storage systems, “sustainability” is the operative word.
“Our customers understand the growing trend to supplement or replace fossil fuels with alternative fuels, such as biomass,” said John Farris, of CST Industries. “Today, we are providing storage systems to accommodate both coal and biomass, as some companies are exclusively using biomass, while others are ‘mixing’ their use to generate heat and power.”
