Coal, Emissions, Gas

Deer Island plant built green throughout

Issue 2 and Volume 100.

FIELD NOTES

Deer Island plantbuilt `green` throughout

The 70-MW on-site thermal power plant serving the Deer Island Waste Water Treatment Facility faced an assortment of challenges, from stringent clean water and air requirements to aesthetic and neighborhood concerns. The 1.3 billion gallon-per-day, $3.5 billion Boston Harbor Project (BHP) is the largest public works project ever undertaken in New England, according to Barrientos Engineers & Architects. The 200-acre site will include 14 buildings and two miles of 30- to 60-foot wide treatment batteries. The BHP, constructed by Massachusetts Water Resources Authority (MWRA), will provide primary wastewater treatment plus oxygen-activated sludge secondary treatment.

The Massachusetts Environmental Protection Agency regulations required two independent sources of electricity be provided to power the treatment facility. Boston Edison Co. will provide primary electric power through a cross-harbor submarine cable system. The plant is designed to be a backup power source. To meet a 70-MW peak load, the site has two 16-MW combustion turbine generators, one 18-MW topping turbine steam generator, two hydroelectric generators in the waste-water outfall and two existing 6-MW diesel generators. The plant will also provide 277 MBtu of thermal energy into a district heating system for the waste-water process and domestic uses.

The plant is founded on 580 precast, prestressed 16-inch concrete piles, each approximately 90 feet long. Each pile is designed to hold 175 tons. Foundation design was complicated by glacial till-bearing soil at a 60-foot depth, requiring special treatment for anticipated pile relation. Another foundation problem involved lateral loading and soil movement, which was expected to occur in a 30-foot-thick clay layer between 30 and 60 feet down. The MWRA has approved construction of a 100-foot-tall berm within 250 feet of the plant. The berm will be constructed after the plant is completed. To prevent failure of the pile, extra reinforcing steel was specified, along with 6,500 psi concrete. Pile driving was completed in two months with no particular problems.

As one of the larger, more prominent buildings visible from the harbor, the plant exterior was designed to blend in with historic power plants of the area. Set in a marine environment with salt air and heavy winds, the project used corrosion resistant materials. Stringent noise limitations were imposed on the power plant as part of the neighborhood negotiations with the town of Winthrop, Mass.

The facility is designed to provide all the energy for the peak domestic heating and process thermal energy for the entire Deer Island complex. The power plant will provide high-temperature water to the primary treatment side of the heating system through use of a highly efficient cogeneration steam cycle. Two boilers will produce high-pressure steam to be expanded through a steam turbine generator to produce low-pressure steam with electricity as a byproduct.

The plant will burn methane-rich digester gas, a byproduct of anaerobic digestion in the waste-water treatment process. The gas, collected in 16 egg-shaped containment structures, is expected to reach an average annual production of 171,000 cubic feet per hour. It will be cleaned of hydrogen sulfide to minimize SO2 emissions. If the digester gas weren`t used by the power plant, it would have to be flared, an unattractive and smelly alternative. Low-sulfur No. 2 oil is the supplemental fuel for the boilers and the only fuel for the combustion turbines. The sulfur-cleaning process will produce pure sulfur which the authority plans to sell on the open market.

The plant uses secondary effluent from the treatment plant instead of potable water for its cooling needs. In addition to the savings from the cost of clean water, the feature allows the plant to avoid cooling towers which would add to operational noise. The design of the two 42-foot-high boilers includes conservative volumetric heat release, low NOx, flue gas recirculation, dual fuel with co-firing capabilities and low emissions of other pollutants. Total NOx and CO pollutants will be nearly half of those from the old Deer Island power facility, despite the substantial increase in thermal and electrical power production.

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An internal view of Deer Island Waste Water Treatment Facility`s on-site thermal power plant.