POWER February 2011 - 60

PLANT DESIGN
Construction of the Virginia City Hybrid
Energy Center began on June 30, 2008.
The plant is projected to begin commercial
operation in summer 2012 (Figure 3).
Direct Firing in a CFB Boiler
Technology developments for the direct firing of
biomass benefited from the Public Utility Regulatory
Policies Act (PURPA), which was passed
as part of the National Energy Act in 1978. It
promoted the conservation of energy, efficient
use of facilities and resources, and equitable
rates for customers. It encouraged greater use of
renewable energy by creating an expanded market
by adding non-utility producers.
A variety of boiler designs were used under
PURPA's regulations, including bubbling
fluidized bed, circulating fluidized bed, fixed
grate stokers, sloping grate, traveling grate
stokers, and water-cooled vibrating grates.
These technologies typically are all available
up to a 50-MW capacity for a single unit.
Over that size, CFB becomes the technology
of choice.
Like PC technology for coal, CFB technology
scales up more easily for coal/biomass
than stokers or grates. Major boiler manufacturers
such as Foster Wheeler, Alstom Power,
and Metso Power are offering 300-MW units
for biomass and planning for the next-largersize
units. (See " FBC Control Strategies for
Burning Biomass, " October 2010.)
Direct firing of biomass can also be added
to a plant that has smaller, older, less-efficient
3. Dominion does biomass. This is an artist's rendering of the Virginia City Hybrid Energy
Center, which will use four identical boiler feed systems that will supply wood chips and forest residuals
to two circulating fluidized bed boilers. Biomass will account for about 20% of the fuel input
to the 558-MW net power plant when it is completed in summer 2012. Courtesy: Dominion
coal-fired units by replacing one or more of
those units with a modern, efficient boiler that
is designed to best burn solid fuels, including
biomass. Public Service of New Hampshire
(PSNH), a subsidiary of Northeast Utilities,
did just that at its Schiller Generating Station
in Portsmouth, N.H. A new 50-MW combination
biomass/coal-fired boiler replaced an
existing, similarly sized unit. The project was
undertaken to qualify for renewable energy
certificates and to comply with lower emissions
limitations.
The retrofit included a totally new biomass
handling/storage/processing system and an
extension from the existing coal-handling
system to the new CFB boiler island. Figure
4 is a view of the boiler island during construction.
In this view, the boiler's cyclones
are visible, before the building and systems
were completed and enclosed. Schiller Station
was awarded POWER's Marmaduke
Award in 2007. A more complete description
of the plant can be found in " PSNH's
Northern Wood Power Project Repowers
Coal-Fired Plant with New Fluidized-Bed
Combustor, " in our August 2007 issue.
For new generation capacity, there are
good reasons to consider a CFB boiler. This
technology is proven and has been used since
the 1980s. Numerous plants equipped with a
CFB boiler utilize agricultural, forest, mill,
and urban biomass products. When forethought
is given to boiler and plant design, a
variety of solid fuels can be used, as needed,
due to availability, market, regulatory, or other
circumstances.
Drax Power has been investigating the ad4.
Work in progress. Public Service of New Hampshire retrofitted Unit 5 at its Schiller Station
with a CFB burning 100% wood or 100% coal. This photo shows the boiler island during erection
of the fluidized-bed combustor, before the building shell was completed. Courtesy: PSNH
dition of three 300-MW biomass-fired CFB
plants in the UK. One would be located adjacent
to the existing 4,000-MW coal-fired
plant and another would be sited in the Port
of Immingham. Sites for the third plant are
being evaluated. A variety of biomass fuels
are being investigated including wood chips,
wood pellets, miscanthus, straw pellets, bagasse,
and logs. Initially, much of the fuel
will be imported while indigenous sources
are developed.
Fuel Issues for Large Boilers
At the outset of any project, the combustion
engineer and boiler manufacturer will seek
to establish a variety of parameters that
define the design basis for the boiler. Site
conditions such as air temperature, humidity,
elevation, and cooling water temperature
affect plant design. Knowing the type
of fuel that will be supplied to the boiler
is very important because fuel properties
and characteristics affect boiler design and
operation. Different solid fuel boilers have
unique design and fuel requirements. For
example, the fuel injected into a PC boiler
60
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POWER February 2011

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