POWER April 2012 - 50

PLANT CONTROLS
tifuel combustion because the heating value and
bulk density of the individual fuels will vary
greatly. Even so, the total energy flow must be
maintained at a consistent rate to make sure
header steam pressure is maintained at setpoint
and energy flow out of the boiler is at required
demand. Changes in energy flow to the boiler
can cause fluctuations in combustion and steam
production. If not precisely controlled, these
fluctuations will reduce power generation and
can risk plant availability.
FBC boilers do not produce high levels of
emissions, such as SOx
and NOx
. However,
those that fire coal require limestone as an additive
to capture SOx
. Limestone is expensive
to use and is an expense to overall operations
that can reach several million dollars each
year, depending upon the size of the boiler.
Because FBC boilers operate at lower temperatures,
NOx
production from an FBC boiler is
minimal, but if the bed temperature rises above
the NOx
threshold, more ammonia is required
as a mediating additive. These emissions are
16-30-00
17-00-00
Two Fluidized Bed Boilers Under Construction in the U.S.
Metso has sold a complete 100-MW net
biomass boiler island and plant automation
system to two wood biomass plants in the
U.S. Both projects are under construction
and will soon share the title of largest biomass
plants in the U.S.
Texas Wood Fires Future
Record-Holder
The Nacogdoches Generating Facility is
owned and operated by Southern Power, a
subsidiary of Southern Co. Southern Pow4.
New title holder. When startup is
completed later this year, the 100-MW Nacogdoches
Generating Facility will become
the largest biomass-fired power plant in the
U.S. Courtesy: Southern Power
er is among the largest wholesale energy
providers in the Southeast, meeting the
electricity needs of municipalities, electric
cooperatives, and investor-owned utilities.
Power from this new plant, located near
Sacul in Nacogdoches County, Texas, will be
sold to Austin Energy, the municipal utility
owned by the City of Austin, under a 20year
contract (Figure 4).
The bubbling bed boiler is designed to burn
about one million tons of combustible forest
residue from the wooded areas of East Texas,
wood processing residues, and clean municipal
wood waste annually. The biomass fuel
will be sourced within a 75-mile radius of the
plant and will normally be delivered by truck.
The fuel-handling system consists of three
truck tippers, two sets of screens and hogs,
an automatic stacker/reclaimer system, and a
manual stacker/reclaimer system to maintain
two 15-day fuel storage piles.
Bubbling fluidized bed boilers are well-suited
to burn high-moisture-content fuels such
as biomass and are generally simpler to operate
than a conventional solid fuel-burning
boiler, as the temperature of the 6-foot-deep
sand bed can be controlled through combustion
air staging. The boiler produces superheated
steam that is used in a single steam
turbine with four stages of feedwater heating.
The steam turbine exhausts axially into the
condenser. A wet evaporative cooling tower
provides condenser cooling. The boiler gas
50
www.powermag.com
17-30-00
18-00-00
O2
Coal flow (0%-100%)
Coal
Steam flow (635,00 - 1.6 Mlb/hr)
O2 (1%-4%)
Biomass flow (50%-250%)
Biomass
3. Well-controlled Nordic biomass plant. The 240-MW Alholmens Kraft Power Station
is the largest biomass power plant in the world and burns a combination of coal and waste
biomass fuels. The plant uses the Fuel Power Compensator (FPC) to optimize FBC boiler performance.
In this recording, the operator is manually adding coal in 10% to 15% steps while
the fuel control is responding to the FPC and driving the biomass flow downward to maintain
constant energy output. Note how the steam flow remains unchanged during the fuel split
change. Source: Metso Automation
Steam flow
side is equipped with a baghouse for particulate
matter control plus an aqueous ammonia
injection selective noncatalytic reduction system
for NOx
control.
American Renewables handled development
and initial planning for the Nacogdoches
project and sold it to Southern Power
in 2009. Construction of the plant began in
2009; commercial service is scheduled to
begin later this summer.
Waste Wood Not Wasted by
Florida Plant
The Gainesville Renewable Energy Center
(GREC) will supply electricity to Gainesville
(Fla.) Regional Utilities (the city-owned
utility) under a 30-year power purchase
agreement. As with the Nacogdoches Generating
Facility, the boiler will use bubbling
fluidized bed technology and annually burn
about a million tons of waste wood from logging
and mills as well as urban wood waste
from clearing, tree trimming, and pallets-
all sourced from a 75-mile plant radius.
Gainesville Renewable Energy Center LLC
was developed by American Renewables.
Zachry Engineering Corp. is providing design
engineering and procurement services
as part of the engineering, procurement,
and construction team. Fagen Inc. is the
construction contractor for both projects.
Commercial operation of this $500 million
Florida plant is scheduled for late 2013.
POWER | April 2012

http://www.powermag.com

POWER April 2012

Table of Contents for the Digital Edition of POWER April 2012