of biomass. Availability of blower trucks and
variability in the condition of the onboard
blower proved to be significant impediments.
Second, the system is not equipped with milling capability and can therefore not process
pelletized fuel.
As the biomass program evolved, it became more apparent that developing the
capability to handle pelletized fuel was essential. Fuel supply economics and flexibility as well as higher generation levels (vs.
cofiring) required pelletized fuel.
Nanticoke’s Direct Milling Program
Direct injection biomass systems are among
the most effective way to employ significant
volumes of biomass via cofiring. However,
it is expected that commercial operation will
require that the biomass is densified in some
form to facilitate long-distance transportation.
The Nanticoke team examined ways that commercial grade wood pellets (see table) could be
directly used within the existing systems. This
review determined that several European utilities possessed experience with handling wood
pellets in modified coal pulverizers. OPG refers to this technique as the dedicated milling
concept to differentiate it from the more familiar co-milling of biomass and coal.
In the dedicated milling concept, pure
wood pellets are handled with the existing
coal-handling systems (conveyors, bunkers,
and gravimetric feeders) and are introduced
into the pulverizers on a pure basis—without
coal. The trials to date at Nanticoke have
been conducted on unmodified pulverizers.
However, the method does require at least
two significant operational changes. First,
the PA employed for wood milling must be
relatively cold, as biomass releases volatile
matter at significantly lower temperatures
than coal. The Nanticoke trials used a target mill inlet temperature of 65C to address
this issue. The second change relates to the
required minimum primary airflow that can
both effectively fluidize the wood dust in the
mill and provide stable pneumatic transport
downstream in the burner lines.
This aspect of the Nanticoke program
commenced with a proof-of-concept test on
one pulverizer, using a single truckload ( 35
Mg) of commercial grade wood pellets. The
initial unloading of the truck into an emergency reclaim hopper produced a significant
quantity of dust, most of which appeared to
have been generated during transportation to
the site. Downstream of this point, the pellets
were conveyed by the existing coal-handling
systems without any major issues.
The mill required a longer time period
(about 30 minutes) to stabilize, and the final
mill differential pressure was higher (similar
to full mill load on coal). The temperature dif-
ferential across the pulverizer was about 20C,
confirming that only a modest level of drying
was necessary with the relatively dry pellets.
The test proceeded uneventfully until the
delivered wood pellet supply was consumed.
At this point, a standard pulverizer cleaning
cycle was started. At Nanticoke, this mill-clearing cycle involves a full 20 minutes of
operation with maximum cold primary airflow in order to blow the mill clear. In practice, this typically only requires between 5 to
10 minutes. However, following a wood pellet test, this clearing operation was seen to
require more than 60 minutes. It is thought
that there was insufficient lift velocity in the
mill body to effectively remove the larger
wood particles from the mill. This has obvious impacts on the flexibility of the unit and
may also represent a potential safety concern
if the recirculating wood dust in the mill begins to generate heat via friction.
The effective throughput of a mill handling
wood pellets is limited by the available cold
PA capacity. The Nanticoke PA system operates at 15 kPa, but the mills are equipped with
relatively small tempering air ducts. Modifications to reduce the mill differential and to
expeditiously transport the wood particles
from the mill are currently under study.
Nanticoke’s Testing Shifts away
from Cofiring with Coal
With the adoption of the coal phase-out
regulation in 2007, OPG’s testing program
changed from focusing on cofiring with coal
to determining generation capability without
the use of coal. A series of wood pellet tests
had been conducted at the Nanticoke GS on
different mill configurations and with various
throughputs and test durations. In 2008 all of
the Unit 4 mills were individually tested with
pellets to confirm and address any anomalies
with the equipment. In November 2008, Nanticoke conducted the first test of a full boiler
operating on biomass fuel.
This larger trial was conducted at a load
of 175 MW with initial operation on coal. A
transition to wood firing was made over the
course of the test as the coal in the bunkers
was exhausted. As before, the gas igniters
were maintained in service for wood firing.
The low load point was chosen, as the team
expected that PA capacity would be the first
limit encountered, and this was indeed the
case. With all five mills in service on wood
pellets, the maximum total biomass furnace
input was 104 Mg/hr. Note that the gas igniters provided a significant quantity of energy
at their minimum (default) settings. In this
case, wood energy represented some 82% of
the total furnace input with the balance from
natural gas. In simple terms, prorating these
inputs yields electrical outputs of about 145
MW from wood and 30 MW from gas.
High hot reheat steam temperatures were
observed, especially at lower loads. It was
necessary to intentionally depress the main
steam temperature setpoint in order to bring
the reheat temperatures under control.
The impact on the air heaters was much
more dramatic. The use of cold PA in all of
the mills created a significant energy imbalance at the PA heater. This has an obvious
negative impact on boiler fuel efficiency, but,
more importantly, the elevated temperatures
are approaching the limits of some downstream components in the electrostatic precipitator (ESP) and the stack.
NOx emissions were basically unchanged
from the base case with coal (at lower excess
air). The test data indicate that there is certainly room to lower excess air with wood
firing, but the current controls configuration
does not allow this due to a low windbox
pressure limit.
Management of dust levels during pellet unloading and conveyance to the coal
bunkers was a significant health and safety
concern. Through the dedicated milling trials, we found that different wood pellets have
very different dusting propensities, despite
similar chemical properties. Consequently,
a number of measures were implemented to
mitigate the health and safety concerns from
dust, including:
■ The application of a light steam to the pellets at transfer points. Management of the
dust with typical coal dust suppression was
a challenge due to the fact that the pellets
needed to be kept dry to avoid pluggage.
■ The use of vacuum trucks at coal transfer
chutes as a rudimentary dust extraction tool.
■ Limiting access to the coal conveyor gallery and the use of respirators and personal
air quality monitors (O2, CO, and CH4) for
required personnel.
■ The use of area dust monitoring throughout the pellet transfer process.
Atikokan Generating Station’s
Test Program
Following the lead of the Nanticoke GS, the
Atikokan GS began setting up its own test
program related to using pelletized biomass
as a fuel source.
The Atikokan GS is located in northwest
Ontario and is equipped with a single Babcock & Wilcox natural circulation boiler of
the opposed-fired design. Five MPS 75G
roll-race pulverizers supply fuel to a total of
15 dual-register low-NOx burners. The boiler
is equipped with a single regenerative secondary air heater and a dedicated PA heater.
Two cold side ESPs provide particulate
capture. The unit is rated at 227 MW with
