POWER July 2013 - 46

PLANT DESIGN
stallation of an SCR due to the inherently
low NOx
emission characteristics of that
boiler design.
The analyses further assumed the plant
capacity factor for this study is 80% (7,008
hours/year). In the current low natural gas
price environment, this 80% capacity factor
is likely higher than many units are currently
operating at. A sensitivity analysis
was performed using a range of lower capacity
factors. The subcommittee believes
that higher capacity factors will likely return
as natural gas prices slowly but steadily
rise in the future.
Determine Upgrades, Benefits,
and Costs
For each case study, the FDS components
were identified and upgrades were proposed.
Next, the potential benefits of the
upgrades were discussed and evaluated.
The list of proposed upgrades was the
source of spirited discussion among subcommittee
members, although a consensus
was always reached that reflected the collective
expertise and diverse experience of
the members.
For each case study, the upgrade's scope
was defined in sufficient detail so that reliable
estimated costs could be prepared.
Fortunately, the various suppliers and
consulting engineers on the subcommittee
were able to provide estimates of the installed
costs for each upgrade considered
based on prior experience.
Assessing the savings produced by the
upgrades was also reached by consensus
over the course of several subcommittee
meetings. Difficulties arose when the economic
benefit of upgrading one component
4. Coal fleet boiler design. The three
case studies were selected from the three
largest categories of boiler design now used
in the fleet. Fluid bed units were not included
because those fuel systems are unique and
often handle opportunity fuels. Source: POWER
and Burns & McDonnell
Vert-ceiling 2%
Fluid bed 3%
Unknown 6%
Cyclone 7%
Opposed 19%
Other 7%
T-fired 37%
Coal pipes
Boiler control system
Project management &
engineering services
Front 20%
Total
Coal-air flow
Burner modernization LNBs & OFA
Neural network
25%
alone was not possible because some upgrades
were contingent on other upgrades.
Also, there were interactions between the
upgrades that were difficult to quantify. The
savings that accrue are not always merely
the sum of the individual upgrades. For
example, an evaluation of the economics
of using neural networks within the boiler
control system produces improved boiler
performance, but much of the performance
improvement came from the burner modifications
required by the neural net system.
Three Case Studies
The subcommittee believed that each case
study 500-MW boiler must be representative
of the boiler design class. Selection of
the case studies was based in large measure
on the spectrum of boiler types that
currently constitute the U.S. utility boiler
fleet, as shown in Figure 4. In considering
the ~500-MW units in the 35- to 45-year
age range, the vertical- and front wall-fired
boiler were eliminated on the basis that few,
if any, would be 500-MW-size boilers.
Cyclone boilers provide an interesting
opportunity for evaluation in this study. Cyclone
boilers lost favor in the 1980s when
they were characterized as high NOx
emitters
and not amenable to combustion modifications.
Supposedly, they were also not
amenable to burning PRB coal. Today there
are approximately 60 cyclone-fired boilers
still in operation, of which 10 are in the 400to
600-MW size range and four are greater
than 600 MW. Many of these boilers have
been successfully converted to burn PRB
coal, usually with retrofitted overfire air
(OFA) ports installed to aid combustion tuning
for reduced NOx
production.
The subcommittee decided to include
a cyclone-fired unit as a separate case
study. However, the FDS boundary was
expanded beyond the individual cyclone
feeder to beyond the coal conveyors, to
the crusher-feeder island, usually located
some distance from the boiler. It was felt
that the crusher, more than any other device,
controlled particle sizing, and the
upgraded feeder provided a more uniform
flow of coal to the crusher, improving
overall crusher performance.
Case Study 1: The Opposed Wall-
Fired Boiler
The candidate opposed wall-fired, natural
circulation boiler was originally designed
for eastern coal and now burns a PRB coal.
It has a retrofit SCR and the original electrostatic
precipitator; plans are in place to
retrofit a wet scrubber. The proposed FDS
components for upgrade include replacement
of original feeders, retrofit dynamic
classifiers on vertical shaft pulverizers, coal
pipe flow-metering devices, burner modernization,
retrofit OFA, airflow metering
devices, and retrofit boiler control system
(BCS) with a neural network. Several of the
suggested upgrades are described below and
summarized in Table 2.
Fuel Feeders. The original, roughly
40-year-old volumetric feeders were replaced
with new gravimetric feeders with
improved metering. They also provide some
increased capacity, because PRB, with its
lower heating value, requires more material
throughput than the original eastern bituminous
coal. Six new feeders were each estimated
at $75,000 x 2 (cost of installation)
= $900,000. Benefits were included with
those of dynamic classifiers (DCs), below.
Pulverizers. DCs are used to increase
coal fineness at a given coal throughput. In
so doing, DCs also reduce the coarse grind,
Table 2. Case Study 1: Opposed wall-fired boiler FDS upgrades benefits
and costs. The investment breakeven point was 15 months. Source: The Fuel Delivery Subcommittee
of the ASME Research Committee on Energy, Environment, and Waste
Component
Feeder
Pulverizer
Upgrade
New feeders
Dynamic classifiers
Cost ($ x 1,000)
Savings ($ x 1,000/year)
$900 Combine with dynamic classifiers
$3,600 5% performance recovery savings, $8,760;
two days' operation, $960.
$700 Combine with boiler control system.
$5,400 Some NOx reduction as NH3 savings. Other
benefits claimed in BCS. NH3 savings: $210.
$300 Efficiency improvements, $596; fan energy
savings, $314; NH3 savings, $85.
$2,725 NA
$13,625 $10,925
Notes: BCS = boiler control system, LNB = low-NOx burners, NA = not applicable, OFA = overfire air.
46
www.powermag.com
POWER | July 2013

http://www.powermag.com

POWER July 2013

Table of Contents for the Digital Edition of POWER July 2013