POWER July 2011 - 68

ELECTRIC POWER 2011: WHERE THE GENCOS MEET
Solid Fuels: Moving Material and
Managing Emissions
In today's solid-fueled power plant, managing emissions and moving materials
more defines the task than the traditional work of making megawatts.
That's the message that emerged from the coal and solid fuels track at this
year's ELECTRIC POWER.
By Kennedy Maize
W
, NOx
hether it's the traditional fuel, coal,
or the hot new trend of cofiring
coal with biomass, moving stuff
around presents major challenges.
The Hidden Costs of Biomass
Ezra Bar-Ziv of Ben-Gurion University described
the experience of mixing raw wood-derived biomass
with coal at two large power plants in Israel.
The aim was to test the environmental benefits of
combining coal with a cleaner fuel. Those benefits,
Bar-Ziv said, were significant: limiting the
growth of CO2
SOx
emissions; substantially lowering
, and rocks (particulates); and the absence
of mercury in the biomass fuel.
Biomass, Bar-Ziv noted, " can be used in
coal-fired boilers without objections from environmental
protection authorities. " Biomass
also qualifies as " sustainable, " Bar-Ziv said,
as long as " the biomass growth rate equals
the rate of biomass burning, " meaning no net
loss of the fuel.
But the technology has little market penetration
so far. According to Bar-Ziv's figures,
there are only 15 commercial projects in the
world that feature cofiring coal and biomass.
Of those, eight are in Europe and only one is
in North America. (There are many more cofiring
pilot and feasibility projects. See " Biomass
Cofiring: A Promising New Generation
Option " in the April 2011 issue of POWER.)
So, asks Bar-Ziv, " if cofiring biomass
is good, why are there so few commercial
plants? " His answer is that moving and mixing
the fuel with coal is tricky and generates
several kinds of costs:
■ Biomass is bulky, which entails expensive
logistics.
■ Its high moisture means it is expensive to
transport.
■ Its low heat value means it requires a lot of
fuel to produce power results.
■ Biomass is hygroscopic, meaning that it
takes up and retains water, burns irregularly,
and produces undesirable tars, which
must be dealt with.
■ It is difficult to pulverize.
68
What to do? Make biomass more like coal
by torrefaction, says Bar-Ziv. Torrefaction involves
heating the biomass to 200C to 300C
in a reduced-oxygen environment. In short,
it is a mild form of pyrolysis. The result of
torrefaction is a fuel that has greater heat
content and a decreased ratio of oxygen to
carbon; it's also a more easily handled product.
" Grindability is improved dramatically,
and the power required for pulverization to
the suitable size range reduces by a factor of
10, reaching the power required to pulverize
coal, " says Bar-Ziv. He called the product
" biocoal " ; others call it biochar (see " Utilities
Increase Renewable Energy Capacity " in
this issue).
Lessons from Biomass
Transferred to Coal
Even some ranks of coal can benefit from the
same basic idea-heating to reduce moisture
and make a fuel that has higher heat value
and is handled more efficiently. That's what
Great River Energy, Minnesota's secondlargest
utility, has done at its two-unit, 1,200MW
Coal Creek Station, the largest power
plant in North Dakota. James Kennedy of the
Worley Parsons consulting firm, described
the Coal Creek project for the coal track.
The mine-mouth plant burns a fuel rated
at 6,200 Btu/lb and 38% moisture. Unfortunately,
noted Kennedy, the plant was designed
to burn 6,800-Btu fuel, meaning that
the two units were burning 9% more fuel
than they should, which lowered efficiency
and increased costs across the board. Using
Department of Energy funding, Great River
Energy mounted a project to use waste heat
from the plant to dry the lignite and improve
plant performance. After running a pilot project
in 2005, the engineers installed full-scale
equipment in 2009. Resulting " performance
[was] right on the predicted curve, " with a
9% reduction in fuel moisture.
After a year of operation, the new equipment
has seen over 90% availability with
more than 90% of the plant's coal processed
through the new dryers. The drying process
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has also produced significant air pollution
reductions: 54% less SO2
mercury, 32% less NOx
, and a CO2
, around 40% less
reduction
of 4% due to more efficient operation.
Because the Coal Creek project was a firstof-a-kind
venture, what lessons can be learned
for those who wish to replicate the technology?
Kennedy suggested two that deal with
fuel handling. The first is crusher location.
" Crushers, " he said, " need to be as close to
the dryer inlets as possible to minimize localized
agglomeration at hoppers along the material
transport system. " The second is fuel size,
he said: " Wet fines need to be minimized to
prevent accumulation and bed elutriation (the
separation of lighter and heavier particles), "
and " Oversize chunks need to be minimized
to prevent bed stagnation zones. "
Kennedy added that Great River Energy
expects even greater gains in the future from
its lignite-drying operations: lower NOx
emissions
as the furnace is retuned based on the
new performance experience and " substantially
reduced routine pulverizer, boiler, and air
quality control system maintenance costs. "
In summary, said Kennedy, the Coal Creek
project produces " coal drying as needed " ; it
avoids the need for protracted fuel storage and
the concomitant risks of spontaneous combustion.
The moisture reduction means lower fuel
throughput, net heat rate improvement, reduced
flue gas volume, and reduced service requirements.
On top of those benefits, the project
lowers air emissions for criteria pollutants and
CO2
as a result of increased efficiency.
Materials-Handling Challenges
One of the major drivers of improved air
quality from coal-fired plants in the past 20
years has been the installation of wet flue gas
desulfurization equipment. But wet scrubbers
have created unique and daunting materialshandling
issues, as described by Richard McCartney
of engineering and construction firm
Roberts & Schaefer Co.
" All of the wet scrubber addition projects
require limestone to be delivered to the power
plants and the gypsum by-products disposed
POWER | July 2011
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POWER July 2011

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