POWER April 2011 - 54

ALTERNATIVE FUELS
3. Lightening the load. This photo shows
an unloading operation at a typical wood chip receiving
facility. Courtesy: Enviro-Burn Inc.
with the combustion of fossil fuels. Given
that cofiring requires large amounts of biomass
fuels, cofiring typically works best at
large coal-fueled plants that have materialshandling
capability already on-site (Figures
3 and 4). However, other industrial users-
such as cement plants-could also make use
of biomass cofiring.
Three types of cofiring are in use around
the world, Hayes explained:
■ Direct cofiring is the simplest of the
three and the most commonly chosen
option. In this version of cofiring, more
than one type of fuel is placed in a furnace/boiler
at a time (Figure 5). The
same or separate feed systems, mills,
and burners can be used, depending on
the fuel characteristics.
4. Tree power. This centralized chipping/
storage yard contains mounds of utility-grade
wood chips ready to be used as fuel. Courtesy:
Enviro-Burn Inc.
■ Indirect cofiring involves converting biomass
from a solid fuel to a gaseous form
before firing. The gas is then burned in the
same furnace/boiler as the coal.
■ Separate biomass firing occurs when a
separate biomass boiler is added to the
overall system to bolster the steam capacity
of an existing boiler.
The Benefits of Cofiring Biomass
with Coal
Dr. Marco J. Castaldi, assistant professor
in the Earth & Environmental Engineering
Department at Columbia University in New
York City, shared with POWER his perspective
on the advantages of cofiring biomass
with coal.
" The benefits primarily are life-cycle car)
reductions, " he said. " Anbon
dioxide (CO2
5. Adding fuel to the fire. Enviro-Burn
Inc. recently conducted a cofiring test burn
of biomass in a utility-scale boiler. Courtesy:
Enviro-Burn Inc.
other benefit of cofiring is that it reduces sulfur
and mercury emissions because biomass is
virtually free of sulfur and mercury. In addition,
higher hydrogen and oxygen content can
give better combustion efficiencies (that is,
better burnout). Finally, cofiring enables fuel
diversity and local supply of fuel if possible
markets for agricultural waste exist. "
Dr. Ashwani Gupta, director of Propulsion
and Energy, AIAA, and professor in the
Department of Mechanical Engineering at
the University of Maryland, also talked to
POWER about the benefits of cofiring biomass
under the right conditions.
" The material-handling cost is significant, "
he said. " So if the material-handling
facility already exists to handle biomass,
then this adds attractive economical benefits
of biomass cofiring. This also reduces the upfront
costs of handling biomass. "
Hayes outlined what the American Coal
Council considers to be the key advantages
that biomass cofiring can offer to operators
of coal-fired power plants under the right circumstances:
54
www.powermag.com
■
Biomass can use existing infrastructure
investments for fossil fuels.
■ Actual fuel costs can be very low-even
free-as biomass producers are often
happy to be rid of what they consider
" waste. "
■ Biomass costs compare favorably with
those of other renewable options.
■ The addition of biomass to a coal-fueled
boiler is likely to have little or no
negative impact on generation efficiency
(depending on the fuel preparation measures
taken).
■ With proper fuel selection and management,
overall emissions of sulfur oxides,
nitrogen oxides (NOx
), and mercury-as
well as net GHG emissions-can be reduced.
Biomass is typically nearly free of
sulfur and mercury, so these emissions are
reduced in proportion to its use; NOx
emissions
may fall or rise slightly, depending
on boiler operation.
■ Biomass is considered to be " carbon-neutral, "
so its use can result in a proportional
reduction in GHG emissions.
Overcoming Operational Challenges
Despite its many benefits, biomass cofiring
also can present challenges due to important
fuel-handling differences compared with
coal. Successful biomass plants have to design
their fuel systems to deal with specific
biomass fuel properties and characteristics
(see " Designing Fuel Systems for Large Biomass
Plants, " February 2011, available in our
archives at www.powermag.com).
Biomass creates issues of boiler slagging
and fouling, according to Gupta. Success in
firing biomass with coal is very boiler-dependent
and fuel-dependent. Unfortunately,
too few full-blown cofiring operations exist
to provide information on how to alleviate
fouling and deposition problems. These
challenges have been handled on an individual
basis, according to experiences with
specific boilers.
" For example, with PRB [Powder River
Basin] coal, there are some experiences
that can be extended to cofiring biomass
with PRB coal, " Gupta said. " The coal industry
is reluctant to invest unless there
is demonstrated technology available that
can be used, or the payback time is relatively
short (less than three years) on the
return on investment. The federal government
must invest in technology development
and demonstration experiences that
can then be used for different kinds of
boilers and loads. "
Gupta noted that problems related to supplying
biomass materials to coal-fired power
plants include sustaining the supply and
quality of biomass over time. Costs associPOWER
| April 2011
http://www.powermag.com http://www.powermag.com
POWER April 2011

Table of Contents for the Digital Edition of POWER April 2011
