POWER May 2014 - 51

RENEWABLES
Biomass Heat Rate Impacts
Biomass cofiring or conversion at the utility scale is typically driven
by environmental concerns. Understanding the corresponding
heat rate impacts of biomass combustion is critical for evaluating
the spectrum of risk and reward possibilities. Heat rate impacts
from biomass cofiring and conversion, relative to coal combustion,
result from three primary causes:
■ Boiler efficiency: A higher fuel moisture and hydrogen content
can lead to higher latent heat losses in the boiler. Changes in
combustion efficiency, furnace heat transfer, and flue gas flow
can lead to higher sensible heat losses. The higher volatility
of biomass fuel can contribute to lower unburned combustible
losses, but oversized fuel particles can negate this benefit, or
even result in greater unburned combustible losses.
■ Station service: Biomass handling, grinding, and preparation
systems can result in greater auxiliary power consumption at
the power plant, even with a reduction in coal equipment use.
■ Turbine heat rate: Changes in sootblowing steam use, steaming
rate and final steam temperatures, and spray flows can result in
2. Impact of increasing biomass use. As biomass replaces
coal, the plant heat rate can suffer significant changes. In this example
a single woody biomass fuel with 20% moisture content was
comilled with the baseline Illinois Basin coal. Source: Black & Veatch
■ Boiler efficiency ■ Net plant heat rate
88
88
87
87
86
86
85
85
0 5 10 15 20 25 30 35 40 45 50
Cofire level % of heat input
plan is to convert three of the six units at
Drax to being exclusively biomass by 2016;
the Unit 2 conversion is already complete.
Biomass Cofiring Project
Challenges and Pitfalls
Biomass utilization at existing coal-fired
power plants can be implemented by several
methods, from cofiring biomass and coal simultaneously
to complete unit conversions to
biomass combustion. Less-common methods
of utilizing biomass include substituting B99
biodiesel for startup fuel oil, using a gasifier to
feed hot fuel gas into an existing boiler, or even
employing a separate standalone biomass unit
to supply heat to the coal unit's steam cycle.
In the last two decades Black & Veatch
has studied the barriers to and concerns over
May 2014 | POWER
10,550
10,500
10,450
10,400
10,350
10,300
10,250
10,200
10,150
10,100
changes to the net turbine heat rate.
Figures 2, 3, and 4 demonstrate some potential heat rate effects
of biomass combustion at a pulverized coal power plant studied
by Black & Veatch.
3. Impact of increasing biomass moisture. As biomass
moisture content increases, boiler efficiency and other losses
increase. In this example a single woody biomass fuel with a moisture
content ranging from 5% to 50% was blended with the baseline
Illinois Basin coal. Source: Black & Veatch
■ Boiler efficiency ■ Net plant heat rate
90
88
86
84
82
80
78
76
74
72
70
0 5 10 15 20 25 30 35 40 45
Wood moisture content (%)
4. Impact of different biomass products. Biomass products
can have an even greater variability in their quality than coal. In this
example, eight different biomass fuels were studied when comilled at
20% with the baseline Illinois Basin coal. Source: Black & Veatch
11,200
11,000
10,800
10,600
10,400
10,200
10,000
Coal
Torrefied
wood
utilizing biomass at more than 70 fossil-fired
units (see sidebar). The following details
some of the most common challenges we
have encountered.
Biomass Supply. Finding a suitable biomass
fuel supply at a competitive price is
difficult for most power plants. Suppliers are
often small " mom and pop companies " and
geographically remote from the power plant.
Supplies can be variable from a single vendor,
mandating contracts with scores of small
biomass suppliers, which greatly complicates
the biomass procurement process. Sometimes
biomass suppliers will promise to meet
a demand for biomass at a reasonable price
during a study, but once the project appears
to be proceeding toward a test burn, suddenly
the price will increase by 50% or more.
www.powermag.com
Straw
pellets
Dried
wood
Bagasse Sewage
sludge
Raw
wood
Arundo
donax
Tree
trimmings
One solution to these problems is for the
utility to take ownership of its biomass fuel.
Such is the case with Drax Power, which is
building biomass pellet plants in Louisiana
and Mississippi and shipping pellets back
to North Yorkshire, England. Designed to
produce more than 900,000 tons per year of
wood pellets, these pellet plants are a first
step toward Drax's ultimate goal of producing
2 million tons per year.
Biomass storage and handling logistics can
be a serious concern for power plants with
limited fuel yard space. Given a woody biomass
of a typical moisture content, one rule
of thumb is to assume that 1 ton per hour of
biomass will be required for 1 MWh of power
generation (less for pellets or torrefied biomass
and more for wet, herbaceous biomass).
51
13,000
12,500
12,000
11,500
11,000
10,500
10,000
NPHR (Btu/kWh)
NPHR (Btu/kWh)
Boiler efficiency (%)
Net plant heat rate (Btu/kWh)
Boiler efficiency (%)
http://www.powermag.com
POWER May 2014

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