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Biomass Co-firing Power Plant Engineering

We have several excellent engineering papers on biomass co-firing:

-- Tampa Electric Co., Co-firing Test Burn Results from the Polk IGCC Unit (coal gasification), Microsoft Word

-- EPRI, Economic Scenarios of Biomass Co-firing,
Excel Spreadsheet

-- Energy Products of Idaho (EPI), Retrofit of Coal Fired Boilers Using Fluidized Bed Biomass Gasification, PDF

-- U.S. Department of Energy, Biomass Co-firing in Coal Power Plants, PDF

-- DOE/EPRI, Renewable Energy Technology Characterizations, Biomass Co-firing Chapter, 1997   PDF

-- National Renewable Energy Laboratory, A Summary of NOx Emissions Reduction from Biomass Co-firing,   Microsoft Word

-- National Renewable Energy Laboratory, A Lifecycle Assessment of Biomass Co-firing in a Coal-fired Power Plant,   PDF

-- Additional NREL Reports/Papers on Bio-Power,   Bio-Power Website

-- Search Engine for the U.S. Dept. of Energy Website on Biomass Co-firing.

Biomass Co-Firing Efforts in Florida:   During the past two years, the Common Purpose Institute and the University of Florida's School of Engineering have partnered with the U.S. Department of Energy and electric utilities in central Florida in collabrative biomass co-firing research at three (3) coal fired power plants.

Lakeland Electric's McIntosh Unit #3 (pulverized coal)
TECO Energy's Gannon Unit #3 (coal-fired cyclone)
TECO Energy's Polk Power Station (coal gasification).

As a result of this effort, two (2) power plants (representing cyclone and pulverized coal boiler technologies) have now received permits from the State of Florida Department of Environmental Protection (DEP) to co-fire biomass fuels.

In December 2001, initial "test burns" commenced at TECO Energy's 250 MW IGCC coal gasification Unit -- hopefully leading to DEP Permitting for biomass co-firing in 2002, or early 2003.

Combined, these three Units have ~900 MWs of generation capacity. At a biomass co-firing rate of 3 percent (by heat input), this would be a Renewable Energy equivalent of installing 54,000 large solar panels.

Since all three of these power plants are high capacity factor, base-load units (i.e., plants that run 24 hours a day), our initial engineering focus has been on operational issues of integrating biomass co-firing without jeopardizing overall Unit availability. Primarily, these efforts have focused on wood fuel pre-processing (mesh size reduction through grinding), and air flows into the boiler.

Test-burn results have shown that wood fuel must be double ground (i.e., through a tub grinder or Montgomery type hog) to a fine mesh size [click here for a picture] for co-firing in pulverized coal and cyclone units. In using larger mesh size biomass (i.e., a single pass through the tub grinder or hog) the material did not burn well as it fell through a pulverized coal boiler's combustion zones (i.e., suspension firing) -- accumulating un-burned wood on the grate. For wet bottom boilers, inadequate suspension firing especially creates a totally un-acceptable operational problem.

For Pulverized Coal Units, controlling air flows with the pneumatic lifting of wood fuel to the boiler's fuel ports has also been shown to be exceedingly important. In initial test-burns at McIntosh Unit 3, even though wood fuel contains almost no sulfur, overall SO2 emissions increased. In our opinion, we believe that this vividly illustrates why research needs to be conducted on large commercial scale units -- compared to small proto-type boilers at research labs.

Future Focus:   We strongly believe that as power plant management becomes more familiar with co-firing, that more advanced engineering methods will be tested, especially those directly addressing NOx formation (i.e., Reburn, Separated Overfire Air, etc.).

In addition to completed and current engineering work, our near term goals are to conduct test burns and permit power plants for biomass co-firing at the following generating stations:

  • Wheelabrator's Ridge Generation Station (stoker unit).
  • TECO Energy's Big Bend Units (pulverized coal).
  • Florida Power's Crystal River Units (pulverized coal).
  • Orlando Utilities' Stanton Units (pulverized coal).
  • Gulf Power's Crist Units (pulverized coal).

Our intermediate goal is to implement an external biomass gasification project at an existing coal or natural gas fired power plant in Florida -- such as the Battelle bio-gas technology being commercially developed by FURCO [pdf document]

Lakeland Electric's McIntosh #3 Unit

Lakeland Electric's McIntosh Unit #3 is a 365 MW Babcock and Wilcox boiler, rated at 2,900,000 pph steam, at 2250 psig. The biomass co-firing technique used at Unit #3 has involved the direct injection of shredded wood fiber material (e.g., closed-loop & open-loop biomass) into a pulverized coal fired furnace. Biomass was shredded to the consistency of fine mulch and blown into the furnace through existing fuel injection ports. Coal and wood handling processing were separate.

In one of several co-firing test burns at Unit #3, in late 1998, approximately 125 tons of shredded/chipped eucalyptus trees were co-fired over a single continuous 6 hour period (~21 tons per hour) -- representing a co-firing level at full load of ~5% (by heating value).

The pre-processed wood fuel was deposited in a live bottom storage bin and then was pneumatically transported to the boiler and injected above the coal burners. Wood particles that did not burn in suspension, fell to a dump grate. Combustion air was introduced below the grate to complete the combustion of the larger fuel particles.

Pulverized coal was fired through existing coal burners. There are a total of four refuse/biomass injection ports, two each on opposing sidewalls above the coal burners. Two opposite wall injection ports were used during the co-firing test burn.

Boiler efficiency was lower with wood co-firing, due predominantly to fuel moisture of ~50% and increased excess air associated with the wood fuel injection. With wood co-firing, boiler efficiency was reduced from baseline condition (100% coal blend) by approximately 1.3 boiler efficiency points.

For the six hour test duration, accumulations of unburned wood were not noted in the air heater. Also, unusual slagging or fouling was not noted.

Fuel Ultimate Analyses:

Element:
Measurement
Eucalyptus
Coal
Carbon
(% wt.)
24.91
74.24
Hydrogen
(% wt.)
2.73
4.57
Oxygen
(% wt.)
19.81
5.33
Nitrogen
(% wt.)
0.11
1.43
Sulfur
(% wt.)
0.04
2.14
Ash
(% wt.)
0.94
7.45
Moisture
(% wt.)
51.46
4.83
Higher Heating Value
(BTU/lb)
4,238
13,305

Stack Test Results:

Emission:
Measurment
Coal Only
Co-Firing
NOx
ppmv,dry basis
249.4
238.6
NOx
lbs/MMBtu
0.504
0.481
PM
grams/DSCF
0.00009
0.00011
PM
lbs/MMBtu
0.0035
0.0040

Effect On Boiler Efficiency:

Test Name:
Measurment
Coal Only
Co-Firing
Dry Gas
%
5.61
5.74
H2 & H2O in Fuel
%
3.94
4.76
Moisture in Air
%
0.15
0.15
Unburned Carbon
%
0.50
0.50
Radiation
%
0.16
0.16
Total Heat Loss
%
10.37
11.32
Boiler Efficiency
%
89.63
88.68

TECO Energy's Gannon Power Generating Station:

Tampa Electric's Gannon Power Generating Station is comprised of 7 Cyclone Units (~1,000 MWs) using 100% coal. During 2000 and 2001, a series of co-firing test burns were conducted at Unit #3 (wet bottom) with Yardwaste Biomass (~5,000 BTUs per pound) and Powder River Basin Coal (~9,000 BTUs per pound).

In 2000, a 72 hour continuous Test Burn was conducted with a biomass fuel blend of approximately 5% (i.e.,5% biomass wood fuel, and 95% coal, by volume). By Heat Input Value, biomass co-firing rate was approximately 2.75%.

Pre-processed biomass wood fuel was delivered "double ground" by initially grinding in a Tub Grinder (4"x 6" openings), screened (1/2" openings), and then re-ground through a Tub Grinder (1/2" grates). The biomass fuel was conveyed (a controlled variable speed conveyor) onto the main coal conveyor belt to be bunkered. Bunkering for Unit #3 (a base load unit) occurred twice per day.

During the Test Burn the focus was primarily on "Power Plant operational issues" associated with blending biomass fuels with the Powder River Basin Coal. While the Unit tripped numerous times during the Test Burn, none of the "trips" appeared to be directly the result of the biomass fuel. We experienced problems where the wood fuel conveyor continued to run, but the coal feed had been tripped. For the Test Burn, we did not mechanically link the separate biomass conveyor to stop when the coal feed tripped. This resulted in the coal conveyor continuing to run (without any coal on the conveyor), and the biomass conveyor continuing to dump. When this has occurred, clumps of 100% wood fuel would occur in the bunker -- which eventually tripped the Unit (after bunkering) because sensors detected low Heat Input Value fuel (i.e., 5,000 BTUs per pound of biomass fuel, versus normal operations of 9,000 to 10,000 BTUs per pound of Powder River Basin coal).