Summary of Chariton Valley 

Switchgrass Co-Fire Testing at 

the Ottumwa Generating Station 

in Chillicothe, Iowa 

Milestone Completion Report 

July 2002 • NREL/TP-510-32424 

W. Amos 

Prepared for the U.S. Department of Energy’s Biomass 

Power Program and the Chariton Valley Biomass 

Project

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle • Bechtel 

Contract No. DE-AC36-99-GO10337 
National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle • Bechtel 

Contract No. DE-AC36-99-GO10337 

July 2002 • NREL/TP-510-32424 

Summary of Chariton Valley 

Switchgrass Co-Fire Testing at 

the Ottumwa Generating Station 

in Chillicothe, Iowa 

Milestone Completion Report 

W. Amos 

Prepared for the U.S. Department of Energy’s Biomass 

Power Program and the Chariton Valley Biomass 

Project

Prepared under Task No. BP01.1026 
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EXECUTIVE SUMMARY 

After several years of planning, the Chariton Valley Biomass Project successfully completed two 

months of switchgrass co-fire testing at the Ottumwa Generating Station (OGS) in Chillicothe, 

Iowa. From November 30, 2000, through January 25, 2001, the switchgrass team co-fired 1,269 

tons (1,151 tonnes) of switchgrass at rates up to 16.8 tons/h (15.2 tonne/h), representing about 

3% heat input to the 725-MW power plant. Stack testing was completed when co-firing 

switchgrass and when burning only coal. Fuel and ash samples were collected for analysis, and 

boiler performance and emissions data were collected. Numerous improvements were made to 

the feed-handling equipment during testing. The co-fire testing was completed with no 

environmental incidents, no injuries to personnel, and no loss in electricity output from OGS. 

The goals of the this—the first of three rounds of co-fire tests—were: to identify the effects of 

co-firing on boiler performance, to measure any changes in emissions during co-firing, and to 

gather information to improve the design of the switchgrass handling equipment. All three of 

these goals were met. The design target for the switchgrass handling system was 12.5 ton/h 

(11.3 tonne/h), which we exceeded after a redesign of the secondary grinder in our system. We 

had hoped to burn over 3,000 tons (2,722 tonnes) of switchgrass during this first round of testing, 

but because of poor equipment performance in December, we were unable to meet this target 

before the planned boiler shutdown in January. There were, however, several days in January 

when we burned more than 100 tons (91 tonnes) of switchgrass. 

Several important lessons were learned about the feed-handling system. Dust control had been a 

concern from the start, but after modifying the equipment to maintain a slight negative pressure 

downstream of the grinding equipment, there were no problems. We also experimented with a 

piece of equipment called the Eliminator for grinding the switchgrass. Unlike a hammer mill, it 

used attrition as the primary means of size reduction. The particle size leaving the unit was 

determined by the residence time and the amount of material flowing through the unit. The 

Eliminator produced an acceptable particle size with relatively low power consumption, but did 

not reduce the size of the switchgrass nodes and initially, it required some modifications to get 

the correct residence time in the unit. 

The results of the fuel and ash analyses held no surprises. The switchgrass, when compared to 

the coal, had higher chlorine and potassium concentrations, but lower sulfur and sodium levels. 

Because the Powder River Basin coal burned at OGS is relatively high in moisture, there wasn’t 

a large difference in heating value between the coal and the switchgrass. 

Analysis of the boiler emissions was problematic. During our co-fire stack testing, a problem 

unrelated to the switchgrass testing caused high carbon monoxide levels in the flue gas, but 

further flue gas sampling showed that carbon monoxide emissions were not significantly affected 

by co-firing. The sulfur emissions, based on continuous emissions monitoring data, decreased 

during co-firing due to the lower sulfur content of the switchgrass. Nitrogen oxide levels were 

higher on average, but upsets in the feed-handling system and differences in boiler load may 

have contributed to the higher nitrogen oxide emissions. 

i 
Bulk switchgrass densities were measured at various points in the system; these data will help 

with the redesign of the feed-handling system for continuous operation. The system used for the 

first round of co-fire testing required four operators and was normally run for just one shift per 

day. The second round of switchgrass testing will evaluate the performance of a system 

designed to run continuously with minimal operator attention. 

After the project team demonstrates that switchgrass can be burned on a continuous basis and 

that the feed-handling system can provide a stable flow of switchgrass to OGS boiler, we will 

begin longer-term boiler studies to evaluate corrosion and fouling effects of co-firing 

switchgrass. 

ii 
TABLE OF CONTENTS 

1.0 PROJECT BACKGROUND .................................................................................................... 1 

1.1 Scope..................................................................................................................................... 1 

1.2 Project Partners ..................................................................................................................... 1 

1.3 Co-Fire Test Objectives ........................................................................................................ 2 

1.4 Ottumwa Generating Station................................................................................................. 3 

1.5 Switchgrass Feed-Handling Equipment................................................................................ 4 

2.0 CO-FIRE TESTING ................................................................................................................. 8 

2.1 Fuel Data............................................................................................................................... 8 

2.2 Stack Testing....................................................................................................................... 12 

2.2 Orsat Gas Analyzer Results ................................................................................................ 13 

2.3 Continuous Emissions Monitoring Results......................................................................... 14 

2.4 Sampling and Ash Analysis................................................................................................ 17

3.0 EQUIPMENT PERFORMANCE........................................................................................... 22 

3.1 Equipment Modifications & Repairs .................................................................................. 22 

3.1.1 Debaler......................................................................................................................... 22 

3.1.2 Air Knife ...................................................................................................................... 22 

3.1.2 Debaler-to-Eliminator Conveyor ................................................................................. 22 

3.1.3 Eliminator Grinder....................................................................................................... 23 

3.1.4 Baghouse...................................................................................................................... 24 

3.1.5 Baghouse Conveyor..................................................................................................... 24 

3.1.6 Surge Bin ..................................................................................................................... 25 

3.1.7 Rotary Airlocks............................................................................................................ 26 

3.1.8 Pneumatic Transport Blowers & Transport Piping...................................................... 26 

3.1.9 Control System............................................................................................................. 27 

3.2 Summary............................................................................................................................. 28 

4.0 LABOR/OPERATING PERSONNEL ................................................................................... 28 

5.0 FIRE PROTECTION.............................................................................................................. 29 

6.0 BOILER PERFORMANCE.................................................................................................... 30 

7.0 RECOMMENDATIONS........................................................................................................ 31 

iii 
FIGURES 

Figure 1.1 – Flowsheet of the Switchgrass Feed-Handling System ............................................... 4 

Figure 1.2 – Picture of Eliminator .................................................................................................. 7 

Figure 1.3– Diagram of Eliminator Discharge Chute..................................................................... 7 

Figure 2.1 – Comparison of SO2 Emissions when Co-Firing....................................................... 15 

Figure 2.2 – Comparison of NOx Emissions when Co-Firing ...................................................... 16 

Figure 2.3 – Comparison of Opacity when Co-Firing .................................................................. 16

TABLES 

Table 2.1 – Daily Switchgrass Totals ............................................................................................. 9 

Table 2.2 – Composition and Fuel Value of Coal and Switchgrass ............................................. 10 

Table 2.3 – Ash & Trace Element Analyses of Coal and Switchgrass......................................... 11 

Table 2.4 – Switchgrass Size Distribution.................................................................................... 12

Table 2.5 – Bulk Switchgrass Densities ....................................................................................... 12

Table 2.6 – Base-line and Co-fire Stack Test Results .................................................................. 13 

Table 2.7 – Orsat Gas Analyzer Results (ppm) ............................................................................ 13 

Table 2.8 – CEM Data for January 2001 ...................................................................................... 14 

Table 2.9 – Fly Ash Data for Baseline Coal-Only Conditions ..................................................... 18 

Table 2.10 – Fly Ash Data from Co-Firing................................................................................... 19 

Table 2.11 – Economizer Ash Analysis........................................................................................ 20 

Table 2.12 – Bottom Ash Analysis............................................................................................... 21 

Table 2.13 – Analysis of Bottom Ash Water................................................................................ 21 

iv
1.0 PROJECT BACKGROUND 

1.1 Scope 

This report summarizes the results of the first switchgrass (Panicum vigatum) co-fire test 

campaign at the Ottumwa Generating Station (OGS), conducted as a part of the Chariton Valley 

Biomass Project (U.S. DOE Contract DE-FC36-96GO10148). This report focuses primarily on 

the co-fire testing conducted at OGS, which is only a part of the larger Chariton Valley Biomass 

Project. The larger project addresses the agronomic, environmental, and economic aspects of 

switchgrass production, in addition to the technical questions answered by the co-fire testing at 

OGS. 

1.2 Project Partners 

The efforts of a number of people were responsible for the successful demonstration of 

switchgrass co-firing at the Ottumwa Generation Station. The following is a partial list of those 

directly involved with the first round of testing and their roles. 

Chariton Valley RC&D, Inc., Centerville, Iowa – Chariton Valley RC&D was the project 

sponsor and provided overall project coordination for the Chariton Valley Biomass Project, 

including the co-fire portion of the project 

Alliant Energy, Cedar Rapids, Iowa – Alliant Energy was a principal project partner in the 

Chariton Valley Biomass Project and hosted the first round of co-fire testing at its 725-MW 

boiler in Chillicothe, Iowa. 

U.S. Department of Energy (DOE), Washington, D.C., and Golden, Colorado – The U.S. DOE 

provided funding for the Chariton Valley Biomass Project and co-fire testing. The Golden Field 

Office managed the cooperative agreement for the DOE. 

Ottumwa Generating Station, Chillicothe, Iowa – OGS hosted the first round of switchgrass cofire testing and provided operating staff for project meetings, sample collection, and stack gas 

testing. 

National Renewable Energy Laboratory (NREL), Golden, Colorado – NREL provided on-site 

startup support and test coordination, in addition to developing the sampling plan for the testing. 

Sandia National Laboratories, Livermore, California & Albuquerque, New Mexico – Sandia 

provided the data acquisition system for collecting boiler performance data, developed the test 

plan for the first round of co-fire testing, and assisted with startup support during co-fire testing. 

Kelderman Manufacturing, Inc., Oskaloosa, Iowa – Kelderman Manufacturing provided 

operating staff for the switchgrass co-fire testing and was responsible for final installation and 

modifications to the switchgrass handling system during testing. 

1
Tech-wise A/S, Fredericia, Denmark – Tech-wise provided engineering and startup support for 

the first round of co-fire testing. Tech-wise has supervised several wheat straw co-firing tests in 

Denmark. 

T. R. Miles Technical Consultants, Inc. – T. R. Miles provided technical assistance, startup 

support, and design review services for the Chariton Valley switchgrass co-fire testing. 

Delta Process Technologies/Dolthan Group, LLC, Mobile, Alabama – Delta Process 

Technologies provided the Eliminator grinder that was used for switchgrass processing at OGS, 

and provided startup and sampling support during testing. 

Sega, Inc., Stilwell, Kansas – Sega provided engineering, procurement, and construction services 

for the switchgrass handling facility at OGS. 

Foster Wheeler Development Corporation, Clinton, New Jersey – Foster Wheeler developed the 

preliminary design of the feed handling system used for co-fire testing and installed the 

switchgrass nozzle inserts for the OGS boiler. 

R. W. Beck, Madison, Wisconsin – R.W. Beck conducted the initial feasibility studies on the 

possibility of co-firing switchgrass in the OGS boiler. 

Prairie Lands Bio-Products, Inc., Lucas, Wayne, Monroe, and Appanoose Counties, Iowa – 

Prairie Lands is a farmer cooperative that provided much of the switchgrass used during testing. 

Many of the operating personnel running the switchgrass handling facility were Prairie Lands 

members. 

MidAmerican Energy Company, Des Moines, Iowa – MidAmerican Energy is a co-owner of 

Ottumwa Generation Station. 

Iowa Department of Natural Resources (DNR), Des Moines, Iowa – The Iowa DNR worked with 

the Chariton Valley Biomass Project and the U.S. Environmental Protection Agency in granting 

variances for co-fire testing at OGS. 

Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee – ORNL provided technical and 

financial support for agronomic and environmental research supporting the switchgrass 

production for the project. 

1.3 Co-Fire Test Objectives 

This report summarizes the results of the first of three proposed co-fire test campaigns planned at 

the Ottumwa Generating Station. There were three chief goals of the first co-fire test campaign. 

First, it was important to see if there were any adverse effects on the boiler operation due to the 

co-firing of switchgrass. Possible problems might include increased slagging and/or fouling of 

heat transfer surfaces, conflicts with fuel and oxygen controls, increased corrosion of heat 

transfer surfaces, or decreased boiler efficiency. The second purpose of the testing was to 

investigate the effects of co-firing on boiler emissions. Because of differences in chemical 

2
composition, one concern was the effect of the switchgrass on the electrostatic precipitator (ESP) 

performance. It was also important to verify the expected decrease in sulfur dioxide emissions 

and determine whether co-firing might reduce nitrogen oxide emissions. The third purpose of 

the co-firing was to learn more about the material handling properties of the switchgrass for the 

proper design of feed-handling systems. Particle-size reduction, the option of using round bales, 

and dust control in the switchgrass building were at the top of the list of questions to be 

answered. 

The second and third planned co-fire test campaigns will address improvements in the feedhandling system and long-term boiler effects. The system used for the first co-fire test campaign 

worked effectively for the purposes stated above, but did not represent a commercial system. A 

minimum of three people was required for system operation with additional personnel required 

for rest breaks and unloading trucks. The system was also only operated one or two shifts per 

day. In the second co-fire test campaign, improvements will be made to allow continuous, 

unattended operation of the switchgrass handling system. This will require an automated balehandling system, upgraded controls, and equipment reliability improvements. The second co-fire 

test campaign will use the current system size of 12.5 tons/h (11.3 tonne/h) and could burn up to 

6,000 tons (5,443 tonnes) of switchgrass. The third co-fire campaign will look at long-term 

boiler effects with the goal of co-firing switchgrass for some 2,000 hours at the 12.5 tons/h (11.3 

tonne/h) feed rate. This testing will answer questions about corrosion and fouling in the boiler. 

The size of the final commercial system will be 25 tons/h (22.7 tonne/h). 

1.4 Ottumwa Generating Station 

Ottumwa Generating Station is a 725-MW gross (675-MW net) tangentially-fired pulverized 

coal boiler burning low-sulfur Wyoming Powder River Basin coal. The plant is one of the 

largest power plants in Iowa and is located in Chillicothe, Iowa, about ten miles northwest of 

Ottumwa. The plant began commercial operation in 1981 and is owned jointly by Alliant Energy 

and MidAmerican Energy. 

The plant has two fireballs (east and west) without a dividing wall between the two fireballs. 

This was one of the first plants built without a dividing wall. Eliminating the dividing wall 

reduced the capital costs, and eliminated problems associated with soot blowing and keeping the 

dividing wall clean. The plant has seven coal mills, with each supplying one row of eight 

burners—four burners on each corner of the two fireballs. Under normal operation, only six of 

the seven coal mills are in operation and the power plant rotates which coal mill is shutdown. 

The gases leaving each fireball pass through the reheat and superheater tubes, then pass down 

through the economizer before making a turn up into the hot-side electrostatic precipitator. 

Some of the larger ash falls out on the upward turn after the economizer. This material is 

referred to as ”economizer ash” in this report and includes unburned fuel, larger pieces of ash, 

pieces of slag, and occasional bits of refractory. The economizer ash is periodically emptied to 

the ash pond through an eduction system. 

3
The east and west flue gas flows on each side of the boiler are further split upon entering the 

ESP. The gases pass through six ESP fields, with every two ESP fields emptying into a fly ash 

hopper. This means that for any path through the ESP, it is possible to get three ash samples: the 

ash removed in the first two fields, the ash removed in the middle two fields and the ash removed 

in the last two fields of the ESP. During testing, fly ash samples were collected from all three fly 

ash hoppers from corresponding rows on both the east and west sides of the boiler. Because of 

the high ESP efficiency, it sometimes was not possible to collect any ash from the final, outlet 

ash hopper. The fly ash from each hopper is pneumatically conveyed to a large ash storage bin, 

where it is loaded into trailer trucks and normally sold as Class C fly ash for concrete 

manufacture. Ash derived from non-coal sources cannot be used for concrete manufacture, per 

ASTM standards, so during co-fire testing, the ash was sold at a loss for other uses. OGS 

produces an average of 300 tons (272 tonnes) of fly ash per day. 

After leaving the ESP, the flue gas passes through a pair of basket air-heaters, which preheat the 

primary and secondary combustion air. The flue gases are pulled through the four parallel 

induced-draft (ID) fans and exit through the 600-ft. (183-m) stack. OGS has continuous 

emissions monitors for SO2, NOx, and opacity. During the winter months, primary combustion 

air is preheated using steam from the de-aerator. 

The steam cycle of the power plant operates at a maximum drum pressure of 2,990 psig (20.6 

MPa) and with a superheat temperature of 1,005°F (541°C). The steam passes through the highpressure turbine and the flow is split and returned to the boiler to be reheated at a maximum of 

680 psi (5.9 MPa) and 1,005°F (541°C). The two separate reheat streams pass through the lowpressure bottoming turbines and the exhaust is condensed using 140,000 gpm (8.8 m3

/s) of water 

from the plant’s two cooling towers. The condensate is heated in low-pressure and high-pressure 

heaters using extraction steam, before passing through the economizer and returning to the steam 

drum. The maximum rated capacity is 4,850,000 lb/h (2,200 tonne/h) of steam with a reheat 

flow of 4,440,000 lb/h (2,000 tonne/h). 

1.5 Switchgrass Feed-Handling Equipment 

Figure 1.1 shows a layout of the switchgrass feed-handling equipment. Several modifications 

were made to the switchgrass feed-handling equipment during testing. What follows is a 

description of the final system design. Details on these modifications are discussed in Section 3 

of the report. 

Figure 1.1 – Flowsheet of the Switchgrass Feed-Handling System 

4
A 12,000 sq. ft. (1,100 m2

) building was constructed behind OGS to house both the feed 

handling equipment and to store approximately 300 tons (272 tonnes) of switchgrass. The 

original hope was to use 3 ft x 3 ft x 8 ft (0.9 m x 0.9 m x 2.4 m) or “3x3” bales, 3 ft x 4 ft x 8 ft 

(0.9 m x 1.2 m x 2.4 m) or “3x4” bales, and 6 ft (1.8 m) round bales during switchgrass testing. 

However, we quickly found that the round bales, because of the variation in density between the 

outside layers and the core, could not be fed evenly into our equipment. For that reason, testing 

was restricted to the 3x3 and 3x4 bales, which weighed 700 lbs (318 kg) and 1,000 lbs (454 kg), 

respectively. 

Bales were delivered to OGS by flatbed trailer, with 54-3x3 or 36-3x4 bales per truck. The bales 

were normally unloaded at night or in the early morning so that the fork truck driver was not 

required to both unload bales from the truck and maintain the feed handling system. The bales 

could be stacked 6 high (18 ft or 5.5 m) in the storage area and three 3x3 or three 3x4 bales could 

be handled at a time, using a telescoping fork truck with a special bale pusher mounted on the 

fork. 

The bales were placed onto a platform with a drag-chain conveyor feeding a Newhouse Big Bale 

Chopper C5000 “debaler.” The debaler platform was 10 ft (3 m) wide and approximately 20 ft 

(6.1 m) long. The bales were placed onto the platform with three 3x3 bales side-by-side or with 

two 3x4 bales side-by-side. After being placed on the platform, the twine was cut with an axe 

and manually pulled off the bale from the top. The twine had a tendency to wrap around the 

rotating parts in the feed-handling system or lodge in the baghouse, so it was removed whenever 

possible. 

The bales were pulled into the debaler where a rotating drum mounted with teeth tore the 

switchgrass straw off the end of the bale one layer at a time. The switchgrass was then pushed 

out of the side of the debaler with a high-speed screw conveyor. The debaler was powered by a 

power take-off (PTO) of a tractor and required approximately 85 hp (63 kW). A variable-speed 

hydraulic drive controlled the speed of the drag-chain conveyor on the debaler. Because the 

entire feed-handling system was operated in a starve-fed mode, the drag-chain conveyor speed 

was the primary method for controlling the switchgrass flow to the boiler. 

The switchgrass exiting the debaler was discharged onto an air knife, designed to allow heavy 

material to drop out through the flow of air, while the lighter switchgrass was blown onto a belt 

conveyor leading to the Eliminator grinder. The air knife opening was about 3 ft (1 m) wide and 

the blower was connected to a variable speed drive that could be adjusted for optimum 

separation. The air knife blower was designed for entrained solids flow, so it did double duty, 

controlling dust by pulling a suction on a hood over the air knife and debaler discharge. Any 

reject material from the air knife was manually removed. 

A smooth belt conveyor took the material from the debaler to the inlet chute of the Dothan, Inc. 

Eliminator grinder. The Eliminator is best described as an attrition mill. It consists of two 

intermeshing rotating shafts running at 1,300 rpm, which throw the material around inside the 

unit, breaking it up. Figure 1.2 shows a picture of the Eliminator with the top removed. Unlike a 

hammer mill, the Eliminator has no outlet screen for classifying the material size—the extent of 

grinding was entirely dependant on the residence time and the amount of material passing 

5
through the unit. As the residence time increased or the amount of material passing through the 

unit increased, the particle size decreased. One benefit of the Eliminator design was that as the 

amount of material passing through the unit increased, the particle size decreased, with no 

blinding of any outlet screen to limit throughput. The Eliminator was driven by a pair of 300-hp 

(224-kW) electric motors. 

At the outlet of the Eliminator, a 26–in. (0.7-m) duct from the baghouse pulled a partial vacuum 

on the Eliminator. The Eliminator was originally designed for granular material, and pulling a 

suction on it was required to help move the lighter switchgrass through the unit. Additional 

transport air was admitted to the duct through an adjustable slide gate located under the 

Eliminator. See Figure 1.3 for the details of this equipment arrangement. Under normal 

operating conditions, the suction at the Eliminator discharge was approximately 8-10 in. of 

water. The suction at the inlet chute of the Eliminator was approximately 2-4 in. of water. 

The air flow from the Eliminator was combined with the flow from several other smaller dust 

collection lines before entering a Camfil Farr “Big Round Filter” model 231BRF10 baghouse 

filter. The baghouse contained 231-10 ft (3 m) 16 oz. polyester bags, 6 in. (15 cm) in diameter. 

The airflow entered the baghouse tangentially, like in a cyclone, so the larger pieces of 

switchgrass dropped to the bottom of the baghouse without contacting the bags, while the bags 

captured the smaller dust particles. The dust cake forming on the bags was continuously 

removed using a pulsed air jet from the clean side of the baghouse. The baghouse and dust 

collection system was rated for 20,000 cfm (9.4 m3

/s) of flow and used a 60-hp (45-kW) Air 

Tech Fan Corporation model 300B1SW3CD3609 centrifugal blower. 

The material was removed from the baghouse using a 30–in. (0.8-m) diameter by 40–in. (1.0-m) 

long rotary valve, custom built by Kelderman Manufacturing. This material was dropped onto a 

totally enclosed tube conveyor, which consisted of a belt lying in a curved trough, driven on the 

upper end. Like all of the equipment downstream of the Eliminator, this conveyor was kept 

under a slight negative pressure to minimize dust levels in the switchgrass building. The 

baghouse was located outside of the switchgrass building because of its height, so the tube 

conveyor passed back inside through the wall of the switchgrass building. 

The baghouse tube conveyor dumped into a 96 in. x 120 in. x 80 in. (2.4 m x 3 m x 2 m) surge 

bin, which had two pair of co-rotating 12–in. (30-cm) diameter 14–ft (4.2-m) long screws in the 

bottom. Each pair of screws fed a Western Pneumatics, Inc. 20 in. x 25 in. (0.5 m x 0.6 m) 

rotary airlock, dumping into a pneumatic transport line operating under a few psi (10 kPa) of 

pressure. Although originally designed to be a surge bin with variable speed screws in the 

bottom, in the final system design, this bin was run in starve-fed mode with the screws near their 

maximum rate. 

The pneumatic transport system consisted of twin 550-ft (168-m) long 10-in. (0.25 m) schedule20 carbon steel pipes, each fed by a 100-hp (75-kW) Gardner Denver model GKGBDCA 

positive displacement rotary lobe blower. After entering the power plant, the pipes went to 

opposing corners of the east fireball. Foster Wheeler switchgrass nozzles injected the 

switchgrass between the third and fourth rows of coal burners from the bottom. A slide gate at 

the boiler nozzle was included for positive shutoff of fuel during boiler trips. 

6
Figure 1.2 – Picture of Eliminator 

Figure 1.3– Diagram of Eliminator Discharge Chute 

Inlet Chute 

Transport Air 

Slide Gate 

To Baghouse 

Eliminator 

Discharge 

Chute 

7
The feed handing system was controlled by an Allen-Bradley SLC 5/3 programmable logic 

controller with two 16-channel inlet modules and two 16-channel outlet modules. In the OGS 

control room, there were indicators for the blowers and feed screws, along with a permissive 

switch and emergency stop for boiler house personnel. In the switchgrass building, there were 

controls for starting and stopping all the equipment, along with two emergency stop buttons 

located on opposite sides of the switchgrass equipment room. The speed control for the debaler 

was a manual hydraulic control, although there was a high-level shutoff tied to ultrasonic level 

detectors in the surge bin. Under normal operation, the surge bin and baghouse equipment were 

controlled from the motor control center (MCC), while the conveyors were operated from the 

control panel in the switchgrass building. 

2.0 CO-FIRE TESTING 

The switchgrass co-fire testing was performed from November 30, 2000, through January 25, 

2001. Over this period of time, there were a total of 26 days when some amount of switchgrass 

was co-fired in the OGS boiler. Normally, no co-firing was done on weekends and there were a 

number of mechanical- and weather-related delays. 

When first started, the system had a very limited throughput of only a few tons per hour. Several 

minor modifications were made, but the feed rate to the Eliminator grinder was still limited to 

about 6 tons/h (5.4 tonne/h). At the beginning of January, we made a major modification that 

allowed us to increase the system capacity to more than 15 tons/h (13.6 tonne/h). A total of 

1,269 tons (1,151 tonnes) of switchgrass was burned during the test period, with the highest 

measured rate being 16.8 tons/h (15.2 tonne/h). Because of our limited capacity to process 

switchgrass in December, our goal of burning 3,000 tons (2,722 tonnes) of switchgrass was not 

met; however, we did exceed the original target rate of 12.5 tons/h (11.3 tonne/h). Table 2.1 lists 

the amounts of switchgrass burned each day along with the approximate co-fire rates, if known. 

The maximum daily switchgrass total was 178 tons on January 22, 2001. This included 384-3x3 

bales and 88-3x4 bales. All weights are given on an “as received” basis. Bale moisture levels 

were consistently below 15% and were estimated to be about 11% on average. 

2.1 Fuel Data 

The boiler was designed to burn low-sulfur (0.5%) Wyoming Powder River Basin (PRB) coal. 

The composition and fuel value are shown in Table 2.2. It is important to note that the lowsulfur PRB coal has a higher moisture content and a lower heating value than bituminous coal. 

Also shown in Table 2.2 is the composition and fuel value of the switchgrass. The switchgrass 

used in the first co-fire test campaign was field-dried to less than 15% moisture and stored 

inside, so its fuel value is close to that of the PRB coal on an as-received basis. Samples sent 

back to NREL for moisture analysis returned values ranging from 9%-16%, with an average 

moisture of 11.2%. Laboratory data from Hazen Research gave lower moistures, but there was a 

delay of several months in sending these samples for analysis. The ash and trace element 

analyses for both fuels are shown in Table 2.3. One fraction of the switchgrass—the nodes— 

was analyzed separately. This data is also shown in Tables 2.2 and 2.3. It is suspected that some 

8
of the ash in the switchgrass samples was associated with dirt picked up during harvesting and 

bale handling. 

Table 2.1 – Daily Switchgrass Totals 

Date 

Switchgrass 

Burned 

(tons) 

Switchgrass 

Burned 

(tonnes) 

Rate 

(ton/h) 

Rate 

(tonne/h) 

30-Nov-00 1.9 2-3 2-3 

1-Dec-00 4.2 6-7 5-6 

4-Dec-00 6.4 5.5 5 

7-Dec-00 9.2 

8-Dec-00 29.9 

13-Dec-00 1.9 

14-Dec-00 15.7 

15-Dec-00 20.9 

18-Dec-00 43.9 

19-Dec-00 36.0 

20-Dec-00 29.2 5 4.5 

21-Dec-00 50.6 

22-Dec-00 30.8 

5-Jan-01 1.9 

8-Jan-01 10.5 15 14 

10-Jan-01 76.2 16.5 15 

11-Jan-01 75.6 13-14 12-13 

12-Jan-01 83.8 

15-Jan-01 4 125.5 12 11 

16-Jan-01 25.5 6-7 5-6 

17-Jan-01 63.5 12-13 11-12 

19-Jan-01 35.4 

22-Jan-01 4 161.8 

23-Jan-01 9 95.1 

24-Jan-01 57.7 

25-Jan-01 58.0 

2.1 

4.6 

7.1 

10.1 

33.0 

2.1 

17.3 

23.0 

48.5 

39.7 

32.2 

55.8 

34.0 

2.1 

11.6 

84.0 

83.4 

92.4 

138.

28.1 

70.0 

39.0 

178.

104.

63.6 

64.0 

9
Table 2.2 – Composition and Fuel Value of Coal and Switchgrass 

As Received: Coal itchgrass Nodes 

Moisture % 33.52 6.34 7.22 

Ash % 5.51 5.35 2.56 

Volatile % 28.98 73.84 75.85 

Fixed Carbon % 32.00 14.48 14.38 

S % 0.36 0.11 0.05 

Btu/lb (HHV) Btu/lb 7,774 7,458 7,631 

MMF Btu/lb Btu/lb 8,268 7,915 7,847 

Dry Basis: 

Ash % 8.24 5.70 2.75 

Volatile % 43.60 78.84 81.75 

Fixed Carbon % 48.16 15.46 15.50 

S % 0.55 0.12 0.05 

Btu/lb (HHV) Btu/lb 11,696 7,965 8,225 

MMF Btu/lb Btu/lb 12,852 8,486 8,477 

MAF Btu/lb Btu/lb 12,747 8,445 8,457 

Ultimate (Dry Basis): 

% 67.98 48.41 49.49 

H % 4.48 5.06 5.35 

N % 1.16 0.56 0.37 

S % 0.55 0.12 0.05 

Ash % 8.24 5.70 2.75 

O % 17.58 40.16 42.00 

Cl % 0.02 0.14 0.06 

Na2O % 0.067 0.003 0.003 

K2O % 0.004 0.809 0.438 

Alkali M Btu 0.12 1.01 0.57 

Ash M Btu 7.06 7.19 3.37 

SO2 M Btu 0.93 0.30 0.13 

Sw

lb/M

lb/M

lb/M

10

 C 
Table 2.3 – Ash & Trace Element Analyses of Coal and Switchgrass 

Ash: Coal itchgrass Nodes 

SiO2 % 35.34 57.52 42.00 

Al2O3 % 20.03 0.70 1.37 

TiO2 % 1.20 0.26 0.11 

Fe2O3 % 5.45 0.40 0.83 

CaO % 20.27 9.10 18.47 

MgO % 3.54 4.64 3.85 

Na2O % 1.22 0.55 0.48 

K2O % 0.50 13.88 17.95 

P2O5 % 1.31 6.56 7.70 

SO3 % 10.24 2.41 1.73 

Cl % <0.01 0.93 0.30 

CO2 % 0.18 1.68 2.59 

Trace Elements: 

As mg/kg 17.7 1.2 1.2 

Ba g/kg 4,727 733 992 

Cr mg/kg 93 58 70 

Cd mg/kg <1 <5 <5 

Pb g/kg 33 40 47 

Hg mg/kg <0.1 <0.1 <0.1 

Ag g/kg 2 <10 2 

Se g/kg 7 4 <2 

Sw

m

m

m

m

One question to be answered by the first round of co-fire testing was how effective the 

Eliminator would be at producing switchgrass with the size distribution necessary for proper 

combustion in the OGS boiler. The ground switchgrass has three fractions: a micron-sized dust; 

a large fraction of thin, flat, and long sections of split switchgrass stalk; and a small fraction of 

round dense nodes that come from where the sections of stem came together. With the 

Eliminator, the nodes were not reduced in size. While these nodes did not cause any operational 

problems and were transported through the feed-handling system, once the nodes entered the 

OGS boiler, they simply dropped to the bottom of the boiler, due to their larger size and higher 

density relative to the rest of the switchgrass. They were clearly visible floating around the 

edges of the ash pond and showed almost no charring. 

During testing, we were able to isolate and collect relatively pure samples of nodes from the 

feed-handling system. We were interested in the composition of these nodes because we knew it 

was possible to pull them out of the main switchgrass stream using an air classifier, if desired. 

The analysis of the nodes is included in Tables 2.3 and 2.4. We were somewhat surprised to find 

that, other than the higher potassium content, the nodes appeared to have favorable fuel 

characteristics, with low ash and low sulfur content. 

The size distribution of the ground switchgrass and the node samples are shown in Table 2.4. It 

is important to note that even though some of the switchgrass pieces are longer than 1 in. (2.5 

cm) in length, they are typically less than 1/16 in. (1.5 mm) in thickness and burn quickly in the 

11
OGS boiler. For comparison, 90% of the pulverized coal leaving the coal mills passes through a 

200-mesh screen. 

Table 2.4 – Switchgrass Size Distribution 

% Retained 

Mesh Size Coal Switchgrass Nodes 

>3 0.75 4.69 

3-48 86.01 94.61 

48-200 10.94 0.68 

200-325 9 1.17 0.02 

325-400 6 0.35 0.00 

<400 57.47 0.78 0.00 

13.5

22.9

During co-fire testing, several samples were taken to determine the bulk densities of the 

switchgrass at different points in the system. This information is important for the design of 

rotary airlocks, screw conveyors, belt conveyors, and surge bins. The density measurements 

varied with not only time, but also with who was conducting the testing. It was difficult to get 

representative samples from the debaler because of the low density. For the baghouse samples, 

the material quickly settled to a higher density after sampling. For these reasons, a range of bulk 

densities is given in Table 2.5. The ground switchgrass settled over time, explaining the high 

bulk density measured for the sample taken from the truck that was used for switchgrass storage 

during startup. The estimated bale densities are included for comparison. The explanation for 

the higher bulk densities after grinding is that the smaller particles of switchgrass fill in the 

spaces between the larger pieces, which results in a higher bulk density. 

Table 2.5 – Bulk Switchgrass Densities 

3x3 Debaler Baghouse Truck 

Bulk Density lb/ft

3

 9.7 10.4 1.3-4.0 6.7-15.6 14.3 13.8-18.3 

Bulk Density kg/m3 156 167 21-64 107-250 229 221-293 

3x4 Nodes 

2.2 Stack Testing 

To meet Iowa Department of Natural Resources requirements, two days of stack testing were 

conducted. On November 15, 2000, baseline coal-only stack testing was performed to collect 

data on carbon monoxide and particulate emissions. Each day of stack testing consisted of three 

tests, each approximately one hour in length. During this time the coal and switchgrass flows 

were measured precisely. This supplemented the continuous emissions monitoring (CEM) data, 

which included SO2, NOx, and opacity measurements. At the same time, an Orsat portable gas 

analyzer was used to sample NOx, CO, CO2, and O2 levels. 

Stack testing was performed when co-firing switchgrass on December 20, 2000. Unfortunately, 

during the co-fire stack testing there was a problem with the OGS boiler, unrelated to the co-fire 

testing, which caused abnormally high CO readings. The night before the stack testing, the 

boiler operators noticed “sparklers” leaving the boiler when burning only coal, indicating 

incomplete combustion. In addition, the operating team didn’t soot blow the night before testing, 

12
resulting in abnormally high ESP temperatures during the following day’s testing. The co-fire 

stack testing results have been included in Table 2.6 for completeness, but it should be 

emphasized that the results do not represent typical boiler performance when co-firing. There 

were also some questions about the reported reduction in particulate emissions during co-firing. 

While this is a desired result, the reduction was due almost entirely to a drop in the condensable 

particulate measurements, which is not an expected result. The particulate emissions from the 

baseline, coal-only testing had initially been processed incorrectly, so we are skeptical of these 

results as well. Note that the co-fire emissions in Table 2.6 include the heat of combustion for 

the switchgrass. 

Table 2.6 – Base-line and Co-fire Stack Test Results 

Coal 

Flow 

Switchgrass 

Flow SO2 NOx PM PM10 CO Load 

(ton/h) (ton/h) (lb/MM Btu) (lb/MM Btu) (lb/MM Btu) (lb/MM Btu) (lb/MM Btu) (MW) 

Base-Line 0.0 0.654 0.352 0.089 0.085 0.0004 711 

Co-Firing 3.8 0.656 0.394 0.046 0.040 0.0043 712 

460 

444 

2.3 Orsat Gas Analyzer Results 

On specific test days, an Orsat portable gas analyzer was used to collect data to supplement the 

CEM results. The Orsat meter recorded NOx, CO, CO2, and O2 levels leaving the economizer, 

entering the air heater, and exiting the air heater. Readings were taken for both the east and west 

sides of the boiler. The Orsat data included three days of baseline, coal-only data. The Orsat 

readings verified that there were abnormally high CO levels during the co-fire stack testing on 

December 20, but the readings also show that the switchgrass did not normally contribute to 

higher CO readings. Table 2.7 shows the average gas analyzer readings for several test days. 

Note that the co-fire rates in January were 3 to 5 times higher than those in December. 

Table 2.7 – Orsat Gas Analyzer Results (ppm) 

Date 15-Nov-00 Dec-00 20-Dec-00 15-Jan-01 16-Jan-01 17-Jan-01 

Fuel Coal Only Coal Only Coal/SWG Coal/SWG Coal/SWG Coal Only 

CO-Economizer Outlet 1.1 15.9 0.9 0.9 0.0 

CO-Air Heater Outlet 0.9 15.0 1.6 4.2 0.5 

NOx-Economizer Outlet 218 243 218 193 232 

NOx-Air Heater Outlet 177 221 217 206 178 

8-

2.3 

2.3 

258 

199 

Note: Numbers in bold are baseline coal-only measurements. 

Date Jan-01 22-Jan-01 23-Jan-01 24-Jan-01 25-Jan-01 

Fuel /SWG Coal/SWG Coal/SWG Coal/SWG Coal/SWG 

CO-Economizer Outlet 0.1 0.4 1.1 1.1 0.6 

CO-Air Heater Outlet 0.4 0.8 1.1 1.5 1.3 

NOx-Economizer Outlet 219 277 255 269 203 

NOx-Air Heater Outlet 166 213 203 207 158 

19-

Coal

13
2.4 Continuous Emissions Monitoring Results 

The continuous emission monitoring equipment at OGS records SO2, NOx, and opacity levels 

leaving the stack. The system records information continuously and it is apparent from 

examining the details of the raw data that major changes up or down in fuel flow result in large 

changes in emissions during the transition period. Table 2.8 shows the average daily SO2, NOx, 

and opacity levels for the month of January, as recorded by the CEM system. Bold numbers 

indicate days when only coal was being burned. The data from January is of the most interest 

because the equipment redesign had been completed, so the switchgrass system could be 

operated at rates of over 15 ton/h (13.6 tonne/h). Data from January 4, 2001, were excluded 

from the averages due to abnormal boiler operation. 

Table 2.8 – CEM Data for January 2001 

Date 2 NOx Opacity Oxygen 

(ppm) (ppm) (%) (%) 

1-Jan-01 164 16.5 6.03 

2-Jan-01 181 16.3 5.93 

3-Jan-01 189 17.0 5.95 

4-Jan-01* 175 128 32.2 8.41 

5-Jan-01 164 17.6 6.12 

6-Jan-01 169 16.0 6.17 

7-Jan-01 170 14.5 6.29 

8-Jan-01 179 14.7 6.12 

9-Jan-01 172 17.3 5.93 

10-Jan-01 179 16.3 6.09 

11-Jan-01 202 18.6 6.02 

12-Jan-01 197 16.0 6.00 

13-Jan-01 180 17.2 6.01 

14-Jan-01 188 17.5 6.09 

15-Jan-01 189 17.1 6.21 

16-Jan-01 199 15.8 6.08 

17-Jan-01 182 17.0 6.02 

18-Jan-01 171 17.9 6.07 

19-Jan-01 176 16.5 5.97 

20-Jan-01 176 15.6 6.23 

21-Jan-01 177 16.6 7.43 

22-Jan-01 214 16.3 6.61 

23-Jan-01 208 16.4 6.13 

24-Jan-01 199 16.1 6.22 

25-Jan-01 150 14.9 6.42 

Average* 7 182 16.48 6.17 

Coal Only* 247.8 176 16.58 6.19 

Co-Firing 7 187 16.41 6.15 

SO

262 

248 

235 

229 

227 

227 

236 

262 

241 

262 

247 

253 

255 

241 

248 

257 

255 

252 

255 

246 

251 

243 

239 

249 

246.

245.

*Several boiler trips occurred on January 4, so these data were not included in the averages. 

14
Because of the variations in emissions caused by weather, fouling, and changes in boiler 

operation from day-to-day, it is also useful to compare the distributions of SO2, NOx, and opacity 

to see the overall effect of co-firing. Figure 2.1 shows a slight decrease in average daily SO2 

emissions when co-firing switchgrass, as expected. Figure 2.2 compares the NOx emissions 

when co-firing with the coal-only values, and there appears to be an increase in NOx emissions, 

although the spread in the data indicates that the boiler was also less stable when running 

switchgrass. This is true because the switchgrass system was not always consistent in supplying 

switchgrass to the boiler so the coal flow ramped up and down in response to the changes in 

switchgrass flow. It is also important to understand that co-firing occurred mostly during the 

week, when the boiler was running at full capacity, and many of the non-co-fire, coal-only days 

were weekends when the boiler was running at reduced load. Because both NOx and SO2 

emissions increase with load, part of the increased emissions in Table 2.8 and in Figure 2.2 are 

due to the higher loads. Figure 2.3 shows that the opacity was virtually unchanged when burning 

switchgrass. 

Figure 2.1 – Comparison of SO2 Emissions when Co-Firing 

Coal Co-Fire 

45% 

40% 

35% 

30% 

25% 

20% 

15% 

10% 

5% 

0% 

200 210 220 230 240 250 260 270 280 290 300 

SO2 Emissions (ppm) 

Frequency (%) 

15
Figure 2.2 – Comparison of NOx Emissions when Co-Firing 

Coal Co-Fire 

60% 

50% 

40% 

30% 

20% 

10% 

0% 

100 120 140 160 180 200 220 240 

NOx Emissions (ppm) 

Figure 2.3 – Comparison of Opacity when Co-Firing 

Coal Co-Fire 

60% 

50% 

40% 

30% 

20% 

10% 

0% 

10 11 12 13 14 15 16 17 18 19 20 

Opacity (%) 

Frequency (%) Frequency (%) 

16
2.5 Sampling and Ash Analysis 

The following samples were collected during selected “test days” during the two-month co-fire

campaign: 

1). Raw coal feed 

2). Pulverized coal feed 

3). Raw switchgrass feed

4). Chopped switchgrass feed 

5). Switchgrass nodes 

6). Boiler bottom ash 

7). Boiler bottom ash water

8). Economizer ash (east and west sides of boiler)

9). Fly ash (east and west sides of boiler, hoppers B, D & F) 

One reason for testing both the raw coal and the pulverized coal is because soda ash is

sometimes added to the coal to optimize ESP performance. All of the raw coal samples were

collected before soda ash addition. The pulverized coal also showed some changes in

composition, possibly due to the high temperature of the primary combustion air. The complete

analysis of the bulk coal, before soda ash addition, is given in Section 2.1 of this report. 

For the switchgrass, three sets of samples were collected. The debaler sample—pulled from 

either the inlet conveyor or the discharge chute of the debaler—was the most representative of 

the switchgrass entering the plant. The second sample point for the switchgrass was downstream 

of the Eliminator. During December, chopped switchgrass samples were collected from the exit 

of the Eliminator; during January, samples were collected from the baghouse discharge. With 

the addition of the suction line to the Eliminator discharge, it became possible to capture a large

fraction of switchgrass nodes for analysis. The thought was that if the nodes proved to be a 

source of high silica or high alkali, it might be desirable to remove them from the fuel stream, 

but this was not the case. The fuel analyses of the debaler samples and the switchgrass nodes are 

given in Section 2.1. 

Economizer and fly ash samples were collected from both the east and west sides of the boiler. 

(Switchgrass was injected into the east fireball.) For the fly ash, samples were collected from the

corresponding sequences of three ash hoppers on each side of the boiler. The flue gases passed 

through six ESP fields, with every two fields emptying into a fly ash hopper. This meant that 

three samples were collected from the east side of the boiler and three samples from the west 

side. The inlet hopper for each side was labeled sample B, with hopper D in the middle, and 

hopper F on the outlet side of the ESP. In some cases, no sample could be collected from the 

outlet hopper F because the ESP was operating too efficiently. Table 2.9 has the average fly ash 

composition data for coal-only conditions. Table 2.10 has the data from the same hoppers during

co-firing. 

17
Table 2.9 – Fly Ash Data for Baseline Coal-Only Conditions 

Fireball East East East West West West 

Hopper Inlet Middle Outlet Inlet Middle Outlet 

Fuel Coal Only Coal Only Coal Only Coal Only Coal Only Coal Only 

LOI 0.49 0.44 0.44 0.91 

Na-Na2O % 0.091 4 0.094 0.132 0.248 

K-K2O % 0.011 1 0.011 0.013 0.016 

SiO2 % 38.33 34.43 38.41 33.06 30.03 

Al2O3 22.36 22.83 22.87 22.24 23.44 

TiO2 % 1.58 1.41 1.46 1.38 

Fe2O3 5.71 4.88 5.46 5.20 

CaO 22.70 24.45 22.15 24.25 24.10 

MgO % 4.13 4.05 4.73 4.41 

Na2O % 2.36 2.38 2.48 2.55 

K2O % 0.47 0.49 0.49 0.50 

P2O5 % 1.52 1.56 2.00 2.63 

SO3 0.88 0.66 1.55 2.13 

Cl % <0.01 01 <0.01 <0.01 <0.01 

CO2 0.03 0.08 0.03 0.03 

As mg/kg 22.3 5 23.1 26.7 31.1 

Ba g/kg 5,550 5 5,570 6,365 7,610 

Cr g/kg 92 5 92.5 101 80 

Cd g/kg 1.3 1.45 2.15 2.7 

Pb g/kg 47 47 67 88 

Hg g/kg <0.1 <0.1 <0.1 <0.1 <0.1 

Ag g/kg <1 <1 <1 <1 

Se g/kg 8 6 10.5 13 

% 0.48 

0.13

0.01

% 

1.51 

% 5.62 

% 

4.41 

2.48 

0.47 

2.00 

% 1.20 

<0.

% 0.04 

23.9

m 6,17

m 100.

m 1.95 

m 64 

m

m <1 

m 10 

18
Table 2.10 – Fly Ash Data from Co-Firing 

Fireball East East East West West West 

Hopper Inlet dle Outlet Inlet dle Outlet 

Fuel Coal/SWG Coal/SWG Coal/SWG Coal/SWG Coal/SWG Coal/SWG 

LOI 0.37 0.18 0.27 0.25 0.29 0.37 

Na-Na2O % 0.128 0.177 0.354 0.138 0.191 0.320 

K-K2O % 0.013 0.010 0.017 0.013 0.016 0.015 

SiO2 % 36.73 34.01 28.07 37.36 33.51 29.03 

Al2O3 22.88 22.20 22.88 22.26 23.33 22.84 

TiO2 % 1.50 1.44 1.36 1.44 1.38 1.42 

Fe2O3 5.63 5.56 5.39 5.62 5.46 5.27 

CaO % 22.33 22.97 24.70 21.07 22.83 24.20 

MgO % 4.17 4.26 4.66 3.93 4.21 4.59 

Na2O % 2.57 2.64 2.81 2.47 2.67 2.78 

K2O % 0.57 0.54 0.48 0.62 0.61 0.54 

P2O5 % 1.72 2.03 2.76 1.68 1.98 2.70 

SO3 0.69 0.84 2.43 0.98 1.31 2.91 

Cl <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 

CO2 0.08 0.09 0.07 0.09 0.06 0.09 

As mg/kg 24.6 27.6 36.6 24.1 25.8 33.6 

Ba g/kg 5,613 6,190 7,545 5,570 5,963 7,323 

Cr mg/kg 79 90 89 79 85 91 

Cd g/kg 1.4 1.8 2.9 1.6 1.7 2.6 

Pb g/kg 49 61 86 47 62 79 

Hg mg/kg <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 

Ag g/kg <1 <1 <1 <1 <1 <1 

Se g/kg 8 11 12 5 8 13 

Mid Mid

% 

% 

% 

% 

% 

% 

m

m

m

m

m

Bottom ash samples were collected from the discharge pipe to the OGS ash pond. The water 

from the ash pond is recirculated to the boiler and re-used for sluicing bottom ash and 

economizer ash, as well as supplying seal water for the bottom of the top-hung boiler. Samples 

of the bottom ash water were collected separately from the bottom ash itself to measure sodium, 

potassium, and trace elements that may have leached out of the ash. Economizer ash samples 

were collected for the extreme east and extreme west ash hoppers at the bottom of the 

economizer section. All ash hoppers were emptied after co-firing was started to assure all ash 

collected was from co-fire periods. Table 2.11 contains the economizer ash analysis when 

burning coal only and when co-firing. Table 2.12 contains the same data for the bottom ash, 

reported on a dry basis. Table 2.13 contains the RCRA trace element, sodium, and potassium 

analyses of the bottom ash water. 

19
Table 2.11 – Economizer Ash Analysis 

Fireball East West East West 

Fuel Coal Only Coal Only Coal/SWG Coal/SWG 

LOI 0.22 0.11 0.71 

Na-Na2O % 0.064 1 0.059 0.052 

K-K2O % 0.013 6 0.009 0.009 

SiO2 % 40.71 39.06 37.83 38.69 

Al2O3 21.93 22.11 21.66 21.22 

TiO2 % 1.51 1.48 1.49 

Fe2O3 5.75 5.64 5.74 

CaO 21.80 21.90 22.13 21.23 

MgO % 3.90 4.00 3.86 

Na2O % 1.87 1.97 1.95 

K2O % 0.44 0.57 0.49 

P2O5 % 1.33 1.43 1.50 

SO3 1.79 2.02 1.68 

Cl % <0.01 01 <0.01 <0.01 

CO2 0.02 0.12 0.11 

As mg/kg 16.5 20.3 17.5 

Ba g/kg 5,215 5 5,097 4,930 

Cr g/kg 88 83 85 

Cd g/kg 3.4 3.3 2.3 

Pb g/kg 29 26 28 

Hg g/kg <0.1 <0.1 <0.1 <0.1 

Ag g/kg <1 <1 <1 

Se g/kg <2 <2 <2 

% 0.29 

0.04

0.00

% 

1.56 

% 5.87 

% 

3.99 

1.85 

0.41 

1.33 

% 1.66 

<0.

% 0.03 

16.2 

m 5,23

m 89 

m 1.4 

m 29 

m

m <1 

m <2 

20
Table 2.12 – Bottom Ash Analysis 

Fuel Coal Only Coal/SWG 

LOI 0.64 0.56 

Na-Na2O % 0.048 0.042 

K-K2O % 0.014 0.014 

SiO2 % 46.82 45.69 

Al2O3 20.16 20.90 

TiO2 % 1.47 1.36 

Fe2O3 5.84 5.56 

CaO 19.05 17.89 

MgO % 3.54 3.38 

Na2O % 1.53 1.74 

K2O % 0.54 0.80 

P2O5 % 1.17 1.30 

SO3 0.20 0.33 

Cl % <0.01 <0.01 

CO2 0.06 0.16 

As mg/kg 3.9 5.2 

Ba g/kg 4,620 4,428 

Cr g/kg 105 68 

Cd g/kg <0.5 <0.5 

Pb g/kg 8 10 

Hg g/kg <0.1 <0.1 

Ag g/kg <1 <1 

Se g/kg <2 <2 

% 

% 

% 

% 

% 

% 

m

m

m

m

m

m

m

Table 2.13 – Analysis of Bottom Ash Water 

Fuel Coal Only Coal/SWG 

Na-Na2O mg/L 422.500 415.000 

K-K2O mg/L 39.350 49.917 

As mg/L 0.0 0.0 

Ba g/L <0.001 <0.001 

Cr g/L <0.04 <0.04 

Cd g/L <0.01 <0.01 

Pb g/L <0.1 <0.1 

Hg g/L <0.005 <0.005 

Ag g/L <0.04 <0.04 

Se g/L <0.2 <0.2 

m

m

m

m

m

m

m

21
3.0 EQUIPMENT PERFORMANCE 

As expected with any test program, there were a number of electrical and mechanical problems 

that interrupted testing. Most of these problems were overcome through changes in operating 

procedures or through equipment modifications, but in some cases it was determined that 

equipment modifications would be too expensive or cause excessive delays in the test program. 

3.1 Equipment Modifications & Repairs 

Section 1.5 of this report describes the switchgrass feed-handling system after several 

modifications were made during testing. Several pieces of the original equipment were modified 

or replaced. These changes are outlined in the following sections. 

3.1.1 Debaler 

No major changes were made to the debaler, but initially the gear ratio of the power take-off was 

changed to achieve a lower idle speed on the tractor. During testing, the teeth on the debaler 

were periodically sharpened. 

3.1.2 Air Knife 

The air knife used a variable frequency drive to control the blower speed. We initially had 

problems starting the blower because it had been wired with a motor starter in addition to the 

variable frequency drive— but only the variable frequency drive was required. The air knife 

worked well at low feed rates, separating corncobs, rocks, and other material from the lighter 

switchgrass, but at higher feed rates, the mat of material was too thick for any separation. While 

metal did not appear to cause any major problems with the Eliminator, we did discover that some 

metal pieces had been ejected from the inlet chute. There was a small baghouse fire after 

shutdown one night, which had presumably been caused by a spark from the Eliminator. In one 

instance, an old tire made it over the air knife and into the Eliminator before causing a plug. 

3.1.2 Debaler-to-Eliminator Conveyor 

To minimize dust from the debaler, a 12-in. (0.3-m) suction line was attached to a hood over the 

debaler outlet. Later a second 12-in. (0.3-m) line was added to the outlet of the hood for 

additional dust control. In order to operate at the higher flows of 15 tons/h (13.6 tonne/h), the 

exit gate on the dust collection hood was increased in size, and 12-in. (0.3-m) sides were added 

to the belt conveyor so the switchgrass from the debaler wouldn’t fall off the sides. At full flow, 

the mat of switchgrass was 4 ft (1.2-m) wide and more than 12-in. (0.3-m) deep on the belt. 

Because the belt was smooth, there was very little friction between the belt and the switchgrass, 

so there were limits to the angle of inclination of the conveyor. Because of the tendency of the 

switchgrass to bridge at the inlet chute of the Eliminator, the distance between the end of the 

conveyor and the inlet chute was adjusted so the switchgrass coming off the belt would fall in an 

arch and land in the mouth of the Eliminator inlet chute without touching the sides. In future 

22
designs using belt conveyors, it will be important to be able to adjust the alignment of the belt 

conveyor and the inlet chute to obtain the proper trajectory of the material into the grinder. 

3.1.3 Eliminator Grinder 

The first problem with the Eliminator was a short circuit in one of the 300-hp (224-kW) motor 

soft-starts. An insulator on the wiring block had apparently come loose during shipment and was 

not noticed after installation. During one of the startups, the starter shorted out, burning out a 

card on the soft-start. A new soft-start was flown in overnight and the entire starter was 

replaced. 

A number of modifications were made to the Eliminator. Initially we could only get about 2 

tons/h (1.8 tonne/h) through the Eliminator. To increase the throughput, parts of three internal 

baffles in the Eliminator were removed. These baffles were originally designed to increase the 

residence time for granular materials, but hindered the flow too much when running switchgrass 

through the unit. Removing portions of these baffles increased the throughput, so later all of 

these baffles were cut out. An additional modification was the installation of twisted blades near 

the inlet of the Eliminator to provide draft to push the material through the grinder. These 

modifications allowed throughputs approaching 6-7 tons/h (5.4-6.4 tonne/h), with some 

occasional bridging of the Eliminator inlet chute. Several minor modifications were also made 

to the inlet chute, including increasing the size of the inlet opening, cutting off the top of the 

chute to allow lower conveyor angles, and adding a lip to capture dust dropping off the debaler 

belt conveyor. 

The original process design simply dumped material out of the Eliminator onto a textured belt 

conveyor going up to the surge bin. A hood was used over this belt to control dust, but because 

the Eliminator produced a slight positive pressure at the outlet, a significant amount of dust blew 

out from under this hood, and was a major source of dust in the switchgrass processing area. 

This conveyor—because of the texture—also had some carryover leaving the surge bin, dumping 

fine material onto the floor below the conveyor. At the end of December, we decided to 

eliminate this conveyor and pull the entire flow of material through the baghouse. A suction on 

the inlet resulted, which provided the means of pulling the switchgrass down into the Eliminator, 

decreasing the frequency of plugs and dramatically increasing the grinder’s capacity. This also 

meant the outlet of the Eliminator was now under negative pressure, so it was no longer a source 

of dust entering the room. Later, the twisted bars in the Eliminator were reversed to provide 

some backpressure against the suction, increasing the residence time slightly to give a smaller 

particle size. 

Separation of metals downstream of the Eliminator was easily accomplished once the suction 

pipe was added because dense materials such as bolts and rocks could not make it up the 30-ft 

(9-m) section of vertical pipe going to the baghouse. Any debris collected in the pipe was 

removed each night after shutdown. The velocity through the 26-in. (0.7-m) suction line leaving 

the Eliminator was estimated at about 30 ft/s (9.1 m/s), which was probably lower than it should 

have been. At this velocity, a slight decrease in transport air to the Eliminator discharge reduced 

the velocity enough that the nodes leaving the Eliminator collected in the bottom of the tube, 

almost filling it in just a few minutes. (Because the transport air intake was at floor level, if two 

23
people stood in front of the intake, it blocked enough air that the nodes would start to 

accumulate.) This pointed out an important lesson for the design of switchgrass systems: the 

minimum transport velocity must be set to transport the nodes, not the bulk switchgrass, which is 

lighter and less dense. This characteristic of our transport system illustrated that it might also be 

possible to selectively remove the nodes, if desired, and it allowed us to get relatively clean 

fractions of nodes for testing. 

At one point after removing the top of the Eliminator for modifications, a hot spot developed on 

the outside casing of the Eliminator. The spot was caused when one of the disks used for dust 

containment around the shaft started rubbing against the outside housing of the unit. This had 

been seen during the startup of other Eliminator units, but was a matter of concern because of the 

combustible switchgrass and high dust levels. We emptied the unit of all combustible material, 

then ran the Eliminator for several hours without any switchgrass flow to allow the high spot in 

the casing to wear away, after which we had no further problems. 

3.1.4 Baghouse 

One early problem with the baghouse was that we periodically had what we termed “dust 

incidents”, where the flow from the baghouse would suddenly switch over to a very fine material 

less than 200 microns in size, which could not be drawn up the screw conveyor. The material 

could be transported as long as there was some longer material present to trap the dust between 

the screws, but this fine material—when by itself—could flow like a liquid down the conveyor 

and out the end, or just sit in the conveyor lifting and dropping as the screw passed under it, 

resembling someone stirring pudding. 

At first we suspected the cleaning arm had jammed, but when we inspected the clean side of the 

baghouse, the arm was turning freely. After the second such dust incident, we discovered that 

the gears had been stripped in the gearbox driving the cleaning arm, because oil had not been 

added to the gearbox after installation. Since no bag cleaning could be accomplished, the dust 

built up to a critical mass in the baghouse and then dropped all at once, overwhelming the screw 

conveyor. A new gear was flown in and after rebuilding the gearbox, we had no further dust 

incidents. 

When we increased the solids flow to the baghouse in January, we quickly exceeded the capacity 

of the original baghouse equipment. As designed, the baghouse necked down to an 18 in. x 40 

in. (0.45 m x 1.0 m) opening with a 9 in. (23 cm) diameter screw conveyor emptying into a 20 in. 

(0.5 m) diameter by 15 in. (0.4 m) long rotary valve. The entire screw and rotary valve assembly 

was replaced by a larger, custom-built 36-in. (1-m) diameter by 40-in. (1-m) long rotary valve. 

3.1.5 Baghouse Conveyor 

The first problem with the conveyor from the baghouse to the surge bin was a basic one: the 

rotation on the screw conveyor from the baghouse to the surge bin was not checked before 

startup. The result was that the switchgrass coming from the baghouse was pushed down to the 

bottom, closed end of the conveyor, causing the belts to fail on the top drive end. Once the cause 

of the problem was discovered, the leads were reversed and the belts were replaced. 

24
There were several times that the screw conveyor from the baghouse to the surge bin became 

overloaded and burned-off the belts on the drive end, finally burning out the motor itself in one 

instance. On closer examination of the screw conveyor, we discovered that it had been installed 

incorrectly. Because of the length of the screw conveyor (35 ft. or 10 m), hanger bearings were 

required every 10 ft. (3.3 m). At the hanger bearings, the individual sections of screw are 

supposed to be aligned at 180° from each other, as if there were an imaginary continuous screw, 

so that the material can transition smoothly under the hanger bearing. When installed, all three 

of the joints had been installed incorrectly with two joints lagging by 90° and one leading by 90°. 

This meant that at the joints lagging by 90°, material would build up longer than it should before 

being moved on by the flight on the next screw. Correcting the problem would have required 

disassembling the screw and then either re-drilling or replacing the stub shafts between the 

individual flights of the screw. To save time, all sections were instead adjusted to lead by 90° to 

minimize problems with the screw conveyor. 

Even with proper installation, the screw conveyor appeared to have been undersized for the peak 

flow from the baghouse, even before we routed the full flow from the Eliminator to it. It is 

possible that the screw conveyor had been sized to handle the average flow and not the peak 

flow. Before the start of testing in January, the screw conveyor was replaced with a “tubeveyor”, which consisted of a belt conveyor lying in a curved trough supporting the belt. A roller 

at the upper end drove the belt, and the entire conveyor was enclosed and operating under a 

slight negative pressure to minimize dust. The lower end was open to allow sampling of the 

material leaving the baghouse, although this allowed excess air to enter the dust collection 

system. The tube conveyor operated without any problems. 

3.1.6 Surge Bin 

The surge bin probably was not the best design for our application. While the bin did have 

diverging sides (wider at the bottom than at the top), there was a pinch point that developed at 

the front wall of the surge bin where the switchgrass was forced into four openings, just slightly 

larger than the diameter of the four screws in the bottom of the bin. The switchgrass passed 

through a short section of tube before emptying into the rotary air locks. The manufacturer said 

that the equipment was designed for handling grains and soybeans, which don’t bridge, and that 

the tubes were meant to minimize dead spots in the equipment. During our testing, if the 

switchgrass level built up, trying to move the stack of material forward toward the outlet moved 

the whole pile of switchgrass up against the front wall, causing the screws to bind and trip out. 

One possible solution would be to add sheer bars just above the screws to disengage the 

switchgrass trapped by the screws from the rest of the pile above it. Our solution was to run the 

surge bin with the feed screws on maximum so we did not build up any significant level of 

switchgrass in the bin. During switchgrass testing at the Southern Company’s Gadsden power 

plant, they observed similar problems where the switchgrass would bind together and cause large 

forces to be exerted backwards against the feed screws. They also resorted to running their bin 

with lower levels of material. 

Another problem with our bin design was that there was a dividing wall between the two pairs of 

feed screws. If the bin had been run as a surge bin with a significant inventory of material, this 

25
would have been fine, but because we were running in starve-fed mode, the result was uneven 

distribution of the flow between the two burners. With no dividing wall between the pair of 

screws, the material could have landed anywhere in the bin and the screws would have leveled 

the material across the bottom of the bin. With the dividing wall, if the material landed mostly 

on one side of the bin, it could not get over the divider to the other set of screws going to the 

other burner. Because the flow leaving the northwest switchgrass burner could not be seen due 

to its location in the boiler, it was difficult to determine if the flow was being split equally. 

However, at times, there were small stones traveling through the transport lines and these stones 

were making a sound in only one of the transport lines, indicating an uneven split in flow. 

The entire surge bin was kept under a negative pressure by a 12-in. (0.3-m) suction line attached 

to the dust collection system. By keeping all of the equipment downstream of the Eliminator 

under a slight negative pressure, dust levels after the January modifications were minimal. 

3.1.7 Rotary Airlocks 

The rotary airlocks caused continual problems. Their purpose in the system was to transfer the 

chopped switchgrass from the surge bin at atmospheric pressure, into the pneumatic transport 

lines going to the boiler, which were operating under several psi (10 kPa) of pressure. They 

were designed with individual pockets with close tolerances to prevent air from the transport line 

from blowing back into the surge bin. The rotary valves were designed with vents that allowed 

the pressure from the transport line to be relieved before refilling the pocket with switchgrass, 

but these were not used, and it is not believed that the blowback from the pockets caused any 

major problems. What we determined to be the biggest problem was the buildup of dust between 

the ends of the rotating pockets and the stationary valve housing. This created enough friction to 

trip out the airlock drives on overload. In some airlock designs, an air purge is used to blow out 

any dust that builds up on the ends. Still other airlocks are designed without an end plate, so this 

buildup does not occur. Discussions with other airlock manufacturers indicated that the rotary 

valves might not have been the proper design for our application. 

When the rotary airlocks tripped, the feed screws sometimes continued running, filling the area 

above the airlock with densely packed material. This required the feed system to be shutdown 

and the airlocks cleared. In addition to numerous motor trips, the housing rotary valves were 

designed to “rock”, so that if something got caught as the pocket came around, the rotary valve 

isn’t damaged. Over the course of testing, both pins that allowed this rocking broke off from the 

forces exerted on the valves. These pins were welded back on and strengthened, but in the end, 

the rotary valves had to be held in place with chain pulls because the pins kept breaking off. 

3.1.8 Pneumatic Transport Blowers & Transport Piping 

The first problem with the pneumatic transport lines was the discovery that some of the flanges 

on the transport lines had not been adequately tightened. We tightened the bolts and the entire 

system was rechecked. 

A design problem with the transport lines was that no expansion joints had been included for 

upward contraction of the boiler when the boiler was shutdown. Expansion joints had been 

26
added for expansion in the north-south and east-west directions, and the transport lines had been 

hung using constant-tension attachments at the top, but only about 6 in. (10 cm) had been left 

between the top of the transport lines and the bottom of the steel I-beams supporting the floor 

above the transport lines. This wasn’t enough clearance for the contraction of the boiler when 

cooled at shutdown. Fortunately the OGS operating personnel noticed the problem and 

disconnected the transport lines before shutdown, preventing any serious damage. 

The positive displacement blowers presented some problems because the air intakes were 

undersized. The blowers consisted of an intake air filter, muffler, blower, outlet damper, and 

check valve at the rotary airlock. Initially the blowers operated fine, but after several weeks of 

testing, we experienced a few bitterly cold days (which coincided with a blizzard that caused us 

to miss two days of testing). On these cold days we were not able to get the blowers started, 

even after changing the pulley ratio on the drives to increase the power to the compressor. 

Initially we thought the problem might be related to the low oil temperature in the gearboxes, so 

we added block heaters, changed the oil, and even tried preheating the inlet air during startup. 

While these methods helped us to start the units, we eventually learned that the air intakes for the 

compressors were undersized. The original pressure drop calculations indicated that the 

compressors should each put out 2,900 scfm (1.4m3

/s) of air at a pressure drop of 4 psi (28 kPa). 

Our system actually had a pressure drop closer to 2 psi (14 kPa), and even though the blowers 

were positive displacement, the normal leakage changes the output slightly, so we were actually 

moving 3,100 scfm (1.5 m3/s) of air through each pipeline. Our air intakes were designed for 

3,000 scfm (1.42 m3

/s), so with the cold and the dust that had collected on the air filters over a 

couple of weeks of testing, the compressors could not handle the additional startup load. The 

vendor believed that the air intakes had probably been ordered based on flange size and not 

based on the airflow, because good design practice would have included a larger safety margin 

on the airflow. We ordered a larger air intake for one unit and on the other unit, we simply 

removed the air filter during startup and reinstalled it once the compressor was running. The 

blower representative also indicated that the installation didn’t meet their company 

specifications, because there was a rigid connection between the compressors and transport line, 

which could increase the stress on the transport lines due to vibration. 

3.1.9 Control System 

There were numerous issues with the control logic used for the switchgrass system. While the 

system operated as designed, in many ways it was not practical and did not meet the needs of the 

operating staff. For example, the control logic allowed for the simultaneous startup of the two 

100-hp (75-kW) blowers and the simultaneous startup of the two 300-hp (224-kW) motors on the 

same circuit. The simultaneous startup of these large motors was not a requirement and did not 

represent good practice. The control logic also started the feed screws immediately after starting 

the transport blowers. We preferred to run the transport blowers for 10 minutes before feeding 

switchgrass to remove any condensation from the transport line that could cause plugging. A 

bigger concern was a section of control logic that allowed remote startup of the equipment in the 

switchgrass building by turning a switch in the OGS control room without any advanced warning 

or alarm in the switchgrass building. 

27
While it would have been possible to re-write the code for the programmable logic controller 

(PLC), it was felt that debugging the code would not be a simple task and that it might delay the 

testing. As a result, much of the equipment was started from the MCC, which limited the ability 

to use interlocks in the feed-handling system. Manual procedures were implemented to prevent 

remote startup of the switchgrass equipment. 

In future system designs, it is important to get operator input on the control designs. For 

example, the boiler room operators needed a positive indication that the switchgrass control 

valves were closed, and while the slide gates at the boiler had proximity switches, which sent a 

signal back to the PLC, no indicator light was provided for the boiler operating personnel. 

3.2 Summary 

Overall, except for the ongoing issues with the rotary airlocks, all of the problems we 

experienced were overcome and the system performed well at rates of 12-15 tons/h (11-14 

tonne/h). The highest measured rate was 16.8 tons/h (15.2 tonne/h) and the Eliminator probably 

had more capacity, if needed. (The Eliminator was delivered with two 300-hp (224-kW) motors, 

which rarely required more than 100 hp (75 kW) when operating.) 

The ability to make on-site equipment modifications and repairs was extremely important to the 

success of our testing at OGS. Any of the above problems could have resulted in a week or more 

of downtime without the experienced maintenance personnel on-site and their suppliers that airfreighted parts overnight from Chicago, preventing potentially lengthy delays. 

4.0 LABOR/OPERATING PERSONNEL 

The system used for this testing was very labor intensive, although it was designed as such to 

minimize capital costs for this short-term testing. When running at 15 tons/h (13.6 tonne/h), 30-

1,000 lb (454 kg) 3x4 bales must be handled every hour, or one every two minutes. Using 3x3 

bales required 43 bales to be handled per hour. That made driving the fork truck a full-time job, 

because little more than 2 tons (1.8 tonnes) of switchgrass could be placed on the debaler inlet 

conveyor at any given time. 

The bale handling was time consuming, but so was twine removal, which was done manually by 

a second person. Each 3x3 bale had 3 strings. Each 3x4 bale had 5 strings, meaning a person 

might be required to cut and pull out about 150 pieces of twine per hour, or one string every 24 

seconds! Ergonomically, this was a job that could not be done for extended periods of time 

because it involved bending, pulling, and walking around on uneven footing. Like driving the 

fork truck, twine removal was a full-time job. 

Another laborer was required to monitor the feed handling system, and at high feed rates, this 

person was responsible for monitoring the inlet chute to the Eliminator. If the chute plugged, the 

operator would temporarily stop the flow of material from the debaler and wait to see if the plug 

cleared itself. If it did not, the operator would stop the flow, manually clear the inlet chute, then 

go back and restart the debaler flow. Depending on the feed rate and the quality of the bales, the 

inlet chute might go hours without plugging, or only minutes. 

28
The above three jobs required full-time attention. In order to provide breaks, an extra person was 

required for relief. That meant a minimum operating crew of four people was needed to man the 

switchgrass building during the first round of testing. This isn’t practical in a commercial 

system, considering the entire 725-MW OGS power plant can be run with fewer people than this. 

The second co-fire test campaign will concentrate on the testing of an automated system for feed 

handing. In Europe, unattended operation has been demonstrated and even OGS has most of 

their coal-handling system designed with monitors that allow it to be safely operated from the 

control room under normal conditions. While there may be people monitoring the operation of 

the feed handling system during the second round of co-fire testing, the system should be 

designed to operate with minimal operator attention. 

5.0 FIRE PROTECTION 

There was one minor incident during the first co-fire test campaign, but there was no damage to 

equipment and no one was injured. At approximately 2:00 a.m. the morning of January 16, OGS 

plant personnel noticed smoke coming from the bottom of the baghouse, found a small fire 

smoldering inside, and quickly extinguished it with water. The fire did not damage any of the 

bags. 

Although the baghouse was emptied every night before being shutdown, upon further 

investigation, it was discovered that some of the twine that had made it through the debaler had 

wrapped around the bag supports in the bottom (dirty-side) of the baghouse. Dust passing 

through the baghouse had built-up on these supports and at some point been ignited by a spark, 

presumably from the Eliminator. 

After this incident, the operating personnel periodically inspected the dirty side of the baghouse 

and removed any debris that had collected. This incident illustrated another issue regarding 

switchgrass testing: the need for a written emergency response plan. Because of the design of 

our system and the requirement of negative pressure for dust control, the baghouse in our system 

would allow any fire to quickly spread through the entire system if the first response of the 

operating personnel is anything other than shutting off the baghouse blower. For future testing, 

it would be advisable to connect the fire pull alarms and flame detectors to a circuit to shutoff the 

baghouse blower in case of a fire. 

For the first co-fire test campaign, there was no sprinkler system in the bale storage area of the 

building, although some limited firefighting equipment was installed. In the event of a fire in the 

bale storage area, some method of quenching the fire will need to be devised and there will likely 

be compartmentalized storage and firewalls included in the final design, to prevent fire from 

spreading to the equipment areas. 

29
6.0 BOILER PERFORMANCE 

When we started testing, we checked to make sure that the boiler responded normally to the 

additional transport air entering through the switchgrass system, and that the boiler fuel flow was 

not adversely affected by the additional switchgrass fuel. As expected, the minor airflow from 

the transport system and the addition of the low flows of switchgrass were undetectable in the 

boiler control room. If it weren’t for the ability to observe the switchgrass leaving the nozzle and 

entering the boiler, there was no way of knowing we were burning any switchgrass at all. 

In the case of the Southern Company’s switchgrass testing in Gadsden, Alabama, they found that 

the cold transport air had a significant effect on boiler efficiency. At OGS, the transport air from 

the switchgrass system amounted to 6,200 scfm (2.9 m3

/s) of air compared to the 1.1 million 

scfm (519 m3

/s) of air normally used for combustion. The operation of our transport blowers had 

no noticeable effect on the boiler. 

As for the fuel flow into the OGS boiler, it must be understood that at our target co-firing rate at 

12.5 ton/h (11.3 tonne/h), the switchgrass represented only about 2.5% heat input to the boiler. 

The commercial system would be 25 ton/h (22.7 tonne/h) or 5% heat input. In December, our 

typical firing rate was 4-6 tons/h (3.6-5.4 tonne/h), or about 1% heat input from switchgrass. In 

the OGS boiler, soot blowing could cause more than a 2% change in fuel flow, so it was difficult 

to link any changes in boiler performance with the low co-firing rates in December. In January, 

on the other hand, it was possible to see changes of up to 3% in coal flow in response to changes 

at the higher switchgrass flows. However, changes in operation by the boiler control room staff 

could still hide these effects. As the boiler operators became comfortable with the switchgrass 

operations—meaning the operators could detect little, if any, effect on their boiler—the operators 

would allow the switchgrass team to ramp up and ramp down the switchgrass flow as desired. 

Toward the end of testing in January, the boiler was losing capacity due to fouling in the reheater 

and superheater sections of the boiler. While this did not appear to affect the heat transfer in the 

boiler, it did cause an increased pressure drop through the boiler tubes, increasing the loads on 

the induced draft fans. This was a normal occurrence just before shutdown, exacerbated by the 

fact that one bank of soot blowers on the non-co-fire side of the boiler had been out of service. 

Unfortunately, no data was available for comparing the fouling while co-firing with the normal 

fouling rate observed in the boiler. During the shutdown immediately after the co-fire testing, 

inspections indicated no unusual fouling, slagging, or deposits that could be linked to the 

switchgrass co-fire testing. 

One observable effect of the co-firing was the collection of unburned nodes in the bottom ash 

pond. The nodes from the joints in the switchgrass stems were not reduced in size by the 

Eliminator. Due to their size and density, they would quickly drop to the bottom of the boiler 

with little or no charring, even though they would pass through two lower rows of coal burners. 

The nodes were easily detected at the ash pond because they would float to the surface, then 

collect around the edges of the ash pond. While they don’t degrade the quality of the ash, which 

was generally used for fill, cover, or landscaping, the nodes did represent unburned fuel. 

30
7.0 RECOMMENDATIONS 

1). For the next round of co-fire testing, a firm with more experience with feed handling 

systems, possibly someone with agricultural processing experience in addition to power plant 

knowledge, would be advantageous. Not only should the firm have experience handling lowdensity materials that are prone to bridging, but the firm should also have operational experience. 

Practical design and flexibility in the equipment controls is important to the operation of the 

switchgrass equipment. 

2). All switchgrass-handling equipment should be covered and kept under a slight negative 

pressure. This prevents any dust from escaping into the room, minimizes labor for 

housekeeping, avoids high-volume dust collection equipment, and minimizes the potential fire 

hazard. 

3). Pulling a suction on grinding equipment or providing some other means of assisted transport 

though the grinder is important. Pulling a suction on the Eliminator more than tripled its 

throughput. One of the hammer mill manufacturers that visited the site agreed and said that 

pulling a suction on a hammer mill will normally double its capacity. 

4). Monitoring equipment should be installed on pneumatic transport lines to sense switchgrass 

flow. It would be helpful to the plant operating personnel to know exactly when switchgrass is 

going to the boiler. During our testing, the boiler room personnel had an indicator for when the 

equipment was running, but if we had a plug, we could go an hour or more without sending fuel 

to the boiler. Having some method of monitoring the switchgrass flow would also help with data 

analysis. Tech-wise has a simple optical method for obtaining qualitative results that they have 

used for testing in Denmark. 

5). A detailed test plan is needed for stack testing. Because this information is some of the only 

certified data submitted to the Iowa DNR, it must be conducted under carefully controlled 

conditions. Procedures should be in place to prepare for the testing and enough time should be 

allotted for conducting the testing, with soot blowing between tests if needed. 

6). Additional time needs to be included for equipment shakedown. Each piece of equipment 

should be tested for normal operation then checked to see if it is operating at its design capacity. 

Several of the early problems due to incorrectly installed equipment could have been identified 

and avoided with a more thorough pre-startup inspection. 

7). The design should ensure that all downstream equipment has the capacity to handle the peak 

flow of the equipment feeding it. This might require limiting the flow of the upstream 

equipment. Interlocks should also be included to shutoff upstream equipment when a piece of 

downstream equipment fails. 

8). The data collection requirements and analysis methods should be determined before the start 

of testing and a sample set of data should be collected and checked before the testing begins. 

This minimizes the amount of excess data collected, reducing the time for data analysis. It also 

assures that the data being collected is of a high quality and that nothing is missing. A daily 

31
check of the data is important, so if there are any process anomalies, someone can check with the 

operators and see if there was a known cause, before this information is forgotten. 

9). Arrangements should be made for on-site moisture determinations. Samples collected and 

sent to NREL by overnight mail gave moistures of about 11%, while moisture measurements at 

Hazen Research of material that had been stored for several months averaged about 6%. The onsite moisture probe was only able to read bale moistures down to 14.5%. This moisture data is 

critical for determining the heat input to the boiler. 

10). More robust record-keeping is required for documenting startup times, shutdown times, 

feed rates, and operating problems. This not only requires an efficient and simple record keeping 

system, but also requires diligence on the part of the test supervisors to check that all the required 

information is being recorded. The best way to accomplish this is to have one technician whose 

sole responsibility is to record operational data during testing. 

11). The support of management at the host facility is extremely important in coordinating and 

providing power plant personnel during testing. It is also important to set aside time to keep the 

operators informed about the project. All of the operators were very interested in the project and 

often had good ideas on how to improve the testing coordination and equipment operation. 

32
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

July 2002 

3. REPORT TYPE AND DATES COVERED 

Milestone Report 

4. TITLE AND SUBTITLE

Summary of Chariton Valley Switchgrass Co-Fire Testing at the Ottumwa Generating 

Station in Chillicothe, Iowa 

6. AUTHOR(S) 

Wade A. Amos 

5. FUNDING NUMBERS

BP01.1026 

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NREL/TP-510-32424 

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13. ABSTRACT (Maximum 200 words)

Results of the switchgrass co-firing tests conducted at the Ottumwa Generating Station in Chillicothe, Iowa as part of the 

Chariton Valley Biomass Project. 

15. NUMBER OF PAGES

14. SUBJECT TERMS 

biomass power, co-firing, switchgrass 16. PRICE CODE

17. SECURITY CLASSIFICATION 

OF REPORT

Unclassified 

18. SECURITY CLASSIFICATION 

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION 

OF ABSTRACT

Unclassified

20. LIMITATION OF ABSTRACT

UL 

 NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) 

Prescribed by ANSI Std. Z39-18 

298-102