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Technical Services, Minnesota Ore Operations Memorandum

Date: April 13, 2012

Subject: Line 6 Biomass Performance Testing on Repaired Kiln Low NOx Burner

During the major repair in March 2012, the Line 6 Low NOx Burner was removed to repair 

the deformed biomass outer pipe on the burner. When the burner was removed, we also 

found two holes in the deformed biomass outer pipe. The photo below shows the deformed 

biomass outer pipe and one of the holes in this pipe. The two holes coincided with where the 

pipe supports for this outer pipe were located. After some discussions with Hatch and Coen, 

it was determined that the pipe supports in the coal annulus were shaped in such a way that 

the coal was being directed at the biomass outer pipe causing excessive wear from coal 

abrasion. To reduce the wear in this area, the new supports installed were modified with a 

beveled leading edge to direct the coal around the supports and not directly at the biomass 

outer pipe. 

Figure 1: Picture of Line 6 Burner Prior to Repair

Deformed 

Biomass 

Outer 

Pipe

Hole in 

biomass 

outer pipe 

from coal 

abrasion at 

supports

Biomass 

Annulus

Atomizing 

Air 

l
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Coen’s analysis of the deformed biomass pipe concluded that it was inadequately supported 

within the burner causing the deformation seen above resulting in reduced biomass 

throughput and plugging issues in the biomass pipe/annulus. The deformed portion of the 

biomass outer pipe (~4 ft) was cut off and replaced with a new pipe and three additional 

supports were added in both the coal and biomass annuluses to properly support the pipe and 

prevent deformation. The picture below shows the repaired burner with the additional three 

supports in both the coal annulus and the biomass annulus. The original design only had 3 

supports 120° apart in each annulus. 

Figure 2: Picture of Line 6 Burner After the Repair

Prior to the repair, at biomass feed rates above 150 lbs/min, we experienced plugging issues 

in the biomass system which we could not confirm were due to the deformed biomass pipe or 

other burner design or biomass feed system issues. On April 11, 2012, biomass testing was 

performed on the repaired Line 6 Kiln Low NOx Burner to determine if we could achieve the 

designed biomass feed rate of 300 lbs/min. Two tests were performed to determine the 

maximum biomass feed rate that could be achieved before plugging issues occurred. Line 6 

was running steady at full production throughout the test period. The fuels burned during the 

testing were a combination of natural gas and biomass. The biomass burned during the test 

period was a combination of oat hulls and wood. For each test, the biomass blower was 

started first and allowed to run for a period of time (~5 minutes) with no biomass feed to 

ensure the line was clear and to get a baseline pipe pressure. The pipe pressure is measured 

Atomizing 

Air 

Biomass 

Annulus

Coal 

Annulus

Combustion 

Air 

Annulus

Six additional 

supports added 

in coal and 

biomass annulus

1 2 3

4

5

6
3

at the feed end of the pipeline and is used to indicate plugging problems in the biomass 

pipeline/burner annulus. The blower and wood auger are interlocked with this pipe pressure 

and will kick out if the pipe pressure exceeds 6.5 psi to prevent severe plugging of the 

biomass pipeline. The following is a list of the burner operating parameters that were used 

and remained constant during the entire test period:

• The burner was in the gas firing position

• Coal was shut off and the primary air fan was running to cool the coal annulus

• Atomizing air was set at 3,000 lbs/hr

• The kiln exit temperature was controlled with natural gas

Figure 3 below is an overview of the entire 2 hr test period showing the relationship between 

the biomass feed rate, VFD wood auger speed and biomass system pipe pressure. As you can 

see from the graph below, for each test we started with 50 lbs/min of biomass and increased 

the feed rate in 50 lb/min increments up to 150 lbs/min. The incremental increase in biomass 

was reduced to 25 lbs/min above 150 lbs/min. At each biomass feed rate, the biomass system 

was allowed to stabilize before further increases in feed rates were made.

Figure 3: Line 6 Biomass System Process Data Trend Chart

0

1

2

3

4

5

6

7

8

0

20

40

60

80

100

120

140

160

180

200

Wood System Pipe Pressure (PSI)

Wood Flow (LBS/MIN) - Wood Auger VFD Speed (%)

Line 6 Biomass Testing 4-11-12

LINE 6 WOOD AUGER VFD SPEED LINE 6 WOOD FLOW (LB/MIN) Line 6 WOOD SYS PIPE PRESS (PSI)

During Test # 1, we ran for an extended time period (34 minutes) at 150 lbs/min due to a 10 

minute period in the middle of the run where the pipe pressure was erratic and was varying 

Start of 

test #1

End of 

test #1 

Start of 

test #2

End of 

test #2 
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between 3.7 and 5.6 psi. During this time period the auger speed was also steadily increasing to 

maintain 150 lbs/min of biomass. Talking with Hillwood Products, they stated that the increase 

auger speed was due to a change in the biomass feed composition from mostly oat hulls to an oat 

hull and wood mixture which changed the density of the biomass feed. After this upset in the 

system, the pipe pressures dropped back down around 3.6 psi and stabilized. The feed rate was 

then increased to 175 lbs/min where it ran relatively stable at 3.9 psi for just over 15 minutes 

before the pipe pressure spiked up above 6.5 psi and the system kicked out. After restarting the 

blower twice, we were able to clear the plug in the biomass system. Table 1 and Figure 4 

summarize the key process data during test #1.

Table 1: Biomass Test # 1 Summary Data

Biomass Feed 

Rate

Wood System 

Pipe Pressure

Wood Auger 

VFD Speed

Air to Fuel 

Ratio 

Setpoint Core Gas 

Annulus 

Gas

Operating Time at 

Biomass Feed Rate

lbs/min PSI % % % min:sec

0 2.2 0.0 15 25 75 6:10

50 2.6 7.9 10.5 25 75 11:05

100 3.0 15.5 9 25 75 11:25

150 3.9 32.1 7.5 38 62 34:30

175 3.9 44.6 7.5 38 62 15:05

Figure 4: Biomass Test #1 Process Data Trend Chart

0

1

2

3

4

5

6

7

8

0

20

40

60

80

100

120

140

160

180

200

Wood System Pipe Pressure (PSI)

Wood Flow (LBS/MIN) - Wood Auger VFD Speed (%)

Line 6 Biomass Testing 4-11-12 - Test #1

LINE 6 WOOD FLOW (LB/MIN) LINE 6 WOOD AUGER VFD SPEED Line 6 WOOD SYS PIPE PRESS (PSI)

Erratic pipe 

pressures
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A second test was performed to confirm the test results from the first test using the same test 

procedures as test #1. During the second test, the pipe pressures at 100 lbs/min biomass feed rate 

were 0.4 psi higher than test #1 (3.4 psi compared to 3.0 psi). When the biomass feed rate was 

increased to 150 lbs/min, the pipe pressure steadily increased to around 5 psi. Less than 3 

minutes after the increase to 150 lbs/min, the pressure spiked over 6.5 psi again causing the 

blower and wood auger to kick out. The blower was restarted and the pipe plug was cleared and 

no further biomass testing was performed. Table 2 and Figure 5 summarize the key process data 

during test #2. 

Table 2: Biomass Test # 2 Summary Data

Biomass Feed 

Rate

Wood System 

Pipe Pressure

Wood Auger 

VFD Speed

Air to Fuel 

Ratio 

Setpoint Core Gas 

Annulus 

Gas

Operating Time at 

Biomass Feed Rate

lbs/min PSI % % % min:sec

0 2.2 0.0 15 25 75 4:35

50 2.6 10.8 10.5 25 75 8:15

100 3.4 25.9 9 25 75 11:00

150 4.5 34.5 7.5 38 62 2:50

Figure 5: Biomass Test #2 Process Data Trend Chart

0

1

2

3

4

5

6

7

0

20

40

60

80

100

120

140

160

Wood System Pipe Pressure (PSI)

Wood Flow (LBS/MIN) - Wood Auger VFD Speed (%)

Line 6 Biomass Testing 4-11-12 - Test #2

LINE 6 WOOD FLOW (LB/MIN) LINE 6 WOOD AUGER VFD SPEED Line 6 WOOD SYS PIPE PRESS (PSI)
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Conclusions

Comparing the test results from April 11th, 2012 with the repaired biomass annulus to the tests 

performed in 2011 with the deformed biomass annulus, there was no improvement in biomass 

throughput with the repaired burner. Plugging issues in the biomass pipe/burner annulus 

continue to occur at feed rates of 150 -175 lbs/min. This leads me to conclude that the deformed 

biomass annulus was not the root cause of the plugging issues before the burner repair and that 

further analysis of the existing burner design and the biomass feed system is needed to determine 

the actual cause of these plugging issues.

Future Work

No further biomass testing is scheduled until we can inspect the burner and biomass pipeline. 

During the next minor repair on line 6, we need to inspect the biomass annulus for deformation 

to ensure the modification made to the biomass annulus were effective. We also need to open up 

the inspection hatch located where the biomass pipeline connects to the burner to determine if 

any large biomass or tramp material built up in this area during the previous biomass testing. 

This is where the 6” biomass pipeline meets the 3⁄4” biomass annulus of the burner, so if we are 

receiving larger biomass or tramp material from our supplier, this is the area where it would 

buildup and restrict the flow of biomass to the kiln.

Once we have confirmed there is no tramp material in the biomass pipeline/annulus and the 

annulus is not deformed, further biomass testing based on analysis and recommendations from 

Coen and Hatch will be performed to determine the root cause of the plugging issues. Additional 

pressure monitoring at various points in the biomass pipeline may be required to pinpoint where 

the plugging is occurring and to ensure any future design modifications are effective. We also 

plan on testing the burner again with a fine ground oat hull biomass to determine how the type of 

biomass affects biomass feed rates. From past experience, fine ground oat hulls are the best 

biomass material to convey through a pipeline and should therefore indicate the maximum 

capacity of the biomass system/burner under ideal conditions.