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Use
of Tire Derived Fuel (TDF) in a fluidized bed hog fuel power boiler at
Pacifica Papers Inc. Alberni
Specialties mill
Authors: Larry Cross, Environmental Supervisor Bob Ericksen, Steam Plant Superintendent INTRODUCTION Pacifica Papers Inc. operates a
specialty paper mill in Port Alberni producing approximately 1200 tonnes/day
of lightweight coated, telephone directory and newsprint .
Furnish for the three paper
machines is supplied by a CTMP and stone ground wood mill and purchased FBK
and de-inked pulps. The mill was purchased by Pacifica Papers Inc. from
MacMillan Bloedel in June 1998. Steam for paper manufacture is produced by three power boilers. #2 and
#3 boilers burn natural gas only to produce approximately 20-40% of the steam
requirements. #4 power boiler produces the remaining 60-80% (seasonal) of the
steam by burning hog fuel. Hog fuel consumption is approximately 700-800
BDtonnes/day or 1800-2000 tonnes/day of wet hog fuel at 60% moisture content.
Hog fuel presses are used to reduce and stabilize the moisture content of the
fuel. The flue gases from all 3 boilers discharge to a electrostatic
precipitator to remove particulate. See Figure 1. Emissions from the
precipitator discharge are regulated by Air Permit PA-1863. #4
POWER BOILER #4 power boiler is a Combustion
Engineering boiler installed in 1977. The boiler was originally equipped with
a traveling grate and designed to produce up to 38 Kg/s (300,000 lb/hr) of 4140
Kpa (600 psi) steam from burning hog fuel or a maximum of 57 Kg/s (450,000
lb/hr) on Bunker C oil. The boiler never consistently achieved the design hog
fuel steam generation rate. Actual hog fuel generation was typically 25 Kg/s
(200,000 lb/hr). The remaining hog fuel was burnt in the older and smaller #2
& #3 power boilers equipped with fixed grates requiring manual cleaning.
The #4 traveling grate proved to be mechanically unreliable. Failure of the
grate system was occurring annually which resulted in high repair costs,
burning of costly fossil fuels during repairs and accumulation of hog fuel
with the associated leachate generation issues. In addition , the mill was
receiving increasing quantities of low quality hog fuel produced from dry land
log sort operations. A reliable system for the combustion of this hog fuel was
required to satisfy environmental disposal requirements. The existing grate
systems were not suited to the higher sand and rock content of this fuel. In 1996, funding was approved to upgrade #4 power boiler to allow all of
the hog fuel to be burnt in a single boiler. Conversion of the boiler to a
bubbling fluidized bed configuration was chosen. The conversion was designed
to consume all of the hog fuel and wood waste available from the Alberni
Valley forest industry operations (sawmills, logging divisions & paper
mill) as well as the sludge from the mills’ effluent treatment plant. Design: ·
43 Kg/s (343,000 lb/hr) steam generation from hog fuel ·
consume 228,000 BDt/year hog fuel @ 60% moisture content ·
no supplemental fossil fuel ·
coarse ash removal system to handle lower quality “dry land sort”
fuels containing sand and rocks As part of the project, the
existing wet flyash handling system would be replaced with a dry system. #2 & #3 power boilers would operate on fossil fuel only to meet the
total mill steam requirements. This would avoid ongoing repairs to the hog
burning systems on these boilers. The total quantity of hog fuel burnt would not increase, but it would
now be burnt in one boiler instead of three, with the associated operating and
maintenance efficiencies. The project was justified based on the significant cost savings from
avoided grate breakdowns and reduced fossil fuel requirements as well as
avoided environmental impacts from stockpiling hog fuel during breakdowns. No
changes to the existing precipitator/emission control system were required. More complete combustion and a reduction in the unburnt carbon content
of the ash was anticipated with the fluidized bed upgrade. Kvaerner Pulping Inc. was awarded the contract for the
fluidized bed conversion project. The installation was completed during
a 7 week shutdown in July 1997. Major components of the conversion were: ·
removal of lower 25 feet of the boiler and replacement ·
reconfiguration of hog feed system ·
start up burner system ·
addition of three level combustion air system ·
coarse ash removal system ·
dry flyash conveying and storage system #4 boiler started up August 11, 1997
on natural gas and August 14, 1997 on hog fuel. Design parameters were met,
with the boiler operating at the Maximum Continuous Rating (MCR) by August 21,
2000. Three months after the startup of the fluidized bed conversion,
significant changes occurred in the mills hog fuel supply. Due to the sudden
collapse of the Japanese lumber market, the sawmill providing the largest
volume of hog fuel to the mill was shutdown for four months. . This eliminated
the highest quality hog fuel source (low moisture, high energy content, low
sand & rock content). At the same time, substantially higher volumes of
hog fuel produced from dryland log sort debris were received. This fuel had
high sand and ash content (10-25 %) and lower heating value. This significant
change in fuel characteristics immediately and negatively impacted the hog
fuel burning rate. It was decided to pursue addition of Tire Derived Fuel (TDF)
to improve the overall energy value of the boiler fuel. TDF
Tire Derived Fuel (TDF) is produced by shredding waste automobile and
truck tires. TDF is utilized in power generation plants and pulp & paper
mills throughout the United States. Approximately 20 pulp & paper mills in
the United States utilize TDF as a supplemental fuel. These mills consume
approximately 35 million tires annually [1]. Tires are shredded to produce “chips” approximately 2” or smaller.
The heavy bead or rim wire is removed prior to shredding. Additional radial
wire is removed magnetically. Alberni Specialties mill personnel
had visited other pulp & paper mills prior to the boiler rebuild to see
similar conversions to existing hog fuel boilers. At one of the mills visited,
(Sheldon, Texas), TDF was being used as a supplemental fuel. The primary
benefit was to assist with the combustion of high moisture de-inking sludge.
Addition rates were typically 10 %. Based on this visit, provision for TDF
burning was included in the design of the #4 boiler upgrade with the addition
of a tertiary air supply. It was anticipated that TDF addition might be
beneficial in the future based on a trend to lower hog fuel quality. The TDF utilized for this trial was
produced by Target Recycling Inc. Target operates a tire recycling plant in
Port Alberni. The plant originally produced crumb rubber for manufacturing
rubber paving bricks but the operation was not economically viable. The
equipment at the plant was reconfigured to produce TDF for the trial. The
Ministry of Environment Tire Diversion Program was supportive of TDF as a
potential tire disposal option. Tires were already being used as a fuel at two
cement kilns in British Columbia. TDF composition is shown in Table I.
TABLE I Tire
Derived Fuel Analysis
Energy
value
33 GJ/tonne (14,300 BTU/lb) %
sulfur
1.6 % %
zinc
1 % %
wire
5 - 7 % %
moisture
~ 10% (average
of 5 samples collected during trials) These values are similar to those found in the literature [2]. The
energy value of TDF is very high compared to that of typical hog fuel at 8 GJ/t
(3400 BTU/lb) of wet hog or 20 GJ/t (8500 BTU/lb) on a dry basis. PERMITTING
#4 power boiler is regulated under
Ministry of Environment Lands and Parks (MOELP) air permit PA-1863. A request
for an approval to burn tire derived fuel was submitted to the MOELP in
December 1997. An addition rate of 2% by weight of BD hog fuel or up to 20 BDt/day
of TDF was requested based on the available capacity of the Target Recycling
facility. #4 power boiler typically burns 700 – 800 BD tonnes/day of hog
fuel. The expected benefits of adding TDF were: ·
improved ability of #4 power boiler to consume poor quality hog fuel
such as that produced from dryland sort wood debris by increasing the overall
fuel energy value ·
increased hog fuel burning & reduce natural gas or fuel oil use ·
reduced inventory of hog fuel stored on the mill site and reduced
leachate generation ·
an environmentally sound & economic solution to waste tire disposal ·
a stable market for TDF for the local Target Recycling plant Potential
Environmental Issues A technical literature review was
conducted on burning low amounts (< 10%) of TDF in pulp & paper mill
hog fuel boilers. The large increase in energy prices in the early 1980’s
resulted in a number of mills trialing the use of TDF as a lower cost
supplemental fuel source. The literature indicated the following potential environmental issues: Particulate emissions: An
efficient particulate control device (eg precipitator) is required to prevent
increased particulate emissions when burning TDF. A proper feed system to provide a consistent and well controlled
TDF feed rate is recommended. Proper
combustion air control on the boiler is required to ensure efficient
combustion of the TDF. Zinc emissions: Rubber tires contain approximately 1% zinc as part
of the tire formulation. The zinc is oxidized to a very fine zinc oxide
particulate in the boiler. Zinc oxide is readily captured by an electrostatic
precipitator due to its electrical resistivity characteristics. As a result,
the zinc content of the flyash will increase. The zinc will tend to remain
insoluble due to the high pH of flyash. The literature indicates that little
or no increase in zinc in the bottom ash from the boiler would be expected
[2]. Sulphur: The sulphur content of TDF will be converted to sulfur dioxide in the
boiler. The significance would depend on the amount of TDF added, sulphur
content of other fuels and specific mill emission permit limits. Other emissions: There was limited information available on other
trace toxic organic emissions from low TDF addition rates to pulp & paper
boilers (most studies related to dedicated tire incinerators or lab scale
studies). A study conducted by
the Washington state Department of Ecology indicated little or no change in
polynuclear aromatic hydrocarbons (PAH) emissions from two wood fired power
boilers burning 2-7% TDF [3]. Local
Environmental Issues:
Air quality had previously been a high profile issue in Port Alberni due to
the limited air dispersion conditions in the winter months (inversions) and
the location of the mill within the community. The installation of the power
boiler precipitator in 1989 and the shutdown of the kraft mill in 1993 had
improved emissions
to the level that the mill was no longer a significant factor in local
air quality. Pacifica Papers, the
MOELP and the community agreed that
TDF addition must not negatively impact emissions
or air quality. Approval
Approval AA-15346 was received
March 24, 1998 to allow addition of a maximum of 20 tonnes/day of TDF (2% by
weight) until March 31, 1999. Following the initial trials, a request
was submitted September 1998 to
increase the allowable TDF addition rate to 40 tonnes/day maximum (5% by
weight) and the approval was amended January 14, 1999. The approval specified the following testing requirements: ·
stack emissions - total particulate, metals, SO2 ·
flyash & bottom ash - metals ·
bottom or coarse ash - Special
Waste Leachate Extraction Procedure (SWEP) An exemption under Section 2 of the Sulphur Content of Fuel Regulation
was also received to allow use of a fuel with a sulphur content in excess of
1.1%. TDF TRIALS
Following receipt of the Approval, several TDF addition trials were
conducted as shown in Table II. TABLE II
Trial Chronology
A total of 1300 tonnes of TDF was consumed during the approval period.
This is equivalent to approximately 150,000 passenger tires. The March and June trials were conducted by feeding the TDF manually (labourer
adding one 12 litre bucket every 30 seconds to the main hog fuel conveyor).
Based on the positive results from these trials, a request was submitted to
mill management to install a feed hopper and variable speed screw feeder for
the TDF. This equipment was installed in early December 1998. The hopper holds
approximately 8 tonnes of TDF. Bridging is minimized using a spike roll
located above the screw, which is operated by a pneumatic cylinder. The screw
feeder was calibrated by collecting and weighing TDF discharged from the
screw. The screw feeder discharges onto the hog fuel conveyor after the hog
presses going to the boiler (see Figure 1). The feed system is interlocked to
the hog fuel conveyor system. RESULTS Boiler
operation: The TDF was confirmed to be an excellent supplemental fuel. Operational benefits observed included: ·
stabilization of boiler operation when burning lower quality hog fuel;
this was especially apparent on weekend operation when the higher quality hog
fuel from the local sawmills is not delivered ·
increased fluidized bed temperature of approximately 55 C (100 F ) ·
approximately 5% increase
in hog fuel burn rate (the significant hour to hour and day to day variations
in hog fuel quality made it difficult to more accurately quantify the
improvement in hog fuel consumption) The maximum benefit of the TDF was during wet, winter operation and
when receiving hog fuel produced from lower quality woodwaste
sources (logging division dryland sorts). During periods when the highest
quality hog fuel (dry, high BTU content) was fed to the boiler, the TDF
addition increased the fluidized bed temperature to the maximum operating
range, limiting the allowable TDF addition rate. The sand in the fluidized bed
will start to clinker or fuse at temperatures of 1090 C (2000 F). The
operating limit is set at 870 C
(1600 F) to provide a safety margin. During the March 1999 test period when
testing requirements necessitated maintaining a 5% addition rate continuously
regardless of hog fuel quality, one or more of the bark presses was bypassed at
times to keep the bed temperature
in range. No negative impacts on boiler operation were experienced. The TDF
blended with the hog fuel with no
problem at the 5% rate. No problems with removal of the bottom ash were
experienced. The residual wire in the TDF was either oxidized or reduced to
very small pieces in the fluidized bed. Total
Particulate and SO2 Emissions Testing was conducted according to the BC Environment Source Testing
Code. Results are shown in Table III. TABLE III
Stack Emission Test Data
There was no impact of TDF addition
on the total particulate emissions. All tests conducted during the trials met
the permit limit of 230 mg/m3
@12% CO2. One of the test runs at the 5% TDF additon rate yielded results significantly
higher than the other 4 tests (177 mg/m3 vs 37-68 mg/m3
respectively). It is believed the higher emissions were due to a problem with
the boiler oxygen meter resulting in insufficient air addition for proper hog
fuel combustion. The data indicates no significant effect on SO2 emissions
from TDF addition. This is reasonable considering the low TDF feedrate
relative to the other fuels. There is some sulfur input to the boiler without
TDF from the effluent treatment plant sludge and the hog fuel. The most
significant potential input of sulfur to the boiler is when fuel oil is burnt
in #2 or #3 boiler due to gas curtailments. No oil was being burnt during any
of the SO2 measurements. Metals
in stack emissions The stack emissions were analyzed
for metals as specified in the approval. Results are shown in Table IV. The data is presented as % by weight of the total particulate emissions.
The majority of the total
particulate is sand (silica) and wood ash. Other metals such as arsenic, cadmium, lead were at or below the
detection level. TABLE
IV Metal content of
particulate emissions
The data indicates no increase in
any of the metals being emitted from the stack compared with the baseline
measurements. Metals
in ash residuals #4 power boiler produces two ash residuals, flyash and bottom ash. Flyash: Lightweight wood ash, salt and fine sand which is emitted from the top
of the boiler. These materials are captured in the boiler ash hoppers,
multiclones and electrostatic precipitator. This ash is collected dry in a
silo and a small amount of water is added just
prior to discharging to a transport trailer to reduce dusting.
Approximately 30-40 tonnes/day of this ash is discharged to the mills’
permitted Block 105 landfill site (PR1751). Bottom ash: Coarse material such as gravel and rocks from the
incoming hog fuel and sand from the fluidized bed is removed by screw
conveyors under the boiler. The boiler bed holds approximately 70 tonnes of
sand and approximately 10-20 tonnes/day are removed and replaced. The bottom
ash is stockpiled and rescreened. The fine sand portion ( < 1 mm) is reused
for sand bed makeup. The over size material (rocks & gravel) goes to the
landfill. The approval specified that the bottom ash sample was to be
collected following at least 15 days of continuous TDF addition. This was to
allow a reasonable equilibrium to be reached in the concentration of metals in
the bed. Metals analysis results for the flyash and unscreened bottom ash are
shown in Both ash residuals were also tested according to the Special Waste
Leachate Extraction Procedure (BC Special Waste Regulations). The leachate
extraction test is used to determine the potential of the ash to release
metals in an acidic environment. This is a “worst case” condition as the
flyash is actually alkaline. TABLE
V Flyash metals analysis (ppm by weight)
TABLE
VI Bottom Ash (unscreened) metals
analysis (ppm by weight)
As expected, the zinc content of
the flyash increased with TDF burning. This is consistent with the technical
literature [1,2]. Zinc in the leachate extraction test increased but remained
well within the Special Waste Regulation limits. Zinc levels in the bottom ash
also increased. Based on limited testing of the re-screened bottom ash, the
fine ( < 1mm) fraction (which is recycled to the boiler) has a higher zinc
concentration than the oversize material. Other metals not listed in this summary, (Arsenic, Mercury) were all
below detection limits for both the “no TDF” and “TDF” samples. Chlorinated Dioxin & Furans in flyash
Combustion of hog fuel containing salt (sodium chloride) from the salt water transportation and storage of logs, is known to produce trace levels of dioxins and furans [4]. Samples of flyash were collected during the March 1999 5 |