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

 

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

Date	TDF addition rate	Comments/details
March 23-25, 1998	1-2% Manual feed system 8 hours / day	Positive impact on boiler operation, no problems caused by TDF in hog or ash conveying systems
June 8-10, 1998	2% 24 hours/day Manual feed system
December 7, 1998 – February 2, 1999	2% Screw feed system	TDF not added at all times depending on TDF supply and hog fuel quality
February 9-10, 1999 March 14-28, 1999	5%	No TDF added Feb. 11 – March 13 to accumulate sufficient TDF to allow continuous burning at 5% rate. Approval expired March 31, 1999

 

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 )
( ie 815 C to 870 C or 1500 F to 1600 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

	No TDF addition	2% addition rate	5% addition rate
# of tests	12	4	5
Total particulate emission (mg/m3 @ 12% CO2 incl. salt )                   Average                  [range]	77   [27 – 170]	65   [44 – 97]	76   [37 – 177]
Hog fuel + TDF steam generation rate  (Kg/s) Average   [range]	38   [33 - 44]	40   [36 - 45]	42   [40 - 43]
SO2 (ppm v)	28 (2 tests)	18 (2 tests)	31 (4 tests)

 

 

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

 

	No TDF addition	2% addition rate	5% addition rate
# of tests	5	1	5
% Zinc	0.32	0.22	0.24
% Iron	8.8	2.1	5.7
% Chromium	0.8	0.1	0.9
% Nickel	1.4	0.03	1.1

 

 

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
Tables V and VI.

 

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)

 

	No TDF addition	2% addition rate	5% addition rate
# of tests	3	4	4
Cadmium	1.0	0.7	0.9
Chromium	53	53	48
Nickel	27	29	25
Iron	24300	37300	40300
Lead	55	40	57
Zinc	413	2640	6360
Leachate extraction test Zinc (mg/l) SW regulation limit = 500	4.6	20.8	104

 

 

TABLE VI  Bottom Ash (unscreened) metals analysis (ppm by weight)

 

	No TDF addition	2% addition rate	5% addition rate
# of tests	3	3	1 (collected after 15 days continous TDF addition)
Cadmium	<	<	<
Chromium	28	29	18
Nickel	20	21	15
Iron	17400	27600	25300
Lead	<	<	<
Zinc	211	1280	2940
Leachate extraction test Zinc (mg/l) SW regulation limit = 500	0.38	8.1	12.9

 

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.
Iron content in both the flyash and bottom ash also increased somewhat, with the source being the residual radial wire in the TDF.

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% TDF addition period  for chlorinated dioxin & furan analysis. Results are shown in Table VII along with data from samples collected in November 1997 with no TDF addition.

 

	No TDF addition November 1997	5% TDF addition March 1999
# of tests	5	4
Average 2,3,7,8 TCDD TEQ (parts per trillion)	823	768
Range (ppt TEQ)	539 – 978	400 - 910

 

The data shows that there was no impact on the trace levels of dioxin & furan in the flyash from TDF addition to the boiler.  This is what would be expected with the efficient combustion conditions in the fluidized bed boiler. Dioxin & furan levels in the collected flyash are considered to be a reasonable  surrogate for overall dioxin formation in the boiler [4]. Stack dioxin and furan emissions were not measured during the TDF trials.

 

Air permit amendment

 

A report detailing the emission testing and ash residual analysis results was submitted to the MOELP following the completion of the trials and the expiration of the twelve month approval.  An application was submitted to amend air permit PA1863 to authorize the use of up to 5% TDF  on an ongoing basis.

 

As part of the permitting process, the permit amendment application was referred by MOELP to local government agencies for comment. Both the mill and MOELP attended meetings to answer questions regarding the potential environmental aspects of TDF combustion.  Government representatives and members of a local environmental group reviewed the trial report and visited  the mill to see the operation of the power boiler and  the TDF addition process.

Concerns raised by the local government agencies and environmental group focussed on two areas:

·        possible future deterioration in particulate emissions (despite the testing results from the trials which showed no impact)

·        potential for trace toxic emissions (PAH and dioxin & furans)

 

These concerns were considered by the MOELP in the final permit amendment.

 

The amended air permit was received in September 1999. Additional permit conditions included:

·        Total Particulate discharge limit requirement of  115 mg/m³ based on a four quarter rolling average in addition to the existing 230 mg/m³ maximum for each quarterly test

·        Monitoring of zinc and iron content of total particulate discharge, flyash and bottom ash (quarterly)

·        Test program for Polycyclic Aromatic Hydrocarbons (PAH) to measure emissions with and without TDF addition

 

Current operation

 

TDF has been routinely added to #4 power boiler since October 1999. The addition rate is dependent on the hog fuel quality and boiler operation. Addition rate for December – March was approximately 200 tonnes per month (or an average of 7 tonnes/day). TDF is normally added during the weekends or other periods when the high energy value sawmill hog fuel is not delivered.

The only negative aspect of TDF has been the potential for contamination of the chip supply to the CTMP operation. This is being addressed by keeping a minimal stockpile of TDF on-site and housekeeping.

 

Conclusions

 

1.      Addition of up to 5% tire derived fuel stabilized the  operation of the fluidized bed boiler during periods of low quality hog fuel. Hog fuel combustion rate increased and fossil fuel requirements decreased.

2.      TDF addition rate is limited by the maximum fluidized bed temperature of  870 C (1600 F). During periods of high heating value hog fuel, TDF addition must be reduced below the 5% rate or discontinued.

3.      Total particulate and SO₂ stack emissions are not affected by 5% TDF addition.

4.      Zinc content of the flyash captured in the electrostatic precipitator increased significantly (410 ppm to 6300 ppm) with TDF addition but continued to meet regulated limits.

5.      The boiler or ash removal system are not adversely affected by the residual steel wire content of the TDF. The wire is either oxidized or reduced to very small pieces in the fluidized bed.

 

 

REFERENCES

1.       COX, J., “Alternative Fuels: US Paper mills are burning rubber”, PIMA’s Papermaker, August 1997

2.       HOPE, M., “Specification Guidelines for Tire Derived Fuel”, TAPPI 1993 Engineering Conference

3.        DRABEK, J. and WILLENBERG, J., “Measurement of PAH and metals from burning tire chips for supplementary fuel”, TAPPI 1987 Environmental Conference

4.       LUTHE, C., KARIDIO, I., ULOTH, V., WEARING, J., STRANG, A. and  PRESCOTT, B., “Sulphur Addition to Control Dioxins Formation in Salt-Laden Power Boilers”, TAPPI 1998 International Environmental Conference