Fuel 89 (2010) 2569–2582

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Fuel

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Efficient and cost effective reburning using common wastes as fuel and additives

Yaxin Su 1, Benson B. Gathitu 2, Wei-Yin Chen *

Department of Chemical Engineering, University of Mississippi, 134 Anderson Hall, University, MS 38677, United States

a r t i c l e i n f o

Article history:Received 2 July 2009Received in revised form 2 December 2009Accepted 9 December 2009Available online 22 December 2009

Keywords:NOReburningNatural gas substitutes

0016-2361/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.fuel.2009.12.009

* Corresponding author. Tel.: +1 662 915 5651; faxE-mail address: cmchengs@olemiss.edu (W.-Y. Che

1 Address: Department of HVAC and Energy EngiShanghai, PR China.

2 Present address: Department of Mechanical Enginesity of Agriculture and Technology, P.O. Box 62000-002

a b s t r a c t

Potential substitutes of natural gas and lignite fly ash as NO and HCN reducing agents, respectively, forheterogeneous reburning were examined in a bench-scale apparatus equipped with a simulated reburn-ing and a burnout furnace. Selection of NO reducing agent is based on fuel volatility and nitrogen func-tionality. HCN reducing agent selection is based on literature data. A wide range of waste materials andindustrial by-products show overall NO reduction efficiency up to 88% at reburning stoichiometric ratio0.90 or 0.95. Mixed fuel containing scrap tire and Fe2O3 is particularly effective. Though its cost is con-strained by the energy-intensive operation of grinding the tire, the estimated raw-material cost is betterthan that of natural gas reburning and highly competitive against SCR. A first-level approximation studyof the selectivities of nitrogen species to form NO in burnout zone reveals the importance of HCN andchar nitrogen reaction mechanisms.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Reburning is a three-stage, in-furnace combustion technologydesigned for the reduction of NO by introducing a small amountof reburning fuel above the primary flame where the majority ofNO is chemically reduced to nitrogen in this fuel rich environment[1]. Reburning is attractive because it can retrofit old boilers at arelatively low operation cost than other post combustion technol-ogies, such as selective catalytic reduction (SCR).

Pilot- and full-scale research over the last three decades, how-ever, has demonstrated a floor, about 60% of NO produced in theprimary flames, in either gas or coal reburning, below which NOcannot be reduced any further [2]. This reduction floor implies thata single reburning device is not sufficient to meet the stringentemission regulations. During natural gas reburning, hydrocarbon-free radicals, including �C, �CH and �CH2, effectively convert NO toHCN, a major reaction product in the reburning zone [3,4]. How-ever, a significant portion of this HCN oxidizes to form NO in theburnout zone that limits the overall NO reduction efficiency.

A multi-functional, mixed fuel containing natural gas for NOreduction, and lignite ash for reducing the reburning intermediate,HCN, has demonstrated remarkably high efficiency in reburning[5]. More recent work has demonstrated that baghouse lignite flyash converts HCN to N2 during methane reburning, and thus breaks

ll rights reserved.

: +1 662 915 7023.n).

neering, Donghua University,

ering, Jomo Kenyatta Univer-00, Nairobi, Kenya.

the 60% NO reduction floor [6]. It appears that heterogeneousreburning by a mixed fuel containing a NO reducing agent, suchas natural gas, and a HCN reducing agent, such as lignite fly ash,is an attractive approach for advanced reburning. However, theprice of natural gas has increased and fluctuated significantly inthe last five years. And the amounts of fly ash required to achieve85% NO reduction in a typical boiler, 720 metric tons per day for a172 MW bituminous coal-fired boiler, are impractically high.Therefore, there is a need for finding their substitutes.

The natural gas and lignite ash substitutes have to be not onlycost-competitive but also widely available and in quantities suffi-cient for use in utility boilers. Waste materials or industrial by-products that consume resources for their proper disposal wouldbe ideal candidates. In addition, the nitrogen functionality in thefuel is of great importance, i.e., they should be highly volatile inreburning so that nitrogen fusion into the carbon ring is limited.Once char nitrogen is incorporated into the ring structure of thecarbon, char nitrogen converts to both N2 and NO in fuel lean con-ditions [7–10]. The selectivity of char nitrogen conversion to N2

over that to NO decreases with decreasing gas phase NO concentra-tion due to NO’s ability to attack and extract surface-bound nitro-gen, C(N), leading to the formation of N2. Thus, the nitrogencontent of the solid fuel entering the burnout zone, where NO con-centration is already low, is likely to be an important factor indetermining the ultimate NO yield from a reburning process. Solidchar entering burnout zone and its nitrogen content have to be aslow as possible.

In this study, five waste materials: tire, pine bark, corn-stoverresidue, paper mill sludge and pine wood, have been selected asnatural gas substitutes based on the above criterions. Tire wasthe first fuel chosen as a substitute for natural gas in this study

2570 Y. Su et al. / Fuel 89 (2010) 2569–2582

for several reasons. According to the EPA, in 2003, there were 270million scrap tires stockpiled in the United States [11]. In this sameyear, of the 290 million scrap tires generated in the US, 130 millionwere used as fuel and 100 million were recycled into new products[12]. This implies that there is a consumption deficit and tire is stillan available fuel for reburning. Previously, Miller et al. have testedtire as a reburning fuel in a pilot-scale facility and achieved up to63% NO reduction at a relatively low temperature (840 �C) andwithout complete burnout [13]. Nimmo et al. recently reportedthat NO reburning using scrap tire powders of <250 lm achieves80% NO reduction in their 80 kW pilot-scale boiler but, like Milleret al., they state not to have achieved proper burnout [14]. Singhet al. confirmed the results earlier achieved by Nimmo et al. andfurther demonstrated the viability of co-firing tire and coal to re-duce NO emission [15]. Adams and Harding investigated reburningusing biomass by computational fluid dynamics, and found theoverall NO reduction varies from 45% to 55% [16]. They then exper-imentally investigated reburning with hardwood and softwood;NO reductions up to 70% were observed [17]. The nitrogen specia-tion from tire and biomass reburning zone, however, has not beenwell examined in these studies, either experimentally or computa-tionally. Indeed, our previous studies suggest that knowledge con-cerning nitrogen speciation in reburning zone is important, as thecontrol of these species, such as HCN and char nitrogen, is a gov-erning factor for the overall reburning performance [4,18].

Literature was critically reviewed to find an adequate substitutefor lignite ash. A good candidate must be capable of converting HCNto amine radicals or N2 at a reasonably low feed rate and not causeproblems, such as slagging and fouling, in a furnace. Fe2O3 and seanodule are chosen in the current study. Prior reburning studies havedemonstrated the effects of Fe2O3 on overall NO reduction [19–21]though the exact mechanisms of its effects, especially that on HCN,have not been revealed or adequately postulated. Since HCN is themajor reaction intermediate in NO reduction during reburning [4],we postulate that Fe2O3 converts HCN to N2 during reburning at theoutset of this study, which can be verified by a single-stage, simu-lated reburning apparatus, such as that in our laboratory. Sea nod-ule is selected for HCN conversion tests for two different reasons. Arepresentative nodule from the Atlantic Ocean contains 19.41%Fe2O3, 20.10% CaCO3 and other minerals [22] that could potentiallycatalytically convert HCN to N2. Nodule has also been considered asa flue gas desulfurization agent in the last four decades [22,23].Thus, if a sea nodule does convert HCN to N2 in reburning, it maybe capable of simultaneously reducing NO and SO2.

This work experimentally tests the effectiveness of a suite ofcarefully selected substitutes for natural gas and lignite fly ash inan apparatus containing independently controlled reburning andburnout stages in series. Addition of a burnout zone from our pre-vious apparatus allows the studies of reactions of reburning prod-ucts in the burnout zone, such as the selectivities of char nitrogenconversion to NO and N2, and HCN conversion to NO. This informa-tion is highly desirable for developing advanced NO control strate-gies, as the ultimate reburning efficiency is governed not only byreburning efficiency, but also by the nitrogen pathways of thereburning products in the burnout zone. Among these reburningproducts, reactions of char nitrogen and HCN in fuel lean condi-tions are the focus of our study because NO entering the burnoutzone is very low and NH3 conversion to NO is minimal.

2. Experimental

2.1. Apparatus and procedure for simulated reburning and burnout

The experimental apparatus previously used by Chen and Gath-itu [6] was modified for the current study. In order to conduct sim-

ulated two-stage experiments that include a reburning zone and aburnout zone, a second furnace was added in series with the pre-vious one. This is a Lindberg/Blue M Model 54494-V furnaceequipped with 10 U-shaped heating elements of 30.48 cm totallength arranged radially. The furnace temperature can be broughtup to 1700 �C by a programmable temperature controller, Lind-berg/Blue M Model 59256-P-B-COM. As shown in Fig. 1, the newand existing furnaces are typically used for reburning and burnoutstudies, respectively. Alumina tubes composed of 99.8% Al2O3 with1.91 cm ID, 2.54 cm OD, from Morgan Advanced Ceramics wereused as reactor tubes for both stages.

To quantify nitrogen speciation from the reburning zone, or thesingle-stage tests, the top furnace temperature was typically set ateither 1150 or 1250 �C, as required, and the bottom furnace at200 �C to avoid condensation of HCN and NH3. The typical gas flowrate was 2 L/min. To conduct two-stage tests, the reburning fur-nace was typically set at 1250 �C and the burnout furnace at1150 �C. In the reburning zone, the residence time in the ‘‘flatzone” at the top of the parabolic temperature profile in the axialdirection is approximately 0.2 s [24]. In the burnout zone, this res-idence time varies with the amount of burnout gas (20% O2 balanceHe) added and can range from approximately 0.14 to 0.16 s.

To investigate the nitrogen speciation in reburning zone, a set ofsingle-stage experiments were conducted. For these simulatedreburning tests, the simulated flue gas had a composition of16.8% CO2, 1.95% O2, and 0.05% NO in a helium base. Two-stageexperiments were carried out to expand our knowledge aboutthe chemistry in the burnout zone. For most of the two-stage tests,the reburning furnace temperature was increased to 1250 �C.Water was introduced for two different sets of experiments, two-staged experiments where Fe2O3, mill scale and sea nodules wereadded to reduce HCN and for single-staged tests to determinethe selectivities of HCN and char nitrogen conversion to NO inthe burnout zone. Water was introduced into the simulated fluegas at a concentration similar to that produced during combustionof Pittsburgh #8 bituminous coal in the primary stage. This alteredthe composition of the flue gas to 16.0% CO2, 1.8% O2, 0.05% NO and6.35% H2O.

Nitrogenous products, HCN and NH3, from reburning zone aretrapped in two parallel 0.5 L impingers filled with a 0.1 N HNO3

aqueous solution for a specified time, typically 30 min. Due tothe acidic nature of the impinger solution, recovery of HCN andNH3 by this method was tested using known standards and foundto be near quantitative for NH3 and 70% for HCN. The capturedsolutions’ pH was adjusted to a level above 10 using a 10 N NaOHaqueous solution and analyzed for CN� and dissolved ammoniawith ion specific electrodes manufactured by Thermo Orion (nowcalled Thermo Fisher Scientific Inc.). Poisoning of the cyanide elec-trode by sulfur ions was prevented by gradual addition of an aque-ous solution of 0.1 N Pb(NO3)2 prior to adding the NaOH, thisprecipitated the sulfide ions in the form of PbS. It should be men-tioned that addition of excessive amounts of Pb(NO3)2 causedinterference in the HCN analysis, by inflating the results, and there-fore, the amount of aqueous 0.1 N Pb(NO3)2 added was determinedbased on the approximate amount of sulfur present in the samplesolution. NO, CO and CO2 analyses were conducted using the sameinstrumentation as that mentioned by Chen and Gathitu [6].

2.2. Humidifier and condenser

To investigate the effects of water vapor in reburning and burn-out zone, a humidifier for the feed stream and a condenser for theproduct stream were fabricated during the course of this study. A1 L Fisher Scientific� ethyl ether aluminum bottle was convertedto a humidifier, or a bubbler. A gas inlet, a gas outlet and a thermo-couple port are fabricated at the top of this unit. It is wrapped with

Burnout Air Port

Buchner flask

Paper Tissue Pouch to Collect Solid Combustion Residue

Furnace heating elements

Reactor Gaseous Effluent to Online

Analytical Instruments (Impinger, NOx

Analyzer, and CO/CO2

Analyzer

Furnace heating elements

Reactants into Reactor (Simulated Flue Gas, Solid and Gaseous Fuel and Water Vapor)

Fig. 1. Experimental setup.

Y. Su et al. / Fuel 89 (2010) 2569–2582 2571

heating tape and insulated using ceramic wool. The inlet port has aTeflon tube running down to the bottom of the bottle so that gascould bubble through the filled length of the bottle before exitingfrom the outlet. The transfer line between the humidifier and thereactor is heat traced using an electrical heating tape. Typically,two-thirds of the humidifier volume is filled with water. To obtainthe targeted feed rate of water vapor, its efficiencies at different Heflow rates were calibrated by measuring the weight of trappedwater vapor in a desiccant unit. At a water bath temperature of70 �C and a helium flow rate of 374 mL/min, the bubbling unitcan achieve 76% relative humidity.

During the reburning experiments, the water vapor entrained inhelium stream merges with the reactant mixture, containing en-trained particles from the particle feeder, inside the reactor. To avoidparticle adhesion to the 0.159 cm ID tube and flow blockage due tothe presence of moisture, the stainless-steel particle feeder tube isextended 3 in. into the reactor, which is below the water injection

port. It should be mentioned that, upon its entry into the hot reactortube above the heated zone of the furnace, the water vapor absorbs alarge amount of heat, induces a thermal shock in the axial directionof the tube and hence tube fracture occurs. This problem happenedat both the reburning and burnout zones entrances and was solvedby insulating the entry portions of the tubes using ceramic wool.

Water vapor has to be removed before the product stream en-ters the online analyzers. Thus, a second bubbler was installedafter the heated inline particulate filter to condense the water va-por. This condenser is similar to the humidifier; but its does nothave a thermocouple and was dipped in a bucket of ice-water dur-ing operation.

2.3. Optimization of burnout

During two-stage studies, the burnout stage stoichiometric ra-tio (SR3) was varied to ensure that enough O2 is fed for complete

2572 Y. Su et al. / Fuel 89 (2010) 2569–2582

CO oxidation. The CO analyzer used for this study has a noise levelof +50 ppm and therefore a CO measurement of less than 50 ppmwas considered successful burnout. For each fuel, the SR3 was ad-justed stepwisely at 0.03 increments until that optimal SR3. Wealso noted that this SR3 is a strong function of residence timeand temperature.

2.4. Samples

This experimental study was conducted using a mixture of or-ganic and inorganic compounds. They include the following:

(1) Chinese tire – this is crumb rubber derived from spent Chi-nese tires processed by Zhenjiang Luyuan Tire Powder Com-pany (LTD), China. One portion of the sample was suppliedin particles of less than 104 lm, and the other, in less than500 lm.

(2) US tire – this is crumb rubber derived from spent US tiresprocessed by Polyvulc. It was supplied in particles of lessthan 180 lm and was ground and then sieved to less than106 lm before experimentation.

(3) Pine bark – this is derived from the de-barking process ofpine logs at the Bowater Newsprint mill. It was dried thenground and sieved to less than 246 lm beforeexperimentation.

(4) Corn-stover residue – this is the biomass to ethanol fermen-tation residue of corn plants. It was dried then ground andsieved to less than 246 lm before experimentation.

(5) Paper mill sludge – these are fibers too short for paper pro-duction, they are derived from the thermal–mechanical pul-ping process of the Bowater Newsprint mill. It was driedthen ground and sieved to less than 246 lm beforeexperimentation.

(6) Pine wood fines – these are the wastes during chipping ofthe pine wood at the Bowater Newsprint mill. They weredried then ground and sieved to between 75 and 106 lmbefore experimentation. This size classification was anattempt to ease feeding and hence use less silica-gel (seethe following section). However, we still needed a ratio offuel to silica-gel of 1:2.

(7) Stanton station baghouse fly ash – this sample was collectedfrom a dust bag of a lignite-fired boiler. The sample was dry,as-received, and it was sieved to less than 106 lm beforeexperimentation.

Table 1Mineral ash analysis of the lignite fly ash and sea nodule (wt.% of mineral).

Sample Na2O MgO Al2O3 SiO2

Stanton station baghouse lignite fly ash 3.2 5.83 16.43 35.51Mn2O3 CaCO3

Blake plateau nodules* 31.20 20.10

* Wt.% of major components as equivalent oxides [22].

Table 2Ultimate analysis of the raw samples (wt.%, dry basis).

Sample C H O N S

MethaneUS tire 81.44 7.03 1.95 0.34 1.77Chinese tire 80.92 6.804 4.293 0.384 1.666Papermill sludge 51.27 6.76 38.73 0.25 0.13Pine bark 51.36 5.96 35.92 0.31 1.05Corn-stover residue 52.5 4.8 26.32 2.35 0.19Pine wood fines 50.84 6.04 42.55 0.11 0.05

(8) Fe2O3 powder – this was purchased from Sigma–Aldrich. Ithas a purity of at least 99% and is sieved to less than 5 lm.

(9) Mill scale – this is iron waste from a steel mill and has aniron content of approximately 90%. It was dried then groundand sieved to less than 246 lm before experimentation.

(10) Sea nodules – these were collected from the Gulf of Mexicoand are rich in iron along with other metals. They wereground and then dried to less than 246 lm beforeexperimentation.

The analytical results of these samples are tabulated in Tables 1and 2. Huffmann Laboratory provided data for most of these sam-ples; data for sea nodules and corn-stover residue were obtainedfrom literature [22,25].

2.5. Sample mixing

The powders of the fuels mentioned above have strong adhe-sion to each other and cannot be fed easily by our particle feeder[26,27]; to assist in this, they have to be mixed with silica-gel ata ratio of 1:2 (fuel:silica-gel). Simple shaking did not produce ahomogeneous mixture of the tire powders and silica-gel due to for-mation of tire crumbs, effective mixing was achieved by hand-rub-bing the mixtures on a clean sheet of paper while wearingneoprene gloves. Effective mixing of Fe2O3, fuel and silica-gel pow-ders also required a special technique; rubbing on a sheet of papercaused loss of Fe2O3 as it adhered upon the paper and neoprenegloves. However, use of a flat metal sheet (aluminum) and a spat-ula proved effective. Chen and Gathitu [6] have previously demon-strated that silica-gel does not affect the reactions duringreburning.

2.6. Collection of char samples

One of our interests is to study char nitrogen conversion to NOin the burnout stage. To achieve this goal, chars from single-stagetests were collected. Their weights and elemental compositionswere determined. The particle collection unit in the down streamof the reactor was modified by tying a pouch made of fine stain-less-steel wire-mesh (width of opening = 10 lm) around the exitof the reactor tube. Filter paper, instead of wire-mesh, was initiallyused, but fine particles adhered to the paper and it was difficult toremove them completely. Only negligible amounts of char werelost when using the wire-mesh. The pouched reactor tube end

P2O5 SO3 K2O CaO TiO MnO2 Fe2O3 LOI

0.18 5.99 1.25 21.17 0.86 0.07 9.53 2.28Fe2O3 Co3O4 CuO NiO

19.41 0.78 0.23 0.82

Mineral matter Atomic H/C ratio Method of oxygen determination

47.47 1.04 By difference7.41 1.01 Merz2.86 1.58 By difference5.21 1.39 Not specified

13.80 1.10 Not specified0.41 1.43 By difference

Y. Su et al. / Fuel 89 (2010) 2569–2582 2573

was inserted into a Buchner flask and the joint sealed using a rub-ber stopper and vacuum grease. The effluent gases were diverteddownstream, as is normally done, to the analytical instruments.

As mentioned previously, the samples were mixed with silica-gel (53–109 lm) to ease feeding. The solid residue collected wassieved on a 53 lm screen to separate silica-gel from the chars. Acontrol test, done using silica-gel particles alone, indicated thatthe size classification by the manufacturer was thorough, andessentially all the silica-gel particles were trapped on a 53 lmsieve.

2.7. Reactor tube cleaning

When reburning was conducted at 1250 �C, the silica-gel soft-ened and stuck on the reactor wall where it progressively accumu-lated and posed a clog up risk. This was avoided by lightly tappingthe reactor tube, on the steel fitting near the injection port, aftereach test until the deposits dropped off which could be visuallyexamined through the open particle injection port. If the tappingdid not work, a 0.159 cm wide metallic wire was used to gentlyloosen off the deposits by rotating it inside the hot tube. To regen-erate the tubes’ normal (inert) activity, the techniques developedpreviously by Chen and Gathitu [6] were used every time the tubewas exposed to lignite fly ash and Fe2O3.

3. Results and discussions

3.1. Reburning at furnace temperatures above 1150 �C

The high-temperature furnace (up to 1700 �C) allows us toinvestigate nitrogen speciation at temperatures closer to the peaktemperature of a practical reburning flame, 1400 �C, which is abovewhat we could have accomplished previously, 1150 �C. Since one ofour goals is to find natural gas substitutes, the nitrogen speciation

SR2 Feed NO =

Furnace Temperatur

0

100

200

300

400

500

600

700

800

1000 1050 1100 1150 1200 12

Furnace Te

Exit

nitr

ogen

spe

cies

con

cent

ratio

ns (p

pm)

NO NH3

Fig. 2. Temperature effects during methane reburning at SR2 = 0.9 without addition of liHCN, but self-diffusion of oxygen through the alumina tube distorts the HCN data at hi1150 or 1250 �C.

from gas reburning in the temperature range 1150–1450 �C is con-sidered baseline data and was investigated at the outset of thisstudy. These results and those of our earlier study in the 900–1150 �C range [28] are illustrated in Fig. 2.

Fig. 2 illustrates that the exit NO concentration decreases withincreasing reburning temperature and reaches a negligible levelat 1400 �C. HCN is the major product from NO conversion. This im-plies that if an effective HCN conversion agent can be found,reburning efficiency can be raised to a very high level, certainlyabove the 60% reduction level observed by many researchers. Onthe other hand, it should be emphasized that reburning residencetime for the current study, 0.2 s, is longer than those in practicalflames. Moreover, NO reactions in flame usually follow kineticallycontrolled mechanisms, and rarely reach chemical equilibrium inpractical flames.

It is also interesting to note that, above 1150 �C, HCN decreasesas the temperature increases. This decrease is likely to be contrib-uted by both thermal decomposition of HCN and reactor wall inter-ference. It is known that O2 diffuses across an alumina reactor tubewall at temperatures above 1300 �C into an oxygen-deficit reactingzone through a self-diffusion mechanism [29]. Chen et al. [30] re-ported that self-diffusion of oxygen results in an addition of about100 ppm of O2, or 200 ppm of oxygen atoms, inside the reactor.This additional source of oxygen leads to HCN oxidation to NH3,as reflected by the NH3 peak at 1350 �C in Fig. 2. Without identify-ing the nature of the wall interference, Song [31] studied thermaldecomposition of HCN up to a furnace temperature of 1400 �C byusing pre-mixed and diffusion-controlled flame configurations toseparate the wall effects. He concluded that the Al2O3 wall reac-tions consume about 25% of the HCN initially fed. The reactor usedin that study had a residence time approximately twice as long asthe one used here, but Song’s reactor has a larger volume-to-sur-face ratio than ours. Thus, it is difficult to assess the exact extentof wall interference on observed HCN conversion at high tempera-

= 0.9 1000ppm

es = 1150 °C to 1450 °C

50 1300 1350 1400 1450 1500

mperature (°C)

HCN TFN

gnite ash for HCN conversion. Higher temperatures seem to favor NO conversion togh temperatures above 1300 �C. Current reburning experiments were conducted at

2574 Y. Su et al. / Fuel 89 (2010) 2569–2582

tures in our reactor. For this reason, we chose a moderate temper-ature, 1250 �C, rather than 1400 �C, for most of our reburningexperiments in the current study. Self-diffusion of oxygen is not se-vere at 1250 �C [30].

Previous work by Chen and Tang [5] checked for the extent ofNO conversion to N2O during reburn with coal chars at experimen-tal conditions similar to the present ones. The yields of N2O wereonly between 7 and 10 ppm in the temperature range between800 and 1100 �C with higher temperatures yielding lower levelsof N2O. At present reactor temperatures, chemical equilibrium fa-vors NO and therefore NO2 will not be an issue. If NO2 were presentat boiler flame conditions, then a scrubber would be used to cap-ture it, much like SO2, but this is not the case.

3.2. Reburning with tire

Similar to natural gas reburning, reburning with tires in a previ-ous study has shown a 60% NO reduction floor [13]. The nitrogenspeciation from tire reburning, however, has not been well exam-ined. Indeed, the aforementioned overall NO reduction efficiency intire reburning resembles that from natural gas reburning. Sincetires are known to be volatile at high temperatures and char nitro-gen is not likely to be a factor contributing to the observed 60% NOreduction floor, we suspect that HCN is the major reaction productfrom reburning.

Fig. 3 illustrates the nominal NO reductions by a mixed fuelcontaining tire and lignite ash from a set of two-stage experimentsat SR2 = 0.9 and SR3 = 1.2. A fraction of fuel-nitrogen may convertto NO in the reburning and the burnout stages, therefore, nominalNO reduction in this paper is defined as:

Nominal NO reduction ¼ 1� Exit NO concentrationFeed NO concentration

ð1Þ

Both reburning and burnout furnace temperatures were set at1150 �C. Interestingly, these NO reduction efficiencies are compa-rable to those by methane and lignite ash over a wide range ofash feed rate. More importantly, 85% NO reduction is achievable.

SR2 = 0.9 aFeed NO

Reburn and Burnout Fu

55%

60%

65%

70%

75%

80%

85%

90%

0 0.025 0.05Feed Rate of Baghous

Nom

inal

NO

Red

uctio

n (%

)

Tire + Ash

Fig. 3. Comparison of two types of two-stage (reburning and burnout) mixed fuel experTheir efficiencies are remarkably similar.

This finding gave us incentives to compare the nitrogen speciationin reburning and to test other industrial by-products as potentialsubstitutes for natural gas.

3.3. NO, HCN and NH3 yields during reburning

Nitrogen speciation in reburning has fundamental importancein determining the NO control strategies. For a comprehensiveevaluation of the effectiveness of different fuels without ligniteash, nitrogen speciation in single-stage reburning tests was mea-sured. As shown in Fig. 4, the ‘‘optimal” SR2 (stoichiometric ratioof the reburning zone) for NO reduction is between 0.9 and 0.95at 1150 �C and four of the five fuels are as effective as methane.The highest SR2s before NO yields take off are considered optimalSR2 because NO is likely to remain stable in the burnout zone andSR2 should be as high as possible to minimize the consumption ofreburning fuel. These optimal SR2s are chosen for the subsequenttwo-stage experiments. It is noted that, except for pine wood finesthat have low NO conversion reactivities, all other fuels haveshown similar NO reduction trends.

Figs. 5 and 6 illustrate the HCN and NH3 yields from six differentfuels in single-staged reburning. All of them have much higherHCN yields than those of methane due to their high contents offuel-nitrogen. Corn-stover residue has particularly high yields ofNH3. These high HCN yields from reburning zone reflect the needfor finding a substitute for lignite ash. It is interesting to note that,since tire produces higher yield of HCN than methane in reburningzone, it results in a lower overall NO reduction efficiency from thetwo-stage experiments without a HCN conversion agent, see thedata at the left end of Fig. 3.

3.4. Effectiveness of Fe2O3 at HCN reduction

Prior reburning studies have demonstrated the effects of Fe2O3

on NO reduction efficiencies though the exact mechanism of its ef-fects, especially that on HCN conversion, has not been revealed oradequately postulated. To investigate Fe2O3’s possible role in NO

nd SR3 = 1.2 = 500 ppmrnace Temps. = 1150 °C

0.075 0.1e Lignite Fly Ash (gm/min)

Methane + Ash

iments: methane and baghouse lignite fly ash, and tire and baghouse lignite fly ash.

Other Fuels vis-à-vis Methane - Reburn OnlyFeed NO = 500 ppm

Furnace Temperature = 1150 °C

0

50

100

150

200

250

300

350

0.6 0.7 0.8 0.9 1 1.1SR2

Exit

NO

(ppm

)

Tire Pine Bark Corn Stover ResiduePaper Mill Sludge Pine Wood Fines Methane

Fig. 4. Exit NO concentration from single-stage reburning with different fuels at different SR2 (no lignite ash is introduced in the feed). Four of the five fuels are as effective asmethane and sludge has an optimal SR2 of 0.95 implying that it has the highest efficiency at NO reduction.

Other Fuels vis-à-vis Methane - Reburn OnlyFeed NO = 500 ppm

Furnace Temperature = 1150 °C

0

100

200

300

400

500

600

700

0.8 0.85 0.9 0.95 1 1.05SR2

Exit

HC

N (p

pm)

Tire Pine Bark Corn Stover ResiduePaper Mill Sludge Pine Wood Fines Methane

Fig. 5. Exit HCN concentration from single-stage reburning with different fuels at different SR2 (no lignite ash is introduced in the feed). All the other fuels besides methanecontain nitrogen and some is shed in the reburning stage in the form of HCN and NH3 (see Fig. 7). This additional HCN makes apparent the need for effective conversion agentif these fuels are to be used effectively for NO reburning.

Y. Su et al. / Fuel 89 (2010) 2569–2582 2575

reduction, a set of experiments were conducted with a helium feedcontaining only 600 ppm of HCN and 1200 ppm of Fe as Fe2O3 inthe temperature range 1150–1300 �C. Fig. 7 illustrates that addi-tion of Fe2O3 indeed effectively converts HCN over a wide rangeof temperatures.

On exposure to HCN during tests, the red-brown color of Fe2O3

(iron III oxide) changed to black that is the color of FeO (iron IIoxide) and Fe3O4 (iron II, III oxide). This observation suggests thatthe 99% pure Fe2O3 was chemically reduced and consequently theHCN was oxidized to form N2 and CO. CO was measured on the CO/CO2 analyzer and, while the CO would react with iron oxides toform CO2, the noise level of our instrument for CO2 measurements,250 ppm, does not allow us to measure such a concentration.

3.5. Effects of water and temperature on Fe2O3 activities

After the effect of HCN conversion by Fe2O3 in a helium basewas demonstrated, its effect in a reburning environment was stud-ied at SR2 = 0.85, reburning furnace temperatures of 1150 and1250 �C and burnout furnace temperature of 1150 �C. As men-tioned earlier, wall effect, or self-diffusion of oxygen, is minimal,i.e., see Figs. 2 and 7, at temperatures below 1250 �C. Initial testswithout water, see Fig. 8, achieved 60% overall NO reduction,which is much lower than the 85% conversion Lissianski et al.[20] reported at a similar SR, though their temperature was higher,1400 �C. Besides the temperature difference, our simulated flue gasdid not contain any water while Lissianki et al.’s reactor configura-

Other Fuels vis-à-vis Methane - Reburn OnlyFeed NO = 500 ppm

Furnace Temperature = 1150 °C

0

50

100

150

200

250

300

350

400

450

500

0.8 0.85 0.9 0.95 1 1.05SR2

Exit

NH

3 (p

pm)

Tire Pine Bark Corn Stover ResiduePaper Mill Sludge Pine Wood Fines Methane

Fig. 6. Exit NH3 concentration from single-stage reburning with different fuels at different SR2 (no lignite ash is introduced in the feed).

HCN Concentration in Helium = 600 ppmFe Concentration = 1200 ppm

Residence Time = 0.2 sec

0

50

100

150

200

250

300

350

1100 1150 1200 1250 1300 1350

Furnace Temperature (°C)

Exit

HC

N (p

pm)

HCN Yields Without Fe2O3 HCN Yields With Fe2O3

Fig. 7. Conversion of HCN by Fe2O3 in a He environment at different temperatures.The plot without Fe2O3 indicates that HCN thermal decomposes in the reactor;nevertheless, the effectiveness of Fe2O3 is evident when both plots are compared.

SR2 = 0.85, SR3 = 1.1Feed NO = 500ppm

Burnout Furnace Temperature = 1150 °C

45%

50%

55%

60%

65%

70%

75%

80%

0 1000 2000 3000 4000 5000Fe Concentration (ppm)

Nom

inal

NO

Red

uctio

n (%

)

1250 °C (without water) 1250 °C (with 6.35% water)1250 °C (with 17% water) 1150 °C (with 6.35% water)

Fig. 8. Effects of water concentration, reburning stage temperature and Feconcentration on the activity of Fe2O3 towards HCN conversion in two-stage testsusing methane as the fuel. Higher temperature and water concentration increaseFe2O3 activity. Water concentration of 6.35% and 17% correspond to the flue gasfrom a bituminous coal and methane primary flame, respectively, operated atSR1 = 1.1.

2576 Y. Su et al. / Fuel 89 (2010) 2569–2582

tion fired natural gas in the primary stage to generate approxi-mately 17% water content in the resulting flue gas entering theirreburning zone. A water addition unit was therefore fabricated,as mentioned in Section 2, to add water into the simulated fluegas. Water at concentrations of 6.35% and 17% was introduced intothe feed, which represent the water vapor concentrations in fluegases from coal- and natural gas-fired boilers, respectively. Two-stage experiments were conducted. The oxygen and hydrogen con-tents of water were taken into account in the SR2 calculation, andit does not change the SR2 values significantly.

Fig. 8 shows that increasing both the water concentration andfurnace temperature enhance Fe2O3 activity during reburning.Water is an oxidant and it appears that water vapor enhancesHCN oxidation to CO and N2. Increasing temperature also favorsFe2O3 oxidative ability.

3.6. Two-stage tests of NO reduction by mixed fuels

With a temperature and water concentration selected, mixedfuels were systematically tested at optimal SR2 to achieve maximalNO reduction with minimal reburning fuel and optimal SR3 toachieve complete burnout. All feed streams contain 6.35% water.Fig. 9 illustrates the effects of Fe concentration on exit NO concen-tration from these two-stage experiments when different reburn-ing fuels are employed. The exit NO concentrations observed inour experiments with methane and Fe2O3 are higher than thoseobserved by Lissianski et al. [20], which may be due to the differ-ences in operating residence time, temperature, water contentand mixing.

Feed NO = 500ppmFurnace Temperatures: Reburn = 1250 °C, Burnout = 1150 °C

40%

45%

50%

55%

60%

65%

70%

75%

80%

85%

90%

0 1000 2000 3000 4000 5000 6000 7000 8000Fe Concentration (ppm)

Nom

inal

NO

Red

uctio

n (%

)

Chinese Tire (SR2 = 0.9, SR3 = 1.2) Pine Bark SR2 = 0.9, SR3 = 1.3)Corn Stover Residue (SR2 = 0.9, SR3 = 1.25) Sludge (SR2 = 0.95, SR3 = 1.3)Wood Fines (SR2 = 0.9, SR3 = 1.3) US Tire (SR2 = 0.9, SR3 = 1.2)Methane (SR2 = 0.9, SR3 = 1.1) Chinese Tire (SR2 = 0.9, SR3 = 1.2) (<500 micron)

Fig. 9. Effects of Fe2O3 on NO reduction efficiencies of various reburning fuels during two-stage (reburning + burnout) tests. A mixture of tire and Fe2O3 can achieve up to 88%NO reduction. Operating reactor variables may have limited methane’s effectiveness while char nitrogen conversion to NO in the burnout stage limited the rest of the fuels.

Y. Su et al. / Fuel 89 (2010) 2569–2582 2577

Results in Fig. 9 show that the combination of tire and Fe2O3

forms an efficient substitute for natural gas and lignite ash. Tirecan achieve up to 88% nominal NO reduction when 4000 ppm ofFe, in the form of Fe2O3, is added into the reactor. This feed rateis equivalent to 185 metric tons of Fe2O3 per day for a 172 MWbituminous coal-fired boiler, which is much more efficient than lig-nite ash. Tire has a relatively low, 27.15%, fixed carbon, and charnitrogen conversion to NO in burnout zone is therefore not a signif-icant issue. For the other fuels, char-N conversion to NO appears tobe limiting the overall NO reduction efficiency.

Purified Fe2O3, like that used above, is an expensive chemicaland would be economically impractical for use in a coal-fired boi-ler; therefore, mill scale, a steel-mill waste rich in iron, was tested,see Fig. 10, in place of the reagent-grade Fe2O3 and achieved 82%nominal NO reduction when feeding it at a Fe concentration of4000 ppm. Its Fe concentration is 90% and therefore the requireddaily amounts increase only slightly from the number mentionedabove.

As illustrated in Fig. 10, 78% nominal NO reduction can beachieved in reburning with tire and sea nodules at a Fe concentra-tion of 4000 ppm. However, with a Fe2O3 concentration of approx-imately 20% the amounts required are five times that of pure Fe2O3

and it may be impractical to inject the excess mass of the noduleinto a boiler. Moreover, the nodules are harvested deep in theocean and, therefore, the economic applicability of sea nodulesfor NOx control in coal-fired boilers is subject to mining costs.

3.7. Selectivities of reburning products to NO in burnout stage

The ultimate reburning efficiency is governed not only by thereactions in reburning stage but also those in the burnout stage.Previous studies on the burnout stage during natural gas reburning

have been conducted [32,33]. However, present knowledge aboutthe selectivities of the fixed nitrogen products from reburningstage, NO, HCN, NH3 and char nitrogen, to form NO in the burnoutzone appears limited. Better understandings of the burnout zonechemistry are desirable in developing new strategies for improvedNO control. We therefore undertook this task in order to determinethe next research direction to take in the course of further improv-ing NO reburning.

We designed a first-level approximation targeting at finding theselectivities of reburning products to NO in burnout stage. We as-sume that the NO yield from a burnout zone is the sum of the con-tributions from the reburning products, i.e.,

NOexiting the burnout stage ¼ a NOfrom the reburning stage

þ b HCNfrom the reburning stage

þ c Char-Nfrom the reburning stage

þ d NH3from the reburning stage ð2Þ

where a, b, c and d are constants representing the selectivities ofreburning products to NO. These parameters are regressed againstobserved concentrations from five different reburning fuels: tire,wood, bark, corn-stover residue and sludge.

Single-stage reburning tests were conducted to collect andquantify char samples, and quantify NO, HCN and NH3. Elementalanalysis of collected chars was also conducted, see Table 3. Theseexperiments were conducted at 1250 �C, with a water concentra-tion of 6.35% and at the optimal SR2 for each sample as discussedin Section 3.3, which are also specified in Fig. 9. Two-staged testswere conducted using the reburning stage parameters mentionedabove, a burnout stage furnace temperature of 1150 �C and optimalSR3 (for complete combustion of char) for each sample, as specified

Feed NO = 500ppmFurnace Temperatures: Reburn = 1250 °C, Burnout = 1150 °C

60%

65%

70%

75%

80%

85%

90%

0 1000 2000 3000 4000 5000 6000 7000 8000Fe Concentration (ppm)

Nom

inal

NO

Red

uctio

n (%

)

Chinese Tire with Mill Scale (SR2 = 0.9, SR3 = 1.2) Methane with Mill Scale (SR2 = 0.9, SR3 = 1.1)

Chinese Tire with Sea Nodules (SR2 = 0.9, SR3 = 1.2) Methane with Sea Nodules (SR2 = 0.9, SR3 = 1.1)

Fig. 10. Effects of mill scale and sea nodules on NO reduction efficiencies of tire and methane as reburning fuels during two-stage tests. A mixture of tire and mill scale canachieve up to 82% NO reduction while tire and sea nodules can achieve 78% NO reduction. Mill scale contains 90% Fe while sea nodules contain only 20% Fe2O3 making millscale the better option.

Table 3Ultimate analysis of the char samples from single-stage (reburning) tests (wt.%, dry basis).

Sample C H O N S Mineral matter Atomic H/C ratio Method of oxygen determination Mass loss during reburn (%)

Chinese tire 46.36 0.68 9.2 0.3 1.62 49.27 0.18 ASTM D5622 79Papermill sludge 22.64 1.03 13.68 0.22 0.58 67.12 0.55 ASTM D5622 44Pine bark 27.72 0.7 10.86 0.24 0.26 62.02 0.30 ASTM D5622 100Corn-stover residue 8.88 0.35 4.09 0.23 0.63 88.09 0.47 ASTM D5622 80Pine wood fines 16.79 0.79 9.68 0.21 0.24 83.32 0.56 ASTM D5622 40

2578 Y. Su et al. / Fuel 89 (2010) 2569–2582

in Fig. 9. Interactions among these nitrogen species are ignored.NO, HCN and char nitrogen concentrations in the burnout zoneare below 60, 200 and 140 ppm, respectively. NH3 contributionsto NO formation were ignored in regression because NH3’s concen-trations in reburning products are all below 10 ppm. In fact, threeof the five reburning fuels yielded NH3 concentrations well below1 ppm.

The regressed selectivities a , b, and c are �0.053, 1.34, and0.422, respectively. Comparisons of experimentally observed andregressed results are shown in Fig. 11. Although the number ofsamples in the current study is small, our analysis is not fueldependent and the SR2 and SR3 for different fuels are different,the recovered selectivities seem to suggest the importance of theHCN and char nitrogen reactions. The selectivity of char nitrogento NO is slightly higher than those observed in coal combustion[34] where the nitrogen concentrations are lower than in the burn-out zone of a reburning process. NO concentration entering theburnout zone is very low, and its small negative selectivity sug-gests either the error in the exercise or the nonlinear nature ofthe reaction, such as NO attack of char nitrogen in the formation

of N2. The high selectivity of HCN to NO formation suggests thecomplex but significant role of HCN in the burnout zone. Selectivitygreater than one may simply be an uncertainty in the regression;nevertheless, it may imply that HCN facilitates the conversion ofother fixed nitrogen, such as char nitrogen, to NO. While literaturecontains scattered works on HCN oxidation in the gas phase[3,35,36] and char nitrogen conversion to HCN and NO [7–10,37–39], additional studies of heterogeneous reaction mechanisms ofnitrogen species in burnout zone will indeed be valuable for fur-ther improvement of reburning.

Since this study was conducted, the authors have formulated anovel means of limiting char-N conversion to NO in the burnoutzone. This is achieved by pretreating coal using supercritical CO2

to change its structure [40]. Consequently, in the reburn stage,the coal sheds char-N in the form of HCN which is then catalyti-cally converted to N2. By using this technique on as-received bitu-minous coal, we have increased the NO reburning efficiency from69% to 78%, with potential for even greater reduction at boilerconditions [41]. Other combustion benefits have also beenrealized.

Feed NO = 500 ppmFurnace Temperatures: Reburn = 1250 °C, Burnout = 1150 °C

0

50

100

150

200

250

300

Methane Tire Bark CSR Sludge WoodReburning Fuels

Nitr

ogen

Spe

cies

Con

cent

ratio

n (p

pm)

Experimental NO Yield from Two-Stage Tests134% of HCN in Burnout Stage42% of Char Nitrogen in Burnout StageSimulated NO Yield from Two-Stage Tests (-5.3% Reburn NO + 134% HCN + 42% Char Nitrogen)

Fig. 11. Estimation of selectivities of reburning products to NO in burnout stage during two-stage reburning tests. A first-level approximation was conducted based on thepremises that all reburning intermediates convert to either nitrogen or NO in the burnout stage. Methane data was not included in the linear regression because it does notpossess char nitrogen.

Y. Su et al. / Fuel 89 (2010) 2569–2582 2579

3.8. Impact of addition of Fe2O3 on boiler performance andmarketability of fly ash

An analysis of slagging and fouling tendency upon the heattransfer surfaces of a bituminous coal-fired boiler after additionFe2O3 (4000 ppm Fe) was conducted using the algorithm devel-oped by Stultz and Kitto [42]. Apparently, at this concentration,slagging and fouling tendency remains low, much like when theboiler is operated using bituminous coal solely.

Fly ash from coal-fired boilers is often used as a substituteraw-material in the cement manufacturing industry; an optionthat provides economic as well as environmental benefits. It istherefore important to ascertain the impact of this reburningtechnology on the marketability of fly ash to cement manufactur-ers. Coal is essentially fossilized plant biomass and the reburningtechnology described in this publication is adding plant biomass(pine wood fines, pine bark, paper mill sludge and corn-stoverresidue) into the boiler. Therefore, addition of these fuels asreburn fuels (up to 20% of total heat-input into the boiler) willnot change the quality of fly ash vis-à-vis that from a boilerfired using coal only to affect the fly ash usability in cementmanufacturing.

With regard to tire, the cement industry is currently using tire-derived fuel to fire their kilns; they even insert the whole tire intothe kiln to avoid the cost of chipping them [12]. The steel beltspresent in the tires are not removed because iron is desirable inthe cement manufacturing process. Therefore, even the additionof Fe2O3 into the boiler, for NO reburning purposes, will ultimatelyserve to increase the value of the fly ash for cement manufacturingpurposes.

According to Cangialosi et al. [43], the most serious issue withregard to the usability of fly ash in cement manufacturing is highlevels of unburned carbon in the fly ash. Therefore, as longas adequate burnout is effected, there should be no issue with

the usability of the fly ash, from our technology, for cementmanufacturing.

3.9. Effect of particle size

Tire is highly volatile at reburning temperatures (65% volatilematters) and the major NO reduction efficiency is likely to be theresult of interactions of tire’s devolatilization products and NO.Thus, we speculate that the particle size may not be a significantfactor governing the NO reduction efficiency.

Discussions with waste tire processors in the US indicate thatthey produce crumb rubber of particle sizes greater than 520 lm(30 Mesh) for traditional markets. As a result, particles that passthrough the 520 lm sieve cost less than larger particles and wouldtherefore be the most cost-effective option for our application.However, the results presented above are for tire particles smallerthan 106 lm and we wanted to test whether larger particle sizes,500 lm and below, could perform just as well.

The particle feeder used in this study so far can only handle par-ticle sizes smaller than 250 lm [27] and we therefore used a newlydeveloped device [44] to feed the tire particles less than 500 lm.We were able to achieve 84% nominal NO reduction at reactor con-ditions identical to those used for the smaller particle size thatachieved 88% nominal NO reduction, see Fig. 9. Therefore, ourcost-analysis below is based on the current price of crumb rubberparticles that are less than 520 lm in size.

3.10. Estimation of raw-material costs of mixed-fuel reburning withtire and Fe2O3

The cost of raw-material has been estimated to compare tradi-tional gas reburning with mixed-fuel reburning using tire and millscale. The analysis is based on the present cost of natural gas, $8/

2580 Y. Su et al. / Fuel 89 (2010) 2569–2582

MMBtu, and the average price of coal delivered to end user [45].Additional factors are listed below:

(a) The heating value of natural gas is 37.8 MJ/m3.(b) The analysis is conducted for power plants that fire bitumi-

nous coal, with a heating value of 12,000 Btu/lb, in the pri-mary stage.

(c) The primary stage of the power plants are operated atSR1 = 1.1 and the reburning stage at SR2 = 0.9.

(d) That tire has a heating value of 15,000 Btu/lb [46].(e) That all the power plants have a similar efficiency of 35%, i.e.,

they convert 35% of the thermal energy supplied by the coaland reburning fuel to electricity.

(f) That the cost of transportation is the same for all cases at $2/mile for a 40,000 lb load of tire and mill scale.

Four power stations in the US [47] are selected for analysis andnearby tire processors (Table 4) and steel mills (Table 5) are se-lected as sources of crumb rubber and mill scale. Since tire as areburning fuel provides 25% of the total heat-input into the boiler,adjustments were made to cater for the use of crumb rubber in-stead of coal to supply this heat into the boiler which leads to over-all fuel cost savings. The major results are summarized in Table 6.

The cost of crumb rubber is the dominant factor, mainly due tothe high cost of grinding soft materials. When using the pricequoted by the tire recycler in the State of New York, $100/ton ofcrumb rubber with a particle size of less than 520 lm [48], theraw-material cost of mixed-fuel reburning for all four cases is morecost effective than that of natural gas reburning. Because of the

Table 4Sources and cost of crumb rubber.

Powerplant

Size(MWe)

Power plantlocation

Tire processor Rubber proceslocation

Hennepin unit 1 80 Hennepin, IL Fennell recycling, LLC Elmira HeightLakeside unit 7 40 Springfield, IL Fennell recycling, LLC Elmira HeightKodak Park 62 Rochester, NY Fennell recycling, LLC Elmira HeightNiles station 108 Niles, OH Fennell recycling, LLC Elmira Height

Table 5Sources and cost of mill scale.

Power plant Steel mill Steel milllocation

Distance(miles)

Micon

Hennepin unit 1 Nucor Bar Mill Group Bourbonnais, IL 111 80Lakeside unit 7 Nucor Bar Mill Group Bourbonnais, IL 159 80Kodak Park Nucor Bar Mill Group Auburn, NY 63 80Niles station AK Steel Corp. Mansfield, OH 108 60

Table 6Cost of raw-material for mixed-fuel reburning.

Name ofpowerplant

Gas reburnheat input(%)

BaselineNOx (lb/MMBtu)

Gas reburnreduction(%)

Gas reburn(lb/MMBtu)

Mixed-fuelreburnreduction (%

Hennepinunit 1

18 0.73 67 0.24 82

Lakesideunit 7

23 0.73 60 0.29 82

KodakPark

18 0.73 56 0.32 82

Nilesstation

8–18 0.73 50 0.37 82

a Based on a natural gas cost of $8/MBtu.b Cost savings derived from replacing coal with tire to provide heat during reburningc Mixed-fuel reburn cost = crumb rubber costs + mill scale costs � overall fuel cost sa

high price of coal in New York and the close proximity of the KodakPark power plant to the tire recycler, in comparison to the otherpower plants, the raw-material cost of mixed-fuel reburning forthis case is $1374/ton NOx that is very competitive. It is importantto note that other tire recyclers closer to the power plants [48]quoted approximately double the price that the tire recycler inNew York did which makes the raw-material costs higher thanthose listed in Table 6.

Moreover, consumers of tire for fuel may enjoy financial incen-tives from local authorities for converting waste to energy and thisfactor, which has not been accounted for in the analysis here, willlower the costs further. Selective catalytic reduction (SCR) that is apopular technology that can achieve similar reduction efficiencyhas an operating cost of approximately $2000–5000/ton NOx in1999 dollars [49]. Therefore, all four power plants are competitiveespecially when inflation, since 1999, is accounted for.

Crumb rubber is processed for use on athletics turf, as a raw-material for various plastic products and in road construction asa sealant and in asphalt. The grinding technology is thereforegeared for these needs whose demand is not as large as that ofelectric power production. Improvements and innovation in grind-ing technology would lower the cost of crumb rubber and makethis technology very attractive.

The estimated costs presented here are sensitive to many oper-ating parameters of the boilers, and the actual cost may vary fromone boiler to another. For instance, Nimmo et al. recently reportedthat, without a HCN reducing agent, reburning by scrap tire pow-ders of <250 lm achieves 80% NO reduction in their 80 kW pilot-scale boiler [14]. This level of NO reduction is notably higher than

sor Distance(miles)

Crumb rubberprice ($/ton)

Crumb rubberrequirements (ton/day)

Crumb rubbercosts ($/ton NOx)

s, NY 734 100 153 4732s, NY 825 100 76 4980s, NY 117 100 118 3048s, NY 294 100 206 3531

ll scale irontent (%)

Mill scale price($/ton)

Mill scalerequirements (ton/day)

Mill scale costs($/ton NOx)

40 61 55840 31 61112 47 20017 110 405

)

Mixed-fuelreburn (lb/MMBtu)

Gas reburncosta ($/ton NOx)

Overall fuel costsavingsb ($/ton NOx)

Mixed-fuelreburn costc ($/ton NOx)

0.13 5888 731 4559

0.13 8402 731 4860

0.13 7045 1874 1374

0.13 7890 1262 2674

; depends on cost of coal at various locations.vings.

Y. Su et al. / Fuel 89 (2010) 2569–2582 2581

what we observed, 65–70%, as illustrated at the left end of the tirereburning curves in Fig. 9. Moreover, Miller et al. used 6.3 mmcrumb tire in their reburning tests [13] and discovered overallNO reduction efficiency, about 60%, similar to what we observed.These discrepancies are probably due to the differences in operat-ing conditions between these laboratories.

4. Conclusions

Substitutes of natural gas and lignite fly ash for effectivereburning have been investigated. Their efficiencies will meet theEnvironmental Protection Agency’s regulation of removing 85%,or up to 0.15 lb/MBtu, of NOx in coal-fired boilers. This technologyutilizes widely available wastes such as scrap tire and mill scale.Mixed fuel containing scrap tire and Fe2O3 is particularly effective.Though its cost is constrained by the energy-intensive operation ofgrinding the tire, the estimated raw-material cost is better thanthat of natural gas reburning and highly competitive against SCR.The current study also reveals that, in order to further improvethe overall reburning efficiency, additional studies of nitrogenreaction chemistry, particularly the mechanisms involvingHCN, char nitrogen and NO, in the burnout zone will be criticallyneeded.

An invention disclosure has been prepared for patentconsideration.

Acknowledgments

The Lignite Research Council, the Great River Energy, and theEnergy and Environmental Research Center of the University ofNorth Dakota provided the ashes of lignite from the utilities. TheBowater Newsprint mill provided the sludge, bark and wood fines.Dr. Clint Williford of University of Mississippi provided the corn-stover residue. Polyvulc provided the US tire. Mr. Martin Baker ofGeneral Recycling provided the mill scale. The late Dr. J. RobertWoolsey of the University of Mississippi provided the sea nodules.Gratitude is also expressed to the ladies and gentlemen who kindlyresponded to inquiries regarding the price and availability ofcrumb rubber and mill scale at various tire processing facilitiesand steel factories in the states of IL, NY, WI, MO and OH. This pro-ject was funded by the National Energy Technology Laboratory ofthe US Department of Energy under Grant DE-FG26-04NT42183.

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