Colloidal Petcoke-in-Water Suspensions as Fuels for PowerGenerationGustavo A. Nu !n"ez,† María I. Bricen"o,*,† Cebers Go !mez,† Takeshi Asa,† Hamid Farzan,‡ Shengteng Hu,‡

and Daniel D. Joseph§

†Nano Dispersions Technology, Incorporated, Building 231, City of Knowledge, Clayton, Panama‡Babcock and Wilcox Power Generation Group, 180 van Buren Avenue, Barberton, Ohio 44203, United States§Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota 55455, United States

ABSTRACT: In this work, it is shown that, despite the low reactivity of petroleum coke (petcoke) and the presence of 40%water, a petcoke suspension having a large colloidal population burned with unprecedented high efficiencies (>99%) without asupport fuel. This paper is an account of the main combustion test results, obtained in a 6330 MJ/h pilot-scale boiler simulatorlocated at the Babcock and Wilcox Research Center. This pilot plant simulates a full-scale utility boiler in many key aspects.Combustion tests of a typical heavy fuel oil (HFO) were carried out to produce baseline data for comparison to the colloidalpetcoke in water suspension (CPW) performance. The CPW fuel showed, besides high particle reactivity during combustion,some advantageous characteristics, such as ease of pumping, metering, and atomization at room temperature, using conventionalequipment designed to handle and fire HFO.

1. INTRODUCTIONHeavy fuel oil (HFO) is still used worldwide in industrial andpower-generating boilers, including facilities in the east andwest coasts of the U.S., where fuel oil can be convenientlyshipped in oil tankers. However, with the rising price of oil andits attached fuel oil price, it has become necessary to find lesscostly alternatives to keep the operational costs of any plant atcompetitive levels.Some alternatives, such as coal and petroleum coke

(petcoke), are quite obvious given their comparatively historicallower cost and wide availability. Some of the shortcomings ofburning pulverized fuels in oil-firing boilers can be overcome bypreparing mixtures of the solid material with water or oil. Thisis not a new idea, and back in the 1970s and 1980s, there wasintensive research and development on coal!water slurry(CWS) or coal!oil slurry (COS).1 However, the interest in thiskind of fuel came to an end in the U.S., mostly because ofeconomic hurdles associated with low oil prices prevalentthrough the 1990s. Nonetheless, to the authors’ knowledge,there was no attempt to prepare petcoke in water slurries foruse as the sole fuel in conventional boilers.At present, the oil price is increasing, and a definite higher

price horizon is foreseen as new large consuming economies,such as China and India, become stronger buyers of fossil fuels.This situation is again becoming a driver to improve coalquality through cleaning procedures and more efficientcombustion processes for the aforementioned technologies.1,2

Our research group has taken a new approach that deals withseveral of the limitations of CWS fuels, by means of themanufacturing of a mostly colloidal suspension of petcokeparticles dispersed in water. The latter characteristic makes agreat difference regarding fuel reactivity and stability. Colloidalparticles have a total surface area many times higher than theparticles present on conventional CWS. Chemical reactions,such as oxidation (combustion) or sulfation, occur on the

surface of solid fuels or minerals. Large increases in surface areaimply higher reaction efficiencies.3!6 A second and alsoimportant aspect is the effect of particle size reduction insuspension stability. Increasing the colloidal fraction slows orprevents altogether sedimentation,3,5 one of the main bottle-necks of CWS in the past.In a previous paper,3 we presented the results of the

preparation and combustion characteristics of a colloidal coal-in-water suspension used as a reburn fuel. As expected, thecolloidal nature of the fuel greatly improved the reburnperformance, outdoing a higher rank pulverized coal and, undersome operational conditions, approaching natural gas behaviorin the same conditions.3 In this work, we present thecombustion results of a colloidal petcoke in water suspension(CPW) used as the sole fuel in a small boiler simulator. Thefuel was prepared in a similar way as the coal suspensiondescribed in our previous publication.3

Petcoke usually has an ash content lower than 1%. A fuelhaving less than 1% ash could be fired, in principle, in a boilerdesigned for HFO, as long as the fuel has combustioncharacteristics similar to or better than fuel oil. However, firingonly petcoke is next to impossible in most conventional boilers,without co-firing with a suitable (supporting) fuel. This fact isattributable to the low reactivity of petcoke related to the lowvolatile content, which is inferior to most commercial coals.Nonetheless, petcoke is an attractive fuel from the point of viewof its cost per heat value (BTU) and from its marketavailability, although its combustion characteristics haverelegated it to mostly being fired in cement furnaces, fluidizedbeds, and other specially designed boilers or being co-fired withother fuels.

Received: July 26, 2012Revised: October 23, 2012

Article

pubs.acs.org/EF

© XXXX American Chemical Society A dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXX

This paper is an account of the main combustion test results,obtained in the 6330 MJ/h pilot-scale small boiler simulator II(SBS-II) located at the Babcock and Wilcox Research Center(BWRC), at Barberton, OH. The SBS-II tests simulated a full-scale utility boiler in many key aspects, such as flame stability,combustion efficiency, and furnace and convection pass heat-transfer characteristics under various operating conditions. Italso included assessing emission characteristics of CPWcombustion, including CO and NOx, which are highlydependent upon combustion parameters, such as in-flameoxygen availability, fuel!air mixing, the nitrogen content offuel, and other factors. For this purpose, about 3000 gallons ofCPW fuel were manufactured in a pilot plant in a semi-batchfashion. Combustion tests of a typical HFO were carried out toproduce baseline data for comparison to the CPW perform-ance.The main purpose of these tests was to collect relevant and

scalable data for commercial power generation.

2. EXPERIMENTAL SECTION2.1. Fuel Preparation and Handling. The petcoke was supplied

by DTE Energy Services (Vicksburg, Mississippi) and previouslymilled to !200 mesh. Table 1 shows some of the most relevantproperties of this fuel-grade petcoke sample.

The manufacturing process was batch-like and would begin by thepreparation of a pre-slurry made of petcoke and a surfactant mixturesolution. Both phases, dry solid and surfactant solution, werecombined by means of a turbine style mixer. The surfactants wererequired to both wet the particles and detach bubbles from thehydrophobic petcoke surface. In this sense, a nonylphenol ethoxylated(NPE) surfactant was used as a wetting agent [0.25% (w/w) based ondry petcoke], and a linear alcohol ethoxylated (LAE) surfactant wasused as a defoamer [0.05% (w/w) based on dry petcoke]. Thesesurfactants were selected after a screening of several commoditysurfactants; the selection criteria were based on the required pre-slurryproperties: viscosity below 1000 mPa s and no foam. The NPEsurfactant was supplied by Oxiteno, and the LAE was supplied byShell. Tap water was used to prepare the surfactant solution.The final composition of each 250 kg pre-slurry batch was on

average 61.5% petcoke. Increasing composition beyond 62% produceda viscous suspension (more than 1500 mPa s) that was not adequatefor further processing.Each pre-slurry batch was pumped into a continuously stirred tank

to avoid sedimentation that otherwise was very fast, taking less than 1h to produce severe settling. Once a 750 kg load was completed, thepre-slurry was processed continuously using a wet-comminutionapparatus (previously described in ref 3). The flow rate was 3 ton/h ofpre-slurry, and the residence time in the device was 5 s. Thetemperature of the output fluid went up to 60 °C in steady-stateconditions because of friction heating, and in consequence, the watercontent diminished slightly (less than 0.5%) because of partial

evaporation. On average, the final petcoke content in the suspensionwas about 62% (w/w).

A total of 12 tons of CPW fuel were manufactured in this fashionand later shipped to BWRC facilities in standard 300 gallonintermediate bulk containers (IBCs). To follow-up the CPWproperties during storage, a couple of samples were kept in one 10gallon container and one 55 gallon drum. As mentioned above, moredetails on the preparation method and a description of the wet-comminution apparatus can be found elsewhere.3

2.2. Evaluation of the Physical Properties. During manufac-ture, the particle size distribution of the petcoke slurry (before wetcomminution) and CPW was measured using a laser diffractionapparatus made by Microtrac. The measurement procedure was asfollows: A small portion of sample was dispersed in 10 mL of sodiumpolyphosphate (0.25%, w/w) and sodium lauryl sulfate (0.75%, w/w)solution. About a third of this suspension was poured into theMicrotrac mixing tank before circulation through the measuring cell. Itis worth mentioning that, while the dry petcoke and petcoke slurrywere relatively easy to measure and results were quite reproducible,CPW particle size characterization was less reproducible. Wet-sievingthe aforementioned solution in a 600-mesh standard sieve (20 μmopening) was used to validate the Microtrac results, and it was foundconsistently that sieving gave way to less mass retention than the laserdiffraction apparatus, although the difference was on the order of 2 or3 mass % below 20 μm.

A gravimetric method was used to measure the water content; aweighed sample of slurry or CPW was placed in an oven and let to dryat 120 °C for at least 6 h. After cooling, the dry sample was weighedand the amount of water lost during the tests would allow us tocalculate the initial moisture content.

A moisture analyzer made by Arizona Instruments, Computracmodel, was used to measure the water content of CPW during thecombustion tests. A Brookfield DV-II+ Pro viscometer, set with a RV3spindle rotating at 100 rpm, was also used to monitor viscosity duringthe tests. The viscosity reading was made 10 s after the start of thespindle’s rotation.

A KinexusPro rheometer, manufactured by Malvern, was used toobtain steady-state flow curves for the CPW. The measuring geometrywas a parallel plate system, 40 mm in diameter and set to a 1 mm gap.

The ultimate analysis and other relevant properties for both CPWand HFO fuels are shown in Table 2.

2.3. Combustion Tests. The first objective of this work was tocorroborate that increasing the colloidal size population would resultin enhanced combustion properties. The second goal was to evaluatecombustion and emission characteristics of CPW to provide relevantand scalable data that would demonstrate the feasibility of using CPWas a fuel for power generation stations. To accomplish these goals, wedecided to do the necessary tests that would (i) demonstrate stablecombustion using CPW fuel at full load (6300 MJ/h) without the use

Table 1. Physical Properties of Fuel-Grade PulverizedPetcokea

petcoke properties typical values

carbon (%) 86hydrogen (%) 3.5sulfur (%) 6.5vanadium (ppm) <2400volatiles (%) 7!13moisture (%) <1high heating value (Btu/lb) 15000particle size (mesh) !200

aInformation provided by DTE.

Table 2. Comparative Chemical and Physical Analyses ofHFOa and CPWb

HFO CPW

carbon (wt %) 87.84 51.6hydrogen (wt %) 9.70 2.10nitrogen (wt %) 0.37 1.20oxygen (wt %) 0.00 0.81sulfur (wt %) 1.92 3.90moisture (wt %) 0.10 40.15ash (wt %) 0.031 0.24high heating value (Btu/lb) 18152 9000viscosity 393 SFS at 50 °C 500!900 cP at 21 °Cspecific gravity at 15.5 °C 1.107 1.21flash point (°F) 221aHFO supplied by New Brunswick Power. bTest analysis carried outat BWRC Laboratories.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXB

of any support fuel and (ii) compare CPW- to HFO-firingperformance.In particular, the following tests were carried out: valuation of the

CPW flame stability and shape under various operating conditions andestimation of combustion efficiency, furnace, and convection pass heat-transfer characteristics also under various operating conditions. COand NOx emissions were characterized during the tests; these gases areclosely related to combustion efficiency (CO) and oxygen availabilityat the flame and flame temperature (NOx).2.3.1. Description of the Combustion Facility. The so-called SBS-

II was used for the pilot-scale tests. This facility belongs to the BWRCand was contracted for this evaluation. The SBS-II is capable of firinggaseous, liquid, and pulverized solid fuels. It is a newly constructed andtotally integrated facility for fuel evaluation, testing, and developmentof fossil fuel combustion hardware and emission control equipment,under commercially representative conditions. To carry out tests forCPW, the furnace was equipped with a single wall-fired XCL low-NOxburner and one I-Jet atomizer. The existing 6330 MJ/h low-NOx XCLburner was modified to accommodate the atomizer positioned alongits centerline. It operates on the principle of controlled mixing of theoxidizer and fuel to minimize NOx and unburned combustibles. Mostof the combustion air was swirled by spin vanes inside the twoconcentric annuli. Burner vane adjustment was the primary way ofadjusting near burner secondary air swirl, to control the fuel!airmixing pattern. The latter was necessary to maintain flame stabilityand, at the same time, control NOx emissions.A small fraction of the combustion air was introduced into the

central core zone, primarily for cooling purposes. The burner wascomprised of a smaller than standard burner throat to enable high exitvelocity of secondary air when operating at full load. This ensured thatthe desired fuel!air mixing pattern was still maintained for flamestability and NOx control under deep-staging operating conditions.The twin-fluid, I-jet atomizer used is a BWRC design. It has four

discharge holes drilled in the spray cap at inclined angles. The use ofcaps with different discharge hole sizes allows for handling various fueland airflow rates to enable atomization air/fuel ratio (A/F)adjustments for both HFO- and CPW-firing tests.The spray angle was generally small compared to the atomizers used

in full-scale units because of the single burner configuration and therelative narrowness of the furnace in the SBS-II. For all of the tests, theatomizer was affixed in such a way that the endcap was flush with theburner exit. A 60° open-blade swirler was mounted behind theatomizer to introduce swirl to the core air and to stabilize the flame. Ahigh-velocity thermocouple (HVT) probe was used for the measure-ment of the furnace exit gas temperature (FEGT).Air and flue gas flow rates were measured with calibrated Venturi

flow meters, corrected for process temperature and pressure and gascomposition (molecular weight). There were gas analyzers for O2,NOx, and CO in the convection pass area and a stack gas analysissystem for O2. After filtering and drying, flue gas composition, inparticular, CO, O2, and NOx concentrations, was measured andrecorded by calibrated analyzers. There was an oxygen concentrationin situ analyzer (wet basis) at the convection pass outlet. Themeasurement principles of the analyzers are as follows: the moisturedevice was a MAC Instruments MAC 125 analyzer (proprietary solid-state process); the measuring probe of the in situ O2 (wet basis)analyzer was a zirconium oxide sensor; the measuring probe of the insitu O2 (dry basis) analyzer was a paramagnetic device; CO wasmeasured by the non-dispersive infrared (NDIR) technique; achemiluminescence probe was used for NOx analysis.Particulate sampling was performed on a horizontal duct at the

convection pass exit using a BWRC custom-designed and proprietary,high volume sampling probe, where flue gas was extracted through sixnozzles along a penetrating tube pseudo-isokinetically. Particulatescollected on the filter paper were analyzed later to determine loss onignition (LOI) or unburned carbon.The flame shape was monitored by means of a FlameView camera.

The latter is able to generate two-dimensional temperature maps fromthe live flame image using the two-color pyrometry principle. A LAND

two-color pyrometer was also used to perform single-point flametemperature measurements at the near burner zone.

2.3.2. Fuel Handling System and Test Procedure. The fuels underscrutiny, first, No. 6 fuel oil (HFO) and, subsequently, CPW, weretransferred to the 300 gallon day tank prior to testing. The tank wasrestocked during tests as needed. HFO was preheated overnight in thetank using electrical belt heaters. The HFO final delivery temperature(74 °C) was adjusted by heat tapes installed outside the pipes. CPWwas stored, pumped, and atomized at room temperature.

A Moyno progressive cavity pump delivered both fuels to theatomizer/burner. The fuel delivered to the XCL burner was atomizedby plant compressed air metered by calibrated orifices using the I-Jetatomizer positioned along the centerline of the burner.

Each test began with a 2 h warming period that involved heating thecombustion air using a trim heater and heating the entire furnace byfiring natural gas through the igniter. Once suitable furnace conditionswere reached, liquid fuel was introduced gradually to the burner,atomized, and ignited by the natural gas support flame. The natural gasheat input was gradually reduced and eventually shut off as the desiredfurnace load was approached. The furnace was then allowed to warmfor at least 1 h more until the convection pass exit temperature and gasspecies concentrations reached steady-state levels before test startup.Computerized data acquisition was used for monitoring and recordingfurnace-operating conditions, such as the flow rate, temperature,pressure, and species composition at various locations. Flue gascomponent concentrations, in particular CO, O2, and NOx, weremeasured continuously at the convection pass exit. Visual observationsof the flame were performed and documented for every test andthroughout the intermediate adjustment.

FEGT and LOI measurements were conducted on selected tests forboth HFO and CPW fuels. During the measurement, the HVT probewas placed perpendicular to the flue gas flow to obtain temperaturemeasurements at five different locations. The collected fly ash sampleswere analyzed for unburned carbon using a modified American Societyfor Testing and Materials (ASTM) D6316 method. It was convertedto combustion efficiency on a heating value basis during data analysis.

Combustion tests were performed at various operating conditions.Test variables were selected to best address the objective via variationsof the main and support fuel heat input, burner stoichiometry ratio,burner swirl, atomizer spray angle, and atomizer A/F. Qualitativeobservation of the flame shape and stability were made during eachtest. It is worth mentioning that there was no clogging of the atomizertip while firing CPW.

3. CPW PHYSICAL PROPERTIES3.1. Particle Size and Stability. The Sauter mean

diameter or D(3,2) of dry petcoke before processing wasabout 15 μm, and as mentioned in section 2, the petcoke pre-slurry would settle almost immediately. The CPW Sauter meandiameter after wet comminution was about 5 μm and was verystable to sedimentation.In fact, the sedimentation pattern was very similar to the

pattern reported for a colloidal coal in water suspension.3

Within 24 h of preparation, a thin, millimeter-sized layer ofclear water would appear at the top; this layer thickness wasindependent of the container size and was likely due to somecondensation of water after manufacturing and storing. Duringthe first 2 weeks, no changes would be observed and the fluidwould be homogeneous from top to bottom. Then, during thenext 10 weeks, the clear water layer would increase in size,reaching a thickness equivalent to about 5% of the total sampleheight, while the suspension underneath would become moreviscous but without a well-defined (bottom) sedimentationlayer. Sample homogenization would be relatively easy bymixing a few minutes with an overhead stirrer.At 6 months after preparation and once the totally clear

water layer had reached a stable thickness (about 15% of total

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXC

height) in the 55 gallon container, samples of the suspensionwere taken at the top, middle, and bottom of the drum and theparticle size was virtually the same at all levels. Hence, thesedimentation behavior can be described as structure-shrinking,while water diffuses to the top or compaction. Notwithstanding,CPW is remarkably more stable than conventional slurries thatrequire additional compounds, such as polymers in solution, toreduce sedimentation.This behavior stems from the microscopic morphology of the

fluid: it can be viewed as a pseudo-fluid consisting of a largecolloidal fraction, in which a spectrum of sizes of non-colloidalparticles is suspended. Under this perspective, the colloidalpseudo-fluid is a “stiffened” phase (strong colloidal interparticleforces) that prevents the settling of larger particles. Thisconcept was first advanced by Probstein7 and was alsodiscussed in our previous publication.3

3.2. Rheological Behavior and Fuel Handling. Asmentioned in section 2, the rheological behavior of the CPWwas evaluated using a rotational rheometer set with parallelplates and a gap wide enough to minimize the particleinteraction with plate walls (1 mm). Steady-state flow curveswere carried out in a wide shear rate range (0.1!1000 s!1) andat 25, 35, and 45 °C. Apparent viscosity as a function of theshear rate and temperature is plotted in Figure 1, and as shown,

viscosity increases with temperature. The material is alsoincreasingly shear-thinning with the temperature, and thecurves tend to converge at high shear. Plotting shear stress as afunction of the shear rate allowed for a clear reading of yieldstress, which was virtually 0 Pa at 25 °C, 60 Pa at 35 °C, and350 Pa at 45 °C.These results point to complex particle interactions at the

molecular and microscopic level; this phenomenon have beenobserved before in coal-in-water slurries.8,9 One plausibleexplanation of the observed behavior is as follows. It is well-known that non-ionic surfactants, such as the surfactants usedto prepare the slurry, become less hydrophilic as temperatureincreases. This effect makes the petcoke particles morehydrophobic as temperature increases, thereby inducing astronger attraction between the particles. This promotes theformation of a stiffer microstructure that, nonetheless, crumblesat high shear. In fact, this effect is reversible in the sense that asample experiencing heating and cooling cycles recovers itsformer lower viscosity when cooled.The macroscopic effect of structuring is observed at the

lowest shear rates; yield stress and viscosity augment about 10-fold each 10 °C change of temperature increase. As the shearrate is increased, the microstructure is gradually destroyed andviscosity is rapidly reduced. Although we could not measure

beyond 1000 s!1, it can be conjectured that the flow curves mayintersect at higher shear rates and the usual behavior ofviscosity versus temperature may be reached.On the other hand, it is also possible that the curve

convergence is due to a slip at the wall, a phenomenonpreviously observed in a colloidal coal-in-water suspension.3

The slip at the wall is produced by migration of the continuousfluid to the wall of the moving disk, forming a so-calledlubricated layer. The formation of this layer marks the onset ofslip that is predominantly a hydrodynamic phenomenon.During the combustion tests, a viscosity index of the fuel was

monitored at room temperature using a Brookfield viscometerrotating at 100 rpm. This type of viscometer is not a rheometerbecause the stress field is not known as a result of the highcomplexity of the flow; in consequence, the viscosity readingcan only be used as a relative ranking of flow characteristics.10

The viscosity index during the tests would vary from 1400 to1800 cP, 10 s after rotation onset. The fuel was discharged intothe day tank and was recirculated before every combustion test.Further, the material was pumped and atomized without anydifficulty, such as plugging by sedimentation buildup.

4. COMBUSTION PERFORMANCE4.1. Baseline HFO Tests. Baseline characterization tests

were carried out by firing No. 6 HFO to obtain reference datafor comparison to the CPW fuel. When burner spin vaneangles, core air flow, and optimum burner settings wereadjusted, attached stable flames were achieved at full load withnominal 3% (v/v) excess oxygen in the convection pass.The effects of burner stoichiometry and fuel heat input

variations were examined in subsequent tests. A fly ash samplewas taken at the optimum condition, with the burnerstoichiometry ratio close to unity. Because of the low ashcontent of the HFO, it was difficult to collect fly ash samples forevery test. FEGT measurements were performed for every testto characterize furnace side heat transfer. Selected data arepresented in later sections in comparison to CPW test results.

4.2. CPW Tests. Performance and emission characteristicsof CPW combustion were evaluated systematically via para-metric testing. Generally speaking, a low-volatile and high watercontent fuel is difficult to ignite, maintain a stable flame, andburn out completely. However, in this case, it was relativelyeasy to achieve good combustion performance, using theexisting burner and atomizers. The startup and shutdownprocedures for both CPW and HFO firing were similar. In thecase of CPW, the fuel temperature was not a concern duringstartup because CPW was fired at room temperature and noheating was required.Firing liquid fuels, including CWS, in commercial boilers

starts by injecting the fuel into the hot furnace in the form of afine spray to increase its surface area and promote fast heatingof the fuel droplets, quickly releasing any moisture and volatilematter associated with the fuel. The latter reaches ignition atsome point, further heating and igniting the remaining fuelresidue (char) until complete burnout, provided that it remainslong enough inside the hot furnace, before reaching the furnaceexit.Steam-assisted atomization using internal mix atomizers is

the preferred method for generating a fine spray in commercialHFO boilers. It also became the norm in CWS firing. Air is avalid alternative and is the fluid assisting atomization in thiswork.

Figure 1. Apparent viscosity as a function of the shear rate andtemperature of CPW. Measurement using a rotational rheometer andparallel plates.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXD

There are other forces at work in CWS or CPW that opposethe breaking of the liquid into a fine spray, not encountered inHFO atomization. These are complex particle!particle andparticle!liquid interactions, which are further affected by thepresence of a surfactant. In addition, CWS rheological behaviorseems to be more important than viscosity during air- or steam-assisted atomization.11 CWS prepared from a fine grindrequires a larger steam/CWS mass ratio to produce the samedroplet size than CWS from a coarser grind.11 Highlyconcentrated CWS also gives coarser sprays than lessconcentrated CWS, keeping all other conditions the same.Steam or air at sonic velocities at the atomizer tip outlet ports isdesirable for the best CWS atomization results.11

A CWS or CPW droplet, suddenly subjected to fast heatinginside the boiler furnace, goes through the above-mentionedsequence: (i) moisture evaporation, (ii) devolatilization,accompanied by swelling and particle agglomeration in somecaking coals (not the case for petcoke), (iii) volatilecombustion, and (iv) char combustion.Step i is longer for CWS and CPW (because of moisture

evaporation) than for the parent dry pulverized petcoke,therefore delaying step ii, and both steps overlap. This effectextends the parent fuel heating time to get to step iii.CPW droplets, once subjected to the hot furnace environ-

ment, reach the water boiling point quickly, remaining at thistemperature until all of the liquid water is consumed. The waterevaporation time is directly proportional to the dropletdiameter squared. Besides being subjected to high heat transferby radiation, convection-enhanced evaporation from the hot gasstrong recirculation toward the flame root also helps to increasethe water evaporation time and, consequently, the flamestability. It follows that it is reasonable to expect that high airswirl should benefit flame stability during CPW firing.Coal particle agglomeration leads to larger, longer burning

chars, and this is responsible for high levels of unburnt carbonin ash when firing coal in HFO-designed boilers. This is muchless of a problem for the non-caking petcoke particles.Therefore, fine grinding, as in CPW, must help to reducechar size and burning time. Colloidal petcoke particles take charsize and burning time to lower values beyond the values foundin conventional CWS or pulverized coal. These two factorscontribute to flame stability and low levels of unburnt carbon,within the in-furnace residence time of about 1 s, for HFO-designed boilers.Another factor that helps CPW and to a less extent CWS to

reduce the unburnt carbon values is through the reactionbetween steam and char, as shown in eq 1.

+ → +C H O CO H(s) 2 (g) (g) 2(g) (1)

This reaction is more relevant for the water-based slurries thanfor pulverized coal combustion,12 and it becomes importantabove 1000 °C, which is a value well within typical furnacetemperatures. It takes place simultaneously with otherreactions, such as

+ →C O CO(s) 2(g) 2(g) (2)

+ →C CO 2CO(s) 2(g) (g) (3)

A detailed account of the parametric testing results is presentednext.4.3. Support Fuel Minimization at Full Load. A

conservative approach was favored for this activity. A flowrate equivalent to 3200 MJ/h of natural gas was used to assist

the CPW first-ignition attempt. The gas load was graduallyreduced accompanied by the gradual increase of the CPW flowrate. During this process, burner vane adjustment wasperformed to keep the flame attached and stable. The burnersettings were 20° and 60° open for inner and outer vanes,respectively. The CPW flame stayed stable and attached whenthe natural gas fuel was cut off completely, which happened atthe very first CPW firing test without requiring any processvariable adjustment. The following patterns were observedduring the first test with CPW firing at full load (6200 MJ/h).CPW produced a relatively translucent flame compared to a

typical coal and HFO counterpart. The radiation emission inthe visible spectral region was weak, although no quantitativemeasurements were conducted. This unique flame appearancewas unexpected for petcoke. It seems to be a distinct feature ofthis new fuel (see Figure 2).

A minimal amount of sparkles was observed at the near-burner zone for this test (and most other tests). No sparkleswere observed at the horizontal furnace section, leading to theconvection pass. In the absence of a quantitative evaluation, thisobservation presents evidence on the ease of atomization andrapid combustion of the CPW fuel. This is most likely relatedto the colloidal-sized particles present in the fuel.The CPW flame temperature showed a slight increase with

the distance from burner throat. The temperature moved to1336 °C at 180 cm from 1312 °C at 20 cm from the burner.However, it was still lower than the HFO temperature (1565°C on average) because of the high water content.In contrast, FEGT measurements indicated that CPW

combustion exhibited heat absorption at the furnace sectionsimilar to HFO combustion. At the outlet of the convectionpass, flue gas temperatures were also similar between CPW-and HFO-firing cases, as shown in Figure 3. This figure depictsFEGT as a function of the distance to the furnace wall for thesame burner stoichiometry ratio (BSR) value (0.85). Asexpected, both curves for CPW and HFO tend to decrease asthe furnace wall is approached. The observed behavior is anindication that convection pass heat absorption characteristicswere similar under the two firing modes. Further, this is also anindication that the increased surface area of CPW is helping toimprove combustion characteristics.The same patterns were observed for 75% of full load. At

50% load (3150 MJ/h), the flame became unstable.

Figure 2. Photograph of the CPW flame at 5900 MJ/h, BSR of 0.85,and A/F ratio of 0.19.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXE

4.4. Burner Optimization Tests. Parametric tests werecarried out to assess the impact of burner vane settings onflame stability and emissions with 100% CPW fuel without anynatural gas support. In general, the inner vane adjustment hadgreater effects on enhancing flame stability than the outer vanesbecause of the more intense swirl generated near the fuelinjection point. However, excessive swirl, especially on theouter flame zones, could cause the flame to flare and flamelength to shorten undesirably.Figure 4 shows comparisons of (a) CO and (b) NOx

emissions of CPW combustion with five different burner vane

settings. The next paragraph gives an outline of theobservations and measured data.Flames were detached from the burner throat when the inner

vanes were set at a 30° opening (first two conditions in Figure4). Flame attachment and stability were significantly improvedwhen the inner vanes were set at a 20° opening (last threeconditions in Figure 4), although with the outer vanes set to65° or more, the flame was detached. With 20° ! 50° openingsettings, the flame was flared because of excessive swirl

introduced by the burner. Overall, 20° ! 60° setting producedthe best flame attachment and length and was used for allsubsequent tests.In comparison to the optimal burner vane setting for HFO

(30° ! 75° open), more swirl was needed to stabilize the CPWflame. However, stable CPW combustion was achieved on thesame burner with no hardware modifications. Further, burnervane settings had little impact on CO emissions (Figure 4),while NOx emissions did vary.

4.5. Atomizer A/F Optimization Tests. CPW atomizationusing steam is commercially more acceptable, but given the lowfuel temperature and high fuel water content, air was used asthe atomization medium. Air was also used for HFOatomization for comparison purposes. From a commercialapplication point of view, there is an interest in reducingatomization gas usage and, therefore, lowering plant operationalcost. We conducted tests to compare combustion and emissionperformances to various atomizer A/F ratios and two atomizers.Figure 5 recaps the combustion efficiencies and major

emission data at various A/F ratios for CPW. Two

configurations of discharge hole sizes (configuration 1, 0.175design A/F; configuration 2, 0.25 design A/F) and similar 50°spray angle were used to vary A/F ratios.Combustion efficiencies were not greatly affected by the

variations in the atomizer A/F ratio, although flames wereslightly detached at the lower ends of the A/F ratio range forboth atomizer configurations. Atomization performance mightbe less than ideal in these cases, but particle burnout was stillexcellent probably because of the increased surface areaassociated with the colloidal-sized particles. This indicatesthat the particles in a single droplet did not burn as anagglomerate during devolatilization and char burnout. It is

Figure 3. FEGT as a function of the distance to the furnace wall for aBSR value of 0.85.

Figure 4. CPW combustion emissions at various burner vane settings.(a) CO emissions in ppm by volume of dry gas and (b) NOx emissionsin ppm by volume of dry gas. Tests conditions: 6300 MJ/h, BSR rangeof 0.92!0.95, and nominal 3% (v/v) excess O2 at the convection pass.

Figure 5. CPW combustion and emission performances at variousatomizer mass A/F ratios. Tests conditions: 6300 MJ/h, BSR range of0.92!0.95, and nominal 3% (v/v) excess O2 at the convection pass.Configuration 1, 0.175 design A/F ratio; configuration 2, 0.25 designA/F ratio. Atomizing angle at 50°.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXF

worth pointing out that CPW combustion efficiencies wereslightly better with natural gas support than without, althoughthe differences were within the measurement uncertainty.CO emissions remained in the range between 30 and 40 ppm

by volume of dry flue gas. Their independence to the atomizerA/F ratio is probably due to the reasons mentioned above.Even though higher atomization air flow for a given atomizerdesign produces more fine droplets that are faster to evaporateand combust, no clear trend was observed for NOx emissionswith the atomizer A/F ratio.This indicates that atomization adjustment played a

secondary role to burner stoichiometry ratio adjustments, interms of NOx control. However, NOx formation tends toincrease when the flame starts to detach, probably because ofthe less effective control of air/fuel mixing at the near burnerzone, in line with Curtis findings.13

5. RESULTS AND DISCUSSIONThe calculated boiler efficiency losses were 17.2 and 13.5% forthe CPW and HFO cases, respectively (or 3.7 percentage losswith respect to HFO). Relative to the HFO-firing data, thegreater boiler efficiency loss for CPW firing was primarily dueto the high moisture content in the flue gas leaving the boiler,which is related to the water present in the CPW fuel. However,there is a certain boiler efficiency gain associated with theincreased flue gas mass and velocity, inherent to CPW firingcompared to HFO, at the same heat input, which increases tosome extent the convection heat transfer to the tube banks ofthe boiler. The measured boiler efficiency is the net result ofthese two counteracting effects.The CPW high combustion efficiency and outstanding

handling characteristics allows for the consideration of CPWas a practical fuel with commercial applications. This opens upthe possibility of using petcoke in commercial boilers designedto fire HFO, without any additional fuel support, even atreduced thermal inputs (75%).The magnitude of the CPW outstanding combustion

performance is better grasped and quantified by the datadepicted in Figure 6. The graph shows actual combustionefficiencies [for dry and ash-free (daf) coal] presented byJuniper and Pohl,14 displaying data obtained from firing coal,for a wide range of volatile matter content, including full-sizeboilers and combustion pilot plants from Australia and theU.S.A.

It should be noted (Figure 6) that there is no data for full-size boilers using coal having a volatile matter content below25%. It is also shown how fast carbon burnout (in pilot plants)deteriorates as the coal volatile matter decreases below 25%.This is the result of the difficulties associated with maintaining astable flame, using coal of volatile content below this level.Carbon burnout values (at 3% excess O2) from the CPW

combustion tests at BWRC were included in the graph (●),showing a completely different behavior from previousexperience, pointing to the fact that colloidal petcoke inwater suspensions should be considered a new category of fuelshowing outstanding combustion properties. This improvedCPW carbon burnout values are the result of the much higherpetcoke colloidal particle count and, consequently, higherparticle surface area in CPW, which translates into an enhancedparticle reactivity. These particles are the particles responsiblefor the much improved flame stability, which likewise directlyleads to elevate the CPW carbon burnout values, as comparedto conventionally pulverized petcoke or coal particles of similaror even higher volatile matter content.

6. CONCLUDING REMARKSIn this work, we have advanced results concerning thecombustion behavior of a new material. The technologypresented here essentially maps solids fuels, such as petcokeand coal, into a gas from the combustion standpoint. This isachieved by taking the specific surface area available forcombustion to values typical of the colloidal domain withoutpaying a great energy expense. Further, it should be said thatthe mapping in question must be made in the presence of waterto prevent the colloidal particles from becoming airborne. Theresulting material is a liquid that flows appropriately at roomtemperature and is thus amenable for power generation inboilers and low-speed diesel engines at expenses that arefractions of currently used fuels, such a HFOs.

! AUTHOR INFORMATIONCorresponding Author*E-mail: briceno@nanodt.com.NotesThe authors declare no competing financial interest.

! ACKNOWLEDGMENTSWe are grateful to DTE Energy for supplying the petroleumcoke for this test. We also acknowledge the support of NDT’spersonnel in manufacturing the samples.

! DEDICATIONTo our knowledge, this is the last contribution advanced byDaniel D. Joseph before his passing on May, 2011. We dedicatethis work to his memory.

! ABBREVIATIONSA/F = air/fuel ratioBSF = boiler simulation furnaceBSR = burner stoichiometry ratioBWRC = Babcock and Wilcox Research CenterCOS = coal!oil slurryCWS = coal!water slurryCPW = colloidal petroleum coke in water suspensiondaf = dry and ash freeFEGT = furnace exit gas temperature

Figure 6. Coal burnout results as a function of the volatile content(daf) from full-size boilers and combustion pilot-scale plants. (■, ◆,and ▲) Data as presented by Juniper and Pohl14. (●) Comparison topetcoke burnout and from this work at 3% excess O2.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXG

HFO = heavy fuel oilHVT = high-velocity thermocoupleLOI = loss on ignition

! REFERENCES(1) Khodakov, G. S. Coal!water suspensions in power engineering.Therm. Eng. 2007, 54 (1), 36!47.(2) Wibberley, L.; Palfreyman, D.; Scaife, P. Efficient Use of CoalWater Fuels; CSIRO Energy Technology, Australian Government’sCooperative Research Centres Program: Pullenvale, Queensland,Australia, 2008; Report 74.(3) Nu !n"ez, G. A.; Bricen"o, M. I.; Go !mez, C.; Joseph, D. D.; Asa, T.Colloidal coal in water suspensions. Energy Environ. Sci. 2010, 3, 629!640.(4) Huang, X.; Jiang, X.; Han, X.; Wang, H. Combustioncharacteristics of fine and micro-pulverized coal in the mixture O2/CO2. Energy Fuels 2008, 22, 3756!3762.(5) Cheng, J.; Zhou, J.; Li, Y.; Liu, J.; Cen, K. Effects of pore fractalstructures of ultrafine coal water slurries on rheological behaviors andcombustion dynamics. Fuel 2008, 87, 2620!2627.(6) Yu, D.; Xu, M.; Sui, J.; Liu, X.; Yu, Y.; Cao, Q. Effect of coalparticle size on the proximate composition and combustion properties.Thermochim. Acta 2005, 439, 103!109.(7) Probstein, R. F. Physicochemical Thermodynamics, 2nd ed.; WileyInterscience: New York, 1994; Chapter 9, p 298.(8) Roh, N. S.; Shin, D. H.; Kim, D. C.; Kim, J. D. Rheologicalbehavior of coal!water mixtures. Fuel 1995, 74 (9), 1313!1318.(9) Tsai, S. C.; Knell, E. W. Viscometry and rheology of coal waterslurry. Fuel 1986, 65 (4), 566!571.(10) Macosko, C. Rheology, Principles, Measurements and Applications;VCH Publishers: New York, 1994; Chapter 5, p 223.(11) Thambimuthu, K. V.; Whaley, H. In Principles of CombustionEngineering for Boilers; Lawn, C. J.. Ed.; Academic Press: New York,1987; Chapter: The Combustion of Coal!Liquid Mixtures, pp 391!392.(12) Hathl, V.; Ramezan, M.; Winslow, J. Industrial and Utility-ScaleCoal!Water Fuel Demonstration Projects; Pittsburgh Energy Technol-ogy Center, U.S. Department of Energy: Pittsburgh, PA, Jan 1993;Technical Report 93/4.(13) Curtis, J. S. Coal Particle Flow Patterns for O2 Enriched, Low NOxBurners; Purdue University: West Lafayette, IN, Sept 2004; FinalReport for the Department of Energy.(14) Juniper, L. A.; Pohl, J. H. Design of pilot scale rigs to simulatecombustion processes. Proceedings of the Australian Symposium onCombustion; Gawler, South Australia, Nov 1995.

Energy & Fuels Article

dx.doi.org/10.1021/ef301249q | Energy Fuels XXXX, XXX, XXX!XXXH