Analysis of Flue Gas in PCC

8/13/2019 Analysis of Flue Gas in PCC

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PULVERIZED COAL COMBUSTION: THE INFLUENCE OF FLAME

TEMPERATURE AND COAL COMPOSITION ON THERMAL AND FUELNO

D. W. PERSHING AND]. O. L WENDTDepartment o Chemical Engineering University o Arizona Tucson Arizona 85721

A laboratory combustor was used to investigate the factors that influence the conversionof fuel nitrogen in coal during coal combustion. Fuel NO was isolated by experimentationutilizing Argon/Oxygen/Carbon Dioxide mixtures as the oxidant, and care was taken tocompare cases with air at matched conditions. For both well mixed and slowly mixed flametypes, fuel NO contributed over 75 of the total NO emissions for all conditions examined.Fuel NO was insensitive to temperature changes except when the adiabatic flame temperatureswere above 2480 K (4000°F). At the highest adiabatic flame temperature, 2580 K (4200°F),a 10 increase in fuel NO was observed.

Four different coals and one coal char were investigated. Fuel NO could not be correlatedwith fuel nitrogen content alone, even though aerodynamic conditions were kept constant.Fuel nitrogen conversion to NO during pulverized char combustion was 12-16% at astoichiometric ratio of 1.15 compared to 28 for a pulverized coal of the same nitrogencontent. Furthermore, in contrast to the coal results, NO emissions from char combustionwere not greatly influenced by changes in injector design. The implication is that although

i f f l i t NO b l ti l l d i h h b i


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39 ENERGY PRODUCTION FROM COAL

and include a detailed discussion of the roleof burner hardware changes only insofar asthese changes allow the above phenomena tobe investigated for two widely different flametypes of practical interest. A detailed discus-sion on the important role of aerodynamicsand mixing on fuel nitrogen conversion fromcoal combustion is outside the scope of thiswork.

Although it has been postulated 4 that thetemperature dependence of homogeneous ox-idation of nitrogenous compounds is slight these results cannot be directly extrapolatedto coal combustion because both particle tem-perature and particle heating rateS influencethe fraction of fuel nitrogen that is devolatil-ized and the fraction that remains in the ch ar 6

during combustion. Indeed variations of thephysical behavior of coal due to differencesin type and rank might also be expected to

COAL DRUM

influence the fate of fuel nitrogen during coalvolatilization .

The fate of fuel nitrogen in char is of interestfor two reasons: first coal char is producedduring coal gaSification and can be utilizedby combustion to raise steam; second the charburnout regime is important during pulverizedcoal combustion and for this reason the roleof char nitrogen has been theoretically investi-gated .7 In this work we sought to determinehow much of the fuel nitrogen in char wasconverted to NO since that information wouldindicate whether it was desirable to modifycoal combustion conditions to alter the nitro-gen distribution between volatiles and char.

Our approach was experimental and utilizeda laboratory combustor which allowed a self-

sustaining pulverized coal flame to be main-tained. A methodology was developed whichallowed the relative level of thermal and fuel


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PULVERIZED COAL COMBUSTION 391

NO to be determined under essentially identical combustion conditions, and which allowed

the factors which influence fuel nitrogen conversion during coal and coal char combustionto be examined.

Combustion Facility

Furnace

The experimental furnace is illustrated inFigure 1. I t was designed to contain the salientfeatures of practical combustion hardware andyet be sufficiently well defined so that onevariable could be changed at a time . Thevertical combustion chamber was 1.86 m (76 )long and 15 cm (6 ) in diameter inside . The

overall outer diameter was approximately 69cm (27 ). Walls consisted of an outer steel shell,two layers of roll board insulation, and about25 cm (10 ) of insulating and high temperaturecastable refractories .

At the full load firing rate of 25 kw (85,000Btu/hr) the cylindrical combustion chamberprovided a nominal residence time of approxi

swirl air inlets and was similar to that usedin previous studies. 3 The axial air entered

through two angled ports into the center pipe .Swirl air entered a vaned swirl chamberthrough two tangential ports, 180 degreesopposed, and passed through curved swirlvanes. Percent swirl was defined as the ratio,in percent, of the volumetric flow of air introduced through the swirl vanes to the totalsecondary air volumetric flow rate. The burner

throat was water cooled and the exit was fittedwith a 30 degree refractory quarl with an L / Dratio of one.

The burner could accept two different fuelinjector types. The first contained three holesangled to distribute the coal away from theaxis of the furnace and was characterized asa rapid mixing injector because it produced

short bulbous flames. t was designed to besimilar to the coal spreader system employedin many commercial systems . The second in-jector contained a single center hole with anarea equal to that of the three holes in thedivergent injector. t produced relatively slowmixing between the primary and secondaryair streams and hence gave a long, very thin


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cooled stainless steel probe. During initial

shakedown the water-cooled stainless steelprobe was compared with both cooled anduncooled quartz probes. No difference in themeasured NO was noted, even with CO andunburned carbon present. t should be noted,however, that the flue gas had cooled to below8oo 0 K at the point of sampling and there wasalways at least 0.5 oxygen present in thesample.

Sample conditioning prior to the instrumentation consisted of a refrigerated dryer(water condenser), two glass wool filters, a60J .L stainless steel filter (283°K), a stainless/teflon sampling pump and a 7J .L stainlessfilter. All sample lines were 6.3 mm 0/4 )teflon and all fittings 316 stainless steel.

Fuels

Analyses of the four coals and the coal char'used in this study are given in Table II. TheColorado coal was the same coal used in a

and nitrogen content to be investigated. All

of the coals were pulverized to approximately75 through 200 mesh. The coal char originated from the FMC-COED coal gasificationprocess, and was subsequently ground to 70through 100 mesh.

Results

uel NO and Thermal NO

Fuel NO was isolated by using, instead ofair, a synthetic oxidant mixture containing 210Z 18 CO 2 and 61 Argon. This allowedtheoretical flame temperatures to be matchedbetween preheated (530 0 K) air and unpreheated Ar / 0 2 / C O Z cases at a stoichiometric ratio,SR, of LIS. Burner sleeves were available toensure that inlet velocities and therefore flowpatterns could also be approximately matched,but our preliminary tests indicated that theywere not required since small variations insecondary air velocity had negligible effect.


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appreciable chemical effect on NO formation.Comparison between total NO with preheated

ai F as the oxidant and fuel NO with the synthetic mixture as the oxidant was thereforeaccomplished under nearly identical conditions. Thermal NO is defined as the differencebetween total NO and fuel NO, on the assumption that thermal fixation of atmosphericnitrogen does not inhibit fuel nitrogen conversion. Data on fuel and thermal NO emissions

as a function of stoichiometric ratio are shownin Figure 2, for the Western Kentucky coalfor both the divergent and single hole injector.All NO emission data are reduced to zeropercent excess O 2 ; i.e. to stoichiometric air.The divergent injector data on total NO (6200Kpreheat, 45% swirl, 14 primary air and 21m / sec (70 ft / sec) throat velocity is consistent

in both magnitude and trend with other pilotand full scale data from wall fired units 9 andwas reproducible over many months testing.The data clearly show that under these conditions over 80% of the total NO is the resultof the oxidation of bound nitrogen in the fuel.Variations in primary air percentage, secondary air swirl and burner throat velocity did

not change this finding,9 and under all conditions examined, fuel NO contributed at least

75% of the total NO emissions.Data from the single hole injector (620 oKpreheat, 45% swirl, 8% primary air) show thatslow mixing drastically lowered total NOemissions. This- is in agreement with pilotdata 10 and field data on tangentially firedunits. However, it is clear that this dramaticreduction was due to a decrease in fuel NO

emissions which again comprised approximately 80% of the total. Thus, although achange in mixing significantly altered totalemission levels, the dominant NO producingmechanism in all cases was still through fuelnitrogen oxidation.

Although the data shown on Figure 2 arefor the Western Kentucky coal they are typical

of the results obtained for the Colorado coal,the Pittsburgh coal and the subbituminousMontana coal. 9 In all cases, under typicalcombustion conditions in our system, fuel NOcontributed over 75% of the total NO emissions.

verall T t d


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FIG. 3. Temperature depe ndence of thermal and fuel NO: Western Kentucky and Colorado coals (15excess air; divergent injector)

TABLE IIIExperimental conditions-Figure 3

ture range of 1285°K through 1450°K. Aboveapproximately 2550 oK however fuel NO


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Analysis of Flue Gas in PCC

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