Carbon Materials for Advanced Technologies Episode 7 ppsx

Effect o f blending coal-derived pitches from WVGS 13421 on elemental composition daf Table 10.. Elemental composition of products from WVGS 13423 NMP-soluble extract 75 25 wt% 450°C hy

Table 9 Effect o f blending coal-derived pitches from WVGS 13421 on elemental composition (daf)

Table 10 Elemental composition of products from WVGS 13423

NMP-soluble extract 75 25 wt% 450°C hydrogenation

Only a limited number of coal-derived pitches were examned by 'H NMR because of their low solubihty in solvents commonly used 111 convenbonal proton magnetic resonance Table 12 reports the distribubon of hydrogen for three of the pitches Unlike coal-tar pitches, which typically have over 85% of the hydrogen bonded to aromatic carbon, the matenals listed in Table 12 are characterized by a high content of aliphabc hydrogen

22 1

Table 11 Characteristics of NMP-soluble pitches from WVGS 13421

+ b y thermal mechanical analysis, * by differential scanning calonrnetry

Table 12 'H NMR characterlzation of coal-derived pitches

of 400 and 500 These values are only slightly higher than those for commercial pitches Moreover, the wdth and shape of the molecular weight distribution curves for the coal extracts are generally simlar to those for commercial pitches

2 3 Ash reduction in coal-derivedpitches

One of the more lmportant considerations in d e t e m m g the end use of synthetic graphte is its contamination with metallic components Metals such

as iron, vanadium, and especially in nuclear applications, boron are deleterious

to the performance of graphte Table 3 presented the extrachon yields of NMP- soluble material for three biturnnous coals For these coals, rmneral matter and insoluble coal residue were separated from the extract by simple filtration through 1-2 pm filter paper Table 13 lists the high-temperature ash content m the dry coal, and m their correspondmg NMP-insoluble and NMP-soluble products The reduced ash content of the extract is typically between 0.1 to 0.3 wt% using traditional filtration techniques for the small-scaled extraction experments

To reduce the quantity of ash in the extracts even further, steps were implemented using a sequential solids removal scheme that entailed a combination of centrifugation and filtration Following extraction of the coal

Table 13 Ash content in WVGS coals and coal products

with NMP, the mixture was placed in a centrifuge during which time the particulates were subjected to 2000 G for two hours The supernatant liquid was removed from the solids by simply decanting and then filtering through 1-2 pm filter paper The resulting filtrate was centrifuged again for an extended period

of time (overnight) at 2000 G Again the supernatant liquid was separated by decanting and then filtered through 0.2 pm filter paper Table 14 shows the results using the above process for WVGS 13421 coal where it can be seen that significant ash reduction is possible

Table 14 Results of de-ashlng experiments using centrifugation and filtration of WVGS

Table 15 Ash content of WVGS 13421 and its products

Raw coal NMP-extract Residue Green coke Calcined coke

223

coal by centrifugation alone at 2000 G for 60 minutes without the use of NMP

The supernatant liquid was decanted and dried under vacuum at 150°C Table

16 lists the yield of products and their ash contents

Table 16 Ash content of raw coal and their hydrogenated coal products, wt%

Supernatant liquid Insoluble solids

The coal-derived pitch precursors for W W - 1 , W W - 2 , and W W - 3 test graphites were de-ashed by the combined centrifugation and filtration method while all of the other pitches were de-ashed by centrifugation alone (2000 G, 90

minutes)

3 Preparation and Characteristics of Cokes Produced from Solvent Extraction

3.1 Preparation of green and calcined cokes

Two reactor types were used to convert coal-derived pitches into green coke A

heavy, carbon-steel pipe (about 0.75 m long by 5 cm inside diameter) was machined at both ends such that plugs could be inserted to seal the system The coking reactor was filled approximately 213 full with pitch, flushed with nitrogen, and then sealed The coking reactor was inserted into a ceramic tube furnace and heated in two stages In the first stage the coal pitch was heated to 400°C In this stage the material becomes a molten mass The tube was kept at

t h s condition for 12 hours In the second stage, the tube reactor contents were then raised to 600°C and held at this temperature for one hour, whereupon the tube was permitted to cool to room temperature The product was then recovered and weighed The green coke precursors for WW-1, W W - 2 , and

W W - 3 test graphites, WVGS 13407 NMP-soluble extract, NMP-soluble extract from 350°C hydrogenated WVGS 13407, and WVGS 13421 NMP-

soluble extract, respectively, were made with this system

All subsequent green coke operations were made in a second coker, which was fashioned from steel pipe approximately 18 cm in diameter and 25 cm in length

A metal plate was welded to one end and a metal collar was welded to the other end such that a steel lid could be bolted to the system Typically, about 250 to

500 g of pitch were sealed under nitrogen in the coker reactor and the system placed in a large temperature-programmable furnace The heat treatment process was as follows The temperature was raised 5"C/min to 350 "C and then l"C/min to 425°C and the temperature held at 425°C for 90 minutes Finally the temperature was raised further at 3"C/min to between 500 and 600"C, and held there for 3 hours The coker was cooled to room temperature and the material recovered to determine green coke yield

The green cokes were calcined by placing a weighed amount of green coke into

an alumina tube The tube was fitted with end caps to allow for a constant purge

of nitrogen The alumina tube was then inserted into a high-temperature furnace and the temperature raised to about 1000°C for a period between 30 and 60

minutes The furnace was turned off, cooled to room temperature, and the product recovered to determine the calcined coke yield

The effect of hydrogenation on the yield of green coke is shown in Table 17 Thermogravimetric analysis (TGA) was also conducted for comparison It can

be seen that the pitch from unhydrogenated coal results in a fairly high yield of green coke As the severity of hydrogenation increased the green coke yield decreased, probably because of molecular weight reduction and loss of low- molecular weight species during coking Also, in general, TGA yields are lower than the yields obtained from the green coking operation Undoubtedly, during the green-coke process in the sealed reactor, some reflux occurred, promoting additional condensation and enhanced carbon yield The TGA experiment, which involves rapid heating under a flowing inert gas atmosphere, tends to promote enhanced distillation of volatile species

Tables 18 and 19 show the effects of blending extracts of hydrogenated coal with those from untreated coal for WVGS 13421 and WVGS 13423, respectively For both coals, the amount of hydrogenated material in the blend causes a reduction in coke yield Again, the TGA yields are generally lower than the yields obtained using the coking reactor This is particularly pronounced for the WVGS 13423 coal following hydrogenation at 450"C, where the TGA yield is only 34 wt% As noted previously, the hydrogenated products from WVGS 13423 are relatively volatile

225 Table 17 Effect of hydrogenation on green coke yields

Table 19 Effect of blending hydrogenated coal-derived pitch and coal extract on green coke yields, WVGS 13423

hydrogen, non-condensable hydrocarbons, and other light gases

3.2 Analysis of cokes by optical microscopy

Polarized light photomicrographs were taken of the green and calcined cokes, as well as their corresponding test graphites The untreated extract cokes are characterized by very small anisotropic domains on the order of 3 microns or less This type of optical structure is believed to be highly desirable for nuclear graphite applications

Table 20 Yield of calcined coke for WVU test graphites

hydrogenation This finding was also substantiated by Seehra et al [22] in a recent publication

3.3 Ash analysis of cokes

Table 21 reports the ash content and ash composition (determined by inductively coupled plasma-atomic emission spectroscopy, ICP-AES) for all of the calcined cokes used to fabricate the test graphites It can be seen that the amount of ash and its make-up are variable, but are within the range observed for petroleum-based calcined cokes Although the ash contents in all of the calcined cokes appear rather high, these materials may still be acceptable because many of the metallic species are driven off during graphitization This aspect is addressed in the next section

227

Figure 1 Optical photomicrographs of green cokes derived from WVGS 13421 pitches: top, EXT; middle, 75:25 EXT:HEXT450; bottom, HEXT450

wvu- wvu- wvu- wvu- wvu- wvu- wvu- wvu- w- ww- wvu- wvu- w w -

13421 13421 13421 13421 13423 13423

75~25 25175 75:25 60:40 25:75 75~25

EXT EXT: 13423

Precur

13407 HEXT 13421 HEXT HEXT HEXT HEXT HEXT HEXT 13423 HEXT HEXT HEXT

229

4 Preparation and Evaluation of Graphite From Coal-Derived Feedstocks

Test graphites were made from calcined coke which was initially milled into a

fine flour so that about 50% passed through a 200 mesh Tyler screen The coke

flour was then mixed with a standard coal-tar binder pitch (1 1 0°C softening

point) at about 155°C The ratio of pitch to coke is about 34:100 parts by

weight After mixing with the liquid pitch, the blend was transferred to the mud cylinder of an extrusion press heated to about 120°C The mix was then extruded into 3-cm diameter by 15-cm long cylinders and cooled These green rods were then packed in coke breeze and baked in saggers to 800°C at a heating rate of 60"Ckour The baked rods were graphitized to about 3000°C in

a graphite tube furnace In most cases, the graphite rods were machined into rectangular specimens 2-cm wide by 15-cm long for measurement of the CTE

4 I Analytical characterization c f graphites

In order to assess the loss of inorganic contaminants during graphibzation, the ash composition of most of the graphites was analyzed by ICP-AES The total ash contents of the W W graphites are compared to those for the precursor

calcined cokes in Table 22 Also included are data for H-45 1 and VNEA, which

are the current qualified nuclear-grade graphites

The elemental ash composition for most of the graphites, as measured by ICP-

A E S , are compiled in Table 23 The results show that most of the inorganic matter is removed during the graphitization process The elemental compositions of the WW graphites are in the same range as the commercial nuclear graphites which have presumably undergone extensive additional halogen purification

Table 22 Ash contents of calcined cokes and thelr processed graphites (ppm)

The results in Table 22 are of crucial importance Indications are that the ash percentage in the calcined cokes produced from coal may already be low enough to yield acceptable graphite The WW graphites have not been halogen purified treated and yet yield metal composition comparable to, or better than H-

451 or VNEA graphite Since the chlorine treatment is quite costly, significant economic advantages may accrue from the production of graphite from coal

4.2 Correlation c f graphite properties with processing methodology

A key factor in the suitability of cokes for graphite production is their isotropy

as determined by the coefficient of thermal expansion After the calcined coke was manufactured into graphite, the axial CTE values of the graphite test bars were determined using a capacitance bridge method over a temperature range of

25 to 100°C The results are summarized in Table 24 Also included in the table are bulk density measurement of calcined cokes and the resistivity values

of their graphites

The degree of isotropy of the graphites varied, as indicated by the CTE, depending upon the characteristics of the starting coal-derived pitches Such control can be exercised in two distinct ways In the first method, the severity

of the hydrogenation conditions to which the raw coal was subjected, was varied

by changing the hydrogenation temperature The higher the reaction temperature the more hydrogen was transferred to the coal-derived pitch The most severe hydrogenation conditions produced the most anisotropic graphites while the least severe, or no hydrogenation at all, produced materials which were more isotropic For example in Figure 2 the effect of hydrogen addition

on the resultant graphite CTE is shown It is apparent that little hydrogen is

required to reduce the CTE value dramatically Furthemore, the addition of more than about 0.5 wt% hydrogen to the coal pitch only reduces the CTE slightly Qualitatively, the degree of isotropy could be easily seen by examination of the photomicrographs of the cokes and graphites

A second method for varying the degree of anisotropy in coal-based graphites was achieved by blending the hydrogenated coal-derived extract with that from the non-hydrogenated raw coal Hence, by varying the proportions of the unhydrogenated and hydrogenated pitch, graphites with controlled CTE can be obtained These CTE values range between the most anisotropic graphites m the case of the pure hydrogenated pitch to the most isotropic graphites in the case of the raw coal extract The effect of blending composition on CTE for pitches derived from WVGS 1342 1 is shown in Figure 3 When the same types

of pitches and graphites were obtained from WVGS 13423 the effect was the same, though the exact functional relationship was different

Table 23 Metals analysis of WVU graphites by ICP-AES (ppm)

Metal WVU-1 WVU-2 WVU-3 WVIJ-4 WVU-5 WVU-6 W W - 7 WVU-8 WVU-9 VNEA H-451

Table 24 Some properties of WW coal-derived calcined cokes and their graphites

5 07 1.09 0.96 CTE 4.42 2.89 5 2 8 1.59 0 7 1 4.52 3.77 1.19 3 12 5.28

0 0 0 5 1 0 1 5 2 0 2 5 3 0

WewM Percent Hydrogen Added to Coal (daq

Figure 2 Effects of hydrogenation on CTE of coal-based graphites

Weight Percent EXT m Blend w t h HEXT450

Figure 3 Effects of blend composition on CTE of graphites manufactured from WVGS

1342 1 derived products

233

These results are significant since they show that the ultimate characteristics of

the graphite product can be unequivocally controlled by the blendmg of pitches Further, the results indicate that a single coal source could be utilized, by appropriate treatment, to provide a slate of different pitches and cokes

5 Summary

It has been demonstrated that a solvent-extraction procedure with N-methyl pyrrolidone is capable of producing coal-derived extract pitches with low-ash contents Moreover, the properties of the pitches can be varied by partial hydrogenation of the coal prior to extraction The yield of the pitches along with the physical and chemical properties of the cokes and graphites vary m an

understandable fashion

By a combination of pitch blending andor hydrogenation, the properties of

calcined cokes and their subsequent graphites can be controlled in a predictable manner Thus by altering processing conditions, graphites ranging from very isotropic to very anisotropic can be produced from a single coal source As

acceptable petroleum supplies dwindle, this technology offers an alternate route for graphite manufacture from the abundant, world-wide reserves of coal

6 Acknowledgments

The authors wish to thank I C Lewis and the UCAR Carbon Company for their assistance in the preparation and characterization of the coal-derived graphites This work was partially funded by a grant from the U S Department of Energy DE-FG02-9 lNP00 159 This support is gratefully acknowledged

Yamada, Y , Imamura, T., Kakiyama, H., Honda, H., Oi, S., and Fukuda, K.,

Characteristux of meso-carbon microbeads separated from pitch, Carbon,

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Edie, D D., and Dunham, M G., Melt spinnmg pitch-based carbon fibers,

Carbon, 1989,27,647 655

Stansberry, P G., Zondlo, J W., Stiller, A H., and Khandare, P M.,

production of coal-derived mesophase pitch In Proceedings c f 22nd Biennial Cofiference O R Carbon, American Carbon Society, San Diego, CA,

Eser, S., and Jenkins, R G., Carbonization of petroleum feedstocks I

relationships between chemical constitution of the feedstocks and mesophase

development, Carbon, 1989,27, 877 887

Lewis, I C., Chemistry of pitch carbonization, Fuel, 1987,66, 1527 1531

Derbyshire, F J., Vitrinite structure: alterations with rank and processing,

Fuel, 199 1, 70, 276 284

Song, C., and Schobert, H H., Non-fuel uses of coals and synthesis of

chemicals and materials In Preprint of Paperspresented at the 209th

American Chemical Society meeting, Vol40(2), Anaheim, CA, 1995,

pp 249 259

Seehra, M S., and Pavlovic, A S., X-ray diffraction, thermal expansion,

electrical conductivity, and optical microscopy studies of coal-derived

graphites, Carbon, 1993,31,557 564

Owen, J., Liquefaction of coal In Coal and Modern Coal Processing A n

Introduction, ed G J Pitt and G R Millward Academic Press, New York,

1979, pp 163 181

Reneganathan, K., Zondlo, J W., Mink, E A., Kneisl, P., and Stiller, A H., Preparation of an ultra-low

Processing Technology, 1988, 18,273 278

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Supplementary Volume, ed H H Lowry John Wiley and Sons, Inc., New York, 1963, pp 1081 1099

Surjit, S., Coke for the steel industry In Proceedings c f the Cor; ference on Coal-Derived Materials and Chemicals,, ed T F Torries and C L Irwin

West Virginia Univesity, Morgantown, WV, 1991, pp 1 14

Habermehl, D., Orywal, F., and Beyer, H D., Plastic properties of coal In

Chemistry c f Coal Utilization, Second Supplementary Volume, ed M A Elliot John Wiley and Sons, Inc., New York, 1981, pp 317 368

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manufacture, Journal c f Materials Science, 1983, 18, 3 16 1 3 176

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pitches as electrode binders, Fuel, 1968,47, 197 212

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bodies In Modern Aspects c f Graphite Technology, ed L C F Blackman Academic Press, New York, 1970, pp 49 78

Shah, Y T., Reaction Engineering in Direct Coal Liquefaction Addison-

Wesley publishing Company, London, 198 I

Given, P H., Cronauer, D C., Spackman, W., Lovell, H L., Davis, A,, and

Biswas, B., Dependence of coal liquefaction behavior on coal characteristics

1 vitrinite-rich samples, Fuel, 1975, 54, 34 39

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graphites, Carbon, 1994, 32,431 435

May 1994

ash coal extract under mild conditions, Fuel

235

Automotive Applications

PHILIP J JOHNSON AND DAVID J SETSUDA

Ford Motor Company

Automotive Components Division

to respiratory related illnesses [2] Due to the unique and severe smog problems

that affected many cities in the state of Califorma, studies of the causes of air pollution were inibakd m the 1950's [3] Based on its fmdmgs, Califomia formed the Motor Vehcle Pollution Control Board m 1960 to regulate pollution fiom automobiles

The generaQon of alr pollutants, including VOC's, from automotive vehicles was

identified to come from two principal sources: vehicle exhaust emssions, and fuel system evaporatwe emissions [4] Evaporative emssions are defimed as the automotwe fuel vapors generated and released from the vehicle's fuel system due

to the interactions of the specific fuel in use, the fuel system characteristics, and environmental factors The sources of the evaporative emissions are discussed below and, as presented rn the remainder of t h s chapter, control of these evaporative emissions are the focus of the application of achvated carbon technology in automotwe systems

Pnor to the implementation of any evaporahve emission controls, fuel vapors were freely vented from the fuel tank to the atmosphere Diurnal, hot soak, running losses, resting losses, and refueling ermssions are the typical evaporatwe contributions from a motor vehicle Diurnal emissions occur while a vehicle 1s

parked and the fuel tank is heated due to dally temperature changes Hot soak

emissions are the losses that occur due to the heat stored in the fuel tank and engine

compartment immediately after a fully warmed up vehicle has been shut down Running loss emissions are the evaporative ermssions that are generated as a result

of fuel heating during driving conditions Resting losses are due to hydrocarbon migration through materials used in fuel system components Refueling ermssions occur due to the fuel vapor that is displaced from the fuel tank as liquid fuel is pumped m

I 2 Development of evaporative emission controls

In 1968 California committed additional resources to fight its unique air polluhon problems with the establishment of the Cahfomia Air Resource Board (CAW) Although the federal government has junsdichon over the states in the area of automotive emission control regulation, California has been given a waiver to implement its own regulations provided that they are more stringent than the federal requirements [ 5 ] California has since been a leader in the development and nnplementation of increasingly stringent automotive ermssion control regulations The Environmental Protection Agency (EPA) was established in 1970 by an act of Congress, the primary purpose of the agency being to promulgate and implement environmental regulations that are mandated by law Congress fiist mandated automotive polluhon control regulations in the Clean Air Act (CAA) Amendments

of 1970 The CAA was amended in 1977 and 1990 to further improve a n quality The primary purpose of the amendments was to push industry mto developmg and mplementmg control technologies The 1990 CAA Amendment also gave other states the option of adoptmg the California regulations

Industry has not always worked in full cooperation with government to meet the technology forcing standards In the early 1970's, the U.S auto mdustry was characterized by slow development of the required technologies to meet the regulations The slow response and seemingly insurmountable technical issues forced congressional and administrative delays to the original regulatory implementahon [l] However, since the late 1970's, the auto industry has responded favorably, allocating enormous resources to meet the mcreasmgly strmgent regulations

237

The buyers of motor vehicles have been substantially positive concerning the need

to have cleaner running vehicles Although the required emission control devices and other mandated safety equipment have increased the cost of new motor vehicles, sales have not been significantly effected The current environmental awareness and concern are evidence of the general population's new found knowledge and acceptance of both mobile and stationary source emission controls

I 3 Evaporative emission control measures

The earliest implementation of evaporative emission control occurred in 1963 when the State of California mandated that crankcase emissions be eliminated This early regulation was easily met by venting crankcase emissions at a metered rate into the air induction system The next areas to be regulated were the hot soak and diurnal losses, which California required starting in the 1970 model year Prior to 1970, the uncontrolled hydrocarbon (HC) emission rate was reported to be 46.6 grams per vehicle for a one hour hot soak plus one hour heat build [6] Canisters containing activated carbon were installed on vehicles to collect the hydrocarbons that were previously freely vented from the vehicles These vapors are later purged (desorbed) fi-om the canister by pulling air through the carbon bed and into the air induction stream

The early test methodology [6] employed activated carbon traps sealed to possible

HC sources, such as the air cleaner and fuel cap, during the test procedure The carbon trap's weight was measured before and after the test procedure to establish the total emissions General Motors [7] developed the Sealed Housing for Evaporative Determination(SHED) as a more precise and repeatable method to measure evaporative losses The SHED method proved to be more accurate at measuring evaporative emissions that had previously escaped through openings other than where the carbon traps were attached The EPA and CARB subsequently changed their test procedures from the carbon trap to the SHED method

The early carbon trap and SHED methods measured two components of evaporative emissions Hot soak emissions were measured for a one hour period immediately after a vehicle had been driven on a prescribed cycle and the engine turned off Diurnal emissions were also measured during a one hour event where the fuel tank was artificially heated The one hour fuel temperature heat build was an accelerated test that was developed to represent a full day temperature heat build

The latest CARBEPA procedures require diurnal emissions to be measured during

a real time, three day test that exposes the complete vehicle to daily temperature fluctuations This test method has been employed to more accurately reflect the real world diumal emissions that occur Running loss emission measurements were also initiated in the latest test procedures Evaporative emissions are measured

whle the vehicle is dnven on a chassis dynamometer with heat applied to the fuel

tank simulating a hot reflective road surface

Onboard Refueling Vapor Recovery (ORVR) regulabons were first proposed 111

1987 but were met with a litany of technical and safety issues that delayed the

requirement The 1990 CAA amendments required the mplementabon of ORVR

and the EPA regulation requires passenger cars to f i s t have the systems starting in

1998 The ORVR test w l l be performed in a SHED and will require that not more

than 0.2 grams of hydrocarbon vapor per gallon of dispensed fuel be released from the vehicle

Fig.1 shows the typical events in the EPA's evaporative emission control test

sequence These test procedures cover the entire range of evaporative emissions, includmg the refueling emissions which are now being addressed through the

ORVR system development Typically, emission regulations are phased in over a

number of years Manufacturers are required to sell a defined percentage of their

fleet each year that meet the requirements Globally, the Umted States has led the

way in terms of technology forcing evaporative emission regulabons

Fuel Draia & 40% Fill

239

The following countries also have evaporatwe emssion regulabons; Canada,, European Economic Community (EEC), Japan, Brazil, Mexico, Australla, South Korea Regulabons in these countries have requirements that are typically less stnngent than the U.S imperakves Table 1 depicts the chronology of evaporative emission regulabon developments in the United States

Table 1 Chronology of U S evaporative emission development [l]

1970 California Carbon Trap 6 grams HC One hour test

1971 49 States Carbon Trap 6 grams HC One hour test

1972 50 States Carbon Trap 2 grams HC One hour test

1978 50 States SHED 6 grams HC One hour test

1995 California VT SHED [9] 2 grams HC Three day test I995 Callfornia Run Loss 0.05 g/mile

1996 50 States VT SHED 2 grams HC Three day test

1996 50 States Run Loss 0.05 g/mtle

200 1 50 States ORVR 0.2 glgal Lt Duty Trucks

1998 50 States ORVR [ 101 0 2 g/gal Passenger cars

2 Activated Carbon

Activated carbon is an amorphous solid with a large internal surface aredpore

structure that adsorbs molecules from both the liquid and gas phase [ 1 11 It has

been manufactured from a number of raw matenials mcluding wood, coconut shell, and coal [ 1 1,121 Specific processes have been developed to produce actwated carbon in powdered, granular, and specially shaped (pellet) forms The key to development of activated carbon products has been the selection of the manufacturing process, raw material, and an understandmg of the basic adsorption process to tailor the product to a specific adsorpbon applicabon

2.1,l Thermal acbvabon processes

Thermal activation is characterrzed by two processing stages: thermal