REFINERY COKE-CONCLUSION POWER, CEMENT INDUSTRIES SHAPE COKE FUTURE

Edward J. Swain
Bechtel Corp.
Houston

The production of petroleum coke by U.S. refineries is expected to continue to increase in the coming years. Process and market trends also indicate the likelihood of further increases in fuel-grade coke production.

COKE GRADES

In this conclusion of a two-part article, the properties and uses of the various grades of petroleum coke, as well as pricing and market trends, will be discussed.

FUEL-GRADE COKE

About 50% of the petroleum coke produced in 1989 was fuel-grade coke. Its production has doubled over the last 10 years because of the declining quality of crude oil processed in U.S. refineries (see Table 8, Part 1, OGJ, May 6, p. 102).

General characteristics of fuel-grade coke are as follows: heat content, 13,000-15,000 BTU/lb; volatile matter, approximately 10%; sulfur, 2.5-5.5 + wt %; ash, 0. 1 0.3 wt %; Hardgrove Index Number, approximately 100; and vanadium, 200-400+ PPM- There are two main markets in the U.S. for fuel-grade coke; cement kilns and power plants. Coal is the primary competitive fuel in these markets.

The biggest problem in burning petroleum coke is its low percentage of volatiles-typically about 10%, compared to 20-40% for coal. Coke's low volatility has several consequences. The most significant one is that coke generally must be mixed with coal in order to be used as a fuel.

In boilers, the amount of coke in the fuel mix is limited to about 20 wt %. In slower burning kilns where the fuel has more time to combust, coke can constitute up to 50% of the fuel mixture. Higher levels of coke in the fuel mixture will result in incomplete combustion and an unstable flame.

Another consequence of coke's low volatility is that it must be finely ground to ensure satisfactory combustion. Both kilns and boilers require coke of small enough particle size to pass 90% through 200 mesh, compared to 65-85% for most bituminous coals. However, because coke is easier to grind than coal, it should generally be possible to pass mixtures of coke and coal through existing pulverizers.

Coke also suffers from high sulfur content, which can exceed 5.5 wt % (coal rarely contains more than 4.0 wt %). Like high-sulfur coal, the use of coke in boilers may be limited or precluded by emissions regulations or scrubber capacity limits.

High-sulfur fuels pose additional problems for cement kilns.

Cement clinkers absorb sulfur and can tolerate high levels of contamination. But sulfur contamination above a threshold level will cause finished concrete to expand and crack. This threshold level is specific to the chemistry of the raw material feed.

Excess sulfur also may limit the kiln operator's control over the cement setting time.

Another potential problem is that in the most modern kilns-dry process units with preheaters-sulfur can combine with alkalies in the kiln feed to form deposits that plug the preheater. This problem is also a function of the raw material's chemistry.

The high vanadium content of petroleum coke, sometimes 500 ppm or greater, poses another problem for cement kilns. Vanadium can cause finished concrete to lose strength through a poorly understood process called "retrogression." It is because of the risks associated with vanadium and sulfur contamination that cement kiln operators almost always require a test burn before buying a refiner's coke.

However, in spite of the preceding problems, coke has some significant advantages over coal, as a fuel. One advantage is coke's high heat content; approximately 14,000 BTU/lb, compared to 10,500-13,000 BTU/lb for bituminous coal,

Another advantage of coke is its low ash content; typically only 0.1 wt % compared to over 10 wt % for most coals. Use of a low ash fuel can reduce boiler maintenance costs, frequency of unscheduled outages, and the volume of ash requiring disposal.

Low ash content feed is also an advantage for cement kilns because cement clinkers absorb coal ash. The charge to a kiln can be formulated to take advantage of the chemistry of the ash. However, ash represents one more variable which must be accounted for in balancing the cement's chemistry. Therefore, the use of a low-ash fuel such as coke largely eliminates the ash variable, and is considered advantageous.

A final advantage of coke is its ease of grinding. It has a Hardgrove Index Number of about 100, while that of coal is closer to 50 (smaller numbers represent a harder material). This means that coke is easier to grind, which is important because its low volatility necessitates a finer grind than does coal's.

It is estimated that during 1989 about 800,000 tons of coke were utilized in cement kilns. Four public utility power

companies purchased 495,000 tons of coke during that year (Table 1).

Pennsylvania Power & Light Co. has been the largest purchaser/consumer of coke over the past 5 years. It burns about 10:90 wt % coke/coal blend in three of its steam-power generating facilities.

The boilers in these three units are of the turbo (or "fantail") type used to burn anthracite coal (anthracite has volatility characteristics similar to those of coke).

The Deepwater cogeneration facility located next to the Lyondell refinery on the Houston ship channel in Pasadena, Tex., is a state-of-the-art power plant. It is one of only a few plants in the world capable of burning 100% delayed petroleum coke. It can do this at as little as 30% of maximum rated capacity. Its boilers produce 1.3 million lb/hr of steam at 1,800 psig and 1,000 F.

A portion of the steam is used to drive power turbines operating at an output of up to 135 106 w. This electric power is sold to Houston Lighting & Power Co.

Turbine steam extracted at 700 psig and 700 F. is sent to the Lyondell refinery for process use, at a maximum rate of 600,000 lb/hr.

The 45,000 b/sd delayed coker at the Lyondell refinery produces an estimated 2,400 tons/day (approximately 800,000 tons/year) of fuel-grade sponge coke. It is reported that the Deepwater facility utilizes about two thirds of the coke output, or about 525,000 tons/year.

The Deepwater plant came on-line in 1986. Foster Wheeler Energy Corp. was responsible for the design, engineering, and erection of the steam generator. Bechtel Power Corp. was the engineer-constructor for the plant, and was also responsible for start-up and operator training.

Five independent power plants based on fired circulating fluidized bed combustor technology came on-line during the last half of 1990. The five plants, each rated at 20 106 w, are the first commercial independent power plants in the U.S. fueled by 100% petroleum coke.

The plants are in the San Francisco Bay area and consume all the fluid petroleum coke produced at Exxon Corp.'s Benecia refinery and Tosco Corp.'s Martinez refinery.

The combined production of fluid petroleum coke from these two refineries is about 2,600 tons/day, or 860,000 tons/year.

ANODE-GRADE COKE

The largest user of calcined coke is the aluminum industry, where the calcined sponge coke is used in the manufacture of carbon anodes. Approximately 0.5 lb calcined sponge coke is used per lb of aluminum. The following are calcined coke consumption values for specific applications:

CALCINED COKE PROCESS USAGE, LB COKE/LB

Aluminum 0.5
Silicon carbide 1.4
Phosphorous 1.8
Calcium carbide 0.69
Graphite 1.25

It is estimated that the U.S. aluminum industry will utilize about 1.65 million tons/year of calcined coke during the 1990-1992 period. The industry is reducing its pot line capacity for several reasons, including increased electric power costs and air pollution problems.

Pot line capacity is gradually moving to Eastern Canada to obtain low cost electric power from the St. James hydro power complex. U.S. pot line capacity is also being reduced because of increased recycling of throwaway aluminum items. For these reasons, anode-grade coke usage in the U.S. aluminum industry should continue to decline.

It is also estimated that the U.S. food-grade phosphorous industry will utilize about 576,000 tons/year of calcined coke during the 1990-1992 period. This industry should continue its mild decline because the use of sodium tripolyphosphate, the most common sodium phosphate, continues to decrease. Further antiphosphate legislation and the encroachment by liquid products on phosphate-based powdered detergents are causes for the declining use of food-grade phosphorous.

The quality of short residue used as feedstock for delayed cokers producing anode-grade coke has been deteriorating. In order to meet the specifications for anode-grade coke, U.S. refiners are hydrotreating coker feedstocks.

The price spread between anode-grade and fuel-grade coke must be sufficient to cover the capital and operating costs for these hydrotreaters, and provide a profit. This price spread is usually sufficient, but the market for anode-grade coke is the major limiting factor.

NEEDLE-GRADE COKE

Table 2 summarizes the production of needle coke and graphite electrodes in major producing countries. Needle coke is presently produced by a relatively small number of operators. The most important consideration in attempting to produce needle coke is the selection of a feedstock.

Needle coke precursors have generally consisted of hydrocarbon streams with:

Low sulfur content catalytic cracker slurry oils
Tars derived from the thermal cracking of refinery gas oils
Hydrodesulfurized catalytic cracker slurry oils
Coal tar pitches.

All feedstocks are characterized by low API gravity, low asphaltene content, and a high degree of aromaticity.

Processing conditions for a coker producing needle coke are somewhat different than those for a coker producing fuel-grade or anode-grade coke.

Operating pressure of a coker producing needle coke is at least 100 psig at the inlet to the coke drums, rather than 20-30 psig for other coking operations.

The operating temperature of a needle-grade coker is at least 50 F. higher.

Calcined needle coke must have certain chemical and physical properties. Table 3 presents a typical listing of needle coke purchasing specifications for the three commercially available grades.

Historically, the most important specification has been the coefficient of thermal expansion (CTE).

This physical property serves as an indicator of the coke's structural alignment (in the form of parallel platelets, or needles). This is important in the resultant electrodes' current-carrying capability and its ability to maintain mechanical integrity when subjected to the enormous temperature differentials (up to 2,900 C.) experienced in electric arc furnace steelmaking.

Table 4 presents a conservative forecast of demands for electric arc furnace steel in major consuming countries, plus the corresponding graphite electrode and calcined needle coke requirements, through 1995. The needle coke market will continue to be characterized by contracted sales opportunities and infrequent spot market activity.

COKE MARKET

About 70% of petroleum coke produced by U.S. refineries is exported. Table 5 lists U.S. coke exports for 1987 through 1989, by Petroleum Administration for Defense (PAD) District. These figures include green fuel-grade coke, green and calcined anode-grade coke, and green and calcined needle coke.

Although no firm, readily available export quantities for each grade of coke are quoted in U.S. government sources, it is reported that the majority of exported coke is of the green fuel-grade type.

Six countries account for about 72% of U.S. coke export receipts.

Table 6 gives the quantities of U.S. coke imported by these countries over the same 3-year period. Japan is the major importing country, using the coke in processing industries and for boiler fuel, as a coal blend.

As the refined petroleum product mix shifts toward transportation fuels and crude oil quality declines, overseas refineries will install cokers, thus reducing the exportation of U.S. coke.

There are little available public pricing data on the three grades of petroleum coke in the U.S. marketplace. However, one can develop some pricing data from the end markets that coke serves.

Fuel-grade coke is saleable as fuel only when priced at a discount as compared to the delivered price of coal. At a minimum, the discount must be large enough to yield a rapid return (less than 3 years) on a dual-energy-handling investment.

The relationship between the price of petroleum coke and the prices of various grades of coal is shown in Table 7 and Fig. 1. The data illustrate this relationship from 1985 to 1989, using average delivered costs to utilities.

The price of petroleum coke is declining more rapidly than the prices of the various coals because of rapidly increasing production. It should be mentioned that bituminous coal is primarily mined and used by power plants in states east of the Mississippi River.

It has been reported that electric utilities and cement producers consider an acceptable discount on coke to be in the range of 20-30%/106 BTU (equivalent to a discount of 10-20%/ton) compared to the delivered price of coal with a comparable sulfur level.

Anode-grade coke will usually sell in the price range of $10-15/ton above fuel-grade coke.

This price differential is usually sufficient to justify the installation of a hydrotreater on the Coker feedstock. The hydrotreater is needed in order to produce green coke that meets anode-grade coke specifications.

However, U.S. demand for anode-grade coke is declining, so it is doubtful that any new or additional coking capacity will be installed for the production of anode-grade coke.

Needle coke commands a high price because it is categorized as a performance product, not a commodity. After being calcined, the relative value of needle coke is determined by its base of processability to graphite electrodes, and its contribution to the electrodes' quality and performance.

Calcined, super-premium needle coke was reported to have sold for about $550/ton during 1989.

Needle coke is commercially available in three grades: super-premium, premium, and intermediate. Super-premium grade coke is used to produce electrodes for the most severe electric arc furnace steelmaking applications, premium-grade coke is used for electrodes destined to less severe operations, and intermediate-grade coke is used for even less critical needs.

Therefore, $550/ton for premium-grade calcined coke is the upper price limit for needle coke.

The additional coke produced as a result of process and market trends will likely be of fuel-grade quality, because the markets for anode and needle-grade coke are limited.

The availability of existing fuel-grade coke and the likelihood of increased production create a source of low-cost energy for the utility power industry and low-cost raw material for the chemical industry.

The uses for fuel-grade coke in these industries depend only on the ingenuity of the potential users.

REFERENCES

"Cost and Quality of Fuels for Electric Utility Plants 1989," Energy Information Administration, July 1990.
Acciari, J.A., and Stockman, G.H., "Demand for super premium needle cokes on upswing," OGJ, Dec. 25, 1989, pp. 118-120.
Petroleum Supply Annuals, 19871989, Department of Energy.

BIBLIOGRAPHY

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