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Figure 4-3. Typical dry process material handling during §

Portland Cement manufacture.3 -

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As stated in Chapter 2, cement kilns incline slightly toward the discharge end and rotate slowly. Feed materials slowly progress to the exit of the kiln by gravity. The majority of the fuel is burned at the discharge end of the kiln, so that the hot gases pass countercurrent to the descending raw feed material. Wet process kilns are typically over 500 feet long, and evaporation of water from the feed occurs in the first 20 to 25 feet of the kiln. Dry process kilns can be 20 to 25 percent shorter than wet process kilns because little or no residence time is needed to evaporate water from the feed and the feed heats faster. After evaporation, the temperature of the feed material increases to about 2700*F during passage through the kiln, and several physical and chemical changes occur. The water of hydration in the clay is driven off, the magnesium carbonate calcinates to MgO and C02, the calcium carbonate calcinates to CaO and C02, and, finally, the lime and clay oxides combine at the firing end of the kiln to form clinker. Figure 4-4 provides a schematic drawing of the typical clinker production process. Section 4.2.6 below discusses the various methods by which tires and TDF are being added to supplement kiln fuel.

4.2.3 Preheaters and Precalciners

Dry process cement production facilities often have several other types of manufacturing equipment designed to increase fuel efficiency. First, many dry process kilns add a preheater to the feed end of the kiln to begin heating of the feed prior to its entrance to the kiln. Two main types of preheaters exist, the suspension preheater and the traveling grate preheater; both use hot, exiting kiln air to facilitate a more efficient heat transfer to the feed than could occur in the feed end of the kiln itself.1 This exhaust stack f

I CONTROL I DEVICE

exit gas primary air and fuel qa8 flow burner material flow raw feed material cooler qa8 flow burner

outlet secondary air / clinker outlet

Figure 4-4. Typical clinker production process during Portland Cement manufacture.3

addition decreases the amount of fuel needed to form one ton of clinker. Compared to a wet process kiln, a dry process kiln with a preheater system can use 50 percent less fuel.3

The second development to increase fuel efficiency in a dry process kiln is a precalciner. For this system, a vessel called a flash precalciner is located between the preheater and the kiln, and is fueled by a separate burner. A discussion of tire use to supplement precalciner fuel is discussed in section 4.2.6 below.

Figure 4-5 shows a four-stage suspension preheater with a precalciner. Feed is blown from stage to stage by the rising countercurrent air, reaching the precalciner after Stage 3 and before being blown into Stage 4. Figure 4-6 shows a traveling grate preheater. About 95 percent of the calcining of the feed occurs in the precalciner. The calciner may use preheated air either from the kiln or the clinker cooler. Precalciners allow several operating advantages. Because calcination is rapid, adjustment to the calcination rate can be made quickly to yield uniform feed calcination. A kiln with a precalciner is shorter, because less distance is needed for calcination. Also, production capacity can be increased over a kiln of identical diameter without a precalciner, because the shorter kiln can be rotated at a higher rate while still maintaining proper operating characteristics of feed residence time and bed depth.

4.2.4 Finished Cement Grinding

Calcined clinker is ground in ball mills, mixed with gypsum, and shipped in bags or bulk. Figure 4-7 depicts finish mill grinding and cement shipping. The type of fuel used to make clinker does not affect these operations.

Figure 4-5. Four-stage suspension preheater with a precalciner at a Portland Cement plant.3

PELLETIZER PAN

TO COLLECTION DEVICE

Figure 4-6. Traveling graté preheater system at a Portland Cement plant.5

ig aii H

AIR CLASSIFIER

CEMENT COLLECTOR .

AIR CLASSIFIER

CEMENT COLLECTOR .

ig aii H

4V. QYPSUM AOOEO

BULK STORAGE

BULK BULK BOX PACKAGING TRUCK TRUCK CAR CAR MACHINE

4V. QYPSUM AOOEO

BULK STORAGE

BULK BULK BOX PACKAGING TRUCK TRUCK CAR CAR MACHINE

Figure 4-7.

Finish mill grinding and' shipping during Portland Cement manufacture.3

4.2.5 Tires as Fuel in the Kiln

Tires or TDF can be used to supplement the kiln fuel and/or the precalciner fuel. When TDF is added to the kiln fuel mix, it is often added at the burner (lower) end of the kiln, near, but not mixed with, the coal feed. At one plant (Holnam/Ideal), TDF is fed in above the coal flame.19 This arrangement permits the chips to be blown further into the kiln and causes the chips to fall through the coal flame to produce much better combustion. In most cases, TDF is added at the feed end (high end) of the kiln. Several kilns have added whole tires at the feed end of the kiln so that burning occurs as the tires move down the kiln; this method is common in Europe.4 However, many kilns in the U.S., particularly wet process kilns, have chains hanging down in the feed end of the kiln to enhance heat exchange. Such equipment forms a barrier to everything but finely ground materials, and precludes use of whole tires at the feed end. Kilns with preheaters provide the best environment for adding TDF or tires at the feed end, because significant preheating of the dry feed has occurred before the feed contacts the tire chips.

Tires have occasionally been used to supplement the primary precalciner fuel (usually coal), with mixed results. Florida Crushed Stone in Brookville, Florida, was feeding TDF into the preheater, but had to discontinue use because of plugging of the preheater (most likely due to oil condensate from the incomplete combustion of the tire chips). The company is in the process of installing a whole tire feeder with weight-belt, computer, variable rate belt, and triple gate chute to feed tires into the kiln.10

Southwestern Portland Cement in Victorville, California, not only adds TDF successfully to the preheater, but concurrently supplements the primary kiln fuel by mixing whole tires in the kiln feed.16 Tire chips are added in the preheater, at the pyroclone (precalciner) unit, right after the tertiary air duct that brings hot air from the clinker cooler.16 The chips burn quickly and go up the air stream into the preheater. Concurrently, whole tires are introduced into the feed end of the kiln with a double gate method. First, the tire is fed upright into a downward chute that slopes 30 to 40 degrees, so that it rolls down and stops at the second gate. The first gate closes and the second gate opens. The tire then rolls across the feed shelf and into the kiln. The double gate method reduces excess air introduction to and heat loss from the kiln.16 Using both kinds of tires concurrently helps maximize the percent of fuel provided by tires. Whole tire use reduces coal used at the firing end of the kiln, but too many whole tires would provide too much heat in the kiln feed end. The TDF replaces coal used in the precalciner, but would not be used in the kiln, because they are more expensive than the whole tires.16

4.3 EMISSIONS, CONTROL TECHNIQUES AND THEIR EFFECTIVENESS

Testing results from three cement facilities and one lime kiln were evaluated for this report. The four facilities are: Ash Grove Cement, Durkee, Oregon; Holnam/Ideal Cement, Seattle, Washington; Calaveras Cement, Redding, California; and Boise Cascade Lime, Wallula, Washington.

Testing performed at Ash Grove Cement in Durkee, Oregon, on October 18 to 20, 1989, evaluated criteria pollutants, aliphatic and aromatic compounds, metals, and specifically examined chloride emissions to assess the possibility of dioxin formation.20 Ash Grove's normal fuel is a mixture of gas and coal. As seen in Table 4-2, emissions of chloride were lower burning some TDF than with normal kiln firing, and; therefore, the Oregon Department of Environmental

Table 4-2. Effect of Burning 9 to 10 percent TDF in a Gas and Oil Co-fired Dry Process, Rotary Cement Kiln Controlled by an ESP20 Ash Grove Cement, Durkee, Oregon

Pollutant

Baseline, 0% TDF

9-10% TDF

Percent Change

Particulate, lb/MMBtu

0.969

0.888

-8

S02, lb/MMBtu

0.276

0.221

-20

CO, ppm

0.049

0.036

-27

Aliphatic compounds, lb/MMBtu

0.0011

0.0009

-18

Nickel, nq

30

DL'

NAb

Cadmium, /¿g

3.0

2.0

-33

Chromium, ng

30

DL'

NAb

Lead, /xg

DL'

DL'

NAb

Zinc, ng

35

35

0

Arsenic, ng

0.2

0.2

0

Chloride, lb/hr

0.268

0.197

-26

Copper, fig

37

13

-65

Iron, jug

400

200

-50

• Below detection limit (DL). b NA - not applicable.

Quality (DEQ) found that the use of TDF as a supplemental fuel at Ash Grove did not enhance the potential for dioxin formation.20 The same report described screening tests performed for 17 specific polynuclear aromatic hydrocarbons (PAH's). Only three PAH's were detected (naphthalene, dibenzofuran, and phenanthrene) and each were detected in all eight samples. However, the highest levels of these compounds were detected while firing normal fuel (gas and coal) , not when burning TDF.20

Testing at Ash Grove also examined total hydrocarbons, vaporous heavy metals, and approximately 115 other PAH's. Emission testing for total hydrocarbons showed results similar when burning TDF and under conditions when TDF was not burned. Since there are no permit limitations on total hydrocarbons, these were not addressed further in the report. For the ten metals tested, emissions during the tire chip burning were equal to or less than emissions when tire chips were hot being burned. The report states that there is no evidence that the emission concentrations found for any of the 10 metals warrant concern. Finally, the screening of the other PAH's did not identify any other compounds of significance. For all PAH's, none of the compounds detected are listed as human carcinogens or possible human carcinogens.20 The Oregon DEQ is requiring Ash Grove in Durkee to conduct a one-year ambient monitoring program for particulate emissions.20

In October, 1990, testing at Holnam/Ideal Cement, in Seattle, Washington, was performed at baseline (100 percent coal-fired), 11 percent TDF, and 14 percent TDF.12 Holnam is a wet process cement plant. The kiln emissions are controlled with an ESP. Particulate, SOz, NOx, VOC, and semi-volatile organic compound emissions decreased significantly from baseline for both 11 and 14 percent TDF use rates. CO emissions increased 30 and 36 percent, respectively, for the 11 and 14 percent tests. Several metals were tested, including cadmium, chromium, copper, leak, and zinc. These also exhibited decreased emissions with the exception of chromium emissions during the 11 percent TDF test, which showed increased emissions.

Figure 4-8 graphs criteria pollutant emissions for each TDF level tested at Holnam's kiln.12 The percent change in emissions of metals at Holnam is shown in Figure 4-9, and the percent change of VOC emissions is shown in Figure 4-10.12 Table 4-3 summarizes the results of hazardous air pollutant (HAP) emission testing performed at Holnam.12

One lime manufacturing plant, Boise Cascade, in Wallula, Washington, burns 15 percent TDF supplementally to natural gas in their rotary kiln.18 Testing was performed in 1986 for metals and organics only. Most significant were the dramatic increases in zinc, chromium, and barium emissions when burning TDF during the test.18 The kiln emissions are controlled by a venturi scrubber, which would not be effective for collecting small metallic particles like zinc oxide. (The collection efficiency of venturi scrubbers decreases as particle size decreases.) Table 4-4 lists results of this test, and Figure 4-11 graphs the percent change in emissions of metals and organics from this kiln.18

Because of the extensive reuse of combustion air in the process at Calaveras' facility, the fabric filter exhaust is the only point of emissions for the kiln, clinker cooler, and raw mill. Exhaust gases from the fabric filter are monitored continuously for carbon monoxide, nitrogen oxides, and hydrocarbons. Calaveras has tested toxic pollutants while burning 20 percent TDF. Table 4-5 summarizes these test results, giving emission factors for metals, hazardous air pollutants, polyaromatic hydrocarbons, dioxins and

3 ffl s

273

#

■--------------- ""

243

234

-

u

-

1.62

-

0.3S

0.36

.—p.1

0.11

0.12

Particulate

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