Part 1 The functions of preheaters, calciners, and clinker coolers are well-known. For example, the calciner performs a major share of the calcination of preheated raw meal, relieving the rotary kiln of this function and increasing the kiln's capacity potential.

Of course, preheater, calciner, and cooler designs evolve over time and new technologies emerge. Therefore, operators of existing kiln systems undertake modernization projects to increase capacity and improve thermal efficiency while guarding against excessively high clinker discharge temperatures. The ever increasing escalation of fuel and power costs, together with increasingly stringent emissions limits, further impact how such modifications are made.

In response to market demand for increasing clinker production while decreasing specific power, fuel, and emissions levels, process equipment suppliers have recently introduced new generations of calciners and clinker coolers.

This article presents a case study of a 1 million-mtpy cement plant and how a partnership between a cement producer and a process equipment supplier results in dramatic savings in the plant's operating costs. These costs are shown to decline in a step-wise manner as new technologies emerge from 1993 through 2000.

Roanoke Cement In 1993, Carolinas Cement Co. signed a $40 million "supply and install" contract with Fuller Company to modernize its Roanoke, Va. plant (also known as Roanoke Cement), not only to cut the costs of maintenance and fuel but also to reduce emissions. Today the Roanoke plant is a world leader in thermal efficiency, availability, and environmental compliance. Three distinct phases of plant modernization took place for the plant to attain such leading status:

Phase 1 (1993-1996): Cut operating costs through modernization/consolidation.

Phase 2 (1999-2000): Increase availability while further increasing output.

Phase 3 (2000-2003): Reduce emissions for permitted capacity of 1.3 million mtpy.

The first phase was simply to cut operating costs. This was accomplished by consolidating the production capacity of four long dry kilns (450,000 mtpy) and an Allis-Chalmer's (A-C) traveling grate preheater kiln (450,000 mtpy) into a single 900,000-mtpy kiln line by converting the A-C traveling grate preheater kiln system to a modern 6-stage preheater/calciner system.

The second phase added a state-of-the-art clinker cooler and new finish mill system as the productivity of the Phase 1 modernization exceeded the guarantee by 20% and as high demand for cement in the U.S. continued.

The third phase, presently in development, is to cope with increasingly stringent U.S. emissions requirements and to permit the plant for upwards of 1.3 million mtpy as an additional 10% increase in capacity has been realized by the performance of the new clinker cooler. Today the pyroprocessing system is capable of producing 1.2 million mtpy at a specific fuel consumption of 673 kcal per kg (at 15% bypass) and at a specific power consumption of 22 kWh per mt.

Plant history Production at the Roanoke plant began in 1951 with two long dry kilns. Two more kilns were added, one in 1953 and the other in 1956, and the plant operated as a four-kiln system until 1976, when a 450,000-mtpy traveling grate preheater kiln and two Fuller roller mills were installed. A cement mill was added by Fuller in 1987. The plant operated as a five-kiln system, producing 900,000 mtpy, until 1995 when major modifications began.

Prior to these modifications, the plant's total daily production was 2,730 mtpd, with an average fuel consumption of 1,200 kcal per kg. This performance serves as the base-line condition for this case study.

Raw materials Limestone and shale are quarried adjacent to the plant and stored under cover. Pyrite is present in some of the limestone formation, while other areas of the formation are covered with a black carbonaceous film containing entrapped pockets of oil. Consequently, there are elevated amounts of sulfur and carbon in the kiln feed.

Fly ash, which also serves as a raw mix component, further increases the carbon content. Additionally, the alkali content in the raw mix is typically above low alkali clinker limits, which requires an average kiln exhaust gas bypass rate of 15% with a maximum to 30% to eliminate the risk of heavy coating and plugging in the preheater vessels.

The raw materials are naturally low in moisture (approximately 3% H subscript 2O). As a result, a 6-stage preheater presents an optimum balance of heat recovery and capital investment while still providing sufficient heat for drying the raw materials.

Phase 1 Maintenance costs were very high, and the run time was poor for the four long dry kilns, as well as for the A-C traveling grate preheater kiln. Moreover, the plant experienced difficulty in conforming with its environmental emissions permits.

In 1993 under new management from Titan (the Greek international cement producer and trader), Carolinas Cement Co. decided to shut down the four long dry kilns permanently and modify the existing A-C traveling grate preheater kiln to a modern preheater/calciner system. The modified kiln system was guaranteed to produce 2,730 mtpd of clinker, which equaled the combined output of the four long dry kilns plus the traveling grate preheater system. To achieve this level of production, the cement producer implemented the following modifications:

- Shut down the four long dry kilns;

- Double capacity of the existing A-C traveling grate preheater kiln by converting it to a Fuller/FLS 6-stage preheater with an In-Line Calciner (ILC);

- Double capacity of the existing A-C reciprocating grate cooler by retrofitting its first drive section to air-beam technology and installing a new cooler vent system;

- Add a tertiary air duct from the retrofitted grate cooler to the new calciner;

- Install a 0% to 30% kiln gas bypass system for the production of low-alkali clinker by using the existing electrostatic precipitator from the former traveling grate's exhaust system;

- Add a second coal mill and modified the existing coal mill from direct to indirect firing; also purchased new coal dosing systems for the existing kiln burner and the new calciner;

- Add new clinker storage silos to eliminate fugitive dust; and

- Replace the control system with state-of-the-art process control.

Performing these plant modifications while minimizing downtime was the ultimate goal. In this regard, the new preheater structure proved to be the greatest design challenge. The new preheater tower was designed to straddle the existing traveling grate preheater building so that the existing kiln systemcould remain in operation during construction. This prerequisite was accomplished by designing steel trusses, 4.25 meters deep, to support the weight of the new preheater.

Such large trusses were required to span the existing 26- x 30-meter preheater building before reaching the new top stage floor level at nearly 100 meters above grade. As a result of this innovative design, nearly all of the calciner and preheater stages were erected without adversely influencing plant operations. The new coal mill and cooler vent systems also were erected while the kiln continued to produce 1,360 mtpd of clinker.

Equipment Selection (Phase 1) By modifying the existing three-support (5.03- x 57.9-meter) kiln, all of Carolinas Cement Co.'s existing market demands (2,730 mtpd) were within reach by consolidating the production of five kiln lines into a single line.

After comparing the costs of construction to anticipated energy savings, the cement producer selected a 6-stage cyclone preheater. The 6-stage preheater offered the best fuel efficiency and still provided enough heat to dry the low moisture raw materials in a pair of existing Fuller vertical roller mills. Over time, the fuel savings gained with the 6-stage preheater far surpassed the initial additional capital costs. The preheater/calciner included Fuller/FLS low pressure drop (LP) cyclones and a conventional in-line calciner (ILC).

The LP cyclone and calciner dimensions are presented in Table 1. The benefits of the LP cyclones and ILC calciner include:

- Ease of operation;

- Minimize build-up potential due to sloped-inlet-shelve;

- Reduce pressure drop with enlarged inlet and outlet openings; and

- High material collection efficiency with 270??? involutes and cast central tube.

The existing A-C reciprocating grate clinker cooler required major modifications to double its rated capacity from 1,350 to 2,730 mtpd. As such, the existing cooler was retrofitted with air-beam technology throughout its relatively small first drive section. The total active grate area of the modified grate cooler was 54 sq meters, consisting of 15 sq meters of direct-aerated grates and 39 sq meters of chamber-aerated grates.

New high-pressure fans replaced all former cooling fans. A new cooler vent system was added to the existing cooler, given that all cooling air input formerly was pulled directly into the traveling grate preheater from the cooler through the kiln. An electrostatic precipitator was chosen for cooler vent dedusting given the staff's familiarity with their operating and maintenance requirements. Following Phase I modifications, the cooler had design parameters as shown in Table 2.

Time-Line (Phase 1) The design work for the Phase 1 upgrade began in September 1994. Equipment deliveries started in April 1995, and the construction contractor mobilized in June 1995. The traveling grate preheater was shut down at the end of December 1995. During the outage, the existing kiln feed, clinker cooler, and main electrostatic precipitators were rebuilt.

During the second quarter of 1996, the new preheater/calciner kiln started up and demonstrated its ease of operation. It was brought up to full production within hours. Performance tests conducted three months after clinker was first produced showed that the newly modified kiln system attained production levels in excess of 3,000 mtpd of clinker. The fuel consumption was consistently around 712 kcal per kg, in spite of its kiln gas bypass of 15%.

Performance results (Phase 1) Shortly after start-up, the modified kiln system proved its ability to run at a capacity of 3,258 mtpd before limitations with main stack emissions, the kiln ID fan, and the clinker cooler became evident. Importantly, this realized capacity exceeded the guarantee nearly 20%. Once the system's capacity had been demonstrated, operators stabilized the system at 3,070 mtpd in order to complete a performance test and end the commissioning phase of the project. Phase 1 performance results are presented in Table 3.

Phase 1 heat balances for kiln system and clinker cooler are presented in Tables 4 and 5, respectively.

Phase 2 The original A-C grate cooler had a capacity of 1,350 mtpd. Upon its modification by Fuller in 1996, the capacity increased to 2,730 mtpd at 50 mtpd per sq meter. Since 1996, however, actual plant performance has exceeded rated capacity by more than 20%. As a result, the modified cooler typically handled 3,300 mtpd on a daily basis. Moreover, the cooler's maximum inlet width of only seven grates resulted in a relatively high grate speed, causing increased maintenance costs. Typically, the clinker cooler required about six days of unscheduled downtime each year for maintenance purposes. Equally important, clinker discharge temperatures of 110??? C above ambient were typical at 3,300 mtpd.

Along with these factors and buoyed by a strong U.S. economy, the cement producer desired to increase the plant's capacity 33% to 3,630 mtpd of clinker. At 3,630 tpd, clinker discharge temperatures were projected to be 150??? C above ambient, which, during the summer months, would negatively impact product quality.

To achieve the higher level of performance, Carolinas Cement selected the cement industry's latest development in clinker cooler technology, the SF Cross-Bar Cooler from Fuller and F.L. Smidth. The cooler was commissioned in March 1999, and a new 82-mtph finish grinding system was commissioned in April 2000. As a result, the plant will ultimately meet and maintain an capacity of 1.2 million mtpy of clinker.

Similar to Phase 1, Phase 2 required a short overall shutdown period to ensure its viability. Fortunately, the SF Cross-Bar Cooler's modular design enabled the new clinker cooler to be installed in 32 days, including demolition of the existing cooler, thereby achieving the goal.

Part 2 of this article presents the results of Phases 2 and 3 of the Roanoke plant's modification project.