Part 2 This second in a two-part case study illustrates how a partnership between a cement producer and a process equipment supplier can result in dramatic savings in the plant's operating costs

Equipment selection (Phase 2) Following Phase 1 modifications, the plant successfully produced more than 3,300 tpd of clinker before facing limitations from the main stack emissions, the kiln I.D. fan capacity, the clinker cooler's mechanical reliability, and elevated clinker discharge temperatures.

With such common challenges, Fuller recognized that additional clinker production could be obtained by further increasing the cooler's recuperative performance, which would lower clinker temperatures, reduce specific fuel consumption, and thereby add operational margin to the kiln I.D. fan capacity and specific emissions levels.

With this knowledge, the plant faced a decision: to modify an existing cooler that it had clearly outgrown or to adopt a completely new clinker-cooling concept. The plant management received the information presented in Table 6 to assist in the decision-making process.

After witnessing the performance of the SF Cross-bar Cooler at three other installations, plant management was convinced that the cross-bar cooler would yield significant improvements in operational run time and maintenance requirements given the following advantages:

- Self-regulating, active air distribution for every grate plate in the cooler;

- Simple operational controls;

- A fixed, non-wearing grate line for air distribution;

- Modular design;

- No moving grates or air-beams; therefore, no internal piping;

- No sealing air fans and fewer overall number of fans;

- Thermal efficiency;

- A clinker conveying mechanism that is separate from the cooling mechanism;

- Easy replacement of wear parts; and

- No clinker fall-through, and no need for hoppers or spillage valves/conveying system.

Time-Line (Phase 2) The contract for the new cooler was signed in November 1998. The cooler was shop-assembled and delivered to the plant site 12 weeks later. The demolition of the existing cooler began on Feb. 2, 1999, and the kiln system with the new cooler was restarted March 5, 1999.

The cooler's modular design enabled the complete replacement of the existing reciprocating cooler with the SF Cross-Bar Cooler in 32 days (from fire-out to fire-in). Reduced installation time and cost were achieved by the arrival of pre-assembled modules from Fuller's manufacturing shop.

Performance results (Phase 2) The cooler demonstrated an ease of operation within hours of start-up. Within days, the control room operators realized the benefits of the cooler's mechanical airflow regulation (MFR), which ensures every grate plate in the cooler is continuously provided with the needed amount of cooling air. As a result, minimal maintenance and attention from the control room operators were required. During the first full year of operation, no unforeseen kiln downtime was attributed to the cooler and the kiln system's availability was significantly higher.

The cooler's recuperative performance proved itself capable of supporting a higher overall kiln system capacity. Production rate was stabilized at 3,600 tpd to complete a performance test on the system. Performance results are presented in Tables 7 and 8.

Table 9 illustrates the benefits realized by the installation of the SF Cross-Bar Cooler. Table 9 also illustrates the importance of calculating the "normalized" VDZ cooler loss so that different coolers may be justifiably compared in terms of efficiency despite operating with different combustion air requirements.

Phase 3 The cement producer commissioned its main stack emissions monitor in September 1996. NOx, CO, and the cooler vent stack opacity were well within the guaranteed numbers. The SO2 emissions, however, needed to be addressed. The process has proven to operate at close to the hourly SO2 level defined by the annual permit limit when both raw mills are in operation. When one or both mills went down, the SO2 emissions would increase by more than 40% within minutes. At the present time, a lime injection system is an effective method of maintaining the required SO2 emissions if one or both raw mills shut down.

The goals for 2000 and beyond are to further increase production to 4,000 tpd of clinker and convert the traditional in-line calciner to a modern low-NOx/low-CO design. The following modifications are proposed to Roanoke Cement Co.:

- Modify the kiln riser duct and relocate the fuel entry point to create a high-temperature, oxygen-deficient atmosphere for destruction of kiln-exit NOx ;

- Add a mixing duct to the top of the existing calciner for increasing retention time and ensuring complete combustion of the fuel;

- Add a pneumatic system for conveying the high carbon-containing fly ash component directly to the calciner, which will eliminate its effect on overall CO emissions and further reduce the specific fuel consumption of the kiln system;

- Enlarge and reroute the tertiary air duct to create a reduction zone in the kiln riser duct and improve overall control of the kiln exit oxygen content;

- Increase the kiln feed system's capacity and install an alleviator on its air-lift for reducing false air introduction with the kiln feed;

- Increase kiln speed; and

- Increase the capacity of an existing coal mills through installation of dynamic classifiers on each mill to meet future fuel demands at up to 1.3 million tpy of clinker.

Equipment selection (Phase 3) The proposed modifications for Phase 3 will increase the calciner's total retention time to a minimum of 3.45 seconds at a production of 4,000 tpd of clinker.

At the Phase 3 capacity of 4,000 tpd, the modifications shown in Table 10 will minimize possibilities for any significant CO to exit the calciner. Emission testing at the plant demonstrates that the overwhelming majority of the CO emission arises from the high amount of carbon in the kiln feed.

The carbon present in the kiln feed (75% of which typically derives from the fly ash component) begins to burn in the upper and middle stages of the preheater tower, where inadequate oxygen and temperature are available for its complete combustion. The majority of stack CO emissions are formed here. Of course, the incomplete combustion of the feed carbon adds heat to the preheater exhaust gas in the form of CO. The higher heat loss, thereby, increases the specific fuel consumption of the kiln system.

In order to meet the permitted CO emission levels at 4,000 tpd (and to further reduce the already low specific fuel consumption), the fly ash component will be introduced directly to the calciner where sufficient oxygen and temperature are available for its complete combustion.

The tertiary air duct modification will provide improved control of the kiln exit oxygen level and create a NOx-reduction zone in the kiln riser duct. The modification will enable the kiln to operate at lower oxygen levels, which will lower kiln exit NOx and improve the effectiveness of the alkali bypass.

Kiln feed modifications will increase transport capacity. The alleviator will instantly reduce the preheater exit volume 5%.

Performance projections (Phase 3) The Virginia Department of Environmental Quality has mandated that the cement producer attain emissions levels of 1.4 kg NOx/ton clinker and 1.0 kg CO/ton clinker by the year 2003. Roanoke Cement can expect to meet these more stringent requirements by introducing the fly ash component directly to the calciner and by implementing Fuller's and F. L. Smidth's proven ILC low-NOx calciner design.

The plant's modified kiln system produces as much as 30% more clinker than the former four long, dry kilns and the traveling grate preheater kiln combined. The preheater/calciner/cooler provide a fuel savings of more than 40% over premodification baseline conditions. A commensurate reduction in specific power consumption has been realized as well. With advances in technology, new and more stringent emissions limits are well within reach. Reduced maintenance requirements of the equipment have significantly improved the plant's run time.

A 33% reduction in cooling fan power consumption is attained by virtue of a 43% reduction in cooling air requirements, while cooling to a significantly lower clinker discharge temperatures as well. The net effect of the cooler modification is a 9.6% increase in clinker production and a 5.5% reduction in specific fuel consumption from an already low figure of 712 kcal/kg at 15% bypass. Pre-modification clinker production at the plant was limited by excessive clinker cooler discharge temperatures. This accounts for the higher-than-expected clinker capacity increase (9.6%), compared to the measured decrease in pyroprocessing fuel consumption (5.5%).

Based on these results, the SF Cross-Bar Cooler installation at the Roanoke plant has a payback period of less than two years. Payback analysis is based on 330 days of operation and the following assumptions:

- 9.6% clinker capacity increase from Phase 1 (assuming US$30.00/ton of clk)

- 4.0 kWh/ton of clk power savings from Phase 1 (using a power cost of US$0.04/kWh)

- 39 kcal/kg clk fuel savings from Phase 1 (assuming a fuel cost of US$45/ton of clk)

The new cooler's simplistic design and increased overall availability of the kiln system further shorten the payback period.

Conclusion This article has presented a case study of how a cement plant originally designed for 900,000 tpy is able to achieve 1.2 million tpy. Roanoke Cement and Fuller originally worked together to double the rated capacity of a mid-1970's kiln system by completing a fast track (Phase 1) project on time. The results are evident in the achievement of the heat, power, and emissions guarantees while exceeding original guaranteed production levels 20%. Then, by implementing emerging clinker cooler technologies in Phase 2, the system has achieved clinker capacities of 33% greater than original guarantees.

Today this kiln system is capable of producing 1.2 million tpy at a specific fuel consumption of 673 kcal/kg (at 15% bypass) and at a specific power consumption of 22 kWh/ton. Therefore, this modernization project serves as an excellent example of the success that is possible through a partnership between a cement producer and a process equipment supplier.