Considerations for large capacity clinker production systems (2)

In the November 2008 issue of Highlights, we began with Part 1 of this 2-part series. Here, we reviewed the overall factors for determining the most practical maximum clinker production level at a plant, and then on the equipment side, examined the considerations in preheater design. Part 2 will now focus on design practicalities of kilns and clinker coolers rated for high capacity.

Despite the current cement market turmoil, many of the fundamental reasons for installing large capacity clinker production systems still exist. In fact, economies of scale and the desire to push operating costs per tonne of production as low as possible may be even more in the forefront. The long-term cement market outlook is also well in line to support modern large capacity systems. But putting aside such economic factors, let us return to some of the key equipment design decisions from the pyro-process perspective.

Kiln design considerations
The heart of a clinker production system is often said to be the rotary kiln. While there may be redundancy in other areas of a large capacity plant, especially in grinding and auxiliary equipment, the kiln itself will certainly be one machine that meets the full production.

FLSmidth offers conventional 3-support kilns and ROTAX-2 (2-support) kilns, which have become a well-proven standard in the industry with tangential tyre suspension, self-aligning supports, and friction drive through the rollers. The largest ROTAX-2 kilns so far in operation are 5.5 metres diameter by 66 metre long size – one now operating for 3½ years and two others for 2 years at production levels up to 7,000 t/d. But the design does not stop there. We also have in the type range ROTAX-2 kilns of 6.0 and 6.6 metres in diameter. On 3-support kilns, we have supplied 11 of 6.0 to 6.6 metres diameter. A 6.6 metre diameter kiln of 3-support or ROTAX-2 design can in most cases support the maximum single kiln capacities designed to date. In general, there are no mechanical design limitations for either type of kiln up to this size.

So then what are the other types of limitations for supplying the very large diameter kilns, and even larger diameter ones? Are there more limitations in either the 3-support or ROTAX-2 kiln? Bear in mind that for a given production level, a ROTAX-2 kiln will have a larger diameter compared to a 3-support kiln due to the lower length to diameter ratio of a 2-support kiln.

• First and foremost, there are few manufacturing facilities in the whole world able to make the large diameter tyres. With the larger diameter, this may have even greater impact with a ROTAX-2 kiln for the largest sizes
• The logistics of transporting the large components to site must be considered, and usually has to be done by water route. Special arrangements must be made for any type of very large kiln
• With increasing kiln diameter, the tapering of the refractory bricks becomes less and the refractory wedges become smaller. Lining installation, therefore, becomes a challenge especially above 6.6 metres diameter. This is the same for either a 3-support or the ROTAX-2 kiln
• Lastly, sufficiently low kiln velocities must be maintained to prevent high dust circulation, even as volumetric and thermal loadings are stretched. Here the favour goes to the larger diameter ROTAX-2 kiln.

There is an equipment weight advantage to the ROTAX-2 kiln up to a production level of about 8,000 t/d. Beyond that, if the 3-support design stays with floating tyres, there is a gradual decrease in that comparison. This is not a completely accurate picture however, because 3-support kilns greater than 6.0 metres diameter would be recommended to have tangential tyre suspension to limit kiln ovality for improved refractory lifetime. When this is considered, we then again see the weight advantage go to the ROTAX-2 kiln.

Clinker cooler design considerations
And now from the kiln to the clinker cooler, which also has single machinery duty for the full production level. Since 1997, FLSmidth has offered the SF Cross-bar cooler. Most recently, the Multi-Movable (MMC) version was developed. With high transport efficiency and resulting horizontal construction, the MMC cooler is especially well-suited for the highest of clinker capacities.

We are now approaching 200 Cross-bar coolers supplied to the industry for a total installed capacity of 240 million tons of clinker. Ten of these Cross-bar coolers are for kilns rated at 8,000 t/d or greater.

With very large clinker capacities, what limitations do we see in the clinker cooler? The standards with respect to the length to width ratio of the cooler versus width loading become stretched. On one hand, we like to maintain a maximum of around 1,500 tpd/m width loading, but on the other hand, there become concerns about the distribution of clinker at the inlet across very wide coolers. There is also the need for high transport efficiency to ensure that the operating drive strokes per minute do not become too high, which ultimately leads to lower life of cooler components.

To overcome the problem with clinker distribution at the inlet, and furthermore to eliminate the formation of “snowmen”, FLSmidth has developed a new design fixed inlet for coolers. It is known as the ABC, or Air Blast Controlled, cooler fixed inlet. Through an internal check valve arrangement, high pressure air blaster air is allowed to eject through each and every grate plate to ensure there is no way for build-up and uneven distribution to occur across the inlet. There is full flexibility in control of blasting area, blasting frequency, and blasting intensity so that specific problem areas can be targeted while minimising overall high pressure air consumption.

This concludes the 2-part series on large capacity clinker production systems. Based on the experience gained over nearly 20 years in design, supply, and commissioning of cement plants with capacity of 10,000 tpd and greater, one can say that FLSmidth knowledge in this area is second to none.