“Burn almost anything” is the tagline that is used in pyro processing nowadays. This line only indicates the challenges faced by the machinery suppliers who are involved in pyro processing. With alternate fuels gaining momentum day by day, the challenge is becoming intense.
The kiln is the heart of a cement plant. Any efficiency gains in the kiln system or pyro process can lead to significant savings in fuel costs and power costs – reducing overall operating expenses. In today’s world, there is a wide range of firing systems, starting from kiln burners to alternative fuel to pre calciners to various combustion devices. One needs to zero down on the requirements to have a perfect system. Pyro processing has remained the important part of plant operations.
The present cement machinery suppliers have been designing the products and services around space utilisation, energy consumption and emission reduction.
In the current issue, we have covered an interview with the Daniel Julien, CEO of Fives Pillard. Fives is a French multinational company having operations in India. The burners designed by Fives have been in use in our country for about three to four decades.
Similarly, pyro processing systems supplied by FLSmidth, KHD and thyssenkrupp have been equally popular. All the three major machinery suppliers like FLSmidth, KHD and thyssenkrupp have their own designs of pyro processing systems that are in use in the cement plants. The burning system suppliers have realised that fuels either to kilns or calciners vary in quality and in size to a large extent. The design of firing system should be such that it can accept all the variations in coal or alternate fuels. Different kind of hardware have been designed for that. These can be added even in the existing plants to feed the alternate fuels. A specially-designed equipment can be opted for the use of alternate fuel in calciners.
Let us take a broad view of the various options available in the pyro processing. The size of the kiln is generally decided based on the limestone reserve available. The other considerations are market size and availability of funds. The kiln size is determined keeping in view future expansion plans. If the market grows; there is always a margin left in the design to expand or may be even double the capacity of the kiln output. The designer of kilns has to take into account the investment cost and operating cost. The systems that score better on these parameters have gained excellent reputation.
In the modern design, the length of the kiln has been reduced because the calcining part of the reaction is done outside the kiln. There is continuous improvement happening on the design of calciners. When AFR is designed to be used in calciners, the design of calciners has to accommodate the use. Attention is paid to get low NOx calciner, which is supposed to be the advanced version. Today Computational Fluid Dynamics (CFD ) is commonly used while designing the hardware like pre heater and pre calciner. The buyer also ensures that he shall get the upgrades at reasonable costs. The low NOx calciner is the need of the hour. The design of calciner should be such that it can be adapted in the old plants as well along with new installations.
Over a period of time there has been a significant improvement in the pre heater and pre calciners of the kiln system so as to reduce the length of the kiln. It serves two-fold purpose 1) the length of kiln is shortened, the cost is reduced and 2) through put of the kiln improves. Day-by-day, the margins in design are getting reduced and the equipment supplied are operating closer to the limit values.
While considering the rotation of a rotary kiln, we have to appreciate that the kiln is lined in side with refractories, and one has to pay attention to the life of the refractory while considering the performance of pyro processing. By a rule of thumb, for the life of refractory, lining is around one year, which may slightly vary depending on the situation. General preference is given to have basic bricks for burning zone and alumina bricks in the rest of the kiln.
The other subject that is debated about the kilns is if it is having two tire stations or three tire stations. A tire station is nothing but the support on which the kiln is resting. The present trend is to go two tire stations. In this case length to diameter ratio of approx. 11:1 to 12:1. The investment cost is approximately lower by 15 per cent. The three-station kiln is an established and proven design and has been thoroughly tried and tested over the decades. The three-station rotary kiln system has relatively a balanced design that ensures stiffness and minimises wobbling and misalignment.
The other important part of the kiln is drive unit. The drive units consist of the main and auxiliary drive. Variable speed AC or DC motors are used as drive motors. The auxiliary drive is furnished with an electric motor and/or a combustion engine. In conjunction with multi-stage or planetary gearboxes and elastic, practically maintenance-free couplings, the drives offer maximum operational safety and efficiency.
The drives for all kilns are designed for the reliable operation of the kiln under any operating conditions. This means that even greater safety factors and torque reserve, are installed for nominal operation. All drive components are designed for the installed motor torque plus all safety factors and not only for effective torque generated at nominal capacity. Kiln suppliers offer various types of drives for kilns operation, depending on the application. The installed reserves and safety factors ensure the extremely flexible operation of the kiln, including higher clinker throughput, without overloading the drive.
FLSmidth advocates that some of the kiln outlet sections to be kept as spares with refractory lining already completed. It says ‘don’t waste time on costly kiln stops to make repairs’. Our replaceable components can prolong the lifetime of kiln outlet sectors and drastically reduce replacement time. Replacing a damaged sector is easy. Reduce the costly downtime by having a second set of replaceable sectors ready to go for scheduled stoppages.
Keep false air out, hot gas in with our innovative graphite kiln seal. This innovative graphite rotary kiln sealing system can make any existing kiln more efficient by optimising pressure stability and reducing the release of hot gasses and dust particles. Its modular frame allows for pre-assembly, which means fast, easy installation with minimal downtime.
The increasing use of alternative fuels has brought the subject of safety to the forefront at many cement plants, especially the hazards related to the storage and transportation of fuels, structural failures of fly ash silos/coal bins, explosion in kilns and coal mills, etc., underline the importance of process safety management that needs to be addressed very seriously.
While Process Safety Management (PSM) is predominantly adopted in process industries such as the oil and gas sectors, its implementation in the cement industry helps plant personnel systematically identify and mitigate process hazards. Various studies have shown that there is enough scope for improving overall process safety in the cement plants.
- VIKAS DAMLE