For the AF system, it is basic requirement for achieving high thermal substitution rate that to utilise all type of available AF like refuse-derived, industrial plastic, biomass, tire chips and hazardous waste effectively, efficiently and economically. Transfer chute is a crucial component of the material handling chain, yet too often also the weakest link and need special attention to ensure smooth handling operation.
Indian cement plants are adopting the alternate fuels (AF) for sustainable development and achieving adequate thermal substitution rate (TSR) as per the Government Guidelines 2016. To achieve the targeted TSR, the Indian cement manufacturers are adopting all the possible alternate fuels according to their geographical availability and economic viability. For AF systems, it is a basic requirement to handle all types of AF utilised by the cement plant like refuse-derived fuel (RDF), industrial plastic waste, biomass, tire chips, hazardous waste, etc. to run the plant effectively, efficiently and economically. The outcome of last decade indicates that the Indian cement industries are now well aware about various equipment available for AF handling, and technical know-how is required only for selection of right equipment at right place for right material. There are several studies that assess the impact of AF on process and main equipment. Another big reason why cement industries do not focus towards increasing TSR is the uncertainty in availability of waste. A single type of AF is not always available and cement plants need to alternate between several AFs (or combine different AFs) to achieve substantial TSR.
Chute transfers are crucial components of the material handling chain, yet too often also the weakest link. They are typically employed in belt conveying systems to transfer bulk materials from feeders, screens, transfer from one conveyor to another or finally discharged into burners/pre-calciner. Chute design often demands more attention and can be the source of more downtime than the conveyors or equipment that precede or follow them. When a generic chute handles AF in combinations with variation of material characteristics or operational requirements often results in problems which translates to loss in productivity and stalled operation due to jamming or unpredictable behavior of chute.
Problem in chutes
To achieve high TSR values, it is required to explore the various type of AFs and study the wide range of variation in their physical properties. Some of them are bulk density, particle size, particle shape, moisture content, cohesion, adhesion, etc. Different AFs possess different characteristics and may require different set-ups for co-processing. These characteristics define different flow ability for AFs. The conventional methods of chute design selection currently used, do not take into account this variation and this may leads to plugging of chutes at the impact points, uncontrolled flow of material, excessive wear on chute surfaces, Excessive generation of dust, attrition or breakdown of particles and finally frequent jamming in chutes. All these problems occur frequently and demands very high attention and so the maintenance will become frequent business. Not only scheduled maintenance, in the event of breakdowns, it became more crucial and urgent attention need to be given for immediate remedy.
It is necessary to handle different AF in an optimised manner. To do this one of the solution could be to find a common chute design which can accommodate all possible AF being utilized in a system with minor setup of mountings. If not possible then certain module can also be found to handle similar type of AF through one channel of handling system.
Modelling of bulk material for which system is being planned can be act as a primary tool to find the characteristics of the flow. The simulation will also help in observing the behavior of a chute towards a particular material flow. The modelling can be done on any CFD (Computational Fluid Dynamics) software which may provide modelling method for bulk granular flow material simulations. A few possible models are described below:
1.Discrete Element method: It considers the material as a collection of particles flowing independently and they move under the influence of rotation, momentum exchange, contact forces and drag and lift forces.
2.Continuum model: When the intermolecular distance is much larger than size of molecule than the properties are not homogenous at all points. To model such materials we consider them continuous and form a continuum model.
3.The Eulerian model: The different phases are treated mathematically as interpenetrating continua. Since volume of one phase cannot be occupies other phases, the concept of phasic volume fraction is utilised.
The outcome of the correct CFD model analysis will provide the direct benefit to the cement manufacturers and the design consultants in the following ways:
Readymade design for selection inline of expected combinations of alternatives fuels.
Least shutdown/stoppage due to jamming of chutes and Safe and environmental friendly operation
Established know-how for particular combination of AFs and Standard guidelines to follow while setting up material handling system, especially the chute proximity subsystem.
The detail analysis on CFD will help the Engineers to run tests without causing much financial burden. An optimum design of chutes can be found and verified with the help of available model. The results can be used for selection of specific chute design for a given requirement and material. More versatile chute design can be used very easily. NCCBM is having latest software of CFD and recently carried out number of CFD studies in various area of cement plant. Outcome of these studies were found very much useful for system design improvement and process optimisation.
About the auther Kapil Kukreja, Dr BN Mohapatra and Ashutosh Saxena are from the National Council for Cement and Building Materials (NCCBM), Ballabgarh, India.