Using modern techniques like CFD and a few laboratory tools, changing from one fuel to the other in Rotary Kilns has become fairly easy and quite predictable.
The DUOFLEX burner is widely used in rotary kilns with a variety of fuels like pulverized coal, coke, or natural gas. The burner is highly customizable, allowing ease of operation with alternate fuels like plastic chips and sewage sludge. Duoflex burner design is based on the proven concept of fuel flowing in a central duct; which is circumscribed by annular ducts for primary air supply. The annular ducts, one providing radial and the other providing axial flow are mixed at the nozzle tip. The tip is retractable, allowing fine control of the nozzle area. The radial to axial air ratio is also adjustable. Thus the burner allows for a high degree of operational flexibility, and provides a stable flame under a wide variety of process conditions. These factors contribute to Duoflex being the burner of choice for cement installations all over the world.
To demonstrate the universal application of the burner, a study was commissioned to explore the feasibility of burning peat as a primary fuel. Peat is the accumulation of partially decayed organic matter and burning it in kiln is not a standard operating condition. Therefore, a Computational Fluid Dynamics (CFD) study was commissioned to evaluate the burner performance. Peat was tested and compared with coal using Thermogravimetric Analysis/Differential Thermal Analysis(TGA/DTA) tests. Figure 2 shows the results from the TGA/DTA tests.
Crucial analysis of peat was carried out in the laboratory to understand the fuel. Based on the results, a CFD model was developed in ANSYS to simulate the burning of peat in a kiln. In the kiln, the fuel undergoes “fast” combustion, limited only by the mixing of fuel and oxidiser. This was incorporated in the CFD model. There are a large number of reference plants for burning coal in the kiln. Using the reference data from the plants and with in-house experience, the CFD model with coal was validated. In the same model, the fuel was changed to peat and the results were compared with each other. Three scenarios were simulated using the CFD model namely, 100% Coal firing, 100% Peat firing and Peat/Natural Gas combination. The CFD model revealed that peat firing is distinctively different from coal fired systems. Figure 2 shows the flame profiles for the three fuel firing in the kiln system. The conclusions from the CFD results were used to design features for the Duoflex burner that would help sustain continued operation for alternate fuel firing.
What is computational fluid dynamics (CFD)?
Computational fluid dynamics (CFD) is the use of applied mathematics, physics and computational software to visualise how a gas or liquid flows — as well as how the gas or liquid affects objects as it flows past. Computational fluid dynamics is based on the Navier-Stokes equations. These equations describe how the velocity, pressure, temperature, and density of a moving fluid are related.
Computational fluid dynamics has been around since the early 20th century and many people are familiar with it as a tool for analysing air flow around cars and aircraft. As the cooling infrastructure of server rooms has increased in complexity, CFD has also become a useful tool in the data centre for analysing thermal properties and modelling air flow. CFD software requires information about the size, content and layout of the data centre. It uses this information to create a 3D mathematical model on a grid that can be rotated and viewed from different angles. CFD modelling can help an administrator identify hot spots and learn where cold air is being wasted or air is mixing.
Simply by changing variables, the administrator can visualise how cold air will flow through the data centre under a number of different circumstances. This knowledge can help the administrator optimise the efficiency of an existing cooling infrastructure and predict the effectiveness of a particular layout of IT equipment. For example, if an administrator wanted to take one rack of hard drive storage and split the hard drives over two racks, a CFD program could simulate the change and help the administrator understand what adjustments would be needed to be made to deal with the additional heat load before any time or money has been spent.
By Arun Appadurai, CFD Specialist, FLSmidth Pvt. Ltd