(406g) Finding the Preferred Safe Operating Condition of a Fluidized Bed Incinerator to be Used for the Disposal of Waste Explosives

Explosives are one of the most important components of missiles systems. In general, they have a limited usable period over which performance can be assured, after which they should be disposed of safely. This is often done by incineration of the particulate explosive, though the process is extremely hazardous due to the highly reactive materials such as TNT (2-Methyl-1,3,5-trinitrobenzene), RDX (1,3,5-Trinitro-1,3,5-triazinane) and HMX (1,3,5,7-Tetranitro-1,3,5,7-tetrazocane). Currently rotary kilns are used for this purpose. However, the method has some problems such as incomplete reaction and local hotspots and fluctuating process conditions. Therefore a new and more efficient type of incinerator is required. Since fluidized bed incinerators are well known for effective heat transfer and well mixed conditions, it was proposed that they be considered as an advanced reactor for this incineration process. It is apparent that fluidizing and mixing characteristics are likely to be quite significant in the design of an effective and safe fluidized bed technology. Thus it was proposed to first study the system using a computational fluid dynamics (CFD) model to evaluate the likely bed performance. For the bed fluid dynamics the Eulerian - Lagrangian coupled method is used to rigorously simulate fluidization behavior of the particles in bed, while allowing the effect of various process conditions such as the air injection rate to be considered. In addition, the decomposition reactions of the explosive are modelled to calculate the temperature and pressure conditions in the incinerator. Physically, the bed is designed as a cylinder 2m in diameter and 9m in height, with a target disposal rate of 20,000kg/year, assuming a uniform particle diameter of 3.0mm. For safety, 40% of fluidized bed is filled with sand to buffer against spikes in pressure and temperature. The simulations are performed over a range of air injection velocities and the maximum temperature change with time in the bed for each velocity is calculated. The objective is to find the condition that gives the least spiking of temperature and pressure. The results of the CFD model showed that fluidization begins to be observed at 1.0 m/s of air injection rate, however, fluidization is weak in that the particulate explosive and sand are not mixed well. At 1.5 m/s of air injection velocity, meaningful fluidization is observed and the maximum temperature in the bed decreases. When the air injection rate is 2.0 m/s, maximum temperature is lowest, before increasing again at 2.5m/s. In this case the fluidization is too strong creating is a lot of voids, therefore, the sand cannot buffer well. Given that high temperature will result a less well controlled reaction and a more hazardous operation, 2.0 m/s of air injection rate is recommended as the preferred safe operating condition. This research will be of help to demilitarize old explosives safely and economically and can be useful data to build incineration facilities.