(461c) Granulation of Reactive Pyrotechnic Composition for Temperature Mapping of Coal-Boilers

Coal is expected to remain a viable energy source for the foreseeable future in some regions of the United States due to its abundance and low cost. However, improving the efficiency of coal-fired boilers is essential to limit the emissions per unit of energy produced. Improved knowledge of the spatial distribution of temperature inside the boiler can provide insights into the boiler performance and can also be used for the tuning and validation of detailed models of combustion within the coal-fired boilers for more efficient operation. The harsh temperatures and reactive environment within a boiler often require novel ways for the determination of a 3-D temperature map of the boiler.

Pyrotechnics are flexible materials that can be tailored to produce a distinct light emission which could therefore enable them to be used as optical sensors. The requisite thermal protection could be provided by coatings designed to shed after specific thermal history. The coated pyrotechnic materials would be fed with the pulverized coal and the light emission can be captured by a camera or spectroscopic techniques to provide the 3-D map of temperature. Given the yellow-orange-red background inside the coal-boiler, a green color would be distinguishable, and thus green light-emitting pyrotechnic compositions were developed. The composition consisted of a mixture of metallic fuel, oxidant, colorant, and binder. These components need to be homogenized into granules having similar particle size to the pulverized coal to enable similar fluidization behavior and for the application of the protective thermal coating layer.

The current work focuses on the agglomeration of optimized green light-emitting pyrotechnic composition in desired-sized granules between 500 and 850 µm (20 – 35 mesh) using a high-shear mixer. The process was optimized by varying different parameters such as inter-impeller distance, type of binder, speed of rotation, and liquid to solid ratio. The granulated material was characterized in size using an optical microscope and standard sieve meshes. It was observed that inter-impeller distance negatively influenced the yields (lower distance gave higher weight% yields) as seen from figure 1. This was because a narrow gap provided more chopping action and prevented large agglomerates from forming. When the distance was increased, larger granules were formed in between the two impellers thereby drastically reducing the yield. In terms of the type of binder, ethylcellulose-based binder provided high yields as compared to conventional pyrotechnic binders: shellac and red gum. This result might have been due to the difference in the viscosity of these binder solutions. Medium speed of rotation (500 rpm) provided adequate granule growth and provided the best yields whereas, a liquid to solid ratio of 0.25 ml/g provided high yields.

Figure 1: Effect of inter-impeller distance on the granulation yield