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The use of solid fuels in compact combustion power systems

A sample of Turbine Bio Fuel was provided by Robert Fulton of CCA, LLC.  The producer of this algae derived solid fuel intends to explore its use as a fuel in aircraft engines or aero-derivative gas turbine power systems.  As an expert in alternative fuel combustion power systems and the fuel feed systems required to support pressurized combustion of bulk solids fuels, I evaluated the powder for feasibility of use in ARL/Penn State’s powder fuel feed system.

The vast majority of compact combustion power systems developed for mobile systems rely on liquid and gaseous fuels as the source of chemical energy.  Under certain circumstances, however, solid fuels might offer particular advantages due to their unique thermodynamic properties or economic attributes.  In order to exploit these advantages, at least two obstacles must be overcome.  The first is to deliver the fuel in a steady manner to the combustor which is typically well above atmospheric pressure.  While delivering gases and liquids to a pressurized combustor with pumps and compressors is routine, doing the same with solids can require specialized equipment and carefully controlled fuel properties.  The second large challenge associated with the use of solid fuels in compact combustion power systems is the rapid ignition and complete combustion within the combustor.  Gases and liquid naturally disperse in air streams in ways that increase reaction rates so that reasonably small combustor sizes can be maintained.  Solids generally tend not to disperse particularly well and solid combustion can often involves relatively slow phase changes.

Solids materials can generally be delivered to pressurized compact combustors and dispersed for rapid, intense combustion if the solids can be fluidized homogeneously.  Homogeneous fluidization, in this context, indicates that the mass of solids in flight through the fuel pipeline has a density that is only slightly less (5-10%) than the powder density at rest and that the cloud of particles flowing through the pipeline and into the combustor is generally of uniform density.  Such fluidizable solids behave very much like liquid fuels and can be considered for use in systems designed for liquid fuels.

The characteristics of fluidizable powders have been studied for some time by a number of researchers.  The Hausner Ratio is a simple but revealing parameter that is indicative of homogenous fluidization behavior.  It is simply the ratio of the tapped or vibrated powder density to the poured density.  For the sample provided, I measured a value of 1.31 indicating a slight degree of cohesion which is generally considered beneficial for stabilizing expanded/aerated masses of powders.  Direct experience indicates that powders with Hausner Ratios of below 1.35 can exhibit homogeneous fluidization.  A Hausner Ratio of 1.25 is generally considered ideal for homogeneous fluidization.  In order to reduce powder cohesion and facilitate further handling experiments, the sample powder was mixed with a very fine, food-additive-grade silica powder.  This step will almost certainly not be necessary to produce homogeneous fluidization if a slight larger particle size distribution can be produced.  Scanning electron microspore images of the sample powder indicate a powder with a relatively high sphericity and a low incidence of irregular particles.  These observations also indicate a good probability that the powder will be able to be handled in the pressurized fuel feed system.

SOURCE: Dr.Tom Cawley
Administrative Area: RESEARCH-DEFENSE REL
The Pennsylvania State University
Applied Research Laboratory
State College, PA 16801

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