Many commercial combustion systems focus on low-cost, readily available, low-grade fuels.  Most of the low-grade fuels (coal, biomass, etc.) form ash. By careful fuel selection, we can reuse materials that would typically be discarded, use alternate energy sources instead of depleting our natural fuel supply, and reduce the harmful pollutants that result from the use of combustion as a power source. Here's a sampling of some of the fuels we commonly burn:

Coal:  one of the nation's primary fuel sources

Biomass: reusing organic materials

Energetic Materials: high energy materials such as TNT and rocket fuel

Black Liquor: a byproduct of the paper-making process

Cofiring: combinations of fuel used to optimize boiler performance and combustion output

Contaminated fuels: Sandia's use of combustion research to aid in restoring contaminated land in Belarus

Pyrolysis Oils: oils with inherently lower burning rates than traditional fuel oils, which have low overall efficiencies and tend to have ash deposition and corrosion problems

Metals: usually mixed with energetic materials in aerospace applications