Crude glycerol and glycerol as fuels and fuel additives in combustion applications

Biodiesels as renewable fuels are important in approaching low-carbon transportation. One critical challenge associated with biodiesel production is the utilization of crude glycerol, an abundant waste byproduct from which glycerol can be obtained via refinement, to maximize economic and environmental benefits. Efforts on crude glycerol and glycerol utilization include direct use (e.g., combustion fuels) and indirect use (e.g., reformation for value-added chemicals). Among different approaches, the direct use as fuels and fuel additives is straightforward and affordable. Recently, significant progress has been made to understand their combustion performance and explore new applications, including fuel additives for reducing emissions and improving the combustion performance of traditional fuels, and fuel for combustion synthesis of valuable materials. The current review discusses these achievements and future outlook on using crude glycerol and glycerol as fuels and fuel additives for combustion applications. Overall, crude glycerol can achieve high-efficiency combustion with the improved design, but ash deposition is still the main challenge. Fortunately, integrating crude glycerol with solid fuels could mitigate ash issues and improve the combustion performance of low-rank solid fuels. In contrast, glycerol is an oxygenated fuel additive for lowering NOx emissions. Furthermore, glycerol is capable as a fuel in combustion material synthesis for manufacturing the raw materials for catalysts, sensors, and battery applications.