Particle Ignition and Char Combustion

The combustion characteristics of solid fuels at 1 atm can be readily characterized and quantified with Sandia’s Optical Entrained Flow Reactor (Figure 1). This unique research facility features a quartz-walled combustion-driven high-speed flow reactor in combination with advanced optical-laser diagnostics and camera systems. We can produce a wide variety of combustion or even gasification environments with the Hencken burner that drives the reactor, over temperatures ranging from 1,200–2,300 K. We use optical diagnostic systems to measure particle temperature-size statistics or collect image data on individual particles or collections of particles. A helium-quench water-cooled probe is used to collect partially reacted particles for off-line analysis.

Figure 1. Schematic of Sandia’s optical entrained flow reactor for investigating the combustion characteristics of solid fuels such as coal and biomass. The optical system shown to the right is used to measure the instantaneous temperature, size, and velocity of burning particles. The photograph in the top right shows a collection of streaks from injected particles as they ignite and burn while flowing upward.

Recent work in this laboratory has focused on measuring the combustion reactivity of lignin residues that remain after biomass is processed and on quantifying the residue’s ignition delay and char combustion rates during oxy-fuel combustion of coal. Approximately one-third of most biomass used for energy is composed of lignin and other constituents that are not amenable to biochemical conversion to fuels. This material must be used effectively through thermochemical processes to increase the efficiency and lower the production costs of second-generation biofuels. Figure 2 shows a photograph of burning residue particles and Figure 3 shows the influence of bulk gas oxygen content on ignition of a stream of coal particles. Oxy-fuel combustion of coal is being developed as a promising technique for capturing CO2 (for subsequent use or sequestration) when burning coal to produce power (see Figure 4). Locally high concentrations of oxygen and elevated concentrations of CO2 and H2O (from flue gas recirculation) create very different physical and chemical properties of the combustion medium, influencing coal ignition and combustion rates. Sandia has been quantifying these effects for inclusion in CFD modeling of full-scale oxy-fuel boilers.

Figure 2. Photograph of burning particles of residue from dilute-acid enzymatic hydrolysis processing of corn stover to make bioethanol.

Figure 3. Photograph of Pittsburgh coal stream particle ignition at 1,150 K for a range of oxygen contents.

 

Figure 4. Schematic of the basic process layout for oxy-fuel combustion with CO2capture.

 

Contact: Chris Shaddix, (925) 294-3840,