(487e) Development of a Bench-Scale Oxy-Fuel Combustion System for the Study of Pollutant Behavior

In July 2008, the U.S. EPA proposed to regulate the underground injection of carbon dioxide (CO₂) for the purpose of geologic sequestration (GS) under the Safe Drinking Water Act (SDWA). Elements of this proposed rule will build upon existing Underground Injection Control (UIC) regulations, with modifications to address the unique nature of CO₂ injection for GS. UIC regulations address technologies developed and refined by the oil and gas, and chemical manufacturing industries over the past several decades, and when applied to CO₂ GS, would regulate the siting, construction, operation, monitoring and testing, and closure of injection wells that could endanger the safety of underground sources of drinking water as required by SDWA. One concern is the possible presence of impurities in the CO₂ stream. These impurities could include residual quantities of hydrogen sulfide, sulfur and nitric oxides (or acids), mercury and other volatile metals, as well as organic and inorganic hazardous pollutants.

Oxygen enriched combustion (oxy-fuel) and specifically, oxy-coal combustion, is a developing, and potentially a strategically key, technology intended to accommodate direct CO₂ recovery for possible sequestration. Oxy-coal combustion is applicable for new as well as for retrofit application to existing coal-fired power plants. During oxy-coal combustion, combustion air is separated into its oxygen (O₂) and nitrogen (N₂) components and the coal is burned in a mixture of O₂ and recycled flue gas. By eliminating the N₂, ~80% of the volumetric flow through the utility plant is eliminated. The resulting effluent is composed primarily of CO₂, H₂O, and small amounts of residual O₂, N₂ (in leakage), and pollutant species (SO_x, NO_x, fly ash, metals, Hg, HCl, organics, etc.) which must be further processed before the CO₂ can be compressed, transported, and sequestered.

The state of the science behind oxy-fuel/oxy-coal combustion technology is far from mature, although a number of academic, governmental, and industrial research organizations are investigating various aspects of the technology. These ongoing efforts are focusing primarily on resolving technical and operational issues such as ignition and flame stability, fouling, slag behavior, corrosion, heat transfer, and efficiency. There is a significant lack of understanding regarding the environmental issues which must also be addressed before oxy-fuel/oxy-coal combustion can be adopted commercially. These include effects on pollutant formation, and how changes in pollutant behavior, concentrations, and the flue gas environment affect existing pollution control devices, and the characteristics of the various process and waste streams. Further, it is unknown how pollutant species will partition within compression and purification units (CPUs) designed to produce a supercritical CO₂ effluent for transportation and sequestration. Key questions include what purity of CO₂ is required to ensure safe long term GS. For oxy-coal combustion, specific questions include whether trace levels of sulfuric and nitric acids and other impurities can be compressed and sequestered with the CO₂, and what are the characteristics of the ash and especially the fly ash and volatile metals such as mercury and selenium (Hg, Se) that would normally penetrate the particle control devices as an air emission, but now may be compressed and sequestered with the CO₂. Other questions include the effect of a CO₂ environment on particle charging and electrostatic precipitator (ESP) operation, as well as similar concerns related to the operation of selective catalytic reduction (SCR) units and acid gas scrubbers.

To address some of these environmental issues, the U.S. EPA's National Risk Management Research Laboratory is modifying several in-house combustion facilities to conduct oxy-fuel experiments. This presentation will describe the design and construction of a small bench-scale (5 cm inside diameter by 1.5 m long) externally heated alumina reactor designed to burn natural gas and coal (~1-2 g/min) with mixtures of O₂ and CO₂. Initial experiments will examine one-pass O₂-CO₂ with natural gas, and evolve in the future to include coal and flue gas recirculation. The system is designed to examine realistic combustion stoichiometries and residence times. Measurements will examine the effects of combustion environment (O₂, CO₂, H₂O, fuel N, fuel S, and temperature) on NO_x and SO_x species (doped natural gas) and ash vaporization (coal). Trace metal behavior (including Hg and Se) are of particular interest. Measurements will include particle size distributions, extent of metal vaporization, and metal speciation. Variations in NO, NO₂, N₂O, and SO₂ emissions as well as efficiency (CO and ash carbon) will be examined and compared to air combustion.

This project is funded in part by the NCSU/EPA Cooperative Training Program in Environmental Science Research, Training Agreement CT833235-01-0 with North Carolina State University.