(419d) Potential Impacts of Oxy-Combustion Retrofit On Boiler Tube Corrosion Rate

The future use of coal in the United States depends on technologies being made available to capture and store CO2 emissions from power plants. A key candidate CO2 capture technology is oxy-firing of coal. Using oxygen instead of air to combust the coal produces a flue gas stream that it composed primarily of CO2 and water, making subsequent CO2 capture relatively easy. However, application of oxy-firing to existing power plants presents unquantified challenges as the characteristics of oxy-firing compared to air-firing have not been fully determined. This presentation will draw results from an on-going DOE-sponsored program on oxy-combustion impacts in existing coal-fired boilers. The primary objective of the program is to develop tools to characterize and predict impacts of CO2 flue gas recycle and burner feed design on flame characteristics (burnout, NOx, SOx and fine particle emissions, heat transfer), slagging, fouling and corrosion, inherent in the retrofit of existing coal-fired boilers for oxy-coal combustion. Key elements to be characterized include: ? Appropriate coal feed, oxygen feed, and flue gas recycle (FGR) design for oxy-fired burners or systems; ? Impacts of oxy-firing system design on boiler flame characteristics (burnout, emissions, and heat transfer), fouling, slagging and steam tube corrosion relative to the expected behavior in existing air-fired systems. The proposed research will accomplish this using: ? New experimental data from Sandia National Laboratories' 0.1 kW Entrained Flow Reactor, University of Utah's 100 kW OxyFuel Combustor (OFC), and University of Utah's 1.5 MW L1500 coal-fired furnace; ? Validated mechanisms describing oxy-firing processes; ? Simulation of oxy-combustion in a pilot-scale furnace and a full-scale utility boiler retrofit using oxy-firing design principles.

Information presented here will focus on the potential impacts of oxy-combustion retrofit on corrosion of tube materials typical in US utility boilers.

During the last twenty years, the introduction of low NOx firing systems (low NOx burners and over fire air) and efforts to improve thermal efficiency though the use of higher pressure/temperature steam conditions have resulted in increasing concern with fireside corrosion and have led to increasing pressure to improve materials technology. Fireside tube corrosion can occur in multiple locations within a coal-fired boiler and the use of oxygen and flue gas recycle has the potential to affect corrosion behavior in a number of different ways.

A simplistic consideration of oxy-coal combustion indicates a number of possible impacts on corrosion tendencies. Thermodynamics indicate that flame temperature is a function of the overall oxygen concentration in the reactants mixture. Therefore the heat flux and temperature gradients in the tube materials will be tied to the flue gas recycle rate. Each of these factors will impact corrosion rates for one or all of the understood corrosion mechanisms mechanisms. Removal of the nitrogen diluent and the resulting increase in concentration of minor species during flue gas recycle provide another potential means for increasing corrosion rates in a boiler. In addition to the thermal and concentration issues noted, deposit chemistry/stability is known to be negatively impacted by the existence of alternating oxidizing and reducing conditions. The potential existence of larger extremes in oxygen concentration during oxy-firing could further magnify any problems that occur.

In order to characterize these effects, experiments have been performed in the University of Utah's 4 MBtu/hr pilot-scale combustor (L1500). The L1500 is a PC-fired research furnace that was designed to simulate combustion in low emission, pulverized coal-fired boilers. This unit has been used for many investigations of technologies for NOx and particulate control, including: staging, reburning, SNCR and burner development. This furnace has recently been retrofit with a flue gas cleaning and recirculation system along with an O2 and CO2 supply and control system, making oxy-combustion experimentation possible. For our program the L1500 was combusted under air- and oxy fired conditions with three diverse coals, including: Utah bituminous, Illinois and PRB.

During these experiments, the real-time corrosion rate was measured using electrochemical noise (ECN) technology. Four ECN probes were developed to simulate both water wall and superheat tubes. Each of these probes has corrosion elements fabricated from boiler tube materials typical in US boilers, including SA210 for the water wall probe and P91, 347h and T22 for the three super heat probes. The water wall probe was installed in the furnace where the flue gas temperatures were about 2400 °F. The superheat probes were installed at a location with gas temperatures of about 1850 °F. Corrosion data was collected for all three coals under air- and oxy fired conditions. The dependence of corrosion rate on probe surface temperature, local gas stoichiometry and deposition characteristics were all investigated. A summary of the key results from this investigation will be presented.