This research program has two primary objectives: 1) to obtain fundamental rate and fuel nitrogen conversion information for oxycoal-fired circulating fluidized bed (CFB) boilers; and 2) to obtain model validation data from pilot-scale operation of an oxycoal-fired CFB.

Fundamental Studies

The fundamental studies are being carried out utilizing both bench-scale experiments and computational chemistry techniques. Previous studies have shown that recirculated CO2 not only reduces flame temperature, but also has a chemical effect that is reflected by increasing the CO formation that modifies oxidative routes, especially the formation and reduction of NO [1]. Another study [2] explored the effect of CO2 on NO emissions, using O2/CO2 mixtures and recirculated flue gas, as compared to conventional air combustion, and found reduced NO levels for the O2/CO2 mixtures. Although this reduction could be attributed to the reduction of Thermal NO (by removal of N2 as well as the use of flue gas recycle for temperature control), it has also been observed [3] that combustion in O2/CO2 mixtures reduces the NOx emission rate from fuel-N, which indicates the potential chemical effect that CO2 may have in NOx formation/reduction chemistry. In particular, the CO2 effect on the heterogeneous reactions that may lead to NO reduction has not yet been investigated. The effect of high CO2 and O2 levels on heterogeneous formation and destruction of NO is the focus of the fundamental studies.

For the experimental work, a single-particle, fluidized bed reactor (SFBR) has been constructed for use in studying char oxidation rates and fuel nitrogen conversion as a function of temperature, PO2 and PCO2. Studies are being carried out with model chars, as well as coal and coal-derived chars. Model chars for use in the char-N conversion studies are created from polyacrylonitrile, with subsequent characterization of the N functionalities in the char by XPS.

The computational work is focused on the interactions of high levels of CO2 found in oxycoal combustion, with fuel nitrogen present in the coal. The calculations are being performed using the Gaussian 03 package, using several different N functionalities in the char, as well as various representative gaseous environments.

Pilot-Scale Studies

Pilot-scale (300 kW) oxycoal CFB data is being obtained for use in validation of the Multiphase Flow with Interphase eXchanges (MFIX) computer model being developed by U.S. DOE/NETL. Measurements include: pressure, temperature and bed voidage mapping, and as well as gas composition and velocity mapping.

This presentation will provide an overview of the fundamental and pilot-scale studies, along with progress and results achieved to date.

REFERENCES

1. Park J., et al., Energy & Fuels, 2007. 21: p. 121-129.

2. Croiset E. and Thambimuthu K.V., Fuel, 2001. 80: p. 2117-2121.

3. Hu Y., et al., Fuel, 2000. 79: p. 1925-1932.