Low cost low nOx

PEi November 2000

As deregulation continues to take hold in the USA, existing coal-fired power plants continue to enjoy a favourable role in baseload generation. This is due to their low cost of electricity resulting from low fuel costs, high availability, low remaining capital depreciation, and high capacity factor.

With the USA facing much tighter air quality standards in the years to come, the Department of Energy (DOE) has implemented a scheme to reduce nitrogen oxide (NOx) emissions through the development of new, low-cost technologies.

The proposed system will improve upon the existing NOx reduction technologies through advances that will address present constraints for achieving lower NOx emissions. The combination of improvements in both components and processes are based on fundamentally sound principles known to lower NOx formation and\or improve NOx destruction, while minimizing the impact on the balance of plant. These improvements are:

* Milling system enhancements, both to the mill internals and coal particle size classification processes, i.e to produce a finer coal product and to provide more rigorous control over the particle size. This will allow more rapid heating of the coal particles in the burner near field, resulting in greater fuel bound nitrogen release, and hence lower NOx.

* Low oxidizing pyrolysis burners will be designed to promote a higher fuel nitrogen release through more rapid heating of coal particles. Operated with the fuel rich fraction of the transport air and coal mixture, these burners will minimize the formation of near field NOx.

* Additional near burner turbulence will be generated to create a more uniform, high intensity, fuel-rich zone. Effective mixing of the combustion products in the fuel-rich lower furnace will result in greater NOx destruction.

* In order to reduce the carbon content of fly ash, a bubbling bed carbon burn-out combustor may be used. It oxidizes the carbon to acceptable levels while also returning energy to the boiler.

* An intelligent control system incorporating advanced neutral network features will be employed to maintain the target NOx emissions by controlling both local and global stoichiometrics over the range of boiler operating loads.

Collectively these advanced control components are expected to help optimize system performance across a range of input parameter variations, providing for minimum NOx operation while simultaneously maintaining desired steam temperatures, carbon in ash levels and optimized plant heat rate.

Ответьте на вопросы

1. How do the coal-fired plants damage the environment?

2. Nevertheless, we don’t refuse from such polluting power generating facilities. Why?

3. How can nitrogen oxide emissions be reduced?