Coal America’s Energy Future phần 2 pdf

Overview of Nitrogen Oxides

Nitrogen oxides are byproducts of the combustion of virtually all fossil fuels. The formation of NOX in the

combustion process is a function of two reactions/sources—thermal NOX originates from the nitrogen found in

the air used for combustion, and fuel NOX originates from organically bound nitrogen found at varied levels in all

coals. Control of NOX emissions is accomplished in PC/CFB units through a combination of in-furnace control of

the combustion process and post-combustion reduction systems.

Combustion NOX Control

Advanced low NOX PC combustion systems, widely used today in utility and industrial boilers, provide dramatic

reductions in NOX emissions in a safe, efficient manner. These systems have been retrofitted to many existing

units and are reducing NOX emissions to levels that in some cases rival the most modern units. The challenges

are considerable, given that the older units were not built with any thought of adding low NOX systems in the

future. Low NOX combustion systems can reduce NOX emissions by up to 80% from uncontrolled levels, with

minimal impact on boiler operation, and they do so while regularly exceeding 99% efficiency in fuel utilization.

Low NOX firing systems are standard equipment on new PC units.

Advanced low NOX systems start with fuel preparation that consistently provides the necessary coal fineness

while providing uniform fuel flow to the multiple burners. Low NOX burners form the centerpiece of the system,

and are designed and arranged to safely initiate combustion and control the process to minimize NOX.

An overfire air (OFA) system supplies the remaining air to complete combustion while minimizing emissions

of NOX and unburned combustibles. Distributed control systems (DCS) manage all aspects of fuel preparation,

air flow measurement and distribution, and flame safety and also monitor emissions. Cutting-edge diagnostic

and control techniques, using neural networks and chaos theory, assist operators in maintaining performance at

peak levels.

For pulverized coal units, uncontrolled NOX emissions from older conventional combustion systems typically

range from 0.4 to 1.6 lb/106 Btu, dependent on the original system designs. Retrofitting of low NOX PC

combustion systems is capable of reducing NOx down to 0.15 to 0.5 lb/106 Btu exiting the combustor; the

performance is highly dependent on the fuel and the ability to modify the existing boiler design. The goal of the

DOE’s low NOX burner program is to develop technologies for existing plants with a NOX emission rate of

0.15 lb/106

Btu by 2007 and 0.10 lb/106

Btu by 2010, while achieving a levelized cost savings of at least 25%

compared to state-of-the-art selective catalytic reduction (SCR) control technology.

New plants which can be designed for optimized reduction of NOX in the firing systems which will achieve

combustor outlet levels at the lower end of this range and designs are in demonstration to drive combustor outlet

NOX levels to 0.1 lb/MMBtu.

Combustion NOX Control Costs

The installed cost of a low NOX combustion system retrofit on a coal-fired unit is in the range of $7 to $15/kW to

achieve NOX reductions of 20 to 70%. Installation of low NOX firing systems is standard procedure on new units,

and the cost is embedded in the firing system cost of the new unit design.

The industry continues to aggressively develop improvements to low NOX burner technology to lessen the NOX

reduction requirements of the post-combustion NOX control equipment (selective catalytic reduction), which can

significantly reduce capital and operating costs.

Post Combustion NOX Control — SCR and SNCR

Advanced PC/CFB plants utilize a combination of combustion and/or post-combustion control for high levels of

NOX reduction. PC plants generally combine low NOX firing with selective catalytic reduction (SCR) to reduce

NOX emissions, while CFB units utilize selective non-catalytic reduction (SNCR).

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