Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 159

1. Coal jet velocity
2. Transport air mass flow
3. Gun position

Burnout problems were found only with a very low coal jet momentum and low
transport air mass flow. Under these conditions the flame was not stable. In all the
other conditions no large differences in the burnout level were noticed.
As expected, an essential condition to achieve low NO x emissions was oxygen
availability in the primary combustion zone. The fuel injection too close to the central
air (fast mixing with the combustion air) and the high coal jet momentum (more
lifted flame) both determine an oxygen-rich primary combustion zone that leads to
high NO x emissions. The lowest NO x emissions were found for the coal gun posi-
tioned farther from the central air both for low- and high-velocity coal injections.
These results are related to the specific coal used in the trial. The coal charac-
teristics are thought to be important for the technique especially the ignition behavior
of the coal. The coal type effects on the above-discussed results should be further
investigated.

2.5.4 C OMBUSTION R ATE OF S OLID C ARBON

With the clarification of the combustion mechanism of gaseous fuels and liquid fuels
to a certain extent, systematic studies on energy saving and low-pollution gas
emission in connection with high temperature air combustion have been conducted.
However, when it comes to solid fuels such as coal, which is used by certain
industries, there have been few basic studies or research on the effects of high
temperature combustion air, partly because the properties of coal differ substantially
depending on the regions where the coal is produced.
This section discusses features of solid carbon (graphite) combustion, placing
emphasis on the combustion of dry-distilled coal, which is the most typical fuel in
coal combustion systems, and clarifies the influence of high temperature air com-
bustion techniques on the combustion mechanism. The following sections present
the results of experiment and analysis.

2.5.4.1 Combustion Field and Solid Carbon Specimens

The high temperature air generator, manufactured by NFK (see Foreword) and shown
in Figure 2.127 , was used for the combustion test in this study. This alternate
changeover combustion type equipment can continuously generate high temperature
airflow on the order of 1280 K for a duration of about 3 min by accumulating heat
in a ceramic honeycomb (regenerative media, in the figure) and subsequently

exchanging the heat with the airflow. The high temperature airflow thus generated
is discharged into the atmosphere through a nozzle (

in the figure, having a diameter
of 12 mm) at a uniform flow rate (max 50 m/s) and a uniform temperature.
Two-dimensional stagnation flow formed with the uniform airflow mentioned
above containing a solid carbon (graphite) was used as the combustion field ( Figure
2.128 ). The flow field in the front stagnation region is uniquely defined by the use