Environmental Engineering Reference
In-Depth Information
2.5.3.6
Summary
During the HTAC99 trial, the high temperature air combustion of coal was examined.
A 580-kW burner was used in IFRF furnace No. 1 in a geometric configuration
similar to the previous HTAC trials (the same distance between central air and fuel
injection). Detailed in- flame measurements were performed in one flame (baseline
flame) and the effects of several parameters on the flue gas emissions were studied.
Analysis of the collected data in the three HTAC trials shows that high temper-
ature air combustion works for NG, LFO, HFO, and coal. The LFO and NG flames
were very similar, and no visible flame was observed. With firing coal and HFO a
visible flame was observed under all the conditions.
All the examined fuels showed similar in-flame gas composition and temperature
fields. The gas composition and temperature tend to be uniform all over the furnace.
The combustion is slow and takes place after the fuel jets have been diluted by the
entrainment of hot combustion products. The in-flame peak temperature is largely
reduced compared with standard combustion techniques with such a highly preheated
air temperature.
The peak in-flame temperature was measured around 1500˚C for NG and coal
(slightly lower for the HFO and LFO). That value is surprisingly low considering
the high preheated level of the comburent between 1300˚C (NG) and 1350˚C (coal).
The temperature rise between air inlet and peak in-flame temperature is as low as
150˚C; in standard combustion technique this value is usually larger than 1000˚C.
The in-flame temperatures are between 1300 and 1500˚C. At this temperature level
the thermal NO
x
mechanism is nearly suppressed.
All along the furnace the measured heat fluxes were flat, high, and in similar
ranges for all the fuels that were tested. The heat flux was slightly lower for LFO.
In particular for NG combustion these heat flux values were surprisingly high.
The particulate concentration in the flue gas for the HFO and so for the coal
flame is probably too high to operate the technique with a regenerative heat
exchanger. As was already concluded in the HTAC98 report, for the HFO the
atomization should be improved to reduce the particulate emissions.
It can be concluded the coal high temperature air combustion is a promising
technique with low emissions and high heat fluxes. The NO
x
emissions were low
compared with standard combustion techniques under similar furnace conditions.
NO
x
emissions were measured in the range 250 to 350 ppm; in standard combustion
techniques a much higher value (on the order of 600 to 700 ppm) may be expected.
Further studies are needed on “air preheating” to apply high temperature air
combustion to coals. The ash deposition in the regenerative part should be investi-
gated or a “different” source of hot air should be tested (as has already been studied
for firing unclean fuels with high preheated temperature
65
).
In high temperature air coal combustion, different conditions have been tested
to determine the influence of the following parameters on the NO
x
and burnout in
the flue gas:
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