Environmental Engineering Reference
In-Depth Information
honeycombs. Each flow path also acts as a flow path for discharging high temperature
burned gases. The high temperature burned gases generated by the one burner are
introduced into the flow path of the other side, and the sensible heat of the burned
gas is stored in the ceramic honeycomb for a while. Then, the operation of the burner
is switched to the other burner, and combustion air is introduced through the ceramic
honeycomb by reversing the flow direction. Preheated air at a temperature of about
1273 K is easily obtained and used for combustion. The high-cycle operation,
typically 30 to 60 s, is adopted to reduce the heat loss escaping with the waste gas.
The principal merit of heat-recirculating combustion is fuel saving, which is
achieved by efficient recovery of waste heat in exhaust gases. Higher preheating
temperature assures less rejection of heat with the exhaust, which results in more
fuel saving. Therefore, heat-recirculating combustion is surely an attractive technol-
ogy for future design of any industrial furnaces as far as energy conservation and
pollution reduction are concerned. However, it was believed that diffusion flames
inevitably harbor near-stoichiometric flame temperature somewhere within their
structure, borrowing the words of Weinberg, 9 which tends to generate increased
levels of nitric oxides even in great excess air ratio. If stricter air quality regulations,
particularly regarding nitric oxides, are applied to furnaces, reduction of nitric oxides
in non-premixed combustion is the first issue to be solved for the future utilization
of heat-recirculating combustion in a variety of furnaces.
Regarding emissions from combustion systems, it has been generally held that
the emission of nitric oxides increases with the temperature rise of combustion air
when preheated air is used. Therefore, any practical trade-off between thermal
efficiency and emission control has always been a critical issue for designers and
engineers. Numerous efforts to strike a balance of furnace between fuel saving and
reduction of nitric oxides emission have been made during the last decade. In
practice, direct injection of fuel into a furnace, 4,11 high momentum ejection of staging
air, 12,13 and mixing control were found to be effective to some extent in reducing
nitric oxide emissions in regenerative combustion. Therefore, the practical extent of
heat recirculation in industrial furnaces has been specified taking account of the
trade-off between energy conservation and tolerance of materials or air quality
regulations.
In the process of developing a high-cycle regenerative furnace, extremely low
nitric oxide emission was reported from an experimental furnace operated with high
temperature combustion air of 1400 K. 5 Because it was difficult to interpret the
results, based on existing knowledge of nitric oxide formation mechanisms, this
motivated an extensive, collaborative study of high temperature air combustion
between industry and academia. Eventually, practical developments and applications
of the concept in industry have achieved great progress in energy saving as well as
reduction of nitric oxide emission. The details of the technology are explained
throughout this topic. The basic concept is the combination of maximum waste heat
recovery by high-cycle regenerator and controlled mixing of highly preheated com-
bustion air with burned gases to yield relatively low temperature flames.
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