This
paper aims to demonstrate an industrial retrofit of NOx reduction
through use of a new technology, high-temperature selective noncatalytic
reduction (abbr. HTSNCR), in a 50 MWe tangentially firing boiler of
pulverized coal. Over the last several years, the characteristics
of HTSNCR were investigated in the bench-scale experiments, as presenting
the temperature window of SNCR can extend to a higher temperature
range in the absence of oxygen. Therefore, HTSNCR provides a promising
option of greater NOx reduction via injecting urea solution or ammonia
into the primary combustion zone specialized with oxygen of nearly
zero in furnace. These retrofit experiments in this paper successfully
showed the ability of HTSNCR. NOx emission of 68 mg/Nm3 was finally achieved through use of the optimum hybrid application
of HTSNCR, SNCR, and OFA. The overall reduction efficiency is approximately
90%, in which 17% is devoted by HTSNCR. The main factors in HTSNCR
were studied extensively, including (1) primary stoichiometric ratio
(SR1) of air staging, (2) normalized stoichiometric ratio
(NSR) of reagent quantity injected, (3) allocation of injectors, e.g.,
on the corners or at the middle of side walls, (4) ammonia slip brought
about by HTSNCR or SNCR, (5) optimized hybrid configuration about
SNCR and HTSNCR. On the basis of the optimum setting of the above
factors, two key features of HTSNCR employed in a tangentially firing
furnace were obtained. First, there is a critical minimum value of
NOx emission in the relationship of NOx emission versus NSR1 of HTSNCR. More NSR1 beyond the critical value, i.e.,
more reagent quantity injected, results in more NOx formation. Secondarily,
the injection of reagent near the corners is beneficial to reach higher
NOx reduction rather than that injected from the side walls, due to
the aerodynamics in the tangentially firing furnace.
A three-dimensional trial bed is
established for a staged combustion
boiler, and a modeling method based on similarity theory is proposed.
The aerodynamic field of the 35 t/h layer combustion—composite
combustion chamber—in the stoker boiler with staged combustion
was evaluated. Further, a three-dimensional calculation model based
on computational fluid dynamics (CFD) was used to simulate the aerodynamic
field of the reformed boiler under normal operation, which facilitated
convenience in the boiler design. Hot-wire anemometer and other instruments
were used for the characteristic test of damper, a velocity field
test in the furnace, wall wind test, temperature balance test at the
outlet of the furnace, etc., and the law of motion for the flow field
in the furnace was obtained. By analyzing the structure of staged
combustion, the emission of nitrogen oxides and the combustion stability
of a novel layer-fired boiler were studied. The calculated results
are in excellent agreement with the experimental data. The results
revealed that the combustion efficiency of the boiler and the reduction
of nitrogen oxides were significantly improved by the staged combustion
technology. There was no erosion on the water wall, and the flow velocity
at the outlet of the furnace was uniform. This modeling method exhibits
good adaptability to the combustion of stratified combustion boilers
and is potentially useful for optimizing furnaces in a variety of
applications.
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