Gas-side fouling, erosion and corrosionof heat exchanger for middle and low temperature flue gas waste heatrecovery
2016
With the ongoing emphasis on the efficient use of energy, much
effort is put on the development and application of various heat recovery
technologies. Heat exchangers play an important role in energy transfer
and utilization and are widely used to recover waste energy from different
kinds of heat sources in many fields such as energy and power engineering,
petroleum and chemical engineering, air conditioning and refrigeration,
aviation and aerospace, etc. In the heat exchangers for waste heat
recovery, the gas-side fluid mediums usually are unclean and corrosive.
The problems of fouling, erosion and corrosion of heat exchangers
in the middle and low temperature flue gas-side caused by the high
ash content, viscous and corrosive components are inevitable, and
how to effectively solve the problems has been the subject of much
research in recent years. In this paper, the researches of fouling,
erosion and corrosion characteristics in middle and low temperature
flue gas heat exchangers are reviewed. The causes and influence factors
are discussed. Firstly, the fouling mechanisms of heat exchangers
are introduced and the existing theoretical models, empirical models,
and numerical methods of particle transport, impact and deposition
are reviewed. The ash fouling characteristics of 6-row tube heat exchangers
are presented and the effects of flue gas velocity, particle diameter,
spanwise tube pitch, longitudinal tube pitch, tube shape and geometry
conditions on fouling rate are discussed. Both decrease of the longitudinal
tube spacing and increase of spanwise spacing can effectively reduce
the deposition of particles. The geometry of elliptical tubes can
fulfill this requirement for fouling reduction with smaller longitudinal
spacing and larger spanswise pitch. The feasibility of using oval
tube is explored for the fouling reduction. Compared with the circular
tube arrangement, the deposition rate of the elliptical tube was decreased
by 68.8%. Secondly, the existing erosion mechanisms, theories and
methods are introduced. Different erosion models, including the model
advantages and disadvantages are also summarized. The dynamics behaviour
of the entrained solid particles in the flow and the deformation of
the tube surface due to erosion are presented. The effects of particle
diameter, particle mass flow rate and fluid Reynolds number are discussed.
Among the three parameters, fluid Reynolds number has the most important
effect on the erosion of tube surface. Erosion mainly occurs at the
first part of the single tube surface. Thirdly, the existing mechanisms,
theories and methods of corrosion characteristics of heat exchangers
are introduced. Then a coupling numerical model, developed in author’s
previous research, is discussed to predict the condensation characteristics
of sulfuric acid vapor on heat exchanger surfaces. The corrosion risk
can be reduced by decreasing the water vapor concentration and increasing
the flue gas temperature. The effects of operating parameters and
geometry conditions on the sulfuric acid condensation and corrosion
characteristics are also discussed. A correlation of Sh number of sulfuric acid versus fin geometries for a 10-row tube
bundle is provided. In order to reveal the anti-corrosion characteristics
of different materials and the coupling mechanisms between the fouling
and dewpoint corrosion, the online water-cooled test system and laboratory
static sample test system on the corrosion characteristics of heat
exchangers are introduced. Some dewpoint corrosion resistant steels
are presented. The mechanism of dewpoint corrosion is revealed, the
relationship between the thickness of corrosion layer and wall temperature
is discussed and the coupling mechanisms between the fouling and dewpoint
corrosion are also discussed. The corrosion products are composed
of the ash and acid reaction products in the outer layer, iron sulfate
in the middle layer, and iron oxide in the inner layer. The innermost
layer is the main corrosion layer. Finally, the research needs and
prospect of fouling, erosion and corrosion research are discussed.
Hope to solve the problems of designing the anti-fouling, anti-erosion
and anti-corrosion heat exchangers, and promote the development of
efficient utilization technologies in middle and low temperature flue
gas heat exchangers.
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