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SCIENCE CHINA Technological Sciences, Volume 59 , Issue 12 : 1874-1881(2016) https://doi.org/10.1007/s11431-016-0013-5

Waste heat recovery and denitrification of flue gases from gas-fired boilers

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  • ReceivedApr 5, 2016
  • AcceptedJun 30, 2016
  • PublishedNov 16, 2016

Abstract

A waste heat recovery and denitrification system was developed for improving energy conservation and emissions control especially for control of PM2.5 particles and haze. The system uses enhanced heat and mass transfer techniques in a packed heat exchange tower with self-rotation and zero-pressure spraying, low temperature NO oxidation by ozone, and neutralization with an alkali solution. Operating data in a test project gave NOx in the exhaust flue gas of less than 30 mg/Nm3 with an ozone addition rate of 8 kg/h and spray water pH of 7.5–8, an average heat recovery of 3 MW, and an average heat supply of 7.2 MW.


Acknowledgment

This work was supported by the National Basic Research Program of China (Grant No. 2013CB228301).


References

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  • Figure 1

    Schematic of waste heat recovery and denitrification process.

  • Figure 2

    Schematic of the packed heat exchange tower.

  • Figure 3

    NOx concentration variations with ozone addition.

  • Figure 4

    NO oxidation efficiency for various O3 to NO mole ratios.

  • Figure 5

    NOx absorptivity for various NaOH concentrations [21].

  • Figure 6

    Packed heat exchange tower.

  • Figure 7

    Recovered flue gas waste heat and heat pump heat supply.

  • Table 1   Main thermodynamic parameters at full thermal load

    Category

    Value

    Flue gas inlet temperature (°C)

    62

    Flue gas outlet temperature (°C)

    23.4

    Circulating water inlet temperature (°C)

    23.3

    Circulating water outlet temperature (°C)

    43.9

    Heat exchange tower pressure difference (Pa)

    203

  • Table 2   NO concentrations in the exhaust gas for various zone flow rates

    Ozone addition (kg/h)

    NOx concentration (mg/m3)

    0(inlet)

    81.8

    6.1

    37.7

    8.1

    21.2

    13.6

    14.3

  • Table 3   Circulating water and drained water quality with/without ozone addition

    Sample

    Unit

    Value

    Drained water

    (without ozone addition)

    pH

    -

    6.7

    Na

    mg/L

    3.29

    S(SO4-2)

    mg/L

    27.0

    N(NO3-)

    mg/L

    1.25

    N(NO2-)

    mg/L

    0.92

    Circulating water

    (without ozone addition)

    pH

    -

    6.8

    Na

    mg/L

    3.90

    S(SO4-2)

    mg/L

    21.0

    N(NO3-)

    mg/L

    1.25

    N(NO2-)

    mg/L

    0.91

    Drained water

    (with ozone addition rate of 8 kg/h)

    pH

    -

    7.9

    Na

    mg/L

    287

    S(SO4-2)

    mg/L

    47.8

    N(NO3-)

    mg/L

    25.4

    N(NO2-)

    mg/L

    2.54

    Circulating water

    (with ozone addition rate of 8 kg/h)

    pH

    -

    8.0

    Na

    mg/L

    267

    S(SO4-2)

    mg/L

    45.9

    N(NO3-)

    mg/L

    23.2

    N(NO2-)

    mg/L

    3.51

  • Table 4   Chemical compositions of drain water and reclaimed water from boilers

    unit

    value

    Standard

    Chromaticity

    °

    <5

    £30

    Turbidity

    °

    <0.5

    £5

    pH

    -

    8.0

    6.5~8.5

    Hardness

    mg/L

    1.02

    £450

    Suspended solids

    mg/L

    4

    £5

    Five-day biochemical oxygen demand

    mg/L

    1.5

    £10

    Chemical oxygen demand

    mg/L

    5.71

    £60

    Anionic surfactant

    mg/L

    <0.05

    £1000

    N

    mg/L

    0.22

    £10

    Fe

    mg/L

    <0.03

    £0.3

    Mn

    mg/L

    <1.00×10-3

    £0.1

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