ВУЗ:
Составители:
Рубрика:
50
products can be evaluated from the fuel chemistry. For methane (CH4) the
complete volumetric analysis would be
methane + air —>carbon dioxide + water vapour + nitrogen
The chemical symbols for these are as follows: oxygen, O2; nitrogen,
N2; carbon dioxide, CO2; water vapour, H2O. Therefore (after complete
combustion) we have
CH
4
+ 20
2
+ N
2
--> C0
2
+ 2H
2
0 + N
2
All measurements are at the same temperature and pressure. It is assumed
that the water vapour is not condensed.
Some condensation is inevitable, however, and when sulphur (S) is pre-
sent in the fuel, it combines with some of the O2 to form sulphur dioxide
(SO2). If the gaseous SO2 comes into contact with condensing water vapour
and further O2, weak sulphuric acid (H2SO4) may be formed in the flue. Co-
agulation of liquid H2SO4 and carbon particles from chimney surfaces leads to
the discharge of acid smuts into the atmosphere, causing local damage to wash-
ing, cars and stonework. Acidic corrosion of the boiler and chimney greatly re-
duce their service period. The flue gas temperature is kept above the acid dew-
point of about 50 °C to avoid such problems.
It can be seen from the methane combustion equation that 2 m
3
of O2 are
required to burn 1 m
3
of CH
4
completely. This O2 is contained in 2/0.21 = 9.52
m
3
of air, and this air contains 9.52 — 2 = 7.52 m
3
N2.
In order to ensure complete combustion under all operating conditions
and to allow for deterioration of boiler efficiency between servicing, excess air
is admitted. This ranges from 30% for a domestic pressure jet oil burner down
to a few per cent in power station boilers where continuous monitoring and
close control are essential. Excess O2 from the excess air appears in the flue
gas analyses. Measurement of O2 and CO2 levels reveals the quantity of excess air.
The presence of carbon monoxide (CO) in the flue gas indicates that
some of the carbon in the fuel has not been completely burnt into CO2 and that
more combustion air is needed. The theoretically correct air-to-fuel ratio is the
stoichiometric ratio.
The CO2 content of oil-fired boiler plant flues will be about 12% at 30%
excess air, the combustion air volume required per kilogram of fuel burnt will
Страницы
- « первая
- ‹ предыдущая
- …
- 48
- 49
- 50
- 51
- 52
- …
- следующая ›
- последняя »
