MODULE IV
Condensing boiler technology Page 30

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11. Transport and neutralisation of the condensate

According to the "general waste water regulations" only waste water/sewage with a pH-value between 6,5 and 8,5 can be introduced into the sewage system.
With gas based condensing boiler systems the pH-value is between approximately 3,8 and 5,4. This range means that the condensate of the condensing process is an acid and so can not be introduced into the sewage system without further treatment/processing.
In regard to its introduction also its composition and its quantity are important .

Depending on the origin of the natural gas its composition can differ. The following applies for natural gas E (former natural gas H):

100% Natural gas E = 93% CH4 + 4,9% CnHm + 1,1% N2 + 1% CO2


For natural gas LL (former natural gas L) the following applies:

100% Natural gas LL = 81,8% CH4 + 3,4% CnHm + 14% N2 + 0,8% CO2


In order to ensure a complete combustion, more air has to be supplied to the combustion process than theoretically would be necessary. A complete combustion can be recognised by a bluish colouring of the flame. An air-to-fuel ratio ranging from 1,25 to 1,30 has proved to yield good results for natural gas.
In the case of complete combustion the following chemical reaction is taking place in the natural gas flame:

CH4 + 2O2 -> CO2 + 2H2O
Methane + Oxygen -> Carbon dioxide + Water steam + Heat


The steam that is created during the combustion process condenses and is transmitting the heat energy it contains to the heating water. The resulting condensate has to be removed. The obtained condensate quantity depends on the condensing boiler design and the operating conditions. Theoretically, the combustion of 1 m of natural gas is resulting in 1,6 litres of condensate with a pH-value of 4,0 and the combustion of 1 litre of fuel oil (EL) is resulting in 0,9 litre of condensate with a pH-value of 2,0. Experience has shown however, that somewhat smaller quantities are resulting in practice.
So, one can say that as a rule of thumb approximately 1 litre of condensate is resulting from the combustion of 1 m of natural gas (annual average). That means that e.g. with a natural gas consumption of 2500m3/year a condensate quantity of 2500l/year or 7l/day is obtained. This corresponds to only 1%-2% of the total yearly and/or daily sewage quantity of a household
With regard to the composition of the condensate, measurements have proved that the percentage of elements like lead, chrome or nickel is well below the limits of the drinking water regulations (see table below).

Substances/elements - water quality Limit values of the drinking water regulation Condensate of a condensing boiler
mg/l
g/l
mg/l
g/l
Lead
0,04
0,00004
< 0,002
< 0,000002
Chrome
0,05
0,00005
< 0,002
< 0,000002
Nickel
0,25
0,00025
< 0,002
< 0,000002
pH-value
6,9-9,5
3,8-5,4

 

 

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