Condensate (heating technology)

In heating technology , condensate is understood to be the condensable components of flue gases .

The pH value of condensate is always in the acidic range due to the ingredients it contains. Since domestic wastewater is mostly basic and helps to neutralize the condensates, condensates from small combustion systems can be discharged into the sewerage system in accordance with the respective officially established discharge conditions.

This creates condensable substances

in the furnaces during the drying, degassing and pyrolysis of the fuels before the burning process
when burning
as recombination products from the exhaust gas components

composition

Depending on the type of fuel, combustion temperatures, oxygen content and other factors (for example the geometry of the combustion chamber and the dwell time of the components in it), different substances arise during combustion or this preceding pyrolysis , which can be found dissolved or undissolved in the condensate after condensation:

Water, it comes mainly from the oxidation of hydrogen atoms from the fuel, part of the fuel moisture and supply air moisture ( air humidity ).
condensable volatile organic compounds
- unburned hydrocarbons
- Tar and its components ( e.g. phenols , naphthalene , anthracene , phenanthrene , ...)
- Wood tar and wood vinegar and their components (e.g. methanol , formic acid , acetic acid , phenol , ...)
- When heating biogenic substances in the course of combustion, volatile organic compounds (such as fatty acids , alcohols , terpenes , hydrocarbons, etc.) can also form and condense in the event of outgassing (gasification | gasification) with incomplete combustion .
Carbon dioxide dissolved in water results in carbonic acid , which in gas boilers makes up the majority of the acid in the condensate and only lowers the pH value to 4.3 under normal conditions.
At elevated temperatures, atmospheric oxygen reacts with nitrogen (from the air or the fuel) to form thermal and fuel nitrogen oxides (NO _x ), which in turn can react with water to form nitrous acid , nitric acid , hydrocyanic acid , etc.
Sulfur and sulfur compounds in the fuel react mainly to form sulfur dioxide , which can be converted into sulphurous acid by further reaction with water or to sulfur trioxide , from which sulfuric acid is formed when dissolved in water
Traces of heavy metals dissolved in the condensate due to corrosion in the exhaust gas or pipe system and from fittings , especially copper and zinc from brass fittings

In addition, the condensate contains easily vaporizable substances such as mercury and cadmium and other heavy metals such as chromium , iron , nickel and lead , as well as chlorides , nitrides , nitrates and sulfates as soluble and insoluble dusts.

Also (pyrolysis) recombination products such as tar, hydrocarbons, polycyclic aromatic hydrocarbons and dioxins or soot (mostly as shining soot ), fly ash and flue dust can be detected in the condensate.

The condensate from wood gasifiers and pellet heating systems has a large proportion of unburned resins, tars, fatty acids and other volatile hydrocarbons, which on the one hand pollute the wastewater and on the other hand lead to sticking in the heat exchanger.

Discharge of condensate from condensing boilers

Situation in Germany

The ATV-DVWK-A 251 worksheet is a non-binding recommendation that many sewage network operators use. In a user note at the beginning it is explicitly stated: "By using it, nobody escapes responsibility for their own actions or for the correct application in a specific case [...]." In case of doubt, the regulations of the local sewage network operator or the lower water authority apply .

According to the recommendation of the DWA worksheet ATV-DVWK -A 251, neutralization must take place if

the domestic wastewater is discharged into a small wastewater treatment plant ,
insufficient dilution takes place by mixing the expected amount of condensate with at least 20 times the volume of other wastewater, or
the drainage pipes do not meet the material requirements (acid resistance).

According to DIN 1986-4, pipes made of the following materials have unlimited resistance to acidic condensation: stoneware, glass, polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), styrene copolymers (ABS / ASA), polyester resin (UP) and rustproof Steel.

If diluted by other wastewater, pipes made of fiber cement, cast iron or steel can also be used.

The public sewage systems mainly consist of concrete pipes, which are usually not acid-resistant, so that the condensate must have been sufficiently diluted at the latest by the time it enters the street sewer.

Damage to non-acid-resistant sewage pipes can occur because the pH value of acid condensate from natural gas-operated condensing boilers is between 2.8 and 4.9. Another source names 3.8-5.3, which roughly corresponds to the pH value of common ( acidic ) rainwater.

Because of the lower sulfur content, the amount of acid produced is lower in gas combustion than in heating oil combustion. A neutralization can be dispensed with under certain conditions if the local regulations for discharge into receiving waters allow this.

The pH value of the condensate of condensing boilers operated with low-sulfur heating oil is 2.2 to 4.2 and that of boilers operated with standard heating oil is 1.8 to 3.7 and, according to the recommendation of the ATV, should always pass before discharge a neutralization device can be brought to a pH-neutral value. To achieve this, granulates are usually used in the neutralization devices, for example consisting of natural substances such as limestone , dolomite , marble , magnesium oxide or mixtures of these substances. The condensate from wet chimneys does not have to be neutralized.

When carbonate-containing granulates are reacted with acid, CO _{2 is} released and the atmosphere is released. In contrast, because of the cleaning agents, detergents and soaps used, domestic wastewater has a rather basic pH value and sufficient acid capacity . The acid capacity is a measure of the buffer capacity (pH value stability) of the water in relation to acids and states how much acid can be introduced before a relevant change in pH value occurs. Mixing the acid condensate from gas condensing boilers with domestic sewage in the sewage system would therefore make more sense for environmental reasons than neutralization with limestone supplied especially for this purpose.

literature

ATV rules for waste water and waste, worksheet ATV-A 251, condensates from condensing boilers, GFA - Verlag für Abwasser, Abfall und Wassererschutz, Hennef 1998 and 2003.
Franz Joos: Technical combustion. Combustion technology, combustion modeling, emissions. Springer-Verlag, Berlin / Heidelberg 2006, ISBN 3-540-34333-4 . (available online at Google Books , last accessed in October 2012)

Individual evidence

↑ Karlheinz Ballschmiter, Reiner Bacher: Dioxins, chemistry, analytics, occurrence, environmental behavior, and toxicology of halogenated dibenzo-p-dioxins and dibenzufurans . VCH, Weinheim 1996, ISBN 3-527-28768-X ( limited preview in Google book search).

↑ Hans Hartmann, Paul Roßmann, Heiner Link, Alexander Marks: Testing the condensing technology in domestic woodchip firing with secondary heat exchangers. Technology and support center in the competence center for renewable raw materials and the Bavarian State Office for Environmental Protection, Straubing, 2004, PDF file, 1003 kB, accessed on February 7, 2012

↑ ^a ^b Dipl.-Ing. (FH), BSc Frank Sprenger: Condensation water from boilers and its neutralization , special edition Buderus Heiztechnik

↑ Thanner, Gerhard / Moche, Wolfgang: Emission of Dioxins, PCBs and PAHs from Small Firings , Austrian Federal Environment Agency, Monographs Volume 153, Vienna, 2002, PDF file accessed on February 7, 2012.

↑ Federal Association of the Schornsteigerfegerhandwerk: Pellets have difficulties with calorific value. ( Memento of August 1, 2012 in the Internet Archive ) PDF file, accessed on February 6, 2012.

↑ Worksheet ATV-DWA-A 251 , p. 2.

↑ DVGW (Ed.): ATV A 251 . 2011.

↑ SHKwissen: neutralization facility .

↑ Worksheet ATV-DWA-A 251 In: 4.1.1 , 2003, p. 9.

↑ Worksheet ATV-DWA-A 251 In: Table 4 , 2003, p. 13

↑ ^a ^b Worksheet ATV-DWA-A 251 In: Table B.1 , 2003, p. 18

↑ Schlapmann: Condensation from condensing boilers , PDF file, accessed on February 6, 2012.