Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2018-02-22.
The oxygen concentration in flue gas is a direct indicator of how well a combustion process is running, in a power plant boiler as much as in a waste incinerator. It affects fuel economy and it affects emissions. Ideal combustion produces only CO2 and water; when the air supply is short, unburnt fuel remains, which wastes money and produces carbon monoxide and hydrocarbons. Plants therefore run with a slight excess of oxygen, enough to show that the furnace has sufficient air for complete combustion. The traditional way to measure it is a zirconia cell. Zirconia is a stable ceramic, ZrO2, with one side exposed to a known oxygen concentration — normally ambient air — and the other to the flue gas. The difference produces an electrochemical voltage that depends on the concentration difference, and the voltage is converted into an oxygen reading. In large plants the real difficulty is the probe, which must survive high temperature, dust and corrosive gas; material selection, dust deflection and purging decide whether it lasts. There is also an optical route: absorption spectroscopy with a laser diode. As in DOAS, the detector reads the fingerprint of oxygen molecules along a path, and the absorption gives the concentration — often together with a very accurate temperature. Such monitors are non-intrusive, which suits reactive flue gases and very high temperatures where zirconia cells would not last long.
In China oxygen is one of the mandatory flue-gas parameters of a CEMS, covered by the HJ 75 and HJ 76 series, and it matters for a reason that is easy to forget: pollutant concentrations are compared with emission limits only after being referenced to a standard oxygen level — 6% for coal-fired power boilers, 3% for oil- and gas-fired boilers, 11% for municipal solid waste incineration, with the current standard text deciding the exact value. Without that correction, dilution air alone would lower a measured concentration. In practice, zirconia probes suit boilers, kilns and furnaces: fast, simple and economic, provided purging and checks are taken seriously. For waste and hazardous-waste incineration, high-chlorine or high-fluorine duties, or very high flue-gas temperatures, the non-contact optical method is the safer choice. Two field points. Oxygen stratifies: readings at the furnace outlet, after the economiser and before the scrubber are not the same, so probe location and cross-checks often influence the conclusion more than the instrument does. And standards are usually written on a dry basis while the analyser measures wet gas — correct humidity and oxygen together, or the numbers will not stand up.
Operating Principle of an Oxygen Monitor — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2018-02-22. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.