Flue gases are the outlet gases obtained when a fuel is burned with air in combustion equipments like engines and boilers. Based on the fuel composition flue gases are formed, a fuel having carbon and hydrogen compounds generates flue gas containing oxides of carbon and hydrogen because the oxygen in air burns with components that release heat energy and resulting the combusted materials. In construction of fire heater, boilers, combustion engines and combuster the outlet is given out with a cylinder shape chimney so that the hot flue gases are eject into the atmosphere. In design and calculating the performance of the above equipments the flue gas is used for analysis which help to determine the composition of the carbon dioxide, oxygen, nitrogen and carbon monoxide. Based on this analysis results the fuel inlet flow rate, air flow rate and burner are controlled to get the efficiency of the fired heaters. By the following presentation the flue gas analysis can be demonstrated by using Orsat apparatus, of course this is a standard piece of equipment. Many high accurate digital and portable devices are available to determine the composition of flue gas.
Modern analyzers derived from Orsat apparatus
Based on the Orsat analyzing technique not only flue gases but process gas like methane, propane and natural gas can be analyzed continuously in a process flow streams. Many new models are reengineered and modified by electronic and automation technology so that they do multi task in a process industry. They measure, control and disengage the process systems. The demand for composition measurement in continuous flow pipeline even at high temperature and pressures fulfilled by latest engineered models that are nitrogen analyzer, oxygen analyzer, carbon dioxide analyzer, methane analyzer.
These modules contain a measuring probe, a signal transmitter and receiver, instrument air sockets, display and controls box. Measuring probes are developed based on the membrane technology, a thin film membrane allow specified molecules to pass through it and these molecules will affect the conductivity of the thermocouple type system which give out signal proportional to the chemical molecules.
Governing chemical absorption reactions in Orsat apparatus:
Typical flue gas analyzers measure the quantity of carbon dioxide, carbon monoxide and oxygen by a chemical absorption principle. Based on the absorption factor of these three components their respective absorbing solutions are selected in three different bulb compartments. When the gas is passed into these bulbs consecutively, where each component is separated in sequence that helps to know the volume drop from initial flue gas volume.
Water vapor in flue gas removed by adsorption on solid calcium chloride and then passed into three pipettes. The sequence of absorption should always be started from CO2, O2 and CO if the absorption solutions are ammoniacal cuprous chloride; alkaline pyrogalic acid and KOH were selected as reagent in the analyser.
- Gas then passed into KOH (potassium hydroxide) solution pipette to absorb CO2 to form potassium carbonate by the reaction 2KOH + CO2 ↔ K2CO3+ H2O at ambient conditions.
- After confirming no change in the volume of reservoir than gas led to alkaline pyrogallic acid containing pipette to absorb oxygen by the reaction:
- 2C6H3(OH)3(pyrogallol) + 2KOH(saturated alkaline)+ O2 ↔ 4H2O + 2C5H3OCOOK and a physical color change is observed.
- Finally carbon monoxide is absorbed by ammoniacal cuprous chloride pipette by the reaction,
- 2CuCl + 2CO →(in NH3 solution)→ [CuCl(CO)]2