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 of the oxygen in air burns with components that release heat energy and resulting in the combusted materials. In the construction of fire heater, boilers, combustion engines and combustor the outlet is given out with a cylinder shape chimney so that the hot flue gases are ejected into the atmosphere. In design and calculating the performance of the above equipments the flue gas is used for analysis which helps 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.
Orsat operation in four steps:
- Filling the flue gas
- Expelling excess gas
- Absorption of the gas through contact with the chemical by raising and lowering the levelling bottle
- Finally measuring the percentage of the component by equating the water levels, repeating (3 and 4) with other two absorber chambers.
Orsat apparatus operation representation with single absorber chamber to understand easily. This is performed with other chemical absorption units for same inlet gas.
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 the 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 pyrogallic acid and KOH were selected as a reagent in the Orsat apparatus analyser.
- The 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 the 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 colour change is observed.
- Finally, carbon monoxide is absorbed by ammoniacal cuprous chloride pipette by the reaction,
- 2CuCl + 2CO →(in NH3 solution)→ [CuCl(CO)]2
The average molecular weight of the flue gas can be calculated using the volume or mole percentage composition determined by the Orsat analysis. Let’s take for example the following composition,CO2 30%, CO 0.2%, SO2 2.2%, N2 63.0%, O2 4.6%.
|Components||Mole percentage, %, MP||Mole fraction, MF= MP/100||Molecular weights, MW||MF * MW|
|Average molecular weight||33.776|
Download: Free excel calculation sheet for finding the flue gas average molecular weight
Enter the gas composition in the coloured space below to calculate the gas average molecular weight
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 Orsat apparatus models are re-engineered and modified by electronic and automation technology so that they do multitask 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 specific molecules to pass through it and these molecules will affect the conductivity of the thermocouple type system which gives out signal proportional to the chemical molecules.
Automatic Orsat apparatus
How would be an Orsat apparatus that can operate automatically rather than manual operations? Let see what are devices required for building the apparatus.
- Suction blower
- Orifice for pressure and flow rate measurement to know the volume of the flue gas entering the apparatus
- Conductive meter to know the concentrations of the components
- Pressure and temperature indicator to know the operation conditions
- Tubes with packing to ensure the high gas-liquid contact
- Automatic control valve with timer and solenoid set up operated by the program of PLC