Determination of Air Demand, Specific Gravitay and Calorific Value - Application
RHADOX(TM) Analysers - The Measuring Procedure
Product sheet for the RHADOX systems
The Measunring Procedure for RHADOX systems
Applications with the RHADOX-systems
Content
Determination of Air Demand, Wobbe-Index and Calorific Value
The factory application
Technical data
Determination of Air Demand, Wobbe-Index and Calorific Value
The measuring procedure for the determinatin of Air Demand and Wobbe-Index of a sample gas is based on the measurement of the Air Demand, if the gas is completely oxidised in a catalytic reactor with air in surplus. Hence the residual Oxygen is a reliable measure for the Air Demand of the oxidised gas, measured with a circonia probe.
Emphasising the particular gas components with their vol-% in the gas mixture concerned, the stoichiometric Air Demand of a gas mixture can be obtained:
| 1 m3 H2 demands 0,5 m3 O2 and thus appr. 2,4 m3 air 1 m3 CO demands 0,5 m3 O2and thus appr. 2,4 m3 air 1 m3 CH4 demands 2,0 m3 O2 and thus appr. 9,6 m3 air 1 m3 C3H8 demands 5,0 m3 O2 and thus appr. 23,9 m3 air |
Additionally one has to taken into account that Hydrogen flows much faster through the burner jets due to its considerably lower density thus demanding a higher air supply for the complete and reliable combustion. This fact leads to the so-called ´Corrected Air Requirement Index´ (CARI).
Accordingly gases with a lower density carry a higher energy load to the burner compared to those with higher density and equal energy load.
Hence the energy flow to the burner represents the Wobbe-Index i.e. the ´Density Corrected Calorific Value` of the gas mixture.
The measuring procedure of the RHADOX reduces the determination of the Corrected Air Requirement Index (CARI) and of the Wobbe-Index to a single measurement, i.e. to the residual Oxygen concentration.
To apply these physical and chemical correlations for the precise and reliable determination of Air Demand and Calorific Value some important requirements for the measuring system have to be met. The parameters pressure, temperature, density and internal friction of the sample gas must be kept constant within narrow margins. The RHADOX design lives up to these pre-conditions through specific mechanical and electronic components:
- a specific mixing chamber for the sample gas and the oxidation air
- a heat exchanger to equalise the temperature of sample gas and oxidation air as well as
- an electronic control unit to equalise the pressure of sample gas and oxidation air.
The density measurement
The determination of the sample gas density relative to air, i.e. the Specific Gravity, is based on Bernoulli´s law (1730). This law describes the dependence of a laminar gas flow through a restriction from its dimensions.
Since in the RHADOX system sample gas and oxidation air temperature and pressure are already equalised, a second mixing chamber is used to determine this proportion of the sample gas flow and oxidation air flow. Engaging this straightforward and precise procedure the gas density can be aquired.
In practice either the dilution of a major component of the sample gas by air or (as in most cases) the dilution of the Oxygen concentration of the air by an addtitional sample gas flow is aquired regarding the measuring instrument´s stiff design and tightly controlled parameters.
A reference measurement compensates the influence of the ambient air pressure, of the temperature as well as of minor local fluctuations of the Oxygen content in the air.
The factory application
Three different stages should be taken into account:
feasibility test
defining the instrument´s parameter
determining the calibration gases
At first the correlation between the Wobbe-Index WI and the Corrected Air Demand Icorr. WI = f (Icorr.)) must be acquired.
In the next step the essential instrument parameters for the process application of the RHADOX analysing system will be defined.
For the RHADOX 3000 system additionally the parameters for the deternination of the Specific Gravity hence the Calorific Value, must be determined.
Based on well known data subsequently the calibration gases are produced employing a computer simulation program.
(C) AMS GmbH 8.0/2007 Subject to technical modifications. Text, drawings and layout: Analytic Journal
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